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J Neurophysiol 80: 2210-2214, 1998;
0022-3077/98 $5.00
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The Journal of Neurophysiology Vol. 80 No. 4 October 1998, pp. 2210-2214
Copyright ©1998 The American Physiological Society

RAPID COMMUNICATION


alpha 2-Adrenoceptors Modulate NMDA-Evoked Responses of Neurons in Superficial and Deeper Dorsal Horn of the Medulla

Kai-Ming Zhang, Xiao-Min Wang, Angela M. Peterson, Wen-Yan Chen, and Sukhbir S. Mokha

Department of Anatomy and Physiology, Meharry Medical College, Nashville, Tennessee 37208

Kai-Ming Zhang, Xiao-Min Wang, Angela M. Peterson, Wen-Yan Chen, and Sukhbir S. Mokha. alpha 2-Adrenoceptors modulate NMDA-evoked responses of neurons in the superficial and deeper dorsal horn of the medulla. J. Neurophysiol. 80: 2210-2214, 1998. Extracellular single unit recordings were made from neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in 21 male rats anesthetized with urethan. NMDA produced an antagonist-reversible excitation of 46 nociceptive as well as nonnociceptive neurons. Microiontophoretic application of a preferential alpha 2-adrenoceptor (alpha 2AR) agonist, (2-[2,6-dichloroaniline]-2-imidazoline) hydrochloride (clonidine), reduced the NMDA-evoked responses of 86% (6/7) of nociceptive-specific (NS) neurons, 82% (9/11) of wide dynamic range (WDR) neurons, and 67% (4/6) of low-threshold (LT) neurons in the superficial dorsal horn. In the deeper dorsal horn, clonidine inhibited the NMDA-evoked responses of 94% (16/17) of NS and WDR neurons and 60% (3/5) of LT neurons. Clonidine facilitated the NMDA-evoked responses in 14% (1/17) of NS, 9% (1/11) of WDR, and 33% (2/6) of LT neurons in the superficial dorsal horn. Idazoxan, an alpha 2AR antagonist, reversed the inhibitory effect of clonidine in 90% (9/10) of neurons, whereas prazosin, an alpha 1-adrenoceptor antagonist with affinity for alpha 2BAR, and alpha 2CAR, were ineffective. We suggest that activation of alpha 2ARs produces a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive neurons in the medullary dorsal horn.




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