Serotonin depletion does not prevent intrinsic sensitization in the leech

Intrinsic sensitization is a form of behavioral facilitation that is distin ct from the extrinsic sensitization normally studied. To examine whether in trinsic and extrinsic sensitization are mediated by different physiological processes, the effects of 5,7-dihydroxytryptamine-induced serotonin (5-HT) depletion on intrinsic sensitization of the leech whole-body shortening re sponse were observed. Previous experiments have shown that 5-HT depletion d isrupts dishabituation and extrinsic sensitization of this behavior in the leech. Intrinsic sensitization was observed in preparations from both contr ol and 5-HT-depleted animals, indicating that this form of behavioral facil itation was not affected by 5-HT depletion. The differences in the effects of 5-HT depletion on intrinsic versus extrinsic sensitization suggest that there are distinct neurophysiological processes mediating these two forms o f behavioral facilitation. In addition, 5-HT depletion appeared to disrupt a putative extrinsic form of habituation of the shortening reflex. These da ta support the hypothesis that both intrinsic and extrinsic processes of ne uromodulation mediate habituation and sensitization.