Blood pressure responses of conscious rats to intravenous administration of enkephalin derivatives (D-ala methionine and leucine enkephalinamide, and methionine and leucine enkephalinamide)

The present study was designed to determine the blood pressure (BP) responses of conscious rats given intravenous (IV) injections of enkephalin derivatives (D-ala²-methionine enkephalinamide, DAMEA; D-ala²-leucine enkephalinamide, DALEA; methionine enkephalinamide, MEA; leucine enkephalinamide, LEA) and the receptor mechanisms mediating the resultant change in BP. IV injection of 1.6–16.0 nmoles of DAMEA or DALEA caused a transient but potent decrease in mean arterial pressure (MAP) and mean heart rate (MHR). LEA and MEA (16.0 nmoles) given IV produced slight pressor responses, which were not associated with concomitant tachycardia whereas 48 nmoles of MEA elicited a hypotensive effect accompanied by a fall in MHR. Pretreatment studies whereby various receptor antagonists (naloxone, diprenorphine, phentolamine, D-L-propranolol or atropine) were given IV 5 min before subsequent IV administration of DAMEA, DALEA, MEA or LEA (16 nmoles) showed that naloxone, diprenorphine and atropine blocked the depressor and bradycardic effects of DALEA and DAMEA. Naloxone and phentolamine suppressed the pressor reponse of both MEA and LEA (16.0 nmoles) while diprenorphine blocked the rise in MAP to only MEA. The results show that DAMEA and DALEA mediate their depressor actions in conscious rats via a negative chronotropic effect through an interaction of muscarinic cholinergic receptors on the myocardium. It is suggested that the pressor response of MEA and LEA may be produced via an -receptor mediated effect on the peripheral vasculature to cause vasoconstriction.     

Hemodynamic responses of conscious rats following intrathecal injections of prodynorphin-derived opioids: independence of action of intrathecal arginine vasopressin

Experiments were conducted (i) to determine the hemodynamic (blood pressure and heart rate) responses of conscious rats following intrathecal (IT) administration of endogenous prodynorphin-derived opioids into the lower thoracic space, (ii) to identify the receptors involved in mediating their cardiovascular responses, and (iii) to reveal any possible hemodynamic interactions with the neuropeptide arginine vasopressin. Male Sprague-Dawley rats were surgically prepared with femoral arterial and venous catheters as well as a spinal catheter (into lower thoracic region, T9-T12). After recovery, hemodynamic responses were observed in conscious rats for 5-10 min after IT injections of artificial cerebrospinal fluid (CSF) solution, prodynorphin-derived opioids (dynorphin A, dynorphin B, dynorphin A (1-13), dynorphin A (1-10), alpha- and beta-neoendorphin, leucine enkephalin (LE), methionine enkephalin (ME), arginine vasopressin (AVP), or norepinephrine (NE)). IT injections of AVP (10 or 20 pmol), dynorphin A (1-13), or dynorphin A (10-20 nmol) caused pressor effects associated with a prolonged and significant bradycardia. Equimolar (20 nmol) concentrations of LE, ME, alpha- and beta-neoendorphin, and dynorphin A (1-10) caused no significant blood pressure or heart rate changes. Combined IT injections of dynorphin A (1-13) and AVP caused apparent additive pressor effects when compared with the same dose of either peptide given alone. IT infusion of the specific AVP-V1 antagonist d(CH2)5Tyr(Me)AVP before subsequent IT AVP, dynorphin A (1-13), or NE administration inhibited only the subsequent pressor responses to AVP. The kappa-opioid antagonist (Mr2266) infused IT blocked the pressor actions of subsequent dynorphin A administration and not AVP or NE.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pressor, tachycardic and feeding responses in conscious rats following i.c.v. administration of dynorphin. Central blockade by opiate and alpha 1-receptor antagonists

Experiments were designed to determine the hemodynamic responses of conscious, unrestrained rats given intracerebroventricular (i.c.v.) injections of dynorphin A-(1-13) and the possible central receptor mechanisms mediating those changes. Male Sprague-Dawley rats (300 gb. wt.) received i.c.v. injections (by gravity flow in a total volume of 3 or 5 microliter) of control solutions of sterile saline (SS) or dimethylsulfoxide (DMSO) or 1.5, 3.0 or 6.1 nmol of dynorphin A-(1-13). Blood pressure and heart rate changes were monitored over 2 h after administration; as well, feeding activity was visually assessed and scored over this period. Other groups of conscious rats were pretreated i.c.v. with equimolar doses (3.0-24.4 nmol) of specific receptor antagonists (naloxone HCl, phentolamine HCl, propranolol HCl, yohimbine HCl or prazosin HCl) 10 min before subsequent i.c.v. administration of SS or DMSO/SS or 6.1 nmol of dynorphin A-(1-13). I.c.v. injection of dynorphin A-(1-13) caused a dose-related pressor response, associated temporally with tachycardia. As well, dynorphin evoked feeding activity and some grooming, which occurred when the rats were hypertensive and tachycardic and decreased as heart rate and blood pressure returned to control levels. I.c.v. pretreatment studies indicated that naloxone HCl (12.2 nmol), phentolamine HCl (12.2 nmol) and prazosin HCl (6.1 nmol) blocked the pressor response, tachycardia as well as feeding activity of rats subsequently given dynorphin. The results suggest the pressor and tachycardic effects of conscious rats following i.c.v. dynorphin administration may, in part, be due to behavioral activation (feeding). As well, these data indicate that both opioid as well as alpha 1-adrenergic receptors within the CNS are involved in mediating the pressor, tachycardic and feeding responses of conscious rats given i.c.v. injections of dynorphin A.     

Effect of intrathecal administration of ET-1, ET-3 and ET(16–21) on blood pressure and micturition reflex in anesthetized rats

In urethane-anesthetized rats, intrathecal administration of endothelin-1 (ET-1), endothelin-3 (ET-3) (3–100 pmol/rat) or the C-terminal hexapeptide ET(16–21) (10–100 nmol/rat) dose-dependently increased mean blood pressure (MBP) and suppressed the supraspinal micturition reflex (SMR). ET(16–21), at 100 nmol, produced a pressor response comparable to that induced by ET-1 at 100 pmol.

Guanethidine and hexamethonium pretreatment significantly reduced the increase of MBP induced by ET-1 but was inactive in antagonizing inhibition of SMR. Neither naloxone nor adrenalectomy were effective in preventing the responses to ET-1.

The high degree of lethality (60–100%), observed with ET-1 (10–100 pmol), was not reduced by guanethidine, naloxone, adrenalectomy or by hexamethonium.

ET-3, at 100 pmol or 1 nmol, induced death in about 50% of cases. The symptoms before death were reduction of the respiratory rate followed by respiratory arrest.

These findings indicate that the pressor response to intrathecally-administered endothelins involves activation of sympathetic preganglionic elements; induction of secretion of catecholamines from adrenal glands was excluded.

Lethality and inhibition of SMR does not involve opioids, sympathetic activation or release of catecholamines from the adrenal glands.     

Adrenomedullary pressor responses to stimulation of the rostral hypothalamus in the rat: influence of adrenaline-induced vasodilation and reflex cardioinhibition

Electrical stimulation (100 Hz, 1 ms, 150 microA, 10 s) of the anterior hypothalamus in chloralose-anesthetized rats evoked a biphasic pressor response consisting of an initial sharp rise in arterial pressure at the onset of stimulation, followed by a second elevation after cessation of the stimulus. This response was accompanied by an increase in plasma noradrenaline and adrenaline levels. Peripheral sympathectomy with guanethidine selectively abolished the primary phase of the biphasic pressor response, while bilateral removal of the adrenal medulla eliminated only the secondary component. After alpha-adrenergic blockade with phentolamine, the primary phase of the stimulation-induced response was reduced while the secondary pressor component was blocked and replaced by a significant hypotension. The intravenous administration of sotalol enhanced the secondary pressor component without affecting the stimulation-induced plasma noradrenaline and adrenaline responses. After treatment with atropine, the secondary pressor effect was also potentiated, as the reflex bradycardia normally associated with the response was eliminated. A subsequent administration of sotalol in these rats further potentiated the secondary pressor component to stimulation. In rats treated with atropine and sotalol, the sympathetic vasomotor and the adrenomedullary pressor responses could be dissociated according to thresholds and stimulus frequency or current-response characteristics. The results suggest that in intact rats, adrenaline-induced vasodilation and reflex cardiac inhibition contribute to either reduce or mask the adrenomedullary component of the biphasic pressor response evoked by stimulation of the anterior hypothalamus. The study also raises the hypothesis of a dual regulation of both components of the sympathetic system in the anterior hypothalamic region.     

Pressor responses in rats following intravenous dynorphin A(1-13) administration are blocked by AVP-V1 receptor antagonism

Hemodynamic (blood pressure and heart rate) experiments were conducted in conscious and/or anesthetized male Sprague-Dawley (S.D.), heterozygous and homozygous Brattleboro rats given intravenous (iv) dynorphin A(1-13), arginine vasopressin (AVP), norepinephrine (HCl, (NE) or sterile saline before and 10 min after an iv bolus injection of a specific receptor antagonist. These receptor blockers (kappa receptor antagonist Mr2266, alpha adrenoceptor antagonist phentolamine HCl or the AVP-V1 receptor antagonist d(CH2)5Tyr-(Me)AVP were given in equimolar concentrations (15 nmol/kg iv). In all conscious S.D. groups, iv injection of AVP (60 pmol/kg), NE (12.5 nmol/kg) and dynorphin A(1-13) (60 nmol/kg) evoked significant increases in mean arterial pressure (MAP) associated with concomitant bradycardia. The hemodynamic responses to 'both' AVP and dynorphin A(1-13) were blocked if given subsequent to AVP-V1 administration but not following phentolamine or Mr2266 pretreatment. The pressor and bradycardic responses of conscious heterozygous and homozygous Brattleboro rats after iv AVP or dynorphin again were only blocked by the AVP-V1 receptor antagonist. Anesthetized heterozygous and homozygous Brattleboro rats again showed pressor responses following iv AVP, NE or dynorphin A(1-13) but with slight or no associated bradycardia. The rise in blood pressure with AVP 'and' dynorphin A(1-13) in these groups also was only blocked by the d(CH2)5Tyr(Me)AVP antagonist. The results indicate that the pressor responses of rats given intravenous dynorphin A(1-13) involve the interaction of AVP-V1 receptors and suggest a functional interaction of these two neuropeptides in the modulation of vascular tone.     

Intravenous morphine infusion (IMF) to drug-naive, conscious rats evokes bradycardic, hypotensive effects, but pressor actions are elicited after IMF to rats previously given morphine

Hemodynamic (blood pressure and heart rate) responses of conscious drug-naive rats were studied following intravenous (i.v.) infusion of sterile saline, morphine sulphate, and then naloxone hydrochloride, as well as of other groups previously injected with morphine sulphate. Those groups chronically given morphine sulphate received twice daily injections of morphine sulphate (5 mg/kg, s.c. per injection) for 3 or 6 days before testing with the i.v. infusion of morphine sulphate. Drugs were infused (135 microL/min) through an indwelling femoral venous catheter via a Harvard infusion pump, and blood pressure was recorded from the abdominal aorta via a femoral arterial catheter. Other pretreatment studies were done to determine the receptor mechanisms mediating the blood pressure responses of drug-naive and chronic morphine-treated rats, whereby equimolar doses (0.32 mumol) of specific receptor antagonists were given as a bolus i.v. injection 5 min after saline but before subsequent infusion with morphine sulphate. Intravenous infusion of morphine sulphate (7.5 mg/kg total over 15 min) to drug-native rats caused a transient but precipitous fall in mean arterial pressure and mean heart rate with an associated rise in mean pulse pressure; these effects were blocked in other groups pretreated with atropine. Interestingly, however, rats chronically injected with morphine sulphate for 3 days previously evoked a transient pressor response when subsequently infused i.v. with morphine sulphate, actions that were blocked in other groups when pretreated i.v. with 0.32 mumol of phentolamine, yohimbine, prazosin, or guanethidine. A greater and persistent pressor response occurred following morphine infusion to groups of rats previously injected over 6 days with morphine sulphate, which was associated with tachycardia during the later stages of the 15-min morphine sulphate infusion period. The prolonged pressor and tachycardic responses of this 6-day chronically injected group were completely blocked in another group pretreated i.v. with both phentolamine and propranolol (0.32 mumol). The results suggest that morphine sulphate infusion to conscious, drug-naive rats evokes classical hypotensive effects due to decreases in mean heart rate caused by activation of parasympathetic vagal activity. With 3 or 6 days of chronic morphine sulphate administration beforehand, subsequent i.v. infusion of morphine sulphate evoked pressor actions felt to be caused by a progressive activation of the sympathetic nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differential cardiovascular effects mediated by mu and kappa opiate receptors in hindbrain nuclei

To further investigate the role of opioid peptides and specific opiate receptor subtypes in central cardiovascular regulation by hindbrain nuclei, mu (D-Ala²,MePhe⁴,Gly-ol⁵ enkephalin, DAGO), delta (D-Ala²,D-Leu⁵ enkephalin, DADL) or kappa (MRZ 2549) agonists were microinjected into hindbrain nuclei of spontaneously or artificially respired, pentobarbital-anesthetized rats. In the nucleus tractus solitarius (NTS), DAGO and DADL (0.3 nmol) elicited pressor responses and tachycardia. MRZ (3.0–16 nmol) depressed blood pressure in spontaneously breathing rats, but accelerated heart rate in artificially ventilated animals. Blood pressure and heart rate of spontaneously breathing animals were not altered following nucleus ambiguus (NA) injection of DAGO or DADL (0.3 nmol), but were elevated in artificially respired animals; MRZ (3.0–10 nmol) injected into the NA depressed blood pressure in both groups. These data suggest that in the absence of respiratory depression, NTS and NA mu receptors mediate pressor responses and tachycardia; kappa receptors in the NA mediate a decrease in blood pressure but cardioacceleration in the NTS.     

The effects of pertussis toxin-treatment on integrated vasoactive response of vascular system in spontaneously hypertensive rats

We investigated the effect of pertussis toxin (PTX) on hypotensive response induced by acetylcholine (ACh) and bradykinin (BK) and on noradrenaline (NA)-induced pressor response in spontaneously hypertensive rats (SHR). Fifteen-week-old Wistar rats and age-matched SHR were used. Half of SHR received PTX (10 microg/kg/i.v.) and the experiments were performed 48 h later. After the anesthesia the right carotid artery was cannulated in order to record blood pressure (BP). The hypotensive response to ACh was enhanced in SHR compared to Wistar rats. After pretreatment of SHR with PTX the hypotensive response to ACh was reduced compared to untreated SHR and it was also diminished in comparison to Wistar rats. Similarly, the hypotensive response to BK was also decreased after PTX pretreatment. The pressor response to NA was increased in SHR compared to Wistar rats. NA-induced pressor response was considerably decreased after PTX pretreatment compared to untreated SHR. In conclusion, the enhancement of hypotensive and pressor responses in SHR was abolished after PTX pretreatment. Our results suggested that the activation of PTX-sensitive inhibitory G(i) proteins is involved in the regulation of integrated vasoactive responses in SHR and PTX pretreatment could be effectively used for modification of BP regulation in this type of experimental hypertension.     

Hypothalamic alpha-adrenergic blockade modifies drinking and blood pressure responses to central angiotensin II in conscious rats

These experiments investigated in the awake rat the involvement of noradrenergic projections to the rostral hypothalamus in the drinking and pressor responses elicited by intracerebroventricular (i.c.v.) injections of 25 ng of angiotensin II. Phentolamine mesylate in doses of 2.5-125 micrograms injected into the rostral hypothalamus produced a dose-dependent depression of both the drinking and pressor responses elicited by i.c.v. administration of angiotensin II. A paradoxical increase in heart rate was associated with a decrease in pressor responses with increasing doses of phentolamine. This response was due to tissue injections, since pretreatment by injecting 12.5 micrograms of phentolamine into the ventricle did not block either the cardiovascular or drinking responses to i.c.v. injections of angiotensin II. Yohimbine (0.33-3.3 micrograms), DL-propranolol (25 micrograms), and atenolol (25 micrograms) did not, but prazosin (0.7 microgram) did significantly alter the pressor responses. Although yohimbine also was without effect on drinking, prazosin reduced the drinking responses. These results suggest that alpha 1-adrenergic receptors in the rostral hypothalamus are involved in the control of both the drinking and pressor responses elicited by i.c.v. injections of angiotensin II. In the case of propranolol and atenolol, beta-adrenergic receptors altered only the drinking response in a nonspecific manner by eliciting competing behaviors. Whether they are involved in modifying the drinking response only remains to be demonstrated.     

Cardiovascular effects of neuropeptide Y in the nucleus tractus solitarius of rats: relationship with noradrenaline and vasopressin

The central haemodynamic effects of neuropeptide Y (NPY), both alone and together with either noradrenaline (NA) or vasopressin (AVP), have been investigated by microinjecting synthetic peptide into the nucleus tractus solitarius (NTS) of anaesthetized rats. NPY alone elicited dose-dependent changes in blood pressure (BP) and heart rate (HR); 470 fmol inducing a pressor response, and 4.7 pmol a fall in BP. The hypotensive response to 20 nmol NA was significantly modified by both simultaneous and prior injection of an ineffective dose (47 fmol) of NPY. Prior injection of a similar dose of NPY also modified the NTS pressor effect of 10 ng AVP. A relationship between the action of AVP and NPY in the NTS was further indicated by the finding that prior injection of an ineffective dose of AVP (1 ng) reduced the hypotensive response to 4.7 pmol NPY, and by the demonstration of contrasting effects of 4.7 pmol NPY in AVP-deficient Brattleboro rats compared to parent strain LE rats. These results, taken together with the recent localization of NPY-like immunoreactivity in the NTS, suggest a role for NPY in central cardiovascular control. In addition, NPY has been shown to exhibit functional interactions with both an amine neurotransmitter and a neuropeptide present in the NTS of rats.     

Cardiovascular effects of the essential oil of Croton zehntneri leaves and its main constituents, anethole and estragole, in normotensive conscious rats

Cardiovascular effects of the essential oil of Croton zehntneri (EOCZ) were investigated in conscious rats. In these preparations, intravenous (i.v.) injections of EOCZ (1-20 mg kg(-1)) and its main constituents anethole and estragole (both at 1-10 mg kg(-1)) elicited brief and dose-dependent hypotension and bradycardia (phase I) that were followed by a significant pressor effect associated with a delayed bradycardia (phase II). The initial hypotension and bradycardia (phase I) of EOCZ were unchanged by atenolol (1.5 mg kg(-1), i.v.) or L-NAME (20 mg kg(-1), i.v.) pretreatment, but were respectively reversed into pressor and tachycardic effects by methylatropine (1 mg kg(-1), i.v.) pretreatment. The subsequent pressor effect and the delayed bradycardia (phase II) remained unaffected by atenolol, but were abolished by L-NAME and methylatropine pretreatment, respectively. In rat endothelium-containing aorta preparations, the vasoconstrictor responses to phenylephrine were enhanced and reduced, respectively, by the lower (1-30 microg mL(-1)) and higher (300-1000 microg mL(-1)) concentrations of EOCZ. Only the enhancement of phenylephrine-induced contraction was abolished by either the incubation with L-NAME (50 microM) or in the absence of the endothelium. These data show, for the first time, that i.v. administration EOCZ induces an initial hypotension followed by a pressor response, two effects that appear mainly attributed to the actions of anethole and estragole. The EOCZ-induced hypotension (phase I) is mediated by a cholinergic mechanism and seems to result mainly from the concomitant bradycardia. The pressor response of EOCZ (phase II) seems to be caused by an indirect vasoconstrictive action of EOCZ most likely through inhibition of endothelial nitric oxide production.     

Effect of phentolamine on the reflex increase in arterial pressure

Experiments were carried out in anaesthetized and curarized cats to study the effects of the alpha-blocker phentolamine on arterial pressor response to stimulation of group I afferent fibers from gastrocnemius-soleus muscles. It has been found that increasing doses of phentolamine given i.v. cause a decreasing pressor response until complete disappearance occurs at 2.5 mg/Kg. It is concluded that pressor response is present only when the adrenergic control system is effective and, therefore, it is due mainly to an increase in peripheral vascular resistances.     

2-Deoxyglucose as a tool for analyzing the humoral component of the cardiovascular reaction to stress

Blood pressure (BP), heart rate (HR), adrenaline (A) and noradrenaline (NA) effects of 2-desoxyglucose (500 mg/kg i.v.) were studied in conscious chronically instrumented wistar rats. A and NA contents in the blood were estimated by HPLC with electrochemical detection. In 15-40 minutes after 2-DG administration it was 13-16-fold increase in A content, 1.7-1.9-fold increase in NA content. At the same time BP fell by 5 mm Hg and HR fell by 100 beats/min. Atropine blocked bradycardia but had no effect on BP. It is concluded that high levels of endogenous A during stress are not responsible for cardiovascular responses usually observed.     

Adrenomedullary secretory response to midbrain stimulation in rat: effects of depletion of brain catecholamines or serotonin

Electrical stimulation of the periaqueductal gray substance (PAG) of the rostral midbrain of the rat produced biphasic or monophasic pressor responses depending on the duration of the stimulus train. Marked increases in plasma noradrenaline (NA) and adrenaline (A) levels accompanied the pressor responses, indicating the participation of the adrenal medulla. Depletion of central catecholamines (CA) by intraventricular administration of 6-hydroxydopamine (6-OHDA) did not affect the primary vasomotor component but markedly depleted adrenal CA levels and attenuated the adrenomedullary component of the response to brain stimulation. The intraperitoneal administration of p-chlorophenylalanine (pCPA) not only depleted brain serotonin (5-HT) levels but also reduced brain CA levels significantly. The adrenaline (A) levels were reduced in the adrenal glands of these rats and the adrenal secretory response to brain stimulation was attenuated. In contrast, the selective destruction of central 5-HT neurons by intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) in rats pretreated with desmethylimipramine (DMI) did not influence either the pressor nor the plasma CA responses to brain stimulation. Furthermore, the adrenal glands of these rats were normal. The results suggest that: (i) the central catecholaminergic neurons play an important role in the regulation of the adrenal glands but are not essential for the activation of the sympathetic vasoconstrictor fiber system: (ii) the pressor and plasma CA responses to PAG stimulation are not dependent on the central serotonergic system.     

Central and peripheral mechanisms underlying gastric distention inhibitory reflex responses in hypercapnic-acidotic rats

We have observed that in chloralose-anesthetized animals, gastric distension (GD) typically increases blood pressure (BP) under normoxic normocapnic conditions. However, we recently noted repeatable decreases in BP and heart rate (HR) in hypercapnic-acidotic rats in response to GD. The neural pathways, central processing, and autonomic effector mechanisms involved in this cardiovascular reflex response are unknown. We hypothesized that GD-induced decrease in BP and HR reflex responses are mediated during both withdrawal of sympathetic tone and increased parasympathetic activity, involving the rostral (rVLM) and caudal ventrolateral medulla (cVLM) and the nucleus ambiguus (NA). Rats anesthetized with ketamine and xylazine or α-chloralose were ventilated and monitored for HR and BP changes. The extent of cardiovascular inhibition was related to the extent of hypercapnia and acidosis. Repeated GD with both anesthetics induced consistent falls in BP and HR. The hemodynamic inhibitory response was reduced after blockade of the celiac ganglia or the intraabdominal vagal nerves with lidocaine, suggesting that the decreased BP and HR responses were mediated by both sympathetic and parasympathetic afferents. Blockade of the NA decreased the bradycardia response. Microinjection of kainic acid into the cVLM reduced the inhibitory BP response, whereas depolarization blockade of the rVLM decreased both BP and HR inhibitory responses. Blockade of GABA(A) receptors in the rVLM also reduced the BP and HR reflex responses. Atropine methyl bromide completely blocked the reflex bradycardia, and atenolol blocked the negative chronotropic response. Finally, α(1)-adrenergic blockade with prazosin reversed the depressor. Thus, in the setting of hypercapnic-acidosis, a sympathoinhibitory cardiovascular response is mediated, in part, by splanchnic nerves and is processed through the rVLM and cVLM. Additionally, a vagal excitatory reflex, which involves the NA, facilitates the GD-induced decreases in BP and HR responses. Efferent chronotropic responses involve both increased parasympathetic and reduced sympathetic activity, whereas the decrease in BP is mediated by reduced α-adrenergic tone.     

A comparison of the central and peripheral antimuscarinic effects of atropine and methylatropine injected systemically and into the cerebral ventricles

We compared the relative abilities of atropine sulfate and methylatropine, injected i.v. and into the cerebral ventricles (icv), to block pharmacological responses mediated through central and peripheral muscarinic receptors. The hypotensive response to i.v. injection of acetylcholine (peripheral muscarinic receptors) was inhibited 50% by i.v. injection of 14.3 nmol (5.5 micrograms)/kg methylatropine and 147.8n molar equivalents (50 micrograms)/kg atropine sulfate. A similar degree of inhibition followed icv injection of 49.4 nmol/kg methylatropine and 384.2 nmol equivalents/kg atropine sulfate, indicating significant leakage out of the ventricular space. The pressor response to icv injection of neostigmine (central muscarinic receptors) also was inhibited more effectively by icv methylatropine than by atropine sulfate. Methylatropine was not effective in blocking central muscarinic receptors when injected i.v.     

Modification by physostigmine of the cardiovascular responses to intracerebroventricular administration of 5-hydroxytryptamine in rats

In rats the effect of inhibition of the brain cholinesterase activity on the pressor and heart rate responses to 5-hydroxytryptamine (5-HT), administered into the lateral cerebral ventricle (l.c.v.) was examined. After administration of physostigmine (twice in a small dose of 2.5 micrograms l.c.v., 20 and 15 min before the second injection of 5-HT), the pressor effect of 5-HT (5 micrograms) was strongly reduced or almost abolished, its pure tachycardia was reduced or reversed into a bradycardia and its pure bradycardia was diminished or reversed into a tachycardia. The type of the cardiovascular response to ACh (5 micrograms l.c.v., 20 min after the second administration of 5-HT) indicates that the modification of the cardiovascular response to 5-HT was accompanied by inhibition of the brain cholinesterase activity. Thus, it seems that a functionally competent cholinesterase in the brain is necessary for the generation of the 5-HT-induced pressor response. The present experiments provide further evidence that there is a cholinergic link in the pathway by which serotonergic mechanisms in the preoptic-anterior hypothalamic area rise blood pressure and support the idea that the same link exists in the pathway(s) mediating the heart rate responses to intracerebroventricular administration of 5-HT.     

Cardioselective profile of AF-DX 116, a muscarine M2 receptor antagonist

AF-DX 116 (see chemical name below) is a competitive antagonist of muscarine receptors in peripheral organs. In contrast to pirenzepine, its behaviour in functional experiments indicates selectivity for the M2 muscarinic subtype. In pithed rats AF-DX 116 inhibits vagally-induced bradycardia, an M2 response, (ED50 32 micrograms/kg i.v.) in preference to the M1-mediated pressor response to McN-A-343 (ED50 211 micrograms/kg i.v.). AF-DX 116 further discriminates among M2 receptors, showing a high affinity for the cardiac muscarine receptors. In isolated preparations, AF-DX 116 has a tenfold higher affinity for the muscarine receptors of the heart (pA2 7.33) than for those in smooth muscles (pA2 6.39-6.44). The same profile appears from animal studies, where the compound is a more potent antagonist of either endogenously or exogenously activated cardiac muscarine responses as compared to vascular, smooth muscle or secretory responses. In general, the ratios of potencies (ED50) observed in cardiac vs. other muscarine mediated functions ranged between 30 and 50. Atropine showed no discrimination, inhibiting all muscarine responses in the same range of doses. In the conscious dog intravenous AF-DX 116 increased basal heart rate, and completely reversed the reflex bradycardia induced by clonidine. Tachycardia was dose-related (ED50 79 micrograms/kg i.v.), and occurred independently of background sympathetic tone. AF-DX 116 clearly distinguishes between M1- and M2-mediated responses; it also emphasizes the long-recognized heterogeneity among the peripheral M2 subtypes. AF-DX 116, for its pronounced cardioselectivity, may have a therapeutic potential in the treatment of sinus bradycardia.     

Chlordiazepoxide inhibition of reflex vagal bradycardia in the cat

The effect of chlordiazepoxide (CDZ) on phenylephrine-induced reflex vagal bradycardia was studied in cats anesthetized with chloralose. The sympathetic component of the reflex was eliminated by either pretreating the animals with propranolol (1 mg/kg, i.v.) or sectioning the spinal cord. In animals pretreated with propranolol, CDZ (3, 10 and 20 mg/kg, i.v.) produced a dose-related inhibition of phenylephrine-induced bradycardia. These doses of CDZ had no significant effect on phenylephrine-induced pressor responses. Similar results were obtained with CDZ in animals with spinal cords transected. Chlordiazepoxide did not prevent bradycardia evoked by electrical stimulation of the peripheral cut-end of the right vagus nerve. These results indicate that CDZ can inhibit reflex vagal bradycardia and that the inhibition involves a central action of the drug.