Acute immobilization stress alters LH and ACTH release in response to administration of N-methyl-D,L-aspartic acid in peripubertal and adult male rats

Changes in Hypothalamus–Pituitary–Adrenal Axis Following Transient Ischemic Attack

Acute brain ischemia caused by transient ischemic attack initiates a complex sequence of events in the central nervous system and hypothalamic–pituitary–adrenal (HPA) axis which may ultimately culminate in neuronal and cell damage. The brain is highly susceptible to ischemia and in response to stress shows changes in morphology and chemistry that are largely reversible. These responses are known to modify the function of the HPA axis, but their mechanisms are not yet clear. Duration and size of the HPA axis activation are regulated by corticotropin-releasing hormone, vasopressin (AVP), and glucocorticoids, including cortisol. Numerous studies suggest that activation of these hormones following brain ischemia can result in neurohormonal dysfunction that can exacerbate long-term prognosis following stroke. These studies represent evidence that changes in the HPA axis play an important role in brain ischemia.

Sex and age differences in the impact of the forced swimming test on the levels of steroid hormones

Compared with the adult disorder, depression in children exhibits differences in its neurobiology, particularly in the HPA axis regulation. The bases of such differences can be evaluated in animal models of depression. The objective of the present study was to determine age and sex differences of Wistar rats in the forced swimming test (FST). The influence of sex and age on corticosterone, estrogens and testosterone serum levels was also determined. Prepubertal rats showed immobility, swimming and climbing behaviors during the pre-test and test sessions. In addition, in the prepubertal animals, no sex differences were found during the pre-test and test sessions. Age comparisons indicated no differences in the female groups, however adult males exhibited more immobility and less swimming than young males, in both FST sessions. The young and female rats showed less immobility behavior and increased levels of estrogens after the FST. The present results indicate that the FST is an animal model suitable to evaluate depressive-like behaviors in prepubertal subjects and to explore behavioral changes related to neurodevelopment.

Acute stress rapidly decreases plasma testosterone in a free-ranging male songbird: potential site of action and mechanism

We used a free-ranging, seasonally breeding adult male songbird, the rufous-winged sparrow, Aimophila carpalis, to investigate the effects of acute stress-induced by capture followed by restraint, on the hypothalamo-pituitary-testicular axis. Intra- and interindividual comparisons revealed that males decreased their plasma testosterone (T) by 37-52% in response to acute stress. The decrease occurred within 15 min of capture and persisted for at least another 15 min. Within 15 min, the decrease in plasma T was not associated with a reduction in plasma luteinizing hormone (LH). Thirty minutes after capture and restraint, the decrease in plasma T either was likewise not associated with decreased plasma LH (intraindividual comparison) or concurred with a reduction in plasma LH (interindividual comparison). These observations indicate that effects of stress may have been mediated at the pituitary gland and also directly at the testicular levels. To address this question, we measured the hormonal response to an injection of the glutamate receptor agonist N-methyl-d,l-aspartate (NMA) to stimulate to stimulate the release of gonadotropin-releasing hormone (GnRH) or of GnRH to stimulate the release of LH. Treatment with NMA did not change plasma LH, presumably because the birds were in breeding condition and already secreting GnRH at a maximum rate. Administration of GnRH increased plasma LH equally in birds that were or were not stressed before the treatment. An injection of purified ovine LH (oLH) increased plasma T equally in birds that were or were not acutely stressed before the hormone injection. Thus, the observed acute stress-induced decrease in plasma T was apparently not mediated by decreased responsiveness of the pituitary gland to GnRH or of the testes to LH. Decreased plasma T following stress may involve a direct impairment of the testicular endocrine function.

Glutamatergic stimulation of luteinising hormone secretion in relatively refractory male songbirds

Seasonal breeding in two Sonoran desert passerines, the Cassin's (Aimophila cassinii) and Rufous-crowned (Aimophila ruficeps) Sparrows is thought to be terminated by the development of a decrease in responsiveness to photostimulation, a condition known as relative photorefractoriness. It was predicted that the development of relative refractoriness is a consequence of a decrease in gonadotrophin-releasing hormone (GnRH) synthesis and associated stores of releasable GnRH. This hypothesis was tested by determining the luteinising hormone (LH) responses to the excitatory amino acid glutamate agonist N-methyl-D,L-aspartate (NMA) in males of the two species subjected to photomanipulations aimed at generating five groups: Fully photosensitive with undeveloped testes on short days (8L : 16D); fully photosensitive with developed testes on 13L : 11D; relatively photorefractory with regressed testes on 13L : 11D, and groups with developed testes held on 15L : 9D or 16L : 8D. LH release was stimulated in the Cassin's Sparrow by NMA most in the 8L group; to a lesser, but similar extent in the two 13L groups; and not at all in the 15L and 16L groups. LH release was not stimulated by NMA in any of the photoperiodic regimes in the Rufous-crowned Sparrow. In both species, NMA induced Fos-like immunoreactivity in the anterior and basal hypothalamus, but not in GnRH cell bodies. It is concluded that the development of relative photorefractoriness in Cassin's Sparrows is a consequence of reduced GnRH synthesis, reflected in a reduction in releasable GnRH. The lack of LH response of the Rufous-crowned Sparrows to NMA administration may be a consequence of high responsiveness to handling stress.

Behavioral effects of d-cycloserine in rats: The role of anxiety level

It has been reported that the glutamatergic N-methyl-D-aspartate (NMDA) receptor is involved in stress responses and that anxiety is the primary response to stress. Although individual differences in anxiety levels of rats have been demonstrated by using the elevated plus-maze (PM) test, the role of NMDA receptor activity in such individuality of anxiety is not clear. Here, we examined whether low (LA) and high (HA) anxiety rats might respond differently to treatment with d-cycloserine (DCS), a partial agonist of the glycine binding site located on NMDA receptors. Male Wistar rats were screened by using the PM and divided into LA and HA subgroups. On the next day, these rats were again tested in the PM, 30 min after the treatment with DCS (5, 10, or 30 mg/kg ip). Five days later, the rats were subjected to a 2-day forced swim (FS) test, receiving the DCS treatment again 30 min before the second day session. The PM data showed that DCS had anxiogenic effects in LA but not HA rats. The immobility of LA or HA rats in the FS test was not affected by DCS. The results indicate that the behavioral effects of DCS depend on the anxiety level of rats and have task-dependent behavioral consequences, suggesting that glycine binding sites on NMDA receptors are involved in individual differences of anxiety level.

Expression of mRNA and proteins for testicular steroidogenic enzymes and brain and pituitary mRNA for glutamate receptors in rats exposed to immobilization stress

The objectives of this study were to determine whether stress attenuates the pituitary LH response to excitatory amino acids by altering expression of glutamate receptor 1 (GluR1) and N-methyl-D-aspartic acid (NMDA) receptor mRNA levels in the hypothalamus or pituitary, and assess whether stress influences testicular levels of mRNA or protein for steroidogenic enzymes. Three hours (h) of immobilization stress was associated with a greater than 7-fold increase in serum corticosterone, and a marked reduction in serum testosterone (T) concentrations. Stress did not significantly alter hypothalamic or pituitary GluR1 and NMDA receptor mRNA levels. Although transcript levels for P450SCC and P45017alpha mRNA in the testis were unchanged in stressed rats, western blotting of testicular fractions revealed reduced amounts of P450SCC and 3beta-HSD, but not P45017alpha. The data suggest that immobilization stress reduces T production by suppressing the translation of transcripts for P450SCC and 3beta-HSD, but the attenuated LH response of stressed animals to NMDA is not mediated by altered hypothalamic or pituitary expression of GluR1 and NMDA receptor levels.