Effect of neuropeptide Y Y2 receptor deletion on emotional stress-induced neuronal activation in mice

Abstract

In different behavioral paradigms including the elevated plus maze (EPM), it was observed previously that deletion of the neuropeptide Y Y2 receptor subtype results in potent suppression of anxiety-related and stress-related behaviors. To identify neurobiological correlates underlying this behavioral reactivtiy, expression of c-Fos, an established early marker of neuronal activation, was examined in Y2 receptor knockout (Y2(-/-)) vs. wildtype (WT) mice. Mice were placed on the open arm (OA) or closed arm (CA) of the EPM for 10 min and the effect on regional c-Fos expression in the brain was investigated. The number of c-Fos positive neurons was significantly increased in both WT and Y2(-/-) lines after OA and CA exposure in 51 of 54 regions quantified. These regions included various cortical, limbic, thalamic, hypothalamic, and hindbrain regions. Genotype influenced c-Fos responses to arm exposures in 6 of the 51 activated regions: the cingulate cortex, barrel field of the primary somatosensory cortex, nucleus accumbens, dorsal lateral septum, amygdala and lateral periaqueductal gray. These differences in neuronal activity responses to the novel environments were more pronounced after OA than after CA exposure. Mice lacking Y2 receptors exhibited reduced neuronal activation when compared to WT animals in response to the emotional stressors. Reduced neuronal excitability in the identified brain areas relevant to the processing of motivated, explorative as well as anxiety-related behaviors is suggested to contribute to the reduced anxiety-related behavior observed in Y2(-/-) mice.

Fig. 1

Schematic diagrams adapted from the mouse brain atlas (Franklin and Paxinos, 1997), showing the 54 brain regions in which c-Fos expression was evaluated. The squares indicate the placement of grids for counting of c-Fos positive cells. Asterisks indicate the regions in which genotype (Y2^−/− vs. WT mice) influenced c-Fos induction to open or closed arm exposure in Experiments 1 and 2. The number in each slide indicates the Bregma level of brain section. Abbreviations in alphabetical order: AcbC, accumbens nucleus, core; AcbSh, accumbens nucleus, shell; Arc, arcuate hypothalamic nucleus; BLA, basolateral amygdaloid nucleus; BSTLP, bed nucleus of the stria terminalis, lateral division, posterior part; BSTMA: bed nucleus of the stria terminalis, medial division, anterior part; BSTMV, bed nucleus of the stria terminalis, medial division, ventral part; cAmy, central nucleus of the amydala; Cg, cingulate cortex; CIC, central nucleus of the inferior colliculus; Cl, claustrum; Cli, caudal linear nucleus of the raphe; CM, central medial thalamic nucleus; dlPAG, dorsolateral periaqueductal gray; DM, dorsomedial hypothalamic nucleus; dmPAG, dorsomedial periaqueductal gray; DR, dorsal raphe nucleus; DRI, dorsal raphe nucleus, inferior part; GrDG, granular layer of the dentate gyrus; IL, infralimbic cortex; lAmy, lateral amygdaloid nucleus; LC, locus coeruleus; LHb, lateral habenular nucleus; lPAG, lateral periaqueductal gray; LPBD, lateral parabrachial nucleus, dorsal part; LPBE, lateral parabrachial nucleus, external part; LPO, lateral preoptic area; LSD, lateral septal nucleus, dorsal part; LSV, lateral septal nucleus, ventral part; M2, secondary motor cortex; MePD, medial amygdaloid nucleus, posterodorsal part; MePV, medial amygdaloid nucleus, posteroventral part; MHb, medial habenular nucleus; MnR, median raphe nucleus; MPO, medial preoptic nucleus; MS, medial septal nucleus; NTS, nucleus of the solitary tract; Op, optic nerve layer of the superior colliculus; PC, paracentral thalamic nucleus; PIL, posterior intralaminar thalamic nucleus; Pir, piriform cortex; PMR, paramedian raphe nucleus; PP, peripeduncular nucleus; PrL, prelimbic cortex; PVA, paraventricular thalamic nucleus, anterior part; PVN, paraventricular hypothalamic nucleus; Py, pyramidal cell layer of the hippocampus; S1BF, primary somatosensory cortex, barrel field; SO, supraoptic nucleus; SuG, superficial gray layer of the superior colliculus; vlPAG, ventrolateral periaqueductal gray; VTA, ventral tegmental area; Xi, xiphoid thalamic nucleus; ZI, zona incerta.

Fig. 2

Representative bright-field photomicrographs showing decreased Fos expression in Y2 knockout (Y2^−/−) compared to wildtype (WT) mice in response to open arm exposure. Cortical areas: prelimbic cortex (PrL), cingulate cortex (Cg), primary somatosensory cortex (S1BF); nucleus accumbens core (AcbC); dorsal lateral septum (LSD) and medial amygdala, posteroventral part (MePV). Scale bar = 200 μm.

Fig. 3

Quantitative analysis of c-Fos-like immunoreactivity in Y2 knockout (Y2^−/−) and wildtype (WT) mice under basal conditions (A), after open arm (OAE; A and B) and closed arm (CAE; B) exposure. Only those brain areas are graphically presented in which the difference in stress-induced c-Fos expression between the two genotypes was statistically significant and consistent in both Experiments 1 (A) and 2 (B). Each column indicates the mean ± SEM number of c-Fos-positive cells in a tissue area of 0.01 or 0.04 mm². n = 4–9 per experimental group. *P < 0.05, **P < 0.01, and ***P < 0.001 Y2^−/− vs. WT mice.