Alpha 1 and alpha 2-adrenergic receptor blockade influences angiotensin II facilitation of avoidance behavior and stereotypy in rats

AT(2) but not AT(1) receptor antagonism abolishes angiotensin II increase of the acquisition of conditioned avoidance responses in rats

In this study we attempted to determine behavioural, including cognitive, consequences of the brain AT(1) (losartan, 2 nmol), AT(2) (PD 123319, 1.5 nmol), and joint AT(1)/AT(2) angiotensin receptors blockade. Male Wistar rats (160-180 g) were injected into the left cerebral ventricle with the above doses of the blockers dissolved in 0.9% NaCl solution (vehicle) or with the vehicle alone. Five minutes later they received, to the right cerebral ventricle, 1 nmol of angiotensin II (Ang II) dissolved in vehicle or the vehicle alone. Ang II consistently increased rate of acquisition of conditioned avoidance response (CARs) and facilitated recall of the passive avoidance behaviour. In one out of the three series of experiments in open field Ang II stimulated rats locomotor activity. Losartan and PD 123319, both ineffective alone, given prior to Ang II abolished all the behavioural changes produced by the peptide except for the Ang II facilitation of CARs acquisition, which was unchanged by losartan. Interestingly, joint injection of losartan and PD 123319 significantly decreased the rate of CARs acquisition both in control and Ang II treated animals. In conclusion, the present data suggest significant though different involvement of both AT(1) and AT(2) angiotensin receptors in cognitive processes.

Baclofen and AII 3-7 on learning and memory processes in rats chronically treated with ethanol

The aim of this study was to determine the possible influence of baclofen, an agonist of the GABA(B) receptor on behavioral activity (recall, acquisition of conditioned reflexes) of angiotensin II fragment 3-7 (AII 3-7) in rats chronically treated with ethanol. Long-term (9 weeks) ethanol intoxication profoundly impaired learning and memory processes in all tests used. The GABA(B) receptor agonist baclofen (0.75 mg/kg i.p.) did not influence exploratory and motor activity in the control rats, but we observed a tendency (without significance) to decrease the psychomotor activity in the alcohol-intoxicated groups of animals, when it was injected together with AII 3-7 (2 microg i.c.v.). Baclofen did not influence the retrieval process in the passive avoidance recall, and when it was given together with AII 3-7 did not change the positive action of this fragment in control groups, but significantly enhanced its action in the animals chronically treated with ethanol. Baclofen showed significant improvement of acquisition in the active avoidance test only in the alcohol-intoxicated groups. Baclofen, injected together with AII 3-7, yielded important attenuation action of AII 3-7 in the control groups in the first 3 days of test, but did not produce any changes during the fourth and fifth day of the experiment. Baclofen did not provoke any changes in activity of AII 3-7 (when it was injected together) in the acquisition of the active avoidance test in the alcohol-intoxicated groups of animals.

Interactions of angiotensin II with central dopamine

There is a large body of evidence to support the concept of a relationship between brain Ang II and catecholamine systems. This interaction may participate in some central actions of Ang II such as cardiovascular control, dipsogenesis, and complex behaviours. It also extends to the nigrostriatal dopaminergic system which bear AT1 receptors, both on their cell bodies in the substantia nigra presynaptically, and on their terminals in the striatum, where Ang II can markedly potentiate DA release. This observation suggests that drugs which modulate central Ang II may be useful in regulating central dopaminergic activity.

Interactions of angiotensin II with central catecholamines

There is a large body of anatomical and functional evidence supporting an interaction between brain angiotensin and central catecholamine systems. Angiotensin II AT1 receptors have been identified on dopamine containing cells in the substantia nigra and striatum of human brain using receptor autoradiography. Using in vivo microdialysis we have demonstrated that locally administered angiotensin II stimulates dopamine release from the striatum of conscious rats. Since some angiotensin receptor antagonists and angiotensin converting enzyme inhibitors can cross the blood brain barrier it is possible that they interact with the brain catecholaminergic systems.

The role of adrenoreceptors in control of stereotyped oral behavior in restricted-fed fowls

Effects on environmentally induced oral stereotypies (object pecking and drinker-directed activity) of preferential antagonists and agonists of adrenoreceptor subtypes were examined in individually caged broiler breeder fowls subjected to chronic food restriction. Three drugs in each category were injected intravenously at three doses, and their effects compared with a saline control treatment. With the antagonists, object pecking was suppressed more by prazosin (alpha 1) and propranolol (beta) than by yohimbine (alpha 2), while drinker-directed activity showed delayed stimulation with yohimbine and propranolol. With the agonists, drinker-directed activity was suppressed more by clonidine (alpha 2) than by isoproterenol (beta) and phenylephrine (alpha 1), while object pecking was inhibited by the high doses of clonidine and isoproterenol but showed delayed stimulation with the low dose of clonidine and (nonsignificantly) the high dose of phenylephrine. Initial suppression of both oral stereotypies by the high doses of yohimbine and isoproterenol, and high and medium doses of clonidine, may have been due to sedation, because in those instances it coincided with increased sitting, an activity not normally seen. Increased standing with clonidine and the medium dose of yohimbine may also reflect sedation. When there were no significant increases in sitting or standing to indicate sedation, responses of both stereotypies were essentially the same with all three adrenoreceptor subtypes; i.e., object pecking was inhibited by the antagonist but not the agonist, while drinker-directed activity was inhibited by the agonist but not the antagonist. It is concluded that alpha 1, alpha 2, and beta adrenoreceptors are all implicated in expression of these stereotypes, and that the two activities may be differentially controlled.

Effects of alpha-2 adrenoceptor antagonists on rough-and-tumble play in juvenile rats: evidence for a site of action independent of non-adrenoceptor imidazoline binding sites

The pharmacological specificity of alpha-2 adrenoceptor involvement in the modulation of rough-and-tumble play behavior was assessed in juvenile rats. The alpha-2 adrenoceptor antagonists idazoxan and RX821002 both increased the frequency of pinning in individually housed rats that were given a brief opportunity to play. Dorsal contacts, a measure of play solicitation, were not consistently affected by these compounds. Since RX821002 shows little affinity for non-adrenoceptor imidazoline binding sites, it is likely that the facilitation of play following administration of these two compounds is due to blockade of alpha-2 receptors. The effect of RX821002 and idazoxan is unlikely to be an artifact associated with using rats that are reared in isolation, as RX821002 also increased pinning, as well as dorsal contacts, in group-housed rats that were isolated for a short period (4h) before the play session. The alpha-1 adrenoceptor antagonist prazosin, which also binds to alpha-2B receptors, reduced the frequency of both pinning and dorsal contacts. There was a strong trend for St 587, a centrally active alpha-1 agonist, to attenuate the effect of prazosin on play. While this leaves open the possibility that prazosin may be reducing play through alpha-1 blockade, antagonist activity at alpha-2B receptors cannot be ruled out. From these data, we conclude that the facilitation of play following idazoxan and RX821002 is likely due to blockade of alpha-2A adrenoceptors. These findings add further support for a specific role of alpha-adrenoceptors in the modulation of playfulness in the juvenile rat.