Peter H. Wu

Nifedipine delays the acquisition of tolerance to ethanol

Rats were trained to remain on a motor-driven belt until they reached a criterion of not exceeding 1% error (1.2 s off the belt) in a 2 min trial. Upon completion of training, animals were treated with nifedipine and/or ethanol and were tested for degree of impairment after an i.p. injection of 2 g/kg ethanol solution on test days. Chronic nifedipine treatment enhances the acute ethanol effects on motor impairment and delays the acquisition of tolerance to ethanol. Results suggest modification of brain calcium channel activities could delay the development of tolerance to ethanol and may prevent ethanol-induced organic brain damage.

Morphine enhances adenosine release from the in vivo rat chemical cortex

Morphine (1 and 5 mg/kg), administered intravenously, increased the rate of efflux of purines from intact rat cerebral cortices prelabelled with 3H-adenosine. Naloxone antagonized morphines action. It is suggested that the depressant actions of morphine on transmission in the central nervous system may be related to this enhancement of extracellular levels of adenosine and the adenine nucleotides.

Characterization of receptor-mediated catecholamine activation of rat brain corticl Na+-K+-ATPase

1. 1. Catechol itself activates Na+-K+-ATPase. The catechol analogues, resorcinol, hydroquinone and gentisic acid are all ineffective in stimulation of the enzyme. Modification of the catecholic moiety by addition of an aldehyde or carboxyl group to the catechol structure reduces the effectiveness of the compound to stimulate Na+-K+ATPase while addition of an ethanolamine side chain enhances its activity. 2. 2. A catechol group is not essential for a compound to activate Na+-K+-ATPase because methoxamine (an α-agonist; α-(l-amino-ethyl-2,5-dimethoxybenzyl alcohol), a non-catecholic compound, can stimulate cerebral cortical Na+K+-ATPase with an EC20 value comparable to that of L-noradrenaline. 3. 3. L-noradrenaline is approximately 10-fold more potent as a Na+-K+-ATPase activator than D-noradrenaline (according to EC20 values). L-isoprenaline is approx 3-fold more active than D-isoprenaline. Activation of Na+-K+-ATPase induced by L-noradrenaline can be antagonized by phentolamine or DL-propranolol. However, enzyme activity stimulated by D-noradrenaline cannot be blocked by phentolamine or DL-propranolol. Enzyme activity evoked by the o- and L-isomers of isoprenaline can be antagonized by DL-propranolol. Stimulation of Na+-K+-ATPase by catechol cannot be blocked by either phentolamine or DL-propranolol. 4. 4. Na+-K+-ATPase solubilized from cell membranes by Triton X-100 no longer responds to noradrenaline stimulation, suggesting the activation of Na+K+-ATPase by noradrenaline is not exerted directly on the enzyme molecule. 5. 5. Modification of the cyclic AMP generating system by treatment with GPP(NH)P failed to alter the sensitivity of Na+-K+-ATPase to noradrenaline, suggesting that the reaction is probably not linked by cyclic AMP. 6. 6. There are at least two mechanisms involved in the noradrenaline activation of Na+K+-ATPase, namely, (a) a receptor (pharmacological characteristics similar to adrenergic receptor)-mediated activation of the enzyme, and (b) a non-specific action of catecholic compounds which may be a result of their chelating properties.

The effect of morphine on purine and acetylcholine release from rat cerebral cortex: Evidence for a purinergic component in morphine's action

Morphine enhances the release of adenosine and its metabolites from the rat cerebral cortex and inhibits the release of acetylcholine. Naloxone antagoinizes the effects of morphine on both purine and acetylcholine release. The adenosine antagonists, caffeine and theophylline, reduce morphines effects on acetylcholine release, and at the same time increase the spontaneous release of acetylcholine. It is suggested that morphine, acting at a naloxone-sensitive site, enhances the level of extracellular adenosine, which in turn inhibits the release of acetylcholine, and that some of morphines actions are mediated by a purinergic step.

Effects of vanadate on brain (Na+K+)ATPase and p-nitrophenylphosphatase-interactions with mono- and di-valent ions and with noradrenaline

Abstract 1. 1. Vanadate inhibits rat brain (Na+K+)ATPase. The inhibition was potentiated by increasing Mg2+ and K+ concentrations. Na+ reversed vanadate inhibition of the enzyme. 2. 2. Co2+ and Mn2+ stimulate (Na+K+)ATPase in concentrations less than 10−4 M. Co2+ potentiated vanadate inhibition of the enzyme, however, Mn2+ antagonized the vanadate effect. 3. 3. High concentrations of vanadate were required to inhibit p-nitrophenylphosphatase. This inhibition was potentiated by increasing Mg2+ concentrations. 4. 4. Kinetic analysis on vanadate inhibition of (NaK+)ATPase showed an uncompetitive inhibition, and vanadate inhibition of p-nitrophenylphosphatase a competitive inhibition, with respect to their substrates. 5. 5. EDTA (10−5M) and noradrenaline (10−4M) reversed vanadate (10−6 M) inhibition of (Na+K+)ATPase. 6. 6. The additive effects of EDTA and noradrenaline suggest that these two compounds activate (Na+K+ATPase by different mechanisms. 7. 7. Na+-activated Mg2+-dependent kinase is probably the primary target for vanadate inhibition.

The Role of Adenosine in Central Neuromodulation

The last decade has witnessed an enormous expansion of the research commitment into the actions of adenosine and its nucleotides on nerve and muscle cells. Interest in adenosine is not, however, a new phenomenon, and over fifty years have elapsed since the first observations of the actions of adenosine and adenosine 5’-monophosphate (AMP) on smooth and cardiac muscles [1]. The intervening years have seen a continuing interest in the peripheral actions of adenosine and its nucleotides, with the publication of an important monograph, Biological Actions of the Adenosine Nucleotides [2] in 1950. This text described the actions of adenine nucleotides and related purines on the cardiovascular and respiratory systems and clearly enunciated some of the structural requirements necessary for activation of purinergic receptors. The modern era of research on adenosine was initiated by the hypothesis, proposed by Burnstock and his colleagues [3], that adenosine 5’-triphosphate (A TP) is the transmitter released from so-called purinergic nerves, which form a third division of the autonomic nervous system. The hypothesis itself and the supporting data were elegantly outlined in a subsequent review [4]. It is curious that, although the status of the purinergic hypothesis remains somewhat uncertain, at least in the form in which it was originally presented, there has been widespread acceptance of the concept of adenosinergic modulation of transmitter release in both the central and peripheral nervous systems.

Nucleoside transport in rat cerebral cortical synaptosomal membrane: A high affinity probe study

1. The nucleoside transport system in rat cerebral cortical synaptosomes was investigated using [H3]p-nitrobenzylthioinosine (NBMPR) as a high affinity probe. 2. There are high affinity and low affinity binding sites for NBMPR on rat synaptosomal membranes. The high affinity sites showed a KD value of 0.05 nM and a Bmax value of 113 fmol/mg protein. 3. Biochemical characterization of the high affinity [H3]NBMPR binding sites indicated that they probably correspond to nucleoside transport sites. 4. Several known adenosine uptake inhibitors including clonazepam were tested for their interaction with this high affinity binding site. 5. The results suggest that hexobendine and papaverine inhibit adenosine uptake by occupying the [H3]NBMPR high affinity binding sites. 6. Clonazepam and dipyridamole appear to inhibit adenosine uptake in rat cerebral cortical synaptosomes via an interaction at a different site.

Morphine enhances the release of 3H-purines from rat brain cerebral cortical prisms.

In vitro experiments have shown that 3H-purines can be released from 3H-adenosine preloaded rat brain cortical prisms by a KCl-evoked depolarization. The KCl-evoked release of 3H-purines is dependent on the concentration of KCl present in the superfusate. At concentrations of 10(-7) approximately 10(-5)M morphine did not influence the basal release of 3H-purines from the prisms, although it enhanced the KCl-evoked release of 3H-purines. The enhancement of KCl-evoked 3H-purine release by morphine was concentration-dependent and was antagonized by naloxone, suggesting the involvement of opiate receptors. Uptake studies with rat brain cerebral cortical synaptosomes show that morphine is a very weak inhibitor of adenosine uptake. Comparisons with dipyridamole, a potent inhibitor of adenosine uptake, suggest that this low level of inhibition of the uptake did not contribute significantly to the release of 3H-purine by morphine seen in our experiments. It is therefore suggested that morphine enhances KCl-evoked 3H-purine release by an interaction with opiate receptors and that the resultant increase in extracellular purine (adenosine) levels may account for some of the actions of morphine.

Increase in the brain regional depolarization-dependent Ca2+ uptake in rats preferring ethanol

The depolarization-dependent Ca2+ uptake system has been suggested to be involved in the release of transmitter and synaptic facilitation. It can be employed as an effective probe to study neurotransmission. Although ethanol has been shown to inhibit or facilitate neurotransmission very little is known about the intrinsic activity of neurotransmission in ethanol-preferring rats. Using the depolarization-dependent Ca2+ uptake system, we demonstrated that synaptic neurotransmission is more active in animals with moderate and high preference for ethanol. Results suggest that there are intrinsic differences in the brain regional neurotransmission among rats showing different degrees of preference for ethanol.