Voltage-Dependent Calcium Currents in Bulbospinal Neurons of Neonatal Rat Rostral Ventrolateral Medulla: Modulation by 2-adrenergic Receptors. Yu-Wen Li, Patrice G. Guyenet and Douglas A. Bayliss. Department of Pharmacology, University of Virginia, Charlottesville VA USA 22908.
APStracts 4:291N, 1997.
ABSTRACT
The properties and modulation by norepinephrine (NE) of voltage-dependent- calcium currents were studied in bulbospinal neurons (n=116) of the rostral ventrolateral medulla (RVLM) using whole-cell patch-clamp techniques in neonatal rat brainstem slices. RVLM bulbospinal neurons were visually identified by their location in slices and by the presence of FITC-tagged microbeads, which were injected into the spinal cord before the experiment; RVLM neurons were filled with Lucifer Yellow during recordings and the slice was processed for detection of tyrosine hydroxylase immunoreactivity (TH-IR). Thirty-four of 42 recovered cells (81%) were positive for TH-IR, indicating that most recorded cells were C1 neurons. Bulbospinal RVLM neurons expressed prominent high-voltage activated (HVA) calcium current, which began to activate at -30 to -40 mV (from a holding potential of -60 or -70 mV), and peaked at 0 mV (0.8 0.1 nA). HVA current comprised predominantly ?-Conotoxin GVIA-sensitive, N-type and ?-Agatoxin IVA-sensitive, P/Q-type components, with smaller dihydropyridine-sensitive, L-type and residual current components. Most RVLM bulbospinal neurons (n=44/52, including 12/14 histologically identified C1 cells) also expressed LVA calcium current. LVA current began to activate at ÿ7E-60 mV (from a holding potential of -100 mV), and was nearly completely inactivated at -50 mV with a half-inactivation potential of -70 2 mV. The amplitude of LVA current at -50 mV was 78 24 pA with Ba2+ and 156 38 pA with Ca2+ as a charge carrier. NE inhibited HVA current in most bulbospinal RVLM neurons (n=70/77) with an EC50 of 1.2 ?M; NE had no effect on LVA current. Calcium current inhibition by NE was mediated by ?2-adrenergic receptors (?2-ARs) as the effect was mimicked by the selective ?2-AR agonist, UK-14,304, and blocked by idazoxan, an ?2-AR antagonist, but unaffected by prazosin and propranolol (?1- and ?-AR antagonists, respectively). Most of the NE-sensitive calcium current was N- and P/Q-type. NE-induced inhibition of calcium current evoked by action potential waveforms (APWs) was significantly larger than that evoked by depolarizing steps (342.5 vs. 232.7%; P<0.05). Although inhibition of calcium current was voltage-dependent and partially relieved by strong depolarizations, when calcium currents were evoked with a 10 Hz train of APWs as a voltage command, the inhibitory effect of NE was maintained throughout the train. In conclusion, bulbospinal RVLM C1 neurons, including C1 cells, express multiple types of calcium currents. Inhibition of HVA calcium current by NE may modulate input-output relationships and release of transmitters from C1 cells. 

Received 28 August 1997; accepted in final form 13 October 1997.
APS Manuscript Number J715-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 29 October 1997