PHOTOINACTIVATION OF AN IDENTIFIED MOTONEURON IN THE LOCUST LOCUSTA-MIGRATORIA

1. The common inhibitory motoneurone 1 (CI1) in the mesothoracic gangl ion of the locust was photoinactivated using a helium-cadmium laser or a mercury lamp as light source. Treated animals showed no signs of ab normal locomotory behaviour over periods of up to 40 days. 2. Photoina ctivation of part of the neurone in the ganglion, i.e. the soma and th e primary neurite, is sufficient to cause irreversible degeneration of all the peripheral extensions of the neurone. Three weeks after photo inactivation, all GABA immunoreactivity had disappeared from the axon branches of the photoinactivated neurone and from their terminals on o ne of the target muscles investigated, the anterior coxa rotator M92, and inhibitory postsynaptic potentials could no longer be elicited thr ough stimulation. This was taken as proof of functional denervation of the muscle with regard to its inhibitory input. By this time, the axo n of CI1 in nerve N3C1, which supplies M92, had also disappeared. 3. A nimals treated during the fourth or fifth instars showed a permanent l oss of the photoinactivated mesothoracic CI1 neurone after moulting in to adulthood, 4. Denervation of M92 in the middle legs of instars and adults by axotomy of N3 always led to rapid functional reinnervation o f the muscle. The first sign of reinnervation (excitatory neuromuscula r activity upon mechanical stimulation of the tarsi) was detected elec trophysiologically as early as 8 days after severing the motor nerve. 5. The eliminination of CI1 by photoinactivation for a period of up to 40 days did not influence parameters of the target muscle, such as si ze, number of fibres and phenotypes of fibres defined histochemically according to their myofibrillar ATPase isoforms, irrespective of wheth er the operation was performed in instars or adults. Similarly, the sh ort period of denervation following axotomy before reinnervation took place did not affect the fibre type composition of the muscle.