Delayed effects of spiperone on serotonin1A receptors in the dorsal hippocampus of rats

The effects of 5-HT1A antagonists spiperone, methiothepin and BMY 7378 on [3H]-8-OH-DPAT binding were determined in vitro and ex vivo in rat hippocampus CA3 membrane preparations, and ex vivo in tissue sections of CA1 and CA3 subfields using quantitative autoradiography. In CA3 membranes from rats sacrificed 1 h or 24 h after administration of 5 mg/kg i.p. spiperone or methiothepin, no decrease in [3H]-8-OH-DPAT Bmax values approached statistical significance. Autoradiograms from identically treated rats showed significant increases in Kd values in both CA1 and CA3 hippocampal subfields 24 h but not 1 h after administration of the drugs, while no changes were observed in the dorsal raphe at either time. In vitro co-incubation of membranes with spiperone (200 or 500 nM) or methiothepin (500 nM) resulted in significant decreases in both affinity and Bmax values. In contrast, co-incubation with BMY 7378 (5 nM) increased only Kd values. GTP gamma S produced a concentration-dependent inhibition of specific [3H]8-OH-DPAT binding. At 0.1 mM of GTP gamma S, Kd values were increased three-fold and Bmax values were significantly decreased. When membranes were co-incubated with GTP gamma S and spiperone or BMY 7378, Kd values increased further. Moreover, the effects of spiperone and GTP gamma S on Bmax values were additive. It is concluded that BMY 7378 acts as a competitive antagonist at hippocampal post-synaptic 5-HT1A receptors, whereas spiperone and methiothepin exert their delayed antagonistic effects at these receptors through a non-competitive mechanism of action, possibly affecting the coupling of the receptors to their Gi/o proteins.