The effects of 5-HT1A/7 receptor ligands on the activity of nucleus tractus solitarius (NTS) neurones in anaesthetized rats

Diana Oskutyte, David Jordan, Andrew Ramage
Departments of Pharmacology and Physiology, University College London, London WC1E 6Bt, United Kingdom

In addition to 5-HT1A receptor blockade, blockade of central 5-HT7 receptors with SB-269970 has been shown to inhibit vagal reflex evoked bradycardias in anaesthetized (Kellett et al., 2005) and conscious rats (Damaso et al., 2007). A site for this action is the nucleus tractus solitarius (NTS) the site of termination of vagal afferents. The present study was carried out to investigate the effects of the 5-HT1A/7 receptor ligands, 5-carboxamidotryptamine (5-CT), 8-OH-DPAT, SB-269970 and WAY-100635 applied by ionophoresis on the ongoing and vagal evoked activity of NTS neurones in vivo.

Male Sprague-Dawley rats (250-330g) were anaesthetized with pentobarbitone sodium (60 mg kg-1, i.v.), neuromuscular blocked (α-bungarotoxin 150 μg kg-1, i.v.) and artificially ventilated with O2-enriched air. NTS neuronal activity was recorded as previously described (see Wang et al., 1998). Depth of anaesthesia was assessed by the stability of arterial blood pressure and heart rate following a noxious stimulus. NTS neurones were identified by orthodromic excitation from stimulation of the vagus nerve (10-400 μA, 0.2-1.0 ms, 0.5-1.0 Hz) and confirmed histologically. A post-hoc analysis was made to determine if these neurones received mono- and polysynaptic inputs from the vagal afferents (see Scheuer et al., 1996). All values are means ± s.e.mean.

54 NTS neurones were recorded with either ongoing activity or activity which had been induced by the excitatory amino acid DL-homocysteic acid. Application of 5-CT (10 mM, pH 5, 5-100nA) a non-selective 5-HT1/7 agonist inhibited by 53±16% in 16/28, increased by 187±61% in 7/28 and had no effect in 5/28 on baseline firing. 8-OH-DPAT (20 mM, pH 4. 10-100nA) a 5-HT1A agonist decreased activity by 66±13% in 7/26, increased by 116±23% in 9/26 had no effect in 10/26 on baseline firing. SB-269970 (10mM, pH 5, 5-50nA) a 5-HT7 antagonist inhibited by 53±14% in 5/16, excited by 87±18% in 2/16 and no effect in 9/16. WAY-100635 (10 mM, pH5, 10-50nA) a 5-HT1A antagonist inhibited by 58±16% in 5/21, excited by 157±15% in 3/21 and had no effect in 13/21. The effects of 5-CT in the presence of SB-269970 were unaffected in 16/16. In the presence of WAY-100635, 5-CT excitations (2/2) were attenuated, although WAY-100635 decreased baseline firing. The 8-0H-DPAT evoked excitation, in the presence of WAY-100635 was attenuated 5/8. On vagal evoked activity WAY-100635 20/20 had no effect while SB-269970 decreased activity in 3/16 neurons. These effects did not seem to depend on whether these neurones received a mono or polysynaptic input. The data indicate that 5-CT excitations are not mediated by 5-HT7 but by 5-HT1A receptor activation, although 5-HT7 but not 5-HT1A receptors are important in afferent transmission at the level of the NTS.

Damaso, EL et al. (2007) Brain Res. 1144, 82-90
Kellett, DO et al. (2005) J. Physiol. 563, 319-331.
Scheuer, DA et al. (1996) J Neurophysiol. 76, 3750-3757.
Wang, Y et al. (1998) 509, 683-69.4

We wish to thank the BHF for support and to acknowledge the sad loss of our colleague David Jordan