A serotonergic innervation of noradrenergic neurons in nucleus locus coeruleus: demonstration by immunocytochemical localization of the transmitter specific enzymes tyrosine and tryptophan hydroxylase

Immunocytochemical localization of the neurotransmitter synthesizing enzymes, tyrosine and tryptophan hydroxylase, was used to determine whether the noradrenergic neurons in the nucleus locus coeruleus of the rat are innervated by serotonergic (5-HT) neurons. Specific antibodies were prepared to tyrosine hydroxylase, purified from the bovine adrenal medulla, and tryptophan hydroxylase, purified from rat midbrain. These were localized by both light and electron microscopy by the use of the peroxidase-antiperoxidase method. In the nucleus locus coeruleus, tyrosine hydroxylase was contained in the cytoplasm, proximal axons, and dendrites of intrinsic neurons. Tryptophan hydroxylase, on the other hand, was only contained within processes surrounding the perikarya and dendrites of the catecholaminergic neurons. The processes labeled with tryptophan hydroxylase were unmyelinated, ranged in size from 0.1 to 1.4 micron, and consisted of terminal varicosities separated by intervaricose segments. Although in close approximation to noradrenergic neurons, these processes, presumably axons, rarely formed synatic contacts with thickened membrane specializations. In processes, tryptophan hydroxylase was associated with subcellular organelles which had size and distribution of microtubules, and small and large synaptic vesicles. These observations provide a morphological basis to support the hypothesis that the activity of noradrenergic neurons may be modulated by a direct action of 5-HT neurons.

Serotonergic neurons in the peripheral nervous system: identification in gut by immunohistochemical localization of tryptophan hydroxylase

A specific antibody to tryptophan hydroxylase [L-tryptophan, tetrahydropteridine:oxygen oxidoreductase (5-hydroxylating), EC 1.14.16.4] has been used to localize the enzyme immunohistochemically in neurons of the mammalian gut. The enzyme was found in perikarya of intestinal neurons of mice, rats, and guinea pigs. Neurons containing the enzyme survived for up to 3 weeks in organotypic tissue culture and were intrinsic to the gut. These neurons are probably serotonergic and are the first such neurons to be found in the peripheral nervous system.

Monoamine-synthesizing enzymes in central dopaminergic, noradrenergic and serotonergic neurons. Immunocytochemical localization by light and electron microscopy

The monoamine-synthesizing enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH) and tryptophan hydroxylase (TrH) were immunocytochemical localized in dopaminergic, noradrenergic and serotonergic neurons of rat brain by light and electron microscopy. In dopaminergic and serotonergic neurons, the respective synthesizing enzymes. TH and TrH, were distributed throughout the cytoplasm of the neuronal perikarya, dendrites, axons and terminals. The most selective accumulation of reaction product for the specific enzyme was associated: (a) in perikarya with endoplasmic reticulum, Golgi apparatus and microtubules, (b) in processes with microtubules, and (c) in terminals with dense granules or clear vesicles. The labeled terminals were characterized by their content of labeled organelles and the absence of synaptic junctions. In noradrenergic neurons, both TH and DBH were localized in the perikarya, similar to TH in dopamine neurons. TH and DBH differed in their localization within proximal axons and dendrites in that TH was associated with microtubules but DBH was not. These results provide ultrastructural evidence to suggest that monoamines may be: (a) synthesized by enzymes which are associated with different organelles depending on the portion of the neuron and the type of enzyme; (b) synthesized in both axons and dendrites and (c) released from terminals without postsynaptic membrane specializations.

Brain tryptophan hydroxylase: purification of, production of antibodies to, and cellular and ultrastructural localization in serotonergic neurons of rat midbrain

Tryptophan hydroxylase [EC 1.14.16.4; L-tryptophan, tetrahydropteridine:oxygen oxidoreductase (5-hydroxylating)], the enzyme catalyzing the rate-limiting step in the biosynthesis of serotonin, was purified 79-fold from the region of the raphe nucleus of rat midbrain by sequential column chromatography and disc-gel electrophoresis. In electrophoresis three bands were distinguished, A, B, and C, which, when separated and submitted individually to electrophoresis, reproduced the same three bands. Bands A and C were enzymatically active and inhibited by para-chlorohenylalanine. Antibodies produced to each of the three bands crossreacted by immuno double diffusion and electrophoresis with each other and homogenates of raphe nuclei; they completely inhibited enzyme activity only of tryptophan hydroxylase. Tryptophan hydroxylase was localized by light and electron immunohistochemistry to serotonin neutrons of the raphe. Ultrastructurally, in cell bodies, the enzyme was distributed in cytoplasm and in association with endoplasmic reticulum and Golgi apparatus. In dendrites and axons, it was associated with microtubules. Tryptophan hydroxylase in brain is only neuronal and cytoplasmic, exists in multiple forms, and is associated with microtubules, suggesting it may be transported from sites of synthesis in cell body into axons.

Ultrastructural localization of tyrosine hydroxylase in noradrenergic neurons of brain

Tyrosine hydroxylase (EC 1.14.16.2), the enzyme catalyzing the rate-limiting step in catecholamine biosynthesis, was localized by electron microscopy within noradrenergic neurons of the nucleus locus coeruleus of the rat brain with a specific antibody to tyrosine hydroxylase labeled by the peroxidase-antiperoxidase immunohistochemical method. Labeled cell bodies and their processes were easily distinguished from unstained neuronal elements in the neuropil. The hydroxylase was only seen in the neuronal cytoplasm. Its distribution in processes was different from that in cell soma. In longitudinal sections of axons and dendrites, the peroxides reaction product appeared as fiber-like strands aligned parallel with the plasma membrane. In cross section, the labeled structures had a diameter of 220 A and exhibited an orderly distribution within the processes. The size and distribution of the peroxidase-stained structures suggest that they are neurotubules. In the perikarya, the cytoplasm was diffusely stained; the reaction was most intense on membranes of endoplasmic reticulum and Golgi apparatus, whereas lysosomes and mitochondria did not stain. The ultrastructural localization of tyrosine hydroxylase is consistent with biochemical data suggesting that the enzyme exists in different states in cell body and processes. The ultrastructural identification of enzymes subserving synthesis of neurotransmitters in central neurons and their processes may provide a useful tool in mapping the distribution of chemically specific synapses on identifiable neurons in brain.

Cellular localization of tyrosine hydroxylase by immunohistochemistry

The enzyme tyrosine hydroxylase (TH) was immunohistochemically localized by the peroxidase-antiperoxidase method in rat to chromaffin cells of the adrenal medulla, large neurons and small darkly staining cells of the superior cervical ganglia and noradrenergic and dopaminergic neurons in brain. As compared with the conjugated peroxidase or immunofluorescence techniques, the peroxidase-antiperoxidase method gave the most selective and specific cytoplasmic localization of TH antisera in every tissue examined. The peroxidase staining with the TH antisera was more intense in dopaminergic than in noradrenergic neurons of the central nervous system. While TH was visualized in cell bodies of both dopaminergic and noradrenergic neurons, it could only be detected in axons and terminals in the dopaminergic system. The perikarya of noradrenergic neurons could be distinguished from dopaminergic neurons by the immunohistochemical demonstration of the enzyme dopamine-beta-hydroxylase only in the former.

Localization of nicotinamide adenine dinucleotide phosphate-dependent dehydrogenases in catecholamine-containing neurons of rat brain. Studies on the nucleus locus ceruleus

Histochemical and cytochemical methods have been used to localize glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, isocitrate (nicotinamide adenine dinucleotide phosphate (NADP)) dehydrogenase and malic enzyme in the nucleus locus ceruleus and other catecholamine-containing neuronal groups of the rat brain stem. The four NADP-dependent dehydrogenases were studied by both quantitative and qualitative histochemical techniques using adjacent tissue sections. Both types of analyses were done on neuronal nuclei known to contain catecholamines in high concentration, the nucleus locus ceruleus and the superior cervical sympathetic ganglion; other known catecholamine-containing nuclei were surveyed by the cytologic technique only. There was intense staining of cell bodies and neuropil of the nucleus locus ceruleus after all four of the NADP dehydrogenase histochemical reactions. In contrast, little staining was observed in the adjacent vestibular nuclei or mesencephalic root nucleus of the trigeminal with the exception of appreciable glucose 6-phosphate dehydrogenase activity present in neuropil elements. Quantitative microchemical determinations in the nucleus locus ceruleus corroborate the histochemical results which indicated high NADP dehydrogenase activities particularly for 6-phosphogluconate dehydrogenase. This same pattern of high NADP enzyme activity as determined by combined cytochemical and quantitative chemical techniques was also observed in the superior cervical ganglion and cytochemically in other catecholamine-containing nuclei of the brain stem. Our findings suggest that a high capacity to generate or utilize nicotinamide adenine dinucleotide phosphate (reduced) may be characteristic of those neurons which either receive adrenergic terminals or synthesize catecholamines.

Cytochemical localization of brain nicotinamide adenine dinucleotide phosphate (oxidized)-dependent dehydrogenases. Qualitative and quantitative distributions

This study compares the histochemical and microchemical localizations of nicotinamide adenine dinucleotide phosphate (reduced) and nicotinamide adenine dinucleotide (reduced) diaphorases and four nicotinamide adenine dinucleotide phosphate (oxidized)-dependent enzymes (glucose 6-phosphate, 6-phosphogluconate, malate and isocitrate dehydrogenases) in areas of rat metencephalon and spinal cord. For the four nicotinamide adenine dinucleotide phosphate (NADP) enzymes, the pattern of localization following use of a modified tetrazolium procedure was compared with quantitative data obtained by microdissection from the same areas in adjacent sections. Optimal experimental conditions for reaction pH, temperature, substrate, cofactor and divalent cation concentrations were used for both the quantitative analysis following microdissection and the histochemical tetrazolium procedure. Consecutive sections were also examined for isocitrate dehydrogenase (nicotinamide adenine dinucleotide (oxidized)) and nicotinamide adenine dinucleotide (reduced) diaphorase activities in addition to seriatim thionine reference sections. Our results indicate that, within the central nervous system, certain characteristic qualitative differences exist in the distribution of the nicotinamide adenine dinucleotide phosphate (oxidized)- and nicotinamide adenine dinucleotide (oxidized)-dependent dehydrogenase enzymes. Nicotinamide adenine, dinucleotide enzymes are visualized predominantly in neuronal cell bodies or neuropil consisting primarily of neuronal processes; in adjacent sections, NADP enzyme activities are visualized almost exclusively in glial elements with two important exceptions. The first is the cerebellar molecular layer where the results from both micro- and histochemical techniques indicate high levels of the NADP enzymes relative to other dehydrogenases and high activity relative to the levels of these NADP enzymes in other nervous system areas. The second exception occurs in those neuronal groups known to contain high levels of catecholamines; these data are the subject of a companion report.