Abstract
Various strains of mice display a reliable increase in motor activity in response to benzodiazepines given at low to moderate doses. This hyperactivity has been described as both an anxiolytic-associated increase in exploratory activity and a nonspecific stimulant effect controlled by central neural mechanisms separate from those involved in the anxiolytic-like effects. The purpose of the current study was to investigate the neural circuitry underlying the hyperactivity effects of benzodiazepines in mice. Specifically, we examined the relationship between anxiety and motor activity after bilateral intra-amygdala or intra-hippocampal microinjections of the nonselective full benzodiazepine receptor agonist midazolam in C57BL/6 mice. Behavioral measures of anxiety and motor activity in open field were examined in mice given localized injections of 0, 2, 8 or 32 nmol of midazolam directed into the amygdala or hippocampus. Midazolam injected into the amygdala at the low dose produced an anxiolytic-like effect, as reflected by an increase in central open field activity. Higher doses injected into the amygdala produced a motor-depressant action, indicative of a drug-induced sedative effect. Infusions into the hippocampus produced a biphasic effect on motor activity with the two lower doses of midazolam producing a motor-stimulant action and the high dose producing a motor-depressant effect. Hippocampus injections produced no anxiolytic-like effects. The current findings demonstrate that injections of midazolam produced a regional dissociation of the anxiety-related and motor-related parameters and provide evidence that the stimulant and anxiolytic effects of benzodiazepines are independent phenomena regulated by different central mechanisms.