Histamine H-3 receptor-mediated depression of synaptic transmission in the dentate gyrus of the rat in vitro

1. The effects of histamine on excitatory synaptic transmission in the dentate gyrus region of rat hippocampal slices were examined using extracellular and whole-cell patch-clamp recording techniques. The GABA(A) receptor antagonist picrotoxin (50 mu M) was present in the bath in all experiments. 2. Histamine (0.7-70 mu M) reversibly depressed field excitatory postsynaptic potentials (fEPSPs) or excitatory postsynaptic currents (EPSCs) recorded intracellularly by up to 30%. The presynaptic fibre volley and EPSC reversal potential were unaffected by histamine, as were responses following pressure ejection of the glutamate receptor agonist S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (S-AMPA) into the slice. 3. Histamine (7 mu M) de-pressed equally the AMPA and N-methyl-D-aspartate (NMDA) components of the dual-component EPSC, recorded at -40 mV. 4. In addition to depressing synaptic transmission, histamine also reduced the magnitude of paired-pulse depression (PPD; 40 ms interpulse interval) of the medial perforant path EPSC. 5. Histamine depressed medial perforant path EPSCs more strongly than lateral perforant path EPSCs. Paired-pulse facilitation (PPP; 40 ms interpulse interval) in the lateral perforant path was enhanced by histamine. 6. The effects of histamine on synaptic transmission and PPD were mimicked by the selective H-3 receptor agonist R-alpha-methylhistamine (0.1-10 mu M) but not by the selective H-2 receptor agonist dimaprit (10 mu M). Similarly, the H-3 receptor antagonist thioperamide (10 mu M) blocked the effect of histamine whereas the H-1 antagonist mepyramine (1 mu M) and the H-2 receptor antagonist cimetidine (50 mu M) n ere ineffective. 7. Histamine actions on synaptic transmission and PPD were not occluded by application of the metabotropic glutamate agonist L-2-amino-4-phosphonobutyrate (AP4). 8. The results indicate that histamine depresses synaptic transmission in the dentate gyrus by binding to histamine H-3 receptors located on perforant path terminals. The mechanism by which histamine depresses transmission is independent of that used by class III metabotropic glutamate receptors.