Pharmacological separation of cannabinoid sensitive receptors on hippocampal excitatory and inhibitory fibers

Our earlier studies demonstrated that in the hippocampus, cannabinoids suppress inhibitory synaptic transmission via CB1 cannabinoid receptors, whereas a novel cannabinoid-sensitive receptor modulates excitatory synapses (Katona, I. et al., Journal of Neuroscience 19 (1999) 4544; Hajos, N. et al., European Journal of Neuroscience 12 (2000) 3239; Hajos, N. et al., Neuroscience 106 (2001) 1). The novel receptor does not correspond to CB2, since this receptor type is not expressed in the brain (Munro, S. et al., Nature 365 (1993) 61). Recent binding experiments revealed that the synthetic cannabinoid WIN 55,212-2 binds with lower affinity to brain membranes of CB1 receptor-knockout mice indicating that pharmacological differences exist between these two types of cannabinoid receptors in the hippocampus (Breivogel et al., Molecular Pharmacology 60 (2001) 155). To analyze this difference in detail, we first determined the EC50 values of WIN 55,212-2 for excitatory and inhibitory transmission in rat hippocampal slices using whole-cell patch-clamp recordings. The estimated EC50 value for inhibitory postsynaptic currents (IPSC) evoked by electrical stimulation in CA1 pyramidal cells was 0.24 muM, whereas for excitatory postsynaptic currents (EPSC) it was 2.01 muM, respectively. The cannabinoid antagonist, AM251, blocked the WIN 55,212-2-induced inhibition of evoked IPSCs, but not of EPSCs, providing evidence for its selectivity for CB1. We then tested the hypothesis of whether the cannabinoid effect on hippocampal excitatory neurotransmission is mediated via receptors with an affinity for vanilloid ligands. Co-application of the vanilloid receptor antagonist capsazepine (10 muM) with cannabinoids (WIN55,212-2 or CP55,940) prevented the reduction of EPSCs, but not of IPSCs. The amplitude of evoked EPSCs was also suppressed by superfusion of the vanilloid receptor agonist capsaicin (10 muM), an effect which could also be antagonized by capsazepine. In contrast, capsaicin did not change the amplitude of evoked IPSCs. These results demonstrate that WIN 55,212-2 is an order of magnitude more potent in reducing GABAergic currents via CB1 than in inhibiting glutamatergic transmission via the new CB receptor. The sensitivity of the new CB receptor (and EPSCs) to vanilloid ligands, but not to the cannabinoid antagonist AM251, represents another pharmacological tool to distinguish the two receptors, since CB1 (and its effect on IPSCs) is not modulated by vanilloids, but is antagonized by AM251. (C) 2002 Elsevier Science Ltd. All rights reserved.