Photoinactivation of an identified motoneurone in the locust Locusta migratoria.

1. The common inhibitory motoneurone 1 (CI1) in the mesothoracic ganglion of the locust was photoinactivated using a helium-cadmium laser or a mercury lamp as light source. Treated animals showed no signs of abnormal locomotory behaviour over periods of up to 40 days. 2. Photoinactivation of part of the neurone in the ganglion, i.e. the soma and the primary neurite, is sufficient to cause irreversible degeneration of all the peripheral extensions of the neurone. Three weeks after photoinactivation, all GABA immunoreactivity had disappeared from the axon branches of the photoinactivated neurone and from their terminals on one of the target muscles investigated, the anterior coxa rotator M92, and inhibitory postsynaptic potentials could no longer be elicited through stimulation. This was taken as proof of functional denervation of the muscle with regard to its inhibitory input. By this time, the axon of CI1 in nerve N3C1, which supplies M92, had also disappeared. 3. Animals treated during the fourth or fifth instars showed a permanent loss of the photoinactivated mesothoracic CI1 neurone after moulting into adulthood. 4. Denervation of M92 in the middle legs of instars and adults by axotomy of N3 always led to rapid functional reinnervation of the muscle. The first sign of reinnervation (excitatory neuromuscular activity upon mechanical stimulation of the tarsi) was detected electrophysiologically as early as 8 days after severing the motor nerve. 5. The elimination of CI1 by photoinactivation for a period of up to 40 days did not influence parameters of the target muscle, such as size, number of fibres and phenotypes of fibres defined histochemically according to their myofibrillar ATPase isoforms, irrespective of whether the operation was performed in instars or adults. Similarly, the short period of denervation following axotomy before reinnervation took place did not affect the fibre type composition of the muscle.