The anti-migraine 5-HT(1B/1D) agonist rizatriptan inhibits neurogenic dural vasodilation in anaesthetized guinea-pigs.

These studies investigated the pharmacology of neurogenic dural vasodilation in anaesthetized guinea-pigs. Following introduction of a closed cranial window the meningeal (dural) blood vessels were visualized using intravital microscopy and the diameter constantly measured using a video dimension analyser. Dural blood vessels were constricted with endothelin-1 (3 microg kg(-1), i.v.) prior to dilation of the dural blood vessels with calcitonin gene-related peptide (CGRP; 1 microg kg(-1), i.v.) or local electrical stimulation (up to 300 microA) of the dura mater. In guinea-pigs pre-treated with the CGRP receptor antagonist CGRP((8-37)) (0.3 mg kg(-1), i.v.) the dilator response to electrical stimulation was inhibited by 85% indicating an important role of CGRP in neurogenic dural vasodilation in this species. Neurogenic dural vasodilation was also blocked by the 5-HT(1B/1D) agonist rizatriptan (100 microg kg(-1)) with estimated plasma levels commensurate with concentrations required for anti-migraine efficacy in patients. Rizatriptan did not reverse the dural dilation evoked by CGRP indicating an action on presynaptic receptors located on trigeminal sensory fibres innervating dural blood vessels. In addition, neurogenic dural vasodilation was also blocked by the selective 5-HT(1D) agonist PNU-142633 (100 microg kg(-1)) but not by the 5-HT(1F) agonist LY334370 (3 mg kg(-1)) suggesting that rizatriptan blocks neurogenic vasodilation via an action on 5-HT(1D) receptors located on perivascular trigeminal nerves to inhibit CGRP release. This mechanism may underlie one of the anti-migraine actions of the triptan class exemplified by rizatriptan and suggests that the guinea-pig is an appropriate species in which to investigate the pharmacology of neurogenic dural vasodilation.

Role of opioid receptors in neurogenic dural vasodilation and sensitization of trigeminal neurones in anaesthetized rats.

Migraine headache is thought to be caused by a distension of meningeal blood vessels, the activation of trigeminal sensory neurones and the the development of a central sensitization within the trigeminal nucleus caudalis (TNC). It has been proposed that clinically effective 5-HT(1B/1D) agonists act peripherally to inhibit the release of calcitonin gene-related peptide (CGRP) and neurogenic dural vasodilation, and to attenuate nociceptive neurotransmission within the TNC. Since opioids are also effective anti-migraine agents the present studies investigated the role of opioids within the trigemino-vascular system in anaesthetised rats. Electrical stimulation of the dura mater evoked neurogenic dural vasodilation which was significantly inhibited by morphine (1 mg kg(-1)) the selective mu-opioid agonist DAGO (10 microg kg(-1)) and the mixed agonist/antagonist butorphanol (1 mg kg(-1)) but not by the kappa- and delta-opioid agonists (+/-) U50488H (100 microg kg(-1)) and DPDPE (1 mg kg(-1)). Morphine had no effect on CGRP-evoked dural vasodilation. In electrophysiological studies morphine (1 - 10 mg kg(-1)) significantly attenuated brainstem neuronal activity in response to electrical stimulation of the dura by 65% at 10 mg kg(-1). Morphine (3 mg kg(-1)) also inhibited the TNC neuronal sensitization following CGRP-evoked dilation. The present studies have demonstrated that opioids block the nociceptive neurotransmission within the trigeminal nucleus caudalis and in addition inhibit neurogenic dural vasodilation via an action on mu-opioid receptors located on trigeminal sensory fibres innervating dural blood vessels. These peripheral and central actions are similar to those of the 'triptan' 5-HT(1B/1D) agonists and could account for the anti-migraine actions of opioids.

Analgesic and toxic effects of ABT-594 resemble epibatidine and nicotine in rats.

The present study directly compared the antinociceptive and toxic effects of the neuronal nicotinic receptor agonist ABT-594 ((R)-5-(2-azetidinylmethoxy)-2-chloropyridine) with (-)-nicotine and (+)-epibatidine. Like (-)-nicotine (0.8 and 1.6 mg/kg s.c.) and (+)-epibatidine (0.005 and 0.01 mg/kg s.c.), ABT-594 (0.05 and 0.1 mg/kg s.c.) increased response latencies in the hot-plate test in rats, indicating that it has antinociceptive activity. In contrast to (-)-nicotine and (+)-epibatidine, ABT-594 did not cause rotarod impairment at antinociceptive doses but did cause hypothermia and life-threatening adverse effects including seizures. ABT-594 (0.01 and 0.1 mg/kg i.v.) also produced a dose-dependent increase in blood pressure resembling that observed with (-)-nicotine (0.03, 0.1 and 0. 03 mg/kg i.v.) and (+)-epibatidine (0.001 and 0.003 mg/kg i.v.). Both the antinociceptive and toxic effects (convulsions and hypertension) were abolished by pretreatment with the brain penetrant neuronal nAChR antagonist mecamylamine (1 mg/kg s.c.; i.v. for cardiovascular studies), demonstrating that these actions of ABT-594 were mediated via activation of neuronal nicotinic receptors. Continuous infusion of ABT-594 (0.2 mg/kg per day s.c.) to rats for 7 days followed by challenge with mecamylamine (1 mg/kg i.p.) induced a nicotine-like abstinence syndrome suggesting that ABT-594 has nicotine-like dependence liability. These findings indicate that the acute safety profile of ABT-594 is not significantly improved over other nicotinic analgesics.

Pathophysiology of migraine--new insights.

Current theories propose that the primary dysfunction in migraine occurs within the CNS and that this evokes changes in blood vessels within pain-producing intracranial meningeal structures that give rise to headache pain. Migraine is now thought of as a neurovascular disorder. It has been proposed that genetic abnormalities may be responsible for altering the response threshold to migraine specific trigger factors in the brain of a migraineur compared to a normal individual. The exact nature of the central dysfunction that is produced in migraineurs is still not clear and may involve spreading depression-like phenomena and activation of brain stem monoaminergic nuclei that are part of the central autonomic, vascular and pain control centers. It is generally thought that local vasodilatation of intracranial extracerebral blood vessels and a consequent stimulation of surrounding trigeminal sensory nervous pain pathways is a key mechanism underlying the generation of headache pain associated with migraine. This activation of the 'trigeminovascular system' is thought to cause the release of vasoactive sensory neuropeptides, especially CGRP, that increase the pain response. The activated trigeminal nerves convey nociceptive information to central neurons in the brain stem trigeminal sensory nuclei that in turn relay the pain signals to higher centers where headache pain is perceived. It has been hypothesized that these central neurons may become sensitized as a migraine attack progresses. The 'triptan' anti-migraine agents (e.g. sumatriptan, rizatriptan, zolmitriptan naratriptan) are serotonergic agonists that have been shown to act selectively by causing vasoconstriction through 5-HT1B receptors that are expressed in human intracranial arteries and by inhibiting nociceptive transmission through an action at 5-HT1D receptors on peripheral trigeminal sensory nerve terminals in the meninges and central terminals in brain stem sensory nuclei. These three complementary sites of action underlie the clinical effectiveness of the 5-HT1B/1D agonists against migraine headache pain and its associated symptoms.

Retinal plasma extravasation in animals but not in humans: implications for the pathophysiology of migraine.

High-intensity electrical stimulation of the trigeminal ganglion is accompanied by mast cell degranulation, vasodilatation, increased endothelial permeability and leakage of albumin from postcapillary venules within the dura mater. Overall, the histological appearance suggests an evolving sterile inflammatory response. This neurogenic inflammation within the meninges has been suggested as a model to explain the pain in migraine and cluster headache, and has been used to characterize the pharmacology of anti-migraine compounds. Using the rat model of neurogenic inflammation, the albumin extravasation ratio (stimulated : unstimulated side) in vehicle-treated animals in the dura and retina was 1.60 +/- 0.11 and 1.76 +/- 0.18, respectively (n = 10; values are mean +/- SEM). Pretreatment with sumatriptan (n = 9) produced a highly significant reduction in the ratio of extravasation within the dura to 1.10 +/- 0.06 (P = 0.002) and in the retina to 0.96 +/- 0.06 (P = 0.001), as did the neurokinin-1 receptor antagonist RP 67580 (n = 12) in the dura (1.04 +/- 0.11, P = 0.002) and retina (1.08 +/- 0.06, P = 0.001). These data demonstrate increased endothelial permeability and leakage of albumin not only in the dura but also in the retina. In a second stage we investigated possible extravasation in the human retina in acute migraine (n = 8) and cluster headache (n = 5) using fluorescein or indocyanine angiography. No increased endothelial permeability or leakage of dye could be found in the human retinal or choroidal vessels during headache attacks or in the headache-free interval in persons suffering from both migraine and cluster headache. These data raise the possibility that neurogenic inflammation is not a major factor in headache attacks in migraine or cluster headache.

Intravital microscope studies on the effects of neurokinin agonists and calcitonin gene-related peptide on dural vessel diameter in the anaesthetized rat.

This study describes a novel intravital microscope technique for direct measurement of dural blood vessel diameter through a closed cranial window in anaesthetized rats. This technique avoids removal of the skull, which can lead to problems of altered vessel reactivity and brain swelling that are encountered with open cranial window techniques. Substance P and calcitonin gene-related (CGRP) evoked increases in dural vessel diameter, which were abolished by the NK1 receptor antagonist, RP67580 and the CGRP receptor antagonist, human-alpha CGRP(8-37) respectively. Neurokinin A produced increases in dural vessel diameter which were unaffected by the NK2 receptor antagonist SR 48968 but were blocked by RP67580, suggesting that neurokinin A can act through NK1 receptors to produce dural vasodilation in rats. The NK3 receptor agonist, senktide, had no effects on dural vessel diameter. All drugs were administered intravenously. In humans, vasodilation within the meningeal vasculature has been implicated in the pathogenesis of migraine, the present experiments indicate that substance P or neurokinin A (both acting through NK1 receptors) or CGRP may be responsible.

Sumatriptan inhibits neurogenic vasodilation of dural blood vessels in the anaesthetized rat--intravital microscope studies.

This study used intravital microscopy to measure the diameter of dural arteries in anaesthetized rats. Electrical stimulation of the surface of a closed cranial window produced increases in dural vessel diameter which were blocked by the CGRP receptor antagonist human-alpha CGRP(8-37) but unaffected by the NK1 receptor antagonist RP67580. Sumatriptan (3 and 10 mg kg-1, i.v.) significantly reduced the response to electrical stimulation. In contrast, sumatriptan (3 mg kg-1) had no effects on the response to exogenously administered CGRP. These results indicate that neurokinins play no role in neurogenic vasodilation in this preparation and that neurogenic vasodilation in rat dural vessels is mediated predominantly by CGRP. Furthermore, the data indicate that sumatriptan attenuates neurogenic vasodilation, probably by inhibiting the release of CGRP from perivascular trigeminal nerve endings innervating the dura. These experimental data parallel the clinical findings that CGRP levels are elevated in migraine and normalized, concomitantly with headache relief, by sumatriptan.

The novel anti-migraine agent rizatriptan inhibits neurogenic dural vasodilation and extravasation.

These studies in anaesthetised rats showed, using intravital microscopy, that the novel anti-migraine agent, rizatriptan, significantly reduced electrically stimulated dural vasodilation but had no effect on increases in dural vessel diameter produced by exogenous substance P or calcitonin gene-related peptide (CGRP). Rizatriptan also significantly inhibited dural plasma protein extravasation produced by high intensity electrical stimulation of the trigeminal ganglion. We suggest that rizatriptan inhibits the release of sensory neuropeptides from perivascular trigeminal nerves to prevent neurogenic vasodilation and extravasation in the dura mater. These prejunctional inhibitory effects may be involved in the anti-migraine action of rizatriptan.

L-733,060, a novel tachykinin NK1 receptor antagonist; effects in [Ca2+]i mobilisation, cardiovascular and dural extravasation assays.

This study investigated the properties of a novel piperidine ether-based tachykinin NK1 receptor antagonist L-733,060, ((2S,3S)-3-((3,5-bis(trifluoromethyl)phenyl)methyloxy)-2-phenyl piperidine and its 2R,3R-enantiomer L-733,061 on [Ca2+]i mobilisation in Chinese hamster ovary cells transfected with human tachykinin NK1 receptors, compared to their effects in rodent cardiovascular and neurogenic plasma extravasation assays. Using FURA-2-imaging techniques, L-733,060 inhibited substance P-induced [Ca2+]i mobilisation with an estimated affinity of 0.8 nM whereas L-733,061 (30-300 nM) did not. No significant effects of L-733,060 were observed on mean arterial blood pressure or heart rate in conscious or anaesthetised rats at doses of < 3000 micrograms kg-1 i.v. L-733,060 also stereoselectively inhibited neurogenic plasma extravasation in rat dura produced by electrical stimulation of trigeminal nerves with an ID50 of 212 +/- 19 micrograms kg-1 i.v. Thus, L-733,060 is a novel antagonist of human tachykinin NK1 receptors which stereoselectively inhibits neurogenic plasma extravasation at doses that do not cause adverse cardiovascular effects.

N-heteroaryl-2-phenyl-3-(benzyloxy)piperidines: a novel class of potent orally active human NK1 antagonists.

The preparation of a series of N-heteroarylpiperidine ether-based human NK1 antagonists is described. Two of the compounds 3-[-(2S,3S)-3-(((3,5-bis(trifluoromethyl)phenyl)methyl)oxy)- 2-phenylpiperidino}methyl]-1,2,4-triazole (11) and 5-[¿(2S,3S)-3-(((3,5-bis(trifluoromethyl)-phenyl)methyl)oxy)-2- phenylpiperidino}methyl]-3-oxo-1,2,4-triazolone (12)), in particular, are orally bioavailable and exhibited significant improvements in potency, both in vitro and in vivo, over the lead (carboxamidomethyl)piperidine ether 1. Rat liver microsome studies on a selected number of compounds from this series show the triazolone heterocycle to be considerably more stable than the others. Furthermore, both 11 and 12 have been profiled in a number of assays that may be predictive of the clinical utility of substance P antagonists.

Calcitonin gene-related peptide as an endogenous vasodilator: immunoblockade studies in vivo with an anti-calcitonin gene-related peptide monoclonal antibody and its Fab' fragment.

1. Calcitonin gene-related peptide (CGRP) is localized in perivascular sensory neurons and is a potent vasodilator. We investigated the utility of immunoblockade as an in vivo technique for probing the role of CGRP as an endogenous vasodilator. 2. The effects of an anti-CGRP monoclonal antibody (MAb; coded C4.19) and its Fab' fragment on CGRP-induced changes in blood pressure and skin blood flow were studied in pentobarbitone-anaesthetized rats. Antidromic skin vasodilatation in the rat hind paw was measured by laser Doppler fluxmetry. 3. The dose-response relationship for the hypotensive effect of intravenous rat alpha CGRP (r alpha CGRP) was similarly shifted rightward by MAb C4.19 IgG (1 mg/rat; intravenously) and Fab' fragment (2 mg/rat; intravenously). The C-terminal fragment of human alpha CGRP (h alpha CGRP8-37) also blocked the hypotensive effect of r alpha CGRP. 4. MAb C4.19 Fab' fragment (2 mg/rat; intravenously) and h alpha CGRP8-37 (100 nmol/kg; intravenously), but not MAb C4.19 IgG (up to 3 mg/rat; intravenously) or normal mouse Fab' fragment (2 mg/rat; intravenously), blocked the increased skin blood flow response to antidromic stimulation of the saphenous nerve. 5. The mean percentage changes in skin blood flow parameters due to MAb C4.19 Fab' fragment were significantly different from those due to normal mouse Fab' fragment (unpaired t-test; P < 0.05) but not from those due to h alpha CGRP8-37. 6. The results demonstrate the pharmacokinetic advantage of Fab' fragment over IgG for immunoblockade studies in vivo and support the role of CGRP in mediating skin vasodilatation.

Comparison of the effects of sumatriptan and the NK1 antagonist CP-99,994 on plasma extravasation in Dura mater and c-fos mRNA expression in trigeminal nucleus caudalis of rats.

Dural plasma extravasation produced by electrical stimulation of the trigeminal ganglion was measured in rats and the concomitant expression of c-fos mRNA produced in the trigeminal nucleus caudalis (NtV) was measured using in situ hybridization techniques. The non-peptide NK1 receptor selective antagonist CP-99,994 (1-3000 micrograms kg-1) and the 5HT1D receptor agonist sumatriptan (1-1000 micrograms kg-1) reduced dural plasma extravasation dose-dependently with ID50S of 52 micrograms kg-1 and 30 micrograms kg-1 respectively. CP-99,994 (1000 micrograms kg-1). a compound known to have good brain penetration, decreased c-fos mRNA expression in the NtV by 37 +/- 7% without disruption of the blood brain barrier (BBB). Sumatriptan (1000 micrograms kg-1), known to be poorly brain penetrant, had no significant effect on c-fos mRNA expression in the NtV unless the BBB was disrupted by infusion of a hyperosmolar mannitol solution after which sumatriptan decreased c-fos mRNA expression by 65 +/- 11%. The results suggest that brain penetrant NK1 receptor antagonists may have anti-migraine effects peripherally through blockade of dural extravasation and centrally by inhibition of nociceptive pathways. Furthermore the data indicates that the anti-migraine action of sumatriptan must be predominantly peripherally mediated, be it via inhibition of plasma extravasation or direct vasoconstriction, since it had little effect on the activation of neurones in the NtV unless the BBB was disrupted.

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.

Can the 5-HT2/1c agonist DOI cause differential sympatho-excitation in nerves supplying the heart in anaesthetized cats?

A comparison of the effects on sympathetic nerve activity to the heart of intravenous administration of the selective 5-HT2/1c agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) alone and in the presence of the peripherally acting 5-HT2/1c antagonist BW501C67 were made in alpha-chloralose anaesthetized cats. Activity in both cardiac sympathetic nerves running in the vagus and in both inferior cardiac nerves was simultaneously recorded. In addition renal and phrenic nerve activity, heart rate, arterial blood pressure, femoral arterial flow and tracheal pressure were also recorded. DOI evoked a rise in blood pressure and increased femoral arterial resistance in both groups of animals. In the BW501C67 pretreated animals, DOI had no effect on heart rate but caused a significant increase in all sympathetic nerve activities. In non-pretreated animals, however, the rise in blood pressure was associated with variable effects on sympathetic nerve activity, a significant rise only occurring in renal nerve activity. In these experiments DOI evoked a bradycardia. The variability in sympathetic nerve activity in the non-pretreated animals may have resulted from the rise in blood pressure evoking a baroreceptor-mediated central sympathoinhibition which would interfere with the central sympathoexcitatory effects of DOI. It is concluded that centrally, DOI will cause sympathoexcitation. In addition, intravenous DOI acting on 5-HT2 receptors on bronchial smooth muscle evokes bronchoconstriction as indicated by the very large rise in tracheal pressure in non-BW501C67-pretreated animals. If not controlled this severely compromises respiration leading to a large overestimate of the sympathoexcitatory effects of stimulation of central 5-HT2/1c receptors.

The non-peptide neurokinin1 receptor antagonist, RP 67580, blocks neurogenic plasma extravasation in the dura mater of rats.

A non-peptide neurokinin1 (NK1) receptor antagonist, RP 67580, that is selective for the rodent subtype of the NK1 receptor, dose-dependently reduced plasma extravasation in the dura mater produced by electrical stimulation of the trigeminal ganglion in rats, with an ID50 of 0.6 micrograms kg-1. Its enantiomer RP 68651 was some 400 fold less active. The results indicate that neurogenic plasma extravasation within the dura mater is NK1 receptor-mediated and suggest that NK1 receptor antagonists may have a role as antimigraine agents.

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.

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.

Actions of 8-OH-DPAT on sympathetic and respiratory drives, blood pressure and heart rate in the rabbit.

The effects of i.v. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on renal and cardiac sympathetic nerve activity, phrenic nerve activity, blood pressure and heart rate were investigated in urethane and pentobarbitone anaesthetised rabbits. Cumulative doses of 8-OH-DPAT (0.5-512 micrograms kg-1) in urethane anaesthetised rabbits, in which simultaneous recordings of renal and phrenic nerve activity were made, caused dose related decreases in mean blood pressure which were maximal after 32 micrograms kg-1 and dose related decreases in heart rate. Effects on renal and phrenic nerve activity were variable. A single dose of 8-OH-DPAT (32 micrograms kg-1) in either urethane or pentobarbitone anaesthetised rabbits, in which simultaneous recordings of cardiac as well as renal and phrenic nerve activity were made, caused significant falls in blood pressure, heart rate and cardiac nerve activity. Renal nerve activity showed no significant change whilst phrenic nerve activity tended to increase but this was only significant in the pentobarbitone anaesthetised rabbits. In all experiments the bradycardia produced by 8-OH-DPAT was unaltered by atropine. These results show that in the anaesthetised rabbit the hypotension and bradycardia caused by 8-OH-DPAT were associated with a decrease in cardiac nerve activity, increased central respiratory drive, and no change in renal nerve activity or vagal tone.

A GABA-mediated inhibition of neurones in the nucleus tractus solitarius of the cat that respond to electrical stimulation of the carotid sinus nerve.

Extracellular recordings were made from 39 single neurones in the nucleus tractus solitarius which were activated following electrical stimulation of the carotid sinus nerve in chloralose anaesthetised cats. Iontophoretic application of GABA abolished or markedly reduced the evoked activity of all neurones. The inhibitory effect of GABA could be antagonised by the simultaneous iontophoretic application of bicuculline. The iontophoretic application of bicuculline alone resulted in a significant increase in the evoked activity of eleven out of seventeen neurones. These results support the notion that a GABAergic inhibitory system, with the potential to modulate sinus nerve reflexes, acts within the nucleus tractus solitarius.

A gamma-aminobutyric-acid-mediated inhibition of neurones in the nucleus tractus solitarius of the cat.

1. Extracellular recordings were made from 123 synaptically activated neurones in the nucleus tractus solitarius of the chloralose-anaesthetized cat. 2. Ninety-one neurones were activated by electrical stimulation of cardiac or pulmonary vagal branches and thirty-two by stimulation of the aortic nerve. 3. Ionophoretic application of GABA abolished or markedly reduced the evoked or spontaneous activity of each neurone tested. These inhibitory effects were antagonized by the simultaneous ionophoretic application of bicuculline. 4. Glycine inhibited the evoked activity of 60% of neurones tested. This inhibition could be antagonized by the simultaneous application of strychnine. 5. Application of bicuculline alone increased the evoked or spontaneous activity of a large proportion of the neurones; strychnine alone had no significant effect on the evoked or spontaneous activity of the neurones. 6. These results are consistent with the hypothesis that GABA acts as an inhibitory transmitter substance within the nucleus tractus solitarius.