Endocannabinoids regulate microglia in developing brain

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Endocannabinoids (EDCs) and the innate immune system are two early signaling systems which independently and together profoundly influence brain development. Critical windows of development are also often sensitive periods for disruption by exogenous stimuli. We have discovered a unique critical period in the neonatal amygdala of the rat during which the innate immune cells of the brain, microglia, actively engulf and kill newborn cells that if left unmolested would have gone on to become astrocytes. Through this mechanism the future density of astrocytes in the amygdala is determined. Most remarkably, the optimal density is lower for males than females in that it is causally responsible for increased neuronal activity many weeks later during epochs of adolescent social play. Equally remarkable, the increased phagocytic activity of microglia in the male amygdala is a direct consequence of a higher EDC tone, which is in turn developmentally programmed by elevated androgens in neonatal males. Changes to either androgen levels or, more importantly, EDC tone, including by exposure to THC, during the critical period permanently alters the neuronal/astrocytic population and playfulness during adolescence. Marijuana use during pregnancy and breastfeeding is prevalent and increasing due to legalization, decriminalization and medicalization but we are largely ignorant of the potential consequences to the fetal and newborn brain. Advances in transcriptomics and adeno-associated virus (AAV) techniques provide new tools for exploration and precision not previously accessible for the rat. A primary goal is a deep mechanistic understanding of endocannabinoid-induced microglia phagocytosis of astrocytic progenitors to illuminate with unprecedented specificity the interaction between immune, neuronal, astrocytic and progenitor cells. An additional goal is to establish a larger framework for understanding development THC exposure that converges on inhibitory neurons in brain regions regulating sex-typic social behaviors. We will achieve these dual ends via the following Specific Aims. Aim 1 will fully characterize the endocannabinoid and innate immune systems of the developing amygdala by high precision cell phenotyping, identification of the source of complement proteins needed for phagocytosis, and transcriptomics to identify genes mediating cell survival. In Aim 2 we will selectively reduce androgen receptor in one cell type at a time to determine the source of the sex difference in endocannabinoid tone and Aim 3 will interrogate the relationship between the effects of THC, EDCs and androgen action with an emphasis on inhibitory neurons across brain regions. Combined, these studies will provide unprecedented clarity in the cellular participants and mechanisms establishing a sex difference in a social circuit and how that process can go awry as a consequence of developmental THC exposure.