Effects of early life adversity and adolescent basolateral amygdala activity on corticolimbic connectivity and anxiety behaviors

Abstract

Early postnatal life is a dynamic period of brain development that involves a coordinated symphony of circuit maturation. As many regions within the brain continue to develop and mature postnatally, external stimuli impact this development and regulate the degree of connectivity to other brain areas. Since subcortical limbic regions undergo substantial synapse formation and pruning during early development while higher-order areas such as the prefrontal cortex (PFC) mature during adolescence, early postnatal life and adolescence are two sensitive periods when limbic and prefrontal regions form functional connections in response to external stimuli and neuronal activity. Thus, activity within limbic regions over the course of development can affect the strength and quantity of efferent connections to cortical areas. Prior work has shown hyper-innervation of glutamatergic basolateral amygdala (BLA) projections to the prefrontal cortex (PFC) in the adolescent time period following rearing in an adverse early life environment, but how these ELA-induced connectivity changes alter the startle circuitry required to execute a startle response and whether the BLA-PFC pathway is responsible for previously reported blunted startle response remains unknown. We directly tested whether BLA activity during a discrete adolescent period altered later anxiety behaviors, and if behavioral changes were driven by altered PFC innervation. To test these questions, we exposed rat pups to an ELA model of maternal separation on postnatal days (P) 2-21. Rats then underwent an injection of an inhibitory DREADD in the BLA in order to silence the excitatory projections during the adolescent time period of P33-39. Finally, rats were tested in an acoustic startle paradigm in late adolescence to assess the effects of reduced BLA-PFC connectivity on the ELA-induced blunted startle response. Density, intensity, and volume of axonal boutons in the BLA projecting PFC neurons were then assessed to elucidate how BLA inhibition during adolescence affects innervation later in life. These results explore how altered early life environments may affect the development of anxiety-related circuitry and anxiety responses later in life, and how rescue of this maladaptive response may be sex and developmentally specific.