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Neurosci Lett. 1988 Dec 19;95(1-3):185-91.

Microinjections of glutamate or morphine at coincident midbrain sites have different effects on nociceptive dorsal horn neurons in the rat.

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II. Physiologisches Institut, Universit├Ąt Heidelberg, F.R.G.


Responses of single lumbar spinal neurons to noxious skin heating (50 degrees C, 10 s) were electrophysiologically recorded in barbiturate-anesthetized rats. Responses of all neurons were suppressed by electrical stimulation in the midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF). Microinjection of glutamate (GLU, 0.1-0.3 microliter, 0.5 M) into the PAG rapidly (within 15 s) suppressed (to 13-55% of control) the responses of 6/16 neurons with recovery within 8 min. The remainder were affected less at even higher doses (0.5-1 microliter). Responses of 4/10 neurons were suppressed following GLU microinjected into the LRF. We also tested effects of microinjection of morphine (MOR, 5 micrograms/0.5 microliter) into GLU-sensitive and insensitive PAG sites. Responses of 4 neurons were unaffected, 4 were enhanced (to 130-155%), and 2 suppressed (to 43 and 57%) following MOR in PAG, with enhancement or suppression beginning within 12-20 min and lasting 40 to over 70 min. The differing effects of GLU and MOR may reflect different mechanisms for the descending modulation of spinal nociceptive transmission.

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