Role of 5-HT2A and 5-HT2C/B receptors in the acute effects of 3,4-methylenedioxymethamphetamine (MDMA) on striatal single-unit activity and locomotion in freely moving rats.

Like amphetamine, a locomotor-activating dose of 3,4-methylenedioxymethamphetamine (MDMA) predominantly excites striatal single-unit activity in freely moving rats. Although both D1- and D2-like dopamine (DA) receptors play important roles in this effect, MDMA, unlike amphetamine, strongly increases both DA and serotonin (5-HT) transmission. This study was conducted to investigate the 5-HT receptor mechanisms underlying the striatal effects of MDMA. We recorded the activity of >200 single units in the striatum of awake, unrestrained rats in response to acute MDMA administration (5 mg/kg) combined with the selective blockade of either 5-HT2A or 5-HT2C/B receptors. Prior administration of SR-46349B (a 5-HT2A antagonist 0.5 mg/kg) blocked nearly all MDMA-induced striatal excitations, which paralleled its significant attenuation of MDMA-induced locomotor activation. Conversely, prior administration of SB-206553 (a 5-HT2C/B antagonist 2.0 mg/kg) had no effect on the amount of MDMA-induced locomotor activation or the distribution of single-unit responses to MDMA. However, a coefficient-of-variation analysis indicated significantly less variability in the magnitude of both MDMA-induced neuronal excitations and inhibitions in rats that were pretreated with SB-206553 compared to vehicle. Analysis of concurrent single-unit activity and behavior confirmed that MDMA-induced striatal activation was not merely due to behavioral feedback, indicating a primary action of MDMA. These results support and extend our previous findings by showing that 5-HT2A and 5-HT2C/B receptors differentially regulate the expression of MDMA-induced behavioral and striatal neuronal responses, either directly or through the modulation of DA transmission.