Subregional Expression of Hippocampal Glutamatergic and GABAergic Genes in F344 Rats with Social Isolation after Weaning

Many studies have shown that postweaning social isolation (pwSI) alters various behavioral phenotypes, including hippocampus-dependent tasks. Here, we report the comprehensive analysis of the expression of glutamatergic and GABAergic neurotransmission-related genes in the distinct hippocampal subregions of pwSI rats. Male F344 rats (age, 4 wk) experienced either pwSI or group housing (controls). At 7 wk of age, the hippocampus of each rat was removed and laser-microdissected into the CA1 and CA3 layers of pyramidal cells and the granule cell layer of the dentate gyrus. Subsequently, the expression of glutamatergic- and GABAergic-related genes was analyzed by quantitative RT-PCR. In the CA1 and CA3 pyramidal cell layers, 18 of 24 glutamate receptor subunit genes were at least 1.5-fold increased in expression after pwSI. In particular, the expression of several N-methyl-D-aspartate and kainate receptors (for example, Grin2a in CA1, Grik4 in CA3) was significantly increased after pwSI. In contrast, pwSI tended to decrease the expression of GABA_A receptor subunit genes, and Gabra1, Gabra2, Gabra4, Gabra5, Gabrb2, Gabrg1, and Gabrg2 were all significantly decreased in expression compared with the levels in the group-housed rats. These results indicate a subregion-specific increase of glutamate receptors and reduction of GABA_A receptors, suggesting that the hippocampal circuits of pwSI rats may be in more excitable states than those of group-housed rats.Abbreviations: AMPA, α-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid, LMD, laser microdissection, NMDA;N-methyl-D-aspartic acid, pwSI, postweaning social isolation, DG;dentate gyrusBehavioral phenotypes differ among strains of laboratory animals, including mice and rats. The acquisition of strain-specific behavioral traits is governed not only by the genetic background but also by the postnatal rearing environment. The level of maternal care is a representative environmental factor. For example, inbred Fischer 344 pups raised by Wistar dams showed Wistar-like behavioral traits in adulthood in socially interactive situations.³⁵ Furthermore, the effect of environmental factors is not limited to neonatal periods. In animals that form a society among conspecifics, such as mice and rats, postweaning social environments have a marked effect on behavioral traits. Many studies have reported that postweaning social isolation (pwSI) alters various strain-specific behavioral phenotypes, including aggressiveness,^15,32,34 novelty preference,²² locomotor activity,²⁹ anxiety-like behavior,²⁰ and learning and memory.^13,15,20,31 In other words, pwSI means the deprivation of several social interactions among conspecifics. The expression of social contact begins fundamentally in the postweaning periods, its frequency increases to a peak at 4 to 5 wk of age, and it declines thereafter until sexual maturity.^2,17,30 Therefore, rats were isolated from their conspecifics to deprive them of social contact during the current study.In the brain, the hippocampus is necessary for the acquisition of episodic and spatial memory,²⁸ but the influences of pwSI on hippocampal functions remain largely unclear. The hippocampus has a lamellar organization of neurons, and the intrinsic neuronal circuit of the hippocampus, termed the trisynaptic circuit, consists of 3 topographically and morphologically distinct neuronal layers: the pyramidal cell layer in subfields CA1 and CA3 and the granule cell layer in the dentate gyrus (DG). Sensory information is carried first to the DG by a perforant pathway that originates in the entorhinal cortex. DG granule cells project to the apical dendrites of the CA3 pyramidal cells through mossy fibers. In turn, CA3 pyramidal cells project to the CA1 layer through Schaffer collaterals.^1,18 Glutamate is a key excitatory neurotransmitter in the hippocampus and plays a central role in the activation of the trisynaptic circuit, whereas the inhibitory neurotransmitter GABA modulates the activated circuit. This excitatory–inhibitory balance is critical for the appropriate functioning of the hippocampal circuit. Here, we comprehensively investigated the effect of pwSI on the expression of glutamatergic and GABAergic neurotransmission-related genes in the 3 hippocampal subregions of inbred F344 rats.