PHARMACOLOGICAL STUDIES ON THE PRIMARY AFFERENT DEPOLARIZATION OF THE TOAD SPINAL CORD

Diversity of inhibitory and excitatory parvalbumin interneuron circuits in the dorsal horn

ABSTRACT

Parvalbumin-expressing interneurons (PVINs) in the spinal dorsal horn are found primarily in laminae II inner and III. Inhibitory PVINs play an important role in segregating innocuous tactile input from pain-processing circuits through presynaptic inhibition of myelinated low-threshold mechanoreceptors and postsynaptic inhibition of distinct spinal circuits. By comparison, relatively little is known of the role of excitatory PVINs (ePVINs) in sensory processing. Here, we use neuroanatomical and optogenetic approaches to show that ePVINs comprise a larger proportion of the PVIN population than previously reported and that both ePVIN and inhibitory PVIN populations form synaptic connections among (and between) themselves. We find that these cells contribute to neuronal networks that influence activity within several functionally distinct circuits and that aberrant activity of ePVINs under pathological conditions is well placed to contribute to the development of mechanical hypersensitivity.

Pioneers in CNS inhibition: 2. Charles Sherrington and John Eccles on inhibition in spinal and supraspinal structures

This article reviews the contributions of the English neurophysiologist, Charles Scott Sherrington [1857-1952], and his Australian PhD trainee and collaborator, John Carew Eccles [1903-1997], to the concept of central inhibition in the spinal cord and brain. Both were awarded Nobel Prizes; Sherrington in 1932 for "discoveries regarding the function of neurons," and Eccles in 1963 for "discoveries concerning the ionic mechanisms involved in excitation and inhibition in central portions of the nerve cell membrane." Both spoke about central inhibition at their Nobel Prize Award Ceremonies. The subsequent publications of their talks were entitled "Inhibition as a coordinative factor" and "The ionic mechanism of postsynaptic inhibition", respectively. Sherrington's work on central inhibition spanned 41 years (1893-1934), and for Eccles 49 years (1928-1977). Sherrington first studied central inhibition by observing hind limb muscle responses to electrical (peripheral nerve) and mechanical (muscle) stimulation. He used muscle length and force measurements until the early 1900s and electromyography in the late 1920s. Eccles used these techniques while working with Sherrington, but later employed extracellular microelectrode recording in the spinal cord followed in 1951 by intracellular recording from spinal motoneurons. This considerably advanced our understanding of central inhibition. Sherrington's health was poor during his retirement years but he nonetheless made a small number of largely humanities contributions up to 1951, one year before his death at the age of 94. In contrast, Eccles retained his health and vigor until 3 years before his death and published prolifically on many subjects during his 22 years of official retirement. His last neuroscience article appeared in 1994 when he was 91. Despite poor health he continued thinking about his life-long interest, the mind-brain problem, and was attempting to complete his autobiography in the last years of his life.

GABAergic transmission in temporal lobe epilepsy: the role of neurosteroids

Modification of GABAergic inhibition is an intensely investigated hypothesis guiding research into mechanisms underlying temporal lobe epilepsy (TLE). Seizures can be initiated by blocking γ amino butyric acid type A (GABAA receptors, GABARs), which mediate fast synaptic inhibition in the brain, and controlled by drugs that enhance their function. Derivatives of steroid hormones called neurosteroids are natural substances that physiologically enhance GABAR function and suppress seizures. GABAR structure, function, expression, assembly, and pharmacological properties are changed in the hippocampus of epileptic animals. These alterations render GABARs less sensitive to neurosteroid modulation, which may contribute to seizure susceptibility. Plasticity of GABARs could play a role in periodic exacerbation of seizures experienced by women with epilepsy, commonly referred to as catamenial epilepsy.

How does a little acronym become a big transmitter?

After an overview of the early, chequered history of the discovery of GABA and its gradual acceptance as inhibitory synaptic transmitter in the brain, the article lists and discusses some of the more unexpected later developments in studies of GABA, especially its role as excitatory transmitter in the immature brain.

Hoffmann reflex from foreleg flexor nerves in cats: lateralization, picrotoxin, strychnine, crossed flexor reflex

Hoffmann (H) reflexes from foreleg flexor nerves were studied in cats. The right and left flexor nerves were stimulated and H reflexes were recorded from the same nerves. Paw preference was assessed by a food reaching test. Stimulation of the right median nerve elicited mono- and polysynaptic reflexes from the left ulnar nerve (crossed flexor nerve). Picrotoxin depressed, and strychnine increased H reflexes from both sides without affecting the spinal motor asymmetry. H reflexes were found to be larger on the left than the right side in right-preferent cats and vice versa in left-preferent cats. The right H-reflex recovery curve was higher than left in right-preferent cats. The inhibitory period of the recovery cycle disappeared after picrotoxin and changed to facilitation for the nonpreferred side. Strychinine caused bilateral, nearly-synchronous motoneuronal discharges from the right and left flexor nerves; the discharges originating from the left side preceded those from the right side in a right-preferent cat. These results indicate that spinal motor activity predominates on the nonpreferred side, which would be a prerequisite for postural adjustments during paw use in cats. This asymmetric motor organization in the forelegs of cats having quadrpedal locomotion seems to be similar to asymmetric motor organization in legs in humans.

Dorsal root potentials in the isolated frog spinal cord: amino acid neurotransmitters and magnesium ions

Sucrose gap techniques recorded dorsal root potentials evoked by supramaximal dorsal root stimulation in in vitro, hemisected frog spinal cords. In 0 mM Mg2+ large (mean 13.0 mV), long lasting (mean 8.1 s) dorsal root potentials were recorded which consisted of two components: (1) an early component sensitive to picrotoxin, bicuculline, and low [Cl-]o and presumably produced by activation of GABAA receptors; and (2) a long-duration second component enhanced and lengthened by picrotoxin, bicuculline and low [Cl-]o and thought to result from increased interneuron discharges resulting from depression of GABA-mediated pre- and postsynaptic inhibition. Both the early and late components were reduced by over 90% in amplitude and duration by 20 mM Mg2+ or by kynurenate and bicuculline. The early component of the dorsal root potential may depend mainly upon activation of non-N-methyl-D-aspartate receptors. Thus, the N-methyl-D-aspartate antagonist D-(-)-2-amino-5- phosphonovalerate caused only a modest reduction in the amplitude of the early dorsal root potential component while the non N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione caused a much more substantial reduction. Exposure of the spinal cord to a "physiological" concentration of Mg2+ (1.0 mM) greatly reduced the duration and somewhat reduced the amplitude of the dorsal root potential. The reduction of dorsal root potentials by 1.0 mM Mg2+ appears to be caused by both pre- and postsynaptic factors.(ABSTRACT TRUNCATED AT 250 WORDS)

General anaesthetics and field currents in unclamped, unmyelinated axons of rat olfactory cortex

1. The effects of seven general anaesthetics and one local anaesthetic having a wide range of physical and chemical properties were studied on nerve terminal Na- and K-mediated currents in slices of olfactory cortex. These currents were measured from the groups of fine unmyelinated axons traversing the surface of the olfactory cortex and which give off synapses en passant. The amplitude of the K-current was visualized by depolarizing the axons via an electrode polarization. 2. The anaesthetics tested were ketamine (0.1-2 mM), pentobarbitone (0.1-5 mM), urethane (5-200 mM), halothane (0.5-5 mM), ether (10-200 mM), alphaxalone (0.001-0.05 mM), diisopropylphenol (0.05-0.5 mM) and lignocaine (0.01-0.5 mM). All had depressant effects on the axonal Na-current (at the higher concentrations tested) and on the K-current (at slightly lower concentrations). The apparent lower potency on the Na-current was considered to be due to a masking of effect as a consequence of the reduction in the K-mediated membrane rectification rather than any real difference in the susceptibilities of the Na and K-currents. 3. Some of the depressant effect of pentobarbitone and alphaxalone was gamma-aminobutyric acid (GABA)-mediated as indicated by the reduced potency in the presence of bicuculline. The actions of ketamine and halothane were unaffected by bicuculline. 4. For some anaesthetics these axonal depressant effects might contribute to general anaesthesia, while for other substances the relatively high concentrations necessary would suggest that this mode of action does not produce effective anaesthesia in vivo.

Barbiturate regulation of kinetic properties of the GABAA receptor channel of mouse spinal neurones in culture

1. Barbiturate regulation of the kinetic properties of gamma-aminobutyric acidA (GABA) receptor channel chloride currents from somata of mouse spinal cord neurones were investigated using whole-cell and excised outside-out patch-clamp recording techniques. 2. GABA (2 microM), GABA (2 microM) plus phenobarbitone (PhB) (500 microM) and GABA (2 microM) plus pentobarbitone (PB) (50 microM), applied by pressure ejection from blunt perfusion micropipettes, evoked inward chloride currents when neurones or patches were voltage clamped at -75 mV and the chloride equilibrium potential was 0 mV. GABA receptor channel currents were increased by PhB and PB. 3. Single GABA receptor channel currents were recorded with a main conductance state of 27 pS and a less frequent subconductance state of 16.5 pS. The conductances of the two states were unchanged by the barbiturates. 4. The main conductance state kinetics were analysed. GABA alone or with the barbiturates gated the channel open singly and in groups of openings. 5. The barbiturates increased GABA receptor channel mean open time and shifted frequency histograms of channel open times to longer times. 6. Three exponential functions were required to fit the frequency histograms of GABA receptor channel open times, suggesting that the channel has at least three open states (O1, O2, O3). The time constants for the exponential functions (0.9, 2.7 and 7.8 ms, respectively) were unchanged by the barbiturates. The increases in mean open times and the shifts of the open-time frequency histograms by the barbiturates were due to a reduction in relative frequency of occurrence of the two short open states (O1 and O2) and to an increase in the relative frequency of occurrence of the longest open state (O3). 7. Frequency histograms of GABA receptor channel closed times were fitted with five exponential functions, suggesting that the channel has multiple closed states. None of the time constants nor areas of the exponential functions were significantly changed by the barbiturates. 8. For analysis, a burst was defined as openings surrounded by closures greater than a critical closed time, tc, of 5 ms. For GABA (2 microM), frequency histograms of GABA receptor channel bursts were fitted with three exponential functions, suggesting that the channel has three burst states (B1, B2, B3). The B1 burst state was probably a single opening to the O1 open state while the B2 and B3 burst states were probably composed of multiple openings to the O2 and O3 open states.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic properties of the pentobarbitone-gated chloride current in frog sensory neurones

1. The kinetic properties of the activation and inactivation (desensitization) phases of pentobarbitone (PB)-induced inward Cl- current (ICl) were studied in isolated frog sensory neurones, following suppression of Na+, K+ and Ca2+ currents, using the concentration jump technique which combines the internal perfusion and the rapid exchange of the external solutions surrounding a neurone with time constants of 2-3 ms. The results were compared with those of the gamma-aminobutyric acid (GABA)-gated ICl. 2. The PB dose-response curve was bell-shaped and the maximum peak value was less than the current induced by 1.7 X 1.5(-5) M-GABA, the concentration at which GABA evoked a half-maximum response. 3. The activation and inactivation phases of PB-induced ICl consisted of double-exponential, fast and slow components, respectively. The time constant of the fast component (tau af) of the activation was relatively stable in a concentration range between 3 X 10(-4) and 6 X 10(-3) M. The time constant of the slow component (tau as) of the activation decreased with increasing PB concentrations. Both the fast and slow components (tau if and tau is) of the inactivation decreased with increasing PB concentrations. 4. Over a wide range of concentrations the tau af and tau as values of the PB-induced ICl were 10-30 times greater than the respective values of GABA-induced ICl. 5. At concentrations below 10(-3) M the PB-induced ICl was voltage dependent at more negative potentials than -20 mV. 6. The PB-induced ICl was blocked by bicuculline and by picrotoxin, but in a different manner. Bicuculline increased the time constants of the activation and inactivation. Picrotoxin had little effect on the activation phase but markedly facilitated the inactivation phase. 7. High concentrations of PB (over 10(-3) M) led to a decline in both the peak and plateau currents of the PB-induced ICl. A transient 'hump' current appeared with wash-out of the external solutions containing high concentrations of PB. This hump current was blocked by bicuculline in a dose-dependent manner. 8. The results suggest the possibilities that the PB receptor-ionophore complexes consist of at least two different components having different affinities and kinetics and that the PB and GABA binding sites are closely located.

Sodium-dependent suppression of gamma-aminobutyric-acid-gated chloride currents in internally perfused frog sensory neurones

1. The effects of the Na+ electrochemical potential gradient on gamma-aminobutyric acid (GABA)-induced Cl- currents (ICl) in frog sensory neurones were studied, using a suction pipette technique with which internal perfusion can be accomplished under current- and voltage-clamp conditions. 2. Under current clamp, the depolarizing response to GABA decreased in the presence of external Na+. A similar external Na+-dependent reduction in the GABA-induced inward ICl was observed under voltage clamp. The reversal potential of GABA-induced ICl (EGABA) was nearly equal to the Cl- equilibrium potential (ECl), irrespective of the presence or absence of external Na+. 3. Varying the Na+ influx by changing the holding membrane potential (VH) altered the GABA response: the GABA-induced ICl decreased progressively as VH became more negative. 4. The effects of changing the external and internal Na+ concentrations ([Na+]o and [Na+]i) on the GABA-induced ICl were also studied. Increasing [Na+]o at a constant [Na+]i reduced this current while increasing [Na+]i at a fixed [Na+]o facilitated it. 5. A high temperature coefficient of about 3 was estimated with respect to the percentage reduction in GABA-induced ICl due to [Na+]o. 6. These results indicate that the [Na+]o-dependent suppression of GABA-induced ICl was mediated chiefly by the uptake of GABA subserved by a Na-GABA co-transport mechanism. 7. GABA dose-response measurements were made with and without external Na+. The [Na+]o-induced suppression was more pronounced in relative amount at lower concentrations and in absolute amount at intermediate concentrations. Analysis of these data indicates, however, that the Na+-coupled GABA influx kept increasing at GABA concentrations high enough to nearly saturate GABA-induced ICl, and the same saturating level was observed as in the Na+-free case. This indicates that the electrogenic co-transport current was much smaller so that our measurements of GABA-induced ICl' were contaminated very little. Thus, the present method based on recording of GABA-induced ICl was legitimate for the analysis of the Na-GABA co-transport. 8. By analysing the [Na+]o-dependent suppression of GABA-induced ICl, the stoichiometric ratio of the underlying co-transport was estimated to be one: one Na+ ion per GABA molecule. 9. The ICl induced by GABA agonists such as beta-alanine, taurine, l-GABOB (l-gamma-amino-beta-hydroxybutyric acid) and muscimol was not affected by the amount of external Na+ present, suggesting difference in the affinity between receptor and transport carrier.

Inhibition of Na+-dependent GABA uptake in isolated frog sensory nerve cell bodies by extra- and intracellular Li+

The effect of Li+ on the Na+-dependent gamma-aminobutyric acid (GABA) uptake was investigated by examining the effects of external and internal Li+ ([Li]o and [Li]i) on GABA-gated Cl- current (ICl) in the frog dorsal root ganglion cells using the suction pipette technique which allows the internal perfusion under voltage-clamp condition. The suppression of GABA responses in the presence of external Na+ ([Na]o) was larger at lower than at higher GABA concentration. Replacement of [Na]o with Li+ completely removed the Na+ suppression, and GABA dose-response curve in Li+ external solution agreed well with that in Na+-free (Tris+) external solution. Either increasing [Li]i) or internal Na+ ([Na]i) at a constant [Na]o equally reduced the Na+-dependent suppression of GABA-gated ICl. The results indicate that both the [Li]o and [Li]i remove in different manner the [Na]o-dependent suppression of GABA-induced ICl:i.e. the [Li]o acts as a blocker of Na+-GABA co-transport mechanism while the increase of [Li]i decreases the Na+ electrochemical potential gradient across the soma membrane as well as [Na]i does.

Glucocorticoid modulates the sensitivity of the GABAA receptor on primary afferent neurons of bullfrogs

With intracellular and voltage-clamp recording techniques, we have demonstrated that the glucocorticoids, prednisolone and hydrocortisone at a concentration of 5 microM to 1 mM, reversibly depressed gamma-aminobutyric acid (GABA)-induced responses on primary afferent neurons of bullfrogs. An analysis with dose-response curves revealed that the glucocorticoids decreased the sensitivity of the GABAA receptor in a non-competitive manner. We suggest that glucocorticoids act as an antagonist of the GABAA receptor on primary afferent neurons, probably by reducing the number of functional GABAA receptor ionic channel complexes.

Identification of extrasynaptic binding sites for [3H]GABA in peripheral nerve

The binding of radiolabelled gamma-aminobutyric acid ( [3H]GABA) to extrasynaptic sites in peripheral nerve was examined in order to characterize the receptor species mediating the depolarizing action of GABA on myelinated axons. Binding of [3H]GABA was carried out under Na+-free conditions with fresh homogenates of amphibian sciatic nerve. The kinetics of association of the radioligand suggested the presence of a rapidly associating, reversible binding site, and a slowly associating, apparently irreversible one. Scatchard plots of the reversible component of binding were linear and yielded a mean affinity (Kd) of 22 nM. This stands in contrast with the ED50 of 90 microM for GABA-evoked depolarization of frog nerve; and therefore the high affinity binding sites are unlikely to be involved in such depolarization. The rank order of potency of agonists that competed with [3H]GABA was muscimol greater than GABA much greater than delta-aminovaleric acid greater than beta-alanine. However, the GABAA analogues 3-aminopropanesulfonic acid, beta-guanidinopropionic acid, and guanidoacetic acid as well as the GABAB agonist baclofen all failed to displace the ligand. Bicuculline methiodide, picrotoxin, and nipecotic acid also did not compete. The bicuculline-insensitive, baclofen-insensitive high affinity binding sites identified here appear to be unique, as they are distinctly different from the classical GABAA and GABAB receptors.

GABA activates different types of chloride-conducting receptor-ionophore complexes in a dose-dependent manner

We report here evidence for 3 new subtypes (alpha 1, alpha 2 and beta) of type-A GABA receptor-channel complexes that conduct chloride ions. The chloride current, ICl, was isolated in the frog sensory neuron by a combination of voltage clamp and internal perfusion. Analysis of the variance of GABA-induced ICl fluctuations shows that the channel population N decreases exponentially with single-channel conductance gamma in such a way that alpha 2 less than alpha 1 less than beta for gamma and alpha 2 much greater than alpha 1 greater than beta for N, and that the population-rank plot fits Zipf's law. Various aspects of the GABA-induced ICl are understood from dose-dependent activation and inactivation of these functionally distinct receptor-channel types. The steady-state ICl is mediated by alpha 1 at low but by beta units at high GABA concentrations, and the pronounced ICl peak at intermediate and high doses reflects the desensitization of alpha 1 and alpha 2 receptors, respectively. Picrotoxin blocks alpha 1 and alpha 2 and has no effect on beta channels. Patch-clamp recordings indicate two distinct classes of GABA-gated chloride conductances that appear to correspond to the alpha 1 and beta types. The presence of these different ICl components explains why the dose-response relationship cannot be fitted well by a single Hill equation; the fitting requires a synthesis of 3 suitable Hill equations.

Interactions of gamma-aminobutyric acid (GABA), pentobarbital, and homopantothenic acid (HOPA) on internally perfused frog sensory neurons

Augmentatory actions among Cl- currents (ICl) induced by gamma-aminobutyric acid (GABA), pentobarbital (PB), and homopantothenic acid (HOPA) were investigated in isolated frog sensory neurons after suppression of Na+, K+, and Ca2+ currents using a suction pipette technique which combines internal perfusion with voltage clamp. GABA-sensitive neurons responded to both PB and HOPA, and the responses behaved as a simple Cl- electrode and reversed at the Cl- equilibrium potential (ECl). The dose-response curve for GABA-induced Cl- conductance was sigmoidal with the GABA concentration producing a half-maximum response (4.2 X 10(-5) M). Both GABA and HOPA dose-response curves shifted to the left in the presence of PB, though the facilitatory action of PB on GABA- and HOPA-induced ICl was more effective in the former. There was a significant facilitatory interaction between GABA- and HOPA-induced ICl. It is concluded that HOPA affects the GABA-GABA or PB-PB receptor interactions.

gamma-Aminobutyric-acid- and pentobarbitone-gated chloride currents in internally perfused frog sensory neurones

gamma-Aminobutyric-acid- (GABA) and pentobarbitone-induced Cl- currents (ICl) were studied in isolated frog sensory neurones after suppression of Na+, K+ and Ca2+ currents using a suction-pipette technique combining internal perfusion with voltage clamp. All GABA-sensitive neurones responded to pentobarbitone. Both GABA- and pentobarbitone-induced ICl reversed at the Cl- equilibrium potential (ECl). The dose-response curve for maxima of GABA-induced ICl was sigmoidal with a mean concentration producing a half-maximum response, Ka of 2 X 10(-5) M at a Hill coefficient of 1.8. In the presence of pentobarbitone, the GABA dose-response curve shifted to the left without affecting the saturating maximum current. At high concentrations, both GABA and pentobarbitone could also potentiate the pentobarbitone- and GABA-induced ICl respectively, while pre-treatment with one of the two markedly attenuated currents induced by the other, indicating a 'cross-desensitization'. In the presence of pentobarbitone, the augmented response was voltage dependent and this augmentation was much greater in the inward-current direction than outward. In producing ICl, pentobarbitone and its stereoisomers were potent in the order of (-) isomer greater than (+/-) racemic mixture greater than (+) isomer. A stereospecific facilitatory action of pentobarbitone on GABA responses was also found in the same order. Responses to GABA, homotaurine, taurine, beta-alanine, 5-aminovaleric acid, (+)- and (-)-gamma-amino-beta-hydroxybutyric acid and muscimol were equally enhanced by pentobarbitone, though its action on glycine-induced ICl was less effective. Picrotoxin inhibited the GABA- and pentobarbitone-induced ICl from either side of membrane, while internal application of GABA and pentobarbitone did not exert any effect. It was concluded that pentobarbitone binds to the 'barbiturate receptors' located close to the GABA receptor-Cl- channel complex, and directly affects the GABA-GABA receptor interactions rather than the ionic channels.

Depolarization of feline primary afferent fibres by acidic amino acids

When administered micro-electrophoretically into the spinal grey matter of cats anaesthetized with pentobarbitone, acidic amino acids known to be neuronal excitants lower the threshold of electrically stimulated muscle and cutaneous primary afferent fibres and terminations. This depolarizing effect was not observed with fibres stimulated in the white matter. Depolarization by micro-electrophoretic potassium and excitant amino acids appeared not be be associated with an alteration in terminal membrane conductance since there was no change in synaptically evoked primary afferent depolarization. Excitant amino acid depolarization was not blocked by the gamma-aminobutyric acid antagonist bicuculline methochloride, but was reduced by selective excitant amino acid antagonists. The results are discussed in relation to the probable absence of specific excitant amino acid receptors on afferent terminals, the depolarizing effect of the amino acids on myelinated fibres and non-myelinated terminals being more likely a consequence of changes in the extracellular medium associated with the depolarization and firing of neurones.

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.

Effects of intrathecally administered pentobarbital and naloxone on the activity evoked in ascending axons of the rat spinal cord by stimulation of afferent A and C fibres. Further evidence for a tonic endorphinergic inhibition in nociception

The effects of intrathecally administered pentobarbital and naloxone on activity in ascending axons were determined in decerebrate rats with the spinal cord transected at the lower thoracic level. Activity in ascending axons of the spinal cord was recorded below the site of transection and evoked by electrical stimulation of afferent A beta, A delta or C fibres in the sural nerve. Pentobarbital 250 micrograms depressed activity evoked by stimulation of non-nociceptive A beta and nociceptive C fibres; it did not change activity in response to stimulation of A delta fibres. A low dose (100 micrograms) had no effect of A beta and C fibre-evoked activity but depressed spontaneous activity in the ascending axons. Naloxone 5 micrograms enhanced the spontaneous and evoked activities only in those ascending axons which responded to C fibre stimulation. Pretreatment with pentobarbital 250 micrograms prevented the facilitation by naloxone of C fibre-evoked activity. Naloxone was ineffective even when it was administered in a dose of 100 micrograms simultaneously with pentobarbital. Intrathecal injections of magnesium chloride depressed spontaneous and C fibre-evoked activities and markedly reduced the facilitatory effect of naloxone. It is concluded that nociceptive C fibre-evoked activity is subject to the inhibitory control of endorphinergic neurones and that naloxone facilitates this activity by producing release from inhibition.

Effect of barbiturates on the GABA receptor of cat primary afferent neurones

1. The effects of the barbiturate anaesthetics, pentobarbitone and thiopentone, on the membrane properties and the gamma-aminobutyric acid (GABA)-induced responses of cat primary afferent neurones were studied with intracellular recording and voltageclamp techniques.2. At low concentrations (10(-7)-10(-5) M) both barbiturates slightly enhanced and prolonged GABA-induced depolarizations or currents without affecting the membrane properties. At these concentrations, barbiturates have no effect on the apparent dissociation constant of the GABA-GABA receptor interaction or the reversal potential for GABA-induced depolarizations or currents.3. At high concentrations (10(-4)-10(-3) M) barbiturates produced a few millivolts reduction in the resting membrane potential. Voltage-clamp analysis revealed that the depolarization was associated with one of the three types of conductance change, i.e., an initial increase followed by a decrease (40% of neurones examined), only an increase (40%) and only a decrease (20%).4. Analysis in different ionic media indicated that the depolarization with a reduced membrane resistance is associated with an increased chloride conductance and that the one with an increased membrane resistance is accompanied by a reduction in potassium conductance. Bath-application of GABA (10(-3) M) or picrotoxin (10(-5) M) inhibited the increase in chloride conductance but not the reduction in potassium conductance.5. Barbiturates at these high concentrations initially caused a marked augmentation and prolongation of GABA responses; this was followed by a depression. The depressant action did not appear to be voltage-dependent. These actions of barbiturates were not accompanied by changes in the apparent dissociation constant of the GABA-current dose-response curve or the reversal potential for GABA currents. In addition, the single exponential decay of GABA current was not changed despite a marked prolongation of its decay time.6. Picrotoxin (10(-5) M) antagonized the depressant effect of barbiturates at high concentrations on GABA currents, and barbiturates (5 x 10(-6) M) reduced the inhibitory action of picrotoxin (5 x 10(-6) M) on the GABA-currents.7. From all these results, it is suggested that the site of barbiturate actions on GABA-responses is mainly the allosteric site (the ionic conductance regulatory subunit) but not the agonist recognition site or the chloride channels linked with GABA receptors.

Pharmacology of GABA-mediated inhibition of spinal cord neurons in vivo and in primary dissociated cell culture

In this paper it is shown that the postsynaptic GABA-receptor chloride ion channel complex is composed of several functional subunits. There are probably at least two stereospecific locations on the receptor for GABA-binding and both must be occupied to obtain an increase in chloride conductance. The interaction between these sites is uncertain but there could be either positive cooperativity between the sites or only a requirement that both sites are occupied without occupation of either site affecting the affinity for GABA of the other site. There is a chloride conductance channel coupled to the GABA receptor which opens for an average of 20 msec and has an average conductance of 18 pS. The GABA-coupled chloride channel may or may not have the same composition as the glycine coupled chloride channel. In addition to the GABA-recognition site and the chloride ion channel, GABA-receptors must have additional binding sites or modulator sites where drugs can bind to modify GABA activation of the GABA receptor. The convulsant PICRO binds to a site which is independent of the GABA site and PICRO reduces GABA responses. Barbiturates and benzodiazepines augment GABA-responses without reducing GABA-binding and thus they must bind to a modulator site independent of the GABA recognition site. Whether or not this is the same site as the PICRO binding site is uncertain. Thus, the GABA-receptor-chloride ion channel complex is composed of at least: 1) two GABA-binding sites; 2) a chloride ion channel; 3) a convulsant binding site (PICRO-binding site) and 4) an anticonvulsant binding site. This organization serves several obvious purposes. First, since two GABA-molecules are required to activate GABA-coupled chloride ion channels, the dose-response relationship for GABA is sigmoidal and steep. Thus minor shifts in GABA affinity will produce large alterations in GABA-responses and the GABA receptor can be easily modulated. Second, since the receptors has binding sites for convulsant and anticonvulsant compounds which decrease and increase GABA-responses, GABAergic inhibition can easily be modulated.

Primary afferent depolarization. Distribution of the gamma-aminobutyric acid system in frog spinal cord

In the frog spinal cord primary afferent depolarization (PAD) constitutes a powerful inhibitory control mechanism. It has been suggested that gamma-aminobutyric acid (GABA) is the transmitter substance involved in the genesis of PAD. In these studies we show that maximal glutamic acid decarboxylase activity is localized roughly 400-600 micrometers from the dorsal surface, and that correlates well with the intraspinal distribution of field potentials associated with PAD. Measurements of GABA in serial spinal cord sections cut in a dorsal--ventral direction shows that high levels of GABA are seen at 400--600 micrometers, with a peak at 800 micrometers from the dorsal surface. Stimulation at frequencies shown to produce PAD augments the release of endogenous GABA from a superfused frog hemicord preparation.

Pentobarbital: stereospecific actions of (+) and (-) isomers revealed on cultured mammalian neurons

Stereoisomers of the barbiturate anesthetic pentobarbital were applied to mouse spinal neurons growing in tissue culture. Intracellular recordings of neuronal membrane properties revealed that the (+) and (-) isomers caused direct changes in membrane potential and conductance on some but not all of the cells tested. The action of the (+) isomer was predominantly excitatory, whereas the (-) isomer produced predominantly inhibitory responses. The (-) isomer was considerably more effective in potentiating inhibitory responses to the transmitter gamma-aminobutyric acid. The results show that pentobarbital has multiple effects on neuronal excitability and demonstrate the presence of stereospecific sites of barbiturate action on central neurons.

Studies on the excitability of sinus nerve afferent terminals

1. The excitability of sinus nerve afferent terminals within the nucleus of the tractus solitarius has been studied in cats and rabbits using the technique of antidromic activation. 2. Conditioning stimuli to the hypothalamic defence area increased the excitability of some glossopharyngeal nerve afferents, though no such effects were observed on sinus nerve terminals. 3. Although the excitability of superior laryngeal nerve afferent terminals was observed to fluctuate in phase with the central respiratory cycle, no equivalent variation in sinus nerve terminal excitability were observed. 4. It is concluded that sinus nerve afferent terminals are not influenced by presynaptic mechanisms. Possible sites for the observed modulations of baroreceptor and chemoreceptor reflexes are discussed in the light of these results.

Enhancement of GABA-mediated postsynaptic inhibition in cultured mammalian spinal cord neurons: a common mode of anticonvulsant action

Murine spinal cord neurons grown in dissociated cell culture were used to study the effects of barbiturate (phenobarbital, mephobarbital) and benzodiazepine (diazepam, chlordiazepoxide( anticonvulsants on amino acid responses. Both types of anticonvulsant augmented GABA-mediated postsynaptic inhibition without augmenting beta-alanine or glycine-mediated postsynaptic inhibition. Barbiturates, but not benzodiazepines, antagonized glutamate-mediated postsynaptic excitation. Augmentation of GABA-mediated inhibition by the anticonvulsants should contribute to their anticonvulsant action; antagonism of glutamate-mediated excitation by barbiturates should also contribute to their anticonvulsant action and could be at least in part responsible for their sedative actions.

Some actions of catechol on synaptic transmission in the isolated spinal cord of the frog

The isolated frog spinal cord was used to investigate the synaptic effects of the convulsant agent catechol. Addition of the compound to the superfusate consistently enhanced orthodromic reflex activity recorded from ventral roots and augmented primary afferent depolarization. Concomitantly catechol altered the polarization changes produced in ventral and dorsal roots by putative neurotransmitter amino acids when these compounds were applied in Mg2+-containing Ringer. Catechol reduced the hyperpolarizations induced in motoneurons by the neutral amino acids, GABA, beta-alanine, taurine and glycine, but did not affect the depolarizations produced by the dicarboxylic amino acids, L-glutamate and L-aspartate. In contrast, catechol increased the dorsal root depolarizations elicited by both neutral and dicarboxylic amino acids and also the depolarizations produced by elevated potassium concentrations. Catechol did not bring about significant changes in the passive electrical properties of motoneurons or dorsal root fibers. In addition, it did not alter either the high affinity uptake or the depolarization-evoked release of tritiated GABA, glycine, L-glutamate and L-aspartate. It appears that the postsynaptic actions of catechol explain its ability to enhance spinal reflexes.

Pentobarbitone pharmacology of mammalian central neurones grown in tissue culture

1. The effects of the barbiturate anaesthetic pentobarbitone on the membrane properties and amino acid pharmacology of mammalian C.N.S. neurones grown in tissue culture were studied using intracellular recording coupled with bath application, extracellular ionophoresis, or focal diffusion. 2. The addition of an anaesthetic concentration of pentobarbitone to the bathing medium abolished all spontaneous synaptic activity, but did not render individual cells electrically inexcitable nor prevent evoked synaptic acitivity. 3. Focal ionophoresis of pentobarbitone or diffusion from blunt micropipettes reversibly increased membrane conductance, effectively dampening excitability without directly affecting individual action potential characteristics. 4. Pentobarbitone-induced membrane conductance was reversibly blocked by picrotoxin. The inversion potential of the pentobarbitone voltage response depended on Cl- ion gradients and was similar to that of GABA. 5. Pentobarbitone reversibly enhanced the conductance increase produced by GABA with a variable slowing of response kinetics, shifting GABA dose-response curves to the left. Responses to glycine and beta-alanine were not affected. 6. Higher ionophoretic currents of pentobarbitone, which measurably increased membrane conductance, attenuated and markedly slowed GABA responses. Similar effects on GABA responses were observed by superimposing GABA pulses on low level GABA currents. 7. Pentobarbitone, in the absence of an increase in membrane conductance, reversibly depressed depolarizing responses to glutamate without changing response kinetics. Slower responses to acetylcholine which were associated with an apparent decrease in membrane conductance were not affected by the drug. 8. Analysis of double-reciprocal plot data suggested a non-competitive type of antagonism between pentobarbitone and glutamate. Pentobarbitone depression of glutamate was not affected by picrotoxin. 9. Both GABA and glutamate responses appeared to be equally sensitive to pentobarbitone. Specific interaction of the drug with amino acid receptor-coupled events is indicated by the requirement for pentobarbitone pipette placement close to the amino acid response site. 10. The results suggest that pentobarbitone depresses neuronal excitability by (1) directly activating post-synaptic GABA-receptor coupled Cl- conductance, (2) potentiating post-synaptic GABA-induced conductance events, probably at the level of the GABA receptor, and (3) depressing post-synaptic glutamate-induced excitation, probably at the level of the conductance mechanism.

Effects of gamma-aminobutyric acid on nerve terminal excitability in a slice preparation of cuneate nucleus

1 Superfusion of a slice preparation of the rat cuneate nucleus with gamma-aminobutyric acid (GABA) depolarized the afferent nerve fibres and increased their excitability. However, before the depolarization had reached its peak the increased excitability reversed to a decreased excitability, an effect which outlasted the depolarization. 2 Both components of the GABA excitability response were dose-related Cl--dependent and antagonized by bicuculline. 3 Possible mechanisms underlying the sequence of excitability changes are discussed.

Different actions of anticonvulsant and anesthetic barbiturates revealed by use of cultured mammalian neurons

Barbiturate anesthetics, but not anticonvulsants, abolish the spontaneous activity of cultured spinal cord neurons; directly increase membrane conductance, an effect which is suppressed by the gamma-aminobutyric acid (GABA) antagonists picrotoxin and penicillin; and are more potent than anticonvulsants in augmenting GABA and depressing glutamate responses. Barbiturate anticonvulsants abolish picrotoxin-induced convulsive activity. These results indicate qualitative and quantitative differences between anesthetic and anticonvulsant barbiturates, which may explain their different clinical effects.

Receptors for gamma-aminobutyric acid (GABA) on Aplysia neurons

Aplysia neurons show 5 different types of response (three excitatory and two inhibitory) to iontophoretic application of gamma-aminobutyric acid (GABA). Four of these are associated with a membrane conductance increase, but one is associated with a conductance decrease. The most common response is a fast hyperpolarization which reverses at about--58 mV and is sensitive to manipulation of external Cl- concentration, and thus is due to a specific increase in Cl- conductance. There is an infrequent, slower hyperpolarizing response which does not reverse above about--80 mV and is insensitive to external Cl-. This response appears to result from a conductance increase to K+. Two types of depolarizing responses are associated with conductance increases. These responses differ in their latency, duration and sensitivity to curare. The more frequent is relatively rapid (peak at 1-2 sec) and is depressed by curare at high concentrations. In other neurons, GABA causes a slower response, peaking at 6-10 sec, which is not curare-sensitive. Usually for both types of response, the voltage and conductance changes are completely abolished by perfusion with Na+-free seawater, and the responses cannot be reversed with depolarization. In other neurons such as L11, the response can be reversed with depolarization, and appears to result from a conductance increase to both Na+ and Cl-. In neuron R15, GABA causes a slow depolarizing response (peak at about 9 sec) which is associated with a decreased membrane conductance, probably to K+. The classical GABA antagonists, picrotoxin and bicuculline, block Cl- responses but no others, while the fast Na+ and Cl- responses are depressed by curare. Strychnine does not affect any GABA response. The multiplicity of GABA responses, the specificity of their organization and the fact that only some neurons have receptors for GABA, argue that GABA may have a role as a neurotransmitter in Aplysia. Furthermore, the existence of several types of excitatory GABA response suggests that GABA may function both as an inhibitory and excitatory neurotransmitter.

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.