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Intersegmental Interneurons and Serotonin‐Facilitated Swimming in the Medicinal Leech

The probability that an animal will respond to a specific, natural stimulus with a particular response is affected by factors such as learning (plasticity) and neuromodulatory state. Thus, interactions between modulatory processes and mechanisms underlying plasticity ultimately bias behavioral output. In the leech, serotonin (5‐HT) promotes swimming behavior, and plays a role in certain types of learning (e.g., sensitization). We examined the habituation of leech swimming in the presence or absence of exogenously applied 5‐HT. We found that treatment of as little as two ganglia with 100 μM 5‐HT significantly increased swim responsiveness, defined as the number of trials in which tactile stimulation of the skin resulted in fictive swimming activity before the preparation habituated to criterion (two consecutive trials without response). Importantly, although habituation of the swim response is segment‐specific (Debski and Friesen, 1985), treatment of a pair of ganglia 9 segments anterior to the stimulated segment significantly increased swim responsiveness. Because recent research (Crisp and Mesce, 2006) suggests that focal application of 5‐HT to the leech head brain increases impulse activity in some fibers of the interganglionic connective, we examined whether or not a similar change in connective activity could be observed when pairs of midbody ganglia were exposed to 5‐HT. We observed an overall increase in impulse activity in the connective within minutes of 5‐HT activation. Units contributing to this increase were identified using offline spike sorting analysis, and the effects of 5‐HT on their firing properties were examined.