Substance use disorders (SUD) are a debilitating group of psychiatric disorders that affect approximately 7% of people in the United States each year (SAMHSA, 2018; Grant et al., 2016). Relapse is a major factor limiting recovery from SUD and points to the strength of memory circuits involved in reward memory processing (Niaura et al., 1988; Carter and Tiffany, 1999; Weiss, 2005; Zironi et al., 2006; Janak and Chaudhri, 2010). Converging evidence from rodent models and human postmortem studies suggest a key role of extracellular matrix (ECM) molecules in the formation and maintenance of reward memories (Brown et al., 2007; Smith et al., 2011, 2015; Xue et al., 2014; Slaker et al., 2015, 2016a; Chioma et al., 2021; Seney et al., 2021; Browne et al., 2023). In addition, ECM abnormalities have been reported in several psychiatric disorders in which dysfunction in memory processing and synaptic regulation are key features, including schizophrenia (Eastwood and Harrison, 2006; Pantazopoulos et al., 2010, 2015; Enwright et al., 2016; Steullet et al., 2018), post-traumatic stress disorder (Gogolla et al., 2009), major depressive disorder (MDD) (Riga et al., 2017; Alaiyed et al., 2020; Blanco and Conant, 2021), and bipolar disorder (BD) (Fatemi et al., 2000; Pantazopoulos et al., 2015; Steullet et al., 2018) – each of which have significant comorbidity with SUD.

Involvement of the ECM in the regulation of reward memories may represent a shared feature across reward processes involved in strengthening memories necessary for survival such as food seeking. Several lines of evidence indicate that food and drugs of abuse activate overlapping brain circuitry (Blum et al., 2012; Volkow et al., 2013a). Furthermore, as the incidence of obesity and metabolic disorders continues to mount in the U.S., there has been an increasing interest in combining habitual overeating and drug abuse within a common diagnostic framework of disorders of addiction (Barry et al., 2009; Alsio et al., 2012; Blum et al., 2012; Baik, 2013; Volkow et al., 2013a,b). Growing evidence supports the involvement of ECM molecules in reward processes involved in high fat, high calorie diets that may contribute to metabolic disorders including obesity. The strength of reward memories contributes to relapse and habit forming, and targeting ECM processes to alleviate reward memory strength represents a promising strategy for the development of new treatments for SUD and obesity (Niaura et al., 1988; Carter and Tiffany, 1999; Lu et al., 2004; Weiss, 2005; Zironi et al., 2006; Janak and Chaudhri, 2010).

A growing number of studies demonstrate a dynamic regulation of the ECM during learning processes, including fear and reward learning (Gogolla et al., 2009; Slaker et al., 2016a). Evidence from over two decades of research indicates that the ECM is regulated by rewarding substances in a complex experience-dependent manner which may impact interpretation of the disease processes in SUD and metabolic disorders as well as treatment strategies. We summarize the current evidence for the involvement of the ECM in reward memory processes, SUD and obesity with an emphasis on how the ECM is modulated in a substance- and time-specific manner.

The brain extracellular matrix (ECM) is a network of chondroitin sulfate proteoglycans (CSPGs), heparan sulfate proteoglycans, glycoproteins, hyaluronan, and other molecules including axon guidance and cell adhesion molecules such as semaphorins and integrins (Table 1; Figure 1). In the brain, ECM molecules form several specialized structures including perineuronal nets (PNNs), perisynaptic ECM, perivascular ECM, and axonal coats (Schuster et al., 2001; Bruckner et al., 2008; Bekku et al., 2009; Dours-Zimmermann et al., 2009; Morawski et al., 2010; Baeten and Akassoglou, 2011; Lendvai et al., 2012; Susuki et al., 2013; Thomsen et al., 2017; Pantazopoulos et al., 2022; Tabet et al., 2022; Miguel-Hidalgo, 2023; Figure 1). These ECM structures are involved in a wide range of processes implicated in SUD including stabilization of synapses, axon guidance during neurodevelopment, regulation of diffusion of molecules such as neurotransmitters, ions, and metabolites, neuronal firing rates, receptor trafficking, protection from oxidative stress, and regulation of the blood-brain barrier (Hartig et al., 1999; Ishii and Maeda, 2008; Maeda, 2015). ECM molecules also have broad, complex roles in neurodevelopmental processes and brain injury [for reviews see Silver and Silver, 2014; Peters and Sherman, 2020; Carulli and Verhaagen, 2021; Fawcett and Kwok, 2022; Siddiqui et al., 2022; Schwartz and Domowicz, 2023]. CSPGs have been the primary focus of preclinical addiction studies and are one of the key proteoglycan families in the central nervous system (CNS). CSPGs are composed of a core protein with a varying number of covalently attached chondroitin sulfated glycosaminoglycan chains consisting of repeated pairs of glucuronic acid (GlcA) and N-acetyl-galactosamine (GalNAc) (Figure 1). The predominant CSPGs within the CNS are aggrecan, brevican, neurocan, phosphacan, neuron-glial antigen 2 (NG2), neuroglycan-C and versican (Milev et al., 1998; Deepa et al., 2006; Giamanco and Matthews, 2012). In addition to the specific functions of the core proteins, the glycosaminoglycan chains can vary in number and length, and are sulfated in various positions, which significantly contributes to the function of CSPGs (Asher et al., 2001; Wang et al., 2008; Maeda, 2010; Karus et al., 2012; Miyata et al., 2012; Alonge et al., 2019). For example, synaptic plasticity is regulated by chondroitin sulfation. CS-4 sulfation inhibits axonal growth (Smith-Thomas et al., 1995; Wang et al., 2008), whereas CS-6 is permissive for axonal growth (Lin et al., 2011; Miyata et al., 2012).

Chondroitin sulfate proteoglycans form several specialized structures in the brain in addition to the diffuse ECM which exists around all cells within the CNS (Figure 1). PNNs represent the most well-studied of these structures. PNNs are composed of highly condensed ternary ECM molecules including CSPGs, hyaluronan, and tenascin and surround the soma, proximal dendrites, and axon initial segments of neurons, typically inhibitory fast-firing interneurons expressing parvalbumin (Figure 2; Hartig et al., 1994, 2022; Adams et al., 2001; Pantazopoulos et al., 2006). PNNs are involved in a broad range of functions including stabilization of synaptic plasticity, protection from oxidative stress, regulation of neuronal firing properties, N-methyl-D-aspartate (NMDA) receptor trafficking, and maintenance of ionic homeostasis (Kalb and Hockfield, 1988; Pizzorusso et al., 2002; Morawski et al., 2004; Sugiyama et al., 2008; Frischknecht et al., 2009; Gogolla et al., 2009; Dityatev et al., 2010; Gundelfinger et al., 2010; Frischknecht and Gundelfinger, 2012; Cabungcal et al., 2013; Wingert and Sorg, 2021). The role of PNNs as stabilizers of synaptic connections places them at the intersection of formation and consolidation of memories. PNNs are commonly labeled using the plant lectin Wisteria floribunda agglutinin (WFA), which binds to non-sulfated N-acetylgalactosamine residues at the terminal ends of CSPG saccharide chains (Hartig et al., 1994; Nadanaka et al., 2020). However, PNNs are complex structures with diverse composition, and labeling with antibodies directed against core CSPG proteins, glycoproteins, or specific sulfation motifs detects varying, partially overlapping populations of PNNs (Ajmo et al., 2008; Pantazopoulos et al., 2015; Dauth et al., 2016; Hartig et al., 2022; Scarlett et al., 2022). Several studies have demonstrated that the distribution of ECM molecules, including CSPGs and PNNs in the brain, varies greatly in a brain region and age specific manner (Mauney et al., 2013; Dauth et al., 2016; Ueno et al., 2017, 2018, 2019; Rogers et al., 2018; Mafi et al., 2020; Lupori et al., 2023).

Chondroitin sulfate proteoglycans also form several distinct structures in the brain in addition to PNNs (Figure 1). For example, ECM molecules including CSPGs condense as perisynaptic ECM aggregates on dendritic spines and contribute to synaptic homeostasis (Schuster et al., 2001). ECM molecules form perinodal structures at nodes of Ranvier along myelinated axons, where they contribute to axonal conductance through regulation of gap junctions by several processes including sodium channel clustering (Bekku et al., 2009; Dours-Zimmermann et al., 2009; Susuki et al., 2013; Pantazopoulos et al., 2022; Miguel-Hidalgo, 2023). CSPGs also interweave within myelin sheaths to form periaxonal aggregates called axonal coats (Bruckner et al., 2008; Morawski et al., 2010; Lendvai et al., 2012; Pantazopoulos et al., 2022). Furthermore, ECM molecules are critically involved in regulation of the blood-brain barrier and immune signaling through perivascular ECM structures (Baeten and Akassoglou, 2011; Thomsen et al., 2017; Tabet et al., 2022).

Recent studies demonstrate that the ECM, once thought to represent a stable component of the brain, is regularly modified in an activity-dependent manner through a complex combination of proteolytic remodeling and ECM recycling processes (Nagy et al., 2007; Brown et al., 2009; Ganguly et al., 2013; Dankovich and Rizzoli, 2022). Matrix metalloproteinases (MMP), a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS), and cathepsins are the putative endogenous proteases involved in degradation of ECMs (Page-McCaw et al., 2007; Mohamedi et al., 2020; Tran and Silver, 2021). These proteases are secreted by astrocytes, microglia, and neurons, indicating that individual neurons may, in part, be able to regulate the composition of their own PNNs (Rossier et al., 2015; Nguyen et al., 2020). MMPs, part of the metzincin superfamily, consist of several functional classes including collagenases, stromelysins, and gelatinases, with the gelatinases MMP-2, and MMP-9 and the stromelysin MMP-3 representing the predominant MMPs in the brain [for review see Rivera et al., 2010 and Brzdak et al., 2017]. ADAMTSs also serve as the substrate for tissue inhibitors of metalloproteinases (TIMP) which bind to these proteases and inhibit ECM proteolysis (Brew et al., 2000). Other proteolytic enzymes include tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA), which cleave and activate plasminogens that can degrade various ECM molecules, including proteoglycans (Saksela, 1985). Endogenous ECM proteases have been reported to modify the ECM during learning, which may facilitate formation of new synapses in response to environmental stimuli (Nagy et al., 2007; Brown et al., 2009; Ganguly et al., 2013). Recent studies have also demonstrated that ECM molecules, including PNNs, are modified in a diurnal manner, which may contribute to memory consolidation processes (Harkness et al., 2019; Pantazopoulos et al., 2020; Gisabella et al., 2021). Taken together, these studies suggest that the ECM is modified in a time-dependent manner which may impact interpretation of studies regarding the role of the ECM in SUD and the development of therapeutic strategies targeting the ECM. Within this context, we review the current literature regarding the involvement of the ECM in reward learning, including drugs of abuse and natural rewards such as high fat diet, and the current evidence for ECM pathology in SUD with an emphasis on time-course dependent findings.

A number of studies have investigated the role of the ECM in animal models and human studies of SUD. Investigations have spanned an array of brain regions, including the prefrontal cortex (PFC) (Goldstein and Volkow, 2011), nucleus accumbens (NAc) (Quintero, 2013), ventral tegmental area (VTA) (Oliva and Wanat, 2016), hippocampus (Kutlu and Gould, 2016), amygdala (Warlow et al., 2017), cerebellum (Moulton et al., 2014), and hypothalamus (Zhang et al., 2020). Together, these brain regions are involved in regulating the rewarding and reinforcing effects of drugs, withdrawal symptoms, and the integration of memories of the rewarding effect of the drug and the associated context. Importantly, several studies cited throughout this review have varying definitions of the terms “acute” and “chronic.” For the purpose of this review, we define acute drug exposures as less than a period of 24-h (<2 h for “immediate”), and chronic drug exposure as greater than 3 consecutive daily sessions.

Despite the growing evidence for a key role of ECM molecules in substance use disorders, there are several knowledge gaps that limit the development of ECM-based pharmacotherapies. First, as indicated in this review, despite the growing number of studies, there is a lack of sufficient information regarding the temporal ECM changes in specific brain regions for specific substances. In addition, despite evidence for WFA labeled PNN alterations, there is a lack of information regarding changes in PNN composition and specific ECM structures such as perisynaptic ECM, axonal coats and/or perivascular ECM. Moreover, few studies thus far have examined the role of the ECM in neurodevelopmental factors prior to the onset of SUD that can contribute to risk of SUD, including genetic factors, chronic stress, exposure to a culture of drug use, traumatic experiences, and socio-economic status. Future studies on the ECM that consider such etiological factors would provide critical insight into the role of the ECM in enhanced risk to SUDs.

Furthermore, the vast majority of current evidence is largely based on animal model studies, there is currently a lack of information regarding these ECM structures in the brain of individuals with SUD. Human postmortem studies may provide critical information that can support preclinical studies and guide development of therapeutic strategies. Polysubstance use disorder is the most common clinical presentation of SUD, thus preclinical studies of polysubstance use may provide key insight into potential interactions of various drugs of abuse on the ECM and allow for development of more translatable therapeutic strategies. In terms of developing or repurposing drugs for ECM based treatments of SUD, the current evidence suggests that MMP inhibitors such as doxycycline may inhibit ECM degradation and formation of reward memories associated with initial drug exposure. In comparison, short-term upregulation of MMPs may represent an effective strategy to treat chronic SUD, especially in combination with CBT. Temporary removal of PNNs may allow for reorganization or weakening of reward associated synapses, and may prevent feedforward modulation of neuronal ensembles by parvalbumin, thus creating more glutamatergic noise and less coherence of the neurons that are associated with drug memories (Brown and Sorg, 2023). Furthermore, several lines of evidence indicate that inhibiting MMP activity may be useful in preventing relapse following a period of abstinence. The development of pharmacotherapies with selectivity for brain ECM structures which can be delivered systemically, provide temporal control, and limit potential off-target side effects remains a major challenge for ECM-based therapeutic strategies.

Regarding the role of the ECM in metabolic disorders, a limitation of rodent models of T2D (e.g., the Zucker rat) is the relatively abrupt development of hyperglycemia, which is not characteristic of the gradual human disease progression (Oltman et al., 2005; Hinder et al., 2018). As rodent models of T2D evolve, examining the role of the ECM in the progression from prediabetic-like states to T2D may provide evidence for temporal ECM alterations that can guide preventative and treatment strategies. As previously mentioned, many individuals with mood disorders often suffer from metabolic disorders and eating disorders, such as binge-eating disorder. In this context, there is currently a lack of evidence providing a link between mood dysregulation, HFD, and the ECM. Studies focused on these associations may enhance our understanding of potential ECM-based precipitating factors for metabolic disorders.

Current evidence suggests that the ECM is at the intersection of synaptic regulation and neuroimmune responses to drug reward and food reward. Synaptic regulation and inflammation are hallmarks of the neuroadaptations induced by both SUD and metabolic disorders (Cui et al., 2014; Hao et al., 2016). These processes may impact overlapping neuroanatomical circuits and cellular mechanisms. Food is essential for survival, and evolutionarily conserved processes may promote strengthening of brain circuits involved in integrating environmental cues associated with biologically relevant rewards such as food, especially food that is high in fat and calories. In comparison, drugs of abuse are not necessary for survival, and initial exposure to drugs most commonly occurs after the closure of critical periods of plasticity associated with maturation of PNNs. Therefore, enhanced PNNs by chronic drug (unnatural) reward may be the result of synaptic reorganization occurring later in development, recruiting additional ECM components to stabilize synapses that participate in integration of novel, potent reward experiences engaging brain circuits that are normally involved in regulating memories for experiences that are necessary for survival. Intriguingly, a similar temporal relationship of PNN alterations was reported in the rat hippocampus across stages of social defeat stress (Riga et al., 2017), suggesting that this temporal regulation of PNNs may be a shared mechanism underlying memory processing across positive and negative experiences. Decreased PNNs in obesity and metabolic disorders may instead reflect increased neuroinflammation associated with these conditions.

In summary, ECM molecules may represent key contributors to the pathogenesis of SUD and metabolic disorders. Acute exposure to substances of abuse render PNNs unstable through the activity of endogenous proteases, and chronic exposure generally increases PNNs and ECM molecules (Figure 3). Pathologically, SUD is an ingrained memory trace that maladaptively triggers activation of brain circuits that recurrently promote relapse. Thus, enhanced PNNs may reflect an inflexibility of memory systems pertaining to reward-associated stimuli which perpetuates the cycle of relapse and addiction.

All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.