Brdn Research Bullerin, Vol. 8, pp. 87-93, 1982. Printed in the U.S.A. Synaptic Inputs and Action Potentials of Magn~cellular Neuropeptidergic Cells: Intracellular Recording and Staining in Slices of Rat Hypothalamus BRIAN A. MAcVICAR,* R. DAVID ANDREW,? F. EDWARD DUDEK-t AND GLENN I. HATTON* ~~earoscie~ce Pr~grum and department of Psychology, ~~c~~ga~ State university, East Lansing, MI 48824 and ~~epurtme~t of Physiology, Tulane U~i~ers~~ School c_$ ~ed~c~ne~ New Orleans, LA 70112 Received 26 September 1981 MAcVICAR , 3. A., R. D. ANDREW, F. E. DUDEK AND G. I. HATTON. Synapric inpufs and actiun ~~fe~t~ff~~ of ~~gn~celi~~ar ~e~ro~ept~dergic cells: ~ntr~ce~i~lar recording and staining in slices of rat hypothalamus. BRAIN RES. BULL. S(1) 87-93, 1982.~-Excitatory postsynaptic potentials (EPSPs) and action potentials of magnocellular neuropep- tidergic cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) were studied with intracellular recording in coronal slices of rat hy~th~~us. The fluorescent dye Lucifer Yellow (LY) was injected intrace~ularly and the cells were subsequently identified as magnocellular (somata >lSx 15 pm). 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 MN& of PVN, which indicates that presynaptic fibers innervating MNCs approach PVN from this region. Short-latency (<l msec) spikes could be evoked in many MNCs of PVN by stimulation near SON, which is consistent with the known projection to the ne~ohy~physis of many MNCs. Action potentials in MNCs of PVN and SON had significantly longer durations at one-third spike height (meanrS.D.=2.06t0.6 msec) than hippocampal CA1 pyramidal cells (1.1’7+0.29 msec). This suggests that neuroendocrine cells in mammals and some lower vertebrates and invertebrates are similar in this regard. Paraventricular neurons Action potential duration Lucifer Yellow injections THE paraventricuiar (PVN) and supraoptic (SON) nuclei of the hy~th~amus contain magnocellul~ ~europeptider~c cells (MNCs) that project to the neurohypophysis and par- vocellular neurons (PARVOs), some of which project to the median eminence [301. Most electrophysiological studies have used extracellular techniques in vivo and have focused on cells which could be activated antidro~c~ly by electrical stimulation of the neurohypophysis [13,231. Intracellular re- cordings from rn~rn~i~ neuroendoc~ne cells, however, have been infrequent. Only a few intracelhzlar recordings have been reported in ia viva studies [17,333, but more re- cently it has been possible to record intracellularly from mammalian hypothalamic cells in tissue culture 191and brain slice preparations 17,201. The present report _ describes intracellular recording studies in vitro in slices of rat hvuo- thalamus from cells identified either as magnocellular or-par- vocellular by intracellular staining with the fluorescent dye, Lucifer Yellow (LY). Our previous intracellular recordings in this system showed that some cells in PVN received excitatory synaptic input from areas lateral to PVN [7]. Specifically, extracellu- far stimuli from an electrode dorsal to the fomix column (Fx) evoked excitatory postsynatpic potentials (EPSPs) in PVN cells. It is highly likely that these recordings were primarily from m~nocelIular somata, since these perikarya were numerous in the area of impalement (the latera PVN) and are presumably the easiest st~ctures to penetrate with a microelectrode. A limited number of intracellular injections with horseradish peroxidase (HRP) also showed that most impalements were from MNCs, although we did not systemat- ically study synaptic inputs in the stained cells in that study. One purpose of the experiments in this report, therefore, was to show more directly that some MNCs receive excitatory synaptic input from regions lateral to PVN. Although neuroendocrine cells of many invertebrates and lower vertebrates have long-duration action potentials, it is unknown whether this is also true for mammals [3, 8, 191, The question is important because prolonged action poten- tials in neu~ndo~e somata would suggest enhanced Ca2+ influx and hormone release in the terminals, if somatic C!a2+ Copyright e? 1982 ANKHO International Inc .-~3~1-923~/82/~1~~7-#7$~3.~0/0