Cardiovascular role of the major noradrenergic cell groups in the rabbit: analysis based on 6-hydroxydopamine-induced transmitter release

We examined the role of the noradrenergic (NA) neurons of the A1, A2, A1 + A2, A5 and A6 + A7 regions on mean arterial pressure (MAP) and heart rate (HR), by comparing the acute responses of chronically lesioned and sham-operated rabbits to intracisternal 6-hydroxydopamine (6-OHDA, 600 micrograms/kg) which induces central release of transmitter. We studied rabbits (1) with intact arterial baroreceptors (non-denervated) and (2) after sino-aortic denervation (SAD). The acute transmitter release response consisted of an early fall in MAP (observed in SAD rabbits) and a late rise in MAP (observed in both non-denervated and SAD rabbits). Medullary lesions had no effect on either MAP component, but A5 and A6 + A7 lesions attenuated both pressor and depressor responses. Normally the transmitter release-induced MAP responses are modified by baroreceptor feedback. The 6-OHDA-induced HR changes were vagal in non-denervated rabbits and were sympathetically mediated in SAD rabbits. In non-denervated rabbits, A1, A2 and A1 + A2 lesions affected mainly the early vagal component, whilst A6 + A7 lesions affected the late vagal component. In SAD rabbits the early bradycardia was due to sympathetic inhibition and the late tachycardia due to sympathetic excitation; A1 + A2 lesions and A5 lesions attenuated the sympathetic bradycardia. We conclude that the various components of the MAP and HR responses are mediated through distinctive NA pathways; the deficits of a given lesion could be due to either to loss of NA cell bodies or of NA fibers of passage.

Vagal and sympathetic components of the heart rate range and gain of the baroreceptor-heart rate reflex in conscious rats

Previous studies in conscious rats have examined the relationship between mean arterial pressure (MAP) and heart rate (HR) only during relatively small or unidirectional changes in blood pressure. We have now examined this relationship more fully in conscious Sprague-Dawley rats using graded bolus i.v. doses of phenylephrine and nitroprusside to alter MAP over a range of 60-160 mm Hg with a view to determining the contribution made by the vagus and sympathetic (after atenolol or methylatropine, respectively). In 25 rats the relationship of HR to MAP followed a sigmoidal rather than a linear function (P less than 0.001) with clear upper and lower HR plateaus. The HR range estimated from a logistic equation was 217 +/- 7 b/min while the average gain between the inflection points was 4.1 +/- 0.2 b/min/mm Hg (1.8 times greater than from a linear fit). The vagus makes a greater contribution to the HR range than the sympathetic (61 vs 39%, respectively) while the converse is the case for the gain (46% vagus, 63% sympathetic). In the presence of both blocking drugs, changes to HR were less than 7% of control. These results suggest that the baroreceptor-HR reflex in the conscious rat is best characterised by a sigmoid curve with approximately equal contributions from both the cardiac vagus and the sympathetic nerves.

Lesions of A1 noradrenergic cells affect AVP release and heart rate during hemorrhage

The role of A1 noradrenergic cells of the ventrolateral medulla in the changes in mean arterial pressure (MAP), heart rate (HR), and plasma arginine vasopressin (AVP) after slow continuous hemorrhage (2% blood vol/min up to 35%) was examined by comparing responses in conscious rabbits before and 3 wk after a sham operation or A1 lesions. In the control experiments, MAP fell minimally up to the withdrawal of 20% of blood volume after which it fell abruptly to 20-30 mmHg below control by the 35% level. Plasma AVP increased nonlinearly during progressive hemorrhage with significant increases occurring only after 25% of blood volume was removed. In contrast, HR increased linearly after the onset of bleeding. After A1 lesions, which destroyed 84% (range 80-94%) of the noradrenergic cells, the amount of AVP released and the tachycardia during hemorrhage were reduced by 83 and 61%, respectively (P less than 0.005), but the fall in MAP was minimally affected. Basal values of MAP, HR, or plasma AVP were not affected by the lesions. These results suggest that during hemorrhage in conscious rabbits A1 noradrenergic neurons are important for the secretion of AVP and the reflex tachycardia but play little role in the maintenance of blood pressure.

Cardiovascular role of A1 catecholaminergic neurons in the rabbit. Effect of chronic lesions on responses to methyldopa, clonidine and 6-OHDA induced transmitter release

We confirmed the findings of previous investigators that bilateral anodal lesions of the A1 region were associated with hypertension, bradycardia, pulmonary edema and a high mortality. All these sequelae (except the bradycardia) no longer occurred after cathodal lesions and these were therefore used to investigate the role of the catecholaminergic (CA) neurons of the A1 region in circulatory regulation. Conscious rabbits were studied 2-4 weeks after A1 lesions or sham-operation, when resting mean arterial pressure (MAP) and heart rate (HR) were closely similar in both groups. We tested for differences in MAP and HR responses between lesioned and sham-operated groups: to intracisternal (i.c.) alpha-methyldopa (MD) and to clonidine; and to the acute effects of i.c. 6-hydroxydopamine (6-OHDA) which elicits central CA release. Since these tests depend on the integrity of the central CA neurons, response differences between lesioned and sham-operated groups denote participation by the CA neurons of the A1 region in the central circulatory pathways. The bradycardia responses in the above tests were all smaller in lesioned than sham-operated rabbits, but there were no differences in MAP responses. Electrical stimulation of the region under alfathesin anaesthesia produced depressor responses at low frequencies and pressor responses at high frequencies. From the results in conscious rabbits CA neurons of the A1 region mainly influence the pathways regulating HR, rather than blood pressure. The changes in MAP during electrical stimulation are thus probably mediated through non-CA neurons.

Effects of 6-hydroxydopamine and the PNMT inhibitor LY134046 on pressor responses to stimulation of the subretrofacial nucleus in anaesthetized stroke-prone spontaneously hypertensive rats

The subretrofacial nucleus of the rostral ventrolateral medulla is an important site for the control of sympathetic vasomotor tone and is the location of the C1 PNMT-containing cell bodies. In the present study the involvement of central monoaminergic neurons in the pressor responses evoked by chemical or electrical stimulation of this nucleus was examined in urethane-anaesthetized stroke-prone spontaneously hypertensive rats (SHRSP). Vehicle-treated rats were compared to animals treated with the PNMT inhibitor LY134046, the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) or a combination of 6-OHDA and the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). LY134046 caused a 43% depletion of adrenaline content in the hypothalamus and medulla but not in the spinal cord but had no effect on the pressor responses to stimulation of the subretrofacial nucleus. However, intraventricular administration of 6-OHDA reduced the pressor responses to subretrofacial nucleus stimulation by 50%. 6-OHDA caused profound depletion of noradrenaline in the brain and spinal cord, and adrenaline in the hypothalamus. Combined treatment with 6-OHDA and 5,7-DHT caused the additional depletion of serotonin to 34% and 13% in the hypothalamus and spinal cord, respectively, but caused no further reduction of pressor responses than with 6-OHDA alone. These results suggest that the pressor responses elicited by subretrofacial nucleus stimulation involve a 6-OHDA-sensitive pathway (presumably catecholaminergic) other than the bulbospinal adrenaline pathway but that serotonergic mechanisms do not contribute.

Localization of the main noradrenergic neuron groups in the pons and medulla of the rabbit and the importance of cathodal lesions for prolonged survival

Methods for stereotaxically localizing the major noradrenergic (NA) cell groups (i.e. A1, A2, A5 and A6 + A7) in the rabbit are described. Using a modified Kopf head holder we used surface landmarks including the obex for making lesions of the A1 and A2 cells in the medulla. Localization of the pontine cell groups was done by mapping intracerebral structures including the facial nerve for A5 and the motor nucleus of the trigeminal nerve for A6 + A7. In the initial experiments we made A1 lesions by passing anodal currents through stainless steel electrodes, which was associated with pulmonary oedema, neurological complications and a high mortality. This syndrome was probably related to toxic effects of ferric ion deposition, and disappeared when cathodal currents were employed. We have now made 106 bilateral cathodal lesions in the different groups, with a 20% intraoperative mortality. But virtually all survivors remained indefinitely in clinically good condition for the 2-4 weeks duration of our experiments. In 65 of these rabbits we achieved greater than 75% of NA cell destruction (average 84%). From the cardiovascular viewpoint 'non-specific' damage by the lesions was relatively small, except after A2 lesions where there was some impairment in the baroreceptor-heart rate reflex, though a considerable amount of residual function remained.

Evidence for a selective localization of voltage-sensitive Ca2+ channels in nerve cell bodies of corpus striatum

Specific binding sites for [3H]nitrendipine, an organic Ca2+ channel antagonist, were abolished in crude synaptosomal membranes of kainic acid-lesioned caudate nuclei. In contrast, specific lesions of dopaminergic or serotonergic axon terminals in caudate nuclei failed to alter the density or the affinity of [3H]nitrendipine binding sites. In addition, the basal and veratridine-stimulated 45Ca2+ accumulations were greatly impaired in slices prepared from kainic acid-lesioned caudate nuclei. The veratridine-elicited accumulation of 45Ca2+ in control slices was attenuated by addition of tetrodotoxin in the incubation medium. The present data provide evidence that most of the [3H]nitrendipine binding sites and the voltage-dependent Ca2+ channels are located in intrinsic neurons or interneurons in caudate nucleus. In contrast, destruction of dopaminergic or serotonergic nerve terminals emanating from other brain areas and innervating the caudate nucleus failed to change the apparent Bmax value for [3H]nitrendipine binding.

Mechanism of the tachycardia following systemic alpha-methyldopa administration in conscious rats

The mechanism of the tachycardia observed in conscious rats following systemic administration of alpha-methyldopa (alpha-MD) was investigated. Heart rate and mean arterial pressure were monitored following peripheral (i.p. and i.v.) and central (lateral and fourth ventricle and cisterna magna) administration of alpha-MD. As little as 25 mg/kg i.p. produced the maximum tachycardia observed: 136 +/- 30 beats/min within 30 min. However, after central administration of alpha-MD--producing similar reductions in blood pressure--only a gradually developing bradycardia occurred (maximum at 3-4 h), suggesting that the tachycardia was peripheral in origin. Tachycardia following administration of 25 mg/kg alpha-MD i.p. was prevented by pretreatment with propranolol, desmethylimipramine, and the dopa-decarboxylase inhibitor R04-4602, but not by pretreatment with pentolinium, guanethidine, the dopamine-beta-hydroxylase inhibitor FLA-63, or by adrenalectomy or depletion of endogenous catecholamines. In isolated spontaneously beating atria, alpha-MD produced a maximum increase in rate similar to that of isoprenaline. This effect of alpha-MD was blocked by propranolol and R04-4602 but not by FLA-63. These results suggest that the tachycardia observed in conscious rats following alpha-MD administration is caused by stimulation of cardiac beta-adrenoceptors following its conversion to alpha-methyldopamine in cardiac sympathetic neurons.

Differential blood pressure responses to intracisternal clonidine, alpha-methyldopa, and 6-hydroxydopamine in conscious normotensive and spontaneously hypertensive rats

The cardiovascular effects of three drugs that activate central alpha-adrenoceptor mechanisms were investigated in conscious normotensive Wistar and Wistar-Kyoto rats, and in spontaneously hypertensive rats (SHR). From dose-response curves, near-maximal intracisternal (i.c.) depressor doses of clonidine, alpha-methyldopa (alpha-MD), and 6-hydroxydopamine (6-OHDA; a neurotoxic agent that acutely releases central noradrenaline) were determined. In all three strains of rats, alpha-MD (1 mg i.c.) produced a 23-25% reduction in blood pressure. Clonidine (2.5 micrograms i.c.) and 6-OHDA (400 micrograms i.c.) produced similar falls in blood pressure in SHR to that observed with alpha-MD, but they were much less effective in normotensive rats (7 and 13%, respectively, in Wistar; 22 and 21% in SHR). Higher doses of clonidine increased blood pressure in Wistar rats, but further reduced it in SHR. The late pressor response observed 2-3 h after 6-OHDA administration in Wistar rats was not observed in SHR. These results support the view that clonidine and 6-OHDA, but not alpha-MD, have central pressor actions in the rat that oppose their antihypertensive action. The absence of this pressor effect in SHR indicates that there are significant differences in central noradrenergic pathways and alpha-adrenoceptor distribution among the three strains of rats.

Cardiovascular responses to central clonidine, alpha-methyldopa, and 6-hydroxydopamine in conscious normotensive and spontaneously hypertensive rats following naloxone

The possible involvement of endogenous opioid peptides in the cardiovascular responses observed following central alpha-adrenoceptor stimulation with clonidine, alpha-methyldopa (alpha-MD), and 6-hydroxydopamine (6-OHDA) was examined in conscious normotensive Wistar and spontaneously hypertensive (SHR) rats. Clonidine [2.5 micrograms intracisternally (i.c.)] produced rapid hypotension (-36 +/- 2 mm Hg) and bradycardia (-53 +/- 5 beats/min) in SHR that were similar to observations in animals given either naloxone (50 micrograms i.c. or 10 mg/kg i.p.) or appropriate saline control injections. Peripheral doses of naloxone (1-2 mg/kg) or saline did not further change arterial pressure or heart rate in either Wistar rats or SHR given alpha-MD (1.0 mg i.c.) 3 h earlier. In addition, central doses of naloxone (3 X 50 micrograms i.c.) given at hourly intervals did not affect the responses to alpha-MD. Central administration of 6-OHDA acutely releases noradrenaline which produces an initial fall in arterial blood pressure and heart rate. Intracisternal 6-OHDA (400 micrograms) produced similar time course and maximum circulatory effects in rats given naloxone (50 micrograms i.c. before and at each subsequent hour) as in saline-treated animals. Naloxone (1 mg/kg s.c.) significantly attenuated morphine-induced analgesia. These findings do not support a critical role of endogenous opioids in mediating the acute antihypertensive actions of clonidine and alpha-MD or in the cardiovascular responses produced by noradrenaline release following central 6-OHDA.

Central and peripheral autonomic mechanisms involved in the circulatory actions of methyldopa

Intracisternal (i.c.) and intravenous (i.v.) administration of methyldopa in conscious rabbits produced closely similar changes in hemodynamics, heart rate, and falls in plasma norepinephrine levels. Two weeks after giving i.c. 6-hydroxydopamine (6-OHDA), when there is widespread destruction of central noradrenergic neurons, the effects of i.c. methyldopa virtually were abolished. This suggests that noradrenergic neurons are the major central site of biotransformation into active metabolites. The circulatory and norepinephrine effects of i.v. methyldopa were attenuated but not completely abolished after giving i.c. 6-OHDA. Hence, in the rabbit about 70% of the action of methyldopa was central and about 30% was peripheral in the human therapeutic range of methyldopa concentrations. Preliminary lesion experiments suggest that the A5 nucleus plays an important role in the bradycardia. Two weeks after giving 5,6-dihydroxytryptamine (5,6-DHT) to destroy serotonergic (5HT) neurons the effects of i.c. methyldopa on mean arterial pressure (MAP) and heart rate were attenuated to approximately 50% of control effects. Therefore, some of the central effects of methyldopa apparently are mediated through 5HT pathways. We also compared the effects of i.c. methyldopa with those of i.c. clonidine (an alpha 2-adrenergic receptor agonist) and with the effects of transmitter release from the endings of noradrenergic and 5HT neurons during the first few hours after either 6-OHDA or 5,6-DHT administration. Our findings suggest that after biotransformation of methyldopa its active metabolites increase the activity of the bulbospinal noradrenergic neurons that control MAP and heart rate and reduce the activity of bulbospinal 5HT neurons.

Effects of sino-aortic denervation on the acute circulatory responses to centrally administered 6-hydroxydopamine and of clonidine in conscious rabbits

In intact animals the circulatory responses evoked by transmitter released from specific neurons of the central nervous system (CNS), or by centrally administered drugs, depend on the direct effects initiated in the CNS plus secondary reflex changes in autonomic activity. We have studied the contribution of the arterial baroreceptor reflexes to the mean arterial pressure (MAP) and heart rate responses produced by intracisternal (i.c.) clonidine and during the first few hours after i.c. 6-hydroxydopamine (6-OHDA), which releases transmitter from noradrenergic neurons. The direct and reflex components were assessed from the differences in responses between intact and sino-aortic denervated (SAD) rabbits, with the latter's responses taken to approximate the direct CNS-induced effects. The direct effects of transmitter release were (a) an early fall in MAP followed by a late pressor effect; and (b) an early bradycardia followed by a late tachycardia. The early responses were similar to the direct effects of i.c. clonidine and to the findings observed in pontine rabbits with intact baroreceptors. In intact rabbits, feedback through the arterial baroreceptors reduced the fall in MAP with clonidine, but had no effect on the heart rate changes. Similarly, it eliminated the early depressor response to 6-OHDA but had little effect on the early bradycardia; however, the late pressor response was little affected, and the late tachycardia was masked by a reflex bradycardia. Our findings suggest that arterial baroreceptor reflexes can modify some effects initiated in the CNS, but some of the pathways are not involved in this interaction.

Contribution of noradrenergic and serotonergic neurons to the circulatory effects of centrally acting clonidine and alpha-methyldopa in rabbits

The effects of intracisternal clonidine (0.04, 0.2, and 1.0 microgram/kg) and of alpha-methyldopa (alpha-MD; 400 micrograms/kg) on mean arterial pressure (MAP) and heart rate (HR) were studied in conscious rabbits before, and 7 and 14 days after, intracisternal injection of (a) vehicle, (b) 6-hydroxydopamine (6-OHDA; 600 micrograms/kg), or (c) 5,6-dihydroxytryptamine (5,6-DHT; 633 micrograms/kg) (n = 6 per group). In the initial control experiment clonidine and alpha-MD produced similar falls in MAP and HR in each group; there was also good reproducibility of responses in vehicle-treated rabbits on the 3 experimental days. But after 6-OHDA or 5,6-DHT administration the circulatory effects of clonidine and alpha-MD were markedly attenuated. On day 14 after injection of 6-OHDA, the clonidine-induced falls in MAP and HR averaged 38 and 18%, respectively, of the control responses (p less than 0.001). On day 14 after 5,6-DHT administration, the falls in MAP and HR after clonidine administration were reduced to 27 and 13% of control, respectively (p less than 0.01), while the corresponding responses after alpha-MD administration were 39 and 61% of control (p less than 0.05). Neurochemical findings suggest that 6-OHDA affected noradrenergic (NA), dopaminergic (DA), but not serotonergic (5HT) neurons, and that 5,6-DHT affected 5HT but not NA and DA neurons. We conclude that the circulatory effects of clonidine and alpha-MD are mediated through both central NA and 5HT neurons.

Effects of intracisternal and intravenous alpha-methyldopa and clonidine on haemodynamics and baroreceptor--heart rate reflex properties in conscious rabbits

We determined the doses of intracisternal (i.c.) and intravenous (i.v.) clonidine and alpha-methyldopa (alpha-MD) that produced near-maximal falls in mean arterial pressure (MAP) in conscious rabbits. We then studied the haemodynamic mechanisms underlying the fall in MAP and changes in the properties of the baroreceptor-heart rate reflex. Intracisternal and intravenous administration of both drugs lowered MAP by approximately 25% of control, and the fall was about half due to the reduction in cardiac output and about half due to a fall in total peripheral resistance. Baroreceptor-heart rate reflex properties were studied by transiently inflating perivascular balloons to alter blood pressure and by deriving sigmoid curves relating MAP to heart period (HP, pulse interval). Both drugs produced very similar vagal facilitation during transient rises in MAP when given by the i.c. and i.v. routes; HP range (between upper and lower plateaus) increased to 145% of control, and gain rose to 190%. The effects of i.c. administration of both drugs on the cardiac sympathetic component of the baroreflex were studied in methscopolamine-treated rabbits. Clonidine produced more pronounced suppression of HP range and gain, while alpha-MD had little effect. These differences between drugs were still present with much larger i.c. doses. Our findings suggest that both drugs influence resting haemodynamics and the vagal component of the baroreflex through similar effects on the central autonomic pathways. But there are some differences in their central actions on cardiac sympathetic motoneurons.

Cardiovascular functions of central noradrenergic and serotonergic neurons in conscious rabbits. Their contributions to the central actions of clonidine

Synaptic release of transmitter from central noradrenergic (NA) and serotonergic (5HT) neurons was studied in intact and pontine decerebrate unanesthetized rabbits, following intracisternal injections of the selective neurotoxic drugs 6-hydroxydopamine (6-OHDA) and 5,6-dihydroxytryptamine (5,6-DHT). The NA and 5HT neurons both raise blood pressure through a suprapontine pathway, with 5HT neurons in series with NA neurons. Descending bulbospinal fibers have antagonistic effects on blood pressure, with NA release lowering blood pressure and 5HT release increasing it. The two transmitters also have antagonistic effects on the cardiac vagus, with NA neurons increasing vagal activity and 5HT neurons inhibiting it. Our results suggest that both NA and 5HT neurons contribute to the cardiovascular effects of clonidine. The actions of clonidine on blood pressure and heart rate mimic the effects of NA and are opposite those of 5HT released at synaptic sites in the bulb and spinal cord.

Cardiovascular functions of brain serotonergic neurons in the rabbit as analysed from the acute and chronic effects of 5,6-dihydroxytryptamine

The effects of intracisternal (i.c.) 5,6-dihydroxytryptamine (5,6-DHT) or creatinine sulphate vehicle on mean arterial pressure (MAP), heart rate, and baroreceptor-heart rate and nasopharyngeal reflex properties were studied in conscious rabbits. For the baroreflex we derived sigmoid MAP-heart period (HP) curves, and for the nasopharyngeal reflex we measured apnea time, rise in HP, and delta MAP. The acute effects occurring over the first few hours in intact and decerebrate rabbits included: (a) tachycardia and a decrease in baroreflex HP range and gain, which were mediated through bulbar or spinal pathways influencing vagal motoneurons; (b) a transient early rise in MAP through a bulbar or spinal pathway and a late suprapontine rise in MAP; and (c) shorter apnea time, less bradycardia, and less well-maintained MAP during nasopharyngeal stimulation. By day 14, when spinal cord serotonin was depleted by 70%, resting MAP and heart rate had recovered, HP range and gain had risen above initial control, but nasopharyngeal apnea time and bradycardia were still reduced. Some of the acute responses were due to synaptic release of serotonin (5HT); since the tachycardia and late component of the pressor response were blocked by pretreatment with i.c. methysergide, the acute changes in baroreflex parameters were opposite to the chronic changes, and a second injection of 5,6-DHT failed to produce these changes. On the other hand, the acute and chronic nasopharyngeal responses were similar, suggesting that the former were due to neuronal block and not 5HT release. These findings indicate that central serotonergic neurons inhibit cardiac vagal activity, increase blood pressure through both suprapontine and bulbar or spinal pathways, and are involved in the nasopharyngeal reflex.

Effects of noradrenergic and serotonergic neurons on blood pressure, heart rate and baroreceptor-heart rate reflex of the conscious rabbit

The acute and chronic effects of intracisternal (i.c.) 6-hydroxydopamine (6-OHDA) and 5,6-dihydroxytryptamine (5,6-DHT) on mean arterial pressure (MAP), heart rate, and baroreceptor-heart rate reflex properties were examined in conscious rabbits. The reflex was studied by deriving MAP-heart period (HP, pulse interval) curves. The acute effects occurring over the first few hours were due to release of transmitter (noradrenaline after 6-OHDA; serotonin after 5,6-DHT) from specific synaptic sites; the changes on day 14 were due to destruction of specific neurons. Noradrenergic bulbar neurons facilitate vagal heart rate motoneurons and serotonergic neurons have an inhibitory effect. Noradrenergic bulbospinal pathways have a depressor effect on MAP and serotonergic bulbospinal activity increases MAP. These effects on heart rate and MAP were mediated through independent parallel pathways. The acute circulatory changes due to each neurotoxin also included a long latency pressor component, which was mediated through a pathway where a serotonergic neuron was in series with a noradrenergic neuron which sends fibers to a suprapontine pressor site. There were no chronic changes in resting MAP or heart rate, but chronic changes in reflex HP range and gain were opposite to the acute effects of transmitter release. These findings indicate that noradrenergic and serotonergic neurons normally participate in the central baroreceptor-heart rate reflex pathways of the intact animal.

Mechanisms of acute hypertension and bradycardia following intracisternal 6-hydroxydopamine in conscious rabbits

Intracisternal (i.c.) 6-hydroxydopamine (6-OHDA) 600 microgram x kg-1 produced a rise in blood pressure and a fall in heart rate, maximal 2-3 h injection. Pretreatment with phentolamine (0.2 or 0.5 mg x kg-1 i.c.) prevented the bradycardia and produced dose-related attenuation of the pressor response. The same dose given 2.1 h after 6-OHDA again abolished the bradycardia but attenuated the pressor response only slightly. We conclude that normally after 6-OHDA both hypertension and bradycardia are due to release of noradrenaline, stimulating alpha-adrenoceptors at distinctive sites in the CNS. The pathway mediating hypertensionis less accessible to the action of phentolamine than that mediating bradycardia.

Effect of 6-hydroxydopamine on blood pressure and heart rate responses to intracisternal clonidine in conscious rabbits

The effects of intracisternal injection (i.c.i.) of clonidine (1 microgram kg-1) on blood pressure and heart rate were studied in conscious rabbits with an implanted catheter in the cisterna magna. Each animal was studied under control conditions and 7 days after i.c.i. of 6-hydroxydopamine (6-OHDA) (1 microgram kg-1; n = 10) or ascorbic acid vehicle (n = 6). In the control experiments blood pressure and heart rate began to fall 1--2 min after i.c.i. of clonidine, with maximum falls at 10--20 min averaging 18 +/- 2 mmHg and 45 +/- 8 b/min and almost complete recovery by 90 min. After vehicle pretreatment neither response was significantly altered. After 6-OHDA the early component of the bradycardia was abolished and only a late fall in heart rate developed 30 min after i.c.i. clonidine. The magnitude of the hypotension was unaffected but the onset was slightly delayed, probably owing to the abolition of the bradycardia. The dose of 6-OHDA reduced spinal cord catecholamines to about 20% of the level observed after vehicle. Central catecholaminergic pathways are thus important in the early predominantly vagal component of the clonidine induced bradycardia, but play little role in the hypotensive response.

Cardiovascular and behavioral effects of intracisternal 6-hydroxydopamine in the rabbit

Acute effects of 6-hydroxydopamine (6-OHDA, 400--900 microgram kg-1 intracisternally, i.c.i.) consisted of bradycardia and hypertension, maximal 2--3 h after injection and preceded after some doses by a phase of hypotension. This pattern was obtained in completely conscious rabbits and after propanidid and sodium pentobarbitone anesthesia. After 600 microgram kg-1 i.c.i. 6-OHDA the peak rise in blood pressure (25 +/- 3.8 mm Hg) was due to a rise in peripheral resistance involving particularly renal and intestinal beds. Suprapontine mechanisms contributed to both hypertension and bradycardia. Giving pontine rabbits 6-OHDA elicited a short-latency fall in blood pressure, resembling the hypotensive phase in intact animals. Chronic effects 7 days after 600 microgram kg-1 included a rapid loss of 10% of body weight associated with reduction in food and water intake. To avoid secondary circulatory effects the rabbits were artificially fed, halving the weight loss. At 7 days blood pressure had fallen by 7.4 +/- 2.3 mm Hg probably owing to this residual weight loss. From experiments involving administration of phenotolamine and clonidine in intact rabbits and the responses of pontine animals it is likely that both descending and ascending catecholaminergic pathways have inhibitory effects on blood pressure, though some of the pathways may also be excitatory. Absence of specific chronic circulatory changes may be due to compensation through parallel pathways involving other transmitters.

Actions of tetrahydropapaveroline on the cardiovascular system and skeletal muscles of the anaesthetized cat and on guinea-pig soleus muscle in vitro

1. Tetrahydropapaveroline (THP) and isoprenaline have been compared for effects on heart rate, diastolic blood pressure and contractions of soleus and tibialis muscles in chloralose-anaesthetized cats and on in vitro guinea-pig soleus muscles. 2. All effects of THP were similar to those of isoprenaline. Maximal responses were the same and responses to both drugs were antagonized by propranolol. 3. THP produced an increase in heart rate, a fall in diastolic blood pressure and a decrease in the tension and degree of fusion of incomplete tetanic contractions of the soleus, being, respectively, 12.9, 14.7 and 19.6 times less potent than isoprenaline. The similarity in potency ratios suggests that THP is a non-selective beta-adrenoceptor agonist. 4. Effects on the guinea-pig soleus in vitro were similar to those in the cat soleus. 5. In the cat, THP enhanced the tension and duration of tibialis twitches, but the tibialis was about ten times less sensitive than the soleus to the effects of THP. 6. THP deepened neuromuscular blockade in partially curarized cat soleus and tibialis preparations. Doses required were ten times greater than for direct depressant effects on the soleus. 7. Agents which depress the tension and fusion of incomplete tetanic contractions of the cat soleus via beta-adrenoceptor stimulation cause tremor in man by peripheral effects on slow muscle fibres. THP is likely to exert such actions. This may explain why levodopa (a precursor of THP) sometimes precipitates a tremor crisis in Parkinsoniam patients, and why Parkinsonian tremor is relatively refractory to levodopa therapy. In such patients combination therapy with levodopa and a peripheral dopa decarboxylase inhibi;or (to reduce levels of THP) or a beta-adrenoceptor antagonist (to block the effects of THP on slow muscle fibres) could be advantageous.