Research reportSubstance P potentiates ATP-activated currents in rat primary sensory neurons
Introduction
It is generally agreed that adenosine-5′-triphosphate (ATP) plays a role of extracellular chemical messenger, either as a neurotransmitter or a co-transmitter acting on peripheral neurons including primary sensory, sympathetic and parasympathetic neurons and on central neurons in a variety of brain areas 1, 10. The receptor for ATP is referred to as P2 purinoceptor which is further classified into P2x and P2y subtypes pharmacologically according to the potency order of the agonists [5]. The activation of ATP receptor which belongs predominantly to the super-family of ligand-gated ion channel receptors results in the opening of nonselective cationic channels, thereby depolarizing its target cells. Growing evidence indicates that ATP exerts an excitatory effect on effector cells innervated by purinergic nerves such as the cardiac and smooth muscle and exocrinic glands while it may also mediate the fast and slow transmission in neuro-neuronal synapses both in the peripheral and central nervous systems 2, 3, 4.
Since the evidence for the existence of ATP receptor in the membrane of sensory neurons including dorsal root ganglion (DRG) neurons first proved by Krishtal et al. [11], a number of works have been carried out in order to elucidate the characteristics, pharmacology and kinetics for ATP receptor. It is well known that many other ligand-gated ion channel receptors such as nAChR, GABAAR, NM-DAR, 5-HT3R, etc., could be modulated by a number of agents [20]and it has been identified in recent years that ATP-activated inward current could be potentiated by micromolar concentration of Zn2+ and inhibited by ETOH 12, 13, 14. Recently, we found that some neurotransmitters can also exert modulatory effects on ATP-activated currents [6]. In the present work the effect of peptide neuro-transmitter substance P (SP) on the ATP-mediated current was studied. A preliminary report has been published elsewhere [8].
Section snippets
Isolation of DRG neurons
2–3-week-old Sprague-Dawley rats, irrespective of sex, were decapitated, and the thoracic and lumbar segments of vertebrate column were dissected and longitudinally divided into two halves along the median lines on both dorsal and ventral sides. The DRGs, together with dorsal and ventral roots and attached spinal nerves, were taken out from the inner side of each half of the dissected vertebrate and transferred immediately into Dulbecco's Modified Eagle's Medium (DMEM, Sigma) at pH = 7.4, 340
ATP- and SP-activated currents in the membrane of DRG neurons
In the present study, the majority of the cells examined responded to externally applied ATP (1–1000 μM) in a concentration-dependent manner (, 96.5%); many of them were also sensitive to substance P. In other words, there was a coexistence of ATP and SP receptor in the membrane of these neurons. The neurons chosen for this experiment were requested to be sensitive both to ATP and SP, the size distribution of them modified from Scroggs et al. [19]were as follows: large cells (> 45 μm) 4;
Discussion
For investigating the modulatory effect of SP on ATP-activated currents it was required that the experiments were carried out on neurons endowed with these two (SP and ATP) receptors. It is practicable because there are a large number of DRG cells in response to SP and ATP according to the data obtained from our previous work, in which the cells sensitive to SP and ATP are 90.6% [18]and 94.5% [6]respectively.
It is evident from Fig. 3 that the enhancement of amplitude of ATP-activated currents
Acknowledgements
We are grateful to Mrs. Y.Z. Fan for preparing isolated neurons, Dr. W.Z. Wei for technical help, and Prof. X.R. Jin for reading the manuscript. This work was supported by Grant 39270244 from the National Natural Science Foundation of China and a fund from the Ministry of Health of China.
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