Immunocytological and electrophysiological study of acetylcholine and octopamine receptors on peripheral mechanosensory neurons of the spider Cupiennius salei

Peripheral mechanosensory neurons of the spider ( Cupiennius salei) receive extensive efferent innervation. Previous work has shown that many of these efferent neurons are immunoreactive to GABA, while ionotropic and metabotropic GABA receptors are found on the mechanosensory neurons themselves. The ACh synthesizing enzyme choline acetyltransferase has also been found in the sensory neurons and in some efferent nerve fibers, suggesting a cholinergic innervation of the mechanosensory neurons. Octopamine immunoreactive neurons have previously been found in the spider CNS making it possible that the sensory neurons are also modulated by octopamine. I investigated the distribution and function of acetylcholine and octopamine receptors on mechanosensilla in the spider leg. Fluorescent antagonists and immunocytochemistry revealed that muscarinic acetylcholine receptors (mAChRs) were present in all the mechanosensory neurons in the spider leg, but intra- and extracellular electrophysiological recordings performed on two types of mechanosensory organs gave no clear responses to muscarinic agonists. However, frequency response analysis revealed that muscarinic agonists abolished a small decay in the rate of firing and information capacity that occurred overtime in control recordings, suggesting that the muscarinic system could attenuate long-term adaptation of the mechanosensory neurons. It is also possible that mAChRs have other functions unrelated to neuronal excitability. Immunocytochemistry against octopamine receptors (OARS) indicated that these receptors are present in all mechanosensory neurons, concentrated in the proximal parts of the somata and axon hillock. Double labeling experiments with an antibody against synapsin suggested that OARs are associated with presynaptic vesicles. Octopamine immunolabeling demonstrated the presence of at least one octopamine immunoreactive efferent fiber in close proximity to the sensory neurons. Using electrophysiology, octopamine and its precursor tyramine greatly increased the firing rate of mechanically stimulated tactile hairs. This effect was inhibited by the OAR blocker phentolamine. Frequency response analysis revealed an increase in sensitivity and information capacity in response to octopamine. Finally, experiments with a cAMP analog and a PKA inhibitor indicated that OARS are positively coupled to adenylyl cyclase and act via a PKA mediated pathway.