In vivo specific binding characteristics and pharmacokinetics of a 1,4-dihydropyridine calcium channel antagonist in the senescent mouse brain

Imaging early embryonic calcium activity with GCaMP6s transgenic zebrafish

Intracellular Ca²⁺ signaling regulates cellular activities during embryogenesis and in adult organisms. We generated stable Tg[βactin2:GCaMP6s]^stl351 and Tg[ubi:GCaMP6s]^stl352 transgenic lines that combine the ubiquitously-expressed Ca²⁺ indicator GCaMP6s with the transparent characteristics of zebrafish embryos to achieve superior in vivo Ca²⁺ imaging. Using the Tg[βactin2:GCaMP6s]^stl351 line featuring strong GCaMP6s expression from cleavage through gastrula stages, we detected higher frequency of Ca²⁺ transients in the superficial blastomeres during the blastula stages preceding the midblastula transition. Additionally, GCaMP6s also revealed that dorsal-biased Ca²⁺ signaling that follows the midblastula transition persisted longer during gastrulation, compared with earlier studies. We observed that dorsal-biased Ca²⁺ signaling is diminished in ventralized ichabod/β-catenin2 mutant embryos and ectopically induced in embryos dorsalized by excess β-catenin. During gastrulation, we directly visualized Ca²⁺ signaling in the dorsal forerunner cells, which form in a Nodal signaling dependent manner and later give rise to the laterality organ. We found that excess Nodal increases the number and the duration of Ca²⁺ transients specifically in the dorsal forerunner cells. The GCaMP6s transgenic lines described here enable unprecedented visualization of dynamic Ca²⁺ events from embryogenesis through adulthood, augmenting the zebrafish toolbox.

The L-type channel antagonist isradipine is neuroprotective in a mouse model of Parkinson's disease

The motor symptoms of Parkinson's disease (PD) are due to the progressive loss of dopamine (DA) neurons in substantia nigra pars compacta (SNc). Nothing is known to slow the progression of the disease, making the identification of potential neuroprotective agents of great clinical importance. Previous studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have shown that antagonism of L-type Ca2+ channels protects SNc DA neurons. However, this was not true in a 6-hydroxydopamine (6-OHDA) model. One potential explanation for this discrepancy is that protection in the 6-OHDA model requires greater antagonism of Cav1.3 L-type Ca2+ channels thought to underlie vulnerability and this was not achievable with the low affinity dihydropyridine (DHP) antagonist used. To test this hypothesis, the DHP with the highest affinity for Cav1.3L-type channels-isradipine-was systemically administered and then the DA toxin 6-OHDA injected intrastriatally. Twenty-five days later, neuroprotection and plasma concentration of isradipine were determined. This analysis revealed that isradipine produced a dose-dependent sparing of DA fibers and cell bodies at concentrations achievable in humans, suggesting that isradipine is a potentially viable neuroprotective agent for PD.

In vivo characterization of muscarinic receptors in peripheral tissues: evaluation of bladder selectivity of anticholinergic agents to treat overactive bladder

The present study was undertaken to characterize in vivo muscarinic receptors in peripheral tissues (urinary bladder, submaxillary gland, colon, stomach, heart) of mice, and further to evaluate bladder-selectivity of anticholinergic agents to treat overactive bladder. Following i.v. injection of [3H]QNB in mice, the radioactivity in peripheral tissues was exclusively detected as the unchanged form. The in vivo specific [3H]QNB binding in particulate fraction of tissue homogenates of mice showed a pharmacological specificity which characterized muscarinic receptors. Binding parameters (Kd and Bmax) for in vivo specific [3H]QNB binding differed between mouse tissues. Oral administration of oxybutynin attenuated significantly in vivo specific [3H]QNB binding in all tissues of mice. From ratios of AUCurinary bladder/AUCother tissues of time-dependent muscarinic receptor occupancy, oral oxybutynin has been shown to exert little urinary bladder selectivity. Following oral administration of propiverine, there was a significant reduction of in vivo specific [3H]QNB binding in the urinary bladder, colon and submaxillary gland, but not in the stomach and heart. From the ratios of AUCurinary bladder to AUCsubmaxillary gland or AUCheart, it has been shown that oral propiverine exerts higher selectivity to muscarinic receptors in the urinary bladder than in the submaxillary gland and heart. Similarly, tolterodine displayed high selectivity to muscarinic receptors in the urinary bladder than in the submaxillary gland. Thus, the present study has demonstrated that [3H]QNB may be a useful ligand for in vivo characterization of muscarinic receptor binding of anticholinergic agents to treat overactive bladder. Propiverine and tolterodine have exhibited in vivo selectivity of muscarinic receptor in the mouse urinary bladder rather than in the submaxillary gland, and such receptor binding specificity may be the reason of lower incidence of dry mouth.

Blood-brain permeability of [3H]-(3-methyl-His2)thyrotropin-releasing hormone (MeTRH) in mice: effects of TRH and its analogues

The present study was undertaken to characterize the transport of (3-methyl-His(2)) thyrotropin-releasing hormone ([(3)H]MeTRH) across the blood-brain barrier in mice and the effects of thyrotropin-releasing hormone (TRH) and its analogues (taltirelin and montirelin) on the transport and brain distribution. Integration plot analysis was used to calculate the influx clearance (CL(in)) of [(3)H]MeTRH after intravenous (i.v.) injection in mice. Furthermore, the capillary depletion method was performed to determine whether [(3)H]MeTRH crossed the blood-brain barrier. The effects of TRH and its analogues on the brain distribution of [(3)H]MeTRH were also examined by co-injection with the radioligand. The brain distribution of [(3)H]MeTRH and [(14)C]sucrose increased with the time after i.v. injection in mice, and the level of [(3)H]MeTRH was significantly higher than that of [(14)C]sucrose 5 min after the injection. The CL(in) value of [(3)H]MeTRH was significantly higher than that of [(14)C]sucrose, and the value of [(3)H]MeTRH was reduced by co-injection with unlabeled MeTRH. Also, capillary depletion showed that [(3)H]MeTRH was distributed largely in the brain parenchyma and this distribution was significantly inhibited by co-injection of TRH and montirelin but not taltirelin. The present study indicates that the transport of [(3)H]MeTRH into the brain may be via a saturable process.

In vivo identification and characterization of binding sites for selective serotonin reuptake inhibitors in mouse brain

The present study was undertaken to identify and characterize in vivo binding sites of selective serotonin reuptake inhibitors (SSRIs) in the mouse brain by using [3H]paroxetine as radioligand. Relatively higher concentration of [3H]paroxetine was detected in the whole brain (minus cerebellum) than in the plasma of mice after the i.v. injection of the radioligand, and the half-life (t1/2) of elimination was much slower. The in vivo specific [3H]paroxetine binding in the mouse brain after the i.v. injection was defined as the difference of particulate-bound radioactivity between the whole brain and cerebellum, and it was dose-dependently attenuated by oral or intraperitoneal administration of fluoxetine (8.68-116 micromol/kg). Furthermore, oral administration of fluvoxamine, fluoxetine, paroxetine and sertraline at the pharmacologically relevant doses reduced significantly (25-94%) in vivo specific [3H]paroxetine binding in the cerebral cortex, striatum, hippocampus, thalamus and midbrain of mice, and their significant decreases were observed up to at least 8 h (fluvoxamine), 24 h (fluoxetine), and 12 h (paroxetine and sertraline) later. The value of area under the curve (AUC) for decrease in [3H]paroxetine binding vs. time in each brain region was largest for fluoxetine among these SSRIs, due to the relatively longer-lasting occupation of brain serotonin transporter. The AUC value in mouse brain after oral administration of each SSRI was 1.2-3.2 times greater in the thalamus and midbrain than in the cerebral cortex, striatum and hippocampus. Thus, the present study has revealed that [3H]paroxetine may be a suitable radioligand for in vivo characterization of brain binding sites and pharmacological effects of SSRIs.

In vivo receptor binding of benidipine and amlodipine in mesenteric arteries and other tissues of spontaneously hypertensive rats

The present study was undertaken to characterize the in vivo 1,4-dihydropyridine (DHP) receptor binding of long-acting 1,4-DHP calcium channel antagonists in the mesenteric artery and other tissues of SHR. In vivo specific binding of (+)-[3H]PN 200-110 in the SHR mesenteric artery was significantly (36.6-49.7 %) reduced 1-8 h after oral administration of benidipine (1.84 micromol/kg). A greater reduction in (+)-[3H]PN 200-110 binding in the mesenteric artery was observed at a higher dose (5.53 micromol/kg) of this drug. This dose of benidipine also reduced significantly the in vivo specific (+)-[3H]PN 200-110 binding in the aorta but not in the myocardium and cerebral cortex. Following oral administration of amlodipine (17.6 micromol/kg), a significant (51.7-94.2 %) reduction in (+)-[3H]PN 200-110 binding was seen at 1-18 h in the mesenteric artery and at 1-12 h in the aorta. Only a slight reduction in myocardial and cerebral cortical (+)-[3H]PN 200-110 binding was seen following amlodipine administration. In contrast, oral administration of nifedipine (28.9 micromol/kg) reduced markedly in vivo (+)-[3H]PN 200-110 binding in all the tissues of SHR at 1-6 h, and the degree and time-course of the reduction did not differ significantly among the tissues. The area under the curve (AUC) for the receptor occupancy vs time was calculated from the reduction rate (%) of in vivo specific (+)-[3H]PN 200-110 binding. The ratios of the AUCmesenteric artery to AUCaorta or AUCmesenteric artery to AUCmyocardium after oral administration of benidipine and amlodipine were greater than the corresponding value for nifedipine. The degree and time-course of arterial receptor occupancy by benidipine and amlodipine agreed well with those of their hypotensive effects in the conscious SHR. In conclusion, the present study demonstrates that benidipine and amlodipine may occupy, in a more selective and sustained manner, 1,4-DHP receptors in arterial tissues than in other tissues of SHR, and thus, such receptor binding specificity may be responsible for the long-lasting hypotensive effects of these drugs.