Inhibition of the LH Surge by Restraint Stress in Cyclic Rats: Studies on the Role of GABAA and GABAB Receptors

There is evidence that stress can alter the activity in the brain of gamma-aminobutyricacid (GABA), a neurotransmitter that has been implicated in the regulation of LH secretion. In the present study the role of GABA in the restraint stress-induced inhibition of the LH surge was investigated in the intact cyclic rat. Intracerebroventricular (icv) administration of the GABAA receptor agonist muscimol (0.1, 0.5 or 1 μg) 5 min before the presumed onset of the pro-oestrous LH surge (at 0900 h) caused a dose dependent suppression of the surge. A single dose of the GABAB receptor agonist baclofen (1 μg; icv) injected at 0855 h postponed the onset of the LH surge, and repeated injections at 0855 and 1130 h suppressed the surge. These data indicate that GABA-ergic activity in the brain can inhibit the LH surge in the cyclic rat via GABAA and GABAB receptors. Pro-oestrous rats were subjected to 5 hrs of restraint starting at 0855 h. Pretreatment with the GABAA receptor antagonist bicuculine (1 μg; icv) at 0840, 0940 and 1040 h or pretreatment with the GABAB receptor antagonist phaclofen (10 μg; icv) at 0840 h were ineffective in preventing the restraint-induced inhibition of the LH surge. The results suggest that GABAA and GABAB receptors are not involved in the inhibitory effect of restraint stress on the LH surge.     

The adaptive response of adult rat Leydig cells to repeated immobilization stress: the role of protein kinase A and steroidogenic acute regulatory protein

The ability of immobilization stress (IMO) to decrease Leydig cell steroidogenesis and serum androgen concentration has been previously observed, but the possible mechanism(s) involved in the adaptation to prolonged or repeated stress have not been identified. In this study, we investigated whether the Leydig cells obtained from adult rats subjected to acute (15 min, 30 min or 2 h) and repeated (2 or 10 days, 2 h daily) IMO show adaptive mechanism(s) in response to stress-impaired steroidogenesis. The results showed that basal and human chorionic gonadotropin-stimulated cAMP production by Leydig cells isolated from rats exposed to both acute and repeated IMO was significantly reduced. Despite the reduced cAMP production, immunoblot analysis revealed increased immunoreactivity for both protein kinase A (PKA) and steroidogenic acute regulatory (StAR) protein in Leydig cells obtained from rats repeatedly exposed to IMO. Also, the phosphorylation and production of mature StAR protein was evident during exposure of rats to repeated IMO treatment. Treatment with cholesterol, the steroid substrate transported into mitochondria by StAR, significantly increased androgen and progesterone production by Leydig cells isolated from rats exposed to repeated IMO. In contrast, when other steroid substrates (22(R)-OH-cholesterol, pregnenolone, progesterone, Delta4-androstenedione) were present in the culture media, Leydig cell steroidogenesis was still reduced by IMO. Thus, PKA-mediated phosphorylation of StAR protein is an important mechanism in the adaptive response of Leydig cells to repeated IMO.