At present, little is known about the mechanisms by which cannabinoids exert their effects on the central nervous system. In this study, fluorescence imaging and electrophysiological techniques were used to investigate the functional relationship between cell surface cannabinoid type 1 (CB1) receptors and GABAergic synaptic transmission in cultured hippocampal neurons. CB1 receptors were labelled on living neurons using a polyclonal antibody directed against the N-terminal 77 amino acid residues of the rat cloned CB1 receptor. Highly punctate CB1 receptor labelling was observed on fine axons and at axonal growth cones, with little somatic labelling. The majority of these sites were associated with synaptic terminals, identified either with immunohistochemical markers or by using the styryl dye FM1-43 to label synaptic vesicles that had undergone active turnover. Dual labelling of neurons for CB1 receptors with either the inhibitory neurotransmitter GABA or its synthesising enzyme glutamate decarboxylase, demonstrated a strong correspondence. The immunocytochemical data was supported by functional studies using whole-cell patch-clamp recordings of miniature inhibitory postsynaptic currents (mIPSCs). The cannabinoid agonist WIN55,212-2 (100 nM) markedly inhibited (by 77±6.3%) the frequency of pharmacologically-isolated GABAergic mIPSCs. The effects of WIN55,212-2 were blocked in the presence of the selective CB1 receptor antagonist SR141716A (100 nM). In conclusion, the present data show that cell surface CB1 receptors are expressed at presynaptic GABAergic terminals, where their activation inhibits GABA release. Their presence on growth cones could indicate a role in the targeting of inhibitory connections during development. (C) 2000 IBRO.
- Growth cones
- Synaptic transmission