Differential effects of acute and chronic 5-HT depletion on Arc expression induced by MDMA J.C. Aston, A.R. Green & J.M. Elliott. Leicester School of Pharmacy, De Montfort University, Leicester LE1 9BH.	Print Abstract Search PubMed for: Aston JC Green AR Elliott JM

Arc (activity-regulated cytoskeleton associated gene) is a functional immediate early gene implicated in synaptic plasticity (Steward & Worley, 2001). Expression of Arc in rat brain is induced by neuronal excitation and by stimulation of 5-HT_2A and D₁ receptors. MDMA (3,4-methylenedioxymethamphetamine) induces a characteristic pattern of behaviour via the release of monoamines, including 5-HT and dopamine (Lyles & Cadet, 2003). In this study we have investigated the effects of acute 5-HT depletion using the tryptophan hydroxylase inhibitor para-chlorophenylalanine (PCPA) on the induction of Arc by MDMA and compared the results to those previously observed following chronic 5-HT depletion via the same mechanism (Aston & Elliott, 2003).

Male Dark Agouti rats (190-210g) were pretreated with saline (1ml/kg) or PCPA (300mg/kg i.p.) daily for 2 days then 24 hours later were injected with MDMA (6mg/kg i.p.) or saline (1ml/kg) (n=4-5 per group). Rats were killed 2 hours later, the brains isolated and Arc mRNA expression analysed by in situ hybridisation histochemistry as described previously (Pei et al., 2000). Tissue 5?HT / 5-HIAA content was analysed by HPLC/EC-detection. Statistical analysis was performed using one-and two-way ANOVA with Newman-Keuls post-hoc test.

In saline-pretreated rats MDMA induced significant Arc expression in frontal cingulate (133±8%) and parietal cortex (89±9%, p<0.05). PCPA significantly reduced the MDMA response in cingulate cortex by 66±8% (p<0.05) compared to saline-pretreated rats whereas in parietal cortex the reduction was smaller (32±10%) and non-significant. Basal Arc expression was not significantly altered by PCPA in cingulate (0±9%) or parietal cortex (9±4%) compared to saline controls. Tissue 5-HT and 5-HIAA were decreased by 92% and 96% respectively in frontal cortex (p<0.05).

The effects of PCPA in the striatum and endopiriform nucleus were qualitatively similar to those observed in the frontal cortex. In the hippocampal CA1 region both MDMA and PCPA did not alter Arc expression.

These results indicate that Arc mRNA expression induced by MDMA in both cortical and sub-cortical regions of rat brain is substantially decreased following acute, severe depletion of tissue 5?HT content by PCPA, although the decrease in Arc response is less marked in the parietal cortex. This suggests that the induction of Arc by MDMA is mediated principally by 5-HT even in areas such as the striatum where MDMA causes substantial release of dopamine. When compared to the effects of chronic depletion of 5-HT by PCPA (Aston & Elliott, 2003) several contrasts are apparent. Firstly, chronic but not acute 5-HT depletion significantly reduced basal Arc expression, suggesting tonic regulation of Arc by 5-HT with a slow response rate. This may have significant implications for the role of 5-HT in the development of synaptic plasticity and its implication in disease states such as depression. Secondly, the generalised decrease in Arc response to MDMA following acute 5-HT depletion is not apparent following chronic 5?HT depletion by PCPA. Indeed, in parietal cortex Arc induction by MDMA is significantly increased. This suggests that prolonged depletion of 5-HT may cause adaptive changes in responses to MDMA associated with synaptic plasticity which may therefore be relevant to the possible neurotoxic effects of MDMA and its illicit use in man.

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Pei, Q. et al (2000) Neuropharmacol. 39, 463-470.
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