Response properties of baroreceptive NTS neurons.

Neurons in the nucleus of the solitary tract (NTS) responding to activation of arterial baroreceptors were recorded intracellularly using patch pipettes in an in situ arterially perfused working heart-brain stem preparation of rat. Seven of 15 (i.e., 46%) of NTS neurons showed adaptive (nonlinear) excitatory synaptic response patterns during baroreceptor stimulation followed by an "evoked hyperpolarization." This evoked hyperpolarization was stimulus intensity dependent and capable of shunting out a subsequent baroreceptor input. We suggest that this adaptive response behavior may be mediated, in part, by calcium-dependent potassium currents (IKCa) since neurons showed spike frequency adaptation during step depolarizations and an after-hyperpolarization after repetitive firing. Furthermore, in in vivo anesthetized rats, NTS microinjections of either charybdotoxin (225 fmol) or apamin (4.5 pmol) to block IKCa increased the baroreceptor reflex gain. Our data purport that the responsiveness of baroreceptive NTS neurons can be regulated by intrinsic membrane conductances such as IKCa. Modulation of such conductances during either physiological (exercise) or pathophysiological (essential hypertension) conditions may lead to changes in both the operating point and gain of the baroreceptor reflex.