Kinetic and pharmacological properties of the GABA-induced chloride current in Aplysia neurones: a 'concentration clamp' study

Independence of and interactions between GABA-, glutamate-, and acetylcholine-activated Cl conductances in Aplysia neurons

In certain Aplysia neurons, glutamate, GABA, and acetylcholine (ACh) all elicit desensitizing Cl-dependent responses. This fact and the finding that the glutamate and GABA responses "cross-desensitize" led to the suggestion (Swann and Carpenter, 1975; King and Carpenter, 1987) that the responses to these transmitters were mediated by the same receptor-channel complex. This hypothesis is incompatible with the demonstration given here that the GABA- and glutamate-gated channels are clearly distinct; the GABA channel, but not the glutamate channel, shows outward rectification (Matsumoto, 1982; King and Carpenter, 1987, 1989) and is selectively blocked by intracellular sulfate. Exploiting these distinctive characteristics and the independent expression of the receptors in some cells, we have been able to reevaluate the so-called cross-desensitization by analyzing the ability of GABA, glutamate, and other agonists to interact with each of the receptor molecules. The cross-desensitization was found to be exclusively attributable to the ability of GABA to interact with the glutamate receptor (Oyama et al., 1990). The GABA receptor is unaffected by glutamate. Nevertheless, in cells expressing both receptors, glutamate can reduce the GABA response by auto-desensitizing the part of the response that is mediated by the glutamate receptor. No interactions were observed between ACh-induced responses and either of the responses elicited by the amino acids. The invertebrate glutamate-gated Cl channels that have been cloned resemble the vertebrate glycine receptor (Vassilatis et al., 1997). Our pharmacological evaluation of the molluscan glutamate receptor points in the same direction.

Independence of and interactions between GABA-, glutamate-, and acetylcholine-activated Cl conductances in Aplysia neurons

In certain Aplysia neurons, glutamate, GABA, and acetylcholine (ACh) all elicit desensitizing Cl-dependent responses. This fact and the finding that the glutamate and GABA responses "cross-desensitize" led to the suggestion (Swann and Carpenter, 1975; King and Carpenter, 1987) that the responses to these transmitters were mediated by the same receptor-channel complex. This hypothesis is incompatible with the demonstration given here that the GABA- and glutamate-gated channels are clearly distinct; the GABA channel, but not the glutamate channel, shows outward rectification (Matsumoto, 1982; King and Carpenter, 1987, 1989) and is selectively blocked by intracellular sulfate. Exploiting these distinctive characteristics and the independent expression of the receptors in some cells, we have been able to reevaluate the so-called cross-desensitization by analyzing the ability of GABA, glutamate, and other agonists to interact with each of the receptor molecules. The cross-desensitization was found to be exclusively attributable to the ability of GABA to interact with the glutamate receptor (Oyama et al., 1990). The GABA receptor is unaffected by glutamate. Nevertheless, in cells expressing both receptors, glutamate can reduce the GABA response by auto-desensitizing the part of the response that is mediated by the glutamate receptor. No interactions were observed between ACh-induced responses and either of the responses elicited by the amino acids. The invertebrate glutamate-gated Cl channels that have been cloned resemble the vertebrate glycine receptor (Vassilatis et al., 1997). Our pharmacological evaluation of the molluscan glutamate receptor points in the same direction.

Effects of isoflurane and hexafluorodiethyl ether on human recombinant GABA(A) receptors expressed in Sf9 cells

Effects of volatile anesthetics and a volatile convulsant on human recombinant gamma-aminobutyric acid (GABA) type A receptor responses were studied using the whole cell configuration of the patch clamp technique. Sf9 cells were transfected with bacuroviruses carrying cDNAs of alpha1beta2, alpha1beta2gamma2s, alpha3beta2 and alpha3beta2gamma2s subunit combinations of the human GABA(A) receptor. Clinical concentrations of isoflurane (a volatile anesthetic) enhanced the GABA-induced current of the alpha1beta2gamma2s and alpha3beta2gamma2s GABA(A) subunit combinations. On the other hand, isoflurane suppressed the current of the alpha1beta2 and alpha3beta2 subunit combinations, indicating that the anesthetic effects depended upon the presence of gamma2s subunit. A high concentration (2 mM) of isoflurane generated a surge current following the washout of GABA and the anesthetic. Hexafluorodiethyl ether (a volatile convulsant) decreased the GABA-response of the both alpha3beta2gamma2s and alpha3beta2 constructs without generating a surge current. The results suggest that volatile agents affect the receptor-ionophore complex via direct interaction with proteins but not through a perturbation of the membrane lipid environment. A hypothetical sequential model for the anesthetic action is presented.

Volatile anesthetics directly activate baseline S K+ channels in aplysia neurons

The actions of halothane on serotonin-sensitive potassium channels (S K+ channels) were studied in sensory neurons of Aplysia. The normalized open probability of S K+ channels was increased by clinical concentrations of halothane in cell-attached and excised patches from neurons of the pleural ventrocaudal cluster. No voltage-dependence of channel activation by halothane was observed. Pre-treatment of neurons with 8-bromo-cAMP (8-Br-cAMP) or nordihydroguaiaretic acid (NDGA) had no effect on the relative level of channel activation by halothane. S K+ channels that were activated by arachidonic acid could also be activated by halothane and exhibited closely similar amplitude distributions of open channel current. Results from these experiments showed that S K+ channel activation by halothane did not depend on second messenger modulation of channel activity. We conclude that it is likely that halothane directly activates S K+ channels.

Ethanol sensitivity of postsynaptic receptors in the abdominal ganglion of Aplysia californica

We examined the effects of ethanol on post-synaptic receptors in the abdominal ganglion of Aplysia californica, including those sensitive to acetylcholine (ACh) and gamma-aminobutyric acid (GABA). Current elicited by direct application of acetylcholine to the cell body was composed of two components; a fast component (peak current - ca. 800 ms) and a slower component (peak current - ca. 12.2 s). These two components were differentially sensitive to ethanol. The fast component, which represents a chloride conductance, was significantly reduced in ethanol concentrations as low as 50 mM, while the slow component of the response, carried by potassium ions, was unaffected by ethanol at to ethanol, showing no change in 50 mM ethanol, and a slight, but statistically significant reduction of the response to GABA in 100 mM ethanol. These results suggest that the cholinergic system warrants further study, since the receptors gating the fast response consistently show sensitivity in the clinically-relevant range. In addition, results obtained studying these receptors may bear on the question of ethanol's site of action, since the receptor-channel complexes associated with the fast and slow responses are differentially sensitive to ethanol, although they are found in the same cell, and therefore would share the same bulk lipid matrix.

Single channels activated by high concentrations of GABA in superior cervical ganglion neurones of the rat

1. Single-channel currents evoked by high concentrations of GABA (10-2000 microM) have been analysed to investigate the characteristics of GABAA receptor channels in outside-out patches from rat sympathetic neurones. When high concentrations of GABA were applied to a patch, channel openings occurred in prolonged clusters (3.8 +/- 3.7 s (mean +/- S.D.) at 50 microM-GABA) consisting, on average, of 350 apparent openings per cluster. Individual clusters were separated by long silent intervals. 2. Channel openings were to many (often ill-defined) conductance states (range 7-36 pS), but the most frequently observed conductance level was approximately 30 pS, (29.6 +/- 0.34 pS). Only these clusters during which the channel was open to this main state conductance for at least 95% of the cluster open time were used in the analysis of probability of being open. Other less frequently observed conductance levels were 15-18 and 22-23 pS, while levels of 33-36 and 7-9 pS were occasionally, but reliably, observed. 3. Bursts within clusters were defined as a series of openings separated by closed intervals shorter than some critical value, tc. At 50 microM-GABA the mean burst length was 439 +/- 434 ms (+/- S.D., tc = 50 ms). 4. The probability of being open, po, during bursts within clusters has been analysed as a function of GABA concentration. As expected, increasing the concentration of GABA resulted in an overall increase in po. However, for a given agonist concentration there was a wide spread in po, far greater than that predicted for a population of identical and independent receptor channels (demonstrated by comparison with stimulated channel activity). 5. The wide range of po values at a particular concentration of GABA was not due to inappropriate selection of tc. At 50 microM-GABA the range of po values was similar for tc of 20-1000 ms, although the overall mean po became lower (0.64 rather than 0.81). 6. On the basis of simulated channel activity, it appears that most clusters which do not contain multiple openings, arise from the activity of one individual channel. Furthermore, there was no detectable tendency for gaps between bursts to be shorter in the middle of a cluster than at its ends. Therefore it is unlikely that variability in po arose from overlapping activity of two or more channels. 7. The values for po, mean open time, and mean shut time for bursts within the same cluster, and between different clusters, were compared by a randomization test.(ABSTRACT TRUNCATED AT 400 WORDS)