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Activity in both divisions of the autonomic nervous system (ANS) can increase during seizures and result in tachy- or bradyarrhythmias. We sought to determine the patterns of ANS activity that led to heart rate (HR) changes and whether the character of ANS and HR changes can impact the seizures themselves. Simultaneous recordings of vagus nerve and cervical sympathetic ganglionic or nerve activity, EEG, ECG, and blood pressure were acquired from 16 urethane-anesthetized rats that received systemic kainic acid to induce seizures. After initial continuous seizure activity, discrete seizures were observed in 11/16 rats. Individual seizures were classified based on HR changes as tachycardic (n = 3), bradycardic (n = 17), or one of two more severe categories in which (a) the seizure appeared to be terminated by severe bradyarrhythmia (n = 5) or (b) the animal died (n = 6). Interestingly, even simple bradycardic seizures had episodes of dramatically increased respiratory effort, which we interpret as evidence of airway occlusion based on muscle artifacts in the recordings with transient blood pressure decreases. In the severe outcomes, ANS activity increased during seizures until it caused a drastic HR reduction (50%), in which case seizure and ANS activity decreased dramatically. Sympathetic activity during this late vulnerable period was important for survival. We conclude that individual seizures produce (a) stereotypical changes in autonomic activity and HR, (b) persistence of sympathetic tone helps to protect against death, and (c) bradycardic seizures may indicate increased risk for seizure-associated obstructive apnea.

A common feature of the primary processing structures of sensory systems is the presence of parallel output “channels” that convey different information about a stimulus. In the mammalian olfactory bulb, this is reflected in the mitral cells (MCs) and tufted cells (TCs) that have differing sensitivities to odors, with TCs being more sensitive than MCs. In this study, we examined potential mechanisms underlying the different responses of MCs vs. TCs. For TCs, we focused on superficial TCs (sTCs), which are a population of output TCs that reside in the superficial-most portion of the external plexiform layer, along with external tufted cells (eTCs), which are glutamatergic interneurons in the glomerular layer. Using whole-cell patch-clamp recordings in mouse bulb slices, we first measured excitatory currents in MCs, sTCs, and eTCs following olfactory sensory neuron (OSN) stimulation, separating the responses into a fast, monosynaptic component reflecting direct inputs from OSNs and a prolonged component partially reflecting eTC-mediated feedforward excitation. Responses were measured to a wide range of OSN stimulation intensities, simulating the different levels of OSN activity that would be expected to be produced by varying odor concentrationsin vivo. Over a range of stimulation intensities, we found that the monosynaptic current varied significantly between the cell types, in the order of eTC > sTC > MC. The prolonged component was smaller in sTCs vs. both MCs and eTCs. sTCs also had much higher whole-cell input resistances than MCs, reflecting their smaller size and greater membrane resistivity. To evaluate how these different electrophysiological aspects contributed to spiking of the output MCs and sTCs, we used computational modeling. By exchanging the different cell properties in our modeled MCs and sTCs, we could evaluate each property's contribution to spiking differences between these cell types. This analysis suggested that the higher sensitivity of spiking in sTCs vs. MCs reflected both their larger monosynaptic OSN signal as well as their higher input resistance, while their smaller prolonged currents had a modest opposing effect. Taken together, our results indicate that both synaptic and intrinsic cellular features contribute to the production of parallel output channels in the olfactory bulb.