EVIDENCE AGAINST AN INVOLVEMENT OF THE HALOPERIDOL-SENSITIVE SIGMA-RECOGNITION SITE IN THE DISCRIMINATIVE STIMULUS PROPERTIES OF (+)-NORMAL-ALLYLNORMETAZOCINE ((+)-SKF-10,047)

The involvement of the haloperidol-sensitive, σ recognition site and the N-methyl-D-aspartic acid (NMDA) receptor in the mediation of the discriminative stimulus properties of (+)-N-allylnormetazocine ((+)-NANM, (+)-SKF 10,047), has been investigated in the rat by use of a two-lever, operant drug discrimination paradigm. Six compounds with nanamolar affinity for the σ recognition site ((±)-pentazocine, (+)-3-(hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP), ditolylguanidine (DTG), haloperidol, (-)-butaclamol and BMY 14802) were investigated for their ability to generalise or antagonise the (+)-NANM discriminative stimulus. Each drug was tested at doses found in an ex vivo radioligand binding assay to displace [3H]-DTG from the central σ recognition site by more than 40%. While (±)-pentazocine (in the presence of naloxone) generalized and (+)-3-PPP partially antagonised the (+)-NANM cue, the other putative σ ligands were ineffective either as agonists or antagonists at doses clearly occupying the σ site in vivo. Dose-dependent generalisation to the (+)-NANM cue was seen with the selective non-competitive NMDA receptor antagonist, MK-801, a compound devoid of significant affinity for the σ recognition site. (±)-Pentazocine was found to antagonise seizures induced in the mouse by NMDLA, a model reflecting antagonism of central NMDA receptors, and a strong correlation was found between the rank order of potency of compounds to generalise to the (+)-NANM discriminative stimulus and their potencies as anticonvulsants. In conclusion, no evidence was found to substantiate the contention that the discriminative stimulus properties of (+)-NANM are mediated by the haloperidol-sensitive σ recognition site. On the other hand, the results are consistent with the interoceptive stimulus being mechanistically based in the NMDA receptor complex.