Electrophysiological properties and glucose responsiveness of guinea-pig ventromedial hypothalamic neurones in vitro

The membrane properties of neurones in the guinea-pig ventromedial hypothalamic nucleus (v.m.h.) were studied in in vitro brain slice preparations. The average resting potential was -62.9 +/- 5.4 mV (mean +/- S.D.), input resistance was 155 +/- 58 M omega, and action potential amplitude was 69.9 +/- 6.3 mV. Three types of neurone were identified. The type A neurones were characterized by a short membrane time constant (7.3 +/- 2.0 ms) and a small after-hyperpolarization (a.h.p.) (2.0 +/- 1.2 mV) with a short half decay time of 67 +/- 55 ms after stimulation with a long outward current pulse. Type B had a long time constant (18.8 +/- 5.7 ms) and a large a.h.p. (6.9 +/- 2.4 mV) with a medium half decay time of 203 +/- 90 ms. Type C was characterized by a long time constant (14.3 +/- 2.3 ms) and a large a.h.p. (6.5 +/- 1.5 mV) with a long half decay time of 478 +/- 230 ms. The slopes of the frequency-current (f-I) plots of the three types were different, particularly for the first spike interval. The slopes for the type A (414 +/- 102 impulses s-1 nA-1) and type B neurones (480 +/- 120 impulses s-1 nA-1) were steeper than that for the type C neurones (178 +/- 41 impulses s-1 nA-1). This difference is probably related to the relatively long first interval observed in the type C neurones. In all type B and a few type C neurones, when the membrane potential was hyperpolarized beyond--65 mV the application of orthodromic or direct stimulation generated a burst of spikes, consisting of a low-threshold response (l.t.r.) of low amplitude and superimposed high-frequency spikes. At the original resting potential, outward current pulses produced a train of low-frequency spikes. In type C neurones maintained in a depolarized state (about -50 mV), inward current pulses produced a specific delay of the return to the original membrane potential. This delayed return was thought to be generated by activation of a transient K+ (IA) conductance. Stimulation at the lateral edge of the v.m.h. produced excitatory post-synaptic potentials (e.p.s.p.s) in type A neurones, e.p.s.p.s with l.t.r. in type B neurones and e.p.s.p.-inhibitory post-synaptic potential sequences in type C neurones. About 20% of v.m.h. neurones, particularly the type C cells, were depolarized by glucose application with an associated increase in the input membrane resistance.(ABSTRACT TRUNCATED AT 400 WORDS)