Abstracts

Rationale: Cortical development is a highly orchestrated process involving neural precursor cell (NPC) expansion within specialized compartments of the fetal brain, radial migration of post-mitotic neurons with distinct laminar fates, and preferential synaptic wiring between radially-aligned clones. Microglia, constituents of innate immunity, play a pivotal role during corticogenesis by regulating NPC numbers in the developing cortex and by mediating synapse elimination postnatally. Systemic maternal infection, a risk factor for epilepsy, alters the in-utero inflammatory milieu leading to aberrant cortical patterning in offspring. It is unknown, however, whether inflammatory insults during gestation contribute to persistent changes in intracortical circuits that lead to epilepsy.Methods: To examine the physiological effect of maternal immune activation (MIA), we administered a single i.p injection of the viral mimic polyinosinic: polycytidylic acid (poly(I:C)) to pregnant dams at E12.5. We assayed prefrontal (PFC) and motor (M1) cortical activity in acute slices from the 4 week old progeny of poly(I:C) and saline injected dams. Multi-channel linear silicon probes were used to simultaneously record extracellular local field potentials (LFPs) from 16 equally spaced sites spanning all cortical layers. Current source density (CSD) analysis localized transmembrane current flow through the circuit. The CSD is a measure of synaptic current leaving (sink) or entering (source) an extracellular space. We computed the maximum CSD to estimate the strength of synaptic inputs to each layer. We also used the relative timing of sinks and sources to monitor signal propagation between layers. This method provides a high-throughput assessment of functional intra- and inter-laminar cortical connectivity, which may be altered by MIA. Electrical stimuli (50-500 microamperes) were delivered to L5 of PFC and L2/3 of M1, both principal input layers. After repeated stimulation, LFP data was averaged across trials, and the CSD profile was derived.Results: We found that PFC and M1 show stereotypic patterns of interlaminar current flow in saline controls. CSD mapping revealed a strong and early synaptic sink in the superficial layers of PFC followed by a delayed and long-lasting source. Synaptic sinks of weaker magnitude were elicited in L6. In M1, synaptic sinks were initially evoked in L2/3 and subsequently present in L5b. A strong and late synaptic source was confined to L5a. Maternal poly(I:C) treatment did not alter the laminar position of synaptic currents nor their relative time of onset. However, the amplitude of current sinks in the deep layers of PFC and M1 were elevated, suggesting that synaptic excitation is enhanced in adult animals after MIA.Conclusions: These data indicate that viral infection during gestation can lead to long-lasting increases in circuit excitability that contribute to the development of epilepsy. Moreover, MIA induced global alterations in cortical activity, suggesting that prenatal inflammation broadly increases seizure susceptibility in affected individuals.