Axonal arborizations of a magnocellular basal nucleus input and their relation to the neurons in the thalamic reticular nucleus of rats

Cell-specific inhibitory modulation of sound processing in the auditory thalamus

Inhibition along the auditory pathway is crucial for processing of acoustic information. Within the auditory thalamus, a key region in the central auditory pathway, inhibition is provided by the thalamic reticular nucleus (TRN), comprised of two large classes of inhibitory neurons, parvalbumin (PV TRN ) and somatostatin (SST TRN ) positive. In the auditory cortex, PV and SST neurons differentially shape auditory processing. We found that the ventral MGB, the thalamic region in the direct ascending auditory pathway, receives inputs predominantly from PV TRN neurons, whereas SST TRN neurons project to the dorso-medial regions of MGB. Consistently, inactivating PV TRN neurons increased sound-evoked activity in over a third of neurons in the vMGB, with another large fraction of neurons being suppressed. By contrast, inactivating SST TRN neuronal activity largely reduced tone-evoked activity in vMGB neurons. Cell type-specific computational models revealed candidate circuit mechanisms for generating the bi-directional effects of TRN inactivation on MGB sound responses. These differential inhibitory pathways within the auditory thalamus suggest a cell-specific role for thalamic inhibition in auditory computation and behavior.

GABABR Modulation of Electrical Synapses and Plasticity in the Thalamic Reticular Nucleus

Two distinct types of neuronal activity result in long-term depression (LTD) of electrical synapses, with overlapping biochemical intracellular signaling pathways that link activity to synaptic strength, in electrically coupled neurons of the thalamic reticular nucleus (TRN). Because components of both signaling pathways can also be modulated by GABA_B receptor activity, here we examined the impact of GABA_B receptor activation on the two established inductors of LTD in electrical synapses. Recording from patched pairs of coupled rat neurons in vitro, we show that GABA_B receptor inactivation itself induces a modest depression of electrical synapses and occludes LTD induction by either paired bursting or metabotropic glutamate receptor (mGluR) activation. GABA_B activation also occludes LTD from either paired bursting or mGluR activation. Together, these results indicate that afferent sources of GABA, such as those from the forebrain or substantia nigra to the reticular nucleus, gate the induction of LTD from either neuronal activity or afferent glutamatergic receptor activation. These results add to a growing body of evidence that the regulation of thalamocortical transmission and sensory attention by TRN is modulated and controlled by other brain regions. Significance: We show that electrical synapse plasticity is gated by GABA_B receptors in the thalamic reticular nucleus. This effect is a novel way for afferent GABAergic input from the basal ganglia to modulate thalamocortical relay and is a possible mediator of intra-TRN inhibitory effects.

Thalamic reticular nucleus in Alligator mississippiensis: Soma and dendritic morphology

The thalamic reticular nucleus (TRN) is a critical structure influencing information transfer to the forebrain. In crocodilians, the TRN shares many features with its mammalian counterpart. One area that has not been explored is how individual neurons in the crocodilian TRN compare with those found in mammals. In mammals, TRN neurons are aligned parallel to the external border of the dorsal thalamus, have their dendrites oriented perpendicular to the fibers in the internal capsule, have fine, filamentous dendritic appendages, are either bipolar or multipolar, and are commonly considered to be a homogeneous morphological population of cells. To investigate the cellular morphology of the TRN complex, a Golgi analysis was undertaken in Alligator mississippiensis. This study examined features that have been used in mammals. In Alligator, the four TRN divisions are the dorsal peduncular nucleus, the perireticular nucleus, the interstitial nucleus, and the neurons in the medial forebrain bundle associated with the interstitial nucleus. In crocodilians, the dorsal peduncular nucleus is homologous to the TRN of mammals. From the 1787 drawn neuron profiles in the traditional three planes of section, the following were concluded. First, neurons in each part of the TRN complex in Alligator were similar in morphology. Second, each part of the TRN complex of Alligator contained a heterogenous population of cells. These variations between the cellular morphology of the dorsal peduncular nucleus of crocodilians and the TRN of mammals are speculated to partly result from differences in forebrain organization.

Activity-dependent long-term potentiation of electrical synapses in the mammalian thalamus

Activity-dependent changes of synapse strength have been extensively characterized at chemical synapses, but the relationship between physiological forms of activity and strength at electrical synapses remains poorly characterized and understood. For mammalian electrical synapses comprising hexamers of connexin36, physiological forms of neuronal activity in coupled pairs have thus far only been linked to long-term depression; activity that results in strengthening of electrical synapses has not yet been identified. Here, we performed dual whole-cell current-clamp recordings in acute slices of P11-P15 Sprague-Dawley rats of electrically coupled neurons of the thalamic reticular nucleus (TRN), a central brain area that regulates cortical input from and attention to the sensory surround. Using TTA-A2 to limit bursting, we show that tonic spiking in one neuron of a pair results in long-term potentiation of electrical synapses. We use experiments and computational modeling to show that the magnitude of plasticity expressed alters the functionality of the synapse. Potentiation is expressed asymmetrically, indicating that regulation of connectivity depends on the direction of use. Furthermore, calcium pharmacology and imaging indicate that potentiation depends on calcium flux. We thus propose a calcium-based activity rule for bidirectional plasticity of electrical synapse strength. Because electrical synapses dominate intra-TRN connectivity, these synapses and their activity-dependent modifications are key dynamic regulators of thalamic attention circuitry. More broadly, we speculate that bidirectional modifications of electrical synapses may be a widespread and powerful principle for ongoing, dynamic reorganization of neuronal circuitry across the brain.NEW & NOTEWORTHY This work reveals a physiologically relevant form of activity pairing in coupled neurons that results in long-term potentiation of mammalian electrical synapses. These findings, in combination with previous work, allow the authors to propose a bidirectional calcium-based rule for plasticity of electrical synapses, similar to those demonstrated for chemical synapses. These new insights inform the field on how electrical synapse plasticity may modify the neural circuits that incorporate them.

Functional Subdivisions of Magnocellular Cell Groups in Human Basal Forebrain: Test-Retest Resting-State Study at Ultra-high Field, and Meta-analysis

The heterogeneous neuronal subgroups of the basal forebrain corticopetal system (BFcs) have been shown to modulate cortical functions through their cholinergic, gamma-aminobutyric acid-ergic, and glutamatergic projections to the entire cortex. Although previous studies suggested that the basalo-cortical projection system influences various cognitive functions, particularly via its cholinergic component, these studies only focused on certain parts of the BFcs or nearby structures, leaving aside a more systematic picture of the functional connectivity of BFcs subcompartments. Moreover, these studies lacked the high-spatial resolution and the probability maps needed to identify specific subcompartments. Recent advances in the ultra-high field 7T functional magnetic resonance imaging (fMRI) provided potentially unprecedented spatial resolution of functional MRI images to study the subdivision of the BFcs. In this study, the BF space containing corticopetal cells was divided into 3 functionally distinct subdivisions based on functional connection to cortical regions derived from fMRI. The overall functional connection of each BFcs subdivision was examined with a test-retest study. Finally, a meta-analysis was used to study the related functional topics of each BF subdivision. Our results demonstrate distinct functional connectivity patterns of these subdivisions along the rostrocaudal axis of the BF. All three compartments have shown consistent segregation and overlap at specific target regions including the hippocampus, insula, thalamus, and the cingulate gyrus, suggesting functional integration and separation in BFcs.

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.

Npas1+-Nkx2.1+ Neurons Are an Integral Part of the Cortico-pallido-cortical Loop

Within the basal ganglia circuit, the external globus pallidus (GPe) is critically involved in motor control. Aside from Foxp2+ neurons and ChAT+ neurons that have been established as unique neuron types, there is little consensus on the classification of GPe neurons. Properties of the remaining neuron types are poorly defined. In this study, we leverage new mouse lines, viral tools, and molecular markers to better define GPe neuron subtypes. We found that Sox6 represents a novel, defining marker for GPe neuron subtypes. Lhx6+ neurons that lack the expression of Sox6 were devoid of both parvalbumin and Npas1. This result confirms previous assertions of the existence of a unique Lhx6+ population. Neurons that arise from the Dbx1+ lineage were similarly abundant in the GPe and displayed a heterogeneous makeup. Importantly, tracing experiments revealed that Npas1+-Nkx2.1+ neurons represent the principal noncholinergic, cortically-projecting neurons. In other words, they form the pallido-cortical arm of the cortico-pallido-cortical loop. Our data further show that pyramidal-tract neurons in the cortex collateralized within the GPe, forming a closed-loop system between the two brain structures. Overall, our findings reconcile some of the discrepancies that arose from differences in techniques or the reliance on preexisting tools. Although spatial distribution and electrophysiological properties of GPe neurons reaffirm the diversification of GPe subtypes, statistical analyses strongly support the notion that these neuron subtypes can be categorized under the two principal neuron classes: PV+ neurons and Npas1+ neurons.SIGNIFICANCE STATEMENT The poor understanding of the neuronal composition in the external globus pallidus (GPe) undermines our ability to interrogate its precise behavioral and disease involvements. In this study, 12 different genetic crosses were used, hundreds of neurons were electrophysiologically characterized, and >100,000 neurons were histologically- and/or anatomically-profiled. Our current study further establishes the segregation of GPe neuron classes and illustrates the complexity of GPe neurons in adult mice. Our results support the idea that Npas1+-Nkx2.1+ neurons are a distinct GPe neuron subclass. By providing a detailed analysis of the organization of the cortico-pallidal-cortical projection, our findings establish the cellular and circuit substrates that can be important for motor function and dysfunction.

Thalamic inhibitory circuits and network activity development

Inhibitory circuits in thalamus and cortex shape the major activity patterns observed by electroencephalogram (EEG) in the adult brain. Their delayed maturation and circuit integration, relative to excitatory neurons, suggest inhibitory neuronal development could be responsible for the onset of mature thalamocortical activity. Indeed, the immature brain lacks many inhibition-dependent activity patterns, such as slow-waves, delta oscillations and sleep-spindles, and instead expresses other unique oscillatory activities in multiple species including humans. Thalamus contributes significantly to the generation of these early oscillations. Compared to the abundance of studies on the development of inhibition in cortex, however, the maturation of thalamic inhibition is poorly understood. Here we review developmental changes in the neuronal and circuit properties of the thalamic relay and its interconnected inhibitory thalamic reticular nucleus (TRN) both in vitro and in vivo, and discuss their potential contribution to early network activity and its maturation. While much is unknown, we argue that weak inhibitory function in the developing thalamus allows for amplification of thalamocortical activity that supports the generation of early oscillations. The available evidence suggests that the developmental acquisition of critical thalamic oscillations such as slow-waves and sleep-spindles is driven by maturation of the TRN. Further studies to elucidate thalamic GABAergic circuit formation in relation to thalamocortical network function would help us better understand normal as well as pathological brain development.

Distinct Thalamic Reticular Cell Types Differentially Modulate Normal and Pathological Cortical Rhythms

Integrative brain functions depend on widely distributed, rhythmically coordinated computations. Through its long-ranging connections with cortex and most senses, the thalamus orchestrates the flow of cognitive and sensory information. Essential in this process, the nucleus reticularis thalami (nRT) gates different information streams through its extensive inhibition onto other thalamic nuclei, however, we lack an understanding of how different inhibitory neuron subpopulations in nRT function as gatekeepers. We dissociated the connectivity, physiology, and circuit functions of neurons within rodent nRT, based on parvalbumin (PV) and somatostatin (SOM) expression, and validated the existence of such populations in human nRT. We found that PV, but not SOM, cells are rhythmogenic, and that PV and SOM neurons are connected to and modulate distinct thalamocortical circuits. Notably, PV, but not SOM, neurons modulate somatosensory behavior and disrupt seizures. These results provide a conceptual framework for how nRT may gate incoming information to modulate brain-wide rhythms.

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.

Unilateral nasal obstruction affects motor representation development within the face primary motor cortex in growing rats

Postnatal growth is influenced by genetic and environmental factors. Nasal obstruction during growth alters the electromyographic activity of orofacial muscles. The facial primary motor area represents muscles of the tongue and jaw, which are essential in regulating orofacial motor functions, including chewing and jaw opening. This study aimed to evaluate the effect of chronic unilateral nasal obstruction during growth on the motor representations within the face primary motor cortex (M1). Seventy-two 6-day-old male Wistar rats were randomly divided into control (n = 36) and experimental (n = 36) groups. Rats in the experimental group underwent unilateral nasal obstruction after cauterization of the external nostril at 8 days of age. Intracortical microstimulation (ICMS) mapping was performed when the rats were 5, 7, 9, and 11 wk old in control and experimental groups (n = 9 per group per time point). Repeated-measures multivariate ANOVA was used for intergroup and intragroup statistical comparisons. In the control and experimental groups, the total number of positive ICMS sites for the genioglossus and anterior digastric muscles was significantly higher at 5, 7, and 9 wk, but there was no significant difference between 9 and 11 wk of age. Moreover, the total number of positive ICMS sites was significantly smaller in the experimental group than in the control at each age. It is possible that nasal obstruction induced the initial changes in orofacial motor behavior in response to the altered respiratory pattern, which eventually contributed to face-M1 neuroplasticity.NEW & NOTEWORTHY Unilateral nasal obstruction in rats during growth periods induced changes in arterial oxygen saturation (SpO₂) and altered development of the motor representation within the face primary cortex. Unilateral nasal obstruction occurring during growth periods may greatly affect not only respiratory function but also craniofacial function in rats. Nasal obstruction should be treated as soon as possible to avoid adverse effects on normal growth, development, and physiological functions.

Cell type-specific long-range connections of basal forebrain circuit

The basal forebrain (BF) plays key roles in multiple brain functions, including sleep-wake regulation, attention, and learning/memory, but the long-range connections mediating these functions remain poorly characterized. Here we performed whole-brain mapping of both inputs and outputs of four BF cell types - cholinergic, glutamatergic, and parvalbumin-positive (PV+) and somatostatin-positive (SOM+) GABAergic neurons - in the mouse brain. Using rabies virus -mediated monosynaptic retrograde tracing to label the inputs and adeno-associated virus to trace axonal projections, we identified numerous brain areas connected to the BF. The inputs to different cell types were qualitatively similar, but the output projections showed marked differences. The connections to glutamatergic and SOM+ neurons were strongly reciprocal, while those to cholinergic and PV+ neurons were more unidirectional. These results reveal the long-range wiring diagram of the BF circuit with highly convergent inputs and divergent outputs and point to both functional commonality and specialization of different BF cell types.

Increased occlusal vertical dimension induces cortical plasticity in the rat face primary motor cortex

Previous studies have demonstrated that functional plasticity in the primary motor cortex (M1) is related to motor-skill learning and changes in the environment. Increased occlusal vertical dimension (iOVD) may modulate mastication, such as in the masticatory cycle, and the firing properties of jaw-muscle spindles. However, little is known about the changes in motor representation within the face primary motor cortex (face-M1) after iOVD. The purpose of the present study was to determine the effect of iOVD on the face-M1 using intracortical microstimulation (ICMS). In an iOVD group, the maxillary molars were built-up by 2mm with acrylic. The electromyographic (EMG) activities from the left (LAD) and right (RAD) anterior digastric (AD), masseter and genioglossus (GG) muscles elicited by ICMS within the right face-M1 were recorded 1, 2 and 8 weeks after iOVD. IOVD was associated with a significant increase in the number of sites within the face-M1 from which ICMS evoked LAD and/or GG EMG activities, as well as a lateral shift in the center of gravity of the RAD and LAD muscles at 1 and 2 weeks, but not at 8 weeks. These findings suggest that a time-dependent neuroplastic change within the rat face-M1 occurs in association with iOVD. This may be related to the animal's ability to adapt to a change in the oral environment.

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.

c-Fos protein expression is increased in cholinergic neurons of the rodent basal forebrain during spontaneous and induced wakefulness

It has been proposed that cholinergic neurons of the basal forebrain (BF) may play a role in vigilance state control. Since not all vigilance states have been studied, we evaluated cholinergic neuronal activation levels across spontaneously occurring states of vigilance, as well as during sleep deprivation and recovery sleep following sleep deprivation. Sleep deprivation was performed for 2h at the beginning of the light (inactive) period, by means of gentle sensory stimulation. In the rodent BF, we used immunohistochemical detection of the c-Fos protein as a marker for activation, combined with labeling for choline acetyl-transferase (ChAT) as a marker for cholinergic neurons. We found c-Fos activation in BF cholinergic neurons was highest in the group undergoing sleep deprivation (12.9% of cholinergic neurons), while the spontaneous wakefulness group showed a significant increase (9.2%), compared to labeling in the spontaneous sleep group (1.8%) and a sleep deprivation recovery group (0.8%). A subpopulation of cholinergic neurons expressed c-Fos during spontaneous wakefulness, when possible confounds of the sleep deprivation procedure were minimized (e.g., stress and sensory stimulation). Double-labeling in the sleep deprivation treatment group was significantly elevated in select subnuclei of the BF (medial septum/vertical limb of the diagonal band, horizontal limb of the diagonal band, and the magnocellular preoptic nucleus), when compared to spontaneous wakefulness. These findings support and provide additional confirming data of previous reports that cholinergic neurons of BF play a role in vigilance state regulation by promoting wakefulness.

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.

The neurophysiology of attention-deficit/hyperactivity disorder

Recent reviews of the neurobiology of Attention-Deficit/Hyperactivity Disorder (AD/HD) have concluded that there is no single pathophysiological profile underlying this disorder. Certainly, dysfunctions in the frontal/subcortical pathways that control attention and motor behavior are implicated. However, no diagnostic criteria or behavioral/neuroimaging techniques allow a clear discrimination among subtypes within this disorder, especially when problems with learning are also considered. Two major Quantitative EEG (QEEG) subtypes have been found to characterize AD/HD. Here we review the major findings in the neurophysiology of AD/HD, focusing on QEEG, and briefly present our previous findings using a source localization technique called Variable Resolution Electromagnetic Tomography (VARETA). These two techniques represent a possible objective method to identify specific patterns corresponding to EEG-defined subtypes of AD/HD. We then propose a model representing the distribution of the neural generators in these two major AD/HD subtypes, localized within basal ganglia and right anterior cortical regions, and hippocampal, para-hippocampal and temporal cortical regions, respectively. A comprehensive review of neurochemical, genetic, neuroimaging, pharmacological and neuropsychological evidence in support of this model is then presented. These results indicate the value of the neurophysiological model of AD/HD and support the involvement of different neuroanatomical systems, particularly the dopaminergic pathways.

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.

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.

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.

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.

The functional neuroanatomy of awareness: with a focus on the role of various anatomical systems in the control of intermodal attention

This review considers a number of recent theories on the neural basis of consciousness, with particular attention to the theories of Bogen, Crick, Llinás, Newman, and Changeux. These theories allot different roles to various key brain areas, in particular the reticular and intralaminar nuclei of the thalamus and the cortex. Crick's hypothesis is that awareness is a function of reverberating corticothalamic loops and that the spotlight of intramodal attention is controlled by the reticular nucleus of the thalamus. He also proposed different mechanisms for attention and intention ("will"). The current review presents a new hypothesis, based on elements from these hypotheses, including intermodal attention and olfaction and pain, which may pose problems for Crick's original theory. This work reviews the possible role in awareness and intermodal attention and intention of the cholinergic system in the basal forebrain and the tegmentum; the reticular, the intralaminar, and the dorsomedial thalamic nuclei; the raphe and locus coeruleus; the reticular formation; the ventral striatum and extended amygdala; insula cortex, and other selected cortical, areas. Both clinical and basic research data are covered. The conclusion is reached that the brain may work by largely nonlinear parallel processing and much intramodal shifts of attention may be effected by intracortical, or multiple corticothalamic mechanisms (small local "flashlights" rather than one major "searchlight"). But this is constrained by the functional anatomy of the circuits concerned and waking "awareness" is modulated by the many "nonspecific" systems (cholinergic from the basal forebrain, noradrenergic from the locus coeruleus, dopaminergic from the substantia nigra and ventral tegmentum, and serotoninergic from the raphe). But the principal agents for intermodal attention shifts, the "searchlight," may be two key nuclei of the cholinergic system in the mesencephalon. Clinical loss of consciousness results from damage to these nuclei but not from damage to the cholinergic nucleus basalis of the basal forebrain.

Ultrastructural characterization of the postnatal development of the thalamic ventrobasal and reticular nuclei in the rat

Electron microscopy has been employed to analyze the normal maturational sequence that characterizes the postnatal development of synaptic circuits in the ventrobasal (VB) and reticular (Rt) thalamic nuclei of rats at different ages (from birth to the end of the third postnatal week). Throughout the first postnatal week, similar signs of immaturity are observed in both nuclei, mainly consisting in scarcity of cytoplasmic organelles, presence of wide extracellular spaces, and absence of myelinated fibers. Several synaptic terminals are however present from birth, thus indicating that some of the afferents have already reached and contacted their thalamic target during embryonic life. Most of the terminals are small and contain only a few round, clear vesicles, and therefore their cytological features do not allow the identification of their origin. In particular, in both nuclei, terminals with flat vesicles and symmetric specialization are only rarely observed, and in VB the ascending terminals are not distinguishable from terminals of other sources as they are in adults. During the second postnatal week, progressive maturational changes in VB and Rt lead to neurons having well-developed cytoplasmic organelles and to an elaborate neuropil containing myelinated fibers and synaptic terminals that are morphologically heterogeneous and resemble the adult ones. The permanence of growth cone-like profiles and of numerous somatic and dendritic protrusions, often contacted by synaptic terminals, indicates that a certain degree of reorganization is still taking place in both nuclei. By the end of the third postnatal week the synaptic organization of VB and Rt is indistinguishable from that observed in adults. This ultrastructural study shows that the appearance of the neuropil of VB and Rt and the morphological complexity of the synaptic arrangements characteristic of the adult rat are not present in neonates, but are gradually acquired during the first three postnatal weeks, and that they result from progressive modifications in circuit organization involving both pre- and postsynaptic elements.

Trends in the anatomical organization and functional significance of the mammalian thalamus

The last decade has witnessed major changes in the experimental approach to the study of the thalamus and to the analysis of the anatomical and functional interrelations between thalamic nuclei and cortical areas. The present review focuses on the novel anatomical approaches to thalamo-cortical connections and thalamic functions in the historical framework of the classical studies on the thalamus. In the light of the most recent data it is here discussed that: a) the thalamus can subserve different functions according to functional changes in the cortical and subcortical afferent systems; b) the multifarious thalamic cellular entities play a crucial role in the different functional states.

Organization of connections between the thalamic reticular and the anterior thalamic nuclei in the rat

The thalamic reticular nucleus (TRN) receives topographically organized input from specific sensory nuclei such as the lateral geniculate nucleus. The present study shows this in the rat. However, the pattern of thalamic connections to the limbic reticular sector is unknown. Injecting biocytin into the ventral parts of anteroventral and anteromedial nuclei labeled neurons and axons in the rostral TRN. Filled axon collaterals and their terminals occupied a rectangular sheet in a plane close to the horizontal, and were confined to the inner zone (the medial portion) of the limbic TRN. Retrogradely filled cells were in the middle of the rostral pole in the same horizontal plane, receiving synapses from surrounding labeled boutons. In electron micrographs, thalamic terminals were found to contain round, densely packed synaptic vesicles and formed asymmetrical synapses onto reticular somata and dendritic profiles. Displacing the injection site along the dorso-ventral and rostro-caudal axis in the anterior nuclei produced corresponding shifts of antero- and retrograde labeling within the inner reticular zone. Projections from the dorsal portions of the anterior nuclei did not follow this pattern. Axons from the anterodorsal nucleus occupied the rostralmost tip of both inner and outer zones of the dorsal limbic sector. In accordance with earlier reports, the limbic sector was found to represent several dorsal thalamic nuclei parallel to each other medio-laterally. A topography is described for the limbic reticulo-thalamic connections, suggesting that the rostral TRN is able to influence circumscribed areas of the limbic thalamus.

Basic mechanisms of generalized absence seizures

Generalized absence seizures are neurophysiologically, pharmacologically, and developmentally unique and comprise the primary seizure type in a number of different absence epilepsy syndromes. Over the last 10 years, the availability of a number of animal models of generalized absence seizures and of sophisticated in vitro electrophysiological techniques that allow investigation of cortical and thalamic networks has begun to shed light on the pathogenesis of this disorder. The basic underlying mechanism appears to involve thalamocortical circuitry and the generation of abnormal oscillatory rhythms from that particular neuronal network. Biochemical mechanisms operative within thalamocortical circuitry during this neuronal oscillation seem to entail phase-locked gamma-aminobutyric acid (GABA)B-mediated inhibition alternating with glutamate-mediated excitation. The basic cellular mechanism operative within this tension between excitation and inhibition appears to involve the T-type calcium current. Local circuitry within the thalamus may influence these oscillatory rhythms by GABAA-mediated inhibition. Pharmacological factors at play external to thalamocortical circuitry include cholinergic, dopaminergic, and noradrenergic mechanisms. Pathways that utilize these various neurotransmitters project onto the thalamus and/or cortex from sites distant to those structures and may modulate the process either up or down. Perturbation of one or more of these neuronal networks may lead to abnormal neuronal oscillatory rhythms within thalamocortical circuitry, with a resultant generation of bilaterally synchronous spike wave discharges that characterize generalized absence seizures. Our increasing understanding of the basic mechanisms that underlie generalized absence seizures promises to allow, for the first time, a rational design of drug treatment for a seizure disorder based on the pathogenesis of that disorder.

Topographic organization of projections from the thalamic reticular nucleus to the anterior thalamic nuclei in the rat

We have investigated connections between the thalamic reticular nucleus (TRN) and the anterior thalamic nuclei (ATN) in the rat, following injections of horseradish peroxidase (HRP) into subnuclei of the ATN and different regions of the rostral TRN. Three nonoverlapping groups of neurons in the dorsal part of the ipsilateral rostral TRN project to, and receive reciprocal projections from, specific subnuclei of the ATN. A vertical sheet of neurons in the most dorsal part of the rostral TRN projects to the dorsal half of the posterior subdivision of the anteroventral thalamic nucleus (AVp), the dorsal region of the medial subdivision of the anteroventral thalamic nucleus (AVm), and the dorsolateral part of the rostral anterodorsal thalamic nucleus (AD). Immediately ventral to this part of TRN, but still within its dorsal portion, are a lateral cluster of neurons and a medially located vertical sheet of neurons. The lateral cluster projects to the ventral part of AVp and to the dorsomedial part of rostral AD. The medial sheet projects to the ventral part of AVm, the ventral part of rostral AD, and to the caudal portions of both AV and AD. There appears to be no input to the anteromedial thalamic nucleus (AM) from the TRN. These findings shed new light on the anatomy of the rostral TRN, the ATN, and the connections between the two, and are relevant to emerging hypotheses about the functional organization of the TRN and reticulo-thalamic projections.

GABAergic projection from the basal forebrain to the visual sector of the thalamic reticular nucleus in the cat

We examined the projection from the basal forebrain to thalamic and cortical regions of the visual system in cats, with particular reference to the visual sector of the thalamic reticular nucleus, the lateral geniculate nucleus, and the striate cortex. First, we made injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the visual sector of the thalamic reticular nucleus and found cells labeled by retrograde transport in the lateral nucleus basalis magnocellularis. Injection of biocytin into the basal forebrain resulted in the anterograde labeling of a dense band of fibers and terminals within the entire thalamic reticular nucleus; this labeling extended through the visual sector including the perigeniculate nucleus. No orthograde labeling was found in the lateral geniculate nucleus. Next, we addressed the issue of putative neurotransmitters used by this pathway using a variety of immunocytochemical and histochemical markers. In this fashion, we identified two populations of cells in the nucleus basalis magnocellularis of the cat; large cholinergic cells that contain choline acetyltransferase, NADPH-diaphorase, and calbindin and that project to striate cortex and smaller cells that contain gamma-aminobutyric acid (GABA), glutamic acid decarboxylase, and parvalbumin and that project to the visual sector of the thalamic reticular nucleus. We also examined at the electron microscopic level terminals in the visual sector of the thalamic reticular nucleus that were labeled from a biocytin injection in the basal forebrain. Most of these terminals form symmetric contacts onto dendrites and were revealed by postembedding immunocytochemical staining to be positive for GABA.

GABAergic and pallidal terminals in the thalamic reticular nucleus of squirrel monkeys

The ultrastructure of synaptic terminals from the external segment of the globus pallidus and of other synaptic terminals positive for gamma-aminobutyric acid (GABA) was examined in the thalamic reticular nucleus (TRN) of squirrel monkeys. Two GABA-positive terminals types were commonly encountered within the TRN neuropil. The most common type of GABAergic terminals (F terminals) are filled with dispersed pleomorphic synaptic vesicles and clusters of mitochondria. These terminals establish multiple symmetric synapses upon the somata and dendrites of TRN neurons. The external pallidal terminals, labeled with WGA-HRP, arise from thinly myelinated axons and correspond to the medium to large F terminals. A less prevalent population of smaller GABAergic synaptic profiles was also identified. The synaptic profiles in this second group contain considerably fewer pleomorphic synaptic vesicles in small irregular clusters and fewer mitochondria, establish symmetric synapses, are postsynaptic to other axonal terminals, are presynaptic to dendrites and soma, and are unlabeled following pallidal injections of WGA-HRP.

A model of the electrophysiological properties of nucleus reticularis thalami neurons

A model of the electrophysiological properties of rodent nucleus reticularis thalami (NRT) neurons of the dorsal lateral thalamus was developed using Hodgkin-Huxley style equations. The model incorporated voltage-dependent rate constants and kinetics obtained from recent voltage-clamp experiments in vitro. The intrinsic electroresponsivity of the model cell was found to be similar to several empirical observations. Three distinct modes of oscillatory activity were identified: 1) a pattern of slow rhythmic burst firing (0.5-7 Hz) usually associated with membrane potentials negative to approximately -70 mV which resulted from the interplay of ITs and IK(Ca); 2) at membrane potentials from approximately -69 to -62 mV, rhythmic burst firing in the spindle frequency range (7-12 Hz) developed and was immediately followed by a tonic tail of single spike firing after several bursts. The initial bursting rhythm resulted from the interaction of ITs and IK(Ca), with a slow after-depolarization due to ICAN which mediated the later tonic firing; 3) with further depolarization of the membrane potential positive to approximately -61 mV, sustained tonic firing appeared in the 10-200-Hz frequency range depending on the amplitude of the injected current. The frequency of this firing was also dependent on the maximum conductance of the leak current, IK(leak), and an interaction between the fast currents involved in generating action potentials, INa(fast) and IK(DR), and the persistent Na+ current, INa(P). Transitions between different firing modes were identified and studied parametrically.

Mechanisms of oscillatory activity in guinea-pig nucleus reticularis thalami in vitro: a mammalian pacemaker

1. The ionic mechanisms of rhythmic burst firing and single spike, tonic discharge were investigated with extracellular and intracellular recordings of single neurones in the guinea-pig nucleus reticularis thalami (NRT) maintained as a slice in vitro. 2. Activation of cortical/thalamic afferents to NRT neurones resulted in a short latency burst of action potentials which could be followed by a rhythmic sequence of oscillatory burst firing. Intracellularly, this oscillatory activity was associated with an alternating sequence of low threshold Ca2+ spikes separated by after-hyperpolarizing potentials. Intracellular injection of short duration hyperpolarizing current pulses resulted in a similar sequence of oscillatory burst firing, suggesting that this activity is an intrinsic property of NRT cells. The frequency of rhythmic burst firing was highly voltage and temperature dependent and was between 7-12 Hz at -65 to -60 mV at 38 degrees C. In addition, at depolarized membrane potentials, oscillatory burst firing was typically followed by a prolonged tail of single spike activity. 3. Application of the Na+ channel poison tetrodotoxin blocked the generation of fast action potentials, but left intact the rhythmic sequence of low threshold Ca2+ spikes separated by after-hyperpolarizing potentials (AHPs). The reversal potential of the AHPs was -94 mV, suggesting that it was mediated by an increase in K+ conductance. Extracellular application of tetraethylammonium or apamin, or intracellular injection of Cs+ or the Ca2+ chelating agent EGTA, blocked the Ca2+ spike AHP, indicating that it is mediated by a Ca(2+)-activated K+ current. 4. Block of the AHP resulted in the marked enhancement of a slow after-depolarizing potential (ADP). The slow ADP occurred only following the generation of low threshold Ca2+ spikes. Replacement of extracellular Ca2+ with Mg2+ or Sr2+ resulted in an abolition of the slow ADP. In addition, the increase in [Mg2+]o resulted in an abolition of the low threshold Ca2+ spike. In contrast, replacement of extracellular Ca2+ with Ba2+ did not abolish the slow ADP. These results indicate that the ADP can be activated by either Ca2+ or Ba2+, but not by Mg2+ or Sr2+. 5. Replacement of extracellular Na+ with choline+ did not abolish the slow ADP, while replacement with N-methyl-D-glucamine+ did, indicating that the slow ADP can be supported by choline+, but not by N-methyl-D-glucamine+. Neither chemical affected the low threshold Ca2+ spike. These results are consistent with the slow ADP being mediated by a Ca(2+)-activated non-selective cation (CAN) current.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology of neurons in the thalamic reticular nucleus (TRN) of mammals as revealed by intracellular injections into fixed brain slices

I have investigated the morphology of neurons in the thalamic reticular nucleus (TRN) by means of intracellular injections in fixed tissue in order to study whether neurons in visual (dorsocaudal part), somatosensory (intermediate part), or limbic/motor (rostral part) sectors in the rat, rabbit, and cat differ morphologically in relation to their different sensory cortical or thalamic inputs. In addition, I have compared the different mammalian species to ask whether there is a morphological difference of TRN neurons according to reported differences in the intrinsic thalamic organisation, for example, due to the presence of GABAergic local circuit neurons in the majority of thalamic nuclei in the cat and the lack of those neurons in most of the rat thalamic nuclei, and presynaptic dendrites in the cat but not in the rat. In all animals investigated so far, neurons in the caudal (visual) and intermediate (somatosensory) part of the TRN have an elongated dendritic morphology in all three species, but some neurons in the rostral part, in particular in dorsal sections, have a distinctive multipolar morphology. Neurons have round, ovoid, or elongated somata ranging in area between 150 and 860 microns 2. In general, 4-8 first order dendrites emerge directly from the two poles of the soma or from a thick stem segment. Most of the dendrites then run parallel to the borders of the nucleus extending for relatively long distances, up to 450 microns, but remain inside the border of the nucleus. Only a few (1-3) dendrites could be observed to run perpendicular to the border of the nucleus and generally only for a short distance (20-70 microns). Some of the smooth first order dendrites give rise to second order dendrites (up to 200 microns in length), which then branch into short (15-70 microns) third order dendrites. Dendritic spines and varicosities, spine-like protusions and/or hair-like processes are mainly found on second and third order dendrites. Surprisingly, the shape, arrangement, and the size of the dendritic field are not strictly related to the shape and size of the nucleus. In mammalian species with a comparatively narrow TRN (rat and cat) the dendritic field size was similar to that in the rabbit with a broad TRN. There was considerable variability in dendritic morphology in the caudal and intermediate parts of TRN. However, in contrast to two recent studies in the rat TRN I have found no obvious basis for classification of neurons in the mammalian TRN according to dendritic morphology.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinergic innervation of the human thalamus: dual origin and differential nuclear distribution

The cholinergic innervation of the human thalamus was studied with antibodies against the enzyme choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr). Acetylcholinesterase histochemistry was used to delineate nuclear boundaries. All thalamic nuclei displayed ChAT-positive axons and varicosities. Only the medial habenula contained ChAT-positive perikarya. Some intralaminar nuclei (central medial, central lateral, and paracentral), the reticular nucleus, midline nuclei (paraventricular and reuniens), some nuclei associated with the limbic system (anterodorsal nucleus and medially situated patches in the mediodorsal nucleus) and the lateral geniculate nucleus displayed the highest density of ChAT-positive axonal varicosities. The remaining sensory relay nuclei and the nuclei interconnected with the motor and association cortex displayed a lower level of innervation. Immunoreactivity for NGFr was observed in cholinergic neurons of the basal forebrain but not in cholinergic neurons of the upper brainstem. The contribution of basal forebrain afferents to the cholinergic innervation of the human thalamus was therefore studied with the aid of NGFr-immunoreactive axonal staining. The anterior intralaminar nuclei, the reticular nucleus, and medially situated patches in the mediodorsal nucleus displayed a substantial number of NGFr-positive varicose axons, presumably originating in the basal forebrain. Rare NGFr-positive axonal profiles were also seen in many of the other thalamic nuclei. These observations suggest that thalamic nuclei affiliated with limbic structures and with the ascending reticular activating system are likely to be under particularly intense cholinergic influence. While the vast majority of thalamic cholinergic input seems to come from the upper brainstem, the intralaminar and reticular nuclei, and especially medially situated patches within the mediodorsal nucleus also appear to receive substantial cholinergic innervation from the basal forebrain.

Noradrenergic innervation of the thalamic reticular nucleus: a light and electron microscopic immunohistochemical study in rats

Fluoro-ruby injections in the rat locus coeruleus result in scattered chain-like arrays of varicose anterogradely labeled axons within the thalamic reticular nucleus of rats. An abundant meshwork of axons giving rise to en passant boutons is detected immunohistochemically within this thalamic nucleus by means of an antibody to dopamine-beta-hydroxylase (DBH). The density of DBH-positive axonal boutons within the reticular nucleus neuropil is greater than that found in the relay nuclei of the dorsal thalamus (with the exception of the anterior group nuclei). Single DBH-positive axons appear to contact both proximal and distal dendrites and occasionally the somata of reticular nucleus neurons. Labeled axons are seen closely juxtaposed not only to the swollen segments of the beaded reticular neuron dendrites, but to the constricted segments as well. Electron microscopic examination of DBH-positive axon terminals within the reticular nucleus neuropil indicates that many of the axonal boutons detected light microscopically participate in asymmetric synaptic contacts. The postsynaptic densities of these synapses are thicker than those of nearby symmetric synapses, but often subtend a shorter length of the postsynaptic membrane than the densities associated with other nearby asymmetric synapses. These observations indicate that the ascending noradrenergic system, in addition to influencing the dorsal thalamus and the cerebral cortex directly, is well situated to influence signal transmission through the nuclei of the dorsal thalamus indirectly via a moderately dense terminal projection upon the thalamic reticular nucleus.

Sources of presumptive glutamatergic/aspartatergic afferents to the magnocellular basal forebrain in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of [3H]-D-aspartate into the magnocellular basal forebrain of the rat was compared with the distribution of neurons labeled by comparable injections of the nonspecific retrograde axonal tracer wheat germ agglutinin conjugated to horseradish peroxidase. Cells retrogradely labeled by wheat germ agglutinin-horseradish peroxidase were found in a wide range of limbic and limbic-related structures in the forebrain and brainstem. In the telencephalon, labeled neurons were seen in the orbital, medial prefrontal, and agranular insular cortical areas, the amygdaloid complex, and the hippocampal formation. Labeled cells were also seen in the olfactory cortex, the lateral septum, the ventral striatopallidal region, and the magnocellular basal forebrain itself. In the diencephalon, neurons were labeled in the midline nuclear complex of the thalamus, the lateral habenular nucleus, and the hypothalamus. In the brainstem, labeled cells were found bilaterally in the ventral midbrain, the central gray, the reticular formation, the parabrachial nuclei, the raphe nuclei, the laterodorsal tegmental nucleus, and the locus coeruleus. A significant fraction of the afferents to the magnocellular basal forebrain appear to be glutamatergic and/or aspartatergic. Only a few of the regions labeled with wheat germ agglutinin-horseradish peroxidase were not also labeled with [3H]-D-aspartate in the comparable experiments. Most prominent among the non-glutamatergic/aspartatergic projections were those from fields CA1 and CA3 of the hippocampus, the hilus of the dentate gyrus, the dorsal subiculum, the tuberomammillary nucleus, and the ventral pallidum. In addition, most of the lateral hypothalamic and brainstem projections to the magnocellular basal forebrain were not significantly labeled with [3H]-D-aspartate. In addition to these inputs, a commissural projection from the region of the contralateral nucleus of the horizontal limb of the diagonal band was confirmed with both wheat germ agglutinin-horseradish peroxidase and the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin. This projection did not label with [3H]-D-aspartate or [3H]-GABA, suggesting that it is not glutamatergic/aspartatergic or GABAergic. Furthermore, double labeling experiments with the fluorescent retrograde tracer True Blue and antibodies against choline acetyltransferase indicate that the projection is not cholinergic.

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

Neurons in the magnocellular nucleus of the caudal basal forebrain extend an axonal projection which arborizes within the reticular nucleus of the thalamus. The present study addresses the ultrastructure and neurochemistry of this projection in rats. Many labeled terminals are apparent within the thalamic reticular nucleus following Phaseolus vulgaris leucoagglutinin injections into the caudal basal nucleus; anterogradely labeled axon terminals most commonly contact both somata and dendrites of reticular nucleus neurons with symmetric membrane specializations. Thus, the majority of the labeled terminals examined contrast with choline acetyltransferase positive terminals which have been previously identified as contacting dendrites and forming asymmetric synapses within this nucleus. Many of the neurons within the caudal basal nucleus which are retrogradely labeled following tracer injections into the thalamic reticular nucleus are gamma-aminobutyric acid (GABA) immunoreactive. In addition, following injections of Phaseolus vulgaris leucoagglutinin or fluoro-ruby into the caudal basal forebrain, some of the labeled axonal swellings and boutons within the thalamic reticular nucleus also contain glutamic acid decarboxylase. These results indicate that a significant component of the projection is GABAergic. These anatomical observations suggest that the projection from the caudal basal nucleus onto the thalamic reticular nucleus could facilitate the relay of information through the dorsal thalamus by inhibiting reticular nucleus neurons, and thus, in turn, disinhibiting thalamic relay neurons.

Alz-50 immunoreactivity in the thalamic reticular nucleus in Alzheimer's disease

Examination of the thalamic reticular nucleus (Rt) with the monoclonal antibody Alz-50 in brains of Alzheimer's disease patients reveals dense extracellular and terminal-like immunoreactivity in the absence of neurofibrillary tangles or neuritic plaques. Similar terminal-like immunoreactivity is not present in other thalamic nuclei of AD brains or in the brains of controls. Based on (1) an immunocytochemical and histopathological analysis of areas known to project to the Rt, (2) that Alz-50 immunocytochemistry reveals immunoreactive neurons, neurofibrillary tangles and neuritic plaques, and (3) evidence that Alz-50 immunoreactivity can be demonstrated in the terminal fields of immunoreactive neurons, the terminal-like immunoreactivity in the Rt probably corresponds to altered preterminal axons and terminals from degenerating basal forebrain neurons. Given the presumed physiological role of the Rt, these selective lesions could alter thalamocortical processing and contribute to the cognitive impairment in Alzheimer's disease.