Rationale: Psychotomimetic drug-induced locomotor hyperactivity is a widely used animal model of psychotic states, such as in schizophrenia. We previously found that serotonergic lesions of the dorsal, but not ventral, hippocampus in rats result in enhanced phencyclidine-induced locomotor hyperactivity.
Objectives: The objective of this study was to investigate the effect of serotonin depletion in the dorsal and ventral hippocampus on hyperlocomotion induced by ketamine, cocaine, 3,4-methylenedioxymethampethamine (MDMA), methamphetamine, and D: -amphetamine.
Materials and methods: Male Sprague-Dawley rats were bilaterally microinjected with vehicle or the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the dorsal or ventral hippocampus using a stereotaxic approach. Separate cohorts of rats were used for each drug of abuse; each rat received saline and a low, medium, and high dose of the drug in a random-sequence, repeated-measures protocol. Locomotor hyperactivity following treatment was measured using automated photocell cages.
Results: Similar to phencyclidine, 5,7-DHT-induced lesions of the dorsal hippocampus enhanced ketamine-induced hyperlocomotion at all doses. They also reduced methamphetamine-induced hyperlocomotion at the high dose only and caused a minor, biphasic modulation of responses to cocaine. Locomotor responses to D: -amphetamine and MDMA were unchanged by lesions of the dorsal hippocampus. Serotonergic lesions of the ventral hippocampus did not significantly alter locomotor hyperactivity induced by any of the drugs investigated.
Conclusions: These findings further implicate a role for serotonin in the dorsal hippocampus in modulating the behavioral effects of dissociative anesthetics, such as ketamine, with more subtle effects on psychostimulant drugs of abuse. The dorsal hippocampus may be a site of serotonergic dysfunction in aspects of schizophrenia.