Role of the nucleus tractus solitarii and the rostral depressive area in the sympatholytic effect of 8-hydroxy-2-(di-n-propylamino)tetralin in the cat

Sympathetic activation and tachycardia in lipopolysaccharide treated rats are temporally correlated and unrelated to the baroreflex

The goal of this study was to investigate the sustained sympatho-excitation which occurs in sepsis and which accompanies the fall in blood pressure and to analyze its time-correlation with heart rate and the role of the baro-chemoreflexes. Rats anesthetized with pentobarbital were treated with lipolysaccharide (LPS) 20 mg/kg/20 min i.v. and mean blood pressure (MBP), heart rate (HR), rectal temperature and renal sympathetic nerve activity (RSNA) were recorded. LPS induced a fall in blood pressure, an increase in HR (+20%) and RSNA (+355%); the arterial PO2 and PCO2 remained stable and the injection was fatal within 4 h. Baroreceptor and chemoreceptor denervation accelerated the fall in MBP but did not change the survival time. Under those conditions; RSNA excitation was slightly more pronounced. During treatment with gallamine and under artificial respiration to avoid possible respiratory changes through the chemoreflex pathway, the effects of LPS remained, except for a decrease in arterial PO2. Electrolytic lesioning of the nucleus tractus solitarius or blocking the effects of baroreflex efferents by either an alpha1 or alpha2-adrenoceptor antagonists failed to alter the effects of LPS. After treatment with a beta-adrenoceptor antagonist, LPS increased RSNA but not HR and the survival time of the rats shortened. LPS administered i.c. (1 mg/kg) induced, with a short latency, effects comparable to those produced by i.v. injection. Surprisingly, the time correlation between RSNA and HR rhythms persisted when MBP dropped after LPS and moreover it reappeared in baroreceptor denervated rats after LPS. Thus under these conditions of altered baroreflex pathway and LPS induced sympathetic activation, the sympathetic output from the medulla appears to play a role in the correlation between heart rate and sympathetic nerve activity. These data indicate that the marked RSNA activation and the tachycardia are correlated and that the baroreflex and chemoreflex are not inhibited during sepsis but appear to be of minor importance in the sympathetic activation and in the blood pressure modifications.

In vivo effects of 5-hydroxytryptamine receptor activation on rat nucleus tractus solitarius neurones excited by vagal C-fibre afferents

The effects of ionophoretically applied 5-hydroxytryptamine (5-HT) and 5-HT receptor agonists were studied on rat nucleus tractus solitarius (NTS) neurones receiving unmyelinated vagal afferent input. 5-HT excited 15 of 34 neurones (44%), inhibited 10 (29%) and had no effect on nine. 8-Hydroxy-2-(di-N-propylamino)tetralin HBr (8-OH-DPAT) excited 23 of 53 neurones (43%), inhibited 24 (45%) and had no effect on six neurones and (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl activated 18 of 37 neurones (49%), inhibited nine (24%) and had no effect on 10. These results demonstrate that activation of 5-HT1A and 5-HT2 receptors can excite or inhibit populations of NTS neurones. Phenylbiguanide, however, excited 20 of 23 neurones (87%), inhibited only one (4%) and had no effect on two indicating that 5-HT3 receptor activation has an excitatory action. NTS neurones receiving cardiac vagal afferent input were more likely to be excited by 5-HT (five of five, 100%) or 8-OH-DPAT (four of five. 80%) than the population as a whole. In conclusion, the data demonstrate that 5-HT1A, 5-HT2, and 5-HT3 receptor subtypes are functionally present on NTS neurones receiving excitatory vagal afferent input. Further, the subpopulation of NTS neurones receiving input from cardiac afferents are excited by 5-HT, possibly by an action on 5-HT1A or 5-HT3 receptors.

Evidence against the involvement of the nucleus tractus solitarii in the sympatholytic effect of 8-hydroxy-2-(di-n-propylamino)tetralin in the cat

In different animal species, microinjections of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into the nucleus tractus solitarii failed to alter arterial blood pressure and sympathetic nerve activity; however, the cardiovascular effects (hypotension, bradycardia, reduction in sympathetic nerve activity) of intravenous administration of 8-OH-DPAT were significantly reduced after blockade of the nucleus tractus solitarii by kainic acid as well as after blockade of the lateral tegmental field by kainic acid. The aim of the present study was to clarify these conflicting results. In the anesthetized cat, inhibition of neurotransmission in the nucleus tractus solitarii by bilateral microinjections of either muscimol (1 nmol in 50 nl) or kynurenic acid (2.5 nmol in 50 nl) suppressed the baroreceptor reflex and abolished the synchronism between the renal sympathetic bursts; however, these procedures did not alter the dose-related hypotension, bradycardia and sympatho-inhibition elicited by cumulative doses of 8-OH-DPAT (1-30 micrograms/kg i.v.). Moreover complete electrolytic destruction of the nucleus tractus solitarii, assessed by a complete loss of the baroreceptor reflex and the cardiac-related bursts of the sympathetic nerves, failed to alter the inhibitory effects of i.v. 8-OH-DPAT. Bilateral microinjections of muscimol into the lateral tegmental field induced a decrease of mean arterial blood pressure, heart rate and renal nerve activity (by respectively -35 +/- 13 mm Hg, -30 +/- 16 beats/min and -53 +/- 14%) and greatly reduced the effects of subsequent i.v. administration of 8-OH-DPAT. The present data indicate that the nucleus tractus solitarii does not play a dominant role in the central action of 8-OH-DPAT whereas they confirm our previous results showing that the lateral tegmental field is involved in this action and in the mecanisms regulating sympathetic tone. The results also suggest that kainic acid lesions are not restricted to the region in which the neurotoxic agent is injected.