Electron microscopic study of the substantia nigra and the strio-nigral projection in the rat

GABAergic nerve terminals decrease in the substantia nigra following hemitransections of the striatonigral and pallidonigral pathways

Glutamic acid decarboxylase (GAD), the enzyme that synthesizes the neurotransmitter, GABA, was immunocytochemically localized in axon terminals as well as in small and medium-sized neurons of the rat substantia nigra. The pattern formed by GAD-containing axon terminals with the dendrites and somata of neurons in the substantia nigra was altered following ipsilateral hemitransections of the striatonigral and pallidonigral pathways. A marked reduction of GAD-positive terminals occurred throughout this brain region, but the ventral fifth of the pars reticulata showed a nearly normal pattern of GAD-positive axon terminals. The results of this investigation are consistent with results from biochemical studies which have indicated that the striatonigral and/or pallidonigral pathways are GABAergic. In addition, these results suggest that the residual GABAergic terminals remaining after hemitransection are derived from intrinsic neurons of the substantia nigra.

Pallidosubthalamic projection in the cat. Electron microscopic study

The degenerative changes within the cat's subthalamic nucleus (Sth) following lesions of the external pallidum were studied by electron microscopy. Four to five days following pallidal lesions a great number of terminals undergoing degenerative changes were encountered in the ipsilateral Sth. The contralateral Sth was free of degeneration. The degenerating terminals show predominantly the light degenerative type, less frequently the dark degenerative pattern, and occasionally exhibit signs of filamentous hyperplasia. The degenerated boutons usually insert on perikarya of the large Sth neurons, on proximal dendrites, and more rarely contact dendritic spines. They were observed neither to perform synaptic contacts with the perikarya of the small Sth neurons nor with other vesicle-containing profiles. On the basis of the ultrastructural aspect of the degenerating terminals, they were identified as F1 terminals, discriminated in a previous study (Romansky et al., 1978). The normal appearance, the synaptic relationships, and the degenerative features of the F1 terminials in the Sth closely resemble the entopeduncular terminals in the thalamus described by Rinvik and Grofová (1974a), and Grofová and Rinvik (1974). The possible contribution of the interrupted passing fibers to the observed degeneration is discussed. The present findings corroborate the relevant morphological, physiological, neurochemical, and neuropharmacological data in the literature.

Ultrastructural localization of acetylcholinesterase activity in primary cultures of rat substantia nigra

Ultrastructural localization of acetylcholinesterase activity was studied in primary cultures of the substantia nigra microdissected from newborn rat brains. Light microscopic observations were also made on the characteristics of dopamine neurones and acetylcholinesterase containing cells in these cultures. Ultrastructurally acetylcholinesterase activity was localized in the nuclear envelope and rough endoplasmic reticulum of neurones, which had deeply infolded, round or oval nucleus, a prominent Golgi apparatus and varying amounts of rough endoplasmic reticulum. In the neuropil acetylcholinesterase activity was seen within microtubules of neuronal processes and in the rough endoplasmic reticulum of dendrites. The enzyme activity was also demonstrated within the nuclear envelope and rough endoplasmic reticulum of probably capillary endothelial cells. Dopaminergic neurones were identified on the basis of the green catecholamine fluorescence they exhibited. Small dopaminergic neurones could be observed and there was indirect evidence that these cells did not stain for acetylcholinesterase.

Dopamine auto- and postsynaptic receptors: electrophysiological evidence for differential sensitivity to dopamine agonists

The responses of dopamine cells in the substantia nigra to iontophoretically administered dopamine and intravenous apomorphine were compared to the responses of spontaneously active neurons in the caudate nucleus. Dopaminergic cells were six to ten times more sensitive to dopamine and intravenous apomorphine than 86 percent of the caudate cells tested. This differential sensitivity of dopamine auto- and postsynaptic receptors may explain the apparently paradoxical behavioral effects induced by small compared to large doses of some dopamine agonists and may provide a means of developing new types of drugs to antagonize dopaminergic influence in the central nervous system.

Is glycine an inhibitory synaptic transmitter in the substantia nigra?

Nigral tissue accumulated 14C-glycine by an energy-, temperature- and sodium-dependent mechanism; the transport process was inhibited by small neutral amino acids and had an apparent Km of 143 microM and Vmax of 787 nmol/g/min. Re-release of accumulated 14C-glycine was initially extremely rapid (40% in first 5 min) and subsequently accelerated by +40 mM K+ in a Ca2+-dependent manner. Intranigral kainate (0.25 microgram) lowered the levels of striatal DA (63%) and nigral GABA (25%) ipsilaterally, but not nigral glycine. Injections of glycine or strychnine (10--100 micrograms) into one SN induced slow ipsiversive or contraversive turning respectively. The evidence for glycine as a neurotransmitter in SN is discussed.

Ultrastructural observations on intercellular contacts of nigral dendrites

Possible morphological correlates of dendritic release sites for dopamine have been investigated in the rat substantia nigra. Electronmicroscopic and freeze etching studies show that nigral dendrites fail to form dendrodendritic contacts but on the contrary are always separated by glial elements, which in turn are connected by multiple gap junctions. No dendro-axonic synapses are present but large specialized symmetrical junctions could provide a possible morphological specialization for a dopamine transfer from presynaptic dendrites to axon endings.

Synaptosomal uptake and release of dopamine in substantia nigra: effects of gamma-aminobutyric acid and substance P

Nigral tissue prepared as synaptosomes demonstrates both high and low affinity uptake of [3H]dopamine. Recently accumulated [3H]dopamine is releasable by 35 mN K+. Substance P increases both uptake and release of dopamine by nigral synaptosomes; gamma-aminobutyric acid (GABA) inhibits release with no effect on uptake at concentrations less than 10-(4) M. In the striatum, substance P inhibits both uptake and release of dopamine. The results support the existence of dopamine-containing terminals in substantia nigra tissue. The differences in response to substance P and GABA found between nigra and striatum may reflect structural differences in dopamine-containing processes in these areas, related to their proposed origin as dendtritic (substantia nigra) and axonal (striatal) terminal.

Effects of para-chlorophenylalanine on amphetamine and haloperidol-induced changes in striatal dopamine turnover

Para-chlorophenylalanine (PCPA) was administered to rats, and the animals were sacrificed 48 h later. Animals received various doses of S(+)-amphetamine (AMPH) 31 min before sacrifice or of haloperidol (HAL) 46 min before sacrifice and an intracerebral injection of [3H]tyrosine 15 min prior to sacrifice. PCPA pretreatment potentiated the increased accumulation of striatal [3H]dopamine (DA) induced by doses of AMPH to 1.5 mg/kg, and inhibited the decrease in striatal [3H]DA accumulation observed at higher doses (greater than 2.5 mg/kg) of AMPH. Further, the AMPH-induced increase in endogenous levels of striatal DA was inhibited by PCPA pretreatment. Similarly, PCPA pretreatment potentiated the HAL-induced increase in striatal [3H]DA accumulation. The data are discussed with respect to an inhibitory serotonergic influence on nigrostriatal DA function, and the relationship of nigrostriatal biochemical events to AMPH-induced behavioral changes.

The smooth endoplasmic reticulum as a possible storage site for dendritic dopamine in substantia nigra neurones

In dendrites of the substantia nigra neurones the monoamine marker 5-hydroxydopamine injected intracerebrally was localized inside of smooth endoplasmic reticulum cisterns. This observation opens the possibility of the existence of an alternative site for dopamine storage in dendrites as opposed to the well-known vesicular storage.

Sites of action of amphetamine intrinsic to catecholaminergic nuclei: catecholaminergic presynaptic dendrites and axons

1. It is now well established that the behavioral actions of amphetamine, especially locomotor and stereotyped behaviors, are dependent in part upon the release of catecholamines in the central nervous system. 2. A variety of empirical evidence has established that the release of catecholamines by amphetamine leads to changes in neuronal activity in regions postsynaptic to catecholaminergic nerve terminals. In addition, release of catecholamines is accompanied by a marked inhibition of neuronal firing in catecholaminergic neurons. 3. At least two conceptions have been advanced to account for this marked decrease in catecholaminergic neuronal activity. (a) In one of these, release of catecholamines from presynaptic terminals is thought to lead to a compensatory decrease in neuronal firing rate by means of a postsynaptic neuronal feedback loop from regions innervated by catecholaminergic neurons. (b) In the other, decreased neuronal activity is hypothesized to result from local release of catecholamines onto or near catecholaminergic neuronal dendrites and somata, a phenomenon that has been characterized as self-inhibition. 4. For dopaminergic neurons, recent biochemical, neuropharmacological and neuroanatomical evidence suggests that the latter process could be subserved by dopamine released from dopaminergic neuronal dendrites, i.e., "presynaptic" dendrites.

Biochemical and behavioral effects of serotonin neurotoxins on the nigrostriatal dopamine system: comparison of injection sites

Unilateral injections of the serotonin neurotoxin, 5,7-dihydroxytryptamine (DHT), at various points along the 5-HT pathway to the forebrain produce a turning syndrome associated with alterations of dopamine synthesis in the ipsilateral striatum. Unilateral injections of DHT into the SN produced an ipsilateral increase in striatal dopamine (DA) turnover and contralateral rotation in response to amphetamine or apomorphine. Injection of DHT into the MFB produced an ipsilateral decrease in striatal DA turnover and tyrosine hydroxylase (TOH) activity, and ipsilateral rotation in response to amphetamine or apomorphine. After the injection of DHT into the SN or MFB, there was a significant correlation between the rates of drug-induced rotation, the decrease in cortical 5-HT turnover, and the change in striatal DA turnover, suggesting that the unilateral change in DA turnover (and, presumably, the increased stimulation of DA receptors) is causally linked to turning. Injection of DHT into the zones of the striatum and GP richest in 5-HT terminals produced the same responses as the MFB-lesioned rats: ipsilateral rotation and a decrease in striatal TOH activity. Injection of DHT into the area of the striatum richest in DA terminals failed to produce rotation or a significant change in TOH activity. We suggest that 5-HT neurons from the raphe nuclei exert a tonic inhibition on the nigrostriatal pathway at the level of the SN through direct synapses on DA neurons, whereas their neostriatal terminals have an indirect effect on DA terminals, perhaps via interaction with cholinergic and GABA-ergic neurons.

Effect of p-chloroamphetamine and 5,7-dihydroxytryptamine on rotation and dopamine turnover

Injections of p-chloroamphetamine (PCA) or 5,7-dihydroxytryptamine (DHT) (after pretreatment with desmethylimipramine) into the median raphe nucleus (MRN) caused depletions of 5-HT and 5-HIAA in the cortex and striatum, and a decrease of cortical 5-HT uptake without affecting NE uptake. Unilateral injections of these neurotoxins into the MRN caused contralateral rotation, which was blocked by haloperidol. The size of the lesion correlated with the rate of rotation and the decrease in 5-HT turnover in the cortex. We also found a significant correlation between the rate of rotation and the decrease in cortical 5-HT turnover, and the increase in striatal DA turnover. Moreover, there was a significant correlation between the decrease in cortical 5-HT turnover and the increase in striatal DA turnover. It was found that injections of DHT into the SN produced similar behavioral and biochemical changes as did the MRN lesions. In this model, amphetamine and apomorphine produce turning in the same direction, whereas they have opposite effects after lesions of the nigrostriatal pathway where postsynaptic DA supersensitivity occurs. Presynaptic changes appear to determine turning in this model. An inhibitory role of the serotonergic MRN on the dopaminergic nigrostriatal pathway mediated via the substantia nigra (SN) is suggested.

Behavioral effects of unilateral basal gangliar lesions in neonatal rats

Two-day-old rats were given unilateral lesions of the caudate-putamen, globus pallidus, or hippocampus and their postural orientation was observed for 30 min immediately after surgery. All groups with damage to the caudate-putamen and/or globus pallidus showed a significant postural deviation toward the side ipsilateral to the lesion. At this age, neurogenesis in the basal ganglia is complete but synapses, neurotransmitters, and enzymes for neurotransmitter synthesis are far below adult levels. These results indicate that the basal ganglia have a role in motor function prior to their full maturation as measured by neuroanatomical and biochemical techniques.

Circling behaviour in the rat following unilateral injections of p-chlorophenylalanine and ethanolamine-O-sulphate into the substantia nigra

Circling behaviour following the unilateral intranigral injection of the gamma-aminobutyric acid (GABA) transaminase inhibitor ethanolamine-O-sulphate, (EOS), or the tryptophan hydroxylase inhibitor p-chlorophenylalanine (pCPA) was compared, both for spontaneous circling and for circling induced by apomorphine administration. EOS-injected rats exhibited spontaneous contralateral rotation for up to 3 h after such injections. pCPA-injected rats showed only a slight ipsilateral postural asymmetry, both groups of rats elicited ipsilateral rotation upon apomorphine challenge. Increased ipsilateral dopamine release within the corpus striatrum accompanied this response. It is proposed that GABA and 5-HT may be closely related in modifying the nigrostriatal impulse flow.

Monoaminergic synapses, including dendro-dendritic synapses in the rat substantia nigra

Intraventricular administration of 1 or 2 mg of the osmiophilic "false transmitter" 5-hydroxydopamine (5-OHDA) was used to label monoamine storage and release sites in the rat substantia nigra. Vesicles containing unusually dense cores indicative of the presence of the marker were seen forming from the Golgi apparatus in the cell bodies of medium-sized neurons of the substantia nigra, pars compacta, and from smooth endoplasmic reticulum in the dendrites of those neurons and in small unmyelinated axons of unknown origin. In serial sections, both axons and dendrites containing synaptic vesicles marked with 5-OHDA were seen to form synapses "en passage" in pars compacta, and some presynaptic dendrites containing vesicles filled by the marker were also observed to form contacts with dendrites in pars reticulata. The only identified postsynaptic elements engaging in monoaminergic synapses in the substantia nigra were dendrites of medium-sized pars compacta neurons.

Evidence concerning the anatomical location of the dopamine stimulated adenylate cyclase in the substantia nigra

The dopamine (DA)-sensitive adenylate cyclase in the substantia nigra was assayed in rats which had been subjected to 3 different kinds of brain lesion: (1) unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle; (2) unilateral lesions of the descending strio-nigral and pallido-nigral projections; (3) total lesions of the serotoninergic raphe-nigral pathway. Lesions of the medial forebrain bundle causing 97% depletion of striatal DA, 72% depletion of nigral tyrosine hydroxylase, and no change in nigral glutamate decarboxylase (GAD), resulted in no change in basal or DA-stimulated cyclic AMP production ipsilateral to the injection. Lesions of the globus pallidus, causing 70% and 79% reductions in GAD and substance P respectively in the ipsilateral nigra, produced a reduction in basal cyclic AMP production and abolished the normal increase in cyclic AMP produced by DA on the side of the lesion. Lesions to the dorsal and median raphe nuclei did not affect the normal DA-sensitive adenylate cyclase response in the nigra. The results suggest that one of the neurotransmitter functions of DA in this brain region may be to modulate the release of psi-aminobutyric acid (GABA) or substance P from synaptic terminals afferent to the nigra.

Effects of peripheral and local administration of picrotoxin on the release of newly synthesized 3H-dopamine in the caudate nucleus of the cat

The release of 3H-DA was estimated in superfusates of a superfusion cannula introduced into the caudate nucleus of "encéphale isolé" cats. 3H-DA was continuously formed from L-3,5-3H-tyrosine introduced into the superfusion medium. Injected at the periphery, picrotoxin (2.5 mg/kg, i.p.) markedly enhanced the release of 3H-DA, the effect being particularly pronounced 90 min after the drug injection. The effects of the local introduction of picrotoxin into the substantia nigra and into the caudate nucleus on 3H-DA release were also examined to ensure the specificity of the result. Picrotoxin (10(5) M) added into the superfusing medium of a second superfusion cannula introduced in to the substantia nigra enhanced by 2 to 5 times the release of 3H-DA during the drug application. No effect could be seen when the tip of the superfusion cannula was not exactly localized into the substantia nigra. Although less pronounced, a stimulation of 3H-DA release was also seen when picrotoxin(10(6) M) was added directly into the superfusion medium of the cannula introduced into the caudate nucleus. These results suggest that some gabaergic neurons are involved in the control of DA release by acting on dopaminergic cell bodies or dendrites into the substantia nigra. Other gabaergic neurons may directly or indirectly act on dopaminergic terminals within the caudate nucleus.

Immunocytochemical localization of glutamate decarboxylase in rat substantia nigra

L-Glutamate decarboxylase (GAD, EC 4.1.1.15), the enzyme which catalyzes the alpha-decarboxylation of L-glutamate to form gamma-aminobutyric acid (GABA), was localized both light and electron microscopically in rat substantia nigra by an immunoperoxidase method. Large amounts of GAD-positive reaction produce were seen throughout the substantia nigra in light microscopic preparations, and it appeared to be localized in punctate structures that were apposed to dendrites and somata. Electron microscopic studies revealed that most of the axon terminals in the substantia nigra were filled with GAD-positive reaction product and formed both axodendritic and axosomatic synapses. Many dendrites were extensively surrounded by GAD-positive terminals which most commonly formed symmetric synaptic junctions, although some formed asymmetric synpatic junctions. The results of this investigation are consistent with biochemical, pharmacological and physiological data which have indicated that neurons of the neostriatum and globus pallidus exert a GABA-mediated, postsynaptic inhibition upon the neurons of the substantia nigra. These findings provide another example in the vertebrate central nervous system where Golgi I projection neurons are inhibitory and use GABA as their neurotransmitter.

Comparison of effects of drugs on dopamine metabolism in the substantia nigra and the corpus striatum of rat brain

The influence of apomorphine, chloral hydrate, haloperidol, morphine, oxotremorine, pargyline, probenecid and promethazine on DOPAC and HVA levels was studied in the substantia nigra (including the ventral tegmental dopaminergic regions) and corpus striatum of the rat brain. The time--effect curves of changes in HVA levels after pretreatment with apomorphine, haloperidol, morphine, oxotremorine or promethazine are presented. The time--effect curves for the substantia nigra showed an initial rapid HVA rise, which was not observed in the corpus striatum. Promethazine treatment caused a small but significant HVA rise in the substantia nigra only. Chloral hydrate, morphine and oxotremorine induced a similar percentage increase in DOPAC and HVA levels in the substantia nigra as well as in the corpus striatum. Haloperidol, however, caused a small percentage change in the metabolite levels in the substantia nigra when compared to the pronounced rise seen in the corpus striatum. The apomorphine-induced HVA decrease observed in both structures provides evidence for the presence of a dopaminergic receptor in the substantia nigra.

Evidence for the existence of a raphe projection to the substantia nigra in rat

The effect of electrical stimulation of the median raphe nucleus on the activity of spontaneously firing single neurones in the substantia nigra and mesencephalic reticular formation (MRF) has been investigated in urethane anaesthetized rats. Depression of activity was the predominant effect observed although this was sometimes accompanied by periods of excitation. Some neurones were only excited. The latency of inhibition of substantia nigra neurones was constant whereas that of MRF neurones was more variable. Microiontophoretically applied 5-hydroxy-tryptamine (5-HT) and dopamine (DA) produced mainly inhibition of neuronal activity, but excitation and biphasic effects were also seen. There was a good correlation between the direction of neuronal responses in the substantia nigra to median raphe stimulation and to the effects of 5-HT but not DA. Discrete electrolytic lesions of the median raphe nucleus were followed by a decrease in 5-HT but not GABA concentrations in the substantia nigra. In addition striatal 5-HT, GABA and NA concentrations were unchanged whereas striatal DA was increased. These observations strongly suggest that the substantia nigra receives a direct inhibitory input from the median raphe nucleus and this pathway uses a 5-HT-like neurotransmitter. This pathway probably contributes to the regulation of nigrostriatal dopaminergic transmission.

The nigrostriatal projection in the cat. Part 1. Silver impregnation study

The course and destination of the degenerating nigrostriatal fibers were studied by selective silver impregnation methods in 37 cats with unilateral lesions in the substantia nigra. The nigrostriatal fibers ascend along the dorsomedial border of the substantia nigra to the prerubral area; they proceed for a short distance through the lateral hypothalamus, enter the medial part of the internal capsule and run in a dorsorostral direction to reach the head of the caudate nucleus and the rostral portion of the putamen. A smaller number of degenerating fibers obliquely cross the peduncular part of the internal capsule and traverse the entopeduncular nucleus and the pallidum to terminate in the central and caudal portions of the putamen. Some features of the topical distribution of the nigrostriatal tract are described. Apparently, the more anterior part of the pars compacta sends axons primarily to the head of the caudate nucleus and to the most rostral putamen. The most medially situated nigral neurons project to the fundus striati. The posteromedial cell groups of the pars compacta innervate primarily the central putamen and the caudal part of the caudate nucleus. The projection of the lateral cell group of the posterior zona compacta to the caudal putamen is sparser than from the other nigral groups, suggesting that a part of them has another destination, possibly lower in the neuraxis. The contribution of the pars of reticulata to the nigrostriatal connections seems to be modest, according to the small number of neurons; they project to the lateral caudate and putamen. Thus, the ascending nigrostriatal fibers mirror the distribution of the descending striatonigral fibers. No convincing evidence for the existence of a nigroentopeduncular and nigropallidal projection was found.

The discrimination of nine different types of synaptic boutons in the fundus striati (Nucleus accumbens septi)

An attempt has been made to discriminate additional types of synapses than have been previously described in the nucleus accumbens septi of the cat, which can, according to Brockhaus (1942), justifiably be termed the fundus striati due to the fact that it possesses all of the morphological and some of the neurochemical features of the striatum. This was undertaken in order to correlate at least one type of synapse with each different afferent pathway. Nine distinct types of synapses could be differentiated electron microscopically: Type I: axo-spinous synapses with sparse, small, round vesicles which seemed to be the nigro-striatal endings (35%). Type II: axo-somatic or axo-dendritic en passant synapses containing small, round vesicles (3%). Type III: axo-spinous synapses filled with densely-packed, small, round vesicles displaying strong postsynaptic thickenings which seem to be cortico-striatal (17%). Type IV: large axo-spinous synapses with densely-arranged, small, round vesicles contacting larger spines branching off a pedicle (9%). Type V: axo-somatic or axo-dendritic synapses containing large pleomorphic vesicles, probably axon collaterals (1%). Type VI: axo-somatic or axo-dendritic synapses with elongated small vesicles (20 X 45 nm) (3%). Type VII: large axo-somatic or axo-dendritic synapses filled by densely-packed, small, round vesicles (11%). Type VIII: large axo-somatic or axo-dendritic synapses containing loosely-arranged, small, round vesicles (8%). Type IX: axo-somatic or axo-dendritic synapses containing large, round vesicles in a translucent axoplasm (13%).