Light and electron microscopic evidence for a GABAergic projection from the caudal basal forebrain to the thalamic reticular nucleus in rats

Elevation of GABA levels in the globus pallidus disinhibits the thalamic reticular nucleus and desynchronized cortical beta oscillations

The external globus pallidus (GP) is a GABAergic node involved in motor control regulation and coordinates firing and synchronization in the basal ganglia-thalamic-cortical network through inputs and electrical activity. In Parkinson's disease, high GABA levels alter electrical activity in the GP and contribute to motor symptoms. Under normal conditions, GABA levels are regulated by GABA transporters (GATs). GAT type 1 (GAT-1) is highly expressed in the GP, and pharmacological blockade of GAT-1 increases the duration of currents mediated by GABA A receptors and induces tonic inhibition. The functional contribution of the pathway between the GP and the reticular thalamic nucleus (RTn) is unknown. This pathway is important since the RTn controls the flow of information between the thalamus and cortex, suggesting that it contributes to cortical dynamics. In this work, we investigated the effect of increased GABA levels on electrical activity in the RTn by obtaining single-unit extracellular recordings from anesthetized rats and on the motor cortex (MCx) by corticography. Our results show that high GABA levels increase the spontaneous activity rate of RTn neurons and desynchronize oscillations in the beta frequency band in the MCx. Our findings provide evidence that the GP exerts tonic control over RTn activity through the GP-reticular pathway and functionally contributes to cortical oscillation dynamics.

Insights on the role of external globus pallidus in controlling absence seizures

Absence epilepsy, characterized by transient loss of awareness and bilaterally synchronous 2-4 Hz spike and wave discharges (SWDs) on electroencephalography (EEG) during absence seizures, is generally believed to arise from abnormal interactions between the cerebral cortex (Ctx) and thalamus. Recent animal electrophysiological studies suggested that changing the neural activation level of the external globus pallidus (GPe) neurons can remarkably modify firing rates of the thalamic reticular nucleus (TRN) neurons through the GABAergic GPe-TRN pathway. However, the existing experimental evidence does not provide a clear answer as to whether the GPe-TRN pathway contributes to regulating absence seizures. Here, using a biophysically based mean-field model of the GPe-corticothalamic (GCT) network, we found that both directly decreasing the strength of the GPe-TRN pathway and inactivating GPe neurons can effectively suppress absence seizures. Also, the pallido-cortical pathway and the recurrent connection of GPe neurons facilitate the regulation of absence seizures through the GPe-TRN pathway. Specifically, in the controllable situation, enhancing the coupling strength of either of the two pathways can successfully terminate absence seizures. Moreover, the competition between the GPe-TRN and pallido-cortical pathways may lead to the GPe bidirectionally controlling absence seizures, and this bidirectional control manner can be significantly modulated by the Ctx-TRN pathway. Importantly, when the strength of the Ctx-TRN pathway is relatively strong, the bidirectional control of absence seizures by changing GPe neural activities can be observed at both weak and strong strengths of the pallido-cortical pathway.These findings suggest that the GPe-TRN pathway may have crucial functional roles in regulating absence seizures, which may provide a testable hypothesis for further experimental studies and new perspectives on the treatment of absence epilepsy.

Role of the Thalamus in Basal Forebrain Regulation of Neural Activity in the Primary Auditory Cortex

Many studies have implicated the basal forebrain (BF) as a potent regulator of sensory encoding even at the earliest stages of or cortical processing. The source of this regulation involves the well-documented corticopetal cholinergic projections from BF to primary cortical areas. However, the BF also projects to subcortical structures, including the thalamic reticular nucleus (TRN), which has abundant reciprocal connections with sensory thalamus. Here we present naturalistic auditory stimuli to the anesthetized rat while making simultaneous single-unit recordings from the ventral medial geniculate nucleus (MGN) and primary auditory cortex (A1) during electrical stimulation of the BF. Like primary visual cortex, we find that BF stimulation increases the trial-to-trial reliability of A1 neurons, and we relate these results to change in the response properties of MGN neurons. We discuss several lines of evidence that implicate the BF to thalamus pathway in the manifestation of BF-induced changes to cortical sensory processing and support our conclusions with supplementary TRN recordings, as well as studies in awake animals showing a strong relationship between endogenous BF activity and A1 reliability. Our findings suggest that the BF subcortical projections that modulate MGN play an important role in auditory processing.

Signal Propagation via Open-Loop Intrathalamic Architectures: A Computational Model

Propagation of signals across the cerebral cortex is a core component of many cognitive processes and is generally thought to be mediated by direct intracortical connectivity. The thalamus, by contrast, is considered to be devoid of internal connections and organized as a collection of parallel inputs to the cortex. Here, we provide evidence that "open-loop" intrathalamic pathways involving the thalamic reticular nucleus (TRN) can support propagation of oscillatory activity across the cortex. Recent studies support the existence of open-loop thalamo-reticulo-thalamic (TC-TRN-TC) synaptic motifs in addition to traditional closed-loop architectures. We hypothesized that open-loop structural modules, when connected in series, might underlie thalamic and, therefore cortical, signal propagation. Using a supercomputing platform to simulate thousands of permutations of a thalamocortical network based on physiological data collected in mice, rats, ferrets, and cats and in which select synapses were allowed to vary both by class and individually, we evaluated the relative capacities of closed-loop and open-loop TC-TRN-TC synaptic configurations to support both propagation and oscillation. We observed that (1) signal propagation was best supported in networks possessing strong open-loop TC-TRN-TC connectivity; (2) intrareticular synapses were neither primary substrates of propagation nor oscillation; and (3) heterogeneous synaptic networks supported more robust propagation of oscillation than their homogeneous counterparts. These findings suggest that open-loop, heterogeneous intrathalamic architectures might complement direct intracortical connectivity to facilitate cortical signal propagation.

Thalamic Reticular Nucleus Parvalbumin Neurons Regulate Sleep Spindles and Electrophysiological Aspects of Schizophrenia in Mice

The thalamic reticular nucleus (TRN) is implicated in schizophrenia pathology. However, it remains unclear whether alterations of TRN activity can account for abnormal electroencephalographic activity observed in patients, namely reduced spindles (10-15 Hz) during sleep and increased delta (0.5-4 Hz) and gamma-band activity (30-80 Hz) during wakefulness. Here, we utilized optogenetic and reverse-microdialysis approaches to modulate activity of the major subpopulation of TRN GABAergic neurons, which express the calcium-binding protein parvalbumin (PV), and are implicated in schizophrenia dysfunction. An automated algorithm with enhanced efficiency and reproducibility compared to manual detection was used for sleep spindle assessment. A novel, low power, waxing-and-waning optogenetic stimulation paradigm preferentially induced spindles that were indistinguishable from spontaneously occurring sleep spindles without altering the behavioral state, when compared to a single pulse laser stimulation used by us and others. Direct optogenetic inhibition of TRN-PV neurons was ineffective in blocking spindles but increased both wakefulness and cortical delta/gamma activity, as well as impaired the 40 Hz auditory steady-state response. For the first time we demonstrate that spindle density is markedly reduced by (i) optogenetic stimulation of a major GABA/PV inhibitory input to TRN arising from basal forebrain parvalbumin neurons (BF-PV) and; (ii) localized pharmacological inhibition of low-threshold calcium channels, implicated as a genetic risk factor for schizophrenia. Together with clinical findings, our results support impaired TRN-PV neuron activity as a potential cause of schizophrenia-linked abnormalities in cortical delta, gamma, and spindle activity. Modulation of the BF-PV input to TRN may improve these neural abnormalities.

Tenuous Inhibitory GABAergic Signaling in the Reticular Thalamus

Maintenance of a low intracellular Cl- concentration ([Cl-]i) is critical for enabling inhibitory neuronal responses to GABAA receptor-mediated signaling. Cl- transporters, including KCC2, and extracellular impermeant anions ([A]o) of the extracellular matrix are both proposed to be important regulators of [Cl-]i Neurons of the reticular thalamic (RT) nucleus express reduced levels of KCC2, indicating that GABAergic signaling may produce excitation in RT neurons. However, by performing perforated patch recordings and calcium imaging experiments in rats (male and female), we find that [Cl-]i remains relatively low in RT neurons. Although we identify a small contribution of [A]o to a low [Cl-]i in RT neurons, our results also demonstrate that reduced levels of KCC2 remain sufficient to maintain low levels of Cl- Reduced KCC2 levels, however, restrict the capacity of RT neurons to rapidly extrude Cl- following periods of elevated GABAergic signaling. In a computational model of a local RT network featuring slow Cl- extrusion kinetics, similar to those we found experimentally, model RT neurons are predisposed to an activity-dependent switch from GABA-mediated inhibition to excitation. By decreasing the activity threshold required to produce excitatory GABAergic signaling, weaker stimuli are able to propagate activity within the model RT nucleus. Our results indicate the importance of even diminished levels of KCC2 in maintaining inhibitory signaling within the RT nucleus and suggest how this important activity choke point may be easily overcome in disorders such as epilepsy.SIGNIFICANCE STATEMENT Precise regulation of intracellular Cl- levels ([Cl-]i) preserves appropriate, often inhibitory, GABAergic signaling within the brain. However, there is disagreement over the relative contribution of various mechanisms that maintain low [Cl-]i We found that the Cl- transporter KCC2 is an important Cl- extruder in the reticular thalamic (RT) nucleus, despite this nucleus having remarkably low KCC2 immunoreactivity relative to other regions of the adult brain. We also identified a smaller contribution of fixed, impermeant anions ([A]o) to lowering [Cl-]i in RT neurons. Inhibitory signaling among RT neurons is important for preventing excessive activation of RT neurons, which can be responsible for generating seizures. Our work suggests that KCC2 critically restricts the spread of activity within the RT nucleus.

Lack of Intrinsic GABAergic Connections in the Thalamic Reticular Nucleus of the Mouse

It is generally thought that neurons in the thalamic reticular nucleus (TRN) form GABAergic synapses with other TRN neurons and that these interconnections are important for the function of the TRN. However, the existence of such intrinsic connections is controversial. We combine two complementary approaches to examine intrinsic GABAergic connections in the TRN of the mouse. We find that optogenetic stimulation of TRN neurons and their axons evokes GABAergic IPSCs in TRN neurons in mice younger than 2 weeks of age but fails to do so after that age. Blocking synaptic release from TRN neurons through conditional deletion of vesicular GABA transporter has no effect on spontaneous IPSCs recorded in TRN neurons aged 2 weeks or older while dramatically reducing GABAergic transmission in thalamic relay neurons. These results demonstrate that except for a short period after birth, the TRN of the mouse lacks intrinsic GABAergic connections.

SIGNIFICANCE STATEMENT

The thalamic reticular nucleus has a critical role in modulating information transfer from the thalamus to the cortex. It has been proposed that neurons in the thalamic reticular nucleus are interconnected through GABAergic synapses and that these connections serve important functions. Our results show that except for the first 2 weeks after birth, the thalamic reticular nucleus of the mouse lacks intrinsic GABAergic connections.

Auditory thalamic circuits and GABAA receptor function: Putative mechanisms in tinnitus pathology

Tinnitus is defined as a phantom sound (ringing in the ears), and can significantly reduce the quality of life for those who suffer its effects. Ten to fifteen percent of the general adult population report symptoms of tinnitus with 1-2% reporting that tinnitus negatively impacts their quality of life. Noise exposure is the most common cause of tinnitus and the military environment presents many challenging high-noise situations. Military noise levels can be so intense that standard hearing protection is not adequate. Recent studies suggest a role for inhibitory neurotransmitter dysfunction in response to noise-induced peripheral deafferentation as a key element in the pathology of tinnitus. The auditory thalamus, or medial geniculate body (MGB), is an obligate auditory brain center in a unique position to gate the percept of sound as it projects to auditory cortex and to limbic structures. Both areas are thought to be involved in those individuals most impacted by tinnitus. For MGB, opposing hypotheses have posited either a tinnitus-related pathologic decrease or pathologic increase in GABAergic inhibition. In sensory thalamus, GABA mediates fast synaptic inhibition via synaptic GABAA receptors (GABAARs) as well as a persistent tonic inhibition via high-affinity extrasynaptic GABAARs and slow synaptic inhibition via GABABRs. Down-regulation of inhibitory neurotransmission, related to partial peripheral deafferentation, is consistently presented as partially underpinning neuronal hyperactivity seen in animal models of tinnitus. This maladaptive plasticity/Gain Control Theory of tinnitus pathology (see Auerbach et al., 2014; Richardson et al., 2012) is characterized by reduced inhibition associated with increased spontaneous and abnormal neuronal activity, including bursting and increased synchrony throughout much of the central auditory pathway. A competing hypothesis suggests that maladaptive oscillations between the MGB and auditory cortex, thalamocortical dysrhythmia, predict tinnitus pathology (De Ridder et al., 2015). These unusual oscillations/rhythms reflect net increased tonic inhibition in a subset of thalamocortical projection neurons resulting in abnormal bursting. Hyperpolarizing de-inactivation of T-type Ca2+ channels switches thalamocortical projection neurons into burst mode. Thalamocortical dysrhythmia originating in sensory thalamus has been postulated to underpin neuropathies including tinnitus and chronic pain. Here we review the relationship between noise-induced tinnitus and altered inhibition in the MGB.

The menagerie of the basal forebrain: how many (neural) species are there, what do they look like, how do they behave and who talks to whom?

The diverse cell-types of the basal forebrain control sleep-wake states, cortical activity and reward processing. Large, slow-firing, cholinergic neurons suppress cortical delta activity and promote cortical plasticity in response to reinforcers. Large, fast-firing, cortically-projecting GABAergic neurons promote wakefulness and fast cortical activity. In particular, parvalbumin/GABAergic neurons promote neocortical gamma band activity. Conversely, excitation of slower-firing somatostatin/GABAergic neurons promotes sleep through inhibition of cortically-projecting neurons. Activation of glutamatergic neurons promotes wakefulness, likely by exciting other cortically-projecting neurons. Similarly, cholinergic neurons indirectly promote wakefulness by excitation of wake-promoting, cortically-projecting GABAergic neurons and/or inhibition of sleep-promoting somatostatin/GABAergic neurons. Both glia and neurons increase the levels of adenosine during prolonged wakefulness. Adenosine presynaptically inhibits glutamatergic inputs to wake-promoting cholinergic and GABAergic/parvalbumin neurons, promoting sleep.

Thalamic Inhibition: Diverse Sources, Diverse Scales

The thalamus is the major source of cortical inputs shaping sensation, action, and cognition. Thalamic circuits are targeted by two major inhibitory systems: the thalamic reticular nucleus (TRN) and extrathalamic inhibitory (ETI) inputs. A unifying framework of how these systems operate is currently lacking. Here, we propose that TRN circuits are specialized to exert thalamic control at different spatiotemporal scales. Local inhibition of thalamic spike rates prevails during attentional selection, whereas global inhibition more likely prevails during sleep. In contrast, the ETI (arising from basal ganglia, zona incerta (ZI), anterior pretectum, and pontine reticular formation) provides temporally precise and focal inhibition, impacting spike timing. Together, these inhibitory systems allow graded control of thalamic output, enabling thalamocortical operations to dynamically match ongoing behavioral demands.

Functions and Mechanisms of Sleep

Sleep is a complex physiological process that is regulated globally, regionally, and locally by both cellular and molecular mechanisms. It occurs to some extent in all animals, although sleep expression in lower animals may be co-extensive with rest. Sleep regulation plays an intrinsic part in many behavioral and physiological functions. Currently, all researchers agree there is no single physiological role sleep serves. Nevertheless, it is quite evident that sleep is essential for many vital functions including development, energy conservation, brain waste clearance, modulation of immune responses, cognition, performance, vigilance, disease, and psychological state. This review details the physiological processes involved in sleep regulation and the possible functions that sleep may serve. This description of the brain circuitry, cell types, and molecules involved in sleep regulation is intended to further the reader's understanding of the functions of sleep.

Pedunculopontine arousal system physiology - Deep brain stimulation (DBS)

This review describes the wake/sleep symptoms present in Parkinson׳s disease, and the role of the pedunculopontine nucleus in these symptoms. The physiology of PPN cells is important not only because it is a major element of the reticular activating system, but also because it is a novel target for deep brain stimulation in the treatment of gait and postural deficits in Parkinson׳s disease. A greater understanding of the physiology of the target nuclei within the brainstem and basal ganglia, amassed over the past decades, has enabled increasingly better patient outcomes from deep brain stimulation for movement disorders.

Open-loop organization of thalamic reticular nucleus and dorsal thalamus: a computational model

The thalamic reticular nucleus (TRN) is a shell of GABAergic neurons that surrounds the dorsal thalamus. Previous work has shown that TRN neurons send GABAergic projections to thalamocortical (TC) cells to form reciprocal, closed-loop circuits. This has led to the hypothesis that the TRN is responsible for oscillatory phenomena, such as sleep spindles and absence seizures. However, there is emerging evidence that open-loop circuits are also found between TRN and TC cells. The implications of open-loop configurations are not yet known, particularly when they include time-dependent nonlinearities in TC cells such as low-threshold bursting. We hypothesized that low-threshold bursting in an open-loop circuit could be a mechanism by which the TRN could paradoxically enhance TC activation, and that enhancement would depend on the relative timing of TRN vs. TC cell stimulation. To test this, we modeled small circuits containing TC neurons, TRN neurons, and layer 4 thalamorecipient cells in both open- and closed-loop configurations. We found that open-loop TRN stimulation, rather than universally depressing TC activation, increased cortical output across a broad parameter space, modified the filter properties of TC neurons, and altered the mutual information between input and output in a frequency-dependent and T-type calcium channel-dependent manner. Therefore, an open-loop model of TRN-TC interactions, rather than suppressing transmission through the thalamus, creates a tunable filter whose properties may be modified by outside influences onto the TRN. These simulations make experimentally testable predictions about the potential role for the TRN for flexible enhancement of cortical activation.

The cortical modulation of stimulus-specific adaptation in the auditory midbrain and thalamus: a potential neuronal correlate for predictive coding

To follow an ever-changing auditory scene, the auditory brain is continuously creating a representation of the past to form expectations about the future. Unexpected events will produce an error in the predictions that should “trigger” the network’s response. Indeed, neurons in the auditory midbrain, thalamus and cortex, respond to rarely occurring sounds while adapting to frequently repeated ones, i.e., they exhibit stimulus specific adaptation (SSA). SSA cannot be explained solely by intrinsic membrane properties, but likely involves the participation of the network. Thus, SSA is envisaged as a high order form of adaptation that requires the influence of cortical areas. However, present research supports the hypothesis that SSA, at least in its simplest form (i.e., to frequency deviants), can be transmitted in a bottom-up manner through the auditory pathway. Here, we briefly review the underlying neuroanatomy of the corticofugal projections before discussing state of the art studies which demonstrate that SSA present in the medial geniculate body (MGB) and inferior colliculus (IC) is not inherited from the cortex but can be modulated by the cortex via the corticofugal pathways. By modulating the gain of neurons in the thalamus and midbrain, the auditory cortex (AC) would refine SSA subcortically, preventing irrelevant information from reaching the cortex.

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.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Target-dependent control of synaptic inhibition by endocannabinoids in the thalamus

Inhibitory neurons in the thalamic reticular nucleus (TRN) play a critical role in controlling information transfer between thalamus and neocortex. GABAergic synapses formed by TRN neurons contact both thalamic relay cells and neurons within TRN. These two types of synapses are thought to have distinct roles for the generation of thalamic network activity, but their selective regulation is poorly understood. In many areas throughout the brain, retrograde signaling mediated by endocannabinoids acts to dynamically regulate synaptic strength over both short and long time scales. However, retrograde signaling has never been demonstrated in the thalamus. Here, we show that depolarization-induced suppression of inhibition (DSI) is prominent at inhibitory synapses interconnecting TRN neurons. DSI is completely abolished in the presence of a cannabinoid receptor 1 (CB1R) antagonist and in mice lacking CB1Rs. DSI is prevented by DAG lipase inhibitors and prolonged by blocking the 2-arachidonoylglycerol (2-AG) degradation enzyme monoacylglycerol lipase, indicating that it is mediated by the release of 2-AG from TRN neurons. By contrast, DSI is not observed at TRN synapses targeting thalamic relay neurons. A combination of pharmacological and immunohistochemical data indicate that the differences in endocannabinoid signaling at the two synapses are mediated by a synapse-specific targeting of CB1Rs, as well as differences in endocannabinoid release between the two target neurons. Together, our results show that endocannabinoids control transmitter release at specific thalamic synapses, and could dynamically regulate sensory information processing and thalamus-mediated synchronous oscillations.

Sleep state switching

We take for granted the ability to fall asleep or to snap out of sleep into wakefulness, but these changes in behavioral state require specific switching mechanisms in the brain that allow well-defined state transitions. In this review, we examine the basic circuitry underlying the regulation of sleep and wakefulness and discuss a theoretical framework wherein the interactions between reciprocal neuronal circuits enable relatively rapid and complete state transitions. We also review how homeostatic, circadian, and allostatic drives help regulate sleep state switching and discuss how breakdown of the switching mechanism may contribute to sleep disorders such as narcolepsy.

Regulation of inhibitory synapses by presynaptic D₄ dopamine receptors in thalamus

Dopamine (DA) receptors are the principal targets of drugs used in the treatment of schizophrenia. Among the five DA receptor subtypes, the D(4) subtype is of particular interest because of the relatively high affinity of the atypical neuropleptic clozapine for D(4) compared with D(2) receptors. GABA-containing neurons in the thalamic reticular nucleus (TRN) and globus pallidus (GP) express D(4) receptors. TRN neurons receive GABAergic afferents from globus pallidus (GP), substantia nigra pars reticulata (SNr), and basal forebrain as well as neighboring TRN neuron collaterals. In addition, TRN receives dopaminergic innervations from substantia nigra pars compacta (SNc); however, the role of D(4) receptors in neuronal signaling at inhibitory synapses is unknown. Using whole cell recordings from in vitro pallido-thalamic slices, we demonstrate that DA selectively suppresses GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) evoked by GP stimulation. The D(2)-like receptor (D(2,3,4)) agonist, quinpirole, and selective D(4) receptor agonist, PD168077, mimicked the actions of DA. The suppressive actions of DA and its agonists were associated with alterations in paired pulse ratio and a decrease in the frequency of miniature IPSCs, suggesting a presynaptic site of action. GABA(A) receptor agonist, muscimol, induced postsynaptic currents in TRN neurons were unaltered by DA or quinpirole, consistent with the presynaptic site of action. Finally, DA agonists did not alter intra-TRN inhibitory signaling. Our data demonstrate that the activation of presynaptic D(4) receptors regulates GABA release from GP efferents but not TRN collaterals. This novel and selective action of D(4) receptor activation on GP-mediated inhibition may provide insight to potential functional significance of atypical antipsychotic agents. These findings suggest a potential heightened TRN neuron activity in certain neurological conditions, such as schizophrenia and attention deficit hyperactive disorders.

Cholinergic stimulation of the nucleus basalis of Meynert and reticular thalamic nucleus affects spike-and-wave discharges in WAG/Rij rats

The role of the cholinergic innervated nucleus basalis of Meynert (NB) and reticular thalamic nucleus (RT) in the generation or modulation of spontaneously occurring spike-and-wave discharges (SWDs) was investigated in the WAG/Rij rat model of absence epilepsy. The cholinergic agonist carbachol and the muscarinic antagonist scopolamine were injected in the NB and RT in the doses of 0.55 and 5.5 nmol while the EEG was recorded. Carbachol injected in the NB decreased the number and the mean duration of SWDs. Scopolamine alone had no influence on SWDs, but could antagonize the effects of carbachol if administered simultaneously in NB. Injections of carbachol in the RT inhibited the occurrence of SWDs, but did not affect the mean duration. Scopolamine administered in the RT had no influence on seizure activity. It is concluded that cholinergic stimulation of the NB or the RT inhibits the cortical synchronous activity characterizing SWDs.

Neuronal networks of the basal ganglia and the value of recording field potentials from them

The basal ganglia constitute parts of highly sophisticated and complex neuronal networks, which represent essential elements of functional circuits, actively involved in the control of movement. The physiologic properties of these networks and their interchange with different brain areas could serve as a model for the pathophysiologic explanation of various movement disorders, particularly Parkinson's disease. Stimulation of these networks and subsequent recording of the evoked Local Field Potentials is currently used not only for understanding the pathophysiology of movement disorders but also for the physiologic localization of the anatomical target during deep brain stimulation procedures. An overview of the currently available research and clinical data from the recording of Local Field Potentials as well as the advantages, the disadvantages and the limitations of this methodology are presented in this chapter.

Thalamic synchrony and dynamic regulation of global forebrain oscillations

The circuitry within the thalamus creates an intrinsic oscillatory unit whose function depends critically on reciprocal synaptic connectivity between excitatory thalamocortical relay neurons and inhibitory thalamic reticular neurons along with a robust post-inhibitory rebound mechanism in relay neurons. Feedforward and feedback connections between cortex and thalamus reinforce the thalamic oscillatory activity into larger thalamocortical networks to generate sleep spindles and spike-wave discharge of generalized absence epilepsy. The degree of synchrony within the thalamic network seems to be crucial in determining whether normal (spindle) or pathological (spike-wave) oscillations occur, and recent studies show that regulation of excitability in the reticular nucleus leads to dynamical modulation of the state of the thalamic circuit and provide a basis for explaining how a variety of unrelated genetic alterations might lead to the spike-wave phenotype. In addition, given the central role of the reticular nucleus in generating spike-wave discharge, these studies have suggested specific interventions that would prevent seizures while still allowing normal spindle generation to occur. This review is part of the INMED/TINS special issue Physiogenic and pathogenic oscillations: the beauty and the beast, based on presentations at the annual INMED/TINS symposium (http://inmednet.com).

Cellular distribution of the GABAA receptor-modulating 3alpha-hydroxy, 5alpha-reduced pregnane steroids in the adult rat brain

The 3alpha-hydroxy,5alpha-reduced pregnane steroids, allopregnanolone and allotetrahydrodeoxycorticosterone, are the most potent endogenous positive modulators of GABA(A) receptor-mediated inhibition. This study presents the first immunohistochemical examination of the cellular distribution of 3alpha-hydroxy,5alpha-reduced pregnane steroids across the brain. We found a widespread distribution in the adult rat, with dense immunolabelling in the olfactory bulb, striatum and cerebral cortex, and lower density labelling in the brainstem reticular formation. In general terms, this distribution accords with the regional concentrations of 3alpha-hydroxy,5alpha-reduced steroids determined, in other laboratories, by brain region sampling and either gas chromatography-mass fragmentography or radioimmunoassay. However, immunohistochemistry allowed for a more detailed examination of regional distribution and cellular specificity. All immunoreactivity was confined to the cell bodies and thick dendrites of neurones; no identifiable glia were labelled. In most brain areas, the location and morphology of labelled cells identified them as excitatory neurones. In addition, cell populations known to be projecting GABAergic neurones (e.g. cerebellar Purkinje cells) were immunoreactive, whereas local inhibitory neurones generally were not. The cellular distribution of 3alpha-hydroxy,5alpha-reduced steroids suggests that sensory, motor, limbic and homeostatic systems can be influenced by neurosteroids at multiple stages of processing.

Fast IPSCs in rat thalamic reticular nucleus require the GABAA receptor beta1 subunit

Synchrony within the thalamocortical system is regulated in part by intranuclear synaptic inhibition within the reticular nucleus (RTN). Inhibitory postsynaptic currents (IPSCs) in RTN neurons are largely characterized by slow decay kinetics that result in powerful and prolonged suppression of spikes. Here we show that some individual RTN neurons are characterized by highly variable mixtures of fast, slow and mixed IPSCs. Heterogeneity arose largely through differences in the contribution of an initial decay component (tau(D) approximately 10 ms) which was insensitive to loreclezole, suggesting involvement of the GABA(A) receptor beta(1) subunit. Single-cell RT-PCR revealed the presence of beta(1) subunit mRNA only in those neurons whose IPSCs were dominated by a rapid and prominent initial decay phase. These data show that brief, beta(1)-dependent, loreclezole-insensitive IPSCs are present in a subpopulation of RTN neurons, and suggest that striking differences in IPSC heterogeneity within single neurons can result from of the presence or absence of a single GABA(A) receptor subunit.

Synaptology of the rostral reticular thalamic nucleus of absence epileptic WAG/Rij rats

The adult WAG/Rij rat is a well-established animal model for human absence epilepsy characterized by the presence of spike-wave discharges (SWDs). The pacemaking activity of the rostral reticular thalamic nucleus (rRTN) has been demonstrated to be essential for SWD maintenance. We investigated if SWD maintenance can be related to the synaptic organization of the rRTN, by studying the ultrastructure of the rRTN of absence epileptic WAG/Rij rats in comparison with that of non-epileptic, age-matched ACI control rats. In WAG/Rij rats, D-, L- and F-type terminals constitute the synaptic organization of the rRTN. D-type synapses, especially axo-dendritic ones, occur frequently. L- and F-type terminals are common but less frequent than D-type terminals. Semi-quantitative observations indicate that all terminal types are present on different parts of the postsynaptic neuron, but in different numbers: they are frequent on dendrites, common on somata and axons, and occur occasionally on dendritic spines. In addition, occasionally an F-type terminal was observed on the axon hillock. The three terminal types are also involved in multiple synaptic configurations, convergent as well as divergent, with dendrites, somata, axon hillocks and axons as postsynaptic structures. Convergent synaptic configurations outnumber divergent ones. The synaptic organization of the rRTN of the non-epileptic ACI rat appears to be very similar to that of the epileptic WAG/Rij rat. This indicates that SWD maintenance in the WAG/Rij rat does not depend on a different synaptic organization of the rRTN.

Structure and connections of the thalamic reticular nucleus: Advancing views over half a century

The advance of knowledge of the thalamic reticular nucleus and its connections has been reviewed and Max Cowan's contributions to this knowledge and to the methods used for studying the nucleus have been summarized. Whereas 50 years ago the nucleus was seen as a diffusely organized cell group closely related to the brain stem reticular formation, it can now be seen as a complex, tightly organized entity that has a significant inhibitory, modulatory action on the thalamic relay to cortex. The nucleus is under the control, on the one hand, of topographically organized afferents from the cerebral cortex and the thalamus, and on the other of more diffuse afferents from brain stem, basal forebrain, and other regions. Whereas the second group of afferents can be expected to have global actions on thalamocortical transmission, relevant for overall attentive state, the former group will have local actions, modulating transmission through the thalamus to cortex with highly specific local effects. Since it appears that all areas of cortex and all parts of the thalamus are linked directly to the reticular nucleus, it now becomes important to define how the several pathways that pass through the thalamus relate to each other in their reticular connections.

Differential Expression of the GABAA Receptor Complex in the Dorsal Thalamus and Reticular Nucleus: An Immunohistochemical Study in the Adult and Developing Rat

The distribution of the GABAA receptor/benzodiazepine receptor/chloride channel complex was investigated in the thalamus of the rat by means of immunohistochemistry in adulthood, as well as during embryonic and postnatal development, using a monoclonal antibody. In adults, the immunoreactivity for the GABAA receptor complex was intensely expressed by neuronal processes throughout the dorsal thalamus. Neuronal perikaryal membranes were frequently outlined by punctate immunostaining; cell bodies, intrathalamic fibre bundles and the internal capsule did not display immunoreactivity for the GABAA receptor. Regional differences in the expression of the receptor were consistently observed: the immunostaining was much lighter in the thalamic reticular nucleus than in the dorsal thalamic nuclei and, among the latter, the anteroventral nucleus and the ventral nuclear complex displayed the most intense immunopositivity. Immunostaining for the GABAA receptor was already expressed in embryos at E14, and was homogeneously distributed throughout the neuropil of the dorsal and ventral thalamic primordia. During the first two postnatal weeks, a regional differentiation of the immunopositivity was appreciable in the thalamus, with a progressive reduction in the reticular nucleus and a parallel increase in the dorsal thalamic structures. Immunoreactive neuronal perikarya were not observed in the thalamus at any developmental stage. The expression of the GABAA receptor complex appeared to have reached a mature configuration by the end of the third postnatal week. These findings indicate that in adults the GABAA receptor is differentially expressed by thalamic nuclear structures, including the reticular nucleus. Furthermore, the maturation of the receptor in the thalamus undergoes a rearrangement during the first postnatal weeks that results in a considerable regression within the reticular nucleus.

Nerve Growth Factor Receptor-immunoreactive Fibres Innervate the Reticular Thalamic Nucleus: Modulation by Nerve Growth Factor Treatment in Neonate, Adult and Aged Rats

Terminal arborizations expressing nerve growth factor receptor (NGF-R) have been detected with immunohistochemistry in the reticular thalamic nucleus of neonate, adult and aged rats. Intracerebroventricular administration of nerve growth factor (NGF) resulted in a dramatic increase in NGF-R immunoreactivity throughout the lifespan. This effect was paralleled by a concomitant increase in NGF-R immunopositivity in the neurons of the basal forebrain, which was here demonstrated also in aged animals, thus indicating that the NGF-R immunoreactivity within the reticular thalamic nucleus derives in all likelihood from cholinergic neuronal cell bodies of the basal forebrain. Our results demonstrate a prominent ability of NGF to up-regulate its receptors within fibres innervating the reticular thalamic nucleus, and show that this up-regulation of NGF-R is maintained throughout the lifetime. Altogether this indicates that the reticular thalamic nucleus may represent a new, important site of action of endogenous NGF or NGF-like molecules within the brain. In view of the crucial role played by the reticular thalamic nucleus in gating thalamocortical information, the autoregulation of NGF-R in this structure may have important concomitants in both physiological and pathological conditions.

A systems model of altered consciousness: integrating natural and drug-induced psychoses

Increasing evidence from neuroimaging and behavioral studies suggests that functional disturbances within cortico-striato-thalamic pathways are critical to psychotic symptom formation in drug-induced and possibly also naturally occurring psychoses. Recent basic and clinical research with psychotomimetic drugs, such as the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, ketamine, and the serotonin-2A (5-HT(2A)) receptor agonist, psilocybin, suggest that the hallucinogenic effects of these drugs arise, at least in part, from their common capacity to disrupt thalamo-cortical gating of external and internal information to the cortex. Deficient gating of sensory and cognitive information is thought to result in an overloading inundation of information and subsequent cognitive fragmentation and psychosis. Cross-species studies of homologues gating functions, such as prepulse inhibition of the startle reflex, in animal and human models of psychosis corroborate this view and provide a translational testing mechanism for the exploration of novel pathophysiologic and therapeutic hypotheses relevant to psychotic disorders, such as the group of schizophrenias.

Discharge profiles of juxtacellularly labeled and immunohistochemically identified GABAergic basal forebrain neurons recorded in association with the electroencephalogram in anesthetized rats

The basal forebrain ostensibly plays a dual role in the modulation of cortical activation and behavioral state. It is essential for stimulating cortical activation in association with waking (and paradoxical sleep), yet also important for attenuating cortical activation and promoting slow wave sleep. Using juxtacellular recording and labeling of neurons with Neurobiotin followed by immunohistochemical staining for glutamic acid decarboxylase (GAD), we studied the discharge properties of identified GABAergic basal forebrain neurons in relation to electroencephalographic (EEG) activity in urethane-anesthetized rats to determine the part or parts that they may play in this dual role. The GABAergic neurons displayed distinct discharge profiles in relation to somatosensory stimulation-evoked cortical activation. Whereas a significant minority increased its average discharge rate, the majority decreased its average discharge rate in association with cortical activation. Moreover, subgroups displayed distinct discharge patterns related to different cortical activities, including very regular high-frequency tonic spiking within a gamma EEG frequency range and rhythmic cluster spiking within a theta-like frequency range during cortical activation. During irregular slow EEG activity in absence of stimulation, one subgroup displayed spike bursts correlated with cortical slow oscillations. As relatively large in size and also antidromically activated from the cortex, many GABAergic neurons recorded were considered to be cortically projecting and thus capable of directly modulating cortical activity. Subgroups of GABAergic basal forebrain neurons would thus have the capacity to promote cortical activation by modulating gamma or theta activity and others to attenuate cortical activation by modulating irregular slow oscillations that normally occur during slow wave sleep.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

EEG correlation of the discharge properties of identified neurons in the basal forebrain

The basal forebrain (BF) is a heterogeneous structure located in the ventral aspect of the cerebral hemispheres. It contains cholinergic as well as different types of noncholinergic corticopetal neurons and interneurons, including GABAergic and peptidergic cells. The BF constitutes an extrathalamic route to the cortex, and its activity is associated with an increase in cortical release of the neurotransmitter acetylcholine, concomitant with electroencephalographic (EEG) low-voltage fast activity (LVFA). However, the specific role of the different BF cell types has largely remained unknown due to the lack of chemical identification of the recorded neurons. Here we show that the firing rate of immunocytochemically identified cholinergic and parvalbumin-containing neurons increase during cortical LVFA. In contrast, increased neuropeptide Y neuron firing is accompanied by cortical slow waves. Our results, furthermore, indicate that BF neurons posses a distinct temporal relationship to different EEG patterns and suggest a more dynamic interplay within BF as well as between BF and cortical circuitries than previously proposed.

Stimulation of NMDA and AMPA receptors in the rat nucleus basalis of Meynert affects sleep

The nucleus basalis of Meynert (NBM), a heterogeneous area in the basal forebrain involved in the modulation of sleep and wakefulness, is rich in glutamate receptors, and glutamatergic fibers represent an important part of the input to this nucleus. With the use of unilateral infusions in the NBM, the effects of two different glutamatergic subtype agonists, namely N-methyl-D-aspartic acid (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) hydrobromide, on sleep and wakefulness parameters were determined in freely moving rats by means of polygraphic recordings. NMDA (5 nmol) and AMPA (0.4 nmol) induced an increase in wakefulness and an inhibition of slow-wave sleep. AMPA, but not NMDA, also caused a decrease in desynchronized sleep. These AMPA- and NMDA-mediated effects were counteracted by a pretreatment with the specific NMDA antagonist 2-amino-5-phosphonopentanoic acid (20 nmol) and the specific AMPA antagonist 6,7-dinitroquinoxaline-2,3-dione (2 nmol), respectively. The results reported here indicate that 1) the NBM activation of both NMDA and AMPA glutamate receptors exert a modulatory influence on sleep and wakefulness, and 2) AMPA, but not NMDA receptors, are involved in the modulation of desynchronized sleep, suggesting a different role for NBM NMDA and non-NMDA receptors in sleep modulation.

Organization of the basal forebrain projection to the thalamus in rats

We have examined the patterns of projections that exist between the different nuclei of the basal forebrain (BF) and the thalamus. Injections of biotinylated dextran were made into different nuclei of the BF (i.e. substantia innominata, nucleus basalis of Meynert, vertical and horizontal limbs of the diagonal band) of Sprague-Dawley rats using stereotaxic coordinates. Our results show that all of the above-mentioned BF nuclei have projections to the thalamus and that projections from different nuclei are rather similar. The bulk of the BF afferents terminate within the intralaminar, midline and mediodorsal nuclei of the dorsal thalamus and the zona incerta and reticular nucleus of the ventral thalamus. Very few terminals are ever seen in the other nuclei of the thalamus. Thus our results indicate that the BF targets particular thalamic nuclei, thereby being in a position to influence distinct thalamo-cortical pathways.

Neurochemical stimulation of the rat substantia innominata increases cerebral blood flow (but not glucose use) through the parallel activation of cholinergic and non-cholinergic pathways

Neurochemical activation of the substantia innominata (SI) in the rat, through the direct injection of the cholinergic agonist carbachol, has been reported to induce large increases in cerebral blood flow (CBF) throughout cortical and subcortical projection regions. The present study aimed to determine whether the vasomotor responses to cholinergic stimulation of the SI were, or were not, the consequence of an increase in metabolic activity. To this end, coupled measurements of CBF and cerebral glucose use (CGU) were undertaken during carbachol-elicited stimulation of the SI. Infusion of carbachol into the basal forebrain induced significant CBF increases in several ipsilateral cortical and subcortical areas including the amygdala. In contrast, CGU increased only in the ipsilateral amygdala and SI. Thus, we tested the hypothesis of a direct neurogenic, rather than metabolic, contribution of the basalocortical system. In this respect, carbachol-elicited stimulation resulted in significant increases in extracellular acetylcholine concentrations in the ipsilateral parietal cortex; systemic pretreatment with the muscarinic receptor antagonist scopolamine completely abolished the increase in cortical CBF elicited by cholinergic stimulation of the SI in the ipsilateral frontoparietal motor cortex while it failed to affect the increase observed in the ipsilateral temporal cortex. Several conclusions can be drawn from the present study. The stimulation of the SI by carbachol induces an increase in CBF that can be dissociated from changes in underlying glucose metabolism. Secondly, these induced changes in cortical CBF are paralleled by an increase in acetylcholine release. Lastly, the failure of scopolamine to block the flow response in all cortical regions would suggest that SI stimulation will evoke the release of vasodilatatory neurotransmitter(s) as well as acetylcholine itself.

An ultrastructural study of p75 neurotrophin receptor-immunoreactive fiber terminals in the reticular thalamic nucleus of young rats

The reticular thalamic nucleus (RT) receives cholinergic fibers from both the basal forebrain and the brainstem. Recent studies have shown that the p75 neurotrophin receptor (p75NTR) is synthesized in cholinergic neurons in the basal forebrain but not in those in the brainstem. In this study, to identify cholinergic fibers originating from the basal forebrain, we used a monoclonal antibody against p75NTR (192-IgG) and characterized the ultrastructure of the immunoreactive fiber terminals in the rostral part of the RT in 3-week-old rats. Light microscopy revealed that p75NTR-immunoreactive fine fibers and varicosities were distributed throughout the nucleus. From electron micrographs, three types of labeled terminals were identified. The first type of labeled fiber terminals (63 out of 106) was consistently small, contained densely packed vesicles, and established asymmetrical synaptic contacts with heavy and bushy postsynaptic thickening on distal dendritic profiles; the second type (18 out of 106) established asymmetrical synaptic contacts with very slight postsynaptic thickening; and the third type (25 out of 106) of labeled terminals contained pleomorphic vesicles and established symmetrical synaptic contacts with more proximal dendritic surfaces than the first two types. In addition to the above, labeled dendritic profiles receiving non-labeled asymmetrical and symmetrical synaptic contacts were identified. These findings suggest that the basal forebrain cholinergic system establishes a variety of synaptic connections in the RT and influences cortical activity indirectly via thalamocortical pathways, as well as via direct projections to the cortex.

Morphological and electrophysiological characteristics of noncholinergic basal forebrain neurons

Cholinergic neurons in the basal forebrain are the focus of considerable interest because they are severely affected in Alzheimer's disease. However, both cholinergic and noncholinergic neurons are intermingled in this region. The goal of the present study was to characterize the morphology and in vivo electrophysiology of noncholinergic basal forebrain neurons. Neurons in the ventral pallidum and substantia innominata were recorded extracellularly, labeled juxtacellularly with biocytin and characterized for the presence of choline acetyltransferase immunoreactivity. Two types of ventral pallidal cells were observed. Type I ventral pallidal neurons had axons that rarely branched near the cell body and tended to have smaller somata and lower spontaneous firing rates than did type II ventral pallidal neurons, which displayed extensive local axonal arborizations. Subtypes of substantia innominata neurons could not be distinguished based on axonal morphology. These noncholineregic neurons exhibited local axon arborizations along a continuum that varied from no local collaterals to quite extensive arbors. Substantia innominata neurons had lower spontaneous firing rates, more variable interspike intervals, and different spontaneous firing patterns than did type II ventral pallidal neurons and could be antidromically activated from cortex or substantia nigra, indicating that they were projection neurons. Ventral pallidal neurons resemble, both morphologically and electrophysiologically, previously described neurons in the globus pallidus, whereas the substantia innominata neurons bore similarities to isodendritic neurons of the reticular formation. These results demonstrate the heterogeneous nature of noncholinergic neurons in the basal forebrain.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

GABA(B) receptor gene expression in monkey thalamus

Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in the macaque monkey thalamus by in situ hybridization, using monkey-specific cRNA probes. GABA(B) transcript expression was widespread and of much higher density in the dorsal thalamus than in the reticular nucleus and other parts of the ventral thalamus and was highest in the epithalamus. In the dorsal thalamus, highest mRNA levels were found in the anteroventral nucleus and in the parafascicular nucleus. Sensory relay nuclei showed moderate GABA(B) mRNA levels. Neurons of all sizes were labeled, suggesting expression in relay cells and interneurons, and there was no labeling of neuroglial cells. Following 10-day periods of monocular deprivation, levels of GABA(B) mRNA were decreased in the deprived magno- and parvo-cellular laminae of the dorsal lateral geniculate nuclei, indicating activity-dependent regulation. High levels of GABA(B) receptors in the dorsal thalamus are likely to reflect the high density of synaptic inputs from the reticular nucleus while low expression in the reticular nucleus implies weak, GABA(B)-mediated intrareticular inhibition.

Immunoreactivity for the GABA transporter-1 and GABA transporter-3 is restricted to astrocytes in the rat thalamus. A light and electron-microscopic immunolocalization

GABA plasma membrane transporters mediate GABA uptake into presynaptic terminals and surrounding glial processes and thus play a key role in shaping the time course and spatial extent of GABA's action. In the present study we have investigated the cellular and subcellular localization of two GABA transporters (1 and 3) in the rat thalamus using affinity-purified polyclonal antibodies. GABA transporter-1 and -3 immunoreactivity, detected with immunoperoxidase and immunofluorescence methods, is present throughout the thalamus in small punctate structures scattered in the neuropil among unlabelled neuronal perikarya. Labelling for GABA transporter-3 is always more intense than that for GABA transporter-1. Astrocytic processes, identified by their immunoreactivity for glial fibrillary acidic protein, express both GABA transporters. Ultrastructural investigations confirm that GABA transporter-1 and -3 labelling is restricted to astrocytes. Labelled astrocytes are adjacent to terminals making either symmetric or asymmetric synaptic contacts, and are close to neuronal profiles that do not form synaptic contacts in the plane of the section. In double-labelled thin sections some GABA transporter-1- or -3-positive astrocytic processes, detected with immunoperoxidase labelling, surround GABA-positive terminals, detected with antibodies to GABA and immunogold labelling. These findings demonstrate that in rat thalamus the GABA uptake system mediated by GABA transporter-1 and -3 is localized exclusively to astrocytes near the synapses and in the neuropil, and absent from GABAergic terminals. Astrocytes play therefore an important role in mediating GABA transmission in the thalamus, compared to cortical regions.