HYPERPOLARIZATION-ACTIVATED CATION CURRENT (I(H)) IN NEURONS OF THE MEDIAL NUCLEUS OF THE TRAPEZOID BODY - VOLTAGE-CLAMP ANALYSIS AND ENHANCEMENT BY NOREPINEPHRINE AND CAMP SUGGEST A MODULATORY MECHANISM IN THE AUDITORY BRAIN-STEM

1. Principal cells in the medial nucleus of the trapezoid body (MNTB) are part of a circuit in the superior olivary complex (SOC) that processes binaural information important for sound localization. MNTB cells have two voltage-dependent currents active near rest that contribute to these cells' highly nonlinear membrane properties and shape their responses to synaptic input. One of these currents, a low-threshold, 4-aminopyridine (4-AP)-sensitive K+ current, has been studied previously under current clamp. Using the single-electrode voltage-clamp technique, we have investigated the other of these currents, a hyperpolarization-activated, mixed cation current (I(h)), in brain slices of the rat SOC. 2. I(h) is responsible for a prominent ''sag'' in the voltage response to a steady hyperpolarizing current recorded under current clamp in MNTB cells. In voltage-clamp recordings, hyperpolarizing voltage steps from the resting potential elicited a large inward current that activated and deactivated with biexponential kinetics. Activation time constants were voltage dependent, with tau1 and tau2 = 246 and 1620 ms at -75 mV and 107 and 560 ms at -100 mV. 3. I(h) was blocked by 1-5 mM cesium and had a reversal potential of -43 mV. Steady-state activation curves derived from tail currents yielded a half-activation voltage of -75.7 mV and slope factor of 5.7 mV, corresponding to < 10% activation of I(h) at rest. 4. Application of norepinephrine (15-20 muM) or 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) (1 mM) caused a depolarizing shift in the steady-state activation curve and decreased the activation time constants. The shift in the activation curve resulted in a large increase in the activation of I(h) at rest, an inward shift in the holding current, and an increase in the resting membrane conductance. In current-clamp recordings, this increase in the resting activation level of I(h) resulted in membrane depolarization of 2-3 mV in the absence of 4-AP, and 5-10 mV in the presence of 4-AP, an increase in the input conductance, and a reduction in the voltage sag in response to hyperpolarizing currents. 5. The resulting change in the resting point of MNTB cells exposed to norepinephrine or 8-Br-cAMP is likely to alter the responses of these cells to synaptic input, both via the direct effect on the resting membrane conductance and by changing the activation of the low-threshold, 4-AP-sensitive potassium current. Thus, activation of I(h) by noradrenergic inputs, as well as other inputs linked to 8-Br-cAMP production, is a potential mechanism for modulating the acoustic signal processing capabilities of MNTB cells.