Axons in the cerebral cortex receive synaptic input at the axon initial segment almost exclusively from gamma-aminobutyric acid-releasing (GABAergic) axo-axonic cells (AACs). The axon has the lowest threshold for action potential generation in neurons; thus, AACs are considered to be strategically placed inhibitory neurons controlling neuronal output. However, we found that AACs can depolarize pyramidal cells and can initiate stereotyped series of synaptic events in rat and human cortical networks because of a depolarized reversal potential for axonal relative to perisomatic GABAergic inputs. Excitation and signal propagation initiated by AACs is supported by the absence of the potassium chloride cotransporter 2 in the axon.

Gamma-aminobutyric acid (GABA) is predominantly released by local interneurons in the cerebral cortex to particular subcellular domains of the target cells1,2. This suggests that compartmentalized, synapse specific action of GABA is required in cortical networks for phasic inhibition2–4. However, GABA released at the synaptic cleft diffuses to receptors outside the postsynaptic density and thus tonically activates extrasynaptic GABAA and GABAB receptors, which include subtypes of both receptor families especially sensitive to low concentrations of GABA3–7. The synaptic and extrasynaptic action of GABA is in line with idea that neurons of the brain use synaptic (or wiring) transmission and nonsynaptic (or volume) transmission for communication8,9. However, reuptake mechanisms restrict the spatial extent of extrasynaptic GABAergic effects10,11 and it was proposed that concerted action of several presynaptic interneurons or sustained firing of individual cells or increased release site density is required to reach ambient GABA levels sufficient to activate extrasynaptic receptors4,9,11–13. Here we show that individual neurogliaform cells release GABA sufficient for volume transmission within the axonal cloud and thus neurogliaform cells do not require synapses to produce inhibitory responses in the overwhelming majority of nearby neurons. Neurogliaform cells suppress connections between other neurons acting on presynaptic terminals which do not receive synapses at all in the cerebral cortex and, moreover, reach extrasynaptic, δ subunit containing GABAA (GABAAδ) receptors responsible for tonic inhibition. We reveal that GABAAδ receptors are localized to neurogliaform cells preferentially among cortical interneurons. Neurosteroids, which are modulators of GABAAδ receptors, alter unitary GABAergic effects between neurogliaform cells. In contrast to the specifically placed synapses formed by other interneurons, the output of neurosteroid sensitive neurogliaform cells represents the ultimate form of spatial unspecificity in GABAergic systems leading to long lasting network hyperpolarization combined with widespread suppression of communication in the local circuit.

Summary CA1 pyramidal cells (PCs) are not homogeneous, but rather can be grouped by molecular, morphological, and functional properties. However, less is known about synaptic sources differentiating PCs. Using paired recordings in vitro, 2-photon Ca2+ imaging in vivo and computational modeling, we found that parvalbumin-expressing basket cells (PVBCs) evoked greater inhibition in CA1 PCs located in the deep compared to superficial layer of stratum pyramidale. In turn, analysis of reciprocal connectivity revealed more frequent excitatory inputs to PVBCs by superficial PCs, demonstrating bias in target selection by both the excitatory and inhibitory local connections in CA1. Additionally, PVBCs further segregated among deep PCs, preferentially innervating the amygdala-projecting PCs but receiving preferential excitation from the prefrontal cortex-projecting PCs, thus revealing distinct perisomatic inhibitory interactions between separate output channels. These results demonstrate the presence of heterogeneous PVBC-PC microcircuits, potentially contributing to the sparse and distributed structure of hippocampal network activity.

Synaptic interactions between neurons of the human cerebral cortex were not directly studied to date. We recorded the first dataset, to our knowledge, on the synaptic effect of identified human pyramidal cells on various types of postsynaptic neurons and reveal complex events triggered by individual action potentials in the human neocortical network. Brain slices were prepared from nonpathological samples of cortex that had to be removed for the surgical treatment of brain areas beneath association cortices of 58 patients aged 18 to 73 y. Simultaneous triple and quadruple whole-cell patch clamp recordings were performed testing mono- and polysynaptic potentials in target neurons following a single action potential fired by layer 2/3 pyramidal cells, and the temporal structure of events and underlying mechanisms were analyzed. In addition to monosynaptic postsynaptic potentials, individual action potentials in presynaptic pyramidal cells initiated long-lasting (37 ± 17 ms) sequences of events in the network lasting an order of magnitude longer than detected previously in other species. These event series were composed of specifically alternating glutamatergic and GABAergic postsynaptic potentials and required selective spike-to-spike coupling from pyramidal cells to GABAergic interneurons producing concomitant inhibitory as well as excitatory feed-forward action of GABA. Single action potentials of human neurons are sufficient to recruit Hebbian-like neuronal assemblies that are proposed to participate in cognitive processes.