THE PUTATIVE 5-HT1A RECEPTOR ANTAGONISTS NAN-190 AND BMY-7378 ARE PARTIAL AGONISTS IN THE RAT DORSAL RAPHE NUCLEUS INVITRO

The present electrophysiological study examined the action of the putative 5-HT1A receptor antagonists NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine) and BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]-decane-7,9-dione dihydrochloride) in the rat dorsal raphe nucleus in vitro. There was no major difference between the effects of the two drugs on any measure investigated. Both compounds reduced neuronal activity in a concentration-dependent manner, with BMY 7378 being slightly more potent than NAN-190. The threshold concentrations eliciting inhibitory effects were 1 nM for BMY 7378 and 3 nM for NAN-190. Complete inhibition occurred at concentrations close to 30 nM. The effects of the 5-HT1A receptor agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) could be antagonized when concentrations of NAN-190 or BMY 7378 were used that were too low to produce a marked inhibition. At concentrations close to threshold both compounds potentiated the inhibitory effects of 3 nM 8-OH-DPAT. The suppression of neuronal firing induced by NAN-190 and BMY 7378 could be completely antagonized with propranolol, indicating that the inhibitory actions of both drugs were not primarily due to alpha(1)-adrenoceptor antagonism. By applying theorems of receptor theory the intrinsic activities for both NAN-190 and BMY 7378 were calculated to be in the range of 0.1-0.3. Thus, NAN-190 and BMY 7378 are partial agonists in the rat dorsal raphe nucleus. The results can be best explained by assuming that a critical threshold of receptor occupancy has to be reached in order to elicit a biological response and by assuming a receptor reserve that may account for the apparent full agonism of NAN-190 and BMY 7378.