Pharmacologically novel GABA receptor in human dorsal root ganglion neurons.
ABSTRACT 1. Whole cell voltage-clamp studies of gamma-aminobutyric acid (GABA) receptors were performed on large (> 80 microns) cultured human dorsal root ganglion (DRG) neurons. 2. GABA and pentobarbital sodium when applied in micromolar concentrations evoked inward Cl- currents in DRG neurons voltage clamped at negative membrane potentials. 3. Diazepam (10 microM) and pentobarbital (10 microM) upmodulated the GABA current by approximately 149 and 168%, respectively. 4. The GABA currents in human DRG cells were unaffected by the classical GABA antagonists picrotoxin and bicuclline (100 microM). In contrast, the GABA responses evoked in adult rat DRG cells cultured in an identical manner were inhibited by both antagonists. The glycine receptor antagonist strychnine (100 microM) did not alter GABA currents in human DRG cells. 5. Human DRG cells did not respond to glycine (10-100 microM) or taurine (10-100 microM). The GABAB agonist baclofen had no effect on the holding current when patch pipettes were filled with 130 mM KCl. The GABAB antagonists saclofen applied either alone or with GABA was without effect. 6. The differences between the GABA receptors described here and GABA receptors in other species may reflect the presence of receptor subunits unique to human DRG cells.
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ABSTRACT: 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. 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 microM) 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 microM) while the GABA(B) agonist baclofen (10-30 microM) was without effect. Increases in excitability produced by GABA (10-30 microM) were blocked by the GABA(A) antagonists gabazine (10-20 microM), bicuculline (10-20 microM) and picrotoxin (10-20 microM). 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.PLoS ONE 01/2010; 5(1):e8780. · 3.73 Impact Factor
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ABSTRACT: Sensory feedback is critical for normal locomotion and adaptation to external perturbations during movement. Feedback provided by group Ia afferents influences motor output both directly through monosynaptic connections and indirectly through spinal interneuronal circuits. For example, the circuit responsible for reciprocal inhibition, which acts to prevent co-contraction of antagonist flexor and extensor muscles, is driven by Ia afferent feedback. Additionally, circuits mediating presynaptic inhibition can limit Ia afferent synaptic transmission onto central neuronal targets in a task-specific manner. These circuits can also be activated by stimulation of proprioceptive afferents. Rodent locomotion rapidly matures during postnatal development; therefore we assayed the functional status of reciprocal and presynaptic inhibitory circuits of mice at birth and compared responses to observations made after one week of postnatal development. Using extracellular physiological techniques from isolated and hemisected spinal cord preparations, we demonstrate that Ia afferent-evoked reciprocal inhibition is as effective at blocking antagonist motor neuron activation at birth as at one week postnatal. In contrast, at birth conditioning stimulation of muscle nerve afferents failed to evoke presynaptic inhibition sufficient to block functional transmission at synapses between Ia afferents and motor neurons, even though dorsal root potentials could be evoked by stimulating the neighboring dorsal root. Presynaptic inhibition at this synapse was readily observed, however, at the end of the first postnatal week. These results indicate Ia afferent feedback from the periphery to central spinal circuits is only weakly gated at birth, which may provide enhanced sensitivity to peripheral feedback during early postnatal experiences.Journal of Neurophysiology 01/2013; · 3.30 Impact Factor
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ABSTRACT: The acid-sensing ion channels (ASICs) are proton-gated cation channels activated when extracellular pH declines. In rodents, the Accn2 gene encodes transcript variants ASIC1a and ASIC1b, which differ in the first third of the protein and display distinct channel properties. In humans, ACCN2 transcript variant 2 (hVariant 2) is homologous to mouse ASIC1a. In this article, we study two other human ACCN2 transcript variants. Human ACCN2 transcript variant 1 (hVariant 1) is not present in rodents and contains an additional 46 amino acids directly preceding the proposed channel gate. We report that hVariant 1 does not produce proton-gated currents under normal conditions when expressed in heterologous systems. We also describe a third human ACCN2 transcript variant (hVariant 3) that is similar to rodent ASIC1b. hVariant 3 is more abundantly expressed in dorsal root ganglion compared with brain and shows basic channel properties analogous to rodent ASIC1b. Yet, proton-gated currents from hVariant 3 are significantly more permeable to calcium than either hVariant 2 or rodent ASIC1b, which shows negligible calcium permeability. hVariant 3 also displays a small acid-dependent sustained current. Such a sustained current is particularly intriguing as ASIC1b is thought to play a role in sensory transduction in rodents. In human DRG neurons, hVariant 3 could induce sustained calcium influx in response to acidic pH and make a major contribution to acid-dependent sensations, such as pain.Journal of Biological Chemistry 10/2010; 285(53):41852-62. · 4.65 Impact Factor