Abstract

Stimulants of abuse such as cocaine and methamphetamine (METH) have dramatic effects on tissue neurotensin (NT) levels in the striatum and nucleus accumbens. Presumably these effects are due to the ability of such drugs to increase dopamine transmission. Because changes in dopamine activity appear to influence NT systems, we examined the effects of increasing doses of METH on extracellular NT levels in the medial striatum and nucleus accumbens using in vivo microdialysis in conscious rats. At the lowest dose tested (0.5 mg/kg), METH almost doubled the extracellular concentration of NT in both regions. When the dose of METH was increased to 5.0 mg/kg, extracellular NT concentration was elevated, but only to approximately 150% of control. At the highest dose examined (15.0 mg/kg), extracellular NT was not altered compared to pretreatment control levels. The role of DA D-1 and D-2 receptors in mediating these effects was determined by combining specific antagonists with the low dose of METH. The D-1 antagonist SCH 23390 blocked the METH-induced increase in extracellular NT levels in the striatum, but not in the nucleus accumbens. Pretreatment with the D-2 antagonist, eticlopride, blocked the increase in extracellular NT in both regions. Changes in striatal NT extracellular levels after a single METH injection were compared to the alterations in tissue NT levels following multiple administrations of the same doses of METH. Tissue levels were significantly elevated with 5 or 15 mg/kg METH in the medial, but not the lateral, striatum. There was not a clear correlation observed between the METH effects on striatal NT tissue levels and extracellular NT concentration.