Biophysical characterization of rat caudal hypothalamic neurons: Calcium channel contribution to excitability

Neurons in the caudal hypothalamus (CH) are responsible for the modulation of various processes including respiratory and cardiovascular output. Previ ous results from this and other laboratories have demonstrated in vivo that these neurons have firing rhythms matched to the respiratory and cardiovas cular cycles. The goal of the present study was to characterize the biophys ical properties of neurons in the CH with particular emphasis in those prop erties 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 MV when recorded in current-clamp mo de in an in vitro brain slice preparation. A large proportion of these neur ons displayed postinhibitory rebound (PIR) that was dependent on the durati on and magnitude of hyperpolarizing current as well as the resting membrane potential of the cell. Furthermore these neurons discharged tonically in r esponse to a depolarizing current pulse at a depolarized resting membrane p otential (more positive than -65 mV) but switched to a rapid burst of firin g 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 mM). These properties suggest that low-voltage- activated (LVA) calcium current is involved in t he PIR and bursting firing of these CH neurons. In addition, high-voltage-a ctivated calcium responses were detected after blockade of outward potassiu m current or in Ba2+-replacement solution. In addition, almost all of the C H neurons studied showed spike frequency adaptation that was decreased foll owing Ca2+ removal, indicating the involvement of Ca2+-dependent K+ current (I-K,I-Ca)in these cells. In conclusion, CH neurons have at least two diff erent 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 s ignificant role in the bursting characteristics that may underlie the rhyth mic firing of CH neurons.