The reinforcing properties of salsolinol in the ventral tegmental area: evidence for regional heterogeneity and the involvement of serotonin and dopamine

Background:  Salsolinol (SAL), the condensation product of acetaldehyde and dopamine, may be a factor contributing to alcohol abuse. Previous research indicated that both ethanol and acetaldehyde are self-administered into the posterior ventral tegmental area (VTA). The current study examined SAL self-infusions into the VTA, and determined the involvement of dopamine neurons and 5-HT₃ receptors in this process.
Methods:  The intracranial self-administration technique was used to determine the self-infusion of SAL into the VTA of adult, male Wistar rats. The rats were placed in 2-lever (active and inactive) experimental chambers, and allowed to respond for the self-infusion of 0, 0.03, 0.1, 0.3, 1.0 or 3.0 μM SAL into the posterior or anterior VTA. In a second experiment, rats self-administered 0.3 μM SAL for the initial 4 sessions, co-administered SAL with ICS-205,930 (a 5-HT₃ receptor antagonist) or quinpirole (a D_2,3 receptor agonist) for sessions 5 and 6, and then only 0.3 μM SAL for session 7.
Results:  Wistar rats, given 0.03 to 0.3 μM SAL, received more infusions per session than did the group given artificial cerebrospinal fluid (aCSF) alone (e.g., 41 infusions for 0.1 μM SAL versus 9 infusions for the aCSF group), and responded more on the active than inactive lever. These effects were observed in the posterior but not in anterior VTA. Co-infusion of 100 μM ICS-205,930, or quinpirole significantly reduced self-infusions and active lever responding.
Conclusions:  SAL produces reinforcing effects in the posterior VTA of Wistar rats, and these effects are mediated by activation of DA neurons and local 5-HT₃ receptors.     

Ethanol increases glutamate neurotransmission in the posterior ventral tegmental area of female wistar rats

Background: The posterior ventral tegmental area (pVTA) mediates the reinforcing and stimulating effects of ethanol (EtOH). Electrophysiological studies indicated that exposure to EtOH increased glutamate synaptic function in the VTA. This study determined the neurochemical effects of both acute and repeated EtOH exposure on glutamate neurotransmission in the pVTA. Methods: Adult female Wistar rats were implanted with microdialysis probes in the pVTA. During microdialysis, rats received acute intraperitoneal (i.p.) injection of saline or EtOH (0.5, 1.0, or 2.0 g/kg), and extracellular glutamate levels were measured in the pVTA. The effects of repeated daily injections of EtOH (0.5, 1.0, or 2.0 g/kg) on basal extracellular glutamate concentrations in the pVTA and on glutamate response to a subsequent EtOH challenge were also examined. Results: The injection of 0.5 g/kg EtOH significantly increased (120 to 125% of baseline), whereas injection of 2.0 g/kg EtOH significantly decreased (80% of baseline) extracellular glutamate levels in the pVTA. The dose of 1.0 g/kg EtOH did not alter extracellular glutamate levels. Seven repeated daily injections of each dose of EtOH increased basal extracellular glutamate concentrations (from 4.1 ± 0.5 to 9.2 ± 0.5 μM) and reduced glutamate clearance in the pVTA (from 30 ± 2 to 17 ± 2%), but failed to alter glutamate response to a 2.0 g/kg EtOH challenge. Conclusions: The results suggest that the low dose of EtOH can stimulate the release of glutamate in the pVTA, and repeated EtOH administration increased basal glutamate transmission in the pVTA, as a result of reduced glutamate clearance.     

Ethanol, glutamate, and the ventral tegmental area--a commentary on: Ding, Engleman, Rodd, and McBride, "ethanol increases glutamate neurotransmission in the posterior ventral tegmental area of female wistar rats"

The recent study by Ding and colleagues (2012) utilized in vivo microdialysis to demonstrate that acute and repeated alcohol administration produces dose-dependent effects on extracellular levels of the glutamate in the posterior ventral tegmental area. These findings have important implications for interactions between glutamatergic and dopaminergic mediation of alcohol reinforcement in the ventral midbrain.     

Operant self-administration of ethanol in Sardinian alcohol-preferring rats

BACKGROUND: "Work" for ethanol, that is, the ability of a laboratory animal to press a lever to gain access to ethanol, has been proposed as (a) a requirement for definition of an animal model of alcoholism and (b) a measure of ethanol-reinforcing properties. The present study evaluated oral self-administration of ethanol under an operant (lever pressing) procedure in selectively bred Sardinian alcohol-preferring (sP) and alcohol-nonpreferring (sNP) rats. METHODS: Rats from both lines were initiated to self-administer 10% ethanol, on a fixed ratio 1 schedule and in daily 30 min sessions, by using the Samson sucrose fading procedure. Subsequently, rats were exposed to increasing concentrations of ethanol up to 30% on a fixed ratio 4 schedule. Finally, the extinction responding for ethanol, defined as the maximal number of lever responses reached by each rat in the absence of ethanol reinforcement, was determined. RESULTS: The results indicated that sP rats acquired and maintained lever pressing for ethanol, self-administering mean amounts of ethanol in the range of 0.6 to 1.1 g/kg/session, which gave rise to mean blood ethanol levels in the 30 to 45 mg% range. Extinction responding for ethanol in sP rats averaged 73. In contrast, once sucrose was faded out, sNP rats displayed minimal levels of responding for ethanol, and extinction responding averaged 6. CONCLUSIONS: The results of the present study extend to the sP/sNP rat lines the finding that ethanol can be established as a reinforcer in selectively bred alcohol-preferring rats, whereas it has modest, if any, reinforcing properties in alcohol-nonpreferring rats.     

Ethanol self-administration is regulated by CB1 receptors in the nucleus accumbens and ventral tegmental area in alcohol-preferring AA rats

Background: Endogenous cannabinoids and their receptors, CB1 receptors in particular, have been implicated in mediation of ethanol reinforcement. Previously, suppression of ethanol drinking by CB1 antagonists has been demonstrated in many experimental paradigms. However, the exact mechanism by which CB1 antagonists modulate ethanol drinking remains elusive. In the present study, we assessed the role of CB1 receptors within the key regions of the mesolimbic dopamine pathway, the nucleus accumbens (NAcc) and ventral tegmental area (VTA), in regulation of ethanol self‐administration. Methods: Adult male alcohol‐prefer AA rats were trained to self‐administer either 10% (w/v) ethanol or 0.1% (w/v) saccharin under an FR1 schedule during daily 30‐minute sessions. Following stable baseline responding, rats were tested after systemic administration of the CB1 antagonist SR141716A (0 to 10 mg/kg) and the agonist WIN55,212‐2 (0 to 2 mg/kg). Separate groups of rats were implanted with bilateral cannulas aimed at the NAcc or VTA, and tested after microinjections of SR141716A (0 to 3 μg) and WIN55,212‐2 (0 to 5 μg) into the NAcc or VTA. The highest intracerebral doses were tested also in rats responding for a 0.1% saccharin solution. Results: SR141617A dose‐dependently suppressed ethanol responding after systemic administration. Microinjections of SR141617A both into NAcc and VTA attenuated ethanol responding. In addition, intra‐NAcc injections of SR141617A suppressed saccharin intake. Although low doses of systemically given WIN55,212‐2 increased ethanol responding, no effects were seen after WIN55,212‐2 microinjections into NAcc or VTA. Conclusions: Bidirectional changes in ethanol self‐administration by the systematically administered CB1 agonist and antagonist show that ethanol reinforcement is controlled by CB1 receptors in alcohol‐preferring AA rats. Replication of the suppressive effects by CB1 antagonism in the NAcc and VTA suggests that endocannabinoids and their receptors mediate ethanol reinforcement through interaction with the mesolimbic dopamine pathway.     

Microdialysis of dopamine in the nucleus accumbens of alcohol-preferring (P) rats during anticipation and operant self-administration of ethanol

BACKGROUND: The present study was designed to test directly whether a contextual stimulus for access to ethanol would acquire the ability to enhance locomotor activity and dopamine efflux in the nucleus accumbens (NAc) of alcohol-preferring (P) rats. The study also explored the association between elevated locomotor activity and NAc dopamine efflux during operant self-administration of ethanol. METHODS: Adult female P rats were randomly assigned to operantly self-administer either 15% (v/v) ethanol or 0.0125% (w/v) saccharin. Both groups were trained in a daily 30-min two-lever concurrent operant task (FR-3) to orally self-administer ethanol or saccharin, with water on the alternate lever. A third (control) group was trained to self-administer water on both levers. All groups were also acclimated in the operant chambers to periods of habituation, anticipation, and postadministration. RESULTS: Compared with controls, the ethanol group, but not the saccharin group, showed significant increases in locomotor activity as well as increased NAc dopamine efflux during the first 10 min of the anticipation period. During the first 10 min of the self-administration period, locomotor activity was significantly increased in both the ethanol and saccharin groups compared with control values. The ethanol group, but not the saccharin group, showed significant increases in NAc dopamine efflux during the 20th and 30th min of the self-administration period and during the first 10 min of the postadministration period. CONCLUSIONS: The findings suggest that acquisition of signal-induced anticipation of self-administered ethanol is associated with increases in locomotor activity and extracellular levels of dopamine in the NAc of P rats. Such associations may be important to the development and maintenance of ethanol-seeking behaviors. The findings also indicate that operant self-administration of ethanol is associated with increases in extracellular levels of dopamine in the NAc of P rats.     

Effects of microinjection of the D2 dopamine antagonist raclopride into the ventral tegmental area on ethanol and sucrose self-administration

From previous microinjection studies, a reciprocal feedback between the nucleus accumbens and the ventral tegmental area (VTA) has been implicated in the reinforcing stimulus actions of ethanol and sucrose. In these studies, the effects of self administration of ethanol or sucrose solutions on maintained responding were similar when a dopamine antagonist was injected in the nucleus accumbens or a dopamine agonist was injected into the VTA. Our study was performed to determine if the effects on responding that had been observed when a dopamine agonist was injected into the nucleus accumbens would occur after an injection of a dopamine antagonist into the VTA. Male, Long-Evans rats were initially trained to lever press using either 10% ethanol or 75% sucrose solutions as the reinforcers. Bilateral guide cannulae were implanted to allow microinjection into the VTA of differing doses of the dopamine D2 antagonist, raclopride. Only at the highest dose tested (10 microg) was any effect observed on responding maintained by either reinforcer. The effect was minimal and different from that observed after the microinjection of a dopamine agonist into the nucleus accumbens. This suggests that either the actions of the nucleus accumbens agonist manipulation involved other processes or that the level of enhanced dopamine release in the nucleus accumbens from the VTA antagonist injection was not sufficient to mimic the effect of the nucleus accumbens agonist injections.     

Ethanol Is Self-Administered Into the Nucleus Accumbens Shell, But Not the Core: Evidence of Genetic Sensitivity

Background:  A previous study indicated that selectively bred alcohol-preferring (P) rats self-administered ethanol (EtOH) directly into the posterior ventral tegmental area at lower concentrations than Wistar rats. The present study was undertaken to determine involvement of the nucleus accumbens (Acb) with EtOH reinforcement, and a relationship between genetic selection for high alcohol preference and sensitivity of the Acb to the reinforcing effects of EtOH.
Methods:  Adult P and Wistar rats were assigned to groups that self-infused 0 to 300 mg% EtOH into the Acb shell (AcbSh) or Acb Core (AcbC). Rats were placed into 2-lever (active and inactive) operant chambers and given EtOH for the first 4 sessions (acquisition), artificial cerebrospinal fluid (aCSF) for sessions 5 and 6 (extinction), and EtOH again in session 7 (reinstatement). Responding on the active lever produced a 100-nl injection of the infusate.
Results:  Alcohol-preferring rats self-infused 75 to 300 mg% EtOH, whereas Wistar rats reliably self-infused 100 and 300 mg% EtOH into the AcbSh. Both P and Wistar rats reduced responding on the active lever when aCSF was substituted for EtOH, and reinstated responding in session 7 when EtOH was restored. EtOH was not self-infused into the AcbC by P or Wistar rats.
Conclusions:  The present results indicate that the AcbSh, but not AcbC, is a neuroanatomical structure that mediates the reinforcing actions of EtOH. The data also suggest that, compared to Wistar rats, the AcbSh of P rats is more sensitive to the reinforcing effects of EtOH.     

Involvement of dopamine D2 autoreceptors in the ventral tegmental area on alcohol and saccharin intake of the alcohol-preferring P rat

BACKGROUND: The ventral tegmental area (VTA) dopamine (DA) system is considered to be involved in mediating the actions of ethanol (EtOH). The objective of the present study was to examine the role of VTA DA D2 receptors in regulating EtOH intake of alcohol-preferring P rats. METHODS: EtOH (10% v/v) and saccharin (SACC, 0.0125% g/v) intake during 2 hr of limited access was assessed after microinjections of the D2 agonist quinpirole and the D2 antagonist sulpiride into the anterior VTA (AVTA) of female P rats. Both EtOH-SACC alternate-day-access conditions and daily availability of EtOH and SACC solutions to separate groups of subjects were used. A second D2 agonist, quinelorane, and coadministration of 2.0 microg sulpiride with 2.0 microg quinpirole were tested in animals given limited access to EtOH. Finally, the effects of quinpirole injected 2 mm dorsal to the VTA and within the posterior VTA (PVTA) were assessed under EtOH-SACC alternate-day-access conditions. RESULTS: Microinjections of 2.0-6.0 microg quinpirole into the AVTA dose dependently decreased EtOH intake 40-80% during the first 30 min of the limited access sessions but did not alter SACC intake. Injections of 2.0-4.0 microg quinelorane into the AVTA also reduced EtOH intake in the first 30 min. Administration of 0.5-2.0 microg sulpiride into the AVTA had no effect on either EtOH or SACC intakes but did attenuate the effects of quinpirole on reducing EtOH intake. Injections of 2.0-4.0 quinpirole 2 mm dorsal to the VTA did not alter EtOH or SACC intakes. Posterior VTA injections of quinpirole decreased EtOH and SACC intakes approximately 25-30% and 60-70%, respectively, in the first 30 min. None of the treatments altered intakes during the 30-120 min period. CONCLUSIONS: The data suggest that DA neuronal activity within the AVTA may be important for maintaining EtOH drinking in P rats, whereas DA neuronal activity within the PVTA may be involved in regulating general drinking and/or motivational behaviors. Overall, the results confirm the involvement of mesolimbic DA in EtOH self-administration and suggest that there is functional heterogeneity within the VTA regulating drinking behavior of the P rat.     

Effect of neuropeptide Y (NPY) on oral ethanol intake in Wistar, alcohol-preferring (P), and -nonpreferring (NP) rats

BACKGROUND: Neuropeptide Y (NPY) deficient mice consume more ethanol than controls, whereas NPY over-expressing mice consume less ethanol than controls. Thus, ethanol drinking may be inversely associated with NPY activity. To determine whether exogenously administered NPY would alter ethanol intake, two experiments were conducted. METHODS: A within-subject design was used with intracerebroventricular (ICV) administration of NPY or artificial cerebral spinal fluid (aCSF) into the lateral ventricles. Infusions were separated by 2 to 7 days. In experiment 1, male Wistar rats (n = 10) were tested for the effects of NPY on an intake of 5% sucrose or 8% (w/v) ethanol during daily 2-hr testing periods with food and water available at all other times. In experiment 2, male alcohol-preferring (P) and alcohol-nonpreferring (NP) rats (n = 8/line) were tested for the effects of NPY on 8% (w/v) ethanol intake. RESULTS: In experiment 1, NPY (5, 10, 20 microg) significantly increased sucrose intake relative to aCSF baseline in Wistar rats, a finding consistent with previous observations of the orexigenic effects of the peptide. However, NPY (10 microg) did not alter ethanol intake in Wistar rats. In experiment 2, NPY (5 and 10 microg) significantly decreased ethanol intake in P rats, but not in NP rats. CONCLUSION: The reduction in ethanol intake seen with the P rats is consistent with the postulated negative relationship between NPY activity and ethanol intake. The lack of effect of NPY on ethanol intake in Wistar and NP rats may be related to the lower baseline levels of ethanol intake in these rats or to differential central nervous system basal NPY activity or sensitivity to the peptide.     

Alcohol-related olfactory cues activate the nucleus accumbens and ventral tegmental area in high-risk drinkers: preliminary findings

BACKGROUND: The mesocorticolimbic dopamine system is implicated in motivation and reward and may be involved in the development of alcoholism. METHODS: We used functional magnetic resonance imaging to study the blood oxygen level-dependent (BOLD) response to alcohol-related olfactory stimuli (AROS; odors of beer and whiskey) and non-alcohol-related olfactory stimuli (NAROS; odors of grass and leather) in 10 high-risk (HR) drinkers (average drinks per week, 19.99; SD, 6.99; all with > or = 2 first- or second-degree alcoholic relatives) and 5 low-risk (LR) social drinking controls (drinks per week, 2.82; SD, 2.87; 1 subject had 1 second-degree alcoholic relative). Data were analyzed with SPM99 and random effects analysis by using regions of interest and corrected cluster statistics (p < 0.05) to focus on the nucleus accumbens (NAc) and ventral tegmental area (VTA). RESULTS: In HR subjects, there was a greater BOLD signal increase in the NAc during AROS than during clean air. BOLD signal increases during AROS were also greater in the NAc than the signal increases induced by NAROS. The AROS signal was significantly greater than the NAROS signal in a small number of voxels in the VTA. Finally, the AROS/NAROS difference signal was larger in HR drinkers in both the NAc and VTA. CONCLUSIONS: Alcoholic olfactory cues may invoke the dopaminergic mesocorticolimbic system to a greater degree than nonalcoholic odors and could be effective tools in exploring the role of the dopamine system in susceptibility to alcoholism.     

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 α‐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.     

Effects of ethanol exposure on subsequent acquisition and extinction of ethanol self-administration and expression of alcohol-seeking behavior in adult alcohol-preferring (P) rats: II. Adult exposure

Background In a preceding study, we reported that ethanol (EtOH) consumption during periadolescence in alcohol‐preferring (P) rats produced significant effects on the acquisition, extinction, Pavlovian spontaneous recovery (PSR), and reacquisition of operant self‐administration of EtOH. The objective of the present study was to determine if EtOH consumption during adulthood produced similar effects on subsequent operant behaviors. Methods Adult female P rats (>135 days of age) were given 24 hr free‐choice access to 15% EtOH for 30 days or were similarly housed and received water only. After a 15 day period of no EtOH access and without any prior training, adult alcohol drinking and adult alcohol‐naïve rats were placed in standard two‐lever (15% EtOH and water) chambers to examine acquisition of EtOH self‐administration. After stable responding was established on a concurrent fixed ratio (FR) 5 FR1 schedule for EtOH versus water, the P rats underwent extinction training for nine sessions. After extinction and a 2 week home cage period (with no operant sessions or access to EtOH), rats were returned to the operant chambers in the absence of reward for seven consecutive sessions to test for PSR. After PSR testing, animals were maintained in their home cage for a week, before being reintroduced to the operant chambers and allowed to respond for EtOH and water. Results Both the adult alcohol‐drinking and adult alcohol‐naïve groups rapidly acquired EtOH self‐administration, expressed a pronounced PSR, which was augmented by EtOH priming and the presence of a discriminative stimulus (odor cue), and increased responding when EtOH was reinstated. Adult pre‐exposure to EtOH did not alter any of the operant measures. Conclusions The results of this study suggest that, unlike the results with EtOH pre‐exposure during periadolescence, chronic alcohol drinking by P rats in adulthood did not produce sufficient long‐lasting changes in neuronal function to alter subsequent operant acquisition of alcohol self‐administration, alcohol relapse, or alcohol‐seeking behavior.     

Neuropeptide Y reduces oral ethanol intake in alcohol-preferring (P) rats following a period of imposed ethanol abstinence

BACKGROUND: Intracerebroventricular infusion of NPY has been shown to reduce ethanol intake in alcohol-preferring (P) rats in a limited access procedure. The purpose of the present investigation was to extend this finding to a two-bottle free-choice continuous access procedure in groups of rats that either did or did not undergo a period of imposed ethanol abstinence and ethanol reinstatement. METHODS: In experiment 1, female P rats were given 6 weeks of continuous access to ethanol (8% w/v) and water. Ethanol was removed for a period of 2 weeks during which the rats were surgically implanted with a cannula into the lateral ventricle. Following the ethanol abstinence period and immediately before ethanol reinstatement, rats received a single infusion of either artificial cerebrospinal fluid or NPY (10 microg). Ethanol and water intake was measured at both 4 hr and 24 hr after infusion, and 24-hr intake measures were taken daily for 13 postinfusion days. Experiment 2 was run in parallel with experiment 1, with the exception that rats did not undergo a period of imposed ethanol abstinence. Also, food intake was measured 4 and 24 hr after infusion. RESULTS: Following 2 weeks of imposed ethanol abstinence (experiment 1), NPY suppressed ethanol intake through postinfusion day 2. After uninterrupted continuous access to ethanol (experiment 2), NPY suppressed ethanol intake to a lesser extent and this effect lasted only 24 hr. NPY increased food intake at the 4-hr but not the 24-hr measure. CONCLUSIONS: Previous findings that central administration of NPY suppresses ethanol intake in P rats are extended by this study to a continuous access procedure, and the effect is amplified following a period of imposed ethanol abstinence. This effect of NPY compares favorably to results obtained with other treatments tested in similar animal models and provides support for a role of NPY in an allostasis model of addiction.     

The effects of neuropeptide S on ethanol drinking and other related behaviors in alcohol-preferring and -nonpreferring rats

Background.  Neuropeptide S (NPS) is a 20-amino-acid peptide, identified in the brain and periphery, that is reported to regulate arousal, anxiety, and feeding behavior. Studies were conducted to determine whether this peptide would alter ethanol intake, sucrose intake, anxiety, and general motor activity in alcohol-preferring (P) and -nonpreferring (NP) rats.
Methods.  Experiment 1 : P and NP rats were given 8 weeks of continuous access to ethanol (15% w/v) and water. All rats were implanted with a cannula aimed at either the left or right lateral ventricle and 1 week later were infused with NPS (0.075, 0.3, 1.2 nmol) or artificial cerebrospinal fluid (aCSF) and tested for ethanol, food, and water intake. Experiment 2 : The same doses of NPS were administered to a group of P rats and intake of 2.5% (w/v) sucrose was measured. Experiment 3 : Infusions of NPS (1.2 nmol) or aCSF were administered to P rats prior to a 5-minute test on an elevated plus maze. Experiment 4 : Ethanol naive P and NP rats were infused with NPS (0.075, 0.15, 0.3, 0.6, and 1.2 nmol) or aCSF prior to a 20-minute test in activity monitors.
Results.  NPS reduced ethanol intake in P, but not in NP rats. It did not influence sucrose solution intake in P rats. However, an increase in food intake was seen in both rat lines following lower doses of the peptide. NPS did neither alter anxiety-like behavior in the elevated plus maze test nor was there an effect on general motor activity; however, there was an increase in the amount of time spent in the center of the activity monitors following infusions of 0.6 nmol of NPS in P, but not in NP rats, indicating anxioltyic actions of the peptide.
Conclusions.  These data suggest a role for NPS in the modulation of ethanol drinking and possibly anxiety-like behavior in rats selectively bred for high alcohol drinking.     

Salsolinol produces reinforcing effects in the nucleus accumbens shell of alcohol-preferring (P) rats

BACKGROUND: The formation of salsolinol (SAL) has been hypothesized to be a factor contributing to alcoholism and alcohol abuse. If SAL is formed under chronic alcohol-drinking conditions, then it may contribute to alcohol addiction by being rewarding itself. Because SAL can be formed by the nonenzymatic condensation of acetaldehyde with dopamine, the reinforcing effects of SAL were tested in the nucleus accumbens shell, a dopamine-rich site considered to be involved in regulating alcohol-drinking behavior. METHODS: The intracranial self-administration technique was used to test the reinforcing properties of SAL. Adult, female alcohol-preferring (P) rats were stereotaxically implanted with guide cannulae aimed at the nucleus accumbens shell. After 7 to 10 days to allow recovery from surgery, P rats were attached to the electrolytic microinfusion transducer system, placed in two-lever experimental chambers, and allowed to respond for the self-infusion of 100 nl of modified artificial cerebrospinal fluid (aCSF) or 0.03, 0.3, 3.0, or 12.5 microM SAL (3-1250 fmol/100 nl). Sessions were 4 hr in duration and were conducted in the dark cycle every 48 hr. The effects of coinfusing 10 to 400 microM sulpiride (given in sessions 5 and 6 after four acquisition sessions) on the intracranial self-administration of 3.0 microM SAL were tested in a separate experiment. RESULTS: P rats given 0.3 to 12.5 microM SAL received significantly more infusions per session than did the group given aCSF alone (e.g., 50 infusions for 3.0 microM SAL versus 10 or fewer infusions for the aCSF group) and responded significantly more on the active than inactive lever. Coinfusion of 100 or 400 microM sulpiride reduced the responding on the active lever (80-100 responses/session without sulpiride) to levels observed for the inactive lever (fewer than 10 responses/session with sulpiride). This effect was reversible because giving SAL alone in session 7 reinstated responding on the active lever. CONCLUSIONS: SAL is reinforcing in the nucleus accumbens shell of P rats at concentrations that are pharmacologically possible, and these reinforcing actions are mediated in part by D2/D3-like receptors.     

Opioidergic modulation of ethanol self-administration in the ventral pallidum

Background: Striatopallidal medium spiny neurons have been viewed as a final common path for drug reward and the ventral pallidum as an essential convergent point for hedonic and motivational signaling in the brain. The medium spiny neurons are GABAergic, but they colocalize enkephalin. Purpose of this study was to investigate the role of the opioidergic mechanisms of the ventral pallidum in ethanol self‐administration behavior. Methods: Effects of bilateral microinjections of μ‐, δ‐, and κ‐opioid receptor agonists and antagonists into the ventral pallidum on voluntary ethanol consumption were monitored in alcohol‐preferring Alko Alcohol (AA) rats using the 90‐minute limited access paradigm. Results: Stimulation of μ‐opioid receptors with DAMGO (0.01 to 0.1 μg) or morphine (1 to 10 μg) in the ventral pallidum decreased ethanol intake dose‐dependently. Conversely, blocking μ‐receptors with CTOP (0.3 to 3 μg) increased ethanol intake significantly. Unlike CTOP, DAMGO also increased locomotor activity. Consumption of ethanol was not modified significantly by a broad‐spectrum opioid receptor antagonist naltrexone, by δ‐opioid receptor agonist DPDPE or antagonist naltrindole, or by a κ‐opioid receptor agonist U50,488H or antagonist nor‐BNI. Conclusions: The study provides evidence for μ‐ but not δ‐ or κ‐opioid receptors in the ventral pallidum playing a role in the regulation of voluntary ethanol consumption. Furthermore, present findings give support to earlier work, suggesting an essential role of pallidal opioidergic transmission in drug reward.     

Chronic ethanol drinking by alcohol-preferring rats increases the sensitivity of the posterior ventral tegmental area to the reinforcing effects of ethanol

BACKGROUND: The ventral tegmental area (VTA) is involved in regulating ethanol drinking, and the posterior VTA seems to be a neuroanatomical substrate that mediates the reinforcing effects of ethanol in ethanol-naive Wistar and ethanol-naive alcohol-preferring (P) rats. The objective of this study was to test the hypothesis that chronic ethanol drinking increases the sensitivity of the posterior VTA to the reinforcing effects of ethanol. METHODS: Two groups of female P rats (one given water as its sole source of fluid and the other given 24-hr free-choice access to 15% ethanol and water for at least 8 weeks) were stereotaxically implanted with guide cannulae aimed at the posterior VTA. One week after surgery, rats were placed in standard two-lever (active and inactive) operant chambers and connected to the microinfusion system. Depression of the active lever produced the infusion of 100 nl of artificial cerebrospinal fluid (CSF) or ethanol. The ethanol-naive and chronic ethanol-drinking groups were assigned to subgroups to receive artificial CSF or 25, 50, 75, or 125 mg/dl of ethanol (n = 6-9/dose/group) to self-infuse (FR1 schedule) during the 4-hr sessions given every other day. RESULTS: Compared with the infusions of artificial CSF, the control group reliably (p < 0.05) self-infused 75 and 125 mg/dl of ethanol but not the lower concentrations. The ethanol-drinking group had significantly (p < 0.05) higher self-infusions of 50, 75, and 125 mg/dl of ethanol than artificial CSF during the four acquisition sessions; the number of infusions of all three doses was higher in the ethanol-drinking group than in the ethanol-naive group. Both groups decreased responding on the active lever when artificial CSF was substituted for ethanol, and both groups demonstrated robust reinstatement of responding on the active lever when ethanol was restored. CONCLUSIONS: Chronic ethanol drinking by P rats increased the sensitivity of the posterior VTA to the reinforcing effects of ethanol.     

Comparison of Alcohol-Preferring and Nonpreferring Selectively Bred Rat Lines. I. Ethanol Initiation and Limited Access Operant Self-Administration

Several lines of alcohol-preferring and alcohol-nonpreferring rats have been developed using selective breeding based on 24-hr homecage ethanol consumption. However, it remains unclear if the selection based on two-bottle choice resulted in similar ethanol self-administration when measured using an operant procedure. In this paper, we compare our previous work using alcohol-accepting (AA) and alcohol-nonaccepting (ANA) rats with data obtained using the identical procedures in the (P) and (NP) rat lines, and both replicate lines of the high alcohol drinking (HAD1 and HAD2) and low alcohol drinking (LAD1 and LAD2) lines. All rats from each line were initiated to self-administer 10% ethanol using the sucrose fading procedure. After initiation, increasing concentrations of ethanol up to 30% ethanol were tested. The results indicated that only in the LAD1 and LAD2 lines was ethanol presentation not able to maintain lever pressing after initiation. Compared with the AA line, the P, HAD1, HAD2, and NP lines all self-administered more ethanol in the operant paradigm after initiation. The ANA line self-administered less ethanol than the AA line, but more than the LAD lines. Correlational analysis of homecage consumption with operant ethanol self-administration suggested that -62% of the genetic variance in operant self-administration resulted from genes selected for the homecage drinking. At the same time, it was clear that there were genetic influences on operant self-administration that were not selected for by homecage ethanol drinking.     

Different sensitivity to ethanol in alcohol-preferring sP and -nonpreferring sNP rats

BACKGROUND AND OBJECTIVES: Clinical research has proposed that initial sensitivity to ethanol may be negatively correlated with levels of subsequent ethanol intake; consistently, alcohol-preferring P rats were found to be less sensitive to the ataxic and sedative/hypnotic effects of ethanol than -nonpreferring NP rats. The present study investigated the initial sensitivity to the ataxic and sedative/hypnotic effects of ethanol and to the sedative/hypnotic effects of pentobarbital and diazepam in selectively bred Sardinian alcohol-preferring sP and -nonpreferring sNP rats. METHODS: In experiment 1, time to lose (onset) and regain (sleep time) the righting reflex after the acute intraperitoneal (ip) administration of 3.0 and 3.5 g/kg ethanol were measured in sP and sNP rats. In experiment 2, sP and sNP rats were required to perform a motor coordination task on a Rota-Rod after the acute intragastric administration of 2.0, 2.5, and 3.0 g/kg ethanol. Experiment 3 assessed onset and sleep time in sP and sNP rats after the acute injection of pentobarbital (40 mg/kg; ip) and diazepam (15 and 20 mg/kg; ip). RESULTS: In experiment 1, sP rats took shorter times to lose the righting reflex and regained this reflex over longer periods of time and at lower blood ethanol levels than sNP rats. In experiment 2, ethanol affected motor coordination to a greater extent in sP than sNP rats. In contrast, results from experiment 3 showed that sP and sNP rats were not differentially sensitive to the sedative/hypnotic effects of pentobarbital and diazepam. CONCLUSIONS: The results of experiments 1 and 2 suggest that sP rats possess a genetically determined, greater sensitivity to the motor impairing and sedative/hypnotic effects of ethanol than sNP rats. Although caution should be adopted before hypothesizing any comparison to humans, these results may feature sP rats as an experimental model of those subsets of human alcoholics with initial high sensitivity to ethanol challenges. Finally, the results of experiment 3 suggest a minimal involvement of the benzodiazepine and barbiturate recognition sites in the differential sensitivity to ethanol of sP and sNP rats.