Figure 3

Effects of argon on amphetamine-induced changes in locomotor activity and mu receptor activity in the nucleus accumbens. (A) When challenged with amphetamine, rats pretreated with amphetamine and argon had lower locomotor activity than rats pretreated with amphetamine and air (AA); in contrast, no significant difference in locomotor activity was found between rats pretreated with saline and argon and those pretreated with saline and air when challenged with amphetamine (SA) or saline (SS). This indicates that argon blocked locomotor sensitization to amphetamine, but had effect neither on locomotor activity induced by acute amphetamine nor on basal locomotor activity. Locomotor activity is expressed in arbitrary units. (B) As assessed immediately after being challenged with amphetamine, rats pretreated with amphetamine and argon had reduced mu receptor activity compared to rats pretreated with amphetamine and air (AA); in contrast, no significant difference in mu receptor activity was found between rats pretreated with saline and argon and those pretreated with saline and air when challenged with amphetamine (SA) or saline (SS). This indicates that argon blocked the increase in mu receptor activity induced by repeated amphetamine, but had effect neither on the increase in mu receptor activity induced by acute amphetamine nor on basal mu receptor activity. The ratio of the dissociation constant (Kd) to the maximal number of binding sites (Bmax) was calculated to assess the activity of mu receptors in the nucleus accumbens; mu receptor activity in controls rats pretreated and challenged with amphetamine was taken as a 100 % value. SS: pretreatment with saline + challenge with saline; SA: pretreatment with saline + challenge with saline; AA: pretreatment with amphetamine + challenge with amphetamine. * P < 0.001 vs AA + Air.