Injected muscimol in pedunculopontine tegmental nucleus alters ethanol self-administration

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

Subregion and sex differences in ethanol activation of cholinergic and glutamatergic cells in the mesopontine tegmentum

Ethanol engages cholinergic signaling and elicits endogenous acetylcholine release. Acetylcholine input to the midbrain originates from the mesopontine tegmentum (MPT), which is composed of the laterodorsal tegmentum (LDT) and the pedunculopontine tegmental nucleus (PPN). We investigated the effect of acute and chronic ethanol administration on cholinergic and glutamatergic neuron activation in the PPN and LDT in male and female mice. We show that ethanol activates neurons of the PPN and not the LDT in male mice. Chronic 15 daily injections of 2 g/kg ethanol induced Fos expression in cholinergic and glutamatergic PPN neurons in male mice, whereas ethanol did not increase cholinergic and glutamatergic neuronal activation in the LDT. A single acute 4 g/kg injection, but not a single 2 g/kg injection, induced cholinergic neuron activation in the male PPN but not the LDT. In contrast, acute or chronic ethanol at either dose or duration had no effect on the activation of cholinergic or glutamatergic neurons in the MPT of female mice. Female mice had higher baseline level of activation in cholinergic neurons compared with males. We also found a population of co-labeled cholinergic and glutamatergic neurons in the PPN and LDT which were highly active in the saline- and ethanol-treated groups in both sexes. These findings illustrate the complex differential effects of ethanol across dose, time point, MPT subregion and sex.

Subregion and sex differences in ethanol activation of cholinergic and glutamatergic cells in the mesopontine tegmentum

Ethanol engages cholinergic signaling and elicits endogenous acetylcholine release. Acetylcholine input to the midbrain originates from the mesopontine tegmentum (MPT), which is composed of the laterodorsal tegmentum (LDT) and the pedunculopontine tegmental nucleus (PPN). We investigated the effect of acute and chronic ethanol administration on cholinergic and glutamatergic neuron activation in the PPN and LDT in male and female mice. We show that ethanol selectively activates neurons of the PPN and not the LDT in male mice. Acute 4.0 g/kg and chronic 15 daily injections of 2.0 g/kg i.p. ethanol induced Fos expression in cholinergic and glutamatergic PPN neurons in male mice, whereas cholinergic and glutamatergic neurons of the LDT were unresponsive. In contrast, acute or chronic ethanol at either dose or duration had no effect on the activation of cholinergic or glutamatergic neurons in the MPT of female mice. Female mice had higher level of baseline activation in cholinergic neurons compared with males. We also found a population of co-labeled cholinergic and glutamatergic neurons in the PPN and LDT which were highly active in the saline- and ethanol-treated groups in both sexes. These findings illustrate the complex differential effects of ethanol across dose, time point, MPT subregion and sex.

The pedunculopontine tegmental nucleus-A functional hypothesis from the comparative literature

We present data from animal studies showing that the pedunculopontine tegmental nucleus-conserved through evolution, compartmentalized, and with a complex pattern of inputs and outputs-has functions that involve formation and updates of action-outcome associations, attention, and rapid decision making. This is in contrast to previous hypotheses about pedunculopontine function, which has served as a basis for clinical interest in the pedunculopontine in movement disorders. Current animal literature points to it being neither a specifically motor structure nor a master switch for sleep regulation. The pedunculopontine is connected to basal ganglia circuitry but also has primary sensory input across modalities and descending connections to pontomedullary, cerebellar, and spinal motor and autonomic control systems. Functional and anatomical studies in animals suggest strongly that, in addition to the pedunculopontine being an input and output station for the basal ganglia and key regulator of thalamic (and consequently cortical) activity, an additional major function is participation in the generation of actions on the basis of a first-pass analysis of incoming sensory data. Such a function-rapid decision making-has very high adaptive value for any vertebrate. We argue that in developing clinical strategies for treating basal ganglia disorders, it is necessary to take an account of the normal functions of the pedunculopontine. We believe that it is possible to use our hypothesis to explain why pedunculopontine deep brain stimulation used clinically has had variable outcomes in the treatment of parkinsonism motor symptoms and effects on cognitive processing. © 2016 International Parkinson and Movement Disorder Society.

Deep brain stimulation of different pedunculopontine targets in a novel rodent model of parkinsonism

The pedunculopontine tegmental nucleus (PPTg) has been proposed as a target for deep brain stimulation (DBS) in parkinsonian patients, particularly for symptoms such as gait and postural difficulties refractory to dopaminergic treatments. Several patients have had electrodes implanted aimed at the PPTg, but outcomes have been disappointing, with little evidence that gait and posture are improved. The PPTg is a heterogeneous structure. Consequently, exact target sites in PPTg, possible DBS mechanisms, and potential benefits still need systematic investigation in good animal models. We have investigated the role of PPTg in gait, developed a refined model of parkinsonism including partial loss of the PPTg with bilateral destruction of nigrostriatal dopamine neurons that mimics human pathophysiology, and investigated the effect of DBS at different PPTg locations on gait and posture using a wireless device that lets rats move freely while receiving stimulation. Neither partial nor complete lesions of PPTg caused gait deficits, underlining questions raised previously about the status of PPTg as a motor control structure. The effect of DBS in the refined and standard model of parkinsonism were very different despite minimal behavioral differences in nonstimulation control conditions. Anterior PPTg DBS caused severe episodes of freezing and worsened gait, whereas specific gait parameters were mildly improved by stimulation of posterior PPTg. These results emphasize the critical importance of intra-PPTg DBS location and highlight the need to take PPTg degeneration into consideration when modeling parkinsonian symptoms. They also further implicate a role for PPTg in the pathophysiology of parkinsonism.

Assessing ethanol's actions in the suprachiasmatic circadian clock using in vivo and in vitro approaches

Research over the past decade has demonstrated substantial interactions between the circadian system and the processes through which alcohol affects behavior and physiology. Here we summarize the results of our collaborative efforts focused on this intersection. Using a combination of in vivo and in vitro approaches, we have shown that ethanol affects many aspects of the mammalian circadian system, both acutely as well as after chronic administration. Conversely, we have shown circadian influences on ethanol consumption. Importantly, we are beginning to delve into the cellular mechanisms associated with these effects. We are also starting to form a picture of the neuroanatomical bases for many of these actions. Finally, we put our current findings into perspective by suggesting new avenues of inquiry for our future efforts.

Acamprosate-responsive brain sites for suppression of ethanol intake and preference

Acamprosate suppresses alcohol intake and craving in recovering alcoholics; however, the central sites of its action are unclear. To approach this question, brain regions responsive to acamprosate were mapped using acamprosate microimplants targeted to brain reward and circadian areas implicated in alcohol dependence. mPer2 mutant mice with nonfunctional mPer2, a circadian clock gene that gates endogenous timekeeping, were included, owing to their high levels of ethanol intake and preference. Male wild-type (WT) and mPer2 mutant mice received free-choice (15%) ethanol/water for 3 wk. The ethanol was withdrawn for 3 wk and then reintroduced to facilitate relapse. Four days before ethanol reintroduction, mice received bilateral blank or acamprosate-containing microimplants releasing ∼50 ng/day into reward [ventral tegmental (VTA), peduculopontine tegmentum (PPT), and nucleus accumbens (NA)] and circadian [intergeniculate leaflet (IGL) and suprachiasmatic nucleus (SCN)] areas. The hippocampus was also targeted. Circadian locomotor activity was measured throughout. Ethanol intake and preference were greater in mPer2 mutants than in wild-type (WT) mice (27 g·kg(-1)·day(-1) vs. 13 g·kg(-1)·day(-1) and 70% vs. 50%, respectively; both, P < 0.05). In WTs, acamprosate in all areas, except hippocampus, suppressed ethanol intake and preference (by 40-60%) during ethanol reintroduction. In mPer2 mutants, acamprosate in the VTA, PPT, and SCN suppressed ethanol intake and preference by 20-30%. These data are evidence that acamprosate's suppression of ethanol intake and preference are manifest through actions within major reward and circadian sites.

Genetical genomic determinants of alcohol consumption in rats and humans

BACKGROUND

We have used a genetical genomic approach, in conjunction with phenotypic analysis of alcohol consumption, to identify candidate genes that predispose to varying levels of alcohol intake by HXB/BXH recombinant inbred rat strains. In addition, in two populations of humans, we assessed genetic polymorphisms associated with alcohol consumption using a custom genotyping array for 1,350 single nucleotide polymorphisms (SNPs). Our goal was to ascertain whether our approach, which relies on statistical and informatics techniques, and non-human animal models of alcohol drinking behavior, could inform interpretation of genetic association studies with human populations.

RESULTS

In the HXB/BXH recombinant inbred (RI) rats, correlation analysis of brain gene expression levels with alcohol consumption in a two-bottle choice paradigm, and filtering based on behavioral and gene expression quantitative trait locus (QTL) analyses, generated a list of candidate genes. A literature-based, functional analysis of the interactions of the products of these candidate genes defined pathways linked to presynaptic GABA release, activation of dopamine neurons, and postsynaptic GABA receptor trafficking, in brain regions including the hypothalamus, ventral tegmentum and amygdala. The analysis also implicated energy metabolism and caloric intake control as potential influences on alcohol consumption by the recombinant inbred rats. In the human populations, polymorphisms in genes associated with GABA synthesis and GABA receptors, as well as genes related to dopaminergic transmission, were associated with alcohol consumption.

CONCLUSION

Our results emphasize the importance of the signaling pathways identified using the non-human animal models, rather than single gene products, in identifying factors responsible for complex traits such as alcohol consumption. The results suggest cross-species similarities in pathways that influence predisposition to consume alcohol by rats and humans. The importance of a well-defined phenotype is also illustrated. Our results also suggest that different genetic factors predispose alcohol dependence versus the phenotype of alcohol consumption.

Behavioral characteristics and neurobiological substrates shared by Pavlovian sign-tracking and drug abuse

Drug abuse researchers have noted striking similarities between behaviors elicited by Pavlovian sign-tracking procedures and prominent symptoms of drug abuse. In Pavlovian sign-tracking procedures, repeated paired presentations of a small object (conditioned stimulus, CS) with a reward (unconditioned stimulus, US) elicits a conditioned response (CR) that typically consists of approaching the CS, contacting the CS, and expressing consummatory responses at the CS. Sign-tracking CR performance is poorly controlled and exhibits spontaneous recovery and long-term retention, effects that resemble relapse. Sign-tracking resembles psychomotor activation, a syndrome of behavioral responses evoked by addictive drugs, and the effects of sign-tracking on corticosterone levels and activation of dopamine pathways resemble the neurobiological effects of abused drugs. Finally, the neurobiological profile of individuals susceptible to sign-tracking resembles the pathophysiological profile of vulnerability to drug abuse, and vulnerability to sign-tracking predicts vulnerability to impulsive responding and alcohol self-administration. Implications of sign-tracking for models of drug addiction are considered.

Intermittent presentations of ethanol sipper tube induce ethanol drinking in rats

AIMS

Intermittent presentations of the ethanol sipper have been reported to induce more ethanol drinking in rats than when the ethanol sipper was continuously available during the session. This intermittent sipper effect was observed in a social drinking situation, in which subjects experienced intermittent opportunities to interact briefly with a conspecific rat. The objective of this study was to evaluate the effects of the intermittent sipper procedure in situations providing for intermittent presentations of food, and, in addition, in situations that do not provide for intermittent presentations of another rewarding event.

METHODS

Four groups of male Long-Evans hooded rats, arranged in a 2 x 2 factorial design with two levels of Sipper Procedure (Intermittent vs Continuous) and two levels of Food procedure (Food vs No Food), were trained in drinking chambers. During each daily session, Intermittent Sipper groups received access to the ethanol sipper during each of 25 trials of 10 s each, while Continuous Sipper groups had access to the ethanol sipper during the entire session (approximately 30 min). During each session, Food groups received 25 presentations of food pellets while No Food groups received no food pellets. Ethanol concentrations in the sipper [3, 4, 6, 8, and 10% (vol./vol.)] increased across sessions.

RESULTS

More rapid escalation of ethanol intake was observed in the Intermittent Sipper groups than in the Continuous Sipper groups, and this effect was observed in both the Food and No Food conditions (P's < 0.05), which did not differ from one another.

CONCLUSIONS

Intermittent Sipper procedures provide less access to the ethanol sipper, yet induced more ethanol drinking than Continuous Sipper procedures. The intermittent sipper effect is not dependent on presentations of food. Implications for schedule-induced polydipsia and Pavlovian autoshaping are discussed.

The effect of baclofen alone and in combination with naltrexone on ethanol consumption in the rat

Naltrexone has been evaluated in preclinical animal models of ethanol consumption and found to be effective in most reports. In clinical use, naltrexone has not proved to be as efficacious in preventing relapse. While naltrexone targets opioid receptors, many other neurotransmitter systems are targeted by ethanol and, to a greater or lesser extent, contribute to modulating ethanol's reinforcing effects. There has been indication that drugs active at the gamma amino butyric acid B (GABAB) receptors can affect the self-administration of many drugs with abuse potential. The experiments reported here evaluated the effect of three doses of baclofen (2.5, 5.0, or 7.5 mg/kg), a GABAB agonist, administered alone or in combination with a single dose of naltrexone (1.0 mg/kg). In Experiment 1, both naltrexone and baclofen, at the two higher doses tested, significantly reduced ethanol consumption in Wistar rats using a limited access procedure on Drug Days 1 and 2. When combined on Drug Days 3 and 4, baclofen/naltrexone was significantly more effective in reducing ethanol consumption than did either drug alone. Neither drug, alone or in combination, had an effect on water consumption. In Experiment 2, both baclofen and naltrexone again significantly reduced ethanol consumption, with no evidence that chronic administration across Drug Days 3 and 4 further reduced consumption compared with Drug Days 1 and 2. The clinical use of multiple pharmacotherapeutic agents in combination may allow for the use of lower doses of individual components, thereby reducing the negative side effects that contribute to lower compliance and higher relapse.

Afferent modulation of dopamine neuron firing differentially regulates tonic and phasic dopamine transmission

The mesolimbic dopamine system is centrally involved in reward and goal-directed behavior, and it has been implicated in multiple psychiatric disorders. Understanding the mechanism by which dopamine participates in these activities requires comprehension of the dynamics of dopamine release. Here we report dissociable regulation of dopamine neuron discharge by two separate afferent systems in rats; inhibition of pallidal afferents selectively increased the population activity of dopamine neurons, whereas activation of pedunculopontine inputs increased burst firing. Only the increase in population activity increased ventral striatal dopamine efflux. After blockade of dopamine reuptake, however, enhanced bursting increased dopamine efflux three times more than did enhanced population activity. These results provide insight into multiple regulatory systems that modulate dopamine system function: burst firing induces massive synaptic dopamine release, which is rapidly removed by reuptake before escaping the synaptic cleft, whereas increased population activity modulates tonic extrasynaptic dopamine levels that are less influenced by reuptake.

Glutamate and GABA modulate dopamine in the pedunculopontine tegmental nucleus

The pedunculopontine tegmental nucleus (PPTg) has an important anatomical position connecting basal ganglia and limbic systems with motor execution structures in the pons and spinal cord. It receives glutamatergic and GABAergic input and has additional reciprocal connections with mesencephalic dopaminergic neurons, suggesting that the PPTg plays a key role in frontostriatal information processing. In vivo microdialysis in freely moving rats, in combination with behavioral analysis, was used in this study to investigate whether the dopaminergic input can be modulated at the level of the PPTg via N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) or GABA(B) receptors. Stimulation of the GABA(B) receptor decreased dopamine release in the PPTg while that of the AMPA and NMDA receptors increased it. A time-related comparison of the effects of NMDA (0.75 and 1 mM) and AMPA (50 and 25 microM) revealed a more long-lasting effect after AMPA stimulation than after NMDA. However, only the infusion of the GABA(B) receptor agonist baclofen (100 and 200 microM) stimulated stereotyped behavior (e.g. sniffing, digging or head movements) and contralateral circling. This study clearly demonstrates that GABAergic as well as glutamatergic terminals in the PPTg are critically involved in the modulation of the dopamine system. Moreover, a decrease in PPTg dopamine via GABA(B) receptor stimulation seems to be behaviorally relevant.

GABA(A) receptor modulation of the rewarding and aversive effects of ethanol

Ethanol has been shown to exert many of its biochemical and behavioral effects through an interaction with the gamma-aminobutyric acid (GABA) receptor system. This review focuses on a subset of studies that has used self-administration, as well as place and taste conditioning, procedures to investigate a role for the GABA(A) receptor system in modulating the rewarding and aversive effects of ethanol. Potential advantages and disadvantages of each procedure are also discussed. A significant amount of evidence supports the suggestion that GABA(A) receptors are important modulators of the motivational effects of ethanol, although most of the findings have been obtained from studies examining oral ethanol self-administration. Relatively fewer studies have investigated ethanol place and taste conditioning. All self-administration studies reviewed used rats, whereas most conditioning studies used mice. Results of these studies show that GABA(A) antagonists and inverse agonists reduce ethanol self-administration under limited-access conditions. The effect of GABA(A) agonists on ethanol self-administration is less clear due to their bidirectional effects. GABA(A) receptor antagonists have been shown to increase ethanol-induced conditioned place preference and conditioned taste aversion in mice and decrease ethanol-induced conditioned taste aversion in rats. Issues related to interpretation and integration of these findings across models and species are considered. The integration of data from self-administration and conditioning procedures is necessary to define the role of GABA(A) receptors in modulating the rewarding and aversive effects of ethanol and may lead to the development of pharmacotherapies that target GABA(A) receptors to treat alcoholism in human beings.

Muscimol injected into the medial prefrontal cortex of the rat alters ethanol self-administration

The role of the rodent prefrontal cortex in the regulation of ethanol self-administration has not been widely explored. Understanding the role of GABAergic transmission in this area in relation to ethanol self-administration is important, as the GABA system may be one of several targets for alcohol's actions in the brain. Rats were initiated to drink 10% ethanol from a dipper using a sucrose-substitution procedure. When baseline behavior was stable, bilateral microinjections of muscimol (a GABA(A) agonist) into the prefrontal cortex were tested at doses of 17.5, 30, 100 and 300 ng/microl. Ethanol self-administration was decreased by approximately 40% at the 30-ng dose and 30% at the 100-ng dose. No effects were observed at either the 17.5- or 300-ng dose. The effect on the pattern of self-administration was to shorten the size of the first run of drinking without affecting the rate of drinking. The hypothesis is put forward that the injections increased glutamatergic output to the nucleus accumbens (nAcc) that in turn increased accumbens output. This increased output is proposed as similar to the effects of dopaminergic (DA) manipulations within this system.