Neuronal responses and topographic organization of feature selectivity in the cerebral cortex are shaped by ascending inputs and by intracortical connectivity. The mammalian primary auditory cortex has a tonotopic arrangement at large spatial scales (greater than 300 microns). This large-scale architecture breaks down in supragranular layers at smaller scales (around 300 microns), where nearby frequency and sound level tuning properties can be quite heterogeneous. Since layer 4 has a more homogeneous architecture, the heterogeneity in supragranular layers might be caused by heterogeneous ascending input or via heterogeneous intralaminar connections. Here we measure the functional 2-dimensional spatial connectivity pattern of the supragranular auditory cortex on micro-column scales. In general connection probability decreases with radial distance from each neuron, but the decrease is steeper in the isofrequency axis leading to an anisotropic distribution of connection probability with respect to the tonotopic axis. In addition to this radial decrease in connection probability we find a patchy organization of inhibitory and excitatory synaptic inputs that is also anisotropic with respect to the tonotopic axis. These periodicities are at spatial scales of ~100 and ~300 μm. While these spatial periodicities show anisotropy in auditory cortex, they are isotropic in visual cortex, indicating region specific differences in intralaminar connections. Together our results show that layer 2/3 neurons in auditory cortex show specific spatial intralaminar connectivity despite the overtly heterogeneous tuning properties.

During the critical period, neuronal connections are shaped by sensory experience. While the basis for this temporarily heightened plasticity remains unclear, shared connections introducing activity correlations likely play a key role. Thus, we investigated the changing intracortical connectivity in primary auditory cortex (A1) over development. In adult, layer 2/3 (L2/3) neurons receive ascending inputs from layer 4 (L4) and also receive few inputs from subgranular layer 5/6 (L5/6). We measured the spatial pattern of intracortical excitatory and inhibitory connections to L2/3 neurons in slices of mouse A1 across development using laser-scanning photostimulation. Before P11, L2/3 cells receive most excitatory input from within L2/3. Excitatory inputs from L2/3 and L4 increase after P5 and peak during P9–16. L5/6 inputs increase after P5 and provide most input during P12–16, the peak of the critical period. Inhibitory inputs followed a similar pattern. Functional circuit diversity in L2/3 emerges after P16. In vivo two-photon imaging shows low pairwise signal correlations in neighboring neurons before P11, which peak at P15–16 and decline after. Our results suggest that the critical period is characterized by high pairwise activity correlations and that transient hyperconnectivity of specific circuits, in particular those originating in L5/6, might play a key role.

Intracellular photorelease of cagedd- myo-inositol 1,4,5-trisphosphate (IP₃), caffeine application, and immunofluorescence confocal microscopy were used to determine thatd- myo-inositol 1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs) coexist in rabbit vagal sensory nodose ganglion neurons (NGNs). ATP, an extracellular physiological signaling molecule, consistently evoked robust transient increases in cytosolic free Ca²⁺concentration (Ca²⁺transients). ATP applied in Ca²⁺-free physiological saline elicited Ca²⁺transients that averaged approximately 70% of the amplitude of transients evoked in the presence of extracellular Ca²⁺. The component of the ATP-evoked Ca²⁺transient that was independent of extracellular Ca²⁺corresponds to Ca²⁺release from intracellular stores. This release component was sensitive to the pharmacological antagonists pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS), U73122 , neomycin, and heparin (13.5–15 kD), indicating that P2 purinoreceptors (P2Y) and the IP₃signaling pathway are required for ATP-evoked Ca²⁺release. Additionally, a portion of ATP-evoked Ca²⁺release was inhibited by ryanodine, a selective blocker of RyRs. The ryanodine-insensitive component (approximately 70%) of ATP-evoked Ca²⁺release corresponds to IP₃-induced Ca²⁺release via IP₃Rs, while the ryanodine-sensitive component (approximately 30%) corresponds to consequent Ca²⁺-induced Ca²⁺release (CICR) via RyRs. These results indicate that functional IP₃Rs and RyRs coexist in nodose neurons and that both IP₃-induced Ca²⁺release and CICR can be activated by ATP.