Effects of methylphenidate on the membrane potential and current in neurons of the rat locus coeruleus

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 microM) 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 Ca(2+) and 11 mM Mg(2+) (Ca(2+)-free ACSF) produced a depolarizing response associated with an increase in spontaneous firing of the action potential. The MPH-induced hyperpolarization was blocked in Ca(2+)-free ACSF. Yohimbine (1 microM) and prazosin (10 microM), antagonists for alpha(2) and alpha(2B/2C) receptors, respectively, blocked the MPH-induced hyperpolarization in LC neurons. Tetrodotoxin (TTX, 1 microM) produced a partial depression of the MPH-induced hyperpolarization in LC neurons. Under the whole cell patch-clamp condition, MPH (30-300 microM) produced an outward current (I(MPH)) with amplitude of 110 +/- 6 pA (n = 17) in LC neurons. The I(MPH) was blocked by Co(2+) (1 mM). During prolonged application of MPH (300 microM 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 microM), 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 alpha(2B/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. Ba(2+) (100 microM) 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.