Carbachol-induced network oscillations in the intact cerebral cortex of the newborn rat

Abstract

In mature cortex, activation of the cholinergic system induces oscillatory network activity and facilitates synaptic plasticity. We used an in vitro preparation of the intact cerebral cortex and cortical slices of the neonatal rat to study carbachol (CCh, ≥30 μM)-induced network oscillations during the early postnatal period. Multi-site extracellular recordings revealed CCh-induced transient beta oscillations with an average duration of 4.6 ± 0.2 s, amplitude of 123 ± 7.4 μV and frequency of 17.7 ± 0.5 Hz. These oscillations propagated uniformly at 0.5-1.5 mm/s over the cortex and were reversibly blocked by tetrodotoxin (TTX) and atropine, indicating that they depended on action potentials and activation of muscarinic receptors. The activity was not blocked by bicuculline methiodide or gabazine, but was reversibly abolished by kynurenic acid, indicating that activation of glutamate receptors, but not GABA-A receptors, was required. CNQX caused a significant decrease in the power of the Fourier frequency spectrum of the CCh-induced oscillations and CPP or MK-801 completely blocked the activity, indicating a contribution of AMPA/kainate receptors and an essential role of NMDA receptors. Oscillations were synchronized between sites separated horizontally by ∼1 mm and for delays of 2-8 ms. Synchronized activity between neighboring recording sites was very stable over repeated applications of CCh. Whole-cell recordings from morphologically identified pyramidal neurons in the intact cortex revealed a close temporal correlation between CCh-induced membrane oscillations and local field potential recordings. In contrast, CCh-induced oscillations recorded in coronal neocortical slices were smaller in amplitude and frequency, suggesting that a widespread network of intracortical axonal connections is required for their generation.