The interactions between biogenic amines and pyridoxal, pyridoxal phosphate, and pyridoxal kinase

Dopamine D1 Receptor Activation Drives Plasticity in the Songbird Auditory Pallium

Vocal learning species must form and extensively hone associations between sounds and social contingencies. In songbirds, dopamine signaling guides song motor production, variability, and motivation, but it is unclear how dopamine regulates fundamental auditory associations for learning new sounds. We hypothesized that dopamine regulates learning in the auditory pallium, in part by interacting with local neuroestradiol signaling. Here, we show that zebra finch auditory neurons frequently coexpress D1 receptor (D1R) protein, neuroestradiol-synthase, GABA, and parvalbumin (PV). Auditory classical conditioning increased neuroplasticity gene induction in D1R-positive neurons. In vitro, D1R pharmacological activation reduced the amplitude of GABAergic and glutamatergic currents and increased the latter's frequency. In vivo, D1R activation reduced the firing of putative interneurons, increased the firing of putative excitatory neurons, and made both neuronal types unable to adapt to novel stimuli. Together, these findings support the hypothesis that dopamine acting via D1Rs modulates auditory association in the songbird sensory pallium.SIGNIFICANCE STATEMENT Our key finding is that auditory forebrain D1 receptors (D1Rs) modulate auditory plasticity, in support of the hypothesis that dopamine modulates the formation of associations between sounds and outcomes. Recent work in songbirds has identified roles for dopamine in driving reinforcement learning and motor variability in song production. This leaves open whether dopamine shapes the initial events that are critical for learning vocalizations, e.g., auditory learning. Our study begins to address this question in the songbird caudomedial nidopallium (NCM), an analog of the mammalian secondary auditory cortex. Our findings indicate that dopamine receptors are important modulators of excitatory/inhibitory balance and sound association learning mechanisms in the NCM, a system that could be a fundamental feature of vertebrate ascending auditory pathways.

gamma-Aminobutyric acid function in the rat striatum is under the double influence of nigrostriatal dopaminergic and thalamostriatal inputs: two modes of regulation?

Glutamic acid decarboxylase (GAD), gamma-[3H]-aminobutyric acid [( 3H]GABA) high-affinity uptake into synaptosomes, and endogenous GABA content were measured in the rat striatum 2-3 weeks following 6-hydroxydopamine injection in the ipsilateral substantia nigra to destroy the nigrostriatal dopaminergic pathway and after kainic acid injection into the centromedial-parafascicular complex of the ipsilateral thalamus to lesion the thalamostriatal input. Both lesions resulted in apparent GAD increase concomitant with a decreased [3H]GABA uptake into striatal synaptosomes. GABA content was increased selectively following the dopaminergic lesion. Kinetic analysis of the uptake process for [3H]GABA showed selectively a decreased Vmax following the dopaminergic lesion; in animals with thalamic lesion, however, the change only concerned the Km, which showed a decreased affinity of the transport sites for [3H]GABA. Determination of Km and Vmax for GAD action on its substrate glutamic acid showed an increased affinity of GAD for glutamic acid in the case of the dopaminergic lesion without any change in Vmax, whereas the thalamic lesion resulted in GAD increase concomitant with a selective increase in Vmax. These data suggest that striatal GABA neurons are under the influence of nigrostriatal dopaminergic neurons which may reduce the GABA turnover, whereas the exact nature of the powerful control also revealed on these neurons following thalamic lesion remains to be determined. Both lesions induced adaptive neurochemical responses of striatal GABA neurons, possibly reflecting in the case of the dopaminergic deprivation an increased GABA turnover.

Dietary pyridoxine interaction with tryptophan or histidine on brain serotonin and histamine metabolism

We studied the metabolic effects of high dietary intakes of pyridoxine and of the substrate-cofactor interaction between dietary histidine or tryptophan and pyridoxine in rat brain. In the substrate-cofactor interaction study, histamine and serotonin levels were determined in rats fed elevated or requirement levels of substrate (histidine: 0.3% and 0.8%, tryptophan: 0.15% and 0.6%) and excess or requirement levels of pyridoxine HCl (7 mg vs. 3,000 mg/kg). Excess pyridoxine intake caused a differential effect on brain histamine concentration--inhibitory with the requirement level of histidine (-29%), and stimulatory (+21%) with the elevated level of histidine. When dietary tryptophan was fed at the requirement level, excess pyridoxine caused essentially no changes in hypothalamic serotonin and 5HIAA (-2%, -2%). With elevated tryptophan intake, excess pyridoxine significantly increased serotonin and 5HIAA (+32%, +20%) in the hypothalamus. These results indicate a clear interaction between substrate and coenzyme precursor which influences brain metabolism of histamine and serotonin.

The status of vitamin B6 metabolism in brains of genetically epilepsy-prone rats

The status of vitamin B6 in brains of genetically epilepsy-prone and epilepsy-resistant rats was ascertained by measuring the concentrations of pyridoxal phosphate and the activities of pyridoxal kinase in the hippocampus, basal ganglia, cerebellum, and cerebral cortex, as well as in the entire brain. The concentrations of pyridoxal phosphate in whole brain of control rats and seizure-prone rats did not vary significantly from each other and were 1.2 +/- 0.02 and 1.2 +/- 0.03 microgram/g, respectively. Similarly, the activities of pyridoxal kinase in brains of the two groups were uniform and had values of 3.6 +/- 0.1 and 3.5 +/- 0.06 microgram pyridoxal phosphate formed/mg protein/15 min, respectively. Unlike in the whole brain, the concentration of pyridoxal phosphate in the cerebral cortex of seizure-prone rats was lower (1.24 +/- 0.026 microgram/g) than that of seizure-resistant rats (1.47 +/- 0.23 microgram/g). The opposite was noted in the cerebellum. The concentrations of pyridoxal phosphate in the hippocampus and in the basal ganglion of seizure-prone and seizure-resistant rats were identical. The activities of pyridoxal kinase in the basal ganglion (3.84 +/- 0.23 vs. 4.99 +/- 0.11 microgram pyridoxal phosphate/mg protein/15 min) and in the cerebral cortex (3.40 +/- 0.06 vs. 3.70 +/- 0.04 microgram pyridoxal phosphate/mg protein/15 min) were lower in seizure-prone rats than in seizure-resistant rats. The reverse took place in the hippocampus and in the cerebellum. No uniform associations among the concentration of pyridoxal phosphate, the activity of pyridoxal kinase, and the susceptibility or lack of susceptibility to seizure could be demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of tryptophan metabolites on the activities of rat liver pyridoxal kinase and pyridoxamine 5-phosphate oxidase in vitro

Pyridoxal kinase was purified 4760-fold from rat liver. The Km values for pyridoxine and pyridoxal were 120 and 190 microM respectively, and pyridoxine showed substrate inhibition at above 200 microM. Pyridoxamine 5-phosphate oxidase was also purified 2030-fold from rat liver, and its Km values for pyridoxine 5-phosphate and pyridoxamine 5-phosphate were 0.92 and 1.0 microM respectively. Pyridoxine 5-phosphate gave a maximum velocity that was 5.6-fold greater than with pyridoxamine 5-phosphate and showed strong substrate inhibition at above 6 microM. Among the tryptophan metabolites, picolinate, xanthurenate, quinolinate, tryptamine and 5-hydroxytryptamine inhibited pyridoxal kinase. However, pyridoxamine 5-phosphate oxidase could not be inhibited by tryptophan metabolites, and on the contrary it was activated by 3-hydroxykynurenine and 3-hydroxyanthranilate. Regarding the metabolism of vitamin B-6 in the liver, the effects of tryptophan metabolites that were accumulated in vitamin B-6-deficient rats after tryptophan injection were discussed.

Dissociation between epileptic seizures induced by convulsant drugs and alteration in the concentrations of pyridoxal phosphate in rat brain regions

Allylglycine increased the concentration of pyridoxal phosphate in cerebral cortex from 1011.4 +/- 25.0 to 1318.0 +/- 66.3 and decreased it in cerebellum from 1289.0 +/- 49 to 1147.7 +/- 119.4 ng/g wet tissue during the preictal period. Mercaptopropionic acid increased the concentration of pyridoxal phosphate in cerebellum from 1525 +/- 91 to 1985.7 +/- 275 ng/g wet tissue. Similar effects were noted in hippocampus and cerebral cortex. Picrotoxin increased the concentration of pyridoxal phosphate in hippocampus from 938.7 +/- 44 to 1043 +/- 118 but decreased it in cerebral cortex from 1124.52 +/- 124 to 979.4 +/- 15 ng/g wet brain. The effects of strychnine were identical to those of allylglycine. Bicuculline reduced the concentration of pyridoxal phosphate in cerebral cortex from 1184 +/- 61 to 1075.14 +/- 78 ng/g wet brain.

The route and significance of endogenous synthesis of alkaloids in animals

There is now substantial evidence that several TIQs and beta-carbolines are present in vivo and increase during certain pathological conditions. It still remains to be determined, however, precisely what roles they play in endogenous functions and whether or not they are critical for the expression of these pathological conditions. Accumulating biochemical information continues to support the notion that these compounds can act as false transmitters. The exciting new findings, which will certainly receive a great deal more attention, concern the interaction of some of the beta-carbolines with the benzodiazepine receptor. Determining if a beta-carboline is an endogenous receptor ligand will attract further research interest on the theoretical and specifically clinically-directed levels. Biochemical, morphological, and behavioral data indicate that some of the condensation products can act as neurotoxins. Very few experiments have included an examination of long-term effects of exposure to one of these alkaloids, so the amount of information on this issue is limited. Chronic rather than acute administration of an alkaloid is more likely to mimic the pathological states in which these compounds are hypothesized to play a role. Biochemically, both the dopaminergic and serotonergic systems have been shown to be affected by chronic treatments with certain alkaloids. Progressive and long-term behavioral alterations also have been reported. Such changes may reflect an adaptation to an increase or decrease in activity of particular systems or a neurotoxic action.

Increased glutamate decarboxylase activity in the red nucleus of the adult cat after cerebellar lesions

Glutamate decarboxylase (GAD)activity, a marker for GABAergic structures, was studied in the cat red nucleus. GAD is more concentrated in the rostral than in the caudal third of the structure. GAD levels were measured after chronic unilateral lesions of the cerebellum. Destruction of the dentate area and of the nucleus interpositus induced increases of GAD in the contralateral but not in the ipsilateral red nucleus. Similar changes also occurred in the denervated nucleus ventralis lateralis (VL) and nucleus ventralis anterior (VA) of the thalamus. Results show that loss of the excitatory cerebellar input could lead to changes in inhibitory GABAergic nerve terminals. This increase may be induced transsynaptically within existing neurons or, more likely, additional GAD-containing nerve terminals may be formed by axonal sprouting.

In vivo formation of histamine phosphopyridoxal cyclic compounds

A possibility of in vivo formation of cyclic compounds between histamine (Hi) given i.p. and endogenous pyridoxal (PL) or pyridoxal 5'-phosphate (PLP) has been studied. Cyclic compounds of Hi with PL or PLP were found in all tissues examined. Although an increase in Hi levels in tissues enhances the formation of cyclic compounds, no simple relationship between the rate of formation and Hi concentration has been observed. The reaction seems to be limited by endogenous PLP. The cyclic products Hi-PL and Hi-PLP were discovered in urine. It is suggested that the process of cyclic compound formation may reduce PLP resources, resulting in a modification of PLP-enzyme activities.

Anticonvulsant activity of muscimol and gamma-aminobutyric acid against pyridoxal phosphate-induced epileptic seizures

The intracerebroventricular injection of pyridoxal phosphate (PLP, 0.125-1.25 mumol/rat) causes epileptic seizures (4 min leads to 1 min) that are preventable or reversible by GABA (1 mumol/rat), by muscimol (0.025 mumol/rat), or by diazepam (1.75 mumol/rat). At the peak of PLP-induced convulsions, the activities of GAD and GABA-T in 14 regions of rat brain remained unaltered, whereas the concentrations of PLP remained elevated. The PLP-induced convulsion was blocked by DABA (10 mumol/rat) but was not altered by beta-alanine (50 mumol/rat). The previous in vitro studies have shown that PLP increases the uptake of [3H]GABA into synaptosomes and inhibits the binding of [3H]GABA to synaptic membranes. These data suggest that PLP-induced convulsion is due to reduced availability of GABA to its recognition sites, rather than to alteration in the activity of GABA metabolizing enzymes, or unavailability of PLP as a coenzyme for GAD and GABA-T. Since the duration of PLP-induced epileptic seizures is short and can be prevented by GABA agonists, PLP may be used as a tool to study the nature of GABA-mediated neuroinhibition and the properties of GABA receptor sites.

Phosphopyridoxal cyclic compounds with histamine and histidine. 7: The properties of pyridoxal and phosphopyridoxal cyclic compounds with histamine and histidine

Cyclic compounds synthesized from histamine (Hi) or histidine (His) with pyridoxal (PL)--[Hi-PL, His-PL] and Hi or His with pyridoxal 5'-phosphate (PLP)--[Hi-PLP, His-PLP] were tested for stability in buffer, acid and base solutions, crude homogenates of various tissues and in the presence of enzymes which metabolize Hi or His. The cyclic pyridoxal compounds were stable in all experimental conditions, whereas phosphopyridoxal cyclic products were degraded by rat intestinal DAO and rat intestine homogenate, apparently enzymatically. In acidic and basic solutions changes in migration velocity and u.v. absorption spectrum occur. The characteristic fluorescence of these cyclic compounds is described.

Phosphopyridoxal complexes with histamine and histidine. (5) The kinetics of cyclic compound formation between histamine and pyridoxal-5'-phosphate in the presence of pig kidney diamine oxidase and rat intestinal histaminase

Pig kidney diamine oxidase (DAO) and rat intestinal histaminase (Hi-ase) activities are inhibited in vitro by high concentrations of both a substrate (histamine) and a coenzyme (pyridoxal-5'-phosphate). This inhibition may be at least partially associated with the formation of a cyclic compound between histamine (Hi) and pyridoxal-5'-phosphate (PLP). The dynamics of this cyclic compound formation in the presence of both enzymes has been examined. In an incubation mixture containing partially purified pig kidney DAO, the rate of cyclization decreased slightly as compared with a buffer. On the contrary, in the presence of crude rat intestinal histaminase, the rate of cyclization was inhibited significantly; this inhibition was proportional to the amount of enzyme preparation present in the incubation mixture. The possible mechanism of the influence of enzyme protein on the rate of cyclic compound formation, and its possible biological significance, are discussed.

Phosphopyridoxal complexes with histamine and histidine. (4) The kinetics of complex formation between histidine and pyridoxal 5' -phosphate in the presence of bacterial histidine decarboxylase

The dynamics of the complex formation between pyridoxal 5'-phosphate (PLP) and histidine in the presence of bacterial histidine decarboxylase was examined. Since PLP is able to form a cyclic product with histidine and histamine, the possibility of complex formation between PLP and histamine formed during a decarboxylation reaction was examined too. It was found that the cyclization reaction between PLP and histidine is equimolecular and the rate of cyclic product formation is not significantly influenced by the presence of enzyme. In the presence of bacterial histidine decarboxylase both the cyclization reaction and cyclic product formation were observed. Predominance of histamine or cyclic product formation was dependent on pH and substrate concentration. In the presence of histidine and enzymatic protein, histamine formed during the decarboxylation reaction was unable to form a cyclic product with PLP.

Phosphopyridoxal complexes with histamine and histidine. (2) The influence of presumed complex on histidine decarboxylase activity in rat gastric mucosa

It was found that in vitro, histidine and histamine form a complex compound with pyridoxal-5'-phosphate (PLP). The purpose of the present experiments was to find whether formation of this complex can influence histidine decarboxylase activity. It was found that excess PLP inhibits rat's gastric mucosa histidine decarboxylase activity in vitro. The inhibitory action of PLP depends on the histidine concentration and is independent on the amount of crude enzyme preparation. The histidine-PLP complex did not influence enzyme activity. The possible mechanisms of the inhibitory action of PLP on histidine decarboxylase activity are discussed.

Phosphopyridoxal complexes with histamine and histidine. (1) The kinetics of complex formation between pyridoxal 5'-phosphate and histamine

Amines and amino acids are able to form cyclic compounds with pyridoxal 5'-phosphate (PLP). The quantitative relationship between histamine and PLP in the cyclization reaction were examined. It was found that one mole of amine reacts with one mole of coenzyme. The velocity of complex formation increased with an excess of either histamine or PLP, without any change in molar ratios. The formation of cyclic compound was confirmed by the use of isotopic method. The separation of cyclic compound from histamine, but not from PLP, was achieved by paper chromatography.

Phosphopyridoxal complexes with histamine and histidine. (3) The influence of presumed complex on activity of rat intestinal histaminase

Histamine in high concentration inhibits the rat intestinal histaminase (diamine oxidase E.C. 1.4.3.6.). The apparent Km is approximately 4.2 X 10(-5) M. This inhibition can be reversed by an addition of PLP. It was also found that excess of PLP inhibits enzyme activity. It is competitive inhibition. Histamine and other amines which were associated with enzyme inhibition form spectrophotometricaly demonstrable complex with PLP. The possible mechanism of the inhibitory action of PLP and complex with histamine and other amines on rat intestinal histaminase activity are discussed.