Neuroanatomical distribution of receptors for a novel voltage-sensitive calcium-channel antagonist, SNX-230 (ω-conopeptide MVIIC)

Abstract

Neuronal voltage-sensitive calcium channels (VSCCs) are a diverse family of proteins that regulate entry of Ca²⁺ into neurons. Selective antagonists of VSCCs have proven to be powerful pharmacological tools for identifying and characterizing these channels. A new VSCC antagonist, SNX-230 (also known as ω-conopeptide MVIIC), binds with high affinity to receptors in rat brain and blocks one or more high-threshold VSCCs that are neither L- nor N-type. We have defined the neuroanatomical distribution of the high-affinity non-L, non-N VSCC receptors for SNX-230 using [¹²⁵I]SNX-230 bound to rat brain sections and compared it with that of [¹²⁵I]SNX-111, a reversible blocker of N-type VSCCs. Highest densities of binding for both ligands were seen in areas rich in synaptic connections, such as the oriens, radiatum and molecular layers of the hippocampus. In general, the density of [¹²⁵I]SNX-230-binding was higher in cerebellum compared with that in forebrain. In contrast, this general distribution of density was reversed for [¹²⁵I]SNX-111. In the glomeruli of the olfactory bulb, binding of [¹²⁵I]SNX-230 was undetectable compared with the high density of [¹²⁵I]SNX-111-binding. Differential localization of the two ligands was also seen in cervical spinal cord. The clearly different localization of [¹²⁵I]SNX-230 compared with that of [¹²⁵I]SNX-111 in the olfactory bulb and spinal cord suggested that the binding sites for [¹²⁵I]SNX-230 in other brain regions, while co-localized macroscopically, are also distinct from those for [¹²⁵I]SNX-111. This was confirmed when addition of saturating concentrations of SNX-111 did not affect the distribution pattern of [¹²⁵I]SNX-230-binding. These observations demonstrate that the high-affinity [¹²⁵I]SNX-230 receptors in the brain and those of the N-type VSCCs are distinct molecular entities. Thus, SNX-230 (ω-MVIIC) offers a useful ligand for investigating the distribution and functions of non-L-, non-N-type VSCCs in the brain.