QUANTITATIVE AUTORADIOGRAPHIC LOCALIZATION IN RAT-BRAIN OF ALPHA(2)-ADRENERGIC AND NONADRENERGIC I-RECEPTOR BINDING-SITES LABELED BY [H-3] RILMENIDINE

alpha(2A)-Adrenergic receptor (AR) and non-adrenergic imidazoline rece ptor (I-R) binding sites have been previously characterized in rat cer ebral cortex membranes using the N-substituted oxazoline, [H-3]rilmeni dine ([H-3]Ril) [King, P.R. et al., fur. J. Pharmacol., 218 (1992) 101 -108]. In the present study, in vitro autoradiography was used to quan tify the regional distribution of these receptors throughout the rat n euroaxis. The distribution and relative density (fmol/mg tissue) of I- Rs was examined in the presence of 1 mu M adrenaline to block the adre nergic component of 40 nM [H-3]Ril binding and non-specific binding wa s measured in the presence of another oxazoline, Bay a6781 (10 mu M). Both alpha(2A)-ARs and I-Rs were broadly, but heterogeneously, distrib uted. In forebrain, high levels of [H-3]Ril-labelled (alpha(2A)-AR sit es were observed in the anterior olfactory nucleus, the piriform, ento rhinal and perirhinal cortices, lateral septum, bed nucleus of the str ia terminalis, several thalamic nuclei, the amygdala and the arcuate, dorsomedial and posterior hypothalamic nuclei. In hindbrain, alpha(2A) -AR sites were concentrated in locus coeruleus, lateral parabrachial n ucleus, nucleus of the solitary tract and area postrema. I-R sites acc ounted for 50% or more of specific [H-3]Ril binding (40 nM) in most co rtical and hypothalamic nuclei, nucleus of the solitary tract, cranial motor nuclei and most spinal cord layers. The highest densities of I- Rs were found in the arcuate, dorsomedial and posterior hypothalamic n uclei, the locus coeruleus, the area postrema, the cranial motor nucle i and associated with spinal motor neurones. A very high concentration of I-Rs was also detected in the pineal gland. The distribution of (a lpha(2)-AR sites determined resembled that reported with [H-3]p-aminoc lonidine which appears to specifically label (alpha,-ARs and not I-1-R sites in rat brain sections, and [H-3]methoxyidazoxan which is a sele ctive alpha(2)-AR antagonist. The regional and cellular distribution o f I-R binding sites was unlike the distribution of putative I-1-R site s labelled by [H-3]clonidine in human brain, although comparable autor adiographic mapping studies in rat brain have not been done using this ligand. The regional and cellular distribution of [H-3]Ril-labelled I -R binding sites had both similarities and differences to that reporte d using the imidazoline ligand, [H-3]idazoxan, with common labelling o f areas such as area postrema, arcuate and interpeduncular nuclei and pineal gland with the two ligands, and differential relative binding l evels ([H-3]Ril > [H-3]idazoxan) associated with hippocampal pyramidal cells and brainstem and spinal motor neurones. Thus, according to the current classification of I-Rs (I-1-, I-2A- or I-2B-subtypes) based o n various pharmacological, biochemical, anatomical and functional char acteristics of the sites, our results suggest that in rat brain [H-3]R il labels a population of binding sites most similar to the described I-2B-subtype of I-R (i.e. a site-labelled by imidazolines and derivati ves, oxazolines and guanidinium compounds; with low affinity for cloni dine and imidazoles; insensitive to amiloride) but may also label addi tional oxazoline-preferring binding sites in some brain regions.