Figure 6.

Endomorphin 1 inhibits oxytocin, but not vasopressin, cells in vitro. (A) The mean firing rate of six oxytocin cells averaged over 1 min before (Control), the second minute during 100 nM–10 μM endomorphin 1 (EM1) superfusion and during the second minute of subsequent 10 μM naloxone (NLX) superfusion. Naloxone reversed endomorphin 1 inhibition of oxytocin cell activity (see Figure 4). (B) The mean firing rate of three oxytocin cells before (Control) synaptic blockade using low Ca²⁺/Co²⁺-containing medium (Low Ca) and during 1–3 μM endomorphin 1 superfusion in low Ca²⁺/Co²⁺-containing medium. The firing rate of these cells was inhibited from an excited level by endomorphin 1 in the presence of synaptic blockade. (C) The mean firing rate of ten vasopressin cells before (Control) and during 10 μM endomorphin 1 superfusion and during subsequent superfusion of four of the cells with 30 μM U50,488H, showing κ-opioid inhibition of vasopressin cells insensitive to endomorphin 1. ^*P<0.05 and ^**P<0.01 versus control; †P<0.05 and ††P<0.01 versus endomorphin 1; one-way RM ANOVA followed by Student-Newman-Keuls tests.