Fiber tracts that contain more opioid binding sites than gray matter does: A quantitative autoradiographic study in the guinea-pig

Abstract

Opioid binding sites were localized in cryostat sections of guinea-pig brain by in vitro autoradiography using (−)-[ 3H]bremazocine. Quantification of binding sites in gray matter was accomplished using standard samples of tritium mixed in brain gray matter. The binding sites in fiber tracts were quantified using standards made from white matter to compensate for the quenching caused by myelin. Binding in gray matter corroborated previous findings that the moderately high densities of opioid sites in the cerebellum are of the kappa type and that the V and VI laminae of the cerebral cortex, the substantia nigra and olfactory bulb contain high levels of opioid sties in the guinea-pig. Several fiber tracts such as cerebellar white matter and the corpus callosum contained densities of (−)-[ 3H]bremazocine binding sites equal to, or higher than, the most densely labeled gray matter areas. The dorsal hippocampal commissure and the splenium of the corpus callosum contained 2200 fmol sites/mg protein, two and one half times more than the most densely labeled gray matter areas, the external plexiform layer of the olfactory bulb. These sites may be receptors in transport, but their density constitutes a massive volume of receptors for which a physiological role still needs to be more clearly defined.