Basal forebrain efferents to the medial dorsal thalamic nucleus in the rhesus monkey

Effects of systemic cholinergic antagonism on reinforcer devaluation in macaques

The capacity to adjust actions based on new information is a vital cognitive function. An animal's ability to adapt behavioral responses according to changes in reward value can be measured using a reinforcer devaluation task, wherein the desirability of a given object is reduced by decreasing the value of the associated food reinforcement. Elements of the neural circuits serving this ability have been studied in both rodents and nonhuman primates. Specifically, the basolateral amygdala, orbitofrontal cortex, nucleus accumbens, and mediodorsal thalamus have each been shown to play a critical role in the process of value updating, required for adaptive goal selection. As these regions receive dense cholinergic input, we investigated whether systemic injections of non-selective nicotinic or muscarinic acetylcholine receptor antagonists, mecamylamine and scopolamine, respectively, would impair performance on a reinforcer devaluation task. Here we demonstrate that in the presence of either a nicotinic or muscarinic antagonist, animals are able to shift their behavioral responses in an appropriate manner, suggesting that disruption of cholinergic neuromodulation is not sufficient to disrupt value updating, and subsequent goal selection, in rhesus macaques.

The dorsal diencephalic conduction system in reward processing: Spotlight on the anatomy and functions of the habenular complex

The dorsal diencephalic conduction system (DDC) is a highly conserved pathway in vertebrates that provides a route for the neural information to flow from forebrain to midbrain structures. It contains the bilaterally paired habenular nuclei along with two fiber tracts, the stria medullaris and the fasciculus retroflexus. The habenula is the principal player in mediating the dialogue between forebrain and midbrain regions, and functional abnormalities in this structure have often been attributed to pathologies like mood disorders and substance use disorder. Following Matsumoto and Hikosaka seminal work on the lateral habenula as a source of negative reward signals, the last decade has witnessed a great surge of interest in the role of the DDC in reward-related processes. However, despite significant progress in research, much work remains to unfold the behavioral functions of this intriguing, yet complex, pathway. This review describes the current state of knowledge on the DDC with respect to its anatomy, connectivity, and functions in reward and aversion processes.

MAPBOT: Meta-analytic parcellation based on text, and its application to the human thalamus

Meta-analysis of neuroimaging results has proven to be a popular and valuable method to study human brain functions. A number of studies have used meta-analysis to parcellate distinct brain regions. A popular way to perform meta-analysis is typically based on the reported activation coordinates from a number of published papers. However, in addition to the coordinates associated with the different brain regions, the text itself contains considerably amount of additional information. This textual information has been largely ignored in meta-analyses where it may be useful for simultaneously parcellating brain regions and studying their characteristics. By leveraging recent advances in document clustering techniques, we introduce an approach to parcellate the brain into meaningful regions primarily based on the text features present in a document from a large number of studies. This new method is called MAPBOT (Meta-Analytic Parcellation Based On Text). Here, we first describe how the method works and then the application case of understanding the sub-divisions of the thalamus. The thalamus was chosen because of the substantial body of research that has been reported studying this functional and structural structure for both healthy and clinical populations. However, MAPBOT is a general-purpose method that is applicable to parcellating any region(s) of the brain. The present study demonstrates the powerful utility of using text information from neuroimaging studies to parcellate brain regions.

Deep Brain Stimulation of Medial Dorsal and Ventral Anterior Nucleus of the Thalamus in OCD: A Retrospective Case Series

BACKGROUND

The current notion that cortico-striato-thalamo-cortical circuits are involved in the pathophysiology of obsessive-compulsive disorder (OCD) has instigated the search for the most suitable target for deep brain stimulation (DBS). However, despite extensive research, uncertainty about the ideal target remains with many structures being underexplored. The aim of this report is to address a new target for DBS, the medial dorsal (MD) and the ventral anterior (VA) nucleus of the thalamus, which has thus far received little attention in the treatment of OCD.

METHODS

In this retrospective trial, four patients (three female, one male) aged 31-48 years, suffering from therapy-refractory OCD underwent high-frequency DBS of the MD and VA. In two patients (de novo group) the thalamus was chosen as a primary target for DBS, whereas in two patients (rescue DBS group) lead implantation was performed in a rescue DBS attempt following unsuccessful primary stimulation.

RESULTS

Continuous thalamic stimulation yielded no significant improvement in OCD symptom severity. Over the course of thalamic DBS symptoms improved in only one patient who showed "partial response" on the Yale-Brown Obsessive Compulsive (Y-BOCS) Scale. Beck Depression Inventory scores dropped by around 46% in the de novo group; anxiety symptoms improved by up to 34%. In the de novo DBS group no effect of DBS on anxiety and mood was observable.

CONCLUSION

MD/VA-DBS yielded no adequate alleviation of therapy-refractory OCD, the overall strategy in targeting MD/VA as described in this paper can thus not be recommended in DBS for OCD. The magnocellular portion of MD (MDMC), however, might prove a promising target in the treatment of mood related and anxiety disorders.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Basal ganglia circuit loops, dopamine and motivation: A review and enquiry

Dopamine neurons located in the midbrain play a role in motivation that regulates approach behavior (approach motivation). In addition, activation and inactivation of dopamine neurons regulate mood and induce reward and aversion, respectively. Accumulating evidence suggests that such motivational role of dopamine neurons is not limited to those located in the ventral tegmental area, but also in the substantia nigra. The present paper reviews previous rodent work concerning dopamine's role in approach motivation and the connectivity of dopamine neurons, and proposes two working models: One concerns the relationship between extracellular dopamine concentration and approach motivation. High, moderate and low concentrations of extracellular dopamine induce euphoric, seeking and aversive states, respectively. The other concerns circuit loops involving the cerebral cortex, basal ganglia, thalamus, epithalamus, and midbrain through which dopaminergic activity alters approach motivation. These models should help to generate hypothesis-driven research and provide insights for understanding altered states associated with drugs of abuse and affective disorders.

The associative and limbic thalamus in the pathophysiology of obsessive-compulsive disorder: an experimental study in the monkey

Obsessive-compulsive disorder (OCD) is a frequent psychiatric disorder characterized by repetitive intrusive thoughts and severe anxiety, leading to compulsive behaviors. Although medical treatment is effective in most cases, resistance is observed in about 30% of patients. In this context, deep brain stimulation (DBS) of the caudate or subthalamic nuclei has been recently proposed with encouraging results. However, some patients were unimproved or exhibited awkward side effects. Therefore, exploration of new targets for DBS remains critical in OCD. In the latter, functional imaging studies revealed overactivity in the limbic and associative cortico-subcortical loops encompassing the thalamus. However, the role of the thalamus in the genesis of repetitive behaviors and related anxiety is unknown. Here, we tested the hypothesis that pharmacological-induced overactivity of the medial thalamus could give rise to abnormal behaviors close to that observed in OCD. We modulated the ventral anterior (VA) and medial dorsal (MD) nuclei activity by in situ bicuculline (GABA(A) antagonist) microinjections in subhuman primates and assessed their pharmacological-induced behavior. Bicuculline injections within the VA caused significant repetitive and time-consuming motor acts whereas those performed within the MD induced symptoms of dysautonomic dysregulation along with abnormal vocalizations and marked motor hypoactivity. These findings suggest that overactivation of the VA and MD nuclei of the thalamus provokes compulsive-like behaviors and neurovegetative manifestations usually associated with the feeling of anxiety in OCD patients. In further research, this translational approach should allow us to test the effectiveness and side effects of these thalamic nuclei DBS in monkey and perhaps, in a second step, to propose a transfer of this technique to severely disabled OCD patients.

Cannabinoid-1 receptors in the mouse ventral pallidum are targeted to axonal profiles expressing functionally opposed opioid peptides and contacting N-acylphosphatidylethanolamine-hydrolyzing phospholipase D terminals

The ventral pallidum (VP) is a major recipient of inhibitory projections from nucleus accumbens (Acb) neurons that differentially express the reward (enkephalin) and aversion (dynorphin)-associated opioid peptides. The cannabinoid-1 receptor (CB1R) is present in Acb neurons expressing each of these peptides, but its location in the VP is not known. To address this question, we used electron microscopic dual immunolabeling of the CB1R and either dynorphin 1-8 (Dyn) or Met(5)-enkephalin (ME) in the VP of C57BL/6J mice, a species in which CB1R gene deletion produces a reward deficit. We also used similar methods to determine the relationship between the CB1R and N-acylphosphatidylethanolamine (NAPE)-hydrolyzing phospholipase D (NAPE-PLD), an anandamide-synthesizing enzyme located presynaptically in other limbic brain regions. CB1R-immunogold was principally localized to cytoplasmic endomembranes and synaptic or extrasynaptic plasma membranes of axonal profiles, but was also affiliated with postsynaptic membrane specializations in dendrites. The axonal profiles included many single CB1R-labeled axon terminals as well as terminals containing CB1R-immunogold and either Dyn or ME immunoreactivity. Dually labeled terminals comprised 26% of all Dyn- and 17% of all ME-labeled axon terminals. Both single- and dual-labeled terminals formed mainly inhibitory-type synapses, but almost 16% of these terminals formed excitatory synapses. Approximately 60% of the CB1R-labeled axonal profiles opposed or converged with axon terminals containing NAPE-PLD immunoreactivity. We conclude that CB1Rs in the mouse VP have subcellular distributions consistent with on demand activation by endocannabinoids that can regulate the release of functionally opposed opioid peptides and also modulate inhibitory and excitatory transmission.

Topographic organization of the ventral striatal efferent projections in the rhesus monkey: an anterograde tracing study

The ventral striatum is considered to be that portion of the striatum associated with the limbic system by virtue of its afferent connections from allocortical and mesolimbic areas as well as from the amygdala. The efferent projections from this striatal region in the primate were traced by using 3H amino acids and Phaseolus vulgaris-leucoagglutinin (PHA-L). Particular attention was paid to the topographic organization of terminal fields in the globus pallidus and substantia nigra, the projections to non-extrapyramidal areas, the relationship between projections from the nucleus accumbens and the other parts of the ventral striatum, and the comparison between ventral and dorsal striatal projections. This study demonstrates that in monkeys a circumscribed region of the globus pallidus receives topographically organized efferent fibers from the ventral striatum. The ventral striatal fibers terminate in the ventral pallidum, the subcommissural part of the globus pallidus, the rostral pole of the external segment, and the rostromedial portion of the internal segment. The more central and caudal portions of the globus pallidus do not receive this input. This striatal output appears to remain segregated from the dorsal striatal efferent projections to pallidal structures. Fibers from the ventral striatum projecting to the substantia nigra are not as confined to a specific region as those projecting to the globus pallidus. Although the densest terminal fields occur in the medial portion, numerous fibers also extend laterally to innervate the dorsal stratum of dopaminergic neurons of the substantia nigra and the retrorubral area. Furthermore, they project throughout the rostral-caudal extent of the substantia nigra. Projections from the medial part of the ventral striatum reach the more caudally located pedunculopontine tegmental nucleus. Thus unlike the above described terminals in the globus pallidus, the ventral striatum project widely throughout the substantia nigra, a fact that indicates that they may contribute to the integration between limbic and other output systems of the striatum. Finally, the ventral striatum projects to non-extrapyramidal regions including the bed nucleus of the stria terminals, the nucleus basalis magnocellularis, the lateral hypothalamus, and the medial thalamus.

Dendritic morphology, local circuitry, and intrinsic electrophysiology of neurons in the rat medial and lateral habenular nuclei of the epithalamus

The habenular complex of the epithalamus in the mammalian brain receives input from the limbic forebrain and pallidum and, in turn, projects to numerous midbrain structures. Traditionally, the habenular complex is divided into the medial nucleus and two divisions of the lateral nucleus. Based on their distinct input and output pathways, the habenula is considered to constitute three, partially overlapping channels that regulate information flow from the limbic forebrain and pallidum to the midbrain. As a step to improve our understanding of how information delivered from the limbic forebrain and pallidum is processed in the habenula, we examined the electrical property and morphology of medial and lateral habenular cells. For this study, we generated live brain slices from rat habenula and performed whole cell recording. During recording, we filled habenular cells with biocytin. Medial habenular cells generate tonic trains of action potentials, whereas lateral habenular cells are capable of producing action potentials in burst mode. Lateral habenular cells produce dendrites that are much longer than those of medial habenular cells. Two distinct intrinsic circuits exist in the medial habenular nucleus, whereas in the lateral habenular nucleus, intrinsic axons travel largely from medial to lateral direction. The connection between the two habenular nuclei is asymmetrical in that only the medial habenula sends projection to the lateral habenula. The differences in the electrical and morphological properties of medial and lateral habenular cells indicate that the two nuclei process and integrate information in distinct fashions that is delivered from the limbic forebrain and pallidum.

Efferent connections of septal nuclei of the domestic chick (Gallus domesticus): An anterograde pathway tracing study with a bearing on functional circuits

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in different subregions of the septum of domestic chicks. The main targets of septal projections comprised the ipsi- and contralateral septal nuclei, including the nucleus of the diagonal band, basal ganglia, including the ventral paleostriatum, lobus parolfactorius, nucleus accumbens, and olfactory tubercle, archistriatum, piriform cortex, and anterior neostriatum. Further diencephalic and mesencephalic septal projections were observed in the ipsilateral preoptic region, hypothalamus (the main regions of afferentation comprising the lateral hypothalamic nuclei, ventromedial, paraventricular and periventricular nuclei, and the mammillary region), dorsal thalamus, medial habenular and subhabenular nuclei, midbrain central gray, and ventral tegmental area. Contralateral projections were also encountered in the septal nuclei, ventral paleostriatum, periventricular and anteromedial hypothalamic nuclei, suprachiasmatic nucleus, and the lateral hypothalamic area. Avian septal efferents are largely similar to those of mammals, the main differences being a relatively modest hippocampal projection arising mainly from the nucleus of the diagonal band (as confirmed by a specific experiment with the retrograde pathway tracer True blue), the lack of interpeduncular projection, and a greater contingent of amygdalar efferents arising from the lateral septum rather than the nucleus of the diagonal band. This pattern of connectivity is likely to reflect an important role of the avian septal nuclei in the coordination of limbic circuits and the integration of a wide variety of information sources modulating the appropriate behavioral responses: attention and arousal level, memory formation, hormonally mediated behaviors, and their affective components (such as ingestive, reproductive, and parental behaviors), social interaction, locomotor modulation, and circadian rhythm.

Distribution and binding parameters of GABAA receptors in the thalamic nuclei of Macaca mulatta and changes caused by lesioning in the globus pallidus and reticular thalamic nucleus

Ascending output from the basal ganglia to the primate motor thalamus is carried by GABAergic nigro- and pallido-thalamic pathways, which interact with intrinsic thalamic GABAergic systems represented in primates by local circuit neurons and axons of the reticular thalamic nucleus. Disease-triggered pathological processes in the basal ganglia can compromise any of these pathways either directly or indirectly, yet the effects of basal ganglia lesioning on its thalamic afferent-receiving territories has not been studied in primates. Two GABA(A) receptor ligands, [(3)H]muscimol and [(3)H]flunitrazepam, were used to study the distribution and binding properties of the receptor in intact monkeys, those with kainic acid lesions in the globus pallidus, and those with ibotenic acid lesions in the reticular nucleus using quantitative autoradiographic technique on cryostat sections of fresh frozen brain tissue. In control monkeys the binding affinities for [(3)H]muscimol averaged 50 nM in the thalamic nuclei and 86 nM in the basal ganglia while the binding densities varied (maximum density of binding sites [Bmax] range of 99.4-1000.1 fmol/mg of tissue). Binding affinities and Bmax values for [(3)H]flunitrazepam averaged 2.02 nM and 81-113 fmol/mg of tissue, respectively. Addition of 100-microM GABA increased average affinity to 1.35 nM whereas Bmax values increased anywhere from 1-50% in different nuclei. Zolpidem (100 nM) decreased binding by 68-80%. Bmax values for both ligands were decreased at the two survival times in both medial and lateral globus pallidus implying involvement of both nuclei in the lesion. Statistically significant, 40% decrease (P=0.055) of Bmax for [(3)H]muscimol was observed in the ventral anterior nucleus pars densicellularis (VAdc, the main pallidal projection territory in the thalamus) 1 week after globus pallidus lesioning and a 36% decrease (P=0.017) 4 months post-lesioning. In contrast, [(3)H]flunitrazepam Bmax values in the VAdc of the same animals were increased by 23% (P=0.021) at 1 week and 28% (P=0.005) 4 months postlesion, respectively. One week after the reticular nucleus lesioning, the binding densities of [(3)H]muscimol and [(3)H]flunitrazepam were decreased in the thalamic nuclei receiving projections from the lesioned reticular nucleus sector by approximately 50% (P<0.05) and 10-33% (P<0.05), respectively. The results suggest that different GABA(A) receptor subtypes are associated with different GABAergic systems in the thalamus which react differently to deafferentation.

Efferent connections of the dorsomedial thalamic nuclei of the domestic chick (Gallus domesticus)

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in the thalamic anterior dorsomedial nucleus (DMA) of domestic chicks. The projections of the DMA covered the rostrobasal forebrain, ventral paleostriatum, nucleus accumbens, septal nuclei, Wulst, hyperstriatum ventrale, neostriatal areas, archistriatal subdivisions, dorsolateral corticoid area, numerous hypothalamic nuclei, and dorsal thalamic nuclei. The rostral DMA projects preferentially on the hypothalamus, whereas the caudal part is connected mainly to the dorsal thalamus. The DMA is also connected to the periaqueductal gray, deep tectum opticum, intercollicular nucleus, ventral tegmental area, substantia nigra, locus coeruleus, dorsal lateral mesencephalic nucleus, lateral reticular formation, nucleus papillioformis, and vestibular and cranial nerve nuclei. This pattern of connectivity is likely to reflect an important role of the avian DMA in the regulation of attention and arousal, memory formation, fear responses, affective components of pain, and hormonally mediated behaviors.

Neural interaction between the basal forebrain and functionally distinct prefrontal cortices in the rhesus monkey

The prefrontal cortex in rhesus monkeys is a heterogeneous region by structure, connections and function. Caudal medial and orbitofrontal cortices receive input from cortical and subcortical structures associated with emotions, autonomic function and long-term memory, while lateral prefrontal cortices are linked with structures associated with working memory. With the aid of neural tracers we investigated whether functionally distinct orbitofrontal, medial and lateral prefrontal cortices have specific or common connections with an ascending modulatory system, the basal forebrain. Ascending projections originated in the diagonal band and the basalis nuclei of the basal forebrain in regions demarcated by choline acetyltransferase. Although the origin of projections from the basal forebrain to lateral, medial and orbitofrontal cortices partially overlapped, projections showed a general topography. The posterior part of the nucleus basalis projected preferentially to lateral prefrontal areas while its rostrally adjacent sectors projected to medial and orbitofrontal cortices. The diagonal band nuclei projected to orbitofrontal and medial prefrontal areas. Cortical and subcortical structures that are interconnected appear to have a similar pattern of connections with the basal forebrain. In comparison to the ascending projections, the descending projections were specific, originating mostly in the posterior (limbic) component of medial and orbitofrontal cortices and terminating in the diagonal band nuclei and in the anterior part of the nucleus basalis. In addition, prefrontal limbic areas projected to two other systems of the basal forebrain, the ventral pallidum and the extended amygdala, delineated with the striatal-related markers dopamine, adenosine 3':5'-monophosphate regulated phosphoprotein of M(r) 32kDa, and the related phosphoprotein Inhibitor-1. These basal forebrain systems project to autonomic nuclei in the hypothalamus and brainstem. We interpret these results to indicate that lateral prefrontal areas, which have a role in working memory, receive input from, but do not issue feedback projections to the basal forebrain. In contrast, orbitofrontal and medial prefrontal areas, which have a role in emotions and long-term memory, have robust bidirectional connections with the basal forebrain. Moreover, orbitofrontal and medial prefrontal cortices target the ventral pallidum and the extended amygdala, through which high-order association areas may activate motor autonomic structures for the expression of emotions.

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.

Prefrontal cortical networks related to visceral function and mood

At least twenty-two architectonic areas can be distinguished within the orbital and medial prefrontal cortex (OMPFC). Although each of these areas has a distinct structure and connections, they can be grouped into two "networks," defined by cortico-cortical connections that primarily interconnect areas within each network. The networks also have different connections to the striatum, medial thalamus, and other brain regions. The orbital network consists of most of the areas in the orbital cortex. It receives several sensory inputs (olfactory, gustatory, visceral afferent, somatic sensory, and visual) that appear to be related to feeding. It also receives many limbic inputs from the amygdala, entorhinal and perirhinal cortex, and subiculum, including a specific projection from the ventrolateral part of the basal amygdaloid nucleus. The orbital network may therefore serve as a substrate to integrate viscerosensory information with affective signals. The medial network consists of areas on the medial frontal surface together with a few select areas in the orbital cortex. These areas have few direct sensory inputs, and their limbic inputs are somewhat different than those to the orbital network (e.g., from the ventromedial part of the basal amygdaloid nucleus). However, they provide the major output from the OMPFC to the hypothalamus and brain stem (especially the periaqueductal gray). The medial network may therefore serve as a visceromotor system to provide frontal cortical influence over autonomic and endocrine function. Connections between the networks presumably allow information flow from viscerosensory to visceromotor systems. In addition to a probable role in eating behavior, this system appears to be involved in guiding behavior and regulation of mood. Lesions of the ventromedial prefrontal cortex result in sociopathic behavior and difficulty in making appropriate choices, whereas functional imaging studies indicate that subjects with unipolar and bipolar depression have abnormal activity in medial and orbital prefrontal areas. Many of these areas also show volume changes and decreased glial number and density in mood-disordered subjects.

GABAergic and cholinergic basal forebrain and preoptic-anterior hypothalamic projections to the mediodorsal nucleus of the thalamus in the cat

The present study examined projections of GABAergic and cholinergic neurons from the basal forebrain and preoptic-anterior hypothalamus to the "intermediate" part of the mediodorsal nucleus of the thalamus. Retrograde transport from this region of the mediodorsal nucleus was investigated using horseradish peroxidase-conjugated wheatgerm agglutinin in combination with peroxidase-antiperoxidase immunohistochemical staining for glutamic acid decarboxylase and choline acetyltransferase. A relatively large number of retrogradely-labelled glutamic acid decarboxylase-positive neurons are located in the basal forebrain, amounting to more than 7% of the total population of glutamic acid decarboxylase-positive cells in this region. Moreover, retrogradely-labelled choline acetyltransferase-positive cells are interspersed among glutamic acid decarboxylase-positive neurons, accounting for about 6% of the total choline acetyltransferase-positive cell population in the basal forebrain. The glutamic acid decarboxylase-positive and choline acetyltransferase-positive retrogradely-labelled neurons are distributed throughout several regions of the basal forebrain, including the medial septum, the diagonal band of Broca, the magnocellular preoptic nucleus, the substantia innominata pars anterior, the substantia innominata pars posterior, and the globus pallidus where only a few retrogradely-labelled neurons were seen. The choline acetyltransferase-positive mediodorsal-projecting neurons are morphologically different from the choline acetyltransferase-positive neurons in the basal forebrain, suggesting that those projecting to the mediodorsal nucleus are a small proportion of the cholinergic neuronal population in the basal forebrain. In the preoptic-anterior hypothalamus, many retrogradely-labelled glutamic acid decarboxylase-positive cells were found, amounting to more than 7% of the total population of glutamic acid decarboxylase-positive cells in this region. These retrogradely-labelled glutamic acid decarboxylase-positive neurons are distributed throughout the preoptic-anterior hypothalamus in a continuous line with those in the basal forebrain, including the lateral preoptic area, the medial preoptic area, the bed nucleus of the stria terminalis, and the anterior and dorsal hypothalamic areas. The highest percentage of mediodorsal-projecting GABAergic neurons is in the anterior lateral hypothalamus where more than 25% of the total population of glutamic acid decarboxylase-positive cells project to the mediodorsal nucleus of the thalamus. Overall, of the large population of retrogradely-labelled neurons in the basal forebrain and preoptic-anterior hypothalamus, a significant proportion are glutamic acid decarboxylase-positive neurons (> 60% in the basal forebrain and > 30% in the preoptic-anterior hypothalamus), while the choline acetyltransferase-positive neurons amount to a smaller percentage of the neurons projecting to the mediodorsal nucleus (< 13% in the basal forebrain and < 2% in the preoptic-anterior hypothalamus). These results provide anatomical evidence of direct GABAergic projections from the basal forebrain and preoptic-anterior hypothalamic regions to the "intermediate" part of the mediodorsal nucleus in the cat. This GABAergic projection field could be the direct pathway by which the basal forebrain directly modulates thalamic excitability and may also be involved in mechanisms modulating electroencephalographic synchronization and sleep through the "intermediate" mediodorsal nucleus.

A population of cholinergic neurons is present in the macaque monkey thalamus

Three new cholinergic markers were employed to study the cholinergic innervation in the thalamus of adult macaque monkeys. They were: two antibodies against choline acetyltransferase (ChAT), one polyclonal and one monoclonal; and a polyclonal antibody against the vesicular transporter of acetylcholine (VAChT), a powerful new marker that colocalizes with ChAT. This approach led to an unexpected finding: the three antibodies positively immunostained a population of neurons in the paracentral nucleus. The immunostained cells are confined to the dorsal region of this nucleus along its rostrocaudal extent. Measurement of the somatic areas of the immunostained neurons indicated that they correspond to a population of large neurons thought to be projection neurons. Because dorsal paracentral neurons are known to project to the dorsal striatum and specific cortical areas involved in visual and visuomotor mechanisms, these structures might be modulated by cholinergic thalamic neurons.

Thalamic input to mesial and superior area 6 in the macaque monkey

The aim of the present study was to define the origin of the thalamocortical projections to each of the mesial and superior area 6 areas. To this purpose, restricted injections of neuronal tracers were made into areas F3, F6, F2, and F7 after physiological identification of the injection sites. The results showed that each of these areas receives afferents from a set of thalamic nuclei and that this set differs, qualitatively and quantitatively, according to the injected area. The main inputs to F3 [supplementary motor area properly defined (SMA-proper)] originate in the nuclei ventral lateral, pars oralis (VLo), ventral posterior lateral, pars oralis (VPLo), and ventral lateral, pars caudalis (VLc) as well as in caudal parts of the VPLo and VLc (VPLo/VLc complex). F6 (pre-SMA) is mainly the target of nucleus ventral anterior, pars parvocellularis (VApc), and area X of Olszewski. The input to F2 originates mainly in the VPLo/VLc complex, in VLc, and in VLo. The dorsal part of F7 (supplementary eye field) mainly receives from area X, VApc, and nucleus ventral anterior, pars magnocellularis (VAmc), whereas the ventral F7 is connected with VApc, area X, VLc, and the VPLo/VLc complex. All of the injected areas receive a strong projection from the medial dorsal nucleus (MD). It is concluded that each cortical area is a target of both cerebellar and basal ganglia circuits. F3 and F2 are targets of the so-called "motor" basal ganglia circuit and a cerebellar circuit originating in dorsorostral sectors of dentate and interpositus nuclei. In contrast, F6 and ventral F7 receive a basal ganglia input mainly from the so-called "complex" circuit and a cerebellar input originating in the ventrocaudal sectors of dentate and interpositus nuclei. Finally, with respect to the rest of F7, dorsal F7 also receives a basal ganglia input from the "oculomotor circuit."

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.

Neuropathology of thiamine deficiency: an update on the comparative analysis of human disorders and experimental models

This paper provides a re-examination of the neuroanatomical consequences of thiamine deficiency in light of more recent studies of human disorders and models of experimental thiamine deficiency. A major goal is to elucidate the relative roles of thiamine deficiency and chronic alcohol consumption in the pathogenesis of Wernicke-Korsakoff syndrome (WKS). Particular emphasis is placed on the role of thiamine deficiency in lesions to basal forebrain, raphe, locus coeruleus, white matter and cortex and their role in the cognitive and memory disturbances of human WKS and experimental models of thiamine deficiency.

Ventral striatopallidothalamic projection: IV. Relative involvements of neurochemically distinct subterritories in the ventral pallidum and adjacent parts of the rostroventral forebrain

Retrograde and anterograde tract-tracing studies were carried out to determine whether the capacity of the nucleus accumbens to influence the thalamic mediodorsal nucleus via ventral striatopallidothalamic connections disproportionately favors the shell over the core subterritory. After injections of Fluoro-Gold into the mediodorsal thalamic nucleus, retrogradely labeled neurons were detected in sections also processed for calbindin-D 28-kD and neurotensin immunoreactivities to facilitate identification of subterritories in the ventral pallidum. Fluoro-Gold-labeled cells were counted in series of sections cut through the ventral pallidum, rostral globus pallidus, nucleus of the vertical limb of the diagonal band, preoptic region, lateral hypothalamus, and the sublenticular gray region, including parts of the extended amygdala. Data were expressed as cells/unit area and as percentages of all labeled forebrain cells. Mediodorsal nucleus-projecting rostroventral forebrain neurons were most numerous in the ventromedial part of the subcommissural ventral pallidum and pallidal parts of the olfactory tubercle. Few were observed in the dorsolateral part of the subcommissural ventral pallidum. In addition, following injections into the ventral pallidum, anterogradely transported biotinylated dextran amine was evaluated in sections processed for calbindin or tyrosine hydroxylase immunoreactivities. Injection into the ventromedial part of the subcommissural ventral pallidum resulted in robust anterograde labeling of the medial segment of the mediodorsal nucleus and ventral tegmental area and weak labeling of the substantia nigra and subthalamic nucleus. Conversely, after injection into the dorsolateral part of the subcommissural ventral pallidum, anterograde labeling was weak in the mediodorsal nucleus and ventral tegmental area, but robust in the substantia nigra and subthalamic nucleus. The results are consistent with a predominant accumbens shell influence on the mediodorsal nucleus and with cortico-ventral striatopallidal-thalamocortical pathways that begin and end in different parts of the frontal lobe.

Ventrolateral orbital cortex and periaqueductal gray stimulation-induced effects on on- and off-cells in the rostral ventromedial medulla in the rat

On- and off-cells of the rostral ventromedial medulla are thought to be involved in bulbospinal inhibition of ascending nociceptive information. Experiments were carried out in lightly anaesthetized rats to assess the effects of prefrontal cortex stimulation on the responses of neurons in the rostral ventromedial medulla. For comparison purposes, effects of periaqueductal gray stimulation were also investigated. Single unit activity was recorded in the rostral ventromedial medulla and on-, off- and neutral-cells were identified based on the tail nocifensor reflex to noxious heat. Short (0.1-1 s) and long (10-15 s) trains of bipolar electrical stimulation (100-300 Hz) were delivered to the ventrolateral orbital cortex of the rat forebrain and the periaqueductal gray. Short-train stimulation of the periaqueductal gray (including dorsolateral, ventrolateral and the dorsal raphé regions) excited 58% (25 of 43) of on-cells and 44% (seven of 16) of off-cells in the rostral ventromedial medulla. Long trains blocked the noxious stimulus-evoked pause of all seven off-cells tested and blocked the excitatory response of two, and enhanced one of three on-cells. Such stimulation also inhibited or abolished the tail-flick reflex at currents below 100 microA. Glutamate microinjections into the periaqueductal gray inhibited the noxious-evoked response of two off- and two on-cells and increased the tail-flick latency. Short-train stimulation of the ventrolateral orbital cortex (100-400 microA) excited eight of 25 on-cells and inhibited the ongoing activity of 10 of 14 off-cells. Long-train ventrolateral orbital cortex stimulation (5-15 s, 100-200 microA, 200-300 Hz) enhanced the noxious evoked responses of 10 of 11 on-cells, prolonged the noxious heat-evoked pause of all of four off-cells and decreased the tail-flick latency (pronociception). The results of this study support the proposed role of on- and off-cells in descending inhibition of nociception from the periaqueductal gray and implicate the ventrolateral orbital cortex in the control of this pathway.

Neurofibrillary tangles in chronic alcoholics

Magnocellular neurons in the cholinergic nucleus basalis appear to be vulnerable in a variety of pathological conditions, including chronic alcoholism. While neurofibrillary degeneration of these neurons has been noted in a number of disorders characterized by dementia, the mechanism of cell death in thiamine-deficient chronic alcoholics has not been identified. In the present post-mortem investigation, multiple brain regions of seven thiamine-deficient chronic alcoholics, three neurologically asymptomatic chronic alcoholics and seven non-alcoholic age matched controls were screened for neurofibrillary pathology using both tau-immunohistochemistry and a modified Bielschowsky silver stain. In chronic alcoholics with thiamine deficiency, neurofibrillary pathology was found in the nucleus basalis, but not any other brain region. Neurofibrillary tangles were not seen in age-matched controls and were infrequent in alcoholics without neuropathological signs of thiamine-deficiency. Neurofibrillary tangles were most numerous in those cases with cell loss in the nucleus basalis. These findings suggest that neurodegeneration of the nucleus basalis in chronic alcoholics proceeds through the formation of neurofibrillary tangles.

Neuronal activity in the peribrachial area: relationship to behavioral state control

Extensive studies have ascribed a role to the brainstem cholinergic system in the generation of rapid eye movement (REM) sleep and ponto-geniculo-occipital (PGO) waves. Much of this work stems from systemic and central cholinergic drug administration studies. The brainstem cholinergic system is also implicated in cortical activation via basal forebrain, thalamic, and hypothalamic relay neurons. This cholinergic ascending reticular activating hypothesis has also been suggested by in vivo experiments under anesthetics and by in vitro studies using cholinergic agonists in thalamic and hypothalamic slices. During the last ten years, brainstem cholinergic neurons have been discovered to be in the peribrachial area (PBL). With the discovery of PBL cholinergic neurons, many studies were devoted to the examination of PBL neuronal activity and their connectivity. This article reviews PBL neuronal activity in behaving animals and the anatomical features of these neurons in relation to behavioral state control. The role of the PBL in the generation of REM sleep, PGO waves, and the ascending reticular activating system (ARAS) has been evaluated at the cellular and neurochemical level. Based on recent literature, tentative mechanisms of REM sleep generation, PGO waves generation, and the cortical activation process are also outlined.

Architectonic subdivisions of the motor thalamus of owl monkeys: Nissl, acetylcholinesterase, and cytochrome oxidase patterns

As the first part of an investigation of the motor thalamus and its cortical connections in the owl monkey, a New World anthropoid primate, we studied thalamic architecture by using stains for Nissl, acetylcholinesterase (AChE), and cytochrome oxidase (CO), in order to identify subdivisions of the ventrolateral thalamic region as well as other nuclei with motor connections. Material was obtained from brains cut in the frontal, horizontal, and parasagittal planes. Our results indicate that the ventrolateral thalamic region (VL) of owl monkeys is a heterogeneous structure composed of several architectonic subdivisions that resemble divisions that have been described in macaques and other Old World anthropoids. All of these subdivisions are more readily distinguished in AChE than in Nissl or CO preparations. The anterior part of VL, VLa (VLo of Olszewski), is characterized by clusters of medium-sized, darkly stained neurons. VLa is also distinguished by AChE-positive cells embedded in a matrix of neurites as well as by a characteristic dark, irregular net of blood vessels. The posterior part of VL is rather uniform cytoarchitectonically and contains large, darkly stained, and sparsely distributed neurons. However, we were able to distinguish three subdivisions of posterior VL that closely correspond to structures described by Olszewski in macaques: a principal segment, VLp (VPLo of Olszewski), a medial segment, VLx ("area X" of Olszewski), and a dorsal segment, VLd (VLc and VLps of Olszewski). In AChE, VLd is much darker than the other divisions. The distinction between VLp and VLx, which together make up the largest part of VL, is less marked, although VLp is somewhat darker and more irregular in appearance in AChE than is VLx.

The organization of the basal ganglia-thalamocortical circuits: open interconnected rather than closed segregated

Anatomical findings in primates and rodents have led to a description of several parallel segregated basal ganglia-thalamocortical circuits leading from a distinct frontocortical area, via separate regions in the basal ganglia and the thalamus, back to the frontocortical area from which the circuit originates. One of the questions raised by the concept of parallelism is whether and how the different circuits interact. The present Commentary proposes that interaction is inherent in the neural architecture of the basal ganglia-thalamocortical circuits. This proposal is based on the re-examination of the data on the topographical organization of the frontocortical-basal ganglia connections which indicates that each circuit-engaged striatal region sends divergent projections to parts of both substantia nigra pars reticulata and the internal segment of the globus pallidus (each ventral striatal region sends divergent projections to parts of ventral pallidum, substantia nigra pars reticulata and globus pallidus), and this segregation is maintained at subsequent thalamic and frontocortical levels. This results in an asymmetry in the frontal cortex-basal ganglia relationships, so that while each frontocortical subfield innervates one striatal region, each striatal region influences the basal ganglia output to two frontocortical subfields. Because of this asymmetry, at least one of the frontocortical targets of a given circuit-engaged striatal region is not the source of its frontocortical input. Since this organization is inconsistent with an arrangement in closed segregated circuits we introduce the concept of a "split circuit". A split circuit emanates from one frontocortical area, but terminates in two frontocortical areas. Thus, a split circuit contains at least one "open" striato-fronto-cortical pathway, that leads from a circuit-engaged striatal region to a frontocortical area which is a source of a different circuit. In this manner split circuits are interconnected via their open pathways. The second striato-fronto-cortical pathway of a split circuit can be another open pathway, or it can re-enter the frontocortical area of origin, forming a closed circuit. On the basis of the available anatomical data we tentatively identified a motor, an associative, and a limbic split circuit, each containing a closed circuit and an open pathway. The motor split circuit contains a closed motor circuit that re-enters the motor and premotor cortical areas and an open motor pathway that terminates in the associative prefrontal cortex. The associative split circuit contains a closed associative circuit that re-enters the associative prefrontal cortex and an open associative pathway that terminates in the premotor cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Contralateral thalamic projections predominantly reach transitional cortices in the rhesus monkey

Connections between the thalamus and the cortex are generally regarded as ipsilateral, even though contralateral connections exist as well in several adult mammalian species. It is not known, however, whether contralateral thalamocortical projections reach particular cortices or whether they emanate from specific nuclei. In the rhesus monkey different types of cortices, ranging from transitional to eulaminate, vary in their cortical connectional pattern and may also differ in their thalamic connections. Because olfactory and transitional prefrontal cortices receive widespread projections, we investigated whether they are the target of projections from the contralateral thalamus as well. With the aid of retrograde tracers, we studied the thalamic projections of primary olfactory (olfactory tubercle and prepiriform cortex) and transitional orbital (areas PAII, Pro, 13) and medial (areas 25, 24, 32) areas, and of eulaminate (areas 11, 12, 9) cortices for comparison. To determine the prevalence of neurons in the contralateral thalamus, we compared them with the ipsilateral in each case. The pattern of ipsilateral thalamic projections differed somewhat among orbital, medial, and olfactory cortices. The mediodorsal nucleus was the predominant source of projections to orbital areas, midline nuclei included consistently about 25% of the thalamic neurons directed to medial transitional cortices, and primary olfactory areas were distinguished by receiving thalamic projections predominantly from neurons in midline and intralaminar nuclei. Notwithstanding some broad differences in the ipsilateral thalamofrontal projections, which appeared to depend on cortical location, the pattern of contralateral projections was consistent with cortical type rather than location. Labeled neurons in the contralateral thalamus were noted in midline, the magnocellular sector of the mediodorsal nucleus, the anterior medial and intralaminar nuclei, and ranged from 0 to 14% of the ipsilateral; they were directed primarily to olfactory and transitional orbital and medial cortices but rarely projected to eulaminate areas. Several thalamic nuclei projected from both sides to olfactory and transitional areas, but issued only ipsilateral projections to eulaminate areas. Though ipsilateral thalamocortical projections predominate in adult mammalian species, crossed projections are a common feature in development. The results suggest differences in the persistence of contralateral thalamocortical interactions between transitional and eulaminate cortices.

Organization of the output of the ventral striatopallidal system in the rat: ventral pallidal efferents

The efferent projections of the ventral pallidum in the rat were studied using anterograde tracing of Phaseolus vulgaris-leucoagglutinin and retrograde tracing of choleratoxin subunit B. The main aim of this study was to determine the degree of topographical organization in the outputs of the ventral pallidum. In the telencephalon, ventral pallidal fibers reach the prefrontal cortex, the ventral striatum, the lateral septum, the basolateral, lateral, and central amygdaloid nuclei, and the lateral entorhinal area. Diencephalic targets of ventral pallidal fibers are the lateral hypothalamus, the reticular nucleus of the thalamus, the mediodorsal thalamic nucleus, the dorsomedial part of the subthalamic nucleus, the medial part of the parafascicular nucleus and the lateral habenula. In the mesencephalon, ventral pallidal fibers terminate in the ventral tegmental area, the substantia nigra, the retrorubral area, the median raphe nucleus, the nucleus raphe magnus, the peribrachial area, the ventromedial part of the central gray substance and the locus coeruleus. The results of the experiments in which retrograde tracers were injected in different nuclei in the mesencephalon allow the distinction of two main areas in the ventral pallidum. Deposits of retrograde tracers in the substantia nigra, pars reticulata result in labeling of cells in the dorsolateral part of the ventral pallidum, located immediately ventral to the anterior limb of the anterior commissure. Retrograde tracer injections in other targets of the ventral mesencephalon, i.e. the dopaminergic cell groups A10, A9 or A8, or nuclei in the peribrachial area result in labeling of neurons in an extensive ventromedial and ventrolateral zone of the ventral pallidum. The medial part of this ventral pallidal zone projects to the ventral tegmental area, whereas ventral and lateral parts connect with more lateral and caudal mesencephalic targets. The projections from the ventral pallidum to the ventral striatum, the subthalamic nucleus and adjacent lateral hypothalamic area, and the mediodorsal thalamic nucleus are distinctly topographically organized. The ventral pallidostriatal projections preserve a medial-to-lateral, a dorsal-to-ventral and, to a lesser degree, a rostral-to-caudal topography. With respect to the subthalamic region, the dorsolateral part of the ventral pallidum projects to the dorsomedial part of the subthalamic nucleus, whereas the ventromedial and ventrolateral parts of the ventral pallidum are topographically connected with the area of the lateral hypothalamus medially adjacent to the subthalamic nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending efferent connections of the sub-pallidal areas in the cat: projections to the subthalamic nucleus, the hypothalamus, and the midbrain

The efferent connections of the sub-pallidal regions to the mediodorsal thalamic nucleus, the subthalamic nucleus, the lateral hypothalamic area, and the midbrain were investigated in the cat, using Phaseolus vulgaris--leucoagglutinin (PHA-L) as an anterograde label. The results indicate that the sub-pallidal regions of the cat project to the (dorso)medial tip of the subthalamic nucleus and the adjoining lateral hypothalamic area as well as to the ventral tegmental area and the greater extent of the dorsolateral tier of the substantia nigra pars compacta. Extensive projections were also found to the peripeduncular nucleus. The central gray as well as the mesencephalic locomotor region receive some input from the basal forebrain too. In contrast only very limited projections were found to the mediodorsal thalamic nucleus. The results are discussed in view of the possible role of these output regions in oro-facial dyskinesia.

The neurons of the human magnocellular septal nuclei: a Golgi study

The morphology of neurons in the magnocellular septal nuclei (medial septal and diagonal band nucleus) were studied in frontal sections of 15 human brains by means of the Golgi method. We classified neurons in the diagonal band nucleus according to their size and morphology into four types: type I--multipolar neurons, type II--fusiform neurons, type III--triangular neurons and type IV--fusiform multipolar neurons. The neurons of the medial septal nucleus we classified into two types: type I--multipolar neurons and type II--fusiform neurons. Our results indicated greater morphological variability of neurons in the human diagonal band nucleus than in the medial septal nucleus.

The organization of the descending ventral pallidal projections in the monkey

This study describes the organization and topography of the descending efferent projections from the monkey ventral pallidum. The main efferent projections from the globus pallidus are to the subthalamus, to the thalamus, and to the substantia nigra. Although these projections have been well established for the dorsal pallidum, the projections of the ventral pallidum have not been explored in primates. The results of this study add an important link in how information from the limbic lobe is channeled through the basal ganglia in monkeys. Anterograde tracers, Phaseolus vulgaris-leucoagglutinin, and tritiated amino acids were injected into various regions of the ventral pallidum. The descending efferent projection from the ventral pallidum in monkeys terminates primarily in the subthalamic nucleus and adjacent lateral hypothalamus, in the substantia nigra, and in the lateral habenular nucleus. Although terminals are also found in the thalamus, these are relatively sparse. The projections to the subthalamic nucleus and the lateral hypothalamus are topographically arranged, while those to the substantia nigra are not. These results suggest that pathways from distinct pallidal regions that receive specific striatal input terminate in distinct regions of the subthalamic/hypothalamic regions, thus maintaining a topographic arrangement. Projections to the substantia nigra, however, overlap extensively, suggesting convergence of terminals from different ventral pallidal regions. The relatively small projection to the thalamus raises the question that without a prominent thalamic projection, is this system parallel to that described for the dorsal globus pallidus?

Cholinergic innervation of the mediodorsal thalamic nucleus in the monkey: ultrastructural evidence supportive of functional diversity

The ultrastructural organization of association nuclei in the primate thalamus is largely unexplored. In the present study we have combined electron microscopy with immunocytochemistry for the acetylcholine synthesizing enzyme choline acetyltransferase (ChAT) to assess the cholinergic synaptic organization of the mediodorsal (MD) nucleus in macaque monkeys. The cholinergic innervation of the MD nucleus showed striking regional variations with the greatest density of immunoreactive axons and varicosities found within the parvicellular division. Electron microscopic examination revealed that these ChAT immunoreactive (ChAT-IR) axons were primarily small and unmyelinated. The majority of immunoreactive synaptic profiles were found within the extraglomerular neuropil (80.5%), with the remainder present in glomerular regions. Within the glomerular and extra-glomerular neuropil ChAT-IR profiles made contact with both conventional, presumably relay cell dendrites (CD), as well as with synaptic vesicle containing dendrites (SVCD) of local circuit neurons. In the glomeruli the frequency of synapses was approximately equal for CDs and SVCDs while in the extraglomerular areas 75% of the synaptic contacts were with CDs. ChAT-IR synaptic profiles had a diversity of junctional complex morphologies. Within glomeruli they made symmetric synapses with CDs and predominantly asymmetric with SVCDs. The majority of extraglomerular contacts (60%) were classified as asymmetric and these as well as the smaller number of symmetric synapses contacted both CDs and SVCDs. In accord with results of physiological studies, these anatomical data indicate that cholinergic input to thalamic nuclei influences relay cell activity both directly and indirectly via local circuit neurons.

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.

The organization of the thalamocortical connections of the mediodorsal thalamic nucleus in the rat, related to the ventral forebrain-prefrontal cortex topography

The medial and central segments of the mediodorsal nucleus of the thalamus (MD) receive afferents from the ventral forebrain, including the piriform cortex, the ventral pallidum, and the amygdaloid complex. Because MD is reciprocally interconnected with prefrontal and agranular insular cortical areas, it provides a relay of ventral forebrain activity to these cortical areas. However, there are also direct projections from the piriform cortex and the amygdala to the prefrontal and agranular insular cortices. This study addresses whether this system has a "triangular" organization, such that structures in the ventral forebrain project to interconnected areas in MD and the prefrontal/insular cortex. The thalamocortical projections of MD have been studied in experiments with injections of retrograde tracers into prefrontal or agranular insular cortical areas. In many of the same experiments, projections from the ventral forebrain to MD and to the prefrontal/insular cortex have been demonstrated with anterograde axonal tracers. The connections of the piriform cortex (PC) with MD and the prefrontal/insular cortex form an organized triangular system. The PC projections to the central and medial segments of MD and to the lateral orbital cortex (LO) and the ventral and posterior agranular insular cortices (AIv and AIp) are topographically organized, such that more caudal parts of PC tend to project more medially in MD and more caudally within the orbital/insular cortex. The central and medial portions of MD also send matching, topographically organized projections to LO, AIv and AIp, with more medial parts of MD projecting further caudally. The anterior cortical nucleus of the amygdala (COa) also projects to the dorsal part of the medial segment of MD and to its cortical targets, the medial orbital area (MO) and AIp. The projections of the basal/accessory basal amygdaloid nuclei to MD and to prefrontal cortex, and from MD to amygdaloceptive parts of prefrontal cortex, are not as tightly organized. Amygdalothalamic afferents in MD are concentrated in the dorsal half of the medial segment. Cells in this part of the nucleus project to the amygdaloceptive prelimbic area (PL) and AIp. However, other amygdaloceptive prefrontal areas are connected to parts of MD that do not receive fibers from the amygdala. Ventral pallidal afferents are distributed to all parts of the central and medial segments of MD, overlapping with the fibers from the amygdala and piriform cortex. Fibers from other parts of the pallidum, or related areas such as the substantia nigra, pars reticulata, terminate in the lateral and ventral parts of MD, where they overlap with inputs from the superior colliculus and other brainstem structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.

Specificity in the projection patterns of accumbal core and shell in the rat

The efferent projections of the core and shell areas of the nucleus accumbens were studied with a combination of anterograde and retrograde tract-tracing methods, including Phaseolus vulgaris-leucoagglutinin, horseradish peroxidase and fluorescent tracers. Both the core and shell regions project to pallidal areas, i.e. ventral pallidum and entopeduncular nucleus, with a distinct topography in the sense that the core projection is located in the dorsolateral part of ventral pallidum, whereas the shell projects to the medial part of the subcommissural ventral pallidum. Both regions of the accumbens also project to mesencephalon with a bias for the core projection to innervate the substantia nigra-lateral mesencephalic tegmentum, and for the shell projection to reach primarily the ventral tegmental-paramedian tegmentum area. The most pronounced differences between core and shell projections exist in regard to the hypothalamus and extended amygdala. Whereas the core projects primarily to the entopeduncular nucleus including a part that invades the lateral hypothalamus, the shell, in addition, projects diffusely throughout the rostrocaudal extent of the lateral hypothalamus as well as to the extended amygdala, especially its sublenticular part. Both the core and shell of the accumbens have unmistakable striatal characteristics both histologically and in their connectional patterns. The shell, however, has additional features that are reminiscent of the recently described extended amygdala [Alheid G.F. and Heimer L. (1988) Neuroscience 27, 1-39; de Olmos J.S. et al. (1985) In The Rat Nervous System, pp. 223-334]; in fact, the possibility exists that the shell represents a transitional zone that seems to characterize most of the fringes of the striatal complex, where it adjoins the extended amygdala.

The relationship between ventral striatal efferent fibers and the distribution of peptide-positive woolly fibers in the forebrain of the rhesus monkey

Peptidergic fibers in the globus pallidus of the monkey appear in the morphological form referred to as woolly fibers. These fibers are composed of a dense plexus of thin beaded axons which ensheath an unstained central core. Such structures are not confined to the globus pallidus, but are also present in the bed nucleus of the stria terminalis, the hypothalamus, the dorsal part of the amygdala, and ventrally in the basal forebrain. The present study describes the relationship between projections from the rostral and ventral striatum and the enkephalin- and substance P-positive woolly fibers. Following injections of either tritiated amino acids or the lectin Phaseolus vulgaris-leucoagglutinin in the ventral striatum, anterogradely labeled fibers and terminals in the forebrain were visualized simultaneously with enkephalin- or substance P immunoreactivity in the same tissue section in order to determine: (i) the extent to which the woolly fiber distribution represents striatal output systems; (ii) whether woolly fibers can be considered as a marker for the entire striatal forebrain projection; and (iii) whether enkephalin and substance P are involved differentially in distinct ventral striatopallidal pathways. Phaseolus vulgaris-leucoagglutinin labeling is seen in the globus pallidus and adjacent structures either as single, beaded fibers or in a profile strikingly similar to that of woolly fibers. In tissue sections treated for a double immunohistochemical protocol, following which the Phaseolus vulgaris-leucoagglutinin-immunoreactive fibers turn black and the peptidergic woolly fibers brown; many of the lectin-positive fibers are seen to enter the peptide-positive woolly fiber plexus. Likewise, following the injections with tritiated amino acids in the ventral striatum, coarse structures that have dimensions resembling those of the woolly fibers are identified. In sections immunohistochemically stained and subsequently treated for autoradiography, peptide-positive woolly fibers can be identified underlying the silver grains. In sections stained for both peptide immunoreactivity and tracer substances, enkephalin or substance P-positive woolly fibers are present in all pallidal regions that receive ventral striatal input. However, the ventral striatum also sends fibers to the hypothalamus, bed nucleus of the stria terminalis, the dorsal part of the amygdala, the septum, the preoptic area, and other areas of the basal forebrain. In these nuclei the peptide-positive woolly fiber distribution is less extensive than the terminal labeling. The distribution of substance P-positive fibers in the subcommissural pallidal region is more limited than the distribution of enkephalinergic fibers.(ABSTRACT TRUNCATED AT 400 WORDS)

Basal forebrain and mesopontine tegmental projections to the reticular thalamic nucleus: an axonal collateralization and immunohistochemical study in the rat

Using a double fluorescence retrograde labeling procedure, the present study sought to determine the degree to which basal forebrain and mesopontine tegmental neurons have axons that innervate both the reticular thalamic nucleus and the cerebral cortex. Immunofluorescence for choline acetyltransferase, somatostatin, and the calcium-binding protein parvalbumin was also performed to elucidate the neurochemical identity of basal forebrain and mesopontine tegmental inputs to the reticular thalamic nucleus. A significant portion (10-15%) of neurons in the basal forebrain and mesopontine tegmentum that were retrogradely labeled from the reticular thalamic nucleus were also found to be retrogradely labeled from the cortex. Many of these neurons stained positively for choline acetyltransferase. Of the basal forebrain neurons retrogradely labeled from the reticular thalamic nucleus, approximately 20% were found to be immunoreactive to choline acetyltransferase, whereas none was stained for somatostatin. A larger portion (up to 50%) of the basal forebrain neurons that were retrogradely labeled from the reticular thalamic nucleus were parvalbumin-immunoreactive, and some of these were also retrogradely labeled from the cortex. These results suggest that a subpopulation of cholinergic and non-cholinergic neurons in the basal forebrain and the mesopontine tegmentum may influence simultaneously the activity of neurons in the reticular thalamic nucleus and the cerebral cortex.