抄録
The vertebrate hindbrain is a segmented structure. The teleost hindbrain contains reticulospinal (RS) neurons that are periodically arranged along the neuraxis. RS neurons sharing a common morphology in adjacent segments are referred to as segmental homologs. The Mauthner (M) cell in the fourth segment (S4) is known to trigger fast escape. MiD2cm in S5 and MiD3cm in S6, serial homologs of the M-cell, are predicted to contribute to this behavior. To investigate whether these homologs are organized as a functional unit for sound-induced escape, we studied the afferent projection from auditory nerve (VIIIn), output firing properties of these neurons and physiological connections among them in adult goldfish. Labeling with different fluorescence tracers showed that the VIIIn projected to nearby the S4-S6 RS neurons. Tone burst and electrical stimulation of VIIIn evoked EPSPs in them. Stepwise depolarization elicited a spike at the onset in the M-cell, but repetitive firing in MiD2cm and MiD3cm. The atypical firing property of M-cell was mediated by dendrotoxin (DTX)-I-sensitive, voltage-gated potassium channels together with recurrent inhibition. The M-cell, but not MiD2cm or MiD3cm, expressed Kv1.2, a DTX-I-sensitive potassium channel subunit. Presynaptic activation of M-cell produced disyaptic IPSPs in MiD2cm and MiD3cm, suggesting inhibitory connection from the M-cell to them. Together, the M-cell and its segmental homologs may sense common auditory information but send different outputs to the spinal circuits and make a hierarchical organization to control adaptive escape behavior. [Jpn J Physiol 54 Suppl:S21 (2004)]
