MATERNAL-CARE DURING INFANCY REGULATES THE DEVELOPMENT OF NEURAL SYSTEMS MEDIATING THE EXPRESSION OF FEARFULNESS IN THE RAT

The mothers of infant rats show individual differences in the frequenc y of licking/grooming and arched-back nursing (LG-ABN) of pups that co ntribute to the development of individual differences in behavioral re sponses to stress. As adults, the offspring of mothers that exhibited high levels of LG-ABN showed substantially reduced behavioral fearfuln ess in response to novelty compared with the offspring of low LG-ABN m others. In addition, the adult offspring of the high LG-ABN mothers sh owed significantly (i) increased central benzodiazepine receptor densi ty in the central, lateral, and basolateral nuclei of the amygdala as well as in the locus ceruleus, (ii) increased alpha(2) adrenoreceptor density in the locus ceruleus, and (iii) decreased corticotropin-relea sing hormone (CRH) receptor density in the locus ceruleus, The express ion of fear and anxiety is regulated by a neural circuitry that includ es the activation of ascending noradrenergic projections from the locu s ceruleus to the forebrain structures. Considering the importance of the amygdala, notably the anxiogenic influence of CRH projections from the amygdala to the locus ceruleus, as well as the anxiolytic actions of benzodiazepines, for the expression of behavioral responses to str ess, these findings suggest that maternal care during infancy serves t o ''program'' behavioral responses to stress in the offspring by alter ing the development of the neural systems that mediate fearfulness.