Differences in chloride gradients allow for three distinct types of synaptic modulation by endocannabinoids

Endocannabinoids can elicit persistent depression of excitatory and inhibitory synapses, reducing or enhancing (disinhibiting) neural circuit output, respectively. In this study, we examined whether differences in Cl^- gradients can regulate which synapses undergo endocannabinoid-mediated synaptic depression vs. disinhibition using the well-characterized central nervous system (CNS) of the medicinal leech, Hirudo verbana. Exogenous application of endocannabinoids or capsaicin elicits potentiation of pressure (P) cell synapses and depression of both polymodal (N_poly) and mechanical (N_mech) nociceptive synapses. In P synapses, blocking Cl^- export prevented endocannabinoid-mediated potentiation, consistent with a disinhibition process that has been indicated by previous experiments. In N_mech neurons, which are depolarized by GABA due to an elevated Cl^- equilibrium potentials (E_Cl), endocannabinoidmediated depression was prevented by blocking Cl^- import, indicating that this decrease in synaptic signaling was due to depression of excitatory GABAergic input (disexcitation). N_poly neurons are also depolarized by GABA, but endocannabinoids elicit depression in these synapses directly and were only weakly affected by disruption of Cl^- import. Consequently, the primary role of elevated E_Cl may be to protect N_poly synapses from disinhibition. All forms of endocannabinoid- mediated plasticity required activation of transient potential receptor vanilloid (TRPV) channels. Endocannabinoid/TRPV-dependent synaptic plasticity could also be elicited by distinct patterns of afferent stimulation with low-frequency stimulation (LFS) eliciting endocannabinoid-mediated depression of N_poly synapses and highfrequency stimulus (HFS) eliciting endocannabinoid-mediated potentiation of P synapses and depression of N_mech synapses. These findings demonstrate a critical role of differences in Cl^- gradients between neurons in determining the sign, potentiation vs. depression, of synaptic modulation under normal physiological conditions.