Reticular Neurons Research Articles

The Global Spike: Conserved Dendritic Properties Enable Unique Ca2+ Spike Generation in Low-Threshold Spiking Neurons.

Low-threshold Ca2+ spikes (LTS) are an indispensible signaling mechanism for neurons in areas including the cortex, cerebellum, basal ganglia, and thalamus. They have critical physiological roles and have been strongly associated with disorders including epilepsy, Parkinson's disease, and schizophrenia. However, although dendritic T-type Ca2+ channels have been implicated in LTS generation, because the properties of low-threshold spiking neuron dendrites are unknown, the precise mechanism has remained elusive. Here, combining data from fluorescence-targeted dendritic recordings and Ca2+ imaging from low-threshold spiking cells in rat brain slices with computational modeling, the cellular mechanism responsible for LTS generation is established. Our data demonstrate that key somatodendritic electrical conduction properties are highly conserved between glutamatergic thalamocortical neurons and GABAergic thalamic reticular nucleus neurons and that these properties are critical for LTS generation. In particular, the efficiency of soma to dendrite voltage transfer is highly asymmetric in low-threshold spiking cells, and in the somatofugal direction, these neurons are particularly electrotonically compact. Our data demonstrate that LTS have remarkably similar amplitudes and occur synchronously throughout the dendritic tree. In fact, these Ca2+ spikes cannot occur locally in any part of the cell, and hence we reveal that LTS are generated by a unique whole-cell mechanism that means they always occur as spatially global spikes. This all-or-none, global electrical and biochemical signaling mechanism clearly distinguishes LTS from other signals, including backpropagating action potentials and dendritic Ca2+/NMDA spikes, and has important consequences for dendritic function in low-threshold spiking neurons.SIGNIFICANCE STATEMENT Low-threshold Ca2+ spikes (LTS) are critical for important physiological processes, including generation of sleep-related oscillations, and are implicated in disorders including epilepsy, Parkinson's disease, and schizophrenia. However, the mechanism underlying LTS generation in neurons, which is thought to involve dendritic T-type Ca2+ channels, has remained elusive due to a lack of knowledge of the dendritic properties of low-threshold spiking cells. Combining dendritic recordings, two-photon Ca2+ imaging, and computational modeling, this study reveals that dendritic properties are highly conserved between two prominent low-threshold spiking neurons and that these properties underpin a whole-cell somatodendritic spike generation mechanism that makes the LTS a unique global electrical and biochemical signal in neurons.

Medullary reticular GABAergic neurons mediate neuropeptide Y-induced inhibition of brown adipose tissue thermogenesis

Medullary reticular GABAergic neurons mediate neuropeptide Y-induced inhibition of brown adipose tissue thermogenesis

Peptide YY signaling in the lateral parabrachial nucleus increases food intake through the Y1 receptor.

Although central PYY delivery potently increases food intake, the sites of action and mechanisms mediating these hyperphagic effects are not fully understood. The present studies investigate the contribution of lateral parabrachial nucleus (lPBN) PYY-Y receptor signaling to food intake control, as lPBN neurons express Y receptors and receive PYY fibers and are known to integrate circulating and visceral sensory signals to regulate energy balance. Immunohistochemical results identified a subpopulation of gigantocellular reticular nucleus PYY-producing neurons that project monosynaptically to the lPBN, providing an endogenous source of PYY to the lPBN. lPBN microinjection of PYY-(1-36) or PYY-(3-36) markedly increased food intake by increasing meal size. To determine which receptors mediate these behavioral results, we first performed quantitative real-time PCR to examine the relative levels of Y receptor expression in lPBN tissue. Gene expression analyses revealed that, while Y1, Y2, and Y5 receptors are each expressed in lPBN tissue, Y1 receptor mRNA is expressed at fivefold higher levels than the others. Furthermore, behavioral/pharmacological results demonstrated that the hyperphagic effects of PYY-(3-36) were eliminated by lPBN pretreatment with a selective Y1 receptor antagonist. Together, these results highlight the lPBN as a novel site of action for the intake-stimulatory effects of central PYY-Y1 receptor signaling.

Turning a Negative into a Positive: Ascending GABAergic Control of Cortical Activation and Arousal.

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. Recent technological advances have illuminated the role of GABAergic neurons in control of cortical arousal and sleep. Sleep-promoting GABAergic neurons in the preoptic hypothalamus are well-known. Less well-appreciated are GABAergic projection neurons in the brainstem, midbrain, hypothalamus, and basal forebrain, which paradoxically promote arousal and fast electroencephalographic (EEG) rhythms. Thus, GABA is not purely a sleep-promoting neurotransmitter. GABAergic projection neurons in the brainstem nucleus incertus and ventral tegmental nucleus of Gudden promote theta (4–8 Hz) rhythms. Ventral tegmental area GABAergic neurons, neighboring midbrain dopamine neurons, project to the frontal cortex and nucleus accumbens. They discharge faster during cortical arousal and regulate reward. Thalamic reticular nucleus GABAergic neurons initiate sleep spindles in non-REM sleep. In addition, however, during wakefulness, they tonically regulate the activity of thalamocortical neurons. Other GABAergic inputs to the thalamus arising in the globus pallidus pars interna, substantia nigra pars reticulata, zona incerta, and basal forebrain regulate motor activity, arousal, attention, and sensory transmission. Several subpopulations of cortically projecting GABAergic neurons in the basal forebrain project to the thalamus and neocortex and preferentially promote cortical gamma-band (30–80 Hz) activity and wakefulness. Unlike sleep-active GABAergic neurons, these ascending GABAergic neurons are fast-firing neurons which disinhibit and synchronize the activity of their forebrain targets, promoting the fast EEG rhythms typical of conscious states. They are prominent targets of GABAergic hypnotic agents. Understanding the properties of ascending GABAergic neurons may lead to novel treatments for diseases involving disorders of cortical activation and wakefulness.

Discontinuous Galerkin finite element method for solving population density functions of cortical pyramidal and thalamic neuronal populations

Compared with the Monte Carlo method, the population density method is efficient for modeling collective dynamics of neuronal populations in human brain. In this method, a population density function describes the probabilistic distribution of states of all neurons in the population and it is governed by a hyperbolic partial differential equation. In the past, the problem was mainly solved by using the finite difference method. In a previous study, a continuous Galerkin finite element method was found better than the finite difference method for solving the hyperbolic partial differential equation; however, the population density function often has discontinuity and both methods suffer from a numerical stability problem. The goal of this study is to improve the numerical stability of the solution using discontinuous Galerkin finite element method. To test the performance of the new approach, interaction of a population of cortical pyramidal neurons and a population of thalamic neurons was simulated. The numerical results showed good agreement between results of discontinuous Galerkin finite element and Monte Carlo methods. The convergence and accuracy of the solutions are excellent. The numerical stability problem could be resolved using the discontinuous Galerkin finite element method which has total-variation-diminishing property. The efficient approach will be employed to simulate the electroencephalogram or dynamics of thalamocortical network which involves three populations, namely, thalamic reticular neurons, thalamocortical neurons and cortical pyramidal neurons.

Synaptic mechanisms underlying cholinergic control of thalamic reticular nucleus neurons.

Neuronal networks of the thalamus are the target of extensive cholinergic projections from the basal forebrain and the brainstem. Activation of these afferents can regulate neuronal excitability, transmitter release, and firing patterns in thalamic networks, thereby altering the flow of sensory information during distinct behavioural states. However, cholinergic regulation in the thalamus has been primarily examined by using receptor agonist and antagonist, which has precluded a detailed understanding of the spatiotemporal dynamics that govern cholinergic signalling under physiological conditions. This review summarizes recent studies on cholinergic synaptic transmission in the thalamic reticular nucleus (TRN), a brain structure intimately involved in the control of sensory processing and the generation of rhythmic activity in the thalamocortical system. This work has shown that acetylcholine (ACh) released from individual axons can rapidly and reliably activate both pre- and postsynaptic cholinergic receptors, thereby controlling TRN neuronal activity with high spatiotemporal precision.

Reproduction of EEG power spectrum over frontal region during the propofol-induced general anesthesia

The present work aims to reproduce certain changes observed experimentally in the EEG power spectrum over the frontal head region during general anesthesia induced by propofol. These observations include increased delta (0-4 Hz) and alpha (8-12 Hz) activities [1]. We extend a previous cortical model [2] and study a neuronal population model of a single thalamo-cortical module consisting of three different populations of neurons, namely cortical excitatory neurons, thalamocortical relay neurons and inhibitory thalamic reticular neurons (Fig. ​(Fig.1).1). Each module obeys a neural mass model. The cortical inhibitory population is neglected in our model to reveal the effect of propofol action in the thalamus. Our model describes well the characteristic spectral changes observed experimentally within the delta- and alpha- frequency bands in frontal and occipital electrodes with increasing concentration of propofol. Figure 1 Schematic of a thalamocortical module. The blue and red lines indicate excitatory and inhibitory connections, respectively. The solid lines represent connections associated with the same delay and the dotted lines denote connections without delay. This shows that neglecting inhibitory action in the cortex but considering thalamic GABAergic action suffices to reproduce the data. From a modeling point of view, our reduced mathematical model is low dimensional and remains analytically treatable while still being adequate to reproduce observed changes in EEG rhythms. Moreover, it is shown that the propofol concentration acts as a control parameter of the system and that propofol-induced changes in the stationary states of the model system lead to changes in the corresponding nonlinear gain function that result in EEG power modulation: increases of power over the frontal region can be caused by an increase in the gain function of thalamocortical network. The results suggest that intra-thalamic inhibition from reticular neurons to relay cells plays an important role in the generation of the characteristic EEG patterns seen during general anesthesia.

Neuronal plasticity during sleep slow wave oscillations

Sleep is critical for regulation of synaptic efficacy, consolidation of memories and learning. It has been proposed that synaptic plasticity associated with sleep rhythms could contribute to consolidation of memories acquired during wakefulness. It was suggested that in the active state of sleep slow wave oscillation, the hippocampal formation activates latent memories stored in the neocortex (replay) and induce permanent changes in synaptic conductances. In this study, we present a thalamocortical network model of the slow-wave sleep activity characterized by repeatable (< 1 Hz) transitions between active (Up) and silent (Down) states of the network. The model consisted a layer of thalamic relay (TC) and reticular (RE) neurons in the thalamus as well as a model of cortical column with pyramidal neurons and inhibitory interneurons. All neurons were modeled based on the Hodgkin-Huxley kinetics. Spike-timing dependent synaptic plasticity was implemented to regulate synaptic efficacy. The slow oscillation was driven by intracortical dynamics; active states were initiated by the spontaneous miniature synaptic releases at different network sites [1] and propagated within the cortical network. The properties of slow oscillations depended on the intrinsic conductances, miniature synaptic releases, and the synaptic strength. In agreement with previous experimental studies, transition from the active to the silent state occurred more synchronously than the active state initiation. We found that the pattern of the active state propagation depended on the spatiotemporal pattern of activity on the previous cycle of slow oscillation and could be influenced by external input. Because of the refractoriness properties of the network, a probability of the next active state initiation was higher for the network site that initiated activity at the previous cycle. Furthermore, even weak external stimulation delivered to the network results in increase probability of up-state induction at the stimulation location. This suggests that spatially and temporally sparse hippocampal input could influence the spatiotemporal pattern of slow oscillation. Location of the initiation site and the pattern of active state propagation determined the relative timing of spiking in cortical neurons. When spike-time dependent synaptic plasticity (STDP) was implemented, there was a net decrease in synaptic strengths and, at the same time, an increase in the strength of specific synapses which were associated with the sequence replay. The change in synaptic weights between any two neurons was determined by the direction of active state propagation and by the distance between the neurons. Our study propose a mechanism of how interaction between cortically generated slow waves and sparse external input, possibly representing input from hippocampal formation, may lead to reorganization of synaptic strength during stage 3/4 sleep.

Phencyclidine Inhibits the Activity of Thalamic Reticular Gamma-Aminobutyric Acidergic Neurons in Rat Brain

The neurobiological basis of action of noncompetitive N-methyl-D-aspartate acid receptor (NMDA-R) antagonists is poorly understood. Electrophysiological studies indicate that phencyclidine (PCP) markedly disrupts neuronal activity with an overall excitatory effect and reduces the power of low-frequency oscillations (LFO; <4 Hz) in thalamocortical networks. Because the reticular nucleus of the thalamus (RtN) provides tonic feed-forward inhibition to the rest of the thalamic nuclei, we examined the effect of PCP on RtN activity, under the working hypothesis that NMDA-R blockade in RtN would disinhibit thalamocortical networks. Drug effects (PCP followed by clozapine) on the activity of RtN (single unit and local field potential recordings) and prefrontal cortex (PFC; electrocorticogram) in anesthetized rats were assessed. PCP (.25-.5 mg/kg, intravenous) reduced the discharge rate of 19 of 21 RtN neurons to 37% of baseline (p < .000001) and the power of LFO in RtN and PFC to ~20% of baseline (p < .001). PCP also reduced the coherence between PFC and RtN in the LFO range. A low clozapine dose (1 mg/kg intravenous) significantly countered the effect of PCP on LFO in PFC but not in RtN and further reduced the discharge rate of RtN neurons. However, clozapine administration partly antagonized the fall in coherence and phase-locking values produced by PCP. PCP activates thalamocortical circuits in a bottom-up manner by reducing the activity of RtN neurons, which tonically inhibit thalamic relay neurons. However, clozapine reversal of PCP effects is not driven by restoring RtN activity and may involve a cortical action.

Direct reticular projections of trigeminal sensory fibers immunoreactive to CGRP: potential monosynaptic somatoautonomic projections.

Few trigeminal sensory fibers project centrally beyond the trigeminal sensory complex, with only projections of fibers carried in its sensory anterior ethmoidal (AEN) and intraoral nerves described. Fibers of the AEN project into the brainstem reticular formation where immunoreactivity against substance P and CGRP are found. We investigated whether the source of these peptides could be from trigeminal ganglion neurons by performing unilateral rhizotomies of the trigeminal root and looking for absence of label. After an 8–14 days survival, substance P immunoreactivity in the trigeminal sensory complex was diminished, but we could not conclude that the sole source of this peptide in the lateral parabrachial area and lateral reticular formation arises from primary afferent fibers. Immunoreactivity to CGRP after rhizotomy however was greatly diminished in the trigeminal sensory complex, confirming the observations of others. Moreover, CGRP immunoreactivity was nearly eliminated in fibers in the lateral parabrachial area, the caudal ventrolateral medulla, both the peri-ambiguus and ventral parts of the rostral ventrolateral medulla, in the external formation of the nucleus ambiguus, and diminished in the caudal pressor area. The nearly complete elimination of CGRP in the lateral reticular formation after rhizotomy suggests this peptide is carried in primary afferent fibers. Moreover, the arborization of CGRP immunoreactive fibers in these areas mimics that of direct projections from the AEN. Since electrical stimulation of the AEN induces cardiorespiratory adjustments including an apnea, peripheral vasoconstriction, and bradycardia similar to those seen in the mammalian diving response, we suggest these perturbations of autonomic behavior are enhanced by direct somatic primary afferent projections to these reticular neurons. We believe this to be first description of potential direct somatoautonomic projections to brainstem neurons regulating autonomic activity.

Subthreshold delta‐frequency resonance in thalamic reticular neurons

The thalamic reticular nucleus (nRt) is an assembly of GABAergic projection neurons that participate in the generation of brain rhythms during synchronous sleep and absence epilepsy. NRt cells receive inhibitory and excitatory synaptic inputs, and are endowed with an intricate set of intrinsic conductances. However, little is known about how intrinsic and synaptic properties interact to generate rhythmic discharges in these neurons. In order to better understand this interaction, I studied the subthreshold responses of nRt cells to time-varying inputs. Patch-clamp recordings were performed in acute slices of rat thalamus (postnatal days 12-21). Sinusoidal current waveforms of linearly changing frequencies were injected into the soma, and the resulting voltage oscillations were recorded. At the resting membrane potential, the impedance profile showed a characteristic resonance at 1.7Hz. The relative strength of the resonance was 1.2, and increased with membrane hyperpolarization. Small suprathreshold current injections led to preferred spike generation at the resonance frequency. Bath application of ZD7288 or Cs(+) , inhibitors of the hyperpolarization-activated cation current (Ih ), transformed the resonance into low-pass behaviour, whereas the T-channel blockers mibefradil and Ni(2+) decreased the strength of the resonance. It is concluded that nRt cells have an Ih -mediated intrinsic frequency preference in the subthreshold voltage range that favours action potential generation in the delta-frequency band.

Adrenoreceptor modulation of oromotor pathways in the rat medulla.

Regulation of feeding behavior involves the integration of multiple physiological and neurological pathways that control both nutrient-seeking and consummatory behaviors. The consummatory phase of ingestion includes stereotyped oromotor movements of the tongue and jaw that are controlled through brain stem pathways. These pathways encompass not only cranial nerve sensory and motor nuclei for processing feeding-related afferent signals and supplying the oromotor musculature but also reticular neurons for orchestrating ingestion and coordinating it with other behaviors that utilize the same musculature. Based on decerebrate studies, this circuit should be sensitive to satiety mechanisms mediated centrally by A2 noradrenergic neurons in the caudal nucleus of the solitary tract (cNST) that are potently activated during satiety. Because the first observable phase of satiety is inhibition of oromotor movements, we hypothesized that norepinephrine (NE) would act to inhibit prehypoglossal neurons in the medullary reticular formation. Using patch-clamp electrophysiology of retrogradely labeled prehypoglossal neurons and calcium imaging to test this hypothesis, we demonstrate that norepinephrine can influence both pre- and postsynaptic properties of reticular neurons through both α1- and α2-adrenoreceptors. The α1-adrenoreceptor agonist phenylephrine (PE) activated an inward current in the presence of TTX and increased the frequency of both inhibitory and excitatory miniature postsynaptic currents. The α2-adrenoreceptor agonist dexmedetomidine (DMT) inhibited cNST-evoked excitatory currents as well as spontaneous and miniature excitatory currents through presynaptic mechanisms. The diversity of adrenoreceptor modulation of these prehypoglossal neurons may reflect their role in a multifunctional circuit coordinating both ingestive and respiratory lingual function.

Intravenous or Local Injections of Flavoxate in the Rostral Pontine Reticular Formation Inhibit Urinary Frequency Induced by Activation of Medial Frontal Lobe Neurons in Rats

The rostral pontine reticular formation has a strong inhibitory effect onmicturition by facilitating lumbosacral glycinergic neurons. We assessed the influence of the rostral pontine reticular formation on the micturition reflex after noradrenaline injection in the medial frontal lobe. We also examined the relation between the medial frontal lobe and the rostral pontine reticular formation. Continuous cystometry was performed in 28 female rats. After the interval between bladder contractions was shortened by noradrenaline injection in the medial frontal lobe we injected glutamate or flavoxate hydrochloride in the rostral pontine reticular formation or intravenously injected flavoxate or propiverine. The change in bladder activity was examined. Noradrenaline injection in the medial frontal lobe shortened the interval between bladder contractions. In contrast to the bladder contraction interval before and after noradrenaline injection in the medial frontal lobe, the interval was prolonged after noradrenaline injection when glutamate or flavoxate was injected in the rostral pontine reticular formation, or flavoxate was injected intravenously. Noradrenaline injection in the medial frontal lobe plus intravenous propiverine injection also prolonged the interval compared to that after noradrenaline injection alone. However, the interval after noradrenaline injection in the medial frontal lobe plus intravenous injection of propiverine was shorter than that before noradrenaline injection only. Medial frontal lobe neurons excited by noradrenaline may facilitate the micturition reflex via activation of inhibitory interneurons, which inhibit descending rostral pontine reticular formation neurons that innervate the lumbosacral glycinergic inhibitory neurons. Therefore, the mechanism of micturition reflex facilitation by the activation of medial frontal lobe neurons involves the rostral pontine reticular formation.

Intrinsic properties and neuropharmacology of midline paraventricular thalamic nucleus neurons.

Neurons in the midline and intralaminar thalamic nuclei are components of an interconnected brainstem, limbic and prefrontal cortex neural network that is engaged during arousal, vigilance, motivated and addictive behaviors, and stress. To better understand the cellular mechanisms underlying these functions, here we review some of the recently characterized electrophysiological and neuropharmacological properties of neurons in the paraventricular thalamic nucleus (PVT), derived from whole cell patch clamp recordings in acute rat brain slice preparations. PVT neurons display firing patterns and ionic conductances (IT and IH) that exhibit significant diurnal change. Their resting membrane potential (RMP) is maintained by various ionic conductances that include inward rectifier (Kir), hyperpolarization-activated nonselective cation (HCN) and TWIK-related acid sensitive (TASK) K+ channels. Firing patterns are regulated by high voltage-activated (HVA) and low voltage-activated (LVA) Ca2+ conductances. Moreover, transient receptor potential (TRP)-like nonselective cation channels together with Ca2+- and Na+-activated K+ conductances (KCa; KNa) contribute to unique slow afterhyperpolarizing potentials (sAHPs) that are generally not detectable in lateral thalamic or reticular thalamic nucleus neurons. The excitability of PVT neurons is also modulated by activation of neurotransmitter receptors associated with afferent pathways to PVT and other thalamic midline nuclei. We report on receptor-mediated actions of GABA, glutamate, monoamines and several neuropeptides: arginine vasopressin, gastrin-releasing peptide, thyrotropin releasing hormone and the orexins (hypocretins). This review represents an initial survey of intrinsic and transmitter-sensitive ionic conductances that are deemed to be unique to this population of midline thalamic neurons, information that is fundamental to an appreciation of the role these thalamic neurons may play in normal central nervous system (CNS) physiology and in CNS disorders that involve the dorsomedial thalamus.

Aspartate‐containing neurons of the brainstem and rostral spinal cord of the sea lamprey Petromyzon marinus: Distribution and comparison with γ‐aminobutyric acid

ABSTRACTThe amino acid L‐aspartate (ASP) is one of the most abundant excitatory neurotransmitters in the mammalian brain, but its distribution in other vertebrates has not yet been well characterized. We investigated the distribution of ASP in the brainstem and rostral spinal cord of the adult sea lamprey by using ASP immunohistochemistry. Our results indicate that ASP is accumulated in specific neurons, but not in glia (tanycytes). ASP‐immunoreactive neuronal populations were rather similar as the glutamatergic populations reported in the adult sea lamprey (Villar‐Cerviño et al. [2013] J Comp Neurol 521:522–557), although some important differences were noted. Characteristically, the largest reticular neurons of the lamprey brainstem (Müller cells) showed ASP immunoreactivity in perikarya and processes, in contrast to the absence or faint glutamate immunoreactivity reported in these perikarya. We also compared the distribution of ASP and γ‐aminobutyric acid (GABA) in brainstem neurons by using double immunofluorescence methods. In regions such as the midbrain tectum, dorsal isthmus, and motor nuclei, ASP and GABA immunoreactivity was mostly located in different neurons, whereas in other nuclei (torus semicircularis, octavolateralis area, parvocellular reticular formation), many of the ASP‐immunonegative neurons displayed colocalization with GABA. These results, together with those of our previous studies of colocalization of glutamate and GABA, suggest that some lamprey neurons may co‐release both excitatory and inhibitory neurotransmitters. Further investigation is needed to elucidate the pathways of uptake and release of ASP by ASP‐immunoreactive neurons. Our results indicate that ASP is a neurotransmitter in the central nervous system representative of agnathans, the earliest vertebrate group. J. Comp. Neurol. 522:1209–1231, 2014. © 2013 Wiley Periodicals, Inc.

Aspartate‐containing neurons of the brainstem and rostral spinal cord of the sea lamprey Petromyzon marinus: Distribution and comparison with γ‐aminobutyric acid

The amino acid L-aspartate (ASP) is one of the most abundant excitatory neurotransmitters in the mammalian brain, but its distribution in other vertebrates has not yet been well characterized. We investigated the distribution of ASP in the brainstem and rostral spinal cord of the adult sea lamprey by using ASP immunohistochemistry. Our results indicate that ASP is accumulated in specific neurons, but not in glia (tanycytes). ASP-immunoreactive neuronal populations were rather similar as the glutamatergic populations reported in the adult sea lamprey (Villar-Cerviño et al. [2013] J Comp Neurol 521:522-557), although some important differences were noted. Characteristically, the largest reticular neurons of the lamprey brainstem (Müller cells) showed ASP immunoreactivity in perikarya and processes, in contrast to the absence or faint glutamate immunoreactivity reported in these perikarya. We also compared the distribution of ASP and γ-aminobutyric acid (GABA) in brainstem neurons by using double immunofluorescence methods. In regions such as the midbrain tectum, dorsal isthmus, and motor nuclei, ASP and GABA immunoreactivity was mostly located in different neurons, whereas in other nuclei (torus semicircularis, octavolateralis area, parvocellular reticular formation), many of the ASP-immunonegative neurons displayed colocalization with GABA. These results, together with those of our previous studies of colocalization of glutamate and GABA, suggest that some lamprey neurons may co-release both excitatory and inhibitory neurotransmitters. Further investigation is needed to elucidate the pathways of uptake and release of ASP by ASP-immunoreactive neurons. Our results indicate that ASP is a neurotransmitter in the central nervous system representative of agnathans, the earliest vertebrate group.

Patch Clamp Electrophysiology and Capillary Electrophoresis–Mass Spectrometry Metabolomics for Single Cell Characterization

The visual selection of specific cells within an ex vivo brain slice, combined with whole-cell patch clamp recording and capillary electrophoresis (CE)–mass spectrometry (MS)-based metabolomics, yields high chemical information on the selected cells. By providing access to a cell’s intracellular environment, the whole-cell patch clamp technique allows one to record the cell’s physiological activity. A patch clamp pipet is used to withdraw ∼3 pL of cytoplasm for metabolomic analysis using CE–MS. Sampling the cytoplasm, rather than an intact isolated neuron, ensures that the sample arises from the cell of interest and that structures such as presynaptic terminals from surrounding, nontargeted neurons are not sampled. We sampled the rat thalamus, a well-defined system containing gamma-aminobutyric acid (GABA)-ergic and glutamatergic neurons. The approach was validated by recording and sampling from glutamatergic thalamocortical neurons, which receive major synaptic input from GABAergic thalamic reticular nucleus neurons, as well as neurons and astrocytes from the ventral basal nucleus and the dorsal lateral geniculate nucleus. From the analysis of the cytoplasm of glutamatergic cells, approximately 60 metabolites were detected, none of which corresponded to the compound GABA. However, GABA was successfully detected when sampling the cytoplasm of GABAergic neurons, demonstrating the exclusive nature of our cytoplasmic sampling approach. The combination of whole-cell patch clamp with single cell cytoplasm metabolomics provides the ability to link the physiological activity of neurons and astrocytes with their neurochemical state. The observed differences in the metabolome of these neurons underscore the striking cell to cell heterogeneity in the brain.

Effects of the Olfactory Bulbs on the Activity of Reticular Neurons of the Bulbar Respiratory Center in Hypoxia

The study reported here seeks to explain the roles of different limbic system formations in controlling respiration in conditions of oxygen insufficiency. The effects of stimulation of the olfactory bulbs (OB) on medulla oblongata respiratory center (RC) neurons during exposure to hypoxia were studied in a laboratory barochamber. Stimulation of the OB in normoxia had a predominantly activatory influence on the baseline activity of neurons. During hypoxia, the responses of medulla oblongata respiratory center neurons to stimulation of the OB were phasic in nature. At the first stage of hypoxia (at an altitude of 4000–5000 m), on the background of hypoxic activation of neurons, the activatory influence of this formation on the spike activity of neurons persisted, albeit weakly. At an altitude of 7500–8000 m, (severe hypoxia phase), on the background of hypoxic suppression of medulla oblongata neuron activity and respiration overall, the activatory effect of stimulation of the OB on the spiked activity of neurons in the bulbar respiratory center was more marked than in the first stage. The results of these experiments provide grounds for the suggestion that the OB is among the limbic system formations with predominantly activatory influences on RC neurons. After lowering the animals to normal atmospheric pressure, there was gradual restoration of initial values for both baseline neuron activity and their responses to stimulation.

Modulation of Short-Term Plasticity in the Corticothalamic Circuit by Group III Metabotropic Glutamate Receptors

Recurrent connections in the corticothalamic circuit underlie oscillatory behavior in this network and range from normal sleep rhythms to the abnormal spike-wave discharges seen in absence epilepsy. The propensity of thalamic neurons to fire postinhibitory rebound bursts mediated by low-threshold calcium spikes renders the circuit vulnerable to both increased excitation and increased inhibition, such as excessive excitatory cortical drive to thalamic reticular (RT) neurons or heightened inhibition of thalamocortical relay (TC) neurons by RT. In this context, a protective role may be played by group III metabotropic receptors (mGluRs), which are uniquely located in the presynaptic active zone and typically act as autoreceptors or heteroceptors to depress synaptic release. Here, we report that these receptors regulate short-term plasticity at two loci in the corticothalamic circuit in rats: glutamatergic cortical synapses onto RT neurons and GABAergic synapses onto TC neurons in somatosensory ventrobasal thalamus. The net effect of group III mGluR activation at these synapses is to suppress thalamic oscillations as assayed in vitro. These findings suggest a functional role of these receptors to modulate corticothalamic transmission and protect against prolonged activity in the network.

Feed-forward and feedback projections of midbrain reticular formation neurons in the cat

Gaze changes involving the eyes and head are orchestrated by brainstem gaze centers found within the superior colliculus (SC), paramedian pontine reticular formation (PPRF), and medullary reticular formation (MdRF). The mesencephalic reticular formation (MRF) also plays a role in gaze. It receives a major input from the ipsilateral SC and contains cells that fire in relation to gaze changes. Moreover, it provides a feedback projection to the SC and feed-forward projections to the PPRF and MdRF. We sought to determine whether these MRF feedback and feed-forward projections originate from the same or different neuronal populations by utilizing paired fluorescent retrograde tracers in cats. Specifically, we tested: 1. whether MRF neurons that control eye movements form a single population by injecting the SC and PPRF with different tracers, and 2. whether MRF neurons that control head movements form a single population by injecting the SC and MdRF with different tracers. In neither case were double labeled neurons observed, indicating that feedback and feed-forward projections originate from separate MRF populations. In both cases, the labeled reticulotectal and reticuloreticular neurons were distributed bilaterally in the MRF. However, neurons projecting to the MdRF were generally constrained to the medial half of the MRF, while those projecting to the PPRF, like MRF reticulotectal neurons, were spread throughout the mediolateral axis. Thus, the medial MRF may be specialized for control of head movements, with control of eye movements being more widespread in this structure.