Modulation of swimming behavior in the medicinal leech

Abstract

The effects of serotonin on the electrical properties of swim-gating neurons (cell 204) were examined in leech (Hirudo medicinalis) nerve cords. Exposure to serotonin decreased the threshold current required to elicit swim episodes by prolonged depolarization of an individual cell 204 in isolated nerve cords. This effect was correlated with a more rapid depolarization and an increased impulse frequency of cell 204 in the first second of stimulation. In normal leech saline, brief depolarizing current pulses (1 s) injected into cell 204 failed to elicit swim episodes. Following exposure to serotonin, however, identical pulses consistently evoked swim episodes. Thus, serotonin appears to transform cell 204 from a gating to a trigger cell. Serotonin had little effect on the steady-state current-voltage relation of cell 204. However, serotonin altered the membrane potential trajectories in response to injected current pulses and increased the amplitude of rebound responses occurring at the offset of current pulses. These changes suggest that serotonin modulates one or more voltage dependent conductances in cell 204, resulting in a more rapid depolarization and greater firing rate in response to injected currents. Thus, modulation of intrinsic ionic conductances in cell 204 may account in part for the increased probability of swimming behavior induced by serotonin in intact leeches.

Serotonin in the Nervous System: Few Neurons Regulating Many Functions

“…Stimulation of Retzius neurons or application of serotonin in the bath also produces swimming episodes [31], because serotonin modulates the activity of some of the oscillating interneurons that may initiate the rhythmic activity of the swimming circuit. For example, through modulating several sodium channels, serotonin changes the excitability (i.e., the likeliness to fire action potentials) of one of the interneurons that initiate the swimming cycle (called cell 204), decreasing the threshold for this neuron to trigger the activity of the circuit [32,33]. Serotonin also modulates the activity of motor neurons that produce swimming, promoting their rhythmic alternated activation [34,35].…”

Neural circuit regulation by identified modulatory projection neurons

“…In the medicinal leech, circulating serotonin increases with hunger, coincident with a decreased threshold for swimming. Although serotonin does not activate swim-activating cell 204, it modulates its intrinsic properties, making it easier for other inputs to activate this neuron and elicit swimming (Angstadt and Friesen, 1993;Kristan et al, 2005). Even if the responsiveness of a modulatory PN does not change, the consequences of its activity may be state-dependent.…”

Acetylcholine as a trigger of the somatic exocytosis in Retzius neurons

“…The Retzius cells in the leech (Rz-neurons) produce and release serotonin (5-HT) that plays an important neuromodulator role and may regulate the RA [4][5][6]. Serotonin may act in three different ways in the nervous system: as a transmitter at the synapses; as a paracrine modulator upon diffusion at a distance from its release sites, and as a hormonal modulator by circulating in the blood stream [7].…”