Glutamate pharmacology and metabolism in peripheral primary afferents: physiological and pathophysiological mechanisms

Abstract

In addition to using glutamate as a neurotransmitter at central synapses, many primary sensory neurons release glutamate from peripheral terminals. Primary sensory neurons with cell bodies in dorsal root or trigeminal ganglia produce glutaminase, the synthetic enzyme for glutamate, and transport the enzyme in mitochondria to peripheral terminals. Vesicular glutamate transporters fill neurotransmitter vesicles with glutamate and they are shipped to peripheral terminals. Intense noxious stimuli or tissue damage causes glutamate to be released from peripheral afferent nerve terminals and augmented release occurs during acute and chronic inflammation. The site of action for glutamate can be at the autologous or nearby nerve terminals. Peripheral nerve terminals contain both ionotropic and metabotropic excitatory amino acid receptors (EAARs) and activation of these receptors can lower the activation threshold and increase the excitability of primary afferents. Antagonism of EAARs can reduce excitability of activated afferents and produce antinociception in many animal models of acute and chronic pain. Glutamate injected into human skin and muscle causes acute pain. Trauma in humans, such as arthritis, myalgia, and tendonitis, elevates glutamate levels in affected tissues. There is evidence that EAAR antagonism at peripheral sites can provide relief in some chronic pain sufferers.

Fig. 1

Primary afferent neuron. Nociceptive and thermally responsive non-nociceptive neurons have free nerve endings distributed in target tissue, e.g., skin. The neuronal soma resides in the dorsal root ganglion or trigeminal ganglion. A primary sensory neuron is a pseudo-unipolar cell with a single axon projecting from the periphery to the spinal cord or brainstem. TG and DRG neurons store neurogenic substances, such as substance P, calcitonin gene-related peptide, and glutamate, in vesicles (white circles) for release in the periphery and spinal cord. Glutaminase, the synthetic enzyme for glutamate, is produced in the cell body, translocated to mitochondria (yellow rectangles), and shipped to nerve terminals. Glutamate, therefore, can be synthesized for neurotransmission at peripheral and spinal nerve endings. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Glutamine cycle in peripheral nervous system. Glutamate (glu) can be taken up by neurons or glia. In neurons, glu is taken up by excitatory amino acid transporter 3 (EAAT). Schwann or satellite cells take up glu via glutamate–aspartate transporter (GLAST) and glutamate transporter 1 (GLT-1) for conversion to glutamine (gln) via glutamine synthetase (GS). Sodium-coupled neutral amino acid transporters (SNAT) transport gln back to neurons for conversion to glu by glutaminase (GLS) and packaging into vesicles by vesicular glutamate transporters (VGLUT). Peripheral glia have glutamate dehydrogenase (GDH) for adding or removing glu from the glutamine cycle. GDH is a bidirectional enzyme for the conversion of 2-oxoglutarate [2-OG] to glu. When 2-OG is removed from the tricarboxylic acid cycle (TCA), pyruvate carboxylase (PC) adds to the glial TCA cycle by converting pyruvate to oxaloacetate. Within neurons, the glutamine cycle interacts with the TCA cycle via aspartate aminotransferase (AT). AT is a bidirectional enzyme for conversion of aspartate and 2-OG to oxaloacetate and glu.

Fig. 3

Phosphate activated glutaminase. Glutamate is produced from the hydrolytic deamidation of glutamine by phosphate-activated glutaminase (GLS; EC 3.5.1.2). GLS is a mitochondrial enzyme that requires inorganic phosphate (P_i) for activation, but also is regulated by its end products, glutamate and ammonia, as well as other intracellular components, 2-oxoglutarate, calcium (Ca²⁺), fatty acids, and fatty acyl-coenzyme A derivatives.

Fig. 4

Cat spinal cord glutaminase mRNA Northern blot and dorsal root ganglion in situ hybridization. Northern blot analysis of cat spinal cord shows two glutaminase mRNAs of 6.0 and 3.4 kb similar to what has been described for rat brain. Northern blot was first evaluated using a ³⁵S-529bp cDNA and then stripped and reprobed with a ³⁵S-1.1 kb cDNA (GLS cDNA vectors supplied by C. Banner, NIH). Using ³⁵S-529bp cDNA, all neuronal profiles (arrows) are labeled with in situ hybridization under stringent conditions.

Fig. 5

Peripheral glutamate mechanisms. Nociceptive free nerve endings store glutamate in neurotransmitter vesicles (white circles) and release glutamate (glu) into peripheral tissue following noxious stimulation (leftward going red arrows). Glu released from the same or a nearby terminal can interact with excitatory amino acid receptors (EAAR; chevrons) to activate or sensitize the terminal. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Dose response relationship of nociceptors to kainate using the in vitro rat glabrous skin–nerve preparation. Ascending concentrations of kainate excite most C fiber nociceptors in normal (naive) rats. In these studies, inflammation is created by injection (i.pl.) of CFA (25 μl) and animals evaluated at 48 h post-injection. An ascending concentration series (0.01–3 mM) of kainate excites most C fiber nociceptors (89%) in a dose dependent manner from naive rats (open circles). Following 48 h of adjuvant induced arthritis, C fiber nociceptors have elevated background activity, but kainate excites most fibers (75%; black circles) in a dose dependent fashion. Used with permission from author; Du et al., Neuroscience, 2006.

Fig. 7

Comparison of differences in the responses in male and female rats to the injection of 0.5 M glutamate into the temporomandibular joint capsule. Glutamate injection in female rats causes a larger median response and longer afferent discharge in slow Aδ afferent fibers than in male rats. EMG activity in digastric and masseter muscles caused by glutamate injection in the temporomandibular joint capsule is greater in female rats than in male rats. Am Physiol Soc, used with permission; Cairns et al., J Neurophysiol, 2001b.

Fig. 8

Antidromic electrical stimulation (ADES) of T₉ dorsal rami causes increased activity in T₁₀ isolated cutaneous branches of Aβ, Aδ, and C fibers (after ADES). Subcutaneous (sc) infusion of MK-801, NMDAR antagonist, into the region of T₁₀ cutaneous branches brings T₉ ADES evoked activity to control levels in T₁₀ Aβ, Aδ and C fibers (after sc MK-801+ADES). Used with permission from author; Cao et al, Brain Res Bull, 2007.

Fig. 9

A. Following injection of 1.0 M glutamate into the masseter muscle, women and men drew their perceived areas of pain. Subjects describe a deep, aching pain that spread to the temporomandibular joint and teeth. Women had a larger area of perceived pain on drawings than men. B. The area of perceived pain was measured and illustrated in the histogram. Women had a larger area of perceived pain than men after a single or two injections of glutamate. Am Physiol Soc, used with permission; Cairns et al., J Neurophysiol, 2001a.