High-order theta harmonics account for the detection of slow gamma

ABSTRACTLocal field potential (LFP) oscillations are the superposition of excitatory/inhibitory postsynaptic potentials. In the hippocampus, the 20-55 Hz range (‘slow gamma’) is proposed to support cognition independent of other frequencies. However, this band overlaps with theta harmonics. We aimed to dissociate the generators of slow gamma versus theta harmonics with current source density and different LFP decompositions. Hippocampal theta harmonic and slow gamma generators were not dissociable. Moreover, comparison of wavelet, ensemble empirical-mode (EEMD), and Fourier decompositions produced distinct outcomes with wavelet and EEMD failing to resolve high-order theta harmonics well defined by Fourier analysis. The varying sizes of the time-frequency atoms used by wavelet distributed the higher-order harmonics over a broader range giving the impression of a low frequency burst (“slow gamma”). The absence of detectable slow gamma refutes a multiplexed model of cognition in favor of the energy cascade hypothesis in which dependency across oscillatory frequencies exists.

Soups and Sparks Revisited

In the famous debate whether neurons communicate via chemical mediators or electrical signals, Henry Dale and Otto Loewi mounted powerful evidence on the mediation of nervous activity by chemical transmitters, while John Eccles led the campaign for the electrophysiologists. Eventually, Eccles converted to chemical transmission, when he identified excitatory and inhibitory postsynaptic potentials initiated by the release of chemical neurotransmitters in the synaptic cleft. This well-known episode from the history of neurophysiology counts as a rare instance of philosophy of science advancing scientific research, because the philosopher Karl Popper had encouraged Eccles to theorize an experiment proving the falsity of his own interpretation – according to Popper’s philosophy of science progressing by falsification. The paper shows how Eccles’ intellectual mobilization was grounded in a series of geographical moves, technological adaptations and re-arrangements of his group. This massive travel of people, ideas, instruments, and techniques mediated between the contradictory views, long before Popper kindled Eccles to reflect about the conflicting paradigms and the new theorizing did hardly change his experimental practice. Popper’s immediate effect was a critical and reflexive distance that enabled Eccles to present his evidence more persuasively, as can be shown from archival sources. The exchanges between Eccles and Popper thus shaped the philosophy of falsification to a powerful strategy for writing science.

Effect of GABAB Receptor Agonist SKF97541 on Cortical and Hippocampal Epileptic Afterdischarges

Activation of GABAB receptors leads to longer inhibitory postsynaptic potentials than activation of GABAA receptors. Therefore GABAB receptors may be a target for anticonvulsant therapy. The present study examined possible effects of GABAB receptor agonist SKF97541 on cortical and hippocampal epileptic afterdischarges (ADs). Epileptic ADs elicited by electrical stimulation of sensorimotor cortex or dorsal hippocampus were studied in adult male Wistar rats. Stimulation series were applied 6 times with 10- or 20-min interval. Either interval was efficient for reliable elicitation of cortical ADs but stimulation at 10-min intervals did not reliably elicit hippocampal ADs, many stimulations were without effect. SKF97541 in dose 1 mg/kg significantly prolonged cortical ADs. Duration of hippocampal ADs was not significantly changed by either dose of SKF97541 in spite of a marked myorelaxant effect of the higher dose. Our present data demonstrated that neither cortical nor hippocampal ADs in adult rats were suppressed by GABAB receptor agonist SKF97541. Proconvulsant effect on cortical ADs indicates a different role in these two brain structures. In addition, duration of refractory period for electrically-induced ADs in these two structures in adult rats is different.

Postnatal development of synaptic properties of the GABAergic projection from the inferior colliculus to the auditory thalamus

The development of auditory temporal processing is important for processing complex sounds as well as for acquiring reading and language skills. Neuronal properties and sound processing change dramatically in auditory cortex neurons after the onset of hearing. However, the development of the auditory thalamus or medial geniculate body (MGB) has not been well studied over this critical time window. Since synaptic inhibition has been shown to be crucial for auditory temporal processing, this study examined the development of a feedforward, GABAergic connection to the MGB from the inferior colliculus (IC), which is also the source of sensory glutamatergic inputs to the MGB. IC-MGB inhibition was studied using whole cell patch-clamp recordings from rat brain slices in current-clamp and voltage-clamp modes at three age groups: a prehearing group [ postnatal day (P)7–P9], an immediate posthearing group (P15–P17), and a juvenile group (P22–P32) whose neuronal properties are largely mature. Membrane properties matured substantially across the ages studied. GABAA and GABAB inhibitory postsynaptic potentials were present at all ages and were similar in amplitude. Inhibitory postsynaptic potentials became faster to single shocks, showed less depression to train stimuli at 5 and 10 Hz, and were overall more efficacious in controlling excitability with age. Overall, IC-MGB inhibition becomes faster and more precise during a time period of rapid changes across the auditory system due to the codevelopment of membrane properties and synaptic properties.

A new inhibitory pathway in the jellyfish Polyorchis penicillatus

Contact of food with the manubrial lips in the genus Polyorchis A. Agassiz, 1862 evokes trains of electrical impulses (E potentials) that propagate to the margin. E potentials are also produced by food stimuli at the margin and tentacle bases. E potentials are shown to be associated with inhibitory postsynaptic potentials (ipsps) in the swimming motor neurons and contribute to the arrest of swimming during feeding. The conduction pathway for E potentials is a nerve plexus located in the endodermal walls of the stomach and radial and ring canals. We have explored the conducting properties of the system; the conduction velocity varies with stimulus frequency but is about 15 cm/s when stimuli are more than 50 s apart. Neurites belonging to the E system run around the margin adjacent to the inner nerve ring, where the swimming pacemaker neurons are located. We suggest that they may make inhibitory synapses on to the swimming motor neurons, but this has yet to be demonstrated anatomically. The reversal potential for ipsps, recorded intracellularly with potassium acetate micropipettes, was estimated to be about –69 mV. Swimming inhibition mediated by this endodermal pathway is distinct from that observed during protective “crumpling” behaviour and that associated with contractions of the radial muscles seen during feeding, though it may accompany the latter.

Localization and function of Ih channels in a small neural network

Subthreshold ionic currents, which activate below the firing threshold and shape the cell's firing properties, play important roles in shaping neural network activity. We examined the distribution and synaptic roles of the hyperpolarization-activated inward current ( Ih) in the pyloric network of the lobster stomatogastric ganglion (STG). Ih channels are expressed throughout the STG in a patchy distribution and are highly expressed in the fine neuropil, an area that is rich in synaptic contacts. We performed double labeling for Ih protein and for the presynaptic marker synaptotagmin. The large majority of labeling in the fine neuropil was adjacent but nonoverlapping, suggesting that Ih is localized in close proximity to synapses but not in the presynaptic terminals. We compared the pattern of Ih localization with Shal transient potassium channels, whose expression is coregulated with Ih in many STG neurons. Unlike Ih, we found significant levels of Shal protein in the soma membrane and the primary neurite. Both proteins were found in the synaptic fine neuropil, but with little evidence of colocalization in individual neurites. We performed electrophysiological experiments to study a potential role for Ih in regulating synaptic transmission. At a synapse between two identified pyloric neurons, the amplitude of inhibitory postsynaptic potentials (IPSPs) decreased with increasing postsynaptic activation of Ih. Pharmacological block of Ih restored IPSP amplitudes to levels seen when Ih was not activated. These experiments suggest that modulation of postsynaptic Ih might play an important role in the control of synaptic strength in this rhythmogenic neural network.