To swim or not to swim: regional effects of serotonin, octopamine and amine mixtures in the medicinal leech.

Focally treating the head brain of the medicinal leech Hirudo medicinalis with various biogenic amines affected the initiation, termination and maintenance of fictive swimming (i.e., the neural correlate of swimming). Application of serotonin to saline surrounding only the head brain inhibited fictive swimming, whereas removing serotonin induced swimming. This contrasts sharply with previous observations that serotonin applied to the nerve cord induces swimming. Although application of octopamine to the brain activated swimming, a mixture of octopamine and serotonin inhibited swimming. Subsequent removal of this mixture from the brain activated robust swimming and was more potent for activating swimming than either the removal of serotonin or the application of octopamine. Swim episodes induced by brain-specific manipulations of octopamine had more swim bursts per episode than those induced by serotonin. These brain-specific effects of the amines on fictive swimming are probably due to the modulation of higher-order circuits that control locomotion in the leech. We observed that serotonin or a mixture of serotonin and octopamine hyperpolarized an identified descending brain interneuron known as Tr2. Removal of the mixture caused Tr2 to exhibit membrane potential depolarizations that correlated in time with the expression of swim episodes.