Neuropharmacological evidence for inhibitory cephalic control mechanisms of stridulatory behaviour in grasshoppers

Abstract

In gomphocerine grasshoppers the neuromuscular patterns of stridulatory hindleg movements are produced by metathoracic rhythm generators under the control of cephalic command neurons. Injections of cholinergic agonists into the protocerebrum activate this command system which induces the performance of stridulatory sequences, resembling natural species specific movements. Injections of GABA, glycine and picrotoxin into the central protocerebrum of the species Omocestus viridulus, Chorthippus mollis and Ch. biguttulus revealed a contribution of inhibitory mechanisms to the control of the stridulatory behaviour. The experiments suggest that inhibition interferes with the cephalic command systems at three levels: (1) sustained inhibition through picrotoxin sensitive receptors acting on all command units while grasshoppers are at rest, and during stridulation on all command units except the one activating the pattern generators of the currently performed movements; (2) premature termination of song sequences, experimentally induced by injections of GABA and glycine; and (3) coupling of a timing mechanism that terminates a song sequence or its subunits with a particular movement pattern after specific durations. These results together with those from previous studies on the pharmacological activation of stridulatory behaviour suggest that a balance of inhibitory and excitatory inputs to the command system selects the appropriate song type and controls its performance.