Noradrenergic regulation of alpha 1-receptors during the postnatal development of the guinea pig

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

Reproductive behavioral responsiveness to noradrenergic stimulation in developing guinea pigs

The stimulatory effects of ovarian hormones on sexual receptivity in guinea pigs may be mediated by norepinephrine. Juvenile females rarely exhibit steroid-induced receptivity and also respond poorly to the lordosis-enhancing action of alpha-noradrenergic receptor stimulation. This experiment was designed to chart the development of behavioral responsiveness to the alpha-noradrenergic agonist, clonidine, and to test the hypothesis that higher doses of estradiol and/or clonidine are required to stimulate lordosis in juvenile compared to adult guinea pigs. Ovariectomized females received estradiol benzoate (10 microg or 50 microg SC) 40-48 h before administration of clonidine (1 mg/kg or 5 mg/ kg IP) or saline at 14-17-day intervals. Regardless of treatment, few animals (0-36%) displayed lordosis at 20, 34, or 48 days of age. At 65 days of age, in both estradiol dose groups significantly more clonidine- (1 mg/kg) than saline-injected animals displayed lordosis (80-91 vs. 0-33%, respectively). Clonidine (5 mg/kg) was ineffective at all ages. These data do not support the hypothesis that behavioral responsiveness to alpha-noradrenergic receptor stimulation in immature females can be elicited by increasing the doses of estradiol and/or clonidine. These results suggest the occurrence of a maturational change in the neural systems governing noradrenergic involvement in steroid-induced sexual behavior in guinea pigs.

Alpha 1-adrenoceptors in the adult rat barrel field: effects of deafferentation and norepinephrine removal

Norepinephrine (NE), acting on brain adrenoceptors, plays an important role in barrel field neuronal activity and plasticity. For this reason, the distribution of alpha 1- and alpha 2-adrenoceptors in the somatosensory cortex barrel field was studied by autoradiographic techniques in rats undergoing plastic change or NE depletion. In layers IV and V of the cortex, the pattern of alpha 1-adrenoceptors (assessed by [3H]prazosin binding) varied across the barrel field. There was relatively low binding within the barrels themselves, with 21% higher binding in the surrounding septa. alpha 2-Adrenoceptor binding (assessed with [3H]paraminoclonidine) was almost homogeneous across the entire barrel field. Two weeks after noradrenergic deafferentation by unilateral lesioning of the locus coeruleus, there was a 16% upregulation of [3H]prazosin binding. This then returned to control levels of by 8 weeks. Peripheral deafferentation of sensory input to the barrel field produced the opposite effect on alpha 1-adrenoceptors. Unilateral removal of all but the central (C3) vibrissa (which induces plastic changes in the cortical representation of the spared virbrissa) caused a 12% decrease in [3H]prazosin binding in the whole barrel field at 2 weeks after surgery which returned to normal by 8 weeks. Therefore, alpha 1-adrenoceptors in the barrel field of the rat are affected in opposite ways by changes in NE content and afferent sensory input. We hypothesize that alpha 1-adrenoceptor levels are modulated after vibrissectomy through either an indirect reaction to reduced cortical gamma-aminobutyric acid levels, or by a reordering of metabolic priorities during plastic change of the cortical neuronal network.

Development of estradiol-induced progestin receptor immunoreactivity in the hypothalamus of female guinea pigs

The inability of young female guinea pigs to display progesterone-facilitated lordosis has been attributed, in part, to a deficiency in the concentration of hypothalamic estradiol-induced progestin receptors, as measured by in vitro binding assays. An immunocytochemical technique was used to ascertain where, within the mediobasal hypothalamus, estradiol-induced progestin receptor levels are lower in immature than in adult females. Adult (greater than 7 weeks) and juvenile (3 weeks) ovariectomized females received 10 micrograms estradiol benzoate, a dose that primes adult, but not immature females to respond behaviorally to progesterone. Progestin receptor-immunoreactive (PR-IR) cells were counted in the arcuate nucleus (ARC) and ventrolateral hypothalamus (VLH), the two regions containing the densest populations of estradiol-induced progestin receptors in the mediobasal hypothalamus. There was no age difference in the number of PR-IR cells in the rostral or caudal VLH, but immunostaining was darker in the rostral VLH of juveniles as compared to adults. We found similar numbers of PR-IR cells in the rostral and mid-ARC, but 35% fewer immunostained cells in the caudal ARC of immature, as compared to adult females. Furthermore, staining intensity was weaker in the mid- and caudal ARC of the juvenile females. These data suggest that the ARC, not the VLH, is a site of fewer estradiol-induced progestin receptors in immature females.

Distribution of alpha 2-adrenergic receptors in the brain of the Japanese quail as determined by quantitative autoradiography: implications for the control of sexually dimorphic reproductive processes

With the use of [3H]p-aminoclonidine (PAC), alpha 2-adrenergic binding sites were mapped in the brain of the Japanese quail (Coturnix coturnix japonica). The sites were labeled with the use of in vitro quantitative autoradiography. Special attention was given to areas implicated in the control of sexually dimorphic reproductive processes including sexual behavior. Preliminary competition experiments found that [3H]PAC binding on tissue sections exhibited a pharmacology appropriate to the alpha 2 receptor. Binding sites were found to be heterogeneously distributed throughout the brain. Some of the highest levels of specific binding were found in several areas regulating reproductive function such as the preoptic area, the supraoptic nucleus, the infundibulum, and the medial mammillary nucleus of the infundibulum. [3H]PAC labeled precisely the morphologically dimorphic preoptic medial nucleus but no sexual dimorphism in density of receptor binding was identified. However, dimorphism in density of receptor binding was identified in two areas: the medial mammillary nucleus and the mesencephalic intercollicular nucleus. The former area appears to be involved in the regulation of gonadotrophin secretion and the latter area has been implicated in the control of vocal behavior. These neurochemical dimorphisms may contribute to the regulation of two sexually dimorphic reproductive processes, gonadotropin secretion and courtship vocalizations.

Electrophysiological and biochemical characterization of the development of alpha 1-adrenergic and 5-HT1 receptors associated with dorsal raphe neurons

Electrophysiological and quantitative autoradiographic receptor binding approaches were used to characterize the postnatal development of alpha 1-adrenergic and serotonergic receptors in the midbrain dorsal raphe nucleus (DRN) in rats. Dose-response data were obtained from extracellular records of the firing of DRN neurons recorded in midbrain slices perfused with artificial cerebral spinal fluid containing varying concentrations of the drugs investigated. These data indicated that DRN neurons, at all postnatal ages, are equally sensitive to the excitatory effects of phenylephrine (an alpha 1-norepinephrine agonist) and the inhibitory effects of serotonin (5-HT). In contrast, LSD (a 5-HT agonist) was considerably more potent in neonates as compared to adults. With one exception, the autoradiographic binding data was found to be in agreement with the electrophysiological findings. There were no observed changes in the number of alpha 1-adrenergic ([3H]prazosin) binding sites at different ages suggesting that cells in this region exhibit functionally mature alpha 1-adrenergic responses at birth. Similarly, the number of [3H]LSD binding sites was found to increase in older animals consistent with the increased potency of LSD in older animals. Paradoxically, the number of [3H]5-HT sites was observed to increase with age, a finding inconsistent with the functional data obtained using 5-HT in the slice. Hypotheses for the observed discrepancy are examined.

Alpha 1- and alpha 2-noradrenergic receptors in steroid-sensitive brain areas: development and response to estradiol-17 beta benzoate in neonatal guinea pigs

Immature female guinea pigs are relatively insensitive to the effects of ovarian steroids on certain reproductive processes including the facilitation of sexual receptivity and the induction of hypothalamic progestin receptors by estrogen. In adult guinea pigs, alpha-receptor-mediated noradrenergic transmission is known to affect both of these processes. In the experiments reported here, we investigated the possibility that age-related differences in alpha 1- and alpha 2-noradrenergic receptors might underlie the insensitivity of neonates to ovarian steroids. With tritium-sensitive film autoradiography, the distribution of alpha 1- and alpha 2-receptors in neonatal and adult female guinea pigs was compared and the effects of exogenous estradiol-17 beta on alpha-receptor binding was examined in neonatal guinea pigs. The results of these experiments showed that in animals not treated with estrogen the binding of both alpha-receptor subtypes differed between adults and neonates in several brain regions including areas known to be involved in the regulation of reproduction. In all regions where differences occurred, alpha-receptor levels were higher in neonatal females than in adult females. In addition, in contrast to previously reported results in adults, estrogen did not affect alpha-receptor binding in any region of neonatal guinea pig brain.

Alpha 1- and alpha 2-noradrenergic receptor binding in guinea pig brain: sex differences and effects of ovarian steroids

Noradrenergic transmission mediated by alpha 1-receptors is involved in the regulation of ovarian steroid-dependent reproductive processes in female guinea pigs. In addition, estradiol treatment is known to affect alpha 2-receptor binding in some steroid-sensitive regions of guinea pig brain. In these experiments, we further assessed the effects of ovarian hormones on alpha-noradrenergic receptors with quantitative autoradiographic methods. Binding of alpha 1- and alpha 2-noradrenergic receptors was measured in male and female guinea pigs treated with either estradiol or oil to determine if sex differences in steroid sensitivity might be related to differences in the distribution or modulation of alpha-receptors. In a second experiment, the possibility was tested that progesterone might alter the sensitivity to norepinephrine by modulating alpha 1- or alpha 2-receptor binding in estradiol-primed female guinea pigs. Results of these studies showed that estradiol treatment reduced alpha 1- and alpha 2-receptor binding of the ventromedial hypothalamic nucleus only in females and increased alpha 2-receptor binding in several preoptic nuclei of both sexes. Progesterone administration, however, had no effect on either alpha-receptor subtype. These studies indicate that estradiol-induced changes in alpha-receptor binding might be involved in steroid-dependent reproductive processes and in the differential responsiveness of males and females to ovarian hormones. Changes in these receptors do not appear to be involved in the stimulatory effects of progesterone on neuroendocrine processes.