Topographical organization of the nigrotectal projection in rat: evidence for segregated channels

Causality methods to study the functional connectivity in brain networks: the basal ganglia - thalamus causal interactions

This study uses methods recently developed to study the complex evolution of atmospheric phenomena which have some similarities with the dynamics of the human brain. In both cases, it is possible to record the activity of particular centers (geographic regions or brain nuclei) but not to make an experimental modification of their state. The study of "causality", which is necessary to understand the dynamics of these complex systems and to develop robust models that can predict their evolution, is hampered by the experimental restrictions imposed by the nature of both systems. The study was performed with data obtained in the thalamus and basal ganglia of awake humans executing different tasks. This work studies the linear, non-linear and more complex relationships of these thalamic centers with the cortex and main BG nuclei, using three complementary techniques: the partial correlation regression method, the Gaussian process regression/distance correlation and a model-free method based on nearest-neighbor that computes the conditional mutual information. These causality methods indicated that the basal ganglia present a different functional relationship with the anterior-ventral (motor), intralaminar and medio-dorsal thalamic centers, and that more than 60% of these thalamus-basal ganglia relationships present a non-linear dynamic (35 of the 57 relationships found). These functional interactions were observed for basal ganglia nuclei with direct structural connections with the thalamus (primary somatosensory and motor cortex, striatum, internal globus pallidum and substantia nigra pars reticulata), but also for basal ganglia without structural connections with the thalamus (external globus pallidum and subthalamic nucleus). The motor tasks induced rapid modifications of the thalamus-basal ganglia interactions. These findings provide new perspectives of the thalamus - BG interactions, many of which may be supported by indirect functional relationships and not by direct excitatory/inhibitory interactions.

Separation of Channels Subserving Approach and Avoidance/Escape at the Level of the Basal Ganglia and Related Brainstem Structures

The basal ganglia have the key function of directing our behavior in the context of events from our environment and/or our internal state. This function relies on afferents targeting the main input structures of the basal ganglia, entering bids for action selection at the level of the striatum or signals for behavioral interruption at the level of the subthalamic nucleus, with behavioral reselection facilitated by dopamine signaling. Numerous experiments have studied action selection in relation to inputs from the cerebral cortex. However, less is known about the anatomical and functional link between the basal ganglia and the brainstem. In this review, we describe how brainstem structures also project to the main input structures of the basal ganglia, namely the striatum, the subthalamic nucleus and midbrain dopaminergic neurons, in the context of approach and avoidance (including escape from threat), two fundamental, mutually exclusive behavioral choices in an animal's repertoire in which the brainstem is strongly involved. We focus on three particularly well-described loci involved in approach and avoidance, namely the superior colliculus, the parabrachial nucleus and the periaqueductal grey nucleus. We consider what is known about how these structures are related to the basal ganglia, focusing on their projections toward the striatum, dopaminergic neurons and subthalamic nucleus, and explore the functional consequences of those interactions.

Pathways from the Superior Colliculus to the Basal Ganglia

The present work aims to review the structural organization of the mammalian superior colliculus (SC), the putative pathways connecting the SC and the basal ganglia, and their role in organizing complex behavioral output. First, we review how the complex intrinsic connections between the SC's laminae projections allow for the construction of spatially aligned, visual-multisensory maps of the surrounding environment. Moreover, we present a summary of the sensory-motor inputs of the SC, including a description of the integration of multi-sensory inputs relevant to behavioral control. We further examine the major descending outputs toward the brainstem and spinal cord. As the central piece of this review, we provide a thorough analysis covering the putative interactions between the SC and the basal ganglia. To this end, we explore the diverse thalamic routes by which information from the SC may reach the striatum, including the pathways through the lateral posterior, parafascicular, and rostral intralaminar thalamic nuclei. We also examine the interactions between the SC and subthalamic nucleus, representing an additional pathway for the tectal modulation of the basal ganglia. Moreover, we discuss how information from the SC might also be relayed to the basal ganglia through midbrain tectonigral and tectotegmental projections directed at the substantia nigra compacta and ventrotegmental area, respectively, influencing the dopaminergic outflow to the dorsal and ventral striatum. We highlight the vast interplay between the SC and the basal ganglia and raise several missing points that warrant being addressed in future studies.

The Neural Basis of Escape Behavior in Vertebrates

Escape is one of the most studied animal behaviors, and there is a rich normative theory that links threat properties to evasive actions and their timing. The behavioral principles of escape are evolutionarily conserved and rely on elementary computational steps such as classifying sensory stimuli and executing appropriate movements. These are common building blocks of general adaptive behaviors. Here we consider the computational challenges required for escape behaviors to be implemented, discuss possible algorithmic solutions, and review some of the underlying neural circuits and mechanisms. We outline shared neural principles that can be implemented by evolutionarily ancient neural systems to generate escape behavior, to which cortical encephalization has been added to allow for increased sophistication and flexibility in responding to threat.

Opponent regulation of action performance and timing by striatonigral and striatopallidal pathways

The basal ganglia have been implicated in action selection and timing, but the relative contributions of the striatonigral (direct) and striatopallidal (indirect) pathways to these functions remain unclear. We investigated the effects of optogenetic stimulation of D1+ (direct) and A2A+ (indirect) neurons in the ventrolateral striatum in head-fixed mice on a fixed time reinforcement schedule. Direct pathway stimulation initiates licking, whereas indirect pathway stimulation suppresses licking and results in rebound licking after stimulation. Moreover, direct and indirect pathways also play distinct roles in timing. Direct pathway stimulation produced a resetting of the internal timing process, whereas indirect pathway stimulation transiently paused timing, and proportionally delayed the next bout of licking. Our results provide evidence for the continuous and opposing contributions of the direct and indirect pathways in the production and timing of reward-guided behavior.

The dynamic of basal ganglia activity with a multiple covariance method: influences of Parkinson's disease

The closed-loop cortico-subcortical pathways of basal ganglia have been extensively used to describe the physiology of these centres and to justify the functional disorders of basal ganglia diseases. This approach justifies some experimental and clinical data but not others, and furthermore, it does not include a number of subcortical circuits that may produce a more complex basal ganglia dynamic than that expected for closed-loop linear networks. This work studied the functional connectivity of the main regions of the basal ganglia motor circuit with magnetic resonance imaging and a new method (functional profile method), which can analyse the multiple covariant activity of human basal ganglia. The functional profile method identified the most frequent covariant functional status (profiles) of the basal ganglia motor circuit, ordering them according to their relative frequency and identifying the most frequent successions between profiles (profile transitions). The functional profile method classified profiles as input profiles that accept the information coming from other networks, output profiles involved in the output of processed information to other networks and highly interconnected internal profiles that accept transitions from input profiles and send transitions to output profiles. Profile transitions showed a previously unobserved functional dynamic of human basal ganglia, suggesting that the basal ganglia motor circuit may work as a dynamic multiple covariance network. The number of internal profiles and internal transitions showed a striking decrease in patients with Parkinson’s disease, a fact not observed for input and output profiles. This suggests that basal ganglia of patients with Parkinson’s disease respond to requirements coming from other neuronal networks, but because the internal processing of information is drastically weakened, its response will be insufficient and perhaps also self-defeating. These marked effects were found in patients with few motor disorders, suggesting that the functional profile method may be an early procedure to detect the first stages of the Parkinson’s disease when the motor disorders are not very evident. The multiple covariance activity found presents a complementary point of view to the cortico-subcortical closed-loop model of basal ganglia. The functional profile method may be easily applied to other brain networks, and it may provide additional explanations for the clinical manifestations of other basal ganglia disorders.

The endogenous opioid system modulates defensive behavior evoked by Crotalus durissus terrificus: Panicolytic-like effect of intracollicular non-selective opioid receptors blockade

BACKGROUND

There is a controversy regarding the key role played by opioid peptide neurotransmission in the modulation of panic-attack-related responses.

AIMS

Using a prey versus rattlesnakes paradigm, the present work investigated the involvement of the endogenous opioid peptide-mediated system of the inferior colliculus in the modulation of panic attack-related responses.

METHODS

Wistar rats were pretreated with intracollicular administration of either physiological saline or naloxone at different concentrations and confronted with rattlesnakes ( Crotalus durissus terrificus). The prey versus rattlesnake confrontations were performed in a polygonal arena for snakes. The defensive behaviors displayed by prey (defensive attention, defensive immobility, escape response, flat back approach and startle) were recorded twice: firstly, over a period of 15 min the presence of the predator and a re-exposure was performed 24 h after the confrontation, when animals were exposed to the experimental enclosure without the rattlesnake.

RESULTS

The intramesencephalic non-specific blockade of opioid receptors with microinjections of naloxone at higher doses decreased both anxiety- (defensive attention and flat back approach) and panic attack-like (defensive immobility and escape) behaviors, evoked in the presence of rattlesnakes and increased non-defensive responses. During the exposure to the experimental context, there was a decrease in duration of defensive attention.

CONCLUSIONS

These findings suggest a panicolytic-like effect of endogenous opioid receptors antagonism in the inferior colliculus on innate (panic attack) and conditioned (anticipatory anxiety) fear in rats threatened by rattlesnakes.

The functional connectivity in the motor loop of human basal ganglia

Basal ganglia interact in a complex way which is still not completely understood. The model generally used to explain basal ganglia interactions is based on experimental data in animals, but its validation in humans has been hampered by methodological restrictions. The time-relationship (partial correlation) of the fluctuations of the blood-oxygen-level-dependent signals recorded in the main basal ganglia was used here (32 healthy volunteers; 18-72 years of age; 16 males and 16 females) to test whether the interaction of the main basal ganglia in humans follows the pattern of functional connectivity in animals. Data showed that most basal ganglia have a functional connectivity which is compatible with that of the established closed-loop model. The strength of the connectivity of some basal ganglia changed with finger motion, suggesting that the functional interactions between basal ganglia are quickly restructured by the motor tasks. The present study with the motor cortico-BG loop centers supports the circling dynamic of the basal ganglia model in humans, showing that motor tasks may change the functional connectivity of these centers.

A Pause-then-Cancel model of stopping: evidence from basal ganglia neurophysiology

Many studies have implicated the basal ganglia in the suppression of action impulses ('stopping'). Here, we discuss recent neurophysiological evidence that distinct hypothesized processes involved in action preparation and cancellation can be mapped onto distinct basal ganglia cell types and pathways. We examine how movement-related activity in the striatum is related to a 'Go' process and how going may be modulated by brief epochs of beta oscillations. We then describe how, rather than a unitary 'Stop' process, there appear to be separate, complementary 'Pause' and 'Cancel' mechanisms. We discuss the implications of these stopping subprocesses for the interpretation of the stop-signal reaction time-in particular, some activity that seems too slow to causally contribute to stopping when assuming a single Stop processes may actually be fast enough under a Pause-then-Cancel model. Finally, we suggest that combining complementary neural mechanisms that emphasize speed or accuracy respectively may serve more generally to optimize speed-accuracy trade-offs.This article is part of the themed issue 'Movement suppression: brain mechanisms for stopping and stillness'.

Connections between the zona incerta and superior colliculus in the monkey and squirrel

The zona incerta contains GABAergic neurons that project to the superior colliculus in the cat and rat, suggesting that it plays a role in gaze changes. However, whether this incertal connection represents a general mammalian pattern remains to be determined. We used neuronal tracers to examine the zona incerta connections with the midbrain tectum in the gray squirrel and macaque monkey. Collicular injections in both species revealed that most incertotectal neurons lay in the ventral layer, but anterogradely labeled tectoincertal terminals were found in both the dorsal and ventral layers. In the monkey, injections of the pretectum also produced retrograde labeling, but mainly in the dorsal layer. The dendritic fields of incertotectal and incertopretectal cells were generally contained within the layer inhabited by their somata. The macaque, but not the squirrel, zona incerta extended dorsolaterally, within the external medullary lamina. Zona incerta injections produced retrogradely labeled neurons in the superior colliculus of both species. In the squirrel, most cells inhabited the lower sublamina of the intermediate gray layer, but in the monkey, they were scattered throughout the deeper layers. Labeled cells were present among the pretectal nuclei in both species. Labeled terminals were concentrated in the lower sublamina of the intermediate gray layer of both species, but were dispersed among the pretectal nuclei. In summary, an incertal projection that is concentrated on the collicular motor output layers and that originates in the ventral layer of the ipsilateral zona incerta is a common mammalian feature, suggesting an important role in collicular function.

The Multiple Correspondence Analysis Method and Brain Functional Connectivity: Its Application to the Study of the Non-linear Relationships of Motor Cortex and Basal Ganglia

The complexity of basal ganglia (BG) interactions is often condensed into simple models mainly based on animal data and that present BG in closed-loop cortico-subcortical circuits of excitatory/inhibitory pathways which analyze the incoming cortical data and return the processed information to the cortex. This study was aimed at identifying functional relationships in the BG motor-loop of 24 healthy-subjects who provided written, informed consent and whose BOLD-activity was recorded by MRI methods. The analysis of the functional interaction between these centers by correlation techniques and multiple linear regression showed non-linear relationships which cannot be suitably addressed with these methods. The multiple correspondence analysis (MCA), an unsupervised multivariable procedure which can identify non-linear interactions, was used to study the functional connectivity of BG when subjects were at rest. Linear methods showed different functional interactions expected according to current BG models. MCA showed additional functional interactions which were not evident when using lineal methods. Seven functional configurations of BG were identified with MCA, two involving the primary motor and somatosensory cortex, one involving the deepest BG (external-internal globus pallidum, subthalamic nucleus and substantia nigral), one with the input-output BG centers (putamen and motor thalamus), two linking the input-output centers with other BG (external pallidum and subthalamic nucleus), and one linking the external pallidum and the substantia nigral. The results provide evidence that the non-linear MCA and linear methods are complementary and should be best used in conjunction to more fully understand the nature of functional connectivity of brain centers.

A GABAergic nigrotectal pathway for coordination of drinking behavior

The contribution of basal ganglia outputs to consummatory behavior remains poorly understood. We recorded from the substantia nigra pars reticulata (SNR), the major basal ganglia output nucleus, during self-initiated drinking. The firing rates of many lateral SNR neurons were time-locked to individual licks. These neurons send GABAergic projections to the deep layers of the orofacial region of the lateral tectum (superior colliculus, SC). Many tectal neurons are also time-locked to licking, but their activity is usually antiphase to that of SNR neurons, suggesting inhibitory nigrotectal projections. We used optogenetics to selectively activate the GABAergic nigrotectal afferents in the deep layers of the SC. Photo-stimulation of the nigrotectal projections transiently inhibited the activity of the lick-related tectal neurons, disrupted their licking-related oscillatory pattern, and suppressed self-initiated drinking. These results demonstrate that GABAergic nigrotectal projections play a crucial role in coordinating drinking behavior.

An integrative role for the superior colliculus in selecting targets for movements

A fundamental goal of systems neuroscience is to understand the neural mechanisms underlying decision making. The midbrain superior colliculus (SC) is known to be central to the selection of one among many potential spatial targets for movements, which represents an important form of decision making that is tractable to rigorous experimental investigation. In this review, we first discuss data from mammalian models-including primates, cats, and rodents-that inform our understanding of how neural activity in the SC underlies the selection of targets for movements. We then examine the anatomy and physiology of inputs to the SC from three key regions that are themselves implicated in motor decisions-the basal ganglia, parabrachial region, and neocortex-and discuss how they may influence SC activity related to target selection. Finally, we discuss the potential for methodological advances to further our understanding of the neural bases of target selection. Our overarching goal is to synthesize what is known about how the SC and its inputs act together to mediate the selection of targets for movements, to highlight open questions about this process, and to spur future studies addressing these questions.

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.

Inhibitory synaptic transmission from the substantia nigra pars reticulata to the ventral medial thalamus in mice

The cortico-basal ganglia-thalamic loop circuit is involved in variety of motor, association and limbic functions. The basal ganglia receive neural information from various areas of the cerebral cortex and transfer them back to the frontal and motor cortex via the ventral medial (VM), and the anterior-ventral lateral thalamic complex. The projection from the basal ganglia to the thalamus is GABAergic, and, therefore, the output from the basal ganglia cannot directly evoke excitation in the thalamic nuclei. The mechanism underlying the information transfer via the inhibitory projection remains unclear. To address this issue, we recorded electrophysiological properties of nigro-thalamic synapses from the VM neuron. We developed a nigro-thalamic slice preparation, in which the projection from the substantia nigra pars reticulata (SNr) to VM nucleus is stored, to enable the selective activation of the projection from the SNr. We characterized synaptic properties and membrane properties of the VM neuron, and developed a VM neuron model to simulate the impacts of SNr inputs on VM neuron activity. Neural simulation suggested that the inhibitory projection from SNr can control neural activity in two ways: a disinhibition from the spontaneous nigral inhibition and a β-band synchronization evoked by combination of excitation and inhibition of SNr activity.

The functional connectivity of intralaminar thalamic nuclei in the human basal ganglia

Projections of the centromedian-parafasicularis neurons of the intralaminar thalamus are major inputs of the striatum. Their functional role in the activity of human basal ganglia (BG) is not well known. The aim of this work was to study the functional connectivity of intralaminar thalamic nuclei with other BG by using the correlations of the BOLD signal recorded during "resting" and a motor task. Intralaminar nuclei showed a marked functional connectivity with all the tested BG, which was observed during "resting" and did not change with the motor task. As regards the intralaminar nuclei, BG connectivity was much lower for the medial dorsal nucleus (a thalamic nucleus bordering the intralaminar nuclei) and for the default mode network (although intralaminar nuclei showed a negative correlation with the default mode network). After the "regression" of intralaminar nuclei activity (partial correlation), the functional connectivity of the caudate and putamen nuclei with other BG decreased (but not with the primary sensorimotor cortex). Present data provide evidence that intralaminar nuclei are not only critical for striatal activity but also for the global performance of human BG, an action involving subcortical BG loops more than cortico-subcortical loops. The high correlation found between BG suggest that, similarly to that reported in other brain centers, the very-slow frequency fluctuations are relevant for the functional activity of these centers.

Disinhibition of the midbrain colliculi unmasks coordinated autonomic, respiratory, and somatomotor responses to auditory and visual stimuli

The midbrain superior and inferior colliculi have critical roles in generating coordinated orienting or defensive behavioral responses to environmental stimuli, and it has been proposed that neurons within the colliculi can also generate appropriate cardiovascular and respiratory responses to support such behavioral responses. We have previously shown that activation of neurons within a circumscribed region in the deep layers of the superior colliculus and in the central and external nuclei of the inferior colliculus can evoke a response characterized by intense and highly synchronized bursts of renal sympathetic nerve activity and phrenic nerve activity. In this study, we tested the hypothesis that, under conditions in which collicular neurons are disinhibited, coordinated cardiovascular, somatomotor, and respiratory responses can be evoked by natural environmental stimuli. In response to natural auditory, visual, or somatosensory stimuli, powerful synchronized increases in sympathetic, respiratory, and somatomotor activity were generated following blockade of GABAA receptors in a specific region in the midbrain colliculi of anesthetized rats, but not under control conditions. Such responses still occurred after removal of most of the forebrain, including the amygdala and hypothalamus, indicating that the essential pathways mediating these coordinated responses were located within the brain stem. The temporal relationships between the different outputs suggest that they are driven by a common population of “command neurons” within the colliculi.

Neuromodulation of whisking related neural activity in superior colliculus

The superior colliculus is part of a broader neural network that can decode whisker movements in air and on objects, which is a strategy used by behaving rats to sense the environment. The intermediate layers of the superior colliculus receive whisker-related excitatory afferents from the trigeminal complex and barrel cortex, inhibitory afferents from extrinsic and intrinsic sources, and neuromodulatory afferents from cholinergic and monoaminergic nuclei. However, it is not well known how these inputs regulate whisker-related activity in the superior colliculus. We found that barrel cortex afferents drive the superior colliculus during the middle portion of the rising phase of the whisker movement protraction elicited by artificial (fictive) whisking in anesthetized rats. In addition, both spontaneous and whisker-related neural activities in the superior colliculus are under strong inhibitory and neuromodulator control. Cholinergic stimulation activates the superior colliculus by increasing spontaneous firing and, in some cells, whisker-evoked responses. Monoaminergic stimulation has the opposite effects. The actions of neuromodulator and inhibitory afferents may be the basis of the different firing rates and sensory responsiveness observed in the superior colliculus of behaving animals during distinct behavioral states.

Dissociable effects of dopamine on learning and performance within sensorimotor striatum

Striatal dopamine is an important modulator of current behavior, as seen in the rapid and dramatic effects of dopamine replacement therapy in Parkinson Disease (PD). Yet there is also extensive evidence that dopamine acts as a learning signal, modulating synaptic plasticity within striatum to affect future behavior. Disentangling these "performance" and "learning" functions is important for designing effective, long-term PD treatments. We conducted a series of unilateral drug manipulations and dopamine terminal lesions in the dorsolateral striatum of rats highly-trained to perform brief instructed head/neck movements (two-alternative forced choice task). Reaction times and accuracy were measured longitudinally to determine if task behavior changed immediately, progressed over time, and/or persisted after drug withdrawal. Enhanced dopamine signaling with amphetamine caused an immediate, nonprogressive, and bilateral decrease in reaction times (RT). The altered RT distributions were consistent with reduced distance to threshold in the linear approach to threshold with ergodic rate (LATER) model of decision-making. Conversely, the dopamine antagonist flupenthixol caused experience-dependent, persistent changes in RT and accuracy indicative of a "learning" effect. These RT distributions were consistent with a slowed rate of approach to decision threshold. Our results show that dopaminergic signaling makes dissociable contributions to current and future behavior even within a single striatal subregion, and provide important clues for both models of normal decision-making and the design of novel drug therapies in PD.

The intralaminar thalamus-an expressway linking visual stimuli to circuits determining agency and action selection

Anatomical investigations have revealed connections between the intralaminar thalamic nuclei and areas such as the superior colliculus (SC) that receive short latency input from visual and auditory primary sensory areas. The intralaminar nuclei in turn project to the major input nucleus of the basal ganglia, the striatum, providing this nucleus with a source of subcortical excitatory input. Together with a converging input from the cerebral cortex, and a neuromodulatory dopaminergic input from the midbrain, the components previously found necessary for reinforcement learning in the basal ganglia are present. With this intralaminar sensory input, the basal ganglia are thought to play a primary role in determining what aspect of an organism's own behavior has caused salient environmental changes. Additionally, subcortical loops through thalamic and basal ganglia nuclei are proposed to play a critical role in action selection. In this mini review we will consider the anatomical and physiological evidence underlying the existence of these circuits. We will propose how the circuits interact to modulate basal ganglia output and solve common behavioral learning problems of agency determination and action selection.

The efference cascade, consciousness, and its self: naturalizing the first person pivot of action control

The 20 billion neurons of the neocortex have a mere hundred thousand motor neurons by which to express cortical contents in overt behavior. Implemented through a staggered cortical "efference cascade" originating in the descending axons of layer five pyramidal cells throughout the neocortical expanse, this steep convergence accomplishes final integration for action of cortical information through a system of interconnected subcortical way stations. Coherent and effective action control requires the inclusion of a continually updated joint "global best estimate" of current sensory, motivational, and motor circumstances in this process. I have previously proposed that this running best estimate is extracted from cortical probabilistic preliminaries by a subcortical neural "reality model" implementing our conscious sensory phenomenology. As such it must exhibit first person perspectival organization, suggested to derive from formating requirements of the brain's subsystem for gaze control, with the superior colliculus at its base. Gaze movements provide the leading edge of behavior by capturing targets of engagement prior to contact. The rotation-based geometry of directional gaze movements places their implicit origin inside the head, a location recoverable by cortical probabilistic source reconstruction from the rampant primary sensory variance generated by the incessant play of collicularly triggered gaze movements. At the interface between cortex and colliculus lies the dorsal pulvinar. Its unique long-range inhibitory circuitry may precipitate the brain's global best estimate of its momentary circumstances through multiple constraint satisfaction across its afferents from numerous cortical areas and colliculus. As phenomenal content of our sensory awareness, such a global best estimate would exhibit perspectival organization centered on a purely implicit first person origin, inherently incapable of appearing as a phenomenal content of the sensory space it serves.

Superior colliculus mediates cervical dystonia evoked by inhibition of the substantia nigra pars reticulata

Cervical dystonia (CD; spasmodic torticollis) can be evoked by inhibition of substantia nigra pars reticulata (SNpr) in the nonhuman primate (Burbaud et al., 1998; Dybdal et al., 2012). Suppression of GABAergic neurons that project from SNpr results in the disinhibition of the targets to which these neurons project. It therefore should be possible to prevent CD by inhibition of the appropriate nigral target region(s). Here we tested the hypothesis that the deep and intermediate layers of the superior colliculus (DLSC), a key target of nigral projections, are required for the emergence of CD. To test this hypothesis, we pretreated the DLSC of four macaques with the GABA(A) agonist muscimol to determine whether this treatment would prevent CD evoked by muscimol infusions in SNpr. Our data supported this hypothesis: inhibition of DLSC attenuated CD evoked by muscimol in SNpr in all four animals. In two of the four subjects, quadrupedal rotations were evoked by muscimol application into SNpr sites that were distinct from those that induced dystonia. We found that inhibition of DLSC did not significantly alter quadrupedal rotations, suggesting that this response is dissociable from the SNpr-evoked CD. Our results are the first to demonstrate a role of DLSC in mediating the expression of CD. Furthermore, these data reveal a functional relationship between SNpr and DLSC in regulating posture and movement in the nonhuman primate, raising the possibility that the nigrotectal pathway has potential as a target for therapeutic interventions for CD.

Segregated anatomical input to sub-regions of the rodent superior colliculus associated with approach and defense

The superior colliculus (SC) is responsible for sensorimotor transformations required to direct gaze toward or away from unexpected, biologically salient events. Significant changes in the external world are signaled to SC through primary multisensory afferents, spatially organized according to a retinotopic topography. For animals, where an unexpected event could indicate the presence of either predator or prey, early decisions to approach or avoid are particularly important. Rodents’ ecology dictates predators are most often detected initially as movements in upper visual field (mapped in medial SC), while appetitive stimuli are normally found in lower visual field (mapped in lateral SC). Our purpose was to exploit this functional segregation to reveal neural sites that can bias or modulate initial approach or avoidance responses. Small injections of Fluoro-Gold were made into medial or lateral sub-regions of intermediate and deep layers of SC (SCm/SCl). A remarkable segregation of input to these two functionally defined areas was found. (i) There were structures that projected only to SCm (e.g., specific cortical areas, lateral geniculate and suprageniculate thalamic nuclei, ventromedial and premammillary hypothalamic nuclei, and several brainstem areas) or SCl (e.g., primary somatosensory cortex representing upper body parts and vibrissae and parvicellular reticular nucleus in the brainstem). (ii) Other structures projected to both SCm and SCl but from topographically segregated populations of neurons (e.g., zona incerta and substantia nigra pars reticulata). (iii) There were a few brainstem areas in which retrogradely labeled neurons were spatially overlapping (e.g., pedunculopontine nucleus and locus coeruleus). These results indicate significantly more structures across the rat neuraxis are in a position to modulate defense responses evoked from SCm, and that neural mechanisms modulating SC-mediated defense or appetitive behavior are almost entirely segregated.

Regulation of Substantia Nigra Pars Reticulata GABAergic Neuron Activity by H₂O₂ via Flufenamic Acid-Sensitive Channels and KATP Channels

Substantia nigra pars reticulata (SNr) GABAergic neurons are key output neurons of the basal ganglia. Given the role of these neurons in motor control, it is important to understand factors that regulate their firing rate and pattern. One potential regulator is hydrogen peroxide (H₂O₂), a reactive oxygen species that is increasingly recognized as a neuromodulator. We used whole-cell current clamp recordings of SNr GABAergic neurons in guinea-pig midbrain slices to determine how H₂O₂ affects the activity of these neurons and to explore the classes of ion channels underlying those effects. Elevation of H₂O₂ levels caused an increase in the spontaneous firing rate of SNr GABAergic neurons, whether by application of exogenous H₂O₂ or amplification of endogenous H₂O₂ through inhibition of glutathione peroxidase with mercaptosuccinate. This effect was reversed by flufenamic acid (FFA), implicating transient receptor potential (TRP) channels. Conversely, depletion of endogenous H₂O₂ by catalase, a peroxidase enzyme, decreased spontaneous firing rate and firing precision of SNr neurons, demonstrating tonic control of firing rate by H₂O₂. Elevation of H₂O₂ in the presence of FFA revealed an inhibition of tonic firing that was prevented by blockade of ATP-sensitive K(+) (K(ATP)) channels with glibenclamide. In contrast to guinea-pig SNr neurons, the dominant effect of H₂O₂ elevation in mouse SNr GABAergic neurons was hyperpolarization, indicating a species difference in H₂O₂-dependent regulation. Thus, H₂O₂ is an endogenous modulator of SNr GABAergic neurons, acting primarily through presumed TRP channels in guinea-pig SNr, with additional modulation via K(ATP) channels to regulate SNr output.

Behavioral pharmacology of orofacial movement disorders

Dysfunction in orofacial movement is evident in patients with schizophrenia, Parkinson's disease and Huntington's disease. In animal studies on orofacial dyskinesia, these neurological disorders have been considered as a starting point to examine the pathophysiology and mechanisms underlying the symptoms. There is circumstantial evidence that orofacial dyskinesia in humans might be the consequence of hyperfunctioning mesolimbic-pallidal circuitry, in which the mesolimbic region occupies a central role, in contrast to typical Parkinson-like symptoms which involve hypofunction in the nigrostriato-nigral circuity. Studies in animals suffer from technical difficulties concerning the assessment of orofacial behaviors. There are some experimental designs that provide detailed information on the amplitude and the frequency of the jaw movements. By using such methods, the involvement of neurotransmitter systems and functional neural connections within the basal ganglia has been studied in rat rhythmical jaw movements. Regarding neurotransmitter systems, dopaminergic, cholinergic, γ-aminobutyric acid (GABA)ergic and glutamaterigic systems have been shown to be involved in rat rhythmical jaw movements. The involved neural connections have also been investigated, focusing on the differential role between the dorsal and ventral part of the striatum, the shell and core of the nucleus accumbens and the output pathways from the striatum and the nucleus accumbens. Taking available clinical and experimental evidence, the orofacial dyskinesias are thought to arise when hierarchically lower order output stations of the mesolimbic region start to dysfunction as a consequence of the arrival of distorted information sent by the mesolimbic region. This review seeks to provide an overview of prior and recent findings across several orofacial movement disorders and interpret new insights in the context of the limitations of behavioral pharmacology and prior knowledge of the regulation of behavior by dopamine receptors and other related neuronal systems.

Interactions between the Midbrain Superior Colliculus and the Basal Ganglia

An important component of the architecture of cortico-basal ganglia connections is the parallel, re-entrant looped projections that originate and return to specific regions of the cerebral cortex. However, such loops are unlikely to have been the first evolutionary example of a closed-loop architecture involving the basal ganglia. A phylogenetically older, series of subcortical loops can be shown to link the basal ganglia with many brainstem sensorimotor structures. While the characteristics of individual components of potential subcortical re-entrant loops have been documented, the full extent to which they represent functionally segregated parallel projecting channels remains to be determined. However, for one midbrain structure, the superior colliculus (SC), anatomical evidence for closed-loop connectivity with the basal ganglia is robust, and can serve as an example against which the loop hypothesis can be evaluated for other subcortical structures. Examination of ascending projections from the SC to the thalamus suggests there may be multiple functionally segregated systems. The SC also provides afferent signals to the other principal input nuclei of the basal ganglia, the dopaminergic neurones in substantia nigra and to the subthalamic nucleus. Recent electrophysiological investigations show that the afferent signals originating in the SC carry important information concerning the onset of biologically significant events to each of the basal ganglia input nuclei. Such signals are widely regarded as crucial for the proposed functions of selection and reinforcement learning with which the basal ganglia have so often been associated.

Basal ganglia control of substantia nigra dopaminergic neurons

Although substantia nigra dopaminergic neurons are spontaneously active both in vivo and in vitro, this activity does not depend on afferent input as these neurons express an endogenous calcium-dependent oscillatory mechanism sufficient to drive action potential generation. However, afferents to these neurons, a large proportion of them GABAergic and arising from other nuclei in the basal ganglia, play a crucial role in modulating the activity of dopaminergic neurons. In the absence of afferent activity or when in brain slices, dopaminergic neurons fire in a very regular, pacemaker-like mode. Phasic activity in GABAergic, glutamatergic, and cholinergic inputs modulates the pacemaker activity into two other modes. The most common is a random firing pattern in which interspike intervals assume a Poisson-like distribution, and a less common pattern, often in response to a conditioned stimulus or a reward in which the neurons fire bursts of 2-8 spikes time-locked to the stimulus. Typically in vivo, all three firing patterns are observed, intermixed, in single nigrostriatal neurons varying over time. Although the precise mechanism(s) underlying the burst are currently the focus of intensive study, it is obvious that bursting must be triggered by afferent inputs. Most of the afferents to substantia nigra pars compacta dopaminergic neurons comprise monosynaptic inputs from GABAergic projection neurons in the ipsilateral neostriatum, the globus pallidus, and the substantia nigra pars reticulata. A smaller fraction of the basal ganglia inputs, something less than 30%, are glutamatergic and arise principally from the ipsilateral subthalamic nucleus and pedunculopontine nucleus. The pedunculopontine nucleus also sends a cholinergic input to nigral dopaminergic neurons. The GABAergic pars reticulata projection neurons also receive inputs from all of these sources, in some cases relaying them disynaptically to the dopaminergic neurons, thereby playing a particularly significant role in setting and/or modulating the firing pattern of the nigrostriatal neurons.

Novelty encoding by the output neurons of the Basal Ganglia

Reinforcement learning models of the basal ganglia have focused on the resemblance of the dopamine signal to the temporal difference error. However the role of the network as a whole is still elusive, in particular whether the output of the basal ganglia encodes only the behavior (actions) or it is part of the valuation process. We trained a monkey extensively on a probabilistic conditional task with seven fractal cues predicting rewarding or aversive outcomes (familiar cues). Then in each recording session we added a cue that the monkey had never seen before (new cue) and recorded from single units in the Substantia Nigra pars reticulata (SNpr) while the monkey was engaged in a task with new cues intermingled within the familiar ones. The monkey learned the association between the new cue and outcome and modified its licking and blinking behavior which became similar to responses to the familiar cues with the same outcome. However, the responses of many SNpr neurons to the new cue exceeded their response to familiar cues even after behavioral learning was completed. This dissociation between behavior and neural activity suggests that the BG output code goes beyond instruction or gating of behavior to encoding of novel cues. Thus, BG output can enable learning at the levels of its target neural networks.

Short-latency visual input to the subthalamic nucleus is provided by the midbrain superior colliculus

The subthalamic nucleus (STN) is one of the principal input nuclei of the basal ganglia. Using electrophysiological techniques in anesthetized rats, we show that the STN becomes responsive to visual stimuli at short latencies when local disinhibitory injections are made into the midbrain superior colliculus (SC), an important subcortical visual structure. Significantly, only injections into the lateral, but not medial, deep layers of the SC were effective. Corresponding disinhibition of primary visual cortex also was ineffective. Complementary anatomical analyses revealed a strong, regionally specific projection from the deep layers of the lateral SC to neurons in rostral and dorsal sectors of the STN. Given the retinocentric organization of the SC, these results suggest that lower-field stimuli represented in the lateral colliculus have a direct means of communicating with the basal ganglia via the STN that is not afforded to visual events occurring in the upper visual field.

Phenotype, compartmental organization and differential vulnerability of nigral dopaminergic neurons

The degeneration of nigral dopaminergic (DA-) neurons is the histopathologic hallmark of Parkinson's disease (PD), but not all nigral DA-cells show the same susceptibility to degeneration. This starts in DA-cells in the ventrolateral and caudal regions of the susbtantia nigra (SN) and progresses to DA-cells in the dorsomedial and rostral regions of the SN and the ventral tegmental area, where many neurons remain intact until the final stages of the disease. This fact indicates a relationship between the topographic distribution of midbrain DA-cells and their differential vulnerability, and the possibility that this differential vulnerability is associated with phenotypic differences between different subpopulations of nigral DA-cells. Studies carried out during the last two decades have contributed to establishing the existence of different compartments of nigral DA-cells according to their neurochemical profile, and a possible relationship between the expression of some factors and the relative vulnerability or resistance of DA-cell subpopulations to degeneration. These aspects are reviewed and discussed here.

Encoding by Response Duration in the Basal Ganglia

Several models have suggested that information transmission in the basal ganglia (BG) involves gating mechanisms, where neuronal activity modulates the extent of gate aperture and its duration. Here, we demonstrate that BG response duration is informative about a highly abstract stimulus feature and show that the duration of “gate opening” can indeed be used for information transmission through the BG. We analyzed recordings from three BG locations: the external part of the globus pallidus (GPe), the substantia nigra pars reticulata (SNr), and dopaminergic neurons from the substantia nigra pars compacta (SNc) during performance of a probabilistic visuomotor task. Most (>85%) of the neurons showed significant rate modulation following the appearance of cues predicting future reward. Trial-to-trial mutual information analysis revealed that response duration encoded reward prospects in many (42%) of the responsive SNr neurons, as well as in the SNc (26.9%), and the GPe (29.3%). Whereas the low-frequency discharge SNc neurons responded with only an increase in firing rate, SNr and GPe neurons with high-frequency tonic discharge responded with both increases and decreases. Conversely, many duration-informative neurons in SNr (68%) and GPe (50%) responded with a decreased rather than an increased rate. The response duration was more informative than the extreme (minimal or maximal) amplitude or spike count in responsive bins of duration-informative neurons. Thus response duration is not simply correlated with the discharge rate and can provide additional information to the target structures of the BG.

Substantia nigra stimulation influences monkey superior colliculus neuronal activity bilaterally

The inhibitory drive arising from the basal ganglia is thought to prevent the occurrence of orienting movements of the eyes, head, and body in monkeys and other mammals. The direct projection from the substantia nigra pars reticulata (SNr) to the superior colliculus (SC) mediates the inhibition. Since the original experiments in the SNr of monkeys the buildup or prelude neuron has been a focus of SC research. However, whether the SNr influences buildup neurons in SC is unknown. Furthermore, a contralateral SNr-SC pathway is evident in many species but remains unexplored in the alert monkey. Here we introduced electrical stimulation of one or both SNr nuclei while recording from SC buildup neurons. Stimulation of the SNr reduced the discharge rate of SC buildup neurons bilaterally. This result is consistent with activation of an inhibitory drive from SNr to SC. The time course of the influence of ipsilateral SNr on the activity of most SC neurons was longer (approximately 73 ms) than the influence of the contralateral SNr (approximately 34 ms). We also found that the variability of saccade onset time and saccade direction was altered with electrical stimulation of the SNr. Taken together our results show that electrical stimulation activates the inhibitory output of the SNr that in turn, reduces the activity of SC buildup neurons in both hemispheres. However, rather than acting as a gate for saccade initiation, the results suggest that the influence of SNr inhibition on visually guided saccades is more subtle, shaping the balance of excitation and inhibition across the SC.

Morphological and physiological properties of parvalbumin- and calretinin-containing gamma-aminobutyric acidergic neurons in the substantia nigra

Evidence for the existence of different populations of gamma-aminobutyric acid (GABA)-ergic neurons in the substantia nigra comes partially from anatomical studies, which have shown there to be little if any overlap between the calcium-binding proteins parvalbumin and calretinin in individual neurons, suggesting that these may represent neuronal subtypes with distinct electrophysiological and/or anatomical properties. We obtained whole-cell recordings from neurons in the substantia nigra pars reticulata in rat brain slices and labeled them with biocytin, followed by immunocytochemical staining for parvalbumin and calretinin. In other cases, neurons were retrogradely labeled from the thalamus or tectum and immunocytochemically identified to determine their projection sites. Intracellularly stained neurons were found to have a variety of somatic sizes and shapes. Reconstructions revealed that all parvalbumin- and calretinin-positive neurons issued at least one axon collateral, which ramified within the substantia nigra pars reticulata and/or pars compacta. Local collaterals were of medium caliber and branched modestly, expressing many long, smooth segments that then issued numerous en passant or terminal boutons, consistent with previous in vivo studies. There were no clear differences in the electrophysiological or morphological properties of neurons expressing parvalbumin or calretinin. Retrograde tracing experiments revealed that both parvalbumin- and calretinin-containing neurons project nonpreferentially to the thalamus or tectum. In sum, the parvalbumin- and calretinin-containing GABAergic neurons of the substantia nigra pars reticulata cannot be differentiated on the basis of their electrophysiological properties, morphological properties, or target nuclei, and both parvalbumin- and calretinin-containing projection neurons issue local axon collaterals that arborize within the substantia nigra.

Projections of somatosensory cortex and frontal eye fields onto incertotectal neurons in the cat

The goal of this study was to determine whether the input-output characteristics of the zona incerta (ZI) are appropriate for it to serve as a conduit for cortical control over saccade-related activity in the superior colliculus. The study utilized the neuronal tracers wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and biotinylated dextran amine (BDA) in the cat. Injections of WGA-HRP into primary somatosensory cortex (SI) revealed sparse, widespread nontopographic projections throughout ZI. In addition, region-specific areas of more intense termination were present in ventral ZI, although strict topography was not observed. In comparison, the frontal eye fields (FEF) also projected sparsely throughout ZI, but terminated more heavily, medially, along the border between the two sublaminae. Furthermore, retrogradely labeled incertocortical neurons were observed in both experiments. The relationship of these two cortical projections to incertotectal cells was also directly examined by retrogradely labeling incertotectal cells with WGA-HRP in animals that had also received cortical BDA injections. Labeled axonal arbors from both SI and FEF had thin, sparsely branched axons with numerous en passant boutons. They formed numerous close associations with the somata and dendrites of WGA-HRP-labeled incertotectal cells. In summary, these results indicate that both sensory and motor cortical inputs to ZI display similar morphologies and distributions. In addition, both display close associations with incertotectal cells, suggesting direct synaptic contact. From these data, we conclude that inputs from somatosensory and FEF cortex both play a role in controlling gaze-related activity in the superior colliculus by way of the inhibitory incertotectal projection.

Substantia nigra osmoregulation: taurine and ATP involvement

An extracellular nonsynaptic taurine pool of glial origin was recently reported in the substantia nigra (SN). There is previous evidence showing taurine as an inhibitory neurotransmitter in the SN, but the physiological role of this nonsynaptic pool of taurine has not been explored. By using microdialysis methods, we studied the action of local osmolarity on the nonsynaptic taurine pool in the SN of the rat. Hypoosmolar pulses (285-80 mosM) administered in the SN by the microdialysis probe increased extrasynaptic taurine in a dose-dependent way, a response that was counteracted by compensating osmolarity with choline. The opposite effect (taurine decrease) was observed when osmolarity was increased. Under basal conditions, the blockade of either the AMPA-kainate glutamate receptors with 6-cyano-7-nitroquinoxaline-2,3-dionine disodium or the purinergic receptors with pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid modified the taurine concentration, suggesting that both receptors modulate the extrasynaptic pool of taurine. In addition, these drugs decreased the taurine response to hypoosmolar pulses, suggesting roles for glutamatergic and purinergic receptors in the taurine response to osmolarity. The participation of purinergic receptors was also supported by the fact that ATP (which, under basal conditions, increased the extrasynaptic taurine in a dose-dependent way) administered in doses saturating purinergic receptors also decreased the taurine response to hypoosmolarity. Taken together, present data suggest osmoregulation as a role of the nonsynaptic taurine pool of the SN, a function that also involves glutamate and ATP and that could influence the nigral cell vulnerability in Parkinson's disease.

The pars reticulata of the substantia nigra: a window to basal ganglia output

Together with the internal segment of the globus pallidus (GP(i)), the pars reticulata of the substantia nigra (SNr) provides a main output nucleus of the basal ganglia (BG) where the final stage of information processing within this system takes place. In the last decade, progress on the anatomical organization and functional properties of BG output neurons have shed some light on the mechanisms of integration taking place in these nuclei and leading to normal and pathological BG outflow. In this review focused on the SNr, after describing how the anatomical arrangement of nigral cells and their afferents determines specific input-output registers, we examine how the basic electrophysiological properties of the cells and their interaction with synaptic inputs contribute to the spatio-temporal shaping of BG output. The reported data show that the intrinsic membrane properties of the neurons subserves a tonic discharge allowing BG to gate the transmission of information to motor and cognitive systems thereby contributing to appropriate selection of behavior.

Topographic and functional neuroanatomical study of GABAergic disinhibitory striatum–nigral inputs and inhibitory nigrocollicular pathways: Neural hodology recruiting the substantia nigra, pars reticulata, for the modulation of the neural activity in the inferior colliculus involved with panic-like emotions

Considering the influence of the substantia nigra on mesencephalic neurons involved with fear-induced reactions organized in rostral aspects of the dorsal midbrain, the present work investigated the topographical and functional neuroanatomy of similar influence on caudal division of the corpora quadrigemina, addressing: (a) the neural hodology connecting the neostriatum, the substantia nigra, periaqueductal gray matter and inferior colliculus (IC) neural networks; (b) the influence of the inhibitory neostriatonigral-nigrocollicular GABAergic links on the control of the defensive behavior organized in the IC. The effects of the increase or decrease of activity of nigrocollicular inputs on defensive responses elicited by either electrical or chemical stimulation of the IC were also determined. Electrolytic or chemical lesions of the substantia nigra, pars reticulata (SNpr), decreased the freezing and escape behaviors thresholds elicited by electrical stimulation of the IC, and increased the behavioral responses evoked by the GABAA blockade in the same sites of the mesencephalic tectum (MT) electrically stimulated. These findings were corroborated by similar effects caused by microinjections of the GABAA-receptor agonist muscimol in the SNpr, followed by electrical and chemical stimulations of the IC. The GABAA blockade in the SNpr caused a significant increase in the defensive behavior thresholds elicited by electrical stimulation of the IC and a decrease in the mean incidence of panic-like responses induced by microinjections of bicuculline in the mesencephalic tectum (inferior colliculus). These findings suggest that the substantia nigra receives GABAergic inputs that modulate local and also inhibitory GABAergic outputs toward the IC. In fact, neurotracing experiments with fast blue and iontophoretic microinjections of biotinylated dextran amine either into the inferior colliculus or in the reticular division of the substantia nigra demonstrated a neural link between these structures, as well as between the neostriatum and SNpr.

Sex-specific control of flurothyl-induced tonic-clonic seizures by the substantia nigra pars reticulata during development

The substantia nigra pars reticulata (SNR) plays an important age- and sex-specific role in control of clonic seizures. Its involvement in control of tonic-clonic seizures is contradictory. We investigated the role of the SNR in the tonic-clonic seizures induced in male, female and neonatally castrated male rats using flurothyl. In adult female rats, vaginal impedance determined the changes in progesterone/estrogen ratio. Rats at various postnatal ages received infusions of muscimol or vehicle in the SNRanterior or SNRposterior. Furthermore, in 15-day-old (P15) and adult male rats, ZAPA (a GABA(A) receptor agonist) or AP7 (an NMDA receptor antagonist) was infused. The developmental profile of tonic-clonic seizure threshold differed between male and female rats possibly due to early postnatal testosterone surge in male rats. On the other hand, changing estrogen/progesterone ratio in cycling adult female rats had no effect on seizure threshold. Intranigral muscimol had proconvulsant effects on tonic-clonic seizures only in immature rats, and this effect was dependent on the perinatal testosterone surge. ZAPA had anticonvulsant effects in P15 rats but was not effective in adult rats. Only AP7 had anticonvulsant effects in both adult and P15 rats. Results indicate that thresholds for flurothyl-induced tonic-clonic seizures develop under the control of postnatal testosterone. Although GABAergic inhibition in the SNR affects tonic-clonic seizures in developing rats, only the NMDA antagonist had consistent anticonvulsant effects throughout development.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

A direct projection from superior colliculus to substantia nigra pars compacta in the cat

Dopaminergic neurons exhibit a short-latency, phasic response to unexpected, biologically salient stimuli. The midbrain superior colliculus also is sensitive to such stimuli, exhibits sensory responses with latencies reliably less than those of dopaminergic neurons, and, in rat, has been shown to send direct projections to regions of the substantia nigra and ventral tegmental area containing dopaminergic neurons (e.g. pars compacta). Recent electrophysiological and electrochemical evidence also suggests that tectonigral connections may be critical for relaying short-latency (<100 ms) visual information to midbrain dopaminergic neurons. By investigating the tectonigral projection in the cat, the present study sought to establish whether this pathway is a specialization of the rodent, or whether it may be a more general feature of mammalian neuroanatomy. Anterogradely and retrogradely transported anatomical tracers were injected into the superior colliculus and substantia nigra pars compacta, respectively, of adult cats. In the anterograde experiments, abundant fibers and terminals labeled with either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin were seen in close association with tyrosine hydroxylase-positive (dopaminergic) somata and processes in substantia nigra pars compacta and the ventral tegmental area. In the retrograde experiments, injections of biotinylated dextran amine into substantia nigra produced significant retrograde labeling of tectonigral neurons of origin in the intermediate and deep layers of the ipsilateral superior colliculus. Approximately half of these biotinylated dextran amine-labeled neurons were, in each case, shown to be immunopositive for the calcium binding proteins, parvalbumin or calbindin. Significantly, virtually no retrogradely labeled neurons were found either in the superficial layers of the superior colliculus or among the large tecto-reticulospinal output neurons. Taken in conjunction with recent data in the rat, the results of this study suggest that the tectonigral projection may be a common feature of mammalian midbrain architecture. As such, it may represent an additional route by which short-latency sensory information can influence basal ganglia function.

Patterns of axonal branching of neurons of the substantia nigra pars reticulata and pars lateralis in the rat

Axons from neurons of the rat substantia nigra pars reticulata (SNr) and pars lateralis (SNl) were traced after injecting their cell body with biotinylated dextran amine. Thirty-two single axons were reconstructed from serial sagittal sections with a camera lucida, whereas four other SNr axons were reconstructed in the coronal plane to determine whether they innervate the contralateral hemisphere. Four distinct types of SNr projection neurons were identified based on their main axonal targets: type I neurons that project to the thalamus; type II neurons that target the thalamus, the superior colliculus (SC), and the pedunculopontine tegmental nucleus (PPTg); type III neurons that project to the periaqueductal gray matter and the thalamus; and type IV neurons that target the deep mesencephalic nucleus (DpMe) and the SC. The axons of the SNl showed the same branching patterns as SNr axons of types I, II, and IV. The coronal reconstructions demonstrated that SNr neurons innervate the thalamus, the SC, and the DpMe bilaterally. At the thalamic level, SNr and SNl axons targeted preferentially the ventral medial, ventral lateral, paracentral, parafascicular, and mediodorsal nuclei. Axons reaching the SC arborized selectively within the deep layers of this structure. Our results reveal that the SNr and SNl harbor several subtypes of projection neurons endowed with a highly patterned set of axon collaterals. This organization allows single neurons of these output structures of the basal ganglia to exert a multifaceted influence on a wide variety of diencephalic and midbrain structures.