Axonterminal uptake and retrograde axonal transport of labeled amino acids and their incorporation into proteins of neuronal perikarya

Ultrastructural analysis of the innervation of TRH-immunoreactive neuronal elements located in the periventricular subdivision of the paraventricular nucleus of the rat hypothalamus

A combination of electron microscopic immunocytochemistry and autoradiography was employed to examine the synaptic organization of thyrotropin-releasing hormone (TRH) neurons in the periventricular subdivision of the paraventricular nucleus of the rat hypothalamus. TRH neurons were identified by immunocytochemistry. Selective uptake of tritiated serotonin (5-HT) was used to identify serotoninergic elements. TRH-immunoreactive axon terminals were found to be in synaptic contact with TRH-immunoreactive dendrites and with unlabeled dendritic branchlets. There were direct appositions between radiolabeled 5-HT terminals and TRH-immunoreactive dendrites, but differential synaptic contacts between 5-HT axonal elements and TRH neurons were not seen. TRH-immunopositive cell bodies and dendrites received a very intense innervation by unlabeled axon terminals or axonal varicosities showing morphologically defined synaptic junctions. These were mostly of the asymmetrical variety and different types could be distinguished. The findings substantiate the view that TRH neurons of the periventricular subvision of the paraventricular nucleus may be influenced by TRH axons, serotoninergic fibers and a large number of unidentified nerve terminals.

Demonstration of serotoninergic axon terminals on somatostatin-immunoreactive neurons of the anterior periventricular nucleus of the rat hypothalamus

Using a combination of electron microscopic autoradiography and immunocytochemistry, the connections between serotoninergic axons and somatostatin neurons of the anterior periventricular nucleus of the rat hypothalamus were examined. The serotoninergic elements were identified after selective uptake of tritiated serotonin and the somatostatin neurons with immunocytochemistry. Synaptic connections between labeled serotoninergic nerve endings and somatostatin-immunoreactive neurons were observed. This finding provides morphological evidence for a direct influence of serotoninergic elements on somatostatin neurons of the anterior periventricular nucleus projecting to the median eminence of the hypothalamus.

Autoradiographic measurement of tritiated agmatine as an indicator of physiologic activity in Hermissenda visual and vestibular neurons

[3H]Agmatine (amino-4-guanidobutane) has been shown to be potentially useful for identifying and assessing the ACh sensitivity of specific neurons. Small cationic amines are able to permeate ACh-activated ion channels in sympathetic neurons and vertebrate endplates. Sensory neurons of the photic pathway in the nudibranch mollusc Hermissenda crassicornis are cholinergic and the synaptic interactions between the photic and vestibular systems have been well characterized electrophysiologically. We have therefore tested the feasibility of using autoradiography with [3H]agmatine, (a) to identify known ACh-responsive postsynaptic cells and (b) to examine its ability to serve as an indicator of physiologic activity within the photic and vestibular pathways under conditions of darkness and light stimulation. Scintillation counting revealed that approximately 70% of the radioactivity was associated with the CNS while approximately 30% was found in the processing fluids, indicating that routine glutaraldehyde-osmium fixation and subsequent processing for epoxy embedding allows retention of substantial amounts of the radiolabel. The autoradiographic results consistently demonstrated that the uptake patterns for [3H]agmatine did reflect some of the known neuronal interactions under the experimental conditions of light and dark. The accuracy extended to the second order cells of the optic ganglion and to putative interneurons along the photic tract in the cerebropleural ganglion. Since all the neurons in these pathways are unipolar with their synaptic interactions occurring only at the terminal endings, the radiolabel accumulated in the somata resulted from retrograde axonal transport. In the photic-vestibular pathways, the highest silver grain densities were found over structures (cell bodies or axon tracts) with increased synaptic activity coupled with higher levels of cellular activity (i.e. increased excitatory postsynaptic potentials or increased spontaneous impulse activity). Slightly less label was found in cells which received increased numbers of inhibitory postsynaptic potentials that produced hyperpolarization and a transient cessation of impulse activity under conditions of illumination. Therefore, the uptake levels of [3H]agmatine as revealed by autoradiography appear to reflect not only changes in sensitivity or density of ACh-activated channels but also changes in cellular activity as indicated by increased amounts of retrograde transport. These results represent the first example of the effective use of this radiolabel as an indicator of synaptic activity in invertebrates and in sensory systems.

Radioautographic evidence that axons from the area of supraoptic nuclei in the rat project to extrahypothalamic brain regions

The axonal efferents of neurons of the supraoptic nucleus area were studied by radioautography in the rat after discrete stereotaxic injections of [3H]leucine into this nucleus. Beside a densely labeled pathway running from the nucleus to the posterior pituitary through the internal median eminence, several of the visualized labeled axonal bundles were found to project into various extrahypothalamic regions, including the olfactory bulb, the cortex, the lateral habenula, the subcommissural organ, the amygdala, the mammillary bodies and the locus coeruleus. These results suggest that part of the vasopressin- or oxytocin-containing perikarya located in the supraoptic nucleus constitute the cells of origin of axons which also contain these peptides and which have already been shown to be present in the above extrahypothalamic areas. This also implies that, like the paraventricular nucleus, the supraoptic nucleus is also involved in central extrahypothalamic regulations.

Synaptic connections between serotoninergic axon terminals and tyrosine hydroxylase-immunoreactive neurons in the arcuate nucleus of the rat hypothalamus. A combination of electron microscopic autoradiography and immunocytochemistry

A combination of electron microscopic autoradiography and immunocytochemistry was used to examine the connections between serotoninergic axons and dopaminergic neurons of the arcuate nucleus of the rat hypothalamus. The serotoninergic elements were identified after selective uptake of tritiated serotonin and the dopaminergic neurons with tyrosine hydroxylase cytochemistry. Synaptic junctions between labeled serotoninergic nerve endings and tyrosine hydroxylase-immunoreactive neurons were observed, providing a structural basis for a direct influence of serotoninergic elements on tuberoinfundibular dopaminergic neurons.

A comparative radioautographic study of the bidirectional axonal and transcellular transport of different amino acids and sugars in the lamprey visual system

Following recent radioautographic evidence for the bidirectional axonal transport of [3H]proline from the eye of Lampetra fluviatilis, the present study examined the anterograde and/or retrograde labeling properties of 11 other amino acids and two sugars after intraocular injections of these tritiated substances in the lamprey. The intraocular injections of aspartic acid, glutamic acid, gamma-aminobutyric acid, arginine, phenylalanine, fucose and acetyl glucosamine resulted in a weak labeling of the primary visual centers: there was no evidence of either the transcellular transport of these tracers or the retrograde labeling of the retinopetal neurons. The primary visual system was found to be heavily labeled 24 h after eye injections of alanine, leucine, glycine, lysine, serine and valine. Among the latter only glycine produced a retrograde somatic labeling of retinopetal neurons. With a longer survival period (7 days), a specific transneuronal labeling was also noted after glycine injections. The significance of the differential uptake and transport of these different tracers in the lamprey visual system and possible mechanisms involved in the axonal retrograde transport of [3H]glycine and [3H]proline in the centrifugal pathways are discussed.

Demonstration of serotoninergic axons terminating on luteinizing hormone-releasing hormone neurons in the preoptic area of the rat using a combination of immunocytochemistry and high resolution autoradiography

The synaptic relationship between serotoninergic terminals and luteinizing hormone-releasing hormone-containing neurons was investigated in the medial preoptic area using a combined technique. Axon terminals selectively taking up 5-[3H]hydroxytryptamine were labelled autoradiographically and luteinizing hormone-releasing hormone-containing neuronal elements were identified by means of immunocytochemistry. Synaptic contacts were observed between tritiated 5-hydroxytryptamine-labelled boutons and luteinizing hormone-releasing hormone-immunoreactive dendrites. About 5% of the boutons which formed synapses with luteinizing hormone-releasing hormone-immunoreactive dendrites were found to be labelled by the tritiated indolamine. Luteinizing hormone-releasing hormone-immunoreactive axon terminals occurred as presynaptic elements in contact with unidentified dendritic spines, shafts or perikarya. These observations provide morphological basis for the idea that 5-hydroxytryptamine-containing neurons can act directly on luteinizing hormone-releasing hormone release. Further, they support the assumption that luteinizing hormone-releasing hormone is not only a neurohormone but may also function as a neurotransmitter or neuromodulator.

Selective retrograde axonal transport of free glycine in identified neurons of Aplysia

The specific retrograde axonal transport of free glycine within the identified neurons R3-14 of Aplysia californica was studied. The soma of the R3-14 neurons are located in the parietovisceral ganglion and their axons project down the branchial nerve to end in a large peripheral field. Using a double-chambered apparatus, the peripheral tissue was incubated in medium containing a 3H-amino acid for 4-48 hr, while the nerve and ganglion were isolated and perfused with plain or chemically altered medium. The nerve and ganglion were then either rapidly frozen for scintillation counting or fixed for autoradiography. When 3H-glycine was used, radioactivity entered the nerve rapidly, reached the ganglion in 3 hr, and was transported largely (greater than 80%) in the free amino acid form [trichloroacetic acid (TCA) soluble]. The right parietovisceral hemiganglion accumulated up to nine times more radioactivity than the left hemiganglion, reflecting the presence of the R3-14 axons and soma. Two phases of radioactivity were observed, a fast component moving at about 3 mm/hr and a slower (but larger) component moving at about 0.4 mm/hr. Light microscope autoradiography on nerves containing 3H-glycine revealed that the R3-14 axons accounted for more than 30% of the total label in the nerve but occupied less than 7% of the total cross-sectional area of the axonal core. Electron microscope autoradiography showed a close association of silver grains and dense core vesicles in the R3-14 axons. Retrograde axonal transport of free glycine was inhibited by (in decreasing order of effectiveness) mercuric chloride, vinblastine, colchicine, Nocodazole, and 2,4-dinitrophenol (2,4-DNP). Comparative studies of other amino acids [3H-leucine, 3H-serine, 3H-glutamic acid, 3H-gamma-aminobutyric acid (3H-GABA), and 3H-alanine] showed that 3H-glycine is the only amino acid that is rapidly axonally transported in large quantities within the R3-14 axons. This work demonstrates, for the first time, that a free amino acid, glycine, is transported in the retrograde direction within a select group of axons. The significance of this transport of glycine is discussed in relation to its use as a neural messenger by neurons R3-14.

Serotoninergic endings on VIP-neurons in the suprachiasmatic nucleus and on ACTH-neurons in the arcuate nucleus of the rat hypothalamus. A combination of high resolution autoradiography and electron microscopic immunocytochemistry

A combination of immunocytochemistry with transmitter specific autoradiography at the electron microscopic level was used to identify vasoactive intestinal polypeptide (VIP)- or ACTH-containing neurons and serotoninergic nerve terminals in the suprachiasmatic (SCN) and arcuate nuclei (AN). We observed nerve terminals showing selective uptake of [3H]serotonin forming synaptic contact with perikarya or dendrites where the postsynaptic structures exhibited VIP-like immunoreactivity in the SCN and ACTH-like immunoreactivity in the AN. The findings provide ultrastructural evidence that serotoninergic nerve fibers terminate on VIP-neurons in the SCN and on ACTH-neurons in the AN.