To Evaluate the Influence of the A118G Polymorphism in the mu Opioid Receptor Gene (OPRM1) on Effects of GSK1521498 and Naltrexone on Physiological and Behavioural Markers of Brain Function in Healthy Social Drinkers

Inclusion Criteria: * Caucasian male or female between 18 and 65 years of age inclusive, at the time of signing the informed consent * BMI in the normal range or greater, which is equal to 22 kilogram (kg) per meter square (m\^2) or above, but otherwise healthy as determined by a responsible and experienced physician, based on a medical evaluation including medical history, physical examination, laboratory tests and cardiac monitoring. A subject with a clinical abnormality or laboratory parameters outside the reference range for the population being studied may be included only if the Investigator and the GSK Medical Monitor agree that the finding is unlikely to introduce additional risk factors and will not interfere with the study procedures. * Self reported alcohol drinking frequency of 3 or more drinks for men (2 or more drinks for women) at least two days per week, on average or a score of 6 or higher on the Alcohol-Use-Disorders-Identification Test (AUDIT). * Aspartate aminotransferase (AST) and alanine transaminase (ALT) \<2xUpper Limit of Normal (ULN); alkaline phosphatase and bilirubin \<=1.5xULN (isolated bilirubin \>1.5xULN is acceptable if bilirubin is fractionated and direct bilirubin \<35%). * A female subject is eligible to participate if she is of child-bearing potential and is abstinent or agrees to use one of the accepted contraception methods for an appropriate period of time (as determined by the product label or investigator) prior to the start of dosing to sufficiently minimize the risk of pregnancy at that point. Female subjects must agree to use contraception until 14 days after receiving the last dose of study medication. * Male subjects with female partners of child-bearing potential must agree to use one of the acceptable contraception methods. This criterion must be followed from the time of the first dose of study medication until 14 days after receiving the last dose of study medication. * Capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form, and capacity to participate in all aspects of the assessment. Exclusion Criteria: * Psychiatric illness and substance abuse: * Current or past history of Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) alcohol or substance dependence or abuse, including treatment-seeking behaviour, as determined by the Investigator or Mini-international neuropsychiatric interview (MINI). * Self administered Beck Depression Inventory II scale total score greater than 13 or suicide question score greater than zero at screening. * Current or past chronic history of neurological disorders. * Current or past history of Axis 1 psychiatric disorders including eating disorders such as anorexia nervosa, bulimia nervosa and binge eating disorder, including treatment seeking behaviour using the MINI. * Subject who, in the investigator/designee's judgement, poses a significant suicide risk. Evidence of serious suicide risk may include any history of suicidal behaviour and/or any evidence of suicidal ideation on any questionnaires e.g. type 4 or 5 on the C-SSRS in the last 5 years. * Concomitant drug use: Positive urine screen for amphetamines, barbiturates, cocaine, opiates, cannabinoids or benzodiazepines at screening. * Unable to refrain from the use of prescription or non-prescription drugs, including vitamins, herbal and dietary supplements (including St John's Wort) within 14 days prior to the first dose of study medication, unless in the opinion of the Investigator and GSK Medical Monitor the medication will not interfere with the study procedures or compromise subject safety. * Miscellaneous: * Special dietary requirements (e.g. vegetarians, vegans, religious, food -intolerantdiets), cannot be accommodated by the experimental design - it is important that all participants are offered the same test meals and snack choices during their in-unit assessments - so people with special dietary requirements will be excluded. * Subjects unsuitable for cannulation. * Any contraindications or logistical complications anticipated in relation to magnetic resonance imaging (MRI) scanning or other endpoint assessments, in the judgment of the Principal Investigator, including: presence of a cardiac pacemaker or other electronic device or ferromagnetic metal foreign bodies as assessed by a standard pre-MRI questionnaire, claustrophobia, inability to lie still on back for approximately an hour. * The subject has participated in a clinical trial and has received an investigational product within the following time period prior to the first dosing day in the current study: 3 months, 5 half-lives or twice the duration of the biological effect of the investigational product (whichever is longer). * QTcB or QTcF \>450 milliseconds (msec). Note that if the initial QTc value is prolonged, the ECG should be repeated two more times (with 5 minutes between ECG readings) and the average of the 3 QTc values used to determine eligibility. * A positive pre-study Hepatitis B surface antigen or positive Hepatitis C antibody or HIV tests result within 3 months of screening. * Exposure to more than four new chemical entities within 12 months prior to the first dosing day. * History of sensitivity to any of the study medications, or components thereof or a history of drug or other allergy that, in the opinion of the investigator or GSK Medical Monitor, contraindicates their participation. * Where participation in the study would result in donation of blood or blood products in excess of 500 milliliter (mL) within a 56 day period. * Pregnant or lactating females. * Occupational use of heavy machinery. * Heavy smokers i.e \>15 cigarettes per day.