Localization of mu- and delta-opioid receptors in cat respiratory areas: an autoradiographic study.

Autoradiography after in vitro binding with selective ligands for either mu ([3H](Tyr-D-Ala-Gly-(NMePhe)-Gly-ol] or delta ([3H](Tyr-D-Thr-Gly-Phe-Leu-Thr)) opioid receptor types revealed the presence of variable amounts of radioactive labeling in the cat brainstem. Areas involved in the respiratory rhythmogenesis were among the most prominently labeled structures. The pneumotaxic center, including the nucleus parabrachialis medialis and the Kolliker-Füse nucleus, contains a very high density of delta binding sites while the dorsal respiratory nucleus which corresponds to the nucleus tractus solitarius, is more heavily labeled by the mu ligand. The neuroanatomical differences in the distribution of opioid receptors correlates well with the pharmacological responses induced by administration of specific mu- or delta ligands.