Various neuromodulators have been shown to be involved in shaping the sensory information available to the brain. Acetylcholine (ACh) modulation, through muscarinic receptors, is a particularly widespread mechanism of controlling sensory information transmission. The precise effects of ACh modulation depend on the subtype of muscarinic ACh receptors that are activated. In weakly electric fish, previous work suggested a role of ACh, via muscarinic receptors, in the modulation of information transmission in the electrosensory lateral line lobe (ELL) of the hindbrain. In this study, we determined which muscarinic receptor (mAChR) subtypes are present in the brain of Apteronotus leptorhynchus as well as their spatial distribution. We partially cloned three subtypes of muscarinic receptors (mAChR2, -3, and -4) from brain tissue of A. leptorhynchus and used in situ hybridization in transverse sections of the brain to determine their distributions. Sites labeled for the three muscarinic receptor mRNAs were found in various brain regions devoted to the processing of different sensory modalities. The mRNA probes for the three receptor types showed differential distribution but also overlapping presence of two or more receptors in particular nuclei. In addition to the presence of mAChR3 in the ELL region, electrosensory nuclei including the nucleus praeeminentialis, dorsal torus semicircularis and optic tectum showed expression of one or more mAChRs. Thus, the overall pattern of mAChR expression found is in agreement with mAChR expression in other species, with additional presence evident in specialized regions of the electrosensory system, which suggests an important modulating role of ACh in this sensory modality.
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