Tonic endocannabinoid-mediated modulation of GABA release is independent of the CB1 content of axon terminals.

The release of GABA from cholecystokinin-containing interneurons is modulated by type-1 cannabinoid receptors (CB1). Here we tested the hypothesis that the strength of CB1-mediated modulation of GABA release is related to the CB1 content of axon terminals. Basket cell boutons have on average 78% higher CB1 content than those of dendritic-layer-innervating (DLI) cells, a consequence of larger bouton surface and higher CB1 density. The CB1 antagonist AM251 caused a 54% increase in action potential-evoked [Ca(2+)] in boutons of basket cells, but not in DLI cells. However, the effect of AM251 did not correlate with CB1 immunoreactivity of individual boutons. Moreover, a CB1 agonist decreased [Ca(2+)] in a cell type- and CB1-content-independent manner. Replica immunogold labelling demonstrated the colocalization of CB1 with the Cav2.2 Ca(2+) channel subunit. Our data suggest that only a subpopulation of CB1s, within nanometre distances from their target Cav2.2 channels, are responsible for endocannabinoid-mediated modulation of GABA release.