PLASTICITY OF SYNAPTIC CONNECTIONS IN SENSORY-MOTOR PATHWAYS OF THE ADULT LOCUST FLIGHT SYSTEM

We investigated possible roles of retrograde signals and competitive i nteractions in the lesion-induced reorganization of synaptic contacts in the locust CNS. Neuronal plasticity is elicited in the adult flight system by removal of afferents from the tegula, a mechanoreceptor org an at the base of the wing. We severed one hindwing organ and studied the resulting rearrangement of synaptic contacts between night interne urons and afferent neurons from the remaining three tegulae (2 forewin g, 1 hindwing). This was done by electric stimulation of afferents and intracellular recording from interneurons (and occasionally motoneuro ns). Two to three weeks after unilateral tegula lesion, connections be tween tegula afferents and flight interneurons were altered in the fol lowing way. I)Axons from the forewing tegula on the operated side had established new synaptic contacts with metathoracic elevator interneur ons. In addition, the amplitude of compound excitatory postsynaptic po tentials elicited by electric stimulation was increased, indicating th at a larger number of afferents connected to any given interneuron. 2) On the side contralateral to the lesion, connectivity between axons f rom the forewing tegula and elevator interneurons was decreased. 3) Th e efficacy of the (remaining) hindwing afferents appeared to be increa sed with regard to both synaptic transmission to interneurons and impa ct on flight motor pattern. 4) Flight motoneurons, which are normally restricted to the ipsilateral hemiganglion, sprouted across the gangli on midline after unilateral tegula removal and apparently established new synaptic contacts with tegula afferents on that side. The changes on the operated side are interpreted as occupation of synaptic space v acated on the interneurons by the severed hindwing afferents. On the c ontralateral side, the changes in synaptic contact must be elicited by retrograde signals from bilaterally arborizing flight interneurons, b ecause tegula projections remain strictly ipsilateral. The pattern of changes suggests competitive interactions between forewing and hindwin g afferents. The present investigation thus presents evidence that the CNS of the mature locust is capable of extensive synaptic rearrangeme nt in response to injury and indicates for the first time the action o f retrograde signals from Interneurons.