Elsevier

Brain Research

Volume 755, Issue 1, April 1997, Pages 101-111
Brain Research

Research report
Synaptic contacts between gonadotropin-releasing hormone-containing fibers and neurons in the suprachiasmatic nucleus and perichiasmatic area: an anatomical substrate for feedback regulation?

https://doi.org/10.1016/S0006-8993(97)00086-3Get rights and content

Abstract

The suprachiasmatic nucleus (SCN) is critically involved in the generation and entrainment of circadian rhythms in mammalian species. Both the occurrence and the timing of the luteinizing hormone surge on the afternoon of proestrus in the female rodent are critically dependent on the integrity of the SCN. Recently, we demonstrated the presence of a monosynaptic pathway from the SCN to the gonadotropin releasing hormone (GnRH) neurons in the preoptic area. In addition, we found that interaction between the SCN and the GnRH system may be found close to the SCN, since we observed apposition of SCN efferents and GnRH fibers at the ultrastructural level in that region. The aim of the present study was to investigate the presence of synaptic contacts between GnRH fibers and structures in the SCN and surrounding perichiasmatic area (periSCN). At the light microscopical level, the immunoreactivity for GnRH showed a considerable overlap with the immunoreactivity for vasopressin and vasoactive intestinal peptide, two neuropeptides synthesized by SCN neurons. At the ultrastructural level, we demonstrated synaptic input of GnRH-containing axons on immunocytochemically unidentified structures in the SCN/peri-SCN region. The present results clearly demonstrate that the SCN and periSCN are postsynaptic targets of GnRH fibers. It is hypothesized that the GnRH input in the SCN region represents an anatomical substrate for feedback-control between these systems.

Introduction

The gonadotropin-releasing hormone (GnRH) system in the brain constitutes the final common pathway for the central regulation of pituitary gonadotropin release. GnRH is synthesized by cell bodies widely spread throughout the ventral forebrain, with concentrations of neurons in the diagonal band of Broca, surrounding the organum vasculosum of the lamina terminals and in the preoptic area [2,37]. A vast majority of these neurons send fibers to the median eminence (ME) [32], were GnRH is released into the portal blood circulation. Tracing studies using peripheral injections of Flourogold suggested that most, if not all GnRH neurons are neuroendocrine in nature [49]. The GnRH-containing fibers reach the ME by two main routes, a lateral one which parallels the base of the brain and a periventricular one that parallels the third ventricle [4,15,16,31]. Other routes reach the ME via the area between the two suprachiasmatic nuclei (SCN) at the basis of the third ventricle (median-SCN) and along the ventral surface of the optic chiasm, forming the subchiasmatic pathway [15]

In the female, synthesis and release of GnRH from the ME and consequently preovulatory luteinizing hormone (LH) release from the pituitary, are under control of a positive feedback by gonadal steroids [10,11,22,23,25]. Also, the SCN is essential for normal control of cyclic LH secretion in the female rodent [5,6,14,19,24,28]. Recent studies have demonstrated the presence of a monosynaptic projection from the SCN to the GnRH neurons in the preoptic area in the female rat, containing vasoactive intestinal peptide-containing (VIP) as a putative transmitter [40,42,43]. This pathway may constitute an anatomical substrate involved in the circadian regulation of preovulatory LH release.

In addition to its role as a neurohormone, GnRH may act as a neurotransmitter within the brain, since GnRH fibers and GnRH receptors are present within various intra- and extra-hypothalamic areas [2,17]. GnRH immunoreactive fibers have been demonstrated in SCN and surrounding area [38], which most probably are axons or axon collaterals of the median and medial pathways to the median eminence, which account for 15–30% of the total GnRH innervation of the median eminence [20]. Recent tracing studies of SCN efferents have shown GnRH-containing axons in apposition to axon- and dendrite-like structures filled with tracer in this region at the ultrastructural level [39].

The objective of the present study was to investigate the occurrence of synaptic interactions between GnRH-containing fibers and neurons in the SCN and peri-SCN. To this end we evaluated the distribution of immunoreactivity for GnRH and for vasopressin (VP) and VIP, two SCN-derived neuropeptides abundantly present in projections of the SCN to the peri-SCN [47], in serial sections through the entire rostrocaudal extent of the SCN at the light microscopical level. Furthermore, we examined the immunoreactivity for GnRH at the ultrastructural level using single labelling and the possible interaction of VP- and VIP-containing structures with GnRH-containing fibers using double labelling immunocytochemistry.

Section snippets

Animals

Adult female Wistar rats obtained from Central Animal Facilities (Utrecht University, The Netherlands) weighing 200-280 g were used for electron- (n = 518) and light- (n = 5) microscopical immunocytochemical studies. Rats were housed under a regular light dark cycle (lights on from 07.00 to 19.00 h) in a temperature controlled room. Food and water were available ad libitum.

Tissue processing

Animals were perfused between 10.00 and 13.00 h under pentobarbital anaesthesia (0.1 ml/100 g body weight, intraperitoneal) with 100

Light microscopical studies

The light microscopical staining for GnRH, VIP and VP was evaluated in alternating sections through the hypothalamus. In the SCN and surrounding peri-SCN, a clear overlap was found in the distribution of the GnRH fibers and that of immunoreactivity for VP and VIP (Fig. 1). Immunoreactivity for all three antigens was present along the entire rostro-caudal extent of the nucleus. VIP-immunoreactive neurons were localized primarily in the ventral part of the SCN, whereas VP-immunoreactive neurons

Discussion

The present study demonstrates a marked overlap in the light microscopical distribution of GnRH-immunoreactive fibers and that of VP- and VIP-immunoreactivity in neurons and fibers in the SCN and surrounding peri-SCN. The GnRH-containing fibers were predominantly localized in the border zone of the SCN and surrounding peri-SCN and between the two suprachiasmatic nuclei as has been reported previously [38]. In addition, GnRH-immunoreactive fibers appeared to be more abundant in the peri-SCN

Acknowledgements

The authors thank Rene Scriwanek, Maurits van Niek-erk and Tom van Rijn (Department of Cell Biology, Medical Faculty, Utrecht University, The Netherlands) for excellent darkroom services. This work was carried out in the framework of the national graduate school ‘Pathophysiology of the Nervous System’.

References (51)

  • J. Barry et al.

    LHRH-containing systems

  • C. Bennet-Clarke et al.

    Immunocytochemical distribution of LHRH neurons and processes in the rat: hypothalamic and extrahypothalamic locations

    Cell Tiss. Res.

    (1982)
  • W. Bishop et al.

    The effects of hypothalamic lesions on the release of gonadotrophins and prolactin in response to estrogen and progesterone treatment in female rats

    Endocrinology

    (1972)
  • K. Brown-Grant et al.

    Abnormalities in reproductive function associated with the destruction of the suprachiasmatic nuclei in female rats

    Proc. R. Soc. London

    (1977)
  • R.M. Buijs et al.

    Antibodies to small transmitter molecules and peptides: production and application of antibodies to dopamine, serotonin, GABA, vasopressin, vasoactive intestinal peptide, neuropeptide Y, somatostatin and substance P

    Biomed. Res.

    (1989)
  • R.M. Buijs et al.

    Ultrastructural evidence for intra- and extra-nuclear projections of GABA-ergic neurons of the suprachiasmatic nucleus

    J. Comp. Neurol.

    (1994)
  • G. Fink

    Gonadotropin secretion and its control

  • G. Fink et al.

    Steroid actions on hypothalamic neurons with special reference to estrogen control of luteinizing hormone-releasing hormone biosynthesis and release

  • F. Gallyas et al.

    High-grade intensification of the end-product of the diaminobenzidine reaction for peroxidase histochemistry

    J. Histochem. Cytochem.

    (1982)
  • G.D. Gray et al.

    Effects of lesions in various structures of the suprachiasmatic-preoptic region on LH regulation and sexual behaviour in female rats

    Neuroendocrinology

    (1978)
  • G.E. Hoffman et al.

    LHRH pathways in rat brain: ‘deafferentation’ spares a sub-chiasmatic LHRH projection to the median eminence

    Neuroscience

    (1982)
  • Y. Ibata et al.

    The location of LH-RH neurons in the rat hypothalamus and their pathways to the median eminence

    Cell Tissue Res.

    (1979)
  • M. Kawakami et al.

    Modulating effect of limbic structures on gonadotropin release

    Neuroendocrinology

    (1973)
  • M. Kawakami et al.

    Loss of oestrogen-induced daily surges of prolactin and gonadotrophins by suprachiasmatic nucleus lesions in ovariectomized rats

    Endocrinology

    (1980)
  • K. Köves et al.

    Effect of various hypothalamic deafferentiations injuring different parts of the GnRH pathway on ovulation GnRH content of the median eminence and plasma LH and FSH levels

    Neuroendocrinology

    (1986)
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