Activation of dihydropyridine receptors differentially regulates temperature responses in rat

Rats receiving the dihydropyridine Ca++ agonist BAY K8644 (0.1-3 mg/kg SC) displayed increasing loss of body temperature. At the highest dose tested (3 mg/kg) rats exhibited decreased motor activity, ataxia, increased vocalization upon handling and increased auditory sensitivity. Nimodipine (1 mg/kg SC) produced antagonism of this response when used as pretreatment at 15 and 30 minutes. The phenylalkylamine, verapamil (5 mg/kg) and the benzothiazepine diltiazem (10 mg/kg) did not alter BAY K8644-induced hypothermia. None of the three Ca++ channel antagonists produced changes in body temperature at the antagonist doses used. BAY K8644 (3 mg/kg SC) produced stimulation of Ca++/Mg++ ATPase activity by 31% in hypothalamus but not in cortex or cerebellum. This stimulation of enzyme activity was selectively prevented by nimodipine but not verapamil or diltiazem. No changes in enzyme activity were observed when Ca++ channel antagonists were used alone. These studies demonstrate that the Ca++ agonist BAY K8644 produces receptor mediated hypothermia which is dihydropyridine receptor dependent. Activation of Ca++ ATPase in the hypothalamus suggests that activation of dihydropyridine receptors may be coupled to Ca++ transport systems in this brain region and may reinforce the Ca++ set point theory of thermoregulation.     

Interaction of kappa receptor agonists with Ca2+ channel antagonists in the modulation of hypothermia

Administration of the opiate U-50,488H (3-20 mg/kg s.c.), a selective kappa receptor agonist, produced a dose-dependent decrease of rectal temperature in rats. This hypothermic effect of U-50,488H was accompanied by an enhanced activity of Ca2+/Mg2+ ATPase in crude synaptosomal (P2) fractions obtained from hypothalamus but not from cortex or cerebellum. Mg2+ ATPase activity in these regions was not altered by U-50,488H (15 mg/kg s.c.). Naloxone (5 mg/kg) partially and MR2266 (5 mg/kg) completely reversed the temperature and enzyme changes. Pretreatment with the calcium channel blockers nimodipine (1 mg/kg s.c.), diltiazem (10 mg/kg s.c.) and verapamil (2.5 mg/kg s.c.) potentiated the hypothermic effect of U-50,488H as well as the stimulation of Ca2+/Mg2+ ATPase in hypothalamus. These observations suggest that kappa agonists may produce opiate receptor mediated hypothermia through changes in intracellular Ca2+ levels in the hypothalamus.     

BAY K 8644 enhances immobility in the mouse behavioral despair test, an effect blocked by nifedipine

The effect of the dihydropyridine calcium channel agonist, BAY K 8644 (0.05, 0.1, 0.5 mg/kg i.p.), in the mouse behavioral despair test was investigated. BAY K 8644 dose dependently prolonged the duration of immobility. The BAY K 8644 (0.1 mg/kg)-induced prolongation of immobility was antagonized by the dihydropyridine calcium channel antagonist, nifedipine, but not by the non-dihydropyridine type antagonists, verapamil and diltiazem. The effect of BAY K 8644 was also antagonized by desipramine and imipramine but not by citalopram and iprindole. Therefore we suggest that central dihydropyridine binding sites play a role in despair behavior.     

Effect of Epomediol on the ATPase and adenylate cyclase activities in plasmamembranes isolated from rat liver

The effect of Epomediol (Clesidren (R)) on the membrane ATPase and Adenylate Cyclase activities has been investigated using highly purified rat liver plasmamembranes. The drug, even at micromolar concentration, showed a significant stimulatory effect on Ca++/Mg++- ATPase, Ca++- ATPase and Adenylate Cyclase activities. In particular the effect on Ca++- ATPase activity suggests that the drug under study may favourably modulate the calcium intracellular homeostasis.     

Preliminary analysis of the discriminative stimuli induced by the calcium channel “agonist” BAY K 8644

Rats were trained in a drug discrimination paradigm to press one of two levers for food reward after injection of the racemic dihydropyridine (DHP) Ca²⁺ -channel activator BAY K 8644 (2.5 mg/kg) and to press the other after vehicle injection. The discrimination was reliably attained in an average of 48 sessions. Thereafter, tests with various doses of BAY K 8544 yielded a dose-dependent selection of the BAY K 8644 lever, with an ED₅₀ of 0.74 mg/kg. The (?)-enantiomer of BAY K 8644 generalized dose-dependently (ED₅₀ = 1.04 mg/kg), while the (+)-enantiomer showed no generalization up to 10 mg/kg. Furthermore, nifedipine pretreatment fully antagonized the BAY K 8644 stimulus. These data indicate that BAY K 8644 produces physiological effects that can readily serve as discriminative stimuli in rats. The results also support the mediation of the BAY K 8644 stimulus through agonistic interaction with the calcium channel DHP receptor.     

Stimulus properties of the L-type calcium channel agonist BAY k 8644 in rats

Calcium (Ca(2+)) channels appear to be involved in the regulation of ethanol (EtOH) intake, as indicated by the effectiveness of both L-type Ca(2+) channel antagonists and agonists in reducing EtOH intake in animals. The present study was aimed to investigate rewarding/aversive and discriminative stimulus effects of the Ca(2+) channel agonist BAY k 8644, a compound showing pronounced anti-alcohol effects in rats. Therefore, a series of conditioned taste aversion (CTA), conditioned place preference (CPP) and two-lever drug discrimination (DD) experiments were conducted in Wistar rats, with (+/-)-BAY k 8644 and its enantiomers. After i.p. application, (+/-)-BAY k 8644 (0.0625-1mg/kg), (-)-BAY k 8644 (0.125-1mg/kg) and (+)-BAY k 8644 (2.5-20mg/kg) all induced a dose-dependent CTA. The minimal effective doses (MED) for (+/-)-, (-)- and (+)-BAY k 8644 were 0.25, 0.25 and 10mg/kg, respectively. In a CPP study, however, (+/-)-BAY k 8644 (0.25-2mg/kg, i.p.) showed neither aversive nor rewarding stimulus properties. Rats were trained to discriminate (-)-BAY k 8644 (0.3mg/kg, i.p.), the enantiomer acting as a high potency Ca(2+) channel agonist, from vehicle, in a two-lever DD procedure (ED(50)) value: 0.05mg/kg); full generalisation: 0.1mg/kg). The (-)-BAY k 8644 cue dose-dependently generalized to (+/-)-BAY k 8644 and (+)-BAY k 8644, the enantiomer acting as a low potency Ca(2+) channel antagonist, with ED(50) values of 0.06 and 0.28mg/kg, respectively. Both (+/-)- and (+)-BAY k 8644 produced full generalization at 1mg/kg, the latter compound showing an inverted U-shaped curve (i.e., this was the only dose showing >80% drug lever selection). The stimulus patterns of BAY k 8644 and its enantiomers appear to resemble the anti-alcohol profiles of these compounds. Therefore, commonalities between the stimulus properties of the agonistic and antagonistic enantiomers might provide a clue for the mechanism underlying the anti-alcohol effects of L-type Ca(2+) channel antagonists and agonists.     

The calcium channel agonist BAY k 8644 reduces ethanol intake and preference in alcohol-preferring AA rats

Applying a 12-h limited access, two-bottle choice procedure, antialcohol effects of the 1,4-dihydropyridine (DHP) L-type calcium (Ca²⁺) channel agonist BAY k 8644 were investigated in alcohol-preferring AA rats. In this Wistar line, selectively bred for a high 10% v/v ethanol (EtOH) preference in a free choice situation, effects on EtOH preference and intake, as well as on food and total fluid intake were evaluated for racemic BAY k 8644 (0.1–1 mg/kg IP; 0.25–2 mg/kg PO), its agonistic (?)-enantiomer (0.1–1 mg/kg IP and PO) and its antagonistic (+)-enantiomer (10–50 mg/kg IP and PO). Irrespective of route of application, BAY k 8644 was found to be effective in reducing both EtOH intake and preference (minimal effective dose: 0.5 mg/kg; maximum effect: approximately 60% of baseline levels). The (+)-enantiomer, acting as a low-potency Ca²⁺ channel antagonist, also reduced EtOH intake and preference, but the effects were not very selective as food intake was also substantially reduced. Moreover, the effects were only obtained at relatively high doses (50 mg/kg). The essential enantiomer involved in the antialcohol effects of BAY k 8644 seems to be the (?)-enantiomer, acting as a strong Ca²⁺ channel agonist. This latter compound was potent (minimal effective dose: 0.3 mg/kg), very effective in reducing EtOH intake (maximum effect: 29% of baseline level) and preference (26% of baseline) and apparently more selective. Although slightly decreasing over days, effects of (?)-BAY k 8644 on EtOH intake and preference were shown to remain after repeated treatment (10 successive days, 0.3 mg/kg IP). Interestingly, the acute antialcohol effects of (?)-BAY k 8644 (0.3–1 mg/kg IP) could not be antagonized with the DHP L-type Ca²⁺ channel antagonists nimodipine (0.01–1 mg/kg IP) and (?)-nimodipine (1–30 mg/kg IP). The present results suggest that a mechanism of action other than L-type Ca²⁺ channel agonism is involved in the antialcohol effects of (±)-and (?)-BAY k 8644. Alternatively, it is possible that the previously described antialcohol effects of DHP Ca²⁺ channel antagonists are not related to antagonistic activity at Ca²⁺ channels. Finally, it cannot be excluded that a mechanism unrelated to Ca²⁺ channels is responsible for the antialcohol effects of both DHP Ca²⁺ channel agonists and antagonists.     

The calcium channel agonist BAY k 8644 reduces ethanol intake and preference in alcohol-preferring AA rats

Applying a 12-h limited access, two-bottle choice procedure, antialcohol effects of the 1,4-dihydropyridine (DHP) L-type calcium (Ca²⁺) channel agonist BAY?k8644 were investigated in alcohol-preferring AA rats. In this Wistar line, selectively bred for a high 10% v/v ethanol (EtOH) preference in a free choice situation, effects on EtOH preference and intake, as well as on food and total fluid intake were evaluated for racemic BAY?k8644 (0.1–1?mg/kg IP; 0.25–2?mg/kg PO), its agonistic (?)-enantiomer (0.1–1?mg/kg IP and PO) and its antagonistic (+)-enantiomer (10–50?mg/kg IP and PO). Irrespective of route of application, BAY?k8644 was found to be effective in reducing both EtOH intake and preference (minimal effective dose: 0.5?mg/kg; maximum effect: approximately 60% of baseline levels). The (+)-enantiomer, acting as a low-potency Ca²⁺ channel antagonist, also reduced EtOH intake and preference, but the effects were not very selective as food intake was also substantially reduced. Moreover, the effects were only obtained at relatively high doses (50?mg/kg). The essential enantiomer involved in the antialcohol effects of BAY?k8644 seems to be the (?)-enantiomer, acting as a strong Ca²⁺ channel agonist. This latter compound was potent (minimal effective dose: 0.3?mg/kg), very effective in reducing EtOH intake (maximum effect: 29% of baseline level) and preference (26% of baseline) and apparently more selective. Although slightly decreasing over days, effects of (?)-BAY?k8644 on EtOH intake and preference were shown to remain after repeated treatment (10 successive days, 0.3?mg/kg IP). Interestingly, the acute antialcohol effects of (?)-BAY?k8644 (0.3–1?mg/kg IP) could not be antagonized with the DHP L-type Ca²⁺ channel antagonists nimodipine (0.01–1?mg/kg IP) and (?)-nimodipine (1–30?mg/kg IP). The present results suggest thata mechanism of action other than L-type Ca²⁺ channel agonism is involved in the antialcohol effects of (±)- and (?)-BAY?k8644. Alternatively, it is possible that the previously described antialcohol effects of DHP Ca²⁺ channel antagonists are not related to antagonistic activity at Ca²⁺ channels. Finally, it cannot be excluded that a mechanism unrelated to Ca²⁺ channels is responsible for the antialcohol effects of both DHP Ca²⁺ channel agonists and antagonists.     

Effects of 5-HT3 receptor antagonists on behavioural measures of naloxone-precipitated opioid withdrawal

The effect of the selective 5-HT3 receptor antagonists, ondansetron and MDL 72,222, against various behaviours elicited by naloxone-precipitated morphine withdrawal were examined. Rats made dependent upon morphine by the subcutaneous implantation of a 75 mg pellet, when challenged with naloxone (0.5 mg/kg SC), 3 or 4 days later exhibited a wide range of behaviours including wet dog shakes, paw shakes, salivation and a marked weight loss. Pre-treatment with ondansetron (0.01-1 mg/kg SC) or MDL 72,222 (1-3 mg/kg SC) failed to affect the incidence of these responses except weight loss, which was attenuated by both treatments. At doses similar to and below those required to elicit the withdrawal syndrome, naloxone produced a single-trial place aversion in morphine dependent rats. The place aversion produced by naloxone (0.05 mg/kg SC) was antagonized by pre-treatment of ondansetron (0.1-1 mg/kg SC) and MDL 72,222 (1 mg/kg SC) prior to conditioning. Chlordiazepoxide (10 mg/kg IP) but not gepirone (3-10 mg/kg SC) was similarly effective. It is concluded that 5-HT3 antagonists may attenuate some but not all behavioural signs associated with morphine withdrawal. Reasons for this apparent selectivity are discussed.     

Dihydropyridines change the uptake of calcium induced by depolarization into primary cultures of cerebellar granule cells

The uptake of calcium (Ca++) into cerebellar granule cells in primary culture was increased by depolarizing the cells with either 60 mM KC1 or veratridine. Nitrendipine, at concentrations of 100 nM or greater, antagonized approximately 40 percent of the depolarization induced Ca++ uptake. The half maximal concentration of nitrendipine was 7nM. Furthermore, another dihydropyridine derivative, BAY K 8644 enhanced the uptake of Ca++ and in the presence of nitrendipine, this facilitation of Ca++ uptake was reduced. Thus, these data indicate the existence of voltage dependent Ca++ channels which are sensitive to dihydropyridines in primary cultures of cerebellar granule cells.     

The behavioral effects of the calcium agonist BAY K 8644 in the mouse: Antagonism by the calcium antagonist nifedipine

Summary Mice injected with the calcium agonist BAY K 8644 (2–4 mg/kg, i. p.) displayed profound behavioral changes including ataxia, decreased motor activity, Straub tail, arched back, limb clonus and tonus, and an increased sensitivity to auditory stimulation. BAY K 8644 significantly impaired rotorod performance in mice with an ED₅₀ of 0.8 mg/kg. The behavioral effects of BAY K 8644 were antagonized by nifedipine, but not by the non-dihydropyridine calcium channel antagonist verapamil or the -adrenoceptor antagonist prazosin. Further, the actions of BAY K 8644 were not mimicked by the -adrenoceptor agonist methoxamine at doses up to 4.5 mg/kg. These observations, coupled with the findings that BAY K 8644 is a potent, competitive inhibitor of [³H]nitrendipine binding to the dihydropyridine binding site in mouse brain (K _i=7.0×10^–9M), suggests that BAY K 8644 may produce its behavioral actions via an interaction with the DHP binding site, which has been linked to the control of calcium flux across membranes in peripheral tissues.     

Role of central ATP-sensitive potassium channels in the hyperthermic effect of morphine in mice

Morphine (10 mg/kg, SC) in combination with ICV vehicle induced a significant hyperthermic effect at 120 min (peak time) after injection compared to ICV vehicle plus SC saline (control group). Glibenclamide (50 µg, ICV), a selective adenosine triphosphate-sensitive potassium (K_ATP) channel blocker, in combination with SC saline hardly affected the rectal temperature compared to the control group. ICV glibenclamide antagonized the hyperthermia induced by SC morphine in a dose-dependent manner. From these results, we demonstrated that K_ATP channels play an important role as modulators of the hyperthermic effect of agonists.     

Antagonism of the acute pharmacological actions of morphine by panax ginseng extract

1. The effects of intraperitoneal administration of a standard extract of Panax ginseng alone and in combination with morphine were determined in male Sprague-Dawley rats. 2. Ginseng extract at 200 mg/kg produced analgesia and hypothermia. These effects of ginseng were not reversed by naltrexone. 3. A dose of morphine (8 mg/kg) produced analgesia and hyperthermia. The analgesic response to morphine was antagonized by 25 and 50 mg/kg doses of ginseng but not by 12.5, 100 and 200 mg/kg doses. 4. Morphine-induced hyperthermia was antagonized by 12.5-200 mg/kg doses of ginseng. 5. Administration of morphine (50 mg/kg) produced cataleptic effect which was antagonized by 25 mg/kg of ginseng. 6. The results suggest that ginseng extract at high doses produces analgesia and hypothermia in the rat by a non-opiate mechanism, and antagonizes the acute pharmacological effects of morphine.     

Effects of dihydropyridine drugs on reversal by imipramine of helpless behavior in rats

The present study was undertaken in order to determine the effects of the dihydropyridine calcium channel blocker, nimodipine and the dihydropyridine calcium channel activator BAY k 8644, in the learned helplessness test in the rat. Nimodipine dose dependently (0.5-2 mg/kg per day) reversed the behavioral deficit induced by inescapable shocks. The reversal of helpless behavior by imipramine (32 mg/kg per day) was antagonized by BAY k 8644 (0.5 and 1 mg/kg per day), and the effects of imipramine 8 and 16 mg/kg per day) were potentiated by a subeffective dose (0.5 mg/kg per day) of nimodipine. These results suggest that central dihydropyridine binding sites may be specifically involved in the modulation of the imipramine reversal of helpless behavior and favor a physiological role for dihydropyridine binding sites in the brain.     

Antagonistic effect of pseudoginsenoside-F11 on the behavioral actions of morphine in mice

The antagonistic effect of pseudoginoside-F11 (PF(11)) on the various actions of morphine was studied in mice. The results demonstrated that PF(11), at the doses of 4 and 8 mg/kg, PO, significantly inhibited morphine (10 mg/kg, SC)-induced memory impairment in the Morris water maze test. PF(11), at 4 mg/kg, PO, did not influence conditioned place preference per se, yet markedly blocked the conditioned place preference to morphine. PF(11), at the doses of 4 and 8 mg/kg, PO, also significantly antagonized morphine (5 mg/kg, SC)-induced analgesia tested by tail pinch method. PF(11), at 4 mg/kg, PO, did not influence locomotor activity per se, yet inhibited the development of the reverse tolerance, as shown by the increase in locomotor activity, to morphine. At the doses of 4 and 8 mg/kg, PO, PF(11) significantly antagonized the development of analgesia tolerance to morphine in the tail pinch test. Thus, the above results demonstrate for the first time that PF(11) can antagonize some actions of morphine. However, the mechanism of action of PF(11) merits further evaluation.     

Calcium agonists and antagonists of the dihydropyridine type: Antinociceptive effects,interference with opiate-μ-receptor agonists and neuropharmacological actions in rodents

The calcium antagonist dihydropyridine derivative nimodipine and its enantiomers BAY N 5247, BAY N 5248, as well as BAY R 4407 (calcium antagonist (+)-enantiomer of the calcium agonist dihydropyridine BAY K 8644) do not exert antinociceptive effects in the rat as measured by the vocalization test in doses up to 100 g/kg IV, and in the mouse as measured by the hot plate test in oral doses up to 100 mg/kg. The calcium agonists BAY K 8644 and BAY R 5417 ((–)-enantiomer of BAY K 8644) are also ineffective in the rat vocalization test but BAY K 8644 increases reaction time in the hot plate test (mouse) dose-dependently (1–10 mg/kg PO). -Receptor agonist (fentanyl) antinociceptive effects are potentiated by simultaneous IV administration of the calcium antagonists, the (–)-enantiomer of nimodipine BAY N 5248 being the most potent. This applies for the rat (vocalization test) and the mouse (hot plate test). The influence on fentanyl antinociception in the rat of the calcium agonist BAY K 8644 and its (–)-enantiomer BAY R 5417 is biphasic: low doses attenuate, high doses potentiate fentanyl antinociception. In the mouse (hot plate test) antinociceptive effects of BAY K 8644 plus fentanyl are less than additive, indicating that the calcium agonist decreases fentanyl effects. The relative potency of calcium antagonists in potentiation of fentanyl antinociception correlates with their relative potency as calcium antagonists as measured by receptor binding studies, effects on vascular and cardiac muscle, and with their neuropharmacological actions (anticonvulsive effects, inhibition of balance and spontaneous motility as well as tranquilizing effects in the mouse). It is concluded that calcium antagonism potentiates -receptor agonist antinociceptive effects, whereas calcium agonism antagonizes -receptor agonist antinociception.Dedicated to Prof. Dr. H. Coper (Berlin) on the occasion of his 60th anniversary     

Different interaction of bepridil and diltiazem with BAY K 8644 in the abolition of autoregulation of renal blood flow

The effect of bepridil and diltiazem on autoregulation of renal blood flow was examined in connection with interaction of BAY K 8644 in perfused kidney of anesthetized dogs. When the perfusion pressure was changed stepwise over the range between 60 and 200 mm Hg, renal blood flow remained nearly constant between 100 and 200 mm Hg. Intra-arterial infusion of diltiazem (50 microgram/min) or bepridil (1 mg/min) increased renal blood flow at perfusion pressure above 100 mm Hg and completely inhibited renal autoregulation. Simultaneous infusion of 5 microgram/min of BAY K 8644 with diltiazem antagonized both the increase of renal blood flow and the impairment of autoregulation, whereas simultaneous infusion of BAY K 8644 (5 micrograms/min) could not block the impairment of renal autoregulation induced by bepridil. These results show that the inhibitory effect of bepridil on autoregulation of renal blood flow is due to some mechanism other than Ca channel blocking action in renal vasculature.     

Comparison of the effects of morphine, pethidine and pentazocine in rabbits pretreated with a monoamine oxidase inhibitor

1. Rabbits were premedicated with pargyline and the changes in rectal temperature measured after the intravenous infusion of morphine, pentazocine and pethidine.

2. Pethidine produced pronounced rises in rectal temperature which were dose dependent. One out of the four rabbits given 1 mg/kg died in hyperthermia. Four out of the four rabbits given 5 mg/kg died in hyperthermia. Doses of 10 mg/kg of pethidine caused no significant change in the rectal temperature of rabbits not pretreated with pargyline.

3. Morphine and pentazocine in doses of 1 mg and 10 mg/kg did not significantly alter the rectal temperature of rabbits pretreated with pargyline except for one rabbit which developed a delayed hyperthermia following the injection of morphine 1 mg/kg.

     

Addictive agents and intracranial stimulation (ICS): novel antagonists and agonists of morphine and pressing for ICS

Rats fixed with chronically indwelling bipolar electrodes pressed for intracranial stimulation (ICS) of the lateral hypothalamus during daily sessions. The effects of two antagonists of morphine (Win 44,441 and naloxone) were then assessed. Naloxone (10 mg/kg) produced its characteristic reduction in pressing. Win 44, 441 produced a reliable increase in pressing at doses as small as 1 mg/kg. Large doses of morphine (10 mg/kg) produced its characteristic effects: depression in pressing when given 1 hr before the test session and facilitation when given 3 hr before the test session. Win 44,441 antagonized morphine's depressive effects. Other compounds (Win 44,156, Win 42,156), having similar structure to Win 44,441 but having agonist and mixed agonist-antagonist activity with respect to analgesia, also facilitated pressing for ICS. All three compounds' effects on pressing for ICS were antagonized by naloxone. It is inferred that opioids' facilitatory effects on pressing for ICS are separable from opioids' other capabilities such as production of analgesia.     

Potentiating effect of morphine upon d-methamphetamine-induced hyperthermia in mice. Effects of naloxone and haloperidol

We have examined changes in rectal temperature of mice after subcutaneous administrations of d-methamphetamine alone or methamphetamine plus morphine. Methamphetamine 5 mg/kg produced slight hyperthermia, while simultaneous administration of morphine (25-100 mg/kg), which alone produces hypothermia, potentiated markedly the increase in body temperature by methamphetamine. Methamphetamine showed a hyperthermic effect in a dose-dependent manner in the presence of morphine. The hyperthermia due to methamphetamine plus morphine was avoided by pretreatment with 10 mg/kg naloxone. When animals were pretreated with 2.5 mg/kg haloperidol, hyperthermia due to methamphetamine alone was completely abolished, while that due to methamphetamine plus morphine was still observed. These results showed that dopamine may be implicated in methamphetamine hyperthermia and a haloperidol-nonsensitive mechanism may be involved in the methamphetamine-morphine hyperthermia.