Abstract
The neuropeptide nociceptin/orphanin FQ (N/OFQ) can be released by stressors and is associated with disorders of emotion regulation and reward processing. N/OFQ and its receptor, NOP, are enriched in dopaminergic pathways, and intra-ventricular agonist delivery decreases dopamine levels in the dorsal striatum, nucleus accumbens (NAc), and ventral tegmental area (VTA). We used whole cell electrophysiology in acute rat midbrain slices to investigate synaptic actions of N/OFQ. N/OFQ was primarily inhibitory, causing outward currents in both immunocytochemically identified dopaminergic (tyrosine hydroxylase positive (TH(+)) and non-dopaminergic (TH(−)) VTA neurons (effect at 1 μM: 20 ± 4 pA). Surprisingly, this effect was mediated by augmentation of postsynaptic GABAAR currents, unlike the substantia nigra pars compacta (SNc), where the N/OFQ induced outward currents were K+ channel dependent. A smaller population, 19% of all VTA neurons, responded to low concentrations N/OFQ with inward currents (10 nM: −11 ± 2 pA). Following 100 nM N/OFQ, the response to a second N/OFQ application was markedly diminished in VTA neurons (14 ± 10% of first response), but not in SNc neurons (90 ± 20% of first response). N/OFQ generated outward currents in medial prefrontal cortex (mPFC)-projecting VTA neurons, but inward currents in a subset of posterior anterior cingulate cortex-projecting VTA neurons. While N/OFQ inhibited NAc-projecting VTA cell bodies, it had little effect on electrically or optogenetically evoked terminal dopamine release in the NAc measured ex vivo with fast scan cyclic voltammetry. These results extend our understanding of the N/OFQ system in brainstem circuits implicated in many neurobehavioral disorders.
Significance statement The neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) are engaged under conditions of stress and are associated with reward processing disorders. Both peptide and receptor are highly enriched in ventral tegmental area (VTA) pathways underlying motivation and reward. Using whole cell electrophysiology in rat midbrain slices we found: 1) NOPs are functional on both dopaminergic and non-dopaminergic VTA neurons; 2) N/OFQ differentially regulates VTA neurons based on neuroanatomical projection target; and 3) repeated application of N/OFQ produces evidence of receptor desensitization in the VTA but not the SNc. These results reveal candidate mechanisms by which the NOP system regulates motivation and emotion.
Competing Interest Statement
Yes, TLW and WJM are employees of and shareholders in BlackThorn Therapeutics, which is evaluating BTRX-246040 for the treatment of neurobehavioral disorders.
Footnotes
JRD, TLW, WJM and EBM designed the studies; JRD, KAM, and EBM collected the data; JRD, KAM, and EBM analyzed the data; JRD, TLW, WJM and EBM wrote the manuscript.
Conflict of interest Yes, TLW and WJM are employees of and shareholders in BlackThorn Therapeutics, which is evaluating BTRX-246040 for the treatment of neurobehavioral disorders.
Funding This work was supported through a sponsored research agreement from BlackThorn Therapeutics, Inc. to the University of California, San Francisco, and by National Institutes of Health grant number R01 DA042025 to E.B.M.
This manuscript has been thoroughly revised and updated. In particular, new mechanism data and SNc recordings have been added, now Figure 5. Kasra Mansourian has also been added as an author.