The Journal of Neurophysiology Vol. 84 No. 6 December 2000, pp. 2896-2903
Copyright ©2000 by the American Physiological Society

Biophysical Characterization of Rat Caudal Hypothalamic Neurons: Calcium Channel Contribution to Excitability

Department of Molecular and Integrative Physiology and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

Fan, Yi-Ping, Eric M. Horn, and Tony G. Waldrop. Biophysical Characterization of Rat Caudal Hypothalamic Neurons: Calcium Channel Contribution to Excitability. J. Neurophysiol. 84: 2896-2903, 2000. Neurons in the caudal hypothalamus (CH) are responsible for the modulation of various processes including respiratory and cardiovascular output. Previous results from this and other laboratories have demonstrated in vivo that these neurons have firing rhythms matched to the respiratory and cardiovascular cycles. The goal of the present study was to characterize the biophysical properties of neurons in the CH with particular emphasis in those properties responsible for rhythmic firing behavior. Whole cell, patch-clamped CH neurons displayed a resting membrane potential of 58.0 ± 1.1 mV and an input resistance of 319.3 ± 16.6 M when recorded in current-clamp mode in an in vitro brain slice preparation. A large proportion of these neurons displayed postinhibitory rebound (PIR) that was dependent on the duration and magnitude of hyperpolarizing current as well as the resting membrane potential of the cell. Furthermore these neurons discharged tonically in response to a depolarizing current pulse at a depolarized resting membrane potential (more positive than 65 mV) but switched to a rapid burst of firing to the same stimulus when the resting membrane potential was lowered. The PIR observed in these neurons was calcium dependent as demonstrated by the ability to block its amplitude by perfusion of Ca²⁺-free bath solution or by application of Ni²⁺ (0.3-0.5 mM) or nifedipine (10 µM). These properties suggest that low-voltage-activated (LVA) calcium current is involved in the PIR and bursting firing of these CH neurons. In addition, high-voltage-activated calcium responses were detected after blockade of outward potassium current or in Ba²⁺-replacement solution. In addition, almost all of the CH neurons studied showed spike frequency adaptation that was decreased following Ca²⁺ removal, indicating the involvement of Ca²⁺-dependent K⁺ current (I_K,Ca) in these cells. In conclusion, CH neurons have at least two different types of calcium currents that contribute to their excitability; the dominant current is the LVA or T-type. This LVA current appears to play a significant role in the bursting characteristics that may underlie the rhythmic firing of CH neurons.