The role of the medial prefrontal cortex in innate fear regulation in infants, juveniles, and adolescents.

In adult animals, the medial prefrontal cortex (mPFC) plays a significant role in regulating emotions and projects to the amygdala and periaqueductal gray (PAG) to modulate emotional responses. However, little is known about the development of this neural circuit and its relevance to unlearned fear in pre-adulthood. To address these issues, we examined the mPFC of 14-d-old (infants), 26-d-old (juveniles), and 38- to 42-d-old (adolescents) rats to represent different developmental and social milestones. The expression patterns of the neuronal marker FOS were used to assess neurological activity. Muscimol, a GABA agonist, was used to inactivate the prelimbic and infralimbic mPFC subdivisions (400 ng in 200 nl). Animals were exposed to either a threatening or nonthreatening stimulus that was ecologically relevant and age specific. Freezing was measured as an indicator of innate fear behavior. The data indicated that the mPFC is neither active nor responsive to innate fear in infant rats. In juveniles, the prelimbic mPFC became responsive in processing aversive sensory stimulation but did not regulate freezing behavior. Finally, during adolescence, inactivation of the prelimbic mPFC significantly attenuated freezing and decreased FOS expression in the ventral PAG. Surprisingly, across all ages, there were no significant differences in FOS levels in the medial and basolateral/lateral amygdala when either mPFC subdivision was inactivated. Together, unlearned fear has a unique developmental course with different brain areas involved in unlearned fear in the immature animal than the adult. In particular, the mPFC neural circuitry is different in young animals and progressively develops more capacities as the animal matures.

Prefrontal cortical inputs to the basal amygdala undergo pruning during late adolescence in the rat.

Transformations in affective and social behaviors, many of which involve amygdalar circuits, are hallmarks of adolescence in many mammalian species. In this study, using the rat as a model, we provide the first evidence that afferents of the basal amygdala (BA) undergo significant structural remodeling during adolescence. We used quantitative tract-tracing and gene expression profiling methods to characterize changes in the medial prefrontal cortical (mPFC) inputs to the BA across ages analogous to the late juvenile period [postnatal day (P) 25], late adolescence (P45), and adulthood (P90) in the rat. As assessed after deposition of Fluorogold into the BA, the number of BA-projecting neurons in the mPFC remained stable between P25 and P45 but decreased by about 50% between P45 and P90. Anterograde tract tracing with biotin dextran amine deposits centered in the ventral prelimbic cortex revealed that, during this period, the density of mPFC-derived axon terminals in the BA also decrease significantly, an effect particularly evident in the dorsal basolateral nucleus. Within the BA, there were also highly significant changes in gene expression indicative of neurite or synaptic plasticity, most notably in the Ras/GTPase superfamily, and in pathways that regulate cytoskeletal dynamics and steroid synthesis/lipid metabolism. These data provide convergent evidence that mPFC inputs to the BA are pruned during late adolescence or early adulthood. Moreover, the structural remodeling within these afferents may be accompanied by significant changes in neurite plasticity within the BA.

The role of the amygdala and olfaction in unconditioned fear in developing rats.

Early in ontogeny, young rats must be able to detect dangerous stimuli and to exhibit appropriate defensive behaviors. Different nuclei of the amygdala mediate unconditioned and conditioned fear responses to threat in adult rats. The aim of this study was to determine the role of the amygdala in unlearned fear behavior in young rats. When exposed to an unfamiliar adult male rat, preweaning rat pups freeze, with peak levels on postnatal day 14 and declining levels on day 18. Pups were made anosmic to block olfactory input to the amygdala, and amygdala activation was assessed by quantifying the neuronal marker c-fos. Anosmic pups did not freeze in the presence of the male rat and had decreased c-fos expression in the medial amygdala on day 14 and in the medial and lateral amygdala on day 18. However, the decrease in freezing between days 14 and 18 was not associated with a decrease in c-fos expression in the medial amygdala. The medial and lateral amygdala were then inactivated by local muscimol infusion on day 14. Muscimol infusion into the medial amygdala decreased freezing to the male rat but not to a loud noise, whereas infusion into the lateral amygdala blocked freezing to a loud noise but not to the male. These findings indicate that different nuclei of the amygdala process sensory information of different modalities, mediate unconditioned freezing, and may be involved in developmental changes in the fear response in young rats.