Use of tissue culture to examine the actions of guanethidine and 6-hydroxydopamine

Prolonged recycling of internalized neurotrophins in the nerve terminal

BACKGROUND

Neurons require contact with their target tissue in order to survive and make correct connections. The retrograde axonal transport of neurotrophins occurs after receptor-mediated endocytosis into vesicles at the nerve terminal. However, the mechanism by which the neurotrophin signal is propagated from axon terminal to cell body remains unclear.

METHODS

Retrograde axonal transport was examined using the transport of I(125)-labeled neurotrophins from the eye to sympathetic and sensory ganglia. The phenomena was further studied by adding rhodamine-labeled nerve growth factor (NGF) to cultures of dissociated sympathetic ganglia and the movement of organelles followed with the aid of video microscopy.

RESULTS

I(125)-labeled neurotrophins were transported from the eye to the sympathetic and sensory ganglia. A 100-fold excess of unlabeled neurotrophin, administered up to 4 h after the labeled material, completely prevented accumulation of labeled neurotrophin in the ganglia. The effect was specific for the labeled neurotrophin as administration of a high concentration of a different neurotrophin failed to inhibit the transport. In dissociated cultures, we found rapid binding of label, to surface membrane receptors, followed by an accumulation of labeled vesicles in the growth cone. Incubation of these cultures with unlabeled NGF led to a rapid loss of label in the growth cones.

CONCLUSIONS

These results suggest that there is a pool of internalized neurotrophin, in vesicles in the nerve terminal, which is in rapid equilibrium with the external environment. It is from this pool that a small fraction of the neurotrophin-containing vesicles is targeted for retrograde transport. Potential models for this system are presented.

Signalling organelle for retrograde axonal transport of internalized neurotrophins from the nerve terminal

The retrograde axonal transport of neurotrophins occurs after receptor-mediated endocytosis into vesicles at the nerve terminal. We have been investigating the process of targeting these vesicles for retrograde transport, by examining the transport of [125I]-labelled neurotrophins from the eye to sympathetic and sensory ganglia. With the aid of confocal microscopy, we examined the phenomena further in cultures of dissociated sympathetic ganglia to which rhodamine-labelled nerve growth factor (NGF) was added. We found the label in large vesicles in the growth cone and axons. Light microscopic examination of the sympathetic nerve trunk in vivo also showed the retrogradely transported material to be sporadically located in large structures in the axons. Ultrastructural examination of the sympathetic nerve trunk after the transport of NGF bound to gold particles showed the label to be concentrated in relatively few large organelles that consisted of accumulations of multivesicular bodies. These results suggest that in vivo NGF is transported in specialized organelles that require assembly in the nerve terminal.

Neuronal degeneration and lipopigment formation in rat sympathetic ganglion after treatment with high-dose guanethidine

Degenerative changes in the rat superior cervical ganglion (SCG) were studied 3 months after treatment with a high-dose of guanethidine using microspectrofluorometry and electron microscopy. The density of the SCG neurons was decreased by 30% and by 60% following 1 and 2 weeks treatment respectively with guanethidine. Intraneuronal lipopigments were increased after the treatment, whereas cytoplasmic catecholamines were decreased. Electron microscopy revealed swollen mitochondria and a novel type of lipo-pigment in surviving neurons. The number and structure of small intensely fluorescent (SIF) cells remained unchanged after the treatment. The results suggest that accelerated lipid peroxidation processes might be involved in guanethidine neurotoxicity and that intraneuronal concentrations of catecholamines may be related to this toxicity.

Subarachnoid hemorrhage in the rat: cerebral blood flow and glucose metabolism after selective lesions of the catecholamine systems in the brainstem

A double-isotope autoradiographic technique was used to evaluate CBF and glucose metabolism 2 days after a subarachnoid hemorrhage (SAH) in rats with lesions in the lower brainstem. Lesioning in the mesencephalon of the ascending catecholamine pathways from locus ceruleus and from the A1 and A2 nuclei, or lesioning in the medulla oblongata of the ascending fibers from A1 and A2, prevents the development of the global changes in flow and metabolism seen in normal animals post SAH. Also the focal low-flow areas with markedly elevated deoxyglucose uptake, which can develop in normal animals 2 days post SAH, were not seen in the lesioned animals after the SAH. The findings indicate that the A1 and A2 nuclei, which project to the hypothalamus-pituitary, are essential for the flow and metabolic changes after an SAH. The lesions per se did not change baseline flow and metabolism as compared with sham-lesioned animals.

Toxicology investigations with cell culture systems

This review concerns some of the cell culture systems that are most frequently used in toxicology investigations. In particular, it sets out to evaluate the effectiveness of these cell culture systems in assessing the toxic potential of chemicals. Metabolic studies and general and specific toxicology investigations are highlighted. Specific toxicology investigations relate to the effects of the tests substances on the highly specialized functions typical of the cell systems chosen. The general toxicology investigations include most of the other studies where differentiated or undifferentiated cells have been used to evaluate the effects of the tested substances on common basic biochemical processes essential for life. Lastly, we have attempted to focus attention on the most promising applications of cell cultures in toxicology studies for the near future and to identify those areas where further research is needed. Because of the several excellent reviews that already exist, we have decided not to consider cell cultures utilized in screening potential mutagens and carcinogens. We have also excluded investigations of drug therapeutic effects and action mechanisms of drugs.

Biochemical differentiation of aggregating cell cultures of different fetal rat brain regions

Rotation-mediated aggregating cell cultures of mechanically dissociated fetal rat brains divided into three (telencephalon, mesencephalon-diencephalon and rhombencephalon), or two (telencephalon and mesencephalon-diencephalon plus rhombencephalon) parts were examined for their biochemical differentiation by measuring the specific activities of choline acetyltransferase, acetylcholinesterase, glutamic acid decarboxylase, tyrosine 3-monooxygenase, aromatic L-amino acid decarboxylase, catechol methyltransferase and monoamine oxidase. The results showed that such parts yielded cultures that were relatively enriched for acetylcholine-synthesizing (telencephalon) or catecholamine-synthesizing (mesencephalon-diencephalon and mesencephalon-diencephalon plus rhombencephalon) enzymes. For cultures which were derived from two brain divisions, the sum of the total activity for each enzyme in the parts after 30 days equalled that in whole brain cultures derived from the same group of embryos, suggesting that development of these enzymes was unaffected by division of the brain in two. In experiments to determine the effects of culture conditions on this development, chronic administration of certain drugs was found to selectively influence the specific activity of certain neurotransmitter metabolizing enzymes. Thus, in cultures of whole brain, ascorbic acid (0.2 mM) decreased tyrosine 3-monooxygenase and aromatic L-amino acid decarboxylase while other enzymes were slightly increased; and in cultures of telencephalon and mesencephalon-diencephalon plus rhombencephalon, N6, O2'-dibutyryladenosine 3',5'-cyclic phosphate (0.2 mM) decreased the specific activities of choline acetyltransferase acetylcholinesterase, glutamic acid decarboxylase and monoamine oxidase. These results demonstrate the feasibility of growing these cultures for pharmacological studies in developmental neurobiology.

Selectivity of neuronal degeneration produced by chronic guanethidine treatment

Chronic guanethidine treatment of rats produced extensive damage to sympathetic neurons of the superior cervical ganglion and pelvic plexus. No ultrastuctural changes were observed in parasympathetic cholinergic neurons in the ciliary ganglion and pelvic plexus, nor in sensory neurons in nodose and dorsal root ganglia. A total of only six nerve cell bodies free of degenerative changes were observed in sections of superior cervical ganglia from 20 rats. This suggests either that the earlier estimates of 5% cholinergic neurons in the superior cervical ganglion based on acetylcholinesterase staining are too high, or implies that sympathetic cholinergic neurons, unlike parasympathetic neurons, are damaged by chronic guanethidine treatment.

Food intake and body weight: effects of specific and non-specific lesions in the midbrain path of the ascending noradrenergic neurons of the rat

Bilateral injections of 4.8 or 12 microgram of 6-hydroxydopamine hydrobromide were made in the midbrain path of the ascending norepinephrine (NE) fibers. Maximum NE depletion was obtained with the 4 microgram dose. Increasing the dose did not increase NE depletion in either the hypothalamus or the telencephalon. Increases in food intake were observed following the 12 microgram dose only. Electrolytic, copper sulfate, or 5,6-dihydroxytryptamine lesions at the same site caused increases in both food intake and body weight with only moderate (-32 to -58%) decreases in NE levels. Behavioral changes were seen in only those groups in which a large amount of non-specific tissue damage was also observed. Therefore, the behavioral effects were attributed to general tissue damage in the area and not the NE fibers specifically.

Intracranial injection of drugs: comparison of diffusion of 6-OHDA and guanethidine

Marked differences in extent of diffusion have been shown with the fluorescence histochemical method between guanethidine and 6-OHDA(64 mug in 2 mul) when injected acutely or chronically into the lateral hypothalamus, the substantia nigra or the amygdala of the rat brain. Cannulation damage up to 1 mm in diameter and attributed to the implantation of cannulae and placebo injection was observed. A further area of generalized damage occurred following the injection of drugs and was far greater for 6-OHDA (2 mm) than for guanethidien (0.3 mm). Guanethidine, but not 6-OHDA, caused specific damage to catecholamine-containing nerurons up to a distance of at least 3 mm and more from the cannula tip. These striking differences between the effects of intracranial injection of 6-OHDA and guanethidine are discussed in terms of the uptake and degradation of the two drugs and the anatomical features of the injection site; they are not explicable in terms of experimental conditions such as concentration, volume of injection, molecular weight or lipid solubility. The different patterns of damage would not easily be distinguished by biochemical analyses and the catecholamine specificity of 6-OHDA in studies of the central nervous system must be seriously questioned. Vascularization of chronically implanted cannula tracks and the presence of anatomical diffusion barriers are also discussed in relation to the diffusion of drugs injected intracranially.