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Involvement of central histamine in amygdaloid kindled seizures in rats
Behavioural Brain Research 124 (2001) 243– 250 www.elsevier.com/locate/bbr
Involvement of central histamine in amygdaloid kindled seizures in rats Chiaki Kamei * Department of Pharmacology, Faculty of Pharmaceutical Sciences, Okayama Uni6ersity, Okayama 700 -8530, Japan Received 30 July 1999; accepted 31 May 2000
Abstract The involvement of central histamine in amygdaloid kindled seizures in rats was investigated using histamine-related compounds. Histamine contents in the amygdala of electrical stimulation site was significantly decreased after development of amygdaloid kindling. Intracerebroventricular (i.c.v.) injection of histamine resulted in inhibition of amygdaloid kindled seizures. The H1-agonists 2-methylhistamine and 2-thiazolylethylamine also inhibited amygdaloid kindled seizures. In addition, intraperitoneal (i.p.) injection of histidine and metoprine inhibited amygdaloid kindled seizures at doses that caused increases in histamine contents of the brain. H1-antagonists (diphenhydramine and chlorpheniramine) attenuated histamine (i.c.v.)-induced inhibition of amygdaloid kindled seizures, however, no significant antagonism was observed with H2-antagonists (cimetidine, ranitidine or zolantidine). Intracerebroventricular injection of H3-antagonists (thioperamide and AQ 0145) resulted in a dose-related inhibition of amygdaloid kindled seizures. The same findings were observed when thioperamide and clobenpropit were injected i.p. The effects of thioperamide (i.p.) and AQ 0145 (i.p.) were inhibited by an H3-agonist [(R)-a-methylhistamine] and H1-antagonists (diphenhydramine and chlorpheniramine). On the other hand, H2-antagonists (cimetidine and ranitidine) showed no antagonistic effects. These findings suggested that a histaminergic mechanism plays an important role in suppressing amygdaloid kindled seizures through histamine H1-receptors. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Amygdala; Kindling; Histamine; Histidine; H1-antagonists; H3-antagonists
1. Introduction Histamine has been shown to play an important role in the pathogenesis of seizure disorders. For example, Tuomisto [14] reported that the central histaminergic neuron system plays an important role in the inhibition of convulsions. Scherkl et al. [12] also found that histidine increased the threshold for pentetrazol-induced seizures. Further evidence of the involvement of histamine has come from studies in which drugs that deplete brain histamine have been found to increase the duration of clonic convulsions induced by maximal electroshock (MES) in mice [16]. In addition, Shimoda
* Tel.: +81-86-2517939; fax: +81-86-2517939. E-mail address: [email protected] Kamei).
(C.
et al. [13] demonstrated that the histamine H1-receptor antagonist diphenhydramine increased focal spikes or spike and wave complexes in patients with grand mal or petit mal epilepsy. Recently, Yokoyama et al. [16] also found that histidine caused inhibition of clonic convulsion induced by MES in mice. On the other hand, H3-receptors are considered to act as autoreceptors in the central nervous system. Thioperamide, clobenpropit and AQ 0145 are specific antagonists of H3-receptors and provide new tools for elucidating the precise functions of histaminergic neurons in the brain [1,9]. The present study was carried out to clarify the involvement of central histamine in amygdaloid kindled seizures in rats using some histamine-related compounds. The participation of postsynaptic histaminergic receptors in amygdaloid kindled seizures was also studied.
0166-4328/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 4 3 2 8 ( 0 1 ) 0 0 2 1 8 - 2
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Fig. 1. Changes in histamine contents of the amygdala after development of kindling in rats. The right amygdala was stimulated to induce kindled seizures. Each value represents the mean 9 S.E.M. of eight rats (Body weight; 280 9 10 g). **, significantly different from sham group (P B0.0 1).
Fig. 2. Effects of histamine (i.c.v.), histidine (i.p.) and metoprine (i.p.) on amygdaloid kindled seizures in rats (A) histamine; (B) histidine; (C) metoprine Stimulation of the amygdala was performed 0.5, 5 and 4 h after administration of histamine, histidine and metoprine, respectively. Each value represents the mean 9S.E.M. of 12 rats. *, **, significantly different from control group (PB 0.05 and 0.01, respectively).
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Fig. 3. Changes in brain histamine contents after intraperitoneal injection of histidine and metoprine Brain histamine contents were measured 5 and 4 h after injection of histidine (1500 mg/kg, i.p.) and metoprine (20 mg/kg, i.p.), respectively. Each value represents the mean 9S.E.M. of eight to ten rats. **, significantly different from control group (PB 0.01).
2. Changes in brain histamine contents after development of kindling Rats received stimulation of the right amygdala once a day. After the development of final stage seizures (stage 5), the stimulation was repeated for 5 more days. Rats were killed 150 h after final stimulation of the amygdala, and brain histamine contents were measured. Histamine content in the hypothalamus was slightly decreased compared with that in the sham group, but no significant difference was observed (data not shown). However, a significant decrease in histamine content was observed in the amygdala, especially at the site of stimulation (Fig. 1). On the other hand, Sato and Nakashima [11] found that whole brain norepinephrine and dopamine contents were decreased in the hippocampi of kindled cats. Callaghan and Schwark [2] also reported decreases in norepinephrine contents in the hippocampus, midbrain, limbic lobe and cortex of amygdaloid kindled rats. On the other hand, Engel and Sharpless [4] reported significant decreases in the levels of both norepinephrine and dopamine only in the stimulated
amygdala of all kindled rats. They [4] suggested that the enduring focal hyperexcitability produced by kindling may be attributable to an enduring stimulusinduced focal decrease in catecholamine-mediated inhibition at the site of stimulation. Watanabe et al. [15] demonstrated that the biogenic amines, norepinephrine and histamine showed similar distribution profiles in the rat brain. Therefore, the observations of Engel and Sharpless [4] may also be applicable to histamine.
3. Effects of histamine (i.c.v.), histidine (i.p.) and metoprine (i.p.) on amygdaloid kindled seizures Intracerebroventricular injection of histamine caused dose-dependent inhibition of amygdaloid kindled seizures 30 min after injection. Significant effects on seizure stage were observed at doses of 5, 10 and 20 mg. AD duration was also shortened by 5, 10 and 20 mg of histamine. Histidine caused dose-dependent inhibition of amygdaloid kindled seizures with reduction of both seizure stage and AD duration. No significant inhibi-
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tion was observed at a dose of 500 mg/kg but at doses of 1000 and 1500 mg/kg histidine showed significant suppression of both seizure stage and AD duration 5 h after administration. Metoprine also caused a dose-dependent inhibition of amygdaloid kindled seizures. Metoprine significantly suppressed the seizure stage and AD duration at doses of 10 and 20 mg/kg 4 h after administration (Fig. 2). Histamine content of the brain was significantly increased after administration of histidine (1500 mg/kg) and metoprine (20 mg/kg) in the cerebral cortex, hippocampus, hypothalamus and amygdala (Fig. 3). Duch et al. [3] reported that metoprine elevated brain histamine level by inhibiting histamine N-methyltransferase. In addition, we found a high correlation coefficient between inhibition of amygdaloid kindled seizures and an increase in histamine contents of the amygdala [7]. Similar findings were reported by Tuomisto and Tacke [14], who obtained a good linear correlation between increases in brain his-
tamine content and protection from seizures induced by MES in rats.
4. Effects of H1-and H2-agonists (i.c.v.) on amygdaloid kindled seizures The H1-agonist 2-methylhistamine showed a dose-related suppressive effect both on seizure stage and AD duration of amygdaloid kindled seizures. At a dose of 5 mg, no significant inhibition was observed, but at doses of 10, 20 and 50 mg, it caused significant suppression of both seizure stage and AD duration 30 min after administration. Similar findings were observed with 2-thiazolylethylamine (Fig. 4). On the other hand, the H2-agonists 4-methylhistamine and dimaprit showed no significant effect even at a dose of 50 mg (data not shown). H1-agonists suppressed amygdaloid kindled seizures in a manner similar to histamine.
Fig. 4. Effects of H1-agonists (i.c.v.) on amygdaloid kindled seizures in rats (A) 2-methylhistamine; (B) 2-thiazolylethylamine Stimulation of the amygdala was performed 0.5 h after drug administration. Each value represents the mean 9 S.E.M. of eight rats. *, **, significantly different from control group (P B 0.05 and 0.01, respectively).
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Fig. 5. Effects of H1-antagonists (i.c.v.) on the inhibition induced by histamine (i.c.v.) (A) diphenhydramine; (B) chlorpheniramine Histamine and H1-antagonists were given simultaneously. Stimulation of the amygdala was performed 0.5 h after drug administration. Each value represents the mean9S.E.M. of eight rats. *, **, significantly different from control group (PB 0.05 and 0.01, respectively).
5. Effects of H1- and H2-antagonists (i.c.v.) on the inhibition induced by histamine (i.c.v.) or histidine (i.p.) Pretreatment with the H1-antagonist diphenhydramine resulted in an antagonizing effect on histamine-induced inhibition of amygdaloid kindled seizures. At doses of 23.0 and 45.9 mg (i.c.v.), diphenhydramine showed a significant antagonizing effect on histamine (20 mg, i.c.v.) induced inhibition of amygdaloid kindled seizures. Similar findings were observed with chlorpheniramine at doses of 24.7 and 49.4 mg (i.c.v.) (Fig. 5). On the other hand, the H2-antagonists cimetidine and ranitidine did not antagonize inhibition of amygdaloid kindled seizures induced by histamine at doses of 45.4 and 56.6 mg (i.c.v.), respectively, (data not shown). Pretreatment with the H1-antagonist diphenhydramine also resulted in a significant antagonizing effect on histidine (i.p.) induced inhibition. At doses of 5 and 10 mg/kg (i.p.), it showed significant effects both on seizure stage and AD duration [7]. Similar findings were observed with chlorpheniramine; at doses of 5 and 10 mg/kg (i.p.), the drug showed a significant antagonistic effect on the inhibition induced by histidine (1500
mg/kg, i.p.). On the other hand, the H2-antagonists cimetidine and ranitidine did not antagonize inhibition of kindled seizures induced by histidine even at a dose of 50 mg/kg (i.p.) [7]. As shown in Fig. 5, the inhibition of amygdaloid kindled seizures induced by histamine (i.c.v.) was antagonized by H1-antagonists but not by H2-antagonists. Yokoyama et al. [17] recently reported that H1-antagonists including pyrilamine and ketotifen accelerated the rate of electrical stimulation to develop fully amygdaloid kindled seizures in rats. These findings suggested that histaminergic mechanisms play a suppressive role in amygdaloid kindled seizures through H1-receptors.
6. Effects of H3-antagonists on amygdaloid kindled seizures Intracerebroventricular injection of thioperamide at doses of 10–50 mg caused dose-dependent inhibition of the amygdaloid kindled seizures. For both seizure stage and AD duration, significant inhibition was observed at doses of 20 mg (60 min) and 50 mg (30 and 60 min).
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Similar findings were obtained with AQ 0145 (Fig. 6). Following i.p. injection, thioperamide was also effective in depressing amygdaloid kindled seizures both in seizure stage and AD duration at doses of 5 and 10 mg/kg. The same results were obtained with clobenpropit [6]. As shown in Fig. 6, the H3-antagonists thioperamide and AQ 0145 significantly inhibited amygdaloid kindled seizures. In the MES seizure test in mice, Yokoyama et al. reported that thioperamide decreased the durations of tonic and clonic convulsions [18]. H3-receptors regulate the release and synthesis of neuronal histamine, and thioperamide, AQ 0145 and clobenpropit are specific antagonists of these receptors [1,9]. Arrang et al. [1] and Sakai et al. [10] reported that brain histamine content was decreased by thioperamide and clobenpropit. The mechanism by which thioperamide and AQ 0145 inhibited amygdaloid kindled seizures although these drugs cause a decrease in brain histamine content is as follows. H3-antagonists accelerate histamine release from histaminergic presynaptic terminals, the released histamine is effective in depressing amygdaloid kindled seizures, and an increase in histamine release results in a decrease in brain histamine content. This was confirmed by Mochizuki et al. [8] using the microdialysis technique in rats.
7. Effect of H3-agonist on the inhibition induced by thioperamide Intracerebroventricular injection of (R)-amethylhistamine resulted in dose-related antagonism of the inhibition induced by thioperamide. Pretreatment with (R)-a-methylhistamine at doses of 10 and 20 mg caused significant antagonism of the thioperamide-induced inhibition of kindled seizures in both seizure stage and AD duration induced by thioperamide (50 mg, i.c.v.). (R)-a-Methylhistamine alone did not affect amygdaloid kindled seizures. Similar findings were observed with AQ 0145 (Fig. 7). The same findings were also reported by Yokoyama et al. [16], who found that the anticonvulsant effect of thioperamide was prevented by pretreatment with (R)a-methylhistamine, although treatment with (R)-amethylhistamine alone did not affect the seizure susceptibility of mice induced by MES. (R)-a-Methylhistamine has been shown to be a highly selective ligand for H3-receptors as this compound has been shown to block histamine release from histaminergic presynaptic terminals [5]. These findings strongly suggest that the effect of thioperamide is mediated via autoreceptors located on histaminergic neurons.
Fig. 6. Effects of H3-antagonists (i.c.v.) on amygdaloid kindled seizures Control, open circles; 10 mg, solid circles; 20 mg, open triangles; 50 mg, solid triangles. Each value represents the mean 9S.E.M. of eight rats. *, **, significantly different from control group (P B0.05 and 0.01, respectively).
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Fig. 7. Effects of H3-agonists (i.c.v.) on the inhibition induced by thioperamide (i.c.v.) or AQ 0145 (i.c.v.) (A) thioperamide; (B) AQ 0145 Stimulation of the amygdala was performed 1 h and 15 min after administration of thioperamide or AQ 0145 and a-methylhistamine, respectively. Each value represents the mean 9S.E.M. of eight rats. *, **, significantly different from control group (P B 0.05 and 0.01, respectively).
Fig. 8. Effects of H1-antagonists (i.p.) on the inhibition induced by thioperamide (i.p.) Stimulation of the amygdala was performed 90 and 30 mm after administration of thioperamide and H1-antagonists. Each value represents the mean 9S.E.M. of eight rats. *, **, significantly different from control group (P B0.05 and 0.01, respectively).
8. Effects of H1-and H2-antagonists on the inhibition induced by thioperamide Intraperitoneal injection of diphenhydramine and chlorpheniramine at 5 and 10 mg/kg showed significant
antagonism of the inhibition of amygdaloid kindled seizures by thioperamide (i.p.) (Fig. 8). On the other hand, cimetidine and ranitidine showed no antagonistic effect even at a dose of 50 mg/kg (data not shown). Interestingly, the inhibition of amygdaloid kindled
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seizures by thioperamide was antagonized by diphenhydramine and chlorpheniramine, representative H1-receptor antagonists. In contrast, cimetidine and ranitidine, which are H2-receptor antagonists, showed no antagonistic effects on the thioperamide-induced inhibition of amygdaloid kindled seizures. The H1-antagonists or H2-antagonists used in this study had no effect on amygdaloid kindled seizures when used separately.
9. Conclusions The results of the present study indicated that histaminergic mechanisms play a suppressive role in amygdaloid kindled seizures through postsynaptic H1 receptors. Presynaptic H3 receptors are also involved in inhibition of amygdaloid kindled seizures, and the inhibition of amygdaloid kindled seizures induced by H3-receptor antagonists is also mediated through postsynaptic H1-receptors.
References [1] Arrang JM, Garbarg M, Schwartz JC. Auto-inhibition of brain histamine release mediated by a novel class (H3) of histamine receptor. Nature 1983;302:832 –7. [2] Callaghan DA, Schwark WS. Involvement of catecholamines in kindled amygdaloid convulsions in the rat. Neuropharmacology 1979;18:541 –5. [3] Duch SD, Bowers SW, Nichol CA. Elevation of brain histamine levels by diaminopyrimidine inhibitors of histamine N-methyltransferase. Biochem Pharmacol 1978;27:1507 –9. [4] Engel J Jr, Sharpless NS. Long-lasting depletion of dopamine in the rat amygdala induced by kindling stimulation. Brain Res 1977;136:381 – 6. [5] Garbarg M, Barbin G, Roderges E, Schwartz JC. Inhibition of histamine synthesis in brain by a-fluoromethylhistidine, a new irreversible inhibitor: in vitro and in vivo studies. J Neurochem 1980;35:1045 – 52.
[6] Kakinoki H, Ishizawa K, Fukunaga M, Fujii Y, Kamei C. The effects of histamine H3-receptor antagonists on amygdaloid kindled seizures in rats. Brain Res Bull 1998;46:461 – 5. [7] Kamei C, Ishizawa K, Kakinoki H, Fukunaga M. Histaminergic mechanisms in amygdaloid-kindled seizures in rats. Epilepsy Res 1998;30:187 – 94. [8] Mochizuki T, Yamatodani A, Okakura K, Takemura M, Inagaki N, Wada H. In vivo release of neuronal histamine in the hypothalamus of rats measured by microdialysis. NaunynSchmiedeberg’s Arch Pharmacol 1991;343:190 – 5. [9] Murakami K, Yokoyama H, Onodera K, Imuma K, Watanabe T. AQ-0145, a newly developed histamine H3 antagonist, decreased seizure susceptibility of electrically induced convulsions in mice. Methods Findings Exp Clin Pharmacol 1995;17:70 – 3. [10] Sakai N, Sakurai A, Sakurai E, Yanai K, Maeyama K, Watanabe T. Effects of the histamine H3 receptor ligands thioperamide and (R)-a-methylhistamine on histidine decarboxylase activity of mouse brain. Biochein Biophys Res Commun 1992;185:121 – 6. [11] Sato M, Nakashima T. Kindling: secondary epileptogenesis, sleep and catecholamines. Can J Neurol Sci 1975;2:439 –46. [12] Scherkl R, Hashem A, Freg H-H. Histamine in brain-its role in regulation of seizure susceptibility. Epilepsy Res 1991;10:111 –8. [13] Shimoda Y, Koizumi A, Tanaka K. Electroencephalographic activation of ‘Diencephalic epilepsy’ with diphenhydramine. Yonago Acta Med 1960;4:99 – 102. [14] Tuomisto L, Tacke U. Is histamine an anticonvulsive inhibitory transmitter? Neuropharmacology 1986;25:955 – 8. [15] Watanabe T, Taguchi Y, Shiosaka S, Tanaka I, Kubota H, Terano Y, Tohyama M, Wada H. Distribution of the histaminergic neuron system in the central nervous system of rats; a fluorescent immunohistochemical analysis with histidine decarboxylase as a marker. Brain Res 1984;295:13 – 25. [16] Yokoyama H, Onodera K, Maeyama K, Yanai K, Imuma K, Tuomisto L, Watanabe T. Histamine levels and clonic convulsions of electrically-induced seizure in mice: the effects of afluoromethylhistidine and metoprine. Naunyn-Schmiedeberg’s Arch Pharmacol 1992;346:40 – 5. [17] Yokoyama H, Sato M, Imuma K, Onodera K, Watanabe T. Centrally acting histamine H1-antagonists promote the development of amygdala kindling in rats. Neurosci Lett 1996;217:194 – 6. [18] Yokoyama H, Onodera K, Imuma K, Watanabe T. Effect of thioperamide, a histamine H3 receptor antagonist, on electrically induced convulsions in mice. Eur J Pharmacol 1993;234:129 –33.
Involvement of central histamine in amygdaloid kindled seizures in rats Chiaki Kamei * Department of Pharmacology, Faculty of Pharmaceutical Sciences, Okayama Uni6ersity, Okayama 700 -8530, Japan Received 30 July 1999; accepted 31 May 2000
Abstract The involvement of central histamine in amygdaloid kindled seizures in rats was investigated using histamine-related compounds. Histamine contents in the amygdala of electrical stimulation site was significantly decreased after development of amygdaloid kindling. Intracerebroventricular (i.c.v.) injection of histamine resulted in inhibition of amygdaloid kindled seizures. The H1-agonists 2-methylhistamine and 2-thiazolylethylamine also inhibited amygdaloid kindled seizures. In addition, intraperitoneal (i.p.) injection of histidine and metoprine inhibited amygdaloid kindled seizures at doses that caused increases in histamine contents of the brain. H1-antagonists (diphenhydramine and chlorpheniramine) attenuated histamine (i.c.v.)-induced inhibition of amygdaloid kindled seizures, however, no significant antagonism was observed with H2-antagonists (cimetidine, ranitidine or zolantidine). Intracerebroventricular injection of H3-antagonists (thioperamide and AQ 0145) resulted in a dose-related inhibition of amygdaloid kindled seizures. The same findings were observed when thioperamide and clobenpropit were injected i.p. The effects of thioperamide (i.p.) and AQ 0145 (i.p.) were inhibited by an H3-agonist [(R)-a-methylhistamine] and H1-antagonists (diphenhydramine and chlorpheniramine). On the other hand, H2-antagonists (cimetidine and ranitidine) showed no antagonistic effects. These findings suggested that a histaminergic mechanism plays an important role in suppressing amygdaloid kindled seizures through histamine H1-receptors. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Amygdala; Kindling; Histamine; Histidine; H1-antagonists; H3-antagonists
1. Introduction Histamine has been shown to play an important role in the pathogenesis of seizure disorders. For example, Tuomisto [14] reported that the central histaminergic neuron system plays an important role in the inhibition of convulsions. Scherkl et al. [12] also found that histidine increased the threshold for pentetrazol-induced seizures. Further evidence of the involvement of histamine has come from studies in which drugs that deplete brain histamine have been found to increase the duration of clonic convulsions induced by maximal electroshock (MES) in mice [16]. In addition, Shimoda
* Tel.: +81-86-2517939; fax: +81-86-2517939. E-mail address: [email protected] Kamei).
(C.
et al. [13] demonstrated that the histamine H1-receptor antagonist diphenhydramine increased focal spikes or spike and wave complexes in patients with grand mal or petit mal epilepsy. Recently, Yokoyama et al. [16] also found that histidine caused inhibition of clonic convulsion induced by MES in mice. On the other hand, H3-receptors are considered to act as autoreceptors in the central nervous system. Thioperamide, clobenpropit and AQ 0145 are specific antagonists of H3-receptors and provide new tools for elucidating the precise functions of histaminergic neurons in the brain [1,9]. The present study was carried out to clarify the involvement of central histamine in amygdaloid kindled seizures in rats using some histamine-related compounds. The participation of postsynaptic histaminergic receptors in amygdaloid kindled seizures was also studied.
0166-4328/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 4 3 2 8 ( 0 1 ) 0 0 2 1 8 - 2
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Fig. 1. Changes in histamine contents of the amygdala after development of kindling in rats. The right amygdala was stimulated to induce kindled seizures. Each value represents the mean 9 S.E.M. of eight rats (Body weight; 280 9 10 g). **, significantly different from sham group (P B0.0 1).
Fig. 2. Effects of histamine (i.c.v.), histidine (i.p.) and metoprine (i.p.) on amygdaloid kindled seizures in rats (A) histamine; (B) histidine; (C) metoprine Stimulation of the amygdala was performed 0.5, 5 and 4 h after administration of histamine, histidine and metoprine, respectively. Each value represents the mean 9S.E.M. of 12 rats. *, **, significantly different from control group (PB 0.05 and 0.01, respectively).
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Fig. 3. Changes in brain histamine contents after intraperitoneal injection of histidine and metoprine Brain histamine contents were measured 5 and 4 h after injection of histidine (1500 mg/kg, i.p.) and metoprine (20 mg/kg, i.p.), respectively. Each value represents the mean 9S.E.M. of eight to ten rats. **, significantly different from control group (PB 0.01).
2. Changes in brain histamine contents after development of kindling Rats received stimulation of the right amygdala once a day. After the development of final stage seizures (stage 5), the stimulation was repeated for 5 more days. Rats were killed 150 h after final stimulation of the amygdala, and brain histamine contents were measured. Histamine content in the hypothalamus was slightly decreased compared with that in the sham group, but no significant difference was observed (data not shown). However, a significant decrease in histamine content was observed in the amygdala, especially at the site of stimulation (Fig. 1). On the other hand, Sato and Nakashima [11] found that whole brain norepinephrine and dopamine contents were decreased in the hippocampi of kindled cats. Callaghan and Schwark [2] also reported decreases in norepinephrine contents in the hippocampus, midbrain, limbic lobe and cortex of amygdaloid kindled rats. On the other hand, Engel and Sharpless [4] reported significant decreases in the levels of both norepinephrine and dopamine only in the stimulated
amygdala of all kindled rats. They [4] suggested that the enduring focal hyperexcitability produced by kindling may be attributable to an enduring stimulusinduced focal decrease in catecholamine-mediated inhibition at the site of stimulation. Watanabe et al. [15] demonstrated that the biogenic amines, norepinephrine and histamine showed similar distribution profiles in the rat brain. Therefore, the observations of Engel and Sharpless [4] may also be applicable to histamine.
3. Effects of histamine (i.c.v.), histidine (i.p.) and metoprine (i.p.) on amygdaloid kindled seizures Intracerebroventricular injection of histamine caused dose-dependent inhibition of amygdaloid kindled seizures 30 min after injection. Significant effects on seizure stage were observed at doses of 5, 10 and 20 mg. AD duration was also shortened by 5, 10 and 20 mg of histamine. Histidine caused dose-dependent inhibition of amygdaloid kindled seizures with reduction of both seizure stage and AD duration. No significant inhibi-
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tion was observed at a dose of 500 mg/kg but at doses of 1000 and 1500 mg/kg histidine showed significant suppression of both seizure stage and AD duration 5 h after administration. Metoprine also caused a dose-dependent inhibition of amygdaloid kindled seizures. Metoprine significantly suppressed the seizure stage and AD duration at doses of 10 and 20 mg/kg 4 h after administration (Fig. 2). Histamine content of the brain was significantly increased after administration of histidine (1500 mg/kg) and metoprine (20 mg/kg) in the cerebral cortex, hippocampus, hypothalamus and amygdala (Fig. 3). Duch et al. [3] reported that metoprine elevated brain histamine level by inhibiting histamine N-methyltransferase. In addition, we found a high correlation coefficient between inhibition of amygdaloid kindled seizures and an increase in histamine contents of the amygdala [7]. Similar findings were reported by Tuomisto and Tacke [14], who obtained a good linear correlation between increases in brain his-
tamine content and protection from seizures induced by MES in rats.
4. Effects of H1-and H2-agonists (i.c.v.) on amygdaloid kindled seizures The H1-agonist 2-methylhistamine showed a dose-related suppressive effect both on seizure stage and AD duration of amygdaloid kindled seizures. At a dose of 5 mg, no significant inhibition was observed, but at doses of 10, 20 and 50 mg, it caused significant suppression of both seizure stage and AD duration 30 min after administration. Similar findings were observed with 2-thiazolylethylamine (Fig. 4). On the other hand, the H2-agonists 4-methylhistamine and dimaprit showed no significant effect even at a dose of 50 mg (data not shown). H1-agonists suppressed amygdaloid kindled seizures in a manner similar to histamine.
Fig. 4. Effects of H1-agonists (i.c.v.) on amygdaloid kindled seizures in rats (A) 2-methylhistamine; (B) 2-thiazolylethylamine Stimulation of the amygdala was performed 0.5 h after drug administration. Each value represents the mean 9 S.E.M. of eight rats. *, **, significantly different from control group (P B 0.05 and 0.01, respectively).
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Fig. 5. Effects of H1-antagonists (i.c.v.) on the inhibition induced by histamine (i.c.v.) (A) diphenhydramine; (B) chlorpheniramine Histamine and H1-antagonists were given simultaneously. Stimulation of the amygdala was performed 0.5 h after drug administration. Each value represents the mean9S.E.M. of eight rats. *, **, significantly different from control group (PB 0.05 and 0.01, respectively).
5. Effects of H1- and H2-antagonists (i.c.v.) on the inhibition induced by histamine (i.c.v.) or histidine (i.p.) Pretreatment with the H1-antagonist diphenhydramine resulted in an antagonizing effect on histamine-induced inhibition of amygdaloid kindled seizures. At doses of 23.0 and 45.9 mg (i.c.v.), diphenhydramine showed a significant antagonizing effect on histamine (20 mg, i.c.v.) induced inhibition of amygdaloid kindled seizures. Similar findings were observed with chlorpheniramine at doses of 24.7 and 49.4 mg (i.c.v.) (Fig. 5). On the other hand, the H2-antagonists cimetidine and ranitidine did not antagonize inhibition of amygdaloid kindled seizures induced by histamine at doses of 45.4 and 56.6 mg (i.c.v.), respectively, (data not shown). Pretreatment with the H1-antagonist diphenhydramine also resulted in a significant antagonizing effect on histidine (i.p.) induced inhibition. At doses of 5 and 10 mg/kg (i.p.), it showed significant effects both on seizure stage and AD duration [7]. Similar findings were observed with chlorpheniramine; at doses of 5 and 10 mg/kg (i.p.), the drug showed a significant antagonistic effect on the inhibition induced by histidine (1500
mg/kg, i.p.). On the other hand, the H2-antagonists cimetidine and ranitidine did not antagonize inhibition of kindled seizures induced by histidine even at a dose of 50 mg/kg (i.p.) [7]. As shown in Fig. 5, the inhibition of amygdaloid kindled seizures induced by histamine (i.c.v.) was antagonized by H1-antagonists but not by H2-antagonists. Yokoyama et al. [17] recently reported that H1-antagonists including pyrilamine and ketotifen accelerated the rate of electrical stimulation to develop fully amygdaloid kindled seizures in rats. These findings suggested that histaminergic mechanisms play a suppressive role in amygdaloid kindled seizures through H1-receptors.
6. Effects of H3-antagonists on amygdaloid kindled seizures Intracerebroventricular injection of thioperamide at doses of 10–50 mg caused dose-dependent inhibition of the amygdaloid kindled seizures. For both seizure stage and AD duration, significant inhibition was observed at doses of 20 mg (60 min) and 50 mg (30 and 60 min).
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Similar findings were obtained with AQ 0145 (Fig. 6). Following i.p. injection, thioperamide was also effective in depressing amygdaloid kindled seizures both in seizure stage and AD duration at doses of 5 and 10 mg/kg. The same results were obtained with clobenpropit [6]. As shown in Fig. 6, the H3-antagonists thioperamide and AQ 0145 significantly inhibited amygdaloid kindled seizures. In the MES seizure test in mice, Yokoyama et al. reported that thioperamide decreased the durations of tonic and clonic convulsions [18]. H3-receptors regulate the release and synthesis of neuronal histamine, and thioperamide, AQ 0145 and clobenpropit are specific antagonists of these receptors [1,9]. Arrang et al. [1] and Sakai et al. [10] reported that brain histamine content was decreased by thioperamide and clobenpropit. The mechanism by which thioperamide and AQ 0145 inhibited amygdaloid kindled seizures although these drugs cause a decrease in brain histamine content is as follows. H3-antagonists accelerate histamine release from histaminergic presynaptic terminals, the released histamine is effective in depressing amygdaloid kindled seizures, and an increase in histamine release results in a decrease in brain histamine content. This was confirmed by Mochizuki et al. [8] using the microdialysis technique in rats.
7. Effect of H3-agonist on the inhibition induced by thioperamide Intracerebroventricular injection of (R)-amethylhistamine resulted in dose-related antagonism of the inhibition induced by thioperamide. Pretreatment with (R)-a-methylhistamine at doses of 10 and 20 mg caused significant antagonism of the thioperamide-induced inhibition of kindled seizures in both seizure stage and AD duration induced by thioperamide (50 mg, i.c.v.). (R)-a-Methylhistamine alone did not affect amygdaloid kindled seizures. Similar findings were observed with AQ 0145 (Fig. 7). The same findings were also reported by Yokoyama et al. [16], who found that the anticonvulsant effect of thioperamide was prevented by pretreatment with (R)a-methylhistamine, although treatment with (R)-amethylhistamine alone did not affect the seizure susceptibility of mice induced by MES. (R)-a-Methylhistamine has been shown to be a highly selective ligand for H3-receptors as this compound has been shown to block histamine release from histaminergic presynaptic terminals [5]. These findings strongly suggest that the effect of thioperamide is mediated via autoreceptors located on histaminergic neurons.
Fig. 6. Effects of H3-antagonists (i.c.v.) on amygdaloid kindled seizures Control, open circles; 10 mg, solid circles; 20 mg, open triangles; 50 mg, solid triangles. Each value represents the mean 9S.E.M. of eight rats. *, **, significantly different from control group (P B0.05 and 0.01, respectively).
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Fig. 7. Effects of H3-agonists (i.c.v.) on the inhibition induced by thioperamide (i.c.v.) or AQ 0145 (i.c.v.) (A) thioperamide; (B) AQ 0145 Stimulation of the amygdala was performed 1 h and 15 min after administration of thioperamide or AQ 0145 and a-methylhistamine, respectively. Each value represents the mean 9S.E.M. of eight rats. *, **, significantly different from control group (P B 0.05 and 0.01, respectively).
Fig. 8. Effects of H1-antagonists (i.p.) on the inhibition induced by thioperamide (i.p.) Stimulation of the amygdala was performed 90 and 30 mm after administration of thioperamide and H1-antagonists. Each value represents the mean 9S.E.M. of eight rats. *, **, significantly different from control group (P B0.05 and 0.01, respectively).
8. Effects of H1-and H2-antagonists on the inhibition induced by thioperamide Intraperitoneal injection of diphenhydramine and chlorpheniramine at 5 and 10 mg/kg showed significant
antagonism of the inhibition of amygdaloid kindled seizures by thioperamide (i.p.) (Fig. 8). On the other hand, cimetidine and ranitidine showed no antagonistic effect even at a dose of 50 mg/kg (data not shown). Interestingly, the inhibition of amygdaloid kindled
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seizures by thioperamide was antagonized by diphenhydramine and chlorpheniramine, representative H1-receptor antagonists. In contrast, cimetidine and ranitidine, which are H2-receptor antagonists, showed no antagonistic effects on the thioperamide-induced inhibition of amygdaloid kindled seizures. The H1-antagonists or H2-antagonists used in this study had no effect on amygdaloid kindled seizures when used separately.
9. Conclusions The results of the present study indicated that histaminergic mechanisms play a suppressive role in amygdaloid kindled seizures through postsynaptic H1 receptors. Presynaptic H3 receptors are also involved in inhibition of amygdaloid kindled seizures, and the inhibition of amygdaloid kindled seizures induced by H3-receptor antagonists is also mediated through postsynaptic H1-receptors.
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