Neurochemical changes following the administration of precursors of biogenic amines

L-tryptophan administration and increase in cerebral serotonin levels: Systematic review

The amino acid tryptophan (2-Amino-3-(lH-indol-3-yl)-propanoic acid; Trp) is a precursor of the neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) that performs various brain functions. The administration of Trp is used in experimental studies to manipulate the serotonergic system, however the dose of Trp required to raise brain 5-HT levels is controversial. The aim of this study was to systemically review the effect of the administration of different doses of Trp on cerebral 5-HT levels. Two independent authors conducted a systematic review in the electronic databases. Twenty-five studies were included in the present review. Trp was administered orally, intraperitoneally or subcutaneous in adult animals. The brain 5-HT levels elevated after Trp administration in different intensities, dependent of the brain region evaluated and the time of administration. Further studies are needed to assess the dose-response of Trp administration to brain 5-HT levels.

Acute amino acids supplementation enhances pituitary responsiveness in athletes

PURPOSE

The purpose of this study was to determine the effect of a mixture of amino acids on pituitary responsiveness to a stimulation test (GnRH + CRH) in athletes.

METHODS

In a double blinded counterbalanced experimental protocol, 10 moderately trained male athletes performed the pituitary stimulation test 60 min after a single oral administration of a placebo (P1-AS) or an amino acid mixture solution (AS) (L-arginine hydrochloride 100 mg x kg(-1) + L-ornithine hydrochloride 80 mg x kg(-1) + L-branched chain amino acids 140 mg x kg(-1): 50% L-leucine, 25% L-isoleucine, 25% L-valine) on two different occasions. Plasma ACTH, LH, FSH, GH, and cortisol were evaluated before (-60, -30, 0 min) and after (+15, +30, +45, +60, +90 min) the stimulation test.

RESULTS

The ACTH, LH and FSH response to CRH + GnRH was significantly higher in AS group both as absolute values and area under curve (AUC) values than in P1-AS group. Pre-test and post-test cortisol AUC levels were significantly higher in P1-AS group although a higher percent increase in post-test cortisol was found in AS group. The total GH-AUC was higher in AS group and, as expected, the absolute GH concentrations at different time points were not influenced by CRH + GnRH administration.

CONCLUSION

The amino acid mixture used enhanced the ACTH, LH, and FSH response to CRH + GnRH.

Carbamazepine increases extracellular serotonin concentration: lack of antagonism by tetrodotoxin or zero Ca2+

Carbamazepine administration causes large increases in extracellular serotonin concentration and dose-related anticonvulsant effects in genetically epilepsy-prone rats (GEPRs). In order to determine the generality of the effect on serotonin, we determined the anticonvulsant ED50 for carbamazepine against maximal electroshock seizures in outbred, non-epileptic Sprague-Dawley rats. We then administered anticonvulsant carbamazepine doses to Sprague-Dawley rats and observed extracellular serotonin concentration in hippocampi by way of microdialysis. We found that administration of carbamazepine, either systemically or through the dialysis probe, resulted in significant and dose-related increases in extracellular serotonin concentration. Basal serotonin release was decreased by tetrodotoxin administration through the dialysis probe. Tetrodotoxin administration through the dialysis probe did not decrease the effect of systemically or focally administered carbamazepine on extracellular serotonin concentration. Similarly, elimination of Ca2+ from the dialysate did not alter the release of serotonin caused by carbamazepine. These findings suggest that the serotonin releasing effect of carbamazepine does not take place by exocytosis and does not require action potentials in the brain area in which the release takes place. Further they suggest that the effect is mediated by an action of carbamazepine directly on serotonergic nerve terminals.

Persistent hypotensive effect of L-dopa given early during development to rats with inherited stress-induced arterial hypertension

A long-lasting decrease of the basal and stress-induced arterial blood pressure was obtained in rats with inherited emotional stress-induced arterial hypertension by means of injections of the dopamine precursor L-DOPA during early development (21-25 days after birth). The restoring effect of L-DOPA was produced through enhancement of synthesis of the brain noradrenaline and, perhaps, adrenaline. The effect was associated with a normalization of the response of the brain adrenergic system to noradrenaline and, presumably, with increase of tyrosine hydroxylase activity in the cortex and hindbrain.

Changes in mouse brain serotonin turnover following chronic imipramine administration

Mice were injected daily for 2 weeks with saline, tryptophan or p-chlorophenylalanine, alone or in combination with the tricyclic antidepressant imipramine. Serotonin turnover in several brain regions was determined by the accumulation of serotonin after pargyline and 5-hydroxyindoleacetic acid after probenecid. Both methods agreed closely. Imipramine tended to depress serotonin turnover. This effect was more marked in hypothalamus, which has high serotonin activity, and was much less in cerebellum, which has low serotonin activity. Chronic imipramine treatment completely abolished the increase in serotonin turnover induced by tryptophan, but had no effect on serotonin turnover in mice treated with p-chlorophenylalanine, which itself reduced serotonin activity.

Effect of increments in the concentration of dopamine in the central nervous system on audiogenic seizures in DBA/2J mice

The effect on audiogenic seizures of drug-induced increments in biogenic amines in the brain was determined in DBA/2J mice. One group of mice was treated with L-dihydroxyphenylalanine (L-DOPA) which caused a large rise in levels of norepinephrine and dopamine in the central nervous system, but did not significantly alter the concentration of 5-hydroxytryptamine. This group of animals exhibited a dramatic reduction in the incidence of tonic extensor seizures. A second group of animals that had been pretreated with diethyldithiocarbamate, a dopamine-beta-hydroxylase inhibitor, was also given L-DOPA. In this group of mice, there was a highly significant rise in the concentration of dopamine in brain but no statistically-significant changes in levels of either norepinephrine or 5-hydroxytryptamine. These animals also had a dramatic decrease in the incidence of tonic extensor seizures. A third group of animals that received only diethyldithiocarbamate did not exhibit any statistically-significant changes in the incidence of seizure or in levels of biogenic amines. The drug-induced reduction in the incidence of seizure in the first two groups correlated with a large increase in levels of dopamine in brain. This reduction in seizures did not correlate with changes in levels of norepinephrine or 5-hydroxytryptamine in brain.

An appraisal of the value of vitamin B12 in the prevention of motion sickness

Unpublished reports have suggested that hydroxycobalamin (B12, i.m.) prevents motion sickness. Some biomedical evidence supports this contention in that B12 influences the metabolism of histidine and choline; dietary precursors to neurotransmitters with established roles in motion sickness. Susceptibility to motion sickness was evaluated after B12 (1000 micrograms, i.m.). Subjects initially completed vestibular function and motion sickness susceptibility tests to establish normal vestibular function. The experimental motion stressor was a modified coriolis sickness susceptibility test. Subjects executed standardized head movements at successively higher RPM until a malaise III endpoint was reached. Following two baseline tests with this motion stressor, subjects received a B12 injection, a second injection two weeks later, and a final motion sickness test three weeks later. No significant differences in susceptibility were noted after B12. Hematological parameters revealed no B12 deficiency before injection. The possibility that patients with B12 deficiencies are more susceptible to motion sickness cannot be ruled out.

Motion sickness: a modulatory role for the central cholinergic nervous system

The present review has extended the general theory of motion sickness proposed by Wood and Graybiel [135, 136] by identifying specific neurophysiological mechanisms that are involved in motion sickness and by interpreting the actions of both scopolamine and amphetamine as effective anti-motion sickness drugs within this neurophysiological context. The neurochemical and neurophysiological effects of scopolamine have been reviewed in relationship to central cholinergic pathways. Cholinergic pathways have been associated with both the perception and expression of normal and excessive levels of motion stimuli. New approaches to the problem of the prevention of motion sickness have been considered. Efferent nicotinic innervation at the primary sensory hair cells and the medial vestibular nucleus were identified as sites where modulation by cholinergic drugs might exert a beneficial influence. However, it was generally conceded that the complexity of the cholinergic system and the interaction of scopolamine with that system left open the possibility that pharmacological doses of drugs specific to the cholinergic system might exert significant modulatory influences at alternative sites, as well.

Additional evidence that L-5-hydroxytryptophan discrimination models a unique serotonin receptor

The purpose of the present studies was to investigate further the role of central serotonin (5-HT) in mediating the L-5-hydroxytryptophan (L-5-HTP) discriminative cue. Rats were trained to discriminate the stimulus properties of 35 mg/kg L-5-HTP combined with RO 4-4602, a peripheral decarboxylase inhibitor. Considering that several 5-HT antagonists were unable to block the L-5-HTP discriminative cue in our earlier studies, we extended these studies to include the two other presumed 5-HT antagonists mianserin and BC-105 (pizotyline). Only BC-105 completely blocked the training dose of L-5-HTP. Furthermore, the blockade of the L-5-HTP cue was both graded and surmountable by increasing the dose of L-5-HTP, suggesting a competitive antagonism. In neurochemical studies, the regional brain levels of 5-HT, norepinephrine and dopamine were determined after the injection of the training dose of L-5-HTP. Marked changes in the levels of 5-HT were found, while the levels of the catecholamines were changed only slightly or not at all. Furthermore, dose-response studies of L-5-HTP demonstrated an orderly dose-related increase in the levels of 5-HT in brain and in the percent responding on the L-5-HTP lever, while no such relationship was found for brain catecholamines. These results agree with previous pharmacological studies and suggest that the L-5-HTP discrimination is mediated by a central 5-HT receptor that has pharmacological properties distinct from those receptors identified in previous behavioral models of 5-HT receptor stimulation.

Effect of 5,7-dihydroxytryptamine on serotonergic control of prolactin secretion and behavior in rats

The intracisternal administration of 5,7-dihydroxytryptamine (5,7-DHT) to rats resulted in a potentiated response to 5-hydroxytryptophan (5-HTP) when the animals were tested 30 days later. The 5-HTP-induced changes include elevation of serum prolactin, decrease in operant responding, and the magnitude of the "serotonin behavioral syndrome" observed after 5-HTP administration. The serotonin concentration in brains of 5,7-DHT-treated animals reached maximum earlier and remained elevated longer than that of controls following administration of 5-HTP. Brain norepinephrine and dopamine concentration were not affected by 5-HTP in either group of animals. The increase in serum prolactin concentration elicited by administration of the serotonergic agonists quipazine or 5-methoxy-N,N-dimethyltryptamine and by the serotonin uptake inhibitor fenfluramine also was potentiated by pretreating rats with 5,7-DHT. These data suggest that both serotonergic receptor supersensitivity and the absence of presynaptic uptake sites contribute to the enhanced responses to 5-HTP occurring in rats previously treated with 5,7-DHT. The findings further demonstrate that both behavioral and hormonal measures can be used to assess the sensitivity of serotonergic receptors and indicate that 5,7-DHT may be useful in evaluating the role of serotonergic neurons in neuroendocrine function.

Characterization of experimental phenylketonuria. Augmentation of hyperphenylalaninemia with alpha-methylphenylalanine and p-chlorophenylalanine

Phenylalanine in conjunction with p-chlorophenylalanine or alpha-methylphenylalanine was administered to suckling rats to induce hyperphenylalaninemia reminiscent of untreated phenylketonuria, and developmental parameters were monitored. The experimental model utilizing p-chlorophenylalanine was found to be unsatisfactory, in that the drug had general deleterious effects on growth, numerous side effects including increased mortality, and affected brain levels of biogenic monoamine neurotransmitters. The model utilizing alpha-methylphenylalanine was relatively free from nonspecific effects and thus, changes observed in the animals were attributable to experimental phenylketonuria. The latter animals had slightly decreased body and brain weights, and exhibited grossly elevated serum phenylalanine and urinary excretion of phenylketone metabolites. Hyperphenylalaninemia produced greatly disrupted brain amino acids at 10 days of age (prior to the formalization of the blood-brain barrier and specific transport systems) which was limited by 30 days of age to changes in glycine, gamma-aminobutyric acid and the aliphatic and aromatic amino acids which compete for uptake in the brain by a common carrier. These animals also exhibited a myelin deficit and changes in proteins from isolated nerve cell preparations. Mature animals which had daily treatment up to 60 days of age exhibited a long-term learning impairment. These observations are consistent with many aspects of the clinical picture of untreated phenylketonuric patients, and suggest that this animal model will be beneficial in studying the disease.

Acute effects of zimelidine and alaproclate, two inhibitors of serotonin uptake, on neuroendocrine function

The accumulation of 14C-5-hydroxytryptamine in human platelets in vitro and plasma levels of a number of hypophyseal hormones and cortisol in healthy male volunteers were determined after acute oral administration of zimelidine and alaproclate, two selective inhibitors of serotonin (5-HT) uptake. Alaproclate (100 mg) significantly inhibited the accumulation of 14C-5-HT by 42% at 90 minutes but showed no significant effect at 4 hours. At 200 mg the decrease in the accumulation was 55% after 90 minutes and 31% after 4 hours. Zimelidine (200 mg) caused a 72% decrease at 90 minutes and 73% at 4 hours. Plasma levels of prolactin, growth hormone, luteinizing hormone, follicle stimulating hormone, and thyroid stimulating hormone remained unchanged after zimelidine and alaproclate, and the levels were comparable to those after placebo. A physiological decline of plasma cortisol levels was noted in the morning during the test period of 4 hours, but there were slight differences in the secretory pattern after the different drugs used.

Serotonergic influences on food intake: effect of 5-hydroxytryptophan on parameters of feeding behaviour in deprived and free-feeding rats

Three experiments were carried out to examine the effect of the serotonin precursor, 5-hydroxytryptophan, upon food intake and the micro-structure of eating in deprived rats, and on the pattern of meal taking in free-feeding animals. The study also investigated the capacity of a peripheral decarboxylase inhibitor (MK-486) to antagonise the effect of 5-HTP in order to identify a central or peripheral mode of action. In deprived rats 5-HTP brought about a dose related inhibition of food intake which was midly antagonised by MK-486. A detailed analysis of the behavioural changes occurring during eating showed that the inhibition of food intake by 5-HTP was reflected in a reduced number of eating bouts and a slower rate of eating. MK-486 did not antagonise the effect of 5-HTP on eating rate. In free-feeding rats whose food comsumption was continuously monitored for 24-hr periods, 5-HTP gave rise to reduction in meal size and a slowing of the intra-meal rate of eating. These findings are in keeping with the effects of other serotonergic manipulations on the patterns of feeding in rats.