κ‐Opioid Receptor Activation Inhibits Post‐Spike Depolarizing After‐Potentials in Rat Supraoptic Nucleus Neurones In Vitro

Abstract

Endogenous agonists acting at kappa-opioid receptors modulate the discharge activity of hypothalamic supraoptic nucleus vasopressin cells in vivo. Phasic activity in vasopressin cells is known to depend critically on intrinsic mechanisms involving post-spike depolarizing after-potentials and we hypothesized that inhibition of phasic bursting by an endogenous kappa-agonist may result from reducing the magnitude of depolarizing after-potentials. To investigate this possibility, intracellular sharp electrode recordings were obtained from supraoptic nucleus cells impaled in superfused explants of rat hypothalamus. Bath application of the selective kappa-agonist, U50,488H (0.1-1 microM), decreased the spontaneous firing rate of magnocellular neurosecretory cells (by 94. 0+/-4.5% at 1 microM, mean+/-SEM; P = 0.02, n = 4). U50,488H did not alter membrane potential (0.9+/-0.8 mV hyperpolarization at 1 microM, P = 0.17, n = 8) or input resistance (11.0+/-4.5% increase at 1 microM, P = 0.09, n = 5). U50,488H (0.1 and 1 microM, both n = 5) reduced depolarizing after-potential amplitude (by 29.9+/-9.3 and 78.0+/-10. 6%, respectively, P<0.001) in eight cells in which the baseline membrane potential was kept constant by dc-current injection and in which a depolarizing after-potential was evoked every 25-40 s by a brief (40-80 ms) train of 3-6 action potentials (the number of spikes in the trains was kept constant for each cell). Thus, kappa-opioid receptor activation reduces depolarizing after-potential amplitude in supraoptic nucleus cells and this may underlie the reduction in burst duration of vasopressin cells caused by an endogenous kappa-agonist in vivo.

Burst Initiation and Termination in Phasic Vasopressin Cells of the Rat Supraoptic Nucleus: A Combined Mathematical, Electrical, and Calcium Fluorescence Study

“…Our analysis supports a recently proposed role of dynorphin in burst termination (Brown et al, 1998(Brown et al, , 1999. We further suggest a mechanism for the action of dynorphin and predict the time course of its effects.…”

“…Other factors, including intracellular ones, that accumulate during firing may have a similar effect. However, our model is consistent both with the most compelling experimental data regarding local release of dynorphin (Brown et al, 1998(Brown et al, , 1999 and also with our own observations of the time course of bulk calcium during bursts (Fig. 3).…”

“…It is known that the successive generation of DAPs on a time scale of several seconds significantly reduces their amplitude , and a mechanism for this inactivation has been proposed by Brown et al (1999). They have shown that the activation of -opioid receptors, by agonists such as dynorphin, can inhibit the DAP.…”

“…It is therefore possible that dynorphin could instead terminate the burst by manipulating calcium rather than by a downstream modulation of the DAP. In fact, direct Ca 2ϩ channel modulation is unlikely because DAPs are still inhibited by -agonist concentrations that are too low to affect calcium channels, and furthermore -agonists do not affect other calcium-dependent processes, such as action-potential broadening (Brown et al, 1999). In addition, aside from Ca 2ϩ influx, the DAP is known to be controlled by release from stores (Li and Hatton, 1997a), and so a direct reduction of influx should only partially inhibit the plateau.…”

“…We show first that each burst is supported by the regenerative activation of a calcium-and voltage-dependent depolarizing afterpotential (the DAP) and second that burst termination can oc-cur via desensitization of the DAP to intracellular calcium. Following the experimental findings of Brown and colleagues (Brown et al, 1998(Brown et al, , 1999, we investigate how the opioid dynorphin (which is colocalized with AVP and released from the somatodendritic region) may regulate phasic firing by autoinhibition of the DAP mechanism during activity.…”