Pharmacology, distribution and development of muscarinic acetylcholine receptor subtypes in the optic tectum of Rana pipiens

Visually evoked behaviors mediated by the Frog optic tectum require choline rgic activity, but the receptor subtypes through which acetylcholine acts a re not yet identified. Using quantitative autoradiography and scintillation spectrometry. we examined the binding of [H-3]pirenzepine and [H-3]AF-DX 3 84 in the laminated optic tectum of the frog. In mammalian systems, these s ubstances bind excitatory (m1 and m3 subtypes) and inhibitory (m2 and mi su btypes) muscarinic acetylcholine receptors. respectively. Pharmacological a nalyses, including the use of specific muscarinic toxins, confirmed the sub type selectivity of the radioligands in the frog brain. Binding sires for [ H-3]pirenzepine were distinct from those for [H-3]AF-DX 384. In the adult t ectum, [H-3]pirenzepine demonstrated specific binding in tectal layers 5-9. [IH]Pirenzepine binding was also present in tadpoles as young as stage V, but all sampled stages of tadpole rectum had significantly less binding whe n compared to adults. Lesioning of the optic nerve had no effect on [H-3]pi renzepine binding. Specific [H-3]AF-DX 384 binding was found in all layers of the adult tectum. All sampled tadpole stages exhibited binding sites for [H-3]AF-DX 384. but the densities of these sites were also significantly h igher in adults than they were in developing stages. Short-term lesions of the optic nerve reduced [H-3]AF-DF; 384 binding in all rectal layers of the deafferented lobe when compared to the afferented one. Long-term lesions d ecreased [H-3]AF-DX 384 sites in both lobes. These results indicate that multiple muscarinic acetylcholine receptor bind ing sites reside in the frog optic tectum at all stages of development, and their pharmacology resembles that of mammalian m1/m3. m2 and ml subtypes. Our data indicate that few, if any, of these receptors are likely to be loc ated on retinal ganglion cell terminals. Furthermore, the expression of inh ibitory muscarinic subtypes seems to be regulated by different mechanisms t han that fur excitatory subtypes. (C) 2001 IBRO. Published by Elsevier Scie nce Ltd. All rights reserved.