Localization of motor- and nonmotor-related neurons within the matrix-striosome organization of rat striatum

The role of nicotinic cholinergic neurotransmission in delusional thinking

Delusions are a difficult-to-treat and intellectually fascinating aspect of many psychiatric illnesses. Although scientific progress on this complex topic has been challenging, some recent advances focus on dysfunction in neural circuits, specifically in those involving dopaminergic and glutamatergic neurotransmission. Here we review the role of cholinergic neurotransmission in delusions, with a focus on nicotinic receptors, which are known to play a part in some illnesses where these symptoms appear, including delirium, schizophrenia spectrum disorders, bipolar disorder, Parkinson, Huntington, and Alzheimer diseases. Beginning with what we know about the emergence of delusions in these illnesses, we advance a hypothesis of cholinergic disturbance in the dorsal striatum where nicotinic receptors are operative. Striosomes are proposed to play a central role in the formation of delusions. This hypothesis is consistent with our current knowledge about the mechanism of action of cholinergic drugs and with our abstract models of basic cognitive mechanisms at the molecular and circuit levels. We conclude by pointing out the need for further research both at the clinical and translational levels.

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

Head direction (HD) cells fire as a function of the animal's directional heading and provide the animal with a sense of direction. In rodents, these neurons are located primarily within the limbic system, but small populations of HD cells are found in two extralimbic areas: the medial precentral cortex (PrCM) and dorsal striatum (DS). HD cell activity in these structures could be driven by output from the limbic HD circuit or generated intrinsically. We examined these possibilities by recording the activity of PrCM and DS neurons in control rats and in rats with anterodorsal thalamic nucleus (ADN) lesions, a manipulation that disrupts the limbic HD signal. HD cells in the PrCM and DS of control animals displayed characteristics similar to those of limbic HD cells, and these extralimbic HD signals were eliminated in animals with complete ADN lesions, suggesting that the PrCM and DS HD signals are conveyed from the limbic HD circuit. Angular head velocity cells recorded in the PrCM and DS were unaffected by ADN lesions. Next, we determined if the PrCM and DS convey necessary self-motion signals to the limbic HD circuit. Limbic HD cell activity recorded in the ADN remained intact following combined lesions of the PrCM and DS. Collectively, these experiments reveal a unidirectional functional relationship between the limbic HD circuit and the PrCM and DS; the limbic system generates the HD signal and transmits it to the PrCM and DS, but these extralimbic areas do not provide critical input or feedback to limbic HD cells. NEW & NOTEWORTHY Head direction (HD) cells have been extensively studied within the limbic system. The lesion and recording experiments reported here examined two relatively understudied populations of HD cells located outside of the canonical limbic HD circuit in the medial precentral cortex and dorsal striatum. We found that HD cell activity in these two extralimbic areas is driven by output from the limbic HD circuit, revealing that HD cell circuitry functionally extends beyond the limbic system.

The neostriatum: two entities, one structure?

The striosome (or patch) was first identified with anatomical techniques as neurons organized in a three-dimensional labyrinth inserted in and interdigitating the rest of neostriatum: the matrix. Striosome and matrix rapidly became known as two neuronal compartments expressing different biochemical markers, embryonic development and afferent and efferent connectivity. In spite of extensive intrinsic neuronal axonal and dendritic extensions supposed to exchange information between matrix and striosomes, evidence suggested the presence of independent areas. Here, we report that indeed these two areas do not exchange synaptic information. We used genetic expression of channel rhodopsin 2 carried by adeno-associated virus serotype 10 (AAVrh10) that only expresses in neurons of the matrix compartment. Whole-cell patch-clamp recordings of matrix neurons activated by light pulses consistently produced inhibitory postsynaptic currents (IPSCs), but the same manipulation did not evoke IPSCs in striosome neurons. The matrix contains both direct and indirect striatal output pathways. By targeting striatal matrix expression of designer receptors exclusively activated by a designer drug (DREADD) hM3di carried by AAVrh10, we were able to inhibit the matrix neuronal compartment of the dorsolateral striatum during performance of a learned single-pellet reach-to-grasp task. As expected, inhibition of matrix neurons by systemic administration of DREADD agonist clozapine-n-oxide interfered with performance of the learned task.

Basal Ganglia disorders associated with imbalances in the striatal striosome and matrix compartments

The striatum is composed principally of GABAergic, medium spiny striatal projection neurons (MSNs) that can be categorized based on their gene expression, electrophysiological profiles, and input–output circuits. Major subdivisions of MSN populations include (1) those in ventromedial and dorsolateral striatal regions, (2) those giving rise to the direct and indirect pathways, and (3) those that lie in the striosome and matrix compartments. The first two classificatory schemes have enabled advances in understanding of how basal ganglia circuits contribute to disease. However, despite the large number of molecules that are differentially expressed in the striosomes or the extra-striosomal matrix, and the evidence that these compartments have different input–output connections, our understanding of how this compartmentalization contributes to striatal function is still not clear. A broad view is that the matrix contains the direct and indirect pathway MSNs that form parts of sensorimotor and associative circuits, whereas striosomes contain MSNs that receive input from parts of limbic cortex and project directly or indirectly to the dopamine-containing neurons of the substantia nigra, pars compacta. Striosomes are widely distributed within the striatum and are thought to exert global, as well as local, influences on striatal processing by exchanging information with the surrounding matrix, including through interneurons that send processes into both compartments. It has been suggested that striosomes exert and maintain limbic control over behaviors driven by surrounding sensorimotor and associative parts of the striatal matrix. Consistent with this possibility, imbalances between striosome and matrix functions have been reported in relation to neurological disorders, including Huntington’s disease, L-DOPA-induced dyskinesias, dystonia, and drug addiction. Here, we consider how signaling imbalances between the striosomes and matrix might relate to symptomatology in these disorders.

Partial ablation of mu-opioid receptor rich striosomes produces deficits on a motor-skill learning task

Basal ganglia striosomes, or patches, are rich in mu opioid receptors (MOR) and form a three-dimensional labyrinth of cells that extend throughout the mid- and anterior striatum in mice. Though previous studies have suggested that striosomes could affect drug-induced motor output in rodents, the functional role of these compartmentalized MOR-rich striosomes is not well understood. To investigate any relationship between the striosomes and motor behavior we used the toxin dermorphin-saporin (DS) to selectively ablate MOR-rich striosomal cells. FVB mice were bilaterally infused with DS in the midstriatum alone or in the mid- and anterior striatum, and were tested on three motor tasks and in an open field. Two volume measurement procedures and stereological cell counts were used to confirm the induced pathology. Mice that received DS injections showed significantly smaller volumes (-26% to -44%) and fewer cells (-30% to -49%) in the striosome compartment compared to mice that received control injections of saline or saporin. Striosome pathology was greatest in the dorsolateral striatum. The extrastriosomal matrix was not significantly affected, resulting in an imbalance in the ratio of striosome-to-matrix cells. Behaviorally, toxin injections caused deficits on an accelerating rotarod task and the deficit was worse in mice that received mid and anterior injections than those that received midstriatal injections alone. However, DS-injected mice did not differ from control mice on other motor tasks. We conclude that the MOR-rich cells of the striosomes are necessary for optimal rotarod performance, including learning and/or improvement on the task.

Behavior-related neuron reactions and the dynamics of neuronal activity

The spike activity of neurons with associative functions, studied as a neural correlate of behavior, constitutes an efferent spike flow which operates continuously throughout each of an animal's actions. However, research practice is dominated by methods based on the "stimulus-response" principle. Analysis from other points of view, directed to investigating the dynamics of neuron activity in monkeys during performance of a multistep behavioral program showed that differences in neuron activity of neurons during performance of different behavioral actions are not so much in terms of the numbers of cells responding or the activation of neurons specialized for the action concerned as in terms of changes in the composition of active neurons, with the formation of different combinations. As each combination of active neurons determines the distribution of efferent signals characteristic of this combination, these data suggest the existence of an addressing mechanism encoding efferent signals.

Basal ganglia neural mechanisms of natural movement sequences

Natural rodent grooming and other instinctive behavior serves as a natural model of complex movement sequences. Rodent grooming has syntactic (rule-driven) sequences and more random movement patterns. Both incorporate the same movements--only the serial structure differs. Recordings of neural activity in the dorsolateral striatum and the substantia nigra pars reticulata indicate preferential activation during syntactic sequences over more random sequences. Neurons that are responsive during syntactic grooming sequences are often unresponsive or have reverse activation profiles during kinematically similar movements that occur in flexible or random grooming sequences. Few neurons could be categorized as strictly movement related--instead they were activated only in the context of particular sequential patterns of movements. Particular sequential patterns included "syntactic chain" grooming sequences of paw, head, and body movements and also "warm-up" sequences, which consist of head and body/limb movements that precede locomotion after a period of quiet resting (Golani 1992). Activation during warm-up was less intense and less frequent than during grooming sequences, but both sequences activated neurons above baseline levels, and the same neurons sometimes responded to both sequences. The fact that striatal neurons code 2 natural sequences which are made up of different constituent movements suggests that the basal ganglia may have a generalized role in sequence control. The basal ganglia are modulated by the context of the sequence and may play an executive function in the complex natural patterns of sequenced behaviour.

Network-level neuroplasticity in cortico-basal ganglia pathways

The striatum, the largest input nucleus of the basal ganglia, receives massive inputs from the neocortex and thalamus, and gives rise to the direct, indirect and striosomal pathways of the basal ganglia. Here, the view is developed that the striatum is a major site for adaptive plasticity in cortico-basal ganglia circuits, affecting in the normal state a broad range of behaviours. This plasticity can become a major source of maladaptive responses in disease states affecting the basal ganglia.

Neuronal activity in the rodent dorsal striatum in sequential navigation: separation of spatial and reward responses on the multiple T task

The striatum plays an important role in "habitual" learning and memory and has been hypothesized to implement a reinforcement-learning algorithm to select actions to perform given the current sensory input. Many experimental approaches to striatal activity have made use of temporally structured tasks, which imply that the striatal representation is temporal. To test this assumption, we recorded neurons in the dorsal striatum of rats running a sequential navigation task: the multiple T maze. Rats navigated a sequence of four T maze turns to receive food rewards delivered in two locations. The responses of neurons that fired phasically were examined. Task-responsive phasic neurons were active as rats ran on the maze (maze-responsive) or during reward receipt (reward-responsive). Neither maze- nor reward-responsive neurons encoded simple motor commands: maze-responses were not well correlated with the shape of the rat's path and most reward-responsive neurons did not fire at similar rates at both food-delivery sites. Maze-responsive neurons were active at one or more locations on the maze, but these responses did not cluster at spatial landmarks such as turns. Across sessions the activity of maze-responsive neurons was highly correlated when rats ran the same maze. Maze-responses encoded the location of the rat on the maze and imply a spatial representation in the striatum in a task with prominent spatial demands. Maze-responsive and reward-responsive neurons were two separate populations, suggesting a divergence in striatal information processing of navigation and reward.

Multiple parallel memory systems in the brain of the rat

A theory of multiple parallel memory systems in the brain of the rat is described. Each system consists of a series of interconnected neural structures. The "central structures" of the three systems described are the hippocampus, the matrix compartment of the dorsal striatum (caudate-putamen), and the amygdala. Information, coded as neural signals, flows independently through each system. All systems have access to the same information from situations in which learning occurs, but each system is specialized to represent a different kind of relationship among the elements (stimulus events, responses, reinforcers) of the information that flows through it. The speed and accuracy with which a system forms a coherent representation of a learning situation depend on the correspondence between the specialization of the system and the relationship among the elements of the situation. The coherence of these stored representations determines the degree of control exerted by each system on behavior in the situation. Although they process information independently the systems interact in at least two ways: by simultaneous parallel influence on behavioral output and by directly influencing each other. These interactions can be cooperative (leading to similar behaviors) or competitive (leading to different behaviors). Experimental findings consistent with these ideas, mostly from experiments with rats, are reviewed.

Differential metabolic activity in the striosome and matrix compartments of the rat striatum during natural behaviors

The striosome and matrix compartments of the striatum are clearly identified by their neurochemical expression patterns and anatomical connections. To determine whether these compartments are distinguishable functionally, we used [14C]deoxyglucose metabolic mapping in the rat and tested whether neutral behavioral states (free movement, gentle restraint, and focal tactile stimulation under gentle restraint) were associated with regions of high metabolic activity in the matrix, in striosomes, or in both. We identified metabolic peaks in the striatum by means of image analysis, striosome-matrix boundaries by [3H]naloxone binding, and primary somatosensory corticostriatal input clusters by injections of anterograde tracer into electrophysiologically identified sites in SI. Peak metabolic activity was primarily confined to the matrix compartment under each behavioral condition. These findings show that during relatively neutral behavioral conditions the balance of activity between the two compartments favors the matrix and suggest that this balance is present in the striatum as part of normal behavior and processing of afferent activity.

Alterations in spontaneous single unit activity of striatal subdivisions during ontogenesis in mutant dystonic hamsters

The pathophysiology of idiopathic dystonia, characterized by sustained twisting movements and postures, is still unknown. Clinically, however, the basal ganglia are thought to be the main causative origin of idiopathic dystonia. In the dtsz hamster, a genetic animal model for idiopathic paroxysmal dystonia, the attacks occur in response to mild stress and the severity of dystonia is age-dependent. Previous autoradiographic studies in the dtsz hamster revealed a decreased dopamine D1 and D2 receptor binding and an increased [3H]-2-deoxyglucose uptake in the dorsomedial caudate-putamen (CPu), a region supposed to be critically involved in dystonia. Therefore, we were interested whether the spontaneous firing rate of dorsomedial striatal neurons is age-dependently altered in comparison to age-matched non-dystonic control hamsters. Extracellular recordings of spontaneous single unit activity of dorsomedial and ventromedial Type II striatal neurons, i.e., biphasic positive-negative action potentials, from fentanyl anesthetized animals revealed a drastically increased firing rate in the dorsomedial CPu of mutants during age of maximum severity of dystonia. In post-dystonic dtsz hamsters, i.e., after remission of stress-inducible dystonia, no significant differences regarding the dorsomedial CPu could be obtained. We conclude that the dorsomedial subregion of the CPu seems to be critically involved in the dystonic syndrome of dtsz hamsters and that a transiently reduced inhibitory control over excitatory cortico-striatal processes, possibly due to an altered development of GABAergic inhibition, occurs during ontogenesis in dtsz hamsters.

Anesthetics eliminate somatosensory-evoked discharges of neurons in the somatotopically organized sensorimotor striatum of the rat

The somatotopic organization of the lateral striatum has been demonstrated by anatomical studies of corticostriatal projections from somatosensory and motor cortices and by single-cell recordings in awake animals. The functional organization in the rat, characterized thus far in the freely moving rat preparation, could be mapped more precisely if a stereotaxic, and possibly an anesthetized, preparation could be used. Because striatal discharges evoked by innocuous somatosensory stimulation are used in mapping, this study tested whether such discharges can be observed during anesthesia, encouraged by responsiveness during anesthesia in somatosensory cortical layers projecting to the striatum. Electrode tracks through lateral striatum of anesthetized rats (pentobarbital or ketamine) revealed spontaneously discharging neurons but no discharges evoked by somatosensory examination (passive manipulation and cutaneous stimulation of 14 body parts). Similar tracks in chronically implanted rats showed evoked firing at numerous sites during wakefulness but not during anesthesia (pentobarbital or urethane). Comparisons of the activity of individual neurons between wakefulness and anesthesia showed that pentobarbital, ketamine, chloral hydrate, urethane, or metofane eliminated evoked firing and suppressed spontaneous firing. Recovery time was greater for neural than for behavioral measures. Thus, mapping as proposed is ruled out, and more importantly, the data show that somatotopically organized lateral striatal neurons stop discharging in response to natural stimulation during anesthesia. Available data indicate they do not reach threshold in response to depolarizations produced by glutamatergic corticostriatal synaptic transmission projected from the somatosensory cortex. These data and demonstrations of anesthetic-induced imbalances in most striatal neurotransmitters emphasize that many results regarding striatal physiology and pharmacology during anesthesia cannot be extrapolated to behavioral conditions, thus indicating the need for more empirical testing in conscious animals.

Cocaine-induced activation of striatal neurons during focused stereotypy in rats

As psychomotor stimulants, both amphetamine and cocaine elicit episodes of repetitive motor activation (focused stereotypy) known to involve the mesostriatal dopamine system. During amphetamine-induced focused stereotypy, motor-related neurons in the striatum respond with either an excitation or inhibition, depending on dose and behavioral pattern, whereas nonmotor-related units are inhibited. To assess striatal activity during the focused stereotypy induced by cocaine, both types of striatal units were recorded in ambulant rats. Either 20 or 40 mg/kg cocaine caused highly focused sniffing and head bobbing, which occurred in conjunction with activation of both motor- and nonmotor-related neurons. The activation of motor-related units was evident even when firing rate was compared during periods of matched pre- and post-drug behavior, arguing against movement as the sole basis for the drug-induced neuronal excitation. Subsequent administration of haloperidol (1.0 mg/kg) reversed but did not completely block the neuronal activation, while the behavioral response shifted away from focused stereotypy toward an increase in ambulation. Thus, the level of activation of both motor- and nonmotor-related striatal neurons may play a critical role in the behavioral response pattern induced by cocaine.

Real-time assessments of dopamine function during behavior: single-unit recording, iontophoresis, and fast-scan cyclic voltammetry in awake, unrestrained rats

Although ample evidence implicates the dopamine (DA) projection to the neostriatum and nucleus accumbens in motor and motivational processes, relatively little information is available on how DA alters neostriatal or accumbal functions under naturally occurring behavioral conditions. Further insight into neuron-behavior relationships can be achieved with the application of single-unit recording techniques, including iontophoresis and fast-scan cyclic voltammetry (FSCV), to awake, unrestrained animals. Single-unit recording has revealed that amphetamine, a widely abused psychomotor stimulant, activates motor-, but inhibits nonmotor-related neurons in neostriatum and nucleus accumbens. Although either response can be blocked by DA receptor antagonists, the amphetamine-induced activation also depends on an intact corticostriatal system, suggesting a role for glutamate (GLU). Both neostriatal and accumbal neurons are sensitive to iontophoretic application of either DA or GLU, but when applied during low-dose application of DA, the GLU signal is enhanced relative to background activity. In effect, DA appears to modulate GLU by strengthening the GLU signal-to-noise ratio. To assess DA release under behaviorally relevant conditions, FSCV has been used to obtain real-time measurements of DA efflux in a free-choice novelty test. DA efflux increased only during the brief period of entry into novelty, and the increase was confined to accumbal shell and the shell-core transition zone, the so-called shore. Neither accumbal core nor the overlying neostriatum showed a novelty-related DA change. Thus, DA release during behavior is not uniform and in the case of novelty appears targeted to the limbic-related area of accumbal shell. Further application of these and other in vivo technologies to ambulant animals is required to identify the complex mechanisms underlying both the release of DA and its effect on neostriatal and accumbal neurons during behavior.

Iontophoresis of amphetamine in the neostriatum and nucleus accumbens of awake, unrestrained rats

When administered systemically to ambulant animals, amphetamine (AMPH) has both excitatory and inhibitory effects on single-unit activity in the neostriatum and nucleus accumbens. To determine the extent to which these results reflect a direct action of the drug, AMPH was applied iontophoretically to neostriatal and accumbal neurons under naturally occurring behavioral conditions. AMPH dose-dependently (5-40 nA) inhibited the vast majority of spontaneously active units. The inhibition, which was evident at low ejection currents (5-10 nA), had relatively short onset (4-12 s) and offset (6-24 s) latencies, and was positively correlated with basal firing rate. Even stronger dose-dependent inhibitory responses were recorded when neurons having no or a very low rate of spontaneous activity were tonically activated by continuous, low-current applications of glutamate (Glu). Systemic injection of either SCH-23390 (0.1 mg/kg) or haloperidol (0.2 mg/kg), relatively selective D1 and D2 receptor antagonists, respectively, blocked the AMPH-induced inhibition. Prolonged AMPH iontophoresis (2-3 min; 5-30 nA) inhibited both spontaneous impulse activity and Glu-induced excitations, resulting in a complete blockade of the Glu response at relatively high AMPH ejection currents (> or = 20 nA). Taken together, these results suggest that although dopamine is largely responsible for the inhibitory effects of iontophoretic AMPH, dopamine alone cannot account for the complex response of neostriatal and accumbal neurons to systemic AMPH administration.

The organization of acromiodeltoid and spinodeltoid motor nuclei in rat spinal cord

The retrograde intraaxonal transport of fluorescent dyes (10% Bisbenzimide or 2% Nuclear Yellow) was used to identify the motoneuron populations that control two prime movers of the rat shoulder joint, the acromiodeltoid (Ad) and spinodeltoid (Sd) muscles. The Ad and Sd motor nuclei were both comprised of an average 30-40 motoneurons distributed between spinal segments C5 and C7, in lateral regions of Rexed's lamina IX. Although there was considerable overlap among them, the center of the Ad motor nucleus was consistently located more rostral and medial than that for the Sd motor nucleus.