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First published online April 18, 2006
Journal of Experimental Biology 209, 1746-1756 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02204

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Beyond the central pattern generator: amine modulation of decision-making neural pathways descending from the brain of the medicinal leech

Kevin M. Crisp^1,2 and Karen A. Mesce^1,2,*

¹ Graduate Program in Neuroscience, University of Minnesota, St Paul, MN 55108, USA
² Departments of Entomology and Neuroscience, University of Minnesota, St Paul, MN 55108, USA

^* Author for correspondence (e-mail: mesce001{at}umn.edu)

Accepted 23 February 2006

The biological mechanisms of behavioral selection, as it relates to locomotion, are far from understood, even in relatively simple invertebrate animals. In the medicinal leech, Hirudo medicinalis, the decision to swim is distributed across populations of swim-activating and swim-inactivating neurons descending from the subesophageal ganglion of the compound cephalic ganglion, i.e. the brain. In the present study, we demonstrate that the serotonergic LL and Retzius cells in the brain are excited by swim-initiating stimuli and during spontaneous swim episodes. This activity likely influences or resets the neuromodulatory state of neural circuits involved in the activation or subsequent termination of locomotion. When serotonin (5-HT) was perfused over the brain, multi-unit recordings from descending brain neurons revealed rapid and substantial alterations. Subsequent intracellular recordings from identified command-like brain interneurons demonstrated that 5-HT, especially in combination with octopamine, inhibited swim-triggering neuron Tr1, as well as swim-inactivating neurons Tr2 and SIN1. Although 5-HT inhibited elements of the swim-inactivation pathway, rather than promoting them, the indirect and net effect of the amine was a reliable and sustained reduction in the firing of the segmental swim-gating neuron 204. This modulation caused cell 204 to relinquish its excitatory drive to the swim central pattern generator. The activation pattern of serotonergic brain neurons that we observed during swimming and the 5-HT-immunoreactive staining pattern obtained, suggest that within the head brain 5-HT secretion is massive. Over time, 5-HT secretion may provide a homeostatic feedback mechanism to limit swimming activity at the level of the head brain.

Key words: gating neuron, command neuron, neuromodulation, locomotion, octopamine, serotonin, behavioral choice, leech, Hirudo medicinalis

This article has been cited by other articles:

				J. G. Puhl and K. A. Mesce Dopamine Activates the Motor Pattern for Crawling in the Medicinal Leech J. Neurosci., April 16, 2008; 28(16): 4192 - 4200. [Abstract] [Full Text] [PDF]