ELECTROPHYSIOLOGY OF THE NEURONS IN THE AREA OF THE ENKEPHALINERGIC MAGNOCELLULAR DORSAL NUCLEUS OF THE GUINEA-PIG HYPOTHALAMUS, STUDIED BYINTRACELLULAR AND WHOLE-CELL RECORDINGS

The electrophysiological characteristics of 103 hypothalamic neurons i n the area of the guinea-pig enkephalinergic magnocellular dorsal nucl eus were studied in a thick slice preparation with sharp microelectrod es (63 neurons) and patch pipettes for whole-cell recordings (40 neuro ns). Of the sampled cells, 79.6% displayed tetrodotoxin-resistant, cal cium-dependent slow-depolarizing potentials when the membrane potentia l was hyperpolarized to -70 mV (type I neurons). Half of them showed r obust slow depolarizing potentials, generating bursts of fast action p otentials. In the remaining neurons, the slow-depolarizing potentials did not cause burst-firing action potentials but triggered single acti on potentials. The other dass of neurons (20.4% of the sample: type II neurons) did not exhibit calcium-dependent slow-depolarizing potentia ls. Resting potential, input resistance and the membrane time constant did not distinguish among the two classes of neurons. Current-voltage relationships were heterogeneous. A transient outward rectification w as observed in the two classes. This was not totally blocked by 2 mM 4 -aminopyridine but was abolished when using perfusion with cobalt inst ead of calcium. Input resistance and the time constant were higher whe n measured in the whole-cell mode but the other electrical parameters and the sampling of the recorded neurons were strikingly similar betwe en the two methods of recording. Intracellular staining of 22 neurons retrogradely labelled from the lateral septum allowed confirmation of their location within the magnocellular dorsal nucleus. The study indi cates that the electrical properties of these neurons did not differ f rom those of neurons found throughout the area explored. It also indic ates the presence of distinct electrophysiological types of cells in t he magnocellular dorsal nucleus, although the nucleus is composed of a single type of enkephalinergic neuron. It provides a basis for the st udy of the regulation of activity of the neurons at the origin of an e nkephalinergic tractus which is involved in neuroendocrine, psychoneur oendocrine and immune processes.