Radioactive choline uptake in the isolated rat phrenic nerve-hemidiaphragm preparation. A biochemical and autoradiographic study

Uptake and metabolism of [3H]choline by the rat phrenic nerve-hemidiaphragm preparation

A whole nerve-muscle preparation (about 160 mg) or an end-plate preparation (about 25 mg) of the rat phrenic nerve-hemidiaphragm were incubated with [3H]choline, to investigate choline uptake and choline metabolism. Choline uptake was measured from the disappearance of choline from the incubation medium during the loading period and from the retention of tritium in the tissue after the loading and washout period. Based on the results obtained with both methods the end-plate preparation takes up three times as much choline than the whole nerve-muscle preparation or a small muscle strip that was cut outside the end-plate region and had a similar size as the end-plate preparation. Choline uptake was not markedly affected by the degree of nerve activity or by a chronic denervation. However, hemicholinium-3 significantly reduced (50%) the choline uptake by the end-plate preparation. Most of the choline (70-88%) taken up was metabolized and incorporated into membrane structures. Phosphatidylcholine was the predominant metabolite in both preparations. The ratio of phosphatidylcholine/lysophosphatidylcholine in the end-plate preparation (16) was significantly lower than in the whole nerve-muscle preparation (31). This might indicate a higher metabolism of phosphatidylcholine in the end-plate preparation. It is suggested that choline uptake by the rat phrenic nerve-hemidiaphragm occurs mainly by the muscle fibres. The innervated part of the muscle fibres can accumulate more choline than the peripheral part outside the end-plate region, probably because of a very active choline phospholipid metabolism within the end-plate region.

Dexamethasone reduces the high-affinity binding of hemicholinium-3 to rat phrenic nerve endings in vitro: an autoradiographic study

Receptor autoradiography has revealed high-affinity binding sites for hemicholinium-3 (HC-3) in the phrenic nerve endings of rat diaphragm. It has been demonstrated that 200 nM dexamethasone (Dex) in vitro reduces the binding of HC-3 to these high-affinity sites. It seems likely that the decrease in inhibitory effects of HC-3 on choline uptake and acetylcholine synthesis in rat diaphragm caused by Dex, should be the result of an interference of Dex with the binding of HC-3 to its high affinity sites.

Retrograde labeling of neurons in the brain stem following injections of [3H]choline into the rat spinal cord

In an attempt to identify cholinergic neurons in the brain stem which project to the spinal cord, [3H]choline (100, 20, 10, 5 or 1 microCi) was injected into the upper cervical spinal cord in 55 rats. The animals were killed 20 h later and the brains processed for autoradiography of diffusible substances. At all doses of [3H]choline, cells were consistently, retrogradely labeled in the medical medullary reticular formation, the lateral vestibular nucleus, the dorsolateral pontine tegmentum and the red nucleus. The retrogradely labeled cells were found to be moderately to darkly stained for acetylcholinesterase. Injection of [3H]noradrenaline (50 microCi) into the upper cervical spinal cord resulted in retrograde labeling of cells in the locus coeruleus, subcoeruleus and the ventrolateral pontine tegmentum, that correspond in position to the neurons of the A6, A7 and A5 catecholamine cell groups, respectively. Injection of [3H]serotonin (20 microCi) into the upper cervical spinal cord was associated with retrograde labeling of cells in the raphe pallidus, obscurus and magnus nuclei that correspond in position to those of the B1, B2 and B3 serotonin cell groups, respectively. Injection of True Blue into the upper cervical spinal cord was followed by retrograde labeling of a large number of cells located in the areas where cells were retrogradely labeled by [3H]choline, [3H]noradrenaline and [3H]serotonin, and additionally, in the solitary tract nucleus, the lateral, parvicellular medullary reticular formation, the caudal and oral pontine reticular formation, the mesencephalic reticular formation and the superior colliculus. These results indicate that from the cervical spinal cord, [3H]choline selectively retrogradely labels a certain population of non-monaminergic, acetylcholinesterase-positive cells localized in the medial medullary, and secondarily the dorsolateral pontine, reticular formation, the lateral vestibular nucleus, and the red nucleus.

Dexamethasone affects radioactive choline uptake in rat diaphragm nerve endings and not in muscle fibres

The initial rate of radioactive choline (Ch) uptake in the endplate-rich area (EPA) of both stimulated and unstimulated hemidiaphragms is significantly increased by 0.2 microM dexamethasone (Dex) in the presence of 10 microM Ch. In autoradiographs, the mean grain densities above the muscle fibres are not altered by Dex. The mean grain densities above the nerve endings are significantly increased in the presence of Dex in stimulated tissue, and slightly but not significantly increased in unstimulated tissue. There is a positive correlation between the initial rate of Ch uptake in the EPA and the amount of isotope in the nerve terminals, in the absence and presence of Dex. Without correcting for the large amount of diffusion which occurs, the ratio of the grain densities above the nerve terminals to that above the muscle fibres in the presence of Dex is 2.12 in stimulated tissue, and 1.40 in unstimulated tissue. The ratio in the stimulated tissue is significantly greater than the control ratio in the absence of Dex (1.66). Therefore, Dex affects radioactive Ch uptake in nerve endings and not in muscle fibres in the rat diaphragm. The stimulation-induced increase in the uptake of isotope into the nerve endings is abolished in a Na+-depleted medium, and in the absence of Ca2+. Dex has no effect on this abolition. We conclude that relatively low concentrations of Dex affect Ch transport in rat diaphragm nerve endings by a mechanism as yet to be defined.

Depletion of total acetylcholine by hemicholinium-3 in isolated rat diaphragm is less in the presence of dexamethasone

Low concentrations of dexamethasone (Dex) stimulate the initial rate of radioactive choline (Ch) accumulation in the endplate-rich area (EPA) of indirectly stimulated hemidiaphragms, while higher concentrations (greater than 0.6 microM) inhibit. This biphasic concentration-effect curve is found even in the presence of 26 microM hemicholinium-3 (HC-3), an inhibitor of Ch accumulation. In incubations (3 min) where the total hemidiaphragm acetylcholine (ACh) content is not altered by 26 microM HC-3, the inhibition by HC-3 of both the Ch accumulation rate and the incorporation of radioactive Ch into ACh in the EPA of stimulated tissues is less in the presence of 0.2 microM Dex. In 120 min incubations with 15 microM HC-3 and without added Ch, the tissue ACh content is depleted in both stimulated and unstimulated hemidiaphragms. In both cases the depletion of ACh is significantly less in the presence of 0.2 microM Dex. In stimulated tissues a comparable depletion of ACh due to 15 microM HC-3 is also found with 1 and 10 microM Ch added to the medium. It is significantly less when 0.2 microM Dex and 1 microM Ch are added to the medium. In 120 min incubations with stimulated tissue, the amount of "bound' ACh is increased by addition of 30 microM Ch to the medium, decreased in the presence of 0.2 microM Dex, and greatly decreased in the presence of 15 microM HC-3. In the presence of Dex plus HC-3, the decrease in the amount of "bound' ACh due to either Dex or HC-3 alone, is abolished provided that 30 microM Ch is also present.(ABSTRACT TRUNCATED AT 250 WORDS)