The medullary cardiovascular effects of imidazolines and some GABA analogues: a review

Imidazoline receptors in cardiovascular and metabolic diseases

The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the rostroventrolateral part of the brainstem. Afterwards, it was shown that imidazolines reduced blood pressure when applied in this area, whereas no catecholamine was capable of such an effect. These data led us to suggest the existence of receptors specific for imidazolines different from the alpha-adrenergic receptors. Soon after, the existence of imidazoline binding sites (IBS) was reported in the brain and in a variety of peripheral tissues including pancreatic gland and kidney. As expected, these specific binding sites do not bind the catecholamines. The IBS are classified in two groups: the I1 type, sensitive to clonidine and idazoxan; and the I2 type, sensitive to idazoxan and largely insensitive to clonidine. Imidazoline receptors were shown to be involved in several physiological regulations and pathological processes such as hypertension, diabetes mellitus and some mood disorders. Evidence for their implication in the nervous regulation of blood pressure and in the insulin secretion control will be presented. The hypotensive effects of clonidine-like drugs involve imidazoline receptors (I1Rs), while their most frequent side-effects only involve alpha2-adrenergic receptors. A new class of centrally acting antihypertensive drugs selective for I1Rs is now available. At hypotensive doses, these drugs are devoid of significant side effects. It was shown that the good acceptability of these drugs is likely due to their selectivity for I1Rs.

Imidazoline receptors: from discovery to antihypertensive therapy (facts and doubts)

The hypothesis and indirect evidence of imidazoline receptors has been promoted since some 15 years ago and it gave a substantial impetus for research in this field, resulting in a better understanding of neuronal and cardiovascular regulatory processes. The nomenclature of the imidazoline receptors has been accepted by international forums but no direct proof for the existence of these receptors has been published. Authors summarise the most important available data, including facts and doubts as far as the discovery, characterisation, and function of imidazoline receptors and their subtypes, the differences between imidazoline receptors and alpha-2 adrenoceptors, and also on their participation in regulatory processes.

Medullary hypotensive effect of endothelin1 in anaesthetized animals

In anaesthetized animals, systemic injection of ET1 at doses from 3 to 100 ng.kg-1 provoked only a transient hypotensive effect. At 300 ng.kg-1 we observed the classical biphasic effect, consisting of a transient lowering of the arterial pressure followed by a long-lasting hypertensive effect. Direct injection of the peptide into the vertebral artery of anaesthetized animals only affected arterial pressure (AP) when the blood-brain barrier was permeabilised. Under these conditions, a dose-dependent decrease in AP was observed, which was not associated with a significant effect on the heart rate. Micro-injections of the peptide in the medullary nucleus reticularis lateralis area (NRL), a medullary vasopressive centre, at doses of 30 to 60 ng.kg-1 led to a significant reduction in mean arterial pressure (MAP) (17 +/- 4% and 36.5 +/- 6%) respectively without a significant change in heart rate. These effects lasted less than 2 hours. These results suggest a possible role of ET1 as a neuromodulator involved in the central regulation of vasomotor tone, in the NRL region.

Baclofen prevents the increase of myocardial oxygen demand indexes evoked by the hypothalamic stimulation in rabbits

The electrical stimulation of the paraventricular nucleus (PVN) of the hypothalamus in anaesthetized rabbits elicited important cardiovascular responses which were mainly characterized by increases in arterial pressure, dP/dtmax, and of the indexes of myocardial oxygen consumption, rate-pressure product (from 34 +/- 2 to 40 +/- 2 mmHg.bpm.10(-3) and triple product (from 102 +/- 12 to 162 +/- 19 mmHg2.s-1.bpm.10(-6). The hemodynamic alterations induced by PVN stimulation were similar to those observed during physical effort and stressful situations. Intracerebroventricular (0.1, 0.3 and 1 microgram.kg-1) or intravenous administration (1, 3 and 10 mg.kg-1) of baclofen, a selective GABAB receptor agonist, induced a dose-related decrease in the peak values of dP/dtmax and of the indexes of myocardial oxygen consumption (rate-pressure and triple products) during the electrical PVN stimulation. After 1 microgram.kg-1 baclofen (i.c.v.), the peak value of the triple product during PVN stimulation was 101 +/- 21 as compared to 149 +/- 15 before treatment. At the 10 mg.kg-1 dose (i.v.), the triple product during stimulation only reached 90 +/- 20 vs. 150 +/- 20 before treatment. These results suggested that a type B GABAergic transmission system is involved in the modulation of central control of the cardiac function. Drugs modulating this system could therefore be designed to blunt the myocardial oxygen demand increases.

GABAB receptor-mediated inhibition of the neurogenic vasopressor response in the pithed rat

1. The effects of gamma-aminobutyric acid (GABA) and related drugs on the vasopressor response induced by electrical stimulation (single pulse of 30 V and 1 ms) of the preganglionic sympathetic nerve fibres or by injection of noradrenaline 0.3 nmol kg-1 were studied in the pithed rat. 2. The electrically-induced increase in diastolic blood pressure was inhibited by GABA and the GABAB-receptor agonist R-(--)-baclofen but was not affected by its S-(+)-enantiomer and by the GABAA-receptor agonists muscimol and 3-aminopropane sulphonic acid. 3. The dose-response curve of R-(--)-baclofen for its inhibitory effect on the electrically-induced vasopressor response was shifted to the right by the GABAB-receptor antagonist 2-hydroxysaclofen, but was not affected by the GABAA-receptor antagonist bicuculline. 2-Hydroxysaclofen and bicuculline by themselves did not affect the electrically-induced vasopressor response. 4. The increase in diastolic blood pressure induced by exogenous noradrenaline was not affected by the GABA-related drugs, which also had no (or very slight) effects on the basal diastolic blood pressure. 5. It is concluded that GABA inhibits catecholamine release in the resistance vessels of the rat via GABAB-receptors, probably located presynaptically on the postganglionic sympathetic nerve fibres.

Role of neurotransmitters in the central regulation of the cardiovascular system

The last decade has seen tremendous progress in determining the nature of the neurotransmitters which regulate central nervous system pathways involved in the regulation of blood pressure. Investigations are now pursuing the identity and functional importance of neurotransmitters contained within pathways shown to be important in cardiovascular regulation. In addition, several key components of the brain stem networks involved in the control of sympathetic activity have been identified. For example, numerous studies indicate the importance of neurons located in the rostral ventrolateral medulla in the regulation of SPN. Indeed, this area contains medullospinal sympathoexcitatory neurons which represent the final site of integration of many brain stem and reflex pathways involved in the regulation of sympathetic nerve activity. The neurotransmitter which is utilized by this medullospinal pathway remains unknown. Epinephrine, substance P and glutamate have all been hypothesized as primary chemical mediators in the descending pathway from the brain stem to SPN. Interestingly, lesions of, or antagonists to, epinephrine, substance P, glutamate and 5-HT neurons all abolish sympathetic activity and reduce blood pressure to a level similar to that in a spinal animal. Clearly, not all these transmitters are primary mediators of sympathetic information carried from the brain stem to the spinal cord. It is likely that monoamines and neuropeptides act in the IML, as in other area of the central nervous system, as neuromodulators to set the level of excitability of SPN rather than relaying sympathetic information over a functionally specific medullospinal pathway. This conclusion is supported by the observation that midline medullary 5-HT neurons provide a tonic excitatory input to SPN, but receive no afferent inputs from other central sympathetic or baroreceptor pathways. However, the firing of 5-HT neurons appears to relate to the state of vigilance of the animal. This suggests that 5-HT neurons may lower the threshold of SPN to sympathetic inputs during states of wakefulness. In addition, the time course of the norepinephrine-mediated slow EPSPs and IPSPs in SPN is consistent with a gain-setting function. By analogy, epinephrine is likely to act as a neuromodulator in the IML rather than to serve as the primary mediator of sympathetic information descending from the rostral ventrolateral medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

The cardiovascular effects of centrally administered taurine in anaesthetised and conscious rats

The effects of pentobarbitone anaesthesia on the cardiovascular changes induced by centrally administered taurine have been investigated in spontaneously hypertensive (SHR) and normotensive rats. Administration of taurine (100-400 micrograms) into the lateral cerebral ventricle (i.c.v.) of anaesthetised SHR and normotensive rats induced a dose-related fall in systemic blood pressure and heart rate, which tended to be of a greater magnitude in the SHR. In anaesthetised rats attached to a ventilator, taurine was not as potent at inducing a fall in systemic blood pressure, approximately double the dose being required to produce the same cardiovascular changes as that which occurred in anaesthetised rats without a ventilator. In conscious normotensive rats taurine had no effect on blood pressure until a dose of 800 micrograms was administered i.c.v., whereas in conscious SHR, a small, but significant depressor response was evident with 400 micrograms. These findings demonstrate that pentobarbitone anaesthesia sensitises the rats to the cardiovascular effects of taurine, partially through a mechanism which involves respiratory depression. Conversely in conscious rats a much higher dose of taurine is required to induce a fall in arterial pressure and heart rate per se.

Production and characterization of anti-clonidine antibodies not cross-reacting with catecholamines

Polyclonal antibodies against clonidine were developed, with para-aminoclonidine coupled to bovine serumalbumin or hemocyanine with glutaraldehyde used as antigens. The selected antibody (from rabbits) cross-reacted with high specificity with clonidine and its structurally closely related analogues but it recognized neither catecholamines nor various endogenous imidazole molecules such as histamine, purine, adenine, and adenosine, thus appearing to be specific for the aminoimidazoline structure. An interesting cross-reactivity was observed with the bovine clonidine displacing substance, the probable endogenous ligand for receptors involved in the hypotensive effect of clonidine-type substances. This suggested that this molecule should contain an aminoimidazoline or guanidine moiety.

The blood pressure effects of alpha-adrenoceptor antagonists injected in the medullary site of action of clonidine: the nucleus reticularis lateralis

We administered a series of alpha-blocking drugs to the nucleus reticularis lateralis (NRL) of the medulla oblongata, the main site for the hypotensive action of clonidine. These experiments were performed on pentobarbital anaesthetized cats. Drugs were injected through a needle which was stereotaxically inserted. Prazosin (6 nmol) was hypertensive (MBP = +25 +/- 8%), corynanthine had no effect and AR-C239 (7 nmol), another alpha 1-blocker, was hypotensive (MBP = -16 +/- 3.5%). The alpha 2-blockers, yohimbine and idaxozan, were hypotensive. The blood pressure effects of alpha-blocking drugs directly microinjected in the nucleus reticularis lateralis cannot be simply related to their selectivity for a particular subtype of alpha-receptors.