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

Abstract

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 (Npoly) and mechanical (Nmech) 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 Nmech neurons, which are depolarized by GABA due to an elevated Cl(-) equilibrium potentials (ECl), endocannabinoid-mediated depression was prevented by blocking Cl(-) import, indicating that this decrease in synaptic signaling was due to depression of excitatory GABAergic input (disexcitation). Npoly 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 ECl may be to protect Npoly 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 Npoly synapses and high-frequency stimulus (HFS) eliciting endocannabinoid-mediated potentiation of P synapses and depression of Nmech 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.

Endocannabinoids Have Opposing Effects On Behavioral Responses To Nociceptive And Non-nociceptive Stimuli

“…1A) 31–33 . Pharmacological inhibitors of TRPV1 blocked both endocannabinoid-mediated potentiation of P cell synapses and depression of the N cell synapses.…”

An annotated CNS transcriptome of the medicinal leech, Hirudo verbana: De novo sequencing to characterize genes associated with nervous system activity

“…Molecular sequence information for the Northwestern European medicinal leech Hirudo medicinalis has more recently been accumulating through studies targeting specific candidate gene sequences [ 9 – 14 ], tissue types [ 15 , 16 ], or life stages [ 17 ]. The Southeastern European medicinal leech Hirudo verbana has not received the same attention in the surge to bring transcriptomic approaches to traditionally “non-genetic” systems, even though it remains the most commonly used species in neurophysiological and behavioral studies [ 18 – 21 ]. While H .…”

Interaction between NMDA Receptor- and Endocannabinoid-Mediated Modulation of Nociceptive Synapses

“…In one recent examples of such a process, induction of LTP in rodent C fiber synapses was found to stimulate the release of cytokines and D-serine by astrocytes and/or microglia which produced heterosynaptic potentiation in non-activated nociceptive synapses 16 . Interestingly, in Hirudo , the same HFS of N cells that produced NMDAR-LTP in nociceptive synapses also produced a heterosynaptic potentiation in pressure (P) cell synapses 37 . In this situation, potentiation of the P cell synapses was due to endocannabinoid-mediated depression of GABAergic input to P cells, resulting in a persistent form of disinhibition.…”

“…1B). This procedure has been used in previous experiments to elicit homosynaptic and heterosynaptic forms of LTP 14,37 . In experiments using BAPTA, 1 mM BAPTA was included in the electrode filling solution and the Ca 2+ chelator was iontophoretically injected into the L motor neuron (-1nA holding current for 5 minutes) prior to HFS.…”