Neuroscience
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The modifications in the hypothalamus-pituitary-adrenal (HPA) axis function induced by repeated unavoidable stress exposure, according to a standardized procedure used for inducing an experimental model of depression, were studied. Rats exposed to this procedure show hyporeactivity to both pleasurable and aversive stimuli and this condition is antagonized by the repeated administration of classical antidepressant drugs. We also studied whether imipramine administration during stress exposure would interfere with the possible modifications in the HPA axis. Rats were exposed to a 4-week stress procedure with and without imipramine treatment and then tested for escape, as compared with non-stressed control animals. Twenty-four hours later all rats were bled through a tail nick for plasma corticosterone measurement before and after dexamethasone (10 microg/kg) or corticotropin-releasing hormone (CRH, 1 microg/kg) administration. Rats were then killed, adrenals and thymus weighed, brain areas dissected out and frozen for glucocorticoid receptors (GRs) and corticotropin-releasing hormone receptor 1 (CRHR1) immunoblotting and for the assessment of hypothalamic corticotropin-releasing hormone levels. ⋯ Rats exposed to a 4-week unavoidable stress showed escape deficit and their basal plasma corticosterone levels were higher than those of control animals. Moreover, they had decreased response to dexamethasone administration, adrenal hypertrophy, and decreased GR expression in the hippocampus, hypothalamus, medial prefrontal cortex and pituitary. No significant modifications in CRHR1 expression were observed in the pituitary nor in different discrete brain areas. CRH levels in the hypothalamus and the plasma corticosterone response to CRH administration were found to be higher in stressed rats than in controls. Imipramine treatment offset all the behavioral and neurochemical stress-induced modifications. In conclusion, the present results strengthen the assumption that the escape/avoidance behavioral deficit induced by inescapable stress exposure is accompanied by steadily increased HPA activity, and that imipramine effect is strongly related to a normalization of HPA axis activity.
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Phosphorylation of specific sites in the second intracellular loop and in the C-terminal domain have previously been suggested to cause desensitization and internalization of the mu-opioid receptor (MOP-R). To assess sites of MOP-R phosphorylation in vivo, affinity-purified, phosphoselective antibodies were raised against either phosphothreonine-180 in the second intracellular loop (MOR-P1) or the C-terminal domain of MOP-R containing phosphothreonine-370 and phosphoserine-375 (MOR-P2). We found that MOR-P2-immunoreactivity (IR) was significantly increased within the striatum of wild-type C57BL/6 mice after injection of the agonist fentanyl. ⋯ Mutant mice selectively lacking all forms of the beta-endorphin peptides derived from the proopiomelanocortin (Pomc) gene did not show increased MOR-P2-IR, decreased morphine antinociception, or reduced morphine CPP following pSNL. In contrast gene deletion of either proenkephalin or prodynorphin opioids did not block the effects of pSNL. These results suggest that neuropathic pain caused by pSNL in wild-type mice activates the release of the endogenous opioid beta-endorphin, which subsequently induces MOP-R phosphorylation and opiate tolerance.
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Prepulse inhibition of the startle response to auditory stimulation (AS) is a measure of sensorimotor gating that is disrupted by the dopamine D1/D2 receptor agonist, apomorphine. The apomorphine effect on prepulse inhibition is ascribed in part to altered synaptic transmission in the limbic-associated shell and motor-associated core subregions of the nucleus accumbens (Acb). We used electron microscopic immunolabeling of dopamine D1 receptors (D1Rs) in the Acb shell and core to test the hypothesis that region-specific redistribution of D1Rs is a short-term consequence of AS and/or apomorphine administration. ⋯ Also in the Acb core, D1R-labeled dendrites were significantly smaller in the VEH+AS group compared with all other groups. These results suggest that alerting stimuli and apomorphine synergistically affect distributions of D1R in Acb shell and core. Thus adaptations in D1R distribution may contribute to sensorimotor gating deficits that can be induced acutely by apomorphine or develop over time in schizophrenia.
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Corticotropin releasing factor (CRF) family peptides play key roles in integrating neural responses to stress. Both major CRF receptors have been pharmacologically identified in the dorsal raphe nucleus (DRN), a stress sensitive and internally heterogeneous nucleus supplying many forebrain regions with serotonergic input. Despite the involvement of chronic stress and serotonergic dysfunction in human mood and anxiety disorders, little is known about the effects of chronic CRF receptor activation on the DRN. ⋯ Changes in 5-HT1A, SERT, and relative Tph2 mRNA abundance were correlated with novel object exploration. These findings suggest chronic intra-DRN administration of CRF agonists decreases exploratory behavior, while producing subregionally limited changes in serotonergic gene expression. These studies may be relevant to mechanisms underlying behavioral changes after chronic stress.
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We investigated whether cortical glutamatergic and GABAergic release machineries can be differentiated on the basis of the proteins they express, by studying the degree of co-localization of synapsin (SYN) I and II, synaptophysin (SYP) I and II, synaptosomal-associated protein (SNAP)-25 and SNAP-23 in vesicular glutamate transporter (VGLUT) 1-, VGLUT2- and vesicular GABA transporter (VGAT)-positive (+) puncta in the rat cerebral cortex. Co-localization studies showed that SYNI and II were expressed in approximately 90% of VGLUT1+, approximately 30% of VGLUT2+ and 30-50% of VGAT+ puncta; SYPI was expressed in approximately 95% of VGLUT1+, 30% of VGLUT2+, and 45% of VGAT+ puncta; SYPII in approximately 7% of VGLUT1+, 3% of VGLUT2+, and 20% of VGAT+ puncta; SNAP-25 in approximately 94% of VGLUT1+, 5% of VGLUT2+, and 1% of VGAT+ puncta, and SNAP-23 in approximately 3% of VGLUT1+, 86% of VGLUT2+, and 22% of VGAT+ puncta. Since SYPI, which is considered ubiquitous, was expressed in about half of GABAergic axon terminals, we studied its localization electron microscopically and in immunoisolated synaptic vesicles: these studies showed that approximately 30% of axon terminals forming symmetric synapses were SYPI-negative, and that immunoisolated VGAT-positive synaptic vesicles were relatively depleted of SYPI as compared with VGLUT1+ vesicles. Overall, the present investigation shows that in the cerebral cortex of rats distinct presynaptic proteins involved in neurotransmitter release are differentially expressed in GABAergic and in the two major types of glutamatergic axon terminals in the cerebral cortex of rats.