title: Methylphenidate potentiates morphine-induced antinociception, hyperthermia, and locomotor activity in young adult rats.
abstract: The goal of this study was to determine if the exaggerated morphine-induced conditioned place preference (CPP) response seen in adult rats after preweanling methylphenidate exposure is unique to reward-mediated behaviors or is indicative of generalized changes in opioid-mediated behaviors. Rats were exposed to saline or methylphenidate (2.0 or 5.0 mg/kg) for 10 consecutive days starting on postnatal (PD) 11 with testing beginning on PD 60. In Experiment 1, morphine-induced (0, 2.5, 5.0 or 10.0 mg/kg) antinociception was assessed using the tail immersion and hot plate tasks. In Experiment 2, morphine-induced (0, 2.5, 5.0, or 10.0 mg/kg) hyperthermia and locomotor activity were measured. Morphine caused an increase in antinociception, with early methylphenidate (5.0 mg/kg) exposure potentiating the effects of 5.0 mg/kg morphine. Rectal temperatures were elevated after morphine, with the greatest increase occurring in male rats. Methylphenidate potentiated the hyperthermic effects of morphine (10.0 mg/kg) but only in males. Moderate doses (2.5 and 5.0 mg/kg) of morphine increased the locomotor activity of adult rats, while a higher dose (10.0 mg/kg) decreased locomotion. Interestingly, methylphenidate-pretreated females showed increased locomotor activity relative to controls. These results suggest that early methylphenidate exposure induces general changes in opioid system functioning that are not specific to reward-mediated behaviors.
authors.affiliation: Department of Psychology, California State University, San Bernardino, CA 92407, USA.
authors.forename: Lindsay R
authors.initials: LR
authors.lastname: Halladay
authors.forename: Sergio D
authors.initials: SD
authors.lastname: Iñiguez
authors.forename: Faiza
authors.initials: F
authors.lastname: Furqan
authors.forename: Matt C
authors.initials: MC
authors.lastname: Previte
authors.forename: Ashley M
authors.initials: AM
authors.lastname: Chisum
authors.forename: Cynthia A
authors.initials: CA
authors.lastname: Crawford
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001831:Body Temperature / Q000187:drug effects; D001835:Body Weight / Q000187:drug effects; D000697:Central Nervous System Stimulants / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D005260:Female; D005334:Fever / Q000139:chemically induced*; D006358:Hot Temperature; D007101:Immersion / Q000503:physiopathology; D008297:Male; D008774:Methylphenidate / Q000494:pharmacology*; D009020:Morphine / Q000494:pharmacology*; D009043:Motor Activity / Q000187:drug effects*; D010147:Pain Measurement / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time / Q000187:drug effects; D017450:Receptors, Opioid, mu / Q000819:agonists
chemical_list: D000701:Analgesics, Opioid; D000697:Central Nervous System Stimulants; D017450:Receptors, Opioid, mu; D008774:Methylphenidate; D009020:Morphine