Heterogeneous 3H-rauwolscine binding sites in rat cortex: two alpha 2-adrenoceptor subtypes or an additional non-adrenergic interaction?

Ligand binding and isolated tissue data have provided evidence for the existence of two, tissue-specific, alpha 2-adrenoceptor subtypes in various rodent and non-rodent species. Thus it has been proposed that the complex binding of alpha 2-antagonists to rat cortical membranes is due to the presence of both subtypes in this tissue. We have previously shown that the alpha 2-antagonist 3H-rauwolscine binds to two sites on rat cortical membranes: a high affinity component characterised pharmacologically as an alpha 2-binding site, and a low affinity, spiperone-sensitive, serotonergic-like component. By the use of computerised non-linear curve-fitting, and the inclusion of (in the incubation buffer of displacement experiments) a concentration of spiperone previously shown to selectively occlude the low affinity component of the 3H-rauwolscine saturation isotherm, we have determined the rank order of affinity at each of the two sites. Whereas the rank order of affinity at the high affinity site retains the pharmacological profile of a single, monophasic alpha 2-binding site, that at the low affinity component is markedly different and is similar to that at the putative 5HT1A subtype. These data, together with the additional, functional serotonergic interactions of rauwolscine and yohimbine, indicate that there is no evidence to support the existence of heterogeneous alpha 2-binding sites, as measured by 3H-rauwolscine binding, on rat cortical membranes. Furthermore, we present evidence that the specific, low affinity serotonergic interaction of 3H-rauwolscine could be avoided by a more judicial estimation of specific binding.