Target cell type-dependent differences in presynaptic release probability (P r ) and short-term plasticity are intriguing features of cortical microcircuits that increase the computational power of neuronal networks. Here, we tested the hypothesis that different voltage-gated Ca 2ϩ channel densities in presynaptic active zones (AZs) underlie different P r values. Two-photon Ca 2ϩ imaging, triple immunofluorescent labeling, and 3D electron microscopic (EM) reconstruction of rat CA3 pyramidal cell axon terminals revealed ϳ1.7-1.9 times higher Ca 2ϩ inflow per AZ area in high P r boutons synapsing onto parvalbumin-positive interneurons (INs) than in low P r boutons synapsing onto mGluR1␣-positive INs. EM replica immunogold labeling, however, demonstrated only 1.15 times larger Cav2.1 and Cav2.2 subunit densities in high P r AZs. Our results indicate target cell type-specific modulation of voltage-gated Ca 2ϩ channel function or different subunit composition as possible mechanisms underlying the functional differences. In addition, high P r synapses are also characterized by a higher density of docked vesicles, suggesting that a concerted action of these mechanisms underlies the functional differences.

Medial septal GABAergic neurons of the basal forebrain innervate the hippocampus and related cortical areas, contributing to the coordination of network activity, such as theta oscillations and sharp wave-ripple events, via a preferential innervation of GABAergic interneurons. Individual medial septal neurons display diverse activity patterns, which may be related to their termination in different cortical areas and/or to the different types of innervated interneurons. To test these hypotheses, we extracellularly recorded and juxtacellularly labeled single medial septal neurons in anesthetized rats in vivo during hippocampal theta and ripple oscillations, traced their axons to distant cortical target areas, and analyzed their postsynaptic interneurons. Medial septal GABAergic neurons exhibiting different hippocampal theta phase preferences and/or sharp wave-ripple related activity terminated in restricted hippocampal regions, and selectively targeted a limited number of interneuron types, as established on the basis of molecular markers. We demonstrate the preferential innervation of bistratified cells in CA1 and of basket cells in CA3 by individual axons. One group of septal neurons was suppressed during sharp wave-ripples, maintained their firing rate across theta and non-theta network states and mainly fired along the descending phase of CA1 theta oscillations. In contrast, neurons that were active during sharp wave-ripples increased their firing significantly during “theta” compared to “non-theta” states, with most firing during the ascending phase of theta oscillations. These results demonstrate that specialized septal GABAergic neurons contribute to the coordination of network activity through parallel, target area- and cell type-selective projections to the hippocampus.

Key points The amplitude of unitary, single action potential‐evoked [Ca2+] transients negatively correlates with GCaMP6f expression, but displays large variability among hippocampal pyramidal cells with similarly low expression levels. The summation of fluorescence signals is frequency‐dependent, supralinear and also shows remarkable cell‐to‐cell variability. The main source of spike inference error is variability in the peak amplitude, and not in the decay or supralinearity. We developed two procedures to estimate the peak amplitudes of unitary [Ca2+] transients and show that spike inference performed with MLspike using these unitary amplitude estimates in weakly GCaMP6f‐expressing cells results in error rates of ∼5%. Abstract Investigating neuronal activity using genetically encoded Ca2+ indicators in behaving animals is hampered by inaccuracies in spike inference from fluorescent tracers. Here we combine two‐photon [Ca2+] imaging with cell‐attached recordings, followed by post hoc determination of the expression level of GCaMP6f, to explore how it affects the amplitude, kinetics and temporal summation of somatic [Ca2+] transients in mouse hippocampal pyramidal cells (PCs). The amplitude of unitary [Ca2+] transients (evoked by a single action potential) negatively correlates with GCaMP6f expression, but displays large variability even among PCs with similarly low expression levels. The summation of fluorescence signals is frequency‐dependent, supralinear and also shows remarkable cell‐to‐cell variability. We performed experimental data‐based simulations and found that spike inference error rates using MLspike depend strongly on unitary peak amplitudes and GCaMP6f expression levels. We provide simple methods for estimating the unitary [Ca2+] transients in individual weakly GCaMP6f‐expressing PCs, with which we achieve spike inference error rates of ∼5%.

Neurological emergencies, such as acute stroke, are especially challenging during the current Coronavirus disease-2019 (COVID-19) pandemic. Symptoms as aphasia or dysarthria are severely impacting cooperation and communication with patients. During physical examination, both the patient and the medical team are fitted routinely with surgical masks to minimize potential exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, such a practice can lead to concealment of particularly relevant physical signs. We report a case series of four acute stroke patients who were transferred for endovascular mechanical thrombectomy to our institute after intravenous thrombolysis was initiated at primary stroke centers. Upon arrival, after removing their masks, we observed oral angioedema, as a reaction to thrombolytic agent alteplase. Symptoms remained obscured by face masks through patient care at the referring stroke unit and during transportation, nevertheless they resolved after treatment. Most probably, there are a number of similar cases encountered at emergency departments and acute stroke units. To improve patient safety, a compromise between ensuring protection against the novel coronavirus and facilitating detection of potentially life-threatening physical signs must be found.