Distribution and morphology of the juxtapositions between growth hormone-releasing hormone-(ghrh)-immunoreactive neuronal elements

Abstract

Previous studies revealed that growth hormone-releasing hormone (GHRH)-immunoreactive (IR) neurons form a circumscribed cell group in the basal infundibulum/median eminence of the human hypothalamus. GHRH from these neurons is released into the hypothalamo–hypophyseal portal circulatory system in a pulsatile manner. It is a common consensus that the pulsatile release of GHRH is the main driving force behind the pulsatile release of growth hormone (GH) and may contribute to the regulation of other hypothalamic functions. The pulsatile release of GHRH requires synchronized activity of GHRH-IR neurons. However, the morphological basis of this synchronization between the GHRH-IR neural elements has not been elucidated yet. Since the utilization of electron microscopy combined with immunohistochemistry is virtually impossible in the human brain due to the long post mortem period, immunohistochemistry, evaluated with oil immersion light microscopy, was used in order to reveal the associations between the GHRH elements. Numerous GHRH–GHRH juxtapositions have been detected in the infundibular area/median eminence, where GHRH-IR axonal varicosities often formed multiple contacts with GHRH-IR perikarya. Examination of these associations with high magnification oil immersion light microscopy revealed (1) axonal swellings at the site of the contacts and (2) no gaps between the contacting elements suggesting that these juxtapositions may be functional synapses. The large number of GHRH–GHRH juxtapositions in the infundibular area/median eminence suggests that these synapse-like structures may represent the morphological substrate of the synchronized activity of GHRH neurons that in turn may result in the pulsatile release of GHRH in human.