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