Modulation of swimming activity in the medicinal leech by serotonin and octopamine

Abstract

1. The monoamines serotonin (5-HT) and octopamine (OA) enhance the expression of swimming activity in the medicinal leech (Willard, 1981; Belanger and Orchard, 1988). We explored further the effects of these monoamines and related agents on swimming activity observed in isolated leech nerve cords. 2. We confirmed that swimming activity is induced reversibly following exposure of the nerve cord to 5-HT (50 μM); the half-maximal rate of swimming activity develops in about 15min. Swimming activity returns to control levels about 30 min after drug washout. 3. Swim-induction by 5-HT is blocked by the presence of 10 μM cyproheptadine (a 5-HT antagonist). 4. Although apparently less effective than 5-HT, OA application to nerve cords also induced swimming activity. 5. Depletion of endogenous amines from nerve cords by acute exposure to reserpine (10–150μM) blocked stimulus-evoked swimming activity within 4 hr. 6. Subsequent application of 5-HT (50 μM) or OA (100 μM) reinstated stimulus-evoked swimming and induced repeated episodes of non-triggered swimming activity. 7. Application of cAMP and cAMP analogs, as well as phosphodiesterase inhibitors (theophylline and IBMX), mimicked the effects of the monoamines, suggesting that 5-HT and OA may activate swimming activity by increasing neuronal cAMP. 8. We obtained episodes of swim-like activity from individual, isolated ganglia exposed to 5-HT or OA. Such episodes were usually brief, with variable cycle period. 9. We conclude that individual nerve cord ganglia contain the complete neuronal circuitry required to generate the rudiments of swimming activity. We propose that these neuronal circuits are modified by swim-inducing drugs to facilitate the expression of rhythmic output.