Binge ethanol-drinking potentiates corticotropin releasing factor R1 receptor activity in the ventral tegmental area.

BACKGROUND: Corticotropin releasing factor (CRF) and urocortin play an important role in many stress responses and also can regulate ethanol (EtOH) intake. Adaptations in CRF signaling in the central amygdala promote EtOH consumption after long-term EtOH intake in dependent animals and also after brief periods of binge EtOH intake. Thus, even brief episodes of EtOH consumption can alter the function of the CRF system, allowing CRF to regulate EtOH intake. Here, we examined whether brief binge EtOH consumption leads to CRF receptor adaptations within the ventral tegmental area (VTA), a structure involved in signaling rewarding and aversive events and important in the development and expression of drug and alcohol addiction. METHODS: We utilized a mouse model of binge drinking known as drinking in the dark (DID), where C57BL/6J mice drink approximately 6 g/kg in 4 hours and achieve blood EtOH concentrations of approximately 100 mg/dl, which is equivalent to binge drinking in humans. We used ex vivo whole-cell recordings from putative VTA dopamine (DA) neurons to examine CRF regulation of NMDA receptor (NMDAR) currents. We also examined the impact of CRF receptor antagonist injection in the VTA on binge EtOH intake. RESULTS: Ex vivo whole-cell recordings from putative VTA DA neurons showed enhanced CRF-mediated potentiation of NMDAR currents in juvenile mice that consumed EtOH in the DID procedure. CRF-induced potentiation of NMDAR currents in EtOH-drinking mice was blocked by administration of CP-154,526 (3 µM), a selective CRF1 receptor antagonist. Furthermore, intra-VTA infusion of CP-154,526 (1 µg) significantly reduced binge EtOH consumption in adult mice. These results were not due to alterations of VTA NMDAR number or function, suggesting that binge drinking may enhance signaling through VTA CRF1 receptors onto NMDARs. CONCLUSIONS: Altered CRF1 receptor-mediated signaling in the VTA promotes binge-like EtOH consumption in mice, which supports the idea that CRF1 receptors may therefore be a promising pharmacological target for reducing binge drinking in humans.

Hypothalamic neurotensin projections promote reward by enhancing glutamate transmission in the VTA.

The lateral hypothalamus (LH) sends a dense glutamatergic and peptidergic projection to dopamine neurons in the ventral tegmental area (VTA), a cell group known to promote reinforcement and aspects of reward. The role of the LH to VTA projection in reward-seeking behavior can be informed by using optogenetic techniques to dissociate the actions of LH neurons from those of other descending forebrain inputs to the VTA. In the present study, we identify the effect of neurotensin (NT), one of the most abundant peptides in the LH to VTA projection, on excitatory synaptic transmission in the VTA and reward-seeking behavior. Mice displayed robust intracranial self-stimulation of LH to VTA fibers, an operant behavior mediated by NT 1 receptors (Nts1) and NMDA receptors. Whole-cell patch-clamp recordings of VTA dopamine neurons demonstrated that NT (10 nm) potentiated NMDA-mediated EPSCs via Nts1. Results suggest that NT release from the LH into the VTA activates Nts1, thereby potentiating NMDA-mediated EPSCs and promoting reward. The striking behavioral and electrophysiological effects of NT and glutamate highlight the LH to VTA pathway as an important component of reward.

Rescuing cocaine-induced prefrontal cortex hypoactivity prevents compulsive cocaine seeking.

Loss of control over harmful drug seeking is one of the most intractable aspects of addiction, as human substance abusers continue to pursue drugs despite incurring significant negative consequences. Human studies have suggested that deficits in prefrontal cortical function and consequential loss of inhibitory control could be crucial in promoting compulsive drug use. However, it remains unknown whether chronic drug use compromises cortical activity and, equally important, whether this deficit promotes compulsive cocaine seeking. Here we use a rat model of compulsive drug seeking in which cocaine seeking persists in a subgroup of rats despite delivery of noxious foot shocks. We show that prolonged cocaine self-administration decreases ex vivo intrinsic excitability of deep-layer pyramidal neurons in the prelimbic cortex, which was significantly more pronounced in compulsive drug-seeking animals. Furthermore, compensating for hypoactive prelimbic cortex neurons with in vivo optogenetic prelimbic cortex stimulation significantly prevented compulsive cocaine seeking, whereas optogenetic prelimbic cortex inhibition significantly increased compulsive cocaine seeking. Our results show a marked reduction in prelimbic cortex excitability in compulsive cocaine-seeking rats, and that in vivo optogenetic prelimbic cortex stimulation decreased compulsive drug-seeking behaviours. Thus, targeted stimulation of the prefrontal cortex could serve as a promising therapy for treating compulsive drug use.

Recombinase-driver rat lines: tools, techniques, and optogenetic application to dopamine-mediated reinforcement.

Currently there is no general approach for achieving specific optogenetic control of genetically defined cell types in rats, which provide a powerful experimental system for numerous established neurophysiological and behavioral paradigms. To overcome this challenge we have generated genetically restricted recombinase-driver rat lines suitable for driving gene expression in specific cell types, expressing Cre recombinase under the control of large genomic regulatory regions (200-300 kb). Multiple tyrosine hydroxylase (Th)::Cre and choline acetyltransferase (Chat)::Cre lines were produced that exhibited specific opsin expression in targeted cell types. We additionally developed methods for utilizing optogenetic tools in freely moving rats and leveraged these technologies to clarify the causal relationship between dopamine (DA) neuron firing and positive reinforcement, observing that optical stimulation of DA neurons in the ventral tegmental area (VTA) of Th::Cre rats is sufficient to support vigorous intracranial self-stimulation (ICSS). These studies complement existing targeting approaches by extending the generalizability of optogenetics to traditionally non-genetically-tractable but vital animal models.

Excitatory transmission from the amygdala to nucleus accumbens facilitates reward seeking.

The basolateral amygdala (BLA) has a crucial role in emotional learning irrespective of valence. The BLA projection to the nucleus accumbens (NAc) is thought to modulate cue-triggered motivated behaviours, but our understanding of the interaction between these two brain regions has been limited by the inability to manipulate neural-circuit elements of this pathway selectively during behaviour. To circumvent this limitation, we used in vivo optogenetic stimulation or inhibition of glutamatergic fibres from the BLA to the NAc, coupled with intracranial pharmacology and ex vivo electrophysiology. Here we show that optical stimulation of the pathway from the BLA to the NAc in mice reinforces behavioural responding to earn additional optical stimulation of these synaptic inputs. Optical stimulation of these glutamatergic fibres required intra-NAc dopamine D1-type receptor signalling, but not D2-type receptor signalling. Brief optical inhibition of fibres from the BLA to the NAc reduced cue-evoked intake of sucrose, demonstrating an important role of this specific pathway in controlling naturally occurring reward-related behaviour. Moreover, although optical stimulation of glutamatergic fibres from the medial prefrontal cortex to the NAc also elicited reliable excitatory synaptic responses, optical self-stimulation behaviour was not observed by activation of this pathway. These data indicate that whereas the BLA is important for processing both positive and negative affect, the glutamatergic pathway from the BLA to the NAc, in conjunction with dopamine signalling in the NAc, promotes motivated behavioural responding. Thus, optogenetic manipulation of anatomically distinct synaptic inputs to the NAc reveals functionally distinct properties of these inputs in controlling reward-seeking behaviours.

Reduced nucleus accumbens SK channel activity enhances alcohol seeking during abstinence.

The cellular mechanisms underlying pathological alcohol seeking remain poorly understood. Here, we show an enhancement of nucleus accumbens (NAcb) core action potential firing ex vivo after protracted abstinence from alcohol but not sucrose self-administration. Increased firing is associated with reduced small-conductance calcium-activated potassium channel (SK) currents and decreased SK3 but not SK2 subunit protein expression. Furthermore, SK activation ex vivo produces greater firing suppression in NAcb core neurons from alcohol- versus sucrose-abstinent rats. Accordingly, SK activation in the NAcb core significantly reduces alcohol but not sucrose seeking after abstinence. In contrast, NAcb shell and lateral dorsal striatal firing ex vivo are not altered after abstinence from alcohol, and SK activation in these regions has little effect on alcohol seeking. Thus, decreased NAcb core SK currents and increased excitability represents a critical mechanism that facilitates motivation to seek alcohol after abstinence.

Voluntary ethanol intake enhances excitatory synaptic strength in the ventral tegmental area.

BACKGROUND: Addiction has been considered a disorder of motivational control over behavior, and the ventral tegmental area (VTA), in conjunction with other limbic brain structures, is thought to play a critical role in the regulation of a number of motivated behaviors including seeking of addictive drugs such as alcohol. Of particular interest is the ability of prolonged exposure of addictive drugs to enhance the function of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamatergic receptors (AMPAR) in the VTA, as glutamate receptor activation can significantly regulate VTA neuron activity. Here, we examined whether voluntary ethanol intake altered VTA AMPAR function. METHODS: We utilized in vitro electrophysiology to examine glutamatergic function in the VTA neurons 12 to 24 hours after the last self-administration bout, which occurred 35 to 50 days after the initiation of ethanol self-administration under a 2-bottle intermittent access model. RESULTS: Voluntary intermittent ethanol intake in a 2-bottle paradigm enhanced postsynaptic AMPAR function, indicated by an increased ratio of evoked AMPAR to N-methyl-d-aspartic acid receptor currents, and by an increase in the amplitude of spontaneous miniature excitatory postsynaptic currents (mEPSCs) measured in the presence of tetrodotoxin to prevent action potential-dependent release. In contrast, ethanol self-administration did not alter evoked presynaptic glutamate release, indicated by no change in the paired-pulse ratio of 2 AMPAR EPSCs evoked 50 ms apart, although spontaneous glutamate release was significantly enhanced, indicated by enhanced mEPSC frequency. CONCLUSIONS: Our results suggest that postsynaptic AMPAR function in VTA neurons was significantly enhanced after ethanol self-administration. As increased VTA AMPAR function can significantly regulate firing and enhance the reinforcing and activating effects of drugs of abuse, the increased AMPAR activity observed here may facilitate the drive to consume ethanol.