1. Reflex patterns were analyzed in spontaneously active postganglionic vasoconstrictor neurons supplying skeletal muscle [muscle vasoconstrictor (MVC) neurons] and hairy skin [cutaneous vasoconstrictor (CVC) neurons] of the rat hindlimb. Postganglionic activity was recorded from single units and from filaments containing the axons of several spontaneously active neurons (multiunit preparations). The animals were freely breathing or artificially ventilated and maintained, in different experiments, under three different types of anesthesia (pentobarbital, chloralose, urethan). Reflexes were elicited by stimulation of arterial baroreceptors, chemoreceptors, cutaneous nociceptors, and cold receptors and visceral receptors from urinary bladder and colon. 2. Spontaneous activity of single postganglionic neurons ranged from 0.3 to 3.6 imp/s (median 1.15 imp/s and 1.0 imp/s in MVC and CVC neurons, respectively). Postganglionic axons conducted at 0.56 +/- 0.15 m/s (mean +/- SD, MVC neurons) and 0.53 +/- 0.11 m/s (CVC neurons). There was almost no difference in the rate of spontaneous activity under the three anesthetics used and whether the animals were artificially ventilated or breathing freely. 3. Stimulation of arterial baroreceptors by increasing arterial blood pressure by > 30 mmHg with intravenous injections of phenylephrine or angiotensin led to a depression of the activity in almost all vasoconstrictor neurons. In simultaneous recordings, with an identical increase of blood pressure, the magnitude of inhibition was greater in MVC neurons than in CVC neurons. Phasic stimulation of the arterial baroreceptors by the pulse pressure wave evoked a pronounced cardiac rhythmicity in the activity of the majority of MVC neurons (78%), but in only a small fraction of CVC neurons (18%). In most CVC neurons the cardiac rhythmicity was weak (33%) or absent (49%). When quantified the difference in the degree of cardiac rhythmicity between simultaneously recorded MVC and CVC neurons was highly significant (P < 0.001). 4. Noxious mechanical stimulation of skin of the ipsilateral hindpaw activated 20/35 MVC preparations (57%) and inhibited 25/47 CVC preparations (53%). Some CVC neurons (19%) were also activated, whereas the remainder of neurons were not affected. The quality of responses to noxious stimulation was correlated with the degree of cardiac rhythmicity that the sympathetic neurons displayed in their activity. A similar reciprocal response pattern in CVC and MVC neurons, albeit less pronounced, was observed to intense cold stimuli (chlor-ethyl spray) applied to the hindlimb. 5. This reciprocal pattern of the responses of MVC and CVC neurons was not observed when nociceptors from the contralateral hindlimb were stimulated and when cold stimuli were applied to the abdominal skin.(ABSTRACT TRUNCATED AT 400 WORDS)

1. The respiratory modulation of activity in postganglionic neurons of the lumbar sympathetic outflow to skeletal muscle and hairy skin of the hindlimb was studied in anesthetized rats, either breathing spontaneously or paralyzed and artificially ventilated, using single- and multifiber recordings. The activity of the neurons was analyzed with respect to the phrenic nerve discharges or with respect to the cycle of artificial ventilation under various experimental conditions. 2. In total, 19 single and 62 multiunit preparations supplying skeletal muscle and 42 single and 95 multiunit preparations supplying skin were analyzed. Qualitatively, both populations and single- and multiunit preparations exhibited similar patterns. The majority (187/218 preparations) exhibited a depression of activity during inspiration and a peak of activity during expiration that was mostly accentuated during early expiration. The remainder exhibited a peak of activity at the transition between expiration and inspiration (n = 15) or showed no respiratory modulation (n = 16). 3. Respiratory modulation in the postganglionic neurons was similar in animals breathing spontaneously and in those that were paralyzed and artificially ventilated. 4. Systemic hypercapnia and asphyxia in most cases enhanced both the peak of activity during early expiration and the depression of activity during inspiration. 5. No peripheral (reflex) component mediated by arterial baroreceptors was found in vagotomized animals that was related to the cycle of artificial ventilation using a tidal volume of 1-2 ml and a ventilation frequency of 70 +/- 7 (SD) strokes, min-1. However, a small ventilation-related rhythm of arterial baroreceptor activity recorded from the aortic nerve was found under these conditions. 6. Respiratory modulation of postganglionic activity was similar before and after bilateral vagotomy. 7. It is concluded that respiratory modulation of sympathetic activity in rats is less differentiated than in cats. The observed differences between neurons supplying different targets are only quantitative. Possible mechanisms involved in the generation of respiratory modulation are discussed.

No abstract

Level III, comparative study.