GABA-induced rise of extracellular potassium in rat dorsal root ganglia: an electrophysiological study in vivo

The pro-algesic effect of γ-aminobutyric acid (GABA) injection into the masseter muscle of healthy men and women

Background and aims

Preclinical studies have reported that activation of peripheral γ-aminobutyric acid A (GABAA) receptors may result in analgesia. The current study was conducted in young healthy men (n = 30) and women (n = 28) to determine whether injections of GABA into the masseter muscle reduce pain in a sex-related manner.

Methods

The effect of injection of GABA alone, or in combination with the non-inflammatory algogen glutamate, was assessed in two separate studies. Lorazepam, a positive allosteric modulator of the GABAA-receptor, was co-injected with GABA in both studies to explore the role of this receptor in muscle pain responses of healthy human volunteers. Masticatory muscle mechanical pain intensity was recorded on an electronic visual analogue scale (VAS) while muscle pain sensitivity was assessed by determining the pressure pain threshold (PPT), tolerance and maximal jaw opening (MJO) of the subjects prior to, and again after the various intramuscular injections.

Results

Intramuscular injection of GABA alone was reported to be significantly more painful, in a concentration related manner, than saline control injections, and this pain was further increased by co-injection of lorazepam with GABA. Co-injection of GABA with glutamate was found to significantly increase glutamate-evoked masseter muscle pain in men, but not in women. There was no effect of injections of either GABA alone, or GABA with glutamate, on PPT, tolerance or maximum jaw opening.

Conclusions

Injection of GABA into the human masseter muscle appears to excite nociceptors to produce muscle pain without a longer term effect on mechanical pain sensitivity in the muscle. The findings suggest that GABA-mediated pain in humans is produced through peripheral GABAA receptor activation. The mechanism underlying the sex-related difference in the effect of GABA on glutamate-evoked muscle pain was speculated to be due to a methodological artifact.

Implications

This study was designed to detect analgesic rather than algesic effects of peripherally administered GABA, and as a result, the concentration of glutamate chosen for injection was close to the maximal pain response for healthy women, based on previously determined pain-concentration response relationships for glutamate. This may explain the finding of greater pain in men than women, when GABA and glutamate were co-injected. Overall, the findings suggest that activation of peripheral GABAA receptors in human masticatory muscle produces pain, possibly due to depolarization of the masticatory muscle afferent fibers.

GABA increases electrical excitability in a subset of human unmyelinated peripheral axons

Background

A proportion of small diameter primary sensory neurones innervating human skin are chemosensitive. They respond in a receptor dependent manner to chemical mediators of inflammation as well as naturally occurring algogens, thermogens and pruritogens. The neurotransmitter GABA is interesting in this respect because in animal models of neuropathic pain GABA pre-synaptically regulates nociceptive input to the spinal cord. However, the effect of GABA on human peripheral unmyelinated axons has not been established.

Methodology/Principal Findings

Electrical stimulation was used to assess the effect of GABA on the electrical excitability of unmyelinated axons in isolated fascicles of human sural nerve. GABA (0.1–100 µM) increased electrical excitability in a subset (ca. 40%) of C-fibres in human sural nerve fascicles suggesting that axonal GABA sensitivity is selectively restricted to a sub-population of human unmyelinated axons. The effects of GABA were mediated by GABA_A receptors, being mimicked by bath application of the GABA_A agonist muscimol (0.1–30 µM) while the GABA_B agonist baclofen (10–30 µM) was without effect. Increases in excitability produced by GABA (10–30 µM) were blocked by the GABA_A antagonists gabazine (10–20 µM), bicuculline (10–20 µM) and picrotoxin (10–20 µM).

Conclusions/Significance

Functional GABA_A receptors are present on a subset of unmyelinated primary afferents in humans and their activation depolarizes these axons, an effect likely due to an elevated intra-axonal chloride concentration. GABA_A receptor modulation may therefore regulate segmental and peripheral components of nociception.

Na+,K+,2Cl- cotransport and intracellular chloride regulation in rat primary sensory neurons: thermodynamic and kinetic aspects

Adult primary afferent neurons are depolarized by GABA throughout their entire surface, including their somata located in dorsal root ganglia (DRG). Primary afferent depolarization (PAD) mediated by GABA released from spinal interneurons determines presynaptic inhibition, a key mechanism in somatosensory processing. The depolarization is due to Cl(-) efflux through GABA(A) channels; the outward Cl(-) gradient is generated by a Na+,K+,2Cl(-) cotransporter (NKCC) as first established in amphibians. Using fluorescence imaging microscopy we measured [Cl(-)]i and cell water volume (CWV) in dissociated rat DRG cells (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and calcein, respectively. Basal [Cl(-)]i was 44.2 +/- 1.2 mM (mean +/- SE), Cl(-) equilibrium potential (E Cl) was -27.0 +/- 0.7 mV (n = 75). This [Cl(-)]i is about four times higher than electrochemical equilibrium. On isosmotic removal of external Cl(-), cells lost Cl(-) and shrank. On returning to control solution, cells reaccumulated Cl(-) and recovered CWV. Cl(-) reaccumulation had Na+-dependent (SDC) and Na+-independent (SIC) components. The SIC stabilized at [Cl(-)]i = 13.2 +/- 1.2 mM, suggesting that it was passive (E(Cl) = -60.5 +/- 3 mV). Bumetanide blocked CWV recovery and most (65%) of the SDC (IC50 = 5.7 microM), indicating that both were mediated by NKCC. Active Cl(-) uptake fell with increasing [Cl(-)]i and became negligible when [Cl(-)]i reached basal levels. The kinetics of active Cl(-) uptake suggests a negative feedback system in which intracellular Cl(-)regulates its own influx thereby keeping [Cl(-)]i constant, above electrochemical equilibrium but below the value that would attain if NKCC reached thermodynamic equilibrium.

Peripheral GABA(A) receptors: evidence for peripheral primary afferent depolarization

We propose that the primary afferent depolarization that follows GABA(A) receptor activation in the spinal cord also occurs in the periphery. As evidence, the present study localizes beta2/beta3 and alpha1 subunits of the GABA(A) receptor on 10-14% of the unmyelinated primary afferents axons in the glabrous skin of the cat paw. Behavioral studies demonstrate that local peripheral injection of the GABA(A) agonist muscimol at a low concentration (2.0 microM) attenuates, and at a high concentration (1 mM) enhances, formalin-induced nociceptive behaviors. Intraplantar injection of muscimol alone at a high dose evokes thermal hyperalgesia. Bicuculline, a GABA(A) antagonist, prevents these muscimol-induced changes in behavior. The muscimol-induced effects are due to local rather than systemic or central activation of GABA(A) receptors, as such effects are not observed in the contralateral paw. We interpret these findings to indicate that activation of GABA(A) receptors by low concentrations of muscimol depolarizes peripheral primary afferent terminals, a phenomenon we call peripheral primary afferent depolarization, in turn reducing the size of the peripheral action potentials and concomitantly reducing the amount of algogenic substances released from the peripheral terminals of these fibers. This sequence of events presumably results in a reduction in nociceptor activation. Higher concentrations of muscimol further depolarize GABA(A) receptor-containing terminals, which then initiates action potentials in nociceptors analogous to the appearance of dorsal root reflexes that arise following activation of GABA(A) receptors on central primary afferent terminals. These latter events reverse the analgesic effects of GABA(A) ligands and lead to potentiation of nociceptive input. Thus, the present study provides anatomical and behavioral evidence supporting a bimodal role for GABA(A) receptors in the modulation of peripheral nociceptive transmission.

Chemically mediated cross-excitation in rat dorsal root ganglia

Primary afferent neurons in mammalian dorsal root ganglia (DRGs) are anatomically isolated from one another and are not synaptically interconnected. As such, they are classically thought to function as independent sensory communication elements. However, it has recently been shown that most DRG neurons are transiently depolarized when axons of neighboring neurons of the same ganglion are stimulated repetitively. Here we further characterize this functional coupling. In electrophysiological recordings made from excised rat DRGs, we found that DRG "cross-depolarization" is excitatory in that it is accompanied by an increase in the probability of spiking in response to otherwise subthreshold test pulses delivered intracellularly. Cross-depolarization contributes to this mutual cross-excitation. However, at least as important a contribution comes from a net increase in the neurons' input resistance (Rin) triggered by the stimulation of neighboring neurons. This change in Rin occurs even when cross-depolarization is absent or is balanced out. The amplitude of cross-depolaration was found to be voltage-dependent, with a reversal potential at approximately -23mV. Reversibility and the change in Rin both indicated that activity of neighboring neurons causes a membrane conductance change that is chemically mediated. Thus, far from being isolated, most DRG neurons participate in ongoing mutual interactions in which neuronal excitability is continuously modulated by afferent spike activity. This intraganglionic dialog appears to be mediated, at least in part, by an activity-dependent diffusable substance(s) released from neuronal somata and/or adjacent axons, and detected by neighboring cell somata and/or axons.

GABA and potassium effects on corticospinal and primary afferent tracts of neonatal rat spinal dorsal columns

The neurotransmitter GABA markedly depresses action potential conduction in neonatal rat spinal dorsal columns. However, GABA sensitivity of the dorsal columns declines with maturation and myelination. At seven to 14 days after birth, the corticospinal tract component of the dorsal columns is immature and unmyelinated compared to the cuneate-gracilis fasciculi. GABA and isoguvacine (a GABAA receptor agonist) were applied to isolated neonatal (seven to 14 days old) dorsal columns during recordings of conducted cuneate-gracilis fasciculi and corticospinal tract action potentials. GABA (10(-4) to 10(-3) M) significantly reduced amplitudes (-28.9% to -69.7%) and increased latencies (+4.8% to +23.9%) of cuneate-gracilis fasciculi responses but had less effect on corticospinal tract response amplitudes (-1.1% to -14.7%) and latencies (+0.9% to +6.2%). Likewise, isoguvacine (10(-5) to 10(-4) M) reduced amplitudes (-26.7% to -37.5%) and increased latencies (+11.2% and +24.0%) of cuneate-gracilis fasciculi responses but had little or no effect on corticospinal tract response amplitudes (-6.2% to -3.8%) or latencies (-0.8% to +1.5%). At 10(-4) and 10(-3) M, GABA rapidly increased extracellular K+([K+]e) from baseline levels of 3.0 mM to 3.7 +/- 0.4 and 6.6 +/- 1.4 mM in cuneate-gracilis fasciculi and increased corticospinal tract [K+]e to 3.9 +/- 0.4 and 4.4 +/- 0.4 mM (mean +/- S.D.). [K+]e declined during drug application and fell below baseline after drug washout. Cuneate-gracilis fasciculi responses, however, did not recover until several minutes after [K+]e returned to baseline. In separate experiments, increasing bath [K+]e concentrations to 3.7 and 6.6 mM reduced cuneate-gracilis fasciculi response amplitudes by only -7.6% and -29.6%. Latencies increased by +1.3% and +3.6% respectively. The results indicate that the cuneate-gracilis fasciculi are more sensitive to GABA than the corticospinal tract and that the GABA effect is not entirely due to [K+]e changes.

Extracellular alkalinization evoked by GABA and its relationship to activity-dependent pH shifts in turtle cerebellum

1. The effect of gamma-aminobutyric acid (GABA) on extracellular pH (pHo) was investigated in the turtle cerebellum, in vitro, using double-barrelled, H(+)-selective microelectrodes. Responses evoked by GABA were compared with pHo shifts evoked by repetitive stimulation of the parallel fibres. 2. In media buffered with 35 mM-HCO3- and 5% CO2, superfusion of GABA (1 mM) elicited an abrupt alkaline shift in the molecular layer, which averaged 0.05 +/- 0.02 pH units (+/- S.D., range 0.02-0.12 pH units). pHo often recovered in the continued presence of GABA, and displayed a rebound acidification upon wash-out. 3. The GABA-evoked alkaline shift was blocked by picrotoxin and was mimicked by the GABAA agonists isoguvacine and muscimol. The GABAB agonist baclofen did not elicit an alkaline shift. Alkaline shifts evoked by stimulation of the parallel fibres were unaffected by picrotoxin. 4. In nominally HCO3(-)-free solutions, buffered with 35 mM-HEPES, superfusion of GABA caused either no pHo change or a slow acid shift. In contrast, the alkaline shift evoked by stimulation of the parallel fibres became enhanced in HEPES-buffered media. 5. The alkaline shift evoked by GABA was accompanied by an increase in extracellular K+ ([K+]o) which averaged 1.7 mM above baseline. Experimental elevation of [K+]o to a comparable level always caused a pure acid shift in the extracellular space. 6. The GABA-evoked alkaline shift persisted when synaptic transmission was blocked using 4 mM-kynurenic acid or saline prepared with nominally zero Ca2+ and 10 mM-Mg2+. The alkaline shift evoked by repetitive stimulation of the parallel fibres was completely abolished in these media. 7. Although the GABA-evoked alkaline shift was blocked in nominally HCO3(-)-free media, substitution of 35 mM-formate for HCO3- restored the GABA response. Superfusion of 1 mM-GABA in formate saline produced an alkaline shift of 0.040 +/- 0.034 pH units. 8. These results indicate that gating of GABAA channels in the vertebrate CNS gives rise to an HCO3- efflux which can significantly increase the pH of the brain microenvironment. However, this mechanism cannot account for the extracellular alkalinization caused by parallel fibre stimulation. Extracellular alkaline shifts capable of modulating local synaptic operations may therefore be a consequence of either excitatory or inhibitory synaptic transmission.

GABA-sensitivity of dorsal column axons: an in vitro comparison between adult and neonatal rat spinal cords

In neonatal rat spinal cord, conduction in the dorsal column is reversibly depressed by GABA. We compared the GABA-sensitivity of dorsal columns in neonate versus adult rats, using in vitro isolated dorsal column preparations. The extracellular compound action potential evoked by submaximal stimuli was recorded with a glass micropipette. GABA (10(-4)-10(-3) M) reversibly depressed the compound action potential of both neonatal and adult rat dorsal columns. The GABA-induced reduction of dorsal column compound action potential amplitudes was blocked by the GABAA antagonist picrotoxin (10(-3) M) and mimicked by the GABAA agonist isoguvacine (10(-4-10(-3) M). The compound action potential reduction by GABA was far less pronounced on adult dorsal columns. The reduction of compound action potential amplitudes by isoguvacine (10(-4)-10(-3) M) was also significantly less in adult dorsal columns. These data suggest that GABAA receptors may play a role in extrasynaptic modulation of spinal long tract conduction in an age-dependent manner.

Changes in extracellular K+ evoked by GABA, THIP and baclofen in the guinea-pig hippocampal slice

Changes in [K+]0 evoked by the inhibitory amino acid transmitter, GABA (gamma-aminobutyric acid) and its agonists were recorded with ion-selective microelectrodes in the CA1 stratum pyramidale of guinea-pig hippocampal slices. Bath applications of GABA (0.1-10 mM) produced dose-dependent increases in [K+]0 (EC50 = 4 mM, Rmax = 1.6 mM), with a peak and decline during exposure, followed by undershoot during recovery. In contrast the selective GABAA agonist, THIP (4,5,6,7-tetrahydroisoxazolo-(5,4-c)-pyridin-3-ol) (0.01-1 mM) showed approximately ten-fold greater potency and evoked only increases in [K+]0 (EC50 = 0.5 mM, Rmax = 2 mM). Reduction of temperature from 34 degrees to 22 degrees C caused a more than two-fold augmentation of the K+0 accumulation evoked by GABA, but no change in that due to THIP. The GABAA antagonist, BMI (bicuculline methiodide) (100 microM) completely blocked responses to THIP and partially antagonized those to GABA. Responses to GABA were synergistically enhanced by pentobarbital (100 microM). Only small, delayed and inconsistent changes could be evoked by relatively high concentrations of the GABAB agonist, DL-baclofen (0.01-1 mM). The K+ changes evoked by GABA appear to be mediated by the activation of GABAA receptors with low affinity and to be related to their depolarizing action. Although the response includes an electrogenic component which suggests the involvement of Na-dependent transmitter uptake/transport, the increase in K+0 probably reflects an outward counter/co-transport of K+ with Cl/HCO3 anion shifts and/or activation of a voltage-dependent K+ conductance.

GABAA receptors modulate axonal conduction in dorsal columns of neonatal rat spinal cord

gamma-Aminobutyric acid (GABA) can influence conduction in a number of axonal preparations from the peripheral and central nervous system. In the spinal cord, the excitability of primary afferent terminals has long been known to be affected by GABA. Whether conduction in the long fiber tracts of the spinal cord can be similarly modulated is unknown. Since GABA causes a pronounced depression of excitability in preparations of unmyelinated axons, and myelination is incomplete in the neonatal rat, we tested whether GABA can modulate conduction in the dorsal columns of 10-17-day-old rats. Experiments were performed in vitro, on isolated dorsal column segments (n = 18). The extracellular compound action potential evoked by submaximal stimuli was recorded with a glass micropipette positioned 0.5-2.0 mm from a stimulating electrode. At concentrations of 10(-4) - 10(-3) M, GABA decreased excitability, reversibly depressing the compound action potential amplitude, and increasing the latency by 47 +/- 11% and 22 +/- 9% (mean +/- S.E.M., n = 5, 10(-3) M), respectively. These effects were blocked by picrotoxin and mimicked by isoguvacine (10(-4) M), which decreased the compound action potential amplitude by 44 +/- 10% and increased the latency by 9 +/- 4% (n = 5). Lower concentrations of these agents caused a modest increase in excitability. At 10(-5) M, GABA increased the compound action potential amplitude by 14 +/- 2% and decreased the latency by 3 +/- 2% (n = 5). Our results demonstrate that functional GABAA receptors are present in neonatal dorsal columns.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of GABA, baclofen and bicuculline on the extracellular K+ activity in the olfactory bulb of the developing rat

gamma-Aminobutyric acid (GABA) and baclofen were iontophoresed into the glomerular and plexiform layers of the olfactory bulb of rats at different ages. Subsequent changes in K+ activity and mitral cell firing rates were monitored to specify GABA receptor activation. Only bicuculline-insensitive receptors are shown to be already present in the glomerular layer of the newborn rat. The possible distribution and maturation of GABA receptors on the olfactory nerve endings or on the dendrites of the mitral or periglomerular cells is discussed.

Nipecotic acid, an uptake blocker, prevents fading of the gamma-aminobutyric acid effect

In rats under urethane, iontophoretic applications of GABA (30-60 nA) in the str. pyramidale of CA1, showed a rapidly fading inhibitory effect. By contrast, GABA had a well-maintained inhibitory effect in str. radiatum. During iontophoresis of nipecotic acid (30-85 nA) identical applications of GABA in str. pyramidale caused a more prominent depression without fading, which suggests that removal of GABA, by uptake, can at least in part account for 'fading'. Nipecotic acid also prolonged the paired-pulse inhibition, presumably by prolonging the duration of inhibitory postsynaptic potentials.

An intracellular analysis of gamma-aminobutyric-acid-associated ion movements in rat sympathetic neurones

Double-barrelled ion-sensitive micro-electrodes were used to measure the changes of the intracellular activities of Cl-, K+, and Na+ (aiCl, aiK, aiNa) in neurones of isolated rat sympathetic ganglia during the action of gamma-aminobutyric acid (GABA). The membrane potential of some of the neurones was manually 'voltage clamped' by passing current through the reference barrel of the ion-sensitive micro-electrode. This enabled us to convert the normal depolarizing action of GABA into a hyperpolarization. A GABA-induced membrane depolarization was accompanied by a decrease of aiCl, aiK and no change in aiNa, whereas a GABA-induced membrane hyperpolarization resulted in an increase of aiCl, aiK and also no change in aiNa. GABA did not change the free intracellular Ca2+ concentration, as measured with a Ca2+-sensitive micro-electrode, whereas such an effect was seen during the action of carbachol. pH-sensitive electrodes, on the other hand, revealed a small GABA-induced extracellular acidification. The inward pumping of Cl- following the normal, depolarizing action of GABA required the presence of extracellular K+ as well as Na+, whereas CO2/HCO3--free solutions did not influence the uptake process. Furosemide, but not DIDS, blocked the inward pumping of Cl-. In conclusion, our data show that only changes in intracellular activities of K+ and Cl- are associated with the action of GABA. Furthermore, they indicate that a K+/Cl- co-transport, and not a Cl-/HCO3- counter-transport, may be involved in the homoeostatic mechanism which operates to restore the normal transmembrane Cl- distribution after the action of GABA.

Different types of potassium transport linked to carbachol and gamma-aminobutyric acid actions in rat sympathetic neurons

Carbachol and gamma-aminobutyric acid depolarize mammalian sympathetic neurons and increase the free extracellular K+-concentration. We have used double-barrelled ion-sensitive microelectrodes to determine changes of the membrane potential and of the free intracellular Na+-, K+- and Cl- -concentrations ( [Na+]i, [K+]i and [Cl-]i) during neurotransmitter application. Experiments were performed on isolated, desheathed superior cervical ganglia of the rat, maintained in Krebs solution at 30 degrees C. Application of carbachol resulted in a membrane depolarization accompanied by an increase of [Na+]i, a decrease of [K+]i and no change in [Cl-]i. Application of gamma-aminobutyric acid also induced a membrane depolarization which, however, was accompanied by a decrease of [K+]i and [Cl-]i, whereas [Na+]i remained constant. Blockade of the Na+/K+-pump by ouabain completely inhibited both the reuptake of K+ and the extrusion of Na+ after the action of carbachol, and also the post-carbachol undershoot of the free extracellular K+-concentration. On the other hand, in the presence of ouabain, no changes in the kinetics of the reuptake of K+ released during the action of gamma-aminobutyric acid could be observed. Furosemide, a blocker of K+/Cl- -cotransport, inhibited the reuptake of Cl- and K+ after the action of gamma-aminobutyric acid. In summary, the data reveal that rat sympathetic neurons possess, in addition to the Na+/K+-pump, another transport system to regulate free intracellular K+-concentration. This system is possibly a K+/Cl- -cotransport.

Properties of the GABA receptors located on spinal primary afferent neurones and hypophyseal neuroendocrine cells of the rat

Electrophysiological techniques have been used to study the pharmacological characteristics of GABA receptors in two in vitro preparations likely to provide the ionic basis for GABAergic inhibition of excitation-secretion coupling. The shortening of Ca2+ spikes duration by GABAB receptors was shown to occur in slow conducting dorsal root ganglion cells, independently of marked depression of inward calcium currents. Ion-selective electrodes (K+ or Ca2+) were used to show the presence of both GABAA and GABAB receptors on the neurosecretory terminals and gland cells from hypophyseal neuro-intermediate lobe (NIL). In this latter preparation, potentiation of hormone release was observed under GABAA receptor activation, whilst inhibition was seen with GABAB agonists.

Comparative study of GABA-mediated depolarizations of lumbar A delta and C primary afferent neurones of the rat

The distribution of GABA receptors on various categories of primary afferents was studied by means of intracellular recordings from rat dorsal root ganglion neurones. Cells were identified on the basis of their conduction velocity and classified as A delta and C neurones. Transient applications of GABA led to a decrease of membrane resistance and a concomitant depolarization. Maximal GABA-induced responses were weaker in C than in A delta and A beta cells. Smaller conductance changes in C cells suggest a lower density of GABAA receptors, and the heterogeneity of the "membrane potential/response amplitude" relationship indicate that the ionic mechanisms underlying GABA-induced responses may not be uniform on all primary afferents; this is supported by the wide range of reversal potential values recorded under voltage-clamp conditions in A delta cells.

Depolarizing action of GABA (gamma-aminobutyric acid) on myelinated fibers of peripheral nerves

The inhibitory neurotransmitter GABA (gamma-aminobutyric acid) has been shown to have a depolarizing action on myelinated axons of both mammalian and amphibian peripheral nerves. In initial in vivo observations intravenous injections of GABA caused an increase in the excitability of the low-threshold, fast conducting fibers of the superficial radial and median nerves of the cat. Similar, graded, reversible effects were confirmed (using changes in the amplitude/integral of the stimulus-evoked A-fiber submaximal compound action potential to assess excitability) in in vitro studies with the isolated, desheathed frog sciatic nerve. GABA caused a mean maximal increase in half-maximal action potential of 29.8% (S.E. +/- 2.7), with an ED50 value of 0.09 mM and Hill coefficient of 0.70. This effect did not appear to desensitize, and could be reversibly antagonized by both bicuculline and picrotoxin. Comparison of agonist sensitivities showed a rank order of potency with muscimol greater than 3-aminopropanesulfonic acid greater than GABA greater than beta-guanidinopropionic acid greater than imidazole-acetic acid greater than guanidoacetic acid greater than delta-aminovaleric acid. With structure activity analysis the maximal activity was found to be related to N+-C separation near the 5 A value. Partial substitution of chloride ions in the superfusate by isethionate reversibly depressed the effect of GABA. These observations support the conclusion that extrasynaptic receptors for GABA are present on the myelinated axons of peripheral nerves.

Role of K+ in GABA (gamma-aminobutyric acid)-evoked depolarization of peripheral nerve

Isolated, desheathed sciatic nerves of the leopard frog or bull frog were used in studies to determine different sources/components of the depolarizing effect of GABA (gamma-aminobutyric acid) on myelinated fibers. During the depolarization induced by 1 mM GABA--which was reflected by an increase of 38.3% (S.E. +/- 2.2) in the amplitude of the evoked half-maximal A-fiber compound action potential--the level of extracellular potassium ([K+]o) measured at depths less than or equal to 200 microns in the nerve with ion-selective microelectrodes, increased by 0.096 mM (S.E. +/- 0.007). Changes in excitability preceded K+]o, and there was a significant difference between their peak latencies. Artificially raised levels of [K+]o, similar to those induced by GABA, caused extremely small changes (less than 10%) in the size of the evoked action potential. From the magnitude and time course of the GABA-evoked augmentation of levels of [K+]o, it can be concluded that potassium ions probably arise indirectly and play a secondary role in what appears to be a mainly receptor-mediated depolarization of axons. A much greater sensitivity to GABA was found for fibers of the dorsal roots in comparison with those of the ventral roots (maximal changes in excitability of 50% and 6% respectively). This suggests that the depolarization of ventral root fibers could be caused by [K+]o accumulation, and that there may be a preferentially localized distribution of receptors for GABA on the sensory axons of peripheral nerve.

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological study with K+- and Ca2+-sensitive micropipettes of GABA receptors in the rat neurointermediate lobe in vitro

An efflux of K+ and a decrease in extracellular Ca2+ activity are to be expected when GABA markedly depolarizes the membrane of unmyelinated axons or secretory cells. Accordingly, we used extracellular recordings of ionic movements to specify GABA receptor presence in parts of the pituitary having GABAergic innervation: the neurohypophysis and intermediate lobe (NIL). We identified a site of action having the same desensitization characteristics and pharmacological criteria as the GABA A receptor which modulates Cl- -conductance. Baclofen, a GABA B agonist, was without effect. The possible distribution of receptors and role of GABAergic synapses in modulating neurotransmitter and hormone release by the NIL are discussed.

GABA 'desensitization' of frog primary afferent fibers

GABA (gamma-aminobutyric acid) depolarizes the terminals of primary afferent fibers of the in vitro hemisected frog spinal cord. During sustained or repetitive exposure to GABA or to muscimol, the amplitude of the depolarization is characterized by a rapid and exponential decline to a steady plateau level (desensitization). Desensitization to muscimol was eliminated by removal of Ca2+ and addition of Mn2+ to the superfusate--a finding consistent with the presence of 'receptor' ('true') desensitization (i.e., receptor inactivation). GABA desensitization was significantly reduced by exposure of the cord to either low Na+, low temperature, ouabain, dinitrophenol, (+/-)-nipecotic acid, or cis-1,3-aminocyclohexanecarboxylic acid. These treatments also significantly decreased the high affinity uptake of GABA when the latter process was studied by incubating frog spinal slices in Ringer's solution containing a low concentration of [3H]GABA. These results suggest that cellular transport processes can influence the form of GABA responses and indicate that neuronal removal of GABA is responsible in part for GABA desensitization.

Extracellular K+ concentration during electrical stimulation of rat isolated sympathetic ganglia, vagus and optic nerves

Recordings of extracellular potassium concentration ( [K+]e) were made in rat isolated sympathetic ganglia, vagus and optic nerves using ion sensitive microelectrodes. Repetitive orthodromic stimulation of ganglia resulted in [K+]e increases of up to 7 mmol/l above resting level (6 mmol/l), which were followed by post-stimulus undershoots. Activation of vagal A and B fibres did not significantly alter [K+]e but C-fibre activity induced rises of up to 5 mmol/l. Repetitive stimulation of the predominantly myelinated optic nerve resulted in [K+]e rises of up to 2.5 mmol/l. In the ganglion and vagus nerve, application of ouabain (30-1000 mumol/l) led to a raised baseline [K+]e concentration, an increase in the peak achieved during stimulation and a reduced undershoot amplitude. The amplitude of the undershoot in normal solution was shown to be dependent on the duration of the preceding stimulation period as well as the amplitude of the preceding [K+]e rise. In ganglia and vagus nerves, bath application of gamma-aminobutyric acid (10-100 mumol/l) and carbachol (10-100 mumol/l) also elevated [K+]e. It is concluded that repetitive activity in rat peripheral and central nerve fibres leads to significant changes in extracellular K+ ion-concentration and that the restoration of these levels is strongly dependent on the intact activity of the membrane Na+/K+ pump.

GABA-mediated changes in excitability of the rat lateral olfactory tract in vitro

1. Conditioning stimulation of the lateral olfactory tract (l.o.t.) in the rat olfactory cortex slice evoked a slow depolarization of the terminal regions of the l.o.t. The depolarization lasted about 150 ms and was abolished by the gamma-aminobutyric acid (GABA) antagonist bicuculline. 2. Excitability testing of the terminal regions of the l.o.t. showed an increase in excitability which lasted for about 150 ms following a conditioning stimulus. This increase in excitability was abolished by the calcium antagonist cadmium and by the GABA antagonists bicuculline and penicillin. 3. Superfused GABA caused a consistent decrease in the excitability of the terminal regions of the l.o.t. within the cortex but an increase in excitability of the axons within the tract itself. These effects were antagonized by bicuculline. K+ caused similar changes in excitability which were not antagonized by bicuculline. 4. The difference between the effects of superfused GABA and the effects of orthodromic conditioning can be explained if a more restricted location of action is assumed for the GABA released by conditioning stimulation. It is suggested that GABA causes a decrease in excitability at its locus of action and that the observed increases in excitability occur in adjacent areas of neuronal membrane.

Action of GABA and glycine on the membrane potential of culture astrocytes and the extracellular concentration

The depolarization of cultured astrocytes by GABA and glycine correlates in amplitude and time course with the increase of the extracellular K+ -concentration during perfusion with these amino acids. It is suggested that the glial depolarization is caused by an efflux of K+ from neighbouring neurones activated by the amino acid transmitters.

Does glial uptake affect GABA responses? AN intracellular study on rat dorsal root ganglion neurones in vitro

1. Using single barrel pipettes, intracellular records were obtained from surface neurones of isolated rat dorsal root ganglia (DRG) impaled under microscopic vision.2. Responses to gamma-aminobutyric acid (GABA) were elicited either by ionophoresis or by placing drops of concentrated GABA solutions directly into the flow of superfusing Ringer. Using this latter method it was estimated that the GABA concentration eliciting threshold ( approximately 1 mV) responses was 3-20 muM.3. Short (</= 1 sec) ionophoretic or drop administrations of GABA elicited depolarizing responses associated with an increased membrane conductance. With longer applications the initial depolarization was not sustained but decayed to a lower plateau level (desensitization) associated with a minimal conductance change.4. Low chloride superfusions did not affect subsequent responses to GABA unless GABA was also administered during the low chloride superfusion, in which case responses declined markedly. This suggests that GABA caused appreciable chloride fluxes when it was administered regularly (e.g. for 1 sec every minute).5. Glial GABA uptake was inhibited by adding 1 mM-beta-alanine or 0.25 mM-chlorpromazine to the bicarbonate-Ringer superfusate or by substituting lithium for sodium in a Tris-Ringer superfusate. Uptake inhibition had no consistent effect on any of the parameters studied, namely membrane potential, input resistance, amplitude and time course of responses to GABA, and GABA desensitization.6. Muscimol and isoguvacine, which are probably not substrates for the glial GABA carrier, elicited responses with time course and desensitization characteristics indistinguishable from those of responses to GABA.7. GABA superfused at concentrations as low as 1 muM could reduce responses to ionophoretic GABA, i.e. cause a desensitization of GABA receptors.8. It is concluded firstly that in DRG, glial uptake does not affect the amplitude or time course of responses to GABA when the neurone under study is close to the source of GABA; and secondly that desensitization can occur independently of GABA uptake.9. The findings are discussed in relation to their possible relevance to GABA systems in the central nervous system.

Effects of 4-aminopyridine and tetraethylammonium on the depolarization by GABA of cultured satellite glial cells

4-Aminopyridine (4-AP) which selectively blocks K+-conductance of excitable membranes, reversibly abolished the depolarization by gamma-aminobutyric acid (GABA) of cultured satellite glial (SG) cells, but did not or only slightly affect the action of GABA on dorsal root ganglion (DRG) neurons. It is therefore suggested that the depolarization of glial cells by GABA is an indirect effect due to the accumulation of K+ which is released from adjacent neurons during their depolarization by the amino acid. Tetraethylammonium (TEA) had no effect on the GABA depolarization at a concentration of 15 mM but produced a slight reduction at higher concentrations (60--70 mM).

Depolarization of neurones in the isolated olfactory cortex of the guinea-pig by gamma-aminobutyric acid

1 Effects of gamma-aminobutyric acid (GABA) on single neurones in slices of guinea-pig olfactory cortex maintained in vitro were recorded with single intracellular microelectrodes. The average resting potential of 52 cells was -75 mV and apparent input resistance ranged from 20 to 200 MOmega.2 Superfusions of GABA over the slice invariably depolarized the neurones and reduced their input resistance. The minimum effective concentration was 50 to 200 muM.3 The reversal potential for the depolarization produced by 0.1 mM GABA (E(g)) was -66 +/- 2 mV. At concentrations >0.1 mM the reversal potential became progressively more positive (-55 to -50 mV).4 Reduction of external chloride, with isethionate as the substitute anion, increased the amplitude of the depolarization.5 GABA reduced the amplitude of the excitatory postsynaptic potential produced by lateral olfactory tract stimulation, and occluded or reversed the subsequent depolarizing recurrent inhibitory postsynaptic potential.6 Action potentials elicited by injection of depolarizing current or by focal antidromic stimulation were slowed and reduced in amplitude by GABA.7 The effects of GABA on membrane conductance (potency = 1) were duplicated by 3-aminopropanesulphonic acid (potency = 20), beta-alanine (0.5), beta-amino-n-butyric acid (0.5), glycine (0.3) and L-2,4-diaminobutyric acid (0.2). For a given conductance change, 3-aminopropanesulphonic acid, glycine and beta-alanine produced less depolarization than did GABA.8 It is concluded that the action of GABA on the neurones is compatible with a role in mediating recurrent postsynaptic inhibition.

Characterization and ionic basis of GABA-induced depolarizations recorded in vitro from cat primary afferent neurones

1. Responses of single cells in the isolated cat spinal ganglion to GABA applied by superfusion or by iontophoresis were recorded using intracellular micro-electrodes. 2. Of the twelve structurally related compounds investigated, GABA was the most effective in its ability to produce a depolarization of the cell membrane. 3. Studies determining concentration-response relationships indicate that two to three molecules of GABA are required to combine with the GABA receptor for activation. 4. Bicuculline and picrotoxin, each act in a non-competitive manner to antagonize the GABA-induced membrane current. 5. The equilibrium potential for iontophoretically induced GABA depolarizations (EGABA) was found to be -23.5 plus or minys 6.1 mV. EGABA was independent upon [cl-]o, but independent of [Na+]o, [K+], or [Ca2+]o. 6. Intracellular injection of twenty antions (Br-, I-, NO2-, NO3-, ClO4-, SCN-, Bf4-, HS-, OCN-, ClO3-, BrO3-, F-, HCO2-, HSO3-, HCO3-, CH3CO2-, SO42-, C6H5O73-) indicated that the activated GABA receptor membrane was permeable to those anions whose hydrated diameter is no larger than that of ClO-3. 7. Restoration of the GABA depolarization to its control level after augmentation by Cl- injection had a mean time constant of 27.8 plus or minus 2.6 min. Picrotoxin did not alter this value. 8. When foreign anions were exchanged for Cl- in the perfusion solution, the ten anaions smaller or equal to ClO3-, decreased the GABA depolarization by 50-90% and increased its time course 1.5-2.0 x control. The only exception having a small radius was Br- which augmented the amplitude 10-30%. 9. The ten anions larger than ClO3- produced a biphasic effect, i.e. an initial augmentation followed by a marked (up to 100%) depression of the response. Experiments with CH3COO-, CH3SO4-, or HOCH2CH2SO3-, indicated that this depression was non-competitive.