Origins of postsynaptic potentials evoked in identified rat neostriatal neurons by stimulation in substantia nigra

Physiological aging at striatal synapses

Mike Levine's body of work guides thinking on how the basal ganglia process information to create coordinated movements and skill learning throughout the life span and in disease. This special issue is a nod to Mike's career and a well-deserved gesture by the neuroscience community thanking him for the impact he has made on many people's careers and the field of basal ganglia physiology. This paper reviews how aging impacts basal ganglia processing with a focus on single cell and synaptic physiology. This review begins with the work Mike did with his collaborators Nat Buchwald, Chester Hull and Jay Schneider. These early studies paved the way for subsequent studies on changes in synaptic processing that occur with aging in the basal ganglia. The primary focus of this review is aging at corticostriatal synapses. Corticostriatal synapses show reduced expression of both short-term and long-term synaptic potentiation. The roles of age-related changes in calcium homeostasis, vesicle cycling, dopamine modulation, and NMDA receptor function in aging's effect on synaptic plasticity are discussed. The article ends with a review of mitochondrial aging theory as it applies to age-induced changes in corticostriatal synaptic function.

Disrupted striatal neuron inputs and outputs in Huntington's disease

Huntington's disease (HD) is a hereditary progressive neurodegenerative disorder caused by a CAG repeat expansion in the gene coding for the protein huntingtin, resulting in a pathogenic expansion of the polyglutamine tract in the N-terminus of this protein. The HD pathology resulting from the mutation is most prominent in the striatal part of the basal ganglia, and progressive differential dysfunction and loss of striatal projection neurons and interneurons account for the progression of motor deficits seen in this disease. The present review summarizes current understanding regarding the progression in striatal neuron dysfunction and loss, based on studies both in human HD victims and in genetic mouse models of HD. We review evidence on early loss of inputs to striatum from cortex and thalamus, which may be the basis of the mild premanifest bradykinesia in HD, as well as on the subsequent loss of indirect pathway striatal projection neurons and their outputs to the external pallidal segment, which appears to be the basis of the chorea seen in early symptomatic HD. Later loss of direct pathway striatal projection neurons and their output to the internal pallidal segment account for the severe akinesia seen late in HD. Loss of parvalbuminergic striatal interneurons may contribute to the late dystonia and rigidity.

Multiplexed neurochemical signaling by neurons of the ventral tegmental area

The ventral tegmental area (VTA) is an evolutionarily conserved structure that has roles in reward-seeking, safety-seeking, learning, motivation, and neuropsychiatric disorders such as addiction and depression. The involvement of the VTA in these various behaviors and disorders is paralleled by its diverse signaling mechanisms. Here we review recent advances in our understanding of neuronal diversity in the VTA with a focus on cell phenotypes that participate in 'multiplexed' neurotransmission involving distinct signaling mechanisms. First, we describe the cellular diversity within the VTA, including neurons capable of transmitting dopamine, glutamate or GABA as well as neurons capable of multiplexing combinations of these neurotransmitters. Next, we describe the complex synaptic architecture used by VTA neurons in order to accommodate the transmission of multiple transmitters. We specifically cover recent findings showing that VTA multiplexed neurotransmission may be mediated by either the segregation of dopamine and glutamate into distinct microdomains within a single axon or by the integration of glutamate and GABA into a single axon terminal. In addition, we discuss our current understanding of the functional role that these multiplexed signaling pathways have in the lateral habenula and the nucleus accumbens. Finally, we consider the putative roles of VTA multiplexed neurotransmission in synaptic plasticity and discuss how changes in VTA multiplexed neurons may relate to various psychopathologies including drug addiction and depression.

Glutamate neurons within the midbrain dopamine regions

Midbrain dopamine systems play important roles in Parkinson's disease, schizophrenia, addiction, and depression. The participation of midbrain dopamine systems in diverse clinical contexts suggests these systems are highly complex. Midbrain dopamine regions contain at least three neuronal phenotypes: dopaminergic, GABAergic, and glutamatergic. Here, we review the locations, subtypes, and functions of glutamatergic neurons within midbrain dopamine regions. Vesicular glutamate transporter 2 (VGluT2) mRNA-expressing neurons are observed within each midbrain dopamine system. Within rat retrorubral field (RRF), large populations of VGluT2 neurons are observed throughout its anteroposterior extent. Within rat substantia nigra pars compacta (SNC), VGluT2 neurons are observed centrally and caudally, and are most dense within the laterodorsal subdivision. RRF and SNC rat VGluT2 neurons lack tyrosine hydroxylase (TH), making them an entirely distinct population of neurons from dopaminergic neurons. The rat ventral tegmental area (VTA) contains the most heterogeneous populations of VGluT2 neurons. VGluT2 neurons are found in each VTA subnucleus but are most dense within the anterior midline subnuclei. Some subpopulations of rat VGluT2 neurons co-express TH or glutamic acid decarboxylase (GAD), but most of the VGluT2 neurons lack TH or GAD. Different subsets of rat VGluT2-TH neurons exist based on the presence or absence of vesicular monoamine transporter 2, dopamine transporter, or D2 dopamine receptor. Thus, the capacity by which VGluT2-TH neurons may release dopamine will differ based on their capacity to accumulate vesicular dopamine, uptake extracellular dopamine, or be autoregulated by dopamine. Rat VTA VGluT2 neurons exhibit intrinsic VTA projections and extrinsic projections to the accumbens and to the prefrontal cortex. Mouse VTA VGluT2 neurons project to accumbens shell, prefrontal cortex, ventral pallidum, amygdala, and lateral habenula. Given their molecular diversity and participation in circuits involved in addiction, we hypothesize that individual VGluT2 subpopulations of neurons play unique roles in addiction and other disorders.

Confocal laser scanning microscopy and ultrastructural study of VGLUT2 thalamic input to striatal projection neurons in rats

We examined thalamic input to striatum in rats using immunolabeling for the vesicular glutamate transporter (VGLUT2). Double immunofluorescence viewed with confocal laser scanning microscopy (CLSM) revealed that VGLUT2+ terminals are distinct from VGLUT1+ terminals. CLSM of Phaseolus vulgaris-leucoagglutinin (PHAL)-labeled cortical or thalamic terminals revealed that VGLUT2 is rare in corticostriatal terminals but nearly always present in thalamostriatal terminals. Electron microscopy revealed that VGLUT2+ terminals made up 39.4% of excitatory terminals in striatum (with VGLUT1+ corticostriatal terminals constituting the rest), and 66.8% of VGLUT2+ terminals synapsed on spines and the remainder on dendrites. VGLUT2+ axospinous terminals had a mean diameter of 0.624 μm, while VGLUT2+ axodendritic terminals a mean diameter of 0.698 μm. In tissue in which we simultaneously immunolabeled thalamostriatal terminals for VGLUT2 and striatal neurons for D1 (with about half of spines immunolabeled for D1), 54.6% of VGLUT2+ terminals targeted D1+ spines (i.e., direct pathway striatal neurons), and 37.3% of D1+ spines received VGLUT2+ synaptic contacts. By contrast, 45.4% of VGLUT2+ terminals targeted D1-negative spines (i.e., indirect pathway striatal neurons), and only 25.8% of D1-negative spines received VGLUT2+ synaptic contacts. Similarly, among VGLUT2+ axodendritic synaptic terminals, 59.1% contacted D1+ dendrites, and 40.9% contacted D1-negative dendrites. VGLUT2+ terminals on D1+ spines and dendrites tended to be slightly smaller than those on D1-negative spines and dendrites. Thus, thalamostriatal terminals contact both direct and indirect pathway striatal neurons, with a slight preference for direct. These results are consistent with physiological studies indicating slightly different effects of thalamic input on the two types of striatal projection neurons.

Glutamate neurons in the substantia nigra compacta and retrorubral field

Dopaminergic neurons of the substantia nigra compacta (SNC), ventral tegmental area (VTA) and retrorubral field (RRF) play a role in reward, motivation, learning, memory, and movement. These neurons are intermingled with GABAergic neurons. Recent evidence shows that the VTA contains glutamatergic neurons expressing vesicular glutamate transporter type 2 (VGluT2); some of them co-express tyrosine hydroxylase (TH). Here, we used a combination of radioactive in situ hybridisation and immunohistochemistry to explore whether any of the vesicular glutamate transporters [vesicular glutamate transporter type 1 (VGluT1), VGluT2, or vesicular glutamate transporter type 3 (VGluT3)] were encoded by neurons in the SNC or RRF. We found expression of VGluT2 mRNA, but not of VGluT1 or VGluT3, in the SNC and RRF. These VGluT2 neurons rarely showed TH immunoreactivity. Within the SNC, the VGluT2 neurons were infrequently found at the rostral level, but were often seen at the medial and caudal levels intercalated in the mediolateral portion of the dorsal tier, at a ratio of one VGluT2 neuron per 4.4 TH neurons. At this level, VGluT2 neurons were also found in the adjacent substantia nigra reticulata and substantia nigra pars lateralis. Within the RRF, the VGluT2 neurons showed an increasing rostrocaudal gradient of distribution. The RRF proportion of VGluT2 neurons in relation to TH neurons was constant throughout the rostrocaudal levels, showing an average ratio of one VGluT2 neuron per 1.7 TH neurons. In summary, we provide evidence indicating that the SNC and RRF, which are traditionally considered to be dopaminergic areas, have neurons with the ability to participate in glutamate signaling.

Loss of corticostriatal and thalamostriatal synaptic terminals precedes striatal projection neuron pathology in heterozygous Q140 Huntington's disease mice

Motor slowing, forebrain white matter loss, and striatal shrinkage have been reported in premanifest Huntington's disease (HD) prior to overt striatal neuron loss. We carried out detailed LM and EM studies in a genetically precise HD mimic, heterozygous Q140 HD knock-in mice, to examine the possibility that loss of corticostriatal and thalamostriatal terminals prior to striatal neuron loss underlies these premanifest HD abnormalities. In our studies, we used VGLUT1 and VGLUT2 immunolabeling to detect corticostriatal and thalamostriatal (respectively) terminals in dorsolateral (motor) striatum over the first year of life, prior to striatal projection neuron pathology. VGLUT1+ axospinous corticostriatal terminals represented about 55% of all excitatory terminals in striatum, and VGLUT2+ axospinous thalamostriatal terminals represented about 35%, with VGLUT1+ and VGLUT2+ axodendritic terminals accounting for the remainder. In Q140 mice, a significant 40% shortfall in VGLUT2+ axodendritic thalamostriatal terminals and a 20% shortfall in axospinous thalamostriatal terminals were already observed at 1 month of age, but VGLUT1+ terminals were normal in abundance. The 20% deficiency in VGLUT2+ thalamostriatal axospinous terminals persisted at 4 and 12 months in Q140 mice, and an additional 30% loss of VGLUT1+ corticostriatal terminals was observed at 12 months. The early and persistent deficiency in thalamostriatal axospinous terminals in Q140 mice may reflect a development defect, and the impoverishment of this excitatory drive to striatum may help explain early motor defects in Q140 mice and in premanifest HD. The loss of corticostriatal terminals at 1 year in Q140 mice is consistent with prior evidence from other mouse models of corticostriatal disconnection early during progression, and can explain both the measurable bradykinesia and striatal white matter loss in late premanifest HD.

Ensemble neural activity of the frontal cortical basal ganglia system predicts reaction time task performance in rats

The question pursued in this study was when neural activity appears in the cortico-basal ganglia system that could predict alternate behavioral responses in a reaction time (RT) task. In this protocol, rats first performed a nose poke to initiate a trial, depressed a lever when presented, and then released the lever after a tone cue. Multiple-channel, single-unit recordings (up to 62 units) were obtained simultaneously from the prefrontal cortex, the dorsal medial striatum, the globus pallidus, and the substantia nigra pars reticulata in a single rat during a session. Results indicated that (1) global alterations of neural activity appeared in clusters, which was associated with different behavioral components and observed in each of the targeted areas; (2) small independent subsets of neurons responded differently between error (lever was released before tone presentation) and correct trials (lever was released within 0.5s after tone onset) during these behavioral episodes; (3) significant correlations between RTs and single units activities were found in the early preparation phases of the task. The results reveal that complex early preparatory activity exists several seconds before the final movements in a RT task, which may determine executive functions leading to rapid decoding of alternate behavioral performances.

Corticostriatal projection neurons - dichotomous types and dichotomous functions

The mammalian striatum receives its main excitatory input from the two types of cortical pyramidal neurons of layer 5 of the cerebral cortex - those with only intratelencephalic connections (IT-type) and those sending their main axon to the brainstem via the pyramidal tract (PT-type). These two neurons types are present in layer 5 of all cortical regions, and thus they appear to project together to all parts of striatum. These two neuron types, however, differ genetically, morphologically, and functionally, with IT-type neurons conveying sensory and motor planning information to striatum and PT-type neurons conveying an efference copy of motor commands (for motor cortex at least). Anatomical and physiological data for rats, and more recent data for primates, indicate that these two cortical neuron types also differ in their targeting of the two main types of striatal projection neurons, with the IT-type input preferentially innervating direct pathway neurons and the PT-type input preferentially innervating indirect pathway striatal neurons. These findings have implications for understanding how the direct and indirect pathways carry out their respective roles in movement facilitation and movement suppression, and they have implications for understanding the role of corticostriatal synaptic plasticity in adaptive motor control by the basal ganglia.

Glutamatergic signaling by mesolimbic dopamine neurons in the nucleus accumbens

Recent evidence suggests the intriguing possibility that midbrain dopaminergic (DAergic) neurons may use fast glutamatergic transmission to communicate with their postsynaptic targets. Because of technical limitations, direct demonstration of the existence of this signaling mechanism has been limited to experiments using cell culture preparations that often alter neuronal function including neurotransmitter phenotype. Consequently, it remains uncertain whether glutamatergic signaling between DAergic neurons and their postsynaptic targets exists under physiological conditions. Here, using an optogenetic approach, we provide the first conclusive demonstration that mesolimbic DAergic neurons in mice release glutamate and elicit excitatory postsynaptic responses in projection neurons of the nucleus accumbens. In addition, we describe the properties of the postsynaptic glutamatergic responses of these neurons during experimentally evoked burst firing of DAergic axons that reproduce the reward-related phasic population activity of the mesolimbic projection. These observations indicate that, in addition to DAergic mechanisms, mesolimbic reward signaling may involve glutamatergic transmission.

Mesocortical dopamine neurons operate in distinct temporal domains using multimodal signaling

In vivo extracellular recording studies have traditionally shown that dopamine (DA) transiently inhibits prefrontal cortex (PFC) neurons, yet recent biophysical measurements in vitro indicate that DA enhances the evoked excitability of PFC neurons for prolonged periods. Moreover, although DA neurons apparently encode stimulus salience by transient alterations in firing, the temporal properties of the PFC DA signal associated with various behaviors is often extraordinarily prolonged. The present study used in vivo electrophysiological and electrochemical measures to show that the mesocortical system produces a fast non-DA-mediated postsynaptic response in the PFC that appears to be initiated by glutamate. In contrast, short burst stimulation of mesocortical DA neurons that produced transient (<4 s) DA release in the PFC caused a simultaneous reduction in spontaneous firing (consistent with extracellular in vivo recordings) and a form of DA-induced potentiation in which evoked firing was increased for tens of minutes (consistent with in vitro measurements). We suggest that the mesocortical system might transmit fast signals about reward or salience via corelease of glutamate, whereas the simultaneous prolonged DA-mediated modulation of firing biases the long-term processing dynamics of PFC networks.

Modulation of an afterhyperpolarization by the substantia nigra induces pauses in the tonic firing of striatal cholinergic interneurons

Striatal cholinergic interneurons, also known as tonically active neurons (TANs), acquire a pause in firing during learning of stimulus-reward associations. This pause response to a sensory stimulus emerges after repeated pairing with a reward. The conditioned pause is dependent on dopamine from the substantia nigra, but its underlying cellular mechanism is unknown. Using in vivo intracellular recording, we found that both subthreshold and suprathreshold depolarizations in cholinergic interneurons induced a prolonged after-hyperpolarization (AHP) associated with a pause in their tonic firing. The AHP duration was dependent on the level of depolarization, whether elicited by intracellular current injection or by activation of excitatory inputs from the cortex. High-frequency stimulation of the substantia nigra induced potentiation of the cortically evoked excitation and increased the prolonged AHP after the stimulus. These findings from anesthetized animals suggest that a substantia nigra-induced AHP produces stimulus-associated firing pauses in cholinergic interneurons. This mechanism may underlie the acquisition of the pause response in TANs recorded from behaving animals during learning.

Dopamine neurons mediate a fast excitatory signal via their glutamatergic synapses

Dopamine neurons are thought to convey a fast, incentive salience signal, faster than can be mediated by dopamine. A resolution of this paradox may be that midbrain dopamine neurons exert fast excitatory actions. Using transgenic mice with fluorescent dopamine neurons, in which the axonal projections of the neurons are visible, we made horizontal brain slices encompassing the mesoaccumbens dopamine projection. Focal extracellular stimulation of dopamine neurons in the ventral tegmental area evoked dopamine release and early monosynaptic and late polysynaptic excitatory responses in postsynaptic nucleus accumbens neurons. Local superfusion of the ventral tegmental area with glutamate, which should activate dopamine neurons selectively, produced an increase in excitatory synaptic events. Local superfusion of the ventral tegmental area with the D2 agonist quinpirole, which should increase the threshold for dopamine neuron activation, inhibited the early response. So dopamine neurons make glutamatergic synaptic connections to accumbens neurons. We propose that dopamine neuron glutamatergic transmission may be the initial component of the incentive salience signal.

Glutamate is a cotransmitter in ventral midbrain dopamine neurons

Interactions between apparently separate dopaminergic and glutamatergic pathways figure prominently in the pathophysiology of Parkinson's Disease. So it is surprising that the ventral midbrain dopamine neurons, which give rise to the dopaminergic pathway, may themselves also be glutamatergic. We have addressed this idea in both rat and monkey brain and found that most ventral midbrain dopamine neurons exhibit glutamate immunoreactivity. We used postnatal cell culture to examine ventral midbrain dopamine neurons more closely. In vitro most dopamine neurons exhibit glutamate immunoreactivity, as well as immunoreactivity for phosphate-activated glutaminase, the enzyme principally responsible for the synthesis of neurotransmitter glutamate; inhibition of glutaminase reduces glutamate staining. In single cell microcultures, dopamine neurons make both dopaminergic and glutamatergic synaptic varicosities. Stimulation of individual dopamine neurons evokes a fast excitatory synaptic response mediated by glutamate; it also evokes dopamine release that inhibits the excitatory response via presynaptic D2 receptors. Thus, dopamine neurons appear to exert rapid synaptic actions via their glutamatergic synapses and slower modulatory actions via their dopaminergic synapses, including possibly inhibition of their own glutamatergic synapses. So, in the setting of dopamine neuron demise, there will be a loss of both dopaminergic and glutamatergic inputs to the striatum; furthermore, glutamate released by dopamine neurons may contribute to an excitotoxic cascade and the death of neighboring dopamine neurons.

Mesoaccumbens dopamine neuron synapses reconstructed in vitro are glutamatergic

The mesoaccumbens projection, formed by ventral tegmental area dopamine neurons synapsing on nucleus accumbens gamma-aminobutyric acid neurons, has been implicated in the pathogenesis of schizophrenia and drug addiction. Despite intensive study, the nature of the signal conveyed by dopamine neurons has not been fully resolved. In addition to several slower, dopamine-mediated, modulatory actions, several lines of evidence suggest that dopamine neurons have fast excitatory actions. To test this, we placed dopamine neurons together with accumbens neurons in microcultures. Surprisingly, most dopamine neurons made excitatory recurrent connections (autapses), which provided a basis for their identification; accumbens gamma-aminobutyric acid neurons were identified by their distinctive size. In 75% of mesoaccumbens cell pairs, stimulation of the dopamine neuron evoked a glutamate-mediated, excitatory synaptic response in the accumbens neuron. Immunostaining revealed dopamine neuron varicosities that were predominantly dopaminergic, ones that were predominantly glutamatergic, and ones that were both dopaminergic and glutamatergic. Despite close appositions of both glutamatergic and dopaminergic varicosities to the dendrites of accumbens neurons, only glutamatergic synaptic responses were seen. In the majority of cell pairs, pharmacologic activation of D2-type dopamine receptors inhibited glutamatergic responses, presumably via immunocytochemically-visualized presynaptic D2 receptors. In some cell pairs, the evoked autaptic and synaptic responses were discordant, suggesting that D2 receptors may be differentially trafficked to different presynaptic varicosities.Thus, dopamine neurons appear to mediate both slow dopaminergic and fast glutamatergic actions via separate sets of synapses. Together with evidence for glutamate cotransmission in serotonergic raphe neurons and noradrenergic locus coeruleus neurons, these results add a new dimension to monoamine neuron signaling that may have important implications for neuropsychiatric disorders.

Corticostriatal activity in primary motor cortex of the macaque

Although input from corticostriatal neurons (CSNs) plays a critical role in basal ganglia functions, little is known about CSN activity during behavior. We compared the properties of antidromically identified CSNs with those of antidromically identified neurons that project via the cerebral peduncle to distant targets. Both types of neurons were recorded in primary motor cortex (M1) of two monkeys as they performed a step-tracking task in which static loads opposed or assisted simple and precued movements of the elbow or wrist. Multiple lines of evidence suggested that CSNs and corticopeduncular neurons (CPNs) belong to distinct populations. No cells were activated from both striatum and peduncle. Compared with CPNs, CSNs had slow conduction velocities and low spontaneous rates, and the activity of most was unmodulated by sensory testing or within the tasks used. CSN activity resembled that described for M1-recipient striatal neurons: perimovement firing was small in magnitude, strongly directional, and rarely showed muscle-like load effects. Contrary to a previous report, perimovement activity in most CSNs began before movement onset. CSN activity was more selective than that of CPNs: CSN sensory responses and perimovement activities were often directionally specific, CSNs were often activated exclusively by sensory stimulation, active movement, or movement preparation, and a substantial fraction of CSNs (19%) was unresponsive to any task or manipulation. Thus, CSNs transmit signals distinct from those sent to spinal cord/brainstem. The highly selective activity of CSNs suggests that a discrete (i.e., sparse) code is used to signal cortical activation states to striatum.

Dopamine neurons make glutamatergic synapses in vitro

Interactions between dopamine and glutamate play prominent roles in memory, addiction, and schizophrenia. Several lines of evidence have suggested that the ventral midbrain dopamine neurons that give rise to the major CNS dopaminergic projections may also be glutamatergic. To examine this possibility, we double immunostained ventral midbrain sections from rat and monkey for the dopamine-synthetic enzyme tyrosine hydroxylase and for glutamate; we found that most dopamine neurons immunostained for glutamate, both in rat and monkey. We then used postnatal cell culture to examine individual dopamine neurons. Again, most dopamine neurons immunostained for glutamate; they were also immunoreactive for phosphate-activated glutaminase, the major source of neurotransmitter glutamate. Inhibition of glutaminase reduced glutamate staining. In single-cell microculture, dopamine neurons gave rise to varicosities immunoreactive for both tyrosine hydroxylase and glutamate and others immunoreactive mainly for glutamate, which were found near the cell body. At the ultrastructural level, dopamine neurons formed occasional dopaminergic varicosities with symmetric synaptic specializations, but they more commonly formed nondopaminergic varicosities with asymmetric synaptic specializations. Stimulation of individual dopamine neurons evoked a fast glutamatergic autaptic EPSC that showed presynaptic inhibition caused by concomitant dopamine release. Thus, dopamine neurons may exert rapid synaptic actions via their glutamatergic synapses and slower modulatory actions via their dopaminergic synapses. Together with evidence for glutamate cotransmission in serotonergic raphe neurons and noradrenergic locus coeruleus neurons, the present results suggest that glutamatergic cotransmission may be the rule for central monoaminergic neurons.

Prolonged and extrasynaptic excitatory action of dopamine mediated by D1 receptors in the rat striatum in vivo

The spatiotemporal characteristics of the dopaminergic transmission mediated by D1 receptors were investigated in vivo. For this purpose dopamine (DA) release was evoked in the striatum of anesthetized rats by train electrical stimulations of the medial forebrain bundle (one to four pulses at 15 Hz), which mimicked the spontaneous activity of dopaminergic neurons. The resulting dopamine overflow was electrochemically monitored in real time in the extracellular space. This evoked DA release induced a delayed increase in discharge activity in a subpopulation of single striatal neurons. This excitation was attributable to stimulation of D1 receptors by released DA because it was abolished by acute 6-hydroxydopamine lesion and strongly reduced by the D1 antagonist SCH 23390. Striatal neurons exhibiting this delayed response were also strongly excited by intravenous administration of the D1 agonist SKF 82958. Whereas the DA overflow was closely time-correlated with stimulation, the excitatory response mediated by DA started 200 msec after release and lasted for up to 1 sec. Moreover, functional evidence presented here combined with previous morphological data show that D1 receptors are stimulated by DA diffusing up to 12 micron away from release sites in the extrasynaptic extracellular space. In conclusion, DA released by bursts of action potentials exerts, via D1 receptors, a delayed and prolonged excitatory influence on target neurons. This phasic transmission occurs outside synaptic clefts but still exhibits a high degree of spatial specificity.

The basal ganglia: focused selection and inhibition of competing motor programs

The basal ganglia comprise several nuclei in the forebrain, diencephalon, and midbrain thought to play a significant role in the control of posture and movement. It is well recognized that people with degenerative diseases of the basal ganglia suffer from rigidly held abnormal body postures, slowing of movement, involuntary movements, or a combination of these a abnormalities. However, it has not been agreed just what the basal ganglia contribute to normal movement. Recent advances in knowledge of the basal ganglia circuitry, activity of basal ganglia neurons during movement, and the effect of basal ganglia lesions have led to a new hypothesis of basal ganglia function. The hypothesis states that the basal ganglia do not generate movements. Instead, when voluntary movement is generated by cerebral cortical and cerebellar mechanisms, the basal ganglia act broadly to inhibit competing motor mechanisms that would otherwise interfere with the desired movement. Simultaneously, inhibition is removed focally from the desired motor mechanisms to allow that movement to proceed. Inability to inhibit competing motor programs results in slow movements, abnormal postures and involuntary muscle activity.

Glutamatergic and GABAergic postsynaptic responses of striatal spiny neurons to intrastriatal and cortical stimulation recorded in slice preparations

Glutamatergic and GABAergic responses of the neostriatal spiny neurons to intrastriatal and cortical stimulation were characterized by intracellular recording in brain slice preparations. This study also demonstrated the role of each response in the spike activity of the spiny neuron. Single neostriatal stimulation induced postsynaptic potentials consisting of multiple components. The early part of the postsynaptic potential, which was isolated by the GABAA antagonist bicuculline methiodide and the N-methyl-D-aspartate antagonist 3-(2-carboxypiperzin-4-yl)-propyl-1-phosphonic acid (CPP), was mainly an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor-mediated response. Perfusion of magnesium-free medium containing bicuculline methiodide and the AMPA/kainate antagonist 3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX) disclosed a large, slow N-methyl-D-aspartate receptor-mediated response. The N-methyl-D-aspartate response in magnesium-containing perfusing medium was small in neurons at the resting membrane potential, but became a significant component when the neurons were depolarized to subthreshold membrane potential. The duration of the N-methyl-D-aspartate response was over 300 ms. The nicotinic antagonists dihydro-beta-erythroidine hydrobromide and mecamylamine failed to change responses to single stimulation. Repetitive intrastriatal stimulation induced a large, long-duration depolarization with action potentials in the spiny neurons. This stimulation-induced response resembles that of the depolarization stage observed in anesthetized animals. Bicuculline methiodide increased the response amplitude. In contrast, CPP reduced the amplitude of the response to the below the spike generation threshold. The CPP-sensitive N-methyl-D-aspartate response was large and lasted several hundred milliseconds after the termination of repetitive stimulation. Responses of the neostriatal neurons to cortical stimulation were similar to those induced after intrastriatal stimulation. CPP greatly reduced both the response amplitude and the number of spikes triggered from the response. Bicuculline methiodide, on the other hand, greatly increased the response amplitude and the number of spikes. The AMPA/kainate response alone, which was isolated by application of bicuculline methiodide and CPP, did not induce sustained depolarization in spiny neurons to repetitive cortical stimulation. Application of NBQX diminished GABAA response to cortical stimulation. This observation indicates that, for neostriatal spiny neurons to respond with GABAA response after cortical stimulation, the AMPA/kainate response must be induced in the GABAergic secondary neurons in the neostriatum. This study indicates that the main synaptic driving forces of neostriatal spiny neurons include AMPA/kainate, N-methyl-D-aspartate and GABAA responses. Although AMPA/kainate response is the main synaptic input, the generation of the action potentials in neostriatal neurons is greatly influenced by both GABAA and N-methyl-D-aspartate responses.

Responses of CA1 pyramidal neurons in rat hippocampus to transient forebrain ischemia: an in vivo intracellular recording study

The electrophysiological responses of CA1 pyramidal neurons to 5 min forebrain ischemia were studied with intracellular recording and staining techniques in vivo. The baseline membrane potential rapidly depolarized to approximately -20 mV about 3 min after the onset of ischemia and began to repolarize 1-3 min after recirculation. The amplitude of this ischemic depolarization (ID) was related directly to the severity of ischemia and its latency of onset was inversely related to brain temperature. Spontaneous synaptic activity ceased shortly after ischemia onset while the evoke synaptic potentials lasted until shortly before the onset of ID. Inhibitory postsynaptic potentials (IPSPs) disappeared earlier than excitatory postsynaptic potentials (EPSPs) and the membrane input resistance of CA1 neurons increased after the onset of ischemia.

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.

Conceptual history of the nigrostriatal dopamine system

The history of the nigrostriatal dopamine system may provide a prime example of the two faces of scientific development. First, a given concept is replaced by another simply as a result of methodologies being improved, and second, successive technical improvements make seemingly settled controversies even more complicated and disputable. The nigrostriatal pathway, which had been unrecognizable with Nauta's silver impregnation method, became apparent by use of the more sensitive silver impregnation method of Fink-Heimer. The sensitivity of the latter method, however, was still insufficient to reveal the whole extent of another ascending dopamine system, the mesocortical dopamine system, until its existence was established through the application of glyoxylic acid fluorescent histochemistry. Electron microscopic analysis of nigrostriatal dopamine synapses in properly fixed tissue was initiated by the demonstration of dark type terminal degeneration, which was induced by either electrolytic lesions or chemical destruction with a specific toxin (6-hydroxydopamine) of the substantia nigra and medial forebrain bundle. The degenerating terminal boutons, thus produced, invariably formed postsynaptic membrane specializations of asymmetric type. However, the asymmetric nature of the synaptic morphology, although later confirmed by the combined study of chemical lesions and autoradiographic anterograde tracing, was seriously challenged with the introduction of electron microscopic immunohistochemistry. The latter method has consistently revealed that symmetric en passant synapses or axonal varicosities with no synaptic membrane specializations are the only tissue compartments immunoreactive to antibodies against dopamine and its synthetic enzyme tyrosine hydroxylase. In view of the fact that more than 95% of the nigrostriatal projection neurons are dopaminergic, it is difficult to satisfactorily interpret all the available and seemingly paradoxical fine structural data. In this context, a novel concept has emerged in the process of eliminating all the possible alternative interpretations. The concept is that single nigrostriatal neurons form two chemically distinct types of synapses, one dopaminergic symmetric en passant bouton and another non-dopaminergic (still chemically unclassified) asymmetric terminal bouton. If the concept is a valid one, it contradicts Dale's long standing principle, as defined by Eccles: at all the axonal branches of a neuron there is liberation of the same transmitter substance or substances. Furthermore, a certain population of substantia nigra pars reticulata neurons has recently been recognized to be immunoreactive to both dopamine synthetic tyrosine hydroxylase and GABA synthetic glutamate decarboxylase. These single neurons send projections to both the striatum and superior colliculus by way of axon collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)

Concentration-dependent actions of stimulated dopamine release on neuronal activity in rat striatum

Voltammetric analysis was combined with single unit recording to measure the effects of endogenous dopamine, released by electrical stimulation of the median forebrain bundle, on neuronal activity in the rat striatum in vivo. Fast differential ramp voltammetry, a more sensitive form of fast cyclic voltammetry, was used to measure extracellular dopamine levels during a 50-ms scan epoch every 500 ms. Using the same carbon fibre microelectrode, neuronal activity was recorded in between the electrochemical epochs. A steady-state electrochemical signal equivalent to about 100 nM dopamine was seen in the unstimulated striatum. The responses of 122 striatal units to stimulated dopamine release were recorded in 37 acute experiments. Ninety-one units which displayed a large spike amplitude (greater than or equal to 50 microV) were recorded during stimulated release of dopamine initially to levels of between 100 and 500 nM. The majority (49) showed a profound excitation, 23 showed inhibition, and nine units gave complex responses. Only 10 units were unresponsive. All the responses of these large units outlasted the transient increase in dopamine levels, often for more than 1 min. In contrast, all the 31 units which displayed a small spike amplitude (less than 50 microV) were powerfully activated by dopamine release within this range. Administration of alpha-methyl-para-tyrosine (250 mg/kg i.p.) abolished both dopamine release and the response of the five large units and four small units examined, indicating that the neuronal response was directly attributable to dopamine. Dopamine release was increased by increasing the stimulus duration over the range 0.25-10 s. With increasing levels of dopamine release the excitatory response of large units rose to a maximum and then decreased until it was eventually transformed entirely into an inhibition at dopamine levels above 1 microM. In contrast, the excitatory response of small units always increased in magnitude with increasing dopamine release to levels greater than 1 microM. The large units that showed inhibition at low levels of dopamine were also inhibited at high levels. Tail-pinch stimuli excited 21/23 large units and all seven small units tested, although this stimulus did not evoke a detectable rise in dopamine levels. We suggest that the fundamental action of dopamine in the striatum is excitation, whether involving D1 or D2 receptors. The small units described here could be inhibitory interneurons which convert the excitatory response of large units into inhibition. Dopamine may regulate striatal function by enhancing particular input-output pathways while also activating lateral inhibitory mechanisms serving to "gate-out" alternative outputs.

Electrophysiological properties of nigrothalamic neurons after 6-hydroxydopamine lesions in the rat

Extracellular recordings were made from electrophysiologically identified nigrothalamic cells in the substantia nigra pars reticulata of anaesthetized rats. The firing rate, firing pattern and responses to striatal stimulation were investigated in normal animals and in animals in which dopamine concentration in the ipsilateral striatum was reduced by more than 90%. At relatively short times after the lesion (less than 10 days) the mean firing rate of the spontaneously active cells in the population was significantly reduced and there was an increase in the occurrence of bursting activity. There was also a significant increase in the number of silent cells, located by antidromic stimulation from the thalamus. In spite of this reduction in mean firing rate the responses of neurons to stimulation of either the ipsilateral striatum or ventromedial thalamus was much larger in cells from lesioned animals. At longer times after the lesion (more than six months) the average firing rate of the neurons had returned to normal but there was still a prevalence of bursting activity and a consequent reduction in mean inter-spike intervals. There was little evidence of the previous hyper-responsiveness to thalamic stimulation but the responsiveness to striatal stimulation was still significantly elevated.

Cortical lesions interfere with behavioral recovery from unilateral substantia nigra lesions induced by brain grafts

Effects of aspiration lesions of the cerebral cortex on the behavioral effects of intraventricular substantia nigra grafts were investigated. Apomorphine-induced rotational behavior consequent to unilateral lesions of the substantia nigra was used as a behavioral measure. Substantia nigra grafts reduced rotational behavior in animals with sham cortical lesions. Cortical lesions also decreased rotational behavior and, in these animals, no additional decrease in rotational behavior was induced by substantia nigra grafts. It is concluded that cortical lesions alter striatal circuitry so as to preclude a behavioral effect of substantia nigra grafts.

An hypothesis regarding the antipsychotic effect of neuroleptic drugs

The antipsychotic effect of neuroleptic drugs appears gradually over the course of several weeks of chronic drug administration. Neuroleptic drugs are thought to act by blocking dopamine receptors; however, the dopamine-blocking effect of neuroleptics appears rapidly. One effect of dopamine antagonists which develops slowly is dopaminergic supersensitivity. It is suggested that this dopaminergic supersensitivity is related to the development of tolerance to some of the acute sedative properties of neuroleptics, but not to the antipsychotic effect. A population of glutamate receptors which are postsynaptic to the cortico-striatal afferents is located on the same neurons as striatal dopamine receptors. These glutamate synapses are located on the heads of dendritic spines of striato-nigral projection neurons, while the dopaminergic synapses are predominantly located on the necks of these same dendritic spines. Similar relationships could exist for mesolimbic and mesocortical dopamine systems. In peripheral systems, postjunctional denervation supersensitivity is known to be nonspecific; in other words, denervation of a single innervation of an excitable cell can alter the response to a range of stimuli. The antipsychotic effect of neuroleptics is therefore suggested to be due to nonspecific postjunctional subsensitivity at glutamate synapses, which develops concomitant with supersensitivity at dopaminergic synapses.

Long-term changes in striatal D1 dopamine receptor distribution after dopaminergic deafferentation

The morphochemical disposition of the adenylate cyclase-linked dopamine receptor (D1 type) in the rat striatum has been assessed at various time points after a neurotoxic lesion of the dopaminergic afferent pathway to the caudate nucleus. D1 receptor binding sites in the caudate nucleus were determined by in vitro autoradiography of the substituted benzazepine D1 antagonists, [3H]SCH 23390 or [125I]SCH 23982, and contrasted to the pattern of striatal immunohistochemical reactivity of the second messenger compound, cyclic 3',5'-adenosine monophosphate. The results demonstrate that the specific association of this dopamine receptor type with cyclic 3',5'-adenosine monophosphate-stained neurons is abolished at 7 days following chemical interruption of the nigrostriatal pathway, and the receptor disruption is persistent for durations as long as 20 weeks. This investigation suggests that once the postsynaptic receptor pathology is produced by deafferentation, it does not recover the selective morphochemical relationship normally established with the target cell containing the second messenger. This is in contrast to modest biochemical recuperation in D1 dopamine receptor binding seen using this experimental paradigm. This change in D1 dopamine receptor morphochemistry is discussed in relation to the neurochemical deficits produced by dopaminergic denervation and in Parkinson's disease.

Striatal grafts in rats with unilateral neostriatal lesions--II. In vivo monitoring of GABA release in globus pallidus and substantia nigra

GABA release was recorded in vivo by push-pull perfusion from the globus pallidus and substantia nigra of control rats, rats with unilateral ibotenic acid lesions of the neostriatum, and rats with embryonic striatal tissue grafts implanted in the lesioned striatum. The lesions reduced baseline levels of GABA release to 5% of control levels in the globus pallidus and to 13% of control levels in the substantia nigra pars reticulata. GABA release was substantially restored in both the globus pallidus and substantia nigra of the grafted rats, to 34 and 60%, respectively. Peripheral injection of the dopaminergic stimulant methamphetamine induced a short (lasting approximately 20 min) 4-5 fold increase in GABA release in the intact globus pallidus and a longer (lasting longer than 80 min) increase in the substantia nigra. The stimulatory effect of methamphetamine on GABA release was completely abolished in both sites by the strial lesions, suggesting that the effect was mediated via a direct or indirect dopaminergic action on striatal output neurons. The grafts reinstated methamphetamine-induced stimulation of GABA release in striatal output targets to a level (as a proportion of baseline) that was similar to that seen in the control rats. The results support the view that activation of the dopaminergic inputs to the striatum is functionally excitatory on the major striatal output projections to the globus pallidus and substantia nigra pars reticulata. The results also support the hypothesis that striatal grafts have the capacity to become functionally incorporated by reciprocal graft-host connections into the neural circuitry of the host brain.

Substantia nigra stimulation evoked antidromic responses in rat neostriatum

Electrical stimulation of the substantia nigra of rats elicits a burst of small amplitude waves with a latency of 4-6 ms that may last for 10-15 ms throughout much of the neostriatum. Frontal cortex stimulation also elicits a burst response, which can occlude the substantia nigra response. The substantia nigra evoked burst response was still present after chronic neocortical ablation or thalamic transection or both treatments combined. The response corresponds to the first sharp negative wave of the substantia nigra evoked neostriatal field potential. Single substantia nigra evoked action potentials were recorded in neostriatum with a mean latency of 9.8 ms, ranging from 4-22 ms. These action potentials were considered to be antidromic because they were occluded during appropriate collision intervals by orthodromic action potentials elicited by frontal cortex stimulation. Subthreshold frontal cortex conditioning stimulation did not alter the threshold for activation from substantia nigra. The refractory period for the axon was at least as long as that for the soma and ranged between 0.8-2.0 ms. The antidromic responses failed to follow low frequency stimulation (less than 40 Hz for 3000 ms). This failure occurred in the axon between substantia nigra and globus pallidus. The burst response and first sharp negative wave of the field potential probably represent the antidromic activation of the ubiquitous and densely packed medium spiny neostriatal projection neurons. These responses occur at the same latency, respond in the same manner to twin pulse and repetitive stimulation and are occluded by frontal cortex stimulation in the same manner as antidromic action potentials.

Electrophysiological study of the effects of D1 and D2 dopamine antagonists on the interaction of converging inputs from the sensory-motor cortex and substantia nigra neurons in the rat

The interaction of stimulation of the cerebral cortex and of the substantia nigra on the activity of neostriatal neurons was investigated in urethane-anesthetized rats. Neurons of the dorsal striatum were activated by single pulse stimulation of the sensory-motor cortex. The effects of nigral conditioning stimulation on this excitatory response of striatal neurons to cortical stimulation were studied in a series of parametric experiments in which the length of the train of pulses and the intensity of the nigral stimulation were varied. One and five pulses of nigral conditioning stimulation had little or no effect. Ten pulses of nigral conditioning stimulation reduced the excitatory response, the magnitude of the reduction being greater with higher current intensities. In another series of experiments, the effects of dopaminergic receptor antagonists on the interaction of cortical and nigral inputs to striatal neurons were studied. Sulpiride, a D2 antagonist, reversed the attenuating effects of nigral conditioning stimulation on the excitatory response of striatal neurons to cortical stimulation, whereas SCH 23390 a D1 antagonist, had no effect. The present findings support the hypothesis that the nigrostriatal dopaminergic pathway modulates the excitatory response of striatal neurons to cortical stimulation by means of dopamine D2 receptors.

Postnatal development of identified medium-sized caudate spiny neurons in the cat

The morphology of intracellularly recorded neurons in the cat caudate nucleus (Cd) was studied during postnatal development. After intracellular recording of evoked responses in these neurons, horseradish peroxidase (HRP) was injected iontophoretically through the recording micropipette. Fifty-eight Cd neurons in cats ranging from 6 days of age through adulthood were identified morphologically. All of the recovered Cd cells were medium-sized spiny neurons. The basic somatodendritic morphology of these neurons was evident in the youngest kittens. The most striking morphological change was the postnatal formation of an extensive local axonal collateral plexus. The development of these local axonal collaterals was also quantified with computer assistance in medium-sized Cd spiny neurons selected from silver-impregnated material. This analysis showed that the major development of the branches of this local plexus occurred between birth and 3-4 months of postnatal age. Data from both the HRP-filled and silver-stained axons indicated that the postnatal growth of the local axonal collaterals of the medium spiny cells was associated with the elaboration and increasing prevalence of evoked inhibitory postsynaptic potentials in Cd neurons.

Neurotransmitter receptor autoradiography in immunohistochemically identified neurons

A method for the simultaneous visualization of neurotransmitter receptor binding sites and immunohistochemically characterized cells is described using in vitro sections of rat striatum. The striatum provides a rich neurochemical environment in which to examine muscarinic acetylcholine receptor relations to peptide-reactive somata, or to assess the cellular locale of the dopamine receptor linked to adenylate cyclase activation. Different modifications of the procedure are employed to determine the localization pattern of 3H-radioligands with slow or rapid receptor dissociation times. The present technique is compatible for use with any combination of immunohistochemical antibody/radioligand probe that can be studied on fresh-frozen, slide-mounted tissue sections.

Active membrane properties of rat neostriatal neurons in an in vitro slice preparation

The active membrane properties of rat neostriatal neurons have been studied in an in vitro slice preparation. All the neurons examined had resting membrane potentials of more than 50 mV and generated action potentials with amplitudes exceeding 70 mV. The morphological characteristics of the neurons identified by intracellular labeling with HRP indicated that they were medium spiny neurons. Depolarizing current injection through the recording microelectrode generated slow depolarizing potentials and repetitive action potentials with frequencies ranging from less than 10 Hz to over 300 Hz. Adaptation of action potentials was observed when long duration depolarizing current was injected. Depolarizing current injections revealed that the membrane of the striatal neuron had an anomalous rectification when the membrane potential was depolarized to the resting potential. A possible bases for the anomalous rectification might involve inactivation of K-conductance and slow inward Ca- and/or Na-currents. Local electrical stimulation evoked depolarizing postsynaptic potentials (DPSPs) followed by long-lasting small depolarizations. In a double stimulation test, a potentiation of the test DPSP was observed at interstimulus time interval of up to 80 ms. Post-tetanic potentiation of DPSPs was also seen in these neurons. Tests utilizing depolarizing current injection, intracellular Cl- injection, and Cl-conductance blocking drugs indicated that the DPSPs were composed of EPSPs and overlapping IPSPs. The nature of the long-lasting small depolarization succeeding the DPSPs could not be conclusively determined. However, available data suggest that the slow inward Ca-current may be responsible for this response.(ABSTRACT TRUNCATED AT 250 WORDS)

Origins of post synaptic potentials evoked in spiny neostriatal projection neurons by thalamic stimulation in the rat

Stimulation of thalamic intralaminar nuclei or structures along the intrathalamic trajectory of thalamostriatal axons evoked complex EPSPs and subsequent hyperpolarizations in rat neostriatal spiny neurons identified by intracellular injection of horseradish peroxidase and/or antidromic activation from substantia nigra. In intact urethane-anesthetized rats, the initial EPSP portion of the response consisted of several components and lasted up to 75 ms. Short (1-10 ms) latency components exhibiting latency variations suggestive of a polysynaptic origin were often observed, and sometimes were the earliest components of the response. However, individual components of the excitatory response could not be clearly distinguished in most neurons and the earliest excitatory component usually appeared to be monosynaptic. After large acute aspiration lesions of ipsilateral cerebral cortex, the early polysynaptic EPSP components of thalamic-evoked EPSPs were absent or greatly attenuated. This suggested that most or all of the short latency polysynaptic EPSP components arose via a thalamo-cortico-striatal route. A short latency (1.6-4.0 ms) monosynaptic EPSP and a second excitatory component with a longer and more variable latency (8-28 ms) remained intact after acute decortication. These were not dependent upon intact corticothalamic or corticostriatal axons, since they were both still present in experiments performed as long as 4 days following ipsilateral hemidecortication. The longer latency excitatory response was shown to be polysynaptic by its latency variation with changes in stimulus intensity and frequency. This component of the response was abolished after acute thalamic hemitransections separating thalamostriatal neurons from their axons. In these experiments, stimulation of thalamostriatal axons rostral to the transection continued to evoke monosynaptic EPSPs in neostriatal spiny neurons. These EPSPs ranged from 1.8 to 3.0 ms in latency, had peak amplitudes up to 11 mV and were 20-37 ms in duration.

Disfacilitation and long-lasting inhibition of neostriatal neurons in the rat

Excitatory postsynaptic potentials evoked in rat neostriatal spiny projections neurons were followed by a long (100-300 ms) period of membrane hyperpolarization, followed in turn by a late depolarization. Concomitant with these changes in membrane potential were inhibition and subsequent excitation of spontaneous firing and excitatory activity evoked from substantia nigra and cerebral peduncle, but not from cortical stimulating sites. Thalamic-evoked excitatory activity was sometimes sensitive and sometimes insensitive to this inhibition, which has previously been believed to result from intrinsic inhibitory synaptic activity among neostriatal neurons. In intracellular recordings from neostriatal neurons in urethane anesthetized rats this long-lasting inhibitory response (1) exhibited alterations with intracellularly applied steady currents comparable to those of the EPSP, (2) failed to respond to intracellular injection of chloride ions, (3) was associated with either a decrease or no detectable change in the input conductance of the neurons, and (4) was abolished after lesions that interrupted polysynaptic pathways to neostriatum through intracortical and intrathalamic synaptic circuits. These findings indicate that the long lasting inhibitory portion of the responses of neostriatal neurons arises from a phasic inhibition of tonically active corticostriatal and thalamostriatal neurons and a concurrent decrease in the excitability of polysynaptic pathways converging on neostriatal neurons.

A short duration GABAergic inhibition in identified neostriatal medium spiny neurons: in vitro slice study

Inhibition in the neostriatum was investigated in rat in vitro slice preparation using intracellular recording and labeling technique. The initial response recorded following local stimulation is a monosynaptically activated EPSP. In 17% of the neurons tested, IPSPs were observed following EPSPs evoked by local stimulation. In paired shock experiments reduction of test EPSP amplitude or action potentials occurred over interstimulus intervals (ISIs) of 3-38 msec. In some neurons, a pulse injection of depolarizing current was used to trigger an action potential which was in a paired shock, used to condition a test monosynaptically induced EPSP. Test EPSPs were shunted over ISIs less than 45 msec. Paired shock performed on the slices perfused with the medium containing GABA antagonists (e.g., bicuculline methiodide, picrotoxin, or penicillin-G) resulted invariably in potentiation of test EPSPs. Inhibition in the neostriatum in vitro is demonstrated as reduction in test amplitude in paired shock tests, by the presence of IPSPs and by the shunting of EPSPs conditioned by an action potential triggered by direct depolarization. Neurons exhibiting these forms of inhibition were intracellularly labelled with HRP and identified as medium spiny neurons. These results indicate that striatal GABAergic medium spiny neurons which are known to have an extensive axon collateral plexus play in a role in a short lasting inhibition observed in the striatum.

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

A theory of the intrinsic, functional organization of the neostriatum and the neostriatal control of voluntary movement is presented. The cell types of neostriatum are described in accordance with the classification scheme of Pasik, Pasik and DiFiglia (ref. 204) and their suspected neurotransmitters are identified. The functional relations between the afferent projections to neostriatum and the intrinsic elements of neostriatum are described. A division of the neostriatal efferents into two functional cell systems is conceived, in which one efferent system, the Spiny I cell matrix, is thought to be represented by a lateral inhibitory network comprised of the common Spiny I neurons which inhibit their targets, while another is termed the Spiny II cell cluster and consists of the far less numerous excitatory efferents of neostriatum (the Spiny II cells) and is represented by clusters of neurons, the members of each cluster consisting of one excitatory efferent neuron and three interneurons. The implications of this view for neostriatal input--output relations are discussed, and the theory is developed by reference to a variety of converging lines of empirical evidence, and is used to interpret the effects of various pharmacologic strategies which have been employed to achieve symptom management in disorders of voluntary movement. In the course of this review, there emerges an integrative theory of the principles governing the neostriatal control of voluntary movement.