ABSTRACT
Behavioral and neurochemical studies suggest that the induction of behavioral sensitization to psychostimulants involves transient changes at the synapses of the ventral tegmental area's dopaminergic neurons (VTA-DA). Differences in the behavioral response to amphetamine (Amph) and methylphenidate (MPD) were observed. In an attempt to understand these behavioral differences at the neuronal level, the dose-response characteristics of these two psychostimulants on electrophysiologically identified VTA-DA neurons at the glutamatergic synapse were investigated. Miniature excitatory post-synaptic currents (mEPSCs) and electrically induced EPSCs were recorded from horizontal midbrain slices of rats that had been pretreated intraperitoneally (i.p.) with saline (control), Amph (2.5, 5.0, 10.0 or 20.0 mg/kg), or MPD (2.5, 5.0, 10.0 or 20.0 mg/kg) 24 h before the recording. Perfusion of Amph through the bath (2.5, 5.0, 10.0 or 20.0 microM) increased the frequency (p<0.01) and the amplitude (p<0.05) of mEPSCs in dose-response characteristics, while MPD perfusion through the bath (2.5, 5.0, 10.0, or 20.0 microM) increased only the frequency (p<0.05) of the mEPSC. Both psychostimulants increased the prefrontal cortex's (PFC) glutamatergic EPSC in the VTA-DA neurons. However, only the higher doses of MPD induced significant effects (p<0.05) on the N-methyl-D-aspartate (NMDA) receptor-mediated EPSC but had no effects on the EPSC mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA/kainate) receptors. Contrarily, Amph increased both kinds of mediated EPSC, but mainly the EPSC mediated by AMPA/kainate receptors (p<0.01). These electrophysiological differences could represent the underlying mechanism responsible for the differences of behavioral effects, such as behavioral sensitization, elicited by MPD and Amph.
Subject(s)
Amphetamine/pharmacology , Dopamine/physiology , Methylphenidate/pharmacology , Receptors, AMPA/physiology , Receptors, N-Methyl-D-Aspartate/physiology , Ventral Tegmental Area/drug effects , Animals , Dose-Response Relationship, Drug , Excitatory Postsynaptic Potentials/drug effects , Excitatory Postsynaptic Potentials/physiology , Male , Motor Activity/drug effects , Motor Activity/physiology , Neurons/drug effects , Neurons/physiology , Rats , Rats, Inbred WKY , Rats, Wistar , Receptors, Glutamate/physiology , Ventral Tegmental Area/physiologyABSTRACT
Treatment of psychostimulants leads to the development of behavioral sensitization, an augmented behavioral response to drug re-administration. The induction of behavioral sensitization to psychostimulants such as amphetamine and cocaine occurs at the ventral tegmental area's dopaminergic neurons (VTA-DA). Currently, there is limited experimental data about the physiological properties of methylphenidate (MPD) on VTA-DA neurons. Behavioral and electrophysiological experiments using male rats were performed before and after MPD treatment. The behavioral experiment included dose-response (0.6, 2.5, and 10.0 mg/kg MPD) study to select the most effective dose for the electrophysiological study. Methylphenidate increased locomotion in typical dose response characteristics. Based on this experiment, the 10.0 mg/kg MPD was used in two types of electrophysiological recordings: 1) intracellular recording of neuronal activity performed on horizontal 275-300 microm brain slices and 2) whole-cell patch clamping before and after electrical stimulation to study post-synaptic currents on neurophysiologically identified VTA-DA neurons. Methylphenidate suppressed the neuronal activity of these neurons for 210 +/- 30 sec. Stimulation of the prefrontal cortex afferent fibers to these VTA-DA neurons in the presence of TTX, saclofen, and picrotoxin led to the conclusion that this input is mediated via NMDA and kainate/AMPA receptors and may participate to induce behavioral sensitization to psychostimulants.
