Modulation of cholinergic synaptic transmission in an identified locust sensory pathway
The monosynaptic connection between the locust forewing stretch receptor (fSR) and the first basalar motoneuron (BAl) is part of a sensory pathway involved in flight. The fSR is a single sensory afferent, triggered during wing elevation, that makes central connections with the wing depressor motoneuron, BAl. The fSR/BAl synapse in Locusta migratoria was used as a model to study the modulation of cholinergic synaptic transmission. Electrophysiological experiments indicate that presynaptic muscarinic cholinergic receptors are involved in the down-regulation of acetylcholine release from the fSR terminals and at least some of these are located on and activate GABAergic intemeurons that inhibit the fSR. These experiments are supported by electron microscopical (EM) immunocytochemical (ICC) studies, which show that the fSR receives synaptic inputs from neurons immunoreactive (IR) for GABA.Additional EM ICC studies reveal that the fSR also receives synaptic inputs from glutamate-IR neurons. The EM ICC studies also showed that neurons that are not immunoreactive for GABA or glutamate are presynaptic to the fSR and its postsynaptic member, suggesting that the fSR/BAl synapse is modulated by other neuromodulators. Further electrophysiological studies revealed that the biogenic amines, octopamine and dopamine are potentially capable of modulating the fSR/BAl synapse, by suppressing the postsynaptic response of BAl to ACh (at least in part). The biogenic amine, 5- hydroxytryptamine (5HT) is also potentially capable of modulating the fSR/Bal synapse but may act through octopamine receptors. This is supported by confocal microscopical ICC studies, which show that the neuropil region containing the fSR/BAl synapses is octopamine-IR but not 5HT-LR. These results indicate that a range of neuromodulators acting via different mechanisms interact to modulate cholinergic synaptic transmission between the fSR and BAl.