Dopaminergic Regulation of Luteinizing Hormone-Releasing Hormone Release at the Median Eminence Level: Immunocytochemical and Physiological Evidence in Hens

Abstract

Theoretically, the most effective inhibitory control of hypophysiotropic luteinizing hormone-releasing hormone (LHRH) release might occur through a presynaptic inhibition of LHRH neuronal terminals at the median eminence (ME) level. Since: (a) we have recently reported the existence of synaptic contacts between dopamine- and LHRH-containing processes in the ewe ME, and (b) nutritional deprivation induces an ovulatory failure in both birds and mammals, we have assessed the possibility that the anovulatory state induced by feed withdrawal (FW) in laying hens, might be caused by a dopaminergic inhibition of LHRH release at the ME level. Laying hens at the start (35 weeks old) and end (75 weeks old) of their commercial egg-laying life were killed at 0, 1,2 and 4 days after FW. Serum luteinizing hormone (LH) and progesterone (P4), in vitro release of LHRH by isolated ME, and LHRH content in ME and preoptic area (POA) were determined by RIA. ME content of dopamine (DA) and its main metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were assessed by LCED. The distribution of LHRH and tyrosine hydroxylase (TH)-containing processes at the ME level of the hen was determined immunocytochemically. In the hen, LHRH-containing cell bodies are localized in the anterior hypothalamus and medial POA. LHRH-containing axons project toward the ME and infundibulum through the ventral-lateral hypothalamus. TH-containing perikarya are concentrated in the arcuate nucleus and in the adjacent part of the periventricular nucleus, dorsal to the arcuate. TH-containing axons converge toward the ME and descend into the infundibulum. Dense concentrations of TH-and LHRH-containing processes are located in the lateral and mediobasal portions of the external layer of the ME, providing opportunities for synaptic interactions between them. Ovulatory failure and regression of the ovary and reproductive tract occurred 2-3 days after FW at the end, but not at the beginning of the hen’s commercial egg-laying life. After FW, hens at the end of their productive life had higher (p < 0.01) tuberoinfundibular DA turnover, produced less LHRH, and had lower serum LH and P4 than hens undergoing FW at the beginning of their productive life. In addition, in vitro release of HRH from dener-vated ME tissue of hens undergoing FW at the end of their commercial egg-laying life was higher and was reversed in a dose-dependent fashion by DA, but not by serotonin. Thus, the ovulatory failure associated with FW in laying hens might be caused by a presynaptic inhibition of in vivo LHRH release at the level of ME hypothalamic neuronal terminals.

Implication of D2-like dopaminergic receptors in the median eminence during the establishment of long-day inhibition of LH secretion in the ewe

“…Nevertheless, a direct action of dopamine on LHRH terminals is suggested by the observation of axo-axonic synaptic contacts between dopaminergic and LHRH terminals within the ME (Kuljis & Advis 1989). Such organization is also found in rats (Ajika 1979, Ugrumov et al 1989) and in hens (Contijoch et al 1992). The ability of dopamine to act directly on LHRH neurons could be determined, for instance, by assessing the presence of D2-like receptors on LHRH neurons by immunocytochemistry at the electron microscopic level, or by testing the action of dopamine in vitro on cultured LHRH neurons.…”

A Relative Depletion of Luteinizing Hormone-Releasing Hormone Was Observed in the Median Eminence of Young but Not Middle-Aged Rats on the Evening of Proestrus

“…This age-relat ed difference also raises questions about the ability of the neurovascular terminals in middle-aged animals to con centrate and store LHRH for release during the preovula tory LH surge. Regulation of LHRH neurosecretion is probably modulated by a balance of excitatory and inhibi tory influences, and the median eminence is a potentially important site for that modulation [28,29]. Effective inhibitory control [29] might be required to limit constitu tive release of the peptide from terminals in the median eminence and to enable the build-up of LHRH stores.…”

“…Regulation of LHRH neurosecretion is probably modulated by a balance of excitatory and inhibi tory influences, and the median eminence is a potentially important site for that modulation [28,29]. Effective inhibitory control [29] might be required to limit constitu tive release of the peptide from terminals in the median eminence and to enable the build-up of LHRH stores. Age-related alterations in neuromodulatory influences on LHRH terminals could therefore result in diminished stores of LHRH available for release in aging females.…”

Effects of Starvation and Refeeding on Gonadotropin and Thyrotropin Subunit mRNAs in Male Japanese Quail

“…In mammals, Bergendahl and Veldhuis (1995) mentioned in their review that inadequate nutritional intake impaired LH secretion mainly via GnRH release from hypothalamus. Similarly, in the chicken, it was suggested that starvation suppressed release of GnRH from the median eminence (Contijoch et al, 1992). However, it was suggested in the chicken that pituitary sensitivity to GnRH declined in an early phase of starvation because administration of GnRH had no effect on the plasma LH level in that phase (Tanabe et al, 1981).…”