Blocking the R-type (Cav2.3) Ca2+ channel enhanced morphine analgesia and reduced morphine tolerance

Morphine is the drug of choice to treat intractable pain, although prolonged administration often causes undesirable side-effects including analgesic tolerance. It is speculated that voltage-dependent Ca(2+) channels (VDCCs) play a key role in morphine analgesia and tolerance. To examine the subtype specificity of VDCCs in these processes, we analysed mice lacking N-type (Ca(v)2.2) or R-type (Ca(v)2.3) VDCCs. Systemic morphine administration or exposure to warm water swim-stress, known to induce endogenous opioid release, resulted in greater analgesia in Ca(v)2.3(-/-) mice than in controls. Moreover, Ca(v)2.3(-/-) mice showed resistance to morphine tolerance. In contrast, Ca(v)2.2(-/-) mice showed similar levels of analgesia and tolerance to control mice. Intracerebroventricular (i.c.v.) but not intrathecal (i.t.) administration of morphine reproduced the result of systemic morphine in Ca(v)2.3(-/-) mice. Furthermore, i.c.v. administration of an R-type channel blocker potentiated morphine analgesia in wild-type mice. Thus, the inhibition of R-type Ca(2+) current could lead to high-efficiency opioid therapy without tolerance.     

Second messenger mechanisms governing opiate peptide transmitter regulation in the rat adrenal medulla

Decreasing transsynaptic activity through surgical adrenal denervation or by medullary explantation, increases Leu-enkephalin immunoreactivity (Leu-Enk) and preproenkephalin mRNA (prepro-EK). Membrane depolarization prevents this rise. To determine whether depolarizing effects are mediated by intracellular movement of calcium ions, explanted medullae were depolarized in the presence of EGTA or the calcium ion 'channel' blockers D600 or verapamil. Inhibition of Ca2+ influx prevented the effects of KCl-induced depolarization on the rise in Leu-Enk and on prepro-EK. Increasing intracellular Ca2+ with the ionophore A23187, in the absence of depolarizing agents, reproduced the effects of depolarization. By contrast, medullae grown in the presence of A23187, but in Ca2+-free medium, showed similar increases in prepro-EK mRNA and Leu-Enk, indicating an absolute requirement for Ca2+. In addition, KCl-inhibitory effects could be partially blocked by the calmodulin and protein kinase-C antagonist, trifluoperazine. However, KCl effects were not antagonized by the preferential calmodulin inhibitors W7, W13 or calmidizolium even at doses 10-fold higher than required to prevent calmodulin-dependent effects. Thus, these data suggest that inhibitory effects of transsynaptic activity and membrane depolarization on adrenal enkephalin occurs through Ca2+ and perhaps through a protein kinase-C dependent pathway, mechanisms known to augment catecholamine biosynthesis. It appears then that the same or similar molecular mechanisms can result in differential regulation of these co-localized transmitter systems.     

Inhibitory influences of FMRFamide and PLG on stress-induced opioid analgesia and activity

The effects of i.c.v. administrations of the peptide FMRFamide (Phe-Met-Arg-Phe-NH₂), as well as i.p. injections of PLG (Pro-Leu-Gly-NH₂) and the opiate antagonist, naloxone, on immobilization-induced analgesia and locomotor activity were examined in CF-1 and C57BL strains of mice. Both naloxone (1.0 mg/kg) and FMRFamide (0.10–1.0 μg) blocked the experimentally induced analgesia and activity, whereas PLG (0.10–10 mg/kg) suppressed only analgesia. These results indicate that FMRFamide (or FMRFamide-like neuropeptides) and PLG may function as differential antagonists of the behavioral and physiological consequences of endogenous opioid activation.     

Modulation of calcium current in snail neurones by dopamine: the role of intracellular free calcium

The inhibition of voltage-gated Ca-current in Limnaea stagnalis (L.) neurones by dopamine cannot be prevented by intracellular administration of the Ca-chelating agent, EGTA. At the same time, substitution of external Ba2+ for Ca2+ substantially weakens the dopamine-induced inhibition. The Ca-ionophore, A23187, also produces an inhibition, but the inhibitory effect of A23187 and of dopamine on Ca-current are non-additive. These findings allow one to suggest that the entry and increase of free Ca concentration presumably in the narrow submembrane space, but not in the bulk of cytoplasm, is important for development of inhibition of neuronal Ca-current by dopamine.     

Magnetic fields and stress: day-night differences

An exposure for 30 min to a 0.5 Hz rotating magnetic field (1.5-90 G) significantly reduced warm water swim stress-induced opioid analgesia in CF-1 male mice. Pre-treatment with naloxone (1.0 mg/kg) had comparable inhibitory effects on warm water swim induced analgesia. The magnetic stimuli also eliminated the day-night rhythm in stress-induced analgesia, with maximum inhibitory effects occurring in the dark period when peak analgesia was present. These results indicate that magnetic stimuli can significantly alter day-night rhythms of stress-induced activation of endogenous opioid systems and their behavioral and physiological consequences. These elevated night time effects may involve actions on the pineal gland, while the day time actions may involve alterations in the distribution and transport of Ca++ and or other divalent ions.     

Calcium channel blockers inhibit the antagonistic effects of PheMetArgPhe-amide (FMRFamide) on morphine- and stress-induced analgesia in mice

Determinations were made of the effects of the calcium channel blockers, nifedipine and verapamil, on the antagonistic effects of FMRFamide (PheMetArgPheNH₂) and naloxone on morphine- and immobilization-induced opioid analgesia in mice. Intraperitoneal (i.p.) administrations of the calcium channel antagonists significantly reduced the inhibitory effects of intracerebroventricular (i.c.v.) FMRFamide, but had no effects on i.p. or i.c.v. naloxone-mediated inhibition of either morphine- or immobilization-induced analgesia. These results suggest that the antagonistic effects of FMRFamide, (or other endogenous FMRFamide-like peptides) on both opiate- and opioid-mediated analgesia in mice may involve alterations in the functioning of calcium channels.     

Morphine-induced analgesia and exposure to low-intensity 60-Hz magnetic fields: inhibition of nocturnal analgesia in mice is a function of magnetic field intensity

In 2 experiments male CF-1 mice were exposed for 60 min, during the mid-dark period of the day-night cycle, to low-intensity (0.5-1.5 gauss, rms) 60-Hz magnetic fields and then tested for levels of analgesia induced by morphine (10 mg/kg) injections. The magnetic field exposures inhibited the degree of morphine-induced analgesia in a field intensity-dependent manner in both experiments (P less than 0.01) with the largest inhibitory effect after exposure to the 1.5-gauss field. Analysis of the combined data from the two experiments revealed a significant (P less than 0.001) linear relationship between level of analgesia and magnetic field intensity. Thus, these data demonstrated a functional relationship between the behavioral effects of morphine in mice and the strength of the 60-Hz magnetic field. Possible mechanisms underlying these effects are discussed.     

Analgesic effects of the putative FSH-suppressing gonadal steroid, 3 alpha-hydroxy-4-pregnen-20-one: possible modes of action

The effects of intracerebroventricular (i.c.v.) administrations of the putative follicle stimulating hormone (FSH) suppressing gonadal steroid, 3 alpha-hydroxy-4-pregnen-20-one (3A4P) on the nociceptive responses of male mice were examined. This allylic steroid elicited significant, dose-dependent (0.001-1.0 micrograms) analgesic responses for 90-150 min after injection. These analgesic effects of 3A4P were stereospecific, the stereoisomer, 3 beta-hydroxy-4-pregnen-20-one (3B4P) failing to affect the nociceptive responses. The analgesic effects of 3A4P were blocked by peripheral administrations of the GABA antagonists, bicuculline and picrotoxin, and reduced by the benzodiazepine antagonist, Ro 15-1788. The exogenous opiate antagonist, naloxone, and the putative endogenous opioid antagonist, Tyr-MIF-1 (Pro-Leu-Gly-amide), also reduced 3A4P-induced analgesia, while i.c.v. administration of 3A4P (0.001 and 0.01 micrograms) itself attenuated the analgesic effects arising from peripheral administrations of opiate receptor agonist, morphine. In addition, the calcium channel antagonists, nifedipine and verapamil, enhanced 3A4P-induced analgesia but had no evident effects on the actions of 3B4P. These results suggest that the central analgesic effects of the FSH-suppressing steroid, 3A4P, arise via benzodiazepine--GABA--opiate mechanisms and calcium channels. These findings also suggest possible central modes of action whereby 3A4P may elicit selective suppression of FSH.     

Chelation of endogenous membrane calcium inhibits gamma-aminobutyric acid uptake in synaptosomes

In a previous work, we have demonstrated that calcium chelators induce the release of gamma-aminobutyric acid (GABA) from synaptosomes in a Na+ -dependent manner and that this release is blocked by cations such as Mg2+, La3+, and ruthenium red. In the present study, we show that treatment of synaptosomes with 0.1 mM EGTA in the absence of both Ca2+ and Mg2+ inhibits the sodium-dependent high-affinity uptake of [3H]GABA by about 50%. This inhibition increased to about 65% with 1.5 mM EGTA, and it was completely prevented by an excess of Ca2+ or by 1.2 mM Mg2+. In contrast, when EDTA was used as a chelator, Mg2+ was unable to reverse the inhibition. The inhibitory effect of 0.1 mM EGTA was also prevented by 250 microM La3+ or by 20 microM ruthenium red. In the absence of chelators and the presence of Ca2+ and Mg2+, 50 microM and 200 microM La3+ inhibited GABA uptake by about 20 and 50%, respectively, whereas 20 microM ruthenium red produced a nonsignificant 25% inhibition and nifedipine was without effect. It is concluded that the membrane-surface negative charges, probably those of the sialic acid molecules that have been implicated in the functioning of the GABA carrier, must be neutralized by endogenous Ca2+ or by another cation in order to permit the adequate function of the transporter. The inhibition by La3+ in the absence of the chelators could be explained by a binding of this cation to the Na+ sites on the GABA carrier.     

Stimulatory influences of calcium channel antagonists on stress-induced opioid analgesia and locomotor activity

The effects of the calcium channel antagonists diltiazem, nifedipine and the calcium channel agonist, BAY K 8644, on immobilization-induced opioid analgesia and locomotor activity were examined in CF-1 and C57BL strains of mice, respectively. The calcium channel antagonists enhanced the experimenatlly induced analgesia and activity, whereas the agonist reduced these responses. In addition, the calcium channel antagonists augmented, while the agonist attenuated, the analgesic effects of the specific mu and delta opioid agonists, DAGO and [D-Pen2,D-Pen5]-enkephalin (DPDPE), respectively. These results indicate that calcium channel antagonists have facilitatory and/or modulatory effects on the behavioral and physiological consequences of endogenous mu and delta opioid activity.     

Regulation of synthesis of the neurosecretory egg-laying hormone of Aplysia: antagonistic roles of calcium and cyclic adenosine 3':5'-monophosphate

The potential role of cyclic nucleotides and calcium as regulators of neuropeptide biosynthesis was examined in the bag cell neurons of Aplysia, which produce and secrete a peptide egg-laying hormone (ELH). Elevated external potassium, which stimulates ELH biosynthesis, increased bag cell cAMP levels when assayed in the presence of a phosphodiesterase inhibitor. Dopamine and serotonin, which increase bag cell cAMP levels, both stimulated ELH synthesis, as did the phosphodiesterase inhibitor isobutylmethylxanthine, the specific adenylate cyclase activator forskolin, and the phosphodiesterase-resistant cAMP analogue 8-benzylthio-cAMP. The stimulatory effect on peptide biosynthesis appears to be specific for cAMP, as bag cell cGMP levels were not altered significantly by high potassium or forskolin, and 8-bromo-cGMP did not stimulate ELH synthesis. In contrast to cAMP, intracellular calcium inhibits ELH production: biosynthesis of the peptide was elevated in a 0 Ca2+/EGTA medium and reduced in the presence of the Ca2+ ionophore A23187. Synthesis was also elevated in the presence of the calmodulin inhibitor calmidazolium. Treatment of intact bag cells with 0 Ca2+/EGTA or A23187 did not alter cAMP levels significantly, suggesting that calcium exerts its effect on peptide synthesis independently of cAMP. The antagonistic effects of cAMP and calcium on ELH synthesis parallel their effects on bag cell excitability, suggesting that, in these cells, neuropeptide synthesis and secretion are co-regulated by the same intracellular messengers.     

Calcium and protein kinase C inhibit biosynthesis of Aplysia egg-laying hormone

Previous studies on the biosynthesis of the peptide egg-laying hormone (ELH) of Aplysia have suggested that the increase in cAMP levels associated with the initiation of a bag cell discharge stimulated ELH synthesis, whereas the calcium influx associated with the discharge inhibits it. This report provides additional documentation of the inhibitory role of calcium. Inhibition by the calcium ionophore A23187 was shown to be dependent on the presence of extracellular calcium. A23187 inhibited ELH synthesis and exposure to 0 Ca2+/2 mM EGTA medium stimulated it in bag cell somata surgically deprived of their sites of synaptic input. The tumor-promoting phorbol ester, TPA, inhibited ELH biosynthesis in a calcium-dependent fashion, whereas the non-tumor-promoting 4 alpha-phorbol did not. These results are consistent with the hypothesis that calcium entry during bag cell discharge may inhibit ELH synthesis via activation of protein kinase C, thus counteracting the stimulation by cAMP early in the discharge. Such a mechanism could precisely regulate the production of ELH molecules to replace those lost by secretion.     

Mechanism of the inhibitory effect of histamine on amygdaloid-kindled seizures in rats

PURPOSE: The mechanism of the inhibitory effect of histamine on amygdaloid-kindled seizures was investigated in rats. METHODS: Under pentobarbital anesthesia, rats were fixed to a stereotaxic apparatus, and bipolar electrodes were implanted into the right amygdala. A guide cannula made of stainless steel tubing was implanted into the right lateral ventricle. Electrodes were connected to a miniature receptacle, which was embedded in the skull with dental cement. EEG was recorded with an electroencephalograph; stimulation of the amygdala was applied bipolarly every day by a constant-current stimulator and continued until a generalized convulsion was obtained. RESULTS: Intracerebroventricular (i.c.v.) injection of histamine at doses of 2-10 microg resulted in a dose-related inhibition of amygdaloid-kindled seizures. I.c.v. injection of calcium chloride at doses of 10-50 microg and A23187 at doses of 2-10 microg also caused dose-dependent inhibition of amygdaloid-kindled seizures. Calcium chloride at a dose of 10 microg, which showed no significant effect on amygdaloid-kindled seizures when used alone, significantly potentiated the effect of histamine. Similar findings were observed with A23187 at a dose of 2 microg. In addition, EGTA and EGTA/AM antagonized the inhibition of kindled seizures induced by histamine. Moreover, the inhibition of kindled seizures induced by histamine was antagonized by KN62. However, calphostin C did not antagonize the inhibitory effect of histamine. CONCLUSIONS: These results indicated that histamine-induced inhibition of amygdaloid-kindled seizures may be closely associated with a calcium calmodulin-dependent protein kinase II activation pathway.     

Analgesic effects of the progesterone metabolite, 3 alpha-hydroxy-5 alpha-pregnan-20-one, and possible modes of action in mice

The effects of intracerebroventricular (i.c.v.) administrations of the progesterone metabolite, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3A5P), on the nociceptive responses of male mice were examined. 3A5P elicited significant, dose-dependent (0.001-1.0 microgram) analgesia for 90-120 min after administration. These effects of 3A5P were significantly more potent than those of progesterone. The stereoisomer, 3 beta-hydroxy-5 alpha-pregnan-20 one (3B5P), failed to affect the nociceptive responses, indicating that the analgesic effect of 3A5P is stereospecific. The analgesic effects of 3A5P were blocked by peripheral administrations of the GABA antagonists, bicuculline and picrotoxin, and reduced by both the opiate and benzodiazepine antagonists, naloxone and Ro 15-788, respectively. The calcium channel antagonists, nifedipine and verapamil, enhanced 3A5P-induced analgesia but had no evident effects on the actions of 3B5P. These results suggest that the central analgesic effects of the progesterone metabolite, 3A5P, may arise via mechanisms involving calcium channels, the GABA-benzodiazepine-chloride complex and endogenous opioid systems.     

泰国眼镜蛇毒cobratoxin的化学修饰物receptin的镇痛作用

目的 在动物疼痛模型上,观察泰国眼镜蛇长链突触后α-神经毒素cobratoxin（CTX）的化学修饰物receptin（REC）的镇痛作用及其阿托品和纳洛酮对其镇痛作用的影响。方法 采用腹腔注射（i．p．,5mg/kg,7．07mg/kg,10mg/kg）或脑室注射（i．c.v．,62．5g/kg）的方法给予REC;采用小鼠热板反应、扭体反应及大鼠甩尾反应试验研究药物的镇痛作用;应用预先给予阿托（atropine,Atr;0．5mg／kg,i．m．或10mg／kg,i．p．）或纳洛酮（naloxone,Nal;3mg／kg,i．p．）研究胆碱能及阿片肽能神经在REC镇痛中的作用;采用mnilnex试验观察REC对小鼠自发活动的影响。结果 REC（5mg／kg,7．07mg/kg及10mg/kg,i．p．）在小鼠热板试验及扭体试验中均呈现出剂量依赖性镇痛作用,并于给药后20小时出现显著镇痛作用。在大鼠甩尾试验中,REC62．5mg/kg（相当于全身给药的1／160,i．c.v．）后产生显著镇痛作用。阿托品或纳洛酮不能阻断REC的镇痛作用。高剂量REC（10mg／kg,i．p．）对小鼠的自发活动无明显影响。结论 REC具有镇痛作用,尽管中枢神经系统参与REC的镇痛作用,但外周神经系统可能亦介导REC的镇痛作用。中枢胆碱能及阿片肽能神经系统可能不参与REC的镇痛作用。     

In vitro dopamine biosynthesis in the median eminence of rat hypothalamic slices: involvement of voltage-dependent Ca2+ channels

It was investigated whether Ca2+ is involved in the regulation of basal depolarization-induced dopamine biosynthesis in tuberoinfundibular dopaminergic neurons. The rate of dopamine biosynthesis was estimated by in vitro dihydroxyphenylalanine (DOPA) synthesis in the median eminence following incubation of rat hypothalamic slices with a DOPA decarboxylase inhibitor. Depolarizing agents such as K+ and veratridine increased the synthesis rate of DOPA in the median eminence in a dose-dependent manner with a maximal synthesis rate obtained at concentrations of 50 mM and 50 microM, respectively. Removal of Ca2+ and addition of EGTA (1 mM) into the medium did not influence basal DOPA synthesis in the median eminence but blocked the K+- and veratridine-induced DOPA synthesis. The Ca2+ channel blockers verapamil (100 microM) and Co2+ (4 mM) were effective in reducing the depolarization-induced DOPA synthesis. A23187 (10 microM), a Ca2+ ionophore, stimulated basal DOPA synthesis in the median eminence. On the other hand, tetrodotoxin (2 microM), a Na+ channel blocker, did not change the basal and K+-induced DOPA synthesis in the median eminence whereas it completely inhibited the veratridine-induced DOPA synthesis. These results suggest that depolarization-induced synthesis of dopamine in tuberoinfundibular neurons requires Ca2+ influx through voltage-dependent Ca2+ channels.     

Regulation of adipokinetic hormone release from locust neuroendocrine tissue: participation of calcium and cyclic AMP

In the locust, cyclic adenosine monophosphate (cAMP) mediates at least part of the effects of octopamine, the neurotransmitter which regulates the release of two adipokinetic hormones (AKHs) from the glandular lobe of the corpus cardiacum (CC). We have examined the requirement for extracellular Ca2+ in the process of AKH release mediated by octopamine and by agents which artificially elevate intracellular cAMP levels. Octopamine and the adenylate cyclase activator forskolin elevate the cAMP content of the glandular lobe in normal saline, in normal saline with the Ca2+ channel blocker, methoxyverapamil, and in Ca2+-free saline during 10-min exposure periods. Octopamine, forskolin, and 8-bromo cAMP mediate release of AKHs in vitro in normal saline, but release is prevented in the absence of extracellular Ca2+. When glands are exposed to these agents in normal saline in the presence of methoxyverapamil, AKH release is curtailed in a similar manner. Lanthanum and EGTA dramatically reduce cAMP production elicited by octopamine and forskolin, and lanthanum prevents octopamine-mediated release of AKHs. The phosphodiesterase inhibitor, IBMX, elevates cAMP content in the presence and absence of extracellular Ca2+, and stimulates normal release of AKHs both in the presence and absence of extracellular Ca2+. However, following extensive washing in Ca2+-free saline, IBMX fails to evoke AKH release. Methoxyverapamil has no effect on IBMX-mediated secretion. These results suggest that IBMX may mobilize intracellular stores of Ca2+ to induce release. Extracellular Ca2+ is apparently required for the process of neurotransmitter-evoked release, as has been shown for release of other peptide hormones. Cyclic AMP is intimately associated with Ca2+ in mediating this process. The release of AKHs is more dependent upon extracellular Ca2+ than is cAMP production under the conditions examined in this study. Ca2+ may provide the signal which initiates the secretory response, although cAMP may modulate this signal or the cells' responsiveness to this signal in some way. Support for this hypothesis is provided by experiments with the Ca2+ ionophore, A23187. This agent provokes release of AKHs in a Ca2+-dependent manner, probably by elevating intracellular Ca2+ levels. A23187 does not elevate cAMP levels in the glandular lobe, indicating that cAMP elevation is not a prerequisite for secretion.     

Stress-induced changes in the analgesic and thermic effects of opioid peptides in the rat

Stress (e.g. restraint) potentiates analgesia and alters changes in body temperature induced by morphine administered either systemically or intracerebroventricularly (i.c.v.) in rats. In order to extend the generality of this phenomenon to opioid peptides, we determined whether the analgesic and thermic effects of i.c.v. D-Ala2-D-Leu5-enkephalin (DADLE) or D-Ala2-N-MePhe4-Gly5(ol)-enkephalin (DAGO), agonists selective for delta- and mu-opioid receptors, respectively, were affected by restraint stress. Analgesia was measured in the tail-flick test and core body temperature by rectal probe. The unstressed rats exhibited a dose-dependent increase in tail-flick latencies after administration of either DAGO or DADLE. Restrained rats treated with DAGO or DADLE had a greater analgesic response to each dose of peptide than did unstressed rats; both the magnitude and duration of the drug effect were increased. The unstressed group of rats responded to all doses of DAGO and DADLE with an increase of core temperature. In contrast, restrained rats showed a decrease of core temperature following injection with either DAGO or DADLE. Thus, restraint stress can significantly modify the effects of DAGO and DADLE on analgesia and body temperature in a manner that is qualitatively and quantitatively similar to that observed previously for morphine administered by the i.c.v. route.     

Strain differences in the magnitude of swimming-induced analgesia in mice correlate with brain opiate receptor concentration

Swimming-induced analgesia was studied in 4 strains of mice differing in central opiate receptor density: C57BL/6By (C57), BALB/cBy (BALB/c), CXBK and CXBH. The degree of 'swim analgesia' significantly differed among strains in the order CXBH greater than BALB/c = C57 greater than CXBK. This order positively correlates with known differences in opiate receptor density in these strains. Naloxone reversed the analgesic effect of swimming in CXBH, C57 and BALB/c, but was ineffective in opiate receptor-deficient CXBK mice. These results suggest that genetic differences in central opiate receptor density influence the analgesic response to stressful stimuli.     

The interactions between plasma membrane depolarization and glutamate receptor activation in the regulation of cytoplasmic free calcium in cultured cerebellar granule cells

The complex modulation of cytoplasmic free calcium concentration ([Ca2+]c) in primary cultures of cerebellar granule cells in response to glutamate receptor agonists has been the subject of several contradictory reports. We here show that 3 components of the [Ca2+]c response can be distinguished: (1) Ca2+ entry through voltage-dependent Ca2+ channels, following KCl- or receptor-evoked depolarization, (2) Ca2+ entry through NMDA receptor channels, and (3) liberation of internal Ca2+ via a metabolotropic receptor. Depolarization with KCl induced a transient [Ca2+]c response (subject to voltage inactivation) decaying to a sustained plateau (largely inhibited by nifedipine). The NMDA response was potentiated by glycine, totally inhibited by (+)5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), and blocked by Mg2+ in a voltage-sensitive manner. Polarized cells displayed small responses to quisqualate (QA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA). Depolarization enhanced a transient response to QA, but not to AMPA. Trans-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD), a selective agonist for the metabolotropic glutamate receptor, caused a transient elevation of [Ca2+]c, which was blocked by prior exposure to QA but not AMPA. The prolonged [Ca2+]c response to kainate (KA) can be resolved into 2 major components: an indirect NMDA receptor-mediated response due to released glutamate and a nifedipine-sensitive component consistent with depolarization-mediated entry via Ca2+ channels. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX), QA at greater than 10 microM, and AMPA (but not trans-ACPD) reversed the KA response, consistent with an inactivation of the KA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)