Abstract
The ventral pallidum (VP) is a major recipient of inhibitory projections from nucleus accumbens (Acb) neurons that differentially express the reward (enkephalin) and aversion (dynorphin)-associated opioid peptides. The cannabinoid-1 receptor (CB1R) is present in Acb neurons expressing each of these peptides, but its location in the VP is not known. To address this question, we used electron microscopic dual immunolabeling of the CB1R and either dynorphin 1–8 (Dyn) or Met5-enkephalin (ME) in the VP of C57BL/6J mice, a species in which CB1R gene deletion produces a reward deficit. We also used similar methods to determine the relationship between the CB1R and N-acylphosphatidylethanolamine (NAPE)-hydrolyzing phospholipase D (NAPE-PLD), an anandamide-synthesizing enzyme located presynaptically in other limbic brain regions. CB1R-immunogold was principally localized to cytoplasmic endomembranes and synaptic or extrasynaptic plasma membranes of axonal profiles, but was also affiliated with postsynaptic membrane specializations in dendrites. The axonal profiles included many single CB1R-labeled axon terminals as well as terminals containing CB1R-immunogold and either Dyn or ME immunoreactivity. Dually labeled terminals comprised 26% of all Dyn- and 17% of all ME-labeled axon terminals. Both single- and dual-labeled terminals formed mainly inhibitory-type synapses, but almost 16% of these terminals formed excitatory synapses. Approximately 60% of the CB1R-labeled axonal profiles opposed or converged with axon terminals containing NAPE-PLD immunoreactivity. We conclude that CB1Rs in the mouse VP have subcellular distributions consistent with on demand activation by endocannabinoids that can regulate the release of functionally opposed opioid peptides and also modulate inhibitory and excitatory transmission.
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