Dopamine receptor-mediated regulation of corticotropin-releasing hormone neurons in the hypothalamic paraventricular nucleus

The present study examined the effects of intraperitoneal administration of selective D1 (SKF 38393) and D2 (quinelorane) dopaminergic receptor agonists on Fos-like immunoreactivity (Fos-LI) and levels of corticotropin-releasing hormone (CRH) mRNA in the paraventricular nucleus of the hypothalamus (PVN) and in the central nucleus of the amygdala (cAMY). Ninety minutes after administration of the D1 agonist SKF 38393, Fos-LI was increased in both the PVN and cAMY. Administration of SCH 39166, a selective D1 antagonist, blocked and attenuated the SKF 38393-induced increase in Fos-LI in the PVN and cAMY, respectively. Similarly, 90 minutes after intraperitoneal injection of the D2 agonist quinelorane, Fos-LI was increased in both PVN and cAMY. Administration of the selective D2 antagonist raclopride prevented the ability of quinelorane to increase Fos-LI in the PVN and cAMY. Both SKF 38393 and quinelorane stimulated the expression of CRH mRNA in the PVN, but failed to alter its expression in the cAMY. Taken together, these results indicate that stimulation of either D1 or D2 dopaminergic receptors activates CRH neurons in the PVN. Stimulation of either D1 or D2 receptors activates neurons in the cAMY, but these changes do not appear to be occurring in CRH neurons.     

Prolonged alcohol intake leads to reversible depression of corticotropin-releasing hormone and vasopressin immunoreactivity and mRNA levels in the parvocellular neurons of the paraventricular nucleus

The ability of alcohol to activate the hypothalamic-pituitary-adrenal (HPA) axis is well documented in investigations based in acute and short-term experimental paradigms. Herein, we have addressed the possibility that the prolonged exposure to ethanol concentrations that are initially effective in stimulating corticosteroid secretion might induce alterations in the response of the HPA axis that cannot be evinced by shorter exposures. Using conventional histological techniques, immunohistochemistry and in situ hybridization, we have examined the medial parvocellular division of the paraventricular nucleus (PVNmp), and the synthesis and expression of corticotropin-releasing hormone (CRH) and vasopressin (VP) by its constituent neurons, in rats submitted to 6 months of ethanol treatment and to withdrawal (2 months after 6 months of alcohol intake). Ethanol treatment and withdrawal did not produce neuronal loss in the PVNmp. However, the total number of CRH- and VP-immunoreactive neurons and the CRH mRNA levels were significantly decreased by ethanol treatment. In withdrawn rats, the number of CRH- and VP-immunostained neurons and the gene expression of CRH were increased relative to ethanol-treated rats and did not differ from those of controls. No significant variations were detected in VP mRNA levels as a result of ethanol treatment or withdrawal. These results show that prolonged alcohol intake blunts the expression of CRH and VP in the parvocellular neurons of the PVN, and that this effect is, partially at least, reversible by withdrawal. They also suggest that the development of tolerance to the effects of ethanol involve changes that take place at the hypothalamic level.     

Corticosterone effects on corticotropin-releasing hormone mRNA in the central nucleus of the amygdala and the parvocellular region of the paraventricular nucleus of the hypothalamus

Using in situ hybridization histochemistry, we report differential expression of corticotropin-releasing hormone (CRH) mRNA in the central nucleus of the amygdala (CEA) and the parvocellular region of the paraventricular nucleus of the hypothalamus (PVN) following systemic treatment with corticosterone (CORT) in adrenally-intact rats. Both injection of low (1 mg/kg/day) and high (5 mg/day) CORT reduced CRH mRNA expression in the PVN in a dose-dependent manner, although it returned to normal at the low dose by 14 days. By contrast, the high dose of CORT increased CRH mRNA transiently in the CEA at 4 days, although the low dose of CORT decreased it at 14 days. In a second experiment, we implanted a slowly-releasing CORT pellet for 2 weeks (200 mg, 60 day release) subcutaneously. This treatment produced an elevation of CRH mRNA in the CEA both at 1 and 2 weeks, whereas CRH mRNA in the PVN was decreased to a large extent as seen in the high CORT group of the first experiment. These results suggest that glucocorticoids can facilitate CRH mRNA expression in the CEA, a site implicated in anxiety and fear, while restraining the hypothalamic-pituitary-adrenal axis as indicated by the reduction in CRH mRNA in the PVN.     

Differential Fos expression in the paraventricular nucleus of the hypothalamus, sacral parasympathetic nucleus and colonic motor response to water avoidance stress in Fischer and Lewis rats

The responsiveness of hypothalamic CRF to various stressors is reduced in the young female Lewis relative to the histocompatible Fischer rat. Whether such a difference impacts the brain-gut response to water avoidance stress was investigated by monitoring Fos immunoreactivity in the brain and sacral spinal cord and fecal pellet output. Exposure for 60 min to water avoidance stress increased the number of Fos positive cells in the paraventricular nucleus of the hypothalamus (PVN), nucleus tractus solitarius (NTS), and the parasympathetic nucleus of the lumbo-sacral spinal cord (L6-S1) in both Lewis and Fischer rats compared with non stress groups. The Fos response was lower by 32.0% in the PVN, and 63% in sacral parasympathetic nucleus in Lewis compared with Fischer rats while similar Fos expression was observed in the NTS. Stress-induced defecation was reduced by 52% in Lewis compared with Fischer rats while colonic motor response to CRF injected intracisternally resulted in a similar pattern and magnitude of defecation in both strains. The CRF receptor antagonist [ -Phe12,Nle^21,38C^aMeLeu³⁷]-CRF_12–41 injected intracisternally antagonized partly the defecation response in Lewis and Fischer rats. These data indicate that a lower activation of PVN and sacral parasympathetic nuclei in Lewis compared with Fisher rats may contribute to the differential colonic motor response and that the blunted CRF hypothalamic response to stress, unlike responsiveness to central CRF plays a role.     

Establishment of stable dominance interactions in prairie vole peers: Relationships with alcohol drinking and activation of the paraventricular nucleus of the hypothalamus

Dominance hierarchies are an important aspect of group-living as they determine individual access to resources. The existence of dominance ranks in access to space has not been described in socially monogamous, communally nesting prairie voles (Microtus ochrogaster). Here, we tested whether dominance could be assessed using the tube test. We also tested whether dominance related to alcohol intake, similar to what has been demonstrated in nonmonogamous species. Same-sex pairs of unfamiliar peers were tested in a series of three trials of the tube test, then paired and allowed individual access to alcohol and water for 4 days, and then tested again in the tube test. For all pairs, the same subjects won the majority of trials before and after alcohol drinking. The number of wins negatively correlated with alcohol intake on the first day of drinking and positively correlated with levels of Fos in the paraventricular nucleus of the hypothalamus following the tube test in a separate group of voles. Dominance was not related to Fos levels in other brain regions examined. Together, these results indicate that prairie voles quickly establish stable dominance ranks through a process possibly involving the hypothalamus and suggest that dominance is linked to alcohol drinking.     

Immunocytochemical evidence for stimulatory control by the ventral noradrenergic bundle of parvocellular neurons of the paraventricular nucleus secreting corticotropin releasing hormone and vasopressin in rats

The regulation, by catecholaminergic innervation, of parvocellular neurons of the paraventricular nuclei (PVN) secreting corticotropin releasing hormone (CRH) and vasopressin (Vp) was studied by immunocytochemical visualization of both neurohormones in control rats and in rats given discrete injections of 6-hydroxydopamine in the ventral noradrenergic ascending bundle (VNAB). In both groups, the changes in immunostaining intensities observed in axon terminals of the external median eminence and in PVN perikarya 48 h after a blockade of axoplasmic transport by intraventricular injections of colchicine, served as an index for hormonal release and synthesis. In controls, this treatment induced a strong decrease in CRH and Vp immunoreactivity within the terminals, together with intense labeling of PVN perikarya containing CRH. By contrast, bilateral VNAB lesions strikingly inhibited both the colchicine-induced reduction of the CRH and Vp immunoreactivity in axons and the accumulation of CRH in the perikarya. Unilateral VNAB lesions induced similar alterations but these were restricted to the ipsilateral PVN and median eminence. Comparison of these immunocytochemical data with earlier physiological observations on the effects of VNAB lesions on ACTH secretion indicates that the catecholaminergic afferents to the PVN conveyed by the VNAB stimulate the release and the synthesis of CRH and Vp by parvocellular neurons projecting into the external median eminence.     

The hypothalamic paraventricular nucleus in two types of Wistar rats with differebt stress responses. II. Differential Fos-expression

The present study investigates the role of corticotropin-releasing hormone (CRH) neurons in stress regulation by a comparison of stress induced Fos-immunoreactivity and CRH-immunoreactivity in the hypothalamic paraventricular nucleus (PVH) of APO-SUS (apomorphine-susceptible), APO-UNSUS (apomorphine-unsusceptible), normal Wistar and adrenalectomized Wistar (ADX) rats. The first two types represent a good model to study the role of the PVH in stress regulation, since they show different stress responses and a differential synaptic organization of the PVH. After placement on an open field for 15 min all rats showed an increase in the number of Fos-immunoreactive nuclei compared to control handling. Interestingly, open field stress, but not control handling, induces significantly fewes Fos-immunoreactive nuclei in the PVH of APO-SUS rats (1255 ± 49) compared to APO-UNSUS rats (1832 ± 201). Experiments with ADX rats revealed that 93% of the CRH-immunoreactive neurons contained a Fos-immunoreactive nucleus, which suggests that the differential Fos-expression in APO-SUS and APO-UNSUS rats represents a differential activation of the CRH neurons. This hypothesis is discussed in relation to reported differences in stress responses, stress-induced ACTH levels and synaptic organization of the PVH.