Discrete states of responsiveness of a locust muscle gamma-aminobutyric acid receptor: the influence of extracellular ion concentrations

Caffeine-induced potentiation of GABA effects on frog spinal cord: an electrophysiological study

A parasagittal slice of the frog spinal cord was kept in vitro for electrophysiological recordings from dorsal and ventral roots. Low concentrations of caffeine (50 microM) which had relatively small effects on baseline electrical activity, increased the depolarizing action of GABA on dorsal root fibres by 50%. A similar result was also obtained when GABA motoneuronal responses were tested. On dorsal roots the potentiation of GABA responses by caffeine was reflected by a significant decrease in GABA ED50 value without change in the maximal response amplitude; this enhancing action of caffeine was not blocked by bicuculline (5 microM) but was abolished by flurazepam (5 microM) or by Ca2+ antagonists (Mn2+ and Cd2+). Blockade of interneuronal activity by procaine left the potentiating action of caffeine unchanged. High doses of caffeine (up to 1 mM) produced a seemingly non-competitive antagonism of GABA responses. We suggest that caffeine can modulate GABA responses through two different mechanisms: a potentiation of GABA effects (seen with low doses of caffeine) probably due to Ca2+ mobilization and an antagonism of GABA responses (typically seen with large doses of caffeine) perhaps caused by block of GABA receptor-activated channels. This novel caffeine-GABA interaction may be useful to interpret some of the effects of caffeine on mammalian behaviour.

The appearance and development of chemosensitivity in Rohon-Beard neurones of the Xenopus spinal cord

1. We have examined the onset and subsequent development of chemosensitivity in Rohon-Beard neurones from the Xenopus spinal cord. These cells become sensitive to bath-applied gamma-aminobutyric acid (GABA) around stage 25 (early tailbud, about 1 d old), and remain so at least until stage 49 (9 d old). In contrast, a number of other neurotransmitter candidates tested caused no potential or conductance change during the same period.2. We examined ionophoretic dose-response relations of the cells at stage 26, a couple of hours after the first acquisition of GABA sensitivity. Sensitivities as high as 450 mV/nC were recorded. Comparable sensitivities were recorded between stages 46-49 (5-9 d old).3. Measurements of ionophoretic sensitivities and input resistances during several periods from stage 26 to maturity show that the underlying conductance change for a given GABA dose is likely to increase steadily during this time. A ;sensitivity index' (ionophoretic sensitivity/input resistance) was calculated, which is low at stage 26, higher at intermediate stages (stages 31-42), and highest for mature cells (stages 46-49; 5-9 d of development).4. The reversal potential of the ionophoretic GABA response is the same at stage 26 (-30 mV) as it is in mature cells. Ion substitution experiments show that Na(+) and K(+), but not Cl(-) or Ca(2+), are involved in the response.5. GABA responses at stage 26 are pharmacologically similar to those of mature cells. The responses are blocked by 10 muM-picrotoxin or curare, and muscimol is an agonist in concentrations as low as 1 muM.6. GABA responses at stage 26 desensitize in a manner similar to that seen for mature cells, either with prolonged bath application of GABA or with repetitive ionophoretic application.7. Nearly half of the cells tested at stage 26 respond to glycine, in concentrations as low as 5 muM. This sensitivity is absent by 3(1/2) d of development.8. The responses of Rohon-Beard neurones to GABA are similar to those of other cells in that they involve a conductance increase, are mimicked by muscimol, and are blocked by picrotoxin. These responses are different in that they do not involve Cl(-) and are blocked by low concentrations of curare.9. Many of the characteristics of GABA receptors, i.e. the reversal potential, desensitization, and pharmacology, are constant during development. However, the sensitivity of the cells to GABA and the spectrum of transmitters to which they are sensitive appear to change.

A novel effect of zinc on the lobster muscle GABA receptor

The effect of zinc (and copper) was investigated on the lobster muscle gamma-aminobutyric acid (GABA) receptor. Zinc (10 microns-1 mM) depressed the GABA-evoked conductance increase in a fully reversible manner by possibly binding to an imidazole group, suggested from pH titration studies on the evoked-chloride conductance. Other transition metal (period 4) divalent cations (up to 500 microM) were inactive in antagonizing GABA responses. Variation of external chloride or anion substitution did not perturb the zinc antagonism; however, decreasing the pH markedly decreased the potency of zinc. A possible explanation for these results is discussed. Although the zinc antagonism resembled that produced by picrotoxinin, combination of these two agents depressed the GABA dose--conductance curve in a manner expected for two antagonists acting on independent sites. The zinc binding site was also discrete from the GABA recognition site; the results are interpreted in terms of a distinct binding site for zinc and H+. The distortion of an agonist dose--response curve by formation of an inactive agonist-divalent cation complex is discussed; however, complexation of GABA did not explain the observed antagonism by zinc. By comparison, zinc had no effect on the GABA responses of rat ganglionic neurons. It is concluded that the zinc binding site, on lobster muscle, may be an important modulatory site for the GABA-evoked chloride conductance.