Phencyclidine increases the affinity of dihydropyridine calcium channel antagonist binding in rat brain

Regional and subcellular distribution of [125I]endothelin binding sites in rat brain

The binding of [125I]endothelin-1 (125I-ET-1) to membranes from whole rat brain, from individual brain regions, and derived from subcellular fractionation of whole rat brain was investigated. 125I-ET-1 binding to whole rat brain membranes was rapid, concentration-dependent, saturable, and characterized as irreversible because it was not displaced by unlabeled endothelin-1 (ET-1) and different concentrations of ligand produced, with time, a similar magnitude of binding. The maximum binding site capacity and second-order forward rate association constant of binding were 1,946 +/- 147 fm/mg protein and 5.53 +/- 1.72 x 10(6) M-1 s-1. Removal of either extramembranal calcium or membrane-bound calcium and calcium binding proteins did not affect the binding of 125I-ET-1 to whole rat brain membranes. The brain stem and cerebellum contained the highest levels of 125I-ET-1 binding sites, whereas the cerebral cortex, striatum, and hippocampus contained binding site levels three- to fourfold less. Subcellular fractionation of whole rat brain and subsequent analyses of the distribution of 125I-ET-1 binding demonstrated a twofold enrichment of binding sites in the synaptosomal fraction compared to the homogenate. The myelin fraction contained a similar density of binding sites compared to the homogenate, while the mitochondrial and microsomal fractions contained considerably less binding sites. The ribosomal fraction did not contain any 125I-ET-1 binding sites. The subcellular distribution of 125I-ET-1 binding sites did not correlate with the distribution of 5'-nucleotidase, cytochrome-C oxidase, phosphodiesterase, and alkaline phosphatase. Depletion of extracellular calcium increased 125I-ET-1 binding in the synaptosomal fraction but not in the myelin and mitochondrial fractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of voltage-dependent calcium channels in canine brain during global ischemia and reperfusion

Elevated intracellular calcium (iCa2+) plays an important role in the pathophysiology of ischemic brain damage. The mechanisms by which iCa2+ increases are uncertain. Recent evidence implicates the voltage-dependent calcium channel (VDCC) as a likely site for the alteration in Ca2+ homeostasis during ischemia. The purpose of this study was to determine whether VDCCs are altered by global ischemia and reperfusion in a canine cardiac arrest, resuscitation model. We employed the radioligand, [3H]PN200-110, to quantitate the equilibrium binding characteristics of the VDCCs in the cerebral cortex. Twenty-five adult beagles were separated into four experimental groups: (a) nonischemic controls, (b) those undergoing 10-min ventricular fibrillation and apnea, (c) those undergoing 10-min ventricular fibrillation and apnea followed by spontaneous circulation and controlled respiration for 2 and (d) 24 h. Brain cortex samples were taken prior to killing of the animal, frozen immediately in liquid nitrogen, and crude synaptosomal membranes isolated by differential centrifugation/filtration. After 10 min of ischemia the maximal binding (Bmax) of [3H]PN200-110 increased to greater than 250% of control values (control Bmax 11.16 +/- 0.98; ischemic 28.35 +/- 2.78 fmol/mg protein; p less than 0.05). Bmax returned to near control values after 2 h of reperfusion but remained significantly greater than the control at 24 h. Although the affinity constant (Kd) (control = 0.12 +/- 0.03 nM) appeared to increase with ischemia and normalize with reperfusion, the changes were not statistically significant. We conclude that the binding of [3H]PN200-110 to L-type VDCCs is increased after 10 min of global ischemia/anoxia produced by ventricular fibrillation and apnea in the dog.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of nimodipine and diltiazem, alone and in combination, on phencyclidine-induced effects in rats: an EEG and behavioural study

The influence of nimodipine and/or diltiazem on the EEG and behavioural effects induced by phencyclidine (PCP) was assessed in adult male Wistar rats. Nimodipine (2 and 10 mg/kg i.p.) and diltiazem (25-100 mg/kg i.p.) significantly potentiated both EEG (increase in background activity voltage, incidence of clustered slow waves) and behavioural (ataxia mean intensity) effects of PCP (5 mg/kg i.p.). A synergistic effect between low, ineffective doses of both nimodipine (0.5 mg/kg i.p.) and diltiazem (5 and 10 mg/kg i.p.) was also found. These data confirm the recent finding of a positive allosteric modulation existing between benzothiazepine (diltiazem) and dihydropyridine (nimodipine) binding sites. They also suggest that the modulation of calcium channels may play a pivotal role in the expression of PCP-induced effects.

The effects of strychnine on the regulation of voltage-dependent calcium channels by dihydropyridines in brain and heart

The effects of strychnine (STR) were investigated on K(+)-stimulated 45Ca2(+)-uptake into mouse brain neurons, the contractile activity of spontaneously beating rat atria and on [3H]nitrendipine and [3H]BAY K 8644 binding to dihydropyridine calcium channel antagonist and agonist binding sites on brain and cardiac membranes. STR (10(-6)-10(-4) M) had no effect on neuronal 45Ca2(+)-uptake. When combined at equimolar concentrations (10(-5) M), STR and nifedipine produced a potent (nM) inhibition (40%) of neuronal 45Ca2(+)-uptake. In the spontaneously beating rat atria, STR produced a dose-dependent (10(-7)-3 x 10(-4) M) decrease in chronotropy but did not affect inotropy. STR (10(-4) M) completely inhibited the positive chronotropic, but did not affect the positive inotropic effects of (-)-S-BAY K 8644. [3H]Nitrendipine and [3H]BAY K 8644 binding to brain and cardiac membranes was enhanced by STR in a concentration-dependent manner (EC50 8 X 10(-6) M). Scatchard analysis revealed that STR increased the affinity (decreased the Kd) of [3H]BAY K 8644 to a greater degree than that of [3H]nitrendipine for dihydropyridine binding sites. STR decreased the Kd of [3H]nitrendipine binding by increasing and decreasing the microassociation and microdissociation constants respectively. STR enhanced [3H]nitrendipine binding to the same extent in the cerebral cortex, striatum, hippocampus, cerebellum, brainstem and spinal cord. The enhancement of [3H]nitrendipine binding in brain was completely inhibited by Ca2+ and partially inhibited by Na+ in a concentration-dependent manner. Glycine (10(-2) M) did not affect the STR enhancement of [3H]nitrendipine binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol withdrawal seizures and the NMDA receptor complex

Prior biochemical and electrophysiological studies have shown that low doses of ethanol inhibited calcium influx through the N-methyl-D-aspartate (NMDA) receptor/ionophore. The present data show that chronic ethanol treatment results in an increase in the number of NMDA receptor/ionophore complexes in the hippocampus, a brain area known to be associated with ethanol withdrawal seizure activity. Treatment during withdrawal with NMDA-exacerbated handling induced withdrawal seizures in the ethanol-dependent mice, while administration of the NMDA receptor-associated calcium channel antagonist MK-801 decreased the occurrence and severity of the withdrawal seizures in a dose-dependent manner. The results are consistent with the hypothesis that the up-regulation of the NMDA receptor systems following chronic ethanol treatment may mediate the seizures associated with ethanol withdrawal in dependent animals.

The iminodipropionitrile (IDPN)-induced dyskinetic syndrome in mice: antagonism by the calcium channel antagonist nifedipine

Chronic administration of IDPN leads to the development of a persistent syndrome which is characterized by lateral and vertical neck dyskinesias, random circling behaviors, and locomotor hyperactivity. Although the dihydropyridine (DHP) calcium channel antagonist nifedipine inhibited all aspects of the syndrome, lateral head dyskinesias (laterocollis) and circling abnormalities were the most significantly affected signs. Dysregulation of calcium-dependent processes might be involved in the pathogenesis of the IDPN-induced dyskinetic abnormalities and clinical disorders of movement in humans.

Calcium antagonist binding in cat brain tolerant to electroconvulsive shock

Cats subjected to daily (25-30 days) electroconvulsive shock (ECS) demonstrated an elevation of their electroconvulsive threshold or tolerance to ECS. [3H] Nitrendipine binding was measured to brain regions from non-tolerant (sham shocked) and ECS tolerant cats 24 hr following the last shock. ECS produced a significant increase (45%) in the density of [3H] nitrendipine binding sites in the cerebral cortex and a significant decrease (33%) in the apparent affinity of [3H] nitrendipine in the cerebellum. No changes in binding were observed in the hippocampus. The effects of ECS were also investigated in the rat, an animal not displaying tolerance to repeated ECS. [3H] Nitrendipine binding to rat brain was measured 10 min and 24 hr following one shock (acute) or ten shocks delivered transauricularly once daily (chronic). Twenty-four hours following chronic ECS, there was a significant increase (19%) and decrease (11%) in the density, but no change in the apparent affinity of [3H] nitrendipine binding sites in the cerebral cortex and hippocampus respectively. No significant change in [3H] nitrendipine binding was observed in rat cerebellum 24 hr following chronic ECS. There were no changes in [3H] nitrendipine binding in the cerebral cortex and hippocampus 10 min and 24 hr following acute ECS. These results indicate that ECS can alter [3H] nitrendipine binding to calcium channel linked dihydropyridine binding sites in the central nervous system. It is suggested that changes in [3H] nitrendipine binding in the cat cerebellum may be involved in the development of tolerance to ECS.

Concomitant regulation of hippocampal calcium antagonist receptors and calcium uptake by substance P

The interaction of various neuropeptides with calcium antagonist binding was investigated in rat hippocampus. Among the peptides examined Substance P selectively increased the binding of phenylalkylamine and dihydropyridine calcium antagonists; this action was receptor mediated. No effect was observed with Substance P in other brain areas and with neurotensin and met-enkephalin in all the areas examined. The modification in calcium antagonist binding is functionally paralleled by an area specific increase in voltage-dependent calcium uptake. These data suggest that in hippocampus Substance P may be an endogenous regulator of voltage sensitive calcium channels.

Multiple calcium channels and neuronal function

Recent investigations have demonstrated that neurons have a number of different types of calcium channels, each with their own unique properties and pharmacology. These calcium channels may be important in the control of different aspects of nerve activity. Some of the possibilities can now be discussed.

Regulation of dihydropyridine calcium antagonist binding sites in the rat hippocampus following neurochemical lesions

The effects of catecholaminergic, cholinergic, serotonergic, and glutaminergic terminal destruction and neurotransmitter depletion on [3H]nitrendipine binding to rat brain membranes were determined using the neurotoxins 6-hydroxydopamine, 5,7-dihydroxytryptamine, and kainic acid and the neurotransmitter-depleting agent reserpine. Following intracisternal injection of 6-hydroxydopamine there were time-dependent increases (14-23%) in the density but not change in the affinity of hippocampal [3H]nitrendipine binding sites. 6-Hydroxydopamine significantly increased [3H]nitrendipine binding in the hippocampus 4 and 10 days following injection. However, no significant change in binding was observed at 16 and 26 days. [3H]Nitrendipine binding in the cerebral cortex, striatum, cerebellum, and brain stem was unaffected by 6-hydroxydopamine. Neither 5,7-dihydroxytryptamine nor kainic acid affected [3H]nitrendipine binding in the hippocampus and cerebral cortex. Acute and chronic reserpinization also did not affect [3H]nitrendipine binding in the hippocampus and cerebral cortex. These results indicate that dihydropyridine calcium antagonist bindings sites in rat brain are subject to brain region-specific regulation following neurochemical lesions and may be present in their largest densities on postsynaptic membranes.

Novel interactions of cations with dihydropyridine calcium antagonist binding sites in brain

The effects of monovalent (Na+, Li+, K+, Rb+) and divalent (Ca2+, Mg2+, Mn2+) cations on dihydropyridine calcium antagonist binding sites in brain and cardiac membranes were investigated using a low ionic strength buffer (5 mM Tris-HCl, pH 7.4), and the dihydropyridine, [3H]-nitrendipine. At 25 degrees C, the monovalent cations Na+, Li+, and K+ (100 mM) but not Rb+ significantly decreased the apparent dissociation constant (KD) but had no effect on the maximum binding site capacity (Bmax) of [3H]-nitrendipine in brain. The divalent cations Ca2+, Mg2+, and Mn2+ (2 mM) significantly increased the Bmax, but did not affect the KD of [3H]-nitrendipine. The effects of cations were concentration-dependent (EC50 monovalent cations 10-25 mM; EC50 divalent cations 50-200 microM) and demonstrated brain region selectivity. The effect of Ca2+, but not Mg2+ or Mn2+ on [3H]-nitrendipine binding was described by a two-site model. At 25 degrees C, neither mono- nor divalent cations altered the characteristics of [3H]-nitrendipine binding to rat cardiac membranes. At 37 degrees C, Na+ (100 mM) but not K+ (100 mM) significantly increased the Bmax of [3H]-nitrendipine in rat brain membranes. Ca2+ (2 mM) significantly increased the Bmax of [3H]-nitrendipine binding to rat brain membranes to a greater extent than at 25 degrees C. Both Na+ and K+ had no effect on [3H]-nitrendipine binding to cardiac membranes, while Ca2+ (2 mM) significantly decreased the KD of [3H]-nitrendipine. It is suggested that the selective effects of mono- and divalent cations on [3H]-nitrendipine binding to rat brain and cardiac membranes may be associated with differences in the calcium current blocking activity of dihydropyridine calcium antagonists in brain and cardiac tissues.

Effects of calcium antagonists on phencyclidine behaviors

The calcium antagonists nifedipine and verapamil were evaluated for their potential behavioral interactions with phencyclidine induced changes in mouse rotarod performance and motor activity. Nifedipine (2 and 10 mg/kg) and verapamil (2 mg/kg) significantly potentiated impairment of rotarod performance produced by phencyclidine. These doses of nifedipine and verapamil did not by themselves affect rotarod performance. This action does not appear to be dependent on the hypotensive properties of these drugs, since hypotensive doses of prazosin did not alter the effect of phencyclidine on rotarod performance. Nifedipine, 4.0 mg/kg, antagonized increases in ambulatory motor activity, and potentiated decreases in vertical motor activity (rearing) induced by phencyclidine. The effects of calcium antagonists to alter the behavioral actions of phencyclidine in mice may occur through an interaction with the dihydropyridine calcium antagonist binding site present in the central nervous system.

Acylating phencyclidines irreversibly enhance brain calcium antagonist binding

Phencyclidine was previously shown to allosterically increase the apparent affinity of the dihydropyridine ( [3H]nitrendipine) calcium antagonist binding site in a lysed synaptosomal membrane preparation of rat forebrain. Treatment of a similar preparation of mouse forebrain with 4-isothiocyanato-1-(1-phenylcyclohexyl) piperidine (FOURPHIT), an acylating phencyclidine derivative, resulted in a concentration dependent (0.1-10 microM), irreversible, increase in the apparent affinity of [3H]nitrendipine in contrast to the effects of phencyclidine which were reversible. The FOURPHIT isomer, 1-[1-(3-isothiocyanatophenyl) cyclohexyl] piperidine (METAPHIT), (10 microM) also irreversibly increased the apparent affinity of [3H]nitrendipine, but was much less efficacious than FOURPHIT. Phencyclidine blocked the irreversible increase in the apparent affinity of [3H]nitrendipine produced by FOURPHIT. The interactions of multivalent cations and the calcium antagonist diltiazem with the [3H]nitrendipine binding site were altered following treatment of membranes with FOURPHIT. These studies suggest that FOURPHIT irreversibly interacts with the same sites as PCP, and thus may be a useful tool with which to further probe both the behavioral and biochemical interactions between phencyclidine and the dihydropyridine calcium antagonist binding site.

Enhancement of brain calcium antagonist binding by phencyclidine and related compounds

The abilities of compounds structurally or pharmacologically related to phencyclidine to increase the apparent affinity of the [3H]dihydropyridine calcium channel antagonist [3H]nitrendipine were examined in lysed synaptosomal membrane preparations of rat brain. The p-bromo analog of phencyclidine (1-(1-(4-bromophenyl)cyclohexyl)piperidine) was the most efficacious compound tested in enhancing the apparent affinity of [3H]nitrendipine. The efficacy of this compound was approximately two-fold greater than PCP. The stereoisomers of PCMP (1-(1-phenylcyclohexyl-3-methylpiperidine) were also more efficacious than phencyclidine, although only a small degree of stereoselectivity was observed. Levoxadrol, dexoxadrol and the enantiomers of ketamine did not potentiate [3H]nitrendipine binding. The enantiomers of SKF 10047 (n-allylormetazocine), dextrorphan, levorphanol and the ion channel toxins histrionicotoxin and pumiliotoxin-B also increased the apparent affinity of [3H]nitrendipine, while several local anesthetics and mu-opiate receptor ligands were without effect. These studies suggest that the ability of phencyclidine and structurally related compounds to increase the apparent affinity of [3H]nitrendipine is not mediated through an interaction with phencyclidine receptors, but may represent a unique site for allosteric modulation of neuronal dihydropyridine calcium channel antagonist binding sites.