Excitotoxic amino acid injections into the medial amygdala facilitate maternal behavior in virgin female rats.

The inhibitory role of the medial amygdala (MA) in maternal behavior control was explored. Injections of N-methyl-D,L-aspartic acid (NMA), an excitotoxic amino acid, into the MA resulted in a dramatic facilitation of maternal behavior in virgin female rats when pups were presented to them 12 days following the injections. This effect was specific to MA in that NMA injections into the basolateral amygdala were ineffective. The facilitatory effect of NMA injections into MA was found to be hormone dependent in that ovariectomies abolished the effect. Subsequent experiments provided evidence that NMA injections into MA induced a pseudopregnant state lasting about 13 days, and that maternal behavior was greatly facilitated only when pups were presented to such females coincident with pseudopregnancy termination. When pups were presented 24 days following injections of NMA into MA, rather than 12 days, only a modest facilitation of maternal behavior was observed. It was concluded that NMA injections into MA influence maternal behavior in two ways: By inducing an hormonal state stimulatory for maternal behavior and by removing neural inhibition over maternal behavior.