In vitro autoradiography of opiate receptors in rat brain suggests loci of "opiatergic" pathways.

Slide-mounted sections of unfixed frozen rat brain can be labeled in vitro with [3H]naloxone to show the mu-like ligand selectivity characterized in previous studies. We have developed an autoradiographic technique using hot paraformaldehyde vapors to prevent diffusion of ligands with reversible binding. Resolution at the light level is sufficient to detect concordance between receptor patterns and terminal fields of axonal projections marked by tract-tracing techniques. The opiate receptor distribution suggests the existence of widespread intrinsic and several longer multisynaptic "opiatergic pathways within sensory and limbic circuits. One multisynaptic pathway may link olfactory structures with limbic circuits in the amygdala and habenula. Another may lie in limbic cortical structures. Opiate receptors are numerous also in sensory systems, and within primary sensory nuclei (visual, auditory, olfactory, somatic) they are found superficially in laminated structures. Together, the opiate receptors are well placed to control incoming sensory and subsequent limbic information processing.