The dorsal tegmental nucleus: an axoplasmic transport study

Angular Head Velocity Cells within Brainstem Nuclei Projecting to the Head Direction Circuit

The sense of orientation of an animal is derived from the head direction (HD) system found in several limbic structures and depends on an intact vestibular labyrinth. However, how the vestibular system influences the generation and updating of the HD signal remains poorly understood. Anatomical and lesion studies point toward three key brainstem nuclei as key components for generating the HD signal-nucleus prepositus hypoglossi, supragenual nucleus, and dorsal paragigantocellularis reticular nuclei. Collectively, these nuclei are situated between the vestibular nuclei and the dorsal tegmental and lateral mammillary nuclei, which are thought to serve as the origin of the HD signal. To determine the types of information these brain areas convey to the HD network, we recorded neurons from these regions while female rats actively foraged in a cylindrical enclosure or were restrained and rotated passively. During foraging, a large subset of cells in all three nuclei exhibited activity that correlated with the angular head velocity (AHV) of the rat. Two fundamental types of AHV cells were observed; (1) symmetrical AHV cells increased or decreased their firing with increases in AHV regardless of the direction of rotation, and (2) asymmetrical AHV cells responded differentially to clockwise and counterclockwise head rotations. When rats were passively rotated, some AHV cells remained sensitive to AHV, whereas firing was attenuated in other cells. In addition, a large number of AHV cells were modulated by linear head velocity. These results indicate the types of information conveyed from the vestibular nuclei that are responsible for generating the HD signal.SIGNIFICANCE STATEMENT Extracellular recording of brainstem nuclei (nucleus prepositus hypoglossi, supragenual nucleus, and dorsal paragigantocellularis reticular nucleus) that project to the head direction circuit identified different types of AHV cells while rats freely foraged in a cylindrical environment. The firing of many cells was also modulated by linear velocity. When rats were restrained and passively rotated, some cells remained sensitive to AHV, whereas others had attenuated firing. These brainstem nuclei provide critical information about the rotational movement of the head of the rat in the azimuthal plane.

Neural dynamics and architecture of the heading direction circuit in zebrafish

Animals generate neural representations of their heading direction. Notably, in insects, heading direction is topographically represented by the activity of neurons in the central complex. Although head direction cells have been found in vertebrates, the connectivity that endows them with their properties is unknown. Using volumetric lightsheet imaging, we find a topographical representation of heading direction in a neuronal network in the zebrafish anterior hindbrain, where a sinusoidal bump of activity rotates following directional swims of the fish and is otherwise stable over many seconds. Electron microscopy reconstructions show that, although the cell bodies are located in a dorsal region, these neurons arborize in the interpeduncular nucleus, where reciprocal inhibitory connectivity stabilizes the ring attractor network that encodes heading. These neurons resemble those found in the fly central complex, showing that similar circuit architecture principles may underlie the representation of heading direction across the animal kingdom and paving the way to an unprecedented mechanistic understanding of these networks in vertebrates.

Angular head velocity cells within brainstem nuclei projecting to the head direction circuit

An animal's perceived sense of orientation depends upon the head direction (HD) system found in several limbic structures and depends upon an intact peripheral vestibular labyrinth. However, how the vestibular system influences the generation, maintenance, and updating of the HD signal remains poorly understood. Anatomical and lesion studies point towards three key brainstem nuclei as being potential critical components in generating the HD signal: nucleus prepositus hypoglossi (NPH), supragenual nucleus (SGN), and dorsal paragigantocellularis reticular nuclei (PGRNd). Collectively, these nuclei are situated between the vestibular nuclei and the dorsal tegmental and lateral mammillary nuclei, which are thought to serve as the origin of the HD signal. To test this hypothesis, extracellular recordings were made in these areas while rats either freely foraged in a cylindrical environment or were restrained and rotated passively. During foraging, a large subset of cells in all three nuclei exhibited activity that correlated with changes in the rat's angular head velocity (AHV). Two fundamental types of AHV cells were observed: 1) symmetrical AHV cells increased or decreased their neural firing with increases in AHV regardless of the direction of rotation; 2) asymmetrical AHV cells responded differentially to clockwise (CW) and counter-clockwise (CCW) head rotations. When rats were passively rotated, some AHV cells remained sensitive to AHV whereas others had attenuated firing. In addition, a large number of AHV cells were modulated by linear head velocity. These results indicate the types of information conveyed in the ascending vestibular pathways that are responsible for generating the HD signal.

Significance Statement

Extracellular recording of brainstem nuclei (nucleus prepositus hypoglossi, supragenual nucleus, and dorsal paragigantocellularis reticular nucleus) that project to the head direction circuit identified different types of angular head velocity (AHV) cells while rats freely foraged in a cylindrical environment. The firing of many cells was also modulated by linear velocity. When rats were restrained and passively rotated some cells remained sensitive to AHV, whereas others had attenuated firing. These brainstem nuclei provide critical information about the rotational movement of the rat's head in the azimuthal plane.

Anatomical projections to the dorsal tegmental nucleus and abducens nucleus arise from separate cell populations in the nucleus prepositus hypoglossi, but overlapping cell populations in the medial vestibular nucleus

Specialized circuitry in the brain processes spatial information to provide a sense of direction used for navigation. The dorsal tegmental nucleus (DTN) is a core component of this circuitry and utilizes vestibular inputs to generate neural activity encoding the animal's directional heading. Projections arising from the nucleus prepositus hypoglossi (NPH) and the medial vestibular nucleus (MVe) are thought to transmit critical vestibular signals to the DTN and other brain areas, including the abducens nucleus (ABN), a component of eye movement circuitry. Here, we utilized a dual retrograde tracer approach in rats to investigate whether overlapping or distinct populations of neurons project from the NPH or MVe to the DTN and ABN. We report that individual MVe neurons project to both the DTN and ABN. In contrast, we observed individual NPH neurons that project to either the DTN or ABN, but rarely to both structures simultaneously. We also examined labeling patterns in other structures located in the brainstem and posterior cortex and observed (1) complex patterns of interhemispheric connectivity between the left and right DTN, (2) projections from the supragenual nucleus, interpeduncular nucleus, and retrosplenial cortex to the DTN, (3) projections from the lateral superior olive to the ABN, and (4) a unique population of cerebrospinal fluid-contacting neurons in the dorsal raphe nucleus. Collectively, our experiments provide valuable new information that extends our understanding of the anatomical organization of the brain's spatial processing circuitry.

The antitussive cloperastine improves breathing abnormalities in a Rett Syndrome mouse model by blocking presynaptic GIRK channels and enhancing GABA release

Rett Syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene. One of the major RTT features is breathing dysfunction characterized by periodic hypo- and hyperventilation. The breathing disorders are associated with increased brainstem neuronal excitability, which can be alleviated with GABA agonists. Since neuronal hypoexcitability occurs in the forebrain of RTT models, it is necessary to find pharmacological agents with a relative preference to brainstem neurons. Here we show evidence for the improvement of breathing disorders of Mecp2-disrupted mice with the brainstem-acting drug cloperastine (CPS) and its likely neuronal targets. CPS is an over-the-counter cough medicine that has an inhibitory effect on brainstem neuronal networks. In Mecp2-disrupted mice, CPS (30 mg/kg, i.p.) decreased the occurrence of apneas/h and breath frequency variation. GIRK currents expressed in HEK cells were inhibited by CPS with IC₅₀ 1 μM. Whole-cell patch clamp recordings in locus coeruleus (LC) and dorsal tegmental nucleus (DTN) neurons revealed an overall inhibitory effect of CPS (10 μM) on neuronal firing activity. Such an effect was reversed by the GABA_A receptor antagonist bicuculline (20 μM). Voltage clamp studies showed that CPS increased GABAergic sIPSCs in LC cells, which was blocked by the GABA_B receptor antagonist phaclofen. Functional GABAergic connections of DTN neurons with LC cells were shown. These results suggest that CPS improves breathing dysfunction in Mecp2-null mice by blocking GIRK channels in synaptic terminals and enhancing GABA release.

Functional and anatomical relationships between the medial precentral cortex, dorsal striatum, and head direction cell circuitry. II. Neuroanatomical studies

An animal's directional heading within its environment is encoded by the activity of head direction (HD) cells. In rodents, these neurons are found primarily within the limbic system in the interconnected structures that form the limbic HD circuit. In our accompanying report in this issue, we describe two HD cell populations located outside of this circuit in the medial precentral cortex (PrCM) and dorsal striatum (DS). These extralimbic areas receive their HD signals from the limbic system but do not provide critical input or feedback to limbic HD cells (Mehlman ML, Winter SS, Valerio S, Taube JS. J Neurophysiol 121: 350-370, 2019.). In this report, we complement our previous lesion and recording experiments with a series of neuroanatomical tracing studies in rats designed to examine patterns of connectivity between the PrCM, DS, limbic HD circuit, and related spatial processing circuitry. Retrograde tracing revealed that the DS receives direct input from numerous structures known to contain HD cells and/or other spatially tuned cell types. Importantly, these projections preferentially target and converge within the most medial portion of the DS, the same area in which we previously recorded HD cells. The PrCM receives direct input from a subset of these spatial processing structures. Anterograde tracing identified indirect pathways that could permit the PrCM and DS to convey self-motion information to the limbic HD circuit. These tracing studies reveal the anatomical basis for the functional relationships observed in our lesion and recording experiments. Collectively, these findings expand our understanding of how spatial processing circuitry functionally and anatomically extends beyond the limbic system into the PrCM and DS. NEW & NOTEWORTHY Head direction (HD) cells are located primarily within the limbic system, but small populations of extralimbic HD cells are found in the medial precentral cortex (PrCM) and dorsal striatum (DS). The neuroanatomical tracing experiments reported here explored the pathways capable of transmitting the HD signal to these extralimbic areas. We found that projections arising from numerous spatial processing structures converge within portions of the PrCM and DS that contain HD cells.

The postrhinal cortex is not necessary for landmark control in rat head direction cells

The rodent postrhinal cortex (POR), homologous to primate areas TH/TF and the human 'parahippocampal place area', has been implicated in processing visual landmark and contextual information about the environment. Head direction (HD) cells are neurons that encode allocentric head direction, independent of the animal's location or behavior, and are influenced by manipulations of visual landmarks. The present study determined whether the POR plays a role in processing environmental information within the HD circuit. Experiment 1 tested the role of the POR in processing visual landmark cues in the HD system during manipulation of a visual cue. HD cells from POR lesioned animals had similar firing properties, shifted their preferred firing direction following rotation of a salient visual cue, and in darkness had preferred firing directions that drifted at the same rate as controls. Experiment 2 tested the PORs involvement in contextual fear conditioning, where the animal learns to associate a shock with both a tone and a context in which the shock was given. In agreement with previous studies, POR lesioned animals were able to learn the tone-shock pairing, but displayed less freezing relative to controls when reintroduced into the environment previously paired with a shock. Therefore, HD cells from POR lesioned animals, with demonstrated impairments in contextual fear conditioning, were able to use a visual landmark to control their preferred direction. Thus, despite its importance in processing visual landmark information in primates, the POR in rats does not appear to play a pivotal role in controlling visual landmark information in the HD system. © 2016 Wiley Periodicals, Inc.

The head direction cell system and behavior: The effects of lesions to the lateral mammillary bodies on spatial memory in a novel landmark task and in the water maze

The head direction system is composed of neurons found in a number of connected brain areas that fire in a sharply tuned, directional way. The function of this system, however, has not been fully established. To assess this, we devised a novel spatial landmark task, comparable to the paradigms in which stimulus control has been assessed for spatially tuned neurons. The task took place in a large cylinder and required rats to dig in a specific sand cup, from among 16 alternatives, to obtain a food reward. The reinforced cup was in a fixed location relative to a salient landmark, and probe sessions confirmed that the landmark exerted stimulus control over the rats’ cup choices. To assess the contribution of the head direction cell system to this memory task, half of the animals received ibotenic acid infusions into the lateral mammillary nuclei (LMN), an essential node in the head direction network, while the other received sham lesions. No differences were observed in performance of this task between the 2 groups. Animals with LMN lesions were impaired, however, in reversal learning on a water maze task. These results suggest that the LMN, and potentially the head direction cell system, are not essential for the use of visual landmarks to guide spatial behavior.

Conserved expression of the GPR151 receptor in habenular axonal projections of vertebrates

The habenula is a phylogenetically conserved brain structure in the epithalamus. It is a major node in the information flow between fronto-limbic brain regions and monoaminergic brainstem nuclei, and is thus anatomically and functionally ideally positioned to regulate emotional, motivational, and cognitive behaviors. Consequently, the habenula may be critically important in the pathophysiology of psychiatric disorders such as addiction and depression. Here we investigated the expression pattern of GPR151, a G protein-coupled receptor (GPCR), whose mRNA has been identified as highly and specifically enriched in habenular neurons by in situ hybridization and translating ribosome affinity purification (TRAP). In the present immunohistochemical study we demonstrate a pronounced and highly specific expression of the GPR151 protein in the medial and lateral habenula of rodent brain. Specific expression was also seen in efferent habenular fibers projecting to the interpeduncular nucleus, the rostromedial tegmental area, the rhabdoid nucleus, the mesencephalic raphe nuclei, and the dorsal tegmental nucleus. Using confocal microscopy and quantitative colocalization analysis, we found that GPR151-expressing axons and terminals overlap with cholinergic, substance P-ergic, and glutamatergic markers. Virtually identical expression patterns were observed in rat, mouse, and zebrafish brains. Our data demonstrate that GPR151 is highly conserved, specific for a subdivision of the habenular neurocircuitry, and constitutes a promising novel target for psychiatric drug development.

The nucleus prepositus hypoglossi contributes to head direction cell stability in rats

Head direction (HD) cells in the rat limbic system fire according to the animal's orientation independently of the animal's environmental location or behavior. These HD cells receive strong inputs from the vestibular system, among other areas, as evidenced by disruption of their directional firing after lesions or inactivation of vestibular inputs. Two brainstem nuclei, the supragenual nucleus (SGN) and nucleus prepositus hypoglossi (NPH), are known to project to the HD network and are thought to be possible relays of vestibular information. Previous work has shown that lesioning the SGN leads to a loss of spatial tuning in downstream HD cells, but the NPH has historically been defined as an oculomotor nuclei and therefore its role in contributing to the HD signal is less clear. Here, we investigated this role by recording HD cells in the anterior thalamus after either neurotoxic or electrolytic lesions of the NPH. There was a total loss of direction-specific firing in anterodorsal thalamus cells in animals with complete NPH lesions. However, many cells were identified that fired in bursts unrelated to the animals' directional heading and were similar to cells seen in previous studies that damaged vestibular-associated areas. Some animals with significant but incomplete lesions of the NPH had HD cells that were stable under normal conditions, but were unstable under conditions designed to minimize the use of external cues. These results support the hypothesis that the NPH, beyond its traditional oculomotor function, plays a critical role in conveying vestibular-related information to the HD circuit.

Vestibular pathways involved in cognition

Recent discoveries have emphasized the role of the vestibular system in cognitive processes such as memory, spatial navigation and bodily self-consciousness. A precise understanding of the vestibular pathways involved is essential to understand the consequences of vestibular diseases for cognition, as well as develop therapeutic strategies to facilitate recovery. The knowledge of the “vestibular cortical projection areas”, defined as the cortical areas activated by vestibular stimulation, has dramatically increased over the last several years from both anatomical and functional points of view. Four major pathways have been hypothesized to transmit vestibular information to the vestibular cortex: (1) the vestibulo-thalamo-cortical pathway, which probably transmits spatial information about the environment via the parietal, entorhinal and perirhinal cortices to the hippocampus and is associated with spatial representation and self-versus object motion distinctions; (2) the pathway from the dorsal tegmental nucleus via the lateral mammillary nucleus, the anterodorsal nucleus of the thalamus to the entorhinal cortex, which transmits information for estimations of head direction; (3) the pathway via the nucleus reticularis pontis oralis, the supramammillary nucleus and the medial septum to the hippocampus, which transmits information supporting hippocampal theta rhythm and memory; and (4) a possible pathway via the cerebellum, and the ventral lateral nucleus of the thalamus (perhaps to the parietal cortex), which transmits information for spatial learning. Finally a new pathway is hypothesized via the basal ganglia, potentially involved in spatial learning and spatial memory. From these pathways, progressively emerges the anatomical network of vestibular cognition.

Resolving the active versus passive conundrum for head direction cells

Head direction (HD) cells have been identified in a number of limbic system structures. These cells encode the animal's perceived directional heading in the horizontal plane and are dependent on an intact vestibular system. Previous studies have reported that the responses of vestibular neurons within the vestibular nuclei are markedly attenuated when an animal makes a volitional head turn compared to passive rotation. This finding presents a conundrum in that if vestibular responses are suppressed during an active head turn how is a vestibular signal propagated forward to drive and update the HD signal? This review identifies and discusses four possible mechanisms that could resolve this problem. These mechanisms are: (1) the ascending vestibular signal is generated by more than just vestibular-only neurons, (2) not all vestibular-only neurons contributing to the HD pathway have firing rates that are attenuated by active head turns, (3) the ascending pathway may be spared from the affects of the attenuation in that the HD system receives information from other vestibular brainstem sites that do not include vestibular-only cells, and (4) the ascending signal is affected by the inhibited vestibular signal during an active head turn, but the HD circuit compensates and uses the altered signal to accurately update the current HD. Future studies will be needed to decipher which of these possibilities is correct.

Vestibular loss as a contributor to Alzheimer's disease

Alzheimer's disease is a complex disorder whose etiology is still controversial. It is proposed that vestibular loss may contribute to the onset of Alzheimer's disease, which initially involves degeneration of cholinergic systems in the posterior parietal-temporal, medial-temporal, and posterior-cingulate regions. A major projection to this system emanates from the semicircular canals of the vestibular labyrinth, with vestibular damage leading to severe degeneration of the medial-temporal region. The vestibular loss hypothesis is further supported by the vestibular symptoms found in Alzheimer's patients as well as in various diseases that are major risk factors for Alzheimer's disease.

The nucleus prepositus predominantly outputs eye movement-related information during passive and active self-motion

Maintaining a constant representation of our heading as we move through the world requires the accurate estimate of spatial orientation. As one turns (or is turned) toward a new heading, signals from the semicircular canals are relayed through the vestibular system to higher-order centers that encode head direction. To date, there is no direct electrophysiological evidence confirming the first relay point of head-motion signals from the vestibular nuclei, but previous anatomical and lesion studies have identified the nucleus prepositus as a likely candidate. Whereas burst-tonic neurons encode only eye-movement signals during head-fixed eye motion and passive vestibular stimulation, these neurons have not been studied during self-generated movements. Here, we specifically address whether burst-tonic neurons encode head motion during active behaviors. Single-unit responses were recorded from the nucleus prepositus of rhesus monkeys and compared for head-restrained and active conditions with comparable eye velocities. We found that neurons consistently encoded eye position and velocity across conditions but did not exhibit significant sensitivity to head position or velocity. Additionally, response sensitivities varied as a function of eye velocity, similar to abducens motoneurons and consistent with their role in gaze control and stabilization. Thus our results demonstrate that the primate nucleus prepositus chiefly encodes eye movement even during active head-movement behaviors, a finding inconsistent with the proposal that this nucleus makes a direct contribution to head-direction cell tuning. Given its ascending projections, however, we speculate that this eye-movement information is integrated with other inputs in establishing higher-order spatial representations.

Head direction cell activity in the anterodorsal thalamus requires intact supragenual nuclei

Neural activity in several limbic areas varies as a function of the animal's head direction (HD) in the horizontal plane. Lesions of the vestibular periphery abolish this HD cell signal, suggesting an essential role for vestibular afference in HD signal generation. The organization of brain stem pathways conveying vestibular information to the HD circuit is poorly understood; however, recent anatomical work has identified the supragenual nucleus (SGN) as a putative relay. To test this hypothesis, we made lesions of the SGN in rats and screened for HD cells in the anterodorsal thalamus. In animals with complete bilateral lesions, the overall number of HD cells was significantly reduced relative to control animals. In animals with unilateral lesions of the SGN, directional activity was present, but the preferred firing directions of these cells were unstable and less influenced by the rotation of an environmental landmark. In addition, we found that preferred directions displayed large directional shifts when animals foraged for food in a darkened environment and when they were navigating from a familiar environment to a novel one, suggesting that the SGN plays a critical role in projecting essential self-motion (idiothetic) information to the HD cell circuit.

Nucleus incertus--an emerging modulatory role in arousal, stress and memory

A major challenge in systems neuroscience is to determine the underlying neural circuitry and associated neurotransmitters and receptors involved in psychiatric disorders, such as anxiety and depression. A focus of many of these studies has been specific brainstem nuclei that modulate levels of arousal via their ascending monoaminergic projections (e.g. the serotonergic dorsal raphé, noradrenergic locus ceruleus and cholinergic laterodorsal tegmental nucleus). After years of relative neglect, the subject of recent studies in this context has been the GABAergic nucleus incertus, which is located in the midline periventricular central gray in the 'prepontine' hindbrain, with broad projections throughout the forebrain. Nucleus incertus neurons express receptors for the stress hormone, corticotropin-releasing factor (CRF), are activated by psychological stressors, and project to key nuclei involved in stress responses and behavioral activation. The nucleus incertus is also a node in neural circuits capable of modulating hippocampal theta rhythm, which is related to control of spatial navigation and memory. A significant population of nucleus incertus neurons express the recently discovered, highly conserved neuropeptide, relaxin-3; and the recent availability of structurally-related, chimeric peptides that selectively activate or inhibit the relaxin-3 receptor, RXFP3, is facilitating studies of relaxin-3/RXFP3 networks and associated GABA and CRF systems. It is predicted that such targeted research will help elucidate the functions of ascending nucleus incertus pathways, including their possible involvement in arousal (sleep/wakefulness), stress reponses, and learning and memory; and in the pathology of related psychiatric diseases such as insomnia, anxiety and depression, and cognitive deficits.

Interspike interval analyses reveal irregular firing patterns at short, but not long, intervals in rat head direction cells

Previous studies have shown that a subset of neurons in the rat anterodorsal thalamus discharge as a function of the animal's head direction (HD) in the horizontal plane, independent of the animal's location and behavior. These cells have consistent firing properties across a wide range of conditions and cell discharge appears highly regular when listened to through a loudspeaker. In contrast, interspike interval (ISI) analyses on cortical cells have found that cell firing is irregular, even under constant stimulus conditions. Here, we analyzed HD cells from the anterodorsal thalamus, while rats foraged for food pellets, to determine whether their firing was regular or irregular. ISIs were measured when the animal's HD was maintained within ± 6° of the cell's preferred firing direction. ISIs were highly variable with a mean coefficient of variation (CV) of 0.681. For each cell, the CV values at HDs ± 24° away from the cell's preferred direction were similar to the coefficient measured at the cell's preferred direction. A second recording session showed that cells had similar coefficients of variation as the first session, suggesting that the degree of variability in cell spiking was a characteristic property for each cell. There was little correlation between ISIs and angular head velocity or translational speed. ISIs measured in HD cells from the postsubiculum and lateral mammillary nuclei showed higher CV values. These results indicate that despite the appearance of regularity in their firing, HD cells, like cortical cells, have irregular ISIs. In contrast to the irregular firing observed for ISIs, analyses over longer time intervals indicated that HD cell firing was much more regular, more nearly resembling a rate code. These findings have implications for attractor networks that model the HD signal and for models proposed to explain the generation of grid cell signals in entorhinal cortex.

Deficits in landmark navigation and path integration after lesions of the interpeduncular nucleus

Experiments were designed to determine the role of the interpeduncular nucleus (IPN) in 3 forms of navigation: beacon, landmark, and path integration. In beacon navigation, animals reach goals using cues directly associated with them, whereas in landmark navigation animals use external cues to determine a direction and distance to goals. Path integration refers to the use of self-movement cues to obtain a trajectory to a goal. IPN-lesioned rats were tested in a food-carrying task in which they searched for food in an open field, and returned to a refuge after finding the food. Landmark navigation was evaluated during trials performed under lighted conditions and path integration was tested under darkened conditions, thus eliminating external cues. We report that IPN lesions increased the number of errors and reduced heading accuracy under both lighted and darkened conditions. Tests using a Morris water maze procedure indicated that IPN lesions produced moderate impairments in the landmark version of the water task, but left beacon navigation intact. These findings suggest that the IPN plays a fundamental role in landmark navigation and path integration.

Head direction cell activity in mice: robust directional signal depends on intact otolith organs

The head direction (HD) cell signal is a representation of an animal's perceived directional heading with respect to its environment. This signal appears to originate in the vestibular system, which includes the semicircular canals and otolith organs. Preliminary studies indicate the semicircular canals provide a necessary component of the HD signal, but involvement of otolithic information in the HD signal has not been tested. The present study was designed to determine the otolithic contribution to the HD signal, as well as to compare HD cell activity of mice with that of rats. HD cell activity in the anterodorsal thalamus was assessed in wild-type C57BL/6J and otoconia-deficient tilted mice during locomotion within a cylinder containing a prominent visual landmark. HD cell firing properties in C57BL/6J mice were generally similar to those in rats. However, in C57BL/6J mice, landmark rotation failed to demonstrate dominant control of the HD signal in 36% of the sessions. In darkness, directional firing became unstable during 42% of the sessions, but landmark control was not associated with HD signal stability in darkness. HD cells were identified in tilted mice, but directional firing properties were not as robust as those of C57BL/6J mice. Most HD cells in tilted mice were controlled by landmark rotation but showed substantial signal degradation across trials. These results support current models that suggest otolithic information is involved in the perception of directional heading. Furthermore, compared with rats, the HD signal in mice appears to be less reliably anchored to prominent environmental cues.

Head direction cell instability in the anterior dorsal thalamus after lesions of the interpeduncular nucleus

Previous research has identified a population of cells throughout the limbic system that discharge as a function of the animal's head direction (HD). Altering normal motor cues can alter the HD cell responses and disrupt the updating of their preferred firing directions, thus suggesting that motor cues contribute to processing the HD signal. A pathway that conveys motor information may stem from the interpeduncular nucleus (IPN), a brain region that has reciprocal connections with HD cell circuitry. To test this hypothesis, we produced electrolytic or neurotoxic lesions of the IPN and recorded HD cells in the anterior dorsal thalamus (ADN) of rats. Direction-specific firing remained present in the ADN after lesions of the IPN, but measures of HD cell properties showed that cells had reduced peak firing rates, large directional firing ranges, and firing that predicted the animal's future heading more than in intact controls. Furthermore, preferred firing directions were moderately less influenced by rotation of a salient visual landmark. Finally, the preferred directions of cells in lesioned rats exhibited large shifts when the animals foraged for scattered food pellets in a darkened environment and when locomoting from a familiar environment to a novel one. We propose that the IPN contributes motor information about the animal's movements to the HD cell circuitry. Furthermore, these results suggest that the IPN plays a broad role in the discharge properties and stability of direction-specific activity in the HD cell circuit.

Anticipation in the Rodent Head Direction System Can Be Explained by an Interaction of Head Movements and Vestibular Firing Properties

The rodent head-direction (HD) system, which codes for the animal's head direction in the horizontal plane, is thought to be critically involved in spatial navigation. Electrophysiological recording studies have shown that HD cells can anticipate the animal's HD by up to 75–80 ms. The origin of this anticipation is poorly understood. In this modeling study, we provide a novel explanation for HD anticipation that relies on the firing properties of neurons afferent to the HD system. By incorporating spike rate adaptation and postinhibitory rebound as observed in medial vestibular nucleus neurons, our model produces realistic anticipation on a large corpus of rat movement data. In addition, HD anticipation varies between recording sessions of the same cell, between active and passive movement, and between different studies. Such differences do not appear to be correlated with behavioral variables and cannot be accounted for using earlier models. In the present model, anticipation depends on the power spectrum of the head movements. By direct comparison with recording data, we show that the model explains 60–80% of the observed anticipation variability. We conclude that HD afferent dynamics and the statistics of rat head movements are important in generating HD anticipation. This result contributes to understanding the functional circuitry of the HD system and has methodological implications for studies of HD anticipation.

The head direction signal: origins and sensory-motor integration

Navigation first requires accurate perception of one's spatial orientation within the environment, which consists of knowledge about location and directional heading. Cells within several limbic system areas of the mammalian brain discharge allocentrically as a function of the animal's directional heading, independent of the animal's location and ongoing behavior. These cells are referred to as head direction (HD) cells and are believed to encode the animal's perceived directional heading with respect to its environment. Although HD cells are found in several areas, the principal circuit for generating this signal originates in the dorsal tegmental nucleus and projects serially, with some reciprocal connections, to the lateral mammillary nucleus --> anterodorsal thalamus --> PoS, and terminates in the entorhinal cortex. HD cells receive multimodal information about landmarks and self-generated movements. Vestibular information appears critical for generating the directional signal, but motor/proprioceptive and landmark information are important for updating it.

Lesions of the tegmentomammillary circuit in the head direction system disrupt the head direction signal in the anterior thalamus

Head direction (HD) cells in the rodent limbic system are believed to correspond to a cognitive representation of directional heading in the environment. Lesions of vestibular hair cells disrupt the characteristic firing patterns of HD cells, and thus vestibular afference is a critical contributor to the HD signal. A subcortical pathway that may convey this information includes the dorsal tegmental nucleus of Gudden (DTN) and the lateral mammillary nucleus (LMN). To test the hypothesis that the DTN and LMN are critical components for generating HD cell activity, we made electrolytic lesions of the DTN or LMN in rats and screened for HD cell activity in the anterior thalamus. Directional activity was absent in all animals with complete LMN lesions and in animals with complete DTN lesions, although a few HD cells were isolated in animals with incomplete lesions. Some DTN-lesioned animals contained cells whose firing rates were modulated by angular head velocity. Although cells with bursting patterns of activity have been observed in the anterior dorsal nucleus of the thalamus of animals with disruption of vestibular inputs, this pattern of activity was not observed in either the LMN- or DTN-lesioned animals. The general absence of direction-specific activity in the anterior thalamus of animals with DTN or LMN lesions is consistent with the view that the DTN-LMN circuit is essential for the generation of HD cell activity.

The supragenual nucleus: a putative relay station for ascending vestibular signs to head direction cells

Head direction (HD) cells located in several regions of the brain, including the postsubiculum, retrosplenial cortex, lateral dorsal thalamic nucleus, anterior dorsal thalamic nucleus, and lateral mammillary nucleus, provide a signal of the rat's momentary directional heading. Experimental evidence suggests that vestibular inputs are critical for the maintenance these cells' directional sensitivity. However, it is still unclear how vestibular information is conveyed to the HD cell-related circuitry. In a recent study, the supragenual nucleus (SG) was suggested as a putative relay of vestibular inputs to this circuitry. In the present study, using anterograde and retrograde tract-tracing methods, we first investigated whether the SG is in a position to convey vestibular inputs. Next, we examined the projections of the SG with the Phaseolus vulgaris leucoagglutinin method. Our results indicate that the SG receives direct inputs from the medial vestibular nucleus and projects to elements of the HD cell-related circuitry, providing a massive input to the contralateral dorsal tegmental nucleus and a moderately dense projection to the shell region of the lateral mammillary nucleus. Overall, the present findings serve to clarify how vestibular inputs reach the HD cell-related circuit and point out the SG as an important interface to this end.

Path integration and lesions within the head direction cell circuit: Comparison between the roles of the anterodorsal thalamus and dorsal tegmental nucleus

Experiments were designed to determine whether 2 regions of the head direction cell circuit, the anterodorsal thalamic nucleus (ADN) and the dorsal tegmental nucleus (DTN), contribute to navigation. Rats were trained to perform a food-carrying task with and without blindfolds prior to receiving sham lesions or bilateral lesions of the ADN or DTN. ADN-lesioned rats were mildly impaired in both versions of the task. DTN-lesioned rats, however, were severely impaired and showed reduced heading accuracy in both task versions. These findings suggest that although both the DTN and ADN contribute to navigation based on path integration and landmarks, disruption of the head direction cell circuit at the level of the DTN has a significantly greater effect on spatial behavior than lesions of the ADN.

Nucleus prepositus

The cytoarchitecture and the histochemistry of nucleus prepositus hypoglossi and its afferent and efferent connections to oculomotor structures are described. The functional significance of the afferent connections of the nucleus is discussed in terms of current knowledge of the firing behavior of prepositus neurons in alert animals. The efferent connections of the nucleus and the results of lesion experiments suggest that it plays a role in a variety of functions related to the control of gaze.

The effects of vestibular lesions on hippocampal function in rats

Interest in interaction between the vestibular system and the hippocampus was stimulated by evidence that peripheral vestibular lesions could impair performance in learning and memory tasks requiring spatial information processing. By the 1990s, electrophysiological data were emerging that the brainstem vestibular nucleus complex (VNC) and the hippocampus were connected polysynaptically and that hippocampal place cells could respond to vestibular stimulation. The aim of this review is to summarise and critically evaluate research published in the last 5 years that has seen major progress in understanding the effects of vestibular damage on the hippocampus. In addition to new behavioural studies demonstrating that animals with vestibular lesions exhibit impairments in spatial memory tasks, electrophysiological studies have confirmed long-latency, polysynaptic pathways between the VNC and the hippocampus. Peripheral vestibular lesions have been shown to cause long-term changes in place cell function, hippocampal EEG activity and even CA1 field potentials in brain slices maintained in vitro. During the same period, neurochemical investigations have shown that some hippocampal subregions exhibit long-term changes in the expression of neuronal nitric oxide synthase, arginase I and II, and the NR1 and NR2A N-methyl-D-aspartate (NMDA) receptor subunits following peripheral vestibular damage. Despite the progress, a number of important issues remain to be resolved, such as the possible contribution of auditory damage associated with vestibular lesions, to the hippocampal effects observed. Furthermore, although these studies demonstrate that damage to the vestibular system does have a long-term impact on the electrophysiological and neurochemical function of the hippocampus, they do not indicate precisely how vestibular information might be used in hippocampal functions such as developing spatial representations of the environment. Understanding this will require detailed electrical stimulation and lesion studies to elucidate the way in which different kinds of vestibular information are transmitted to various hippocampal subregions.

Angular path integration by moving "hill of activity": a spiking neuron model without recurrent excitation of the head-direction system

During spatial navigation, the head orientation of an animal is encoded internally by neural persistent activity in the head-direction (HD) system. In computational models, such a bell-shaped "hill of activity" is commonly assumed to be generated by recurrent excitation in a continuous attractor network. Recent experimental evidence, however, indicates that HD signal in rodents originates in a reciprocal loop between the lateral mammillary nucleus (LMN) and the dorsal tegmental nucleus (DTN), which is characterized by a paucity of local excitatory axonal collaterals. Moreover, when the animal turns its head to a new direction, the heading information is updated by a time integration of angular head velocity (AHV) signals; the underlying mechanism remains unresolved. To investigate these issues, we built and investigated an LMN-DTN network model that consists of three populations of noisy and spiking neurons coupled by biophysically realistic synapses. We found that a combination of uniform external excitation and recurrent cross-inhibition can give rise to direction-selective persistent activity. The model reproduces the experimentally observed three types of HD tuning curves differentially modulated by AHV and anticipatory firing activity in LMN HD cells. Time integration is assessed by using constant or sinusoidal angular velocity stimuli, as well as naturalistic AHV inputs (from rodent recordings). Furthermore, the internal representation of head direction is shown to be calibrated or reset by strong external cues. We identify microcircuit properties that determine the ability of our model network to subserve time integration function.

A continuous attractor network model without recurrent excitation: maintenance and integration in the head direction cell system

Motivated by experimental observations of the head direction system, we study a three population network model that operates as a continuous attractor network. This network is able to store in a short-term memory an angular variable (the head direction) as a spatial profile of activity across neurons in the absence of selective external inputs, and to accurately update this variable on the basis of angular velocity inputs. The network is composed of one excitatory population and two inhibitory populations, with inter-connections between populations but no connections within the neurons of a same population. In particular, there are no excitatory-to-excitatory connections. Angular velocity signals are represented as inputs in one inhibitory population (clockwise turns) or the other (counterclockwise turns). The system is studied using a combination of analytical and numerical methods. Analysis of a simplified model composed of threshold-linear neurons gives the conditions on the connectivity for (i) the emergence of the spatially selective profile, (ii) reliable integration of angular velocity inputs, and (iii) the range of angular velocities that can be accurately integrated by the model. Numerical simulations allow us to study the proposed scenario in a large network of spiking neurons and compare their dynamics with that of head direction cells recorded in the rat limbic system. In particular, we show that the directional representation encoded by the attractor network can be rapidly updated by external cues, consistent with the very short update latencies observed experimentally by Zugaro et al. (2003) in thalamic head direction cells.

Polysynaptic pathways from the vestibular nuclei to the lateral mammillary nucleus of the rat: substrates for vestibular input to head direction cells

The activity of some neurons in the lateral mammillary nucleus (LMN) of the rat corresponds with the animal's current head direction (HD). HD cells have been studied extensively but the circuitry responsible for the generation and maintenance of the HD signal has not been established. The present study tested the hypothesis that a polysynaptic pathway connects the vestibular nuclei with the LMN via one or more relay nuclei. This circuitry could provide a substrate for the integration of sensory input necessary for HD cell activity. This hypothesis is based upon the prior demonstration that labyrinthectomy abolishes HD selectivity in thalamic neurons. Viral transneuronal tracing with pseudorabies virus (PRV) was used to test this hypothesis. We injected recombinants of PRV into the LMN and surrounding nuclei of adult male rats and defined the patterns of retrograde transneuronal infection at survival intervals of 60 and 72 h. Infected medial vestibular neurons (MVN) were only observed at the longest postinoculation interval in animals in which the injection site was localized largely to the LMN. Robust infection of the dorsal tegmental nucleus (DTN) and nucleus prepositus hypoglossi (PH) in these cases, but not in controls, at both survival intervals identified these nuclei as potential relays of vestibular input to the LMN. These data are consistent with the conclusion that vestibular information that contributes to the LMN HD cell activity is relayed to this caudal hypothalamic cell group via a polysynaptic brainstem circuit.

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.

Cytoarchitecture and efferent projections of the nucleus incertus of the rat

The nucleus incertus is located caudal to the dorsal raphe and medial to the dorsal tegmentum. It is composed of a pars compacta and a pars dissipata and contains acetylcholinesterase, glutamic acid decarboxylase, and cholecystokinin-positive somata. In the present study, anterograde tracer injections in the nucleus incertus resulted in terminal-like labeling in the perirhinal cortex and the dorsal endopyriform nucleus, the hippocampus, the medial septum diagonal band complex, lateral and triangular septum medial amygdala, the intralaminar thalamic nuclei, and the lateral habenula. The hypothalamus contained dense plexuses of fibers in the medial forebrain bundle that spread in nearly all nuclei. Labeling in the suprachiasmatic nucleus filled specifically the ventral half. In the midbrain, labeled fibers were observed in the interpeduncular nuclei, ventral tegmental area, periaqueductal gray, superior colliculus, pericentral inferior colliculus, pretectal area, the raphe nuclei, and the nucleus reticularis pontis oralis. Retrograde tracer injections were made in areas reached by anterogradely labeled fibers including the medial prefrontal cortex, hippocampus, amygdala, habenula, nucleus reuniens, superior colliculus, periaqueductal gray, and interpeduncular nuclei. All these injections gave rise to retrograde labeling in the nucleus incertus but not in the dorsal tegmental nucleus. These data led us to conclude that there is a system of ascending projections arising from the nucleus incertus to the median raphe, mammillary complex, hypothalamus, lateral habenula, nucleus reuniens, amygdala, entorhinal cortex, medial septum, and hippocampus. Many of the targets of the nucleus incertus were involved in arousal mechanisms including the synchronization and desynchronization of the theta rhythm.

Does the vestibular system contribute to head direction cell activity in the rat?

Head direction cells (HDC) located in several regions of the brain, including the anterior dorsal nucleus of the thalamus (ADN), postsubiculum (PoS), and lateral mammillary nuclei (LMN), provide the neural substrate for the determination of head direction. Although activity of HDC is influenced by various sensory signals and internally generated cues, lesion studies and some anatomical and physiological evidence suggest that vestibular inputs are critical for the maintenance of directional sensitivity of these cells. However, vestibular inputs must be transformed considerably in order to signal head direction, and the neuronal circuitry that accomplishes this signal processing has not been fully established. Furthermore, it is unclear why the removal of vestibular inputs abolishes the directional sensitivity of HDC, as visual and other sensory inputs and motor feedback signals strongly affect the firing of these neurons and would be expected to maintain their directional-related activity. Further physiological studies will be required to establish the role of vestibular system in producing HDC responses, and anatomical studies are needed to determine the neural circuitry that mediates vestibular influences on determination of head direction.

Prominent expression of Narp in central vestibular pathways: selective effect of labyrinth ablation

Recent in vitro studies demonstrated that Narp, a secreted immediate early gene (IEG) product, induces AMPA receptor clustering. Accordingly, Narp has been implicated in mediating activity-dependent changes in synaptic efficacy. To help define the role of Narp in vivo, we conducted immunohistochemical studies of Narp in rat brain. Unexpectedly, we found robust Narp expression in several discrete areas linked to the vestibular system: the anterodorsal nucleus (ADN) of the thalamus, which relays head orientation information to the cortex, the lateral vestibulospinal (Deiters') nucleus and Purkinje cells in the flocculonodular lobe of the cerebellum. Although strong Narp expression in Deiters' nucleus and the cerebellum was present consistently, Narp expression in the ADN displayed a high degree of variability among animals. To check if this variability in ADN Narp expression reflects its dependence on fluctuating levels of vestibular input, we monitored Narp immunostaining following bilateral labyrinth ablation. This procedure significantly suppressed Narp immunostaining in the ADN, indicating that it is stimulated by naturally occurring vestibular input. In contrast, labyrinth ablation did not affect Narp staining in Deiters' nucleus or the flocculonodular lobe of the cerebellum, presumably because these areas are driven by inputs from multiple systems. As previous studies implicate Narp in synaptic plasticity, these findings suggest that this IEG may mediate ongoing adjustments in synaptic strength or connectivity in several pathways linked to the vestibular system.

Mapping the oculomotor system: the power of transneuronal labelling with rabies virus

Neuronal networks underlying and related to horizontal eye movements were visualized by retrograde transneuronal tracing with rabies virus from the left medial rectus muscle in guinea pigs. Time-sequenced labelling revealed distinct circuitries involved in particular oculomotor functions, i.e. vestibulo-ocular reflex and saccade generation (brainstem circuitry), adaptive plasticity (cerebellar modules) and possibly motivation and navigation (limbic, hippocampal and cortical structures). Our results provide a first comprehensive road map of the oculomotor system that is unsurpassed by any previous tracing study. We report a number of unexpected findings that illustrate a much vaster and more complicated network for the control of the relatively simple horizontal eye movements than had been envisioned previously.

Human relaxin gene 3 (H3) and the equivalent mouse relaxin (M3) gene. Novel members of the relaxin peptide family

We have identified a novel human relaxin gene, designated H3 relaxin, and an equivalent relaxin gene in the mouse from the Celera Genomics data base. Both genes encode a putative prohormone sequence incorporating the classic two-chain, three cysteine-bonded structure of the relaxin/insulin family and, importantly, contain the RXXXRXX(I/V) motif in the B-chain that is essential for relaxin receptor binding. A peptide derived from the likely proteolytic processing of the H3 relaxin prohormone sequence was synthesized and found to possess relaxin activity in bioassays utilizing the human monocytic cell line, THP-1, that expresses the relaxin receptor. The expression of this novel relaxin gene was studied in mouse tissues using RT-PCR, where transcripts were identified with a pattern of expression distinct from that of the previously characterized mouse relaxin. The highest levels of expression were found in the brain, whereas significant expression was also observed in the spleen, thymus, lung, and ovary. Northern blotting demonstrated an approximately 1.2-kb transcript present in mouse brain poly(A) RNA but not in other tissues. These data, together with the localization of transcripts in the pars ventromedialis of the dorsal tegmental nucleus of C57BLK6J mouse brain by in situ hybridization histochemistry, suggest a new role for relaxin in neuropeptide signaling processes. Together, these studies describe a third member of the human relaxin family and its equivalent in the mouse.

Connections of the nucleus incertus

The nucleus incertus (NI) is a distinct cell group in caudoventral regions of the pontine periventricular gray, adjacent to the ventromedial border of the caudal dorsal tegmental nucleus. Recent interest in the NI stems from evidence that it represents one of the periventricular sites with the highest expression levels of mRNA encoding the type 1 corticotropin-releasing hormone (CRH) receptor, which has a high affinity for naturally occurring CRH, perhaps accounting for some of the extrapituitary actions of the peptide on autonomic and behavioral components of the stress response. However, almost nothing is known about NI function and hodological relationships. In this paper, we present the results of a systematic analysis of NI inputs and outputs using cholera toxin B subunit as a retrograde tracer and Phaseolus vulgaris-leucoagglutinin as an anterograde tracer. Our retrograde tracer experiments indicate that the NI is in a strategic position to integrate information related to behavioral planning (from the prefrontal cortex), lateral habenular processing, hippocampal function, and oculomotor control. Based on its efferent connections, the NI is in a position to exert significant modulating influences on prefrontal and hippocampal cortical activity, and the nucleus is also in a position to influence brain sites known to control locomotor behavior, attentive states, and learning processes. Overall, the present results support the idea that the NI is a distinct region of the pontine periventricular gray, and together with the superior central (median raphé) and interpeduncular nuclei the NI appears to form a midline behavior control network of the brainstem.

Neural correlates for angular head velocity in the rat dorsal tegmental nucleus

Many neurons in the rat lateral mammillary nuclei (LMN) fire selectively in relation to the animal's head direction (HD) in the horizontal plane independent of the rat's location or behavior. One hypothesis of how this representation is generated and updated is via subcortical projections from the dorsal tegmental nucleus (DTN). Here we report the type of activity in DTN neurons. The majority of cells (75%) fired as a function of the rat's angular head velocity (AHV). Cells exhibited one of two types of firing patterns: (1) symmetric, in which the firing rate was positively correlated with AHV during head turns in both directions, and (2) asymmetric, in which the firing rate was positively correlated with head turns in one direction and correlated either negatively or not at all in the opposite direction. In addition to modulation by AHV, some of the AHV cells (40.1%) were weakly modulated by the rat's linear velocity, and a smaller number were modulated by HD (11%) or head pitch (15.9%). Autocorrelation analyses indicated that with the head stationary, AHV cells displayed irregular discharge patterns. Because afferents from the DTN are the major source of information projecting to the LMN, these results suggest that AHV information from the DTN plays a significant role in generating the HD signal in LMN. A model is proposed showing how DTN AHV cells can generate and update the LMN HD cell signal.

The anatomical and computational basis of the rat head-direction cell signal

As a rat navigates through space, neurons called head-direction (HD) cells provide a signal of the rat's momentary directional heading. Although partly guided by landmarks, the cells also show a remarkable ability to track directional heading based on angular head movement. Theoretical models suggest that the HD cells are linked together to form an attractor network, and that cells which signal angular velocity update the directional setting of the attractor. Recently, cell types similar to those required theoretically have been discovered in the lateral mammillary and dorsal tegmental nuclei. Lesion and anatomical data suggest these nuclei might constitute the postulated attractor-path integration mechanism, and that they provide the HD cell signal to cortical areas where it has been observed.

Role of locus coeruleus in foot shock-evoked Fos expression in rat brain

The robust activation of locus coeruleus neurons in response to a variety of stressors, in conjunction with the widespread outputs of the locus coeruleus, suggest that the locus coeruleus may be important in mediating responses to stress. Previous studies in rats have demonstrated that exposure to foot shock elicits Fos expression, a marker of neuronal activation, in the locus coeruleus and other brain sites. In order to evaluate the involvement of the locus coeruleus in foot shock-induced activation of other brain sites, shock-induced Fos expression was examined in the locus coeruleus and other brain areas known to be activated by foot shock, following direct inhibition of the locus coeruleus by local infusion of muscimol, a GABA agonist, prior to foot shock. Control rats received infusions of artificial cerebrospinal fluid into the locus coeruleus or muscimol into areas outside of locus coeruleus. Rats infused with artificial cerebrospinal fluid and then exposed to foot shock had significant increases in Fos expression in several brain areas, including locus coeruleus, nucleus O, several subdivisions of the hypothalamus, subnuclei of amygdala, bed nucleus of the stria terminalis and cingulate cortex. Inhibition of the locus coeruleus prior to foot shock significantly inhibited Fos expression in the locus coeruleus, nucleus O, some subdivisions of the hypothalamus including the magnocellular and medial parvicellular paraventricular hypothalamic nucleus, subnuclei of amygdala, and cingulate cortex. In contrast, inhibition of the locus coeruleus did not affect shock-induced Fos expression in other areas, including certain subdivisions of the hypothalamus and bed nucleus of the stria terminalis. We suggest that foot shock may activate multiple pathways, with activation of certain discrete nuclei requiring input from the locus coeruleus and activation of others occurring independently of locus coeruleus input.

Localization of amino acids, neuropeptides and cholinergic neurotransmitter markers in identified projections from the mesencephalic tegmentum to the mammillary nuclei of the rat

Retrograde labelling has been combined with immunohistochemistry to localize neurons containing GABA, glutamate, choline acetyltransferase, leu-enkephalin, neurotensin and substance P-like immunoreactivity in the projection pathways from the midbrain tegmental nuclei to the mammillary nuclei in the rat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the medial mammillary nucleus resulted in retrogradely labelled neurons in the ventral tegmental nucleus of Gudden, whereas injections into the lateral mammillary nucleus resulted in large numbers of retrogradely labelled neurons in the ipsilateral dorsal tegmental nucleus of Gudden and in the laterodorsal tegmental nucleus. In the ventral tegmental nucleus, moderate to small numbers of retrogradely labelled neurons were also immunolabelled for GABA and approximately ten to 18 WGA-HRP-labelled neurons per section were immunoreactive for leu-enkephalin. In addition, small numbers of WGA-HRP-labelled neurons in the principal subnucleus of the ventral tegmental nucleus were immunoreactive for Glu whereas small numbers of retrogradely labelled neurons in the compact subnucleus of the central superior nucleus displayed neurotensin-like immunoreactivity. In the ventral subnucleus of the dorsal tegmental nucleus, moderate to small numbers of retrogradely labelled neurons were also GABA-immunoreactive and approximately ten to 14 WGA-HRP labelled neurons per section were immunoreactive for leu-enkephalin. The ventral subnucleus of the dorsal tegmental nucleus also contained small numbers of retrogradely labelled neurons that displayed either glutamate or substance P-like immunoreactivity. In addition, moderate to small numbers of WGA-HRP-labelled neurons (five to 20 per section) in the laterodorsal tegmental nucleus were immunoreactive for choline acetyltransferase. These results are compatible with the possibility that tegmentomammillary projection neurons use several different neurochemicals as neurotransmitter(s) and/or neuromodulator(s).

Role of the lateral mammillary nucleus in the rat head direction circuit: a combined single unit recording and lesion study

We recorded head direction (HD) cells from the lateral mammillary nucleus (LMN) and anterior thalamus (ATN) of freely behaving rats and also made bilateral lesions of LMN while recording HD cells from ATN. We discovered that the tuning functions of LMN HD cells become narrower during contraversive head turns, but not ipsiversive head turns, compared to when the head is not turning. This narrowing effect does not occur for ATN HD cells. We also found that the HD signal in LMN leads that in ATN by about 15-20 ms. When LMN was lesioned bilaterally, HD cells in ATN immediately lost their directional firing properties and never recovered them. Based on these findings, we argue that LMN may be an essential component of an attractor-integrator network that participates in generating the HD signal.

Firing properties of rat lateral mammillary single units: head direction, head pitch, and angular head velocity

Many neurons in the rat anterodorsal thalamus (ADN) and postsubiculum (PoS) fire selectively when the rat points its head in a specific direction in the horizontal plane, independent of the animal's location and ongoing behavior. The lateral mammillary nuclei (LMN) are interconnected with both the ADN and PoS and, therefore, are in a pivotal position to influence ADN/PoS neurophysiology. To further understand how the head direction (HD) cell signal is generated, we recorded single neurons from the LMN of freely moving rats. The majority of cells discharged as a function of one of three types of spatial correlates: (1) directional heading, (2) head pitch, or (3) angular head velocity (AHV). LMN HD cells exhibited higher peak firing rates and greater range of directional firing than that of ADN and PoS HD cells. LMN HD cells were modulated by angular head velocity, turning direction, and anticipated the rat's future HD by a greater amount of time (approximately 95 msec) than that previously reported for ADN HD cells (approximately 25 msec). Most head pitch cells discharged when the rostrocaudal axis of the rat's head was orthogonal to the horizontal plane. Head pitch cell firing was independent of the rat's location, directional heading, and its body orientation (i.e., the cell discharged whenever the rat pointed its head up, whether standing on all four limbs or rearing). AHV cells were categorized as fast or slow AHV cells depending on whether their firing rate increased or decreased in proportion to angular head velocity. These data demonstrate that LMN neurons code direction and angular motion of the head in both horizontal and vertical planes and support the hypothesis that the LMN play an important role in processing both egocentric and allocentric spatial information.

The fasciculus retroflexus controls the integrity of REM sleep by supporting the generation of hippocampal theta rhythm and rapid eye movements in rats

The fasciculus retroflexus (FR) fiber bundle comprises the intense cholinergic projection from the medial division of the habenula nucleus (Hbn) of the epithalamus to the interpeduncular nucleus (IPN) of the limbic midbrain. Due to the widespread connections of the Hbn and IPN, it could be surmised that the FR is integrated in the processings of various subsystems that are known to be involved in the sleep-wake mechanisms; relevant sites include the limbic forebrain and midbrain areas and more caudal pontine structures. Consequently, the present study addressed the significance of the FR in the spontaneous sleep-wake stage-associated variations of the different activity patterns of frontal cortex and hippocampal electroencephalograms (EEGs), the electrooculogram, and body movements, in freely behaving rats that had been subjected to either bilateral electrolytic lesioning of the FR or control operations. The evolution of different state combinations was assessed by the combinatory analysis of different activity stages appearing on the 6-h records. As compared to the control-operated group, the FR lesioning substantially reduced the time spent in rapid eye movement (REM) sleep by 79%, moderately decreased the duration of the intermediate state of sleep by 29%, and quiet waking state by 44%, but had virtually no effects on the durations of different types of non-REM sleep (i.e., drowsiness that which involved quiet sleep or slow-wave sleep containing delta and spindle state components) or on the times of active waking behavior that corresponded to the body movements. Quantitative decomposition analyses revealed marked variations in the frontal cortex and hippocampal activity as well as REM during the course of the extracted sleep-wake stages described and there were also some group differences. Of those individual features that were used to determine different sleep-wake stages, the overall hippocampal theta time (41% decrease) and single REM frequency (71% reduction during the REM sleep) were most affected. In contrast, the various properties of desynchronization/synchronization patterns of frontal cortex EEGs were consistently hardly influenced by the FR lesioning. Therefore, the present data suggest the involvement of the FR in the REM sleep processes by establishing prominent associations with the limbic and REM control mechanisms that involve the hippocampus and plausibly pontine ocular activity networks.

Head direction cells and the neurophysiological basis for a sense of direction

Animals require two types of fundamental information for accurate navigation: location and directional heading. Current theories hypothesize that animals maintain a neural representation, or cognitive map, of external space in the brain. Whereas cells in the rat hippocampus and parahippocampal regions encode information about location, a second type of allocentric spatial cell encodes information about the animal's directional heading, independent of the animal's on-going behaviors. These head direction (HD) cells are found in several areas of the classic Papez circuit. This review focuses on experimental studies conducted on HD cells and describes their discharge properties, functional significance, role in path integration, and responses to different environmental manipulations. The anterior dorsal thalamic nucleus appears critical for the generation of the directional signal. Both motor and vestibular cues also play important roles in the signal's processing. The neural network models proposed to account for HD cell firing are compared with known empirical findings. Examples from clinical cases of patients with topographical disorientation are also discussed. It is concluded that studying the neural mechanisms underlying the HD signal provides an excellent opportunity for understanding how the mammalian nervous system processes a high level cognitive signal.

Comparisons of head direction cell activity in the postsubiculum and anterior thalamus of freely moving rats

Single cells in the rat anterior thalamic nucleus (ATN) and postsubiculum (PoS) discharge as a function of the rat's directional heading in the horizontal plane, independent of its location. A previous study that compared cell firing during clockwise and counterclockwise head turns concluded that ATN 'head direction' (HD) cell discharge anticipates the rat's future directional heading, while PoS HD cell discharge is in register with the rat's current directional heading (Blair and Sharp [1995] J Neurosci 15:6260-6270). In the current study we extend these findings by using a different method of analysis. HD cells in the ATN and PoS were first characterized by three different measures: peak firing rate, range width, and information content. We then examined how these measures varied when cell firing was aligned with past (negative time shift) or future (positive time shift) head direction of the rat. We report that all three measures were optimized when ATN cell firing was aligned with the animal's future directional heading by about +23 msec. In contrast, PoS HD cell firing was optimized when cell firing was aligned with the rat's past head direction by about -7 msec. When the optimal value was plotted as a function of the amount of time spikes were shifted relative to head orientation, the mean ATN function was shifted to the right of the PoS function only at negative time shifts; at positive time shifts the two functions overlapped. Analysis of two recording sessions from the same cell indicated that each cell in a particular brain area is 'tuned' to a specific time shift so that all cells within a brain area are not uniformly tuned to the same time shift. Other analyses showed that the clockwise and counterclockwise tuning functions were not skewed in the direction of the head turn as postulated by Redish et al. ([1996] Network: Computation in Neural Systems 7:671-685) and Blair et al. ([1997] J Neurophysiol 17:145-159). Additional analysis on episodes when the rat happened to continually point its head in the preferred direction indicated that HD cell firing undergoes little adaptation. In the Discussion, we argue that these results are best accounted for by a motor efference copy signal operating on both types of HD cells such that the copy associated with the PoS HD cells is delayed in time by about 30 msec relative to the copy associated with ATN HD cells.

Vestibular-hippocampal interactions

Since the early 1960s, researchers have speculated that the vestibular system, the sensory system concerned with the perception of balance and self-motion, contributes to spatial information processing and the development of spatial memory in the hippocampus. Anatomical studies have suggested that various parts of the thalamus are likely to transmit vestibular information to the hippocampus, perhaps via the parietal cortex; however, more direct pathways are possible. Over the last 2-3 years there have been a number of direct electrophysiological demonstrations that vestibular stimulation affects head direction cells in the anterior thalamic nuclei and place cells in the hippocampus. These studies demonstrate the importance of vestibular-hippocampal interactions for hippocampal function but also raise the possibility that the hippocampus may be important for compensation of vestibular function following peripheral or central vestibular lesions.