Role of serotonin and cyclic AMP on facilitation of the fast conducting system activity in the leech Hirudo medicinalis.

In the nervous system of the leech Hirudo medicinalis it has been possible to study short-term plastic changes. Depression and facilitation have been demonstrated in the fast conducting system (FCS) activity; this pathway consists of a chain of electrically linked neurons present in each ganglion. In semi-intact animals or in preparation of nerve cord and segments of body wall, both electrical stimulation of peripheral roots and tactile stimulation of the skin induced, after repetitive stimulation (0.1/s) a prolonged decrement of FCS response. Strong nociceptive stimulation applied onto the head or the body wall produced a sustained facilitation of the waned response. The same potentiation has been observed by perfusing the isolated ganglion with serotonin (5 x 10(-5) M). Such a potentiation is abolished by preincubation with methysergide, an antagonist of serotonin, and with imidazole, a cAMP-phosphodiesterase activator. Such an effect is mimicked by an analog of cAMP, db-cAMP. Simultaneous recordings of both T neurons (intracellularly) and FCS firing discharge showed that, during FCS response decrement, the T cell activity remained unchanged and no modification of conductance occurred, excluding therefore a detectable involvement of sensory neurons in the depression. These results suggest that short-term plastic changes of the FCS of the leech are due to a prolonged potentiation of synaptic transmission as a result of serotonin-mediated increase in cAMP.