Chronic psychosocial stress regulates the expression of both GR and MR mRNA in the hippocampal formation of tree shrews

A persistent hyperactivity of the hypothalamic-pituitary-adrenal axis and thus elevated glucocorticoid levels are main neuroendocrine features of depressive symptomatology in humans. The broad range of effects that are set off by glucocorticoids is mediated by glucocorticoid receptors (GRs) and mineralocorticoid receptors (MRs), which themselves are subject to autoregulation. In order to investigate the impact of long-lasting psychological stress on corticosteroid receptor mRNA expression in the hippocampal formation, we employed the psychosocial stress paradigm in male tree shrews (Tupaia belangeri). By in situ hybridization studies and semiquantitative evaluation of stress-induced changes of GR and MR mRNA expression at the single-cell level, brain tissue from subordinate animals which were exposed to 27 days (1 h/day) of social confrontation was compared to that of nonstressed animals. Four weeks of stress exposure resulted in a downregulation of GR mRNA in the dentate gyrus and hippocampal subfields CA1 and CA3 of subordinate male tree shrews compared to controls. The MR mRNA content in these subfields of the anterior hippocampus was also clearly reduced. On the contrary, in a more posterior location on the longitudinal axis of the tree shrew hippocampus, the MR message was increased in subfields CA1 and CA3 and in the dentate gyrus. These results suggest a relevance of the stress-induced regulation of both corticosteroid receptor subtype mRNAs in a naturalistic challenging situation. Moreover, the differential regulation of MR mRNA along the rostrocaudal axis of the hippocampus adds another feature to the heterogenous composition of this structure.

Social isolation after a single defeat reduces striatal dopamine transporter binding in rats

A single social defeat in male rats has long lasting physiological and behavioural consequences, which are similar to those seen in depressive patients. In addition, the housing conditions after social defeat appear to be crucial for the development of depression-like symptoms. Because the dopaminergic system is thought to be altered in depressive illness, we investigated the impact of individual and group housing on the temporal development of changes of dopamine transporter (DAT) binding in male rats after a single social defeat. The number of striatal DAT binding sites was reduced in animals that remained isolated after being defeated. The isolation length after social defeat amplified this effect, indicating a temporal development of the changes on the striatal DAT. In animals which returned to the familiar group after social defeat the density of striatal DAT binding sites was not affected. We conclude that social isolation after a single defeat reduces the number of DAT binding sites. In contrast, a familiar environment after a single social defeat appears to prevent the stress-induced alterations on the dopaminergic system. This finding suggests that housing conditions are critical when investigating the central nervous effects of social defeat in male rats.

Interaction between substance P and TRH in the control of prolactin release

Substance P (SP) may participate as a paracrine and/or autocrine factor in the regulation of anterior pituitary function. This project studied the effect of TRH on SP content and release from anterior pituitary and the role of SP in TRH-induced prolactin release. TRH (10(-7) M), but not vasoactive intestinal polypeptide (VIP), increased immunoreactive-SP (ir-SP) content and release from male rat anterior pituitary in vitro. An anti-prolactin serum also increased ir-SP release and content. In order to determine whether intrapituitary SP participates in TRH-induced prolactin release, anterior pituitaries were incubated with TRH (10(-7) M) and either WIN 62,577, a specific antagonist of the NK1 receptor, or a specific anti-SP serum. Both WIN 62,577 (10(-8) and 10(-7) M) and the anti-SP serum (1:250) blocked TRH-induced prolactin release. In order to study the interaction between TRH and SP on prolactin release, anterior pituitaries were incubated with either TRH (10(-7) M) or SP, or with both peptides. SP (10(-7) and 10(-6) M) by itself stimulated prolactin release. While 10(-7) M SP did not modify the TRH effect, 10(-6) M SP reduced TRH-stimulated prolactin release. SP (10(-5) M) alone failed to stimulate prolactin release and markedly decreased TRH-induced prolactin release. The present study shows that TRH stimulates ir-SP release and increases ir-SP content in the anterior pituitary. Our data also suggest that SP may act as a modulator of TRH effect on prolactin secretion by a paracrine mechanism.

Chronic psychosocial stress reduces the density of dopamine transporters

The effect of different types of physical stress on brain dopaminergic function has been well established in rodents; however, the role of the dopaminergic system in more naturalistic stress situations is poorly understood. Therefore, the aim of the current study was to investigate the effect of chronic psychosocial stress on the dopamine transporter, which is an important component in the regulation of dopaminergic neurotransmission. For this purpose, we used the well-characterized paradigm of subordination stress in male tree shrews (Tupaia belangeri). In the present study, the animals were subjected to psychosocial stress for 28 days. Animals were daily videotaped and locomotor activity was quantified. In subordinate animals, urinary cortisol and noradrenaline, as well as adrenal weight, were increased, whereas body weight, locomotor activity and testicular function were decreased. Brain dopamine transporter binding sites were quantified by in vitro autoradiography using [3H] WIN 35,428 as ligand. Chronic stress reduced the number of binding sites (Bmax) in the caudate nucleus and the putamen without affecting the affinity (Kd). Stress did not influence the binding parameters in the nucleus accumbens, the substantia nigra or the ventral tegmental area. Furthermore, we found a positive correlation between locomotor activity and the Bmax values for [3H] WIN 35,428 binding in the caudate nucleus, the putamen and the nucleus accumbens. The present study shows that a naturalistic stressor, such as chronic psychosocial conflict, decreases dopamine transporter binding sites in motor-related brain areas, suggesting that the reduction in locomotor activity in subordinate tree shrews is related to the downregulation of dopamine transporter binding sites.

Localization of dopamine receptors in the tree shrew brain using [3H]-SCH23390 and [125I]-epidepride

The tree shrew is a mammalian species, which is phylogenetically related to insectivores and primates. The aim of the present study was to investigate the distribution of dopamine receptor D1- and D2-like binding sites in the brain of this non-rodent, non-primate mammal. Using in vitro autoradiography and employing the radioligands [3H]-SCH23390 and [125I]-epidepride, dopamine receptors were mapped and quantified. Significant findings with regard to the D1-like binding pattern include the presence of a "patchy" binding in the striatum. In the cortex, D1-like binding sites were observed in both the superficial and the deep layers. In the hippocampal formation, D1-like binding sites were seen primarily in the CAI region and not in the dentate gyrus. These characteristics of the D1 pattern in the tree shrew brain are shared by cat and monkey and human brain, but not by rodent brain. Significant findings with regard to the D2-like binding pattern include the presence of D2-like binding in the claustrum. In addition, the striatum demonstrated "patchy" D2-like binding. These characteristics of the D2 pattern in the tree shrew brain are shared by cat and monkey and human brain, but not by rodent brain. On the other hand, the significant densities of D2-like binding sites in the glomerular layer of the tree shrew olfactory bulb is a finding that discriminates tree shrews from higher evolutionary species who lack such binding. Overall, the evidence coincides with the view that tree shrews are phylogenetically related to primates.

Involvement of hypothalamic substance P in the effect of prolactin on dopamine release

In order to examine the role of hypothalamic SP in the feedback regulation of prolactin, we studied the effect of prolactin and dopamine on SP concentration and release, and the effect of SP on dopamine release. Hypothalamic fragments from male Wistar rats were incubated in the presence of prolactin, dopamine or SP under basal and K(+)-stimulated conditions. SP (10(-7) M) stimulated dopamine release, while dopamine (10(-7) M) decreased SP content and release. Prolactin (100 ng ml-1) increased SP content and release. An increase in hypothalamic SP content was also found during suckling. In addition, a specific antagonist for SP, Win 62,577, blocked the effect of prolactin and dopamine release. These results show an interaction between SP and dopamine at the hypothalamic level and suggest that SP could mediate the feedback action of prolactin on dopamine release.