Possible mechanisms involved in the stereotyped behavior elicited by amphetamine.

In a variety of animals, amphetamine administration produces an increase in locomotor behavior and an induction of repetitive, stereotyped behaviors. There is now considerable evidence to suggest that the induction of stereotyped behaviors is accomplished, in part, by alterations in catecholaminergic transmission in the central nervous system. By recording the spontaneous activity of neurons in the rat brain substantia nigra, reticular formation, basal ganglia, and elsewhere during systemic administration of amphetamine and related drugs, or during administration by means of microinfusions directly into these brain regions, relationships may be drawn between the biochemical and behavioral effects of these drugs and drug-induced changes in neuronal activity in the central nerovous system. Current evidence, for example, suggests that amphetamine produces an inhibition of neuronal activity in the neostriatum and pars compacta of the substantia nigra by means of dopamine released from dopaminergic terminals in the neostriatum and dopaminergic dendrites in the substantia nigra respectively. In addition, current evidence suggests the possibility of a GABA-mediated functional antagonism between excitatory cortical and/or thalamic input to the neostriatum and dopaminergic input from the substantia nigra which could be involved in the apparently mutually exclusive occurrence of amphetamine-induced locomotion and stereotyped behaviors that follow amphetamine administration. Such evidence may also have relevance to a variety of behavioral disorders involving the basal ganglia and catecholaminergic transmission in the central nervous system.

A mechanism for the involvement of colocalized neuropeptides in the actions of antipsychotic drugs.

Evidence has accumulated to implicate neuropeptides localized within midbrain dopamine neurons (cholecystokinin, neurotensin, acetylcholinesterase) in synaptic transmission, mental disease, and pharmacotherapy. We suggest a means by which antipsychotic drugs alter the dynamics between dopamine and colocalized peptides: the intrinsic ability of these agents to stimulate dopamine neuronal activity while blocking dopamine receptors modulates the ratio of catecholaminergic to peptidergic transmission within the mesotelencephalic system. Imbalances of peptide and dopamine cotransmission and their modulation by neuroleptics may be relevant to the pathogenesis and pharmacotherapy of schizophrenia.

Subsensitivity of catecholaminergic neurons to direct acting agonists after single or repeated electroconvulsive shock.

Spontaneous firing rates and changes in firing rate in response to an intravenously administered dose of apomorphine were measured after various electroconvulsive shock (ECS) treatment regimens from dopaminergic cells of the substantia nigra in urethane-anesthetized rats. Similar measurements were obtained from noradrenergic neurons of the locus coeruleus before and after intravenous injection of clonidine. A significant decrement in the inhibition of spontaneous firing in response to intravenous administration of these agonists was observed following multiple or single ECS treatment in both substantia nigra and locus coeruleus cells. There was a consistent but nonsignificant tendency for cells in both areas of the brain from treated animals to display higher rates of spontaneous firing than their respective sham-shocked controls. Both the effects on base-line rates of spontaneous activity and on the depression of firing rate in response to drug administration were found to be independent of repeated treatment. A significant negative correlation was obtained between base-line firing rate and percentage depression to the autoreceptor agonist, but this correlation alone was insufficient to account for the observed differences in the drug response. These results are discussed with respect to possible mechanisms of action of electroconvulsive therapy in the treatment of depression.

Fine structure of rat locus coeruleus.

Locus coeruleus of the rat was studied in material prepared by aldehyde-osmium fixation. Cell bodies of locus coeruleus neurons possess large nuclei with a prominent nucleolus, a homogeneous karyoplasm of moderate density, and occasional indentations of the nuclear membrane. The cytoplasm is rich in organelles, including an extensive network of endoplasmic reticulum which forms well organized Nissl bodies. The highly developed Golgi apparatus surrounds the nucleus and extends into large dendritic trunks. In coronal section, cell bodies appear elongated along an approximate dorso-ventral axis, and most dendrites as well as axons appear in cross-section. In parasagittal sections the cells are very elongate, with dendrites and axons in the neuropil mostly cut longitudinally. Thus, locus coeruleus neurons possess disc-shaped dendritic fields parallel to the anterior-posterior axis of the brainstem, with predominantly longitudinal axo-dendritic synaptic configurations. Presynaptic profiles in locus coeruleus neuropil were classified according to the characteristics of their vesicle populations and other features. The most frequently encountered synaptic ending was characterized by small, round, densely packed synaptic vesicles, and comprised approximately 41% of the total sample of 775 synapses. Another group having large, rounded synaptic vesicles, which could be traced in a number of instances to large myelinated axons, accounted for 20% of the sample. Synaptic endings having large, flattened vesicles were also numerous, comprising 23% of the total. Another category of presynaptic endings was identified as those possessing numerous, small, flattened vesicles and comprising about 11% of the sample. Presynaptic endings having many vesicles of mixed sizes accounted for 2% of the total, and another group of the same proportion having small, rounded synaptic vesicles but also an unusually large number of larger, dense-cored vesicles was also present. Two other categories of synaptic endings were encountered, each comprising less than 1% of the total. One of these was derived from small, unmyelinated axons and contained clusters of pleomorphic synaptic vesicles. The other consisted of dendro-dendritic synapses between locus coeruleus neurons and also displayed small clusters of pleomorphic synaptic vesicles near the zone of synaptic apposition. Quantitative analysis revealed that most afferents to the nucleus synapse onto dendrites ranging between 0.5 and 2.5 micrometers in diameter and onto spine-like appendages derived from somata and dendrites. There were no significant differences between different categories of afferent terminals and their spatial distribution onto various postsynaptic targets of locus coeruleus neurons.

Monoaminergic presynaptic axons and dendrites in rat locus coeruleus seen in reconstructions of serial sections.

Locus coeruleus was studied in rats pretreated with intraventricularly administered 5-hydroxydopamine 1/2 to 3 hours prior to conventional aldehydeosmium fixation. Presynaptic profiles in locus coeruleus neuropil were classified according to the characteristics of their vesicle populations and other features, as in our previous report. Similar categories of endings were observed, and the sites of postsynaptic innervation were identical to those described previously, that is, a majority of synapses were made with dendrites between 0.5 and 2.5 micrometers in cross-sectional diameter, a significant proportion was seen ending onto somatic and dendritic spines, with a relative paucity of synapses made with spine-free membrane of somata and large dendritic trunks. There were no significant differences between different morphological categories of afferent terminals and their spatial distribution onto various postsynaptic targets on locus coeruleus neurons. In addition to various membrane-bound compartments of the cytoplasm, three categories of synaptic endings were labelled by the synaptic marker, while all others were unlabelled. One of these was identified previously as containing small, rounded synaptic vesicles and an unusually large number of large, dense core vesicles. The synaptic vesicles were lightly labelled with scattered, small, eccentrically placed opaque cores inside the synaptic vesicles, apparently randomly distributed throughout the terminal. This terminal is thought to be serotonergic. A second category of labelled synapse has been previously identified as that derived from small, unmyelinated axons having clusters of pleomorphic synaptic vesicles in which the vesicles are heavily labelled by 5-hydroxydopamine. These are believed to represent catecholaminergic synaptic endings derived from recurrent collaterals as well as extrinsic catecholaminergic innervation of locus coeruleus. A final category of heavily labelled profile was identified as presynaptic dendrites, which, along with recurrent catecholaminergic axon collaterals, probably play an important part in the intrinsic regulation of nucleus locus coeruleus. When 59 labelled synapses were examined in adjacent serial sections, every vesicle-containing profile was associated with a synaptic contact having characteristic membrane specializations. A similar result was obtained when 132 other unlabelled terminals of different categories were examined in serial sections.

Fine structure and synaptic connections of the common spiny neuron of the rat neostriatum: a study employing intracellular inject of horseradish peroxidase.

Medium-sized spiny neurons of the neostriatum, identified by intracellular injection of horseradish peroxidase, were examined at both light and electron microscopic levels. These neurons were characterized by their heavy investment of dentritic spines, beginning about 20 micron from the soma and continuing to the tips of the dendrites. Their axons arose from the soma or from a large dendritic trunk very near the soma, and tapered rapidly to form a main axonal branch from which arose several smaller initial collaterals. These arborized extensively throughout an area of about the same size as, and highly overlapping with, the dendritic field of the cell, while the main axon could be followed for distances of up to 1 mm in the direction of the globus pallidus. Three major synaptic types were seen in contact with spiny neurons. Boutons containing small round synaptic vesicles formed synapses exclusively with spiny regions of the dendrites, and most of these were axo-spinous. Small, very pleomorphic synaptic vesicles characterized a second bouton type of unknown origin, which made contacts with somata, initial segments, and dendrites, but not dendritic spines. Boutons containing large pleomorphic synaptic vesicles had the most widespread distribution, contacting all regions including dendritic spines. Spines receiving these contacts also were postsynaptic to boutons containing small round vesicles. Axon collaterals of spiny cells formed synapses with large pleomorphic vesicles and made synapses with somata, initial segments of axons, dendrites, and dendritic spines of striatal neurons, including other spiny cells.

Quantitative Golgi study of anatomically identified subdivisions of motor thalamus in the rat.

A Golgi study of neurons in the ventroanterior-ventrolateral complex (VAL) and ventromedial (VM) nucleus in the dorsal thalamus of rats was performed. To facilitate the delineation of subdivisions of these nuclei, some animals received injections of horseradish peroxidase (HRP) into the afferent and efferent fields of VAL and VM, and alternate sections were processed for the histochemical detection of HRP. As an adjunct to subjective observations, a multivariate statistical analysis of morphometric variables was performed to provide an objective assessment of neuronal morphology. All Golgi-stained neurons in VAL and VM were tentatively identified as projection neurons; no cells with morphological features commonly ascribed to thalamic interneurons were impregnated. Four classes of morphologically distinct neurons were identified in VAL. Type 1 neurons, the most commonly impregnated cell, were found throughout the extent of VAL and resembled "tufted" or "multipolar bush" neurons described previously in many thalamic nuclei. The remaining three neuronal types differed in a number of morphometric parameters and were differentially distributed throughout VAL. Type 2 neurons, distinguished in part by dendritic spine morphology and elongated bipolar dendritic fields, were found only in the rostral sector of the dorsal division of VAL (VALD). Type 3 neurons, characterized by a large and evenly distributed dendritic field, were situated in rostral VAL (all subdivisions). Type 4 neurons had small soma and dendritic dimensions and were located in the ventromedial aspect of the ventral division of VAL (VALV) adjacent to VM. In contrast, the vast majority of neurons in VM were considered to be a single morphological class (similar in form to type 4 neurons in VAL), although a rarely impregnated second type of neuron was also observed. The apparent scarcity of interneurons in VAL and VM is consistent with previous evidence that the synaptic organization of motor thalamus in the rat is markedly different from that of higher-order mammals. Speculation about the functional attributes of the neuronal types in VAL and VM is necessarily restricted to considerations of afferent and efferent relations, since "motor modality" functions of neurons in these nuclei have yet to be elucidated.

The substantia nigra of the rat: a Golgi study.

Three variants of the Golgi method were employed to examine the cell types, their dendritic fields and organization and axonal trajectories within the substantia nigra of albino and hooded rats. In both sagittal and coronal sections, large, medium and small neurons were classified on the basis of soma size, extent of dendritic fileds and dendritic caliber. In general nigral cells have three to five primary dendrites that branch relatively infrequently. Some dendrites of all cell types have thinly scattered spines or varicosities. Small cells, found in all areas of the nucleus, have thin dendrites and small, nondirectional dendritic fields. These are considered to be interneurons. The medium cells found in pars compacta, presumed to be the dopaminergic cells of the nigroneostriatal pathway, send long dendrites into pars reticulata perpendicular to the course of pars compacta. In addition, these cells have a number of dendrites which remain in pars compacta. These cells have axons that run medio-dorsally. No axon collaterals were detected. Both large and medium cells are found in pars reticulata. Cells in the dorso-medial aspect of pars reticulata orient rostro-caudally and roughly perpendicular to the course of pars compacta, while cells in the peripeduncular area show a strict orientation which is parallel to the crus cerebri. Some pars reticulata cells emit axon collaterals while others remain unbranced for their observable lenght. Both large and medium cells are also seen in pars lateralis. These cells send long dendrites ventrally into pars reticulata where they run parallel to the crus cerebri, while some shorter dendrites remain in pars lateralis. In total, the substantia nigra appears to have a layered organization: the superior layer is the cellular pars compacta, the second is the dorso-medial area of pars reticulata where both pars compacta and pars reticulata dendrites run rostro-caudally and dorso-ventrally and the third layer is the peripeduncular region where dendrites from all areas run parallel to the crus cerebri.

Amphetamine alteration of amplitude and timing of cortical-neostriatal interactions.

Amphetamine (0.1 to 5.0 mg/kg, IV) altered frontal cortex stimulation evoked neostriatal potentials in rats. The amplitude of wave P1, which corresponds to an initial intracellular excitatory postsynaptic potential, was reduced, as was the latency to wave N3, which corresponds to the late rebound depolarization. Repetitive electric stimulation of the mesencephalic reticular formation at low currents (0.05 to 0.5 mA, 0.2-ms duration, 60 Hz square waves) produced similar effects. The peripherally acting sympathomimetics, norepinephrine (3 and 10 micrograms/kg, IV) and vasopressin (10 mU/kg, IV), increased blood pressure but did not alter the neostriatal evoked response. In rats with medial thalamic lesions induced by kainic acid, wave N3 was eliminated, and the effects of amphetamine and mesencephalic reticular formation stimulation on neostriatal evoked responses were reduced or eliminated. Thus many of the effects of systemic amphetamine on frontal cortex evoked neostriatal potentials may be mediated via extrastriatal sites, including the mesencephalic reticular formation and the medial thalamus.

Modulation of dopaminergic terminal excitability by D1 selective agents.

The effect of the active R (+) enantiomer of the dopaminergic selective D1 agonist 1-Phenyl-2,3,4,5-tetrahydrol-(1H)-3-benzazepine-7,8-diol HCL (R-SKF 38393) was examined on the excitability of antidromically identified nigro-striatal dopaminergic neurons. Striatal infusions of R-SKF 38393 produced a decrease in terminal excitability, which was reversed by subsequent infusion of the Dopaminergic D1 selective antagonist R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-o l-HCL (SCH 23390). The actions of these D1 specific agents are localized to the terminal fields of the dopaminergic neurons and are not nonspecific effects on the axonal membrane since application along the axons of these cells in the medial forebrain bundle produced no change (R-SKF 38393) or only a slight decrease (SCH 23390) in excitability. The results suggest that the terminal excitability of antidromically identified nigro-striatal dopaminergic neurons can be modified with specific Dopamine D1 receptor agents via a receptor mediated mechanism. Previous studies from this laboratory have demonstrated that the electrical excitability of nigro-striatal dopaminergic terminals is reduced by the dopaminomimetics apomorphine and amphetamine and is increased by the dopamine antagonists haloperidol, fluphenazine and sulpiride (Groves, Fenster, Tepper, Nakamura, and Young 1981; Tepper, Nakamura, Young and Groves 1984). Since, with the exception of sulpiride, these compounds affect both the D1 and D2 subclasses of dopamine receptors, it can not be concluded from these reports, which, if only one, of these receptors mediates the effect on terminal excitability.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of dopaminergic terminal excitability by D1 selective agents: further characterization.

We have previously shown that stimulation of striatal D1 receptors affects dopaminergic nigrostriatal terminal excitability, which is thought to be an index of biophysical events resulting from the activation of receptors on the presynaptic membrane. The experiments presented here further examine the locus and bases of these D1 effects in the rat. We now report that striatal administration of the D1 receptor selective antagonist R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazapine+ ++-7-ol-HCl (SCH 23390) produces a paradoxical agonist-like decrease in dopaminergic terminal excitability. This effect is blocked by pretreatment with the dopamine synthesis inhibitor, alpha-methyl-paratyrosine, suggesting that the action of SCH 23390 is dependent upon endogenous dopamine. Further, haloperidol pretreatment also prevents the SCH 23390-induced decrease in terminal excitability, confirming that dopamine, acting through a dopamine receptor, is responsible for this agonist-like action. Striatal application of the active R-(+) enantiomer of the dopaminergic D1-selective agonist 1-phenyl-2,3,4,5-tetrahydrol-(1H)-3-benzazepine-7,8-diol-HCl (R-SKF 38393) decreases terminal excitability in the alpha-methyl-paratyrosine pretreated animal, indicating that dopamine is not required for the agonist action. In an effort to ascertain the presynaptic or postsynaptic location of these actions, an extensive destruction of postsynaptic neurons in the neostriatum was produced by local administration of the neurotoxin, kainic acid. It was observed that the neurotoxin-induced neostriatal neuronal loss did not disrupt the action of R-SKF 38393 nor its reversal by SCH 23390.(ABSTRACT TRUNCATED AT 250 WORDS)

A GABA immunocytochemical study of rat motor thalamus: light and electron microscopic observations.

A light and electron microscopic study of GABA-immunoreactive neurons and profiles in the ventroanterior-ventrolateral and ventromedial nuclei of rat dorsal thalamus was conducted using antiserum raised against GABA. Less than 1% of the neurons in these motor-related nuclei exhibited GABA immunoreactivity, confirming previous reports that these nuclei are largely devoid of interneurons. Immunoreactive neurons in the ventral anterior-ventral lateral complex and ventromedial nucleus were bipolar or multipolar in shape, and tended to be smaller than non-immunoreactive neurons. GABA immunoreactivity in the neuropil consisted of labeled axon terminals and myelinated and unmyelinated axons, and was lower in the ventral anterior-ventral lateral complex and ventromedial nucleus than in neighboring thalamic nuclei. The density of neuropil immunolabeling was slightly higher in ventral anterior-ventral lateral complex than in ventromedial nucleus. GABA-immunoreactive axon terminals, collectively termed MP boutons for their medium size and pleomorphic vesicles (and corresponding to "F" profiles of some previous studies of thalamic ultrastructure), formed symmetric synapses and puncta adhaerentia contacts predominantly with large and medium-diameter (i.e. proximal) non-immunoreactive dendrites. Approximately 12 and 18% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, were GABA-immunopositive. Many of these immunoreactive profiles probably arose from GABAergic neurons in the thalamic reticular nucleus, substantia nigra pars reticulata and entopeduncular nucleus. Two types of non-immunoreactive axon terminals were distinguished based on differences in morphology and synaptic termination sites. Boutons with small ovoid profiles and round vesicles that formed prominent asymmetric synapses onto small-diameter dendrites were observed. Mitochondria were rarely observed within these boutons, which arose from thin unmyelinated axons. These boutons composed approximately 82 and 74% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, and were considered to arise predominantly from neurons in the cerebral cortex. In contrast, boutons with large terminals that contained round or plemorphic vesicles and formed multiple asymmetric synapses predominantly with large-diameter dendrites were also observed. Puncta adhaerentia contacts were also common. Mitochondria were numerous within large boutons with round vesicles, which arose from myelinated axons. Many of the large boutons were likely to have originated from neurons in the cerebellar nuclei. Approximately 6% of the boutons in the ventral anterior-ventral lateral complex and 8% in ventromedial nucleus were of the large type.(ABSTRACT TRUNCATED AT 400 WORDS)

Striatal cells containing aromatic L-amino acid decarboxylase: an immunohistochemical comparison with other classes of striatal neurons.

In a previous study, we described a population of striatal cells in the rat brain containing aromatic L-amino acid decarboxylase, the enzyme involved in the conversion of L-DOPA into dopamine. We have also presented evidence that these cells produce dopamine in the presence of exogenous L-DOPA. In this paper, we further characterize these striatal aromatic L-amino acid decarboxylase-containing cells in order to determine whether they form a subclass of one of the known categories of striatal neurons or if they represent a novel cell type. Using immunohistochemical methods, we compared the morphology and distribution of the aromatic L-amino acid decarboxylase-immunolabeled cells with those of other classes of striatal neurons. Our results show that both the morphology and distribution of aromatic L-amino acid decarboxylase-immunolabeled cells are very distinctive and do not resemble those of cells labeled for other striatal neuronal markers. Double-labeling procedures revealed that aromatic L-amino acid decarboxylase cells do not co-localize somatostatin or parvalbumin, and only a very small percentage of them co-localize calretinin. However, the population of aromatic L-amino acid decarboxylase cells label intensely for GABA.Overall, our results suggest that these aromatic L-amino acid decarboxylase-containing cells represent a class of striatal GABAergic neurons not described previously.

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.

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: light and electron microscopy.

The morphology and synaptic organization of neurons in the ventroanterior-ventrolateral nucleus of rats was examined using in vivo intracellular staining techniques. Neurons were characterized electrophysiologically based on intrinsic membrane properties and synaptic responses to stimulation of motor cortex and cerebellar nuclei, as described in the companion paper. Cerebellar-responsive neurons were stained intracellularly with either horseradish peroxidase or biocytin. All stained ventroanterior-ventrolateral nucleus neurons were identified as thalamocortical neurons on anatomical (and often electrophysiological) grounds, consistent with previous findings that rat ventroanterior-ventrolateral nucleus is interneuron-sparse. Ventroanterior-ventrolateral nucleus neurons had three to eight thick primary dendrites. Proximal dendrites often exhibited a tufted branching pattern, from which many thinner, higher order dendrites arose. Dendrites branched to form a funnel-like infiltration of the neuropil that resulted in a spherical, roughly homogeneous dendritic field. The axon originated from the cell body or a proximal dendrite and coursed laterally and dorsally to innervate motor cortex. One to five axon collaterals were emitted in the rostral dorsolateral sector of the thalamic reticular nucleus; collaterals were not observed in the ventroanterior-ventrolateral nucleus or other nuclei in dorsal thalamus. The synaptic organization of the ventroanterior-ventrolateral nucleus was examined with electron microscopy, including two intracellularly labeled ventroanterior-ventrolateral nucleus neurons that were shown electrophysiologically to receive monosynaptic inputs from the cerebellum. The neuropil of rat ventroanterior-ventrolateral nucleus lacked the complexity and diversity found in corresponding thalamic nuclei of felines and primates, due to the paucity of interneurons. Vesicle-containing dendrites, dendrodendritic synapses and glomeruli were not observed. Three broad classes of presynaptic terminals were identified. (1) Small round boutons: small boutons containing densely-packed, small round vesicles that formed asymmetric synapses predominantly with the distal dendrites of thalamocortical neurons. These were the most prevalent type of bouton in the ventroanterior-ventrolateral nucleus (78% of presynaptic elements) and likely arose from the cerebral cortex. (2) Large round boutons: large terminals with loosely packed small round vesicles that made multiple asymmetric synapses with proximal and intermediate dendrites. Large round boutons comprised 8% of the neuropil, and likely arose from the cerebellar nuclei. (3) Medium size boutons with pleomorphic vesicles: medium-sized profiles containing pleomorphic vesicles that formed symmetric synapses with proximal, intermediate and distal dendrites and, less frequently, with cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in noradrenergic terminal excitability induced by amphetamine and their relation to impulse traffic.

The effects of amphetamine upon the terminal excitability of noradrenergic neurons of the nucleus locus coeruleus were studied in urethane anesthetized rats. Terminal excitability was measured by determining the stimulus currents necessary to evoke antidromic responses in locus coeruleus neurons from terminals in the frontal cortex. In most cases, terminal excitability was decreased following local infusion of amphetamine into the frontal cortex, while intravenous administration of the drug tended to increase terminal excitability. The decreased terminal excitability induced by local infusion of amphetamine appeared to be due to activation of alpha-adrenergic receptors located on the terminals of locus coeruleus neurons, since this effect mimics that of clonidine, a direct acting alpha-adrenergic agonist, and since the effect was abolished by pretreatment with alpha-methyl-p-tyrosine which disrupts the catecholamine liberating properties of amphetamine. Phentolamine, a direct acting alpha-adrenergic receptor antagonist was also found to block or reverse the effect of amphetamine. The changes in terminal excitability following intravenous injection of amphetamine appeared to be related to changes in the spontaneous activity of locus coeruleus neurons. A large decrease in spontaneous activity following intravenous administration of amphetamine was associated with increased terminal excitability, whereas when smaller changes in spontaneous activity occurred, terminal excitability was found to be decreased. These results are discussed with respect to the pharmacological properties of catecholaminergic neurons and the mechanisms of action of amphetamine.

Cholinergic neurons are distributed preferentially in areas rich in substance P-like immunoreactivity in the caudate nucleus of the adult cat.

The distribution of cells stained immunocytochemically for the cholinergic marker choline acetyltransferase was compared to the pattern of substance P immunoreactivity in the caudate nucleus of adult cats using a double-label immunocytochemical protocol and three-dimensional reconstructions of adjacent sections single-labeled for either substance P or choline acetyltransferase. Substance P immunoreactivity was distributed in a highly complex mosaic within the caudate nucleus of the cat. In the dorsal caudate nucleus, substance P-rich zones consisting of either clusters of substance P-positive cell bodies or fibers were seen against a lighter staining background. The density of cholinergic neurons was found to be significantly greater within these substance P-rich patches in comparison to surrounding regions. The pattern of substance P immunoreactivity within the ventral caudate nucleus differed from that in more dorsal regions. Clear substance P-rich patches were not seen in this region, but a large substance P-rich area consisting of a dense plexus of substance P-containing fibers was visible. Embedded within this substance P-rich area were fairly discrete patches of light substance P staining. As in the dorsal caudate nucleus, increased numbers of cholinergic neurons and processes were associated with substance P-rich regions in the ventral caudate nucleus. Choline acetyltransferase-positive perikarya also appeared to be concentrated in substance P-rich areas in the nucleus accumbens and olfactory tubercle. The results of this study suggest that a close relationship exists between the distribution of substance P fibers and cholinergic perikarya in the striatum of the cat.

Electrophysiological characteristics of cells within mesencephalon suspension grafts.

Both spontaneous and evoked extracellular electrophysiological activity of neurons within fetal mesencephalon suspension grafts to the dopamine-depleted striatum of rats were examined. In some cases, extracellular recording was combined with intracellular labeling to identify recorded neurons. Grafted rats displaying a complete cessation of ipsilateral rotations following amphetamine administration were examined at post-implantation time intervals of two, four, five, eight and nine months. Four separate classes of neurons were distinguished within the transplanted striatum based on electrophysiological properties. The first of these groups, the type I cells, appeared to be non-grafted striatal neurons. When spontaneously active, these striatal-like cells fired bursts of action potentials separated by periods of decreased activity. Evoked responses in these cells were characteristic of striatal cells. Type I cells which were intracellularly labeled were found outside the grafts and displayed the characteristic morphology of the medium spiny neuron of the neostriatum. The other three cell classes displayed electrophysiological properties similar to neurons recorded in situ within the reticular formation, substantia nigra pars compacta and substantia nigra pars reticulata. Neurons from these three groups which were labeled with an intracellular marker were found to lie within the suspension grafts. The spontaneous activity of the pars compacta dopaminergic-like neurons was predominantly irregular, with some cells also firing in a regular or pacemaker-like pattern. Infrequently, irregular firing dopaminergic-like neurons displayed episodes of doublet bursting. Many of the grafted neurons responded to electrical stimulation of prefrontal cortex and striatum, indicating that the graft was receiving functional inputs from host neurons. Comparison of the firing rate and pattern of grafted neurons to in situ mesencephalic neurons as a function of time following grafting suggested that the grafted neurons and/or the neuronal circuitry is slowly developing within the host environment. A prolonged time-course for the maturation of the graft may be reflected in the time required to achieve improvements in some behavioral deficits following transplantation. However, the relatively rapid recovery of drug-induced rotational asymmetry following grafting suggests that this form of recovery may not require mature functioning of the grafted neurons.

Glutamate-dependent long-term presynaptic changes in corticostriatal excitability.

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

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: in vivo electrophysiology.

In vivo intracellular recordings were obtained from identified thalamocortical neurons in the ventroanterior-ventrolateral complex in urethane-anesthetized rats. This thalamic nucleus has few interneurons. Neurons that responded to cerebellar stimulation were injected intracellularly with horseradish peroxidase or biocytin and examined with light and electron microscopy (see companion paper). Intrinsic membrane properties and voltage-dependent rhythmic activity of cerebellar-responsive ventroanterior-ventrolateral neurons were similar to those described previously for thalamic neurons. Thus, in addition to conventional "fast" Na(+)-dependent spikes, rat ventroanterior-ventrolateral neurons had "slow" Ca(2+)-mediated low-threshold spikes and membrane conductances that supported rhythmic oscillations. Two modes of spontaneous activity were observed: (i) a tonic firing pattern that consisted of irregularly occurring fast spikes that predominated when the membrane potential was more positive than about -60 mV, and (ii) a rhythmic firing pattern, observed when the membrane potential was more negative than about -65 mV, composed of periodic (4-8 Hz) membrane hyperpolarizations and ramp depolarizations that often produced a low-threshold spike and a burst of fast spikes. In some neurons, spontaneous fast prepotentials were also observed, often with a relatively constant rate (up to 70 Hz). Cerebellar stimulation elicited excitatory postsynaptic potentials that in some cases appeared to be all-or-none and were similar in form to fast prepotentials. Stimulation of ipsilateral motor cortex elicited a short-latency antidromic response followed by a monosynaptic excitatory postsynaptic potential, which had a slower rise time than excitatory postsynaptic potentials evoked from cerebellum, suggesting that cortical inputs were electrotonically distal to cerebellar inputs. In the presence of moderate membrane hyperpolarization, the cortically evoked excitatory postsynaptic potential was followed by a long-lasting hyperpolarization (100-400 ms duration), a rebound depolarization and one or two cycles resembling spontaneous rhythmic activity. Membrane conductance was increased during the initial component of the long hyperpolarization, much of which was probably due to an inhibitory postsynaptic potential. In contrast, membrane conductance was unchanged or slightly decreased during the latter three-quarters of the long hyperpolarization. The amplitude of this component of the long hyperpolarization usually decreased when the membrane was hyperpolarized with intracellular current injection. Thus, both disfacilitation and an inhibitory postsynaptic potential may have contributed to the latter portion of the cortically-evoked long hyperpolarization. The cortically-evoked inhibitory postsynaptic potentials likely originated predominantly from feedforward activation of GABAergic neurons in the thalamic reticular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)