Multiple peptides infrequently coexist in progesterone receptor-containing neurons in the ventrolateral hypothalamic nucleus of the guinea-pig: an immunocytochemical triple-label analysis of somatostatin, neurotensin and substance P

Somatostatin in the rat periventricular nucleus: sex differences and effect of gonadal steroids

In the rat, the sexual dimorphism in growth hormone release is driven by sex steroids, and is suggested to result mainly from differences in somatostatin (SOM) release patterns from the median eminence. We studied the effect of gonadal steroids on SOM peptide-containing cells in the periventricular nucleus (PeVN) of ovariectomized (OVX) female rats, and compared these data with data from intact male rats. Adult female rats were treated with estradiol (E₂) and/or progesterone (P), 3 months (long-term) or 2 weeks (short-term) after ovariectomy (OVX). Perfusion-fixed brains were sliced and stained, and the number of SOM-immunoreactive (-ir) cells and total SOM-ir area (in μm²) were determined using computer assisted analysis. SOM-ir cells in the PeVN showed a very characteristic rostro-caudal distribution and localization in relation to the third ventricle. Both the number of SOM-ir cells and total SOM-ir area in the PeVN were higher in male compared to OVX female rats. Neither the number of SOM-ir cells, nor the total SOM-ir area in the PeVN was affected by E₂ or P treatment alone. Treatment with both gonadal steroids, however, did increase total SOM-immunoreactivity. This study is the first to describe SOM cell distribution within the rat PeVN in great detail. A clear sex difference exists in SOM peptide content in the rat PeVN. In addition, E₂ and P may act synergistically to affect SOM cells in the female PeVN, suggesting that both gonadal steroids may be involved in the generation of the typical feminine SOM release pattern.

Localization of neurotensin immunoreactivity in neurons and organ of corti of rat cochlea

The distribution of neurotensin-containing cell bodies and fibers has been observed in the central and peripheral nervous system, including sensory ganglia, but no description has been found in the peripheral auditory system. Here, we investigated the presence of neurotensin immunoreactivity in the cochlea of the adult Wistar rat. Strong neurotensin immunoreactivity was detected in the cytoplasm of the inner hair cells (IHC) and Deiters' cells of the organ of Corti. Outer hair cells (OHC) show weak immunoreaction. Neurotensin immunoreactivity was also found in the neurons and fibers of the spiral ganglia. Quantitative microdensitometric image analysis of the neurotensin immunoreactivity showed a strong immunoreaction in the hair cells of organ of Corti and a moderate to strong labeling in the spiral ganglion neurons. A series of double immunolabeling experiments demonstrated a strong neurotensin immunoreactivity in the parvalbumin immunoreactive IHC and also in the calbindin immunoreactive Deiters' cells. Weak neurotensin immunoreactivity was seen in the calbindin positive OHC. Neurofilament and parvalbumin immunoreactive neurons and fibers in the spiral ganglia showed neurotensin immunoreactivity. Calbindin immunoreactivity was not detected in the spiral ganglion neurons, which are labeled by neurotensin immunoreactivity. The presence of neurotensin in the cochlea may be related to its modulation of neurotransmission in the peripheral auditory pathway.

Role of endogenous opioid peptides in mediating progesterone-induced disruption of the activation and transmission stages of the GnRH surge induction process

How progesterone blocks the E2-induced GnRH surge in females is not known. In this study we assessed whether the endogenous opioid peptides (EOPs) that mediate progesterone negative feedback on pulsatile GnRH secretion also mediate the blockade of the GnRH surge. We treated ovariectomized ewes with physiological levels of E2 and progesterone to stimulate and block the GnRH surge, respectively, using LH secretion as an index of GnRH release. A pilot study confirmed that blocking opioidergic neurotransmission with the opioid receptor antagonist, naloxone (NAL; 1 mg/kg.h, i.v.), could prevent the suppression of pulsatile LH secretion by progesterone in our model. By contrast, antagonizing EOP receptors with NAL did not restore LH surges in ewes in which the E2-induced GnRH surge was blocked by progesterone treatment during the E2-dependent activation stage (Exp 1) of the GnRH surge induction process. However, in ewes treated with progesterone during the E2-independent transmission stage (Exp 2), NAL partially restored blocked LH surges, as indicated by increased fluctuations in LH that, in some cases, resembled LH surges. We conclude, therefore, that the EOPs that mediate progesterone negative feedback on pulsatile GnRH secretion are not involved in blockade of activation of the E2-induced GnRH surge by progesterone, but do appear to be part of the mechanism by which progesterone disrupts the transmission stage.

Genomic and membrane actions of progesterone: implications for reproductive physiology and behavior

Progesterone, produced by the ovaries and adrenal glands, regulates reproductive behavior and the surge of luteinizing hormone which precedes ovulation by acting on neurons located in different parts of the hypothalamus. The study of the activation of these reproductive functions in female rats has allowed to explore the different mechanisms of progesterone action in the brain. It has allowed to demonstrate that new actions of the hormone, which have been observed in particular in vitro systems, are also operational in vivo, and may thus be biologically relevant. This mainly concerns the direct actions of progesterone on receptors of neurotransmitters such as oxytocin and GABA. Activation of the progesterone receptor in the absence of ligand by phosphorylation may also play a role.

Quantitative studies of progesterone receptor and nitric oxide synthase colocalization with somatostatin, or neurotensin, or substance P in neurons of the guinea pig ventrolateral hypothalamic nucleus: an immunocytochemical triple-label analysis

Previous double-label studies have demonstrated that progesterone receptors (PR) may coexist with nitric oxide synthase (NOS) and also with neuroactive peptides such as somatostatin (SOM), neurotensin (NT) and substance P (SP) in neurons of the ventrolateral hypothalamic nucleus (VL) of the guinea pig. In the present study, triple-label immunofluorescence histochemistry was used to determine whether neurons co-expressing both PR and NOS also contain one neuropeptide (SOM, NT or SP) and to quantify these colocalization relationships. Ovariectomized guinea pigs were primed with estradiol to induce PR immunoreactivity and treated with colchicine to visualize immunoreactive (IR) peptidergic neurons. Using three primary antibodies raised in different species and labeled with three different markers, it is possible to recognize which type(s) of immunoreactivity each cell contains on the same sections. We observed that PR, NOS and SOM co-occurred extensively, whereas PR, NOS and NT or PR, NOS and SP were colocalized infrequently in neurons of the VL. Combining our various quantitative observations, we found that SOM immunoreactivity appeared in 33% of the PR/NOS-IR neurons in the caudal aspect of the nucleus and that nearly all neurons containing SOM and NOS contained PR. The occasional localization between PR, NOS and SP seen in the rostral subdivision of the VL represented a very low proportion of the PR/NOS-IR neurons but 48% of NOS/SP-IR neurons. The relatively high proportion of PR/NOS-IR cells expressing SOM in the VL suggests that these neurons are part of a neural circuitry involved in a variety of steroid-dependent functions.

Nitric oxide synthase in the guinea pig preoptic area and hypothalamus: distribution, effect of estrogen, and colocalization with progesterone receptor

Nitric oxide (NO) may function as an intercellular messenger in the hypothalamus and may play a role in the control of gonadotropin-releasing hormone (GnRH) secretion and sexual behavior. Progesterone also plays an important role in the regulation of reproductive functions. Recent experiments have shown that progesterone-induced sexual behavior in ovariectomized, estrogen-primed rats was caused by the release of NO from nitric oxide synthase (NOS)-containing neurons and the subsequent stimulation of the release of GnRH. To provide further neuroanatomical support for the role of NO in these gonadal steroid-dependent behavioral and physiological processes, we determined (1) the distribution of the nicotinamide-adenosine-dinucleotide phosphate-diaphorase (NADPHd) and NOS enzymes in the guinea pig preoptic area and hypothalamus, regions that contain steroid receptors; (2) the effect of estrogen on NADPHd activity in these regions; and (3) the neuroanatomical relationship between NOS and the progesterone receptor (PR). For this purpose, single-(NADPHd) and double- (NADPHd with NOS or NADPHd with PR or NOS with PR) staining techniques were applied to sections of brains of guinea pigs. The studies showed scattered NADPHd-positive neurons in most parts of the preoptic area and heavily stained cells in the hypothalamus. In these regions, the pattern and density of NOS immunoreactivity closely corresponded to the pattern of NADPHd staining. Quantitative analysis showed an increase in the number of NADPHd-positive neurons in the ventrolateral nucleus of ovariectomized animals primed with estradiol. Approximately 16% of the NOS-immunoreactive (IR) cells in the rostral preoptic area and 55% of NOS-IR cells in the ventrolateral nucleus displayed PR immunoreactivity. These results suggest that NOS may be regulated by gonadal steroids and provide neuroanatomical evidence that progesterone may exert its effect directly on more than half of NOS-synthesizing cells in the ventrolateral nucleus, a key region in the control of sexual behavior.

Estrogen modulation of neuropeptides: somatostatin, neurotensin and substance P, in the ventrolateral and arcuate nuclei of the female guinea pig

In the guinea pig, steroid target cells reside in the ventrolateral hypothalamic nucleus (VLH), an important site in the mediation of female receptive behavior, and in the arcuate nucleus (AR), a structure essential for stimulation effects of ovarian hormones on gonadotropin secretion. However, the mechanisms by which these steroid-dependent reproductive neuroendocrine processes occur are only partially understood. Estrogen is known to affect the hypothalamus content of certain neuropeptides. In the present study, we investigated the effects of estradiol benzoate (EB) on immunoreactivity of neurons containing one of three following neuropeptides: somatostatin (SOM), neurotensin (NT) and substance P (SP) in VLH and AR. The number of immunoreactive (IR)-neurons was quantified in anatomically matched sections through VLH and AR of ovariectomized (OVX), OVX + EB and OVX + oil-treated guinea pigs. Analysis of variance revealed that the number of SOM-IR and SP-IR neurons significantly increased in all regions of VLH of OVX + EB-treated guinea pigs as compared to OVX or OVX + oil-treated animals (P < 0.01) but showed no EB effect on the number of NT-IR neurons. Although the number of SOM-IR and NT-IR neurons slightly increased following treatment with EB in AR, analysis of variance revealed no significant change. The present results provide additional information relevant to possible involvement of these neuropeptides in facilitation of female typical sexual behavior.