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.

Mixtures of octopamine and serotonin have nonadditive effects on the CNS of the medicinal leech.

It is well established that neural networks respond to a wide variety of modulatory substances by which they can become reconfigured, yet few studies have examined the effects of neurotransmitter mixtures on such networks. In a previous study of the medicinal leech using triple intracellular recordings, we found that stimulation of identified mechanosensory neurons activated both the serotonergic cell 21 (a swimgating neuron) and the dorsal lateral octopamine (DLO) cell. Because these findings suggested that serotonin (5-HT) and octopamine (OA) may be released together, we investigated the effects of 5-HT and OA mixtures on isolated nerve cords of Hirudo medicinalis (which contained both head and tail brains). Fifty micromolar OA, 50 microM 5-HT, or a mixture of 50 microM OA and 50 microM 5-HT was bath applied to the nerve cord under constant perfusion conditions. Additional experiments were performed with combinations of either 25 or 100 microM OA and 5-HT. Neural activity was examined specifically in the segmentally repeated dorsal posterior (DP) nerve because it has been shown to contain identified swim motor units. Nonadditive effects of amine combinations were most apparent in their ability to decrease overall activity in the DP nerve and to alter patterned motor activity in the form of fictive swimming. Whereas swim burst activity has been previously shown to increase in nerve cords bathed in either 5-HT or OA solutions alone, we demonstrated that a mixture of the two amines resulted in a robust decrease in the number of swim bursts expressed and an inhibition of swim activity in preparations already swimming. Most compelling was the observation that when the amine mixture was replaced with normal saline, swim burst activity increased dramatically. We discuss that the effects of amine mixtures may be due to their interaction with descending interneurons known to trigger and inhibit swimming as the mixture-induced effects were not observed in nerve cords lacking the head and tail brains. Because the net effect of the two amines was not simply additive (i.e., 5-HT or OA is known to activate swimming, yet the mix inhibits swimming), this result reveals yet another layer of complexity inherent in "simpler" invertebrate nervous systems.