Donald J. Jenden

A cholinergic mechanism in the brainstem reticular formation: Induction of paradoxical sleep

Abstract Microinjection of oxotremorine (a muscarinic agent) or carbachol into the pontomesencephalic reticular formation in conscious cats causes atonia, cortical desynchronization and synchronization of hippocampal activity. Some animals present a behavioral and electrographic picture indistinguishable from paradoxical sleep (PRS), while others, judging from EEG patterns and eye activity, appear to be alert despite the absence of muscular tone. These effects may be induced by as little as 0.2 of either drug, and persist for 45–60 min; they are antagonized by atropine sulfate administered either focally into the same sites, or systemically. These findings constitute evidence for the existence of a cholinergic mechanism in this area of the reticular formation. It is concluded that the receptor sites involved are muscarinic, and that the effects described most probably occur as a result of the selective activation of inhibitory neurones. A method for recording sleep cycles, based upon the measurement of hippocampal theta wave activity, is presented.

Simultaneous measurement of endogenous and deuterium-labeled tracer variants of choline and acetylcholine in subpicomole quantities by gas chromatography-mass spectrometry.

Abstract A method is described for the simultaneous analysis of endogenous and d 4 -labeled tracer variants of choline and acetylcholine by a gas chromatograph/mass spectrometer system operated in the specific ion detection mode, using d 9 -labeled variants of both choline and acetylcholine as internal standards. The detection limit is approximately 10 −13 moles of both tracer and endogenous variants.

In vivo proton magnetic resonance spectroscopy of the normal aging human brain

The effect of age on brain metabolite concentrations was evaluated using localized proton magnetic resonance spectroscopy. This technique allows in vivo measurements of N-acetyl compounds (NA), total creatine (CR), choline-containing compounds (CHO), myo-inositol (MI), glutamate and glutamine (GLX), as well as the percentage of cerebrospinal fluid (CSF) and the brain water content within the brain region studied. Frontal gray matter and frontal white matter brain regions were examined in 36 normal healthy volunteers (19-78 years of age). Using a rigorous absolute quantitation method, with an external reference and atrophy correction, we found relatively stable concentrations of NA, a neuronal marker. In contrast, CR, CHO, MI, and the percentage of CSF increased in the gray matter with age. However, the brain water content decreased significantly with age (r = -0.72; p < 0.0001). No significant age-related changes in metabolite concentrations, CSF or brain water content were observed in the white matter regions. These findings demonstrate that biochemical alterations are associated with aging in the frontal gray matter. There might be an increase in the brain density as indicated by increased metabolite concentrations and decreased brain water content with aging.

Plasma choline: its turnover and exchange with brain choline.

—The kinetics of plasma choline (Ch) and the uptake of plasma Ch into the brain were studied by means of intravenous infusion of [2H4]Ch at various rates into anaesthetized and conscious rats. [2H4]Ch levels in both arterial and venous plasma at steady state were linearly related to the infusion rate; however, unlabelled Ch levels were independent of infusion rate. [2H4]Ch levels were higher in the arterial plasma than in the venous plasma, while unlabelled Ch levels were higher in the venous plasma than in the arterial plasma. It was concluded that Ch is being generated in the brain and is released into the venous effluent. The supply of Ch to the plasma is not decreased if the plasma Ch level is increased. The clearance and turnover of Ch in the compartment of its initial distribution are 75 ml kg‐1 min‐1 and 716 nmol kg‐1 min‐1, respectively. The uptake of plasma Ch into the brain is not saturated even at very high levels of plasma Ch.

The source of choline for acetylcholine synthesis in brain.

Abstract More is known about the synthesis and metabolism of acetylcholine (ACh) than other choline (Ch) containing compounds in the brain in spite of the fact that ACh represents only a small fraction of the total Ch esters. This review will attempt to summarize the evidence for the source of Ch in the brain and its relation to the turnover of ACh. Ch is a precursor not only for ACh but also for phosphoryl Ch and phospholipids. It appears that in the rat a bound form of Ch in the brain can produce free Ch which can leave the brain, be converted to ACh or be reutilized for phospholipid synthesis. There is evidence that one of the sources of free Ch that is utilized for ACh synthesis is outside the cholinergic nerve terminal.

Lecithin increases plasma free choline and decreases hepatic steatosis in long-term total parenteral nutrition patients

Plasma-free choline levels have previously been found below normal in patients receiving long term parenteral nutrition (TPN). In a group of 15 patients receiving home TPN who had low plasma free choline levels (6.3 +/- 0.8 mmol/L), we found 50% had hepatic steatosis. These patients were given oral lecithin or placebo in a double-blind randomized trial for 6 weeks. Lecithin supplementation led to an increase in plasma free choline of 53.4% +/- 15.4% at 2 weeks (P = 0.04), which continued at 6 weeks. The placebo group had no change in plasma-free choline at 2 weeks, but a significant decrease of 25.4% +/- 7.1% (P = 0.01) at 6 weeks. A significant and progressive decrease in hepatic fat was indicated by increased liver-spleen CT Hounsfield units at 2 and 6 weeks (7.5 +/- 1.7 units, P = 0.02; 13.8 +/- 3.5 units, P = 0.03) in the lecithin supplemental group. Nonsignificant changes were seen in the placebo group. It was concluded that hepatic steatosis in many patients receiving long term TPN is caused by plasma-free choline deficiency and may be reversed with lecithin supplementation. Choline is a conditionally essential nutrient in this population.

Hypothermia following systematic and intracerebral injection of oxotremorine in the rat

Abstract Intravenous administration of oxotremorine leads to marked hypothermia in the rat. The fall in core temperature can be reproduced by microinjection of oxotremorine into the preoptic/anterior hypothalamic nuclei. The hypothermie response to intracerebral oxo-tremorine is blocked by prior intraperitoneal administration of atropine or trihexyphenidyl. Injection of carbachol into the anterior hypothalamus led to a fall in temperature of the same order as that produced by oxotremorine. It is concluded that the hypothermie action of oxotremorine is due to a direct effect on the thermoregulatory centers. These results also suggest that the effect of oxotremorine on body temperature is due to a muscarinic action of the drug.

The attenuation of a specific cue-to-consequence association by antiemetic agents.

Previous research has shown that rats develop a conditioned taste aversion after a single pairing of a distinct taste and subsequent toxicosis. The experiments reported here test the hypothesis that the expression of a taste aversion may reflect classically conditioned nausea mediated by activation of brainstem emetic centers by taste stimuli. Rats were allowed to drink a saccharin solution (1 g/l) and 10 min later were intubated with LiCl (180 mg/kg) to produce nausea. When control rats were posttested for saccharin preference they consumed less than 50% of their pretest intake. Experimental rats were injected with one of four pharmacologically distinct antiemetic drugs 30 min prior to their posttest with saccharin. Each drug significantly attenuated the aversion to saccharin at one dose level. The antiemetic drugs we used were scopolamine HBr, cyclizine, prochlorperazine dimaleate, and trimethobenzamide. These drugs had no effect on the conditioned fear of a noise that signaled foot shock or on a natural aversion to a bitter fluid (quinine monohydrochloride, 100 mg/l). Our data suggest that pharmacological suppression of the neural mechanisms of emesis selectively disrupts conditioned taste aversions, and that moderate dose levels are critical for obtaining this effect.

Further characterizations of the nature of the behavioral and neurochemical effects of lesions to the nucleus basalis of Meynert in the rat.

Converging lines of evidence indicate an important role for the basal forebrain cholinergic system in memory processes. The principal origin of the cholinergic projection to the neocortex appears to be the magnocellular neurons in the region of the nucleus basalis of Meynert (NbM). We examined the effects of bilateral lesions of the NbM on retention of shock avoidance training by stereotaxically injecting rats with 0.5 microliter of ibotenic acid (14 micrograms/microliter) into the NbM. Two weeks later rats were given passive avoidance training and tested for retention of the original avoidance habit 5 min, 30 min, or 24 hr later. Rats with lesions of the NbM showed significantly impaired shock avoidance performance compared to non-operated controls at both 30 min and 24 hr, but not at 5 min after training. Lesioned animals also showed a significant decrease in cortical choline acetyltransferase (CAT) and acetylcholinesterase (AChE) activities. No differences in muscarinic receptor binding or plasma cholinesterase activity was observed. The results demonstrate the usefulness of NbM lesions as a model for studying the role of the basal forebrain cholinergic system in memory processes.

Acetylcholine turnover estimation in brain by gas chromatography/mass spectrometry

Abstract A gas chromatograph/quadrupole mass spectrometer system has been employed to estimate the turnover of acetylcholine in mouse brain in vivo following a pulse intravenous injection of choline, using discrete deuterium labelled variants of choline and acetylcholine as tracer and internal standards. There appear to be two functional pools with turnover rates of 1.4 and 7.9 nmol gm −1 min −1 .