Elsevier

Brain Research

Volume 871, Issue 2, 21 July 2000, Pages 181-191
Brain Research

Research report
Inflammation-induced changes in primary afferent-evoked release of substance P within trigeminal ganglia in vivo

https://doi.org/10.1016/S0006-8993(00)02440-9Get rights and content

Abstract

Substance P (SP) is synthesized in a subset of nociceptive sensory neurons and is released from their peripheral and central terminals. Here we demonstrate with the use of in vivo microdialysis and radioimmunoassay techniques that SP is also released within trigeminal ganglia following intraganglionic application of KCl, veratridine or capsaicin, and after electrical stimulation of peripheral afferent fibers. Both the basal and KCl-evoked release of SP are shown to be dependent on extracellular calcium. Using the turpentine-induced model of unilateral orofacial inflammation we also show that both the basal and KCl-evoked release of SP within trigeminal ganglia are greatly increased on the inflamed side 48 h after induction of inflammation. Coupled with previous demonstrations of excitatory effects of SP on sensory neurons, these results suggest that SP fulfils the role of a non-synaptically released diffusible chemical messenger that may modulate the somatic excitability of neurons within sensory ganglia in inflammatory pain states.

Introduction

The pseudounipolar somata of neurons in mammalian sensory ganglia are generally devoid of synaptic contacts and traditionally have been thought to play only supportive roles in the maintenance of their axons which transmit sensory information from the periphery to their central terminals [20]. Transmission of nociceptive signals is achieved by activating the peripheral terminals of neurons possessing unmyelinated C-fibers or the thinly myelinated Aδ-fibers, which synapse in the spinal cord and brainstem [21]. A large subset of these cells synthesizes the undecapeptide substance P (SP), which is released at both central and peripheral terminations following activation of nociceptive fibers [31].

Despite their seemingly passive supportive roles, the somata of sensory neurons may become a source of abnormal action potential discharge after axonal injury [16], [18], [43]. Although devoid of synaptic contacts, the somata of sensory neurons respond to many neurotransmitters and neuromodulators [20]. A large subset of spinal and trigeminal sensory neurons depolarize in response to application of synthetic SP [10], [37], [38]. These depolarizing responses with concomitant increases in neuronal excitability suggested that the effects of SP on somata of primary afferents may have a functional role, and led us to propose the possibility of neuropeptide release within sensory ganglia [39]. The present study directly addresses this hypothesis by measuring SP release within sensory ganglia and examines changes in SP release in a model of unilateral orofacial inflammation. Preliminary results were presented in abstract form [27], [28].

Section snippets

Microdialysis probes

Probes were of concentric cannula design, either purchased from CMA/Microdialysis AB (CMA/12, stainless steel shaft, outer diameter (O.D.) 0.64 mm, type SS 316, 4 mm active length polycarbonate membrane, O.D. 0.5 mm, molecular weight cutoff=20 kD), or made in our laboratory (24-gauge thin-wall stainless steel external cannulae, O.D. 510 μm, 5 mm active length polyacrylonitrile membrane, O.D. 300 μm, molecular weight cutoff=40 kD (AN69, Hospal)). Prior to the experiments, probes were immersed in

Release of SP in trigeminal ganglia by local depolarizing stimuli

To quantify SP release within trigeminal ganglia we adapted the technique of in vivo microdialysis coupled with solid-phase RIA for SP detection in dialysate samples [22], [23], [24]. Baseline microdialysis samples from the TRG were collected for 3 h following which the control microdialysate solution was switched to one containing 100 mM KCl for 20 min (Fig. 1A, C) to depolarize TRG neurons. A 5-fold reversible increase (F2,13=4.11) in the amount of SP-like immunoreactivity (SP-LI) detected by

Discussion

The major findings of this study are: 1) changes in SP levels within trigeminal ganglia of guinea pigs may be monitored using in vivo microdialysis and radioimmunoassay techniques; 2) chemical or electrical stimulation of thin trigeminal primary afferent fibers or their somata evoke release of SP within the TRG; 3) KCl-evoked release of SP is dependent on extracellular calcium levels, 4) SP release in TRG is greatly increased 48 h after unilateral orofacial inflammation.

Action potential

Acknowledgements

We thank Dr. Christopher J. Evans for the generous gift of the substance P antibody. This work was supported by the NIH grants DE07212, DE00408, NS-05685 and the Whitehall Foundation.

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