William M. Doyon

Dopamine Activity in the Nucleus Accumbens During Consummatory Phases of Oral Ethanol Self-Administration

UNLABELLED BACKGROUND This present study was designed to clarify the role of dopamine in the nucleus accumbens during operant ethanol self-administration by separating bar pressing (ethanol seeking) from ethanol consumption. Furthermore, we sought to define the relationship between ethanol in the brain and the accumbal dopamine response after oral self-administration of ethanol. METHODS Two separate groups of male Long-Evans rats were trained to bar press with 10% ethanol or water. Rats were trained to elicit an escalating number of bar presses across daily sessions before gaining access to the drinking solution for 20 min. Microdialysis was performed before (during a waiting period), during, and after bar pressing and drinking. A handling control group was included, but did not receive training. RESULTS A significant increase in dopamine occurred during placement of the rats into the operant chamber in trained rats and handling controls. The lever-pressing period did not produce an increase in dialysate dopamine. Accumbal dopamine was increased in the first 5 min of ethanol, but not water, consumption. Ethanol appeared in the dialysate sample following ethanol availability, and peak concentrations were reached at 10 min. Most of the ethanol and water consumption occurred within 5 min of fluid access. The probes were distributed in the core (32%), shell (32%), and core plus shell (36%) regions of the nucleus accumbens. CONCLUSIONS The enhancement of dopamine during transfer into the operant chamber does not depend on anticipation or operant training with ethanol or water reinforcement. Furthermore, the difference between the time course of accumbal dopamine and ethanol in dialysates suggests that the dopamine response is not solely due to pharmacological effects of ethanol. The dopamine response may be associated with the stimulus properties of ethanol presentation, which would be strongest during consumption.

Controls of tonic and phasic dopamine transmission in the dorsal and ventral striatum.

Dopamine (DA) release varies within subregions and local environments of the striatum, suggesting that controls intrinsic and extrinsic to the DA fibers and terminals regulate release. While applying fast-scan cyclic voltammetry and using tonic and phasic stimulus trains, we investigated the regulation of DA release in the dorsolateral to ventral striatum. The ratio of phasic-to-tonic-evoked DA signals varied with the average ongoing firing frequency, and the ratio was generally higher in the nucleus accumbens (NAc) compared with the dorsolateral striatum. At the normal average firing frequency, burst stimulation produces a larger increase in the DA response in the NAc than the dorsolateral striatum. This finding was comparable whether the DA measurements were made using in vitro brain slices or were recorded in vivo from freely moving rodents. Blockade of the dopamine transporters and dopamine D2 receptors particularly enhanced the tonic DA signals. Conversely, blockade of nicotinic acetylcholine receptors (nAChRs) containing the β2 subunit (β2*) predominantly suppressed tonic DA signals. The suppression of tonic DA release increased the contrast between phasic and tonic DA signals, and that made the frequency-dependent DA dynamics between the dorsolateral striatum and NAc more similar. The results indicate that intrinsic differences in the DA fibers that innervate specific regions of the striatum combine with (at least) DA transporters, DA receptors, and nAChRs to regulate the frequency dependence of DA release. A combination of mechanisms provides specific local control of DA release that underlies pathway-specific information associated with motor and reward-related functions.

Withdrawal from Chronic Nicotine Exposure Alters Dopamine Signaling Dynamics in the Nucleus Accumbens

BACKGROUND Unaided attempts to quit smoking commonly fail during the first 2 weeks of the nicotine withdrawal syndrome. Alterations in dopamine (DA) signaling correlate with withdrawal from chronic nicotine exposure, but those changes have not been well-characterized. METHODS Mice were administered nicotine in their drinking water for 4 or 12 weeks. Then nicotine was withheld for 1 to 10 days while DA signaling was characterized with in vivo microdialysis or fast-scan cyclic voltammetry. RESULTS Upon withdrawal of nicotine, the basal DA concentration in the nucleus accumbens decreased as measured by microdialysis. The length of time that the low basal DA state lasted depended on the length of the chronic nicotine treatment. Microdialysis indicated that acute re-exposure to nicotine during withdrawal temporarily reversed this hypodopaminergic state. Voltammetry measurements supported the microdialysis results by showing that nicotine withdrawal decreased tonic and phasic DA release. The basal DA concentration and tonic DA signals, however, were disproportionately lower than the phasic DA signals. Therefore, the phasic/tonic DA signaling ratio was increased during the withdrawal period. CONCLUSIONS The relative increase in the sensitivity of DA release to phasic stimulation suggests an increase in the signal-to-noise relationship of DA signaling during the withdrawal period. Therefore, the DA signal produced by acute nicotine re-exposure produces a DA response that might reinforce relapse to drug use (i.e., smoking). Because the basal DA concentration is low during withdrawal, therapies aimed at elevating the background DA signal represent a reasonable treatment strategy for nicotine-dependent individuals attempting to quit.

Nicotine decreases ethanol-induced dopamine signaling and increases self-administration via stress hormones.

Tobacco smoking is a well-known risk factor for subsequent alcohol abuse, but the neural events underlying this risk remain largely unknown. Alcohol and nicotine reinforcement involve common neural circuitry, including the mesolimbic dopamine system. We demonstrate in rodents that pre-exposure to nicotine increases alcohol self-administration and decreases alcohol-induced dopamine responses. The blunted dopamine response was due to increased inhibitory synaptic transmission onto dopamine neurons. Blocking stress hormone receptors prior to nicotine exposure prevented all interactions with alcohol that we measured, including the increased inhibition onto dopamine neurons, the decreased dopamine responses, and the increased alcohol self-administration. These results indicate that nicotine recruits neuroendocrine systems to influence neurotransmission and behavior associated with alcohol reinforcement.

Potential substrates for nicotine and alcohol interactions: a focus on the mesocorticolimbic dopamine system.

Epidemiological studies consistently find correlations between nicotine and alcohol use, yet the neural mechanisms underlying their interaction remain largely unknown. Nicotine and alcohol (i.e., ethanol) share many common molecular and cellular targets that provide potential substrates for nicotine-alcohol interactions. These targets for interaction often converge upon the mesocorticolimbic dopamine system, where the link to drug self-administration and reinforcement is well documented. Both nicotine and alcohol activate the mesocorticolimbic dopamine system, producing downstream dopamine signals that promote the drug reinforcement process. While nicotine primarily acts via nicotinic acetylcholine receptors, alcohol acts upon a wider range of receptors and molecular substrates. The complex pharmacological profile of these two drugs generates overlapping responses that ultimately intersect within the mesocorticolimbic dopamine system to promote drug use. Here we will examine overlapping targets between nicotine and alcohol and provide evidence for their interaction. Based on the existing literature, we will also propose some potential targets that have yet to be directly tested. Mechanistic studies that examine nicotine-alcohol interactions would ultimately improve our understanding of the factors that contribute to the associations between nicotine and alcohol use.

κ-Opioid receptor modulation of accumbal dopamine concentration during operant ethanol self-administration

Our study examined ethanol self-administration and accumbal dopamine concentration during kappa-opioid receptor (KOPr) blockade. Long-Evans rats were trained to respond for 20 min of access to 10% ethanol (with sucrose) over 7 days. Rats were injected s.c. with the long-acting KOPr antagonist, nor-binaltorphimine (NOR-BNI; 0 or 20 mg/kg) 15-20 h prior to testing. Microdialysis revealed a transient elevation in dopamine concentration within 5 min of ethanol access in controls. NOR-BNI-treated rats did not exhibit this response, but showed a latent increase in dopamine concentration at the end of the access period. The rise in dopamine levels correlated positively with dialysate ethanol concentration but not in controls. NOR-BNI did not alter dopamine levels in rats self-administering 10% sucrose. The transient dopamine response during ethanol acquisition in controls is consistent with previous results that were attributed to ethanol stimulus cues. The altered dopamine response to NOR-BNI during ethanol drinking suggests that KOPr blockade temporarily uncovered a pharmacological stimulation of dopamine release by ethanol. Despite these neurochemical changes, NOR-BNI did not alter operant responding or ethanol intake, suggesting that the KOPr is not involved in ethanol-reinforced behavior under the limited conditions we studied.

Acute in vivo nicotine administration enhances synchrony among dopamine neurons.

Altered functional interactions among midbrain dopamine (DA) neurons contribute to the reinforcing properties of environmental stimuli and addictive drugs. To examine correlations among DA neurons, acute nicotine was administrated to rats via an intraperitoneal catheter and unit activity was measured using multi-tetrode in vivo recordings. Nicotine administration enhanced the correlated activity of simultaneously recorded DA neurons from the ventral tegmental area (VTA). The strength of the correlations between DA neuron pairs, as measured by cross covariance among two spike trains, showed dynamic changes over time. Nicotine produced a gradual rise in firing rate and burst activity that reached a stable plateau approximately 20 min after the intraperitoneal nicotine infusion. Shortly after that time the cross correlations measured using 5-ms bins increased significantly above baseline. In addition, nicotine increased the firing rates of DA neurons in the posterior VTA more than in the anterior VTA. Unlike nicotine, eticlopride administration also boosted DA neuron firing activity but did not enhance synchronization, indicating that the cross correlations induced by nicotine were not due to a non-specific increase in firing rate. The overall results show that nicotine induces nearly synchronous firing by a subset of DA neurons, and those changes in correlative firing will enhance the DA signal that contributes to nicotine-induced behavioral reinforcement.

Stress Increases Ethanol Self-Administration via a Shift toward Excitatory GABA Signaling in the Ventral Tegmental Area

Stress is a well-known risk factor for subsequent alcohol abuse, but the neural mechanisms underlying interactions between stress and alcohol remain largely unknown. Addictive drug reinforcement and stress signaling involve common neural circuitry, including the mesolimbic dopamine system. We demonstrate in rodents that pre-exposure to stress attenuates alcohol-induced dopamine responses and increases alcohol self-administration. The blunted dopamine signaling resulted from ethanol-induced excitation of GABA neurons in the ventral tegmental area. Excitation of GABA neurons was mediated by GABAA receptor activation and involved stress-induced functional downregulation of the K+, Cl- cotransporter, KCC2. Blocking stress hormone receptors, enhancing KCC2 function, or preventing excitatory GABA signaling by alternative methods all prevented the attenuated alcohol-induced dopamine response and prevented the increased alcohol self-administration. These results demonstrate that stress alters the neural and behavioral responses to alcohol through a neuroendocrine signal that shifts inhibitory GABA transmission toward excitation.

Route of nicotine administration influences in vivo dopamine neuron activity: habituation, needle injection, and cannula infusion.

Mesolimbic dopamine (DA) systems play a critical role in tobacco addiction driven by nicotine. Nicotine activates midbrain DA neurons and, consequently, elevates DA concentrations in targets, especially in the nucleus accumbens (NAc) of the ventral striatum. The route of drug administration influences the impact of addictive drugs. Here, we examine whether the nature of the administration alters DA neuron activity and DA concentrations in the NAc. Using unhabituated naïve freely moving rats, microdialysis measurements showed that nicotine administered via needle injection caused greater DA release in the NAc than the same dose administered via an implanted chronic cannula. After habituation to the needle injections, however, there was no significant difference in DA signaling between the needle and cannula routes of administration. Consistent with these microdialysis results after habituation, our in vivo tetrode unit recordings showed no significant difference in midbrain DA neuron activity in response to nicotine delivered by needle or cannula as long as predictive cues were avoided.

Quantitative unit classification of ventral tegmental area neurons in vivo

Neurons in the ventral tegmental area (VTA) synthesize several major neurotransmitters, including dopamine (DA), GABA, and glutamate. To classify VTA single-unit neural activity from freely moving rats, we used hierarchical agglomerative clustering and probability distributions as quantitative methods. After many parameters were examined, a firing rate of 10 Hz emerged as a transition frequency between clusters of low-firing and high-firing neurons. To form a subgroup identified as high-firing neurons with GABAergic characteristics, the high-firing classification was sorted by spike duration. To form a subgroup identified as putative DA neurons, the low-firing classification was sorted by DA D2-type receptor pharmacological responses to quinpirole and eticlopride. Putative DA neurons were inhibited by the D2-type receptor agonist quinpirole and returned to near-baseline firing rates or higher following the D2-type receptor antagonist eticlopride. Other unit types showed different responses to these D2-type receptor drugs. A multidimensional comparison of neural properties indicated that these subgroups often clustered independently of each other with minimal overlap. Firing pattern variability reliably distinguished putative DA neurons from other unit types. A combination of phasic burst properties and a low skew in the interspike interval distribution produced a neural population that was comparable to the one sorted by D2 pharmacology. These findings provide a quantitative statistical approach for the classification of VTA neurons in unanesthetized animals.