Neurons in the perinuclear zone (PZ) of the supraoptic nucleus (SON) are thought to serve as interneurons and may mediate changes in neurohypophysial hormone release in response to physiological changes in blood pressure. However, the morphology and electrophysiological characteristics of PZ neurons are unknown. In the present study, PZ neurons from male and female rats were recorded intracellularly to determine some membrane properties, then filled with biocytin or biotinamide for morphological analysis. In general, PZ neurons had faster spikes than magnocellular SON neurons, and the great majority were characterized by a subthreshold depolarizing hump when depolarized from a hyperpolarized (less than -80 mV) membrane potential. In most neurons, this hump was similar to low-threshold spikes described in other CNS regions. Near-threshold, fast action potentials were clustered near the onset of these depolarizations. Conspicuously absent in all PZ neurons was the strong transient and subthreshold outward rectification characteristic of vasopressin and oxytocin neurons of the SON. These results suggest that PZ neurons are electrophysiologically distinct from neurosecretory neurons of the SON. No differences were found between male and female rats in any of the basic properties examined, including input resistance, membrane time constant, spike height, spike width, spike threshold, and the size of the spike afterhyperpolarization. Morphologically, PZ neurons were diverse but were divided into spiny and aspiny groups. Three spiny neurons and one aspiny neuron contributed an axonal projection to the SON characterized by varicosities suggestive of terminals. In the case of the three spiny neurons, the SON projection was clearly a minor collateral projection. The axon arborized in the PZ, but one or more branches were cut at the edge of the explant, indicating a longer projection. In the remaining neurons, no axonal projection to the SON was detected and several had axons leaving the explant. Some portion of the dendritic tree penetrated the SON in several neurons. The morphology of PZ neurons was thus heterogeneous and suggests that, for some cells at least, the projection to the SON may be a minor collateral component of a much wider axonal projection.
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