AUTHOR=Haidar Mouna , Tin Kimberly , Zhang Cary , Nategh Mohsen , Covita João , Wykes Alexander D. , Rogers Jake , Gundlach Andrew L. 

TITLE=Septal GABA and Glutamate Neurons Express RXFP3 mRNA and Depletion of Septal RXFP3 Impaired Spatial Search Strategy and Long-Term Reference Memory in Adult Mice

JOURNAL=Frontiers in Neuroanatomy

VOLUME=Volume 13 - 2019

YEAR=2019

URL=https://www.frontiersin.org/journals/neuroanatomy/articles/10.3389/fnana.2019.00030

DOI=10.3389/fnana.2019.00030

ISSN=1662-5129

ABSTRACT=Relaxin-3 is a highly conserved neuropeptide abundantly expressed within neurons of the nucleus incertus, which project to nodes of the septohippocampal system including the medial septum/diagonal band of Broca (MS/DB) and dorsal hippocampus, as well as to limbic circuits. High densities of the Gi/o-protein-coupled receptor for relaxin-3, known as relaxin-family peptide-3 receptor (RXFP3) are expressed throughout the septohippocampal system, further suggesting a role for relaxin-3/RXFP3 signalling in modulating learning and memory processes that occur within these networks. Therefore, this study sought to gain further anatomical and functional insights into relaxin-3/RXFP3 signalling in the mouse MS/DB. Using Cre/LoxP recombination methods, we assessed locomotion, exploratory behaviour, and spatial learning and long-term reference memory in adult C57BL/6J Rxfp3loxP/loxP mice with targeted deletion of Rxfp3 in the MS/DB. Following prior injection of an AAV(1/2)-Cre-IRES-eGFP vector into the MS/DB to conditionally delete/deplete Rxfp3 mRNA/RXFP3 protein, mice tested in a Morris water maze displayed an impairment in allocentric spatial learning during acquisition, as well as an impairment in long-term reference memory on probe day. However, ‘RXFP3-depleted’ and control mice displayed similar motor activity in a locomotor cell and exploratory behaviour in a large open-field test. A quantitative characterisation using multiplex, fluorescent in situ hybridisation identified a high level of co-localisation of Rxfp3 mRNA and vesicular GABA transporter (vGAT) mRNA in MS and DB neurons (~87% and ~95% co-expression, respectively). Rxfp3 mRNA was also detected, to a correspondingly lesser extent, in vesicular glutamate transporter 2 (vGlut2) mRNA-containing neurons in MS and DB (~13% and ~5% co-expression, respectively). Similarly, a qualitative assessment of the MS/DB region, identified Rxfp3 mRNA in neurons that expressed parvalbumin mRNA (reflecting hippocampally-projecting GABA neurons), whereas choline acetyltransferase mRNA-positive (acetylcholine) neurons lacked Rxfp3 mRNA. These data are consistent with a qualitative immunohistochemical analysis that revealed relaxin-3-immunoreactive nerve fibres in close apposition with parvalbumin immunoreactive neurons in the MS/DB. Together these studies suggest relaxin-3/RXFP3 signalling in the MS/DB plays an important role in modulating specific learning and long-term memory associated behaviours in adult mice via effects on GABAergic neuron populations known for their involvement in modulating hippocampal theta rhythm and associated cognitive processes.