Studies on glutamate decarboxylase of the mammalian brain

Evidence for GABAergic inhibition of a hypothalamic sympathoexcitatory mechanism in anesthetized rats

The hypothesis that endogenous gamma-aminobutyric acid (GABA) suppresses the activity of a latent hypothalamic sympathoexcitatory mechanism was tested by examining the effects of stereotaxic intrahypothalamic microinjection of drugs influencing GABAergic inhibition in anesthetized rats. Bicuculline methiodide (BMI) 1-25 ng, a competitive antagonist at post-synaptic GABA receptors, as well as isoniazid (INH) 35 and 70 micrograms and 3-mercaptopropionic acid (3MP) 0.02 microliter, inhibitors of GABA synthesis, all evoked marked increases in heart rate and modest pressor responses. However, while the effects of BMI appeared almost immediately and peaked within 10 min of injection, changes caused by INH or 3MP developed much more slowly, attaining a maximum 35-40 and 19 min after injection, respectively. The effects of BMI on heart rate were blocked by pretreatment with propranolol 2 mg/kg i.v. or hexamethonium 20 mg/kg i.v. plus atropine 2 mg/kg i.v. and were shown to be highly localized to the posterior hypothalamic nucleus and the adjacent lateral hypothalamus. In addition to the cardiovascular effects, BMI also elicited dose-related increases in respiratory rate which were independent of the heart rate changes although they followed a similar time course. The results support the notion that hypothalamic GABA inhibits a local mechanism capable of generating cardiorespiratory arousal.

Radiation inactivation of brain [35S]t-butylbicyclophosphorothionate binding sites reveals complicated molecular arrangements of the GABA/benzodiazepine receptor chloride channel complex

[35S]t-Butylbicyclophosphorothionate ([35S]TBPS), a bicyclic cage convulsant, binds to the anion gating mechanism of the GABA/benzodiazepine receptor chloride channel complex. Using a carefully calibrated radiation inactivation technique, the molecular weight of [35S]TBPS binding complexes from frozen rat cerebral cortex was estimated to be 137,000 daltons. The GABA agonist muscimol reduced [35S]TBPS binding to 0-10% of the control value, in a way which is independent of the radiation dose. This shows that the GABA receptor (Mw = 55,000 daltons) is included in the 137,000-dalton [35S]-TBPS binding complex; the [35S]TBPS binding protein alone accounts for 137,000-55,000 = 82,000 daltons. The pyrazolopyridazine etazolate (SQ 20.009) and etomidate in appropriate concentrations both reduced specific binding of [35S]TBPS. The ability of SQ 20.009 and etomidate to reduce [35S]TBPS binding was greatly reduced by exposure to low radiation doses, suggesting that SQ 20.009 and etomidate reduce [35S]TBPS binding by an allosteric mechanism requiring a molecular structure of 450,000-500,000 daltons. Benzodiazepine agonists (ethyl 4-methoxymethyl-6-benzyloxy-beta-carboline-3-carboxylate, ZK 93423) and inverse agonists (methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, DMCM) enhance and reduce [35S]TBPS binding, respectively, in repeatedly frozen and washed membrane preparations. The effects of ZK 93423 and DMCM on [35S]TBPS binding disappeared upon exposure of membranes to low radiation doses. This suggests that the benzodiazepine receptor site interacts allosterically with the [35S]TBPS binding site, requiring a molecular complex of at least c. 400,000 daltons. The [35S]TBPS site alone in these latter conditions of membrane preparation (repeatedly frozen/washed) revealed a molecular weight of 221,000 daltons (TBPS-site + GABA receptor + unknown structures). The number of binding sites for [35S]TBPS (145 pmol/g tissue) was only slightly higher than for [3H]flunitrazepam (130 pmol/g tissue) in cerebral cortex. These results are all consonant with the conclusion that the GABA/BZ receptor chloride channel complex is composed of highly integrated multimeric subunits, tentatively accounted for by a tetramic complex of molecular weight 548,000 daltons.

Ultrastructural studies on catecholaminergic terminals and GABAergic neurons in nucleus tractus solitarius of the rat medulla oblongata

Synaptogenesis of catecholamine (CA) boutons in the nucleus tractus solitarius (NTS) was compared between spontaneously hypertensive (SHR) rats and Wistar-Kyoto (WKY) rats at different ages. On the average, there were about 32 CA varicosities per 2200 microns2 area of the NTS in both SHR and WKY rats as revealed by glyoxylic acid fluorescence microscopic (FM) morphometric study. The FM analysis indicated that there were no significant changes in the CA varicosity density between SHR and WKY rats. The CA boutons were labeled with 5-hydroxydopamine and appeared to contain small granular vesicles at the electron microscopic (EM) level. A total of 1402 CA boutons were studied in a 540,000 micron2 area of the NTS. The number of CA boutons involved in synaptic contacts vs the number of total CA boutons was used to obtain synaptic frequency which was taken as an index for synaptogenesis. A reduction of approximately 18% and 14% of synaptogenesis of CA boutons was observed in the NTS of SHR rats at 4 weeks (prehypertensive stage) and 12 weeks (early hypertensive stage) of age respectively, as compared to age-matched WKY rats. No significant difference of synaptogenesis of CA neurons was found between SHR and WKY rats at 16 weeks of age, a stage in which hypertension is well established and maintained in SHR rats. These results suggest that CA neurons with fewer synaptic contacts in the NTS may play a more important role in the initiation than in the maintenance of hypertension in the SHR rats. In addition to CA terminals, there were numerous GABAergic cell bodies in the NTS which were identified by immunocytochemistry using antibodies to the GABA synthesizing enzyme, L-glutamate decarboxylase (GAD). GABAergic dendrites with GAD-positive reaction were often seen to receive several GAD-negative synapses at EM random profiles. In the text, a viewpoint is thus discussed that emphasizes that a synaptic abnormality of CA terminals with fewer synaptic contacts affecting GABAergic neurons may participate in the pathogenesis of hypertension. However, it remains to be determined as to whether or not there is a direct contact between CA boutons and GABAergic dendrites.

Blockade of GABA receptors in periventricular forebrain of anesthetized cats: effects on heart rate, arterial pressure, and hindlimb vascular resistance

Pharmacologic antagonism of GABAergic inhibition in the periventricular forebrain of anesthetized cats caused dose related sympathetically mediated increases in heart rate and arterial pressure in vagotomized cats, and suppression of reflex vagal activation in vagus-intact preparations where sympathetic effects are prevented by cervical spinal cord transection. In the present study performed in cats with intact autonomic pathways, similar administration of the GABA antagonist bicuculline methiodide (BMI) produced dose-related increases in arterial pressure, heart rate, and, when perfusion pressure in the autoperfused hindlimb was measured, vascular resistance. Furthermore, the relationship between heart rate and hindlimb vascular resistance changes suggested that the effects on heart rate reflected the combined sympathetic and vagal baroreflex effects of BMI. Intraventricular administration of muscimol, a potent GABA agonist, elicited abrupt and parallel reversal of BMI-induced effects on systemic arterial pressure and vascular resistance while attempts to acutely denervate the hindlimb at the height of the BMI response by cutting the femoral and sciatic nerves sharply reversed the increase in hindlimb vascular resistance in most experiments. The results suggest that the cardiovascular changes observed represent an integrated pattern consisting of excitation of sympathetic nerves innervating the heart and vasculature along with suppression of reflex evoked vagal excitability.

Assay and properties of glutamic acid decarboxylase in homogenates of crayfish nervous tissue

The activity of glutamic acid decarboxylase (GAD) was measured in homogenates of crayfish nervous tissue. Radioactive GABA and CO2 were formed from radioactive glutamic acid in approximately equimolar amounts. Product formation was linear for 9.5 hr at 11-32 degrees C with about 1-30 micrograms homogenate protein. Enzyme activity remained high at pH 7-10 but declined steeply above pH 10.5 and below pH 7. Enzyme activity was stimulated by pyridoxal phosphate, 2-mercaptoethanol, and potassium phosphate; at higher than optimal concentrations of each the activity was reduced. Sodium phosphate altered the stimulatory effect of potassium phosphate. Crayfish GAD behaves like a typical neural GAD but is distinguishable biochemically from GAD of other species.

Properties of gamma-aminobutyric acid synthesis by rat renal cortex

Substantial synthesis of gamma-aminobutyric acid occurs in rat renal cortex. Renal glutamate decarboxylase activity (24.3 +/- 2.9 (S.E.) nmols/mg protein per h) is 15% of that in brain; renal gamma-aminobutyric acid content (39.5 +/- 5.3 (S.E.) nmols/g wet wt.) is 5% of the whole brain concentration. Properties of glutamate decarboxylase were studied in homogenates of rat renal cortex and rat brain under conditions for which gamma-aminobutyric acid formation from [2,3-3H]glutamate and CO2 release from [1(-14)C]glutamate were equal. Several properties of renal glutamate decarboxylase distinguish it from the corresponding brain enzyme: (1) renal glutamate decarboxylase is selectively inhibited by cysteine sulfinic acid (Ki = 5X10(-5) M); (2) renal glutamate decarboxylase is less sensitive (Ki = 3-5X10(-5) M) to inhibition by aminooxyacetic acid than is the brain enzyme (Ki = 1X10(-6) M); (3) brain but not renal glutamate decarboxylase activity can be substantially stimulated in vitro by the addition of exogenous pyridoxal 5'-phosphate; (4) renal glutamate decarboxylase is significantly decreased in renal cortex from rats on a low-salt diet. Proximal tubules are enriched in glutamate decarboxylase compared to the activity in whole renal cortex or glomeruli (42, 22 and 14 nmols/mg protein per h, respectively). We speculate that renal gamma-aminobutyric acid synthesis does not reflect the presence of GABAergic renal nerves, but may serve a function in proximal tubular cells.

Ontogeny and distribution of specific succinic semialdehyde reductase apoenzyme in the rat brain

The ontogeny and distribution in rat brain of specific succinic semialdehyde reductase is described. This enzyme is probably responsible for the synthesis of gamma-hydroxybutyrate in brain. The highest activities and levels of apoenzyme are found in cerebellum, olfactory bulb, septum and median hypothalamus. During neonatal development, the enzyme activity remains stable at least until 63 days of age. As the levels of other enzymes of the GABA shunt pathway increase during this same period, this result indicates that there is a relative decrease in the reductive pathway of succinic semialdehyde catabolism during development leading to gamma-hydroxybutyrate synthesis, compared to the oxidative pathway leading to succinate.