Cellular mechanisms underlying two muscarinic receptor-mediated depolarizing responses in relay cells of the rat lateral geniculate nucleus

Mechanism of an Intrinsic Oscillation in Rat Geniculate Interneurons

Depolarizing current injections produced a rhythmic bursting of action potentials - a bursting oscillation - in a set of local interneurons in the lateral geniculate nucleus (LGN) of rats. The current dynamics underlying this firing pattern have not been determined, though this cell type constitutes an important cellular component of thalamocortical circuitry, and contributes to both pathologic and non-pathologic brain states. We thus investigated the source of the bursting oscillation using pharmacological manipulations in LGN slices in vitro and in silico. 1. Selective blockade of calcium channel subtypes revealed that high-threshold calcium currentsI L andI P contributed strongly to the oscillation. 2. Increased extracellular K+ concentration (decreased K+currents) eliminated the oscillation. 3. Selective blockade of K+ channel subtypes demonstrated that the calcium-sensitive potassium current (I A H P ) was of primary importance. A morphologically simplified, multicompartment model of the thalamic interneuron characterized the oscillation as follows: 1. The low-threshold calcium currentI T provided the strong initial burst characteristic of the oscillation. 2. Alternating fluxes through high-threshold calcium channels andI A H P then provided the continuing oscillation's burst and interburst periods respectively. This interplay betweenI L andI A H P contrasts with the current dynamics underlying oscillations in thalamocortical and reticularis neurons, which primarily involveI T andI H , orI T andI A H P respectively. These findings thus point to a novel electrophysiological mechanism for generating intrinsic oscillations in a major thalamic cell type. Because local interneurons can sculpt the behavior of thalamocortical circuits, these results suggest new targets for the manipulation of ascending thalamocortical network activity.

The organization of cholinergic projections in the visual thalamus of the mouse

Cholinergic projections from the brainstem serve as important modulators of activity in visual thalamic nuclei such as the dorsal lateral geniculate nucleus (dLGN). While these projections have been studied in several mammals, a comprehensive examination of their organization in the mouse is lacking. We used the retrograde transport of viruses or cholera toxin subunit B (CTB) injected in the dLGN, immunocytochemical labeling with antibodies against choline acetyltransferase (ChAT), brain nitric oxide synthase (BNOS), and vesicular acetylcholine transporter (VAChT), ChAT-Cre mice crossed with a reporter line (Ai9), as well as brainstem virus injections in ChAT-Cre mice to examine the pattern of thalamic innervation from cholinergic neurons in the pedunculopontine tegmental nucleus (PPTg), laterodorsal tegmental nucleus (LDTg), and the parabigeminal nucleus (PBG). Retrograde tracing demonstrated that the dLGN receives input from the PPTg, LDTg, and PBG. Viral tracing in ChAT-Cre mice and retrograde tracing combined with immunocytochemistry revealed that many of these inputs originate from cholinergic neurons in the PBG and PPTg. Most notable was an extensive cholinergic projection from the PBG which innervated most of the contralateral dLGN, with an especially dense concentration in the dorsolateral shell, as well as a small region in the dorsomedial pole of the ipsilateral dLGN. The PPTg was found to provide a sparse somewhat diffuse innervation of the ipsilateral dLGN. Neurons in the PPTg co-expressed ChAT, BNOS, and VAChT, whereas PBG neurons expressed ChAT, but not BNOS or VAChT. These results highlight the presence of distinct cholinergic populations that innervate the mouse dLGN.

Visual hallucinations, thalamocortical physiology and Lewy body disease: A review

One of the core diagnostic criteria for Dementia with Lewy Bodies (DLB) is the presence of visual hallucinations. The presence of hallucinations, along with fluctuations in the level of arousal and sleep disturbance, point to potential pathological mechanisms at the level of the thalamus. However, the potential role of thalamic dysfunction in DLB, particularly as it relates to the presence of formed visual hallucinations is not known. Here, we review the literature on the pathophysiology of DLB with respect to modern theories of thalamocortical function and attempt to derive an understanding of how such hallucinations arise. Based on the available literature, we propose that combined thalamic-thalamic reticular nucleus and thalamocortical pathology may explain the phenomenology of visual hallucinations in DLB. In particular, diminished α7 cholinergic activity in the thalamic reticular nucleus may critically disinhibit thalamocortical activity. Further, concentrated pathological changes within the posterior regions of the thalamus may explain the predilection for the hallucinations to be visual in nature.

Muscarinic Receptor Modulation of the Cerebellar Interpositus Nucleus In Vitro

How the cerebellum carries out its functions is not clear, even for its established roles in motor control. In particular, little is known about how the cerebellar nuclei (CN) integrate their synaptic and neuromodulatory inputs to generate cerebellar output. CN neurons receive inhibitory inputs from Purkinje cells, excitatory inputs from mossy fibre and climbing fibre collaterals, as well as a variety of neuromodulatory inputs, including cholinergic inputs. In this study we tested how activation of acetylcholine receptors modulated firing rate, intrinsic properties and synaptic transmission in the CN. Using in vitro whole-cell patch clamp recordings from neurons in the interpositus nucleus, the acetylcholine receptor agonist carbachol was shown to induce a short-term increase in firing rate, increase holding current and decrease input resistance of interpositus CN neurons. Carbachol also induced long-term depression of evoked inhibitory postsynaptic currents and a short-term depression of evoked excitatory postsynaptic currents. All effects were shown to be dependent upon muscarinic acetylcholine receptor activation. Overall, the present study has identified muscarinic receptor activation as a modulator of CN activity.

Development, form, and function of the mouse visual thalamus

The dorsal lateral geniculate nucleus (dLGN) of the thalamus is the exclusive relay of retinal information en route to the visual cortex. Although much of our understanding about dLGN comes from studies done in higher mammals, such as the cat and primate, the mouse as a model organism has moved to the forefront as a tractable experimental platform to examine cell type-specific relations. This review highlights our current knowledge about the development, structure, and function of the mouse dLGN.

Synaptic organization of the dorsal lateral geniculate nucleus

A half century after Ray Guillery's classic descriptions of cell types, axon types, and synaptic architecture of the dorsal lateral geniculate nucleus, the functional organization of this nucleus, as well as all other thalamic nuclei, is still of enormous interest. This review will focus on two classic papers written by Ray Guillery: 'A study of Golgi preparations from the dorsal lateral geniculate nucleus of the adult cat', and 'The organization of synaptic interconnections in the laminae of the dorsal lateral geniculate nucleus of the cat', as well as the studies that most directly followed from the insights these landmark manuscripts provided. It is hoped that this review will honor Ray Guillery by encouraging further investigations of the synaptic organization of the dorsal thalamus.

Inward rectifier potassium current IKir promotes intrinsic pacemaker activity of thalamocortical neurons

Slow repetitive burst firing by hyperpolarized thalamocortical (TC) neurons correlates with global slow rhythms (<4 Hz), which are the physiological oscillations during non-rapid eye movement sleep or pathological oscillations during idiopathic epilepsy. The pacemaker activity of TC neurons depends on the expression of several subthreshold conductances, which are modulated in a behaviorally dependent manner. Here we show that upregulation of the small and neglected inward rectifier potassium current I_Kir induces repetitive burst firing at slow and delta frequency bands. We demonstrate this in mouse TC neurons in brain slices by manipulating the Kir maximum conductance with dynamic clamp. We also performed a thorough theoretical analysis that explains how the unique properties of I_Kir enable this current to induce slow periodic bursting in TC neurons. We describe a new ionic mechanism based on the voltage- and time-dependent interaction of I_Kir and hyperpolarization-activated cationic current I_h that endows TC neurons with the ability to oscillate spontaneously at very low frequencies, even below 0.5 Hz. Bifurcation analysis of conductance-based models of increasing complexity demonstrates that I_Kir induces bistability of the membrane potential at the same time that it induces sustained oscillations in combination with I_h and increases the robustness of low threshold-activated calcium current I_T-mediated oscillations. NEW & NOTEWORTHY The strong inwardly rectifying potassium current I_Kir of thalamocortical neurons displays a region of negative slope conductance in the current-voltage relationship that generates potassium currents activated by hyperpolarization. Bifurcation analysis shows that I_Kir induces bistability of the membrane potential; generates sustained subthreshold oscillations by interacting with the hyperpolarization-activated cationic current I_h; and increases the robustness of oscillations mediated by the low threshold-activated calcium current I_T. Upregulation of I_Kir in thalamocortical neurons induces repetitive burst firing at slow and delta frequency bands (<4 Hz).

Seizures and Sleep in the Thalamus: Focal Limbic Seizures Show Divergent Activity Patterns in Different Thalamic Nuclei

The thalamus plays diverse roles in cortical-subcortical brain activity patterns. Recent work suggests that focal temporal lobe seizures depress subcortical arousal systems and convert cortical activity into a pattern resembling slow-wave sleep. The potential simultaneous and paradoxical role of the thalamus in both limbic seizure propagation, and in sleep-like cortical rhythms has not been investigated. We recorded neuronal activity from the central lateral (CL), anterior (ANT), and ventral posteromedial (VPM) nuclei of the thalamus in an established female rat model of focal limbic seizures. We found that population firing of neurons in CL decreased during seizures while the cortex exhibited slow waves. In contrast, ANT showed a trend toward increased neuronal firing compatible with polyspike seizure discharges seen in the hippocampus. Meanwhile, VPM exhibited a remarkable increase in sleep spindles during focal seizures. Single-unit juxtacellular recordings from CL demonstrated reduced overall firing rates, but a switch in firing pattern from single spikes to burst firing during seizures. These findings suggest that different thalamic nuclei play very different roles in focal limbic seizures. While limbic nuclei, such as ANT, appear to participate directly in seizure propagation, arousal nuclei, such as CL, may contribute to depressed cortical function, whereas sleep spindles in relay nuclei, such as VPM, may interrupt thalamocortical information flow. These combined effects could be critical for controlling both seizure severity and impairment of consciousness. Further understanding of differential effects of seizures on different thalamocortical networks may lead to improved treatments directly targeting these modes of impaired function.SIGNIFICANCE STATEMENT Temporal lobe epilepsy has a major negative impact on quality of life. Previous work suggests that the thalamus plays a critical role in thalamocortical network modulation and subcortical arousal maintenance, but its precise seizure-associated functions are not known. We recorded neuronal activity in three different thalamic regions and found divergent activity patterns, which may respectively participate in seizure propagation, impaired level of conscious arousal, and altered relay of information to the cortex during focal limbic seizures. These very different activity patterns within the thalamus may help explain why focal temporal lobe seizures often disrupt widespread network function, and can help guide future treatments aimed at restoring normal thalamocortical network activity and cognition.

RNA-Seq Analysis of Human Trigeminal and Dorsal Root Ganglia with a Focus on Chemoreceptors

The chemosensory capacity of the somatosensory system relies on the appropriate expression of chemoreceptors, which detect chemical stimuli and transduce sensory information into cellular signals. Knowledge of the complete repertoire of the chemoreceptors expressed in human sensory ganglia is lacking. This study employed the next-generation sequencing technique (RNA-Seq) to conduct the first expression analysis of human trigeminal ganglia (TG) and dorsal root ganglia (DRG). We analyzed the data with a focus on G-protein coupled receptors (GPCRs) and ion channels, which are (potentially) involved in chemosensation by somatosensory neurons in the human TG and DRG. For years, transient receptor potential (TRP) channels have been considered the main group of receptors for chemosensation in the trigeminal system. Interestingly, we could show that sensory ganglia also express a panel of different olfactory receptors (ORs) with putative chemosensory function. To characterize OR expression in more detail, we performed microarray, semi-quantitative RT-PCR experiments, and immunohistochemical staining. Additionally, we analyzed the expression data to identify further known or putative classes of chemoreceptors in the human TG and DRG. Our results give an overview of the major classes of chemoreceptors expressed in the human TG and DRG and provide the basis for a broader understanding of the reception of chemical cues.

The role of two-pore-domain background K⁺ (K₂p) channels in the thalamus

The thalamocortical system is characterized by two fundamentally different activity states, namely synchronized burst firing and tonic action potential generation, which mainly occur during the behavioral states of sleep and wakefulness, respectively. The switch between the two firing modes is crucially governed by the bidirectional modulation of members of the K2P channel family, namely tandem of P domains in a weakly inward rectifying K(+) (TWIK)-related acid-sensitive K(+) (TASK) and TWIK-related K(+) (TREK) channels, in thalamocortical relay (TC) neurons. Several physicochemical stimuli including neurotransmitters, protons, di- and multivalent cations as well as clinically used drugs have been shown to modulate K2P channels in these cells. With respect to modulation of these channels by G-protein-coupled receptors, PLCβ plays a unique role with both substrate breakdown and product synthesis exerting important functions. While the degradation of PIP2 leads to the closure of TREK channels, the production of DAG induces the inhibition of TASK channels. Therefore, TASK and TREK channels were found to be central elements in the control of thalamic activity modes. Since research has yet focused on identifying the muscarinic pathway underling the modulation of TASK and TREK channels in TC neurons, future studies should address other thalamic cell types and members of the K2P channel family.

Thalamic neuromodulation and its implications for executive networks

The thalamus is a key structure that controls the routing of information in the brain. Understanding modulation at the thalamic level is critical to understanding the flow of information to brain regions involved in cognitive functions, such as the neocortex, the hippocampus, and the basal ganglia. Modulators contribute the majority of synapses that thalamic cells receive, and the highest fraction of modulator synapses is found in thalamic nuclei interconnected with higher order cortical regions. In addition, disruption of modulators often translates into disabling disorders of executive behavior. However, modulation in thalamic nuclei such as the midline and intralaminar groups, which are interconnected with forebrain executive regions, has received little attention compared to sensory nuclei. Thalamic modulators are heterogeneous in regards to their origin, the neurotransmitter they use, and the effect on thalamic cells. Modulators also share some features, such as having small terminal boutons and activating metabotropic receptors on the cells they contact. I will review anatomical and physiological data on thalamic modulators with these goals: first, determine to what extent the evidence supports similar modulator functions across thalamic nuclei; and second, discuss the current evidence on modulation in the midline and intralaminar nuclei in relation to their role in executive function.

Chemosensory information processing between keratinocytes and trigeminal neurons

Trigeminal fibers terminate within the facial mucosa and skin and transmit tactile, proprioceptive, chemical, and nociceptive sensations. Trigeminal sensations can arise from the direct stimulation of intraepithelial free nerve endings or indirectly through information transmission from adjacent cells at the peripheral innervation area. For mechanical and thermal cues, communication processes between skin cells and somatosensory neurons have already been suggested. High concentrations of most odors typically provoke trigeminal sensations in vivo but surprisingly fail to activate trigeminal neuron monocultures. This fact favors the hypothesis that epithelial cells may participate in chemodetection and subsequently transmit signals to neighboring trigeminal fibers. Keratinocytes, the major cell type of the epidermis, express various receptors that enable reactions to multiple environmental stimuli. Here, using a co-culture approach, we show for the first time that exposure to the odorant chemicals induces a chemical communication between human HaCaT keratinocytes and mouse trigeminal neurons. Moreover, a supernatant analysis of stimulated keratinocytes and subsequent blocking experiments with pyrodoxalphosphate-6-azophenyl-2',4'-disulfonate revealed that ATP serves as the mediating transmitter molecule released from skin cells after odor stimulation. We show that the ATP release resulting from Javanol® stimulation of keratinocytes was mediated by pannexins. Consequently, keratinocytes act as chemosensors linking the environment and the trigeminal system via ATP signaling.

The interplay of seven subthreshold conductances controls the resting membrane potential and the oscillatory behavior of thalamocortical neurons

The signaling properties of thalamocortical (TC) neurons depend on the diversity of ion conductance mechanisms that underlie their rich membrane behavior at subthreshold potentials. Using patch-clamp recordings of TC neurons in brain slices from mice and a realistic conductance-based computational model, we characterized seven subthreshold ion currents of TC neurons and quantified their individual contributions to the total steady-state conductance at levels below tonic firing threshold. We then used the TC neuron model to show that the resting membrane potential results from the interplay of several inward and outward currents over a background provided by the potassium and sodium leak currents. The steady-state conductances of depolarizing Ih (hyperpolarization-activated cationic current), IT (low-threshold calcium current), and INaP (persistent sodium current) move the membrane potential away from the reversal potential of the leak conductances. This depolarization is counteracted in turn by the hyperpolarizing steady-state current of IA (fast transient A-type potassium current) and IKir (inwardly rectifying potassium current). Using the computational model, we have shown that single parameter variations compatible with physiological or pathological modulation promote burst firing periodicity. The balance between three amplifying variables (activation of IT, activation of INaP, and activation of IKir) and three recovering variables (inactivation of IT, activation of IA, and activation of Ih) determines the propensity, or lack thereof, of repetitive burst firing of TC neurons. We also have determined the specific roles that each of these variables have during the intrinsic oscillation.

Muscarinic modulation of high frequency oscillations in pedunculopontine neurons

We previously reported that persistent application of the non-specific cholinergic agonist carbachol (CAR) increased the frequency of calcium channel-mediated oscillatory activity in pedunculopontine nucleus (PPN) neurons, which we identified as dependent on voltage-gated, high-threshold P/Q-type channels. Here, we tested the hypothesis that M2 muscarinic receptors and G-proteins associated with M2 receptors mediate the increase in oscillatory frequency in PPN neurons. We found, using depolarizing ramps, that patch clamped 9-12 day old rat PPN neurons (n = 189) reached their peak oscillatory activity around -20 mV membrane potential. Acute (short duration) application of CAR blocked the oscillatory activity through M2 muscarinic receptors, an effect blocked by atropine. However, persistent (long duration) application of CAR significantly increased the frequency of oscillatory activity in PPN neurons through M2 receptors [40 ± 1 Hz (with CAR) vs. 23 ± 1 Hz (without CAR); p < 0.001]. We then tested the effects of the G-protein antagonist guanosine 5'-[β-thio] diphosphate trilithium salt (GDP-β-S), and the G-protein agonist 5'-[γ-thio] triphosphate trilithium salt (GTP-γ-S). We found, using a three-step protocol in voltage-clamp mode, that the increase in the frequency of oscillations induced by M2 cholinergic receptors was linked to a voltage-dependent G-protein mechanism. In summary, these results suggest that persistent cholinergic input creates a permissive activation state in the PPN that allows high frequency P/Q-type calcium channel-mediated gamma oscillations to occur.

Muscarinic acetylcholine receptors enhance neonatal mouse hypoglossal motoneuron excitability in vitro

In brain stem slices from neonatal ( postnatal days 0–4) CD-1 mice, muscarinic ACh receptors (MAChRs) increased rhythmic inspiratory-related and tonic hypoglossal nerve discharge and depolarized single hypoglossal motoneurons (HMs) via an inward current without changing input resistance. These responses were blocked by the MAChR antagonist 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP; 100 nM). MAChRs shifted voltage-dependent activation of the hyperpolarization-activated cation current to more positive levels. MAChRs increased the HM repetitive firing rate and decreased rheobase, with both effects being blocked by 4-DAMP. Muscarinic agonists reduced the afterhyperpolarization of single action potentials (APs), suggesting that small-conductance Ca2+-dependent K+ current inhibition increased the HM firing rate. Muscarinic agonists also reduced the AP amplitude and slowed its time course, suggesting that MAChRs inhibited voltage-gated Na+ channels. To compare muscarinic excitation of single HMs to muscarinic excitatory effects on motor output in thicker brain stem slices requiring higher extracellular K+ for rhythmic activity, we tested the effects of muscarinic agonists on single HM excitability in high-K+ artificial cerebrospinal fluid (aCSF). In high-K+ aCSF, muscarinic agonists still depolarized HMs and altered AP size and shape, as in standard aCSF, but did not increase the steady-state firing rate, decrease afterhyperpolarization, or alter threshold potential. These results indicate that the basic cellular response of HMs to muscarinic receptors is excitatory, via a number of distinct mechanisms, and that this excitatory response will be largely preserved in rhythmically active brain stem slices.

Cholinergic modulation of fast inhibitory and excitatory transmission to pedunculopontine thalamic projecting neurons

The pedunculopontine nucleus (PPN) is part of the cholinergic arm of the reticular activating system, which is mostly active during waking and rapid-eye movement sleep. The PPN projects to the thalamus and receives cholinergic inputs from the laterodorsal tegmental nucleus and contralateral PPN. We employed retrograde labeling and whole cell recordings to determine the modulation of GABAergic, glycinergic, and glutamatergic transmission to PPN thalamic projecting neurons, and their postsynaptic responses to the nonspecific cholinergic agonist carbachol. M2 and M4 muscarinic receptor-modulated inhibitory postsynaptic responses were observed in 73% of PPN output neurons; in 12.9%, M1 and nicotinic receptor-mediated excitation was detected; and muscarinic and nicotinic-modulated fast inhibitory followed by slow excitatory biphasic responses were evident in 6.7% of cells. A significant increase in the frequency of spontaneous excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents during carbachol application was observed in 66.2% and 65.2% of efferent neurons, respectively. This effect was blocked by a M1 antagonist or nonselective muscarinic blocker, indicating that glutamatergic, GABAergic, and/or glycinergic neurons projecting to PPN output neurons are excited through muscarinic receptors. Decreases in the frequency of miniature EPSCs, and amplitude of electrical stimulation-evoked EPSCs, were blocked by a M2 antagonist, suggesting the presence of M2Rs at terminals of presynaptic glutamatergic neurons. Carbachol-induced multiple types of postsynaptic responses, enhancing both inhibitory and excitatory fast transmission to PPN thalamic projecting neurons through muscarinic receptors. These results provide possible implications for the generation of different frequency oscillations in PPN thalamic projecting neurons during distinct sleep-wake states.

Two distinct populations of projection neurons in the rat lateral parafascicular thalamic nucleus and their cholinergic responsiveness

The lateral parafascicular nucleus (lPf) is a member of the intralaminar thalamic nuclei, a collection of nuclei that characteristically provides widespread projections to the neocortex and basal ganglia and is associated with arousal, sensory, and motor functions. Recently, lPf neurons have been shown to possess different characteristics than other cortical-projecting thalamic relay neurons. We performed whole cell recordings from lPf neurons using an in vitro rat slice preparation and found two distinct neuronal subtypes that were differentiated by distinct morphological and physiological characteristics: diffuse and bushy. Diffuse neurons, which had been previously described, were the predominant neuronal subtype (66%). These neurons had few, poorly-branching, extended dendrites, and rarely displayed burst-like action potential discharge, a ubiquitous feature of thalamocortical relay neurons. Interestingly, we discovered a smaller population of bushy neurons (34%) that shared similar morphological and physiological characteristics with thalamocortical relay neurons of primary sensory thalamic nuclei. In contrast to other thalamocortical relay neurons, activation of muscarinic cholinergic receptors produced a membrane hyperpolarization via activation of M(2) receptors in most lPf neurons (60%). In a minority of lPf neurons (33%), muscarinic agonists produced a membrane depolarization via activation of predominantly M(3) receptors. The muscarinic receptor-mediated actions were independent of lPf neuronal subtype (i.e. diffuse or bushy neurons); however the cholinergic actions were correlated with lPf neurons with different efferent targets. Retrogradely-labeled lPf neurons from frontal cortical fluorescent bead injections primarily consisted of bushy type lPf neurons (78%), but more importantly, all of these neurons were depolarized by muscarinic agonists. On the other hand, lPf neurons labeled by striatal injections were predominantly hyperpolarized by muscarinic agonists (63%). Our results indicate two distinct subpopulations of lPf projection neurons, and interestingly lPf neurons respond differentially to muscarinic receptor activation based on their axonal target.

Cholinergic responses and intrinsic membrane properties of developing thalamic parafascicular neurons

Parafascicular (Pf) neurons receive cholinergic input from the pedunculopontine nucleus (PPN), which is active during waking and REM sleep. There is a developmental decrease in REM sleep in humans between birth and puberty and 10-30 days in rat. Previous studies have established an increase in muscarinic and 5-HT1 serotonergic receptor-mediated inhibition and a transition from excitatory to inhibitory GABA(A) responses in the PPN during the developmental decrease in REM sleep. However, no studies have been conducted on the responses of Pf cells to the cholinergic input from the PPN during development, which is a major target of ascending cholinergic projections and may be an important mechanism for the generation of rhythmic oscillations in the cortex. Whole cell patch-clamp recordings were performed in 9- to 20-day-old rat Pf neurons in parasagittal slices, and responses to the cholinergic agonist carbachol (CAR) were determined. Three types of responses were identified: inhibitory (55.3%), excitatory (31.1%), and biphasic (fast inhibitory followed by slow excitatory, 6.8%), whereas 6.8% of cells showed no response. The proportion of CAR-inhibited Pf neurons increased with development. Experiments using cholinergic antagonists showed that M2 receptors mediated the inhibitory response, whereas excitatory modulation involved M1, nicotinic, and probably M3 or M5 receptors, and the biphasic response was caused by the activation of multiple types of muscarinic receptors. Compared with CAR-inhibited cells, CAR-excited Pf cells showed 1) a decreased membrane time constant, 2) higher density of hyperpolarization-activated channels (I(h)), 3) lower input resistance (R(in)), 4) lower action potential threshold, and 5) shorter half-width duration of action potentials. Some Pf cells exhibited spikelets, and all were excited by CAR. During development, we observed decreases in I(h) density, R(in), time constant, and action potential half-width. These results suggest that cholinergic modulation of Pf differentially affects separate populations, perhaps including electrically coupled cells. Pf cells tend to show decreased excitability and cholinergic activation during the developmental decrease in REM sleep.

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

Although EEG alpha (8-13 Hz) rhythms are traditionally thought to reflect an "idling" brain state, they are also linked to several important aspects of cognition, perception, and memory. Here we show that reactivating cholinergic input, a key component in normal cognition and memory operations, in slices of the cat primary visual and somatosensory thalamus, produces robust alpha rhythms. These rhythms rely on activation of muscarinic receptors and are primarily coordinated by activity in the recently discovered, gap junction-coupled subnetwork of high-threshold (HT) bursting thalamocortical neurons. By performing extracellular field recordings in combination with intracellular recordings of these cells, we show that (1) the coupling of HT bursting cells is sparse, with individual neurons typically receiving discernable network input from one or very few additional cells, (2) the phase of oscillatory activity at which these cells prefer to fire is readily modifiable and determined by a combination of network input, intrinsic properties and membrane polarization, and (3) single HT bursting neurons can potently influence the local network state. These results substantially extend the known effects of cholinergic activation on the thalamus and, in combination with previous studies, show that sensory thalamic nuclei possess powerful and dynamically reconfigurable mechanisms for generating synchronized alpha activity that can be engaged by both descending and ascending arousal systems.

Differences in Response to Muscarinic Activation Between First and Higher Order Thalamic Relays

The mammalian thalamus is composed of two types of thalamocortical relay. First order relays receive information from subcortical sources and relay it to cortex, whereas higher order relays receive information from layer 5 of one cortical area and relay it to another. Recent reports suggest that modulatory inputs to first and higher order relays may differ. We used rat thalamic brain slices and whole cell recordings from relay cells in various first order (the lateral geniculate nucleus, the ventral posterior nucleus, and the ventral portion of the medial geniculate body) and higher order (the lateral posterior, the posterior medial nucleus, and the dorsal portion of the medial geniculate body) relays to explore their responses to activation of muscarinic receptors. We found that, whereas all first order relay cells show a depolarizing response to muscarinic activation, ∼20% of higher order relay cells respond with hyperpolarization. The depolarization is accompanied by an overall increase in input resistance, whereas the hyperpolarization correlates with a decrease in resistance. Because activation of cholinergic brain stem afferents to thalamus increases with increasing behavioral vigilance, the findings suggest that increased vigilance will depolarize all first order and most higher order relay cells but will hyperpolarize a significant subset of higher order relay cells. Such hyperpolarization is expected to bias these relay cells to the burst firing mode, and so these results are consistent with evidence of more bursting among higher order than first order relay cells.

Immunolocalization of muscarinic receptor subtypes in the reticular thalamic nucleus of rats

In this study, to identify the precise localization of the muscarinic receptor subtypes m2, m3 and m4 in the rostral part of the rat reticular thalamic nucleus (rRt), namely, the limbic sector, we used receptor-subtype-specific antibodies and characterized the immunolabeled structures by light, confocal laser scanning, and electron microscopies. The m2-immunolabeling was preferentially distributed in the distal dendrite region where cholinergic afferent fibers tend to terminate and in the peripheral region of somata, whereas the m3-immunolabeling was more preferentially distributed in a large part of somata and in proximal dendrite shafts than in the distal dendrite region. Dual-immunofluorescence experiments demonstrated that majority of rRt neurons with parvalbumin immunoreactivity contain both m2 and m3. Neither m2 nor m3 was detected in presynaptic terminals or axonal elements. No m4-immunolabeling was detected in the rostral part of the thalamus including rRt. These results show the different distributions of m2 and m3 in rRt neurons, and strongly suggest that m2 is more closely associated with cholinergic afferents than m3.

Modulation of cyclic nucleotide-regulated HCN channels by PIP(2) and receptors coupled to phospholipase C

Recent results indicate that phosphoinositides, including phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), directly enhance the opening of hyperpolarization-activated, cyclic nucleotide-regulated (HCN) channels by shifting their activation gating to more positive voltages. This contrasts with the action of phosphoinositides to inhibit the opening of the related cyclic nucleotide-gated (CNG) channels involved in sensory signaling. We both review previous studies and present new experiments that investigate whether HCN channels may be regulated by dynamic changes in PI(4,5)P(2) levels caused by the receptor-mediated activation of phospholipase C (PLC). We coexpressed HCN1 or HCN2 channels in Xenopus oocytes with the PLC-coupled bradykinin BK(2) receptor, the muscarinic M1 receptor, or the TrkA receptor. Activation of all three receptors produced a positive shift in HCN channel voltage gating, the opposite of the effect expected for PI(4,5)P(2) depletion. This action was not caused by alterations in cAMP as the effect was preserved in HCN mutant channels that fail to bind cAMP. The receptor effects were mediated by PLC activity, but did not depend on signaling through the downstream products of PI(4,5)P(2) hydrolysis: IP(3) or diacylglycerol (DAG). Importantly, the modulatory effects on gating were blocked by inhibitors of phosphatidylinositol (PI) kinases, suggesting a role for increased PI(4,5)P(2) synthesis. Finally, we found that bradykinin exerted a similar PI kinase-dependent effect on the gating of native HCN channels in cardiac sinoatrial node cells, suggesting that this pathway may represent a novel, physiologically relevant mechanism for enhancing HCN channel function.

HCN pacemaker channel activation is controlled by acidic lipids downstream of diacylglycerol kinase and phospholipase A2

Hyperpolarization-activated pacemaker currents (I(H)) contribute to the subthreshold properties of excitable cells and thereby influence behaviors such as synaptic integration and the appearance and frequency of intrinsic rhythmic activity. Accordingly, modulation of I(H) contributes to cellular plasticity. Although I(H) activation is regulated by a plethora of neurotransmitters, including some that act via phospholipase C (PLC), the only second messengers known to alter I(H) voltage dependence are cAMP, internal protons (H+(I)s), and phosphatidylinositol-4,5-phosphate. Here, we show that 4beta-phorbol-12-myristate-13-acetate (4betaPMA), a stereoselective C-1 diacylglycerol-binding site agonist, enhances voltage-dependent opening of wild-type and cAMP/H+(I)-uncoupled hyperpolarization-activated, cyclic nucleotide-regulated (HCN) channels, but does not alter gating of the plant hyperpolarization-activated channel, KAT1. Pharmacological analysis indicates that 4betaPMA exerts its effects on HCN gating via sequential activation of PKC and diacylglycerol kinase (DGK) coupled with upregulation of MAPK (mitogen-activated protein kinase) and phospholipase A2 (PLA2), but its action is independent of phosphoinositide kinase 3 (PI3K) and PI4K. Demonstration that both phosphatidic acid and arachidonic acid (AA) directly facilitate HCN gating suggests that these metabolites may serve as the messengers downstream of DGK and PLA2, respectively. 4BetaPMA-mediated suppression of the maximal HCN current likely arises from channel interaction with AA coupled with an enhanced membrane retrieval triggered by the same pathways that modulate channel gating. These results indicate that regulation of excitable cell behavior by neurotransmitter-mediated modulation of I(H) may be exerted via changes in three signaling lipids in addition to the allosteric actions of cAMP and H+(I)s.

Synaptic bombardment modulates muscarinic effects in forelimb motor cortex

Neocortical neurons in vivo exist in an environment of continuous synaptic bombardment, receiving a complex barrage of excitatory and inhibitory inputs. This background activity (by depolarizing neurons, increasing membrane conductance, and introducing fluctuations) strongly alters many aspects of neuronal responsiveness. In this study, we asked how it shapes neuromodulation of postsynaptic responses. Specifically, we examined muscarinic modulation of forelimb motor cortex, a brain area in which cholinergic stimulation is known to be necessary for modifications during motor skill learning. Using a dynamic clamp system to inject simulated conductances into pyramidal neurons in motor cortical slices, we mimicked in vivo-like activity by introducing a random background of excitatory and inhibitory inputs. When muscarinic receptors were stimulated with the agonist oxotremorine-M, several previously described currents were modified, and excitability was increased. However, the presence of the background conductances strongly attenuated most muscarinic agonist effects, with the notable exception that sustained firing responses to trains of inputs were well preserved. This may be important for promoting plasticity in vivo.

Computing with thalamocortical ensembles during different behavioural states

A series of recent studies have indicated that ensembles of neurones, distributed within the neural structures that form the primary thalamocortical loop (TCL) of the trigeminal component of the rat somatosensory system, change the way they respond to similar tactile stimuli, according to both the behavioural strategy employed by animals to gather information and the animal's internal brain states. These findings suggest that top-down influences, which are more likely to play a role during active discrimination than during passive whisker stimulation, may alter the pattern of neuronal firing within both the distinct layers of the primary somatosensory cortex (S1) and the ventral posterior medial nucleus (VPM). We propose that through this physiological process, which involves concurrent dynamic modulations at both cellular and circuit levels in the TCL, rats can either optimize the detection of novel or hard to sense stimuli or they can analyse complex patterns of multi-whisker stimulation, during natural exploration of their surrounding environment.

Dynamic shifting in thalamocortical processing during different behavioural states

Recent experiments in our laboratory have indicated that as rats shift the behavioural strategy employed to explore their surrounding environment, there is a parallel change in the physiological properties of the neuronal ensembles that define the main thalamocortical loop of the trigeminal somatosensory system. Based on experimental evidence from several laboratories, we propose that this concurrent shift in behavioural strategy and thalamocortical physiological properties provides rats with an efficient way to optimize either the detection or analysis of complex tactile stimuli.

Thalamocortical [correction of Thalamcortical] optimization of tactile processing according to behavioral state

We propose a conceptual model that describes the operation of the main thalamocortical loop of the rat somatosensory system. According to this model, the asynchronous convergence of ascending and descending projections dynamically alters the physiological properties of thalamic neurons in the ventral posterior medial (VPM) nucleus as rats shift between three behavioral states. Two of these states are characterized by distinct modes of rhythmic whisker movements. We posit that these simultaneous shifts in exploratory behavioral strategy and in the physiological properties of VPM neurons allow rats to either (i) optimize the detection of stimuli that are novel or difficult to sense or (ii) process complex patterns of multi-whisker stimulation.

Effects of activation of the histaminergic tuberomammillary nucleus on visual responses of neurons in the dorsal lateral geniculate nucleus

We investigated the effects of the central histaminergic system on afferent sensory signals in the retinogeniculocortical pathway in the intact brain. Extracellular physiological recordings in vivo were obtained from neurons in the cat dorsal lateral geniculate nucleus (LGN) in conjunction with electrical activation of the histamine-containing cells in the tuberomammillary nucleus of the hypothalamus. Tuberomammillary activation resulted in a rapid and significant increase in the amplitude of baseline activity and visual responses in LGN neurons. Geniculate X- and Y-cells were affected similarly. LGN cells that exhibited a burst pattern of activity in the control condition switched to a tonic firing pattern during tuberomammillary activation. Effects on visual response properties were assessed using drifting sinusoidal gratings of varied spatial frequency. The resultant spatial tuning curves were elevated by tuberomammillary activation, but there was no change in tuning curve shape. Rather, the effect was proportionate to the control response, with the greatest tuberomammillary effects at spatial frequencies already optimal for the cell. Tuberomammillary activation caused a small phase lag in the visual response that was similar at all spatial frequencies, consistent with the induced shift from burst to tonic firing mode. These results indicate a significant histaminergic effect on LGN thalamocortical cells, with no clear effect on thalamic inhibitory neurons. The histaminergic system appears to strengthen central transmission of afferent information, intensifying but not transforming the retinally derived signals. Promoting sensory input may be one way in which the histaminergic system plays a role in arousal.

Muscarinic regulation of dendritic and axonal outputs of rat thalamic interneurons: a new cellular mechanism for uncoupling distal dendrites

Inhibition is crucial for sharpening the sensory information relayed through the thalamus. To understand how the interneuron-mediated inhibition in the thalamus is regulated, we studied the muscarinic effects on interneurons in the lateral posterior nucleus and lateral geniculate nucleus of the thalamus. Here, we report that activation of muscarinic receptors switched the firing pattern in thalamic interneurons from bursting to tonic. Although neuromodulators switch the firing mode in several other types of neurons by altering their membrane potential, we found that activation of muscarinic subtype 2 receptors switched the fire mode in thalamic interneurons by selectively decreasing their input resistance. This is attributable to the muscarinic enhancement of a hyperpolarizing potassium conductance and two depolarizing cation conductances. The decrease in input resistance appeared to electrotonically uncouple the distal dendrites of thalamic interneurons, which effectively changed the inhibition pattern in thalamocortical cells. These results suggest a novel cellular mechanism for the cholinergic transformation of long-range, slow dendrite- and axon-originated inhibition into short-range, fast dendrite-originated inhibition in the thalamus observed in vivo. It is concluded that the electrotonic properties of the dendritic compartments of thalamic interneurons can be dynamically regulated by muscarinic activity.

Properties of a hyperpolarization-activated cation current in interneurons in the rat lateral geniculate nucleus

A hyperpolarization-activated cation conductance contributes to the membrane properties of a variety of cell types. In the thalamus, a prominent hyperpolarization-activated cation conductance exists in thalamocortical cells, and this current is implicated in the neuromodulation of complex firing behaviors. In contrast, the GABAergic cells in the reticular nucleus in the thalamus appear to lack this conductance. The presence and role of this cation conductance in the other type of thalamic GABAergic cells, local interneurons, is still unclear. To resolve this issue, we studied 54 physiologically and morphologically identified local interneurons in the rat dorsal lateral geniculate nucleus using an in vitro whole-cell patch recording technique. We found that hyperpolarizing current injections induced depolarizing voltage sags in these geniculate interneurons. The I-V relationship revealed an inward rectification. Voltage-clamp study indicated that a slow, hyperpolarization-activated cation conductance was responsible for the inward rectification. We then confirmed that this slow conductance had properties of the hyperpolarization-activated cation conductance described in other cell types. The slow conductance was insensitive to 10 mM tetraethylammonium and 0.5 mM 4-aminopyridine, but was largely blocked by 1-1.5 mM Cs+. It was permeable to both K+ and Na+ ions and had a reversal potential of -44 mV. The voltage dependence of the hyperpolarization-activated cation conductance in interneurons was also studied: the activation threshold was about -55 mV, half-activation potential was about -80 mV and maximal conductance was about 1 nS. The activation and deactivation time constants of the conductance ranged from 100 to 1000 ms, depending on membrane potential. The depolarizing voltage sags and I-V relationship were further simulated in a model interneuron, using the parameters of the hyperpolarization-activated cation conductance obtained from the voltage-clamp study. The time-course and voltage dependence of the depolarizing voltage sags and I-V relationship in the model cell were very similar to those found in geniculate interneurons in current clamp. Taken together, the results of the present study suggest that thalamic local interneurons possess a prominent hyperpolarization-activated cation conductance, which may play important roles in determining basic membrane properties and in modulating firing patterns.

Muscarinic receptor subtypes in the lateral geniculate nucleus: a light and electron microscopic analysis

Neural activity in the dorsal lateral geniculate nucleus of the thalamus (DLG) is modulated by an ascending cholinergic projection from the brainstem. The purpose of this study was to identify and localize specific muscarinic receptors for acetylcholine in the DLG. Receptors were identified in rat and cat tissue by means of antibodies to muscarinic receptor subtypes, ml-m4. Brain sections were processed immunohistochemically and examined with light and electron microscopy. Rat DLG stained positively with antibodies to the m1, m2,and m3 receptor subtypes but not with antibodies to the m4 receptor subtype. The m1 and m3 antibodies appeared to label somata and dendrites of thalamocortical cells. The m1 immunostaining was pale, whereas m3-positive neurons exhibited denser labeling with focal concentrations of staining. Strong immunoreactivity to the m2 antibody was widespread in dendrites and somata of cells resembling geniculate interneurons. Most m2-positive synaptic contacts were classified as F2-type terminals, which are the presynaptic dendrites of interneurons. The thalamic reticular nucleus also exhibited robust m2 immunostaining. Cat DLG exhibited immunoreactivity to the m2 and m3 antibodies. The entire DLG stained darkly for the m2 receptor subtype, except for patchy label in the medial interlaminar nucleus and the ventralmost C laminae. The staining for m3 was lighter and was distributed more homogeneously across the DLG. The perigeniculate nucleus also was immunoreactive to the m2 and m3 subtype-specific antibodies. Immunoreactivity in cat to the m1 or m4 receptor antibodies was undetectable. These data provide anatomical evidence for specific muscarinic-mediated actions of acetylcholine on DLG thalamocortical cells and thalamic interneurons.