Previous studies focusing on amphetamine (AMPH), methamphetamine (METH) and methylphenidate (MPH) neurotoxicity have almost exclusively been conducted in rodents during the light cycle, which is when most rodents sleep. There are virtually no studies that have simultaneously compared the effects of these three stimulants on body temperature and also determined serum stimulant levels during exposure. The present study compared the effects of MPH, AMPH and METH treatment on body temperature and neurotoxicity during the waking (dark) cycle of the rat. This was done to more effectively replicate stimulant exposure in waking humans and to evaluate the relative risks of the three stimulants when taken inappropriately or non-therapeutically (e.g., abuse). Four subcutaneous injections (4×), at 2 h intervals, were used to administer each dose of the stimulants tested. Several equimolar doses for the three stimulants were chosen to produce plasma levels ranging from 3 times the highest therapeutic levels (no effect on body temperature) to those only attained by accidental overdose or intentional abuse in humans. Either 4×2.0 mg/kg AMPH or 4×2.2 mg/kg METH administered during the waking cycle resulted in peak serum levels of between 1.5 and 2.5 μM (4 to 5 times over maximum therapeutic levels of METH and AMPH) and produced lethal hyperthermia, 70% striatal dopamine depletions, and neurodegeneration in the cortex and thalamus. These results show that METH and AMPH are equipotent at producing lethal hyperthermia and neurotoxicity in laboratory animals during the wake cycle. Administration of either 4×2.2 or 4×3.3 mg/kg METH during the sleep cycle produced lower peak body temperatures, minimal dopamine depletions and little neurodegeneration. These findings indicate that administration of the stimulant during the waking cycle compared to sleep cycle may significantly increase the potency of amphetamines to produce hyperthermia, neurotoxicity and lethality. In contrast, body temperature during the waking cycle was only significantly elevated by MPH at 4×22 mg/kg, and the serum levels producing this effect were 2-fold (approximately 4.5 μM) greater on a molar basis than hyperthermic doses of AMPH and METH. Thus, AMPH and METH were equipotent on a mg/kg body weight basis at producing hyperthermia and neurotoxicity while MPH on a mg/kg body weight basis was approximately 10-fold less potent than AMPH and METH. However, the 10-fold lower potency was in large part due to lower plasma levels produced by MPH compared to either AMPH or METH.
Published by Elsevier Inc.