Cellular dynamics of cholinergically induced alpha (8-13 Hz) rhythms in sensory thalamic nuclei in vitro

Temporal framing of thalamic relay-mode firing by phasic inhibition during the alpha rhythm

Several aspects of perception, particularly those pertaining to vision, are closely linked to the occipital alpha (α) rhythm. However, how the α rhythm relates to the activity of neurons that convey primary visual information is unknown. Here we show that in behaving cats, thalamocortical neurons in the lateral geniculate nucleus (LGN) that operate in a conventional relay-mode form two groups where the cumulative firing is subject to a cyclic suppression that is centered on the negative α rhythm peak in one group and on the positive peak in the other. This leads to an effective temporal framing of relay-mode output and results from phasic inhibition from LGN interneurons, which in turn are rhythmically excited by thalamocortical neurons that exhibit high-threshold bursts. These results provide a potential cellular substrate for linking the α rhythm to perception and further underscore the central role of inhibition in controlling spike timing during cognitively relevant brain oscillations.

GABAergic neuron distribution in the pedunculopontine nucleus defines functional subterritories

gamma-Aminobutyric acid (GABA)ergic neurons are widely distributed in brainstem structures involved in the regulation of the sleep-wake cycle, locomotion, and attention. These brainstem structures include the pedunculopontine nucleus (PPN), which is traditionally characterized by its population of cholinergic neurons that have local and wide-ranging connections. The functional heterogeneity of the PPN is partially explained by the topographic distribution of cholinergic neurons, but such heterogeneity might also arise from the organization of other neuronal populations within the PPN. To understand whether a topographical organization is also maintained by GABAergic neurons, we labeled these neurons by in situ hybridization for glutamic acid decarboxylase mRNA combined with immunohistochemistry for choline acetyltransferase to reveal cholinergic neurons. We analyzed their distribution within the PPN by using a method to quantify regional differences based on stereological cell counts. We show that GABAergic neurons of the rat PPN have a rostrocaudal gradient that is opposite to that of cholinergic neurons. Indeed, GABAergic neurons are predominantly concentrated in the rostral PPN; in addition, they form, along with cholinergic neurons, a small, high-density cluster in the most caudal portion of the nucleus. Thus, we provide evidence of heterogeneity in the distribution of different neuronal populations in the PPN and show that GABAergic and cholinergic neurons define neurochemically distinct areas. Our data suggest that the PPN is neurochemically segregated, and such differences define functional territories.

Establishing a physiological environment for visualized in vitro brain slice recordings by increasing oxygen supply and modifying aCSF content

Our insights into the basic characteristics of neuronal function were significantly advanced by combining the in vitro slice technique with the visualization of neurons and their processes. The visualization through water immersion objectives requires keeping slices submerged in recording chambers where delivering artificial cerebro-spinal fluid (aCSF) at flow rates of 2-3 ml/min results in a limited oxygen supply [Hájos N, Ellender TJ, Zemankovics R, Mann EO, Exley R, Cragg SJ, et al. Maintaining network activity in submerged hippocampal slices: importance of oxygen supply. Eur J Neurosci 2009;29:319-27]. Here we review two methods aimed at providing sufficient oxygen levels to neurons in submerged slices to enable high energy consuming processes such as elevated firing rates or network oscillations. The use of these methods may also influence the outcome of other electrophysiological experiments in submerged slices including the study of intercellular signaling pathways. In addition, we also emphasize the importance of various aCSF constituents used in in vitro experiments.

ATP-dependent infra-slow (<0.1 Hz) oscillations in thalamic networks

An increasing number of EEG and resting state fMRI studies in both humans and animals indicate that spontaneous low frequency fluctuations in cerebral activity at <0.1 Hz (infra-slow oscillations, ISOs) represent a fundamental component of brain functioning, being known to correlate with faster neuronal ensemble oscillations, regulate behavioural performance and influence seizure susceptibility. Although these oscillations have been commonly indicated to involve the thalamus their basic cellular mechanisms remain poorly understood. Here we show that various nuclei in the dorsal thalamus in vitro can express a robust ISO at ∼0.005–0.1 Hz that is greatly facilitated by activating metabotropic glutamate receptors (mGluRs) and/or Ach receptors (AchRs). This ISO is a neuronal population phenomenon which modulates faster gap junction (GJ)-dependent network oscillations, and can underlie epileptic activity when AchRs or mGluRs are stimulated excessively. In individual thalamocortical neurons the ISO is primarily shaped by rhythmic, long-lasting hyperpolarizing potentials which reflect the activation of A1 receptors, by ATP-derived adenosine, and subsequent opening of Ba²⁺-sensitive K⁺ channels. We argue that this ISO has a likely non-neuronal origin and may contribute to shaping ISOs in the intact brain.

Novel modes of rhythmic burst firing at cognitively-relevant frequencies in thalamocortical neurons

It is now widely accepted that certain types of cognitive functions are intimately related to synchronized neuronal oscillations at both low (alpha/theta) (4-7/8-13 Hz) and high (beta/gamma) (18-35/30-70 Hz) frequencies. The thalamus is a key participant in many of these oscillations, yet the cellular mechanisms by which this participation occurs are poorly understood. Here we describe how, under appropriate conditions, thalamocortical (TC) neurons from different nuclei can exhibit a wide array of largely unrecognised intrinsic oscillatory activities at a range of cognitively-relevant frequencies. For example, both metabotropic glutamate receptor (mGluR) and muscarinic Ach receptor (mAchR) activation can cause rhythmic bursting at alpha/theta frequencies. Interestingly, key differences exist between mGluR- and mAchR-induced bursting, with the former involving extensive dendritic Ca2+ electrogenesis and being mimicked by a non-specific block of K+ channels with Ba2+, whereas the latter appears to be more reliant on proximal Na+ channels and a prominent spike afterdepolarization (ADP). This likely relates to the differential somatodendritic distribution of mGluRs and mAChRs and may have important functional consequences. We also show here that in similarity to some neocortical neurons, inhibiting large-conductance Ca2+-activated K+ channels in TC neurons can lead to fast rhythmic bursting (FRB) at approximately 40 Hz. This activity also appears to rely on a Na+ channel-dependent spike ADP and may occur in vivo during natural wakefulness. Taken together, these results show that TC neurons are considerably more flexible than generally thought and strongly endorse a role for the thalamus in promoting a range of cognitively-relevant brain rhythms.

The role of oscillations and synchrony in cortical networks and their putative relevance for the pathophysiology of schizophrenia

Neural oscillations and their synchronization may represent a versatile signal to realize flexible communication within and between cortical areas. By now, there is extensive evidence to suggest that cognitive functions depending on coordination of distributed neural responses, such as perceptual grouping, attention-dependent stimulus selection, subsystem integration, working memory, and consciousness, are associated with synchronized oscillatory activity in the theta-, alpha-, beta-, and gamma-band, suggesting a functional mechanism of neural oscillations in cortical networks. In addition to their role in normal brain functioning, there is increasing evidence that altered oscillatory activity may be associated with certain neuropsychiatric disorders, such as schizophrenia, that involve dysfunctional cognition and behavior. In the following article, we aim to summarize the evidence on the role of neural oscillations during normal brain functioning and their relationship to cognitive processes. In the second part, we review research that has examined oscillatory activity during cognitive and behavioral tasks in schizophrenia. These studies suggest that schizophrenia involves abnormal oscillations and synchrony that are related to cognitive dysfunctions and some of the symptoms of the disorder. Perspectives for future research will be discussed in relationship to methodological issues, the utility of neural oscillations as a biomarker, and the neurodevelopmental hypothesis of schizophrenia.