Pattern of forebrain activation in high novelty-seeking rats following aggressive encounter

Abstract

We have previously demonstrated that selectively-bred High (bHR) and Low (bLR) novelty-seeking rats exhibit agonistic differences, with bHRs acting in a highly aggressive manner when facing homecage intrusion. In order to discover the specific neuronal pathways responsible for bHRs' high levels of aggression, the present study compared c-fos mRNA expression in several forebrain regions of bHR/bLR males following this experience. bHR/bLR males were housed with female rats for 2 weeks, and then the females were replaced with a male intruder for 10 min. bHR/bLR residents were subsequently sacrificed by rapid decapitation, and their brains were removed and processed for c-fos in situ hybridization. Intrusion elicited robust c-fos mRNA expression in both phenotypes throughout the forebrain, including the septum, amygdala, hippocampus, cingulate cortex, and the hypothalamus. However, bHRs and bLRs exhibited distinct activation patterns in select areas. Compared to bHR rats, bLRs expressed greater c-fos in the lateral septum and within multiple hypothalamic nuclei, while bHRs showed greater activation in the arcuate hypothalamic nucleus and in the hippocampus. No bHR/bLR differences in c-fos expression were detected in the amygdala, cortical regions, and striatum. We also found divergent 5-HT1A receptor mRNA expression within some of these same areas, with bLRs having greater 5-HT1A, but not 5-HT1B, receptor mRNA levels in the septum, hippocampus and cingulate cortex. These findings, together with our earlier work, suggest that bHRs exhibit altered serotonergic functioning within select circuits during an aggressive encounter.

Figure 1

Location of templates used to sample integrated optical density measurements within specific brain regions. Abbreviations: Acc - nucleus accumbens; AH - anterior hypothalamus; Arc - arcuate nucleus; BLA - basolateral amygdala; cc – corpus callosum; CeA - central amygdala; Cg - cingulate cortex; CPu – caudate-putamen; DG - dentate gyrus; DMH - dorsomedial hypothalamus; f fornix; LaA - lateral amygdala; LH - lateral hypothalamus; LPO - lateral preoptic area; LSD - dorsal lateral septal nucleus; LSI - intermediate lateral septal nucleus; LSV - ventral lateral septal nucleus; M1,2 - motor cortex 1 and 2; MeA - medial amygdala; MPA - medial preoptic area; opt – optic tract; ox – optic chiasm; PH - posterior hypothalamus; Pir - piriform cortex; PMD - dorsal premammillary nucleus; PMV - ventral premammillary nucleus; PVN - paraventricular nucleus; TC - tuber cinereum; VMH - ventromedial hypothalamus.

Figure 2

Photomicrographs from X-ray films illustrating the distribution of c-fos mRNA within several forebrain regions from Selectively-bred High Reponder (bHR) and bred Low Responder (bLR) rats. Coronal brain sections are arranged in a rostrocaudal sequence (A, most rostral through E, most caudal), with a representative bLR brain shown in the left column and bHR brain shown at right. Homecage intrusion evoked robust mRNA expression in several brain regions, including the lateral septum, ventral portion (LSV), medial preoptic area (MPA), paraventricular nucleus of the hypothalamus (PVN), dorsomedial hypothalamus (DMH), arcuate nucleus of the hypothalamus (Arc), and the central (CeA) and medial amygdala (MeA).

Figure 3

High-power photomicrographs of cresyl violet-stained sections (left column) and similar sections processed for in situ hybridization to detect c-fos mRNA expression in a representative bLR and bHR rat (middle and right columns, respectively). C-fos levels were assessed within multiple amygdalar nuclei, including: lateral (LaA), basolateral (BLA), central (CeA), and medial (MeA; A). The arcuate nucleus of the hypothalamus (Arc) was one hypothalamic region where we observed greater c-fos expression in bHR rats compared to bLRs (B). Three subdivisions of the lateral septum (LS) were analyzed: the dorsal (LSD), ventral (LSV), and intermediate (LSI; C). C-fos mRNA levels were also assessed in 4 subregions of the hippocampus: Cornu Ammonis fields CA1–CA3 and the dentate gyrus (DG) (D).

Figure 4

Homecage intrusion elicted robust region-specific patterns of c-fos mRNA expression in multiple forebrain regions of bHR and bLR animals. The homecage intrusion experience differentially activated subdivisions of the lateral septum (LS), with the greatest activation within the ventral subdivision (LSV) and the least activation within the dorsal subdivision (LSD) (data collapsed across bHR and bLR groups; A). Compared to bHRs, bLR males exhibited greater c-fos mRNA expression in the intermediate and ventral portions of the lateral septum (LSI and LSV, respectively) following the homecage intrusion experience (B). Homecage intrusion also produced a nuclei-specific response in the amygdala, with the greatest activation occurring in the medial nucleus of the amygdala compared to the other nuclei (data collapsed across groups; C). Surprisingly, there were no bHR/bLR differences in c-fos expression within these nuclei (D). Of the thirteen hypothalamic nuclei examined, the paraventricular nucleus of the hypothalamus (PVN) showed the greatest activation compared to the other nuclei (data collapsed across groups; E). Abbreviations for the other hypothalamic nuclei are the same as those defined in Figure 1C. bLRs showed greater intrusion-evoked c-fos activation within the posterior hypothalamus (PH), lateral preoptic area (LPO), and medial preoptic area (MPA) compared to bHRs, although bHRs showed greater activation in the arcuate nucleus (Arc). There were no phenotypic differences in the paraventricular nucleus (PVN) or ventromedial hypothalamus (VMH) (F). Finally, while the intrusion experience produced robust activation of the hippocampus, all subregions showed similarly high levels of c-fos expression (data collapsed across groups; G). bHR animals exhibited greater overall c-fos activation across all hippocampal subfields compared to bLRs (H). *** -- p<0.001; ** -- p<0.01; * -- p<0.05.

Figure 5

bHR and bLR males exhibit differences in 5-HT1A receptor mRNA levels within select forebrain regions. Panels A and B depict photomicrographs from X-ray films illustrating the distribution of 5-HT1A receptor mRNA (A) and 5-HT1B receptor mRNA (B) within several forebrain regions. 5-HT1A receptor mRNA levels are particularly enriched in the septum and hippocampus (A), while 5-HT1B receptor mRNA is highly expressed in the caudate-putamen and the CA1 subfield of the hippocampus (B). We found increased 5-HT1A receptor mRNA expression in the CA1 region of the hippocampus (C), dorsal and intermediate portions of the lateral septum (LSD and LSI, respectively; D), and the cingulate cortex (E) in bLR animals compared to bHRs. There were no bHR/bLR differences in the expression of 5-HT1B receptor mRNA (data not shown). ** -- p<0.001; * -- p<0.05.