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The Journal of Neuroscience, November 15, 1998, 18(22):9480-9488

-Opioid Regulation of Neuronal Activity in the Rat Supraoptic Nucleus In Vivo

Colin H. Brown, Mike Ludwig, and Gareth Leng

Department of Physiology, University Medical School, Edinburgh EH8 9AG, United Kingdom

We investigated the influence of endogenous -opioids on the activity of supraoptic neurons in vivo. Administration of the -antagonist nor-binaltorphimine (200 µg/kg, i.v.), increased the activity of phasic (vasopressin), but not continuously active (oxytocin), supraoptic neurons by increasing burst duration (by 69 ± 24%) and decreasing the interburst interval (by 19 ± 11%). Similarly, retrodialysis of nor-binaltorphimine onto the supraoptic nucleus increased the burst duration (119 ± 57% increase) of vasopressin cells but did not alter the firing rate of oxytocin cells (4 ± 8% decrease). Thus, an endogenous -agonist modulates vasopressin cell activity by an action within the supraoptic nucleus. To eliminate -agonist actions within the supraoptic nucleus, we infused the -agonist U50,488H (2.5 µg/hr at 0.5 µg/hr) into one supraoptic nucleus over 5 d to locally downregulate -receptor function. Such infusions reduced the spontaneous activity of vasopressin but not oxytocin cells and reduced the proportion of cells displaying spontaneous phasic activity from 26% in vehicle-infused nuclei to 3% in U50,488H-infused nuclei; this treatment also prevented acute inhibition of both vasopressin and oxytocin cells by U50,488H (1000 µg/kg, i.v.), confirming functional -receptor downregulation. In U50,488H-infused supraoptic nuclei, vasopressin cell firing rate was increased by nor-binaltorphimine (100 and 200 µg/kg, i.v.) but not to beyond that found in vehicle-treated nuclei, indicating that these cells were not U50,488H-dependent. Thus, normally functioning -opioid mechanisms on vasopressin cells are essential for the expression of phasic firing.

Key words: electrophysiology; dendrites; oxytocin; vasopressin; opioid; dynorphin; U50,488H; nor-binaltorphimine; naloxone

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Copyright © 1998 Society for Neuroscience  0270-6474/98/18229480-09$05.00/0

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