For rats (Carli et al., 1983) (Figure 1) the requirement during the 5-CSRT task performance is to sustain spatial attention divided among five locations to detect a brief visual stimulus over a large number of trials. Performance is characterized in terms of accuracy of visual discrimination, omissions, speed of responding and by different aspects of executive control such as premature and perseverative responses (see Robbins, 2002 for a detailed description and discussion of these performance measures). The main measure of the selective spatial attention in the 5-CSRT task is accuracy of visual discrimination. Correct responses are rewarded by a food pellet while incorrect responses or failure to respond within the allotted time (omission) result in few seconds of darkness (time out period). Accuracy is independent of omissions and it is relatively impervious to potential confounds such as changes in motor activity or motivation (see Robbins, 2002). Premature responses that occur before the onset of visual stimulus may arise as a consequence of animal not being able to wait for a reward related cue. These “impulsive” responses measure an aspect of response inhibition that is related to response selection but also to action restraint during waiting and could be considered a type of motor impulsivity (Evenden, 1999; Dalley et al., 2011). Nose poke responses after the correct target detection has been performed are defined as perseverative responses and are considered an indicator of “compulsivity.” Perseverative responses constitute persistence in an initially rewarded behavior such as nose poke (even though is no longer rewarded) and may be regarded as inability to alter behavior in reaction to changing task demands thus representing a measure of behavioral flexibility. Premature and perseverative response result in time out. Responses during time out are not reported usually even if they may constitute an additional parameter reflecting compulsivity (Amitai and Markou, 2010). Finally, a measure of response latency (i.e., mean latency to make a correct response) likely reflects decision time as long as changes in motivation and motor status are ruled out.

Numerous studies in rodents show that acute or repeated administration of NMDA antagonists such as PCP, dizocilpine and ketamine consistently lead to disinhibition of the firing of pyramidal neurons (Jackson et al., 2004) most probably by decreasing the activity of GABA interneurons (Homayoun and Moghaddam, 2007) whose response depends on the firing pattern of pyramidal cells (Thomson, 2000; Shi and Zhang, 2003). NMDA receptor antagonists increase glutamate, 5-HT and NE release in the PFC (Moghaddam et al., 1997; Moghaddam and Adams, 1998; Abekawa et al., 2006; Lena et al., 2007; Lopez-Gil et al., 2007). Systemic PCP and dizocilpine also reduce extracellular GABA in the mPFC (Yonezawa et al., 1998) and there is evidence that glutamate release is inhibited by GABA (Pende et al., 1993; Bonanno et al., 1997; Perkinton and Sihra, 1998). Similarly, intra-mPFC infusion of R-CPP to conscious rats increased glutamate efflux within this brain area (Ceglia et al., 2004; Abekawa et al., 2006; Calcagno et al., 2006, 2009) and lowered GABA levels (Calcagno et al., 2009; Agnoli et al., 2013). The glutamate increase elicited by R-CPP is suppressed by TTX added to the medium perfusing the microdialysis probe suggesting that neuronal activity is required. Thus, the effect of R-CPP on extracellular glutamate may be mediated by direct or indirect suppression of cortical GABAergic transmission, which in turn enhances the release of glutamate. Employing dual-probe microdialysis technique we confirmed and extended these finding showing that R-CPP infused in the mPFC raised also extracellular levels of cortical DA whereas in the dm-STR extracellular levels of GABA were increased together with those of glutamate and DA (Agnoli et al., 2013). These data are summarized in Table 2.

The activation of glutamate neurotransmission in the mPFC and increased firing activity of pyramidal-projecting neurons may drive the increase in endogenous glutamate release in the dm-STR, which in turn may increase GABA and DA release. The NMDA/GABA interaction, regulate DA levels in the PFC and striatum (Balla et al., 2009). Reducing GABA transmission in the mPFC with GABA_A receptor antagonist bicuculline or infusion of glutamate increases DA release in the dorsal-STR and these effects are abolished by intracortical infusion of dizocilpine or the GABA_A agonist muscimol (Matsumoto et al., 2003). It is conceivable that glutamate by activating NMDA receptors on striatal medium spiny GABA neurons or interneurons facilitates GABA release (Morari et al., 1993, 1994, 1996; Young and Bradford, 1993).

Systemic administration of NMDA receptor antagonists such as ketamine and PCP had no significant effect on extracellular glutamate in the striatum (Lillrank et al., 1994; Moghaddam et al., 1997), and caused no changes or increased basal but inhibited K⁺ evoked GABA release (Lillrank et al., 1994; Hondo et al., 1995). These findings suggest that different NMDA receptor antagonists may have different effects on extracellular glutamate and GABA depending on the route of administration and brain region considered.

These findings in rats are paralleled by data from functional magnetic resonance imaging (fMRI) in human subjects showing that NMDA receptor antagonist such as ketamine at doses that cause specific behavioral impairment in the executive component of a working memory task (Honey et al., 2003), increases BOLD response in a brain system comprising frontal cortex, parietal cortex, putamen, and caudate nucleus (Honey et al., 2004) and increases glutamine, a putative marker of glutamate release (Rowland et al., 2005). Some more recent studies assessing PFC activation and global connectivity within a working memory network during rest or during task performance have reported an increased or decreased ketamine-associated activation, respectively (Driesen et al., 2013a,b).

The functions of 5-HT are afforded by the concerted actions of multiple 5-HT receptor subtypes and as shown repeatedly 5-HT through its receptor subtypes exert diverse, often antagonistic actions on the same behavioral response. Several lesion and pharmacological studies have attempted to define the role of 5-HT and its various receptors in different aspects of 5-CSRT task (for a review of these studies see Robbins, 2002).

The mPFC receives extensive 5-HT innervation from the dorsal (DR) and median (MR) raphè nuclei and contains several 5-HT receptors, with particular abundance of 5HT_1A and 5-HT_2A and 5-HT_2C subtypes (Azmitia and Segal, 1978; Steinbusch, 1984; Blue et al., 1988; Jakab and Goldman-Rakic, 1998, 2000; Barnes and Sharp, 1999; Clemett et al., 2000; Pandey et al., 2006). In the PFC the 5-HT1A and 5-HT2A receptors are expressed throughout cortical regions with a greater proportion of expression on pyramidal rather than GABA interneurons (Santana et al., 2004). The 5-HT_2C receptors are mainly expressed on pyramidal neurons (Clemett et al., 2000; Puig et al., 2010) and not in fast-spiking interneurons (Puig et al., 2010) but another immunohistochemical study using a different antibody shows more than 50% of the 5-HT_2C receptors on GABA neurons (Liu et al., 2007). These 5-HT receptors have been extensively characterized in terms of their localization to pyramidal and GABA interneurons as well as biochemically and electrophysiologically and a detailed review of their impact on cortical neuron activity can be found in Celada et al. (2013).

Stimulation of 5-HT_1A receptors by 8-OH-DPAT inhibits NMDA-mediated synaptic excitation in the rat visual cortex (Edagawa et al., 1998) and suppresses glutamate signaling in the PFC by reducing NMDA and AMPA receptor currents (Cai et al., 2002). In vitro studies show that activation of 5-HT_1A receptor reduces NMDA-evoked glutamate release elevation while their blockade has opposite effects (Matsuyama et al., 1996; Maura and Raiteri, 1996). In in vivo studies PFC application of 8-OH-DPAT does not affect NMDA-evoked glutamate release, while the 5-HT_1A receptor antagonist WAY100135 enhance basal and NMDA-evoked glutamate release in the striatum (Dijk et al., 1995). Additionally, the 5-HT_1A partial agonists and full antagonists attenuate working memory deficits as well as psychotomimetic effects induced by NMDA antagonists (Harder and Ridley, 2000; Wedzony et al., 2000). Stimulation of 5-HT_1A somatodendritic autoreceptors in the DR or blockade of post-synaptic 5-HT_1A receptors in the hippocampus remediate the spatial learning deficit induced by blockade of NMDA receptors (Carli et al., 1998).

Activation of 5-HT_2A/_2C receptors by DOI enhances the firing of pyramidal neurons (Puig et al., 2003) and 5-HT release dependent on activation of AMPA receptors (Martin-Ruiz et al., 2001) and increases glutamate levels in the somatosensory cortex (Scruggs et al., 2000, 2003). Activation of 5-HT_2A/_2C receptors in the PFC modulates GABAA receptor currents (Feng et al., 2001) and increases GABA release (Abi-Saab et al., 1999). Blockade of 5-HT_2A receptors reduces NMDA antagonists-induced fos expression (Habara et al., 2001), motor hyperactivity (Gleason and Shannon, 1997; Martin et al., 1997, 1998; Swanson and Schoepp, 2002), forced swimming immobility (Corbett et al., 1999) and pre-pulse inhibition (PPI) (Varty et al., 1999). Blockade of 5-HT_2C receptors enhances NMDA antagonists-induced motor hyperactivity and DA release (Hutson et al., 2000).

The behavioral manifestation of the functional interaction between 5-HT_1A and 5-HT_2A and 5-HT_2C with NMDA receptors in the mPFC is the demonstration that selective agonist at 5-HT_1A receptor 8-OH-DPAT and antagonist at 5-HT_2A receptor M100907 as well as 5-HT_2C receptor agonist Ro60-0175 recovered attentional performance deficit due to blockade of NMDA receptor in the mPFC, albeit in a distinct manner (Mirjana et al., 2004; Carli et al., 2006a; Calcagno et al., 2009). Microinjections of 8-OH-DPAT or M100907 in the mPFC prevent accuracy deficit (Carli et al., 2006a). Clearly, the functional opposition between the two 5-HT receptor subtypes on accuracy suggest that the improvement produced by M100907 and 8-OH-DPAT might reside in their opposite activity on common cellular substrates (Araneda and Andrade, 1991; Ashby et al., 1994; Celada et al., 2013). The 5-HT_1A but not 5-HT_2A or 5-HT_2C receptors appear to be involved in decision processes in this task as 8-OH-DPAT but not M100907 or Ro60-0175 reduced correct response latency and omissions (Mirjana et al., 2004; Carli et al., 2006a; Calcagno et al., 2009). DA system and in particular D₁ receptor in the PFC and in the dm-STR have been shown to impact decision processes in this task (Granon et al., 2000; Robbins, 2002); (see Table 2 in Agnoli et al., 2013). The fact that 8-OH-DPAT infused in the mPFC increases DA efflux in this cortical region (Sakaue et al., 2000) may at least in part explain its effects on speed and omissions.

Injections of 8-OH-DPAT and M100907 in the mPFC in control rats had no effect on accuracy, which is in contrast to what reported by other studies. The effects of 5-HT_1A agonists on accuracy in normal rats performing the task under basal conditions depend on whether the 5-HT_1A somatodendritic autoreceptors or post-synaptic receptors are activated. Systemic 8-OH-DPAT impaired accuracy and this effect was abolished by 5,7-dihydroxytryptamine lesion or blockade of 5-HT receptors in the DR by a selective 5-HT_1A antagonist WAY100635 (Carli and Samanin, 2000). In contrast, Winstanley et al. (2003a) reports a facilitation of accuracy after systemic or intra-cortical injections of 8-OH-DPAT. The role of 5-HT_2A receptors in accuracy is much less clear as systemic M100907 had no effect (Winstanley et al., 2004a) and intra-mPFC injection facilitated accuracy at long but not short stimulus duration (Winstanley et al., 2003a). The 5-HT_2C receptor do not appear to control accuracy in normal rats as no effect on accuracy has been reported after 5-HT_2C receptor agonists or antagonists (Higgins et al., 2003a; Winstanley et al., 2004a; Fletcher et al., 2007, 2011).

In contrast to the effects of 5-HT_2A receptor antagonist, which reduced premature responses but not perseverative over-responding either after systemic or intra-cortical injection (Mirjana et al., 2004; Carli et al., 2006a), activation of 5-HT_1A receptors in the mPFC had no effect on premature but decreased perseverative over-responding (see Table 3). 5-HT acting on 5-HT_2A receptors segregated to apical dendrites of pyramidal neurons (Jakab and Goldman-Rakic, 1998) and to GABA interneurons specialized in the perisomatic inhibition of pyramidal cells (Jakab and Goldman-Rakic, 2000) can affect excitatory input (Aghajanian and Marek, 1997) and by acting on 5-HT_1A receptors in the axon hilloc (DeFelipe et al., 2001; Czyrak et al., 2003) can suppress the generation of action potential along the axon and influence the activity in subcortical projection areas. Thus, by finely tuning the complex activity of glutamatergic pyramidal neurons, 5-HT may differently influence distinct aspects of executive control. These results clearly demonstrate the selectivity of executive control processes and indicate that impulsivity and compulsivity may be dissociated by 5-HT_1A and 5-HT_2A receptor mechanisms in the mPFC.

The effects of systemic M100907 and Ro60-0175 on R-CPP-induced impulsivity (Mirjana et al., 2004; Calcagno et al., 2009) are consistent with studies showing similar effects of these compounds on impulsivity but not compulsivity induced by systemic injections of NMDA antagonists dizocilpine and Ro63-1908 (Higgins et al., 2003b; Fletcher et al., 2011). In contrast the 5-HT_2C antagonist SB242084 increased premature responses already in control rats and tended to enhance dozocilpine-induced impulsivity (Higgins et al., 2003b).

Previous studies have suggested that enhanced impulsivity in the 5-CSRT task is associated with increased 5-HT turn-over (Puumala and Sirvio, 1998) and release in the PFC (Dalley et al., 2002) and activation of 5-HT_2A/_2C receptors by DOI (Koskinen et al., 2000). However, global forebrain 5-HT depletion consistently results in enhanced impulsivity (Soubrié, 1986; Harrison et al., 1997; Carli and Samanin, 2000; Mobini et al., 2000). This apparent discrepancy may be resolved by 5-HT exerting inhibitory activity on impulsivity through 5-HT_2C but not 5-HT_2A receptors since decreasing their activity leads to impulsivity (Higgins et al., 2003b; Winstanley et al., 2004a; Fletcher et al., 2007). This suggestion is further supported by findings that activation of 5-HT_2C receptors decreases while their suppression increases premature responding in the 5-CSRT task under various conditions such as when the waiting period is increased (Carli, 2006b; Fletcher et al., 2007) or premature responding is enhanced by NMDA receptor antagonists (Higgins et al., 2003b; Calcagno et al., 2009).

Like systemic NMDA receptor antagonists, intra-mPFC infusion of R-CPP enhances DA release in the mPFC (Table 2) (Moghaddam et al., 1997; Del Arco and Mora, 1999; Feenstra et al., 2002). Increasing DA transmission by d-amphetamine increases perseverative responses in the 5-CSRT task (Baunez and Robbins, 1999). Although microdialysis studies show that 8-OH-DPAT increases DA release in the mPFC (Arborelius et al., 1993; Sakaue et al., 2000) it actually reduces the rise in cortical DA release induced by d-amphetamine, stress and isolation rearing (Rasmusson et al., 1994; Kuroki et al., 1996; Ago et al., 2002) and attenuate d-amphetamine-induced motor activation (Przegalinski and Filip, 1997). The D₂ receptor antagonist haloperidol also decreases R-CPP-induced perseverative responding (Baviera et al., 2008) (Table 3). It is plausible that 8-OH-DPAT could decrease perseverative responding through its action on DA mechanisms. However, the effects of 8-OH-DPAT were due to activation of 5-HT_1A receptors in the mPFC as a selective 5-HT_1A antagonist WAY100635 completely blocked the effects of 8-OH-DPAT on accuracy deficit and perseverative responding (Carli et al., 2006a).

It is worth noting that the effects of M100907 on R-CPP-induced impairments in 5-CSRT task performance and the increase in glutamate release in the mPFC (see Table 3) are mimicked by mGlu_2/3 receptor agonist LY379268 (Pozzi et al., 2011). 5-HT-evoked excitatory post-synaptic currents are similarly inhibited by 5-HT_2A antagonist M100907 and by mGlu_2/3 receptor agonists (1S,3S)-ACPD and LY354740 and enhanced by the mGlu_2/3 antagonist LY341495 (Aghajanian and Marek, 1999, 2000; Marek et al., 2000). Activation of 5-HT_2A receptors by DOI or LSD increases excitatory post-synaptic currents and potentials, glutamate release, c-fos in PFC, and induces head-twitch response (Aghajanian and Marek, 2000; Gewirtz and Marek, 2000; Klodzinska et al., 2002; Zhai et al., 2003; Gonzalez-Maeso et al., 2008). All these effects are blocked by 5-HT_2A antagonists or by mGlu_2/3 agonists. This functional analogy may be based in part on anatomical overlap of mGlu₂ particularly in apical dendrites of lamina V with the riches distribution of 5-HT_2A receptors (Blue et al., 1988; Aghajanian and Marek, 1999; Marek et al., 2000, 2001) but may also derive from the mGlu₂ receptor forming a complex through the specific transmembrane helix with 5-HT_2A receptor (Gonzalez-Maeso et al., 2008).

The 5-HT afferents arising mainly in the DR nucleus (Steinbusch, 1984) innervate all components of the basal ganglia circuitry (Lavoie and Parent, 1991). The fact that 5-HT modulates not only DA but also GABA and glutamate neurotransmission in the dorsal striatum and output regions of the basal ganglia (Nicholson and Brotchie, 2002) suggest a 5-HTergic regulation of action selection and motor control (Di Matteo et al., 2008) but little is known about their contribution to cognitive function.

Among the various 5-HT receptor subtypes present within dorsal striatum the 5-HT_2A and 5-HT_2C receptors are particularly abundant (Barnes and Sharp, 1999). They are equally distributed on medium spiny neurons (MSN) forming the direct striatonigral and the indirect striatopallidal output projections but also on GABA and cholinergic (ACh) interneurons (Ward and Dorsa, 1996; Eberle-Wang et al., 1997). These 5-HT₂ receptor subtypes play a prominent role in the modulation of striatal DA function (Abdallah et al., 2009; Navailles and De Deurwaerdere, 2011) and excite striatal ACh and fast spiking GABA interneurons (Blomeley and Bracci, 2005, 2009). The 5-HT₂ receptor antagonists administered within the striatum block DA-mediated oral activity (Plech et al., 1995), synergize D₁-induced locomotor activity (Bishop and Walker, 2003) and cause retrograde amnesia in rats (Prado-Alcala et al., 2003a,b). Loss of 5-HT_2C receptors enhances behavioral sensitivity to D₁ receptor activation (Abdallah et al., 2009).

Activation of 5-HT_2C or blockade of 5-HT_2A receptors in the dm-STR reduce accuracy deficit induced by R-CPP. These data concur with the above discussed data showing opposing roles of these receptors on the neurochemical processes that support the 5-CSRT task performance deficits induced by NMDA receptor antagonists. However, cortical 5-HT_2A receptors exert a much more effective control over attention as much higher dose of M100907 had to be administered in the dm-STR than in the mPFC to achieve an effect on accuracy. The PFC shows much higher levels of 5-HT_2A hybridization signals than the dorsal striatum (Pompeiano et al., 1994) and there is a substantial and reciprocal control of the activity of DR cortical 5-HT neurotransmission by PFC (Hajos et al., 1998, 1999; Celada et al., 2001). This control has an important functional role; for example 5-HT depletion abolishes the facilitatory effects of M100907 on accuracy and its ability to prevent R-CPP-induced glutamate release in the mPFC (Winstanley et al., 2004a; Calcagno et al., 2009) but also in stress-induced activation of DR (Amat et al., 2005). The DR from which originates the 5-HT projection to the dorsal striatum does not receive reciprocal innervation from the striatum indicating no direct modulation by striatal feedback (Casanovas et al., 1999).

In contrast to the lack of effect of systemic or intracortical M100907 and Ro-60-0175 on perseverative responding, M100907 and Ro60-0175 administered locally in the dm-STR reduced perseverative responding caused by R-CPP. As systemic injections of these 5-HT₂ agents had no effect on R-CPP-induced perseverative over-responding it is conceivable that the reduction of perseverative responding in one brain area such as dm-STR, is compensated by opposite effects in other brain regions. In fact, M100907 injected in the ventral tegmental area (VTA) further enhanced perseverative responding caused by blockade of NMDA receptors in the mPFC (Agnoli and Carli, 2012). These findings are in keeping with evidence that different 5-HT receptor subtypes have distinct roles in the modulation of perseverative responses, depending on the type of cognitive process engaged by the task and brain area. For example, 5-HT in the PFC is not essential for higher-order shifting of attentional set while it is critical for the flexible responding in a reversal learning task (Clarke et al., 2005). The 5-HT_2A and 5-HT_2C receptors exert functionally opposing action on perseverative responding in a spatial-reversal task (Boulougouris et al., 2008) while suppression of 5-HT_2C receptors in the orbitofrontal cortex but not in PFC decreases perseverative errors in a similar reversal-learning task (Boulougouris and Robbins, 2010).

It is worth noting that the ability of intra-STR M100907 and Ro60-0175 to remove impulsivity and compulsivity induced by blockade of mPFC NMDA receptors is remarkably similar to what found after systemic or intra dm-STR injections of a D₂ receptor antagonist haloperidol (Baviera et al., 2008; Agnoli et al., 2013). It should be noted that blockade of NMDA receptors in the mPFC increases glutamate, GABA, and DA release in the dm-STR (see Table 2). As pointed above substantial neurochemical and behavioral evidence support the suggestion that 5-HT can influence DA's effects in the striatum, which may be of relevance for the observed analogy in the behavioral effects 5-HT_2A and 5-HT_2C receptor agents and D₂ receptor antagonists. However, as M100907 and Ro60-0175 but not haloperidol had additional effects on accuracy deficit other likely non-D₂, mechanisms in the dm-STR may also contribute. Local infusion of M100907 has been shown to decrease basal and MPTP-stimulated glutamate levels in the dorsal striatum and ameliorate behavioral impairment of MPTP-treated mice (Ansah et al., 2011).

Activation of 5-HT receptors in the striatum elicits predominantly inhibitory responses in the medium spiny (MS) projection neurons (el Mansari et al., 1994; el Mansari and Blier, 1997). 5-HT though 5-HT_2C excites striatal ACh interneurons, which in turn inhibit the glutamatergic input to MS projection neurons (Pakhotin and Bracci, 2007). Notably changes in firing activity of ACh interneurons encode behaviorally relevant information (Morris et al., 2004; Yamada et al., 2004). Activation of 5-HT_2C receptors strongly increases the firing of GABAergic interneurons in the striatum, which potently inhibit striatal output (Blomeley and Bracci, 2009). Thus, 5-HT₂ receptor subtypes through a likely action on glutamate, ACh and GABA mechanisms in the dm-STR may integrate the glutamate cortico-striatal inputs critical for the different aspects of performance in the 5-CSRT task.

DA receptors are broadly expressed in the brain with a distribution largely matching the density of innervating DA fibers (Bentivoglio and Morelli, 2005). Among the DA receptors the D₁ and D₂ receptor subtypes display the widespread distribution and the highest expression levels. They are most prominent in the dorsal and ventral striatum, olfactory tubercle, and cortex (Bentivoglio and Morelli, 2005).

In the striatum the D₁ and D₂ receptors are segregated to the two MSN output populations of neurons forming direct striato-nigral and indirect striato-pallidal patways, respectively. However, both D₁ and D₂ receptors are expressed in a subset of MSN neurons in the striatum. Whether cooperative effects of D₁ and D₂ receptors observed in some studies (Perreault et al., 2011) arise from complex network interactions or from their co-localization in some MSN neurons is unclear. Striatal interneurons although proportionally small (5–10% of total neuronal population) exert powerful influence on striatal output. Five distinct GABA subtypes (distinguished by different neuropeptide expression, synthetic enzymes and calcium binding proteins) and one type of ACh interneurons are present. D₂ and D₁-like (D₅) receptors are expressed in ACh interneurons while some GABA interneurons express D₁-like (D₅) receptors. D₂ receptor is expressed also on presynaptic DA terminals of DA afferents as well as in glutamatergic cortical and thalamic afferents. D₁ receptors have also been found in a small number of presynaptic glutamatergic terminals (Bentivoglio and Morelli, 2005).

Numerous studies have demonstrated that DA through pre- and post-synaptic D₁ and D₂ receptors modulate the probability of release at glutamate, GABA and ACh terminals, ionotropic glutamate, and GABA receptor function and trafficking, post-synaptic excitability and synaptic integration in striatal projecting neurons and interneurons as well as in cortical pyramidal cells and interneurons. DA bi-directionally modulates synaptic NMDA receptors through its D₁- and D₂-like receptors, but the responses of individual neurons across brain areas and the intracellular pathways recruited vary greatly. These studies (for a comprehensive review see Surmeier et al., 2010; Tritsch and Sabatini, 2012) reveal the complex nature and consequences of this modulation on neural networks implicated in motor, cognitive, and motivational processes (Di Chiara, 2005; Dunnett, 2005; Robbins, 2005; Berridge, 2007; Salamone and Correa, 2012).

In rats in which accuracy was impaired by intra-mPFC injections of R-CPP (50 ng/side) suppression of D₁-like receptor activity in the dm-STR by an antagonist such as SCH23390 prevented accuracy deficit. It could be argued that in rats performing the 5-CSRT task at a very high level of efficiency (i.e., high accuracy) the activity at dorsal striatal D₁-like receptors may be already at a maximum and any further activation such as that operated by intra-mPFC injections of R-CPP (Table 2) may have detrimental effects. On the other hand, concomitant infusion of SCH23390 and R-CPP in the dm-STR at individually ineffective doses had detrimental effects on accuracy (Agnoli and Carli, 2011). Thus, suppression of D₁ receptor function in the dm-STR has positive or detrimental effects on accuracy depending on whether cortico-striatal neurotransmission is increased or decreased. Interestingly, another study also reported that systemic SCH23390 tend to improve accuracy of rats with excitotoxic lesion to mPFC (Passetti et al., 2003b).

These findings stand rather alone as in the majority of published studies the effects of D₁-like agents was examined in normal rats performing the task under baseline conditions. The picture that emerges is that detrimental effects of SCH23390 on accuracy of rats performing the task under basal conditions depend on dose, brain area, and baseline level of accuracy (Granon et al., 2000; Pezze et al., 2007). In rats performing the task at relatively high level of accuracy (between 80 and 90%) SCH23390 injected in the mPFC or NAC impairs accuracy (Granon et al., 2000; Pezze et al., 2007) while the same dose (100 ng) injected in the dm-STR had no effect (Agnoli and Carli, 2011). Similarly the effect of SKF38393 a D₁-like receptor agonist on accuracy is also baseline dependent; injected in the dm-STR or given systemically impairs accuracy of rats performing at high levels of efficiency (about 90%correct) whereas injected in the mPFC boosts accuracy but only in poorly performing rats (about 70% correct) (Granon et al., 2000). SKF38393 injected in the NAC boost accuracy of rats performing at less than 80% correct but only at the lowest dose tested (100 ng) while the same dose injected in the dorsolateral striatum had no effect. These discrepancies in the effects of D₁ receptor agents on accuracy are far from surprising as it has been repeatedly shown that the effects of D₁ receptor manipulation on task performance depend on the optimal levels of DA for the particular task (Sawaguchi and Goldman-Rakic, 1991; Arnsten, 1997; Zahrt et al., 1997; Granon et al., 2000; Pezze et al., 2003; Chudasama and Robbins, 2004; Robbins, 2005). This is reminiscent of the Yerkes-Dodson principle based on the inverted U-shaped function relating levels of arousal/activation with efficiency of behavioral performance (Robbins, 2005) but also to the inverted U-shaped function relating D₁ receptor activation and NMDA-EPSC changes (Seamans and Yang, 2004; Trantham-Davidson et al., 2004; Tritsch and Sabatini, 2012). As for PFC an inverted U-shaped function may relate D₁ receptor stimulation in the dm-STR to the efficiency of attentional functioning.

Contrasting with the findings above, suppression of D₂-like receptor activity in the dm-STR by local injections of haloperidol has no effect in control rats and is unable to recover accuracy deficit caused by blockade of NMDA receptors in the mPFC (Agnoli et al., 2013). Similar lack of effects on R-CPP-induced accuracy deficit is observed after systemic haloperidol (Baviera et al., 2008). However, the doses used in these studies were effective in reversing other R-CPP-induced effects (see below). That D₂-like receptors in the dorsal striatum are unlikely to be involved in governing accuracy is supported by data showing that their activation by quinpirole an agonist at these receptors has no effect either injected in the dorsomedial or dorsolateral striatum (Pezze et al., 2007; Agnoli et al., 2013). Doses of haloperidol and quinpirole higher than those reported in the studies by Baviera et al. (2008) and Agnoli et al. (2013) cannot be tested in the 5-CSRT task as rats stop responding or make mostly omissions.

Systemic haloperidol do not allows for the precise definition of the locus of D₂ receptor suppression. However, after systemic administration haloperidol binds comparable proportion of D₂ receptors in the striatum (caudate-putamen) and in the frontal cortex (Mukherjee et al., 2001) and the protein structure of the D₂ receptors throughout the brain are similar and so is their in vitro affinity (Seeman and Ulpian, 1983). It is worth noting that a chemically different D₂-like receptor antagonist such as l-sulpiride injected in the mPFC had no effect on accuracy (Granon et al., 2000). Although these findings may suggest that cortical D₂-like receptors do not contribute to accuracy further studies are necessary to better delineate the role of PFC D₂ receptors in attention. However, l-sulpiride given systemically or in the NAC impairs accuracy in control rats but prevents accuracy deficit in rats bearing excitotoxic lesions of the mPFC (Passetti et al., 2003b; Pezze et al., 2009). L-sulpiride does not discriminate between D₂ and D₃ receptor subtypes (Missale et al., 1998). The D₃ receptors are present at very low levels in the mPFC and dorsal striatum but are particularly abundant in the NAC and limbic regions (Sokoloff et al., 1990; Bentivoglio and Morelli, 2005) and hence it could not be excluded the possibility that D₃ receptors in the NAC may account for the effect of l-sulpiride. However, as systemic or intra-NAC nafadotride, a preferential D₃ (compared to D₂) receptor antagonist (Sautel et al., 1995) has no effect on accuracy (Besson et al., 2010) the precise contribution of D₃ receptors for the control of accuracy has yet to be fully disclosed.

Intra-dm-STR injections of haloperidol or SCH23390 did not reduce the R-CPP-induced increase in omissions and correct response latencies. It is unlikely that the increased proportion of omissions was due to a change in motivation as the latency to collect the food, which is a more direct measure of motivation was not affected. This increase in omissions may indicate an inability to maintain voluntary control over sustained performance due to motor hyperactivity. However, haloperidol and SCH23390 did not reduce R-CPP-induced motor hyperactivity (Agnoli, 2011). SKF38393 speeded correct responses and decreased omissions when injected in the dm-STR (see Table 2 in Agnoli et al., 2013) similarly to what reported after intra-mPFC injection of this compound while systemic SKF38393 decrease correct response latencies (Passetti et al., 2003a). These finding are broadly consistent with a general performance scaling function of tonic DA activity (Cagniard et al., 2006) and with evidence from other reaction time tasks that striatal DA is implicated in decisional processes (Carli et al., 1985, 1989; Robbins and Brown, 1990; Brown and Robbins, 1991).

Dorsomedial striatal D₁-like and D₂-like receptors play an important role in the expression of impulsivity in the 5-CSRT task as both SCH23390 and haloperidol injected in the dm-STR dose-dependently reversed R-CPP-induced premature over-responding, a proxy of impulsivity. Blockade of these D₁- and D₂-like receptors in control conditions had no effect or decrease premature responses depending on the dose employed and the number of premature responses made by rats under the control condition (Agnoli and Carli, 2011; Agnoli et al., 2013) while their activation by SKF38393 and quinpirole, respectively increase premature responses (Agnoli et al., 2013). The finding that R-CPP-induced motor hyperactivity was not affected by SCH23390 and haloperidol (Agnoli, 2011) suggest that their ability to decrease R-CPP-induced impulsivity is not due to alteration in motor activity and helps dissociating impulsivity from changes in motor activity.

These findings question the prevailing hypothesis that impulsivity can be mostly attributed to the mesolimbic not the nigrostriatal DA system as d-amphetamine-induced impulsivity in the 5-CSRT task was abolished by ventral striatal but not dorsal striatal DA depletion (Cole and Robbins, 1989; Baunez and Robbins, 1999). In addition other studies had shown that D₁- but not D₂-like receptors in the NAC core contribute to impulsivity under basal conditions (Pezze et al., 2007) whereas D₂-like receptors in the NAC appear to come into play only under perturbed conditions such as those induced by amphetamine or in rats made impulsive by excitotoxic lesion of the PFC (Cole and Robbins, 1987; Pattij et al., 2007; Pezze et al., 2009). In the case of high-impulsive rats D_2/3 antagonist nafadotride alleviated or exacerbated impulsivity depending whether injected in the core or shell sub-region of the NAC, respectively (Besson et al., 2010). However, DA depletion in the dorsal striatum reversed impulsivity in the 5-CSRT task induced by lesions to the sub-thalamic nucleus (Baunez and Robbins, 1999) and D₂ receptor availability in the dorsal striatum was associated with impulsiveness in methamphetamine-dependent subjects (Lee et al., 2009). Thus, the modulation of impulsivity by DA mechanisms in the dorsal striatum may be detected in particular conditions.

Haloperidol but not SCH23390 injected in the dm-STR reduce perseverative responding induced by blockade of NMDA in the mPFC. The effects of systemic and intra-dm-STR injected haloperidol are remarkably similar; both reduce perseverative and premature over-responding but not accuracy deficit (see, Tables 3, 4). These findings are in accord with a study showing that the “compulsive” stimulus bound perseveration of monkeys after frontal ablation is also alleviated by haloperidol (Ridley et al., 1993). However, other studies report that l-sulpiride either after systemic or intra-NAC injections had no effect in rats made compulsive by excitotoxic lesions of mPFC (Passetti et al., 2003b; Pezze et al., 2009). The causes for the lack of effect of this D₂/D₃ antagonist are not clear and may depend on various factors such as whether emitting a perseverative responses leads to behavioral consequence or not, brain area or else; for example it has been repeatedly shown that similar pharmacological manipulations increase perseverative responding when these lead to time-out but not when they are without consequences (Harrison et al., 1999; Robbins, 2002; Mirjana et al., 2004; Winstanley et al., 2004a; Murphy et al., 2005).

On the other hand, in normal rats performing the 5-CSRT task at baseline conditions activation of D₂-like receptors in the dm-STR dose-dependently increase perseverative responding (Agnoli et al., 2013) similarly to what found after injections of similar doses of quinpirole in the NAC core but not after injections in the dorsolateral striatum (Pezze et al., 2007). That the perseverative responses in the 5-CSRT task may be modulated by nigrostriatal DA system is also suggested by paradoxical increase in these responses after dorsal striatal DA depletion (Baunez and Robbins, 1999) most likely due to the supersensivity of D₂-receptors after 6-hydroxydopamine lesion (Ungerstedt, 1971). These findings are in agreement with other studies linking changes in D₂ receptors function at various nodes of cortico-striatal circuit to flexible modification of behavior. Although it could not be assumed that perseverative errors in the 5-CSRT task and those made in other tasks such as for example in set-shifting, reversal learning or working memory represent the same psychological process, Floresco et al. (2006) report increased number of perseverative errors after blockade of D₂-like receptors in the mPFC in a maze based set-shifting task while Goto and Grace (2005) report that PFC-dependent perseveration in a task requiring an egocentric response strategy depends on tonic DA release and D₂-like receptor stimulation in the striatum. In addition, mice over-expressing D₂ receptors in the striatum make more perseverative errors in a working memory task (Kellendonk et al., 2006). D₂-receptor stimulation by quinpirole increases preseverative but not learning errors of rats performing a spatial reversal task (Boulougouris et al., 2009). The probability of perseverative responses of monkeys performing a three-choice reversal task is also related to D₂-receptor availability in the dorsal striatum (Groman et al., 2011).

The lack of effects of D₁-like receptor agents injected either systemically, in the mPFC, NAC or dm-STR on perseverative responding in the 5-CSRT task (Granon et al., 2000; Pezze et al., 2007; Agnoli and Carli, 2011; Barnes et al., 2012a; Agnoli et al., 2013) contrast with evidence that D₁-like receptors in the mPFC or NAC control perseverative type errors in set-shifting and working memory tasks (Zahrt et al., 1997; Ragozzino, 2002; Haluk and Floresco, 2009). Thus, both DA receptor subtypes act in a cooperative manner to control a component of set-shifting such as ability to disengage from the previously effective but now inappropriate strategy whereas in the 5-CSRT task they appear to control separate cognitive processes such as those engaged by accuracy of visual discrimination and perseverative responding. However, the perseverative over-responding in the 5-CSRT task may result from a deficit in the selection and integration of an adequate response in a long sequence, leading to reward rather than the inability to flexibly adapt to the shifts between rules, strategies and sets. The organization of complex sequences of actions and the ordering of movements within a sequence implicate dorsal striatum with its DA afferents (Graybiel, 1998; Hikosaka et al., 1998; Bailey and Mair, 2006; Jin and Costa, 2010; Jin et al., 2014). Notably, the D₁-nigrostriatal and D₂-striatopallidal basal ganglia pathways show concomitant activity during action selection and initiation but behave differently during the execution of action sequences (Jin et al., 2014).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.