Synaptology of luteinizing hormone-releasing hormone neurons in the preoptic area of the male rat: effects of gonadectomy

Vasoactive intestinal polypeptide contacts on gonadotropin-releasing hormone neurones increase following puberty in female rats

Successful reproduction requires precise temporal coordination among various endocrine and behavioural events. The circadian system regulates daily temporal organization in behaviour and physiology, including neuroendocrine rhythms. The main circadian pacemaker in mammals is located in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. The SCN sends direct efferents to the reproductive axis via monosynaptic projections to gonadotropin-releasing hormone (GnRH) neurones. This communication generates circadian endocrine rhythms as well as the preovulatory luteinizing hormone (LH) surge necessary for successful ovulation. One SCN peptide thought to be important for the regulation of oestrous cycles is vasoactive intestinal polypeptide (VIP). VIP neurones from the SCN contact GnRH cells, and these cells are preferentially activated during an LH surge in rats. Unlike adult rats, prepubertal females do not exhibit oestrous cycles, nor do they exhibit an LH surge in response to oestradiol positive-feedback. The present study was undertaken to determine the extent to which the development of a 'mature' reproductive axis in female rats is associated with modifications in VIP contacts on GnRH neurones. The brains of diestrus adult (approximately 60 days of age) and prepubertal (21 days of age) female rats were examined using double-label fluorescence immunohistochemistry for VIP and GnRH, with light and confocal microscopy. Although the total number of GnRH-immunoreactive neurones did not differ between adult and prepubertal females, adults had a significant increase in the percentage of GnRH cells receiving VIP contacts compared to juveniles. These data suggest that the development of reproductive hormone rhythms and oestrous cyclicity may be, in part, due to modifications of VIP input to the GnRH system.

Prenatal testosterone masculinizes synaptic input to gonadotropin-releasing hormone neurons in sheep

In sheep, the control of tonic and surge GnRH secretion is sexually differentiated by testosterone in utero. However, GnRH neurons are not sexually dimorphic with respect to number, distribution, or gross morphology. Therefore, this study tested the hypothesis that prenatal steroids influence synaptic input to GnRH neurons. We compared the number of synapses on GnRH neurons from male, female, and androgenized female lambs (n = 5 each). Androgenized females were exposed to testosterone during mid-gestation. Yearling lambs were perfused, and GnRH neurons were visualized using the LR-1 antibody. Five to seven GnRH neurons from the rostral preoptic area in each animal were viewed at the ultrastructural level. Afferent synapses and glial ensheathment on each neuron were counted in a single section through the plane of the nucleus. GnRH neurons from females received approximately twice as many contacts (3.6 +/- 0.7 synapses/100 microm plasma membrane) as those from male lambs (1.6 +/- 0.3; p < 0.05), similar to previous reports in rats. In addition, the number of synapses on GnRH neurons from androgenized female lambs (1.5 +/- 0.5) was similar to that from male lambs, suggesting that prenatal steroids give rise to sex differences in synaptic input to GnRH neurons.

Comparison of ultrastructural characteristics of gonadotropin-releasing hormone neurons in prepubertal and adult male rats

Gonadotropin-releasing hormone neurons from prepubertal (29-day-old) and adult (three-month-old) male rats were demonstrated immunocytochemically using the LR1 antibody, and prepared for electron microscopic examination. Gonadotropin-releasing hormone neurons were equally immunoreactive in the two age groups, but there were heavy deposits of reaction product in the outer nuclear envelope of these neurons in prepubertal animals. Point count stereology on electron micrographic montages of gonadotropin-releasing hormone neurons at x25,000 was used to compare the relative proportion of cytoplasm containing various subcellular organelles. More of the cytoplasm was occupied by Golgi apparatus and secretory vesicles in the prepubertal animals. The representation of mitochondria was equal in the two age groups, while there were more lysosomes in the gonadotropin-releasing hormone neurons from adult animals. The density of synaptic input to the neurons was estimated using quantitative morphometrics on electron micrographs of three levels of section through the neuron, magnified x25,000. The percentage of the perikaryal membrane with synaptic contacts was greater in the gonadotropin-releasing hormone neurons from adults. Most strikingly, there were gonadotropin-releasing hormone terminals on gonadotropin-releasing hormone soma of these neurons in prepubertal animals, but not in the adults. The highly immunoreactive outer nuclear envelope and relative larger representation of Golgi and secretory vesicles in gonadotropin-releasing hormone neurons in prepubertal animals suggest that these cells are actively synthesizing peptides, including gonadotropin-releasing hormone. The large representation of Golgi apparatus may also reflect the active biosynthesis of membrane in association with the elaboration of neuronal processes.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural changes in gonadotropin-releasing hormone neurons as a function of age and ovariectomy in rats

In this study we examined the effects of aging on various aspects of the ultrastructure of gonadotropin-releasing hormone neurons in female rats, including the density of synaptic input and the volume fraction of various subcellular organelles. In addition, we explored the possibility that removal of estrogen might provide a protective effect on the aging of the gonadotropin-releasing hormone neuron as exposure to gonadal steroids alters the time course of reproductive aging. Our experimental groups included four- and 18-20-month-old virgin female rats divided as follows: young intact, young short-term ovariectomized, old intact, old short-term ovariectomized and old long-term ovariectomized. Brain tissue was processed for immunocytochemical detection of gonadotropin-releasing hormone neurons and selected cells from the preoptic area were chosen for electron microscopic examination. The percentage of plasma membrane containing synaptic modification was quantified using a morphometrics program, and the volume fraction of lysosomes/lipofuscin, rough endoplasmic reticulum and Golgi apparatus were estimated using point count stereology. Whereas we had previously found a significant increase in the density of synaptic input to gonadotropin-releasing hormone neurons in aged virgin male rats, the density of synaptic input to gonadotropin-releasing hormone cells in the virgin female was not affected by age. The volume fraction of lysosomes/lipofuscin was increased in all age groups. Aging produced a dramatic decrease in the volume fraction of rough endoplasmic reticulum as well as a decrease in Golgi, suggesting a general decrease in biosynthetic activity of the cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased synaptic input to gonadotropin-releasing hormone neurons in aged, virgin, male Sprague-Dawley rats

Using double-label ultrastructural immunocytochemistry, we found the synaptic input to gonadotropin-releasing hormone (GnRH) neurons in the preoptic area of aged (20 months old), virgin, male Sprague-Dawley rats to be denser than that in young adults (3 months old). These results confirmed earlier observations on F-344 virgin male rats. The aging F-344 rat, however, is prone to testicular tumor and so it was essential to see if the phenomenon was reproducible in another rat strain. In the first study, a portion of the increase in synaptic density was due to an increase in the proportion of synapses containing pleiomorphic vesicles, frequently associated with the neurotransmitter GABA. We tested the possibility directly using a double-label protocol for GnRH and glutamic acid decarboxylase (GAD). However, in the present study the density of input by GABA did not change with age. This inhibitory amino acid represented about 10% of the total innervation in young animals; but, in aged animals, because the total synaptic input was greater, GABA represented only about 4% of the innervation. Synaptic vesicles within GAD-immunoreactive terminals were uniformly clear and spherical, suggesting that pleiomorphic vesicle shape is not an appropriate criterion for GABAergic innervation.

Reproductive history affects the synaptology of the ageing gonadotropin-releasing hormone system in the male rat

This study is an examination of the density of synaptic input to gonadotropin-releasing hormone (GnRH) neurons in young adult and aged retired breeder male rats. In earlier experiments on aged virgin male rats we observed an increase in synaptic input to this specific neuronal population, ascribable in part to synapses containing flattened vesicles, suggesting GABAergic input. The present study utilized retired breeders in order to dissect the effects of ageing from those associated with reproductive behavioral history. Tissue from the preoptic area was treated for the simultaneous electron microscopic immunocytochemical demonstration of GnRH with tetramethylbenzidine and glutamic acid decarboxylase (the essential enzyme in the production of GABA) using 3,3'-diaminobenzidine. Estimates of the density of synaptic input to the soma of GnRH neurons were made by calculating the percentage of perikaryal membrane with postsynaptic modification. Five GnRH neurons per animal were measured using computerized morpho-metrics and differences in the percent of membrane with synaptic modification between experimental groups were tested using the Mann-Whitney U non-parametric statistic. There was no difference in the total density of synaptic input to GnRH neurons in the young and old animals, or in the proportion of this input that was immunoreactive for glutamic acid decarboxylase. Similar measurements were made on random, non-identified neurons in the same region and a significant decrease with ageing in total synaptic input was found, though the glutamic acid decarboxylase component was unchanged. The present results are in contrast to our earlier findings on virgin males and suggest that reproductive behavioral experience affects the connectivity of GnRH neurons.

Gonadal steroids affect LHRH preoptic cell number in a sex/role changing fish

In diandric sex-reversing fishes, sexually active males and females (primary phase) regularly transform into an alternative reproductive morph, terminal-phase males, that are morphologically and behaviorally distinct. The transformation from primary to terminal phase is associated with a twofold increase in the number of luteinizing hormone-releasing hormone (LHRH) immunopositive cells in the forebrain preoptic area, a region involved in both the initial development and daily control of reproductive physiology and behavior. We now show that implants of 11-ketotestosterone induce increases in LHRH cell number in both primary phase sexes to the level observed in field-collected terminal phase males. Conversely, gonadal steroids had no effect on the number of LHRH preoptic cells in terminal phase males, suggesting that this is indeed a terminal stage in the development of this species. These results demonstrate that transition to the terminal phase by both sexes involves a parallel and convergent change in LHRH cell number, which utilizes an evolutionarily conserved mechanism of sexual differentiation: the inductive effects of gonadal steroid hormones.

Ultrastructural differences between smooth and thorny gonadotropin-releasing hormone neurons

It has been suggested that gonadotropin-releasing hormone neurons with irregular contours receive more synaptic input than do those with smooth contours. To test this hypothesis, a morphometric analysis of gonadotropin-releasing hormone neurons of differing shapes was made. Male Sprague-Dawley rats were perfused and 40-microns tissue sections from the preoptic area were treated for the demonstration of gonadotropin-releasing hormone immunoreactivity, using the avidin-biotin horseradish peroxidase method with 3,3'-diaminobenzidine as the chromogen. Gonadotropin-releasing hormone neurons with either a smooth or a highly irregular outline, sampled at three depths ultrastructurally, were compared for density of synaptic innervation and for relative content of various subcellular organelles. Point counting stereology comparisons of five gonadotropin-releasing hormone neurons of the two shapes in four rats revealed that smooth neurons contained less cytoplasm than "thorny" neurons, but the size of their nuclei was the same. There were more and larger nucleoli in smooth cells. Thorny neurons contained more Golgi apparatus and more mitochondria, but the amount of rough endoplasmic reticulum was the same in neurons of the two morphological types. There was no difference in the incidence of synapses or in the density of synaptic input as measured per unit of neuronal membrane in smooth and thorny gonadotropin-releasing hormone neurons. These results suggest that smooth-contoured gonadotropin-releasing hormone neurons may be more actively transcribing a message while thorny neurons are more actively engaged in peptide processing and packaging.(ABSTRACT TRUNCATED AT 250 WORDS)