A theory of the functional organization of the neostriatum and the neostriatal control of voluntary movement

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.

Quinpirole inhibits striatal and excites pallidal neurons in freely moving rats

The activity of single neurons in the striatum of freely moving rats was recorded in response to systemic administration of dopamine agonists selective for either the D1 or D2 family of receptors. At a dose that induced behavioral activation, SKF-38393 (5.0 mg/kg s.c.), a D1 agonist, had no consistent effect on striatal activity, whereas quinpirole (1.0-5.0 mg/kg s.c.), a D2 agonist, inhibited the great majority of striatal neurons. In addition, quinpirole (1.0 mg/kg s.c.) excited neurons of the globus pallidus, which receives an inhibitory projection from the striatum. These results are consistent with models of the basal ganglia in which dopamine, via D2 receptors, inhibits striatopallidal activity, resulting in a disinhibition of neurons in globus pallidus.

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.

Functional injury of cholinergic, GABAergic and dopaminergic systems in the basal ganglia of adult rat with kaolin-induced hydrocephalus

Structural and/or functional injury of the basal ganglia can lead to motor functional disabilities, abnormal gait and posture, and intellectual/emotional impairment, disorders also frequently seen in hydrocephalus. Previous reports have documented changes in dopamine levels in the neostriatum in experimental hydrocephalus. The present study was designed to investigate possible functional injury of cholinergic, GABAergic and dopaminergic systems in the basal ganglia immunohistochemically in a model of kaolin-induced hydrocephalus. Hydrocephalus was induced in 12 Wistar rats by intracisternal injection of 0.05 ml volume of 25% kaolin solution under microscopic guidance. Four controls received an equal volume of sterile saline. The animals were killed at 2, 4 and 8 weeks after injection. The numbers of choline acetyltransferase (ChAT)- and glutamic acid decarboxylase (GAD)-immunoreactive (IR) neostriatal neurons and tyrosine hydroxylase (TH)-IR nigral neurons, were counted in 60-micron thick representative sections and the IR cellular densities (counted cell number/neostriatal area) were calculated in the neostriatum. The number of total neostriatal neurons was also counted in 15-micron thick sections stained by cresyl violet (Nissl staining) to calculate the cellular density. The number and cellular density of neostriatal ChAT-IR neurons were significantly reduced at 2, 4, and 8 weeks after injection (P < 0.05), while those of GAD-IR neurons decreased at 4 and 8 weeks (P < 0.05). There was a linear correlation between degree of ventricular enlargement, and reduction in number of ChAT- and GAD-IR neurons (P < 0.001) as well as in the cellular density (P < 0.001). However, Nissl staining revealed no reduction in the cellular density of total neostriatal neurons (P < 0.001). TH immunoreactivity was reduced in neostriatal axons and in nigral compacta neurons, particularly in the medial portion of the dopaminergic nigrostriatal pathway. These findings suggest that progressive hydrocephalus results in functional injuries of cholinergic and GABAergic neurons in the neostriatum and dopaminergic neurons in the substantia nigra compacta by mechanical distortion. The disturbance in balance of these neurotransmitter systems in the basal ganglia may explain some of motor functional disabilities in hydrocephalus.

Bromocriptine enhances feeding behavior without changing dopamine metabolism

Bromocriptine is an ergot derivative and has been thought to act as a selective D2 receptor agonist, but its effects on dopamine release in vivo have not been confirmed. We administered bromocriptine into the striatum of rats and studied the effects on feeding behavior and dopamine release. Bromocriptine was perfused via a microdialysis probe into the ventrolateral striatum of rats fasted for 22 h, and the rats were then allowed to feed freely for 6 h. Bromocriptine perfusion increased food intake in a dose-dependent manner, whereas the extracellular concentrations of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) did not change. Perfusion of (-) sulpiride, a selective D2 receptor antagonist, decreased food intake, but increased dopamine release and the levels of DOPAC and HVA. Pretreatment with (-)sulpiride perfusion for 1 h prior to bromocriptine perfusion inhibited the increase of food intake induced by bromocriptine, and it increased dopamine release and the levels of DOPAC and HVA. These findings suggest that bromocriptine directly perfused into the ventrolateral striatum acts selectively on postsynaptic D2 receptors and enhances feeding behavior.

Synaptic plasticity and physiological interactions between dopamine and glutamate in the striatum

Several electrophysiological studies have addressed the interaction between glutamate and dopamine within the striatum. Although the results obtained from these studies were often conflicting, more recently the characterization of new forms of synaptic plasticity in the basal ganglia provided a possible integrative explanation of the different electrophysiological data regarding the interaction between these transmitters. In this review we will try to summarize and discuss the available data concerning the possible impact of the functional role of D1 and D2 receptor activation on the modulation of the glutamatergic corticostriatal pathway. Moreover, we will also describe the function of the striatum in the integration of glutamatergic and dopaminergic inputs to produce long-term changes of synaptic efficacy (long-term depression, long-term potentiation). Finally, we will consider the implication of the interaction between dopamine and glutamate in the regulation of energetic metabolism whose failure is responsible for neuronal death.

Activation of group III metabotropic glutamate receptors depresses glutamatergic transmission at corticostriatal synapse

Intracellular recordings were performed from a rat corticostriatal slice preparation in order to characterize the effects of group III metabotropic glutamate receptor (mGluR) agonists on excitatory transmission at corticostriatal synapses. The amplitude of excitatory postsynaptic potentials (EPSPs), evoked by cortical stimulation, was significantly decreased by agonists acting at group III metabotropic glutamate receptors. Both L-2-amino-4-phosphonobutanoate (L-AP4) and L-serine-O-phosphate (L-SOP) were effective in reducing the amplitude of cortically evoked EPSPs, in a dose-dependent manner. The EC50 value for the effect of L-SOP and L-AP4 was 0.89 microM and 9.95 microM, respectively. Both L-AP4 and L-SOP had negligible effects on the intrinsic membrane properties of the recorded neurons and did not alter the postsynaptic response to focal application of glutamate, suggesting a presynaptic site of action. The presynaptic inhibition of both L-SOP and L-AP4 was fully antagonized by 250 microM (s)-2-methyl-2-amino-4-phosphonobutanoate (MAP4), whilst it was unaffected by 500 microM RS-methyl-4-carboxyphenylglycine (MCPG). Conversely, the presynaptic inhibitory effect on the EPSP amplitude exerted by 10 microM 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) was antagonized by 500 microM MCPG, whilst it was not blocked by 250 microM MAP4. Finally, the reduction of the EPSP amplitude produced by a saturating dose of L-SOP was further increased by 10 microM 1S,3R-ACPD, suggesting an additive effect of these compounds. The present results are consistent with the idea that group III mGluRs exert a presynaptic inhibitory modulation of the excitatory glutamatergic transmission at corticostriatal synapses.

Enhancement of NMDA responses by group I metabotropic glutamate receptor activation in striatal neurones

1. The interactions between N-methyl-D-aspartate (NMDA) and metabotropic glutamate receptors (mGluRs) were investigated in striatal slices, by utilizing intracellular recordings, both in current- and voltage-clamp mode. 2. Bath-application (50 microM) or focal application of NMDA induced a transient membrane depolarization, while in the voltage-clamp mode, NMDA (50 microM) caused a transient inward current. Following bath-application of the non-selective mGluR agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 10 microM), NMDA responses were reversibly potentiated both in current (197 +/- 15% of control) and voltage-clamp experiments (200 +/- 18% of control). 3. Bath-application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (3,5-DHPG, 10-300 microM) resulted in a dose-dependent potentiation of NMDA-induced membrane depolarization (up to 400 +/- 33% of control). This potentiation was either prevented by preincubation with (RS)-alpha-methyl-4-carboxyphenylglycine (RS-alpha-MCPG, 300 microM), or blocked when applied immediately after 3,5-DHPG wash-out. 4. Neither (2S,1'S,2'S)2-(2'-carboxycyclopropyl)glycine (L-CCG I, up to 100 microM) nor (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)-glycine (DCG-IV, 1 microM), agonists for group II mGluRs caused any change in NMDA responses. Likewise, L-serine-O-phosphate (L-SOP, 30 microM), agonist for group III mGluRs, did not affect the NMDA-induced depolarization. 5. The enhancement of the NMDA responses was mimicked by phorbol-12,13-diacetate (PDAc, 1 microM) which activates protein kinase C (PKC). The 3,5-DHPG-mediated potentiation of the NMDA-induced depolarization was prevented by preincubation with staurosporine (100 nM) or calphostin C (1 microM), antagonists of PKC. 6. Electrophysiological responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor activation were not affected by agonists for the three-classes of mGluRs. 7. The present data suggest that group I mGluRs exert a positive modulatory action on NMDA responses, probably through activation of PKC. This functional interaction in the striatum appears of crucial importance in the understanding of physiological and pathological events, such as synaptic plasticity and neuronal death, respectively.

Dopamine-glutamate interactions in the striatum: behaviourally relevant modification of excitotoxicity by dopamine receptor-mediated mechanisms

The two most important afferent projections to the striatum contain glutamate and dopamine, respectively. Excitotoxic damage resulting from excessive stimulation of the N-methyl-D-aspartate subtype of glutamate receptor has been implicated in pathophysiology of ischaemic stroke, hypoglycaemic brain damage and Huntington's disease. We studied the ability of the dopamine system to modify the anatomical, neurochemical and behavioural consequences of glutamatergic toxicity in the striatum. In a first set of experiments, the specific N-methyl-D-aspartate receptor agonist quinolinate was injected unilaterally into the striatum of rats pretreated with one of (i) intraperitoneal (i.p.) saline (controls); (ii) i.p. haloperidol, a D2 dopamine receptor agonist; or (iii) 6-hydroxydopamine lesion of the ipsilateral nigrostriatal tract. Quinolinate-induced striatal damage, as assessed by morphometric and neurochemical criteria, was significantly attenuated in the animals with 6-hydroxydopamine lesions and in those pretreated with haloperidol, compared with saline-pretreated controls. There were no significant differences between the 6-OHDA and haloperidol groups. In a second set of experiments, animals received (i) bilateral intrastriatal quinolinate plus perioperative i.p. saline; (ii) bilateral intrastriatal quinolinate plus i.p. haloperidol; or (iii) bilateral intrastriatal saline. Again, the quinolinate-lesioned animals treated with perioperative haloperidol had significantly less striatal damage than the bilateral quinolinate rats. Behavioural assessment in the Morris Water Maze showed the bilateral quinolinate+haloperidol group to be significantly less impaired on a spatial acquisition task than the bilateral quinolinate animals. Measures of spontaneous daytime motor activity showed significant differences in average speed and rest time between the bilateral quinolinate+haloperidol rats and the bilateral quinolinate group. The performance of the bilateral quinolinate+haloperidol group was not significantly different from that of controls on any of the behavioural tasks. These results indicate an important role for D2 dopamine receptor-mediated mechanisms in striatal excitotoxicity. Since the excitotoxic process involves the same fundamental signalling mechanism that is involved in normal glutamatergic transmission, these findings imply an ability of D2 receptor blockade to modify glutamate signalling in the striatum. These results may have implications for treatment strategies in ischaemic stroke, hypoglycaemic brain damage and schizophrenia.

Dual character, asynaptic and synaptic, of the dopamine innervation in adult rat neostriatum: a quantitative autoradiographic and immunocytochemical analysis

Dopamine (DA) axon terminals (varicosities) in the neostriatum of adult rats were examined for shape, size, content, synaptic incidence, type of junction, synaptic targets, and microenvironment after electron microscopic identification either by [3H]DA uptake autoradiography or by immunocytochemistry with monoclonal antibodies against DA-glutaraldehyde-protein conjugate. Both approaches yielded comparable results. Whether they were from the paraventricular or the mediodorsal neostriatum, respectively, the [3H]DA-labeled and DA-immunostained varicosities were generally oblong and relatively small; more than 60% contained one or more mitochondria. Sixty to seventy percent were asynaptic, and 30-40% were endowed with a synaptic membrane differentiation (junctional complex), as inferred by stereological extrapolation from single thin sections (both approaches) or observed directly in long, uninterrupted series of thin sections (immunocytochemistry). The synaptic DA varicosities always displayed symmetrical junctions: 67% with dendritic branches, 30% with dendritic spines, and 2-3% with neuronal cell bodies. DA varicosities juxtaposed to one another were frequent. Other axonal varicosities were more numerous in the immediate vicinity of DA varicosities than around randomly selected, unlabeled terminals. The respective microenvironments of DA and unlabeled varicosities also showed enrichment in the preferred synaptic targets of both groups of varicosities, with dendritic branches for DA and dendritic spines for the unlabeled ones. These data suggest a dual mode of operation that is diffuse as well as synaptic for the nigrostriatal DA system. In such a densely DA-innervated brain region, they also lead to the hypothesis that a basal level of extracellular DA might be maintained permanently around every tissue constituent and, thus, contribute to the mechanisms of action, properties, and functions (or dysfunctions) of DA within the neostriatum itself and as part of the basal ganglia circuitry.

Adenylate cyclase in striatal cholinergic interneurons regulates acetylcholine release

Fractional [3H]ACH efflux from dissociated rat striata tested whether tonic inhibition prevents stimulation of acetylcholine (ACH) release by adenylate cyclase. Forskolin stimulated release from the dissociated cells (threshold at 300 nM; EC50 > or = 1 MicroM). Release was also stimulated by 3-isobutyl-1-methylxanthine and was additive with forskolin. The 1,9-dideoxy forskolin analog that lacks cyclase-stimulating activity was ineffective. Thus, stimulation of adenylate cyclase within striatal cholinergic interneurons increases ACH secretion but is tonically inhibited by endogenous striatal transmitters. Disinhibition of the excitatory cyclase by denervation of striatal cholinergic interneurons in situ could contribute to supersensitivity without receptor upregulation.

A D1 dopamine agonist stimulates acetylcholine release from dissociated striatal cholinergic neurons

We tested the hypothesis that a D1 dopamine agonist could stimulate acetylcholine release directly from striatal cholinergic neurons. A suspension of viable dissociated striatal cells was made enzymatically and mechanically from normal adult male rats. The heterogeneous suspension was incubated in [3H]choline to allow synthesis of [3H]acetylcholine selectively by cholinergic neurons. Fractional [3H]acetylcholine release from the cholinergic cells in the suspension was recorded during continuous dynamic perifusion. The D1 agonist, 50 microM (+/-) SKF 38393, increased the basal rate of release from the cholinergic cells by 50% and the action was inhibited by the D1 antagonist, SKF 83566. Stimulation of [3H]acetylcholine secretion was recorded as low as 500 nM SKF 38393. The (S, -) SKF 38393 stereoisomer was significantly less effective than the (R, +) isomer in stimulating release. The D1-mediated stimulation of acetylcholine secretion was abolished in a low-calcium environment that also inhibited basal release. The data suggest that striatal cholinergic cells express D1 receptors functionally coupled to the regulation of acetylcholine release. These D1 actions in the absence of synaptic circuitry imply that such circuitry is not required in situ. In vivo however, indirectly mediated D1 actions and those of other transmitters may modify the manifestations of this direct cholinergic stimulation.

Glutamate-dependent long-term presynaptic changes in corticostriatal excitability

We have previously shown that brief high frequency stimulation of the anteromedial prefrontal cortex induces a long-term decrease in excitability of the glutamatergic corticostriatal terminal field. In contrast, a long-term increase in presynaptic corticostriatal excitability may be induced by presenting two brief cortical tetanizing stimuli separated by 2-3 min such that the second tetanus coincides with a period of increased excitability elicited by the first. In the present study, we examined the glutamate receptor subtypes involved in these long-term changes in presynaptic excitability. A specific glutamate receptor antagonist was infused into the rat striatum 10-25 min prior to either a single or double cortical tetanic stimulation. To eliminate the participation of intrinsic striatal cells, a subset of animals received a striatal kainic acid lesion eight to 20 days before the recording experiment. Antagonists of the N-methyl-D-aspartate and metabotropic glutamate receptor subtypes were effective in blocking the decrease in excitability induced by single cortical tetanic stimulation whereas an antagonist of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptor did not prevent the induction of a long-term reduction in excitability. In contrast, each of these antagonists prevented the induction of a long-term increase in excitability. These long-term modifications in excitability of the presynaptic glutamate axon terminals appear to be induced by similar mechanisms to those postulated to operate in long-term potentiation and depression. These enduring changes in presynaptic excitability are likely to represent important mechanisms for the selective modification of information processing in the striatum.

Heterogeneous distribution of benzodiazepine receptors among rat neostriatal neurones

1. The effects of benzodiazepine receptor (BZR) agonist were investigated in dissociated rat neostriatal neurones by a conventional whole-cell patch recording configuration at room temperature. 2. The dissociated neurones, with a longest somatic diameter of larger than 25 microns, were classified as 'large neurones', while those having soma measuring less than 15 microns were described as 'small neurones'. Large neurones were intensely positive for acetylcholinesterase staining, whereas the small ones were not. 3. CL218,872 enhanced the GABA response in both the large and small neurones with similar EC50S. However, the potentiation efficacy of CL218,872 in large neurones was larger than that of small ones. 4. Zolpidem also potentiated the GABA response in both neuronal populations with similar EC50S. This compound also enhanced the GABA response more strongly in large neurones than in small ones. 5. Zopiclone exerted a prominent potentiation in large neurones, although no difference was seen in the EC50S in the large and small neurones. 6. It was concluded that the BZR in large neurones had a different pharmacological property from that in small ones and that the BZR agonists showed a prominent difference, not in EC50, but in the potentiation efficacy between these neuronal populations.

Modulatory action of dopamine on acetylcholine-responsive striatal and accumbal neurons in awake, unrestrained rats

In ambulant rats, iontophoresis of low concentrations of dopamine (DA) enhances the response of neurons in striatum and nucleus accumbens to iontophoretic glutamate. In an extension of this line of investigation, we tested the effects of acetylcholine (ACh), a presumed modulator of neuronal function in these same brain regions, and assessed possible DA-ACh interactions. Data were obtained from spontaneously active neurons known to respond to ACh (5-30 nA) when the animals rested quietly with no overt movement. ACh iontophoresis either excited or inhibited striatal and accumbal activity but excitatory effects predominated in both areas. With multiple applications of ACh, especially at the lowest currents tested, either response often was interspersed with instances of no change in firing rate. Responsiveness to ACh also diminished during periods of spontaneous movement when basal firing showed phasic increases in activity. In fact, neurons with the highest rates of basal activity showed the smallest magnitude response to ACh. Prolonged applications (120-180 s) of DA attenuated basal firing as well as the iontophoretic effects of ACh both during the DA application itself and for up to 1 min after DA ejection offset. The result of these inhibitory effects was no net change in the relative magnitude of the ACh response. Thus, although ACh can modulate striatal and accumbal neuronal activity, DA does not regulate this effect in the same way that it regulates the neuronal responsiveness to glutamate.

Total number of neurons in the neostriatal, pallidal, subthalamic, and substantia nigral nuclei of the rat basal ganglia: a stereological study using the cavalieri and optical disector methods

The total number of neurons within six subdivisions of the rat basal ganglia was estimated using unbiased stereological counting methods and systematic random sampling techniques. Six young adult rats were perfuse-fixed, their right cerebral hemispheres were embedded in glycolmethacrylate, and a complete set of serial 40-mu m sections was cut through each hemisphere. After a random start, a systematic subset (e.g., every tenth) of these sections was used to estimate the total volume of each subdivision using Cavalieri's method. The same set of sampled sections was used to estimate the number of neurons in a known subvolume (i.e., the Nv) by the optical disector method. The product of the total volume and the Nv by these methods yields an unbiased estimate of the total number of neurons. It was found that the right basal ganglia consisted, on average, of 2.79 million neostriatal or caudate-putamen neurons (with a coefficient of variation of 0.07), 46,000 external globus pallidus neurons (0.11), 3,200 entopeduncular/internal globus pallidus neurons (0.10), 13,600 subthalamic neurons (0.10), 7,200 substantial nigra pars compacta neurons (0.15), and 26,300 substantia nigra pars reticulata neurons (0.07).

Dynamic regulation of striatal dopaminergic grafts during locomotor activity

The present experiment was designed to estimate the neurochemical activity of dopaminergic grafts in hemiparkinsonian model rats during locomotion and to examine the functional importance of dynamic regulation of the grafted neurons in the host brain. Rats were trained to run on a straight treadmill at various speeds (300, 660, 1200, 1800 cm/min), and extracellular dopamine (DA) and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were measured by in vivo microdialysis during and after running. Grafted rats were divided into two groups depending on their running ability and data were compared with those of normal and lesioned controls. Although the tonic level of extracellular DA in grafted rats recovered to 70% of control, levels of DOPAC and HVA remained 15-20% of controls. A small number of grafted rats showed full recovery in treadmill running tasks. In these animals, the percentage increase in DOPAC and HVA showed similar time courses and magnitudes as those in normal rats. Most grafted rats showed partial recovery in locomotor ability. The percentage increase in DOPAC and HVA in these animals remained at a lower level than that in normal rats, though the tonic levels of DA, DOPAC and HVA were not lower than those of fully recovered rats. Data suggest that grafted DAergic cells in functionally well recovered rats were dynamically regulated in the host brain in an actual behavior and that well-controlled release of DA might be involved in the recovery of complex motor behavior, such as high speed locomotion.

Attention, configuration, and hippocampal function

We present a neural network that characterizes a remarkably large number of classical conditioning paradigms and describes the effects of many neurophysiological manipulations. First, the network 1) describes behavior in real time 2) comprises simple and configural stimulus representations, and 3) includes attentional control of storage and retrieval. Second, mapping of the network onto the brain can be summarized by several information processing loops: 1) a hippocampal-cortical configural loop, 2) a hippocampal-cerebellar conditioned-response loop, 3) a hippocampal-accumbens-thalamic attentional loop, and 4) a hippocampal-medial raphe-medial septum error loop. Third, within this global view of brain function, it is assumed that the hippocampal formation computes 1) the aggregate prediction of environmental events and 2) the error signals for cortical learning. These assumptions are supported by rigorous computer simulations consistent with a large body of data on hippocampal and septal neural activity, induction and blockade of hippocampal long-term potentiation, administration of cholinergic agonists and antagonists, administration of haloperidol, and selective and nonselective hippocampal and cortical lesions.

Correlating mind and body

Gray's integration of the different levels of description and explanation in his theory is problematic: (1) The introduction of consciousness into his theorising consists of the mind-brain identity assumption, which tells us nothing new. (2) There need not be correlations between levels of description. (3) Gray's account does not extend beyond “brute” correlation. Integration must be achieved in a principled, mutually constraining way.

Consciousness beyond the comparator

Gray's comparator model fails to provide an adequate explanation of consciousness for two reasons. First, it is based on a narrow definition of consciousness that excludes basic phenomenology and active functions of consciousness. Second, match/mismatch decisions can be made without producing an experience of consciousness. The model thus violates the sufficiency criterion.

Ultimate differences

Gray unwisely melds together two distinguishable contributions of consciousness: one to epistemology, the other to evolution. He also renders consciousness needlessly invisible behaviorally.

Human consciousness: One of a kind

To avoid teleological interpretations, it is important to make a distinction between functions and uses of consciousness, and to address questions concerning the consequences of consciousness. Assumptions about the phylogenetic distribution of consciousness are examined. It is concluded that there is some value in identifying consciousness an exclusively human attribute.

Context and consciousness

The commentary argues (1) that we cannot be sure that human consciousness has survival value and (2) that in order to understand the origins and, perhaps, the function of consciousness, we should examine the behavioural and neural precursors to consciousness in nonhumans. An example is given of research on the role of context in decisions regarding fleeing from probable predators in the Mongolian gerbil.

Electrophysiological actions of felbamate on rat striatal neurones

1. We have investigated the effects of the anticonvulsant drug, felbamate (FBM), on striatal neurones, recorded in vitro by using both intracellular and extracellular conventional recordings in slices and whole-cell recordings in acutely isolated neurones. 2. FBM, at therapeutically relevant concentrations (30-300 microM) showed multiple mechanisms of action. Like other antiepileptic drugs, FBM (30-300 microM) showed a direct inhibitory action on current-evoked firing discharge of striatal neurones. A patch-clamp analysis of this effect revealed a dose-related reduction of voltage-dependent sodium (Na+) currents (10-100 microM), with a half inhibiton dose (IC50) value of 28 microM. 3. We also tested whether FBM affected corticostriatal glutamate transmission. In control medium (1.2 mM external magnesium), both extracellularly recorded field potentials and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) evoked by cortical stimulation were no affected by bath application of 30-300 microM FBM. 4. When magnesium was removed from the perfusing solution, a procedure which reveals a N-methyl-D-aspartate (NMDA)-mediated component in the corticostriatal synaptic potential, FBM (30-300 microM) produced a dose-dependent reduction of the amplitude of both the field potential and the e.p.s.p. 5. FBM reduced the inward currents produced either by bath or by focal applications of 30 microM NMDA, finding consistent with the hypothesis that the observed reduction of the NMDA-mediated component of the synaptic potentials may be caused at postsynaptic level. 6. The reduction of the NMDA-mediated component of the synaptic transmission by FBM and its depressant effect on the voltage-dependent Na+ channels, may account for the antiepileptic action of this drug. Moreover, the pharmacological properties of FBM might render this drug interesting as a neuroprotectant agent.

Effect of muscimol on haloperidol-induced alteration of neurotensin gene expression in the striatum and nucleus accumbens in the rat

Acute neuroleptic administration increases the expression of neurotensin/neuromedin (NT/N) gene in rat dorsolateral striatum and shell sector of the nucleus accumbens. The purpose of this study was to examine modulation of neuroleptic induction of NT/N and the proto-oncogene c-fos expression by the GABAA agonist muscimol. Adult male Sprague-Dawley rats were treated with saline, haloperidol (1 mg/kg); muscimol (3.2 mg/kg); or haloperidol (1 mg/kg) plus muscimol (3.2 mg/kg). Animals were sacrificed 1 h after drug administration. Expression of NT/N and c-fos mRNA was examined by in situ hybridization using 35S-antisense probes. Muscimol alone had no measurable effect on basal levels of NT/N or c-fos mRNA in either the dorsolateral striatum or the nucleus accumbens. However, co-administration of muscimol with haloperidol reduced haloperidol-induced increases in NT/N as well as c-fos mRNA in the dorsolateral striatum. In contrast, NT/N mRNA expression in accumbal shell induced by haloperidol was not modulated by co-administration of muscimol. These data suggest that GABAA receptors may be involved in regulation of NT/N gene expression in the DLSt, but not in the nucleus accumbens.

Interactions of glutamate and dopamine in a computational model of the striatum

A network model of simplified striatal principal neurons with mutual inhibition was used to investigate possible interactions between cortical glutamatergic and nigral dopaminergic afferents in the neostriatum. Glutamatergic and dopaminergic inputs were represented by an excitatory synaptic conductance and a slow membrane potassium conductance, respectively. Neuronal activity in the model was characterized by episodes of increased action potential firing rates of variable duration and frequency. Autocorrelation histograms constructed from the action potential activity of striatal model neurons showed that reducing peak excitatory conductance had the effect of increasing interspike intervals. On the other hand, the maximum value of the dopamine-sensitive potassium conductance was inversely related to the duration of firing episodes and the maximal firing rates. A smaller potassium conductance restored normal firing rates in the most active neurons at the expense of a larger proportion of neurons showing reduced activity. Thus, a homogeneous network with mutual inhibition can produce equally complex dynamics as have been proposed to occur in a striatal network with two neuron populations that are oppositely regulated by dopamine. Even without mutual inhibition it appears that increased dopamine concentrations could partially compensate for the effects of reduced glutamatergic input in individual neurons.

Ascorbate potentiates amphetamine-induced conditioned place preference and forebrain dopamine release in rats

In order to evaluate the effects of ascorbate, which is known to modulate dopamine neurotransmission, on the reinforcing effects of amphetamine, we coadministered ascorbate and amphetamine during the acquisition of conditioned place preference (CPP) in rats. Our results indicate that 100 mg/kg ascorbate potentiates the CPP induced by 0.5 mg/kg, but not 1.0 mg/kg, amphetamine. A higher dose of ascorbate (500 mg/kg) did not influence the CPP induced by either dose of amphetamine. In vitro release assays revealed that, whereas ascorbate alone (0.01-1.0 mM) did not influence striatal dopamine levels, this vitamin potentiated amphetamine-induced dopamine release in both the nucleus accumbens and neostriatum. Collectively, these results raise the possibility that ascorbate potentiates amphetamine-induced CPP by increasing the ability of this psychostimulant to release dopamine.

Effects of local connectivity on striatal function: stimulation and analysis of a model

Neuronal population activity was investigated by computer simulation of a network model based on the neostriatum. Three network topologies were studied, based on different assumptions about the synaptic connectivity among medium spiny neurons. In all networks neurons were interconnected by inhibitory synapses. The connectivity was either symmetric, in which case all connections between cells were reciprocal and equal in strength; or asymmetric. Simulations showed that networks with symmetric connectivity receiving randomly distributed afferent excitation produced stationary spatial activity patterns. In contrast, asymmetric connectivity in homogeneous networks produced slow travelling-wave activity across the network. We suggest that the shape of the medium spiny neurons is an important determinant of synaptic connectivity and that changes in the shape of these neurons caused by Huntington's disease would result in asymmetric connectivity. Slow travelling-wave activity produced by asymmetric connectivity in the neostriatum could explain some aspects of the choreic movement and some electromyographic features seen in Huntington's patients.

Iontophoresis in the neostriatum of awake, unrestrained rats: differential effects of dopamine, glutamate and ascorbate on motor- and nonmotor-related neurons

The neostriatum and its major afferent transmitters, dopamine and glutamate, play a critical role in behavior, but relatively little information is available on their postsynaptic effects in behaving animals. As a first step in addressing this shortcoming, single-unit electrophysiology was combined with iontophoresis in the neostriatum of awake, unrestrained rats. Relative to periods of quiet rest, most neurons (58 of 77) changed discharge rate in close temporal association with movement, while the remainder showed no such relationship. When animals resumed a resting posture, iontophoretic current-response curves were established for dopamine and glutamate as well as for ascorbate, a modulator of neostriatal function released from glutamatergic terminals. Application of either glutamate or ascorbate produced current-dependent increases in activity in all neurons, although this effect was somewhat less pronounced for nonmotor cells. In both types of neurons, the excitatory effect of ascorbate either diminished or shifted to an inhibition at high ejection currents. Dopamine, on the other hand, routinely excited motor-related, but inhibited nonmotor-related neurons. Further assessment of motor-related neurons revealed that in most cases the excitatory effects of either glutamate or dopamine alone were supra-additive when these compounds were either administered together or co-administered with ascorbate. Our results suggest that the response of neostriatal neurons to glutamate or dopamine depends, at least in part, on the motor responsiveness of these cells. Motor-related neurons, moreover, respond to the co-administration of glutamate and dopamine with synergistic increases in firing rate. Ascorbate also influences neostriatal activity, but the postsynaptic action of this substance cannot be explained as a simple interaction with either glutamatergic or dopaminergic mechanisms.

Long-term enhancement of dopamine release by high frequency tetanic stimulation via a N-methyl-D-aspartate-receptor-mediated pathway in rat striatum

We studied the effects of high frequency tetanic stimulation of the striatum on the KCl (20 mM)-evoked dopamine release in rat striatal slices. The KCl-evoked dopamine release was potentiated by high frequency tetanic stimulation (10-20 Hz) of the striatum including the corticostriatal fibers, and this potentiation was observed until 3 h after high frequency tetanic stimulation. Potentiation of dopamine release after high frequency tetanic stimulation was induced not only by KCl but also by glutamate in Mg(2+)-free medium, N-methyl-D-aspartate in Mg(2+)-free medium, and by DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid. 2-amino-5-phosphovalerate, 3-[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonate or dibenzocycloheptaneimine, N-methyl-D-aspartate receptor inhibitors, abolished enhancement by tetanus, whereas, 6,7-dinitroquinoxaline-2,3-dione, an antagonist of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid ionotropic receptors, or L-2-amino-4-phosphonobutyrate, an antagonist of glutamate metabotropic receptors, showed no effect. Moreover, pretreatment with glutamate or N-methyl-D-aspartate in the absence of Mg2+ also facilitated dopamine release evoked by KCl concentrations. When extracellular Ca2+ was removed from the medium during pretreatment, potentiation by glutamate disappeared. We conclude that activation of N-methyl-D-aspartate receptors on dopaminergic nerve terminals in the striatum produces the long-term changes in efficacy of the response to KCl or glutamatergic agents. That is, plastical phenomena could exist at presynaptic levels between glutamatergic neurons and dopaminergic neurons in striatum.

Quantitative investigations into the histostructural nature of the human putamen. I. Staining, cell classification and morphometry

Combined staining with aldehyde and cresyl violet allows a reliable morphological distinction to be made between seven different types of neurons in the human putamen. We examined the age distribution of nearly 42,000 neurons in 27 normal putamina, using a semiautomatic morphometric procedure on defined tissue blocks. For morphometric evaluation and stereological calculations a section thickness of 20 microns is recommended. We modified routine aldehyde fuchsin cresyl violet combination staining for nervous tissue, since Braak's original method (Braak 1978, 1980) was developed for thick sections. The results show that neuronal density varies with age for the different types of neurons.

Glutamate/dopamine D1/D2 balance in the basal ganglia and its relevance to Parkinson's disease

The recent availability of selective ligands for NMDA and AMPA receptors has enabled neuroscientists to test the hypothesis that Parkinson's disease is a glutamate hyperactivity disorder and hence treatable with glutamate antagonists. This review takes a critical look at the motor characteristics of this new class of drugs in rodent and primate models of parkinsonism and assesses the clinical potential and pitfalls of this radical new approach. Monotherapy of Parkinson's disease with glutamate antagonists appears impractical at the present time, due to their low efficacy and unacceptable side effects, but polypharmacy with L-DOPA and a glutamate antagonist as adjuvant is a more realistic prospect. This review will focus on the ways in which glutamate receptor blockade facilitates motor recovery with L-DOPA and will examine whether the basis for this beneficial effect can be traced to a specific interaction with dopamine at D1 or D2 receptors, and therefore to discrete motor pathways within the basal ganglia.

Role of the striatal cholinergic system in the regulation of learned manipulation in rats

The experiments were performed on adult Wistar male rats trained to push with the forepaw on a fixed piston inside a narrow tube. It was found that after localized intracerebral injection of a cholinergic antagonist into the dorso-lateral (but not medial) neostriatum (i.e., the caudato-putamen) the behavioral performance requiring brief innate movements remained unchanged, but the performance requiring a prolonged pushing movement (> 50 msec) became disrupted. Micoinjection of carbacholine (0.03-3 mu g/l microliters) did not affect the performance of the acquired movements, whereas scopolamine (3 mu g/l microliters) led to the significant decrease in pushing time. We conclude that changes in the state of the dorso-lateral neostriatal cholinergic system result only in disturbances of the sensory-controlled component of a complex instrumental movement.

Contribution of L-type Ca2+ channels to long-term enhancement of high K(+)-evoked release of dopamine from rat striatal slices

Pre-treatment with BAY K 8644, an L-type voltage-gated Ca2+ channels agonist, produced long-term enhancement (LTE) of over 2 h of high K(+)-evoked dopamine release from rat striatal slices. Exposure to BAY K 8644 under Ca(2+)-free conditions did not enhance this release. Thus, a transient activation of L-type voltage-gated Ca2+ channels followed by Ca2+ entry can trigger LTE of the evoked DA release from striatal tissues. This type of LTE in the striatum underlines the importance of presynaptic mechanisms, including L-type voltage-gated Ca2+ channels of 'long-term potentiation' expression.

Characteristics of the influences of the centrum medianum of the thalamus on the neuronal activity of the caudate nucleus

The specific character of the relationship of the baseline cellular activity of the caudate nucleus to the features of the impulse activity of the centrum medianum of the thalamus has been identified in chronic microelectrode experiments. Reciprocal changes in the bioelectrical activity were recorded in the first group of cells, with a low action potential repetition frequency, while unidirectional changes with reorganizations in the intralaminar nucleus were recorded in the second. The activation of the cells of the centrum medianum of the thalamus was accompanied during the formation of an instrumental conditioned defense reflex by the formation of a pattern of impulse activity in neurons of the neostriatum which coincided in sign with the reorganizations which were characteristic during the investigation of the baseline activity. It is hypothesized that the features of the internal structure of the caudate nucleus underlie such a relationship.

The dopaminergic, glutamatergic, GABAergic bases for the action of amphetamine and cocaine

The present experiments were designed to evaluate pharmacologically the role of three neuroanatomically related systems--dopamine, glutamate and GABA--in the motor-stimulant response to amphetamine and cocaine. The data indicate that stimulant-induced stereotypy is blocked by antagonists of all three systems and that agonists of all three systems administered into the striatum induce stereotypy. Furthermore, the interaction among them occurs in the striatum; and the reaction sequence, as determined by the effect of the relatively selective antagonists on agonist-induced stereotypy, appears to be a dopaminergic activation of a glutamatergic system which in turn activates a GABAergic system. Because the GABAergic system represents the major efferents from the striatum, the evidence suggests that the motor-stimulatory effects of amphetamine and cocaine result from a disinhibition of inhibitory systems in the thalamus, resulting in facilitation of excitation in the cortex.

Impaired prepulse inhibition of acoustic and tactile startle response in patients with Huntington's disease

The corpus striatum serves a critical function in inhibiting involuntary, intrusive movements. Striatal degeneration in Huntington's disease results in a loss of motor inhibition, manifested by abnormal involuntary choreiform movements. Sensorimotor inhibition, or "gating", can be measured in humans using the startle reflex: the startle reflex is normally inhibited when the startling stimulus is preceded 30-500 ms earlier by a weak prepulse. In the present study, prepulse inhibition (PPI) was measured in patients with Huntington's disease to quantify and characterise sensorimotor gating. Compared with age matched controls, patients with Huntington's disease exhibit less PPI. Startle gating deficits are evident in patients with Huntington's disease when startle is elicited by either acoustic or tactile stimuli. Even with stimuli that elicit maximal PPI in normal subjects, patients with Huntington's disease exhibit little or no PPI, and their pattern of startle gating does not show the normal modulatory effects usually elicited by changing the prepulse interval or intensity. Startle amplitude and habituation and latency facilitation are largely intact in these patients, although reflex latency is significantly slowed. In patients with Huntington's disease, startle reflex slowing correlates with cognitive impairment measured by the dementia rating scale, and with the performance disruptive effects of interference measured by the Stroop test. These findings document a profound disruption of sensorimotor gating in patients with Huntington's disease and are consistent with preclinical findings that identify the striatum and striatopallidal GABAergic efferent circuitry as critical substrates for sensorimotor gating of the startle reflex.

Reflection of internal and external determinants of behavior in the neuronal activity of the neostriatum of monkeys

It has been demonstrated that some neurons of the neostriatum of monkeys (Macaca nemestrina) participate in supporting the reaction of the animal to the appearance of standard situations in which a decision is made on the basis of already existing skills. Other neurons (primarily non-baseline active), by contrast, are activated in nonstandard situations, when the use of existing skills is ineffective. It is hypothesized that their activity is associated with the shift of the animal to a different activity which consists in the rejection of the use of existing skills for the formation of new skills.

Neuropharmacology of the nucleus accumbens: systemic morphine effects on single-unit responses evoked by ventral pallidum stimulation

Extracellular recordings of neurons in the nucleus accumbens septi of anesthetized rats have previously shown systemic opiates to have mixed effects on rates of unit activity. However, these effects become more predictable as nucleus accumbens septi neurons are functionally categorized by their responses to afferent stimulation. In the present study, the effects of systemic opiates on individual nucleus accumbens septi neurons categorized by their response patterns to ventral pallidum stimulation were examined. Across all nucleus accumbens septi units tested, these experiments showed that morphine either inhibited (42%, n = 91), excited (15%, n = 32) or had no effect (43%, n = 93) on these unit responses. Further experiments were conducted in which nucleus accumbens septi neurons were categorized on the basis of their response patterns to concurrent fimbria and ventral pallidum stimulation. In these studies, if the neuron was orthodromically evoked to respond by both fimbria stimulation and ventral pallidum stimulation, the neurons' responses to ventral pallidum stimulation (but not their fimbria-evoked responses) were consistently inhibited by morphine. In contrast, nucleus accumbens septi unit responses that were orthodromically activated by ventral pallidum stimulation but unaffected by fimbria stimulation were consistently unaffected by morphine. The major observation of this work is that the effect of systemically administered morphine on individual nucleus accumbens septi neurons can be predicted by that neuron's evoked responses to stimulation of different efferent and afferent circuits. Since these studies involve systemic administration of morphine, the results do not elucidate site of action.(ABSTRACT TRUNCATED AT 250 WORDS)

Compartmental organization of the peptide network in the human caudate nucleus

The mammalian striatum may be divided into a striosomal compartment and a surrounding matrix region. We have examined the distribution of leucine enkephalin (LENK) and substance P (SP) immunoreactivity in relation to striosomes defined by calbindin-D (CABD) staining in alternate 70 microns serial sections from the human caudate nucleus. The distribution of LENK immunoreactivity showed a transition from dorsal to ventral striatum: dorsally, LENK-rich patches were present in a lightly stained matrix; mid-ventrally, annular patches of LENK staining with a lighter core were seen. These patches corresponded to striosomal regions defined by CABD-poor zones. In contrast, in the ventral caudate and nucleus accumbens, LENK-poor zones matched CABD-defined striosomes. CABD staining in the matrix was intense in the dorsal caudate, diminishing ventrally. SP-rich zones in dorsal caudate and SP-poor areas in the mid-ventral region overlapped striosomes. In the ventromedial sector, the SP staining pattern was complex and did not consistently correlate with striosomes. Computer-assisted three-dimensional reconstruction of the striosomal system in the human, based on regions of either high LENK or low CABD immunoreactivity, revealed the existence of considerable long-range order. Patches appeared aligned over several millimeters to form long, horizontal structures in the caudate nucleus, with occasional orthogonal interconnecting crossbridges. Our results are in accord with previous work in the human and in other species. These three-dimensional networks are strikingly similar across individuals and may relate to the segregation of and interactions between striatal circuits.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional interactions between glutamate and dopamine in the rat striatum

The functional role of NMDA receptors in a spontaneous (locomotion) and a conditioned behaviour (reaction-time task) known to preferentially involve dopamine transmission in the ventral or the dorsal part of the striatum, respectively, was studied in the rat. The non-competitive NMDA receptor antagonist MK-801 systemically injected produced a dose-dependent increase in locomotor activity and impaired the performance of the animals trained to release a lever after a visual stimulus within a time limit by increasing the number of anticipatory errors (lever releases occurring before the stimulus onset). Similar behavioural changes were obtained after bilateral striatal microinjections of the competitive NMDA-antagonist APV into the ventral or dorsal striatum, respectively, suggesting that MK-801-induced behavioral effects after systemic injection might be mediated through a blockade of EAA transmission within the striatum. Dopamine injected in the same striatal locations induced effects similar to APV on locomotion and reaction-time performance, in agreement with the proposal for a functional antagonism between the glutamatergic and the dopaminergic transmission at striatal level. The conjoint administration of APV and dopamine directly into the striatum did not alter the behavioural effect induced by each compound injected alone showing that these effects are not additive. This latter observation actually suggests the occurrence of a functional interaction between the two neuronal systems probably acting on a common striatal target relaying dopaminergic and glutamatergic antagonistic influences on locomotion and conditioned motor behaviours.

Neostriatal modulation of motor cortex excitability

The influence of the basal ganglia motor loop on motor cortex function was examined by pharmacologically altering neostriatal activity while monitoring the electrical stimulation thresholds for eliciting movements of the ipsilateral and contralateral motor cortex in ketamine anesthetized rats. Repeated unilateral intraneostriatal infusions (1-3) of the glutamate agonist, kainic acid (0.1 microliter, 75 ng), or glutamate (0.3 microliter, 1.65 micrograms) reliably increased ipsilateral but not contralateral cortical thresholds. Single infusions of kainic acid (0.3 microliter, 150 or 225 ng) elevated ipsilateral cortical thresholds for 30-45 min; with glutamate (0.3 microliter, 1.65 micrograms), the change lasted less than 10 min. Antidromically identified striatonigral projection neurons (n = 8) located approximately 500 microM from the infusion cannula, showed either increased firing (n = 4) for less than 10 min following glutamate infusion or no change from their non-firing state (n = 4). Non-antidromically activated neurons (n = 3) were all excited by the infusion, although an interval of inhibition preceded or followed the excitation in two cases. Infusions (0.3 microliter) of inhibitory agents (GABA, 31 and 310 ng; muscimol 34.2 ng; and DNQX 34.2 ng) did not alter cortical threshold, nor did saline vehicle. Lesion of the ventrolateral but not ventromedial thalamic nucleus prevented the modulation of cortical thresholds following intraneostriatal infusion of 225 ng kainic acid. Thus the neostriatal alteration of cortical thresholds indicates a modulation of cortical excitability via thalamic projections and not the outcome of competing descending cortical and neonstriatal influences converging on motorneurons. These results suggest that tonic feedforward modulation of the motor cortex and the pyramidal tract by the basal ganglia can be inhibitory.

Influence of glutamate on the content and metabolism of dopamine in the nigrostriatal system of rats distinguished by capacity for learning

The effect of repeated (over the course of nine days) intrastriatal microinjections of glutamate (5 or 0.5 micrograms in 0.75 microliters of physiological solution daily) were investigated in 42 male Sprague-Dawley rats. Twenty-nine rats were preliminarily trained in a Skinner box using food reinforcement. It was demonstrated that the administration of glutamate to rats not subjected to training increases the content of homovanillic acid in the striatum. A similar influence in rats that are capable of learning leads to an increase in the content of dopamine and a decrease in the level of homovanillic acid in this nucleus, while it does not induce changes in the biochemical indicators under investigation in those rats that are incapable of learning. Microinjections of glutamate also do not alter the capacity for learning in any of the groups of animals. The possible causes for the different influence of intrastriatal microinjections of glutamate on the activity of the nigrostriatal system of rats differing by capacity for learning are discussed.

5-hydroxydopamine-labeled dopaminergic axons: three-dimensional reconstructions of axons, synapses and postsynaptic targets in rat neostriatum

Previous studies employing 5-hydroxydopamine to identify nigrostriatal dopaminergic axons and their synapses found that labeled axons made few synapses or that asymmetric contacts predominated. In contrast, recent studies using tyrosine hydroxylase or dopamine antibody techniques indicate that presumed dopaminergic axons form small symmetric contacts. We re-examined 5-hydroxydopamine-labeled material from the rat neostriatum using serial three-dimensional reconstruction techniques to characterize the morphology of labeled axons, synapses and postsynaptic targets. This ultrastructural analysis revealed a class of heavily labeled axons that are small (0.06-1.5 microns in diameter) and lack large varicosities. These axons form small (0.011-0.09 microns 2), en passant, symmetric synapses, mainly onto dendritic spines and spiny dendritic shafts and, in some cases, onto aspiny dendritic segments near branch points. The sites of these synapses along the axon appeared unrelated to the locations of axonal enlargements, suggesting that counting varicosities may not be an accurate indication of the extent of dopaminergic innervation in the neostriatum. The characteristics of these 5-hydroxydopamine-labeled elements correspond in all respects to axons and synapses identified as dopaminergic by immunohistochemistry in previous studies. In tissue in which all labeled and unlabeled synapses were classified, approximately 9% of all synapses were identified as dopaminergic by this type of label. Three-dimensional reconstructions provided additional insight concerning the interaction of dopaminergic afferents with postsynaptic striatal targets and their relation to other afferents to these neurons. They reveal that a short, unbranched dopaminergic axonal segment can make multiple synapses onto dendritic spines, shafts and branch points of one or more dendrites. In addition, one dendrite can receive contacts from several labeled axons. Dopamine synapses onto spines are always associated with unlabeled, asymmetric synapses onto the same spine. Synapses of various morphologies with a distinctly different, lighter form of labeling were much rarer, and may represent other aminergic afferents to the neostriatum. The presence of this second form of label in earlier 5-hydroxydopamine studies may have contributed to the long-standing controversy over the appearance of dopaminergic synapses examined by different techniques. Our results help to resolve this controversy and confirm that the nigrostriatal projection makes small symmetric synapses with a variety of striatal targets.

Topographical organization of the sources of discrete cortical projections within the striatum as determined by a retrograde fluorescence tracing technique in the cat

The projections from the neostriatum and the paleostriatum to the cerebral cortex in the cat were examined by means of retrogradely transported fluorescent tracers primuline, Fast Blue, Nuclear Yellow and Evans Blue injected into different neocortical fields. In all cases after dye injections only large labelled cells of sources of striatocortical ipsilateral projections were observed. The main projections from the caudate nucleus and the putamen are directed to the auditory and neighbouring "associative" cortex, and more numerous projections from the globus pallidus are addressed to the motor cortex. No sources of cortical projections within the entopeduncular nucleus were found. Simultaneous injections of Fast Blue and primuline into even closely located and tightly bound functional regions of parietal or temporal cortex failed to reveal double-labelled neurons in the caudate nucleus, internal capsule, putamen and globus pallidus. Thus, our findings on cats are consistent with recent studies on rats and monkeys that suggest that striatal neurons innervate relatively small, restricted fields of the neocortex. Again, the results show evidence for a significant contribution to cholinergic cortical innervation not only of magnocellular neurons of the basal forebrain but also of large neo- and paleostriatal cells.