Possible involvement of L-type voltage-gated calcium channels in release of dopamine in the striatum of irradiated rats.

The object of this study was to determine the effect of exposure to gamma radiation on potassium chloride (KCl)-stimulated release of dopamine (DA) in the striatum of the rat. In addition, the effect of some calcium channel blockers [nicardipine, a blocker of the L-type voltage-gated N-type VGCC; Omega-agatoxin TK, a selective blocker of P-type VGCC; and nickel chloride (NiCl(2)), which preferentially blocks the T-type VGCC] on KCl-stimulated release of DA in the striatum in sham-irradiated and irradiated rats was determined. Exposure of rats to 1-10 Gy (60)Co gamma rays had no significant effect on KCl-stimulated release of DA in the striatum in comparison to sham-irradiated animals. Administering 100, 300 and 500 nM of Omega-agatoxin TK or 50, 100 and 200 nM of Omega-conotoxin GVIA significantly decreased the release of DA stimulated by KCl in both irradiated and sham-irradiated animals in a dose-dependent manner. However, 10, 30 and 50 microM of nicardipine decreased the release of DA in irradiated animals but not in sham-irradiated animals. It is unknown why doses of 5-20 microM NiCl(2) had no effect on the release of DA in sham-irradiated and irradiated animals. The results demonstrate that the doses of radiation used in this study had no effect on release of DA in the striatum. Multiple calcium channel types coexist to regulate release of DA. P- and N-type VGCCs are involved in release of DA in sham-irradiated and irradiated animals, whereas only L-type VGCCs are involved in release of DA in irradiated animals.

Effect of ionizing radiation on the release of cholecystokinin in the hypothalamus of the rat.

This study was designed to identify the mechanisms underlying the reduction in food intake in rats. Measurements were made of the release of cholecystokinin (CCK) stimulated by potassium chloride in the hypothalamus after (a) gamma irradiation (60Co), (b) treatment with the CCK-A and CCK-B antagonists L-364,718 and L-365,260 with and without radiation, (c) bilateral abdominal vagotomy, and (d) vagotomy with and without radiation and with and without L-364,718. The concentrations of CCK in hypothalamus perfusate were measured by a radioimmunoassay. Exposure of rats to 1, 3, 5 and 10 Gy (1 Gy/min) increased release of CCK in the hypothalamus in a manner that was dependent on dose. A dose of 5 Gy was chosen for further studies. Intraperitoneal (i.p.) administration of 10, 20 and 50 microg/kg of L-364,718 did not induce significant changes in release of CCK in sham-irradiated animals. However, the drug decreased the release of CCK induced by radiation in a dose-dependent manner. In contrast to L-364,718, 20-50 microg/kg of L-365,260 decreased the release of CCK in the hypothalamus in sham-irradiated animals but did not decrease release of CCK induced by exposure to radiation. Vagotomy produced an insignificant reduction in release of CCK compared to that in sham-irradiated controls. However, vagotomy decreased release of CCK in irradiated rats compared to the irradiated rats without vagotomy. Vagotomy and i.p. administration of 10, 20 and 50 microg/kg of L-364,718 decreased release of CCK in irradiated rats compared to that in irradiated rats without vagotomy. However, i.p. administration of 10, 20 and 50 microg/kg of L-364,718 did not induce significant decreases in release of CCK in the hypothalamus of vagotomized and irradiated animals compared to those in rats that were vagotomized and irradiated but not treated with L-364,718. These results demonstrate that radiation increases the release of CCK in the hypothalamus, and that this effect is inhibited by vagotomy and the administration of a CCK-A receptor antagonist. A CCK-A receptor antagonist may be used to mitigate a radiation-induced deficit in food intake.

Effect of Bay K 8644, calcimycin and phorbol ester on radiation-induced decreases in the release of norepinephrine in the hippocampus in rats.

Gamma radiation (10 Gy at 10 Gy/min from a 60Co source) induces a decrease in the release of norepinephrine in the hippocampus 48 h after exposure. The purpose of this study was to determine the effect of Bay K 8644 [methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-2(trifluoromethylphenyl)py rid ine-5-carboxylate], an agonist for L- and N-type calcium channel subtypes; calcimycin [6S-[6alpha(2S*,3S*),8beta(R*),9beta,11alpha]-5( methylamino)-2-[[3,9,11-trimethyl-8-[1-methyl-2-oxo-2(1H-pyrrol-2-yl)eth yl]-1,7-dioxaspiro[5,5]undec-2-yl]methyl]-4-benzoxazoleca rboxylic acid], a calcium ionophore which increases intracellular Ca2+; and phorbol 12-O-tetradecanoate 13-acetate (TPA), which stimulates protein kinase C (PKC) alone or in combination with decreases in the release of norepinephrine in the hippocampus 48 h after irradiation. Neither Bay K 8644 (1-100 nM), calcimycin nor TPA prevented the radiation-induced decreases in the release of norepinephrine in the hippocampus. However, 10 nM of Bay K 8644 or calcimycin in combination with 1-100 nM of TPA or 10 nM of TPA in combination with 1-100 nM of Bay K 8644 or calcimycin did prevent the radiation-induced decreases in the release of norepinephrine. These results suggest that stimulation of PKC by TPA and mobilization of calcium by Bay K 8644 or calcimycin are necessary to prevent the radiation-induced decreases in the release of norepinephrine in the hippocampus.

Possible involvement of tumor necrosis factor alpha in radiation-induced hyperthermia in rats.

Exposure of rats to 1-15 Gy of 60Co gamma radiation induces hyperthermia. Because tumor necrosis factor alpha/cachectin (TNF-alpha) is considered to be an important mediator of the biological response to ionizing radiation, its role in radiation-induced hyperthermia was investigated; in addition, the levels of TNF-alpha in serum before and after radiation exposure were determined. Male Sprague-Dawley rats weighing 250-300 g were irradiated with gamma rays from a 60Co source (10 Gy, 10 Gy/min). Irradiation increased the levels of TNF-alpha in serum. Intraperitoneal (i.p.) doses of 10-50 microg/kg of recombinant human TNF-alpha (hTNF-alpha) induced hyperthermia; i.p. administration of 1-3 mg/kg of indomethacin, an inhibitor of prostaglandin synthesis, or 10-50 microg/kg of polyclonal rabbit anti-human TNF-alpha (anti-hTNF-alpha) attenuated the hyperthermia induced by 30 microg/kg of hTNF-alpha or exposure to 10 Gy of gamma rays. These results suggest that irradiation increases TNF-alpha in serum and that TNF-alpha is involved in gamma-radiation-induced hyperthermia. TNF-alpha and gamma-radiation-induced hyperthermia are mediated by prostaglandins.

Effect of ondansetron and ICS 205-930 on radiation-induced hypothermia in rats.

This study determined the effects of the 5-hydroxytryptamine (5-HT) serotonin antagonists ondansetron and (3 alpha-tropanyl]-1H-indole-3-carboxylic acid ester HCl (ICS 205-930) on hypothermia induced in rats by irradiation and by administration of a 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2-Me-5-HT). Intraperitoneal (i.p.) administration of 50-200 micrograms/kg of ondansetron and intraventricular administration of 5-20 micrograms of ondansetron attenuated hypothermia induced by 20 Gy gamma rays. However, the same doses of ondansetron administered i.p. or intraventricularly did not antagonize the hypothermia induced by 10 micrograms 2-Me-5-HT. In contrast, i.p. administration of 50-200 micrograms/kg of ICS 205-930 and intraventricular administration of 5-20 micrograms of ICS 205-930 attenuated hypothermia induced by radiation and 2-Me-5-HT. These results indicate that ICS 205-930 attenuates hypothermia induced by radiation and 2-Me-5-HT. However, the doses of ondansetron that attenuated radiation-induced hypothermia did not attenuate hypothermia induced by 2-Me-5-HT.

Indomethacin attenuation of radiation-induced hyperthermia does not modify radiation-induced motor hypoactivity.

Exposure of rats to 5-10 Gy of ionizing radiation produces hyperthermia and reduces motor activity. Previous studies suggested that radiation-induced hyperthermia results from a relatively direct action on the brain and is mediated by prostaglandins. To test the hypothesis that hypoactivity may be, in part, a thermoregulatory response to this elevation in body temperature, adult male rats were given indomethacin (0.0, 0.5, 1.0, and 3.0 mg/kg, intraperitoneally), a blocker of prostaglandin synthesis, and were either irradiated (LINAC 18.6 MeV (nominal) high-energy electrons, 10 Gy at 10 Gy/min, 2.8 microseconds pulses at 2 Hz) or sham-irradiated. The locomotor activity of all rats was then measured for 30 min in a photocell monitor for distance traveled and number of vertical movements. Rectal temperatures of irradiated rats administered vehicle only were elevated by 0.9 +/- 0.2 degree C at the beginning and the end of the activity session. Although indomethacin, at the two higher doses tested, attenuated the hyperthermia in irradiated rats by 52-75%, it did not attenuate radiation-induced reductions in motor activity. These results indicate that motor hypoactivity after exposure to 10 Gy of high-energy electrons is not due to elevated body temperature or to the increased synthesis of prostaglandins.

Effect of ionizing radiation on in vivo striatal release of dopamine in the rat.

The time-course effect of ionizing radiation on the levels of basal and KCl-stimulated striatal release of dopamine (DA) was examined in vivo using microdialysis techniques. The basal level of extracellular DA in sham-irradiated controls was 0.172 +/- 0.042 pmol/sample (n = 9), and it increased 7.1-fold after the stimulation by 30 mM KCl (20microliters). However, the release of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), two metabolites of DA, was reduced significantly by 30 mM KCl (P < 0.05). In the presence of 10 microM forskolin, an activator of adenylate cyclase, a second stimulation by 30 mM KCl increased the release of DA 6.9-fold. Radiation exposure, at a dose of 10 Gy at 10 Gy/min, had no significant effect on the levels of either basal or KCl-stimulated release of DA or on the release of DOPAC and HVA. Striatal DA release increased in response to two consecutive challenges of KCl. However, the release of DA in response to the second challenge of KCl was significantly smaller than that after the first challenge (543 +/- 110% compared to 794 +/- 164%, P < 0.05; Student's paired t test). Pretreatment with 10 muM forskolin, which by itself had no significant effect on the level of basal release of DA, prevented the decreased response of DA to the second challenge of KCl. Our results suggest that radiation exposure at the dose we used has no significant effect on the level of the basal release of DA or the release of DA stimulated by 30mM KCl in the rat striatum, and that a reduced release of DA in response to repeated KCl stimulation might involve the cAMP effector system.

Effect of chloral hydrate on in vivo KCl-induced striatal dopamine release in the rat.

The release of striatal dopamine (DA) and its metabolites in response to locally-induced K+ depolarization was investigated in vivo in chloral hydrate-anesthetized and freely moving rats. KCl at concentrations of 30, 50, and 100 mM induced significant dose-dependent increases in extracellular DA overflow in both chloral hydrate-anesthetized and freely moving rats (P < 0.05). Extracellular levels of dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were decreased. The DA overflow in response to 30 mM KCl stimulation in anesthetized rats was significantly greater than that in freely moving rats (P < 0.05). In addition, chloral hydrate anesthesia resulted in a significant decrease in extracellular levels of DOPAC and significant increases in extracellular levels of HVA and 5-HIAA in comparison with freely moving rats (P < 0.05). Furthermore, the basal level of extracellular HVA in chloral hydrate-anesthetized rats was significantly higher than that in freely moving rats. These results suggest that chloral hydrate anesthesia could have significant effects on the pharmacological response of the striatal dopaminergic neurons.

Possible involvement of prostaglandins in increases in rat plasma adrenocorticotropic hormone and corticosterone levels induced by radiation and interleukin-1 alpha alone or combined.

Exposing rats to 1-10 Gy of ionizing radiation increased plasma adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels. In both irradiated and nonirradiated rats, recombinant human interleukin-1 alpha (rhIL-1 alpha; 1 hr before radiation/sham exposure) enhanced plasma ACTH and CORT levels. Indomethacin, a cyclooxygenase inhibitor, attenuated plasma ACTH and CORT levels induced by radiation. Indomethacin also attenuated ACTH and CORT levels induced by radiation and interleukin-1 alpha alone or combined. These results suggest that prostaglandins are involved in the increase in plasma ACTH and CORT levels induced by radiation and rhIL-1 alpha alone or combined.

Iron-56 irradiation diminishes muscarinic but not alpha 1-adrenergic-stimulated low-Km GTPase in rat brain.

Initial findings from our laboratory have indicated that muscarinic enhancement of K(+)-evoked release of dopamine from perifused striatal slices is reduced after exposure to 56Fe-particle irradiation. This finding suggested that there is a radiation-induced deficit in muscarinic receptor sensitivity. Subsequent findings have indicated that at least part of the loss in sensitivity may occur as a result of alterations in the initial steps of the signal transduction process and involve muscarinic receptor-G protein coupling/uncoupling. The present study was carried out to localize this deficit further by determining carbachol-stimulated low-Km guanosine triphosphatase (GTPase) activity in striatal and hippocampal tissue obtained from rats exposed to 0, 0.1 or 1.0 Gy of 56Fe-particle irradiation. In addition, to examine the specificity of the effect of 56Fe-particle irradiation, alpha 1-adrenergic-stimulated low-Km GTPase activity was also examined in these tissues. The results showed that there was a high degree of specificity in the effects of 56Fe particles. Decrements were observed in muscarinic-stimulated low-Km GTPase in striatum but not in hippocampus, and 56Fe-particle irradiation did not affect alpha 1-adrenergic low-Km GTPase activity in either brain tissue.

Behavioral endpoints for radiation injury.

The relative behavioral effectiveness of heavy particles was evaluated. Using the taste aversion paradigm in rats, the behavioral toxicity of most types of radiation (including 20Ne and 40Ar) was similar to that of 60Co photons. Only 56Fe and 93Nb particles and fission neutrons were significantly more effective. Using emesis in ferrets as the behavioral endpoint, 56Fe particles and neutrons were again the most effective; however, 60Co photons were significantly more effective than 18 MeV electrons. These results suggest that LET does not completely predict behavioral effectiveness. Additionally, exposing rats to 10 cGy of 56Fe particles attenuated amphetamine-induced taste aversion learning. This behavior is one of a broad class of behaviors which depends on the integrity of the dopaminergic system and suggests the possibility of alterations in these behaviors following exposure to heavy particles in a space radiation environment.

Exposure to heavy charged particles affects thermoregulation in rats.

Rats exposed to 0.1-5 Gy of heavy particles (56Fe, 40Ar, 20Ne or 4He) showed dose-dependent changes in body temperature. Lower doses of all particles produced hyperthermia, and higher doses of 20Ne and 56Fe produced hypothermia. Of the four HZE particles, 56Fe particles were the most potent and 4He particles were the least potent in producing changes in thermoregulation. The 20Ne and 40Ar particles produced an intermediate level of change in body temperature. Significantly greater hyperthermia was produced by exposure to 1 Gy of 20Ne, 40Ar and 56Fe particles than by exposure to 1 Gy of 60Co gamma rays. Pretreating rats with the cyclo-oxygenase inhibitor indomethacin attenuated the hyperthermia produced by exposure to 1 Gy of 56Fe particles, indicating that prostaglandins mediate 56Fe-particle-induced hyperthermia. The hypothermia produced by exposure to 5 Gy of 56Fe particles is mediated by histamine and can be attenuated by treatment with the antihistamines mepyramine and cimetidine.

Involvement of superoxide dismutase and glutathione peroxidase in attenuation of radiation-induced hyperthermia by interleukin-1 alpha in rats.

Pretreatment with recombinant human interleukin-1 alpha (rhIL-1 alpha) 20 h before irradiation attenuates radiation-induced hyperthermia. Experiments were conducted to determine the role of antioxidant enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GSHPx) in rhIL-1 alpha-induced attenuation of radiation-induced hyperthermia. Radiation exposure increased SOD and decreased GSHPx levels in the hypothalamus, while treatment with rhIL-1 alpha increased GSHPx levels and had no effect on SOD levels. However, rhIL-1 alpha and irradiation together increased hypothalamic SOD level but prevented the fall in GSHPx level. Our results suggest that attenuation of radiation-induced hyperthermia by rhIL-1 alpha may involve stimulation of SOD and GSHPx because rhIL-1 alpha treatment and irradiation together increased hypothalamic GSHPx and SOD levels, and intracerebroventricular administration of SOD and GSHPx attenuated the radiation-induced hyperthermia.

Effect of prostaglandins, inositol 1,4,5-trisphosphate, and phorbol esters on radiation-induced decreases in calcium influx in rat brain synaptosomes.

Gamma irradiation (60Co) reduced KCl-stimulated voltage-dependent uptake of 45Ca2+ in rat whole-brain synaptosomes. Prostaglandins, inositol 1,4,5-trisphosphate, and phorbol esters were tested for their ability to inhibit radiation-induced decreases in calcium influx. None of the compounds tested alone completely prevented radiation-induced decreases in calcium uptake, but some drug combinations did inhibit decreases. Results suggest that radiation-induced decreases in calcium uptake are due to impairment of protein kinase C activity and mobilization of calcium by these drugs.

Relationship between linear energy transfer and behavioral toxicity in rats following exposure to protons and heavy particles.

Rats were exposed to protons (155 MeV) or to helium (165 MeV/amu), neon (522 MeV/amu) or argon (670 MeV/amu) particles to evaluate the behavioral toxicity of these types of radiations. Behavioral toxicity was assessed using the conditioned taste aversion paradigm. Exposure to all types of radiation produced dose-dependent increases in the intensity of the acquired taste aversion. However, the intensity of the aversions, measured as the dose that produced a 50% decrease in the intake of the sucrose-conditioned stimulus, did not show significant variation as a function of the linear energy transfer (LET) of the radiation. The results are discussed in terms of the relationship between LET and behavioral toxicity.

Role of neurotensin in radiation-induced hypothermia in rats.

The role of neurotensin in radiation-induced hypothermia was examined. Intracerebroventricular (ICV) administration of neurotensin produced dose-dependent hypothermia. Histamine appears to mediate neurotensin-induced hypothermia because the mast cell stabilizer disodium cromoglycate and antihistamines blocked the hypothermic effects of neurotensin. An ICV pretreatment with neurotensin antibody attenuated neurotensin-induced hypothermia, but did not attenuate radiation-induced hypothermia, suggesting that radiation-induced hypothermia was not mediated by neurotensin.