Carnitine,carnitine acyltransferases,and rat brain function

The concentrations of free, short chain, and long chain acylcarnitines and the enzyme activities of carnitine acetyltransferase (CAT) and carnitine palmityltransferase (CPT) were studied in different rat brain regions. The fate of tritium-labeled carnitine was studied in different brain regions in vivo after i.p. injection in 3-month-old Sprague-Dawley rats. The tritium counts were particularly high in the hypothalamic region. At 24 h after hydrocortisone injection, a significant increase in counts was observed in the hypothalamus (P < 0.01). A high concentration of total carnitine was found in the hypothalamus (4.00 nmol/mg noncollagen protein) and in other regions such as the spinal cord (1.29 nmol/mg noncollagen protein), cerebellum (1.19), and olfactory tracts (0.66) carnitine concentration was much lower. Carnitine content was proportional to CPT, an inner mitochondrial enzyme. The activity of the enzyme CAT was found to be high in rat hippocampus and hypothalamus. This enzyme in brain may be involved in the transport of acyl groups outside the mitochondria and in the regulation of pyruvate utilization, contributing to acetylcholine synthesis or regulation.     

Estimation of brain sensitivity to the convulsive effects of choline and changes induced by chronic barbital treatments in the rat

A threshold method to test sensitivity to the convulsive effect of choline has been developed. Choline was infused at a constant rate in the tail vein of rats pretreated with methylatropine (2 mg/kg). The dose needed to induce a clear-cut convulsion was determined. The threshold dose was found to be reproducible. The optimal rate of the choline infusion was around 30 mg/kg/min. The threshold could be increased by pretreatment with atropine (8 mg/kg i.p. 1.5 h before the choline threshold). On day 3 of abstinence after long-term barbital treatment (32–33 weeks) no change in choline threshold was found. At that time other signs of abstinence such as spontaneous convulsions reached a maximum. However, later in the abstinence period (day 10–12) there was an increased sensitivity to choline. This increased sensitivity late in the abstinence period could be reduced by a single dose of atropine given slightly more than a week earlier (on day 3 of abstinence). Experiments with pilocarpine showed that sensitivity to the convulsive effect was increased on day 3 of abstinence. A similarly increased sensitivity to the temperature reducing effect of pilocarpine had been shown earlier. The pilocarpine-induced convulsions as such could reduce the tolerance to a hexobarbital threshold recorded in rats previously treated with barbital. The time difference between the action of choline and of pilocarpine during abstinence indicates that there could be more than one cholinergic mechanism involved in the changes during the abstinence after barbiturates.     

Aminergic and cholinergic systems in the dorsolateral pontine tegmentum

Topographic relationship between cholinergic and aminergic neuronal somata has been studied in the dorsolateral pontine tegmentum by using three histochemical techniques, acetylcholinesterase (AchE) histochemistry in rats having received systemic injection of diisopropylfluorophosphate, choline acetyltransferase (CAT) immunohistochemistry and catecholamine fluorescence histochemistry. Based on comparing the data obtained from these different techniques, it seems evident that all noradrenergic neurons contain AchE. The remaining population of AchE-containing somata appears to correspond with CAT-containing, therefore, cholinergic neuronal cell bodies. No AchE-positive perikarya were detected in neuronal structures other than cholinergic and aminergic neurons. In addition, coexistence of noradrenaline and acetylcholine in a single cell seems improbable, at least, in the dorsolateral pontine tegmentum.     

Direct inhibition of choline acetyltransferase activity by a monoclonal antibody raised against the plasma membrane of cholinergic nerve terminals

A monoclonal antibody raised against cholinergic synaptosomal plasma membranes isolated from Torpedo electric organ, inhibited completely amphiphilic and hydrophilic choline acetyltransferase (ChAT) activities extracted and separated by using Triton X-114 phase partition of synaptosomes. We tested whether ChAT inhibition was direct or not. We found that the antibody was inhibiting ChAT in preparations of very low purity as well as ChAT that was immunoprecipitated by using a non-inhibitory anti-ChAT polyclonal antibody. Also, inclusion of acetylcoenzyme A at 20 times its K_m during incubation of ChAT and antibody, completely prevented ChAT inhibition. This same concentration of the ChAT substrate could significantly but not completely dissociate the complex enzyme-antibody. These results spoke in favour of a direct inhibition of ChAT; the antibody most probably binds to an epitope that may be located at or near the acetylcoenzyme A binding site. The inhibitory effect on hydrophilic and amphiphilic ChAT was dependent on the antibody concentration, but amphiphilic activity required higher concentrations to be affected to the same extent as hydrophilic activity. This was found not only with Torpedo, but also with rat and human ChAT activities. Thus, the antibody appears to be able to distinguish the two forms of ChAT activity.     

Lack of correlation between cortical levels of choline acetyltransferase and learning scores in rats with globus pallidus lesions

Previous work has shown that rats with lesions of the globus pallidus (GP) exhibit a generalized learning impairment. Data are presented suggesting that this impairment is not due to inadvertent damage to the nucleus basalis magnocellularis. Rats with GP lesions evidenced a significant visual discrimination learning loss and a significant reduction in cortical choline acetyltransferase (ChAT) activity. However, there was no significant correlation between the severity of the learning loss and the amount of reduction of cortical ChAT activity.     

Neural regulation of glucose 6-phosphate dehydrogenase in rat muscle: effect of denervation and reinnervation

We tested the hypothesis that glucose 6-phosphate dehydrogenase (G6PD) in rat extensor digitorum longus (EDL) muscle is under neural control by studying changes in G6PD activity in EDL muscles following nerve crush-induced denervation and reinnervation. Changes in G6PD were correlated with choline acetyltransferase activity, as well as with neurological function, muscle weights, and muscle isometric twitch tension. The data show a dramatic increase in G6PD following denervation. The gradual recovery of enzyme activity toward normal levels correlates with the return of functional synaptogenesis manifested by the return of neurological function, choline acetyltransferase, and muscle twitch tension. We conclude, therefore, that muscle G6PD is under neural control. G6PD activity provides a facile biochemical indicator of muscle reinnervation.     

Brain renin: localization in rat brain synaptosomal fractions

The distribution of brain renin activity was determined in subcellular fractions of rat brain prepared by discontinuous density gradient centrifugation. The highest amounts of brain renin activity occurred in both the light and heavy synaptosomal fractions, while the activity of choline acetyltransferase was elevated only in the light synaptosomal fraction. These results indicate an intraneuronal localization of brain renin.     

Deoxyglucose uptake and choline acetyltransferase activity in cerebral cortex following lesions of the nucleus basalis magnocellularis

The uptake of [3H]2-deoxyglucose (2-DG) into various brain regions of rats with unilateral or bilateral lesions of the nucleus basalis magnocellularis (nBM) was measured. The activity of choline acetyltransferase (ChAT) in these brain regions was also determined. Lesions of the nBM caused a significant decrease in cortical ChAT activity but had no effect on 2-DG accumulation. Pentobarbital treatment reduced 2-DG accumulation in all brain areas examined and these reductions were not influenced by the nBM lesions. The results indicate that a decrease in the cholinergic innervation of the cortex does not influence cortical glucose utilization. It appears unlikely, therefore, that the reported decrease in cortical glucose utilization in Alzheimer's disease is related to degeneration of the nBM-cortical cholinergic projection.     

Estrogen treatment suppresses forebrain p75 neurotrophin receptor expression in aged,noncycling female rats

There is increasing evidence that estrogen has beneficial effects on cognition, both in humans and in rodents, and may delay Alzheimer's disease onset in postmenopausal women. Several rodent studies have utilised the ovariectomy model to show estrogen regulation of the p75 neurotrophin receptor, TrkA, and markers of acetylcholine synthesis in the cholinergic basal forebrain. We studied estrogenic effects in aged (16-17-month-old), noncycling rats. Estrogen treatment for 10 days drastically reduced p75(NTR) immunoreactivity in the rostral parts of the basal forebrain. The number of p75(NTR)-immunoreactive neurons was decreased, and those neurons remaining positive for p75(NTR) showed reduced p75(NTR) staining intensity. In vehicle-treated rats, almost all choline acetyltransferase-immunoreactive neurons were p75(NTR) positive (and vice versa), but, in estrogen treated rats, large numbers of choline acetyltransferase-immunoreactive cells were negative for p75(NTR). Similar levels of p75(NTR) down-regulation in the rostral basal forebrain were found when estrogen treatment was extended to 6 weeks. There was no reduction in the number of p75(NTR)-immunoreactive neurons in the caudal basal forebrain after 10 days of treatment. After 6 weeks of treatment, however, there was evidence of p75(NTR) down-regulation in the caudal basal forebrain. There was no evidence of hypertrophy or atrophy of cholinergic neurons even after 6 weeks of estrogen treatment. Considering the evidence for the role of p75(NTR) in regulating survival, growth and nerve growth factor responsiveness of cholinergic basal forebrain neurons, the results indicate an important aspect of estrogen's effects on the nervous system.