A microiontophoretic study on the nature of the putative synaptic neurotransmitter involved in the pedunculopontine-substantia nigra pars compacta excitatory pathway of the rat

Decreased habituation of midlatency auditory evoked responses in Parkinson's disease

The P1 midlatency auditory evoked potential was studied in patients with Parkinson's disease and compared to that in age-matched controls. Habituation of the potential was determined by using a two-click stimulus paradigm in which the stimuli were presented at 250-, 500-, and 1,000-ms interstimulus intervals. Results showed that habituation of the P1 potential had a statistically significant decrease at the 250-ms and 500-ms interstimulus intervals in patients with Parkinson's disease compared to normal controls. The degree of decreased habituation was found to increase with severity of the disease such that stage 5 patients showed greater decreases in habituation compared to stage 4, as did stage 4 compared to stage 3. These findings may be explained by the presence of a dysregulation of sensory processing, possibly by elements of the reticular activating system, including the pedunculopontine nucleus, in Parkinson's disease.

Modulation of dopamine neuronal activity by glutamate receptor subtypes

In vitro and in vivo electrophysiological studies have been used to assess the effects of glutamate, as well as specific agonists and antagonists for ionotropic, N-methyl-D-aspartate (NMDA), (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate, and metabotropic subtypes of the glutamate receptor, on the neuronal firing activity of midbrain, substantia nigra zona compacta (A9) and ventral tegmental area (A10), dopamine neurons. In in vitro experiments, agonists for all glutamate receptor subtypes depolarize the membrane and increase firing rate. In in vivo experiments, iontophoretic application of these agonists increases the firing rate and induces burst-firing. Studies with subtype selective antagonists suggest that a tonic glutamate tone, acting via NMDA receptors, may modulate the firing activity of some dopamine neurons. Glutamatergic afferents from the subthalamus, pedunculopontine nucleus and frontal cortex can modulate the firing activity of dopamine neurons. The role(s) of the different glutamate receptor subtypes and pathways in mediating the physiological and pathological effects on dopamine systems is an area for further investigation.

Excitotoxic lesions of the pedunculopontine tegmental nucleus produce contralateral hemiparkinsonism in the monkey

Dopaminergic nigrostriatal neurons, degeneration of which causes Parkinson's disease, are known to receive excitatory input almost exclusively from the pedunculopontine tegmental nucleus (PPN). We report here that excitotoxic lesions of the PPN produce abnormal motor signs relevant to hemiparkinsonism in the macaque monkey. Under the guidance of extracellular unit recordings, the electrophysiologically identified PPN was injected unilaterally with kainic acid. These PPN-lesioned monkeys exhibited mild to moderate levels of flexed posture and hypokinesia in the upper and lower limbs contralateral to the lesion. In most of the monkeys, such pathophysiological events were gradually improved and became stationary in 1-2 weeks. The hemiparkinsonian symptoms observed after PPN destruction might be ascribed to a decrease in nigrostriatal neuron activity due to excitatory input ablation.

Dopamine neurons and their role in reward mechanisms

Information related to rewards is processed by a limited number of brain structures. Recent studies have demonstrated that dopamine neurons respond to appetitive events, such as primary rewards and reward-predicting stimuli. Rather than responding unconditionally, these neurons signal deviations from the prediction of future appetitive events. These reward-related responses correspond formally to concepts of behavioral and computational learning theories and may thus constitute teaching signals for appetitive learning.

AMPA and NMDA glutamate receptor subunits in midbrain dopaminergic neurons in the squirrel monkey: an immunohistochemical and in situ hybridization study

The objective of the present study was to analyze the cellular and subcellular localization of ionotropic glutamate receptor subunits in midbrain dopaminergic neurons in the squirrel monkey. This was achieved by means of immunohistochemistry at light and electron microscopic levels and in situ hybridization histochemistry. Colocalization studies show that nearly all dopaminergic neurons in both the ventral and dorsal tiers of the substantia nigra compacta (SNc-v, SNc-d) and the ventral tegmental area (VTA) are immunoreactive for AMPA (GluR1, GluR2/3, and GluR4) and NMDAR1 receptor subunits, but not for NMDAR2A/B subunits. The immunoreactivity of the receptor subunits is associated mainly with perikarya and dendritic shafts. Apart from the intensity of immunolabeling for the GluR4 subunit, which is quite similar for the different groups of midbrain dopaminergic neurons, the overall intensity of immunostaining for the other subunits is higher in the SNc-v and SNc-d than in the VTA. In line with these observations, in situ hybridization shows that the average level of labeling for the GluR2 and NMDAR1 subunit mRNAs is significantly higher in the SNc-v than in the VTA, and for the NMDAR1 subunit, higher in the SNc-v than in the SNc-d. In contrast, no significant difference was found for the level of GluR1 mRNA labeling among the three groups of midbrain dopaminergic neurons. At the subcellular level in the SNc-v, AMPA (GluR1 and GluR2/3) and NMDAR1 receptor subunit immunoreactivity is preferentially associated with the postsynaptic densities of asymmetric synapses, but occasionally some immunoreactivity is found along nonsynaptic portions of plasma membranes of dendrites. A small number of preterminal axons, axon terminals, and glial cell processes are also immunoreactive. Our observations indicate that the different groups of midbrain dopaminergic neurons in primates exhibit a certain degree of heterogeneity with regard to the level of expression of some ionotropic glutamate receptor subunits. The widespread neuronal and glial localization of glutamate receptor subunits suggests that excitatory amino acids may act at different levels to control the basal activity and, possibly, to participate in the degeneration of midbrain dopaminergic neurons in Parkinson's disease.

A slow excitatory postsynaptic current mediated by G-protein-coupled metabotropic glutamate receptors in rat ventral tegmental dopamine neurons

Dopamine neurons in the substantia nigra and ventral tegmental area express metabotropic glutamate receptors, but activation of these receptors by synaptic release of neurotransmitter has not been demonstrated thus far. Patch pipettes were used to record membrane currents under voltage clamp from presumed dopamine-containing neurons in the whole-cell configuration in the rat brain slice. A short train of electrical stimuli delivered to bipolar electrodes placed in the slice evoked a slow excitatory postsynaptic current (EPSC; 50-300 pA at -70 mV) which peaked 560 ms after onset and lasted several seconds, with a decay time-constant of 630 ms. This slow EPSC was voltage-dependent, and was abolished by tetrodotoxin (0.5 microM) or by perfusate containing low calcium (0.5 mM) and high magnesium (10 mM). The metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG; 300 microM) blocked the slow EPSC, but L(+)-2-amino-3-phosphonopropionic acid (AP3; 300 microM) had no effect. The slow EPSC was largely occluded by inward current produced by the metabotropic receptor agonist trans-(+/-)-1-amino-1, 3-cyclopentanedicarboxylic acid (t-ACPD; 300 microM), and the EPSC was reduced > 90% during acute desensitization produced by prolonged perfusion with t-ACPD. (+/-)-2-Amino-4-phosphonobutyric acid (AP4; 300 microM), another metabotropic receptor agonist, reduced the slow EPSC but had no effect on currents evoked by t-ACPD applied by pressure-ejection from micropipettes. The slow EPSC was progressively reduced in amplitude when pipettes contained the G-protein inhibitor GDP-beta-S (0.5 mM). When pipettes contained GTP-gamma-S (0.5 mM), a non-hydrolysable analogue of GTP, onset of the slow EPSC was more rapid and its decay was significantly prolonged. These results demonstrate that a slow EPSC mediated by G-protein-coupled metabotropic glutamate receptors can be evoked in dopamine neurons.

N-methyl-D-aspartate receptors mediate a slow excitatory postsynaptic potential in the rat midbrain dopaminergic neurons

Repetitive local application of a short train of stimuli to the rat substantia nigra and ventral tegmental area elicited a predominant depolarizing, slow, long-lasting synaptic response in the dopaminergic cells intracellularly recorded in vitro. This slow excitatory postsynaptic potential ranged between 13 and 27 mV at holding potentials of about-75 mV and lasted for 0.2-6 s. It was not greatly affected by the perfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (10-20 microM), while it was potentiated in the presence of bicuculline methiodide (30 microM) or picrotoxin (50-100 microM) and 2-hydroxysaclofen (100-300 microM). In contrast, a substantial component of the slow excitatory postsynaptic potential was reversibly depressed, in a concentration-dependent manner, by the application of the N-methyl-D-aspartate receptor antagonists D,1-2-amino-5-phosphonovalerate (10-100 microM). Furthermore, the slow excitatory postsynaptic potential was reversibly increased by the superfusion of nominally magnesium-free solution. It was graded, increasing in amplitude with increased stimulus intensity, and was blocked by tetrodotoxin (0.5 microM). We suggest that a sustained activation of synaptic terminals containing excitatory amino acids mediates a slow excitatory postsynaptic potential in the dopaminergic cells of the midbrain. N-Methyl-D-aspartate receptors participate in the generation of this slow potential, while the alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate/kainate receptors do not seem to contribute substantially to this potential. This N-methyl-D-aspartate-mediated synaptic event could be implicated in the release of dopamine as well as in the excitotoxic injury of the dopaminergic neurons.

Cholinergic and noncholinergic tegmental pedunculopontine projection neurons in rats revealed by intracellular labeling

Morphological features of rat pedunculopontine projection neurons were investigated in in vitro preparation by using intracellular labeling with biocytin combined with choline acetyltransferase (ChAT) immunohistochemistry. These neurons were classified into two types (Type I and II), based on their electrical membrane properties: Type I had low-threshold Ca2+ spikes, and Type II had A-current. All Type I neurons (n = 17) were ChAT immunonegative (ChAT-). Type II neurons were either ChAT immunopositive (ChAT+; n = 49) or ChAT- (n = 20). In terms of topography in the tegmental pedunculopontine nucleus (PPN), Type I neurons were dispersed throughout the extent of the nucleus, whereas Type II neurons tended to be located more in the rostral and middle sections. Both Type I and II neurons consisted of small (long axis < 20 microns), medium (20-35 microns), and large (> 35 microns) cells. The small cells were round or oval; medium cells were round, triangular, or fusiform; and the large cells were primarily fusiform in shape. In terms of the soma size, there was a difference in Type I (15-38 microns) and Type II (11-50 microns) neurons, but no significant difference was found between Type II ChAT+ and ChAT- cells. Both types of neurons had three to six primary dendrites, but the dendritic field was more prominent in Type II neurons. Most of the axons originated from one of the primary dendrites, which gave off axon collaterals, some of which projected out of the nucleus. The intrinsic collaterals were thin and branched partly within the dendritic field of the parent cell. The extrinsic collaterals were thicker and could be grouped into three categories: 1) collaterals arborizing in the substantia nigra; 2) collaterals ascending mainly toward the thalamus, pretectal, and tectal area; and 3) collaterals descending toward the mesencephalic and/or pontine reticular formation. It was noted that the collaterals of both ChAT+ and ChAT-neurons were traced into the substantia nigra. There was no significant difference in antidromic latencies between Type I (m = 1.47 msec) and Type II (m = 1.36 msec) neurons following electrical stimulation of the substantia nigra.

Growth factor-induced c-fos expression defines distinct subsets of midbrain dopaminergic neurons

Growth factors are considered pivotal for the development, maintenance, and function of mesencephalic dopaminergic neurons. Recent studies have identified a plethora of growth factors which support the survival and differentiation of embryonic dopaminergic neurons. However, the exact cellular targets of these growth factors, and, thus, their precise mechanisms of action, remain largely unknown. To identify these cellular targets, we analysed, at the single cell level, growth factor-induced c-fos expression in dissociated mesencephalic cell cultures derived from a fos-lac Z transgenic mouse line. Pharmacological interference with cell-cell communication was utilized to control for direct growth factor effects. beta-Galactosidase-expressing cells were phenotypically characterized by immunocytochemistry to specific neural cell markers. Glia cell line-derived neurotrophic factor, basic fibroblast growth factor, brain-derived neurotrophic factor, and neurotrophin-3 directly induced Fos expression in differently sized, yet overlapping, populations of tyrosine hydroxylase-immunoreactive dopaminergic neurons. In an additional subpopulation of dopaminergic neurons, neurotrophin-3 induced fos-lac Z expression indirectly through a glutamate-mediated activation of N-methyl-D-aspartate receptors. Consistent with their proposed glial-mediated mode of action, transforming growth factor alpha and platelet-derived growth factor induced Fos expression predominantly in glia but only in a very small number of dopaminergic neurons. These findings demonstrate that individual dopaminergic neurons represent the direct targets of different sets of extracellular growth factors. Our findings further establish that growth factors affect dopaminergic neurons by indirect mechanisms which require specific cell-cell communication. These data also suggest a potential role for growth factors in the establishment of the morphological and functional diversity of midbrain dopaminergic neurons.

Stimulation of the prefrontal cortex in the rat induces patterns of activity in midbrain dopaminergic neurons which resemble natural burst events

Evidence suggests that excitatory amino acid-containing afferents from the prefrontal cortex (PFC) play an important role in the induction of burst firing in midbrain dopaminergic (DA) neurons. In the present study, the extracellular activity of individual DA neurons (A10 and A9 cell groups) was recorded during single pulse electrical stimulation (0.25 and 1 mA) of the PFC. The majority of cells were responsive, and two main patterns of activity were elicited: responses characterised by an initial excitation (E responses; 41.8% of responses at 0.25 mA and 26.6% at 1 mA; cell groups combined) and responses characterised by excitation following an initial inhibition (IE responses; 43.3% of responses at 0.25 mA and 56.6% at 1 mA; cell groups combined). Burst analysis performed on the excitatory phase of E and IE responses revealed that the excitation contained events which fulfilled the criteria for natural bursts in DA neurons. A procedure was developed for assessing whether these bursts were time-locked to the stimulus. This showed that 27.9% of E responses and 33.3% of IE responses were accompanied by time-locked bursts (currents and cell groups combined). It is argued that time-locked bursts during IE responses were produced by rebound activation of a low threshold calcium conductance, whereas time-locked bursts during E responses were produced by excitatory afferents. Since natural bursts in DA neurons also seem to involve cortically induced excitation, the hypothesis that the PFC plays a role in the production of natural bursts in DA neurons is strengthened.

Glutamatergic inputs from the pedunculopontine nucleus to midbrain dopaminergic neurons in primates: Phaseolus vulgaris-leucoagglutinin anterograde labeling combined with postembedding glutamate and GABA immunohistochemistry

To verify the possibility that the pedunculopontine nucleus is a source of glutamatergic terminals in contact with midbrain dopaminergic neurons in the squirrel monkey, we used the anterograde transport of Phaseolus vulgaris-leucoagglutinin in combination with preembedding immunohistochemistry for tyrosine hydroxylase and for calbindin D-28k and postembedding immunocytochemistry for glutamate and for gamma-aminobutyric acid. Following tracer injections in the pedunculopontine nucleus, numerous anterogradely labeled fibers emerged from the injection sites to innervate densely the pars compacta of the substantia nigra and ventral tegmental area. The major type of labeled fibers were thin with multiple collaterals and varicosities that established intimate contacts with midbrain dopaminergic neurons. At the electron microscopic level, the anterogradely labeled boutons were medium sized (maximum diameter between 0.9 microns and 2.5 microns) and contained numerous round vesicles and mitochondria. Postembedding immunocytochemistry revealed that 40-60% of anterogradely labeled terminals were enriched in glutamate and formed asymmetric synapses with dendritic shafts of substantia nigra and ventral tegmental area neurons. In triple-immunostained sections, some of the postsynaptic targets to these terminals were found to be dopaminergic. In addition, 30-40% of the anterogradely labeled terminals in both regions displayed immunoreactivity for gamma-aminobutyric acid and, in some cases, formed symmetric synapses with dendritic shafts. In conclusion, our results provide the first ultrastructural evidence for the existence of synaptic contacts between glutamate-enriched terminals from the pedunculopontine nucleus and midbrain dopaminergic neurons in primates. Our results also show that the pedunculopontine nucleus is a potential source of gamma-aminobutyric acid input to this region. These findings suggest that the pedunculopontine nucleus may play an important role in the modulation of the activity of midbrain dopaminergic cells by releasing glutamate or gamma-aminobutyric acid as neurotransmitter.

Synaptic innervation of midbrain dopaminergic neurons by glutamate-enriched terminals in the squirrel monkey

The excitatory amino acid, glutamate, has long been thought to be a transmitter that plays a major role in the control of the firing pattern of midbrain dopaminergic neurons. The present study was aimed at elucidating the anatomical substrate that underlies the functional interaction between glutamatergic afferents and midbrain dopaminergic neurons in the squirrel monkey. To do this, we combined preembedding immunocytochemistry for tyrosine hydroxylase and calbindin D-28k with postembedding immunostaining for glutamate. On the basis of their ultrastructural features, three types (so-called types I, II, and III) of glutamate-enriched terminals were found to form asymmetric synapses with dendrites and perikarya of midbrain dopaminergic neurons. The type I terminals accounted for more than 70% of the total population of glutamate-enriched boutons in contact with dopaminergic cells in the dorsal and ventral tiers of the substantia nigra pars compacta as well as in the ventral tegmental area, whereas 5-20% of the glutamatergic synapses with dopaminergic neurons involved the two other types of terminals. The major finding of our study is that the glutamate-enriched boutons were involved in 70% of the axodendritic synapses in the ventral tegmental area. In contrast, less than 40% of the boutons in contact with dopaminergic dendrites were immunoreactive for glutamate in the dorsal and ventral tiers of the substantia nigra pars compacta. Approximately 50% of the terminals in contact with the perikarya of the different populations of midbrain dopaminergic neurons displayed glutamate immunoreactivity. In conclusion, our findings provide the first evidence that glutamate-enriched terminals form synapses with midbrain dopaminergic neurons in primates. The fact that the proportion of glutamatergic boutons in contact with dopaminergic cells is higher in the ventral tegmental area than in the substantia nigra pars compacta suggests that the different groups of midbrain dopaminergic neurons are modulated differently by extrinsic glutamatergic afferents in primates.

Effects of electrical stimulation of the central nucleus of the amygdala on the in vivo electrophysiological activity of rat nigral dopaminergic neurons

The central nucleus of the amygdala (CeA) receives a dopaminergic (DA) innervation from the midbrain. Among its many efferent projections, the CeA innervates the substantia nigra. The possibility that the CeA influences the activity of nigral DA neurons was evaluated. The effects of electrical stimulation of the CeA on the firing rate and pattern of nigral DA neurons were investigated in anesthetized rats. Poststimulus time histograms revealed that nigral DA cells were either inhibited (N = 15), excited (N = 13), or unresponsive (N = 17) to CeA stimulation (250 stimuli at 0.5 Hz). The mean (+/- SEM) latency to inhibition (24 +/- 9 msec) was significantly shorter than that for excitation (65 +/- 10 msec); the duration of inhibition (200 +/- 29 msec) was also significantly greater than the duration of excitation (86 +/- 11 msec) (P < 0.01 for both). DA cells that were excited had basal firing rates significantly lower than those of the inhibited or unresponsive cells (P < 0.05). Preliminary data suggest that DA cell burst-firing increases or decreases, respectively, in association with stimulation-evoked increases or decreases in firing rate. The relatively long latencies for stimulation-evoked responses suggest that CeA projection neurons indirectly affect nigral DA neurons via polysynaptic pathways. These results demonstrate that the CeA has the ability to influence the activity of nigral DA neurons, consistent with the putative role of the CeA as an interface between the limbic and extrapyramidal systems. Given the crucial role of the amygdala in anxiety states, these findings suggest that DA cell function may also be affected in such disorders.

Distribution of pontomesencephalic cholinergic neurons projecting to substantia nigra differs significantly from those projecting to ventral tegmental area

Locations of pontomesencephalic cholinergic projection neurons from the laterodorsal tegmental (LDTg) and pedunculopontine tegmental (PPTg) nuclei to midbrain dopaminergic nuclei were mapped. Stereotaxic microinjections of Fluoro-Gold- or rhodamine-labeled microspheres were made either to substantia nigra (SN) or ventral tegmental area (VTA) in rat. Choline acetyltransferase was visualized immunohistochemically. Labeled cells were digitally mapped at multiple levels of the nuclei using an interactive computer/microscope system. SN-projecting neurons were distributed predominantly ipsilaterally in distinct regions of the PPTg: either at its rostral pole or caudally in an area ventromedial to the superior cerebellar peduncle. Few SN-projecting neurons were found in LDTg. VTA-projecting neurons were distributed bilaterally throughout the cholinergic group, primarily in the densest regions of the LDTg and caudal PPTg. Neurons were not strictly segregated into these patterns. Scattered cells belonging to either projection could be found throughout the cholinergic group on either side. Hierarchical log-linear analysis showed these differences in topographic distribution to be statistically significant. Subtraction of cell density images demonstrated well delineated regions of the cholinergic group where the projections were predominately either to SN or VTA. These data indicate a high degree of internal organization within the pontomesencephalic cholinergic group based on the location of efferent projections to SN or VTA. These findings support the concept that this cholinergic group is functionally organized in a manner which selectively innervates motor (SN) and limbic (VTA) dopaminergic nuclei.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Evidence for N-methyl-D-aspartate and AMPA subtypes of the glutamate receptor on substantia nigra dopamine neurons: possible preferential role for N-methyl-D-aspartate receptors

The present studies utilized extracellular single-unit recordings in chloral hydrate-anesthetized rats to evaluate the contribution of N-methyl-D-aspartate (NMDA) and (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) subtypes of glutamate receptors to the excitatory effects of glutamate on substantia nigra dopamine neurons. Iontophoretic administration of NMDA, AMPA and glutamate increased the firing rate and amount of burst-firing of dopamine neurons. Iontophoretic application of the NMDA antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid (CPP) inhibited the excitatory effect of NMDA and glutamate, but not that of AMPA. Iontophoretic application of the AMPA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (NBQX), inhibited the excitatory effect of AMPA and glutamate, but not that of NMDA. CPP produced a greater antagonism of the glutamate excitation than did NBQX. In addition, CPP, but not NBQX, reduced the firing rate and burst-firing of a subpopulation of DA neurons. These data indicate that both NMDA and AMPA receptors are present on substantia nigra dopamine neurons and suggest that NMDA receptors may be more sensitive than AMPA receptors to endogenous glutamate and that a tonic glutamate tone, acting via NMDA receptor stimulation, may modulate the firing rate and burst-firing activity of some dopamine neurons.

Intrastriatal and intrasubthalamic stimulation of metabotropic glutamate receptors: a behavioral and Fos immunohistochemical study

Prior work has shown that intrastriatal injection of the metabotropic glutamate receptor agonist 1S,3R-ACPD results in pronounced contralateral rotation, and the basis for this effect is thought to be increased activity of dopaminergic nigrostriatal neurons. We tested this hypothesis by determining the expression of Fos-like immunoreactivity after intrastriatal injection of 1S,3R-ACPD. Intense Fos-like immunoreactivity was noted in the globus pallidus, entopeduncular nucleus, subthalamic nucleus and substantia nigra pars reticulata. Ablation of the subthalamic nucleus 10 days prior to intrastriatal injection of 1S,3R-ACPD abolished rotational behaviour but not Fos-like immunoreactivity in the globus pallidus, entopeduncular nucleus and substantia nigra. Intrasubthalamic injection of 1S,3R-ACPD produced marked contralateral rotation and a pattern of Fos-like immunoreactivity similar to that seen after intrastriatal 1S,3R-ACPD injection. These results suggest that stimulation of striatal metabotropic glutamate receptors inhibits striatal projection neuron activity, while stimulation of subthalamic metabotropic glutamate receptors increases subthalamic nucleus activity. Increased subthalamic nucleus activity is necessary and sufficient for the expression of rotational behavior. These results also suggest that metabotropic glutamate receptor antagonists may be useful in the treatment of Parkinson's disease.

Glutamatergic and cholinergic inputs from the pedunculopontine tegmental nucleus to dopamine neurons in the substantia nigra pars compacta

Postsynaptic responses of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) to stimulation of the pedunculopontine tegmental nuclei (PPN) were studied in in vitro slice preparations in the rat. The recorded neurons were intracellularly injected with biocytin and also identified as DA neurons by an immunocytochemical technique. PPN stimulation induced in DA neurons monosynaptic excitatory postsynaptic potentials (EPSPs) that consisted of early transient and slow components. An application of anti-glutamatergic agents (1 mM kynurenic acid and/or 30 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)) in the bathing media partially suppressed the EPSPs, indicating that PPN inputs to SNc DA neurons are glutamatergic and non-glutamatergic. Anti-glutamatergic resistant EPSPs were suppressed by applications of anti-cholinergic agents such as atropine, mecamylamine, and pirenzepine. These data indicate a convergence of glutamatergic and cholinergic excitatory inputs from the PPN to SNc DA neurons and that both nicotinic and muscarinic receptors are involved in the cholinergic transmission.

A role for non-NMDA excitatory amino acid receptors in regulating the basal activity of rat globus pallidus neurons and their activation by the subthalamic nucleus

We have investigated the hypothesis that excitatory amino acid (EAA) receptors in the globus pallidus (GP) play a significant role in maintaining the firing rates of GP neurons under basal conditions and following activation of the subthalamic nucleus (STN). Drugs were infused directly into the GP and/or STN while the extracellular single unit activity of Type II GP neurons was recorded in ketamine-anesthetized rats. Local infusions of the EAA agonists NMDA (30-300 pmol/200 nl) or AMPA (0.1-1 pmol/200 nl) elicited increases in the firing rate of GP neurons in a dose-dependent fashion. Infusion of the GABAA receptor antagonist bicuculline methiodide (1-10 pmol/100 nl) into the STN also elicited dose-related increases in the firing rate of GP neurons. Intrapallidal infusion of the non-NMDA (AMPA/kainate) receptor antagonist NBQX (0.1-1.0 nmol) reduced the basal firing rate of GP neurons by 40%. In contrast, the NMDA antagonist MK-801 (0.01-0.1 nmol) produced no significant effect on basal firing rate. Intrapallidal infusion of the non-selective EAA receptor antagonist kynurenic acid or NBQX reversed or blocked the increase in firing rate of GP neurons following bicuculline-induced activation of the STN. Similar treatment with MK-801, however, had no significant effect on this response. These results indicate that tonic stimulation of non-NMDA receptors plays an important role in maintaining the basal activity of GP neurons and in mediating the effects of increased excitatory input from subthalamic afferent neurons.

Immunocytochemical localization of the quinolinic acid synthesizing enzyme, 3-hydroxyanthranilic acid oxygenase, in the rat substantia nigra

Quinolinic acid, an endogenous excitatory amino acid receptor agonist, may play a role in several brain diseases. In the present study, the immunocytochemical localization of 3-hydroxyanthranilic acid oxygenase (3HAO), the enzyme responsible for the synthesis of quinolinic acid, was examined in the adult rat substantia nigra at the light and electron microscopic levels. 3HAO-immunoreactivity was detected exclusively in astrocytes. Labeling was present in cell bodies and in fine glial processes, which frequently encircled capillaries and partially enveloped neuronal somata. Notably, 3HAO-labeled processes were in close contact with several types of synaptic profiles. Often, they partially engulfed asymmetric synapses, characteristic of excitatory neurotransmission. In addition, they were found in apposition to putative dopaminergic cell bodies. These data provide an anatomical basis for the idea that functional interactions may occur between glial processes which synthesize quinolinic acid, and synaptic profiles, many of which presumably utilize excitatory neurotransmitters.

Pedunculopontine nucleus in the squirrel monkey: cholinergic and glutamatergic projections to the substantia nigra

The distribution and chemospecificity of the pedunculonigral neurons have been studied in squirrel monkeys (Saimiri sciureus) with cholera toxin subunit B (CTb) and fluorogold (FG) as retrograde tracers combined with immunohistochemistry for choline acetyltransferase (ChAT), glutamate, and the calcium binding protein calbindin D-28k. The injection of either CTb or FG into the substantia nigra produces prominent retrograde cell labeling in the mesopontine tegmentum. Labeled neurons are particularly numerous at the level of the decussation of the superior cerebellar peduncle, where they abound principally in the pars dissipata of the pedunculopontine nucleus (PPN). A significant proportion of retrogradely labeled neurons in the PPN display ChAT immunoreactivity. Within the entire PPN, approximately 25% of the retrogradely labeled neurons express ChAT immunoreactivity, but proportions of doubly labeled neurons are about 35%, 25%, and 15% in the rostral, middle, and caudal thirds of the PPN, respectively. These doubly labeled neurons are scattered among numerous retrogradely labeled neurons that are ChAT-negative and whose number increases along the rostrocaudal extent of the PPN. Several retrogradely labeled neurons in the PPN display glutamate immunoreactivity, but very few express calbindin. This study provides the first direct evidence for the involvement of cholinergic and glutamatergic neurons in the pedunculonigral projection in primates. Furthermore, the fact that some neurons of the PPN display both ChAT and glutamate immunoreactivity indicates that single neurons in the mesopontine tegmentum may exert a two-fold effect upon dopaminergic neurons of the substantia nigra. This dual cholinergic and glutamatergic pedunculonigral projection may play a crucial role in the functional organization of primate basal ganglia.

Pedunculopontine nucleus in the squirrel monkey: projections to the basal ganglia as revealed by anterograde tract-tracing methods

The efferent projections of the pedunculopontine nucleus (PPN) to the basal ganglia have been studied in the squirrel monkey (Saimiri sciureus) with [3H]leucine and Phaseolus vulgaris-leucoagglutinin (PHA-L) as anterograde tracers. Following unilateral injections of [3H]leucine or PHA-L in the central portion of the PPN, numerous autoradiographic linear profiles or PHA-L-labeled fibers ascend to the forebrain, both ipsilaterally and contralaterally. These fibers form a compact bundle that courses in the central portion of the mesopontine tegmentum. At rostral mesencephalic levels, this bundle splits into ventromedial and dorsolateral fascicles that arborize in basal ganglia and thalamic nuclei, respectively. The substantia nigra and the subthalamic nucleus are by far the most densely innervated structures of the basal ganglia. In these two nuclei, labeled fibers arborize profusely ipsilaterally and less abundantly contralaterally. The labeled fibers in the substantia nigra are thin and varicose and arborize almost exclusively in the pars compacta, where they closely surround the soma and proximal dendrites of dopaminergic neurons. In the subthalamic nucleus, labeled fibers are also thin and appear to contact more than one neuron along their course. Numerous labeled fibers also occur in the pallidal complex, where they arborize most profusely in the internal segment. Several thick, labeled fibers oriented dorsolaterally in the pallidal complex give rise to thinner fibers that closely surround the soma and proximal dendrites of pallidal neurons. Some labeled fibers are also scattered in the striatum. These fibers abound in the peripallidal and ventral portions of the putamen, are more sparsely distributed in the remaining portion of the putamen as well as in the caudate nucleus, and are virtually absent in the ventral striatum. These results reveal that the PPN gives rise to a massive and highly ordered innervation of the basal ganglia in the squirrel monkey. This nucleus may thus act as an important relay in the basal ganglia circuitry in primates.

Excitatory amino acid-induced excitation of dopamine-containing neurons in the rat substantia nigra: modulation by kynurenic acid

Kynurenic acid (KYNA), an endogenous antagonist of ionotropic excitatory amino acid (EAA) receptors, was tested for its ability to modulate N-methyl-D-aspartate (NMDA)- and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced excitation of dopamine (DA)-containing neurons in the zona compacta of the rat substantia nigra (SNc). Experiments were conducted using extracellular recording techniques in conjunction with an in vitro brain slice preparation. Bath application of NMDA (1-20 microM) or AMPA (0.5-10 microM) produced a concentration-dependent increase in the firing rate of SNc DA neurons but had no effect on firing pattern. The highest concentration of both agonists produced a rapid and reversible cessation of activity that was attributed to acute induction of depolarization block. Addition of glycine (GLY) (up to 100 microM) to the bathing solution had no effect on either basal firing rate or the increase in activity produced by NMDA. KYNA (10 microM-1 mM) antagonized the excitatory effects of both NMDA (15 microM) and AMPA (3 microM) in a concentration-dependent fashion (IC50:102 microM and 64 microM, respectively) without affecting basal firing rate. Perfusion of tissue slices with a modified Ringer's solution containing low Mg2+ (0.12 mM) increased NMDA-induced excitation but did not affect the antagonist properties of KYNA. D-serine (100 microM) reversed the ability of KYNA to block the excitatory effects of NMDA, suggesting that KYNA attenuates NMDA-induced excitation of SNc DA neurons via blockade of the GLY allosteric site on the NMDA receptor. The ability of KYNA to modulate the excitatory effects of both NMDA and non-NMDA agonists implies that endogenous KYNA may play a physiological role in regulating DA cell excitability.

Lesions of the pedunculopontine tegmental nucleus block drug-induced reinforcement but not amphetamine-induced locomotion

It has been proposed that the positive reinforcing and motor stimulating effects of drugs involve the activation of a common neural substrate. Reinforcing effects of food, drugs and brain stimulation are blocked by lesions of the pedunculopontine tegmental nucleus (PPTg), which is a component of the mesencephalic locomotor region. This has suggested that the PPTg may be involved in both positive reinforcement and forward locomotion. In four separate experiments, rats were prepared with NMDA (0.5 microliters of 0.1 M solution) or sham lesions of the PPTg. Animals in the first two experiments were tested for the development of a conditioned place preference (CPP) to morphine (2 mg/kg x 3 pairings) or amphetamine (1.5 mg/kg x 3 pairings). Ten days later, spontaneous motor activity (SMA) was assessed in these animals following a subcutaneous injection of saline or amphetamine (1.5 mg/kg). In two further experiments, drug-naive lesioned and control animals were tested for SMA only (saline or 1.5 mg/kg amphetamine in Experiment 3, and saline, 0.5 mg/kg, or 3 mg/kg amphetamine in Experiment 4). Lesions of the PPTg blocked the development of a CPP to both morphine and amphetamine. In contrast, lesions had no effect on saline or amphetamine-stimulated SMA. The PPTg, therefore, appears to be involved in the reinforcing effects of amphetamine and morphine, but is not necessary for the expression of amphetamine-induced activity.

Effects of pedunculopontine tegmental nucleus lesions on morphine-induced conditioned place preference and analgesia in the formalin test

The development of a conditioned place preference to morphine (2 mg/kg; three pairings) and the analgesic effect of morphine (0, 4 or 8 mg/kg) in the formalin test were studied in rats with sham or neurotoxin lesions of the pedunculopontine tegmental nucleus. Lesions were induced by bilateral infusions of N-methyl-D-aspartate (0.5 microliter of 0.1 M solution) or vehicle over 10 min. No anti-seizure medication was administered in the first experiment, whereas animals in the second experiment were injected with diazepam (1 mg/kg) immediately after surgery. In Experiment 1, behaviour in the conditioned place preference and formalin tests was assessed in separate groups of lesioned and control rats. In Experiment 2, the same animals received both sets of tests. In both experiments lesions of the pedunculopontine tegmental nucleus blocked the development of a conditioned place preference to morphine, but had no effect on the behavioural response to formalin, or on its inhibition by morphine. Examination of cholinergic-stained cells found no correlation between the magnitude of behavioural effects and the number of acetylcholine cells destroyed by the lesions. These results confirm that the pedunculopontine tegmental nucleus mediates the development of a morphine-induced conditioned place preference, but not the analgesic effect of morphine.

Activation of metabotropic glutamate receptors induces an inward current in rat dopamine mesencephalic neurons

To investigate the electrophysiological effects of the stimulation of the metabotropic excitatory amino acid receptors, we applied trans-1-amino-cyclopentane-1,3-dicarboxylate, an agonist of this type of receptors, on presumed rat dopamine cells intracellularly recorded in vitro. Trans-1-amino-cyclopentane-1,3-dicarboxylate (3-30 microM, t-ACPD) caused a sustained increase of the spontaneous firing rate and a depolarization. When the membrane potential was held at about the resting level (-50, -60 mV), by the single-electrode voltage-clamp technique, t-ACPD induced an inward current. In 57% of the tested cells the inward current was associated with a decrease of the apparent input conductance. In the remaining cells no obvious changes in membrane conductance were observed. The active form of t-ACPD, (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylate [3-50 microM, (1S,3R)-ACPD] also produced a reversible inward current on the dopaminergic cells and this was antagonized by (S)-4-carboxy-3-hydroxyphenylglycine (300 microM), a selective antagonist of the (1S,3R)-ACPD-induced depolarization on central neurons. The (1S,3R)-ACPD-induced inward current was not antagonized by L-2-amino-3-phosphonopropionic acid (100 microM), an antagonist of the t-ACPD-induced activation of inositide synthesis. 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), an alfa-amino-3-hydroxy-5- methyl-isoxazole propionic acid/kainate antagonist, DL-amino-5-phosphonopentanoic acid (30 microM), an N-methyl-D-aspartate antagonist, and scopolamine (10 microM), a muscarinic antagonist, did not significantly affect the actions of t-ACPD. A block of synaptic transmission obtained by applying tetrodotoxin failed to prevent the action of t-ACPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Topographic heterogeneity of substantia nigra neurons: diversity in intrinsic membrane properties and synaptic inputs

The passive and active membrane properties of substantia nigra neurons were recorded in vitro at various locations throughout its anterior-posterior extent and their responses to extracellular electrical stimulation within the pars reticulata were analysed. One class of nigral pars compacta cell showed the well-established electrophysiological characteristics of mesencephalic dopaminergic neurons, i.e. spontaneous discharge in a very rhythmic, pacemaker fashion without bursting activity and with broad action potentials. However, these neurons could be subdivided further according to differences in electrophysiological properties which correlated with their position within the substantia nigra. Thus, neurons recorded from the anterior part of the substantia nigra, at the level of the mammilary bodies displayed a significantly higher firing rate and shorter action potential than those located in posterior slices at the level of the accessory optic tract. The location of the cell was also a critical factor in its response to stimulation of the pars reticulata: in anterior slices only 45.5% of the cells responded with inhibitory postsynaptic potentials to stimulation, while in posterior slices inhibitory postsynaptic potentials occurred in 85.7% of the neurons (n = 44). In addition, anteriorly located neurons were more sensitive to direct electrical stimulation than posteriorly located cells and they also exhibited excitatory postsynaptic potentials (33%) on pars reticulata stimulation. However, the actual properties of inhibitory postsynaptic potentials were essentially the same in these neurons irrespective of whether they were located either in the anterior or posterior part of the nigra: reversal potentials of inhibitory postsynaptic potentials were found at two distinct potentials indicating involvement of both GABAA and GABAB receptors. This deduction is also supported by additional pharmacological findings: application of the GABAA antagonist, bicuculline methiodide and/or GABAB antagonist, 2-hydroxysaclofen blocked both the inhibitory postsynaptic potentials and the cessation of spontaneous firing activity of the cells to stimulation of the pars reticulata. The other type of pars compacta neuron recorded discharges phasically and was located exclusively in the anterior pole of the substantia nigra. These cells showed a wide range of spontaneous firing activity, a non-rhythmic, irregular pattern of firing, a shorter action potential width and the presence of a low-threshold calcium conductance. These "phasic" neurons also differed greatly from other compacta neurons in their response to pars reticulata stimulation: spontaneous activity of these cells was not inhibited nor did they show inhibitory postsynaptic potentials. Instead, the majority was preferentially activated by direct stimulation of the dendrites, although excitatory postsynaptic potentials could also be evoked.(ABSTRACT TRUNCATED AT 400 WORDS)

AMPA glutamate receptor subunits are differentially distributed in rat brain

To demonstrate the regional, cellular and subcellular distributions of non-N-methyl-D-aspartate glutamate receptors in rat brain, we generated antipeptide antibodies that recognize the C-terminal domains of individual subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-preferring glutamate receptors (i.e. GluR1, GluR4, and a region highly conserved in GluR2, GluR3 and GluR4c). On immunoblots, antibodies detect distinct proteins with mol. wts ranging from 102,000 to 108,000 in homogenates of rat brain. Immunocytochemistry shows that glutamate receptor subunits are distributed abundantly and differentially within neuronal cell bodies and processes in cerebral cortex, basal ganglia, limbic system, thalamus, cerebellum and brainstem. The precise patterns and cellular localizations of glutamate receptor subunit immunoreactivities are unique for each antibody. In neocortex and hippocampus, pyramidal neurons express GluR1 and GluR2/3/4c immunoreactivities; many non-pyramidal, calcium-binding, protein-enriched neurons in cerebral cortex are selectively immunoreactive for GluR1. In striatum, the cellular localizations of GluR1, GluR2/3/4c and GluR4 immunoreactivities are different; in this region, GluR1 co-localizes with many cholinergic neurons but is only present in a minor proportion of nicotinamide adenine dinucleotide phosphate diaphorase-positive striatal neurons. GluR1 co-localizes with most dopaminergic neurons within the substantia nigra. In several brain regions, astrocytes show GluR4 immunoreactivity. Within the cerebellar cortex, cell bodies and processes of Bergmann glia express intense GluR4 and GluR1 immunoreactivities; perikarya and dendrites of Purkinje cells show GluR2/3/4c immunoreactivity but no evidence of GluR1 or GluR4. Ultrastructurally, GluR subunit immunoreactivities are localized within cell bodies, dendrites and dendritic spines of specific subsets of neurons and, in the case of GluR1 and GluR4, in some populations of astrocytes. This investigation demonstrates that individual AMPA-preferring glutamate receptor subunits are distributed differentially in the brain and suggests that specific neurons and glial cells selectively express glutamate receptors composed of different subunit combinations. Thus, the co-expression of all AMPA receptor subunits within individual cells may not be obligatory for the functions of this glutamate receptor in vivo.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Ascending afferent regulation of rat midbrain dopamine neurons

Standard, extracellular single-unit recording techniques were used to examine the electrophysiological and pharmacological responsiveness of midbrain dopamine (DA) neurons to selected, ascending afferent inputs. Sciatic nerve stimulation-induced inhibition of nigrostriatal DA (NSDA) neurons was blocked by both PCPA (5-HT synthesis inhibitor) and 5,7-DHT (5-HT neurotoxin), suggesting mediation by a serotonergic (5-HT) system. Direct stimulation of the dorsal raphe (which utilizes 5-HT as a neurotransmitter and inhibits slowly firing NSDA neurons) inhibited all mesoaccumbens DA (MADA) neurons tested. Paradoxically, DPAT, a 5-HT1A agonist which inhibits 5-HT cell firing, enhanced sciatic nerve stimulation-induced inhibition of NSDA neurons. MADA neurons were not inhibited by sciatic nerve stimulation and, therefore, could not be tested in this paradigm. In contrast to the dorsal raphe, electrical stimulation of the pedunculopontine tegmental nucleus preferentially excited slowly firing NSDA and MADA neurons. Thus, both excitatory and inhibitory ascending afferents influence the activity of midbrain DA neurons, and intact 5-HT systems are necessary for sciatic nerve stimulation to alter DA cell activity. However, the role that 5-HT plays in mediating peripheral sensory input remains unclear.

A voltage-clamp analysis of NMDA-induced responses on dopaminergic neurons of the rat subtantia nigra zona compacta and ventral tegmental area

The effects of NMDA-receptor activation on dopaminergic neurons of the rat substantia nigra zona compacta and ventral tegmental area were studied by using in vitro intracellular electrophysiological recordings (current and voltage-clamp). NMDA depolarized the membrane and increased the firing activity. A voltage-dependent inward current and a reduction of the apparent input conductance were observed in voltage-clamp experiments. Interestingly, the peak amplitude of the inward current occurred at approximately -60 mV. The NMDA-induced responses were reduced by the application of DL-2-amino-5-phosphonovaleric acid (APV). The NMDA-induced current was unaffected by potassium channel blockers, was present in low-sodium solutions or in solutions treated with TTX; but was reduced or blocked in low-calcium solutions containing cobalt. In addition, no reduction of the apparent input conductance was observed either in the solutions without magnesium or in those with low-sodium. Our data indicate that the activation of NMDA receptors produces a powerful excitatory stimulus on the dopaminergic neurons of the ventral mesencephalon and this may be primarily the result of a voltage-dependent influx of calcium ions. The degeneration of the dopaminergic cells after application of neurotoxins may be explained by their peculiar response to NMDA.

Electrophysiological effects of MK-801 on rat nigrostriatal and mesoaccumbal dopaminergic neurons

The electrophysiological effects of the non-competitive N-methyl-D-aspartate (NMDA) antagonist (+)-MK-801 (MK-801) on nigrostriatal and mesoaccumbal dopaminergic (DA) neurons were evaluated in chloral hydrate-anesthetized rats. MK-801 (0.05-3.2 mg/kg, i.v.) stimulated the firing rates of 14 (74%) of 19 nigrostriatal DA (NSDA) neurons and all 16 mesoaccumbal DA (MADA) neurons tested. Stimulatory effects of the drug were more prominent on MADA neurons. Interspike interval analysis revealed that MK-801 also regularized DA neuronal firing pattern. Acute brain hemitransection between the midbrain and forebrain attenuated the stimulatory effects of MK-801 on firing rate and blocked the effects on firing pattern. Similar to MK-801, hemitransection itself increased NSDA and MADA cell firing rates and regularized firing pattern. Both i.v. and iontophoretic MK-801 blocked the excitatory effects of iontophoretic NMDA but did not affect excitations caused by the non-NMDA glutamatergic receptor agonists quisqualate and kainate. Iontophoretic MK-801 had no effect alone. These results suggest that the excitatory effects of i.v. MK-801 on DA neuronal activity are not due to direct actions on DA neurons. Glutamatergic projections originating anterior to the hemitransection appear to play a role in the effects of MK-801 on DA neuronal activity.

Membrane properties of identified mesencephalic dopamine neurons in primary dissociated cell culture

Dopamine (DA)-containing neurons in primary dissociated cell cultures derived from the embryonic mouse mesencephalon (day E13) were studied by histochemical and electrophysiological techniques. DA neurons exhibited two distinct morphologies, fusiform and multipolar, tended to reside in groups and organize dendrites into common fascicles. While these neurons expressed the cell-surface marker acetylcholinesterase, the presence of this enzyme could not be used to identify DA neurons unequivocally, since it was also observed in nondopaminergic cells. Neurons were therefore identified as DA by their distinct morphology, and this identification was validated with a double-labeling procedure that entailed the intracellular deposition of a fluorescent dye (Lucifer yellow or ethidium bromide), followed by processing for tyrosine hydroxylase immunocytochemistry. DA neurons identified in this manner were observed to have resting membrane potentials between -50 and -75 mV, input resistances of 50-360 M omega, and membrane time constants of 4.1-14.1 msec. Forty-seven percent of these cells displayed spontaneous activity that was irregular in nature and often contained bursts (burst length was between two and six action potentials). The DA neurons displayed a variety of ionic conductances, including (1) a Na+ conductance (gNa) that underlies the action potential, (2) Ca2+ conductances (gCa) that mediate the nonsomatic low- and high-threshold spikes observed, and (3) at least three K+ conductances (gK). Voltage-clamp analysis revealed several distinct transmembrane ionic currents, including (1) a large, rapidly inactivating tetrodotoxin-sensitive inward Na+ current (INa), (2) a 4-aminopyridine-sensitive, transient early outward K+ current that required a conditioning hyperpolarization of the membrane to be activated by a subsequent depolarization (A-current, IA), (3) a slowly developing inward current that was seen only after a conditioning hyperpolarization of the membrane and that was dependent on the presence of external Ca2+ ions (ICa), and (4) a late-onset, noninactivating K+ current. Between 25% and 54% of the late-onset K+ current was Ca(2+)-dependent and was not affected by tetraethylammonium ions. This current was termed IAHP. The remaining current was not sensitive to changes in the extracellular Ca2+ concentration but was blocked by external tetraethylammonium. This current was termed IK. The direct pressure application of DA (1-200 microM) onto the soma dose-dependently hyperpolarized these neurons; this effect was potentiated by the presence of the catecholamine reuptake blocker cocaine hydrochloride (10-200 microM). Under voltage-clamp conditions, DA was observed to increase IK significantly and had little effect on IAHP.(ABSTRACT TRUNCATED AT 400 WORDS)

Iontophoretically administered drugs acting at the N-methyl-D-aspartate receptor modulate burst firing in A9 dopamine neurons in the rat

Extracellular single-unit recording and iontophoresis were used to examine the effect of N-methyl-D-aspartate (NMDA) and the competitive NMDA antagonist (+/-)-4-(3-phosphonopropyl)-2-piperazine carboxylic acid (CPP) on the firing rate and firing pattern of A9 dopamine (DA) neurons in the rat. Administration of NMDA produced a dose-dependent increase in firing rate (up to nearly 300% of baseline at the highest ejection current), which could be blocked by iontophoretic CPP. Low currents (less than 10 nA) were sufficient to induce apparent depolarisation inactivation in some neurons. In addition to this effect on firing rate, NMDA also caused a dramatic increase in burst firing, which was also dose dependent; cells made more bursts, and each burst consisted of more spikes. The only measured aspect of burst morphology that was not affected was the mean burst interspike interval. All nonbursting cells (n = 10) were converted to burst firing by the drug. CPP administered alone was found to reduce burst firing, without affecting the firing rate. These data suggest that a tonically active excitatory amino acid input to A9 DA neurons is responsible for inducing burst firing in vivo and that this input seems to operate via the NMDA receptor, possibly by virtue of its link to a Ca2+ ionophore.

Organization and physiology of the substantia nigra

A comprehensive review of the literature on the anatomy, electrophysiology and pharmacology of the substantia nigra is presented. A diagram is developed taking into account the interneuronal interactions of neurotransmitters and receptors that control firing rates and neurotransmitter releases. The central features of the diagram are a positive dopaminergic feedforward process and a positive feedback mechanism mediated by extrasynaptic substance P diffusing from striatal terminals to dopaminergic dendrites of the zona compacta neurons. This loop can enhance the transmission of information from the striatum through the pars reticulata output neurons. The loop is controlled at the level of zona compacta neurons by a negative feedback supported by the dendritic release of dopamine and boosted by pedunculopontine activation mediated by muscarinic receptors. The output of the loop is controlled by two negative feedforward processes, both involving GABAergic striatonigral afferents. Application of the model to pharmacological studies of diverse behaviors including seizures, turning, and conditioned behaviors reveals unforseen relationships and may offer insights into, and directions for, further analysis of the mechanisms and functions involved.

Excitatory amino acid binding sites in the basal ganglia of the rat: a quantitative autoradiographic study

Quantitative receptor autoradiography was used to determine the distribution of excitatory amino acid binding sites in the basal ganglia of rat brain. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid, N-methyl-D-aspartate, kainate, quisqualate-sensitive metabotropic and non-N-methyl-D-aspartate, non-kainate, non-quisqualate glutamate binding sites had their highest density in striatum, nucleus accumbens, and olfactory tubercle. Kainate binding was higher in the lateral striatum but there was no medial-lateral striatal gradient for other binding sites. N-Methyl-D-aspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid binding sites were most dense in the nucleus accumbens and olfactory tubercle. There was no dorsal-ventral gradient within the striatal complex for the other binding sites. Other regions of the basal ganglia had lower densities of ligand binding. To compare binding site density within non-striatal regions, binding for each ligand was normalized to the striatal binding density. When compared to the striatal complex, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and metabotropic binding sites had higher relative density in the globus pallidus, ventral pallidum, and subthalamic nucleus than other binding sites. Metabotropic binding also had a high relative density in the substantia nigra. Non-N-methyl-D-aspartate, non-kainate, non-quisqualate glutamate binding sites had a high relative density in globus pallidus, ventral pallidum, and substantia nigra. N-Methyl-D-aspartate binding sites had a low relative density in pallidum, subthalamic nucleus, substantia nigra and ventral tegmental area. Our data indicate heterogeneous distribution of excitatory amino acid binding sites within rat basal ganglia and suggest that the character of excitatory amino acid-mediated neurotransmission within the basal ganglia is also heterogeneous.

Dopamine transmission in the initiation and expression of drug- and stress-induced sensitization of motor activity

Progress has been made over the last 10 years in determining the neural mechanisms of sensitization induced by amphetamine-like psychostimulants, opioids and stressors. Changes in dopamine transmission in axon terminal fields such as the nucleus accumbens appear to underlie the expression of sensitization, but the actions of drugs and stressors in the somatodendritic regions of the A10/A9 dopamine neurons seem critical for the initiation of sensitization. Manipulations that increase somatodendritic dopamine release and permit the stimulation of D1 dopamine receptors in this region induce changes in the dopamine system that lead to the development of long-term sensitization. However, it is not known exactly how the changes in the A10/A9 region are encoded to permit augmented dopamine transmission in the terminal field. One possibility is that the dopamine neurons of sensitized animals have become increasingly sensitive to excitatory pharmacological and environmental stimuli or desensitized to inhibitory regulation. Alternatively, changes in cellular activity or protein synthesis may result in a change in the presynaptic regulation of axon terminal dopamine release.