Diacylglycerol lipase disinhibits VTA dopamine neurons during chronic nicotine exposure

Abstract

Chronic nicotine exposure (CNE) alters synaptic transmission in the ventral tegmental area (VTA) in a manner that enhances dopaminergic signaling and promotes nicotine use. The present experiments identify a correlation between enhanced production of the endogenous cannabinoid 2-arachidonoylglycerol (2-AG) and diminished release of the inhibitory neurotransmitter GABA in the VTA following CNE. To study the functional role of on-demand 2-AG signaling in GABAergic synapses, we used 1,2,3-triazole urea compounds to selectively inhibit 2-AG biosynthesis by diacylglycerol lipase (DAGL). The potency and selectivity of these inhibitors were established in rats in vitro (rat brain proteome), ex vivo (brain slices), and in vivo (intracerebroventricular administration) using activity-based protein profiling and targeted metabolomics analyses. Inhibition of DAGL (2-AG biosynthesis) rescues nicotineinduced VTA GABA signaling following CNE. Conversely, enhancement of 2-AG signaling in naïve rats by inhibiting 2-AG degradation recapitulates the loss of nicotine-induced GABA signaling evident following CNE. DAGL inhibition reduces nicotine self-administration without disrupting operant responding for a nondrug reinforcer or motor activity. Collectively, these findings provide a detailed characterization of selective inhibitors of rat brain DAGL and demonstrate that excessive 2-AG signaling contributes to a loss of inhibitory GABAergic constraint of VTA excitability following CNE.T he mesocorticolimbic dopamine (DA) system provides a critical link between the brain regions that process cognitive information and those controlling motor behavior. Precise control of these ventral tegmental area (VTA) projections facilitates seeking rewarding stimuli, retreating from aversive stimuli, constraint of motivational state, and behavioral flexibility necessary for survival. GABAergic signaling provides robust inhibition that gates VTA DA cell excitability (1, 2), and loss of this inhibition leads to pathological dysregulation of mesocorticolimbic circuitry (3, 4).Endocannabinoids (eCBs) regulate DAergic activity through retrograde signaling from DA cell bodies onto presynaptic cannabinoid type 1 (CB 1 ) receptors expressed on both inhibitory and excitatory inputs. Although both 2-arachidonoylglycerol (2-AG) and anandamide (AEA) function as endogenous CB 1 agonists in the brain (5-7), these lipids exhibit distinct pharmacological profiles in vivo (8, 9) and mediate differential behavioral effects (10, 11). Endocannabinoids are produced and degraded ondemand, and the primary enzymes responsible for eCB degradation have been well-characterized using selective pharmacological tools that inactivate monoacylglycerol lipase (MAGL) or fatty acid amide hydrolase (FAAH) (11-13). However, a complete evaluation of the influence of eCB signaling in the brain has been hampered by the lack of appropriate corresponding tools for selectively inactivating on-demand eCB biosynthesis.Substantial evidence implicates eCB signaling in the etiology of nicotine ...

Constitutive Increases in Amygdalar Corticotropin-Releasing Factor and Fatty Acid Amide Hydrolase Drive an Anxious Phenotype

“…Samples were snap-frozen in liquid nitrogen, and stored at −80°C. Membrane proteomes were processed in phosphate buffered saline as previously described (42). …”

“…Amygdalar proteomes (1 mg/mL) were evaluated for eCB metabolic enzyme activity using ABPP as previously described (42). Details of the method are provided in Supplemental Information.…”

“…One week later, microdialysis probes were inserted and secured into the guide cannulas. Dialysis and quantification of eCBs and neurotransmitters were performed in separate experiments as previously established (42, 44). Details are included in Supplemental Information.…”

Enduring effects of perinatal nicotine exposure on murine sleep in adulthood

“…In mouse models of early-life nicotine, cigarette smoke, or e-cigarette vapor exposure, developmental abnormalities in locomotor behavior (39), depressive-like behavior (43), and cognitive function (2) have been observed extending into adolescence and adulthood. These changes may be due to deregulated cholinergic signaling during development, as nicotine is a powerful neuromodulator acting through nicotinic acetylcholine receptors (nAChRs), with off-target actions on dopaminergic transmission (7), hypocretin/orexin signaling (36), the circadian clock (9,32), and others.…”

Acute Nicotine Exposure Alters Ventral Tegmental Area Inhibitory Transmission and Promotes Diazepam Consumption

“…The specific mechanisms by which nicotine increases vulnerability for subsequent addiction to other substances are not fully understood, but likely involve nicotine-induced adaptations in the reward system of the brain. Accumulating evidence indicates that chronic nicotine exposure dysregulates GABAergic signaling in the ventral tegmental area (VTA), a key dopaminergic brain area involved in the rewarding properties of virtually all addictive drugs (Vihavainen et al, 2008b;Li et al, 2014;Buczynski et al, 2016;Thomas et al, 2018). To date, nicotine-induced changes in VTA GABAergic transmission have been shown to influence dopamine (DA) signaling and behavioral responses to alcohol and morphine (Vihavainen et al, 2008a,b;Thomas et al, 2018).…”