Behavioral and biochemical studies in rats following prenatal treatment with beta-adrenoceptor antagonists

Atenolol developmental toxicity: animal-to-human comparisons

BACKGROUND

Atenolol, 4-2'-hydroxy-3'-isopropyl-aminopropoxy) phenylacetamide, is a beta-adrenoreceptor blocker used for treatment of hypertension in pregnancy. Beta-blockers are reported to cause fetal harm (such as decreased birth weight) when administered to a pregnant woman. We evaluate published human and animal evidence of atenolol developmental toxicity and compare the manifestations in humans and in routinely-used animal models.

METHODS

The comparison is based on the following criteria: comparability of pharmacokinetic/pharmacodynamic characteristics, type of adverse outcome, lowest adverse effect levels, and specificity and selectivity of effect.

RESULTS

Manifestations of atenolol prenatal toxicity (placental changes, intrauterine growth retardation and changes in fetal weight in the absence of structural malformations) are similar in the tested animal species (rats and rabbits) and humans. The human seems to be more sensitive, however, because adverse embryo-fetal effects are reported at doses much lower than those in the tested species. In humans and rats, adverse embryo-fetal effects are induced by doses that are not maternally toxic. In the rabbit, however, such effects are seen only at maternally toxic doses, suggesting that in this species, developmental toxicity may be maternally mediated.

CONCLUSIONS

The available data suggest animal-human concordance with regard to the nature and manifestations of atenolol prenatal toxicity. The animal models "predicted" developmental toxicity manifests as placental changes, intrauterine growth retardation and fetal weight decrease in the absence of structural malformations. Thus far, this is concordant with the data from humans, in whom intrauterine growth retardation has been observed but not structural abnormalities.

Regulation of the neurotensin NT(1) receptor in the developing rat brain following chronic treatment with the antagonist SR 48692

The aim of the present study was to investigate the role of neurotensin in the regulation of NT(1) receptors during postnatal development in the rat brain. Characterization of the ontogeny of neurotensin concentration and [(125)I]neurotensin binding to NT(1) receptors in the brain at different embryonic and postnatal stages showed that neurotensin was highly expressed at birth, reaching peak levels at postnatal day 5 (P5) and decreasing thereafter. The transient rise in neurotensin levels preceded the maximal expression of NT(1) receptors, observed at P10, suggesting that neurotensin may influence the developmental profile of NT(1) receptors. Using primary cultures of cerebral cortex neurons from fetal rats, we showed that exposure to the neurotensin agonist JMV 449 (1 nM) decreased (-43%) the amount of NT(1) receptor mRNA measured by reverse transcription-PCR, an effect that was abolished by the nonpeptide NT(1) receptor antagonist SR 48692 (1 microM). However, daily injection of SR 48692 to rat pups from birth for 5, 9, or 15 days did not modify [(125)I]neurotensin binding in brain membrane homogenates. Moreover, postnatal blockade of neurotensin transmission did not alter the density and distribution of NT(1) receptors assessed by quantitative autoradiography nor NT(1) receptor mRNA expression measured by in situ hybridization in the cerebral cortex, caudate-putamen, and midbrain. These results suggest that although NT(1) receptor expression can be regulated in vitro by the agonist at an early developmental stage, neurotensin is not a major factor in the establishment of the ontogenetic pattern of NT receptors in the rat brain.

Increased alpha 2-adrenoceptor mediated regulation of adult rat brain noradrenaline overflow after chronic neonatal exposure to propranolol; a microdialysis study

Direct and persistent effects of chronic neonatal administration of the beta-adrenoceptor antagonist propranolol on brain noradrenergic activity were investigated by measuring tissue concentrations of noradrenaline and its metabolites and in vivo overflow of noradrenaline during adulthood. Rat pups were chronically treated with propranolol from postnatal day 1 to day 10. Determination of monoamine metabolism after the last injection showed an increase in noradrenaline metabolism in frontal cortex, limbic system and hippocampus of propranolol-exposed rats, but 47 days after this last injection it was apparent that these effects were not long-lasting. Moreover, basal noradrenaline overflow in vivo in the hippocampus of 40-55 day-old propranolol-exposed rats did not differ from that in controls. However, the regulation of noradrenaline release seemed to have been altered, since a pharmacological challenge with the alpha 2-adrenoceptor antagonist idazoxan induced an enhanced increase in the in vivo noradrenaline overflow in propranolol-exposed rats compared to controls. It is suggested that the neonatal beta-blockade induced a supersensitivity of the presynaptic alpha 2-adrenoceptor. The precise mechanism underlying this effect has to be elucidated.

Developmental changes in rat brain monoamine metabolism and beta-adrenoceptor subtypes after chronic prenatal exposure to propranolol

The aim of this study was to investigate whether a chronic prenatal beta-blockade can alter the maturation of the noradrenergic system in the rat brain. Pregnant female and adult male rats were treated for 10 days with the beta-antagonist propranolol dissolved in the drinking water (40-50 mg/kg/day). Direct and long-term effects on beta-adrenoceptors and monoamine metabolism in various rat brain regions were determined. After the prenatal treatment the propranolol level in the foetal brain was 0.9 micrograms/g, while in the adult brain 2.0 micrograms/g was present. The foetal beta 1-receptors were significantly up-regulated by propranolol (200%), whereas the beta 2-receptor number remained unaltered. On postnatal days 4 and 21 the number of both beta-subtypes was the same as that of controls. Noradrenaline, its metabolite 3-methoxy-4-hydroxyphenylglycol and their ratio were unaltered directly after the prenatal treatment. In the PN 21 offspring, however, the metabolite level had increased in the frontal cortex (+ 17%) and hippocampus (+ 32%), and the ratio in the hippocampus (37%) and medulla pons (+ 34%). Prenatal treatment also induced a significant increase of the 5-hydroxyindoleacetic acid.5-hydroxytryptamine ratio (+ 15%) in the medulla pons at GD 21. No direct or lasting effects were found on dopamine metabolism. Propranolol treatment of adult rats gave no direct changes in monoamine metabolism. We concluded that chronic prenatal propranolol exposure (a) reversibly up-regulates foetal beta 1-adrenoceptors, and (b) increases the NA activity in the brain in later life.

Postnatal exposure to the beta-adrenoceptor agonist clenbuterol has regionally selective direct and long-term effects on rat brain beta-adrenoceptors and monoamine metabolism

The effects of chronic postnatal beta 2-adrenoceptor activation on the maturation of the rat brain noradrenergic system have been studied. For that purpose, rat pups have been treated twice daily during the first 10 days of life with the beta 2-agonist clenbuterol-HCl (2.5 mg/kg s.c.), and the effects on the beta-adrenoceptor number and monoamine metabolism have been determined directly after the treatment and in adulthood. On postnatal day 10, 90 min after the last clenbuterol injection 4.5 micrograms/g of the drug was present in the brain. At the end of the treatment the beta-receptor binding had decreased in the cerebellum (35%), but not in the frontal cortex or mesolimbic system. Clenbuterol significantly increased the steady-state brain levels of noradrenaline (NA) in the striatum 90 min after the last injection, whereas the levels in the frontal cortex, meso-limbic system, medulla pons and cerebellum were unaffected. The NA metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG), had significantly increased in the frontal cortex and striatum. The serotonergic (5-HT) and dopaminergic (DA) system were not altered. In general, no long-lasting effects on beta-adrenoceptor number and affinity or monoamine metabolism were measurable, except for the frontal cortex which showed a sustained increase of MHPG, a decrease of 5-HT and an increase of 5-HIAA/5-HT on PN 60. In conclusion, chronic postnatal activation of beta 2-adrenoceptors by clenbuterol treatment selectively causes changes in the setting of the neurochemical parameters investigated in the frontal cortex.