Biochemical and pharmacological actions of imidazoleacetic acid

Pharmacology and function of imidazole 4-acetic acid in brain

1. Imidazole 4-acetic acid (IMA) is a naturally occurring metabolite in brain, although it is unclear what biochemical pathways are involved in its biosynthesis and breakdown. Some evidence, however, suggests that IMA is an oxidation product of histamine. 2. The compound has pronounced neuropharmacological properties, many of which are consistent with an activation of GABA(A) receptors. Indeed, IMA is able to displace [3H]GABA from GABA(A) sites in a potent manner. 3. IMA displays definite partial agonist characteristics as an enhancer of benzodiazepine binding to the GABA(A) receptor complex in membrane preparations. In addition, it has an affinity for GABA(C) receptors, where it seems to act as an antagonist, and perhaps as a weak partial agonist. A third recognition site for IMA in brain is the I1-imidazoline receptor. 4. Parenteral administration to experimental animals leads to a sleep-like state which can often be accompanied by seizures. In addition, central application of IMA has been associated with a dose-related reduction in arterial pressure and sympathetic nervous discharge. 5. No specific receptor site or uptake system for IMA has yet been discovered, adding uncertainty to its role in central nervous system function. Yet the possibility cannot be overlooked that IMA plays a role in regulating blood pressure.

Gamma hydroxybutyrate is not a GABA agonist

Gamma hydroxybutyrate (GHB) is primarily known and used as a relatively specific inhibitor of central DA release. However, it is also widely assumed to be an agonist or prodrug of gamma-aminobutyric acid (GABA) and its central activity has been attributed to an action exerted at GABA receptors. Nevertheless, there is compelling evidence that: (1) GHB formation may occur independently of GABA; (2) GHB is behaviorally, biochemically and physiologically distinct from GABA in many ways, and does not consistently effect GABAA or GABAB agonist induced responses; (3) GHB has little effect on either GABAA or GABAB receptors at less than millimolar concentrations. Consequently, GHB does not appear to be either a GABA prodrug or a GABA agonist. However, the GHB metabolite gamma butyrolactone (GBL) may possess some limited GABA agonist activity.

Deuterium isotope effect in gamma-aminobutyric acid transamination: determination of rate-limiting step

The rate of transamination of gamma-aminobutryic acid (GABA) catalyzed by hog brain gamma-aminobutyrate aminotransferase was substantially reduced when the hydrogen at the gamma-carbon position was replaced by deuterium. The deuterium isotope effect of this reaction has been substantiated by fluorometric, radiometric, and mass spectrometric procedures and assessed kinetically. The ratios of Vmax of the nonlabeled substrate/Vmax of the deuterated substrate obtained under different conditions ranged from 6 to 7. This indicates that the cleavage of the hydrogen from the gamma-carbon is the rate-determining step in GABA transamination. Similar isotope effects have also been shown to occur in the peripheral system in vivo.

The effects of imidazole-4-acetic acid upon the energy metabolism of normoxic and hypoxic rat brain

The metabolic effects of intravenous administration of 200 mg/kg imidazole-4-acetic acid (IMA) on normoxic, hypoxemic and hypoxemic-oligemic rat brain were assessed by measurement of the cerebral contents of energy phosphates and selected glycolytic-citric acid cycle intermediates. In normoxic brain IMA was associated with unaltered adenylates, increased glucose and aspartate, and decreased pyruvate, lactate, citrate, alpha-ketoglutarate and malate; a pattern which is compatible with cerebral metabolic depression. In hypoxemic brain IMA was associated with a lower accumulation of lactate and a higher level of tissue glucose; again conforming to patterns documented for classical cerebral depressants. In hypoxemic-oligemic brain IMA was associated with a gross deterioration of the tissue ATP content and with a massive lactacidosis which indicated a detrimental action in this hypoxic state.

Effects of baclofen and gamma-hydroxybutyrate on rat striatal and mesolimbic 5-HT metabolism

The effects of D, L-baclofen and gamma-hydroxybutyric acid (GHBA) on endogenous 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the rat striatum and the mesolimbic area were investigated and compared with their effects on dopamine (DA) concentrations in the same regions. Both baclofen (5-50 mg/kg i.p.) and GHBA (300--1500 mg/kg i.p.) led to dose-dependent increases in the striatal levels of 5-HT and 5-HIAA by up to 200%. Maximal effects of both drugs occurred about 3 h after treatment. Normalization was reached 6 h after GHBA and 16 h after baclofen. The effect of GHBA in the mesolimbic area was qualitatively similar to, but weaker than, its effect in the striatum. Baclofen had little or no effect in the mesolimbic area, the cortex or the brain stem. The two drugs affected striatal 5-HT and DA metabolism similarly. Their effects on the latter are thought to be due to cessation of impulse flow. The effects on striatal 5-HT may be related to more complex phenomena.

Centrally-acting drugs and the formation of brain gamma-aminobutyric acid

1. The effects of a number of centrally-acting compoinds were tested on the levels of GABA and glutamate and on the formation of GABA from glutamate in rat brain. 2. Pargyline and desipramine increased the brain concentration of GABA but were without effect on GABA production. 3. Imidazoleacetic acid increased GABA levels, decreased glutamate levels but did not alter GABA production, whereas gamma-hydroxybutyric acid had an effect only on GABA formation. 4. Isonicotinc acid hydrazine caused a marked decrease in the formation of GABA from glutamate. 5. Chlorpromazine, diphenylhydantoin, bicucculine, and L-DOPA had no effect on any of the biochemical parameters measured. 6. The central effects of these drugs are discussed in relation to the above findings.