Cannabinoids are known to be powerful modulators of inhibition in neocortical and hippocampal networks. In particular, depolarization in postsynaptic cells results in a CB1 cannabinoid receptor-dependent, short-term inhibition of GABA release from cholecystokinin-positive (CCK+) interneurons, including basket cells, through a process known as depolarization-induced suppression of inhibition (DSI) (Llano et al. 1991; Pitler & Alger, 1992; Kreitzer & Regehr, 2001; Ohno-Shosaku et al. 2001; Wilson & Nicoll, 2001). While considerable attention has focused on DSI-related, phasic cannabinoid actions on GABA release, less is known about how CB1 receptors may regulate GABAergic transmission in a tonic manner. However, there are interesting clues from several neuronal systems that indicate that a major function of CB1 receptors may be the tonic regulation of GABA release. For example, a recent study demonstrated that hypothalamic proopiomelanocortin neurons release endocannabinoids continuously under basal conditions, and that the released endocannabinoids significantly depress GABA release onto these cells (Hentges et al. 2005). There are also indications that such processes may be at play in the hippocampus as well, as a subpopulation of CCK+ interneurons in the stratum lucidum in the CA3 region was found to be tonically silenced by presynaptic CB1 cannabinoid receptors (Losonczy et al. 2004). However, it was not clear if tonic, CB1 receptor-dependent inhibition of GABA release in the hippocampus was a unique property of these ‘muted’ stratum lucidum interneuronal subpopulations, or whether other CCK+ interneurons outside the CA3 stratum lucidum were also under tonic control by CB1 receptors.
The mechanism of tonic CB1 receptor-dependent inhibition of GABA release in hippocampal circuits is also not known. GABA release may be tonically depressed because of the constitutive activity of the CB1 receptors that is thought to occur even in the absence of endocannabinoid ligands (Bouaboula et al. 1997; Pan et al. 1998; Vasquez & Lewis, 1999; Guo & Ikeda, 2004; Pertwee, 2005). In principle, such constitutive activity may be revealed using compounds that act as inverse agonists at the CB1 receptor. Alternatively, postsynaptic neurons may continuously release endocannabinoids resulting in a CB1 receptor-mediated basal depression of GABA release. The latter mechanism was shown to underlie the tonic inhibition of GABA release by CB1 receptors in the case of the hypothalamic proopiomelanocortin neurons (Hentges et al. 2005), as well as in the basolateral amygdala (Zhu & Lovinger, 2005), by demonstrating that CB1 receptor antagonists/inverse agonists do not enhance GABA release when the postsynaptic cells were filled with high concentrations of the calcium chelator BAPTA.
A better understanding of the tonic control of GABA release is also important in light of reports that several major neurotransmitter systems, including muscarinic acetylcholine and metabotropic glutamate receptors, exert their effects on GABA release from CCK+ cells indirectly, through the modulation of the synthesis and release of endocannabinoids from the postsynaptic pyramidal cells (Varma et al. 2001; Kim et al. 2002). For example, whereas low concentrations (<0.5 μm) of the muscarinic agonist carbachol enhance DSI without altering basal, stimulation-evoked IPSCs, higher concentrations of carbachol (≥1 μm) persistently depress evoked IPSCs mainly through the mobilization of endocannabinoids (Kim et al. 2002). However, because these studies were carried out using compound, electrical stimulation-evoked IPSCs that originate from a mixture of CB1 receptor-expressing and CB1 receptor-lacking fibres, it has not been unequivocally established whether either muscarinic or metabotropic glutamate receptors can directly modulate GABA release from CCK+ basket cells. For example, application of carbachol at 25 μm resulted in a significant depression of evoked IPSCs even in the presence of the CB1 receptor antagonist AM251 or in CB1 receptor knockout mice (Kim et al. 2002), but it is unclear whether this inhibition acts directly on presynaptic muscarinic receptors on the CCK+ basket cell terminals or only on the other components of the multifibre-evoked response. This issue is particularly relevant because parvalbumin-positive (PV+) terminals express muscarinic receptors (Hajos et al. 1998; Freund, 2003; Seeger et al. 2004).
In order to gain a better insight into the existence and mechanism of tonic CB1 receptor-mediated control of GABA release, we have performed paired, whole-cell patch-clamp recordings from identified CCK+ basket cells and postsynaptic pyramidal cells in the CA1 of the rat hippocampus. The results reveal a widespread, but highly variable influence of tonic, homosynaptic, CB1 receptor-dependent modulation of GABA release in these connections, and identify the postsynaptic release of endocannabinoid ligands as the main cause for the endocannabinoid tone. In addition, the data show that transmitter release at these connections is directly modulated by CB1 and GABAB receptors, but not by opioid, serotonergic, muscarinic or metabotropic glutamate receptors.