Methylphenidate increases cortical excitability via activation of alpha-2 noradrenergic receptors

Abstract

Although methylphenidate (MPH), a catecholaminergic reuptake blocker, is prescribed for attention-deficit/hyperactivity disorder, there is a dearth of information regarding the cellular basis of its actions. To address this issue, we used whole-cell patch-clamp recordings to investigate the roles of various catecholamine receptors in MPH-induced changes in cortical neuron excitability. We bath-applied dopamine or noradrenaline receptor antagonists in combination with MPH to pyramidal cells located in deep layers of the infralimbic and prelimbic prefrontal cortices. Application of MPH (10 microM) by itself increased cortical cell excitability in slices obtained from juvenile rats. This MPH-mediated increase in excitability was lost when catecholamines were depleted with reserpine prior to recording, demonstrating the requirement for a presynaptic monoamine component. Antagonist studies further revealed that stimulation of alpha-2 noradrenergic receptors mediates the MPH-induced increase in intrinsic excitability. Dopamine D1 receptors played no observable role in the actions of MPH. We therefore propose that MPH is acting to increase catecholaminergic tone in the PFC, and thereby increases cortical excitability by mediating the disinhibition of pyramidal cells through mechanisms that may include activation of alpha-2 adrenoreceptors located in interneurons.

Figure 1

Bath-administration of MPH (10 μM) results in significant increases in cortical excitability. (a1) MPH mediates a significant increase in cortical excitability (n = 30, p < 0.0007). In contrast, administration of lower concentrations of MPH (1 μM) did not evoke significant changes in cortical excitability. The bar graphs depict the average number of spikes/pulse. (a2) Control-recovery experiments show that repetitive injection of current pulses does not induce an increase in the evoked activity of cortical cells. In addition, this experiment provides control for the temporal aspects of the experiments, demonstrating that the length of the recording does not affect intrinsic excitability. (b1) Representative traces of the average number of spikes evoked during control recordings and (b2) after 5 min of MPH (10 μM) administration.

Figure 2

MPH did not increase cortical excitability in reserpine-treated tissue. Bath-administration of MPH (10 μM) mediates significant increases in cortical excitability by increasing catecholaminergic tone. In contrast, if the tissue is pretreated with reserpine, MPH is unable to increase cortical excitability. The histogram represents the increases in average number of spikes/pulse.

Figure 3

Alpha-2 antagonists block the MPH-mediated increases in cortical excitability. (a1) The alpha-2 antagonist yohimbine (2 μM) blocks the MPH-mediated effects in cortical excitability. (a2) Representative traces showing the preventive effects of yohimbine administration. (b1) The alpha-1 antagonist prazosin (1 μM) did not prevent the increase in excitability mediated by MPH (10 μM, p < 0.004). (b2) Representative traces showing the MPH-mediated increases in cortical excitability following prazosin administration. (c) The beta antagonist propanolol (10 μM) induce a significant increase in cortical excitability (p < 0.001) that is sustained following MPH administration (10 μM). (d) The DA D1 antagonist SCH 23390 (5 μM) did not prevent the MPH-mediated increases in cortical excitability.

Figure 4

The specific alpha-2 agonist clonidine mimicked the increase in cortical excitability mediated by MPH. (a) The histogram indicates that both the reuptake blocker MPH and the specific alpha-2 agonist clonidine increased cortical excitability. (b1) Representative traces of the average number of spikes evoked during control recordings and following MPH (10 μM) administration. (b2) Representative traces of the average number of spikes evoked during control recordings and following clonidine (10 μM) administration.

Figure 5

When the GABA_A and glutamate blockers, bicuculline (10 μM), CNQX (10 μM), and CPP (10 μM), were added to the bath, MPH administration did not increase cortical excitability.