Comparison of the sedative, cognitive, and analgesic effects of nitrous oxide, sevoflurane, and ethanol

Combined effects of mild hypothermia and nitrous-oxide-induced narcosis on manual and cognitive performance

Divers are at enhanced risk of suffering from acute cognitive deterioration because of the low ambient temperatures and the narcotic action of inert gases inspired at high pressures. Yet, the behavioral effects of cold and inert gas narcosis have commonly been assessed in isolation and during short-term provocations. We therefore evaluated the interactive influence of mild hypothermia and narcosis engendered by a subanesthetic dose of nitrous oxide (N2O; a normobaric intervention analog of hyperbaric nitrogen) on cognitive function during prolonged iterative exposure. Fourteen men partook in two ∼12-h sessions (separated by ≥4 days), wherein they performed sequentially three 120-min cold (20°C) water immersions (CWIs), while inhaling, in a single-blinded manner, either normal air or a normoxic gas mixture containing 30% N2O. CWIs were separated by a 120-min rewarming in room-air breathing conditions. Before the first CWI and during each CWI, subjects performed a finger dexterity test, and the Spaceflight Cognitive Assessment Tool for Windows (WinSCAT) test assessing aspects of attention, memory, learning, and visuospatial ability. Rectal and skin temperatures were, on average, reduced by ∼1.2 °C and ∼8 °C, respectively (P < 0.001). Cooling per se impaired (P ≤ 0.01) only short-term memory (∼37%) and learning (∼18%); the impairments were limited to the first CWI. N2O also attenuated (P ≤ 0.02) short-term memory (∼37%) and learning (∼35%), but the reductions occurred in all CWIs. Furthermore, N2O invariably compromised finger dexterity, attention, concentration, working memory, and spatial processing (P < 0.05). The present results demonstrate that inert gas narcosis aggravates, in a persistent manner, basic and higher-order cognitive abilities during protracted cold exposure.

At the intersection of alcohol use disorder and chronic pain

OBJECTIVE

Alcohol use disorder (AUD) and chronic pain are widespread conditions with extensive public health burden. This review seeks to describe neuroanatomical links and major mediating influences between AUD and chronic pain, in the service of identifying factors that predict the risk of chronic pain in precipitating or facilitating AUD.

METHOD

We review the neural bases of pain and the influence of AUD on processes involved in pain perception. We propose potential mechanisms involved in the development of chronic pain in AUD, and we consider implications for pain management in recovery from AUD.

RESULTS

Pain is a multidimensional and subjective experience that, in its acute form, is essential for survival, but in chronic form, pain is a disorder that negatively impacts quality of life. Neural substrates involved in initiating and maintaining chronic pain include dysfunction in descending pain pathways and reward network circuitry. AUD involves preoccupation or craving, intoxication, withdrawal, and negative affect. Neural substrates of AUD involve widespread mesocorticolimbic and cerebrocerebellar networks. Both conditions involve dysfunction of extended reward and oversight circuitry, particularly prefrontal cortex.

CONCLUSIONS

The interrelationship between chronic pain and AUD resides in the intersection of etiological influences, mental experiences, and neurobiological processes. Characterization of the connection between brain and behavioral abnormalities in AUD's precipitation of chronic pain-and vice versa-allows for early detection and treatment of patients at risk for developing either or both of these conditions and for preemptive interventional approaches to reduce the risk of consequent vulnerabilities and harm. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Colonoscopy quality with Entonox®vs intravenous conscious sedation: 18608 colonoscopy retrospective study

AIM

To compare colonoscopy quality with nitrous oxide gas (Entonox®) against intravenous conscious sedation using midazolam plus opioid.

METHODS

A retrospective analysis was performed on a prospectively held database of 18608 colonoscopies carried out in Lothian health board hospitals between July 2013 and January 2016. The quality of colonoscopies performed with Entonox was compared to intravenous conscious sedation (abbreviated in this article as IVM). Furthermore, the quality of colonoscopies performed with an unmedicated group was compared to IVM. The study used the following key markers of colonoscopy quality: (1) patient comfort scores; (2) caecal intubation rates (CIRs); and (3) polyp detection rates (PDRs). We used binary logistic regression to model the data.

RESULTS

There was no difference in the rate of moderate-to-extreme discomfort between the Entonox and IVM groups (17.9% vs 18.8%; OR = 1.06, 95%CI: 0.95-1.18, P = 0.27). Patients in the unmedicated group were less likely to experience moderate-to-extreme discomfort than those in the IVM group (11.4% vs 18.8%; OR = 0.71, 95%CI: 0.60-0.83, P < 0.001). There was no difference in caecal intubation between the Entonox and IVM groups (94.4% vs 93.7%; OR = 1.08, 95%CI: 0.92-1.28, P = 0.34). There was no difference in caecal intubation between the unmedicated and IVM groups (94.2% vs 93.7%; OR = 0.98, 95%CI: 0.79-1.22, P = 0.87). Polyp detection in the Entonox group was not different from IVM group (35.0% vs 33.1%; OR = 1.01, 95%CI: 0.93-1.10, P = 0.79). Polyp detection in the unmedicated group was not significantly different from the IVM group (37.4% vs 33.1%; OR = 0.97, 95%CI: 0.87-1.08, P = 0.60).

CONCLUSION

The use of Entonox was not associated with lower colonoscopy quality when compared to intravenous conscious sedation using midazolam plus opioid.

Sevoflurane vs. propofol in post-operative catheter-related bladder discomfort: a prospective randomized study

BACKGROUND

Post-operative catheter-related bladder discomfort (CRBD) causes increased emergence agitation. Muscarinic receptor activation is a major mechanism in CRBD development. Experimental studies showed that sevoflurane has anti-muscarinic effects whereas propofol does not. Our hypothesis was that sevoflurane anaesthesia would reduce the incidence of CRBD following bladder surgery.

METHODS

In total, 82 patients undergoing transurethral bladder tumour excision (TURBT) were assigned randomly to two groups according to the maintenance anaesthetic agent received: sevoflurane (n = 41) or propofol (n = 41). The incidence of CRBD was evaluated at 0, 1, 6 and 24 h post-operatively. The number of patients treated with a rescue medication (tramadol) for CRBD was noted.

RESULTS

The incidence of CRBD at post-operative 1 h was lower in the sevoflurane group than that in the propofol group (59% vs. 85%; P = 0.007). The differences in CRBD were 27% and 22% at 0 and 6 h post-operatively (P = 0.008 and 0.047, respectively). CRBD occurred in 27 (66%) patients in the sevoflurane group vs. 38 (93%) in the propofol group from 0 to 24 h post-operatively (P = 0.005). The number of patients treated with tramadol was lower in the sevoflurane group (13 [22%] vs. 22 [54%]; P = 0.044).

CONCLUSION

Sevoflurane, as a maintenance in general anaesthesia, decreased the incidence of early post-operative CRBD and tramadol requirements in patients undergoing TURBT, compared with propofol.

Analgesic Effects of Alcohol: A Systematic Review and Meta-Analysis of Controlled Experimental Studies in Healthy Participants

Despite the long-standing belief in the analgesic properties of alcohol, experimental studies have produced mixed results. This meta-analysis aimed to clarify whether alcohol produces a decrease in experimentally-induced pain and to determine the magnitude of any such effect. PubMed, PsycINFO, and Embase databases were searched from inception until April 21, 2016 for controlled studies examining the effect of quantified dosages of alcohol on pain response to noxious stimulation. Eighteen studies involving 404 participants were identified providing alcohol versus no-alcohol comparisons for 13 tests of pain threshold (n = 212) and 9 tests of pain intensity ratings (n = 192). Random effects meta-analysis of standardized mean difference (SMD) provided robust support for analgesic effects of alcohol. A mean blood alcohol content (BAC) of approximately .08% (3-4 standard drinks) produced a small elevation of pain threshold (SMD [95% CI] = .35 [.17-.54], P = .002), and a moderate to large reduction in pain intensity ratings (SMD [95% CI] = .64 [.37-.91], P < .0001), or equivalently, a mean reduction of 1.25 points on a 0- to 10-point pain rating scale. Furthermore, increasing BAC resulted in increasing analgesia, with each .02% BAC increment producing an increase of SMD = .11 for pain threshold and SMD = .20 for reduced pain intensity. Some evidence of publication bias emerged, but statistical correction methods suggested minimal impact on effect size. Taken together, findings suggest that alcohol is an effective analgesic that delivers clinically-relevant reductions in ratings of pain intensity, which could explain alcohol misuse in those with persistent pain despite its potential consequences for long-term health. Further research is needed to corroborate these findings for clinical pain states.

PERSPECTIVE

This meta-analysis provides robust evidence for the analgesic properties of alcohol, which could potentially contribute to alcohol misuse in pain patients. Strongest analgesia occurs for alcohol levels exceeding World Health Organization guidelines for low-risk drinking and suggests raising awareness of alternative, less harmful pain interventions to vulnerable patients may be beneficial.

Opioid Toxicity

In recent years, there has been a substantial increase in opioid use and abuse, and in opioid-related fatal overdoses. The increase in opioid use has resulted at least in part from individuals transitioning from prescribed opioids to heroin and fentanyl, which can cause significant respiratory depression that can progress to apnea and death. Heroin and fentanyl may be used individually, together, or in combination with other substances such as ethanol, benzodiazepines, or other drugs that can have additional deleterious effects on respiration. Suspicion that a death is drug-related begins with the decedent's medical and social history, and scene investigation, where drugs and drug paraphernalia may be encountered, and examination of the decedent, which may reveal needle punctures and needle track marks. At autopsy, the most significant internal finding that is reflective of opioid toxicity is pulmonary edema and congestion, and frothy watery fluid is often present in the airways. Various medical ailments such as heart and lung disease and obesity may limit an individual's physiologic reserve, rendering them more susceptible to the toxic effects of opioids and other drugs. Although many opioids will be detected on routine toxicology testing, more specialized testing may be warranted for opioid analogs, or other uncommon, synthetic, or semisynthetic drugs.

Recreational nitrous oxide use: Prevalence and risks

Nitrous oxide (N2O; laughing gas) is clinically used as a safe anesthetic (dentistry, ambulance, childbirth) and appreciated for its anti-anxiety effect. Since five years, recreational use of N2O is rapidly increasing especially in the dance and festival scene. In the UK, N2O is the second most popular recreational drug after cannabis. In most countries, nitrous oxide is a legal drug that is widely available and cheap. Last month prevalence of use among clubbers and ravers ranges between 40 and almost 80 percent. Following one inhalation, mostly from a balloon, a euphoric, pleasant, joyful, empathogenic and sometimes hallucinogenic effect is rapidly induced (within 10 s) and disappears within some minutes. Recreational N2O use is generally moderate with most users taking less than 10 balloons of N2O per episode and about 80% of the users having less than 10 episodes per year. Side effects of N2O include transient dizziness, dissociation, disorientation, loss of balance, impaired memory and cognition, and weakness in the legs. When intoxicated accidents like tripping and falling may occur. Some fatal accidents have been reported due to due to asphyxia (hypoxia). Heavy or sustained use of N2O inactivates vitamin B12, resulting in a functional vitamin B12 deficiency and initially causing numbness in fingers, which may further progress to peripheral neuropathy and megaloblastic anemia. N2O use does not seem to result in dependence. Considering the generally modest use of N2O and its relative safety, it is not necessary to take legal measures. However, (potential) users should be informed about the risk of vitamin B12-deficiency related neurological and hematological effects associated with heavy use.

The effect of nonrecurring alcohol administration on pain perception in humans: a systematic review

Purpose

Alcohol is believed to have pain-dampening effects and is often used as self-medication by persons with pain problems; however, experimental evidence confirming this effect is scarce. We conducted a systematic review of experimental studies on the effects of nonrecurring alcohol administration on pain perception in healthy human subjects and the underlying mechanisms.

Method

Three databases (PubMed, PsycINFO, and Web of Science) were searched for relevant studies using a predefined algorithm. In a next step, irrelevant articles were excluded by screening titles and abstracts. Finally, articles were checked regarding a set of methodological criteria; only publications meeting these criteria were selected for this review. A total of 14 experimental studies were identified.

Results

Overall, most of the studies were able to show a pain-dampening effect of alcohol. However, many of them had methodological shortcomings (eg, lack of placebo control, insufficient blinding, or very small sample sizes). In addition, comparability is limited due to considerable variations in alcohol administration and pain measurement. More importantly, potential mechanisms of action and moderating variables have scarcely been investigated.

Conclusion

Despite the frequent use of alcohol as self-medication by persons with pain problems, there are to date only a few experimental investigations of alcohol effects on pain perceptions. The results of these studies suggest that alcohol does in fact have pain-dampening effects. However, the mechanisms implicated in these effects are still unknown, and experimental research has been limited to pain-free subjects. Future research should provide more knowledge about alcohol effects on pain, especially in chronic pain patients.

Sevoflurane versus propofol sedation during periocular anesthetic injections in oculoplastic procedures: An open-label randomized comparison

PURPOSE

The purpose of the current investigation was to make an objective controlled comparison of pain tolerance, patient satisfaction and potential complications during the injection of local anesthesia in oculoplastic procedures under short-term sedation using inhalational versus parenteral sedatives.

METHODS

This was an open-label, randomized clinical trial where patients were randomized to 3 groups. Group I: Sedation with intravenous propofol. Group II: Sedation with inhaled sevoflurane. Group 3: Control group receiving no sedation.

RESULTS

A total of 396 patients were randomly assigned, and 375 were included in the final analysis. Study groups were similar in age, gender, and distribution of operative procedures performed. There was no statistically significant difference in the adjusted primary composite outcome measure between propofol and sevoflurane (pain scores and patient satisfaction). Significantly more patients in group I required restraining during periocular injections than group II or III (p < 0.001). Significantly more patients sneezed in group I than group II (p < 0.001) and none in the control group. Three patients in group II suffered severe excitation-disinhibition during emergence from sedation which was rapidly reversible, and 3 more suffered a severe bout of postoperative nausea and vomiting (PONV).

CONCLUSION

Sevoflurane and propofol during periocular anesthetic injections produce an equally favorable experience. Sevoflurane is introduced painlessly, and offers better patient control with less induction of the sneezing reflex which may provide a higher safety profile, however short-term aggression/disinhibition and PONV may be an issue in some patients.

A prospective, randomized controlled trial of conscious sedation using propofol combined with inhaled nitrous oxide for dental treatment

PURPOSE

Adverse reactions during propofol sedation include a decrease in arterial blood pressure, propofol-induced pain on injection, and airway complications. The purpose of this study was to investigate whether combined use of intravenous propofol and inhaled nitrous oxide could decrease the hypotensive and other adverse effects of propofol.

PATIENTS AND METHODS

We designed and implemented a prospective, randomized controlled trial. Patients undergoing dental procedures requiring intravenous sedation were randomly allocated to 2 groups: group P comprised those receiving sedation with propofol alone, and group N+P comprised those receiving sedation with 40% nitrous oxide inhalation and propofol. During the dental procedures, the sedation level was maintained at an Observer's Assessment of Alertness/Sedation scale score of 4 by adjusting propofol's target plasma concentration. Nitrous oxide inhalation was the predictor variable, whereas the hemodynamic changes, amount and concentration of propofol, and adverse events were the outcome variables.

RESULTS

Eighty-eight patients were successfully analyzed without any complications. The total amount of propofol was significantly less in group N+P (249.8 ± 121.7 mg) than in group P (310.3 ± 122.4 mg) (P = .022), and the mean concentration of propofol was significantly less in group N+P (1.81 ± 0.34 μg/mL) than in group P (2.05 ± 0.44 μg/mL) (P = .006). The mean blood pressure reduction in group N+P (11.0 ± 8.0 mm Hg) was significantly smaller than that in group P (15.8 ± 10.2 mm Hg) (P = .034). Pain associated with the propofol injection and memory of the procedure were less in group N+P (P = .011 and P = .048, respectively). Nitrous oxide did not affect respiratory conditions or recovery characteristics.

CONCLUSIONS

The results of this study suggest that nitrous oxide inhalation combined with propofol sedation attenuates the hypotensive effect and pain associated with propofol injections, along with potentiating the amnesic effect.

Direct measurement of nitrous oxide kinetics

BACKGROUND

Using conscious subjects, measurement of the effects of low concentrations of anaesthetic agents can allow the dynamics of onset and offset of the agent to be measured and kinetic values estimated. However, the tests have to be rapid and preferably assess cerebral function.

METHODS

We used a short version of the digit symbol substitution test (DSST) that allowed frequent measurement of the impairment caused by nitrous oxide. We compared 10 min of onset and offset of breathing 5% and 30% nitrous oxide in 30% oxygen, compared with 30% oxygen only. End-tidal nitrous oxide concentrations were used to predict the concentration in a central compartment, according to a range of T(1/2) values chosen to be consistent with possible cerebral blood flow values.

RESULTS

We studied 19 volunteers and estimated a mean response. Only 30% nitrous oxide decreased the DSST. When DSST scores were related to the values in the predicted central compartment, the best dose-effect relationship was found when the T(1/2) was 37 s, consistent with a regional blood flow of about 120 ml 100 g(-1) min(-1).

CONCLUSIONS

The onset of nitrous oxide effect on DSST is rapid, consistent with the perfusion of metabolically active cerebral cortical tissues. The rate of onset is greater than previous measures based on a motor test which involved the function of subcortical structures in the central nervous system.

Xenon and sevoflurane provide analgesia during labor and fetal brain protection in a perinatal rat model of hypoxia-ischemia

It is not possible to identify all pregnancies at risk of neonatal hypoxic-ischemic encephalopathy (HIE). Many women use some form of analgesia during childbirth and some anesthetic agents have been shown to be neuroprotective when used as analgesics at subanesthetic concentrations. In this study we sought to understand the effects of two anesthetic agents with presumptive analgesic activity and known preconditioning-neuroprotective properties (sevoflurane or xenon), in reducing hypoxia-induced brain damage in a model of intrauterine perinatal asphyxia. The analgesic and neuroprotective effects at subanesthetic levels of sevoflurane (0.35%) or xenon (35%) were tested in a rat model of intrauterine perinatal asphyxia. Analgesic effects were measured by assessing maternal behavior and spinal cord dorsal horn neuronal activation using c-Fos. In separate experiments, intrauterine fetal asphyxia was induced four hours after gas exposure; on post-insult day 3 apoptotic cell death was measured by caspase-3 immunostaining in hippocampal neurons and correlated with the number of viable neurons on postnatal day (PND) 7. A separate cohort of pups was nurtured by a surrogate mother for 50 days when cognitive testing with Morris water maze was performed. Both anesthetic agents provided analgesia as reflected by a reduction in the number of stretching movements and decreased c-Fos expression in the dorsal horn of the spinal cord. Both agents also reduced the number of caspase-3 positive (apoptotic) neurons and increased cell viability in the hippocampus at PND7. These acute histological changes were mirrored by improved cognitive function measured remotely after birth on PND 50 compared to control group. Subanesthetic doses of sevoflurane or xenon provided both analgesia and neuroprotection in this model of intrauterine perinatal asphyxia. These data suggest that anesthetic agents with neuroprotective properties may be effective in preventing HIE and should be tested in clinical trials in the future.

Inert gas narcosis has no influence on thermo-tactile sensation

Contribution of skin thermal sensors under inert gas narcosis to the raising hypothermia is not known. Such information is vital for understanding the impact of narcosis on behavioural thermoregulation, diver safety and judgment of thermal (dis)comfort in the hyperbaric environment. So this study aimed at establishing the effects of normoxic concentration of 30% nitrous oxide (N(2)O) on thermo-tactile threshold sensation by studying 16 subjects [eight females and eight males; eight sensitive (S) and eight non-sensitive (NS) to N(2)O]. Their mean (SD) age was 22.1 (1.8) years, weight 72.8 (15.3) kg, height 1.75 (0.10) m and body mass index 23.8 (3.8) kg m(-2). Quantitative thermo-tactile sensory testing was performed on forearm, upper arm and thigh under two experimental conditions: breathing air (air trial) and breathing normoxic mixture of 30% N(2)O (N(2)O trial) in the mixed sequence. Difference in thermo-tactile sensitivity thresholds between two groups of subjects in two experimental conditions was analysed by 3-way mixed-model analysis of covariance. There were no statistically significant differences in thermo-tactile thresholds either between the Air and N(2)O trials, or between S and NS groups, or between females and males, or with respect to body mass index. Some clinically insignificant lowering of thermo-tactile thresholds occurred only for warm thermo-tactile thresholds on upper arm and thigh. The results indicated that normoxic mixture of 30% N(2)O had no influence on thermo-tactile sensation in normothermia.

Sevoflurane to alleviate pain on propofol injection

PURPOSE

Since the introduction of propofol, several drugs and methods have been used to alleviate the pain on its injection. This study was designed to evaluate the effect of adding sevoflurane 3% during preoxygenation in alleviation of pain on propofol injection.

METHODS

In this randomized single-blinded study, 100 patients were randomly allocated equally into five groups: sevoflurane-lidocaine-tourniquet (SLT), sevoflurane-lidocaine (SL), lidocaine-tourniquet (LT), lidocaine (L), and sevoflurane (S). Approximately 10 min before the induction of anesthesia, midazolam 1-2 mg was administered intravenously to all patients. All patients received fentanyl 1 µg/kg as pretreatment and a full induction dose of propofol. A blinded anesthesia nurse assessed pain and hand movements throughout the injection of propofol.

RESULTS

In the SLT group, all patients (100%) were pain free and had no hand movements. There was no significant difference in pain grade or in hand movements between the L and the S groups, or between the SLT and the SL groups. However, significant differences were observed in pain grade between the SLT and the L groups as well as between the SLT and the S groups. In addition, a significant difference in hand movement was observed only between the SLT and the S groups.

CONCLUSION

The addition of 3% sevoflurane at the time of preoxygenation for 1 min along with routine use of lidocaine-tourniquet completely prevented pain upon propofol injection, whereas sevoflurane by itself provided similar analgesia to premixed lidocaine with propofol.

Nitrous oxide inhalation among adolescents: prevalence, correlates, and co-occurrence with volatile solvent inhalation

Few studies have examined the prevalence of nitrous oxide (NO) inhalation or co-occurrence of NO and volatile solvent (VS) use in adolescents. Study aims were to (1) describe the independent and conjoint prevalence of NO and VS use in incarcerated youth, (2) compare adolescent users of both NO and VS inhalants (NO+VS) to users of NO-only, VS-only, and nonusers of NO and VS (NO/VNS nonusers) with regard to demographic, psychological, and behavioral characteristics, and (3) conduct logistic regression analyses identifying correlates of NO use. Residents (N = 723) of Missouri Division of Youth Services were assessed with standardized psychosocial measures. Participants averaged 15.5 (SD = 1.2) years of age, were ethnically diverse and predominantly male. Lifetime prevalence of NO use was 15.8%. NO+VS users evidenced greater impairments compared to NO+VS nonusers. VS-only users evidenced impairments that were similar in kind but at lower prevalences compared to those displayed by NO+VS users, whereas NO-only youth had profiles that were similar to those of NO/VS nonusers. Psychiatric disorders, polydrug use, and temperamental fearlessness were correlates of NO use. NO+VS users were at high risk for behavioral and emotional problems. Screening and interventions for NO and VS inhalant use should be implemented in juvenile justice facilities.

[Use of nitrous oxide in Uruguay]

BACKGROUND

Nitrous oxide (N2O) is a medicinal gas that has been used for anesthesia for over a century and a half. As evidence of the adverse effects of N2O have been contrasted with the good safety profiles of new anesthetic agents, use of the older gas has declined.

OBJECTIVE

To describe and analyze the use of N2O for anesthesia in the Eastern Republic of Uruguay. To describe the use of other drugs for general anesthesia.

METHODS

The study had 2 phases. In the first phase, information on the use of N2O and other drugs for general anesthesia was obtained from the medical records of a sample of patients who underwent surgery between November 2007 and November 2008 at nationally important hospitals. In the second phase 80 physician anesthesiologists were surveyed to obtain information on their reported use of N2O for general anesthesia. The respondents were found working during the first half of 2008.

RESULTS

A total of 104 patients' records were read and 80 anesthesiologists were surveyed. We found that 31 (29.8%) patients received N2O during anesthesia. The survey showed that 62 (77.5%) of the anesthesiologists had used N2O in the last 6 months. Nine (50%) of the anesthesiologists who had not used N2O said their reason was that it was unavailable. The other 9 referred either to the risk of postoperative nausea and/or vomiting (n = 6) or the fact of having better alternatives (n = 9).

CONCLUSION

This study shows that N2O is little used in current anesthesia practice. The tendency in Uruguay continues to be toward declining use.

The dopamine response in the nucleus accumbens core-shell border differs from that in the core and shell during operant ethanol self-administration

BACKGROUND

Ethanol self-administration has been shown to increase dopamine in the nucleus accumbens; however, dopamine levels in the accumbal subregions (core, shell, and core-shell border) have not yet been measured separately in this paradigm. This study was designed to determine if dopamine responses during operant ethanol self-administration are similar in the core, core-shell border, and shell, particularly during transfer from the home cage to the operant chamber and during consumption of the drinking solution.

METHODS

Six groups of male Long-Evans rats were trained to lever-press for either 10% sucrose (10S) or 10% sucrose + 10% ethanol (10S10E) (with a guide cannula above the core, core-shell border, or shell of the accumbens). On experiment day, 5-minute microdialysis samples were collected from the core, core-shell border, or shell before, during, and after drinking. Dopamine and ethanol concentrations were analyzed in these samples.

RESULTS

A significant increase in dopamine occurred during transfer of the rats from the home cage into the operant chamber in all 6 groups, with those trained to drink 10S10E exhibiting a significantly higher increase than those trained to drink 10S in the core and shell. No significant increases were observed during drinking of either solution in the core or shell. A significant increase in dopamine was observed during consumption of ethanol in the core-shell border.

CONCLUSIONS

We conclude that dopamine responses to operant ethanol self-administration are subregion specific. After operant training, accumbal dopamine responses in the core and shell occur when cues that predict ethanol availability are presented and not when the reinforcer is consumed. However, core-shell border dopamine responses occur at the time of the cue and consumption of the reinforcer.

The shell of the nucleus accumbens has a higher dopamine response compared with the core after non-contingent intravenous ethanol administration

Dopamine increases in the nucleus accumbens after ethanol administration in rats, but the contributions of the core and shell subregions to this response are unclear. The goal of this study was to determine the effect of various doses of i.v. ethanol infusions on dopamine in these two subregions of the nucleus accumbens. Male Long-Evans rats were infused with either acute i.v. ethanol (0.5, 1.0, 1.5 g/kg), repeated i.v. ethanol (four 1.0 g/kg infusions resulting in a cumulative dose of 4.0 g/kg), or saline as a control for each condition. Dopamine and ethanol were measured in dialysate samples from each experiment. The in vivo extraction fraction for ethanol of probes was determined using i.v. 4-methylpyrazole, and was used to estimate peak brain ethanol concentrations after the infusions. The peak brain ethanol concentrations after the 0.5, 1.0 and 1.5 g/kg ethanol infusions were estimated to be 20, 49 and 57 mM, respectively. A significant dopamine increase was observed for the 0.5 g/kg ethanol group when collapsed across subregions. However, both the 1.0 g/kg and 1.5 g/kg ethanol infusions produced significant increases in dopamine levels in the shell that were significantly higher than those in the core. An ethanol dose-response effect on dopamine in the shell was observed when saline controls, 0.5, 1.0, and 1.5 g/kg groups were compared. For the cumulative-dosing study, the first, second, and fourth infusions resulted in significant increases in dopamine in the shell. However, these responses were not significantly different from one another. The results of this study show that the shell has a stronger response than the core to i.v. ethanol, that dopamine in the shell increases in a dose-dependent manner between 0.5-1.0 g/kg doses, but that the response to higher ethanol doses reaches a plateau.