C.J.A. Game

STRUCTURE AND BIOLOGICAL ACTIVITY OF A SERIES OF CONFORMATIONALLY RESTRICTED ANALOGUES OF GABA

—Microelectrophoretic methods were used to study the effects on spinal neurones of a series of conformationally restricted analogues of GABA, most of which are structurally related to musci‐mol (3‐hydroxy‐5‐aminomethylisoxazole). 3‐Hydroxy‐5‐(l‐aminoethyl)isoxazole and 3‐hydroxy‐5‐(2‐aminoethyl)isoxazole were GABA‐like depressants comparable in effectiveness with GABA. The inhibitors of GABA uptake 4,5,6,7‐tetrahydroisoxazolo[4,5‐c]pyridin‐3‐ol and nipecotic acid (piperidine‐3‐carboxylic acid) reversibly enhanced the depressant action of GABA. 3‐Hydroxy‐5‐dimethylaminomethly‐isoxazole, 5,6,7,8‐tetrahydro‐4H‐isoxazolo[4,5‐d]azepm‐3‐ol, 4,5,6,7‐tetrahydroisoxazolo[4,5‐c]pyridin‐3‐ol, and nipecotic acid reversibly antagonized the postsynaptic action of glycine.

Central effects of β-(p-chlorophenyl)-γ-aminobutyric acid

(1) Electrophoretically administered β-(p-chlorophenyl)-GABA depressed the firing of spinal interneurones, pyramidal tract neurones and Purkinje cells of anaesthetised cats. Although less potent than GABA, the action of β-p-CPG was more prolonged. (2) Renshaw cells were relatively insensitive to β-p-CPG. (3) Since the depressant effects of β-p-CPG were not reduced by either biculline methochloride or strychnine, activation of GABA or glycine receptors seems an unlikely explanation of the depressant action. (4) β-p-CPG inhibited GABA transaminase activity and reduced the high affinity uptake ofl-glutamate by rat brain slices. (5) β-p-CPG may activate neuronal receptors for an inhibitory transmitter chemically related to phenylethylamine.

Cis- and trans-4-aminocrotonic acid as GABA analogues of restricted conformation.

Cis‐4‐aminocrotonic acid, an analogue of GABA in a folded conformation, appears not to act as a GABA analogue with respect to bicuculline‐sensitive postsynaptic receptors, ‘high affinity’ GABA uptake and GABA: 2‐oxoglutarate aminotransferase in the mammalian central nervous system. On the other hand, trans‐4‐aminocrotonic acid, an analogue of GABA in an extended conformation, acts as efficiently as GABA with respect to each of the above systems, indicating that extended rather than folded conformations of GABA are likely to be important in the interaction of GABA with the specific macromolecules concerned.

The pharmacology of the inhibition of dorsal horn neurones by impulses in myelinated cutaneous afferents in the cat.

SummaryThe effects of microelectrophoretic strychnine and bicuculline methochloride were studied on the time course of synaptic inhibitions of single dorsal horn neurones in the lumbar spinal cord of cats anaesthetized with pentobarbitone. The inhibitions, evoked by volleys in mixed myelinated cutaneous afferents, varied in latency and duration. In general, early inhibitions (latency less than 12 msec: duration less than 36 msec) were reduced by microelectrophoretic strychnine whereas late inhibitions (latency more than 16 msec and more prolonged in duration) were usually sensitive to bicuculline. These results can be interpreted in terms of glycine and GABA as the inhibitory transmitters of early and late inhibitions respectively.

The differential sensitivity of spinal interneurones and Renshaw cells to kainate and N-methyl-D-aspartate

SummaryWith sensitivity to N-methyl-D-aspartate as the basis for comparison, spinal interneurones were relatively more sensitive than Renshaw cells to kainate, a conformationally restricted analogue of glutamate. These findings are consistent with proposed transmitter roles for L-glutamate and L-aspartate in the spinal cord.

The in vivo inactivation of GABA and other inhibitory amino acids in the cat nervous system.

SummaryIn cats anaesthetised with pentobarbitone, the effect of inhibitors of the in vitro cellular uptake of GABA were tested on the responses of single central neurones to GABA and other depressant amino acids. (+)- And (-)-nipecotic acid, (+)-2,4-diaminobutyric acid (DABA) and 2,2-dimethyl-β-alanine, enhanced the action of GABA on spinal, cerebellar and cerebral cortical neurones.In the spinal cord DABA, and to a less extent (-)-nipecotic acid, enhanced the action of β-alanine, whereas the actions of glycine and taurine were unaffected by DABA and reduced by (-)-nipecotic acid. In the cerebellum and cerebral cortex, these two substances enhanced the action of GABA, usually to a greater extent than that of β-alanine and taurine, although this specificity was not marked.The GABA-mediated basket cell inhibition of Purkinje cells in the cerebellum was unaffected by DABA and (-)-nipecotic acid, and neither substance appears suitable for determining the role of uptake processes in the inactivation of synaptically released GABA.Quantitatively these in vivo results agree more closely with recent in vitro uptake studies in cat tissue than the previously published data on rat cerebral cortex and dorsal root ganglia, and the observations provide further evidence for the importance of cellular uptake in maintaining low extraneuronal concentrations of inhibitory amino acid transmitters.

Benzodiazepines and central glycine receptors.

1 In cats, anaesthetized with pentobarbitone, intravenous diazepam (minimum dose 3.0 mg/kg) enhanced dorsal root potentials but did not significantly diminish the reduction by electrophoretic strychnine of the inhibitory action of electrophoretic glycine on dorsal horn interneurones. 2 In mice, intraperitoneal diazepam (2.5 mg/kg) had no appreciable effect on the potency of strychnine as a convulsant, although providing some protection against bicuculline. 3 These observations, together with the failure of chlordiazepoxide to either inhibit the firing of spinal interneurones or reduce antagonism between strychnine and glycine when administered locally, provide no support for the interaction between benzodiazepines and mammalian central glycine receptors which has been proposed on the basis of in vitro studies of strychnine binding.

ANTAGONISM OF INHIBITORY AMINO ACID ACTION BY TUBOCURARINE

A comparison has been made of the antagonism by microelectrophoretically administered (+)‐tubocurarine, bicuculline methochloride and strychnine of the inhibition of spinal interneurones and Renshaw cells in the cat by glycine and γ‐aminobutyric acid. The results indicate that (+)‐tubocurarine would be of little use in assessing which of these amino acids was the transmitter at central inhibitory synapses.