Abstract

Stereotaxic injections of 10 µg of either d- or l-methadone, into a lateral cerebral ventricle of rats, produced analgesia only in those animals injected with the pharmacologically active l-isomer of methadone. The intraventricular injection of C¹⁴-labeled d- or l-methadone with H³-mannitol as an inert marker resulted in the accumulation of both d- and l-methadone in the region of the anterior hypothalamus before and during the period of maximal analgesia. There was no significant difference in the accumulation of the two isomers. Sagittal sectioning of the anterior hypothalamus showed no significant difference in the rates of diffusion of the two isomers of methadone, although the greater diffusabiity of mannitol laterally from the third ventricle compared to methadone could be demonstrated.