A5 noradrenergic neurons and the carotid sympathetic chemoreflex.
Inhibition of neural activity in the caudal ventrolateral pons (A5 area) by microinjection of muscimol (Mus) attenuates (-65%) the carotid sympathetic chemoreflex (SChR) without altering the concomitant activation of the phrenic nerve (PND). The present study, performed in urethan-anesthetized rats, explores the possibility that activation of the noradrenergic (NE) neurons of the A5 area is involved in the SChR. The NE neuron-selective toxin 6-hydroxydopamine (6-OHDA) was microinjected bilaterally into the spinal cord at T2 level (4 micrograms). This dose reduced the SChR by 55% (n = 5) 90 min after injection, while 0.4 microgram of 6-OHDA produced no effect (n = 5). In seven rats that had received 250 micrograms 6-OHDA intracisternally 2 wk before, Mus injections into the A5 area failed to attenuate the SChR. These rats also had a lower resting mean arterial pressure than controls (97 vs. 112 mmHg). Spinal intrathecal injection of alpha-adrenergic receptor antagonists (prazosin, 10 and 20 micrograms) or phentolamine (20 and 40 micrograms) attenuated resting sympathetic nerve discharge (SND) and SChR in a roughly proportional manner (25-40%); the beta-adrenergic antagonist nadolol (10 and 20 microgram(s) intrathecally) attenuated the SChR selectively but modestly (-10%). The results are generally compatible with the hypothesis that A5 NE neurons and particularly their spinal cord projection could play a facilitating role in the SChR. However, clear evidence that A5 cells contribute selectively to sympathoactivation during chemoreceptor stimulation by releasing NE in the spinal cord could not be obtained.
Koshiya
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N
Department of Pharmacology, University of Virginia Health Sciences Center, Charlottesville 22908.
Guyenet
P G
PG
D000818:Animals; D002339:Carotid Arteries / Q000294:innervation*; D002628:Chemoreceptor Cells / Q000502:physiology*; D007278:Injections, Spinal; D008297:Male; D009474:Neurons / Q000502:physiology*; D009638:Norepinephrine / Q000502:physiology*; D016627:Oxidopamine / Q000494:pharmacology; D011149:Pons / Q000166:cytology / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012018:Reflex / Q000502:physiology*; D013116:Spinal Cord / Q000187:drug effects; D013564:Sympathetic Nervous System / Q000502:physiology*; D013565:Sympatholytics / Q000494:pharmacology; D013997:Time Factors
D013565:Sympatholytics; D016627:Oxidopamine; D009638:Norepinephrine