137. Microglial Pruning of Dendritic Spines in the Prefrontal Cortex During Adolescence

Abstract

Background: Aberrant developmental synaptic elimination during adolescence has been suggested to contribute to both the reduced spine density of pyramidal cells (PCs) in the prefrontal cortex (PFC) and the cognitive deficits in schizophrenia. Recent data indicates that microglia are critically involved in synaptic pruning during early postnatal periods1-3. However, the role of microglia in the structural and functional maturation of the PFC, which is characterized by a protracted postnatal development with a later maturation than other cortices4,5, has not yet been examined. We therefore determined if microglia contribute to the developmentally-specific pruning of dendritic spines in the PFC, with a focus on the periadolescent period. Method(s): Equal numbers of male and female SpragueDawley rats, weaned at 21 days of age, were used as subjects. Randomly selected Layer 5 PCs from the prelimbic area of the PFC of one hemisphere were intracellularly filled with the fluorescent dye Lucifer Yellow for dendritic spine density measurements in rats sacrificed on postnatal days 24 and 30 (a time of increasing PFC PC spine density), 35 (posited peak spine density), and 39 and 50 (decreasing spine density). The contralateral hemisphere was used to quantitatively determine immune-histochemical colocalization of microglia (identified by Iba1 expression) and PFC PC dendritic spines (revealed by PSD-95-immunoreactivity) at these five periadolescent ages. Result(s): Dendritic spine density on L5 PFC PCs peaked between P30 and P35. We observed a significant increase in Iba1 and PSD-95 colocalization in the PFC at P39 relative to P24 and P30, consistent with active microglial pruning of dendritic spines on PFC PCs. We also noted a suggestion of sex differences in the timing of the increase in microglial engulfment of spines: males displayed a non-significant trend (p =0.06) toward an increase at P39 relative to earlier time points, while females showed no such trend. Conclusion(s): Our data suggests that the developmentallyspecific pruning of dendritic spines on PCs in the PFC occurs during adolescence, substantially later than in other areas of the brain; current efforts are examining if the presynaptic components of synapses are also engulfed by microglia during adolescence. These studies serve as a foundation for understanding the processes governing developmental PFC synaptic elimination, which is thought to be disrupted in schizophrenia and other disorders. The age at which we observe a sharp increase in microglial sculpting of PFC neurons is thought to correspond to early adolescence. Prodromal symptoms emerge in human adolescents and subsequently may evolve into a psychotic episode. An exaggeration of synaptic pruning in the PFC by microglia supports the hypothesis of Feinberg6 that a deranged enhancement of synaptic elimination is a key pathophysiological feature of schizophrenia. Our work may guide the identification of pharmacological targets aimed at early intervention to attenuate or prevent the cortical pathology and emergence of cognitive dysfunction in schizophrenia. In future experiments we will treat mice that conditionally express the diphtheria toxin (DT) receptor in microglia with DT to ablate microglia during critical pruning periods and assess in the adult the structural and functional consequences of adolescent microglial ablation, including the effects on spine density and on PFC-dependent behaviors (for example working memory) that are disrupted in schizophrenia.