SYNCHRONIZATION of neuronal activity is fundamental in the operation of cortical networks. With respect to an ongoing synchronized oscillation, the precise timing of action potentials is an attractive candidate mechanism for information coding. Networks of inhibitory interneurons have been proposed to have a role in entraining cortical, synchronized 40-Hz activity. Here we demonstrate that individual GABAergic interneurons can effectively phase spontaneous firing and subthreshold oscillations in hippocampal pyramidal cells at 0 frequencies (4-7 Hz). The efficiency of this entrainment is due to interaction of GABAA-receptor-mediated hyperpolarizing synaptic events with intrinsic oscillatory mechanisms tuned to this frequency range in pyramidal cells. Moreover, this GABAergic mechanism is sufficient to synchronize the firing of pyramidal cells. Thus, owing to the divergence of each GABAergic interneuron, more than a thousand pyramidal cells may share a common temporal reference established by an individual interneuron.

Acetylcholine is vital for cognitive functions of the brain. Although its actions in the individual cell are known in some detail, its effects at the network level are poorly understood. The hippocampus, which receives a major cholinergic input from the medial septum/diagonal band, is important in memory and exhibits network activity at 40 Hz during relevant behaviours. Here we show that cholinergic activation is sufficient to induce 40-Hz network oscillations in the hippocampus in vitro. Oscillatory activity is generated spontaneously in the CA3 subfield and can persist for hours. During the oscillatory state, principal neurons fire action potentials that are phase-related to the extracellular oscillation, but each neuron fires in only a small proportion of the cycles. Both excitatory and inhibitory synaptic events participate during the network oscillation in a precise temporal pattern. These results indicate that subcortical cholinergic input can control hippocampal memory processing by inducing fast network oscillations.

Neural organoids have the potential to improve our understanding of human brain development and neurological disorders. However, it remains to be seen whether these tissues can model circuit formation with functional neuronal output. Here, we have adapted air-liquid interface culture to cerebral organoids leading to improved neuronal survival and axon outgrowth. The resulting thick axon tracts display various morphologies including long-range projection within and away from the organoid, growth cone turning, and decussation. Single-cell RNA-sequencing reveals various cortical neuronal identities, and retrograde tracing demonstrates tract morphologies that match proper molecular identities. These cultures exhibit active neuronal networks, and extracortical projecting tracts can innervate mouse spinal cord and evoke contractions of adjacent muscle in a manner dependent on intact organoid-derived innervating tracts. Overall, these results reveal a remarkable self-organization of corticofugal and callosal tracts with a functional output, providing new opportunities to examine relevant aspects of human CNS development and disease.

Coherent network oscillations in several distinct frequency bands are seen in the hippocampus of behaving animals. To investigate how different neuronal types within this network respond to oscillatory inputs we made whole‐cell current clamp recordings from three different types of neurones in the CA1 region of rat hippocampal slices: pyramidal cells, fast‐spiking interneurones and horizontal interneurones, and recorded their response to sinusoidal inputs at physiologically relevant frequencies (1‐100 Hz). Pyramidal neurones showed firing preference to inputs at theta frequencies (range 2‐7 Hz; n= 30). They showed subthreshold resonance in the same frequency range (2‐7 Hz; mean 4.1 ± 0.4 Hz; n= 19). Interneurones differed in their firing properties. Horizontal interneurones in the stratum oriens showed firing preference to inputs at theta frequencies (range 1.5‐10 Hz; n= 10). These interneurones also showed resonance at low frequencies (range 1‐5 Hz; mean 2.4 ± 0.5 Hz; n= 7). In contrast, fast‐spiking interneurones with cell bodies in the pyramidal cell layer fired preferentially at input frequencies in the gamma band (range 30‐50 Hz; n= 10/12). These interneurones showed resonance at beta‐gamma frequencies (10‐50 Hz; mean 26 ± 5 Hz; n= 7/8). Thus, in the hippocampus, different types of neurones have distinct frequency preferences. Therefore, in the CA1 layer of the hippocampal network, a compound oscillatory input may be segregated into distinct frequency components which are processed locally by distinct types of neurones.

Gamma frequency (30 -100 Hz) network oscillations occur in the intact hippocampus during awake, attentive behavior. Here, we explored the underlying cellular mechanisms in an in vitro model of persistent gamma-frequency oscillations, induced by bath application of 20 M carbachol in submerged hippocampal slices at 30 Ϯ 1°C. Current-source density analysis of the field oscillation revealed a prominent alternating sink-source pair in the perisomatic and apical dendritic regions of CA3. To elucidate the active events generating these extracellular dipoles, we examined the firing properties of distinct neuron types. Visually guided unit recordings were obtained from individual CA3 neurons followed by intracellular labeling for anatomical identification. Pyramidal cells fired at 2.82 Ϯ 0.7 Hz, close to the negative peak of the oscillation (0.03 Ϯ 0.65 msec), and often in conjunction with a negative spike-like component of the field potential. In contrast, all phase-coupled interneurons fired after this negative peak. Perisomatic inhibitory interneurons fired at high frequency (18.1 Ϯ 2.7 Hz), shortly after the negative peak (1.97 Ϯ 0.95 msec) and were strongly phase-coupled. Dendritic inhibitory interneurons fired at lower frequency (8.4 Ϯ 2.4 Hz) and with less fidelity and a longer delay after the negative peak (4.3 Ϯ 1.1 msec), whereas interneurons with cell body in the stratum radiatum often showed no phase relationship with the field oscillation. The phase and spike time data of individual neurons, together with the current-source density analysis, support a synaptic feedback model of gamma oscillations primarily involving pyramidal cells and inhibitory cells targeting their perisomatic region.