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

The serotonin 2A receptor agonist 25CN-NBOH increases murine heart rate and neck-arterial blood flow in a temperature-dependent manner

BACKGROUND

Serotonin 2A receptors, the molecular target of psychedelics, are expressed by neuronal and vascular cells, both of which might contribute to brain haemodynamic characteristics for the psychedelic state.

AIM

Aiming for a systemic understanding of psychedelic vasoactivity, here we investigated the effect of N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine - a new-generation agonist with superior serotonin 2A receptor selectivity - on brain-supplying neck-arterial blood flow.

METHODS

We recorded core body temperature and employed non-invasive, collar-sensor based pulse oximetry in anesthetised mice to extract parameters of local blood perfusion, oxygen saturation, heart and respiration rate. Hypothesising an overlap between serotonergic pulse- and thermoregulation, recordings were done under physiological and elevated pad temperatures.

RESULTS

N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine (1.5 mg/kg, subcutaneous) significantly increased the frequency of heart beats accompanied by a slight elevation of neck-arterial blood flow. Increasing the animal-supporting heat-pad temperature from 37°C to 41°C enhanced the drug's effect on blood flow while counteracting tachycardia. Additionally, N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine promoted bradypnea, which, like tachycardia, quickly reversed at the elevated pad temperature. The interrelatedness of N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine's respiro-cardiovascular effects and thermoregulation was further corroborated by the drug selectively increasing the core body temperature at the elevated pad temperature. Arterial oxygen saturation was not affected by N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine at either temperature.

CONCLUSIONS

Our findings imply that selective serotonin 2A receptor activation modulates systemic cardiovascular functioning in orchestration with thermoregulation and with immediate relevance to brain-imminent neck (most likely carotid) arteries. As carotid branching is a critical last hub to channel cardiovascular output to or away from the brain, our results might have implications for the brain haemodynamics associated with psychedelia.

Central cardiovascular regulation and 5-hydroxytryptamine receptors

Evidence is provided to support the view that central 5-HT(1A) and 5-HT(2) receptors are the major receptor subtypes important in cardiovascular regulation. Data are also provided to implicate 5-HT(1B/1D/1F) receptors in central cardiovascular regulation. Activation of 5-HT(2) receptors generally causes sympathoexcitation and a rise in blood pressure and this is mainly mediated by 5-HT(2A) receptors. However, presympathetic vasomotor neurones located in the hindbrain (RVLM), controlling sympathetic outflow to the heart, are not activated in the same way as other presympathetic vasomotor neurones, although activation of 5-HT(2) receptors located in the midbrain can activate sympathetic outflow to the heart. Furthermore, at least in the rat, these midbrain 5-HT(2A) receptors are also responsible for the release of vasopressin by activation of a central angiotensinergic pathway. The ability of vasopressin directly and/or indirectly to modify renal sympathetic outflow involves the activation of central 5-HT(2B) receptors, which in turn, when activated via the i.c.v. route, can cause selective renal sympathoexcitation. Evidence is also provided which indicates that the reflex control of parasympathetic outflow to the heart and to other organs involves central 5-HT(1A) receptors located in the vicinity of these preganglionic vagal neurones. Finally, 5-HT(3) receptors are implicated in the afferent regulation of central sympathetic and parasympathetic tone.

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.

Activation of bulbospinal serotonergic neurons during cold exposure

In a four-part study, we expand on our previous report that bulbospinal serotonin (5HT) neuronal activation occurs with 24 h of cold exposure. To characterize temporal aspects, rats were exposed to 3°C or were maintained at 22°C for 2, 8, 48, or 96 h (experiment 1) or for 15, 30, or 60 min (experiment 2). To ensure that cold-induced changes in 5HT activity were not due to disturbances in diurnal pattern, rats in experiment 3 were exposed to cold (8 h) during the dark cycle. To explore the hypothesis that cold-induced 5HT activation is part of a broad metabolic response that includes activation of the sympathetic nervous system, metabolically impaired (hypothyroid) rats were exposed to 8°C in experiment 4. Significant increments in 5-hydroxyindoleacetic acid (5HIAA) concentration were evident by 60 min of cold exposure and existed at all later time points measured. These findings were most robust in spinal cord and rostral brainstem. Activation in spinal cord was also found when rats were exposed to 8 h of cold during the dark cycle, the active period for rats. In experiment 4, hypothyroid rats exhibited significantly greater norepinephrine excretion compared with control rats exposed to the same cold stimulus; this finding was accompanied by significantly greater increments in 5HIAA concentration in rostral brainstem and spinal cord of hypothyroid rats. In addition, significant elevations in tryptophan concentration were noted throughout the brainstem and spinal cord of cold-exposed, hypothyroid rats relative to room temperature, hypothyroid rats. This finding suggested that elevations in 5HIAA concentration in these rats were due to increases in precursor availability. The implications of these findings relative to autonomic and metabolic control are discussed.Key words: serotonin, spinal cord, raphe, cold, sympathetic nervous system.

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.

Cardiac neurones of autonomic ganglia

The properties of the postganglionic sympathetic neurones supplying the heart and arising in the stellate and adjacent paravertebral ganglia of various species are discussed with respect to their location, morphology, synaptic input and membrane characteristics. Results from our laboratory on the morphology of rat stellate neurones projecting to the heart were obtained either by intracellular injection of hexammine cobaltic (III) chloride or by retrograde labelling of cells using cobalt-lysine complex. Intracellular recordings were made from cells using electrodes filled either with potassium chloride plus hexammine cobaltic chloride or potassium acetate. Neurones which projected axons into cardiac nerve branches arising from the stellate ganglion were termed putative cardiac neurones, because of the possibility that some supply pulmonary targets. Putative cardiac neurones had unbranched axons and were ovoid or polygonal in shape, but showed considerable variation in soma size and in the complexity of dendritic trees. The mean two-dimensional surface area was 463 microns2 and the mean number of primary dendrites was seven. Other studies have found that the morphology of rat stellate ganglion neurones is similar to that of superior cervical ganglion cells. However, in strains of rat displaying spontaneous hypertension, dendritic length may be increased. Histochemical studies do not, as yet, seem to have demonstrated a distinctive neurochemical profile for stellate cardiac neurones, but various types of peptide-containing intraganglionic nerve fibres have been identified in the guinea pig. In our electrophysiological studies, putative cardiac neurones were found to receive a complex presynaptic input arising from the caudal sympathetic trunk and from T1 and T2 thoracic rami. In addition, 16% of cardiac neurones received a synaptic input from the cardiac nerve. The properties of postganglionic parasympathetic neurones distributed in the cardiac plexus and termed intrinsic cardiac neurones are discussed, including the results of studies on cultures of these neurones.

Spinal 5-HT2 receptor-mediated facilitation of pudendal nerve reflexes in the anaesthetized cat

1. 5-Hydroxytryptamine (5-HT) is intimately associated with central sympathetic and somatic control of the lower urinary tract. The sympathetic and somatic innervation of the lower urinary tract is conveyed through efferent axons of the hypogastric and pudendal nerves, respectively. 2. The present study examined the effects of 2,5-dimethoxy-4-iodophenylisopropylamine (DOI), a 5-HT2 receptor subtype-selective agonist, on evoked potentials recorded from the central ends of the hypogastric and pudendal nerves in response to electrical stimulation of afferent fibres in the pelvic and pudendal nerves, respectively. Various spinalization paradigms were employed to localize the site of action. All cats were pretreated with xylamidine (1 mg kg-1), a peripherally-restricted 5-HT2 receptor antagonist. 3. In acute spinal cats, DOI (0.01-3 mg kg-1, i.v.) reliably produced dose-dependent increases in the pudendal nerve reflex (to 228 +/- 31% of control). These increases were reversed by the 5-HT2 receptor-selective antagonist, LY53857 (0.3-3 mg kg-1, i.v.). On the other hand, in spinally-intact cats, DOI produced no significant changes in the pudendal reflex. However, within minutes of spinalization of DOI-pretreated cats, a marked increase (to 221 +/- 16% of control) in the pudendal reflex was observed which could be reversed by LY53857. No significant effects were observed on hypogastric reflexes in either acute spinal or spinally-intact cats following DOI administration. No effects were seen in either spinally-intact or acute spinal animals when LY53857 was administered as the initial drug. 4. These results indicate that activation of spinal 5-HT2 receptors facilitates pudendal reflexes. In spinally-intact cats, it is hypothesized that DOI activates supraspinal pathways that mediate inhibition of the pudendal reflexes and counteracts the facilitatory effects of spinal 5-HT2 receptor activation.

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.

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.

Systemic and regional haemodynamic effects of 1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI) and alpha-methyl-5-HT, in the anaesthetised rat

The present experiment was performed to investigate the haemodynamic effects of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT) in the anaesthetised normotensive rat. DOI (1-300 micrograms/kg i.v.) increased mean arterial pressure (MAP), total peripheral resistance (TPR) and decreased cardiac output (CO) and heart rate (HR). DOI increased all vascular resistances investigated (hindquarters, mesenteric and renal). Alpha-methyl-5-HT (10-300 micrograms/kg i.v.) dose-dependently increased MAP, TPR, all regional vascular resistances and decreased CO and HR. The bradycardia induced by alpha-methyl-5-HT was suppressed by bivagotomy. Both DOI and alpha-methyl-5-HT were more effective on renal vascular bed than hindquarters and mesenteric vascular beds. The effects of DOI and alpha-methyl-5-HT were antagonised by spiperone (10 or 100 micrograms/kg i.v.) and LY 53857 (10 micrograms/kg i.v.). Intracerebroventricular administration of DOI (100 micrograms/kg) increased MAP, TPR, regional vascular resistances and did not change HR and CO. Pretreatment with xylamidine (10 micrograms/kg i.v.), a selective peripheral 5-HT2 receptor antagonist, blocked i.v. and i.c.v. effects of DOI. These results suggest that: 1) the increase in MAP induced by DOI and alpha-methyl-5-HT is due to an increase in TPR. All regional vascular beds and in particular the renal vascular bed participate in the increase of TPR. 2) Peripheral--and may be--central 5-HT2 receptors seem to be implicated in the control of regional vascular resistances. 3) Cardiac effects of alpha-methyl-5-HT are baroreflexly-mediated whereas those of DOI are--at least in part--centrally mediated.