Synaptic inputs and action potentials of magnocellular neuropeptidergic cells: intracellular recording and staining in slices of rat hypothalamus.

Excitatory postsynaptic potentials (EPSPs) and action potentials of magnocellular neuropeptidergic cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) were studied with intracellular recording in coronal slices of rat hypothalamus. The fluorescent dye Lucifer Yellow (LY) was injected intracellularly and the cells were subsequently identified as magnocellular (somata greater than 15 x 15 micrometer). These cells generally had a large cytoplasm-to-nucleus ratio. In PVN it was frequently possible to trace filled dendrites to the ependyma of the third ventricle, and occasionally dendritic spines could be seen. Electrical stimuli in areas dorsolateral and ventrolateral to the fornix column evoked EPSPs in some anatomically identified MNCs of PVN, which indicates that presynaptic fibers innervating MNCs approach PVN from this region. Short-latency (less than 1 msec) spikes could be evoked in many MNCs of PVN by stimulation near SON, which is consistent with the known projection to the neurohypophysis of many MNCs. Action potentials in MNCs of PVN and SON had significantly longer durations at one-third spike height (mean +/- S.D. = 2.06 +/- 0.6 msec) than hippocampal CA1 pyramidal cells (1.17 +/- 0.29 msec). This suggests that neuroendocrine cells in mammals and some lower vertebrates and invertebrates are similar in this regard.