Differential release of neurotransmitters from superficial and deep layers of the dorsal horn in response to acute noxious stimulation and inflammation of the rat paw

Abstract

Experimental evidence suggests that release of neurotransmitters in response to acute noxious stimulation and inflammation can differ in superficial and deeper dorsal horn (DH) laminae. Using two different microdialysis probes, we studied changes in levels of glutamate, aspartate, arginine and GABA in dialysates collected from the surface of the spinal cord and within the DH induced by pinching the paw or paw inflammation. In penthotal anaesthetized rats, a flexible microdialysis probe was placed on the dorsal surface of the L4-L5 or L6-S2 spinal segments. In other rats, a rigid microdialysis probe was implanted within the DH of the same segments. Samples were collected every minute before, during and after pinching the hind paw (acute pain), and every half an hour after injecting either carrageenan or saline into the same paw (inflammation-induced pain). Amino acids were measured by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIFD). Pinching the paw induced a significant but short lasting increase in extracellular glutamate and aspartate in dialysates from the surface of the DH. Carrageenan, but not saline, injected into the paw significantly increased concentrations of glutamate, aspartate and arginine both on the surface and within the DH of L4-L5 and also within the DH of the L6-S2 segments. The GABA level was significantly increased following carrageenan only within the DH. The maximum increase on the surface was detected 60-120 min after the onset of inflammation whereas the response within the DH reached a maximum between 150 and 180 min after carrageenan. These results indicate that unlike acute mechanical noxious stimulation which enhances amino acid neurotransmitters in surface dialysate, inflammation induced neurotransmitter release in all layers of the DH suggesting sensitization of the DH.

Fixative Composition Alters Distributions of Immunoreactivity for Glutaminase and Two Markers of Nociceptive Neurons, Nav1.8 and TRPV1, in the Rat Dorsal Root Ganglion

“…Background fluorescence did not increase appreciably as less formaldehyde was used, indicating that a loss of specificity did not account for the higher estimates of GLS expression. Expression of GLS in all DRG neurons is consistent with the need of most, if not all, DRG neurons to have a means for synthesizing neurotransmitter glutamate at their central terminals (Skilling et al 1988;De Biasi and Rustioni 1990;Zahn et al 2002;Li et al 2003b;Dmitrieva et al 2004;Brumovsky et al 2007). The higher GLS IR of small and medium DRG neurons ( Figures 1A-1E) may correspond to the added ability of small-diameter nociceptive DRG neurons to release glutamate from their peripheral terminals (deGroot et al 2000;Jin et al 2006;Brumovsky et al 2007).…”

“…Glutamate is widely accepted as the major excitatory neurotransmitter of the central nervous system and is a candidate for use by pseudounipolar primary sensory neurons at all central terminals (Skilling et al 1988;De Biasi and Rustioni 1990;Zahn et al 2002;Li et al 2003a;Dmitrieva et al 2004;Brumovsky et al 2007) and at some peripheral terminals (deGroot et al 2000;Jin et al 2006;Brumovsky et al 2007). One marker of glutamatergic function is the enzyme glutaminase [(GLS) EC 3.5.1.2], which synthesizes glutamate from glutamine.…”

Increased Release of Serotonin in the Spinal Cord During Low, But Not High, Frequency Transcutaneous Electric Nerve Stimulation in Rats With Joint Inflammation

“…Several explanations could account for the differences between these studies. Distinct differences occur in the spinal cord and RVM after tissue injury particularly for opioids and γ-aminobutyric acid [25][26][27] and thus different mechanisms of action could exist for TENS between animals with tissue injury and those without. Differences between the decerebrated and the intact animal, or species differences, could also contribute to these observations of a lack of change in serotonin in the inflamed rat treated with 100Hz TENS to the inflamed knee joint.…”

Glutaminase Immunoreactivity and Enzyme Activity Is Increased in the Rat Dorsal Root Ganglion Following Peripheral Inflammation

“…Acutely, glutamate is released from central primary afferent terminals following noxious stimulation [13–16]. Acute glutamate release, along with SP and CGRP, is responsible for sensitization of spinal neurons leading to persistent or chronic changes [2].…”