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

Correlation between dopaminergic neurons, acetylcholinesterase and nicotinic acetylcholine receptors containing the α3- or α5-subunit in the rat substantia nigra

The aim of this study was to investigate the relationship between the cells possessing the alpha3 or alpha5 nicotinic acetylcholine receptor subunits and the enzyme acetylcholinesterase, with respect to tyrosine hydroxylase immunoreactive dopaminergic neurons in the rat substantia nigra. Most, but certainly not all, acetylcholinesterase immunoreactive cells were located in the pars compacta. In the substantia nigra pars compacta there were in turn two populations of acetylcholinesterase containing neurons: those that were tyrosine hydroxylase reactive and those that were not. Double label studies, that included an antibody immunoreactive against a common immunogen on alpha1 of muscle and alpha3 and alpha5 neuronal nicotinic acetylcholine receptor subunits, revealed that nearly all nicotinic receptor positive cells were also tyrosine hydroxylase neurons. However, a minority non-tyrosine hydroxylase population was alpha3- and/or alpha5-nAChR positive and these were always AChE-immunoreactive. In summary, there appears to be a close correlation between nicotinic receptors and acetylcholinesterase in the substantia nigra, irrespective of the transmitter phenotype in different neuronal subpopulations.

A noncholinergic action of acetylcholinesterase (AChE) in the brain: from neuronal secretion to the generation of movement

1. In various brain regions, there is a puzzling disparity between large amounts of acetylcholinesterase and low levels of acetylcholine. One such area is the substantia nigra. Furthermore, within the substantia nigra, a soluble form of acetylcholinesterase is released from the dendrites of dopamine-containing nigrostriatal neurons, independent of cholinergic transmission. These two issues have prompted the hypothesis that acetylcholinesterase released in the substantia nigra has an unexpected noncholinergic function. 2. Electrophysiological studies demonstrate that this dendritic release is a function, not of the excitability of the cell from which the acetylcholinesterase is released, but of the inputs to it. In order to explore this phenomenon at the behavioral level, a novel system has been developed for detecting release of acetylcholinesterase "on-line." It can be seen that release of this protein within the substantia nigra can reflect, but is not causal to, movement. 3. Once released, the possible actions of acetylcholinesterase can be studied at both the cellular and the behavioral level. Independent of its catalytic site, acetylcholinesterase has a "modulatory" action on nigrostriatal neurons. The functional consequences of this modulation would be to enhance the sensitivity of the cells to synaptic inputs. 4. Many basic questions remain regarding the release and action of acetylcholinesterase within the substantia nigra and, indeed, within other areas of the brain. Nonetheless, tentative conclusions can be formulated that begin, in a new way, to provide a link between cellular mechanisms and the control of movement.

Organotypic slice cultures of dopaminergic neurons of substantia nigra

Morphological methods were used to study the plasticity of target-deprived mammalian dopaminergic (DA) neurons. Slices of substantia nigra (SN) were taken from the midbrain of rats aged one to twelve days, and cultured for one to two weeks. Localization of tyrosine hydroxylase (TH) was used to examine the distribution and shapes of DA neurons. Histochemical staining for acetylcholinesterase (AChE) was carried out to estimate both survival and biosynthesis of SN neurons. We found that some DA neurons can survive in vitro without their usual target neurons. This was demonstrated by injecting rhodamine-conjugated microspheres (RD) into the caudate putamen, a SN target area, at 6 to 8 days prior to culturing. RD-labeled cells survived in SN cultures and some of them were doubly labeled with AChE. TH neurons had different shapes and their axon terminals formed close contacts with adjacent nondopaminergic neurons. These findings suggest that a subset of DA neurons may switch targets, but the majority of them require target interactions with the caudate putamen for survival in vitro.

Electrophysiology of pars compacta cells in the in vitro substantia nigra--a possible mechanism for dendritic release

The electrophysiological properties of neurons in the pars compacta of the substantia nigra have been examined in vitro in guinea pig mesencephalic slices. These cells display a set of voltage- and Ca2+-dependent ionic conductances which confer upon them rather unique electrophysiological characteristics. In particular, the results indicate the presence of two separate dendritic Ca2+ conductances. One is inactivated at rest membrane potential and is de-inactivated by membrane hyperpolarization. The second resembles that responsible for dendritic spikes in other neurons. These conductances fulfil most of the physiological and pharmacological requirements for the ionic mechanisms underlying Ca2+-dependent dendritic release.

Neurons cultured from developing rat brain attach and spread preferentially to laminin

Neurons, mechanically dissociated from newborn rat brain and identified by immunostaining for neurofilaments, attached preferentially to laminin-coated coverslips without need of an underlying glial monolayer. The most extensive neurite outgrowth was seen when 20-30 micrograms/ml of laminin was used to coat the coverslips. Higher concentrations of laminin (greater than 30 micrograms/ml) supported single neurons to spread on the coverslips. Fibronectin coating of the coverslips allowed glial cells to attach more rapidly than on uncoated surfaces, but it did not support neuronal spreading or neurite outgrowth. Spreading of neurons and neurite outgrowth were completely inhibited by preincubation of laminin-coated coverslips with laminin antibodies but were unaffected by fibronectin antibodies. These results indicate that laminin is an attachment and spreading factor for central neurons in culture and suggest the presence of a laminin receptor on the neuronal cell surfaces.

Evoked release of proteins from central neurons in vivo

Push-pull cannulae were implanted in both substantiae nigrae and caudate nuclei of the halothane-anesthetized cat. The release of total protein, acetylcholinesterase, and nonspecific cholinesterases was examined. Following direct application of potassium to one substantia nigra, changes occurred in the local release of total protein and acetylcholinesterase, but not nonspecific cholinesterases; changes also were observed in both caudate nuclei and the contralateral substantia nigra. The local evoked release of acetylcholinesterase and of total protein differed in the extent to which they were calcium-dependent. Control studies suggest that release of these compounds, both spontaneous and evoked, is related, at least in part, to neuronal activity. The significance of the neuronal release of proteins is discussed.

Laminin is produced by early rat astrocytes in primary culture

The production of laminin by early rat astrocytes in primary culture was investigated by double immunofluorescence staining for laminin and the glial fibrillary acidic protein (GFAP), a defined astrocyte marker. In early cultures (3 d in vitro; 3 DIV) cytoplasmic laminin was detected in all the GFAP-positive cells which formed the major population (80%) of the nonneuronal cells present in cultures from 20-21-d embryonic, newborn, or 5-d-old rat brains. Monensin treatment (10 microM, 4 h) resulted in accumulation of laminin in the Golgi region, located using labeled wheat germ agglutinin. Laminin started gradually to disappear from the cells with the time in culture, was absent in star-shaped, apparently mature astrocytes, but remained as pericellular matrix deposits. The disappearance of cellular laminin was dependent on the age of the animal and the time in culture so that it started earlier in cultures from 5-d-old rat brains (5 DIV) and approximately following the in vivo age difference in cultures from newborn (12 DIV) and embryonic (14 DIV) rat brains. Our results indicate that laminin is a protein of early astrocytes and also deposited by them in primary culture, thus suggesting a role for this glycoprotein in the development of the central nervous system.

Origin of various enzymes released from the substantia nigra and caudate nucleus: effects of 6-hydroxydopamine lesions of the nigro-striatal pathway

Various enzymes are released from the nigro-striatal pathway. The hypothesis that these enzymes--i.e., acetylcholinesterase, nonspecific cholinesterase, aminopeptidase, and lactate dehydrogenase--originate from dopaminergic nigro-striatal neurons was investigated. Following a 6-hydroxydopamine lesion of the rat nigro-striatal pathway, the spontaneous release of all four enzymes was reduced, in both the caudate nucleus and substantia nigra. In both structures the reduction in release of aminopeptidase and lactate dehydrogenase was less marked than that seen for acetylcholinesterase and nonspecific cholinesterase. In nonlesioned rats, application of 50 mM KCl to the substantia nigra led to an increase in enzyme release locally, accompanied by a decrease in release from the caudate nucleus. These changes were observed in release of acetylcholinesterase, aminopeptidase, and lactate dehydrogenase, but nonspecific cholinesterase remained unchanged. In rats with 6-hydroxydopamine lesions, nigral application of potassium no longer induced a significant decrease in release from the caudate nucleus in any of the enzymes. In perfusates of the treated group, however, a potassium-evoked increase in nigral aminopeptidase and lactate dehydrogenase release was still detectable. Nigral acetylcholinesterase release was not affected by potassium in animals with 6-hydroxydopamine lesions. It appears that lactate dehydrogenase and aminopeptidase are released together from both dopamine- and non-dopamine-containing cells in the caudate nucleus and substantia nigra; this release may be related to neuronal depolarization. A high proportion of nonspecific cholinesterase is released from nigro-striatal dopamine-containing cells but appears not to be dependent on depolarization. Acetylcholinesterase, which is released in the caudate nucleus and substantia nigra following neuronal depolarization, originates almost exclusively from dopamine-containing nigro-striatal neurons.

Release of acetylcholinesterase and aminopeptidase in vivo following infusion of amphetamine into the substantia nigra

Push-pull cannulae were implanted acutely in both substantiae nigrae and both caudate nuclei of the rabbit under urethane anesthesia. Direct infusion of d-amphetamine (10(-6) M) to one substantia nigra evoked a release of acetylcholinesterase and aminopeptidase not only locally, but also from the contralateral caudate nucleus and substantia nigra. The spontaneous release of acetylcholinesterase was reduced in the ipsilateral caudate nucleus. Gel electrophoresis showed that only one molecular form of each enzyme was released. Since the electrophoretic mobilities of these two enzymic activities were different, the released acetylcholinesterase and aminopeptidase belong to separate molecular species. As amphetamine is known to modify dopamine metabolism the evoked release of these two enzymes is probably related to the dopamine-containing nigrostriatal system.

Glial uptake of monoamines in primary cultures of rat median raphe nucleus and cerebellum. A combined monoamine fluorescence and glial fibrillary acidic protein immunofluorescence study

Glial uptake of serotonin and dopamine was studied in primary cultures of the median raphe nucleus and cerebellum by using consecutive demonstration of monoamine fluorescence and glial fibrillary acidic protein immunofluorescence. Most of the glial cells taking up monoamines were glial fibrillary acidic protein positive. Astrocytes with a strong immunoreactivity exhibited monoamine fluorescence only occasionally, although such cells did take up L-dopa readily. Glial fibrillary acidic protein negative cells--morphologically identified as astrocytes--were seen to exhibit monoamine fluorescence after exposure. Glial uptake of serotonin at a concentration of 10(-4) M was detected in cerebellar cultures but not in cultures from the median raphe nucleus. When the concentration was 10(-3) M uptake of serotonin took place in both the areas but was weaker in cultures from the median raphe nucleus. At concentrations greater than 10(-5) M glial uptake of dopamine was detected in cultures from both the regions studied. No region dependent differences in glial uptake of dopamine was demonstrated, however. Based on these observations astrocytes and astrocyte-like glial cells take up dopamine and serotonin. Also glial cells with a remarkably high content of the glial fibrillary acidic protein are more resistant to monoamine uptake than cells exhibiting less intense or no glial fibrillary acidic protein immunofluorescence. The existence of regional differences in uptake of serotonin between the median raphe nucleus and cerebellum suggests that glial uptake of monoamines is not an entirely passive mechanism but may be actively controlled by glial cells in a region dependent manner.