NEUROPHARMACOLOGICAL EVIDENCE FOR INHIBITORY CEPHALIC CONTROL MECHANISMS OF STRIDULATORY BEHAVIOR IN GRASSHOPPERS

In gomphocerine grasshoppers the neuromuscular patterns of stridulator y hindleg movements are produced by metathoracic rhythm generators und er the control of cephalic command neurons. Injections of cholinergic agonists into the protocerebrum activate this command system which ind uces the performance of stridulatory sequences, resembling natural spe cies specific movements. Injections of GABA, glycine and picrotoxin in to the central protocerebrum of the species Omocestus viridulus, Chort hippus mollis and Ch. biguttulus revealed a contribution of inhibitory mechanisms to the control of the stridulatory behaviour. The experime nts suggest that inhibition interferes with the cephalic command syste ms at three levels: (1) sustained inhibition through picrotoxin sensit ive receptors acting on all command units while grasshoppers are at re st, and during stridulation on all command units except the one activa ting the pattern generators of the currently performed movements; (2) premature termination of song sequences, experimentally induced by inj ections of GABA and glycine; and (3) coupling of a timing mechanism th at terminates a song sequence or its subunits with a particular moveme nt pattern after specific durations. These results together with those from previous studies on the pharmacological activation of stridulato ry behaviour suggest that a balance of inhibitory and excitatory input s to the command system selects the appropriate song type and controls its performance.