Influences of isoproterenol pretreatment on cerebral cortical bindings of [3H]clonidine and [3H]dihydroalprenolol in infant and adult rats

Development and isoproterenol-induced regulation of adrenoceptor binding in cultured rat neocortical explants is seen only with the beta-1, not with the beta-2 subtype

The presence and time-course of beta-adrenoceptor density in cultured explants of neocortex obtained from 6-day-old rat pups were investigated using a [125I]ICYP binding assay. A delayed, but more pronounced, increase in the receptor expression was observed as compared to the situation previously described in vivo. These changes only occurred for the beta 1-subtype of the receptor, whereas the beta 2-subtype binding remained constant up to 3 weeks in vitro. The delay of beta 1-adrenoceptor expression may be due to the incomplete presence of the proper maturational input, and the late enhancement of receptor expression to upregulation related to the absence in vitro of noradrenergic input. Decreased beta-adrenoceptor levels could be induced by chronic treatment of the beta-agonist isoproterenol (1 microM) introduced either for 3 or 13 days. Again, changes in density were found only for the beta 1-adrenoceptor binding sites. There is no reduction of receptor density following return to control conditions for 10 days after a 3-day treatment with isoproterenol, demonstrating the ability of this model to attain its final receptor density notwithstanding the developmental insult.

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.

Changes in adrenoceptors and monoamine metabolism in neonatal and adult rat brain after postnatal exposure to the antihypertensive labetalol

1. The purpose of the present study was to investigate the acute (single injection), direct (chronic treatment) and the long-lasting effects after exposure to the alpha 1/beta-adrenoceptor antagonist labetalol during rat brain development on adrenoceptors and monoamine metabolism. 2. In 10-day-old rat pups, subcutaneously administered labetalol (10 mg kg-1) passed the blood-brain barrier, reaching a level of 2.1 micrograms g-1 tissue in the brain 90 min after injection. 3. Chronic labetalol treatment (10 mg kg-1, s.c., twice daily) during the first 10 days of life significantly increased alpha 1-adrenoceptor binding in the hypothalamus (+39%), but not in the occipital cortex. 4. This chronic postnatal labetalol treatment did not result in long-lasting changes in alpha 1- and beta-receptors measured on day 60. 5. A single labetalol injection (10 mg kg-1, s.c.) on postnatal day 10 significantly increased noradrenaline (NA) metabolism in all brain regions tested (+25 to 105%), but had no effects on 5-hydroxytryptamine (5-HT) or dopamine metabolism. 6. Chronic labetalol treatment between postnatal (PN) days 1 and 10 also increased NA metabolism on PN 10 (3-methoxy-4-hydroxyphenylglycol (MHPG)/NA, +20 to 100%), suggesting that tolerance to the acute effect of labetalol did not occur. A slight increase in 5-HT metabolism (20%) was induced by the chronic labetalol treatment in the hippocampus and meso-limbic system. 7. In general, long-lasting effects on NA metabolism could not be detected on day 60 more than one month after the treatment. However, 5-HT metabolism was significantly increased in all four brain regions measured (+20 to 70%). 8. We conclude that chronic labetalol exposure during early postnatal rat brain development does not cause long-lasting changes in beta-receptor number or NA metabolism, but appears to be critical for the rate of 5-HT metabolism in later life.

beta-Adrenoceptor stimulation-induced increase in the number of alpha 2-adrenoceptors of cerebral cortical membranes in rats

Effects of pre-treatment of synaptic membranes with beta-adrenoceptor agonists and cholera toxin on [3H]clonidine and [3H]yohimbine binding were examined in rat cerebral cortex. Pre-incubation of cerebral cortical membranes with isoproterenol (10 or 200 microM) or dobutamine (1, 10 or 200 microM) at 37 degrees C for 40 min caused a significant elevation of specific [3H]clonidine binding but treatment with salbutamol (10 or 200 microM) did not. Scatchard analysis showed that 200 microM isoproterenol treatment resulted in a significant elevation of high affinity component of [3H]clonidine binding which was significantly decreased by the addition of 10 microM GTP. A significant elevation in high affinity [3H]clonidine binding was observed by treatment with 100 micrograms/ml cholera toxin, while a significant decrease in low affinity one was by the treatment. Specific [3H]yohimbine binding was also elevated by 10 or 200 microM isoproterenol treatment. It is suggested that stimulation of beta-receptors, presumably beta 1-subtype, could elevate the number of agonist and antagonist binding sites in alpha 2-receptors in synaptic membranes by partially mediated by stimulatory and/or inhibitory GTP binding regulatory proteins.

The interaction between beta- and alpha 2-adrenoceptors in cerebral cortical membranes: isoproterenol-induced increase in [3H]clonidine binding in rats

The pretreatment of rat cerebral cortical membranes with 10 or 100 microM isoproterenol at 37 degrees C for 40 min caused a significant elevation of the Bmax value of [3H]clonidine binding but pretreatment at 4 degrees C did not affect the value. The isoproterenol-induced increase in the Bmax value of the binding was higher in 50 mM Tris-HCl buffer (pH 7.7) than in Krebs-Ringer solution. In 50 mM Tris-HCl buffer (pH 7.7), treatment with isoproterenol reduced the Bmax value of [3H]dihydroalprenolol binding but neither Kd nor Bmax of [3H]WB 4101 binding was affected by this treatment. Fifty microM propranolol or 100 microM GTP produced a significant reduction in isoproterenol-induced elevation of the Bmax value of [3H]clonidine binding. In contrast, 100 microM cyclic AMP did not affect the control binding and 0.1 or 1 mM theophylline did not affect the isoproterenol-induced elevation of the binding. The only Bmax value in high affinity binding of [3H]clonidine was increased by isoproterenol. It is suggested that isoproterenol increases the density of alpha 2-adrenoceptors in a temperature-dependent manner. The direct interaction between beta- and alpha 2-receptor molecules and/or their indirect interaction, mediated by GTP regulatory proteins, would exist in the cerebral cortical membranes of rats.

Developmental changes of cerebral cortical [3H]clonidine binding in rats: influences of guanine nucleotide and cations

Cerebral cortical [3H]clonidine binding and influences of GTP and cations were investigated in developing rats. The results from Scatchard plots were compatible with the presence of two populations of binding sites [high-affinity binding (KD = 0.59 nM) and low-affinity binding (KD = 7.12 nM)] in 70-day-old rats but only high-affinity binding (KD = 0.27 nM) on day 1. Low-affinity binding was detectable on day 7. KD values in high- and low-affinity binding were not significantly changed during development after 7 days. Bmax of high-affinity binding reached a peak on day 15, and the value of low-affinity binding gradually increased with age. The addition of 10 microM GTP caused a significant reduction in Bmax of high-affinity binding after day 7. Neither KD nor Bmax of low-affinity binding was affected by 10 microM GTP during development. NaCl (10 and 100 mM) diminished the binding on days 7 and 70. MnCl2 (0.1 and 1.0 mM) markedly increased the binding on days 15 and 70 but not on day 7. It is suggested that: (1) single binding sites of alpha 2-adrenoceptors with higher affinity seem to be present on day 1; (2) low-affinity binding appears on day 7; (3) the number of high-affinity binding sites reaches a peak on day 15, followed by changes in populations of high-affinity as well as low-affinity sites without changing affinity; (4) the regulatory mechanism in alpha 2-receptors by guanine nucleotide reaches functional maturity between days 1 and 7; and (5) the involvement of Na+ and Mn2+ in alpha 2-receptor binding becomes functional by days 7 and 15, respectively.