Table 4. Genetically determined differences in the effect of mu opioid receptor antagonist naltrexone at 1 mg/kg (A) and 2 mg/kg (B) on alcohol drinking in nPE1 male mice.

The groups assigned as the vehicle- or naltrexone-treated mice in each genotypes had similar alcohol intake 24 hours before the test day (data not shown). On the test day, alcohol (15%) was presented 10 min after a single i.p. injection of naltrexone in saline or vehicle, and then alcohol and water intake values were recorded after 4, 8 and 24 hours of alcohol access. In these experiments, mice were assigned to one of four treatment groups: (1) nPE1^+/+ with vehicle as control; (2) nPE1^+/+ with naltrexone; (3) nPE1^−/− with vehicle as control; and (4) nPE1^−/− with naltrexone. Data are presented at all the 3 recording time points. Genotype difference: *p<0.05 vs. nPE1^+/+ at the same time point after the same treatment; Naltrexone treatment difference: +p<0.05 vs. vehicle control in the same genotype at the same time point by 3-way ANOVA with Newman-Keuls post-hoc tests. Data presented as mean ± SEM.

A. No effects of naltrexone at 1 mg/kg on either nPE1+/+ or nPE1−/− mice
Genotype		nPE1+/+ (n = 5)		nPE1−/− (n = 5)
Treatment		vehicle	1 mg/kg naltrexone	vehicle	1 mg/kg naltrexone
Alcohol intake g/kg	4h	5.4 ± 1.4	4.8 ± 1.5	2.6 ± 1.4	3.0 ± 0.41
	8h	9.8 ± 2.6	8.8 ± 2.2	4.2 ± 0.9 *	3.5 ± 0.57 *
	24h	17.6 ± 2.4	18.1 ± 2.9	8.9 ± 2.2 *	7.3 ± 2.1 *
Alcohol preference	4h	0.80 ± 0.04	0.74 ± 0.11	0.59 ± 0.07	0.54 ± 0.15
	8h	0.71 ± 0.06	0.70 ± 0.05	0.57 ± 0.07	0.60 ± 0.11
	24h	0.65 ± 0.05	0.62 ± 0.04	0.50 ± 0.03	0.55 ± 0.13
B. Effects of naltrexone at 2 mg/kg on nPE1+/+, but not nPE1−/−, mice
Genotype		nPE1+/+ (n = 5)		nPE1−/− (n = 5)
Treatment		vehicle	2 mg/kg naltrexone	vehicle	2 mg/kg naltrexone
Alcohol intake g/kg	4h	6.1 ± 0.6	3.0 ± 0.5 +	2.8 ± 1.2	2.4 ± 0.91
	8h	10.1 ± 2.1	6.8 ± 2.1	3.7 ± 1.0 *	2.9 ± 0.60 *
	24h	18.0 ± 2.5	12.2 ± 1.3 +	9.1 ± 1.2 *	7.8 ± 1.8 *
Alcohol preference	4h	0.82 ± 0.02	0.50 ± 0.06	0.61 ± 0.08	0.53 ± 0.10
	8h	0.70 ± 0.05	0.62 ± 0.04	0.56 ± 0.05	0.53 ± 0.15
	24h	0.66 ± 0.05	0.64 ± 0.03	0.49 ±0.11	0.55 ± 0.12