No abstract

1 We show that a portion of the TM2 domain regulates the sensitivity of beta subunit-containing rat neuronal nicotinic AChR to the ganglionic blocker mecamylamine, such that the substitution of 4 amino acids of the muscle beta subunit sequence into the neuronal beta4 sequence decreases the potency of mecamylamine by a factor of 200 and eliminates any long-term e ects of this drug on receptor function. 2 The same exchange of sequence that decreases inhibition by mecamylamine produces a comparable potentiation of long-term inhibition by nicotine. 3 Inhibition by mecamylamine is voltage-dependent, suggesting a direct interaction of mecamylamine with sequence elements within the membrane ®eld. We have previously shown that sensitivity to TMP (tetramethylpiperidine) inhibitors is controlled by the same sequence elements that determine mecamylamine sensitivity. However, inhibition by bis-TMP compounds is independent of voltage. 4 Our experiments did not show any in¯uence of voltage on the inhibition of chimeric receptors by nicotine, suggesting that the inhibitory e ects of nicotine are mediated by binding to a site outside the membrane's electric ®eld. 5 An analysis of point mutations indicates that the residues at the 6' position within the beta subunit TM2 domain may be important for determining the e ects of both mecamylamine and nicotine in a reciprocal manner. Single mutations at the 10' position are not su cient to produce e ects, but 6' 10' double mutants show more e ect than do the 6' single mutants.

We previously demonstrated that oxytocin stimulates LH release from rat pituitary cells in vitro and advances follicular development and ovulation in mice in vivo. This study reports an investigation of rat LH levels following in-vivo administration of oxytocin. Injection of oxytocin (10 mIU/g, i.p.) to rats at 07.00, 08.00 and 09.00 h of pro-oestrus or at 09.00, 10.00 and 11.00 h of pro-oestrus advanced the onset of the LH surge (P less than 0.005) and attainment of peak concentrations of LH (P less than 0.02) in peripheral blood. On the other hand, the descending phase of the LH surge and the surge amplitude were not altered by oxytocin. Treatment at 05.00, 06.00 and 07.00 h of pro-oestrus or at 11.00, 12.00 and 13.00 h of pro-oestrus had no effect on the LH profile. A higher oxytocin dose (20 mIU/g) inhibited LH release when treatment was begun at 05.00, 07.00 or 09.00 h of pro-oestrus. A lower dose (5 mIU/g) was ineffective in altering LH concentrations. In addition, injections of oxytocin (10 mIU/g) at oestrus, metoestrus or dioestrus had no effect on the release of LH. Thus the efficacy of oxytocin in altering concentrations of LH was dose dependent and also critically affected by the day of the oestrous cycle and the time of pro-oestrus. Removal of endogenous oxytocin activity by the use of an oxytocin receptor antagonist abolished the pro-oestrous LH surge, indicating that oxytocin is a vital physiological component of the LH-releasing mechanism in rats. The study provides unequivocal evidence that oxytocin induces LH release in vivo, but the manifestation of oxytocin activity is dependent upon conditions of exposure.

Neurohypophysial hormones have been implicated in the control of anterior pituitary function, and oxytocin has been shown to stimulate gonadotrophin excretion and ovarian follicular development in certain species. To determine the role of neurohypophysial peptides in the control of gonadotrophin release, their actions on LH and FSH secretion were analysed in rats in vivo and in vitro. In adult female rats, administration of oxytocin during early pro-oestrus advanced the spontaneous LH surge and markedly increased peripheral LH levels at 15.00 h compared with control animals. In cultured pituitary cells from adult female rats, oxytocin and vasopressin elicited dose-related increases in LH and FSH release. Such responses were not affected by a potent gonadotrophin-releasing hormone (GnRH) antagonist that abolished GnRH agonist-induced release of LH and FSH. Oxytocin did not enhance GnRH agonist-stimulated gonadotrophin release to the same extent as it increased basal secretion, but at low concentrations of GnRH agonist the effects were additive. The gonadotrophin responses to oxytocin and vasopressin were inhibited by the specific neurohypophysial hormone antagonists, [d(CH2)5D-Ile2,Ile4,Arg8]vasopressin and [d(CH2)5Tyr (Me),Arg8]vasopressin. These results provide direct evidence that neurohypophysial hormones can stimulate gonadotrophin secretion through a receptor system distinct from the GnRH receptor. Such a mechanism could represent a complementary hypothalamic control system for long-term modulation of LH and FSH secretion by exerting a basal or tonic influence on gonadotrophin production.

To study the effects of exogenous growth hormone (GH) administration to the mother on fetal growth, dwarf rats with an isolated GH deficiency were given daily s.c. injections of GH throughout pregnancy. Fetuses were heavier in GH-treated mothers (p < 0.001), and pups from GH-treated mothers continued to grow faster (p < 0.001) than pups from control mothers throughout the postnatal growth period through to weaning. In normal Wistar rats, administration of a potent antiserum to somatostatin to pregnant rats increased the mean birth weight of the offspring (p < 0.01). Administration of GH to the mothers also significantly increased birth weight (p < 0.05), but administration of antiserum to rat GH resulted in a significant retardation of both fetal and placental growth (p < 0.001). These data suggest that maternal GH status is significantly involved in the growth of the fetal rat.