RB101(S), a dual inhibitor of enkephalinases does not induce antinociceptive tolerance, or cross-tolerance with morphine: a c-Fos study at the spinal level.

In behavioural tests, RB101 (N-[(S)-2-benzyl-3[(S)(2-amino-4-methyl-thio)butyldithio]-1-oxopropyl]-L-phenylalanine benzyl ester), a mixed inhibitor of enkephalin-degrading enzymes, induces antinociceptive effects without producing tolerance, or cross-tolerance with morphine. In the present experiments, the acute or chronic effects of enantiomer RB101(S) were examined on the response of spinal cord neurons to nociceptive inflammatory stimulation (intraplantar injection of carrageenin) using c-Fos studies in awake rats. The number of c-Fos immunoreactive nuclei was evaluated in the lumbar spinal cord 90 min after carrageenin. c-Fos-immunoreactive nuclei were preferentially located in the superficial (I-II) and deep (V-VI) laminae of segments L4-L5 (areas containing numerous neurones responding exclusively, or not, to nociceptive stimuli). In the first experimental series, acute RB101(S) (30 mg/kg, i.v.), morphine (3 mg/kg, i.v.), or respective vehicles were injected in rats chronically treated with RB101(S) (160 mg/kg/day for 4 days, s.c.). In chronically treated RB101(S) rats, both acute RB101(S) and morphine reduced the total number of carrageenin-evoked c-Fos-immunoreactive nuclei. In the second experimental series, acute RB101(S) (30 mg/kg, i.v.) reduced the total number of carrageenin-evoked c-Fos-immunoreactive nuclei with similar magnitude in naive and in morphine-tolerant (100 mg/kg/day for 3 days, s.c.) rats. These data provide further evidence that different cellular mechanisms occurred after chronic stimulation of opioid receptors by morphine or endogenous enkephalins.