Localization of M₁ muscarinic receptors in rat brain using selective muscarinic toxin-1

Mambas, African snakes of the genus Dendroaspis, produce several types of toxins that are of pharmacological interest. The novel muscarinic toxin-1 (MT-1), from the green mamba Dendroaspis angusticeps, binds specifically to muscarinic M₁ receptors in homogenates of rat cerebral cortex. Iodination of the toxin, ¹²⁵I-muscarinic toxin-1 (¹²⁵I-MT-1), renders the toxin selective for M₁ muscarinic receptors. Quantitative measurement of ¹²⁵I- MT-1 autoradiography in rat brain sections indicated highest labeling in the nucleus accumbens, striatum, and dentate gyrus. High densities of ¹²⁵I- MT-1 binding sites were located in the CA1 region of the hippocampus, frontal, and parietal cortices. Moderate densities of binding sites were seen in temporal cortex, and hippocampal subregions CA2, CA3, and CA4, whereas low labeling was observed in the cerebellum and spinal cord.