Apamin Increases Post-spike Excitability of Supraoptic Nucleus Neurons in Anaesthetized Morphine-naïve Rats and Morphine-dependent Rats: Consequences for Morphine Withdrawal Excitation

Supraoptic nucleus (SON) oxytocin neurons develop morphine dependence when chronically exposed to this opiate and undergo excitation when morphine is subsequently withdrawn. Morphine withdrawal excitation is evident as an increased action potential (spike) firing rate and is associated with an increased post-spike excitability that is consistent with the expression of an enhanced post-spike afterdepolarization (ADP) during withdrawal. Here, we administered apamin (which inhibits the medium afterhyperpolarization [mAHP] in vitro and unmasks an ADP) into the SON of urethane-anaesthetized rats to determine its effects on oxytocin neurons in vivo. As predicted, intra-SON apamin administration increased the propensity to fire a spike soon (<100 ms) after each spike (post-spike excitability) more in oxytocin neurons recorded from morphine-treated rats than in morphine-naïve rats. However, intra-SON apamin did not alter the overall firing rate of oxytocin neurons recorded from morphine-treated rats or morphine-naïve rats, indicating that an increase in post-spike excitability alone is not sufficient to trigger withdrawal excitation of oxytocin neurons. Nevertheless, bilateral intra-SON apamin infusion increased oxytocin secretion (which depends on firing pattern as well as firing rate) by 90 ± 46% in morphine-dependent rats (P < 0.01 compared to aCSF). Hence, an increase in post-spike excitability does not appear to drive morphine withdrawal-induced increases in oxytocin neuron firing rate, but does contribute to withdrawal-induced hyper-secretion of oxytocin.