Ultrastructural localization of radiolabelled L-dopa in the endocrine hypothalamus of the rat

The fine structural organization of tyrosine hydroxylase immunoreactive neurons in rat arcuate nucleus

The fine structure of the tyrosine hydroxylase (TH) immunoreactive neurons of the hypothalamic arcuate nucleus was examined by means of immunocytochemistry [peroxidase-antiperoxidase (PAP) method], utilizing an antibody against TH. Immunolabeled axon terminals were observed infrequently and were located predominantly in the lateral region, whereas numerous labeled perikarya and dendrites were found throughout the nucleus. The labeled terminals, containing primarily clear and occasionally dense core vesicles, were never observed in synaptic contact. On the other hand, unlabeled axon terminals were frequently seen synapsing on labeled dendrites. In addition, the labeled dendrites were often seen in direct apposition to other neuronal elements such as both labeled and unlabeled perikarya. In contrast, unlabeled dendrites were never seen apposed to labeled perikarya. Labeled dendrites also occurred in direct contact with one another and with unlabeled dendrites. Moreover, numerous labeled dendrites were encountered along tanycytic processes. Dendrites engaged in tanycytic appositions were occasionally partially encompassed by thin sheaths emanating from the tanycytic process. The extensive contact made by the labeled dendritic profiles on both labeled perikarya and dendrites suggests that tubero-infundibular dopaminergic (TIDA) cells may communicate with each other by means of dendritic release of dopamine. The presence of appositions between labeled dendrites and both unlabeled perikarya and dendrites suggests that the TIDA system also influences other neuronal populations through its dendrites. Finally, the dendrotanycytic relationship suggests that the TIDA system may play some role in the regulation of tanycytic function.

Autoradiographic analysis of [3H]dopamine and [3H]dopa uptake in the turtle olfactory bulb

Uptake and retention of exogenous tritiated dopamine and L-dopa was observed within turtle olfactory bulb slices. In the more superficial layers, periglomerular and superficial tufted cells, as well as their processes, and intraglomerular dendrites were recognized as labeled. Within the deeper part of the bulb, some labeled cells between the tanycytes, as well as nerve fibers and terminals within the granule cell layer, are reported. The results confirm the presence of specific intrinsic dopaminergic cells within the reptilian olfactory bulb.

Uptake and retention of [3H]adrenaline by central monoaminergic neurons: a light- and electron-microscope radioautographic study after intraventricular administration in the rat

Paraventricular and paracisternal regions of adult rat central nervous system were investigated by light- and electron-microscope radioautography after intraventricular administration of tritiated adrenaline. In tissue primarily fixed by glutaraldehyde perfusion and post-fixed by immersion in osmium tetroxide, there were no aggregates of silver grains indicative of intraneuronal accumulation of the tracer, except over perivascular nerve terminals at the base of the brain. In contrast, when both fixation and postfixation were carried out by rapid vascular perfusion, preferentially labeled nerve cell bodies and axonal varicosities (i.e. terminals) were detected in various anatomical areas known to contain dopaminergic and/or noradrenergic neurons. Serotoninergic axonal varicosities in the supraependymal plexus and subcommissural organ, as well as a small group of nerve cell bodies of undetermined chemical identity in the n. paraventricularis thalami were also found to be labeled. Addition of a ten-fold higher concentration of non-radioactive serotonin to the solution of [3H]adrenaline suppressed the reactivity in the subcommissural organ and the supraependymal plexus but had no such effect elsewhere in brain. Lesioning of the nigrostriatal dopaminergic system with 6-hydroxydopamine prior to [3H]adrenaline injection eradicated axon terminal labeling in the ipsilateral neostriatum. Electron-microscopic examination of [3H]adrenaline-labeled varicosities in the neostriatum, lateral septum, arcuate nucleus and median eminence extended earlier observations on the ultrastructure of the catecholaminergic innervation of these regions. It was concluded that both dopaminergic and noradrenergic neurons as well as certain serotonin-containing axon terminals can take up and retain [3H]adrenaline, although they probably have lesser affinity for this amine than for their own transmitter. Due to the fact that presumptive adrenergic neurons are intermingled with dopaminergic and noradrenergic elements, further work will be needed to determine to which extent they also contributed to [3H]adrenaline uptake in the present experimental conditions.

Radioautographic investigation of monoaminergic neurons: an evaluation

This review outlines the most relevant contributions currently available on the detection and ultrastructural characterization of monoaminergic neurons by radioautography after administration of radiolabeled monoamines. It includes methodological considerations and then a critical analysis of the diagnostic value of the radioautographic method for catecholaminergic and serotoninergic neurons, emphasizing in particular its recent applications to the visualization of dopaminergic axon terminals. An attempt is then made to evaluate the method in terms of specificity, sensitivity and resolution and its possibilities with regard to quantitative analysis. Lastly, its value for approaching the dynamic and metabolic properties of monoaminergic neurons is stressed.

Quantative radioautographic light and electron microscopic analysis of the localization of monoamines in the median eminence of the rat

Serotonin containing structures in the median eminence of the rat have been studied by quantitative light and electron microscopic radioautography following intraventricular infusion of tritiated 5-hydroxytryptophan. One hour after injection of the tracer the highest density of silver grains was recorded in the ependymal and external zones, especially in the lateral palisade zone. The proportion of labelled neurosecretory terminals was also larger in the lateral palisade zone (29%) as compared with the medial palisade zone (13%), although the mean number of developed silver grains per one terminal was higher in the latter. On the average, 16% of neurosecretory terminals sequestered radiolabelled 5-hydroxytryptophan in the external zone of the rat median eminence. It is suggested that serotonin, like catecholamines, is discharged from neurosecretory terminals localized in the external zone and via the portal circulation affects the function of the anterior pituitary. The sites of origin of serotoninergic structures of the median eminence as well as the possible role of monoamine (catecholamine and indolamine) neurohormones in a dual peptidergic and monoaminergic control of anterior pituitary functions are discussed.