Neurophysiologic effects of folate compounds in hippocampus, in vitro

Benefits and risks of folic acid to the nervous system

During three decades of neurological practice I have witnessed a remarkable change in attitudes to the benefits and risks of folic acid therapy in nervous system disorders. In the 1960s all that was known and taught was that folic acid was harmful to the nervous system, especially in precipitating or exacerbating the neurological complications of vitamin B12 deficiency. So deeply held was this view that the possibility of neuropsychological benefits from this vitamin was initially viewed with considerable scepticism.

Localization of the murine reduced folate carrier as assessed by immunohistochemical analysis

The reduced folate carrier (RFC1) is a major route for the transport of folates in mammalian cells. The localization of RFC1 in murine tissues was evaluated by immunohistochemical analysis using a polyclonal antibody to the C-terminus of the carrier. There was expression of RFC1 in the brush-border membrane of the jejunum, ileum, duodenum and colon. RFC1 was localized to the basolateral membrane of the renal tubular epithelium. Carrier was detected on the plasma membrane of hepatocytes but not in bile duct epithelial cells. In the choroid plexus RFC1 was highly expressed at the apical surface. It was also expressed in axons and dendrites and on the apical membrane of cells lining the spinal canal. In spleen, RFC1 was detected only in the cells of the red pulp. These data provide insights into the role that RFC1 plays in folate delivery in a variety of tissues. In particular, the localization of carrier may elucidate the role of RFC1 in the vectorial transport of folates across epithelia. The data also indicate that in kidney tubules and choroid plexus the sites of RFC1 expression are different from what has been reported previously for the folate receptor; and while RFC1 is expressed in small intestine, folate receptor is not.

Methylfolate modulates potassium evoked neuro-secretion: evidence for a role at the pteridine cofactor level of tyrosine 3-hydroxylase

We have previously shown that 5-methyltetrahydrofolate influences neuro-secretion. The present study more precisely characterises the processes involved and considers one probable site of action. Focusing on the tyrosine-noradrenalin axis in cerebellum we showed 5-methyltetrahydrofolate causes a significant reduction in the apparent K+ evoked secretion of noradrenalin to only 12.9% of control release. Evidence supports the idea that this could actually be due to increased synthesis leading to; depletion of reserves, possibly through leakage, exocytotic inhibition via activation of presynaptic receptors or end product inhibition by noradrenalin at the pteridine cofactor level of tyrosine hydroxylase: a) concomitant decreased measurement of perfusate and intracellular tyrosine with released noradrenalin following 5-methyltetrahydrofolate treatment supports the idea of increased transmitter turn over; b) kinetic studies indicate that at saturating concentrations of tyrosine and in the presence of an inhibitor of L-DOPA decarboxylase, 5-methyltetrahydrofolate partially duplicates the rate limiting behaviour of a synthetic pteridine cofactor--DL,2-amino-4-hydroxy-6,7,dimethyltetrahydropteridine. We debate whether, in vivo, CSF 5-methyltetrahydrofolate might interact at the tetrahydrobiopterin cofactor level of tyrosine hydroxylase and other aromatic amino-acid hydroxylases.

Modulation of potassium evoked secretory function in rat cerebellar slices measured by real time monitoring: evidence of a possible role for methylfolate in cerebral tissue

The real time dynamics of K+ evoked neurosecretion in cerebellar slices has been monitored electrochemically. In the presence of 5-methyltetrahydrofolate a statistically significant diminution in secretory response occurs. Agonists to probe the pharmacological basis for this indicate it is not due to voltage sensitive Ca2+ channel blockade, nor does it show any similarity of effect with kainate, whose receptor is a putative binding site for 5-methyltetrahydrofolate. The method is fully validated, although no account is taken of individual molecular species. High performance liquid chromatography combined with off line microbiological assay could only detect 5-methyltetrahydrofolate in cerebrospinal fluid. We therefore discuss our findings in relation to possible cerebral roles for cerebrospinal fluid 5-methyltetrahydrofolate in the context of both membrane and transmitter related interactions.

Folate interactions with cerebral G proteins

Intracerebral folate injections produce convulsions and brain lesions, folic acid itself and tetrahydrofolate being more potent toxins than 5-methyltetrahydrofolate, the primary folate of mammalian extracellular fluids. Folates are known to excite neurons, by unknown mechanisms Folates stimulate GTP binding and GTPase activity in slime molds. We observed folate stimulation of GTP gamma S binding and inhibition of high affinity GTPase activity in rat brain membranes. Three fold stimulation of GTP gamma S binding was observed in cerebellar membranes treated with 50 microM FA. Folic acid (FA), dihydrofolate (DHF) and tetrahydrofolate (THF) were much more potent than 5-methyltetrahydrofolate in this regard. The effect varies between brain regions and was greatest in cerebellar and hippocampal membranes. Folates inhibit GTPase activity, with DHF and FA being the most potent and maximum inhibition being to 33% of control values. We find high affinity guanine nucleotide sensitive binding of [3H]FA in cerebellar membranes, another response typical of G protein coupled membrane receptors. Folates were also shown to stimulate the release of [3H]GDP from brain membranes. These effects are seen in washed brain membranes and can not be explained by any known folate metabolic or coenzyme functions. They resemble the effects of cholera toxin, except for their reversibility. They may be relevant to known folate neuroexcitant effects of folates.

Alterations in hippocampal extracellular amino acids and purine catabolites during limbic seizures induced by folate injections into the rabbit amygdala

The effects on hippocampal extra- and intracellular amino acids of focal injection of folic acid into the amygdala in the rabbit were studied with brain dialysis. Folate seizures were accompanied by pronounced elevations of extracellular alanine and phosphoethanolamine. The increase of extracellular alanine was related to an enhanced level of this amino acid in total hippocampal tissue, whereas phosphoethanolamine was unaltered in tissue biopsies. Folate seizures did not significantly affect extracellular aspartate and extracellular glutamate was only slightly elevated (50-75% over baseline values). The tissue concentration of glutamate remained at control levels during the seizures and tissue aspartate was decreased by 28%. Extracellular glutamine decreased rapidly after folate injection with a concomitant increase of total hippocampal glutamine. Neither extra- nor intracellular taurine was affected by folate epilepsy. The experiments also encompassed measurements of the extracellular purine catabolites inosine, hypoxanthine and xanthine. Folic acid-induced epilepsy produced changes in these compounds indicative of moderately accelerated degradation of adenosine 5'-triphosphate. The findings support the view of glutamate as a mediator of epilepsy-related brain damage. It is, however, questionable if the small enhancement of extracellular glutamate is enough to provoke neuronal necrosis associated with folate epilepsy.

Folic acid has a disinhibitory action in the rat hippocampal slice preparation

The effects of folic acid on synaptic transmission in the hippocampal slice have been studied. Application of folic acid (0.1-1 mM) increased the size of population spikes recorded extracellularly in the CAl pyramidal cell layer and caused the appearance of multiple population spikes. Intracellular recording revealed that folic acid had no consistent effect on the membrane potential, but greatly reduced the rapid chloride-mediated phase of the inhibitory postsynaptic potential (IPSP) evoked by ortho- and antidromic stimulation. The slower, potassium-mediated phase of the IPSP was usually enhanced. Furthermore, folic acid abolished spontaneous IPSPs recorded with potassium chloride-filled microelectrodes. All of these effects were quickly reversible when the drug was washed from the chamber. Finally bath-applied folic acid reduced the hyperpolarization produced by iontophoretically applied GABA. Based on these results, we conclude that folic acid produces its excitatory effects on hippocampal pyramidal cells by a disinhibitory action which involves a postsynaptic blockade of GABA responses.