Impaired object recognition memory following methamphetamine, but not p-chloroamphetamine- or d-amphetamine-induced neurotoxicity

Repeated moderate doses of methamphetamine (mAMPH) damage forebrain monoaminergic terminals and nonmonoaminergic cells in somatosensory cortex, and impair performance in a novelty preference task of object recognition (OR). This study aimed to determine whether the memory deficit seen after a neurotoxic mAMPH regimen results from damage to dopamine (DA) and/or serotonin (5-HT) terminals. Animals were given a neurotoxic regimen of mAMPH, p-chloroamphetamine (PCA, preferentially damages 5-HT terminals), d-amphetamine (d-AMPH, preferentially damages DA terminals), or saline. After 1 week, animals were trained and tested for OR memory. Rats treated with mAMPH showed no recognition memory during the short-term memory (STM) test, whereas both PCA- and d-AMPH-treated rats showed OR STM scores comparable to controls. After behavioral testing, the specificity of monoaminergic lesions was determined by postmortem [I-125]RTI-55 binding to dopamine (DAT) and serotonin (SERT) transporter proteins. Tissue from a separate group of animals killed 3 days after drug treatment was processed for Fluoro-Jade (F-J) fluorescence histochemistry to detect damaged cortical neurons. mAMPH-treated rats showed reductions in striatal DAT and hippocampal (HC) and perirhinal (pRh) SERT, as well as degeneration of neurons in primary somatosensory cortex. In PCA-treated rats, HC and pRh SERT were substantially depleted, but striatal DAT and cortical neuron survival were unaffected. By contrast, d-AMPH-treated animals showed marked depletions in stratal DAT and cortical neurodegeneration, but HC and pRh SERT were unaffected. This pattern of results indicates that no single feature of mAMPH-induced neurotoxicity is sufficient to produce the OR impairments seen after mAMPH treatment.