Regional and laminar distribution of the dopamine and serotonin innervation in the macaque cerebral cortex: a radioautographic study

Antipsychotic treatment induces alterations in dendrite- and spine-associated proteins in dopamine-rich areas of the primate cerebral cortex

BACKGROUND

Mounting evidence indicates that long-term treatment with antipsychotic medications can alter the morphology and connectivity of cellular processes in the cerebral cortex. The cytoskeleton plays an essential role in the maintenance of cellular morphology and is subject to regulation by intracellular pathways associated with neurotransmitter receptors targeted by antipsychotic drugs.

METHODS

We have examined whether chronic treatment with the antipsychotic drug haloperidol interferes with phosphorylation state and tissue levels of a major dendritic cytoskeleton-stabilizing agent, microtubule-associated protein 2 (MAP2), as well as levels of the dendritic spine-associated protein spinophilin and the synaptic vesicle-associated protein synaptophysin in various regions of the cerebral cortex of rhesus monkeys.

RESULTS

Among the cortical areas examined, the prefrontal, orbital, cingulate, motor, and entorhinal cortices displayed significant decreases in levels of spinophilin, and with the exception of the motor cortex, each of these regions also exhibited increases in the phosphorylation of MAP2. No changes were observed in either spinophilin levels or MAP2 phosphorylation in the primary visual cortex. Also, no statistically significant changes were found in tissue levels of MAP2 or synaptophysin in any of the cortical regions examined.

CONCLUSIONS

Our findings demonstrate that long-term haloperidol exposure alters neuronal cytoskeleton- and spine-associated proteins, particularly in dopamine-rich regions of the primate cerebral cortex, many of which have been implicated in the psychopathology of schizophrenia. The ability of haloperidol to regulate cytoskeletal proteins should be considered in evaluating the mechanisms of both its palliative actions and its side effects.

Event-related potentials. Do they reflect central serotonergic neurotransmission and do they predict clinical response to serotonin agonists?

The increasing knowledge concerning anatomical structures and cellular processes underlying event-related potentials (ERP) as well as methodological advances in ERP data analysis (e.g. dipole source analysis) begin to bridge the gap between ERP and neurochemical aspects. Reliable indicators of the serotonin system are urgently needed because of its role in pathophysiology and as target of pharmacotherapeutic interventions in psychiatric disorders. Converging arguments from preclinical and clinical studies support the hypothesis that the loudness dependence of the auditory evoked N1/P2-response (LDAEP) is regulated by the level of central serotonergic neurotransmission. Dipole source analysis represents an important methodological advance in this context, because the two N1/P2-subcomponents, generated by the primary and secondary auditory cortex known to be differentially innervated by serotonergic fibers, can be separated. A pronounced LDAEP of primary auditory cortices is supposed to reflect low central serotonergic neurotransmission, and vice versa. LDAEP is a parameter with potential clinical value since subgroups of patients with a serotonergic dysfunction can be identified and can be treated more specifically. In depressed patients, a significant relationship between strong LDAEP, indicating low serotonergic function, and a favorable response to SSRI has been found. Additionally, there is evidence from several studies with patients with affective disorders implicating a strong LDAEP as a predictor of favorable response to a preventive lithium treatment.

The role of D1-dopamine receptors in working memory-guided movements mediated by frontal cortical areas

Like the striatum, the frontal motor cortices receive dopaminergic fibers from midbrain dopamine cells and contain high levels of dopamine receptors. Among frontal cortical areas, the dorsolateral PFC (PFd1) and the dorsal premotor cortex (PMd) have strong neural connections and play a major role for working memory-guided directional movements. To reveal the role of dopamine in this cognitive motor function, dopamine antagonists (SCH23390 for D1 receptors and sulpiride for D2 receptors) were applied locally or iontophoretically to the PFd1 and PMd in monkeys that performed delayed-response tasks with memory-guided directional movements. Applications of SCH23390, but not sulpiride, to these areas had significant effects at both the behavioral and neuronal levels. In the PFd1 and at the behavioral level, local injections of SCH23390 induced specific errors for memory-guided saccades, whereas it had no effects on visually guided saccades. In the PMd, local injections of SCH23390 induced directional errors and increased reaction time and movement time in memory-guided reaching movements. At the neuron level, iontophoretic applications of SCH23390 attenuated directional tuning of neurons of the PFd1 and PMd, which showed directional activities during the delay-and/or response-period(s). These findings suggest that the activation of D1-dopamine receptors in these frontal cortical areas plays a facilitating role in a series of neuronal processes of working memory-guided directional movements; the working memory process for guiding motor act in the PFd1 and preparation/control of directional manual movements in the PMd. In addition, our findings may provide insight into symptoms of schizophrenia and Parkinson's disease; the dysfunction of D1-dopamine receptors in the PMd1 and PMd may contribute to some symptoms, such as bradyphrenia and bradykinesia, in these disorders.

Neuronal coding of prediction errors

Associative learning enables animals to anticipate the occurrence of important outcomes. Learning occurs when the actual outcome differs from the predicted outcome, resulting in a prediction error. Neurons in several brain structures appear to code prediction errors in relation to rewards, punishments, external stimuli, and behavioral reactions. In one form, dopamine neurons, norepinephrine neurons, and nucleus basalis neurons broadcast prediction errors as global reinforcement or teaching signals to large postsynaptic structures. In other cases, error signals are coded by selected neurons in the cerebellum, superior colliculus, frontal eye fields, parietal cortex, striatum, and visual system, where they influence specific subgroups of neurons. Prediction errors can be used in postsynaptic structures for the immediate selection of behavior or for synaptic changes underlying behavioral learning. The coding of prediction errors may represent a basic mode of brain function that may also contribute to the processing of sensory information and the short-term control of behavior.

Dopamine innervation of monkey entorhinal cortex: postsynaptic targets of tyrosine hydroxylase-immunoreactive terminals

Dopamine (DA) has been demonstrated to play an important role in regulating cortical activity in both neocortical and periallocortical regions. However, marked differences between these two types of cortices in the laminar pattern of DA axons, the types and distribution of DA receptors, and the postnatal development of the DA innervation suggest that DA may have region-specific effects. Such regional specialization may also include the types of cortical cells apposed to DA terminals. In neocortical regions, such as the prefrontal and motor cortices, the majority of structures apposed to DA terminals appear to be the dendritic spines and shafts of pyramidal cells, and a minority are dendrites immunoreactive for gamma-amino butyric acid (GABA). However, the identity of the neural elements apposed to DA terminals in the entorhinal cortex, a periallocortical region, is unknown. In this study, we used immunocytochemical techniques and antibodies against tyrosine hydroxylase (TH) and GABA, visualized with preembedding immunoperoxidase and immunogold-silver labels, respectively, to examine DA terminals and their targets with electron microscopy. In the superficial layers of the monkey entorhinal cortex, TH-immunoreactive (IR) terminals varied greatly in size and formed thin, symmetric synapses. The majority of dendritic structures apposed to these TH-terminals were not GABA-IR, and included both dendritic shafts (64%) and spines (14%). A minority (22%) of the apposed dendrites were GABA-IR. A similar distribution of targets was observed for the subset of TH-IR terminals with identifiable synaptic specializations. In addition, the proportions of GABA-labeled and unlabeled dendrites apposed to TH terminals did not differ from those previously reported for monkey prefrontal cortex. These findings indicate that DA terminals provide direct input to both excitatory and inhibitory cells in the monkey entorhinal cortex and suggest that the effects of DA are mediated through a set of targets that are common to both neo- and periallocortex.

Eye fields in the frontal lobes of primates

Two eye fields have been identified in the frontal lobes of primates: one is situated dorsomedially within the frontal cortex and will be referred to as the eye field within the dorsomedial frontal cortex (DMFC); the other resides dorsolaterally within the frontal cortex and is commonly referred to as the frontal eye field (FEF). This review documents the similarities and differences between these eye fields. Although the DMFC and FEF are both active during the execution of saccadic and smooth pursuit eye movements, the FEF is more dedicated to these functions. Lesions of DMFC minimally affect the production of most types of saccadic eye movements and have no effect on the execution of smooth pursuit eye movements. In contrast, lesions of the FEF produce deficits in generating saccades to briefly presented targets, in the production of saccades to two or more sequentially presented targets, in the selection of simultaneously presented targets, and in the execution of smooth pursuit eye movements. For the most part, these deficits are prevalent in both monkeys and humans. Single-unit recording experiments have shown that the DMFC contains neurons that mediate both limb and eye movements, whereas the FEF seems to be involved in the execution of eye movements only. Imaging experiments conducted on humans have corroborated these findings. A feature that distinguishes the DMFC from the FEF is that the DMFC contains a somatotopic map with eyes represented rostrally and hindlimbs represented caudally; the FEF has no such topography. Furthermore, experiments have revealed that the DMFC tends to contain a craniotopic (i.e., head-centered) code for the execution of saccadic eye movements, whereas the FEF contains a retinotopic (i.e., eye-centered) code for the elicitation of saccades. Imaging and unit recording data suggest that the DMFC is more involved in the learning of new tasks than is the FEF. Also with continued training on behavioural tasks the responsivity of the DMFC tends to drop. Accordingly, the DMFC is more involved in learning operations whereas the FEF is more specialized for the execution of saccadic and smooth pursuit eye movements.

Identifying psychiatric patients with serotonergic dysfunctions by event-related potentials

The increasing knowledge concerning anatomical structures and cellular processes underlying event-related potentials (ERP) as well as methodological advances in ERP data analysis (e.g. dipole source analysis) is beginning to bridge the gap between ERP and neurochemical aspects. Reliable indicators of the serotonin system are urgently needed because of its role in pathophysiology and as target of pharmacotherapeutic interventions in psychiatric disorders. Converging arguments from preclinical and clinical studies support the hypothesis that the loudness dependence of the auditory evoked N1/P2-response (LDAEP) is regulated by the level of central serotonergic neurotransmission. Dipole source analysis represents an important methodological advance in this context, because the two N1/P2-subcomponents, generated by the primary and secondary auditory cortex known to be differentially innervated by serotonergic fibres, can be separated. A pronounced LDAEP of primary auditory cortices is supposed to reflect low central serotonergic neurotransmission, and vice versa. LDAEP is a parameter with potential clinical value since subgroups of patients with a serotonergic dysfunction can be identified and can be treated more specifically. In depressed patients, a significant relationship between strong LDAEP, indicating low serotonergic function, and a favourable response to SSRI has been found. Additionally, there is evidence from several studies with patients with affective disorders that a strong LDAEP predicts favourable response to a preventive lithium treatment.

Response differences in monkey TE and perirhinal cortex: stimulus association related to reward schedules

Anatomic and behavioral evidence shows that TE and perirhinal cortices are two directly connected but distinct inferior temporal areas. Despite this distinctness, physiological properties of neurons in these two areas generally have been similar with neurons in both areas showing selectivity for complex visual patterns and showing response modulations related to behavioral context in the sequential delayed match-to-sample (DMS) trials, attention, and stimulus familiarity. Here we identify physiological differences in the neuronal activity of these two areas. We recorded single neurons from area TE and perirhinal cortex while the monkeys performed a simple behavioral task using randomly interleaved visually cued reward schedules of one, two, or three DMS trials. The monkeys used the cue's relation to the reward schedule (indicated by the brightness) to adjust their behavioral performance. They performed most quickly and most accurately in trials in which reward was immediately forthcoming and progressively less well as more intermediate trials remained. Thus the monkeys appeared more motivated as they progressed through the trial schedule. Neurons in both TE and perirhinal cortex responded to both the visual cues related to the reward schedules and the stimulus patterns used in the DMS trials. As expected, neurons in both areas showed response selectivity to the DMS patterns, and significant, but small, modulations related to the behavioral context in the DMS trial. However, TE and perirhinal neurons showed strikingly different response properties. The latency distribution of perirhinal responses was centered 66 ms later than the distribution of TE responses, a larger difference than the 10-15 ms usually found in sequentially connected visual cortical areas. In TE, cue-related responses were related to the cue's brightness. In perirhinal cortex, cue-related responses were related to the trial schedules independently of the cue's brightness. For example, some perirhinal neurons responded in the first trial of any reward schedule including the one trial schedule, whereas other neurons failed to respond in the first trial but respond in the last trial of any schedule. The majority of perirhinal neurons had more complicated relations to the schedule. The cue-related activity of TE neurons is interpreted most parsimoniously as a response to the stimulus brightness, whereas the cue-related activity of perirhinal neurons is interpreted most parsimoniously as carrying associative information about the animal's progress through the reward schedule. Perirhinal cortex may be part of a system gauging the relation between work schedules and rewards.

Dopamine-mediated stabilization of delay-period activity in a network model of prefrontal cortex

The prefrontal cortex (PFC) is critically involved in working memory, which underlies memory-guided, goal-directed behavior. During working-memory tasks, PFC neurons exhibit sustained elevated activity, which may reflect the active holding of goal-related information or the preparation of forthcoming actions. Dopamine via the D1 receptor strongly modulates both this sustained (delay-period) activity and behavioral performance in working-memory tasks. However, the function of dopamine during delay-period activity and the underlying neural mechanisms are only poorly understood. Recently we proposed that dopamine might stabilize active neural representations in PFC circuits during tasks involving working memory and render them robust against interfering stimuli and noise. To further test this idea and to examine the dopamine-modulated ionic currents that could give rise to increased stability of neural representations, we developed a network model of the PFC consisting of multicompartment neurons equipped with Hodgkin-Huxley-like channel kinetics that could reproduce in vitro whole cell and in vivo recordings from PFC neurons. Dopaminergic effects on intrinsic ionic and synaptic conductances were implemented in the model based on in vitro data. Simulated dopamine strongly enhanced high, delay-type activity but not low, spontaneous activity in the model network. Furthermore the strength of an afferent stimulation needed to disrupt delay-type activity increased with the magnitude of the dopamine-induced shifts in network parameters, making the currently active representation much more stable. Stability could be increased by dopamine-induced enhancements of the persistent Na(+) and N-methyl-D-aspartate (NMDA) conductances. Stability also was enhanced by a reduction in AMPA conductances. The increase in GABA(A) conductances that occurs after stimulation of dopaminergic D1 receptors was necessary in this context to prevent uncontrolled, spontaneous switches into high-activity states (i.e., spontaneous activation of task-irrelevant representations). In conclusion, the dopamine-induced changes in the biophysical properties of intrinsic ionic and synaptic conductances conjointly acted to highly increase stability of activated representations in PFC networks and at the same time retain control over network behavior and thus preserve its ability to adequately respond to task-related stimuli. Predictions of the model can be tested in vivo by locally applying specific D1 receptor, NMDA, or GABA(A) antagonists while recording from PFC neurons in delayed reaction-type tasks with interfering stimuli.

The dopaminergic innervation of the avian telencephalon

The present review provides an overview of the distribution of dopaminergic fibers and dopaminoceptive elements within the avian telencephalon, the possible interactions of dopamine (DA) with other biochemically identified systems as revealed by immunocytochemistry, and the involvement of DA in behavioral processes in birds. Primary sensory structures are largely devoid of dopaminergic fibers, DA receptors and the D1-related phosphoprotein DARPP-32, while all these dopaminergic markers gradually increase in density from the secondary sensory to the multimodal association and the limbic and motor output areas. Structures of the avian basal ganglia are most densely innervated but, in contrast to mammals, show a higher D2 than D1 receptor density. In most of the remaining telencephalon D1 receptors clearly outnumber D2 receptors. Dopaminergic fibers in the avian telencephalon often show a peculiar arrangement where fibers coil around the somata and proximal dendrites of neurons like baskets, probably providing them with a massive dopaminergic input. Basket-like innervation of DARPP-32-positive neurons seems to be most prominent in the multimodal association areas. Taken together, these anatomical findings indicate a specific role of DA in higher order learning and sensory-motor processes, while primary sensory processes are less affected. This conclusion is supported by behavioral findings which show that in birds, as in mammals, DA is specifically involved in sensory-motor integration, attention and arousal, learning and working memory. Thus, despite considerable differences in the anatomical organization of the avian and mammalian forebrain, the organization of the dopaminergic system and its behavioral functions are very similar in birds and mammals.

Neurochemical evidence for at least two regional subdivisions within the homing pigeon (Columba livia) caudolateral neostriatum

The distributions of one neurotransmitter, two neurotransmitter-related substances, and five neuropeptides were examined within the homing pigeon caudolateral neostriatum (NCL). All eight neuroactive substances were found within a tyrosine hydroxylase (TH)-dense region that defines the NCL. Overall regional variation in the relative density of these substances suggested at least two neurochemically distinct portions of NCL. Dorsal NCL contained relatively dense staining for TH, choline acetyltransferase, and substance P, whereas vasoactive intestinal polypeptide was more abundant in ventral portions of NCL. Serotonin and cholecystokinin were found to be densest in intermediate portions of NCL. Somatostatin and leucine-enkephalin were homogeneously distributed throughout NCL. The results suggest that NCL may consist of multiple subdivisions. Investigations into the behavioral importance of these regions are necessary to clarify the role of this brain region in avian behavior.

Ovarian hormones differentially influence immunoreactivity for dopamine beta- hydroxylase, choline acetyltransferase, and serotonin in the dorsolateral prefrontal cortex of adult rhesus monkeys

Recent studies have shown that ovariectomy reduces, and subsequent hormone replacement restores the density of axons immunoreactive for tyrosine hydroxylase in the dorsolateral prefrontal cortex of adult female rhesus monkeys. The present study indicates that three additional extrathalamic frontal lobe afferents are also sensitive to changes in the ovarian hormone environment. Specifically, the combination of hormone manipulation with qualitative and quantitative analysis of immunocytochemistry for dopamine beta-hydroxylase, choline acetyltransferase, and serotonin in the primate prefrontal cortex revealed quantitative responses in both cholinergic and monoaminergic axons to changing ovarian hormone levels. However, whereas ovariectomy produced a modest net decrease in the density of fibers immunoreactive for choline acetyltransferase, this same treatment markedly increased the density of axons immunoreactive for dopamine beta-hydroxylase and for serotonin. Further, the effects of ovariectomy on these afferent systems were differentially attenuated by estrogen verses estrogen plus progesterone hormone replacement. Estrogen was as effective as estrogen plus progesterone in stimulating normal prefrontal immunoreactivity for choline acetyltransferase and dopamine beta-hydroxylase. The dual replacement of estrogen plus progesterone, however, was a much more potent influence than estrogen alone for serotonin immunoreactivity. Thus, ovarian hormones appear to provide stimulation that differentially affects each of four chemically identified extrathalamic prefrontal afferent systems examined to date, and may have roles in maintaining the normal balance and functional interactions between these neurotransmitter systems.

Distribution of the catecholaminergic neurons in the central nervous system of human embryos and fetuses

The catecholaminergic cell groups in the human brain, denominated from A1 to A17, display some striking anatomical differences with those described in the rodent. These differences are essentially observed in the extent of the dopaminergic neurons and especially their axonal fields in the telencephalon. Immunocytochemistry for tyrosine-hydroxylase and dopamine-ss-hydroxylase allowed the visualization of the precocious human catecholaminergic groups as early as 4.5 postovulatory weeks. Maps of tyrosine-hydroxylase positive neurons generated in the different rhombomeres, midbrain, and prosomeres are shown following the prosomeric model introduced by Puelles and Rubenstein [(1993) Trends Neurosci. 16:472-476]. Such a description is convenient to compare catecholaminergic systems in different mammalian species and provide clear anatomical landmarks of the embryonic substantia nigra (midbrain and prosomeres 1 and 2), that are necessary for transplantation of neural tissue in Parkinson's disease. The development and early specification of the dopaminergic neurons expressing calbindin D28K phenotype in the substantia nigra and in the ventral tegmental area are described. The catecholaminergic axons enter the anlage of the cerebral cortex just after the formation of the cortical plate, from 7 postovulatory weeks on. They invade the subplate layer where they wait for 4 weeks before penetrating the cortical plate. At midgestation, the different areas and layers of the frontal cerebral wall are invaded by the catecholaminergic axons, before the layering of the cortex is completed, in a pattern of fiber distribution similar to that described in the adult human brain. The early pattern of development of the catecholamine systems appeared to be phylogenetically well preserved in mammals, but specific features emerging during the differentiation period are unique to humans.

A neurocomputational theory of the dopaminergic modulation of working memory functions

The dopaminergic modulation of neural activity in the prefrontal cortex (PFC) is essential for working memory. Delay-activity in the PFC in working memory tasks persists even if interfering stimuli intervene between the presentation of the sample and the target stimulus. Here, the hypothesis is put forward that the functional role of dopamine in working memory processing is to stabilize active neural representations in the PFC network and thereby to protect goal-related delay-activity against interfering stimuli. To test this hypothesis, we examined the reported dopamine-induced changes in several biophysical properties of PFC neurons to determine whether they could fulfill this function. An attractor network model consisting of model neurons was devised in which the empirically observed effects of dopamine on synaptic and voltage-gated membrane conductances could be represented in a biophysically realistic manner. In the model, the dopamine-induced enhancement of the persistent Na+ and reduction of the slowly inactivating K+ current increased firing of the delay-active neurons, thereby increasing inhibitory feedback and thus reducing activity of the "background" neurons. Furthermore, the dopamine-induced reduction of EPSP sizes and a dendritic Ca2+ current diminished the impact of intervening stimuli on current network activity. In this manner, dopaminergic effects indeed acted to stabilize current delay-activity. Working memory deficits observed after supranormal D1-receptor stimulation could also be explained within this framework. Thus, the model offers a mechanistic explanation for the behavioral deficits observed after blockade or after supranormal stimulation of dopamine receptors in the PFC and, in addition, makes some specific empirical predictions.

Sensitivity to ageing of the limbic dopaminergic system: a review

The limbic system includes the complex of brain centres, nuclei and connections that provide the anatomical substrate for emotions. Although the presence of small amounts of dopamine (DA) in several limbic structures has been recognized for a long time, for many years it was thought that limbic DA represented a precursor of noradrenaline in the biosynthetic pathway of catecholamines. More recent evidence has shown that limbic centres and nuclei are supplied with a dopaminergic innervation arising from the ventral tegmental area (field A10) and in smaller amounts from the mesencephalic A9 field. The dopaminergic limbic system is sensitive to ageing. Parameters of dopaminergic neurotransmission (DA levels, biosynthetic and catabolic markers and DA receptors) undergo age-related changes which depend on the structure and species investigated and are characterized mainly by a decline of different parameters examined. In this paper, the influence of ageing on DA biosynthesis, levels, metabolism and receptors are reviewed in laboratory rodents, monkeys and humans as well as in cases of Alzheimer's disease and Parkinson's disease. The possibility that changes of dopaminergic neurotransmission markers in the limbic system are associated with cognitive impairment and psychotic symptoms affecting the elderly is discussed. Better knowledge of dopaminergic neurotransmission mechanisms in the so-called physiological ageing and in senile dementia may provide new insights in the treatment of behavioural alterations frequently occurring in old age.

Occlusion and levodopa-carbidopa treatment for childhood amblyopia

PURPOSE

The purpose of the current study was to compare the effects of levodopa-carbidopa with and without part-time occlusion on visual function in older amblyopic children.

METHODS

Thirteen older amblyopic children were randomly assigned to receive or not receive part-time occlusion (3 h/day) combined with 7 weeks of oral dosing with levodopa-carbidopa (1.02 mg/0.25 mg/kg body weight three times daily). Visual acuity, contrast sensitivity, and fusion were measured at baseline; 1, 3, 5, and 7 weeks during the treatment regimen; and 4 weeks after termination of all treatment. At these same times health status was assessed with standard laboratory blood tests, physical examination, and subjective questionnaire.

RESULTS

From baseline to the follow-up test trial, both groups improved in visual acuity in the amblyopic eyes (occlusion group 20/116 to 20/76, P < .001; no occlusion group 20/90 to 20/73, P < .01) and dominant eyes (occlusion group 20/18 to 20/15, P > .05; no occlusion group 20/20 to 20/16, P < .01). The occlusion group exhibited a significant decrease in the difference in acuity between the dominant and amblyopic eyes of 1.3 lines (P < .02), whereas the no occlusion group revealed no significant effect. A comparison between groups revealed a significantly greater improvement in visual acuity in the amblyopic eye in the occlusion group compared with the no occlusion group (P = .01). In contrast, there was no significant difference between groups in terms of the change in visual acuity in the dominant eye (P = .15). Mean log contrast sensitivity in the amblyopic eye significantly improved in the occlusion group and did not significantly change in the no occlusion group. Fusion changed similarly in both groups. The improvements in visual function were maintained 4 weeks after the termination of all treatment. Adverse side effects were minimal in both groups.

CONCLUSION

The combination of levodopa-carbidopa and occlusion improves visual function more than levodopa-carbidopa alone in older amblyopic children.

Predictive reward signal of dopamine neurons

The effects of lesions, receptor blocking, electrical self-stimulation, and drugs of abuse suggest that midbrain dopamine systems are involved in processing reward information and learning approach behavior. Most dopamine neurons show phasic activations after primary liquid and food rewards and conditioned, reward-predicting visual and auditory stimuli. They show biphasic, activation-depression responses after stimuli that resemble reward-predicting stimuli or are novel or particularly salient. However, only few phasic activations follow aversive stimuli. Thus dopamine neurons label environmental stimuli with appetitive value, predict and detect rewards and signal alerting and motivating events. By failing to discriminate between different rewards, dopamine neurons appear to emit an alerting message about the surprising presence or absence of rewards. All responses to rewards and reward-predicting stimuli depend on event predictability. Dopamine neurons are activated by rewarding events that are better than predicted, remain uninfluenced by events that are as good as predicted, and are depressed by events that are worse than predicted. By signaling rewards according to a prediction error, dopamine responses have the formal characteristics of a teaching signal postulated by reinforcement learning theories. Dopamine responses transfer during learning from primary rewards to reward-predicting stimuli. This may contribute to neuronal mechanisms underlying the retrograde action of rewards, one of the main puzzles in reinforcement learning. The impulse response releases a short pulse of dopamine onto many dendrites, thus broadcasting a rather global reinforcement signal to postsynaptic neurons. This signal may improve approach behavior by providing advance reward information before the behavior occurs, and may contribute to learning by modifying synaptic transmission. The dopamine reward signal is supplemented by activity in neurons in striatum, frontal cortex, and amygdala, which process specific reward information but do not emit a global reward prediction error signal. A cooperation between the different reward signals may assure the use of specific rewards for selectively reinforcing behaviors. Among the other projection systems, noradrenaline neurons predominantly serve attentional mechanisms and nucleus basalis neurons code rewards heterogeneously. Cerebellar climbing fibers signal errors in motor performance or errors in the prediction of aversive events to cerebellar Purkinje cells. Most deficits following dopamine-depleting lesions are not easily explained by a defective reward signal but may reflect the absence of a general enabling function of tonic levels of extracellular dopamine. Thus dopamine systems may have two functions, the phasic transmission of reward information and the tonic enabling of postsynaptic neurons.

The dopaminergic innervation of the pigeon telencephalon: distribution of DARPP-32 and co-occurrence with glutamate decarboxylase and tyrosine hydroxylase

Dopaminergic axons arising from midbrain nuclei innervate the mammalian and avian telencephalon with heterogeneous regional and laminar distributions. In primate, rodent, and avian species, the neuromodulator dopamine is low or almost absent in most primary sensory areas and is most abundant in the striatal parts of the basal ganglia. Furthermore, dopaminergic fibres are present in most limbic and associative structures. Herein, the distribution of DARPP-32, a phosphoprotein related to the dopamine D1-receptor, was investigated in the pigeon telencephalon by immunocytochemical techniques. Furthermore, co-occurrence of DARPP-32-positive perikarya with tyrosine hydroxylase-positive pericellular axonal "baskets" or glutamate decarboxylase-positive neurons, as well as co-occurrence of tyrosine hydroxylase and glutamate decarboxylase were examined. Specificity of the anti-DARPP-32 monoclonal antibody in pigeon brain was determined by immunoblotting. The distribution of DARPP-32 shared important features with the distribution of D1-receptors and dopaminergic fibres in the pigeon telencephalon as described previously. In particular, DARPP-32 was highly abundant in the avian basal ganglia, where a high percentage of neurons were labelled in the "striatal" parts (paleostriatum augmentatum, lobus parolfactorius), while only neuropil staining was observed in the "pallidal" portions (paleostriatum primitivum). In contrast, DARPP-32 was almost absent or present in comparatively lower concentrations in most primary sensory areas. Secondary sensory and tertiary areas of the neostriatum contained numbers of labelled neurons comparable to that of the basal ganglia and intermediate levels of neuropil staining. Approximately up to one-third of DARPP-32-positive neurons received a basket-type innervation from tyrosine hydroxylase-positive fibres in the lateral and caudal neostriatum, but only about half as many did in the medial and frontal neostriatum, and even less so in the hyperstriatum. No case of colocalization of glutamate decarboxylase and DARPP-32 and no co-occurrence of glutamate decarboxylase-positive neurons and tyrosine hydroxylase-basket-like structures could be detected out of more than 2000 glutamate decarboxylase-positive neurons examined, although the high DARPP-32 and high tyrosine hydroxylase staining density hampered this analysis in the basal ganglia. In conclusion, the pigeon dopaminergic system seems to be organized similar to that of mammals. Apparently, in the telencephalon, dopamine has its primary function in higher level sensory, associative and motor processes, since primary areas showed only weak or no anatomical cues of dopaminergic modulation. Dopamine might exert its effects primarily by modulating the physiological properties of non-GABAergic and therefore presumably excitatory units.

Reinforcement learning by Hebbian synapses with adaptive thresholds

A central problem in learning theory is how the vertebrate brain processes reinforcing stimuli in order to master complex sensorimotor tasks. This problem belongs to the domain of supervised learning, in which errors in the response of a neural network serve as the basis for modification of synaptic connectivity in the network and thereby train it on a computational task. The model presented here shows how a reinforcing feedback can modify synapses in a neuronal network according to the principles of Hebbian learning. The reinforcing feedback steers synapses towards long-term potentiation or depression by critically influencing the rise in postsynaptic calcium, in accordance with findings on synaptic plasticity in mammalian brain. An important feature of the model is the dependence of modification thresholds on the previous history of reinforcing feedback processed by the network. The learning algorithm trained networks successfully on a task in which a population vector in the motor output was required to match a sensory stimulus vector presented shortly before. In another task, networks were trained to compute coordinate transformations by combining different visual inputs. The model continued to behave well when simplified units were replaced by single-compartment neurons equipped with several conductances and operating in continuous time. This novel form of reinforcement learning incorporates essential properties of Hebbian synaptic plasticity and thereby shows that supervised learning can be accomplished by a learning rule similar to those used in physiologically plausible models of unsupervised learning. The model can be crudely correlated to the anatomy and electrophysiology of the amygdala, prefrontal and cingulate cortex and has predictive implications for further experiments on synaptic plasticity and learning processes mediated by these areas.

Attenuation of preparatory activity for reaching movements by a D1-dopamine antagonist in the monkey premotor cortex

To examine the role of dopamine receptors in the function of the premotor cortex (PM) for preparing for reaching movements, dopamine antagonists (SCH23390 for D1 receptors and sulpiride for D2 receptors) were applied iontophoretically to neurons of the PM of monkeys that performed a delayed-reaching (DR) task with their arms. In the DR task, the monkey made a reaching movement to one of three target levers (left, upper, and right), which had been cued by a visuospatial stimulus before a delay period of 4 s. We focused on neurons (n = 56) that showed a sustained increase in activity during the delay period (delay-period activity; i.e., "set-related" activity), because such activity is considered to play a central role in preparing for forelimb movements. Iontophoretic application of SCH23390 (usually with a current of 50 nA) significantly decreased the activity of most of these neurons (n = 44/56, 79%), and delay-period activity was attenuated during its application. In contrast, application of sulpiride or SCH23388 (an inactive analogue of SCH23390), using the same current intensity, had no effect on most of the neurons tested with these drugs (n = 31/33 and n = 21/23, respectively), despite the fact that their activity was decreased by SCH23390. Furthermore, for neurons that were affected by SCH23390, the percent decrease in delay-period activity was significantly greater than the percent change in background activity. In more than one-half of the neurons (n = 26/44, 59%), background activity was either increased (n = 10) or not affected (n = 16) by SCH23390, even though it significantly attenuated delay-period activity. These findings suggest that the activation of D1-dopamine receptors plays a modulatory role in PM function in preparing for reaching movements.

Auditory-evoked potentials as indicator of brain serotonergic activity--first evidence in behaving cats

Due to the increasing importance of the central serotonergic neurotransmission for pathogenetic concepts and as a target of pharmacotherapeutic interventions in psychiatry, reliable indicators of this system are needed. Several findings from basic and clinical research suggest that the stimulus intensity dependence of auditory evoked potentials (AEP) may be such an indicator of behaviorally relevant aspects of serotonergic activity (Hegerl and Juckel 1993, Biol Psychiatry 33:173-187). In order to study this relationship more directly, epidural recordings over the primary and secondary auditory cortex were conducted in chronically implanted cats under intravenous (i.v.) administration of drugs influencing the serotonergic and other modulatory systems (8-OH-DPAT, m-CPP, ketanserin, DOI, apomorphine, atropine, clonidine). The intensity dependence of the cat AEP component with the highest functional similarity to this of the N1/P2-component in humans was significantly changed by influencing 5-HT1a and 5-HT2 receptors, but not 5-HT1c receptors. This serotonergic modulation of the intensity dependence was only found for the primary auditory cortex which corresponds to the known different innervation of the primary and secondary auditory cortex by serotonergic fibers. Our study supports the idea that the intensity dependence of AEP could be a valuable indicator of brain serotonergic activity; however, this indicator seems to be of relative specificity because at least cholinergic effects on the intensity dependence were also observed.

Cortical dopamine in schizophrenia: strategies for postmortem studies

Many of the symptoms of schizophrenia appear to involve dysfunction of the cognitive processes mediated by the neural circuitry of the cerebral cortex. The application of modern neuroscience techniques to the study of postmortem human brain specimens provides a powerful approach for exploring the manner in which cortical circuitry may be disrupted in schizophrenia. In this paper, we describe a strategy for the conduct of postmortem investigations of schizophrenia that involves (1) the use of a nonhuman primate model to guide the design and interpretation of studies in humans; (2) the detailed characterization of the normal organization of neural systems in the human cerebral cortex, and the range of inter-individual variations in that organization; and (3) the testing of specific hypotheses about the disruption of that organization in schizophrenia. The application of this strategy, and its value in overcoming some of the potential pitfalls of postmortem studies, is demonstrated in a series of investigations designed to test the hypothesis that dopamine neurotransmission is impaired in the entorhinal cortex in schizophrenia.

Dopamine, the prefrontal cortex and schizophrenia

Dysfunction of the prefrontal cortex (PFC) in schizophrenia has been suspected based on observations from clinical, neuropsychological and neuroimaging studies. Since the PFC receives a dense dopaminergic innervation, abnormalities of the mesocortical dopamine system have been proposed to contribute to the pathophysiology of schizophrenia. In this review, aspects of the anatomy, physiology and pharmacology of the mesencephalic-frontal cortical dopamine system as they may relate to schizophrenia are described, and evidence for altered dopaminergic neurotransmission in the frontal cortex of schizophrenic patients is presented.

The anatomical relationships of the prefrontal cortex with limbic structures and the basal ganglia

This paper briefly discusses the anatomical criteria that have been used to delineate the prefrontal cortex (PFC) from the (pre)motor cortical areas in the frontal lobe. Single anatomical criteria, such as cytoarchitecture, connectivity with the mediodorsal thalamic nucleus or a dopaminergic innervation, are insufficient to unequivocally define the PFC. It is argued that, with respect to a number of structural aspects, the prefrontal and the (pre)motor cortical areas must be viewed as a continuum, whereas a (functional) differentiation is based on the type of information that is being processed in different parts of the frontal lobe. The involvement of the PFC, like the premotor cortex, in a number of basal ganglia-thalamocortical circuits may be interpreted in the same way. The paper also summarizes the organization of the inputs from midline/intralaminar thalamic nuclei, the basal amygdaloid complex and the hippocampus into the PFC-ventral striatal system. The results of tracing studies in rats indicate that these thalamic and limbic inputs both at the level of the PFC and the ventral striatum show various patterns of convergence and segregation. This leads to the conclusion that the PFC-ventral striatal system consists of a number of smaller modules.

Transient uptake and storage of serotonin in developing thalamic neurons

Serotonin (5-HT) has been shown to affect the development and patterning of the mouse barrelfield. We show that the dense transient 5-HT innervation of the somatosensory, visual, and auditory cortices originates in the thalamus rather than in the raphe: 5-HT is detected in thalamocortical fibers and most 5-HT cortical labeling disappears after thalamic lesions. Thalamic neurons do not synthesize 5-HT but take up exogenous 5-HT through 5-HT high affinity uptake sites located on thalamocortical axons and terminals. 3H-5-HT injected into the cortex is retrogradely transported to thalamic neurons. In situ hybridization shows a transient expression of the genes encoding the serotonin transporter and the vesicular monoamine transporter in thalamic sensory neurons. In these glutamatergic neurons, internalized 5-HT might thus be stored and used as a "borrowed transmitter" for extraneuronal signaling or could exert an intraneuronal control on thalamic maturation.

The serotonergic innervation of the cerebral cortex in man and its changes in focal cortical dysplasia

We present the morphology and the laminar distribution of the serotonin (5-hydroxytryptamine, 5-HT) innervation of the cerebral cortex of patients who underwent cortical resection for partial seizures. The limits of the resections were established by stereoelectroencephalography. The 5-HT innervation was mapped by using an antiserum anti-5-HT. Two patients had cryptogenic epilepsies and two others had seizures related to focal cortical dysplasia. 5-HT immunoreactive axons were morphologically heterogeneous and projected diffusely to the cerebral cortex with regional-specific densities. Two types of terminal axon were demonstrated. Type I had large and spherical (intensely immunoreactive) varicosities and was distributed sparsely with a characteristic predominance in the molecular layer. Type II had fine and pleiomorphic varicosities (granular or fusiform) and was distributed through all cortical layers. The distribution of the 5-HT innervation varied according to the different architectonic areas investigated. The granular cortical areas characterized by a highly developed layer IV (primary somatosensory, primary visual and prefrontal cortices) had the highest density of 5-HT-ir fibers distributed from layer I to layer V. The agranular primary motor cortex had the lowest density with fibers preferentially seen in layers I, IIIa and V-VI. The orbital cortex with a poorly defined layer IV had an intermediate density with a laminar repartition predominant in the supragranular layers. In patients with cryptogenic epilepsies, the brain epileptogenic tissue was histologically normal as well as the serotonergic innervation. In contrast, in patients with focal cortical dysplasia, the dysplastic epileptogenic tissue was characterized by a serotonergic hyperinnervation. In agreement with previous data in primates, we give morphological evidence for two morphologically distinct serotonergic subsystems and for regional specific densities in the human cerebral cortex. Moreover, we previously reported an altered pattern of the catecholaminergic innervation in the same dysplasia areas. All these results provide evidence that this development epileptogenic lesion involves several sets of neurons which may contribute to epileptogenic activity.

Serotonergic axons in monkey prefrontal cerebral cortex synapse predominantly on interneurons as demonstrated by serial section electron microscopy

Anatomical approaches were used to describe the distribution, appearance, and synaptic interactions of serotonin (5-HT)-immunoreactive axons in monkey prefrontal cortex. A plexus of 5-HT axons was found throughout the gray matter, with an especially high density in layer I and a slight increase in layer IV. They were strikingly heterogeneous, with a gradient of morphologies ranging from fine and nonvaricose to highly varicose or thick and nonvaricose. Electron microscopy showed that both varicose and nonvaricose axons were typically filled with clear vesicles and less abundant dense core vesicles. A serial section analysis of 5-HT varicosities in layers I, III, and V showed consistent results across layers. Only about 23% of labeled varicosities formed identifiable synapses. These synapses were consistently asymmetric and were 2-5 serial sections (or 0.08-0.38 mu) in diameter. Targets of identified 5-HT synapses were dendritic shafts with the exception of one cell soma. Followed in serial sections, postsynaptic dendrites typically had morphological features of interneurons, i.e. they lacked spines, had a high density of synaptic inputs, and often had a varicose morphology. Only 8% of postsynaptic shafts were classified as pyramidal dendrites. This is in striking contrast to our previous study in this cortex of dopamine axons, which synapsed predominantly on pyramidal dendrites. These are the first results to indicate that interneurons are the major recipient of identifiable 5-HT synapses in the monkey prefrontal cortex.

Visual hallucinosis: the major clinical determinant of distorted chromatic contour perception in Parkinson's disease

Recently distorted chromatic contour perception has been demonstrated in Parkinson's disease (PD). The aim of our study is to determine the clinical factors which influence chromatic contour perception in PD. Chromatic and achromatic contour perception, colour discrimination and clinical data were evaluated in 73 patients with PD. We used a computer-aided method to determine the chromatic fusion time (CFT) which indicates the acuity of monochromatic contour perception. Chromatic CFT was generally shortened in patients as compared to controls (p < 0.01), whereas achromatic CFT was not significantly different. Variance analysis revealed the ability of colour discrimination and the risk of visual hallucinations as statistically significant (p < 0.05) variables influencing contour perception of certain stimuli. In contrast, disease stage, disease duration and disease severity have no relevant effect on chromatic contour perception in Parkinson's disease. On the basis of those properties one may suggest that distorted chromatic contour perception is due to an impairment at a central stage of visual processing in PD and an imbalance of the serotonergic system. Whether CFT is a reliable method to predict the individual risk of hallucinosis in PD has to be evaluated.

Axon terminals immunolabeled for dopamine or tyrosine hydroxylase synapse on GABA-immunoreactive dendrites in rat and monkey cortex

Dopamine afferents to the cortex regulate the excitability of pyramidal neurons via a direct synaptic input. However, it has not been established whether dopamine also modulates pyramidal cell activity indirectly through synapses on gamma-aminobutyric acid (GABA) interneurons, and whether such inputs differ across cortical regions and species. We sought to address these issues by an immunocytochemical electron microscopic approach that combined peroxidase staining for dopamine or tyrosine hydroxylase (TH) with a pre-embedding gold-silver marker for GABA. In the deep layers of the rat prefrontal cortex and in the superficial layers of the monkey prefrontal and primary motor cortices, terminal varicosities immunoreactive for dopamine or TH formed primarily thin, symmetric synapses on distal dendrites. Both GABA-immunoreactive dendrites as well as unlabeled spines and dendrites were contacted by dopamine- or TH-immunoreactive terminals. Synaptic specializations were detected at some, but not all of these contacts. The relative frequency of these appositional and synaptic contacts did not appear to differ between the rat and monkey prefrontal cortex, or between the monkey prefrontal and motor cortices. Across regions and species, labeled and unlabeled targets of dopamine- or TH-positive terminals received additional synaptic input from unlabeled, and occasionally GABA-immunoreactive terminals. Close appositions between dopamine- or TH-immunoreactive and GABA-positive terminals were observed only rarely. These findings indicate that dopamine afferents provide direct synaptic inputs to GABA local circuit neurons in a consistent fashion across cortical regions and species. Thus, dopamine's cellular actions involve direct as well as modulatory effects on both GABA interneurons and pyramidal projection neurons.

The ascending neuromodulatory systems in learning by reinforcement: comparing computational conjectures with experimental findings

A central problem in cognitive neuroscience is how animals can manage to rapidly master complex sensorimotor tasks when the only sensory feedback they use to improve their performance is a simple reinforcing stimulus. Neural network theorists have constructed algorithms for reinforcement learning that can be used to solve a variety of biological problems and do not violate basic neurophysiological principles, in contrast to the back-propagation algorithm. A key assumption in these models is the existence of a reinforcement signal, which would be diffusively broadcast throughout one or several brain areas engaged in learning. This signal is further assumed to mediate up- and downward changes in synaptic efficacy by acting as a multiplicative factor in learning rules. The biological plausibility of these algorithms has been defended by the conjecture that the neuromodulators noradrenaline, acetylcholine or dopamine may form the neurochemical substrate of reinforcement signals. In this commentary, the predictions raised by this hypothesis are compared to anatomical, electrophysiological and behavioural findings. The experimental evidence does not support, and often argues against, a general reinforcement-encoding function of these neuromodulatory systems. Nevertheless, the broader concept of evaluative signalling between brain structures implied in learning appears to be reasonable and the available algorithms may open new avenues for constructing more realistic network architectures.

Postnatal maturation of the dopaminergic innervation of monkey prefrontal and motor cortices: a tyrosine hydroxylase immunohistochemical analysis

The mature functional architecture of the primate prefrontal cortex arises during a protracted period of postnatal development. Although catecholaminergic afferents arrive in the primate cortex quite early during fetal development, several lines of evidence suggest that substantial changes in the dopaminergic innervation of prefrontal cortex may occur during postnatal development. In this study, we used immunocytochemical techniques and antibodies against tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, to examine the precise time course from birth to adulthood of the maturational changes of tyrosine hydroxylase-labeled axons in prefrontal cortical areas 9 and 46 and primary motor cortex (area 4) of rhesus monkeys. In area 9, the densities of tyrosine hydroxylase-labeled axons and varicosities in the superficial and deep cortical layers remained relatively constant during postnatal development. In contrast, marked developmental changes in innervation density occurred in the middle cortical layers. For example, in deep layer III, the density of tyrosine hydroxylase-positive varicosities was relatively low and uniform in animals under 1 month of age but then increased by a factor of three in animals 2-3 months of age. The density of labeled varicosities continued to increase, reaching a peak (sixfold greater than in the youngest animals) in animals 2-3 years of age before declining to stable adult levels. Similar laminar-specific patterns of change also occurred in areas 46 and 4, although regional differences were present in the magnitude and precise time course of these developmental changes. These findings demonstrate that the innervation of monkey frontal cortex by tyrosine hydroxylase-immunoreactive axons undergoes a protracted, laminar-specific pattern of change during postnatal development that continues through adolescence and into early adulthood. These developmental refinements may interact with other modifications of cortical circuitry that underlie the functional maturation of these regions.

Dopaminergic innervation of the telencephalon of the pigeon (Columba livia): a study with antibodies against tyrosine hydroxylase and dopamine

The dopaminergic structures in the telencephalon of the pigeon were investigated with antisera against glutaraldehyde-conjugated dopamine (DA) and tyrosine hydroxylase (TH). Our goal was to describe the morphological patterns of the labelled axons and to provide a detailed map of the density and regional distribution of the dopaminergic innervation in relation to cytoarchitectonic areas. DA- and TH-like fibers reached their highest density in the paleostriatum augmentatum and the lobus parolfactorius of the basal ganglia. The paleostriatum primitivum was characterized by a dichotomous DA-positive innervation with a diffuse fiber network contacting enpassant granular cells and a more specific input that completely wrapped up large cells, which probably represent relay neurons. Two distinct DA-positive pathways could be followed back from the forebrain leading to the dopaminergic cell groups of the nucleus tegmenti pedunuculopontinus pars lateralis and the area ventralis tegmentalis. The primary sensory areas of the visual, auditory, somatosensory, and trigeminal systems within the forebrain of the pigeon were virtually devoid of DA-like fibers and demonstrated only TH-positive axons, probably of a noradrenergic nature. Among the limbic structures, the neostriatum caudolaterale (a possible equivalent of the mammalian prefrontal cortex), the septum, the nucleus accumbens, and parts of the archistriatum were heavily labelled by DA-like axons. A highly characteristic morphological feature of the catecholaminergic innervation was the presence of "baskets," which are constituted by TH- and DA-positive fibers coiled up around large perikarya, so that the surrounded somata were virtually visible by the presence of labelled axons. The density of basket and nonbasket type innervations seemed to be independently regulated, so that each forebrain structure could be characterized by a mixture consisting of the individual degrees of these two features. Our results demonstrate that the dopaminergic innervation of the forebrain of the pigeon is widespread but shows important regional variations. Similar to mammals, associative and motor structures are heavily innervated by dopaminergic fibers, whereas sensory areas are dominated by their noradrenergic input. The basket and nonbasket type innervations observed in virtually all of these subdivisions of the telencephalon may indicate the presence of two main classes of catecholaminergic afferents with different mechanisms of modulation of forebrain activity patterns.

Levodopa/carbidopa treatment for amblyopia in older children

The purpose of the present study is to determine how long visual function improves during levodopa/carbidopa with part-time occlusion treatment in older amblyopic children. A 7-week open clinical trial of levodopa (0.55 mg/kg, three times daily) with 25% carbidopa combined with part-time occlusion (3 hours/day), was undertaken in 15 older (mean age, 8.87 years) amblyopic children. Visual acuity, fusion, and stereo acuity were measured at baseline, at weeks 1, 3, 5, and 7 during treatment, and 6 weeks following the end of all treatment. Health status was assessed by physical examination, questionnaire, and standard laboratory tests (CHEM 20, complete blood cell count [CBC], and differential). The results revealed that visual acuity in the amblyopic eye improved for 5 weeks, from 20/170 at baseline to 20/107, then stabilized for the last 2 weeks. Visual acuity also improved in the dominant eye by 0.6 lines, from 20/19 to 20/16. At the 6-week follow up, visual acuity in the amblyopic eye remained at 20/107 (paired t = 4.78, df = 14, P < .001), a 37% improvement. Thirty-three percent of the subjects demonstrated improved fusion from baseline to the 7-week trial that was maintained at follow up (chi 2 = 3.97, P < .05). Stereo acuity did not significantly change. Physical exam (blood pressure, body temperature, respiration, heart rate) and standard laboratory tests remained normal and side effects were minimal.(ABSTRACT TRUNCATED AT 250 WORDS)

D1- and D2 dopaminergic receptors in the developing cerebral cortex of macaque monkey: a film autoradiographic study

Film autoradiography was used to study the distribution of D1- and D2-dopaminergic receptors in the prefrontal association, somatosensory, primary motor and visual regions in the developing cerebral cortex of macaque monkeys. D1 receptors were labeled with [125I]SCH23982, while D2 sites were visualized with [125I]epidepride. D1- and D2-dopaminergic sites are already present in all cortical areas at embryonic day 73, the earliest age observed in this study. In contrast to the adult cortex, where D1 and D2 receptors have different distributions, during development there are substantial similarities in the laminar patterns of these sites. In particular, both D1 and D2 receptors tend to concentrate in the marginal zone and layer V of the developing cortical plate. The autoradiograms also show a high density of D1-dopaminergic sites in the transient ventricular and subventricular zones, where cortical neurons are generated. Although there is a significant rearrangement of the early laminar patterns, the adult distribution of both dopaminergic receptors in most cortical areas is achieved prenatally, soon after all cortical neurons assume their final positions. An early presence in the cerebral wall, a high density in the proliferative zones and fast maturation of the laminar distribution suggests that dopaminergic receptors may be involved in the regulation of cortical development.

Afferent connections of the medial frontal cortex of the rat. II. Cortical and subcortical afferents

In order to compare the frontal cortex of rat and macaque monkey, cortical and subcortical afferents to subdivisions of the medial frontal cortex (MFC) in the rat were analyzed with fluorescent retrograde tracers. In addition to afferent inputs common to the whole MFC, each subdivision of the MFC has a specific pattern of afferent connections. The dorsally situated precentral medial area (PrCm) was the only area to receive inputs from the somatosensory cortex. The specific pattern of afferents common to the ventrally situated prelimbic (PL) and infralimbic (IL) areas included projections from the agranular insular cortex, the entorhinal and piriform cortices, the CA1-CA2 fields of the hippocampus, the subiculum, the endopiriform nucleus, the amygdalopiriform transition, the amygdalohippocampal area, the lateral tegmentum, and the parabrachial nucleus. In all these structures, the number of retrogradely labeled cells was larger when the injection site was located in area IL. The dorsal part of the anterior cingulate area (ACd) seemed to be connectionally intermediate between the adjacent areas PrCm and PL; it receives neither the somatosensory inputs characteristic of area PrCm nor the afferents characteristic of areas PL and IL, with the exception of the afferents from the caudal part of the retrosplenial cortex. A comparison of the pattern of afferent and efferent connections of the rat MFC with the pattern of macaque prefrontal cortex suggests that PrCm and ACd areas share some properties with the macaque premotor cortex, whereas PL and IL areas may have characteristics in common with the cingulate or with medial areas 24, 25, and 32 and with orbital areas 12, 13, and 14 of macaques.

Development of the catecholamine neurons in human embryos and fetuses, with special emphasis on the innervation of the cerebral cortex

The cathecholaminergic (CA) systems have been described as appearing early in the development of the mammalian central nervous system (CNS), but their exact distribution in humans has been studied only following gestational week (g.w.) 13. Furthermore, it is not known when CA fibers initially penetrate the developing cerebral cortex. In this study, the CA cells groups and fibers are described in the human central nervous system from 6 to 13 g.w. as revealed with immunocytochemical techniques, with antibodies raised against three synthetic enzymes of the catecholamine (CA) pathway: tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT). At 6 g.w., TH-like immunoreactive (TH-IR) cell groups were widespread through the caudorostral extension of the CNS corresponding to the different dopaminergic mesencephalic and hypothalamic groups. Noradrenergic groups also were labeled in the medulla oblongata and in the locus coeruleus as well as in other areas in the pons. Additional TH-IR cell groups might represent a transient developmental expression of TH similar to that observed in the rat. DBH immunoreactivity labeled primarily the noradrenergic pontic cell groups and, to a lesser extent, groups located in the medulla oblongata. Rare PNMT-IR neurons were detected in the medulla oblongata only at 13 g.w. The main CA bundles described in the adult were also observed in human embryos and fetuses. At 6 g.w., TH-IR pathways extended caudorostrally within the central tegmental tract and the dorsal tegmental bundle, the latter merging with the dopaminergic mesotelencephalic pathway giving rise to the medial forebrain bundle in the basal forebrain. At 7-8 g.w., TH-IR fibers extended to the basal ganglia and the telencephalic wall. The first TH-IR and, to a much lesser extent, DBH-IR fibers penetrated the frontal lateral cortical anlage through the intermediate zone and sparsely through the marginal zone but not through the thin cortical plate. A second stream entered the telencephalic anlage frontomedially, ventral to the septal area. At 11 g.w., numerous TH-IR fibers invaded the subplate layer, but they penetrated the cortical plate only at 13 g.w. At that time, TH-IR and DBH-IR fibers had reached the occipital cortex in a rostrocaudal gradient. The appearance of well-organized CA system already in embryonic stages in humans could be of great importance for normal shaping of the nervous system as well as for development of cortical circuitry.

Frontal and parietal premovement slow brain potentials in Parkinson's disease and aging

During the anticipation of a stimulus that induces a predetermined pattern of behavior, a slowly increasing negative electric potential can be recorded from the human scalp at central and parietal electrodes and has been named contingent negative variation (CNV). We used a simple and a choice reaction time paradigm to investigate premovement potentials in patients with Parkinson's disease (PD) and in normal controls. There was a clear CNV in young subjects whereas it was negligible in the elderly control subjects and absent in the patients. In addition, we found a slowly increasing positive frontal potential. In normals the steepness of this potential decreased with the complexity of the task (simple vs. choice) and with age. This difference was abolished in the patients: If a slowly increasing positivity was observed at all, it was, on average, larger in the choice task. Reaction times of the patients were disproportionally prolonged in the simple compared to the complex task. These findings support the hypothesis that storing or initiating a simple preprogrammed motor response is more impaired in PD than selecting and initiating a motor response of a more complex task. The electrophysiological recordings suggest that impaired activation of the frontal lobes may be responsible for this deficit.

Somatosensory evoked potentials at rest and during movement in Parkinson's disease: evidence for a specific apomorphine effect on the frontal N30 wave

Studies attempting to relate the abnormalities of the frontal N30 components of the somatosensory evoked potentials (SEPs) to motor symptoms in Parkinson's disease (PD) have shown contradictory results. We recorded the frontal and parietal SEPs to median nerve stimulation in 2 groups of PD patients: a group of 17 patients presenting the wearing-off phenomenon, and a group of 10 untreated PD patients. The results were compared with a group of 13 healthy volunteers of the same age and with a group of 10 non-parkinsonian patients. All parkinsonian and non-parkinsonian patients were studied before ("off" condition) and after a subcutaneous injection of apomorphine ("on" condition). The gating effects of a voluntary movement (clenching of the hand) on the SEPs were also studied for the wearing-off group of PD patients (in states off and on) in comparison with the healthy subjects. At rest and in the off condition the amplitude of the frontal N30 was significantly reduced in the 2 groups of PD patients. We demonstrate that the movement gating ability of the PD patient is preserved in spite of the reduced amplitude of the frontal N30. This result suggests that the specific change in the frontal N30 in PD is not the consequence of a continuous gating of the sensory inflow by a motor corollary discharge. Clinical motor improvement induced by apomorphine was associated with a significant enhancement of the frontal N30 wave. In contrast, the subcortical P14 and N18 waves and the cortical N20, P22, P27 and N45 were not statistically modified by the drug. Apomorphine infusion did not change the absolute reduced voltage of the N30 reached during the movement gating. While the frontal N30 component of the non-parkinsonian patients was significantly lower in comparison to healthy subjects, this wave did not change after the apomorphine administration. In the wearing-off PD patient group the frontal N30 increment was positively correlated with the number of off hours per day. This specific apomorphine sensitivity of the frontal N30 was interpreted as a physiological index of the dopaminergic modulatory control exerted on the neuronal structures implicated in the generation of the frontal N30.

Expression and distribution of two isoforms of tyrosine hydroxylase in macaque monkey brain

In humans, the RNA for tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines, can undergo alternative splicing to produce four different types of mRNA. Each of the predicted TH protein forms has been shown to be expressed in human brain, but constraints inherent in human postmortem studies require the use of an animal model to further explore the functional significance of multiple TH isoforms. Since the anatomical organization of catecholaminergic systems in Old World macaque monkeys appears to accurately predict their organization in humans, we used antibodies that selectively recognize each of the human TH isoforms to determine the expression and distribution of TH isoforms in monkey brain. Blot immunolabeling and immunocytochemical techniques demonstrated that catecholaminergic neurons of monkey brain express both type 1 and type 2 TH, but not type 3 or type 4. Thus, monkeys differ in the number of TH isoforms expressed in brain both from rodents (type 1 TH only) and humans (types 1-4 TH). In some catecholaminergic neurons and axon terminals of human brain, only type 1 TH immunoreactivity was detectable, suggesting that the selective expression of a single isoform could occur or that TH isoforms might differ in their accessibility to or engagement with cell trafficking mechanisms. However, in monkeys there was no evidence for the selective expression or distribution of either type 1 or type 2 TH in any of the catecholaminergic cell bodies or terminal fields examined. In addition, quantitative blot immunolabeling studies demonstrated that type 1 and type 2 TH were present in the same relative abundances in monkey substantia nigra and caudate nucleus. Furthermore, studies in monkeys designed to model the human postmortem state revealed that the apparently selective distribution of TH isoforms seen in the human study could be created by postmortem effects. In summary, in contrast to nonprimate species, multiple protein isoforms of TH are expressed in monkey brain as well as in human brain. Although the precise functional role of each isoform has not been determined, their presence suggests that the regulation of catecholamine biosynthesis may be more complex and subject to alternative modes of regulation in primates than in nonprimate species. In addition, the difference between monkeys and humans in the number of TH isoforms may provide insight into a molecular basis for species differences in cognitive and other brain functions. However, both type 1 and type 2 TH, the two predominant isoforms of human TH, are expressed in monkey brain.(ABSTRACT TRUNCATED AT 400 WORDS)

6-[18F]fluoro-L-dihydroxyphenylalanine metabolism and positron emission tomography after catechol-O-methyltransferase inhibition in normal and hemiparkinsonian monkeys

Increased and sustained central delivery of L-dihydroxyphenylalanine (L-DOPA) is a desirable therapeutic strategy in Parkinson's disease. We investigated the effects of peripheral catechol-O-methyltransferase (COMT) inhibition, by the non-toxic drug nitecapone on the metabolism of 6-[18F]fluoro-L-dihydroxyphenylalanine (6FD) and on its positron emission tomography (PET) imaging in non-human primates. Nitecapone produced a dose-dependent inhibition in the formation of 3-O-methyl-6-[18F]fluorodihydroxyphenylalanine (OMFD). This inhibition of OMFD formation was attended by increased production of other metabolites, in particular 6-[18F]fluorodopamine (6FDA), 6-[18F]fluorodihydroxyphenylacetic acid (FDOPAC), 6-[18F]fluorohomovanillic acid (FHVA) and [18F]-sulfated conjugates (FSC). Although nitecapone had no effect on plasma 6FD pharmacokinetics, high-dose nitecapone increased contrast of cerebral 18F uptake and retention between regions with high (striatum) versus sparse (parieto-occipital lobes) dopaminergic innervation. 18F uptake contrast was also improved between structures known to possess an intermediate dopaminergic innervation, including the upper brainstem, frontal and temporal lobes, versus sparsely innervated regions. This increased contrast was secondary to decreased activity in sparsely innervated structures and not to increased activity in highly innervated structures. Contrast was correlated inversely with the plasma OMFD/6FD concentration ratio, OMFD being the main 6FD metabolite which can cross the blood brain barrier. We conclude that nitecapone is an effective inhibitor of COMT in non-human primates. This inhibition results in increased 6FD flux through other catabolic pathways. Because of decreased OMFD formation, however, COMT inhibition improves the specificity of 6FD-PET and facilitates in-vivo detection of a wide range of dopaminergic innervation densities in cerebral structures.

Immunocytochemical evidence of well-developed dopaminergic and noradrenergic innervations in the frontal cerebral cortex of human fetuses at midgestation

The catecholaminergic (CA) innervation of the frontal lobe was visualized in 20- to 24-week-old human fetuses with immunocytochemical techniques, by use of antibodies raised against three synthetic enzymes of the CA pathway, tyrosine-hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT). DBH-like immunoreactivity (IR) was probably labeling the noradrenergic (NA) fibers and terminals in the cerebral cortex since no PNMT-IR fibers were detected. In double-labeling TH-DBH experiments, 92-95% of the DBH-IR afferents were not labeled with TH antibodies, indicating that TH-like immunoreactivity (TH-IR) was found primarily in dopaminergic (DA) fibers. Although cortical layering had not yet occurred at this stage, the widespread CA innervation observed in the different areas and layers of the fetal frontal cortex was comparable to that previously described in the adult (Gaspar, Berger, Febvret, Vigny, and Henry: J. Comp. Neurol. 279:249-271, '89). At midgestation, the distribution of CA innervation was region and laminar specific: 1) The densest dopaminergic innervation in the cerebral cortex was located caudal to the genu of the corpus callosum: TH-IR fibers were abundant throughout all layers, from the medial telencephalon (future cingulate) to the dorsal areas (presumed motor cortices) and the lateral insular areas; 2) TH-IR fibers were less dense in the rostral prefrontal cortical anlage; 3) DBH-IR noradrenergic afferents were less numerous than the dopaminergic ones in all the cortical areas studied; 4) in all areas, the highest amount of TH and DBH-IR terminals was found in the upper subplate and in the lower part of the cortical plate, followed by the molecular layer and the intermediate zone. The deep subplate exhibited a lower number of positive fibers but contained TH-IR cell bodies. The presence of dense CA innervation in the immature cortical anlage of the human frontal lobe does not exclude a reorganization of DA and NA innervations within the different cortical layers and areas during the protracted pre- and postnatal period of development.

Topography and collateralization of the dopaminergic projections to motor and lateral prefrontal cortex in owl monkeys

The sources and histochemical characteristics of dopaminergic projections to motor and premotor areas of cortex were investigated in owl monkeys in which information from related studies was used to subdivide cortex into motor fields. Brainstem projections to frontal cortex were identified by injections of different fluorescent dyes in the primary motor cortex (M1) and the supplementary motor area (SMA), first identified by microstimulation. Injections were also placed in dorsal premotor cortex and lateral prefrontal cortex. The distribution of retrogradely labeled neurons was related to the location of tyrosine hydroxylase immunolabeled neurons on the same or alternate brain sections to identify the dopamine (DA) neurons. All DA cortically projecting neurons were located in the A8-A10 complex, largely in its dorsal components, including the parabrachial pigmented n. of the ventral tegmental area (VTA), pars gamma of the substantia nigra compacta, and the dorsal part of the retrorubral area (A8). Fewer cells were in the midline groups of VTA (n. linearis rostralis and caudalis) and in the n. paranigralis. DA neurons projecting to M1, SMA, and prefrontal cortex were largely intermixed, and some of these neurons were double or triple labeled by the fluorescent dyes, indicating collateralization to two or three fields; DA cells projecting to M1 were more numerous than to the other locations. The dorsal components of the A8-A10 complex from which arose the DA mesocortical projection were also characterized by the presence of calbindin-immunoreactive neurons and by a dense neurotensin and noradrenergic terminal innervation. Compared to rodents or felines, the DA neurons projecting to the lateral frontal lobe of primates appear to be shifted dorsally and laterally in the nigral complex. The topographic overlap, partial collateralization, and common histochemical characteristics of the DA mesocortical neurons projecting to different fields of the lateral frontal lobe suggest that some degree of functional unity exists within this projection.

Intensity dependence of auditory evoked potentials as an indicator of central serotonergic neurotransmission: a new hypothesis

Because of the increasing importance of the central serotonergic neurotransmission for pathogenetic concepts and its role as a target of pharmacotherapeutic interventions in psychiatry, reliable indicators of this system are needed. It is proposed that the stimulus intensity dependence of auditory evoked N1/P2-component, which is probably modulated by cortical serotonergic innervation, may be a useful and noninvasive indicator of behaviorally relevant aspects of serotonergic activity. Converging evidence from our own studies as well as from the literature suggests that a pronounced intensity dependence of auditory evoked N1/P2-component reflects low central serotonergic neurotransmission. Recent findings concerning general functional aspects of the brain serotonin system reveal that this system is well qualified for adjusting individual levels of sensory processing ("set the tone"), especially in the primary auditory cortex in which the N1/P2-component is mainly generated. Dipole source analysis represents an important methodological advance in this context because it allows the separation of N1/P2-subcomponents generated in the primary auditory cortex from those generated in secondary auditory areas.