ELECTRICAL-PROPERTIES OF OXYTOCIN NEURONS IN ORGANOTYPIC CULTURES FROM POSTNATAL RAT HYPOTHALAMUS

1. Intracellular recordings were performed on immunocytochemically ide ntified oxytocin (OT) neurons (n = 101) maintained for 2-7 wk in hypot halamic organotypic cultures derived from 4- to 6-day-old rat neonates . The neurons displayed a resting potential of -58.9 +/- 6.8 mV (mean +/- SD, n = 74), an input resistance of 114 +/- 26.8 M Omega (n = 66), and a time constant of 9.6 +/- 1.4 ms (n = 57). Voltage-current (V-I) relations, linear at resting potential, showed a pronounced outward r ectification when depolarized from hyperpolarized membrane potentials. At these hyperpolarized potentials, depolarizing current pulses induc ed a delayed action potential. 2. Action potentials had an amplitude o f 73.4 +/- 9.7 mV and a duration of 1.9 +/- 0.2 ms. Each action potent ial was followed by an afterhyperpolarization of 7.9 +/- 2.0 mV in amp litude lasting 61.7 +/- 11.3 ms. The depolarizing phase of action pote ntials was both Na+ and Ca2+ dependent, whereas repolarization was due to a K+ conductance increase. 3. When Ba2+ was substituted for Ca2+ i n the medium, OT neurons displayed prolonged sustained depolarizations . In the presence of tetrodotoxin (TTX), these depolarizations were tr iggered by depolarizing current pulses and arrested by hyperpolarizing current pulses or by local application of Ca2+, Co2+, Cd2+. NO sustai ned depolarization was obtained when nifedipine was added to the mediu m. These data suggest that OT cells in organotypic culture possess L-t ype Ca2+ channels. 4. All OT neurons generated spontaneous action pote ntials at resting potential. Of 59 neurons, 29 showed a slow, irregula r firing pattern (less than or equal to 2.5 spikes/s), 24 generated a fast continuous firing pattern (less than or equal to 2.5 spikes/s), a nd 6 cells displayed a bursting pattern of activity consisting of alte rnating periods of spike discharge and quiescence. None of the burstin g cells exhibited regenerative endogenous potentials (plateau potentia ls). On the contrary, in four of these cells, the bursting activity wa s clearly due to patterned synaptic activity. 5. The cultured OT cells responded to exogenous gamma-aminobutyric acid (GABA) and muscimol wi th a hyperpolarization and an increase in membrane conductance. These effects still were observed in the presence of TTX, indicating that th ey were due to direct activation of GABA receptors in the cells. The G ABA-induced response was mediated by GABA, receptors because it was bl ocked by bicuculline, but not by GABA, receptors, because baclofen and hydroxysaclofen had no effect on membrane potential and input resista nce. 6. OT neurons responded to exogenous glutamate, quisqualate, and kainate with a depolarization concomitant with an increase in membrane conductance. N-methyl-D-aspartate depolarized the cells in Mg2+-free medium. These effects were observed in the presence of TTX, suggesting that OT cells expressed ionotropic glutamate receptors. rans-(1S,3R)- 1-amino-1,3-cyclopentane-dicarboxylic acid and (+/-)-alpha-amino-4-car boxymethylphenylglycine had no effect on OT cells, thus excluding the presence of metabotropic glutamate receptors. 7. Taken together, our o bservations demonstrate that hypothalamic slice cultures from 4- to 6- day-old rat neonates contain well-differentiated OT neurons that displ ay electrical properties similar to those shown by adult neurons in vi tro. Such cultures provide a reliable model to investigate membrane pr operties of adult OT neurons and a useful means to study the long-term modulation of their electrical behaviour by various agents known to a ffect OT cells in vivo.