High resolution analysis of [3H]2-deoxyglucose incorporation into neurons and glial cells in invertebrate ganglia: Histological processing of nervous tissue for selective marking of glycogen

Summary The 2-deoxyglucose (2-DG) autoradiographic technique, in conjunction with³H-labelled 2-DG and glutaraldehyde fixation, was applied to the isolated ganglia of the leech and the snail in order to analyse its potential use for the understanding of energy utilization in these simple nervous systems. Approximately 50% of the label is retained in these tissues after histological processing, and the method can be satisfactorily applied at the subcellular level. In both species most neurons progressively accumulate radioactivity over 15 min-1 h, although to different extents. In the leech the glial cells showed greater uptake than the neurons. Paired homologous neurons in contralateral buccal ganglia of the snail were equally labelled. In all structures studied by electron-microscope autoradiography the label was positively associated with glycogen. Freshly extracted glycogen from ganglia previously exposed to 2-DG was significantly labelled (2–10% of total radioactivity in the ganglia); thus the glutaraldehyde fixation did not appear responsible for artifactual binding of the label. The significance of the glycogen labelling and the potential of the technique for metabolic mapping in these nervous systems are discussed.     

Choline uptake, acetylcholine synthesis and release by Limulus abdominal ganglia

Isolated ganglia from the ventral nerve cord of the horseshoe crab, Limulus polyphemus, were incubated in [³H]choline and subsequently analyzed for choline uptake, conversion of choline to acetylcholine and the release of the newly synthesized acetylcholine. The ganglia readily accumulated radioactivity when incubated in Chao's solution containing 2 μM [³H]choline. The rate of uptake was 0.08 pmols/min/mg tissue and 26% of the [³H]choline taken up during a 1 h period was converted to [³H]acetylcholine. A 15 min exposure of ganglia to 90 mM K⁺ prior to incubation in [³H]choline caused an 89% increase in choline uptake and a 150% increase in its conversion to [³H]acetylcholine. The presence of unlabeled acetylcholine in the uptake medium inhibited both uptake and conversion of [³H]choline.There was a 5-fold increase in the efflux of radioactivity when ganglia incubated in 2 μM [³H]choline were superperfused with 90 mM K⁺. The increased efflux of radioactivity was Ca²⁺-dependent and was inhibited by Mg²⁺ (44%) and by Co²⁺ (72%). Similarly, addition of veratridine caused a Ca²⁺ and Na⁺-dependent release of radioactivity from prelabeled ganglia. Analysis of the superperfusate revealed that virtually all of the released radioactivity was [³H]acetylcholine.The abdominal ganglia of Limulus take up choline at micromolar concentrations, convert substantial amounts of it to acetylcholine and possess a depolarization-triggered, Ca²⁺-requiring mechanism for the specific release of acetylcholine. These results give further support to the view that the abdominal ganglia of Limulus contain a population of cholinergic terminals.     

Radioautographic study of 5-hydroxytryptamine-containing nerve terminals in central ganglia of Planorbis corneus: comparison with other species and characteristics of the serotoninergic nerve terminal.

Summary Central ganglia fromPlanorbis corneus were incubated with [³H] 5-hydroxytryptamine and [³H]-leucine. After fixation, sections were examined by light and electron microscopic autoradiography. With [³H] 5-hydroxytryptamine there was a high level of uptake into a small number of axons and terminal processes in the neuropil. The terminal processes contained granular vesicles (diameter 50–120 nm), some of which possessed eccentrically placed cores, and agranular vesicles (mean diameter 60 nm). No membrane synaptic specializations were observed. With [³H] leucine there was a general distribution of radioactivity throughout the ganglia.The vesicular inclusions and non-specialized nature of the labelled terminal processes were very similar to presumed 5-hydroxytryptamine-containing terminals in other invertebrates and in the mammalian brain. It appears that several anatomical features of serotoninergic nerve terminals are common in all animals studied, although there is no specific characteristic which allows positive identification.     

Mapping monoaminergic neurons with [3H]reserpine by autoradiography.

The synthesis of [³H]reserpine of high specific activity is described. The accumulation of radioactivity in peripheral sympathetically innervated organs and in the brain after intravenous injection of [³H]reserpine to rats was measured biochemically and its localization studied by light-microscopic autoradiography. In most of the organs and tissues investigated minute quantities of [³H]reserpine persisted up to 10 days after injection. By autoradiography, it was observed that silver grains were unevenly distributed in various brain regions and peripheral organs 18 h and up to 10 days after administration of [³H]reserpine. In the brain the radiolabel selectively accumulated in distinct areas known to have a high monoamine content including the substantia nigra, nucleus (n.) raphe, locus coeruleus, n. dorsomedialis hypothalami and the n. arcuatus, where cell bodies as well as the neuropil were labelled; whereas in the neostriatum, n. septi lateralis, n. accumbens, n. tractus solitarii, eminentia mediana, hippocampus and cortex the label was confined to the neuropil. Supra-ependymal 5-hydroxytryptamine-containing nerves in the cerebroventricular system and the interplexiform layer of the retina also selectively accumulated the label, as did a network of nerve fibres in the iris, cells and paracellular areas of the superior cervical ganglia and chromaffin cells of the adrenal medulla. Pretreating (but not post-treating) animals with non-radioactive reserpine prevents, by up to 80%, the accumulation of radiolabel and abolishes to a great extent the autoradiographic localization. The fact that the persistently bound radiolabel is confined to monoaminergic neurons suggests that it is irreversibly bound to its target site, the amine-storing vesicle. Support for this interpretation comes from studies demonstrating a fast anterograde axonal transport of [³H]reserpine in the nigrostriatal tract after intranigral injection of the radiolabel.Altogether, these findings are in line with a wealth of neuropharmacological, endocrinological and clinical observations relating the effects of reserpine to its interaction with dopaminergic, noradrenergic and serotonergic neuronal systems. They demonstrate that [³H]reserpine of high specific activity is a useful tool in studies designed to map monoaminergic pathways in the brain and to further characterize amine-storing mechanisms.     

Autoradiographic localization of monoamine uptake in the central nervous system of a marine mollusc (Mactra stultorum L., pelecypoda)

Uptake of [³H]serotonin and [³H]dopamine has been investigated in the central nervous system of the marine mussel, Mactra stultorum L. by means of light and electron microscopic autoradiography. It was established that among the axon profiles of the neuropil of the ganglia, axons containing dense-core vesicles with 700–1500Ådiameter play the primary role in the uptake of both serotonin and dopamine. Although the nerve cell bodies in the cortical layer were not extensively labelled, glial cells and processes in this layer took up large amounts of the labelled amines.This autoradiographic study shows that transmitter re-uptake is a possible means of transmitter inactivation in the central nervous system of marine mussels and that glial elements might also participate in the uptake and inactivation of transmitter.     

The persistence of donor-derived cells in thymus grafts, lymph nodes and spleens of recipient mice

Thymus was labelled in vivo by injecting newborn C3H/Bi mice with three or four doses of 2 μc of [³H]thymidine. The labelled thymus was grafted into the kidney capsule of 22–25-day-old intact and thymectomized syngeneic recipients which were killed 18, 41 or 42 days later. The grafts and lymphoid tissue were sectioned and examined by autoradiography after exposure times of up to 26 weeks.

Small and medium labelled lymphocytes were seen in the thymus-dependent areas of the lymph nodes and spleen of all recipients provided the sections were exposed for a sufficiently long period. More labelled cells were seen in recipients killed at 18 days than at 41 or 42 days after grafting. It was concluded that these cells were the direct descendants of those cells labelled in the graft. There was no evidence of re-utilization of the isotope label.

Most of the cells originally labelled in the graft either lost their label by multiple divisions or were replaced by unlabelled cells; although the epithelial-reticular cells remained heavily labelled. The importance of this last finding is discussed.

     

Labelling of schwann and satellite cells by [3H]dexamethasone in a rat sympathetic ganglion and sciatic nerve

Light-microscopic autoradiograms of superior cervical ganglia of adult rats after injection of [³H]dexamethasone showed a preferential accumulation of radioactivity in the nuclei of satellite and Schwann cells. The total radioactivity found in the ganglion at 10 min after injection was high compared to the cerebral cortex of the brain, but declined rapidly. Only Schwann and satellite cell nuclei retained the label for up to 2 h. The labelling was specific for dexamethasone, since it could be prevented by an excess of unlabelled dexamethasone but not by corticosterone. Administration of [³H]corticosterone resulted in only a weak labelling of the ganglion and no specific labelling of Schwann and satellite cells could be observed. As in the ganglion, accumulations of [³H]dexamethasone by Schwann cell nuclei was also observed in the sciatic nerve.It is possible, therefore, that satellite and Schwann cells represent an important target for glucocorticoids in the nervous system.     

High activity neurons in the reticular formation of the medulla oblongata: a high-resolution autoradiographic 2-deoxyglucose study

Accumulation of radioactivity in the medulla oblongata of the rat was assessed with high resolution autoradiography after injection of [3H]2-deoxyglucose or [14C]2-deoxyglucose. The autoradiographic approach employed allowed analysis at the cellular level. A salient feature of autoradiograms was the presence of foci of very high silver grain density in the reticular formation. These heavily labeled foci were shown to be associated with neurons by combined Acridine Orange staining and autoradiography. The high activity neurons in the ventral lateral medulla were predominantly located in the caudal portion of the paragigantocellular reticular nucleus. In addition, a group of high activity neurons was present in the dorsal reticular formation. The potential involvement of the high activity neurons in cardiovascular and respiratory regulation is discussed. In dry-mount autoradiograms produced after injection of [3H]2-deoxyglucose, certain small cells, presumably glial cells, were observed throughout the brainstem to accumulate radioactivity to a greater extent than the surrounding tissue. High activity neurons and high activity glial cells were observed for 5 min and 45 min survival intervals after intravenous injection of [3H]2-deoxyglucose. The similar appearance of autoradiograms at 5 min and 45 min after injection of [3H]2-deoxyglucose indicates that the utility of a 5-min survival period deserves further evaluation for assessment of functional activity patterns in brain.     

Central transport and distribution of labelled glutamic and aspartic acids to the cochlear nucleus in cats: an autoradiographic study.

Tritiatedl-glutamic acid orl-aspartic acid were injected unilaterally into the cochleas of adult cats, and 4 h-7 days later the localization of label was studied by light-microscopic autoradiography in sections of the brain stem. All experiments with [³H]glutamate and most with [³H]aspartate produced visibly denser grains and quantitatively higher grain counts in the cochlear nucleus of the injected side than in the cochlear nucleus of the noninjected side. In general, [³H]glutamate produced dense label ipsilaterally in all major subdivisions of the cochlear nucleus except the molecular layer of the dorsal division and the granular cell cap of the ventral division, in patterns like those after injections of [³H]leucine. The most heavily labelled areas were those that received the densest inputs of primary cochlear fibers. By contrast, [³H]aspartate produced dense grains in the ventral, but not in the dorsal, cochlear nucleus. The results with [³H]glutamate closely resembled those after [³H]leucine or [¹⁴C]leucine injections, but the results with [³H]aspartate showed consistent differences from those after [³H]leucine. After [³H]aspartate, label was densely located around cell bodies and basal dendrites, but not around primary dendrites of octopus cells, in contrast to the dense perisomatic and peridendritic label after [³H]glutamate. Also, relatively low grain counts occurred in the nerve root entry zone and in the deep dorsal cochlear nucleus after [³H]aspartate, although high counts occurred in those same zones after either [³H]glutamate or [³H]leucine injections.Consistent differences in labelling after glutamate and after aspartate suggest differences in their uptake, metabolic conversion and/or transport to the cochlear nucleus by cochlear fibers. The morphological differences shown here agree with the distribution of those two amino acids in the cat cochlear nucleus as shown by microchemical analyses.     

Tunicamycin does not inhibit transport of phosphatidylinositol toXenopus rod outer segments

Summary Tunicamycin inhibits the dolichol pathway forN-linked glycosylation of proteins, including photoreceptor opsin, and causes a buildup of tubulo-vesicular profiles in the intersegmental space between photoreceptor rod inner and outer segments associated with disruption of new disc assembly. We tested the hypothesis that a tunicamycin lesion in photoreceptors would block lipid transport into the outer segment. AdultXenopus retinas were preincubated in dim red light with 20 g ml^–1 of tunicamycin for one hour followed by incubation in the light for 2–6 h with tunicamycin plus either [³H]mannose, [³H]leucine, [2-³H]glycerol or [³H]myo-inositol. Tunicamycin caused accumulation of tubulo-vesicular membranes in the intersegmental space and significantly reduced both [³H]leucine and [³H]mannose incorporation into the basal region of rod outer segments. However, tunicamycin had no effect on [³H]glycerol incorporation into the rod outer segment phospholipids. After 5 h incubation with [³H]glycerol, radiolabel in outer segment fractions was associated primarily with phosphatidylinositol in both control and tunicamycin treated retinas. Quantitative light microscope autoradiography of both [³H]glycerol and [³H]inositol labelled retinas showed diffuse labelling over the entire rod outer segment in both control and tunicamycin treated retinas with no accumulation of radioactivity in the basal discs of control retinas or in the tubulo-vesicular structures in the intersegmental space of tunicamycin treated retinas. Our results indicate that despite the morphological disruption and inhibition of glycoprotein transport to outer segments after tunicamycin treatment, transport of labelled phosphatidylinositol occurs normally. These data add to a growing body of evidence separating the lipid and protein transport pathways to the outer segment.     

Selective uptake of [3H]glutamine and [3H]glutamate into neurons and satellite cells of dorsal root ganglia in vitro

The uptake of [³H]glutamate and [³H]glutamine into rat dorsal root ganglia has been examined by autoradiography and thin-layer chromatography. [³H]glutamate was selectively accumulated by satellite glial cells and after 10 min, 53% of this had been converted to [³H]glutamine. [³H]glutamine, on the other hand, entered neuronal perikarya and 40% was converted to [³H]glutamate. It is suggested that these selective uptake processes provide supporting evidence for the existence of a neuronal-glial glutamine cycle in dorsal root ganglia. Small dark (B) cells accumulated 6 times as much [³H]glutamine as did large light (A) cells. The reasons for this marked difference in the metabolism of the two main types of dorsal root ganglion neurone are discussed.     

Initiation of fast axonal transport: involvement of calcium during transfer of proteins from Golgi apparatus to the transport system.

Fast axonal transport of [³H]fucose-labelled glycoproteins was examined in vitro in a preparation of spinal sensory neurons of the bullfrog. Rapidly transported glycoproteins were labelled when dorsal root ganglia were exposed to [³H]fucose, but not when a localized region of spinal nerve trunk was exposed to the labelled sugar; these results are consistent with the general finding that fucosylation occurs predominantly in the Golgi apparatus. Incubation of ganglia and nerve trunks in medium containing 0.18 mM CoCl₂ depressed the amount of rapidly transported, [³H]fucose-labelled glycoprotein by approximately 80%. This level of cobalt impairs neither protein synthesis nor incorporation of [³H]fucose into glycoprotein, suggesting that the inhibition of axonal transport by cobalt occurs at a step subsequent to fucosylation. When a 4 mm region of spinal nerve was desheathed, to facilitate access of ions to the axons, cobalt had no effect on fast axonal transport of glycoproteins at the level of the axon: the amount of [³H]fucose-labelled protein was similar in the regions of nerve trunk proximal and distal to the desheathed area.Thus, the cobalt-sensitive step in the transport of glycoproteins is likely to occur in the somata during transfer of the proteins from the Golgi apparatus to the transport system; it is suggested that cobalt antagonizes the action of intracellular calcium ions. An analogy is considered between the processing of fast-transported proteins by neuronal somata and the processing of secretory proteins by pancreatic exocrine cells.     

Production and secretion of Leishmania braziliensis proteins

The kinetics of secretion of proteins by Leishmania braziliensis was followed by incorporation of [³H]leucine into macromolecules produced by the cells which are released into the growth medium. About 10% of the total protein synthesized by actively growing cells is secreted. Cycloheximide (100 μg/ml) and puromycin (0.5 mM) inhibited the incorporation of labelled leucine by 85 and 99%, respectively. The secreted proteins do not seem to results from cell lysis since, first, the kinetics of production are linear and, secondly, less than 1% the thymidine or uridine incorporated by the cells is found in the medium. Cells grownh with [³H]leucine and then transferred to fresh medium show two phases of secretion. During the first six hours, it is slow and reaches a plateau. The release increases about ten-fold during the next six hours. An analysis of the secreted material showed that following precipitation with methanol and sodium acetate, three isotopically labelled peaks were eluted from Sephadex G-120–150. The first of these, containing 50% of the radioactivity, did not react with anti-leishmanial serum, while the last two did. Since the last two fractions could be labelled with [³H]glucosamine as well as [³H]leucine it is suggested that they are glycoprotein in nature and are similar to the products released by other species of Leishmania.     

The localization of antigen in lymph nodes and its relation to specific antibody-producing cells. II. Comparison of iodine-125 and tritium labels

A branched multichain polypeptide of the type p(Tyr,Glu)-pAla--pLys was synthesized from L-lysine, L-tyrosine, L-glutamic acid and tritium labelled DL-alanine; the final product, [³H](T,G)-A--L, had a specific radioactivity about 3 mc/mg. Its immunological behaviour in mice was compared with that of another preparation of (T,G)-A--L trace labelled with ¹²⁵I at a specific radioactivity of 2 mc/mg.The [³H](T,G)-A--L proved to be only very weakly immunogenic compared with [¹²⁵I](T,G)-A--L. This was not attributable to its radioactivity, but probably to its relatively low tyrosine content. However, detailed autoradiographical studies of the localization of the two materials in the draining lymph nodes after injection into the footpads of previously primed and of unprimed mice revealed no qualitative differences between their behaviour, which resembled that previously described for [¹²⁵I](T,G)-A--L in similar experiments. When a preparation of (T,G)-A--L, labelled on the same molecules with both ³H and ¹²⁵I, was studied in respect of gross retention of each label in lymph nodes, selective retention of ³H relative to ¹²⁵I was observed. This was explained by greater susceptibility to peptidase activity of L-tyrosine residues at the ends of the side chains compared with that of the underlying polymeric DL-alanine. It is concluded that in studies of the fate of iodine-labelled peptides or proteins detection of the radioactive label is likely to indicate the presence of intact molecules or of large portions of them, but that failure to detect iodine cannot be taken to denote their absence.The autoradiographs suggested the existence of fine channels containing antigen penetrating through the cortical and intermediate zones of lymph nodes.     

On the presence of serotonin in the gut lumen and possible release mechanisms

The possible presence of a luminal release of serotonin (5-HT) from gut enterochromaffin cells (EC) of the rat. was studied after the injection of the tritiated 5-HT precursor 5-hydroxytryptophan [³H]-5-HTP by electron microscopic autoradiography. The uptake of 5-HTP into gut epithelial cells was also studied by fluorescence histochemistry according to the Hillarp-Falck technique at the same post-injection interval as in the autoradiography experiments. 3 h after the injection of 5-HTP (100 mg/kg i. v.) the fluorescence intensity of EC was increased and numerous, probably enteroendocrine, cells had an increased yellow tryptamine induced fluorescence due to an uptake of 5-HTP and probably decarboxylation to 5-HT. However, the labelled precursor [³H]-5-HTP was taken up not only into granules of enteroendocrine cells but also incorporated into the cytoplasm and nucleus of nonendocrine cells when studied by autoradiography. After 10 min of efferent electrical stimulation of the vagal nerve much of the label was found in the gut lumen suggesting a release of the amine. The hypothesis of a luminal release of 5-HT was further corroborated in starved cats, where considerable amounts of 5-HT were detected by fluorimetric assays in the lumen of isolated jejunal loops under resting conditions. The experiments demonstrate that: (i) 5-HTP is taken up not only into typical EC but also into other enteroendocrine cells, and most probably decarboxylated to 5-HT. (ii) Also intestinocytes take up [³H]-5-HTP and incorporate the amino acid into peptides to a certain extent, (iii) Following vagal nerve stimulation labelled material, probably 5-HT, is secreted into the gut lumen, (iv) 5-HT normally occurs in the gut lumen.     

Ribonucleotides in newly synthesized DNA of herpes simplex virus

Newly synthesized DNA of herpes simplex virus type 1 (HSV-1), obtained from primary rabbit kidney cells pulse-labeled with [³H]thymidine or [³H]uridine at 6 hr postinfection, was purified by two cycles of centrifugation in CsCl density gradients. These intracellular viral DNA preparations hybridized specifically with homologous HSV-1 DNA but not with host cell DNA or E. coli DNA. Upon denaturation by alkali, the [³H]thymidine-labeled HSV-1 DNA cleaved to smaller pieces. The alkali-labile material in the viral DNA was identified as ribonucleotides on the basis of the following observations: (1) When labeled with [³H]uridine for short periods, the labeled viral “DNA” was susceptible to RNase and NaOH, and all the radioactivity was confined to the nucleoside [³H]uridine; however, upon longer labeling periods (up to 20 hr), the [³H]uridine-labeled viral “DNA” became more susceptible to DNase, as most but not all of the [³H]uridine was converted to deoxyribonucleosides. (2) Denaturation of [³H]uridine-labeled double-stranded HSV-1 “DNA” (?_Cs2SO4 = 1.45 g/cm³) by heat shifted the buoyant density to single-stranded DNA region (?_Cs2SO4 = 1.48?1.50 g/cm³) but not to single-stranded RNA region; however, treatment with hot NaOH considerably reduced the radioactivity of this “DNA.” Treatment with DNase, but not with pronase, shifted the buoyant density to the heavier RNA region of the gradient. Heat-denatured DNA but not the native DNA was susceptible to single-strand specific nuclease S1.     

Autoradiography of dendritic acetylcholine receptors: A method for study of isolated neurons of the adult central nervous system in the turtle

Adult neurons were isolated from turtle retina in order to study the subcellular distribution of muscarinic acetylcholine receptors. Retinas were perfused in vitro with [³H]quinuclidinylbenzilate [³H]QNB) to label muscarinic receptors, and individual neurons with extensive dendritic arborization were obtained for autoradiography by use of papain and trituration. Papain caused no change in the number of muscarinic receptor sites nor in their affinity for [³H]QNB. Receptors, indicated by the positions of silver grains, were localized to dendritic processes.     

ACTH controls [3H]dopamine uptake in the adrenal chromaffin cell

Effect of hypophysectomy on the disposition of [³H]dopamine-derived radioactivity in the mouse adrenal medulla was examined by autoradiography. Chromaffin cells, both A and NA type, incorporated less radioactivity in the hypophysectomized mice (50 days after operation) than in normal control mice: this decrease in the [³H]dopamine uptake was restored by intraperitoneal injection of ACTH (for 26 days; starting from 24th day after hypophysectomy; 40 μg/gbw/day). In the control mice chromaffin cells near the cortico-medullary junction showed a higher radioactivity than those in the center: hypophysectomy obscured this specific distribution pattern which reappeared after ACTH treatment.     

Autoradiographic and biochemical analysis of photoreceptor membrane renewal inOctopus retina

Summary Using autoradiographic and biochemical methods, we have demonstrated the renewal of light-sensitive membranes and photopigments inOctopus visual cells. After the injection ofOctopus with [³H]leucine, electron microscope autoradiography revealed an intracellular pathway similar to that in vertebrates for the synthesis and transport of nascent protein from the inner segments to the rhabdomes. However, migration of labelled protein from synthetic sites to the light-sensitive rhabdomes took longer inOctopus than the equivalent process in vertebrates. Biochemical analysis of [³H]leucine-labelled retinas identified some of the labelled protein observed in autoradiographs of the rhabdomes as the visual pigment, rhodopsin. We have shown that retinochrome, a second photopigment in cephalopod retinas, is also renewed. Biochemical analysis 8 h after injection of [³H]leucine revealed heavy labelling of this photoprotein. Light microscope autoradiography ofOctopus retina 8 h after injection of [³H]retinol showed labelling of both the rhabdomes and the myeloid bodies of the inner segments. Biochemical data gathered 8 h after injection of [³H]retrnol indicated chromophore addition to both rhodopsin and retinochrome with retinochrome being more heavily labelled than rhodopsin. Thus, silver grains observed over the rhabdomes and inner segments could arise from one or both photopigments. These data suggest that retinal is stored in the myeloid bodies of the photoreceptor inner segments. Retinal could then be transferred, perhaps via retinochrome, to newly synthesized opsin before the visual pigment is assembled into new rhabdomeric membranes. Alternatively, retinochrome may serve to transport retinal from the myeloid bodies to the rhabdomes to regenerate rhodopsin as previously proposed.     

Autoradiographic localization of [3H]reserpine in rat brain: Correlation with distribution of monoaminergic neurones

The in vivo localization of [³H]reserpine in rat brain was studied by autoradiography using unfixed, frozen sections prepared 18 h after intravenous injection of the radiolabel. Radioactivity was localized in those areas of the brain where monoaminergic cell bodies and nerve terminals have been describes. The radiolabel found was chromatographically identical with reserpine. Treatment of animals with the unlabelled drug 4 h before administration of [³H]reserpine prevented the specific localization of radioactivity in the brain.