Reduction in Tryptophan Hydroxylase (TPH) Activity
The activitv of TPH, the rate limiting enzyme in the synthesis of 5-HT, has been shown to decrease both in vivo and ex vivo following MDMA treatment (Schmidt and Taylor, 1987; Schmidt and Taylor, 1988; Stone et al., 1987; Stone et al., 1986). MDMA is reported to decrease the Vmax for TPH without an apparent change in Km for either the substrate or any of the enzyme cofactors (Schmidt and Taylor, 1987Stone ct al., 1989a). MDMA decreases TPH activity at both neurotoxic and non-neurotoxic doses (Schmidt and Taylor, 1987; Stone et al., 1987). The S-enantiomer of MDMA is a more potent inhibitor than the R-enantiomer (Johnson ct al., 1987; Schmidt ct al., 1987).
One mechanism that has been proposed for the MDMA-induced inhibition of TPH suggests a role for oxidative stress. Stone ct al. (1989b) demonstrated that hthiothreitol and reduced iron could reverse the MDMA induced inhibition of TPH 3 hours, but not 18 hours after treatment with MDMA. The results suggest that the acute decrease in TPH activity may result from oxidation of thiol groups within the TPH molecule. However, the source of this oxidative stress is unknown. That report is consistent with an earlier observation that TPH is an extremely labile enzyme (Kuhn ct al., 1980), where it was shown that the sulfhydryl groups within the TPH molecule are sensitive to oxidation, including that by molecular oxygen. in vitro studies have shown that MDMA does not directly inhibit TPH (Schmidt and Taylor, 1987) suggesting either that a metabolite of MDMA may be responsible for this inhibition, or that inhibition only occurs in the context of the neuron existing in a functional environment.
Another mechanism suggested for lowered TPH activity is that the 5HT released by MDMA activates a 5HT feedback system, resulting in a down-regulation of PH. Support for this hypothesis has been generated by data showing that 5-HT uptake inhibitors such as fluoxetine not only block the short term depiction of 5-HT but also attenuate the inhibition of TPH by MDMA (Schmidt et al., 1987; Schmidt and Taylor, 1987; Schmidt and Taylor, 1990). Ketanserin, the 5-HT,,,_ receptor antagonist, and methiothepin, the 5-HT autoreceptor antagonist, also have been shown to attenuate the inhibition of TPH when given three hours following MDMA treatment (Schmidt and Taylor, 1987).
There appears to be a time-dependent nature to the reversibility of TPH inactivation, it can only be completely restored within 3 hours following MDMA treatment. Between 3 and 6 hours, enzyme activity can only be partially restored; and after 6 hours, the loss of TPH activity cannot be reversed (Stone ct al., 1989b). The long term irreversible reduction in TPH activity is thought to reflect axonal degeneration and the short-term reversible phase is thought to be an indication of enzyme inhibition.
In summary, three potential mechanisms have been proposed for the inhibition of TPH by MDMA: (1) some form of oxidative stress within the neuron may oxidize functional thiol groups within the TPH molecule; (2) a toxic metabolite of MDMA may be responsible for TPH inhibition; or (3) activation of the autoreceptor by released 5-HT may decrease TPH activity. Of the three possibilities, the first appears most consistent with the current evidence.
