Ca(2+)-antagonistic action of bevantolol on hypothalamic neurons in vitro: its comparison with those of other beta-adrenoceptor antagonists, a local anesthetic and a Ca(2+)-antagonist

Paradoxical effect of noradrenaline-mediated neurotransmission in the antinociceptive phenomenon that accompanies tonic-clonic seizures: role of locus coeruleus neurons and α(2)- and β-noradrenergic receptors

The postictal state is generally followed by antinociception. It is known that connections between the dorsal raphe nucleus, the periaqueductal gray matter, and the locus coeruleus, an important noradrenergic brainstem nucleus, are involved in the descending control of ascending nociceptive pathways. The aim of the present study was to determine whether noradrenergic mechanisms in the locus coeruleus are involved in postictal antinociception. Yohimbine (an α(2)-receptor antagonist) or propranolol (a β-receptor antagonist) was microinjected unilaterally into the locus coeruleus, followed by intraperitoneal administration of pentylenetetrazole (PTZ), a noncompetitive antagonist that blocks GABA-mediated Cl(-) influx. Although the administration of both yohimbine and propranolol to the locus coeruleus/subcoeruleus area resulted in a significant decrease in tonic or tonic-clonic seizure-induced antinociception, the effect of yohimbine restricted to the locus coeruleus was more distinct compared with that of propranolol, possibly because of the presynaptic localization of α(2)-noradrenergic receptors in locus coeruleus neurons. These effects were related to the modulation of noradrenergic activity in the locus coeruleus. Interestingly, microinjections of noradrenaline into the locus coeruleus also decrease the postictal antinociception. The present results suggest that the mechanism underlying postictal antinociception involves both α(2)- and β-noradrenergic receptors in the locus coeruleus, although the action of noradrenaline on these receptors causes a paradoxical effect, depending on the nature of the local neurotransmission.

Pharmacological evidence for the involvement of diacylglycerol lipase in depolarization-induced endocanabinoid release

Depolarization-induced suppression of inhibition (DSI) or excitation (DSE) is a well-known form of endocannabinoid-mediated short-term plasticity that is induced by postsynaptic depolarization. It is generally accepted that DSI/DSE is triggered by Ca(2+) influx through voltage-gated Ca(2+) channels. It is also demonstrated that DSI/DSE is mediated by 2-arachidonoylglycerol (2-AG). However, how Ca(2+) induces 2-AG production is still unclear. In the present study, we investigated molecular mechanisms underlying the Ca(2+)-driven 2-AG production. Using cannabinoid-sensitive inhibitory synapses of cultured hippocampal neurons, we tested several inhibitors for enzymes that are supposed to be involved in 2-AG metabolism. The chemicals we tested include inhibitors for phospholipase C (U73122 and ET-18), diacylglycerol kinase (DGK inhibitor 1), phosphatidic acid phosphohydrolase (propranolol), and diacylglycerol lipase (DGL; RHC-80267 and tetrahydrolipstatin (THL)). However, unfavorable side effects were observed with these inhibitors, except for THL. Furthermore, we found that RHC-80267 hardly inhibited the endocannabinoid release driven by G(q/11)-coupled receptors, which is thought to be DGL-dependent. By contrast, THL exhibited no side effects as long as we tested, and was confirmed to inhibit the DGL-dependent process. Using THL as a DGL inhibitor, we demonstrated that DGL is involved in both hippocampal DSI and cerebellar DSE. To test a possible involvement of PLCdelta in DSI, we examined hippocampal DSI in PLCdelta1, delta3 and delta4-knockout mice. However, there was no significant difference in the DSI magnitude between these knockout mice and wild-type mice. The present study clearly shows that DGL is a prerequisite for DSI/DSE. The enzymes yielding DG remain to be determined.

Vasodilating beta-adrenoceptor blockers as cardiovascular therapeutics

beta-Adrenoceptor blocking agents (beta-blockers) have been established as therapeutics for treatment of patients with hypertension, ischemic heart diseases, chronic heart failure, arrhythmias, and glaucoma. However, their clinical use is limited because some patients are adversely affected by their side effects. The discovery of cardioselective (beta(1)-selective) blockers has overcome some of the problems. Current retrospective studies have revealed that vasodilating beta-blockers (so-called beta-blockers of the third generation) have advantages over the conventional type of beta-blockers in terms of minimizing the adverse effects and improving the disease-derived dysfunction, thus enhancing the quality of life variables. Some of the possible advantages include improvement of insulin resistance, decrease in low-density lipoprotein cholesterol in association with increase in high-density lipoprotein cholesterol, attenuation of bronchial asthma attack and respiratory dysfunction, alleviation of coronary vasospasm provocation, peripheral circulatory disturbances, and erectile dysfunction, and better patient compliance. Release of nitric oxide, antioxidant action, beta(2)-adrenoceptor activation, Ca(2+) entry blockade, and other mechanisms underlying the vasodilating action may be responsible for the beneficial therapeutic effects of these agents.

Effect of betaxolol on aspartate aminotransferase activity in hypoxic rat retina in vitro

We investigated the effect of betaxolol on the decrease of mitochondrial aspartate aminotransferase (mAAT) activity in rat retinas induced by hypoxia in vitro. It is reported that mAAT decreases in ischemic or hypoxic retina and that the decrease is caused by Ca(2+)-dependent proteases such as calpain. Betaxolol is a compound that has beta(1)-adrenergic receptor blocking and voltage-dependent calcium channel blocking properties. The rat eye cups were maintained with Locke's solution saturated with 95% air - 5% CO(2). The eye cups were immersed in glucose-free Locke's solution saturated with 95% N(2) / 5% CO(2) (hypoxic solution). Ninety minutes of hypoxia caused a 20% decrease in mAAT activity. The eye cups incubated with the hypoxic solution containing 1 mM EGTA, 10 micro M MK-801 or 100 micro M betaxolol were protected from the decrease in mAAT activity, so that the residual mAAT activity was 50%, 45% or 40%, respectively, compared to the eye cups incubated in hypoxic solution alone, which had a 100% decrease in mAAT activity. In addition, co-incubation with EGTA and betaxolol had a greater protective effect against the mAAT decrease than a single application. This additive effect of betaxolol was dose-dependent. These results suggested that betaxolol had a protective effect against the decrease of mAAT caused by hypoxia and indicated that betaxolol might inhibit the Ca(2+) release from intracellular Ca(2+) stores.

Topically applied betaxolol attenuates NMDA-induced toxicity to ganglion cells and the effects of ischaemia to the retina

The present results show that topically applied Betoptic(R)(0.5% betaxolol) to the rabbit or rat eye reaches the retina and can counteract the detrimental effects caused by ischaemia/reperfusion or N -methyl- d -aspartate (NMDA)-induced insults to the retina. Betaxolol is a beta(1)-adrenergic blocker but its neuroprotective action is generally thought to be due to its calcium channel blocking properties. Support for this view comes from studies on cultures of cortical neurones where it was found that betaxolol attenuated the NMDA-induced influx of(45)Ca(2+)while beta-adrenoreceptor agonists were ineffective. Topically applied Betoptic(R)to the rabbit eye was observed to reach the retina in maximal amounts within 60 min. Some of the substance was also found in the contralateral retina of the untreated eye suggesting that the agent reaches the retina by local systemic and retinal circulation. Concurrent treatment with Latanoprost(R)did not result in a greater amount of betaxolol reaching the retina. An ophthalmodynamometric procedure, which raises the intraocular pressure, was used to apply an ischaemic insult to the rabbit retina. After three days of reperfusion the b-wave of the electroretinogram was reduced by an average of 59% and the choline acetyltransferase immunoreactivity in the retina was almost obliterated. However, when experiments were carried out on animals which had been treated with one drop of Betoptic(R) twice daily for 4 weeks before ischaemia and also during the reperfusion phase, the reductions in both the b-wave of the electroretinogram and retinal choline acetyltransferase immunoreactivity due to ischaemia/reperfusion were greatly attenuated. Intravitreal injection of NMDA into the rat eye caused a decrease in the immunostaining for Thy-1 antigen which is associated with ganglion cells. The Thy-1 mRNA level was also reduced as was the mRNA for the common subunit of the NMDA receptor, the NR1 subunit. However, in animals subjected to a topical Betoptic(R)regime, before and after intravitreal injection of NMDA, the decreases in the mRNA levels of Thy-1 and NR1 were significantly attenuated.