Nuclei containing cardiac vagal preganglionic neurones (CVPNs), the nucleus ambiguus (nA) and dorsal vagal nucleus, are densely innervated by 5-HT immunoreactive terminals, some of which make synaptic contact with CVPNs (Izzo et al. 1993). Reflex activation of CVPNs can be attenuated by antagonising central 5-HT1A receptors in the nA (Wang & Ramage, 2001). However, the location of the 5-HT-containing cell bodies involved in this control is unknown. Since 5-HT-containing neurones projecting to the nA are located in raphe magnus, obscurus and pallidus (Haxhiu et al. 1993), the present experiments have been carried out to identify sites within the medullary raphe that may be involved in modulating cardiac vagal drive.
Male Sprague-Dawley rats (300-400 g) were anaesthetised with isoflurane (induction) and α-chloralose (80 mg kg-1 I.V.; maintenance), neuromuscularly blocked (α-bungarotoxin 150 µg kg-1 I.V.; Jones et al. 2002), atenolol pretreated (1 mg kg-1 I.V. for cardiac sympathoadrenal blockade), and instrumented for measurement of BP, HR, integrated phrenic (PNA) and integrated renal sympathetic nerve activity (RNA). Cardiovascular variables were used to monitor the depth of anaesthesia. Distinct areas of the medullary raphe were stimulated using a 3-barrel glass micropipette containing 50 mM DL-homocysteic acid (DLH), pontamine sky blue, and either 5 mM DLH, saline, or Wood’s metal for electrical stimulation. At the end of experiments, animals were killed by an overdose of pentobarbitone.
Electrical stimulation (10-50 Hz, 0.2 ms, 50-100 µA, 10 s) produced variable results. Chemical stimulation (2.5 nmol DLH in 50 nl) of the raphe 0.5 mm rostral to obex evoked significant (Student’s paired t test; P < 0.05) changes in baseline PNA (+116 ± 21% mean ± S.E.M.) and RNA (-45 ± 8% n = 6). At 1.5 mm rostral, RNA reduction was smaller (-29 ± 8% n = 7). At 2.5 mm rostral (raphe magnus/pallidus), DLH evoked significant changes in HR (-42 ± 6 beats min-1), PNA (-60 ± 6 %) and RNA (+46 ± 6% n = 7). These DLH effects were dose dependent and lateral (0.5-1 mm) microinjection evoked much smaller responses. Saline microinjection had no effect. The 5-HT1/2/7 receptor antagonist methiothepin (3 mg kg-1 I.V.) had no significant effect on the evoked bradycardia (n = 4).
The present data are consistent with the hypothesis that the caudal raphe have an excitatory action on CVPNs.
D.O.K. is a BHF PhD student.