Cocaine Directly Inhibits α6-Containing Nicotinic Acetylcholine Receptors in Human SH-EP1 Cells and Mouse VTA DA Neurons

Behavioral and neuronal extracellular vesicle biomarkers associated with nicotine's enhancement of the reinforcing strength of cocaine in female and male monkeys

While the majority of people with cocaine use disorders (CUD) also co-use tobacco/nicotine, most preclinical cocaine research does not include nicotine. The present study examined nicotine and cocaine co-use under several conditions of intravenous drug self-administration in monkeys, as well as potential peripheral biomarkers associated with co-use. In Experiment 1, male rhesus monkeys (N = 3) self-administered cocaine (0.001-0.1 mg/kg/injection) alone and with nicotine (0.01-0.03 mg/kg/injection) under a progressive-ratio schedule of reinforcement. When nicotine was added to cocaine, there was a significant leftward/upward shift in the number of injections received. In Experiment 2, socially housed female and male cynomolgus monkeys (N = 14) self-administered cocaine under a concurrent drug-vs-food choice schedule of reinforcement. Adding nicotine to the cocaine solution shifted the cocaine dose-response curves to the left, with more robust shifts noted in the female animals. There was no evidence of social rank differences. To assess reinforcing strength, delays were added to the presentation of drug; the co-use of nicotine and cocaine required significantly longer delays to decrease drug choice, compared with cocaine alone. Blood samples obtained post-session were used to analyze concentrations of neuronally derived small extracellular vesicles (NDE); significant differences in NDE profile were observed for kappa-opioid receptors when nicotine and cocaine were co-used compared with each drug alone and controls. These results suggest that drug interactions involving the co-use of nicotine and cocaine are not simply changing potency, but rather resulting in changes in reinforcing strength that should be utilized to better understand the neuropharmacology of CUD and the evaluation of potential treatments.

Integrative Pharmacology in the Treatment of Substance Use Disorders

The detrimental physical, mental, and socioeconomic effects of substance use disorders (SUDs) have been apparent to the medical community for decades. However, it has become increasingly urgent in recent years to develop novel pharmacotherapies to treat SUDs. Currently, practitioners typically rely on monotherapy. Monotherapy has been shown to be superior to no treatment at all for most substance classes. However, many randomized controlled trials (RCTs) have revealed that monotherapy leads to poorer outcomes when compared with combination treatment in all specialties of medicine. The results of RCTs suggest that monotherapy frequently fails since multiple dysregulated pathways, enzymes, neurotransmitters, and receptors are involved in the pathophysiology of SUDs. As such, research is urgently needed to determine how various neurobiological mechanisms can be targeted by novel combination treatments to create increasingly specific yet exceedingly comprehensive approaches to SUD treatment. This article aims to review the neurobiology that integrates many pathophysiologic mechanisms and discuss integrative pharmacology developments that may ultimately improve clinical outcomes for patients with SUDs. Many neurobiological mechanisms are known to be involved in SUDs including dopaminergic, nicotinic, N-methyl-D-aspartate (NMDA), and kynurenic acid (KYNA) mechanisms. Emerging evidence indicates that KYNA, a tryptophan metabolite, modulates all these major pathophysiologic mechanisms. Therefore, achieving KYNA homeostasis by harmonizing integrative pathophysiology and pharmacology could prove to be a better therapeutic approach for SUDs. We propose KYNA-NMDA-α7nAChRcentric pathophysiology, the "conductor of the orchestra," as a novel approach to treat many SUDs concurrently. KYNA-NMDA-α7nAChR pathophysiology may be the "command center" of neuropsychiatry. To date, extant RCTs have shown equivocal findings across comparison conditions, possibly because investigators targeted single pathophysiologic mechanisms, hit wrong targets in underlying pathophysiologic mechanisms, and tested inadequate monotherapy treatment. We provide examples of potential combination treatments that simultaneously target multiple pathophysiologic mechanisms in addition to KYNA. Kynurenine pathway metabolism demonstrates the greatest potential as a target for neuropsychiatric diseases. The investigational medications with the most evidence include memantine, galantamine, and N-acetylcysteine. Future RCTs are warranted with novel combination treatments for SUDs. Multicenter RCTs with integrative pharmacology offer a promising, potentially fruitful avenue to develop novel therapeutics for the treatment of SUDs.

Distinct sub-second dopamine signaling in dorsolateral striatum measured by a genetically-encoded fluorescent sensor

The development of genetically encoded dopamine sensors such as dLight has provided a new approach to measuring slow and fast dopamine dynamics both in brain slices and in vivo, possibly enabling dopamine measurements in areas like the dorsolateral striatum (DLS) where previously such recordings with fast-scan cyclic voltammetry (FSCV) were difficult. To test this, we first evaluated dLight photometry in mouse brain slices with simultaneous FSCV and found that both techniques yielded comparable results, but notable differences in responses to dopamine transporter inhibitors, including cocaine. We then used in vivo fiber photometry with dLight in mice to examine responses to cocaine in DLS. We also compared dopamine responses during Pavlovian conditioning across the striatum. We show that dopamine increases were readily detectable in DLS and describe transient dopamine kinetics, as well as slowly developing signals during conditioning. Overall, our findings indicate that dLight photometry is well suited to measuring dopamine dynamics in DLS.

Nicotinic acetylcholine receptor subtype expression, function, and pharmacology: Therapeutic potential of α-conotoxins

The pentameric nicotinic acetylcholine receptors (nAChRs) are typically classed as muscle- or neuronal-type, however, the latter has also been reported in non-neuronal cells. Given their broad distribution, nAChRs mediate numerous physiological and pathological processes including synaptic transmission, presynaptic modulation of transmitter release, neuropathic pain, inflammation, and cancer. There are 17 different nAChR subunits and combinations of these subunits produce subtypes with diverse pharmacological properties. The expression and role of some nAChR subtypes have been extensively deciphered with the aid of knock-out models. Many nAChR subtypes expressed in heterologous systems are selectively targeted by the disulfide-rich α-conotoxins. α-Conotoxins are small peptides isolated from the venom of cone snails, and a number of them have potential pharmaceutical value.

Cocaine potently blocks neuronal α3β4 nicotinic acetylcholine receptors in SH-SY5Y cells

Cocaine is one of the most abused illicit drugs worldwide. It is well known that the dopamine (DA) transporter is its major target; but cocaine also acts on other targets including nicotinic acetylcholine receptors (nAChRs). In this study, we investigated the effects of cocaine on a special subtype of neuronal nAChR, α3β4-nAChR expressed in native SH-SY5Y cells. α3β4-nAChR-mediated currents were recorded using whole-cell recordings. Drugs were applied using a computer-controlled U-tube drug perfusion system. We showed that bath application of nicotine induced inward currents in a concentration-dependent manner with an EC50 value of 20 µM. Pre-treatment with cocaine concentration-dependently inhibited nicotine-induced current with an IC50 of 1.5 μM. Kinetic analysis showed that cocaine accelerated α3β4-nAChR desensitization, which caused a reduction of the amplitude of nicotine-induced currents. Co-application of nicotine and cocaine (1.5 μM) depressed the maximum response on the nicotine concentration-response curve without changing the EC50 value, suggesting a non-competitive mechanism. The cocaine-induced inhibition of nicotine response exhibited both voltage- and use-dependence, suggesting an open-channel blocking mechanism. Furthermore, intracellular application of GDP-βS (via recording electrode) did not affect cocaine-induced inhibition, suggesting that cocaine did not alter receptor internalization. Moreover, intracellular application of cocaine (30 µM) failed to alter the nicotine response. Finally, cocaine (1.5 μM) was unable to inhibit the nicotine-induced inward current in heterologous expressed α6/α3β2β3-nAChRs and α4β2-nAChRs expressed in human SH-EP1 cells. Collectively, our results suggest that cocaine is a potent blocker for native α3β4-nAChRs expressed in SH-SY5Y cells.

Cholinergic Receptor Blockade in the VTA Attenuates Cue-Induced Cocaine-Seeking and Reverses the Anxiogenic Effects of Forced Abstinence

Drug relapse after periods of abstinence is a common feature of substance abuse. Moreover, anxiety and other mood disorders are often co-morbid with substance abuse. Cholinergic receptors in the ventral tegmental area (VTA) are known to mediate drug-seeking and anxiety-related behavior in rodent models. However, it is unclear if overlapping VTA cholinergic mechanisms mediate drug relapse and anxiety-related behaviors associated with drug abstinence. We examined the effects of VTA cholinergic receptor blockade on cue-induced cocaine seeking and anxiety during cocaine abstinence. Male Sprague-Dawley rats were trained to self-administer intravenous cocaine (~0.5 mg/kg/infusion, FR1 schedule) for 10 days, followed by 14 days of forced abstinence. VTA infusion of the non-selective nicotinic acetylcholine receptor antagonist mecamylamine (0, 10, and 30 μg/side) or the non-selective muscarinic receptor antagonist scopolamine (0, 2.4 and 24 μg /side) significantly decreased cue-induced cocaine seeking. In cocaine naïve rats, VTA mecamylamine or scopolamine also led to dose-dependent increases in open arm time in the elevated plus maze (EPM). In contrast, rats that received I.V. cocaine, compared to received I.V. saline rats, displayed an anxiogenic response on day 14 of abstinence as reflected by decreased open arm time in the EPM. Furthermore, low doses of VTA mecamylamine (10 μg /side) or scopolamine (2.4 μg /side), that did not alter EPM behavior in cocaine naive rats, were sufficient to reverse the anxiogenic effects of cocaine abstinence. Together, these data point to an overlapping role of VTA cholinergic mechanisms to regulate relapse and mood disorder-related responses during cocaine abstinence.