Reflex patterns in preganglionic sympathetic neurons projecting to the superior cervical ganglion in the rat

Reflex patterns in preganglionic neurons projecting in the cervical sympath etic trunk (CST) were analyzed in response to stimulation of various affere nt systems. We focused on the question whether these preganglionic neurons can be classified into functionally distinct subpopulations. Reflex respons es were elicited by stimulation of trigeminal and spinal nociceptive, therm oreceptive as well as baroreceptor and chemoreceptor afferents. Multi- and single fiber preparations were studied in baroreceptor intact and sino-aort ically denervated animals. Spontaneous activity of 36 preganglionic single neurons ranged from 0.2 to 3.5 imp/s (median=1.11 imp/s). The degree of car diac rhythmicity (CR) in the activity of sympathetic neurons was 69.5+/-13% (mean+/-S.D.; N=52; range=39-95%). Noxious stimulation of acral skin activ ated the majority (67%) of sympathetic preparations by 37-1-25% (N=35) abov e pre-stimulus activity; 15% were inhibited. In these neurons the response to noxious stimulation of acral skin was significantly correlated with the degree of CR (P<0.001, N=52) in that neurons showing the strongest excitati on to noxious stimulation displayed the strongest CR. Noxious mechanical st imulation of body trunk skin (N=60) inhibited the majority (80%) of fiber p reparations tested (by 34+/-18% of pre-stimulus activity, N=48); an activat ion was not observed. Cold stimulation of acral (N=9) and body trunk skin ( N=42) activated most fiber preparations. Trigeminal stimulation evoked a un iform reflex activation of preganglionic neurons (+79+/-73% of pre-stimulus activity, N=32). Chemoreceptor stimulation by systemic hypercapnia elicite d inhibitory (-31+/-19%, N=8) as well as excitatory (+59+/-5%, N=4) respons es. These results show that preganglionic sympathetic neurons projecting to target organs in the head exhibit distinct reflex patterns to stimulation of various afferent systems; however, a clear classification into different functional subgroups did not emerge. Furthermore, reflex patterns showed a segmental organization to noxious cutaneous stimulation of acral parts and body trunk reflecting a differential central integration of spinal afferen t input. Compared with the cat the reflex organization of sympathetic neuro ns projecting to the head seems to be less differentiated in the anesthetiz ed rat. (C) 2000 Elsevier Science B.V. All rights reserved.