Nigral and striatal dopamine release under sensory stimuli

Experience-dependent regulation of dopaminergic signaling in the somatosensory cortex

Dopamine critically influences reward processing, sensory perception, and motor control. Yet, the modulation of dopaminergic signaling by sensory experiences is not fully delineated. Here, by manipulating sensory experience using bilateral single-row whisker deprivation, we demonstrated that gene transcription in the dopaminergic signaling pathway (DSP) undergoes experience-dependent plasticity in both granular and supragranular layers of the primary somatosensory (barrel) cortex (S1). Sensory experience and deprivation compete for the regulation of DSP transcription across neighboring cortical columns, and sensory deprivation-induced changes in DSP are topographically constrained. These changes in DSP extend beyond cortical map plasticity and influence neuronal information processing. Pharmacological regulation of D2 receptors, a key component of DSP, revealed that D2 receptor activation suppresses excitatory neuronal excitability, hyperpolarizes the action potential threshold, and reduces the instantaneous firing rate. These findings suggest that the dopaminergic drive originating from midbrain dopaminergic neurons, targeting the sensory cortex, is subject to experience-dependent regulation and might create a regulatory feedback loop for modulating sensory processing. Finally, using topological gene network analysis and mutual information, we identify the molecular hubs of experience-dependent plasticity of DSP. These findings provide new insights into the mechanisms by which sensory experience shapes dopaminergic signaling in the brain and might help unravel the sensory deficits observed after dopamine depletion.

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

Rhythmic oscillations in the midbrain dopaminergic nuclei in mice

Introduction

Dopamine release in the forebrain by midbrain ventral tegmental nucleus (VTA) and substantia nigra pars compacta (SNc) neurons is implicated in reward processing, goal-directed learning, and decision-making. Rhythmic oscillations of neural excitability underlie coordination of network processing, and have been reported in these dopaminergic nuclei at several frequency bands. This paper provides a comparative characterization of several frequencies of oscillations of local field potential and single unit activity, highlighting some behavioral correlates.

Methods

We recorded from optogenetically identified dopaminergic sites in four mice training in operant olfactory and visual discrimination tasks.

Results

Rayleigh and Pairwise Phase Consistency (PPC) analyses revealed some VTA/SNc neurons phase-locked to each frequency range, with fast spiking interneurons (FSIs) prevalent at 1-2.5 Hz (slow) and 4 Hz bands, and dopaminergic neurons predominant in the theta band. More FSIs than dopaminergic neurons were phase-locked in the slow and 4 Hz bands during many task events. The highest incidence of phase-locking in neurons was in the slow and 4 Hz bands, and occurred during the delay between the operant choice and trial outcome (reward or punishment) signals.

Discussion

These data provide a basis for further examination of rhythmic coordination of activity of dopaminergic nuclei with other brain structures, and its impact for adaptive behavior.

Chronotherapies for Parkinson's disease

Parkinson's disease (PD) is the second-most common progressive neurodegenerative disorder. Although the clinical diagnosis of PD is still based on its cardinal motor dysfunctions, several non-motor symptoms (NMS) have been established as integral part of the disease. Unlike motor disorders, development of therapies against NMS are still challenging and remain a critical unmet clinical need. During the last decade, several studies have characterised the molecular, physiological and behavioural alterations of the circadian system in PD patients. As a consequence, and given the ubiquitous nature of circadian rhythms in the entire organism, the biological clock has emerged as a potential therapeutic target to ease suffering from both motor and NMS in PD patients. Here we discuss the emerging field of using bright light, physical exercise and melatonin as chronotherapeutic tools to alleviate motor disorders, sleep/wake alterations, anxiety and depression in PD patients. We also highlight the potential of these readily available therapies to improve the general quality of life and wellbeing of PD patients. Finally, we provide specific data- and mechanisms-driven recommendations that might help improve the therapeutic benefit of light and physical exercise in PD patients.

Spinal pathways involved in somatosensory inhibition of the psychomotor actions of cocaine

Previous studies have demonstrated that somatosensory stimuli influence dopamine transmission in the mesolimbic reward system and can reduce drug-induced motor behaviors, craving and dependence. Until now, the central links between somatosensory and brain reward systems are not known. Here, we show that the dorsal column (DC) somatosensory pathway contains projections that convey an inhibitory input from the periphery to mesolimbic reward circuits. Stimulation of the ulnar nerve under HT7 acupoint suppressed psychomotor response to cocaine, which was abolished by disruption of the DC pathway, but not the spinothalamic tract (STT). Low-threshold or wide-dynamic range neurons in the cuneate nucleus (CN) were excited by peripheral stimulation. Lesions of dorsal column or lateral habenula (LHb) prevented the inhibitory effects of peripheral stimulation on cocaine-induced neuronal activation in the nucleus accumbens (NAc). LHb neurons projecting to the ventral tegmental area (VTA)/rostromedial tegmental nucleus (RMTg) regions were activated by peripheral stimulation and LHb lesions reversed the inhibitory effects on cocaine locomotion produced by peripheral stimulation. These findings suggest that there exists a pathway in spinal cord that ascends from periphery to mesolimbic reward circuits (spino-mesolimbic pathway) and the activation of somatosensory input transmitted via the DC pathway can inhibit the psychomotor response to cocaine.

Hitchhiker's Guide to Voltammetry: Acute and Chronic Electrodes for in Vivo Fast-Scan Cyclic Voltammetry

Fast-scan cyclic voltammetry (FSCV) has been used for over 20 years to study rapid neurotransmission in awake and behaving animals. These experiments were first carried out with carbon-fiber microelectrodes (CFMs) encased in borosilicate glass, which can be inserted into the brain through micromanipulators and guide cannulas. More recently, chronically implantable CFMs constructed with small diameter fused-silica have been introduced. These electrodes can be affixed in the brain with minimal tissue response, which permits longitudinal measurements of neurotransmission in single recording locations during behavior. Both electrode designs have been used to make novel discoveries in the fields of neurobiology, behavioral neuroscience, and psychopharmacology. The purpose of this Review is to address important considerations for the use of FSCV to study neurotransmitters in awake and behaving animals, with a focus on measurements of striatal dopamine. Common issues concerning experimental design, data collection, and calibration are addressed. When necessary, differences between the two methodologies (acute vs chronic recordings) are discussed. The topics raised in this Review are particularly important as the field moves beyond dopamine toward new neurochemicals and brain regions.

Dopamine D1 Receptor Immunoreactivity on Fine Processes of GFAP-Positive Astrocytes in the Substantia Nigra Pars Reticulata of Adult Mouse

Substantia nigra pars reticulata (SNr), the major output nucleus of the basal ganglia, receives dopamine from dendrites extending from dopaminergic neurons of the adjacent nucleus pars compacta (SNc), which is known for its selective degeneration in Parkinson's disease. As a recipient for dendritically released dopamine, the dopamine D1 receptor (D1R) is a primary candidate due to its very dense immunoreactivity in the SNr. However, the precise location of D1R remains unclear at the cellular level in the SNr except for that reported on axons/axon terminals of presumably striatal GABAergic neurons. To address this, we used D1R promotor-controlled, mVenus-expressing transgenic mice. When cells were acutely dissociated from SNr of mouse brain, prominent mVenus fluorescence was detected in fine processes of glia-like cells, but no such fluorescence was detected from neurons in the same preparation, except for the synaptic bouton-like structure on the neurons. Double immunolabeling of SNr cells dissociated from adult wild-type mice brain further revealed marked D1R immunoreactivity in the processes of glial fibrillary acidic protein (GFAP)-positive astrocytes. Such D1R imunoreactivity was significantly stronger in the SNr astrocytes than that in those of the visual cortex in the same preparation. Interestingly, GFAP-positive astrocytes dissociated from the striatum demonstrated D1R immunoreactivity, either remarkable or minimal, similarly to that shown in neurons in this nucleus. In contrast, in the SNr and visual cortex, only weak D1R immunoreactivity was detected in the neurons tested. These results suggest that the SNr astrocyte may be a candidate recipient for dendritically released dopamine. Further study is required to fully elucidate the physiological roles of divergent dopamine receptor immunoreactivity profiles in GFAP-positive astrocytes.

Nonsulfated cholecystokinins in cerebral neurons

Cholecystokinin (CCK) is a widely expressed neuropeptide system originally discovered in the gut. Both cerebral and peripheral neurons as well as endocrine I-cells in the small intestine process proCCK to tyrosyl-O-sulfated and α-carboxyamidated peptides. Recently, we reported that gut endocrine I-cells also synthetize nonsulfated CCK in significant amounts. Accordingly, we have now examined whether porcine and rat cerebral tissues (four cortical regions, hypothalamus and cerebellum) also synthesize nonsulfated CCK. A new, specific radioimmunoassay showed that all brain samples from pigs (n=15) and rats (n=6) contained nonsulfated CCK. The highest concentrations were measured in the neocortex; 4.7±0.25pmol/g (7.4%) in the rat and 4.3±1.88pmol/g (2.3%) in the pig. Chromatography of porcine cortical extracts revealed that 96.4% of the CCK was O-sulfated CCK-8. A higher fraction of the larger peptides (CCK-58 and CCK-33) was nonsulfated in comparison with the shorter forms (CCK-22 and CCK-8), i.e., 8.1% and 4.3% versus 0.9% and 1.5%. Immunohistochemical analysis of the rat brain showed an overall similar distribution pattern in selected regions when comparing the antibody specific for nonsulfated CCK-8 with an antibody recognizing both sulfated and nonsulfated CCK. However, nonsulfated CCK immunoreactivity was stronger than that of sulfated CCK in cell bodies and weaker in nerve terminals. We conclude that only a small fraction of neuronal CCK is nonsulfated. The intracellular distribution of nonsulfated CCK in neurons suggests that they contribute only modestly to the CCK transmitter activity.

Alteration of daily and circadian rhythms following dopamine depletion in MPTP treated non-human primates

Disturbances of the daily sleep/wake cycle are common non-motor symptoms of Parkinson's disease (PD). However, the impact of dopamine (DA) depletion on circadian rhythms in PD patients or non-human primate (NHP) models of the disorder have not been investigated. We evaluated alterations of circadian rhythms in NHP following MPTP lesion of the dopaminergic nigro-striatal system. DA degeneration was assessed by in vivo PET ([(11)C]-PE2I) and post-mortem TH and DAT quantification. In a light∶dark cycle, control and MPTP-treated NHP both exhibit rest-wake locomotor rhythms, although DA-depleted NHP show reduced amplitude, decreased stability and increased fragmentation. In all animals, 6-sulphatoxymelatonin peaks at night and cortisol in early morning. When the circadian system is challenged by exposure to constant light, controls retain locomotor rest-wake and hormonal rhythms that free-run with stable phase relationships whereas in the DA-depleted NHP, locomotor rhythms are severely disturbed or completely abolished. The amplitude and phase relations of hormonal rhythms nevertheless remain unaltered. Use of a light-dark masking paradigm shows that expression of daily rest-wake activity in MPTP monkeys requires the stimulatory and inhibitory effects of light and darkness. These results suggest that following DA lesion, the central clock in the SCN remains intact but, in the absence of environmental timing cues, is unable to drive downstream rhythmic processes of striatal clock gene and dopaminergic functions that control locomotor output. These findings suggest that the circadian component of the sleep-wake disturbances in PD is more profoundly affected than previously assumed.

Randomized, proof-of-principle clinical trial of active transcranial magnetic stimulation in chronic migraine

BACKGROUND

High-frequency repetitive transcranial magnetic stimulation of the left dorsolateral prefrontal cortex (rTMS-DLPFC) is an effective treatment for depression. Preliminary studies indicated beneficial effects of rTMS-DLPFC on pain relief in patients treated for depression, and in patients with chronic migraine.

METHODS

In this randomized, double-blind, parallel-group, single-center, proof-of-principle clinical trial, we tested the hypothesis that 23 sessions of active rTMS-DLPFC delivered over eight weeks would be feasible, safe and superior to sham rTMS to decrease the number of headache days in 18 patients with chronic migraine without severe depression. Per-protocol analysis was performed.

RESULTS

rTMS-DLPFC applied over eight weeks was feasible and safe in patients with chronic migraine. Contrary to our primary hypothesis, the number of headache days decreased significantly more in the sham group than in the group treated with active rTMS-DLPFC at eight weeks. Average decrease in headache days was >50% in the sham group, indicating a powerful placebo response. Pain intensity improved in both groups to a similar extent.

CONCLUSIONS

Positive results of M1 stimulation in other studies, and the absence of significant benefits of active high-frequency rTMS of the DLPFC in the present study, point to M1 as a more promising target than the DLPFC, for larger trials of noninvasive brain stimulation in patients with chronic migraine.

Peripheral stimulation in treating Parkinson's disease: Is it a realistic idea or a romantic whimsicality?

Parkinson's disease (PD) is a common, however, intractable neurodegenerative disorder in the aging population. Levodopa (l-dopa) administration is regarded as the most effective strategy in treating PD with prominent motor side-effects after undergoing long-term treatment. Surgical therapies such as deep brain stimulation (DBS) show certain efficacy, yet there are several limitations in adopting such surgical procedures. Therefore, performing electrical stimulation out of the brain, namely peripheral stimulation for PD has been a dream of many clinicians. Recently, the efficacy of dorsal column stimulation was verified in animal PD models; on the other hand, tons of acupunctural studies from East Asia claim good efficacy in treating PD both in bench and clinical studies. This review will introduce the progress of peripheral stimulation for PD, and will discuss the potential mechanisms involved in these strategies.

Role of the thalamus in the bilateral regulation of dopaminergic and GABAergic neurons in the basal ganglia

In halothane-anaesthetized cats implanted with several push-pull cannulae the release of dopamine from nerve terminals and dendrites of the two nigrostriatal dopaminergic pathways is shown to be asymmetrically or symmetrically regulated bilaterally when dopaminergic or GABAergic drugs are infused into one side of the nigra. Nigrothalamic GABAergic neurons may intervene in the asymmetrical regulation, as sagittal section of the thalamic massa intermedia blocks the contralateral effects induced by dopaminergic drugs. The role of thalamic nuclei in the bilateral regulation of dopamine and GABA release in the caudate nucleus and substantia nigra is further demonstrated by studies showing that (1) electrical stimulation of thalamic motor nuclei induces bilateral asymmetrical changes in dopamine release resembling the changes evoked by unilateral sensory stimuli or stimulation of cerebellar nuclei; (2) electrical stimulation of intralaminar or some midline thalamic nuclei leads to bilateral (or contralateral) symmetrical changes in dopamine release, some of these effects being comparable to those induced by unilateral stimulation of the motor cortex; (3) unilateral lesions of motor or intralaminar thalamic nuclei reverse the changes in GABA release in the contralateral caudate nucleus or substantia nigra induced by unilateral infusion of muscimol into the nigra; and (4) unilateral infusion of GABA into thalamic motor nuclei induces bilateral symmetrical regulation of dopamine release in caudate nuclei by means of presynaptic facilitatory influences.

Inhibition of stimulated dopamine release and hemodynamic response in the brain through electrical stimulation of rat forepaw

The subcortical response to peripheral somatosensory stimulation is not well studied. Prior literature suggests that somatosensory stimulation can affect dopaminergic tone. We studied the effects of electrical stimulation near the median nerve on the response to an amphetamine-induced increase in synaptic dopamine. We applied the electrical stimulation close to the median nerve 20 min after administration of 3mg/kg amphetamine. We used fMRI and microdialysis to measure markers of dopamine (DA) release, together with the release of associated neurotransmitters of striatal glutamate (Glu) and gamma-aminobutyric acid (GABA). Changes in cerebral blood volume (CBV), a marker used in fMRI, indicate that electrical stimulation significantly attenuated increased DA release (due to AMPH) in the striatum, thalamus, medial prefrontal and cingulate cortices. Microdialysis showed that electrical stimulation increased Glu and GABA release and attenuated the AMPH-enhanced DA release. The striatal DA dynamics correlated with the CBV response. These results demonstrate that electrical stimulation near the median nerve activates Glu/GABA release, which subsequently attenuate excess striatal DA release. These data provide evidence for physiologic modulation caused by electroacupuncture at points near the median nerve.

Robust syntaxin-4 immunoreactivity in mammalian horizontal cell processes

Horizontal cells mediate inhibitory feed-forward and feedback communication in the outer retina; however, mechanisms that underlie transmitter release from mammalian horizontal cells are poorly understood. Toward determining whether the molecular machinery for exocytosis is present in horizontal cells, we investigated the localization of syntaxin-4, a SNARE protein involved in targeting vesicles to the plasma membrane, in mouse, rat, and rabbit retinae using immunocytochemistry. We report robust expression of syntaxin-4 in the outer plexiform layer of all three species. Syntaxin-4 occurred in processes and tips of horizontal cells, with regularly spaced, thicker sandwich-like structures along the processes. Double labeling with syntaxin-4 and calbindin antibodies, a horizontal cell marker, demonstrated syntaxin-4 localization to horizontal cell processes; whereas, double labeling with PKC antibodies, a rod bipolar cell (RBC) marker, showed a lack of co-localization, with syntaxin-4 immunolabeling occurring just distal to RBC dendritic tips. Syntaxin-4 immunolabeling occurred within VGLUT-1-immunoreactive photoreceptor terminals and underneath synaptic ribbons, labeled by CtBP2/RIBEYE antibodies, consistent with localization in invaginating horizontal cell tips at photoreceptor triad synapses. Vertical sections of retina immunostained for syntaxin-4 and peanut agglutinin (PNA) established that the prominent patches of syntaxin-4 immunoreactivity were adjacent to the base of cone pedicles. Horizontal sections through the OPL indicate a one-to-one co-localization of syntaxin-4 densities at likely all cone pedicles, with syntaxin-4 immunoreactivity interdigitating with PNA labeling. Pre-embedding immuno-electron microscopy confirmed the subcellular localization of syntaxin-4 labeling to lateral elements at both rod and cone triad synapses. Finally, co-localization with SNAP-25, a possible binding partner of syntaxin-4, indicated co-expression of these SNARE proteins in the same subcellular compartment of the horizontal cell. Taken together, the strong expression of these two SNARE proteins in the processes and endings of horizontal cells at rod and cone terminals suggests that horizontal cell axons and dendrites are likely sites of exocytotic activity.

Therapeutic application of transcranial magnetic stimulation in Parkinson's disease: the contribution of expectation

Repetitive transcranial magnetic stimulation (rTMS) is a valuable probe of brain function. Ever since its adoption as a research tool, there has been great interest regarding its potential clinical role. Presently, it is unclear whether rTMS will have some role as an alternative treatment for neuropsychiatric and neurological disorders such as Parkinson's disease (PD). To date, studies addressing the contribution of placebo during rTMS are missing. The placebo effect has been shown to be associated either with release of dopamine in the striatum or with changes in brain glucose metabolism. The main objective of this study was to test whether, in patients with PD, the expectation of therapeutic benefit from rTMS, which actually was delivered only as sham rTMS (placebo-rTMS) induced changes in striatal [11C] raclopride binding potentials (BP) as measured with positron emission tomography (PET). Placebo-rTMS induced a significant bilateral reduction in [11C] raclopride BP in dorsal and ventral striatum as compared to the baseline condition. This reduction BP is indicative of an increase in dopamine neurotransmission. The changes in [11C] raclopride binding were more evident in the hemisphere contralateral to the more affected side supporting the hypothesis that the more severe the symptoms, the greater the drive for symptom relief, and therefore the placebo response. This is the first study addressing the placebo contribution during rTMS. While our results seem to confirm earlier evidence that expectation induces dopaminergic placebo effects, they also suggest the importance of placebo-controlled studies for future clinical trials involving brain stimulation techniques.

Striatal dopamine during sensorial stimulations: a [18F]FDOPA PET study in human and cats

Sensory stimulations of the forelimb in cats are known to increase dopamine release in the ipsilateral striatum and to decrease it in the homologous contralateral structure. Using positron emission tomography in both humans and cats, the present study shows that such sensory stimulations greatly reduce [(18)F]FDOPA accumulation ipsilateral to the stimulation (by 40.4% and 26.4% in the human caudate and putamen, respectively, and by 33.3% in the cat striatum). This decrease in striatal [(18)F]FDOPA uptake suggests a reduced DA storage resulting from the increased amine release. No change was observed in the contralateral striatum in neither human or cat suggesting, in contrast, that [(18)F]FDOPA accumulation is not facilitated by decreased DA release. These results support the hypothesis that sensory stimulations activate a non-synaptic mode of dopamine release.

Sensory stimulation accelerates dopamine release in the basal ganglia

We report herein the modulation of dopamine release in the basal ganglia during peripheral electrical stimulation in animals. The endogenous dopamine release during electrical stimulation was measured in anesthetized cats by positron emission tomography (PET) using the D2 receptor agonist [11C]-raclopride. Binding potential (BP) parametric maps were calculated using a simplified reference region model. The regional dopamine release evoked by electrical stimulation was estimated both by region of interest (ROI) analysis and statistical parametric mapping (SPM 99). Both ROI analysis and statistical parametric mapping showed significant release of endogenous dopamine in the nucleus accumbens and the striatum contralateral to the stimulated side as compared to the resting condition as well as the ipsilateral side. Accordingly, we suggest that the activity of the dopaminergic neurons in the midbrain projecting to the nucleus accumbens and the striatum is modulated by the input from the afferent nerves. This provides an in vivo evidence for the importance of the basal ganglia in the processing of peripheral information required for normal movement. This may also explain the clinically observed sensory system abnormalities in patients with movement disorders.

Effect of sensory stimulus on striatal dopamine release in humans and cats: a [11C]raclopride PET study

BACKGROUND

Sensory stimulation of the forelimb extremities constitutes a well-established experimental model that has consistently shown to activate dopamine (DA) neurotransmission in the mammals' forebrain.

OBJECTIVES

To visualize in vivo this modification of striatal DA release in healthy human volunteers using Positron Emission Tomography (PET) and [(11)C]raclopride. Experiments in humans were paralleled by experiments in anesthetized cats. Changes in endogenous DA release were assessed through its competition with [(11)C]raclopride binding (BP(raclo)), a radioligand probing DA D2-receptors.

RESULTS

In humans no significant difference of BP(raclo) in caudate (with sensory stimulation: 2.0 +/- 0.3 versus without sensory stimulation: 2.2 +/- 0.3; P = 0.3) or putamen (2.6 +/- 0.3 versus 2.6 +/- 0.2; P = 0.9) ipsilateral to the stimulus was disclosed as a result of sensory stimulation. Similarly, no change of BP(raclo) was observed contralaterally to the stimulation in the caudate nucleus (with sensory stimulation: 2.0 +/- 0.4 versus without sensory stimulation: 2.1 +/- 0.2; P = 0.5) and the putamen (2.5 +/- 0.4 versus 2.6 +/- 0.2; P = 0.4). In cats the same results were obtained in the ipsilateral to stimulation striatum (with sensory stimulation: 2.5 +/- 0.03 versus without sensory stimulation: 2.4 +/- 0.05; P = 0.7). No change was also observed contralaterally to the stimulation (2.4 +/- 0.04 versus 2.5 +/- 0.06; P = 0.6). The [(11)C]raclopride binding remained unchanged by sensory stimuli in both humans and cats.

CONCLUSION

This suggests that the DA release induced by sensory stimulus is mostly extrasynaptic whereas the synaptic DA release is probably small, which fits well with the absence of [(11)C]raclopride displacement. The mechanism of this extrasynaptic DA release could be related to a local action of glutamate on dopaminergic terminals via a thalamo-cortico-striatal loop. Present results also underline homology between cat and human responses to sensory stimuli and validate the use of cat brain to find physiological concepts in humans.

Why do restless legs occur at rest?—pathophysiology of neuronal structures in RLS. Neurophysiology of RLS (part 2)

Restless legs syndrome (RLS) is a heterogeneous disorder encompassing genetically caused types with early onset and acquired varieties occurring later in life. Genetic studies in the near future will most likely discover more than one causative gene. The acquired cases too have different etiologies ranging from idiopathic types to secondary forms with uremia, iron depletion, polyneuropathy and others. Here we aim to correlate typical RLS symptoms, such as the sensory symptoms at rest, the reduction of the complaint in response to movement or other physical stimuli, the dominant involvement of the legs, pain, circadian rhythm, and the responsiveness to dopaminergic drugs with neurophysiological features of the central nervous system. We outline the complexity of the neural structures involved and their connections. A diversity of hypothetical affections of different neuronal levels might lead to various combinations of RLS symptomatology. No single pathophysiological explanation has yet been developed that covers all clinical features.

Somatodendritic dopamine release in rat substantia nigra influences motor performance on the accelerating rod

The physiological role of somatodendritic dopamine release in the rat substantia nigra was evaluated with a combination of dual probe microdialysis and simultaneous motor performance tests on an accelerating rod. Three main findings support a modulating influence of somatodendritic dopamine release on motor coordination. (1) The rod performance tests were associated with an increase in extracellular dopamine but not 5-hydroxytryptamine concentrations in substantia nigra and with increases in both dopamine and 5-hydroxytryptamine concentrations in the striatum. (2) Nigral application of dopamine antagonists without intrinsic activity resulted in changed performances on the accelerating rod. The response to nigral perfusion with low concentrations (0.1, 1.0 microM) of the D(2)/D(3)-antagonist raclopride consisted of an impairment in rod performance to 63% of the pre-perfusion performance. Higher concentrations (10, 100 microM), however, were not associated with impaired rod performance, but with increased striatal dopamine concentrations. Perfusion of the substantia nigra with 1, 10 and 100 microM of the D(1)/D(5)-antagonist SCH 23390 dose-dependently impaired rod performance. SCH 23390 consistently increased dopamine and 5-hydroxytryptamine concentrations in substantia nigra but did not change the dialysate in the striatum. (3) In unilaterally 6-hydroxydopamine-lesioned rats, a dose-dependent improvement in rod performance was observed during perfusion of the substantia nigra with the non-selective dopamine agonist apomorphine.

Facilitatory effect of unilateral lesion of the ventral tegmental area on locomotor response to stimulation of the contralateral ventral tegmental area: involvement of GABAergic transmission

It was found previously that in the rat, unilateral electrolytic lesion of the ventral tegmental area (VTA) facilitated feeding induced by electrical stimulation of the homologous VTA tissue in the contralateral hemisphere. In the present work, VTA stimulation-induced locomotor response was tested in male Wistar rats using a latency to move/stimulation frequency curve shift paradigm in order to check for functional generality of the "contralateral facilitation effect" and also with the aim of elaborating an easy and reliable behavioral model to study this phenomenon. In a further step, the hypothesis was tested that enhancement of function of the intact VTA results from elimination of tonic GABAergic influence derived normally from the lesioned VTA. GABA(A) (bicuculline, doses 0, 0.5 and 5.0 ng) and GABA(B) (phaclofen, doses 0, 500 and 1000 ng) receptors antagonists, and for comparison, a GABA(A) receptor agonist (muscimol, doses 0, 12.5, 25. 0 and 50.0 ng), were injected unilaterally to VTA and their effect on locomotor response elicited by electrical stimulation of the contralateral VTA was tested in a latency/frequency paradigm. It was found that similar to feeding, locomotor response evoked by unilateral electrical stimulation of the VTA was facilitated after contralateral VTA lesion which manifested as a decrease of the locomotion threshold and a leftward shift of the function relating latency to move to stimulation frequency. The effect was immediate, long-lasting and specific to the VTA destruction; lesions outside the VTA area caused gradual impairment of the locomotor response to stimulation. The facilitatory effect of the electrolytic lesion could be replicated by bicuculline, which significantly facilitated stimulation-induced behavior. Phaclofen exerted slight facilitating influence only at a low dose. No effect of muscimol on the locomotion threshold was found. We conclude that "the contralateral facilitation effect" at the level of VTA reflects the interhemispheric regulation of activity of the dopaminergic (DA) cells in which GABA(A)-mediated interhemispheric communication plays a significant role.

Release of endogenous dopamine in cultured mesencephalic neurons: influence of dopaminergic agonists and glucocorticoid antagonists

Several electrochemical techniques allow the measurement of dopamine release in freely moving animals and brain slices. In this report, we applied one of these techniques, coulometry, coupled to high-performance liquid chromatography (HPLC), to the study of dopamine release in primary cultures of embryonic mesencephalic dopaminergic neurons. Between day 9 and 33 of culture, concentrations of dopamine, above the detection threshold, were found in the incubation buffer (Krebs ringer buffer, KRB). Concentrations of dopamine in the incubation buffer reflected neuronal release as they were: (i) positively correlated with the number of tyrosine hydroxylase-positive dopamine neurons in the culture; (ii) tetrodotoxin (TTX) sensitive and Ca2+ dependent; (iii) increased by a depolarizing stimulus, e.g. K+ (20 mM), or by the indirect dopamine agonists amphetamine and cocaine; (iv) decreased by a hyperpolarizing stimulus, e.g. the dopamine D2-like receptor agonist quinpirole. Dopamine release in this model was also sensitive to the manipulation of glucocorticoids, potent modulators of dopamine release in vivo. Long-term treatment of the cell cultures with RU 39305, a selective antagonist of glucocorticoid receptors (GR), but not with spironolactone, a selective antagonist of mineralocorticoid receptors (MR), dose-dependently decreased K+-stimulated dopamine release. In conclusion, these results demonstrate an in vitro model that allows the studying of the release of endogenous dopamine in cell cultures and the effects of glucocorticoid hormones on the release dynamics.

Burst firing in midbrain dopaminergic neurons

Midbrain dopaminergic (DA) neurons fire bursts of activity in response to sensory stimuli, including those associated with primary reward. They are therefore conditional bursters - the bursts conveying, amongst other things, motivationally relevant information to the forebrain. In the forebrain, bursts give rise to a supra-additive release of dopamine, and possibly favour the release of co-localised neuropeptides. Evidence is presented that in rat DA neurons, bursts are engendered by the activity of cortically-regulated afferents. Certain factors are identified which, in combination, lead to burst production: (1) A burst of activity in EAAergic afferents to DA neurons arising from non-cortical sources, but controlled by the medial prefrontal cortex; (2) N-methyl-D-aspartate receptor activation, producing a slow depolarising wave in the recipient neuron; (3) activation of a high threshold, dendritically located calcium conductance which produces a 'plateau potential'; (4) activation of a calcium-activated potassium conductance, which terminates the burst. These factors are argued to operate in the context of an 'optimal' level of intracellular calcium buffering for bursting. Other factors which appear to be involved in bursting in other systems, in particular a low threshold calcium conductance, are rejected as being necessary for bursting in DA neurons. The factors which do play a crucial role in burst production in DA neurons are integrated into a theory from which arises a series of hypotheses amenable to empirical investigation. Additional factors are discussed which may modulate bursting. These may either act indirectly through changes in membrane potential (or intracellular calcium concentration), or they may act directly through an interaction with certain conductances, which appear to promote or inhibit burst firing in DA neurons.

Ultrastructural localization of the vesicular monoamine transporter-2 in midbrain dopaminergic neurons: potential sites for somatodendritic storage and release of dopamine

Midbrain dopaminergic neurons are known to release dopamine from somata and/or dendrites located in the substantia nigra (SN) and the ventral tegmental area (VTA). There is considerable controversy, however, about the subcellular sites for somatodendritic dopamine storage in these regions. In the present study, we used dual-labeling electron microscopic immunocytochemistry to localize the vesicular monoamine transporter-2 (VMAT2), a novel marker for sites of intracellular monoamine storage, within identified dopaminergic (tyrosine hydroxylase-containing) neurons in the rat SN and VTA. In dopaminergic perikarya, immunogold labeling for VMAT2 was localized to the Golgi apparatus, tubulovesicles that resembled smooth endoplasmic reticulum (SER), and the limiting membranes of multivesicular bodies. In dopaminergic dendrites, VMAT2 was extensively localized to tubulovesicles that resembled saccules of SER, and less frequently localized to isolated small synaptic vesicles (SSVs) or large dense-core vesicles (DCVs). In rare cases, VMAT2-immunoreactive SSVs were clustered within the cytoplasm of an SN or a VTA dendrite. Dopaminergic dendrites in the VTA contained a significantly higher number of immunogold particles for VMAT2 per unit than those in the SN. Together, these observations support the proposal that dopamine is stored in and may be released from dendritic SSVs and DCVs, but suggest that the SER is the major site of dopamine storage within midbrain dopaminergic neurons. In addition, they provide new evidence that dopaminergic dendrites in the VTA may have greater potential for reserpine-sensitive storage and release of dopamine than those in the SN.

Adrenergic neural connections between the bilateral supraoptic nuclei of the rat hypothalamus

Our previous study has demonstrated that unilateral microinjection of norepinephrine (NE) into the right supraoptic nucleus (SON) of anesthetized hydrated rats elicited dose-dependent decreases in the urine outflow rate. This was antagonized by pretreatment with phenoxybenzamine (an alpha-antagonist) and timolol (a beta-antagonist) in the same SON. In the present study, we examined the effects of NE, microinjected into the right, left and bilateral SON, on the urine outflow rate in order to investigate neural connections between the bilateral SON. NE administered by those three routes dose-dependently decreased the urine outflow rate. The order for the antidiuretic potency was as follows: the effect elicited by the intrabilateral-SON microinjection > the intra-left-SON microinjection = the intra-right-SON microinjection. The antidiuresis of NE microinjected into the right SON was inhibited by an electrolytic left-SON lesion and by pretreatment with phenoxybenzamine (20 nmol) and timolol (100 nmol), but not by atropine (300 nmol) in the left SON. These findings suggest adrenergic neural connections from the right to left SON, contributing to the regulation of urine production. Furthermore, there is a possibility that stimulation of endogenously-released NE in the bilateral SON is amplified through these neurons and elicits more potent effects than those produced in either the right or left nucleus.

Functional repair of striatal systems by neural transplants: evidence for circuit reconstruction

Intrastriatal grafts of nigral and adrenal tissues have been found to be effective in alleviating many of the simple motor and sensorimotor deficits associated with lesions of the nigrostriatal dopamine system. However, the mechanisms by which such grafts exert their effects may be less specific than originally conceived, and both pharmacological and trophic actions play an essential role. Damage to intrinsic cortico-striatal circuits are unlikely to prove similarly amenable to such diffuse mechanisms of repair. Nevertheless, striatal grafts have been found to alleviate cognitive and motor deficits after excitotoxic lesions of the neostriatum. Accumulating evidence suggests that in this particular case many aspects of functional recovery may indeed be attributable to the striatal grafts providing an effective functional reconstruction of damaged neuronal circuits within the host brain.

Behavioural recovery of rats grafted with dopamine cells after partial striatal dopaminergic depletion in a conditioned reaction-time task

The functional effects of grafts of dopamine-rich ventral mesencephalic suspension transplanted in a partially dopamine-depleted striatum were studied in rats performing a reaction-time motor task. The animals were trained to depress a lever, hold it down and release it within a limited period of time (700 ms) after the onset of a visual conditioned stimulus to obtain a food reward. The animals' performances were tested daily for up to two months after transplantation and for up to three months in the case of the animals with lesion only (bilateral striatal 6-hydroxydopamine injection). The baseline performances of the sham-operated control animals tended to improve, whereas the performances of the lesioned rats were significantly disrupted throughout the three months test. The majority of the animals (13/21) in the lesion group showed severe deficits mainly reflected in an increase in the number of the anticipated responses (premature release of the lever before the visual stimulus), and also in the number of the delayed responses (lever release after the time limit) recorded after dopamine depletion. The remaining animals (8/21) exhibited mild deficits (delayed responses only). These differences in the performance deficits appeared to be in relation to the extent of the dopamine denervation within the striatum assessed by the tyrosine hydroxylase immunostaining. Grafted animals showed a large number of dopamine fibers in the reinnervated striata and most of them (73%) significantly improved the reaction-time performance after transplantation. In the most severely impaired animals the number of anticipated errors was totally reversed within one month post-grafting, while the number of delayed responses remained high after transplantation. The performances of the less severely impaired animals returned more rapidly (within three weeks) to the pre-operative levels. The results show that intrastriatal ventral mesencephalic transplants are able to induce substantial or complete recovery in a complex reaction-time task. In the present model for partial dopamine depletion of the striatum, the mechanisms underlying the graft-induced recovery probably involve the participation of endogenous dopamine neurons acting in addition to, and/or in synergy with the dopamine-rich grafted tissue so that a functional level of dopaminergic transmission is restored in transplanted animals.

Behavioral and neurochemical indices of barrel cortex-basal ganglia interaction

Previous experiments from our laboratory have shown a wide variety of time-dependent lateralized changes in behavior and nigrostriatal function following unilateral manipulation of the mystacial vibrissae of rats. The present experiment investigated the effects of unilateral radiofrequency lesion of the cortical vibrissae representation (the barrel fields) in light of these results. We measured lateralized changes in behavior as well as tissue monoamines in neostriatum and substantia nigra, between 1 and 16 days post-lesion. Short-term asymmetries in exploratory behavior (thigmotactic scanning) and neostriatal serotonin metabolism that lasted up to day 6 were seen. In substantia nigra, time-related asymmetries in dopamine concentrations were found with higher ipsilateral values on day 3 and higher contralateral values on day 6. After day 6, the animals had recovered from these acute effects and thereafter, neostriatal dopamine metabolism became asymmetrical. Also during this time, they showed a directional bias in spontaneous and apomorphine-induced turning. Finally, neostriatal serotonin was bilaterally elevated on day 16. These results parallel some of the effects previously seen following unilateral removal of the vibrissae, indicating that the barrel cortex is a critical link in the functional interaction between the vibrissae and basal ganglia.

Eating and drinking cause increased dopamine release in the nucleus accumbens and ventral tegmental area in the rat: measurement by in vivo microdialysis

Dopamine (DA) release was simultaneously monitored in the nucleus accumbens (NAC) and ventral tegmental area (VTA) of conscious rats using in vivo microdialysis. During dialysis perfusion, rats were allowed access to food or water for 20 min following a 36 h food and water deprivation period. DA release increased significantly in the NAC and VTA in response to eating and drinking. The increases in both regions continued until 20-60 min after the end of the feeding or drinking session. These results show that the mesolimbic DA pathway is activated in response to ingestive behavior, and that DA release occurs in the cell body (A10) region as well as in the mesolimbic DA nerve terminals.

Early and widespread normalization of dopamine-neuropeptide Y interactions in the rat striatum after transplantation of fetal mesencephalon cells

Graft-to-host interactions were examined at cellular level, by measuring changes in the immunoreactivity of striatal interneurons expressing neuropeptide Y after dopamine denervation and transplantation of fetal mesencephalon neurons into the striatum of adult rats. Mesencephalic cell suspensions were implanted unilaterally into the dorsal part of the striatum in rats two weeks after intranigral injection of 6-hydroxydopamine. One month and three to four months later, rats showing abolition of amphetamine-induced turning were perfused. Serial brain sections containing intrastriatal grafts were treated for tyrosine hydroxylase and neuropeptide Y immunocytochemistry, and neuropeptide Y-immunoreactive neurons were quantified in various parts of the striatal surface and compared with the striatum of controls and age-matched rats with lesions. Biochemical analyses of dopamine and dihydroxyphenyl acetic acid tissue levels and [3H]dopamine uptake were also performed on striatal samples from similar groups of normal, lesioned and transplanted rats. As early as one month post-grafting, a complete reversal of the increase in the number of neuropeptide Y-immunoreactive neurons occurring after 6-hydroxydopamine lesion was observed in dopamine-grafted animals, although a partial restoration of the tyrosine hydroxylase immunostaining and a recovery of 8% dopamine tissue level were observed in the striata of grafted as compared to normal rats. This effect on the host immunoreactivity was found to be specific to dopamine grafts, since no reversal was observed in sham-spinal cord-transplanted rats. Moreover, similar degrees of normalization were recorded either in the total striatum, or in the area immediately adjacent to the graft, or even in the zone most sensitive to dopamine denervation in terms of neuropeptide Y immunoreactivity. No more pronounced functional effects were observed three to four months after transplantation. These data suggest that grafted dopamine neurons are able to induce rapid and extensive host responsiveness, possibly by means of mechanisms involving synaptic and diffuse release of dopamine and adaptive changes in the host brain. These data may provide a cellular basis for interpreting larger behavioural recoveries than those expected to occur with dopamine grafts in view of the partial restoration of the dopaminergic innervation.

Activity in monkey substantia nigra neurons related to a simple learned movement

Single cell activity was recorded in the pars compacta (SNc) and pars reticulata (SNr) of the substantia nigra (SN) in 4 unanesthetized Macaca fascicularis to determine the motor role of the nucleus. Animals were trained to perform a simple task that involved moving a lever by elbow flexion-extensions, in the horizontal plane using the hand contralateral to the recording site. Two monkeys learnt to execute the task on both sides. Electromyograms (EMG) of limb muscles were recorded simultaneously with SN neurons. Discharge rate modulation related to specific movement phases was present in 35% of the neurons. A significant positive correlation of the discharge rate with movement velocity and amplitude was found in SNc and SNr neurons. Some SNr cells discharged in anticipation of the EMG, suggesting a participation of the nucleus in the preparation of movement. The activity of SNr neurons was also related to movement of the left and right upper limb. In conclusion, the SN seems to play an important role in the control of specific motor mechanisms, probably modulating movement velocity, amplitude and direction, with little participation of somatosensory feedback. The involvement of the SNr in the coordination of bilateral arm activity is discussed.

Enhanced dopamine metabolism in accumbens leads to motor activity and concurrently to increased output from nondopamine neurons in ventral tegmental area and substantia nigra

We previously have reported that nondopamine (non-DA) neurons in substantia nigra (SN) and ventral tegmental area (VTA) of the rat show increased discharge rates during amphetamine (AMPH) and apomorphine (APO)-induced motor activity. The present study represents an attempt to determine the contribution of nucleus accumbens (ACC) dopaminergic activity to these effects, and to ascertain whether the effects in VTA differ from those seen in SN when dopaminergic activity is enhanced locally in ACC. The experiments were carried out in male albino rats (300-400 g) chronically implanted with multiple fine wire electrodes (62 microns) aimed at the pars reticulata of SN (SNR) and VTA. Unit activity was recorded extracellularly in the behaving rat, from neurons identified on the basis of the properties of their action potentials as representing the output of the non-DA neurons in these two structures. In each drug session, unit activity was recorded in parallel from several probes, while motor activity was measured with the open-ended wire technique. But with the recording technique used, a unit represented in most instances the output of a small family of neurons (3-10). Each animal underwent a series of tests given on consecutive days. During these tests, motor and unit activity were measured for 90 min before the drug was administered, and for 135 min after. The first test was of the effects of AMPH, 5 mg/kg, given by the systemic route. The second was of the effects of saline containing 0.1% ascorbic acid (the vehicle) injected bilaterally in ACC, in a volume of 2 microliters per side. The third and all subsequent tests were of the effects of a mixture containing 40 micrograms AMPH, 20 micrograms DA, and 20 micrograms pargyline (P) dissolved in 2 microliters of the vehicle, injected bilaterally in ACC. The results showed that systemic AMPH made the animal hyperactive and at the same time, increased the discharge rate of the non-DA neurons. The bilateral injections of the vehicle in ACC, increased motor activity for about 7 min, an effect interpreted as a rebound from the restraint of the animal during the intracerebral injections, and then depressed motor throughout the 135 min of the postinjection recording period. The effect of the vehicle was to depress unit activity. The effects of injecting the mixture in ACC was to increase motor activity, but with the magnitude and duration of the increase depending on the number of treatments received.(ABSTRACT TRUNCATED AT 400 WORDS)

Compartmental organization of the thalamostriatal connection in the cat

The compartmental organization of the thalamostriatal connection in the cat was studied by labelling thalamic fibers in anterograde axonal transport experiments and comparing their striatal distributions with the arrangement of striosomes and matrix tissue identified by histochemical staining methods. When analyzed according to their principal compartmental targets in dorsal striatum, the thalamic deposits indicated the existence of medial and lateral divisions within the thalamostriatal projection. Nuclei of the medial division, which includes parts of the thalamic midline, projected primarily to striosomes. The lateral division, which embraces the anterior and posterior intralaminar groups, the rostral ventral tier nuclei, and parts of the posterior lateral nuclear complex, predominantly innervated matrix tissue. In the dorsal division of the nucleus accumbens, the medial system preferentially terminated in zones that stain heavily in butyrylcholinesterase and substance P preparations, but fibers from both the medial and the lateral systems largely avoided the histochemically marked compartments such as the border islands of the nucleus accumbens that are seen elsewhere in the ventral striatum. Medial division: Thalamic deposits involving the paraventricular and rhomboid nuclei of the thalamic midline elicited labelling of striosomes and, invariably, ventral extrastriosomal matrix, the nucleus accumbens, and the amygdala. This projection was topographically organized: rostral thalamic deposits elicited labelling in the medial caudate nucleus and the medial nucleus accumbens. More caudal injections produced more lateral labelling. Lateral division: The lateral division is composed of at least three projection systems distinguished by their patterns of matrix innervation. Deposits involving the anterior intralaminar nuclei and the striatally projecting cells located lateral to the stria medullaris (anterior intralaminar complex) produced an even, diffuse labelling of the matrix tissue and weak labelling of the striosomes. Injections placed in the ventroanterior, ventrolateral, and ventromedial nuclei (rostral ventral complex) elicited fibrous labelling of matrix tissue that often showed nonstriosomal inhomogeneities. Deposits involving the centromedian and parafascicular nuclei (posterior intralaminar complex) produced a highly variable pattern of matrix labelling that included both homogeneous and decidedly patchy innervations of the extrastriosomal matrix. Each of these lateral thalamostriatal systems showed a similar spatial organization, whereby dorsoventral and mediolateral thalamic axes were roughly preserved in the projection to striatum.

Lateralized changes in behavior and striatal dopamine release following unilateral tactile stimulation of the perioral region: a microdialysis study

Intracranial microdialysis was used to measure dopamine (DA) release in the ventrolateral neostriatum of freely moving rats before and after unilateral tactile stimulation was applied to the orofacial region. Several behavioral parameters which have been linked to changes in nigrostriatal DA transmission (scanning, or snout contact with the walls of the observation chamber, turning and locomotion) were measured as well. Orofacial stimulation was followed by an asymmetrical increase in DA release with concentrations of transmitter higher in the neostriatum ipsilateral to the side of stimulation. Asymmetrical scanning behavior was observed during the time period when DA release was asymmetric, with rats favoring use of the side of the face contralateral to increased DA release. Increases in the DA metabolites DOPAC and HVA were found in the striatum ipsilateral to stimulation, but were delayed 40 min following the increase in DA.

Sex-related olfactory stimuli induce a selective increase in dopamine release in the nucleus accumbens of male rats. A voltammetric study

Changes in the dopaminergic (DA) transmission in the nucleus accumbens were investigated in male rats exposed to sociosexual olfactory stimuli from different conspecifics: receptive female, non-receptive female and intact male. DAergic transmission was assessed by measurement of extracellular levels of DA and dihydroxyphenylacetic acid (DOPAC). Both compounds were recorded by using differential normal pulse voltammetry (DNPV) with electrochemically pretreated carbon fiber electrodes and numerical analysis of the catechol peak. Exposition to receptive female odors induced a marked and selective increase in DA release compared to control values. Exposition to non-receptive female odors and male odors induced an increase in DA release not significantly different from that following the change of environment. In conclusion, mesencephalic DAergic neurons reaching the nucleus accumbens appear to be involved in the perception of behaviorally significant olfactory cues.

Long-term survival of intrastriatal dopaminergic grafts: modulation of acetylcholine release by graft-derived dopamine

The nigrostriatal dopaminergic system of rats was unilaterally lesioned with 6-hydroxydopamine. Part of the animals was grafted 2 weeks later with fetal dopaminergic cells on the lesioned side; untreated rats of the same strain served as controls. Both 3 and 12-14 months after surgery the striatal dopamine (DA) content and the in vivo rotational response following injection of D-amphetamine showed significant changes in grafted as compared to lesioned animals. At 12-14 months after transplantation, the electrically evoked release of tritiated DA and acetylcholine (ACh) in slices (preincubated with [3H]DA or [3H]choline, respectively) of striata of intact, lesioned, or grafted animals was also investigated. Electrical field stimulation of striatal slices of the lesioned side did not evoke any significant [3H]DA overflow, whereas a marked [3H]DA release was observed in slices of grafted and control striata. Moreover, both DL-amphetamine (3 microM) and nomifensine (10 microM) strongly enhanced basal 3H outflow in these slices. Electrically evoked [3H]ACh release was significantly reduced in slices from all striatal tissues by 0.01 microM apomorphine. In slices from denervated striata a clearcut hypersensitivity for this action of apomorphine was present, indicating supersensitivity of DA receptors on cholinergic terminals; this hypersensitivity was significantly reduced in graft-bearing striata. Furthermore, because this hypersensitivity was unchanged in slices of lesioned striata under stimulation conditions (four pulses/100 Hz) avoiding inhibition by endogenously released DA, it is concluded that lesion-induced DA receptor supersensitivity is caused by an increase in receptor density or efficacy rather than by a decreased competition between endogenous and exogenous agonists. Both reuptake blockade of DA with nomifensine (10 microM) and release of endogenous DA by DL-amphetamine (3 microM) potently reduced [3H]ACh release only in control and grafted but not in lesioned tissue. In experiments using potassium-evoked [3H]ACh release, tetrodotoxin had no effect on the inhibitory activity of amphetamine and nomifensine, indicating that the DA receptors involved in their indirect inhibitory action are located directly on the cholinergic terminals.

The basal ganglia-orofacial system: studies on neurobehavioral plasticity and sensory-motor tuning

We have employed the unilateral removal of the vibrissae as a tool to examine ensuing behavioral changes in relation to concomitant changes in the central nervous system. In this paper we review a series of studies showing that unilateral removal of the vibrissae leads to behavioral asymmetries (e.g., in thigmotactic scanning) from which rats recover over time. Time-related to these behavioral changes we found neuronal alterations in striatal afferents, that is, in uncrossed and crossed projections from the substantia nigra and the tuberomammillary nucleus. The involvement of dopaminergic mechanisms was indicated by results showing that dopaminergic agonists can induce asymmetries in thigmotactic scanning and turning; the direction of these asymmetries was also dependent on time after vibrissae removal. Furthermore, it was shown that endogenous preferential use of one vibrissae side in thigmotactic scanning interacts with the expression of spontaneous and drug-induced behavioral asymmetries exhibited after unilateral vibrissae removal. Neurochemical studies indicated that both unilateral vibrissae removal and unilateral perioral stimulation can have lateralized effects on biogenic amines in the brain. Finally, using electrical stimulation of the substantia nigra, evidence was found for a lateralized and bidirectional interaction between basal ganglia and the orofacial systems, indicating an involvement in mechanisms of motivation and particular stimulation. These results are important from several perspectives. One, they indicate functional links between the orofacial systems and the basal ganglia. Two, they raise the possibility that unilateral removal of the vibrissae can serve as a model (a) to investigate the dynamics of recovery of function after CNS insults, in general, and specifically, (b) to study neuronal plasticity in the nigrostriatal and tuberomammillary-striatal pathways, and (c) to investigate the neuropharmacology of catecholamine systems in the brain.

Dopamine release in the nucleus caudatus and in the nucleus accumbens is under glutamatergic control through non-NMDA receptors: a study in freely-moving rats

Perfusion with quisqualate (5 x 10(-6) M) and kainate (5 x 10(-7) M), selective agonists of glutamate receptors, enhanced the release of dopamine in both caudate and accumbens nuclei of freely-moving rats, measured by the transcerebral microdialysis technique. In contrast, N-methyl-D-aspartate (NMDA) did not affect dopamine release, except at very high concentrations (10(-2) M). The quisqualate-kainate antagonist, FG 9041 (DNQX), antagonized the elevation of dopamine release induced by quisqualate and, furthermore, reduced that of kainate. CPP, a selective NMDA antagonist, did not counteract the quisqualate- or kainate-induced stimulation of dopamine release. The enhancement of dopamine release after quisqualate and kainate was accompanied by behavioural stimulation characterized by grooming, rearing, hypermotility with sniffing and confined sniffing. This behavioural syndrome could be blocked by haloperidol. Conversely, perfusion with NMDA did not activate behaviour even at high concentrations. These results indicate that the dopaminergic system, within the caudate and the accumbens nuclei, is under glutamatergic control through kainate and quisqualate receptors, while the NMDA receptors do not appear to be involved.

The role of D1 and D2 receptors in dopamine agonist-induced modulation of affective defense behavior in the cat

The role of D1 and D2 dopamine (DA) receptor subtypes in mediating DAergic modulation of affective defense behavior in the cat has been investigated in the present study. Feline affective defense, characterized mainly by autonomic arousal, ear retraction, hissing and paw striking, was elicited by electrical stimulation of the ventromedial hypothalamus. Following the establishment of a stable threshold current for eliciting the hissing response of the behavior, the effect of systemic (IP) administration of various DAergic agonists and antagonists on the hissing threshold was determined. The injection of the nonselective DA agonist apomorphine (1.0, 0.3 and 0.1 mg/kg) facilitated hissing in a dose-related manner. This effect was mimicked by the D-2 selective agonist LY 171555 (0.1, 0.03 and 0.01 mg/kg) but not by the D1-selective agonist SKF 38393 (1.0, 5.0 and 10.0 mg/kg), and was blocked by the nonselective and the D2-selective antagonists haloperidol (0.1 and 0.5 mg/kg) and spiperone (0.2 mg/kg), respectively. The D1-selective antagonist SCH 23390 blocked apomorphine-induced facilitation only at a high dose (0.5 mg/kg). In addition, the injection of haloperidol (1.0 mg/kg), spiperone (0.2 mg/kg) or SCH 23390 (0.1 mg/kg) alone inhibited the behavior. It was therefore concluded that DAergic facilitation of affective defense behavior is mainly mediated by the D2 receptors, but that activation of the D1 receptors may play a "permissive" role. The interaction between the D1 and D2 receptors in mediating this facilitation and the behavioral specificity of the effect are discussed.

Effects of hemivibrissotomy in the rat: time-dependent asymmetries in turning and biogenic amines induced by apomorphine

Behavioral and neurochemical changes accompanying unilateral removal of vibrissae were investigated in the rat. Rats were tested either 4 hours or 10 days after hemivibrissotomy. A systemic injection of apomorphine (0.5 mg/kg) induced turning behavior towards the intact vibrissae side in rats tested 4 hours after hemivibrissotomy. Compared to these animals, apomorphine induced more turning towards the side of vibrissae removal and less turning towards the intact side in animals tested 10 days after vibrissae removal. This reversal is suggestive of time-dependent changes in dopamine receptor sensitivity. Analysis of biogenic amines (dopamine, norepinephrine, serotonin) in the hemispheres ipsi- and contralateral to the side of vibrissae removal revealed evidence for neurochemical changes in apomorphine- and amphetamine-treated rats. Lateralized and bilateral differences were found in the neostriatum, septum and ventral mesencephalon, which were dependent on the side and duration of hemivibrissotomy. These results are discussed with respect to the behavioral and neural analogy between hemivibrissotomy and unilateral 6-hydroxydopamine lesions of the nigrostrial system.

Functional organization of striatum as studied with neural grafts

The function of the striatum has proved elusive. A structure which, at the gross microscopic level, appears homogeneous is now revealed to be heterogeneous in terms of its afferent and efferent relationships with cortex, limbic system and mid brain. Cerebral cortex projects topographically to caudate/putamen. Lesions to different cortical areas result in different behavioural impairments which are mirrored by selective neuronal or neurochemical lesions to the sectors of striatum receiving input from the cortex. Foetal neurones prepared from substantia nigra or striatum grafted to a damaged area of adult striatum reverse the lesion-induced behavioural impairments. Within different sectors of striatum the neurones and their afferent and efferent connections are defined to striosomes and matrix representing a finer grain of intrastriatal organization, the functional significance of which is unclear. It remains a challenge within such complex anatomical circuitry to discover the full extent of anatomical reintegration and functional compensation that can be achieved with grafts of foetal neurones.

"On-line" measurement of acetylcholinesterase release from the substantia nigra of the freely-moving guinea-pig

Acetylcholinesterase is released from dopaminergic cells within the substantia nigra. The functional significance of this phenomenon has been studied in the freely-moving animal by a novel system for measuring acetylcholinesterase release from the substantia nigra "on-line" and in vivo. In the unanaesthetized guinea-pig the amount of acetylcholinesterase released was significantly greater than during anaesthesia, and release occurred in a more pulsatile manner. In addition, release of acetylcholinesterase could be evoked by either pharmacological or physiological manipulations, i.e. (1) a depolarizing concentration of potassium ions administered locally; (2) metamphetamine, administered systematically, which also resulted in increased locomotor activity; (3) drinking behaviour, elicited by presentation of a water bottle. Although all three treatments were accompanied by an increase in acetylcholinesterase release within the substantia nigra, potassium-evoked release did not cause any detectable change in behaviour. It is therefore suggested that release of the protein acetylcholinesterase within the substantia nigra is not necessarily a direct cause of locomotor activity: rather, it reflects diverse sensorimotor events.

Asymmetries in crossed and uncrossed nigrostriatal projections dependent on duration of unilateral removal of vibrissae in rats

The influence of unilateral removal of vibrissae on the crossed and uncrossed nigrostriatal projections was examined with the horseradish peroxidase tract tracing technique. Hemivibrissotomy mainly affected the projections arising from the rostral part of the substantia nigra. One to three days after clipping the vibrissae, rats were found to have more labeled neurons in the crossed projection to the caudate-putamen (CPU) on the same side as vibrissae removal than in the crossed projection to the CPU opposite to vibrissae removal. A reversed asymmetry was seen in rats examined 4-20 days after vibrissae removal. These animals had more labeled cells in the crossed and uncrossed projections terminating in the CPU opposite to the shaved side, i.e. in the hemisphere deprived of vibrissal sensory input. This time-course of neural alterations is similar to that of the recovery from behavioral asymmetries seen after hemivibrissotomy. Similar time-dependent alterations in the nigrostriatal projection had been found after unilateral injection of 6-OHDA into the substantia nigra.

Differential control by N-methyl-D-aspartate and kainate of striatal dopamine release in vivo: a trans-striatal dialysis study

Using the technique of trans-striatal dialysis in halothane-anesthetized rats, we have studied the effects of intrastriatally infused N-methyl-D-aspartate (NMDA), kainate, and quisqualate on the liberation of endogenous striatal dopamine. The striatal infusion of NMDA (10(-3)-10(-2) M) or kainate (10(-4)-10(-2) M) but not of quisqualate (up to 10(-2) M) for one 20-min fraction provoked a dramatic increase in striatal dopamine efflux up to a maximum of 1,200 and 3,400% of basal levels for NMDA and kainate, respectively. NMDA (10(-3) M) evoked liberation of striatal dopamine was totally blocked by coinfusion of 2-amino-5-phosphonovalerate (2-APV; 5 X 10(-4) M) and by the systemic injection of phencyclidine (3 mg/kg i.p.). The effects of NMDA (10(-3) M) were also totally antagonized in a dose-dependent manner by the striatal coinfusion of atropine (10(-7)-10(-4) M), and abolished in rats that had received bilateral striatal ibotenate lesions (10 micrograms/1 microliter) 1 week prior to implantation of the dialysis fiber. The striatal infusion of tetrodotoxin (10(-6) M) reduced basal dopamine efflux by 60-70% and abolished the NMDA (10(-3) M)-evoked liberation of striatal dopamine. The effects of kainate (10(-3) M) on striatal dopamine efflux were only partially reduced by doses of 2-APV or atropine that totally blocked the NMDA response, and were also partially resistant to tetrodotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

The response of non-dopamine neurons in substantia nigra and ventral tegmental area to amphetamine and apomorphine during hypermotility: the striatal influence

The effects of haloperidol pretreatment in striatum on the motor response, and on concurrently recorded unit responses of nondopamine (DA) neurons in substantia nigra (SN) and ventral tegmental area (VTA) to systemic amphetamine and apomorphine, were investigated with the objective of determining the role of the striatum in the output of putative DA output neurons. Unit and motor activity were recorded in the male rat, chronically implanted with 9 electrodes in SN and VTA and with two cannulae for bilateral injections into striatum. The recording electrodes were 3 bundles of 3 wires, each wire in the bundle of a different length, but all 3 aimed at SN, pars reticulata, or VTA. In each recording session, unit activity was derived from 7 wires while gross motor activity was recorded with the open-ended wire technique. The subjects were tested under two conditions. In the first, the vehicle was injected bilaterally into striatum 90 min before one of the DA agonists was injected by the intraperitoneal route. In the second, the DA antagonist haloperidol was injected bilaterally into striatum before the systemic treatment with the DA agonist. In subjects which received injections of the vehicle into striatum, amphetamine induced a large motor response, and concurrently, a large increase in the rate of discharge of a portion of the identified non-DA neurons in SN and VTA. In subjects which received injections of haloperidol into striatum, amphetamine induced a smaller behavioral response, a smaller increase in the rate of discharge of these neurons in SN but not in VTA where the increase was of the same magnitude as controls. In control subjects, apomorphine induced an increase in motor activity and concurrently, an increase in the rate of firing of the identified non-DA neurons in SN and VTA. But the increases were of somewhate smaller magnitude and much shorter duration than the increases induced by amphetamine. In subjects which had been pretreated with haloperidol in striatum, apomorphine induced an increase in motor activity that was of the same magnitude as the insion that the striatum has the capacity to influence the output of non-DA neurons only in SN but also in VTA, indicating that, if there is a specialization of function, it is only relative.(ABSTRACT TRUNCATED AT 400 WORDS)