Non-synaptic depolarizing potentials in rat supraoptic neurones recorded in vitro.

Intracellular recordings obtained from eighty-two supraoptic nucleus neurones in perfused explants of rat hypothalamus revealed a mean resting membrane potential of -66 +/- 5 mV (S.D.) and spike amplitudes of 70-106 mV. When recorded with K acetate-filled micropipettes, non-spike membrane voltage fluctuations included spontaneous depolarizing and hyperpolarizing potentials. Spontaneous hyperpolarizing potentials peaked in 3-5 ms and decayed exponentially with a mean time constant of 20.2 +/- 0.1 ms, 1.6 times the membrane time constant of 13.8 +/- 0.1 ms. These potentials were identified as spontaneous inhibitory post-synaptic potentials, and all demonstrated a common reversal potential near -80 mV, a depolarizing shift of this reversal potential during intracellular Cl- accumulation, and reversible blockade by raising [Mg2+] to 15 mM in the perfusate. Depolarizing potentials with features typical of spontaneous excitatory post-synaptic potentials i.e. brief (8-20 ms) depolarizing transients, were rarely recorded with K acetate-filled micropipettes. Instead, most neurones demonstrated what are termed non-synaptic depolarizing potentials (n.s.d.p.s) lasting 20-125 ms (mean 86.4 +/- 8.6 ms (S.E. of mean)) with a rise time 21.1 +/- 2.8 ms and a decay time of 16.3 +/- 2.8 ms (n = 28 measured). Unlike typical spontaneous post-synaptic potentials, these events could sustain a constant peak amplitude for most of their duration. These n.s.d.p.s displayed a strong voltage-dependent behaviour and were detected only at membrane potentials within 5-7 mV of the threshold for spike initiation. Spontaneous slow depolarizing membrane shifts preceding or following phasic bursts, or any manipulation (e.g. current step, sinusoid, depolarizing after-potential) causing the membrane potential to enter this range of activation, prompted their appearance. N.s.d.p.s were completely insensitive to the presence of 15 mM-Mg2+ but they were reduced in size and frequency when Ca2+ were replaced with Co2+ or Mn2+. They were detected at a more positive membrane potential when Na+-dependent action potentials were blocked with tetrodotoxin. The size, voltage-dependent and non-synaptic nature of these depolarizing potentials raises the possibility that they reflect the activity of individual (or small clusters of) ionic channels carrying inward current. Their ability to serve as prepotentials to trigger spikes is deemed to be particularly important for promoting the onset of phasic bursts in supraoptic neurosecretory neurones.