Cardiovascular effects of activation of central alpha7 and alpha4beta2 nAChRs: a role for vasopressin in anaesthetized rats.

BACKGROUND AND PURPOSE: Central application of nicotine causes the release of vasopressin and affects blood pressure. Involvement of the 5 neuronal nicotinic receptor groups, alpha2(*)-alpha7(*) in these effects is unknown. The availability of selective agonists for alpha7 (PSAB-OFP) and alpha4beta2 (TC-2559) nACh receptors allowed their role to be investigated. EXPERIMENTAL APPROACH: Recordings were made of arterial blood pressure, heart rate and renal sympathetic nerve activity in anaesthetized male rats with neuromuscular blockade and artificial respiration. Effects of the agonists, PSAB-OFP (1-10 micromol kg(-1)) and TC-2559 (1-10 micromol kg(-1)) on these variables given intracerebroventricularly (i.c.v.) and intracisternally (i.c.) in the presence or absence of the antagonists, DhbetaE (10 micromol kg(-1)) and MLA (0.5 micromol kg(-1)), for the appropriate nicotinic receptor subtypes, respectively, and a V(1) receptor antagonist, given i.v. or centrally, were investigated. KEY RESULTS: Both agonists given i.c.v. caused a delayed rise in blood pressure and renal nerve activity which could be blocked only with the appropriate antagonist. The agonists had an earlier onset of action when given i.c., favouring the brainstem as the major site of action. The effects of these agonists were also attenuated by the V(1) receptor antagonist given i.v. and blocked when this antagonist was given centrally. Antagonists had no effect on baseline variables. CONCLUSIONS AND IMPLICATIONS: Activation of alpha4beta2 and alpha7 receptors in the brainstem is mainly responsible for the cardiovascular effects of activating these receptors, which have a similar profile of action. These actions, although independent, are mediated by the central release of vasopressin.

Investigation of the role of 5-HT2 receptor subtypes in the control of the bladder and the urethra in the anaesthetized female rat.

BACKGROUND AND PURPOSE: Micturition is controlled by central 5-HT-containing pathways. 5-HT2 receptors have been implicated in this system especially in control of the urethra, which is a drug target for treating urinary incontinence. This study investigates the role of each of the three subtypes of this receptor with emphasis on sphincter regulation. EXPERIMENTAL APPROACH: Recordings of urethral and bladder pressure, external urethral sphincter (EUS) EMG, as well as the micturition reflex induced by bladder distension along with blood pressure and heart rate were made in anaesthetized rats. The effects of agonists and antagonists for 5-HT2 receptor subtypes were studied on these variables. KEY RESULTS: The 5-HT2C agonists Ro 60-0175, WAY 161503 and mCPP, i.v., activated the EUS, increased urethral pressure and inhibited the micturition reflex. The effects of Ro 60-0175 on the EUS were blocked by the 5-HT2C antagonist SB 242084 and the 5-HT2A antagonists, ketanserin and MDL 100907. SB 242084 also blocked the inhibitory action on the reflex, while the 5-HT2B antagonist RS 127445 only blocked the increase in urethral pressure. The 5-HT2A receptor agonist DOI given i.v. or i.t. but not i.c.v. activated the EUS. CONCLUSIONS AND IMPLICATIONS: 5-HT2A/2C receptors located in the sacral spinal cord activate the EUS, while central 5-HT2C receptors inhibit the micturition reflex and 5-HT2B receptors, probably at the level of the urethra, increase urethral smooth muscle tone. Furthermore, 5-HT2B and 5-HT2C receptors do not seem to play an important role in the physiological regulation of micturition.

Activation of 5-HT1B and 5-HT1D receptors in the rat nucleus tractus solitarius: opposing action on neurones that receive an excitatory vagal C-fibre afferent input.

BACKGROUND AND PURPOSE: Central 5-HT-containing pathways are known to be important in cardiovascular regulation and a crucial area for this regulation is the nucleus tractus solitarius (NTS), which contains many of the known 5-HT receptor subtypes. In this study the role of 5-HT(1B) and 5-HT(1D) receptors, targets for the antimigraine drugs known collectively as triptans, was examined in the NTS. EXPERIMENT APPROACH: Extracellular recordings were made, in anaesthetized rats, from 109 NTS neurones that were excited by electrical stimulation of the vagus and drugs were applied ionophoretically to these neurones. KEY RESULTS: The 5-HT(1B/1D) receptor agonist sumatriptan applied to 64 neurones produced a 64% reduction in the firing rate of 54 of these neurones. Ketanserin, a 5-HT(1D/2A) receptor antagonist, alone had little effect, but co-applied with sumatriptan significantly attenuated this inhibition, whilst co-application of the 5-HT(1B) receptor antagonist GR55562 resulted in potentiation of this inhibition. Sumatriptan also caused a 25% reduction in vagal afferent evoked activity as well as that caused by stimulation of cardiopulmonary afferents. In another 41 neurones the 5-HT(1B) receptor agonist CP-93 129 produced a doubling of the background firing rate in 31 of these neurones and a significant increase in both vagal afferent evoked activity and that evoked by cardiopulmonary afferent activation. CONCLUSIONS AND IMPLICATIONS: Activation of 5-HT(1B) and 5-HT(1D) receptors have opposing actions on NTS neurones of excitation and inhibition, respectively. As both receptors are negatively coupled to adenylate cyclase this would indicate that they have different anatomical locations within NTS.

The effect of propranolol and other antihypertensive drugs on preganglionic sympathetic activity in the cat.

Preganglionic sympathetic nerve activity was recorded from the white rami of T3/T4 along with blood pressure, heart rate and femoral arterial conductance in vagotomised, atropinised cats under alpha-chloralose/pentobarbitone anaesthesia, and paralysed by gallamine. Injection of propranolol or trimetaphan caused falls in mean blood pressure and concomitant rises in preganglionic sympathetic nerve activity. Propranolol was then infused over 3 hr with a stepwise increase in dose rate. This infusion caused a secondary fall in blood pressure associated with a delayed increase in preganglionic sympathetic nerve activity. Infusion of clonidine and sodium nitroprusside over 3 hr with a stepwise increase in dose rate caused falls in blood pressure, but clonidine caused a decrease while sodium nitroprusside caused an increase in preganglionic sympathetic nerve activity. It is therefore suggested that propranolol interferes at more than one site, along the reflex arc involved in compensation for the propranolol-induced fall in cardiac output.

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.

The role of central 5-HT receptors in the bronchoconstriction evoked by inhaled capsaicin in anaesthetised guinea-pigs.

The effects of intracisternal (i.c) injections of the 5-HT1A receptor agonists, buspirone and 8-OH-DPAT, and the antagonists WAY-100635; and (-)-pindolol, the 5-HT1B/1D receptor agonist sumatriptan and antagonist GR127935, the 5-HT2 receptor agonist DOI and the antagonist cinanserin, the 5-HT3 receptor antagonist granisetron, the alpha-adrenoceptor agonist clonidine and the antagonist idazoxan, the D2 receptor antagonists (-)-sulpiride and the 5-HT uptake inhibitor fluoxetine on capsaicin-evoked increase in tracheal inflation pressure (bronchoconstriction) were investigated in alpha-chloralose anaesthetised, neuromuscularly blocked, artificially ventilated guinea-pigs. Buspirone, 8-OH-DPAT and fluoxetine significantly potentiated while WAY-100635 (-)-pindolol and sumatriptan attenuated the evoked bronchoconstriction when applied i.c. Granisetron attenuated the response when applied i.v. but not when given i.c. The 5-HT2, alpha2-adrenoceptor and D2 dopamine receptor ligands did not have any significant effect on the evoked bronchoconstriction. Pretreatment i.v. with WAY-100635 alone had no effect on the capsaicin-evoked bronchoconstriction but blocked the potentiating action of i.c. buspirone. The effects of sumatriptan could be completely blocked by pretreatment i.v. with GR127935. Only DOI, in the presence (i.v.) of the peripheral acting 5-HT2 receptor antagonist BW501C67, caused a significant increase in baseline tracheal inflation pressure. It is concluded that activation of central 5-HT1A and 5-HT1B/1D receptors have opposing roles, facilitation and inhibition respectively, on the reflex activation of bronchoconstrictor vagal preganglionic neurones.

In vivo modulation of nucleus tractus solitarius (NTS) neurones by activation of 5-hydroxytryptamine(2) receptors in rats.

In in vivo experiments, 5-hydroxytryptamine (5-HT) and (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl (DOI), a 5-HT(2) receptor agonist, were applied by ionophoresis to rat nucleus tractus solitarius (NTS) neurones identified by their vagal and cardiopulmonary afferent inputs to test whether the response of NTS cells to 5-HT(2) receptor activation was related to whether they received mono- or polysynaptic vagal inputs and their presumed function as defined by their afferent input. Cells were classified on the basis of the variability of the latency of the vagal-evoked spikes: this varied by less than 3 ms for Group 1, from 3 to 5 ms for Group 2, and more than 5 ms for Group 3. Both 5-HT and DOI inhibited most Group 1 cells (16/18) and inactive (without ongoing activity) cells (8/13) in Group 2. Cells inhibited by DOI were also inhibited by cardiopulmonary afferent stimulation, evoked by atrial phenylbiguanide administration. By contrast, application of 5-HT and DOI excited the majority of Group 3 cells (14/19) and Group 2 with ongoing activity (7/9). Cells excited by DOI were also activated by cardiopulmonary stimulation. Both actions of DOI were reversed by application of ketanserin (n=15). In conclusion, these data demonstrate that activation of 5-HT(2) receptors in the NTS produces different effects dependent on whether the neurones received mono- or polysynaptic vagal input and their response to cardiopulmonary afferent stimulation.

The mechanism of the sympathoinhibitory action of urapidil: role of 5-HT1A receptors.

1. An investigation was carried out to determine if the sympathoinhibition caused by urapidil is due to activation of 5-HT1A receptors by investigating whether it could be reversed by the non-selective 5-HT1A receptor antagonist spiperone. To control for the possibility of functional antagonism by spiperone, the ability of spiperone to reverse the sympathoinhibition caused by clonidine was also investigated. These experiments were carried out in anaesthetized prazosin-pretreated cats to prevent the alpha 1-adrenoceptor antagonist action of urapidil and spiperone from masking any effects observed. 2. Cats were anaesthetized with alpha-chloralose and simultaneous recordings were made of whole cardiac, splanchnic and renal nerve activities, blood pressure, heart rate and femoral arterial flow (from which conductance was derived). All animals were initially pretreated with prazosin (1 mg kg-1, i.v.) given in divided doses (0.75 followed 10 min later by 0.25 mg kg-1), then either urapidil (0.75 mg kg-1, i.v.) or clonidine (10 micrograms kg-1, i.v. in two divided doses) followed by 3 separate injections of spiperone (1 mg kg-1, i.v.). In another set of experiments urapidil was given followed by injections of the appropriate vehicle for spiperone, while in another set urapidil was replaced with an injection of the appropriate vehicle followed by injections of spiperone. In the experiments with clonidine, the alpha 2-adrenoceptor antagonist Wy 26392 (0.3 mg kg-1) was given after the last injection of spiperone. 3. The prazosin pretreatment caused a fall in blood pressure associated with femoral vasodilatation, a small bradycardia and little change in cardiac, splanchnic or renal nerve activities. Urapidil or clonidine injection after prazosin caused sympathoinhibition associated with an additional bradycardia. However, only urapidil caused an additional fall in blood pressure. Spiperone injections reversed the sympathoinhibition caused by urapidil but not that caused by clonidine. The sympathoinhibition caused by clonidine was reversed by the alpha 2-adrenoceptor antagonist Wy 26392. 4. These results show that the sympathoinhibition caused by urapidil in prazosin-pretreated cats can be reversed by spiperone. The reversal of this sympathoinhibition is not due to functional antagonism. It is concluded that urapidil can cause sympathoinhibition by activation of 5-HTlA receptors.

Evidence that activation of central 5-HT(2B) receptors causes renal sympathoexcitation in anaesthetized rats.

The effects of injections i.c.v. of alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (BW723C86; 0.02 - 2 micromol kg(-1)), a 5-HT(2B) receptor agonist, on renal sympathetic and phrenic nerve activity, mean arterial blood pressure and heart rate were investigated in alpha-chloralose anaesthetized rats pretreated with a peripherally acting 5-HT(2) receptor antagonist. BW723C86 i.c.v. caused a dose-related increase in renal nerve activity reaching a maximum of 67+/-6%, which at the highest dose was associated with a small and maintained fall in mean arterial blood pressure of 7+/-3 mmHg. These changes were not associated with any significant changes in heart rate or phrenic nerve activity. BW723C86-evoked increases in renal nerve activity and hypotension were attenuated by pretreatment (i.c.v.) with SB204741 (300 nmol kg(-1); a 5-HT(2B) receptor antagonist) but not by the same dose (i.c.v.) of ketanserin (a 5-HT(2A) receptor antagonist) or RS102221 (a 5-HT(2C) receptor antagonist). None of these antagonists alone had any effect on the variables being measured. It is concluded that central 5-HT(2B) receptors may play a selective role in the control of sympathetic supply to the kidney, which could be important in the central mechanisms involved in blood volume regulation. British Journal of Pharmacology (2000) 129, 177 - 183

The central action of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on cardiac inotropy and vascular resistance in the anaesthetized cat.

Experiments were carried out to determine the effects of the application of the selective 5-HT2 receptor agonist DOI intravenously (in the presence of the peripherally acting 5-HT2 receptor antagonist, BW501C67, 1 mg kg(-1), i.v.) or to the 'glycine sensitive area' of the ventral surface (30 microg each side) on the left ventricular inotropic (left ventricular dP/dt max) and vascularly isolated hindlimb responses in anaesthetized cats. For the ventral surface experiments, NMDA (10 microg each side) was applied to act as a positive control. In all experiments heart rate and mean arterial blood pressure were held constant to exclude any secondary effects caused by changes in these variables. DOI (n=6) i.v or on the ventral surface had no effect on left ventricular dP/dt max but caused a significant increase in hindlimb perfusion pressure of 40+/-9 and 50+/-14 mmHg, respectively. Respiration was unaffected. NMDA (n=6), applied to the ventral surface, caused significant increases in both left ventricular dP/dt max and hindlimb perfusion pressure of 1,950+/-349 mmHg s(-1) and 69+/-17 mmHg respectively, with no associated change in left ventricular end-diastolic pressure. The amplitude of respiratory movements increased. It is concluded that activation of 5-HT2 receptors at the level of the rostral ventrolateral medulla (RVLM) excites sympathetic premotor neurons and/or their antecedents controlling hindlimb vascular resistance but not those controlling the inotropic effects on the left ventricle.

Evidence that different regional sympathetic outflows vary in their sensitivity to the sympathoinhibitory actions of putative 5-HT1A and alpha 2-adrenoceptor agonists in anaesthetized cats.

1. An investigation was carried out to determine whether the centrally acting hypotensive drugs whose mechanisms of action are due either to activation of 5-HT1A receptors (flesinoxan, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and urapidil--also an alpha 1-adrenoceptor antagonist) or to activation of alpha 2-adrenoceptors (clonidine and moxonidine) cause differential sympathoinhibition. 2. Cats were anaesthetized with alpha-chloralose and simultaneous recordings were made of whole cardiac, splanchnic and renal nerve activity, blood pressure and heart rate. Cumulative dose-response (i.v.) curves were constructed in separate experiments for the above hypotensive agents on these parameters. 3. Renal nerve activity was found to be more sensitive to the sympathoinhibitory action of flesinoxan and 8-OH-DPAT when compared with cardiac nerve activity, whereas the reverse was observed for clonidine and moxonidine, cardiac being more sensitive than renal nerve activity. Splanchnic nerve activity was similarly affected by all drugs. Furthermore at the highest dose, all drugs tended to cause complete inhibition in all regional sympathetic nerve outflows. 4. Urapidil differed from all the above hypotensive drugs in that it caused a similar degree of sympathoinhibition in all sympathetic outflows at all doses. It is suggested that this may be due to the ability of urapidil to block central alpha 1-adrenoceptors in addition to stimulation of 5-HT1A receptors.

Evidence for a role for central 5-HT2B as well as 5-HT2A receptors in cardiovascular regulation in anaesthetized rats.

1. The effects of injections i.c.v. of quipazine, (2 micromol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 2 micromol kg-1) on renal sympathetic and phrenic nerve activity, mean arterial blood pressure (MAP) and heart rate were investigated in alpha-chloralose anaesthetized rats pretreated with a peripherally acting 5-HT2 receptor antagonist. 2. Quipazine or DOI caused a rise in MAP which was associated with a tachycardia and renal sympathoinhibition in rats pretreated (i.c.v.) with the antagonist vehicle 10% PEG. These effects of quipazine were completely blocked by pretreatment with cinanserin (a 5-HT2 receptor antagonist) and attenuated by spiperone (a 5-HT2A receptor antagonist). However, pretreatment with SB200646A (a 5-HT2B/2C receptor antagonist) only blocked the sympathoinhibition, while pretreatment with SB204741 (a 5-HT2B receptor antagonist) reversed the sympathoinhibition to excitation as it also did for DOI. Quipazine also caused renal sympathoexcitation in the presence (i.v.) of a vasopressin V1 receptor antagonist. 3. Injection (i.v.) of the V1 receptor antagonist at the peak pressor response evoked by quipazine alone and in the presence of SB204741 caused an immediate fall in MAP. For quipazine alone the renal sympathoinhibition was slowly reversed to an excitation, while the renal sympathoexcitation observed in the presence of SB204741 was potentiated. In both, the quipazine-evoked tachycardia was unaffected. 4. The data indicate that cardiovascular responses caused by i.c.v. quipazine and DOI are primarily due to activation of central 5-HT2A receptors, which causes the release of vasopressin and a tachycardia. This released vasopressin appears to suppress a 5-HT2A receptor-evoked central increase in sympathetic outflow, which involves the activation of central 5-HT2B receptors indirectly by the released vasopressin.

Investigation of the effects of IVth ventricular administration of the 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), on autonomic outflow in the anaesthetized cat.

1. The effects of IVth ventricular injections of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on renal, splanchnic and cardiac sympathetic nerve activities, phrenic nerve activity, arterial blood pressure, heart rate, femoral arterial conductance, tracheal and intragastric pressures were investigated in alpha-chloralose anaesthetized, neuromuscular blocked and artificially ventilated cats. 2. Cumulative doses of DOI (80, 160 and 320 nmol kg-1) injected into the IVth ventricle caused an increase in mean arterial blood pressure, a fall in femoral arterial conductance, an increase in tracheal pressure and a decrease in the rate of phrenic nerve bursts but did not affect any of the other variables recorded. 3. Even after i.v. administration of the peripheral 5-HT2 antagonist BW501C67 (2 mg kg-1) following the highest dose of DOI there was still a significant pressor response, a fall in femoral arterial conductance and small increase in tracheal pressure. 4. In control experiments, intravenous infusion of noradrenaline to raise blood pressure to the levels obtained during the cumulative doses of DOI caused large falls in renal, splanchnic and cardiac nerve activities which were all significantly lower than those recorded during the cumulative doses of DOI. 5. The results of this study provide evidence for a brainstem site of action of DOI in producing hypertension and further support the hypothesis that central 5-HT2 receptors are involved in the control of skeletal muscle and skin vascular beds.

Mediation by 5-HT3 receptors of an excitatory effect of 5-HT on dorsal vagal preganglionic neurones in anaesthetized rats: an ionophoretic study.

1. Extracellular recording were made from 141 vagal preganglionic neurones in the dorsal vagal nucleus (DVN). The effects of ionophoretic administration of 5-hydroxytryptamine (5-HT), the 5-HT3 receptor agonist, phenylbiguanide (PBG) and the antagonists, granisetron and tropisetron (ICS 205-930) on these vagal preganglionic neurones were studied in pentobarbitone sodium anaesthetized rats. 2. Ionophoretic application of 5-HT at low currents (< 10 nA) increased the activity in 46 (73%) of 63 neurones tested. Application of granisetron (5-20 nA) or tropisetron (5-20 nA) abolished or attenuated the 5-HT excitatory responses in 8 out of 11 and 5 out of 5 neurones respectively. At the currents used, neither antagonist had any effect on baseline firing rate. 3. Ionophoresis of the selective 5-HT3 receptor agonist, phenylbiguanide (0-40 nA) excited 54 (82%) of the 66 vagal neurones tested, whilst the remaining 12 neurones were unaffected. 4. Granisetron applied either ionophoretically (8/11) or intravenously (3/3),abolished or attenuated the excitations evoked by PBG. Similarly, tropisetron administered either ionophoretically (2/3) or intravenously (2/2), attenuated the PBG excitation. In contrast, the PBG excitations were unaffected by the 5-HT2 receptor antagonist, cinanserin (2/2), and the selective 5-HT1A receptor antagonist, WAY- 100802 (6/6). 5. In conclusion, excitation of vagal preganglionic neurones evoked by ionophoretic application of 5- HT is mediated in part by 5-HT3 receptors and activation of 5-HT3 receptors on and/or in the vicinity of vagal motoneurones causes excitation of these neurones.

Comparison of the effects of IVth ventricular administration of some tryptamine analogues with those of 8-OH-DPAT on autonomic outflow in the anaesthetized cat.

1 The present study compares the effects on representative autonomic outflows of IVth ventricular application of tryptamine analogues which act at 5-HT1 receptors with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). 2 Cumulative doses of 8-OH-DPAT, N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT) and 5-carboxamidotryptamine (5-CT, 2.5-40 nmol kg-1), sumatriptan (10-160 nmol kg-1), indorenate (100-800 nmol kg-1), 5-hydroxytryptamine (5-HT, 20-640 nmol kg-1) both alone and in the presence of cinanserin (0.1 mg kg-1) were given into the IVth ventricle of cats which were anaesthetized with a mixture of alpha-chloralose and pentobarbitone sodium, neuromuscularly blocked and artificially ventilated. Recordings were made of arterial blood pressure, heart rate, renal, cardiac, splanchnic and phrenic nerve activities, femoral arterial flow, tracheal and intragastric pressures. 3 Central application of each of the agonists evoked significant falls in arterial blood pressure. In addition 8-OH-DPAT, DP-5-CT, 5-CT and 5-HT all evoked a differential inhibition of sympathetic nerve activities, with renal nerve activity being the most sensitive and cardiac nerve activity the least sensitive. In the dose-ranges used, administration of sumatriptan evoked reductions only in renal and splanchnic nerve activities whilst indorenate reduced activity in all three sympathetic nerves to a similar extent. 4. The effect of the agonists on heart rate was more inconsistent than the effects on sympathetic outflow.IVth ventricular application of 5-CT and sumatriptan were without effect on heart rate whilst 8-OH-DPAT, DP-5-CT, indorenate and 5-HT alone and in the presence of cinanserin all evoked significant bradycardias. However, whilst atropine partially reversed the bradycardias evoked by 8-OHDPAT and only slightly reversed those caused by indorenate, atropine was without effect on those evoked by DP-5-CT or 5-HT.5. None of the analogues tested had significant effects on gut motility, phrenic nerve discharge or tracheal pressure. 8-OH-DPAT, DP-5-CT, indorenate and 5-HT were without effect on femoral arterial conductance. However, following pretreatment with cinanserin, 5-HT evoked a significant reduction in femoral arterial conductance. At its highest dose, sumatriptan evoked a significant increase in femoral arterial conductance as did 5-CT at the 20 nmol kg-1 dose.6. It is concluded that the present data support the view that 5-HT1A receptors at the level of the brainstem are involved in the central sympathoinhibitory effects caused by intravenous administration of 5-HT1A agonists. Further, brainstem 5-HT1A receptors play an important role in the control of renal sympathetic outflow while brainstem 5-HT2 receptors are involved in the control of skeletal muscle and/or skin blood flow. Selective tryptamine agonists for 5-HT1A receptors differ from non-tryptamine agonists in that they do not cause an increase in central cardiac vagal tone.

Central administration of 5-HT activates 5-HT1A receptors to cause sympathoexcitation and 5-HT2/5-HT1C receptors to release vasopressin in anaesthetized rats.

1. The effects of intracerebroventricular injections to the right lateral ventricle (i.c.v.) of 5-hydroxytryptamine (5-HT, 40 and 120 nmol kg-1), N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT; 3 nmol kg-1), 5-carboxamidotryptamine (5-CT; 3 nmol kg-1), 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT; 3, 40 and 120 nmol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 40 and 120 nmol kg-1) on renal sympathetic nerve activity, blood pressure, heart rate and phrenic nerve activity were investigated in normotensive rats anaesthetized with alpha-chloralose. 2. 5-HT caused a long lasting pressor response which was associated with an initial bradycardia and renal sympathoinhibition followed by a tachycardia and renal sympathoexcitation. Pretreatment with the 5-HT2/5-HT1C receptor antagonists, cinanserin (300 nmol kg-1, i.c.v.) or LY 53857 (300 nmol kg-1, i.c.v.) reversed the initial bradycardia and sympathoinhibition to tachycardia and sympathoexcitation. Combined pretreatment with LY 53857 (300 nmol kg-1, i.c.v.) and the 5-HT1A antagonist, spiroxatrine (300 nmol kg-1, i.c.v.), blocked the effects of 5-HT on all the above variables. 3. Pretreatment with the vasopressin V1-receptor antagonist, beta-mercapto-beta,beta-cyclopentamethylene-propionyl1, O-Me-Tyr2, Arg8-vasopressin [(d(CH2)5Tyr(Me)AVP, 10 micrograms kg-1, i.v.] did not affect the magnitude but reduced the duration of the pressor response produced by i.c.v. 5-HT and reversed the initial bradycardia and renal sympathoinhibition to tachycardia and sympathoexcitation. 4. 1-(2,5-Di-methoxy-4-iodophenyl)-2-aminopropane (DOI) caused a pressor effect which was associated with a bradycardia and sympathoinhibition. These effects were blocked by pretreatment with BWSOlC67 (0.1 mg kg-', i.v.), a peripherally acting 5-HT2/5-HTc receptor antagonist. However,BWSOlC67 (0.1 mg kg-', i.v.) failed to block the effects of i.c.v. 5-HT.5. DP-5-CT, 5-CT and 8-OH-DPAT (3 nmol kg-', i.c.v.) caused sympathoexcitation, tachycardia and a rise in blood pressure. Pretreatment with methiothepin (1 mg kg-', i.v.) or spiroxatrine (300 nmol kg-',i.c.v.) attenuated the response to i.c.v. DP-5-CT.6. It is concluded that i.c.v. administration of 5-HT activates 5-HTlA receptors to cause sympathoexcitation and 5-HT2 or 5-HT1c receptors to cause the release of vasopressin.

Microinjections of 5-HT1A agonists into the dorsal motor vagal nucleus produce a bradycardia in the atenolol-pretreated anaesthetized rat.

1. The effects of microinjections (100 nl) into the dorsal motor vagal nucleus of the 5-HT1A receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and flesinoxan, the 5-HT2 receptor agonist (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), the 5-HT3 receptor agonist phenylbiguanide (PBG), the alpha 2-adrenoceptor agonist clonidine and the excitatory amino acid glutamate on heart rate, blood pressure, tracheal pressure and phrenic nerve activity were investigated in atenolol-pretreated rats anaesthetized with sodium pentobarbitone. 2. Microinjections of glutamate (2.5 nmol) caused decreases in blood pressure, heart rate and phrenic nerve activity. In contrast, microinjections of 5-HT (1.2 nmol), 8-OH-DPAT (1.2 nmol) and flesinoxan (1.3 nmol) all caused a bradycardia but had no effect on blood pressure. In addition, 8-OH-DPAT and flesinoxan caused an increase in phrenic nerve activity. 3. Microinjections of DOI, PBG and clonidine had no significant effect on any of the variables recorded. None of the drugs used had any significant effect on tracheal pressure. 4. These results support the hypothesis that activation of 5-HT1A receptors causes excitation of cardiac vagal motoneurones and suggest that these receptors are also important in the control of central respiratory drive.

Pressor effects following microinjection of 5-HT1A receptor agonists into the raphe obscurus of the anaesthetized rat.

1. The effects of electrical stimulation and microinjections (90 nl) of the 5-HT1A receptor agonists, flesinoxan and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), and glutamate into the raphe obscurus on blood pressure, heart rate and phrenic nerve activity (central inspiratory drive) were investigated in rats anaesthetized with alpha-chloralose. 2. Electrical stimulation of the raphe obscurus caused a rise in blood pressure which was associated with bradycardia, while glutamate (2.7 nmol) caused only a rise in blood pressure. 3. Flesinoxan (1.3 nmol) and 8-OH-DPAT (0.7 nmol) increased blood pressure by 9 +/- 1 and 14 +/- 2 mmHg, respectively and did not affect heart rate. For both agonists the effect on blood pressure was shown to be dose-dependent; again no effect on the heart rate was observed over the dose-ranges chosen. 4. Microinjections of the non-selective 5-HT1A receptor antagonists, (+/-)-pindolol (2.7 nmol) or methiothepin (5.2 nmol), into the raphe obscurus prevented the increase in blood pressure caused by microinjection of flesinoxan. However, (+/-)-pindolol caused a sustained rise in blood pressure of 15 +/- 1 mmHg while methiothepin caused a transient rise in blood pressure. Neither drugs affected heart rate. The ability of methiothepin to attenuate the pressor effect of flesinoxan was found to be partially reversed after 30 min. 5. It is suggested that activation of 5-HT1A receptors within the raphe obscurus can cause sympatho-excitation.

Evidence that 5-HT1D receptors mediate inhibition of sympathetic ganglionic transmission in anaesthetized cats.

In anaesthetized cats, 5-carboxamidotryptamine (5-CT) or 5-hydroxytryptamine (5-HT) (0.3-300 micrograms kg-1,i.v.) inhibited the postganglionic compound action potential evoked by preganglionic electrical stimulation (0.5 Hz) with a similar potency in the stellate and splanchnic ganglia. In the 5-HT experiments transmission thorough the inferior mesenteric ganglia was also recorded. The maximal inhibitory effect of 5-HT was greater on the stellate and splanchnic ganglia (60 +/- 4 and 52 +/- 5%) than on the inferior mesenteric (15 +/- 2%). The effects of 5-HT were unaffected by pretreatment with antagonists (1 mg kg-1;i.v.) for 5-HT2 (BW501C67), 5-HT1A (WAY-100635) and 5-HT3 receptors (ondansetron). However, responses to both 5-HT and 5-CT were attenuated significantly by GR127935 (1 mg kg-1) except the responses to 5-HT at the inferior mesenteric ganglia. These results are consistent with the involvement of 5-HT1D receptors mediating inhibition of sympathetic ganglionic transmission in vivo.

Evidence that activation of 5-HT2 receptors in the forebrain of anaesthetized cats causes sympathoexcitation.

1. The aim of the present experiments was to determine whether the effects of lateral ventricular application of 5-HT on cardiovascular and respiratory variables in anaesthetized cats are mediated by forebrain 5-HT2 receptors. This was carried out by determining whether the effects of 5-HT are blocked by the 5-HT2 antagonist, cinanserin and if they are mimicked by the selective 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). 2. Cats were anaesthetized with a mixture of alpha-chloralose and pentobarbitone sodium, neuromuscularly blocked and artifically ventilated. The following cardiovascular and respiratory variables were recorded: renal, splanchnic and cardiac sympathetic nerve activities, phrenic nerve activity, heart rate, arterial blood pressure, femoral arterial conductance and tracheal pressure. All drugs were administered via the lateral ventricle and the action of these agonists was restricted to forebrain sites by a cannula placed in the Aqueduct of Sylvius. 3. Cumulative doses of 5-HT (10-160 nmol kg-1) and DOI (80-320 nmol kg-1) injected into the lateral ventricle caused significant increases in blood pressure, heart rate, cardiac and splanchnic sympathetic nerve activity and a decrease in femoral arterial conductance. DOI and 5-HT caused a greater increase in cardiac compared with splanchnic nerve activity and failed to change renal nerve activity. 5-HT but not DOI significantly increased the magnitude and the number of phrenic bursts as well as significantly increasing tracheal pressure. The effects of 5-HT also differed from DOI in that 5-HT evoked maximal pressor and near maximal sympathoexcitatory effects after the first dose, whereas the pressor and sympathoexcitatory effects of DOI were graded over the complete dose-range.4 The 5-HT2 antagonist, cinanserin (265 nmol kg-1, i.c.v.) caused significant falls in blood pressure,heart rate and cardiac nerve activity and an increase in femoral arterial conductance. Splanchnic andrenal sympathetic nerve activity, phrenic nerve activity and tracheal pressure were unaffected by cinanserin. After pretreatment with cinanserin all cardiovascular and respiratory effects of 5-HT were significantly attenuated.5 It is concluded that in the cat, as DOI and 5-HT have similar effects on the cardiovascular variables recorded and as the effects of 5-HT are blocked by cinanserin, 5-HT can act on 5-HT2 receptors located in the forebrain to cause differential sympathoexcitation and a rise in arterial blood pressure. Further,the sympathoexcitatory effects mediated by 5-HT2 receptors located in the forebrain differ from those located in the hindbrain in that they mediate increases in cardiac nerve activity and heart rate and also have no effect on renal nerve activity.