Involvement of central 5-HT1A receptors in the reflex activation of pulmonary vagal motoneurones by inhaled capsaicin in anaesthetized cats

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Characterization of cardiovascular reflexes evoked by airway stimulation with allylisothiocyanate, capsaicin, and ATP in Sprague-Dawley rats

Acute inhalation of airborne pollutants alters cardiovascular function and evidence suggests that pollutant-induced activation of airway sensory nerves via the gating of ion channels is critical to these systemic responses. Here, we have investigated the effect of capsaicin [transient receptor potential (TRP) vanilloid 1 (TRPV1) agonist], AITC [TRP ankyrin 1 (TRPA1) agonist], and ATP (P2X2/3 agonist) on bronchopulmonary sensory activity and cardiovascular responses of conscious Sprague-Dawley (SD) rats. Single fiber recordings show that allyl isothiocyanate (AITC) and capsaicin selectively activate C fibers, whereas subpopulations of both A and C fibers are activated by stimulation of P2X2/3 receptors. Inhalation of the agonists by conscious rats caused significant bradycardia, atrioventricular (AV) block, and prolonged PR intervals, although ATP-induced responses were lesser than those evoked by AITC or capsaicin. Responses to AITC were inhibited by the TRP channel blocker ruthenium red and the muscarinic antagonist atropine. AITC inhalation also caused a biphasic blood pressure response: a brief hypertensive phase followed by a hypotensive phase. Atropine accentuated the hypertensive phase, while preventing the hypotension. AITC-evoked bradycardia was not abolished by terazosin, the α1-adrenoceptor inhibitor, which prevented the hypertensive response. Anesthetics had profound effects on AITC-evoked bradycardia and AV block, which was abolished by urethane, ketamine, and isoflurane. Nevertheless, AITC inhalation caused bradycardia and AV block in paralyzed and ventilated rats following precollicular decerebration. In conclusion, we provide evidence that activation of ion channels expressed on nociceptive airway sensory nerves causes significant cardiovascular effects in conscious SD rats via reflex modulation of the autonomic nervous system.

Agonist of 5-HT1A/7 receptors but not that of 5-HT2 receptors disinhibits tracheobronchial-projecting airway vagal preganglionic neurons of rats

The vagus nerves supply the major cholinergic tone to airway smooth muscles physiologically and play critical roles in the genesis of airway hyperreactivity under some pathological conditions. Postganglionic airway cholinergic tone relies largely on the ongoing activity of medullary airway vagal preganglionic neurons (AVPNs), of which the tracheobronchial-projecting ones are primarily located in the external formation of the nucleus ambiguus (eNA). AVPNs are regulated by 5-HT, and 5-HT(1A/7) and 5-HT(2) receptors have been indicated to be involved. But the mechanisms at synaptic level are unknown. In the present study, tracheobronchial-projecting AVPNs (T-AVPNs) were retrogradely labeled from the trachea wall; fluorescently labeled T-AVPNs in the eNA were recorded with whole-cell voltage patch clamp; and the effects of 5-HT(1A/7) receptor agonist (±)-8-Hydroxy-2-(dipropylamino) tetralin hydrobromide (8-OH-DPAT) (1 μmol L(-1)) and 5-HT(2) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (10 μmol L(-1)) on the synaptic inputs were examined. 8-OH-DPAT significantly inhibited the GABAergic and glycinergic spontaneous inhibitory postsynaptic currents (sIPSCs) of T-AVPNs in both the frequency and amplitude but had no effect on the GABAergic and glycinergic miniature inhibitory postsynaptic currents (mIPSCs). The 8-OH-DPAT inhibition of the GABAergic and glycinergic sIPSCs was prevented by 5-HT(1A/7) receptor antagonist N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY-100635) (1 μmol L(-1)). 8-OH-DPAT had no effect on the glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) and caused no alterations in the baseline current and input resistance of T-AVPNs. DOI had no effect on any types of the synaptic inputs of T-AVPNs. These results suggest that 5-HT(1A/7) receptor agonist causes "disinhibition" of T-AVPNs, which might, in part, account for the reflex increase of airway resistance.

Sensory nerves and airway irritability

The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.

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.

Central 5-HT7 receptors are critical for reflex activation of cardiac vagal drive in anaesthetized rats

5-Hydroxytryptamine (5-HT; serotonin)-containing neurones contribute to reflex activation of parasympathetic outflow in a number of species, but the 5-HT receptors mediating these effects have yet to be fully determined. The present experiments demonstrate that central 5-HT7 receptors are involved in the vagal bradycardia evoked during the cardiopulmonary reflex, baroreflexes and the chemoreflex, as well as other autonomic changes caused by these reflexes. The experiments examined the effects of the selective 5-HT7 receptor antagonists SB-269970 and SB-656104 on these reflexes. For the cardiopulmonary reflex, when compared to time-matched vehicle control experiments, intracisternal application of SB-269970 (30-300 microg kg(-1), i.c.) dose-dependently attenuated the evoked bradycardia. At the highest dose, SB-269970 also attenuated the reflex hypotension and sympathoinhibition. The structurally different 5-HT7 receptor antagonist SB-656104 (100 microg kg(-1), i.c.) similarly attenuated the reflex bradycardia and hypotension. SB-269970 (100 microg kg(-1), i.c.) also attenuated the bradycardias evoked by electrical stimulation of aortic nerve afferents and the baroreflex evoked by the pressor response to phenylephrine (3-25 microg kg(-1), i.v.). The gain of the baroreflex was also significantly attenuated (0.15 +/- 0.06 versus 0.34 +/- 0.06 ms mmHg(-1)). Finally, SB-269970 (100 microg kg(-1), i.c.) significantly attenuated both the bradycardia and sympathoexcitation evoked by the chemoreflex. These data indicate that central 5-HT7 receptors play an important facilitatory role in the reflex activation of vagal outflow to the heart.

Evidence for the involvement of central 5-HT7 receptors in the micturition reflex in anaesthetized female rats

(1) The effects of the selective 5-HT7 receptor antagonists SB-269970 (3-300 microg kg-1; n=5-6) and SB-656104 (30 microg kg-1; n=5) administered centrally (i.c.v.) were investigated on the 'micturition reflex' in the urethane anaesthetized female rat. (2) In cystometric recordings, SB-269970 caused significant increases in volume of 58+/-15 and 138+/-33% and pressure of 140+/-46 and 149+/-60% thresholds at 10 and 30 microg kg-1. These changes were associated with significant decreases in distension-induced bladder contraction of 62+/-14 and 60+/-11%, respectively. However, there was no change in residual volume. At the higher doses, SB-269970 blocked the micturition reflex. SB-656104 had similar effects to SB-269970 but in addition significantly increased the residual volume. (3) SB-269970 (10 microg kg-1; n=5) given i.v. had no effect on the micturition reflex. (4) SB-269970 (30 microg kg-1; n=4) given intrathecally (i.t.) had no effect on micturition reflex, although the selective 5-HT1A receptor antagonist WAY-100635 given i.t. after SB-269970 caused a significant increase in the volume threshold. (5) Using an isovolumetric method in which urethral changes were measured, SB-269970 (30 microg kg-1; n=4; i.c.v.) failed to have any effect on these urethral-evoked changes although they significantly reduced the amplitude of the bladder contraction. (6) These data demonstrate that 5-HT7 receptors located supraspinally in the rat are involved in the control of micturition.

Modulation of reflexly evoked vagal bradycardias by central 5-HT1A receptors in anaesthetized rabbits

1. The role of central 5-HT(1A) receptors in the control of the bradycardia and changes in central respiratory drive, renal nerve activity and blood pressure evoked by stimulating cardiopulmonary afferents with phenylbiguanide, baroreceptors by electrical stimulation of the aortic nerve and chemoreceptors by injections of sodium cyanide (NaCN) in atenolol-pretreated anaesthetized rabbits were studied. 2. Buspirone (100 micro g kg(-1); i.c.) potentiated the bradycardia (increase in R-R interval) and the changes in blood pressure and renal nerve activity evoked by all three reflexes. These effects could be attenuated by pretreatment with the 5-HT(1A) receptor antagonist WAY-100635 (100 micro g kg(-1)); i.v.), which alone had no effect on these reflex-evoked changes. However, WAY-100635 (100 micro g kg(-1); i.c.) did attenuate these reflex-evoked responses produced by activation of cardiopulmonary and aortic baroreceptors but not that caused by stimulation of chemoreceptors. When given i.v., buspirone was less effective in modulating the responses evoked by these three reflexes. 3. The present data are consistent with the view that central 5-HT(1A) receptors play a role in the reflex activation of cardiac preganglionic vagal motoneurones. However, although antagonists of 5-HT(1A) receptors affected the responses evoked by cardiopulmonary and aortic nerve afferents, they were not effective on chemoreceptor reflex-evoked changes. This suggests that 5-HT(1A) receptors play a different role in chemoreceptor pathways compared to that for the other reflexes. This may relate to the fact that the chemoreceptor afferents travel in the IXth (glossopharyngeal) nerve whilst the other afferents travel in the Xth (vagus) nerve and thus may use different central circuitry and neurotransmitters.

The role of central 5-HT(1A) receptors in the control of B-fibre cardiac and bronchoconstrictor vagal preganglionic neurones in anaesthetized cats

1. Experiments were performed to determine whether 5-HT(1A) receptors (a) modulate the activity of cardiac and bronchoconstrictor vagal preganglionic neurones (CVPNs and BVPNs) in the nucleus ambiguus (NA) and (b) are involved in pulmonary C-fibre afferent-evoked excitation of CVPNs, by right-atrial injections of phenylbiguanide (PBG). These experiments were carried out on alpha-chloralose-anaesthetized, artificially ventilated and atenolol (1 mg kg(-1))-pretreated cats. 2. The ionophoretic application of 8-OH-DPAT (a selective 5-HT(1A) receptor agonist) influenced the activity of 16 of the 19 CVPNs tested. 8-OH-DPAT tended to cause inhibition at low currents (40 nA) and excitation at high currents (120 nA). The activity of 15 of these neurones increased in response to the application of 8-OH-DPAT. In six of the CVPNs tested, this excitatory action of 8-OH-DPAT was attenuated by co-application of the selective 5-HT(1A) receptor antagonist WAY-100635. 3. The pulmonary C-fibre afferent-evoked excitation of eight CVPNs was attenuated by ionophoretic application of WAY-100635. 4. In three out of four CVPNs, the ionophoretic application of PBG caused excitation. 5. In five out of the nine identified BVPNs that were tested with ionophoretic application of 8-OH-DPAT, excitation was observed that was attenuated by WAY-100635. 6. WAY-100635 (I.V. or intra-cisternally) also reversed bradycardia, hypotension and the decrease in phrenic nerve activity evoked by the I.V. application of 8-OH-DPAT (42 microg kg(-1)). 7. In conclusion, the data indicate that 5-HT(1A) receptors located in the NA play an important role in the reflex activation of CVPNs and BVPNs, and support the view that overall, these receptors play a fundamental role in the reflex regulation of parasympathetic outflow.

Modulation of the vagal bradycardia evoked by stimulation of upper airway receptors by central 5-HT1 receptors in anaesthetized rabbits

1. The effects of central application of 5-HT1A and 5-HT1B/1D receptor ligands on the reflex bradycardia, apnoea, renal sympathoexcitation and pressor response evoked by stimulating upper airway receptors with smoke in atenolol-pretreated anaesthetized rabbits were studied. 2. Intracisternal administration of the 5-HT1A receptor antagonists WAY-100635 (100 microg kg(-1)) and (-)pindolol (100 microg kg(-1)) significantly reduced the smoke-induced bradycardia, attenuated the pressor response and in the case of (-)pindolol, sympathetic nerve activity. The same dose of WAY-100635 i.v. was without effect. 3. Buspirone (200 microg kg(-1), i.c.) potentiated the reflex bradycardia. This action was prevented if the animals were pretreated with WAY-100635 (100 Hg kg(-1), i.v.) 4. (+)8-OH-DPAT (25 microg kg(-1), i.c.) attenuated the evoked bradycardia, pressor response, apnoea and renal sympathoexcitation. The attenuation of the apnoea and renal sympathoexcitation, but not the bradycardia or pressor response was prevented in animals pretreated with WAY-100635 (100 microg kg(-1), i.v.). The attenuation of the reflex bradycardia and the reduction in the renal sympathoexcitation were reduced by pretreatment with the 5-HT1B/1D receptor antagonist GR127935 (100 microg kg(-1), i.v.). 5. In WAY-100635 (100 microg kg(-1), i.v.) pretreated animals, sumatriptan (a 5-HT1B/1D receptor agonist) reduced the reflex bradycardia and the pressor response. The 5-HT1B/1D receptor antagonist GR127935 (20 microg kg(-1), i.c. or 100 microg kg(-1), i.v.) had no effect on the reflex responses. 6. In conclusion, the present data are consistent with the hypothesis that activation of central 5-HT1A receptors potentiate whilst activation of 5-HT1B/1D receptors attenuate the reflex activation of cardiac preganglionic vagal motoneurones evoked by stimulation of upper airway receptors with smoke in rabbits.

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.

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.