The effects of benzodiazepines in newborn rats suggest a function for type 2 receptors.

In day 4 female rats lorazepam, diazepam and clonazepam produced dose-related increases in forward walking and loss of righting and diazepam produced a dose-related increase in paddling. Lorazepam and diazepam increased jerks of the fore- and hind-limbs and the whole body, and clonazepam increased the latter two; these increases were not dose-related. Some doses of lorazepam and the lowest dose of diazepam increased tonic-clonic movements. Thus the benzodiazepines were observed to have two kinds of stimulant effect in day 4 rats. One is to cause hyperactivity and this effect is dose-related. The other is to cause a type of seizure-like behavior, although this action is not dose-related and the responses can be distinguished from those caused by convulsant compounds. The effects of the benzodiazepine antagonist Ro 15-1788 resembled those of the benzodiazepines. It increased hind-limb and whole body jerks, forward walking, paddling, loss of righting and tonic-clonic movements. CL 218,872, which is selective for type 1 benzodiazepine receptors, was devoid of significant effects. This suggests that the behavioral changes observed with the other compounds were mediated by the type 2 receptors.