Sensitization and dishabituation of swim induction in the leech Hirudo medicinalis: role of serotonin and cyclic AMP

In this paper the role of serotonin (5HT) and cyclic AMP (cAMP) in sensitization and dishabituation of swim induction (SI) has been investigated in the leech Hirudo medicinalis. Electrical stimulation of the body wall evokes swimming activity with a constant latency. In animals with a disconnection between head ganglion and segmental ganglia, repetitive stimulation induces habituation of swimming whereas brushing on the dorsal skin provokes sensitization of a naïve response or dishabituation of a previously habituated response. Our findings indicate that 5HT is the neurotransmitter underlying both sensitization and dishabituation of SI. Injection of the 5HT receptor blocking agent methysergide impaires the onset of sensitization and dishabituation induced by brushing. Moreover, injection of 5HT mimics these forms of nonassociative learning, whereas injection of dopamine does not. Finally, the effects of 5HT are mediated by cAMP: (1) after injections of specific adenylate cyclase inhibitors such as MDL 12.330A or SQ22536, brushing becomes ineffective in facilitating the SI in either non-habituated or habituated animals. (2) 8Br-cAMP application mimics both sensitization and dishabituation of SI.