Central 5HT1A-receptor binding in normotensive and spontaneously hypertensive rats

Involvement of serotonin1A receptors in cardiovascular responses to stress: a radio-telemetry study in four rat strains

We studied the effect of treatment with the serotonin-1A (5-HT1A) receptor ligands buspirone, 8-hydroxy-di-propyl-aminotetralin (8-OH-DPAT), and (8-[2-(2,3-dihydro-1,4-benzodioxin-2-yl-methylamino)ethyl]-8-azaspiro[4,5]decane-7,9-dione methyl sulphonate (MDL73,005EF) on blood pressure and heart rate increases to open field stress. We compared Spontaneously Hypertensive Rats (SHR), Fawn-Hooded (FH) rats, Wistar-Kyoto (WKY) rats, and Sprague-Dawley (SD) rats instrumented with radio-telemetry probes. Buspirone treatment reduced the blood pressure increase in SHR, FH rats, and WKY rats and heart rate increase in FH rats and WKY rats. 8-OH-DPAT treatment reduced the blood pressure increase in FH rats and WKY rats, but had no effect in SHR and enhanced the pressor response in SD rats. This treatment reduced the heart rate increase in FH rats and WKY rats only. Similarly, MDL73,005EF treatment reduced the blood pressure increase in FH rats and WKY rats, but had no effect in SHR and enhanced this response in SD rats. Little effect of this treatment was seen on heart rate changes. For comparison, diazepam treatment abolished the pressor response in SD rats and reduced it in FH rats and WKY rats, but not SHR. Differential effects of the treatments were also seen between strains for locomotor activity in the open field, although behavioural changes could not explain the effects of the drugs on cardiovascular responses. These data suggest that 5-HT1A receptors are involved in cardiovascular stress responses; however, the extent of this involvement differs between rat strains and the drugs used. These results could be important for our understanding of possible anxiolytic properties of antipsychotic drugs with affinity for the 5-HT1A receptor.

Effects of repeated fluoxetine on anxiety-related behaviours, central serotonergic systems, and the corticotropic axis axis in SHR and WKY rats

In keeping with the anxiolytic property of selective serotonin reuptake inhibitors (SSRIs) in humans, we have examined in the spontaneously hypertensive rat (SHR) and the Wistar-Kyoto (WKY) rat, which display low and high anxiety, respectively, some psychoneuroendocrine effects of a repeated treatment with the SSRI fluoxetine (5 or 10 mg/kg daily, for 3 weeks). Two days after the last injection, plasma levels of fluoxetine were not detectable whereas those of its metabolite, norfluoxetine, were present to similar extents in both strains. By means of the elevated plus-maze test (29-30 h after the 13th administration of fluoxetine) and an open field test (48 h after the last injection of fluoxetine), it was observed that fluoxetine pretreatment did not yield anxiolysis; hence, some, but not all, behaviours were indicative of anxiety and hypolocomotion (as assessed through principal component analyses and acute diazepam studies). In both strains, the 10 mg/kg dose of fluoxetine decreased hypothalamus 5-HT and 5-HIAA levels, and reduced midbrain and/or hippocampus [3H]citalopram binding at 5-HT transporters, but did not affect [3H]8-hydroxy-2-(di-N-propylamino)tetralin binding at hippocampal 5-HT1A receptors. However, the fluoxetine-elicited reduction in hippocampal 5-HT transporter binding was much more important in WKY than in SHR rats, this strain-dependent effect being associated in WKY rats with a reduction in cortical [3H]ketanserin binding at 5-HT2A receptors. Lastly, in WKY rats, repeated fluoxetine administration increased adrenal weights and the plasma corticosterone response to open field exposure, but did not affect the binding capacities of hippocampal mineralocorticoid and glucocorticoid receptors. These data show that key psychoneuroendocrine responses to repeated fluoxetine administration may be strain-dependent, and that repeated fluoxetine administration does not yield anxiolysis, as assessed by two standard tests of emotivity.

Stress and emotionality: a multidimensional and genetic approach

The use of behavioural tests aiming to assess the psychological components of stress in animals has led to divergent and sometimes arbitrary interpretations of animal behaviour. This paper presents a critical evaluation of behavioural methods currently used to investigate stress and emotionality. One of its main goals is to demonstrate, through experimental evidence, that emotionality may no longer be seen as a unidimensional construct. Accordingly, following a discussion about concepts, we propose a multiple-testing approach, paralleled by factor analyses, as a tool to dissociate and study the different dimensions of emotionality. Within this multidimensional context, genetic studies (illustrated here by different rat models) are shown to be particularly useful to investigate the neurobiology of stress/emotionality. A genetic approach can be used (i) to broaden and dissect the variability of responses within and between populations and (ii) to search for the molecular bases (i.e. genes and gene products) which underlie such a variability.

Differential cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, 5-methyl-urapidil and MDL 75,608A in conscious spontaneously hypertensive rats

The effects of intravenous (i.v.) administration of four agonists at central 5-HT1A receptors were investigated and compared. Acute iv injection of 0.1 mg/kg of 8-OH-DPAT induced a decrease in blood pressure both in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The maximal hypotensive effect was observed 15 and 10 min after injection, respectively, but the effect was greater and longer-lasting in the SHR. 8-OH-DPAT significantly decreased heart rate in WKY and, to a lesser extent, in SHR. The i.v. injection of 1 mg/kg of flesinoxan caused a similar fall in blood pressure and heart rate in SHR and WKY. The i.v. administration of 1 mg/kg of 5-methyl-urapidil or MDL 75,608A caused a fall in blood pressure which was significantly more pronounced in SHR than in WKY. 5-methyl-urapidil induced a significant tachycardia in WKY, but had little effect on heart rate in SHR. MDL 75,608A caused a short-lasting tachycardia in SHR and WKY. In conscious SHR, the intracerebroventricular (icv) injection of 10 micrograms of 8-OH-DPAT or 100 micrograms of either flesinoxan or MDL 75,608A caused a decrease in blood pressure and heart rate. The icv injection of 100 micrograms of 5-methyl-urapidil caused only a decrease in blood pressure. Chronic pre-treatment with these compounds, by daily i.v. injection, did not significantly influence the hypotensive or bradycardic effects in an acute experiment. The involvement of alpha 1-adrenoceptors in the effects of these compounds was studied by administering phenylephrine (1 microgram/i.v.) at 5-min intervals before and after the i.v. injection of the experimental compounds. The injection of phenylephrine reproducibly increased blood pressure by 35-40 mm Hg after saline pre-treatment, and these responses were not affected by the i.v. injection of 0.1 mg/kg of either 8-OH-DPAT or 1 mg/kg of flesinoxan. In contrast, the phenylephrine-induced pressor responses were markedly diminished at 5 min after treatment with 1 mg/kg of either 5-methyl-urapidil or MDL 75,608A, but slowly recovered thereafter. These results show that the 5-HT1A receptor agonists 8-OH-DPAT, flesinoxan, 5-methyl-urapidil and MDL 75,608A show antihypertensive properties in conscious SHR after iv or icv injection. However, the mechanism of action of the compounds differs: 8-OH-DPAT and flesinoxan may act predominantly as 5-HT1A receptor agonists, where as 5-methyl-urapidil and MDL 75,608A also seem to have an effect on peripheral alpha 1-adrenoceptors.