A role for muscarinic excitation: Control of specific singing behavior by activation of the adenylate cyclase pathway in the brain of grasshoppers

Muscarinic acetylcholine receptors exert slow and prolonged synaptic effect s in both vertebrate and invertebrate nervous systems. Through activation o f G proteins, they typically decrease intracellular cAMP levels by inhibiti on of adenylate cyclase or stimulate phospholipase C and the turnover of in ositol phosphates. In insects, muscarinic receptors have been credited with two main functions: inhibition of transmitter release from sensory neuron terminals and regulation of the excitability of motoneurons and interneuron s. Our pharmacological studies with intact and behaving grasshoppers reveal ed a functional role for muscarinic acetylcholine receptors as being the ba sis for specific arousal in defined areas of the brain, underlying the sele ction and control of acoustic communication behavior. Periodic injections o f acetylcholine into distinct areas of the brain elicited songs of progress ively increasing duration. Coinjections of the muscarinic receptor antagoni st scopolamine and periodic stimulations with muscarine identified muscarin ic receptor activation as being the basis for the underlying accumulation o f excitation. In contrast to reports from other studies on functional circu its, muscarinic excitation was apparently mediated by activation of the ade nylate cyclase pathway. Stimulation of adenylate cyclase with forskolin and of protein kinase A with 8-Br-cAMP mimicked the stimulatory effects of mus carine whereas inhibition of adenylate cyclase with SQ22536 and of protein kinase A with H-89 and Rp-cAMPs suppressed muscarine-stimulated singing beh avior. Activation of adenylate cyclase by muscarinic receptors has previous ly been reported from studies on membrane preparations and heterologous exp ression systems, but a physiological significance of this pathway remained to be demonstrated in an in vivo preparation.