Endocannabinoid 2-AG and intracellular cannabinoid receptors modulate a low-threshold calcium spike-induced slow...

In rat paraventricular thalamic nucleus (PVT) neurons, activation of low-threshold calcium (Ca(2+)) channels triggers a low-threshold spike (LTS) which may be followed by slow afterpotentials that can dramatically influence action potential patterning. Using gluconate-based internal recording solutions, we investigated the properties of a LTS-induced slow afterdepolarization (sADP) observed in a subpopulation of PVT neurons recorded in brain slice preparations. This LTS-induced sADP required T-type Ca(2+) channel opening, exhibited variable magnitudes between neurons and a voltage dependency with a maximum near -50 mV. The area under the sADP remained stable during control monitoring, but displayed gradual suppression in media where strontium replaced Ca(2+). The sADP was suppressed following bath application of 2-APB or ML204, suggesting engagement of transient receptor potential canonical (TRPC)-like channels. Further investigation revealed a reversible suppression during bath applications of membrane permeable cannabinoid receptor (CBR) blockers rimonabant, AM630 or SR144528 suggesting the presence of both CB1Rs and CB2Rs. Similar results were achieved by intracellular, but not bath application of the membrane impermeant CB1R blocker hemopressin, suggesting an intracellular localization of CB1Rs. Data from pharmacologic manipulation of endocannabinoid biosynthetic pathways suggested 2-arachidonlyglycerol (2-AG) as the endogenous cannabinoid ligand, derived via hydrolysis of diacylglycerol (DAG), with the latter formed from the pathway involving phosphatidylcholine-specific phospholipase D and phosphatic acid phosphohydrolase. The sADP suppression observed during recordings with pipettes containing LY294002, a PI3-kinase inhibitor, suggested a role for PI3kinase in the translocation of these TRPC-like channels to the plasma membrane. Drug-induced attenuation of the availability of 2-AG influences the number of action potentials that surmount the LTS evoked in PVT neurons, implying an ongoing intracellular CBR modulation of neuronal excitability during LTS-induced bursting behavior.