GABAB Receptors Tonically Inhibit Motoneurons and Neurotransmitter Release from Descending and Primary Afferent Fibers.

Motoneurons receive thousands of excitatory and inhibitory synapses from descending tracts and primary afferent fibers. The excitability of these neurons must be precisely regulated to respond adequately to the requirements of the environment. In this context, GABAA and GABAB receptors regulate motoneuron synaptic strength. GABAA and GABAB receptors are expressed on primary afferent fibers and motoneurons, while in the descending afferent fibers, only the GABAB receptors are expressed. However, it remains to be known where the GABA that activates them comes from since the GABAergic interneurons that make axo-axonic contacts with primary afferents have yet to be identified in the descending afferent terminals. Thus, the main aim of the present report was to investigate how GABAB receptors functionally modulate synaptic strength between Ia afferent fibers, excitatory and inhibitory descending fibers of the dorsolateral funiculus, and spinal motoneurons. Using intracellular recordings from the spinal cord of the turtle, we provide evidence that the GABAB receptor antagonist, CGP55845, not only prevents baclofen-induced depression of EPSPs but also increases motoneuron excitability and enhances the synaptic strength between the afferent fibers and motoneurons. The last action of CGP55845 was similar in excitatory and inhibitory descending afferents. Interestingly, the action of baclofen was more intense in the Ia primary afferents than in the descending afferents. Even more, CGP55845 reversed the EPSP depression induced by the increased concentration of ambient GABA produced by interneuron activation and GABA transporter blockade. Immunofluorescence data corroborated the expression of GABAB receptors in the turtle's spinal cord. These findings suggest that GABAB receptors are extrasynaptic and tonically activated on descending afferent fibers and motoneurons by GABA released from astrocytes and GABAergic interneurons in the cellular microenvironment. Finally, our results also suggest that the antispastic action of baclofen may be due to reduced synaptic strength between descending fibers and motoneurons.

The role of spinal cord extrasynaptic α5 GABAA receptors in chronic pain.

Chronic pain is an incapacitating condition that affects a large population worldwide. Until now, there is no drug treatment to relieve it. The impairment of GABAergic inhibition mediated by GABAA receptors (GABAA R) is considered a relevant factor in mediating chronic pain. Even though both synaptic and extrasynaptic GABAA inhibition are present in neurons that process nociceptive information, the latter is not considered relevant as a target for the development of pain treatments. In particular, the extrasynaptic α5 GABAA Rs are expressed in laminae I-II of the spinal cord neurons, sensory neurons, and motoneurons. In this review, we discuss evidence showing that blockade of the extrasynaptic α5 GABAA Rs reduces mechanical allodynia in various models of chronic pain and restores the associated loss of rate-dependent depression of the Hoffmann reflex. Furthermore, in healthy animals, extrasynaptic α5 GABAA R blockade induces both allodynia and hyperalgesia. These results indicate that this receptor may have an antinociceptive and pronociceptive role in healthy and chronic pain-affected animals, respectively. We propose a hypothesis to explain the relevant role of the extrasynaptic α5 GABAA Rs in the processing of nociceptive information. The data discussed here strongly suggest that this receptor could be a valid pharmacological target to treat chronic pain states.

γ-Aminobutyric acid (GABA) from satellite glial cells tonically depresses the excitability of primary afferent fibers.

Primary afferent fibers express extrasynaptic GABAA and GABAB receptors in the axons and soma. However, whether these receptors are tonically activated by ambient GABA and the source of the neurotransmitter is presently unknown. Here, we show that GABA release from dorsal root ganglia (DRG) does not depend on extracellular calcium, but depends upon calcium released from intracellular stores, and is mediated by Best1 channels. Using a preparation consisting of the spinal nerve in continuity with the DRG and the dorsal root, we found that endogenous GABA tonically activates GABA receptors, depressing the excitability of the primary afferents. In addition, using HPLC we found that GABA is released in the DRG, and by immunofluorescence microscopy we show the presence of GABA, the Best1 channel, and some enzymes of the putrescine pathway of GABA biosynthesis, in glutamine synthase- and GFAP-positive satellite glial cells. Last, we found that the blockade of the Best1 channel activity reduced the excitability of primary afferents and prevented the activation of the GABA receptors. These results suggest that satellite glial cells may be the source of endogenous GABA released in the DRG via Best1 channels, which tonically activates extrasynaptic GABA receptors.

α5GABAA receptors play a pronociceptive role and avoid the rate-dependent depression of the Hoffmann reflex in diabetic neuropathic pain and reduce primary afferent excitability.

Diabetic neuropathy is an incapacitating complication in diabetic patients. The cellular and molecular mechanisms involved in this pathology are poorly understood. Previous studies have suggested that the loss of spinal GABAergic inhibition participate in painful diabetic neuropathy. However, the role of extrasynaptic α5 subunit-containing GABAA (α5GABAA) receptors in this process is not known. The purpose of this study was to investigate the role of α5GABAA receptors in diabetes-induced tactile allodynia, loss of rate-dependent depression (RDD) of the Hoffmann reflex (HR), and modulation of primary afferent excitability. Intraperitoneal administration of streptozotocin induced tactile allodynia. Intrathecal injection of α5GABAA receptor inverse agonist, L-655,708, produced tactile allodynia in naive rats, whereas it reduced allodynia in diabetic rats. In healthy rats, electrical stimulation of the tibial nerve at 5 Hz induced RDD of the HR, although intrathecal treatment with L-655,708 (15 nmol) abolished RDD of the HR. Streptozotocin induced the loss of RDD of the HR, while intrathecal L-655,708 (15 nmol) restored RDD of the HR. L-655,708 (15 nmol) increased tonic excitability of the primary afferents without affecting the phasic excitability produced by the primary afferent depolarization. α5GABAA receptors were immunolocalized in superficial laminae of the dorsal horn and L4 to L6 dorsal root ganglion. Streptozotocin increased mean fluorescence intensity and percentage of neurons expressing α5GABAA receptors in dorsal horn and L4 to L6 dorsal root ganglia in 10-week diabetic rats. Our results suggest that spinal α5GABAA receptors modulate the HR, play an antinociceptive and pronociceptive role in healthy and diabetic rats, respectively, and are tonically active in primary afferents.