PhD Candidate, University of Toronto
The neural mechanisms initiated by chronic drugs that transform hedonic liking into a habitual need
In substance dependence, drugs are sought not for their pleasurable qualities but, rather, for their ability to alleviate withdrawal. For example, smokers often report that they do not like cigarettes but that they, nevertheless, want them. This transition from drug-liking to drug-wanting is by no means limited to nicotine dependence; it is a hallmark of addiction. What remains a mystery, however, is how exactly the brain determines whether a substance will produce pleasure, discomfort or merely alleviate withdrawal. This is precisely what I am investigating.
The ventral tegmental area (VTA) is a brainstem region that dictates whether we seek or avoid a stimulus. Long-term drug use initiates a series of molecular events in this region that alter our experience and, therefore, the very reason we seek the drug. Our lab has pinpointed the biochemical events that elicit withdrawal. We have shown that artificially inducing these events via pharmacological or genetic manipulations is sufficient to produce feelings of withdrawal in rodents with no prior exposure to the drug. We use an assay that can explain why the animal seeks morphine: for the buzz or to overcome withdrawal. Our most recent work demonstrates that a population of inhibitory neurons in the VTA contain a protein that serves as an enhancer of electrical conductance between these cells, thereby facilitating the spread of electrical signals from one neuron to the next. We have shown that this protein - called connexin-36 - has to be expressed for animals to become drug-dependent. Blocking these electrically conductive links by injecting a drug directly into the VTA reverts drug dependent rats to a drug-nondependent state wherein opiates are sought for their pleasurable qualities, rather than withdrawal alleviation. Furthermore, mice lacking the connexin-36 gene are perpetually drug-nondependent: they never experience withdrawal and the rewarding properties of opiates never diminish. We are currently studying the events that unfold downstream of the VTA when electrical coupling is prevented. Our data suggest that two different pathways can be activated in response to drugs: a drug-liking and a drug-wanting pathway. The latter pathway is never active in mice lacking connexin-36, thereby explaining their dependence-resistance.
All in all, these experiments suggest that the VTA needs to transmit synchronous signals with minimal electrical impedance to trigger withdrawal and, consequently, dependence.
Abstract: Communication between neurons in the mammalian brain is primarily through chemical synapses; however, evidence is accumulating in support of electrical synaptic transmission between some neuronal types in the mature nervous system. The authors have recently demonstrated that the gap junction (GJ) blocker quinidine suppresses stimulus-induced and dopamine-evoked coupling of gamma amino butyric acid (GABA) neurons in the ventral tegmental area (VTA) of mature rats (Stobbs et al., 2004). The aim of this study was to evaluate the role of connexin-36 (Cx36) GJs in mediating electrical coupling between VTA GABA neurons in P50-80 rats in vivo and P25-50 rats in vitro. Single stimulation of the internal capsule (IC) evoked VTA GABA neuron spike couplets in mature rats when activated antidromically, and multiple poststimulus spike discharges (PSDs) when activated with brief high-frequency stimulation of the IC (ICPSDs). The Cx36 GJ blocker mefloquine (30 mg/kg) suppressed VTA GABA neuron ICPSDs in mature freely behaving rats. VTA GABA neurons recorded via whole-cell patch clamp in the midbrain slice preparation of P25-50 rats showed robust expression of Cx36 transcripts when tested with single-cell quantitative reverse transcription polymerase chain reaction. In P50-80 rats, Cx36 protein immunoreactivity was evident in the VTA and surrounding structures. Dye-coupling between VTA neurons was observed following Neurobiotin labeling of VTA GABA neurons, as well as with the fluorochrome Alexa Fluor 488 using real-time video fluorescent microscopy. Thus, mature VTA GABA neurons appear to be connected by electrical synapses via Cx36 GJs, whose coupling is enhanced by corticotegmental input and by dopamine.
Pub.: 01 Apr '06, Pinned: 31 Aug '17
Abstract: The neural substrate of brain stimulation reward (BSR) has eluded identification since its discovery more than a half-century ago. Notwithstanding the difficulties in identifying the neuronal integrator of BSR, the mesocorticolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) of the midbrain has been implicated. We have previously demonstrated that the firing rate of a subpopulation of gamma-aminobutyric acid (GABA) neurons in the VTA increases in anticipation of BSR. We show here that GABA neurons in the VTA, midbrain, hypothalamus, and thalamus of rats express connexin-36 (Cx36) gap junctions (GJs) and couple electrically upon DA application or by stimulation of the internal capsule (IC), which also supports self-stimulation. The threshold for responding for IC self-stimulation was the threshold for electrical coupling between GABA neurons, the degree of responding for IC self-stimulation was proportional to the magnitude of electrical coupling between GABA neurons, and GJ blockers increased the threshold for IC self-stimulation without affecting performance. Thus, a network of electrically coupled GABA neurons in the ventral brain may form the elusive neural integrator of BSR.
Pub.: 26 May '07, Pinned: 31 Aug '17
Abstract: Connexin-36 (Cx36) gap junctions (GJs) appear to be involved in the synchronization of GABA interneurons in many brain areas. We have previously identified a population of Cx36-connected ventral tegmental area (VTA) GABA neurons that may regulate mesolimbic dopamine (DA) neurotransmission, a system implicated in reward from both natural behaviors and drugs of abuse. The aim of this study was to determine the effect mefloquine (MFQ) has on midbrain DA and GABA neuron inhibition, and the role Cx36 GJs play in regulating midbrain VTA DA neuron activity in mice. In brain slices from adolescent wild-type (WT) mice the Cx36-selective GJ blocker mefloquine (MFQ, 25 μM) increased VTA DA neuron sIPSC frequency sixfold, and mIPSC frequency threefold. However, in Cx36 KO mice, MFQ only increased sIPSC and mIPSC frequency threefold. The nonselective GJ blocker carbenoxolone (CBX, 100 μM) increased DA neuron sIPSC frequency twofold in WT mice, did not affect Cx36 KO mouse sIPSCs, and did not affect mIPSCs in WT or Cx36 KO mice. Interestingly, MFQ had no effect on VTA GABA neuron sIPSC frequency. We also examined MFQ effects on VTA DA neuron firing rate and current-evoked spiking in WT and Cx36 KO mice, and found that MFQ decreased WT DA neuron firing rate and current-evoked spiking, but did not alter these measures in Cx36 KO mice. Taken together these findings suggest that blocking Cx36 GJs increases VTA DA neuron inhibition, and that GJs play in key role in regulating inhibition of VTA DA neurons. Synapse, 2011. © 2011 Wiley-Liss, Inc.
Pub.: 11 Jan '11, Pinned: 31 Aug '17
Abstract: Ventral tegmental area (VTA) GABA neurons appear to be critical substrates underlying the acute and chronic effects of ethanol on dopamine (DA) neurotransmission in the mesocorticolimbic system implicated in alcohol reward. The aim of this study was to examine the role of midbrain connexin-36 (Cx36) gap junctions (GJs) in ethanol intoxication and consumption. Using behavioral, molecular, and electrophysiological methods, we compared the effects of ethanol in mature Cx36 knockout (KO) mice and age-matched wild-type (WT) controls. Compared to WT mice, Cx36 KO mice exhibited significantly more ethanol-induced motor impairment in the open field test, but less disruption in motor coordination in the rotarod paradigm. Cx36 KO mice, and WT mice treated with the Cx36 antagonist mefloquine (MFQ), consumed significantly less ethanol than their WT controls in the drink-in-the-dark procedure. The firing rate of VTA GABA neurons in WT mice was inhibited by ethanol with an IC₅₀ of 0.25 g/kg, while VTA GABA neurons in KO mice were significantly less sensitive to ethanol. Dopamine neuron GABA-mediated sIPSC frequency was reduced by ethanol (30 mM) in WT mice, but not affected in KO mice. Cx36 KO mice evinced a significant up-regulation in DAT and D2 receptors in the VTA, as assessed by quantitative RT-PCR. These findings demonstrate the behavioral relevance of Cx36 GJ-mediated electrical coupling between GABA neurons in mature animals, and suggest that loss of coupling between VTA GABA neurons results in disinhibition of DA neurons, a hyper-DAergic state and lowered hedonic valence for ethanol consumption.
Pub.: 04 Jun '11, Pinned: 31 Aug '17
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