Neuroendocrine mechanisms for reproductive senescence in the female rat: gonadotropin-releasing hormone neurons.

Reproductive aging in female rats is characterized by profound alterations in the neuroendocrine axis. The preovulatory luteinizing hormone (LH) surge is attenuated, and preovulatory expression of the immediate early gene fos in gonadotropin-releasing hormone (GnRH) neurons is substantially reduced in middle-aged compared with young rats. We tested the hypothesis that alterations in GnRH gene expression may be correlated with the attenuation of the LH surge and may be a possible mechanism involved in neuroendocrine senescent changes. Sprague-Dawley rats ages 4 to 5 mo (young), 12-14 mo (middle-aged), or 25 to 26 mo (old) were killed at 10:00 AM or 3:00 PM on proestrus, the day of the LH surge, or diestrus I in cycling rats, and on persistent estrus or persistent diestrus in acyclic rats. RNase protection assays of GnRH mRNA and GnRH primary transcript were performed. GnRH mRNA levels increased significantly with age, whereas GnRH primary transcript levels, an index of GnRH gene transcription, decreased in old compared to young and middle-aged rats. This latter result suggests that an age-related change in GnRH mRNA levels occurs independently of a change in gene transcription, indicating a potential posttranscriptional mechanism. On proestrus, GnRH mRNA levels increased significantly from 10:00 AM to 3:00 PM in young rats. This was in contrast to proestrous middle-aged rats, in which this afternoon increase in GnRH mRNA levels was not observed. Thus, the normal afternoon increase in GnRH mRNA levels on proestrus is disrupted by middle age and may represent a substrate for the attenuation of the preovulatory GnRH/LH surge that occurs in rats of this age, prior to reproductive failure.

Cellular distribution of the calcium-binding proteins parvalbumin, calbindin, and calretinin in the neocortex of mammals: phylogenetic and developmental patterns.

The three calcium-binding proteins parvalbumin, calbindin, and calretinin are found in morphologically distinct classes of inhibitory interneurons as well as in some pyramidal neurons in the mammalian neocortex. Although there is a wide variability in the qualitative and quantitative characteristics of the neocortical subpopulations of calcium-binding protein-immunoreactive neurons in mammals, most of the available data show that there is a fundamental similarity among the mammalian species investigated so far, in terms of the distribution of parvalbumin, calbindin, and calretinin across the depth of the neocortex. Thus, calbindin- and calretinin-immunoreactive neurons are predominant in layers II and III, but are present across all cortical layers, whereas parvalbumin-immunoreactive neurons are more prevalent in the middle and lower cortical layers. These different neuronal populations have well defined regional and laminar distribution, neurochemical characteristics and synaptic connections, and each of these cell types displays a particular developmental sequence. Most of the available data on the development, distribution and morphological characteristics of these calcium-binding proteins are from studies in common laboratory animals such as the rat, mouse, cat, macaque monkey, as well as from postmortem analyses in humans, but there are virtually no data on other species aside of a few incidental reports. In the context of the evolution of mammalian neocortex, the distribution and morphological characteristics of calcium-binding protein-immunoreactive neurons may help defining taxon-specific patterns that may be used as reliable phylogenetic traits. It would be interesting to extend such neurochemical analyses of neuronal subpopulations to other species to assess the degree to which neurochemical specialization of particular neuronal subtypes, as well as their regional and laminar distribution in the cerebral cortex, may represent sets of derived features in any given mammalian order. This could be particularly interesting in view of the consistent differences in neurochemical typology observed in considerably divergent orders such as cetaceans and certain families of insectivores and metatherians, as well as in monotremes. The present article provides an overview of calcium-binding protein distribution across a large number of representative mammalian species and a review of their developmental patterns in the species where data are available. This analysis demonstrates that while it is likely that the developmental patterns are quite consistent across species, at least based on the limited number of species for which ontogenetic data exist, the distribution and morphology of calcium-binding protein-containingneurons varies substantially among mammalian orders and that certain species show highly divergent patterns compared to closely related taxa. Interestingly, primates, carnivores, rodents and tree shrews appear closely related on the basis of the observed patterns, marsupials show some affinities with that group, whereas prototherians have unique patterns. Our findings also support the relationships of cetaceans and ungulates, and demonstrates possible affinities between carnivores and ungulates, as well as the existence of common, probably primitive, traits in cetaceans and insectivores.

Perinatal changes in hypothalamic N-methyl-D-aspartate receptors and their relationship to gonadotropin-releasing hormone neurons.

During the neonatal period, the brain is subject to profound alterations in neuronal circuitry due to high levels of synaptogenesis and gliogenesis. In neuroendocrine regions such as the preoptic area-anterior hypothalamus (POA-AH), the site of GnRH perikarya, these changes could affect the maturation of GnRH neurons. Because the GnRH system is developmentally regulated by glutamatergic neurons, we hypothesized that changes in the N-methyl-D-aspartate (NMDA) receptor system begin early in postnatal development, before the onset of puberty, thereby playing a role in establishing the appropriate environment for the subsequent maturation of GnRH neurons. To this end, we determined developmental changes in NMDA receptors, alterations in GnRH gene expression, and the regulation of GnRH neurons by the NMDA receptor system in developing male and female rats. In Exp I, NMDA receptor subunit (NR) 1 mRNA levels in the POA-AH were found to increase significantly (approximately 5-fold) from E18 through P10 in both males and females. NR2b mRNA increased significantly between P0 and P5 in both males and females. In contrast, NR2a subunit mRNA, which was in very low abundance in both males and females, increased only in males between P10 and P15. In Exp II we determined that GnRH gene expression changes differentially in developing male and female rats, with increases from P0 to P5 in males, and decreases from P5 to P10 in females. This latter effect in females is attributed to a change in GnRH gene transcription because GnRH primary transcript RNA levels paralleled changes in GnRH mRNA levels. In Exp III, we tested effects of treatment with an NMDA receptor analog on GnRH mRNA levels and found that only P5 and P10 male rats responded to NMDA receptor activation with an increase in GnRH mRNA levels, via a posttranscriptional mechanism. This greater responsiveness of males to NMDA receptor stimulation may be due to differences in the composition and levels of NMDA receptor subunits. Exp IV examined the localization of NR1 in the POA-AH during neonatal development. No GnRH neurons were immunopositive for NR1, indicating that effects of glutamate on GnRH neurons are mediated by interneurons or other glutamate receptor subunits or types. Taken together, these data indicate that glutamatergic inputs to the POA-AH change dramatically during the early postnatal period, before puberty and before the GnRH system is fully responsive to glutamate, consistent with the hypothesis that the maturation of inputs to GnRH neurons, and the establishment of the proper neurotransmitter "milieu" enabling the activation of GnRH neurons, occurs before the onset of puberty.