A BSTRACT : Medullary neurons containing pituitary adenylate cyclase-activating polypeptide (PACAP) and noradrenalin (NA) project to the hypothalamus and they are involved in the regulation of arginine vasopressin (AVP) neurons. At the ultrastructural level, PACAP immunoreactivity was detected in the granular vesicles in catecholaminergic nerve terminals that made synaptic contact with AVP neurons. Both PACAP (at least 1 nM) and NA (at least 1 M) induced large increases in the cytosolic Ca 2+ concentration ([Ca 2+ ] i ) in isolated AVP cells. PACAP at 0.1 nM and NA at 0.1 M had little effects, if any, on [Ca 2+ ] i . However, when 0.1 nM PACAP and 0.1 M NA were combined, they evoked large increase in [Ca 2+ ] i in AVP neurons. An inhibitor of protein kinase A (PKA) completely inhibited the PACAP-induced increase in [Ca 2+ ] i , but only partly inhibited the NA-induced increase in [Ca 2+ ] i . In AVP cells that were prelabeled with quinacrine, PACAP and NA acted synergistically to induce a loss of quinacrine fluorescence, indicating secretion of neurosecretory granules in AVP neurons.The results suggest that PACAP and NA, coreleased from the same nerve terminals, act in synergy to evoke calcium signaling and secretion in AVP neurons, and that the synergism is mediated by the interaction between cAMP-PKA pathway an as yet unidentified factor "X" linked to L-type Ca 2+ channels. The synergism between PACAP and NA may contribute to the regulation of AVP secretion under physiological conditions.
The innervation of neurons containing oxytocin by gamma-aminobutyric acid (GABA)-containing neurons was examined by electron microscopy using a method of pre-embedding peroxidase-antiperoxidase (PAP) to show GABA with post-embedding immunocolloidal gold staining (IGS) to show oxytocin in the same tissue sections. GABA-like immunoreactive (GABA-LI) terminals were distributed throughout the rat hypothalamus and were abundant in all parts of the paraventricular nucleus (PVN). Ultrastructural observation revealed that GABA-LI axon terminals, containing immunoreactive granular vesicles (70-80 nm in diameter), formed synapses with cell bodies and processes of the magnocellular and parvocellular neurosecretory neurons in the PVN. In the combined PAP-IGS preparations, PAP-labeled GABAergic axon terminals appeared at times making synaptic contacts with IGS-labeled oxytocin-immunoreactive cell bodies and processes. These findings provide morphological evidence for direct synaptic influence of GABAergic elements on the secretory activity of oxytocin-containing neurons in the rat hypothalamic PVN.
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