Effects of repeated morphine on locomotion, place preference and dopamine in heterozygous glial cell line‐derived neurotrophic factor knockout mice

Glial cell line‐derived neurotrophic factor (GDNF) has been shown to be involved in the maintenance of striatal dopaminergic neurons. Neurotrophic factors are crucial for the plasticity of central nervous system and may be involved in long‐term responses to drug exposure. To study the effects of reduced GDNF on dopaminergic behaviour related to addiction, we compared the effects of morphine on locomotor activity, conditioned place preference (CPP) and extracellular accumbal dopamine in heterozygous GDNF knockout mice (GDNF+/−) with those in their wild‐type (Wt) littermates. When morphine 30 mg/kg was administered daily for 4 days, tolerance developed towards its locomotor stimulatory action only in the GDNF+/− mice. A morphine 5 mg/kg challenge dose stimulated locomotor activity only in the GDNF+/− mice withdrawn for 96 h from repeated morphine treatment, whereas clear and similar sensitization of the locomotor response was seen after a 10 mg/kg challenge dose in mice of both genotypes. Morphine‐induced CPP developed initially similarly in Wt and GDNF+/− mice, but it lasted longer in the Wt mice. The small challenge dose of morphine increased accumbal dopamine output slightly more in the GDNF+/− mice than in the Wt mice, but doubling the challenge dose caused a dose‐dependent response only in the Wt mice. In addition, repeated morphine treatment counteracted the increase in the accumbal extracellular dopamine concentration we previously found in drug‐naive GDNF+/− mice. Thus, reduced endogenous GDNF level alters the dopaminergic behavioural effects to repeatedly administered morphine, emphasizing the involvement of GDNF in the neuroplastic changes related to long‐term effects of drugs of abuse.