Table 4. Behavioral effects of nicotine in opioid receptor and peptide knockout mice.

Data are shown for each knockout (gene KO) mouse line. Behavioral tests are detailed in the Box. Unchanged: no genotype effect; increased: KO shows higher response compared to wild-type (WT); decreased: KO shows lower response compared to WT; abolished: no response in KO. CPP: Conditioned Place Preference; d: day; SA: self-administration; FR: fixed ratio; PR: progressive ratio.

Gene KO	Behavioral test	Drug of abuse dose, route	Genotype effect	Ref
mu	CPP	0.5 or 0.7 mg/kg, s.c.	abolished	Berrendero et al., 2002
	CPP	1 mg/kg, i.p.	abolished	Walters et al., 2005
		2 mg/kg, i.p.	unchanged
	withdrawal	10 mg/kg/d, minipump (6d)	decreased	Berrendero et al., 2002
delta	CPP	0.17 mg/kg, s.c.	abolished	Berrendero et al., 2012
	SA	15 µg/kg/infusion 10d, i.v. FR1	unchanged	Berrendero et al., 2012
		30 µg/kg/infusion 10d, i.v. FR1	decreased
		30 µg/kg/infusion, i.v. PR	decreased
	withdrawal	8.77 mg/kg/d, minipump (6d)	unchanged	Berrendero et al., 2012
βend	CPP	0.5 mg/kg, s.c.	abolished	Trigo et al., 2009
	withdrawal	10 mg/kg/d, minipump (6d)	unchanged	Trigo et al., 2009
pEnk	CPP	0.5 mg/kg, s.c.	abolished	Berrendero et al., 2005
	withdrawal	25 mg/kg/d, minipump (6d)	decreased	Berrendero et al., 2005
pDyn	CPP	0.5 mg/kg, s.c.	unchanged	Galeote et al., 2009
	SA	5.2–85.5 mg/kg/infusion, i.v. FR1	leftward shift	Galeote et al., 2009
		5.2, 10.6, 21.3 or 85.5 mg/kg/infusion, i.v. PR	unchanged
		42.7 mg/kg/infusion, i.v. PR	decreased
	withdrawal	25 mg/kg/d, minipump (6d)	unchanged	Galeote et al., 2009