Effects of Methylphenidate on the Membrane Potential and Current in Neurons of the Rat Locus Coeruleus

Abstract

Effects of methylphenidate (MPH), a therapeutic agent used in children presenting the attention deficit hyperactivity disorder (ADHD), on the membrane potential and current in neurons of the rat locus coeruleus (LC) were examined using intracellular and whole cell patch-clamp recording techniques. Application of MPH (30 μM) to artificial cerebrospinal fluid (ACSF) produced a hyperpolarizing response with amplitude of 12 ± 1 mV ( n = 29). Spontaneous firing of LC neurons was blocked during the MPH-induced hyperpolarization. Superfusion of LC neurons with ACSF containing 0 mM Ca2+ and 11 mM Mg2+ (Ca2+-free ACSF) produced a depolarizing response associated with an increase in spontaneous firing of the action potential. The MPH-induced hyperpolarization was blocked in Ca2+-free ACSF. Yohimbine (1 μM) and prazosin (10 μM), antagonists for α2 and α2B/2Creceptors, respectively, blocked the MPH-induced hyperpolarization in LC neurons. Tetrodotoxin (TTX, 1 μM) produced a partial depression of the MPH-induced hyperpolarization in LC neurons. Under the whole cell patch-clamp condition, MPH (30–300 μM) produced an outward current ( I MPH) with amplitude of 110 ± 6 pA ( n = 17) in LC neurons. The I MPH was blocked by Co2+ (1 mM). During prolonged application of MPH (300 μM for 45 min), the hyperpolarization gradually decreased in the amplitude and eventually disappeared, possibly because of depression of norepinephrine (NE) release from noradrenergic nerve terminals. At a low concentration (1 μM), MPH produced no outward current but consistently enhanced the outward current induced by NE. These results suggest that the MPH-induced response is mediated by NE via α2B/2C-adrenoceptors in LC neurons. I MPH was associated with an increase in the membrane conductance of LC neurons. The I MPH reversed its polarity at −102 ± 6 mV ( n = 8) in the ACSF. The reversal potential of I MPH was changed by 54 mV per decade change in the external K+ concentration. Current-voltage relationship showed that the I MPH exhibited inward rectification. Ba2+ (100 μM) suppressed the amplitude and the inward rectification of the I MPH.These results suggest that the I MPH is produced by activation of inward rectifier K+channels in LC neurons.