Prepulse inhibition of the startle reflex in rats: effects of compounds acting at various sites on the NMDA receptor complex.

Prepulse inhibition (PPI) of the startle reflex in rats is disrupted by N-methyl-D-aspartate (NMDA) receptor non-competitive antagonists (phencyclidine-like compounds). In order to explore more thoroughly the control exerted by NMDA receptors on PPI, we assessed the effects of i.p. administration, in Sprague-Dawley rats, of compounds acting as antagonists or agonists at the five binding sites of the NMDA receptor complex. The non-competitive NMDA receptor antagonists phencyclidine (1-6 mg/kg) and MK-801 (dizocilpine: 0.05-0.2 mg/kg) robustly and dose-dependently disrupted PPI. A similar effect was obtained with the competitive NMDA receptor antagonists CGS 19755 (1-20 mg/kg) and CPP (3-20 mg/kg), but not with the cation Mg2+ (100 and 200 mg/kg), the glycine/NMDA binding site antagonist L-701,324 (1-10 mg/kg), or the polyamine/NMDA binding site antagonist eliprodil (3-20 mg/kg). Potentiation of glutamatergic neurotransmission by NMDA (10-50 mg/kg), and the glycine/NMDA site partial agonist d-cycloserine (1-30 mg/kg) also failed to modify PPI, though d-cycloserine diminished PPI at higher doses (50-200 mg/kg). Co-administration of sub-threshold doses of CPP (3 mg/kg) and phencyclidine (2 mg/kg) resulted in an additive effect, disrupting PPI. In contrast, co-administration of L-701,324 (6 mg/kg) with phencyclidine (2 mg/kg), eliprodil (20 mg/kg), or CPP (3 mg/kg), did not disrupt PPI. These results demonstrate that PPI-disrupting effects can only be obtained with phencyclidine-like compounds and NMDA receptor competitive antagonists. Treatment with compounds that potentially augment glutamatergic tone were without effect. Finally, despite the permissive control of the glycine/NMDA binding site on glutamatergic neurotransmission, the glycine/NMDA binding site antagonist L-701,324 did not produce synergistic activity when combined with antagonists at the glutamate, polyamine/NMDA or phencyclidine-like compound binding sites.