ABSTRACT
Previous studies have shown that excitotoxic hippocampal lesions in rats attenuate the ability of different doses of haloperidol, but not of clozapine, to suppress locomotor activity. The purpose of the present study was to determine if kainic acid-induced hippocampal damage reduces the degree of locomotor suppression produced by two relatively newer antipsychotic drugs, risperidone and olanzapine. Young adult male rats received bilateral intracerebroventricular infusions of the excitotoxin, kainic acid (KA), or vehicle and were tested for locomotor responses to drug treatment 30 days later. Infusions of KA produced neuronal loss in the CA3 region of the dorsal hippocampus in every rat. As reported previously, KA lesions reduced the ability of haloperidol (0.35 mg/kg) to completely suppress the locomotor activity elicited by amphetamine (1.5 mg/kg) relative to the effect of haloperidol in non-lesioned controls. Lesioned animals treated with a moderate dose of risperidone (1.4 mg/kg) also exhibited significantly more locomotor activity after amphetamine treatment in comparison to control animals. A trend toward greater activity was also observed in the lesioned group relative to the control group after olanzapine (3.0 mg/kg) injection (p =.09, 2-tailed). The locomotor effects of lower and higher doses of risperidone and olanzapine were not altered by kainic acid lesions. These data suggest that the locomotor-suppressive effects of moderate doses of risperidone and, perhaps, olanzapine involve hippocampal neurons, but that higher doses of each drug can suppress activity in a hippocampal-independent manner.
Subject(s)
Excitatory Amino Acid Agonists/pharmacology , Hippocampus/physiology , Motor Activity/drug effects , Pirenzepine/analogs & derivatives , Pirenzepine/pharmacology , Risperidone/pharmacology , Animals , Benzodiazepines , Dose-Response Relationship, Drug , Hippocampus/drug effects , Kainic Acid/pharmacology , Male , Motor Activity/physiology , Olanzapine , Rats , Rats, Sprague-DawleyABSTRACT
The degree to which the neonatal hippocampus is resistant to the effects of excitotoxins, such as kainic acid (KA) remains uncertain. Previously, we showed delayed loss of hippocampal neurons during pubescence in neonatal rats subjected to intracerebroventricular (i.c.v.) KA administration (10 nmol) at postnatal day 7 (P7). To further characterize the time course as well as the underlying mechanisms of this neuronal loss, we administered i.c.v. KA (10 or 50 nmol) to P7 preweanling rats. Brain sections were then examined at several neurodevelopmental time points (i.e., P8, P14, P25, P40, P60 and P75) using thionin staining and three-dimensional, non-biased cell counting to assess neuronal loss, and immunohistochemistry and electron microscopy to search for evidence of necrosis and apoptosis. Dose-dependent acute neuronal loss was observed at P8-P14 in hippocampal subfields CA3a and CA3c. Transient heat shock protein (HSP-70) immunostaining accompanied this acute neuronal loss. Progressive neuronal loss then continued in CA3 until P75, but without concomitant HSP-70 immunostaining. Progressive neuronal cell loss was also observed in the CA1 subfield of the hippocampus beginning at pubescence (i.e., P40) and continuing until P75. The appearance of TUNEL-positive hippocampal neurons accompanied the delayed neuronal loss in both CA3 and CA1 and electron micrographs confirmed that neurons in these subfields were undergoing apoptosis. KA administration (i.c.v.) to preweanling rats caused both immediate and delayed damage to hippocampal neurons. The effect of KA was dose-dependent, and the delayed neuronal damage occurred through an apoptosis-mediated mechanism. These findings may be relevant to the pathogenesis of some neuropsychiatric disorders, where early CNS injury is not apparent until the onset of clinical symptoms in young adulthood.
