Development of sensory-motor circuit associated with behaviour in the leech.

ANTONIA MARIN BURGIN; W. B. KRISTAN JR; K.A. FRENCH

New Orleans

Congreso; Meeting of the society for neuroscience; 2003

Society for Neuroscience

Behaviors appear in a stereotyped order during embryogenesis, and this ordering should reflect progressive development of connectivity among the neurons. Embryonic behaviors in the leech Hirudo medicinalis appear in sequence, beginning with spontaneous behaviors, arising at 51% of embryonic development (ED), and followed by mechanically evoked behaviors starting at 54%ED. (100%ED requires 30 days.) To investigate changes in the nervous system underlying these behaviors, we studied development of neuronal connections in the circuit that produces local bending, which begins at 60%ED. This circuit consists of 3 layers of neurons: 4 mechanosensory P cells, 17 identified interneurons, and approximately 24 inhibitory and excitatory motor neurons that innervate longitudinal muscles. To characterize electrical connectivity, we injected neurobiotin (a small molecule that crosses gap junctions) into identified neurons at different stages of development (45%-100%ED). Early in development (46%ED), when P cells have innervated the skin but do not branch in the CNS, they were electrically coupled to glial cells. This coupling disappeared as central branches grew (50%ED) leaving P cells uncoupled to other cells for about 1 day. However, at the same stage, motor neurons were electrically coupled to other motor neurons. At 54% ED, P cells began to electrically couple with interneurons, and some of the local bending interneurons were coupled to other interneurons and to motor neurons. Interestingly, connections between motor neurons are established just as spontaneous behaviors first appear. One day later, when mechanically evoked behaviors first appear, the sensory neurons and interneurons first show electrical connections. These results suggest that an electrically coupled motor network carries out the early spontaneous behaviors, and as sensory neurons connect to interneurons, these spontaneous behaviors give way to evoked behaviors.