Thyrotropin-releasing hormone in the treatment of intractable epilepsy

Review Articles

Thyrotropin-Releasing Hormone in the Treatment of Intractable Epilepsy Michael J. Kubek, PhD*, and Bhuwan P. Garg, MB, BS† Intractable seizures remain a significant therapeutic challenge despite current advances in the treatment of epilepsy. Thyrotropin-releasing hormone, the first neuroendocrine releasing factor to be isolated and fully characterized, was also the first releasing factor investigated as a possible neurotransmitter/neuromodulator outside the hypothalamus. Basic and clinical research has revealed a distinct neuroanatomic distribution and a neurochemical role for thyrotropin-releasing hormone in seizure modulation. Thyrotropin-releasing hormone and selected analogs were reported to have antiepileptic effects in several animal seizure paradigms, including kindling and electroconvulsive shock. Clinically, thyrotropin-releasing hormone treatment has been reported to be efficacious in such intractable epilepsies as infantile spasms, Lennox-Gastaut syndrome, myoclonic seizures, and other generalized and refractory partial seizures. Herein, we review evidence that suggests that thyrotropin-releasing hormone and selected thyrotropin-releasing hormone analogs may represent a new class of novel antiepileptic drugs, namely, antiepileptic neuropeptides and provide insights into potential new treatments for the intractable epilepsies. © 2002 by Elsevier Science Inc. All rights reserved. Kubek MJ, Garg BP. Thyrotropin-releasing hormone in the treatment of intractable epilepsy. Pediatr Neurol 2002; 26:9-17.

Introduction The tripeptide, thyrotropin-releasing hormone (TRH) (l-pyroglutamyl-l-histidyl-l-prolinamide; TRH; protirelin), the first hypothalamic-releasing hormone to be isolated, subserves a key role in feedback regulation of the

From the *Departments of Anatomy and Cell Biology, and Psychiatry; and †Section of Pediatric Neurology; Department of Neurology; Indiana University School of Medicine; Indianapolis, Indiana.

© 2002 by Elsevier Science Inc. All rights reserved. PII S0887-8994(01)00321-6 ● 0887-8994/02/$—see front matter

hypothalamic-pituitary-thyroid axis in maintaining thyroid homeostasis. TRH stimulates the release of prolactin in addition to thyroid-stimulating hormone, and these effects are used diagnostically to test the integrity of hypothalamus and pituitary. The discovery of thyrotropin-releasing hormone, TRH mRNA, and associated receptor(s) and metabolic enzymes outside the hypothalamus quickly established the significance of TRH in several nonendocrine functions [1-3]. TRH and its analogs are known to modulate the actions of such classical neurotransmitters as acetylcholine, noradrenaline, dopamine, serotonin (5-hydroxytryptamine), and glutamate [3], indicating a broad range of neurophysiologic functions for TRH. A role for TRH has been explored in affective and cognitive disorders, and in some neurodegenerative disorders, such as amyotrophic lateral sclerosis, progressive cerebellar ataxia, acute neurotrauma, and alcohol withdrawal [3]. In 1981, we began to study the role of TRH in generalized electroconvulsive seizures. We found that electroconvulsive seizures in rats were associated with substantial and prolonged increases in TRH levels that outlasted the stimulus and seizures for days. This effect was seen exclusively in forebrain structures, such as the amygdala, piriform cortex, entorhinal cortex, hippocampal formation, and specific neocortical regions [4]. We hypothesized that postictal TRH accumulation was the result of either seizure-induced synthesis or decreased metabolism. Furthermore, we postulated that TRH may modulate neurotransmitter activity in these limbic regions when it was sufficiently elevated [4]. Concurrently, Inanaga and Inoue reported that an analog of TRH (DN 1417), was effective in treating a patient with myoclonic epilepsy [5]. Since these initial studies, evidence has continued to accumulate suggesting TRH, its analogs, and other neuropeptides (such as neuropeptide Y) as a potentially new class of antiepileptic drugs [6]. The purpose of this review

Communications should be addressed to: Dr. Kubek; Department of Anatomy and Cell Biology, MS 5035; Indiana University School of Medicine; 635 Barnhill Drive; Indianapolis, IN 46202-5120. Received February 26, 2001; accepted May 24, 2001.

Kubek and Garg: Thyrotropin-Releasing Hormone in Epilepsy 9

Table 1.

Summary of clinical studies of TRH in intractable epilepsies

Clinical Trial [Ref]

Infantile spasms* Matsumoto et al. [7] Matsumoto et al. [8] Takeuchi et al. [9] Matsuishi et al. [10] Lennox-Gastaut* Matsumoto et al. [7] Matsumoto et al. [8] Takeuchi et al. [9] Takeuchi et al. [11] Matsuishi et al. [10] Inanaga et al. [5] Miscellaneous seizure types Matsumoto et al. [7] Matsumoto et al. [8] Inanaga et al. [5] Tanaka et al. [12]

No. of Patients

Patients Seizure-free*

13 15 12 2

7 8 3

10 6 6 3 7 98†

1 3 3 2

4 5 47‡ 1

Clinical Outcomes No. of Patients With Decreased Seizures*

2 2 1

No Change

Worse

6 7 7

1 2 2

8 3 2 1 2

2 1

2 4

1

1

* See text for details. † Seizure frequency reported in only 85 patients. See text for details. ‡ Data reported as % only. See text for details.

is to present current clinical and basic research related to the role of TRH in epilepsy and its proposed mechanism(s) of action. Clinical Studies and Thyrotropin-Releasing Hormone Safety A number of add-on studies and case reports suggest an antiepileptic role for TRH in intractable epilepsies including West syndrome, Lennox-Gastaut syndrome, and other refractory epileptic syndromes. However, no placebocontrolled, double-blind trials of TRH in any type of epilepsy syndrome or seizure disorder have been reported. Such studies would enhance and complement existing reports reviewed here (Table 1) and strengthen the epileptic syndrome most likely to respond to TRH therapy. Matsumoto et al. [7] evaluated the effectiveness of TRH compared with adrenocorticotropic hormone treatment in patients with infantile spasms. They studied a total of 33 patients with infantile spasms; 13 patients received TRH, whereas 20 patients received adrenocorticotropic hormone. Patients receiving TRH were given 0.5-1 mg of TRH-tartrate IV on day 1, and the same dose intramuscularly every morning for the subsequent 1-4 weeks. Synthetic Cortrosyn-z (ACTH-Z ) in a dose of 0.125-0.75 mg/day given every morning for 1-4 weeks was used for patients undergoing adrenocorticotropic hormone therapy. Both TRH-tartrate and Cortrosyn-z were add-on drugs, and patients continued to receive their antiepileptic drugs. TRH-treated patients were monitored with video-electro-

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encephalogram (EEG) at the beginning to accurately classify seizures and at the end of treatment to evaluate the efficacy of treatment. Complete seizure control was achieved in seven of 13 (53.8%) patients receiving TRHtartrate and in 15 of 20 (75%) patients who received Cortrosyn-z. Side effects were significantly less in patients receiving TRH-tartrate as compared with Cortrosyn-z. In another study the same authors reported that eight of 15 (53.3%) patients with infantile spasms became seizurefree after treatment with TRH [8]. Takeuchi et al. [9] treated 12 West syndrome patients with TRH-tartrate at a dose of 0.05 mg/kg given IV on the first day of treatment and intramuscularly on subsequent days for a total of 4 weeks. They observed that three of 12 patients became seizure-free, two had reduction in seizures without being seizure free, and in the remaining seven patients, there was no effect on seizure frequency. These authors noted reduced effectiveness of TRH in patients who had been treated previously with adrenocorticotropic hormone. Additionally, Takeuchi et al. reported a 50% or greater reduction in seizure frequency in four of eight patients with West syndrome treated with TRH at a dose of 0.05 mg/kg/day. Matsuishi et al. [10] used DN 1417, a TRH analog, in two patients with West syndrome and observed only a slight effect on seizure frequency and no change in the EEG pattern of hypsarrhythmia. Effectiveness of TRH in Lennox-Gastaut syndrome has been evaluated by a number of investigators. Matsumoto et al. [7] found improvement in seizure frequency in two of 10 patients treated with 0.5-1.0 mg/day of TRH for 1-4 weeks. One patient had complete control of his seizures,

whereas another had a mild to moderate decrease in his seizure frequency. The other eight patients had no change in their seizure frequency. Most patients (eight of 10) in this group had tonic seizures; one had atypical absence, and another had generalized tonic-clonic seizures. The same group of investigators found that three of six patients with atypical absence seizures had complete control of seizures after TRH treatment [8]. Takeuchi et al. [9] treated six patients with 0.05 mg/kg/day of TRH-tartrate for 4 weeks. Three patients had complete control of seizures for 1 year subsequent to treatment. One patient had reduced frequency of seizures, whereas in two patients there was no change in seizure frequency. Another study of three patients with Lennox-Gastaut syndrome treated with TRH documented a more-than 50% reduction in seizure frequency in two of three patients and no change in the third patient [11]. The central nervous system-selective TRH analog DN 1417 has also been used in the treatment of patients with Lennox-Gastaut syndrome. DN 1417 was given intramuscularly in a dose of 0.03-0.05 mg/kg/day for 14 days to seven patients with Lennox-Gastaut syndrome. Four of seven patients had an improvement in seizure frequency; two patients had complete control of seizures; one patient had a 50-99% reduction in seizure frequency, and one patient had a 10-49% reduction in seizure frequency. There was no effect in two patients (less than 10% reduction in seizure frequency), and seizures worsened in one patient [10]. Inanaga et al. [5] used DN 1417 as an oral add-on drug in 98 patients older than 2 years of age with Lennox-Gastaut syndrome. Patients were randomly assigned to two dose groups; 48 patients received a low dose of 0.4 mg/kg/day (maximum 20 mg/day), and 50 patients were in the high-dose group and received 1.6 mg/kg/day (maximum 80 mg/day) for 8 weeks. Seizure frequency was graded as markedly improved (decrease in frequency by more than 75%), moderately improved (50-74% decrease), slightly improved (25-49% decrease), or no change when there was less than a 24% decrease in seizure frequency. There was no clear dose-related difference between the two dose groups, with 15% of the low-dose group and 20% of the high-dose group demonstrating a marked to moderate improvement. There was slight improvement in 21% in each dose group. However, when patients were stratified by seizure type, a positive effect was seen in patients with atypical absence; response was poor in tonic seizures. Atypical absence patients in the high-dose group seemed to do better when compared with the low-dose group with P ⬍ 0.01. In addition, Lennox-Gastaut syndrome patients without a history of West syndrome tended to do better. TRH has been evaluated to a limited extent in partial seizures, myoclonic seizures, and other seizure types, including tonic and atonic seizures. Two of four patients in one study and one of five patients in another study had some improvement after TRH treatment [7,8] In the study by Inanaga et al. [5], 17% in the low-dose group and 29% in the high-dose group of patients with partial epilepsy

reported marked to moderate improvement (a more-than 50% decrease in seizure frequency) after treatment with DN 1417. A single report also has found TRH to be effective in treating myoclonic seizures [12]. In summary, it seems that TRH may have efficacy in the treatment of infantile spasms and Lennox-Gastaut syndrome in children. The studies suggest that TRH may benefit atypical absence seizures more than tonic seizures in Lennox-Gastaut syndrome. In the limited studies of partial seizures, a role for TRH is hinted at, although data are too meager to draw any definite conclusions. Clearly, continued controlled studies with different dosage and treatment paradigms in larger selected patient populations would determine the seizure syndrome(s) most responsive to TRH treatment. TRH and its analogs have been associated with few side effects. Clinical studies using various intrathecal, IV, intramuscular, subcutaneous, or parenteral doses in amyotrophic lateral sclerosis [13], senile dementia [14], women at risk for delivering premature infants [15], and intractable epilepsy [8,16] have reported infrequent and transient side effects. These have included urinary retention, irritability, sleepiness, mild hypertension, tachycardia, appetite loss, nausea and vomiting, sweating, shivering, and cold and warm sensations. Zegher et al. [17] performed follow-up thyroid function studies in 26 offspring 6 years after their mothers had received prenatal TRH (400 ␮g every 6 hours for 4 days) to improve lung maturation. Thyroid function seemed to be normal in all 26 children, indicating no adverse affect on the hypothalamo-pituitarythyroid axis. A recent long-term TRH trial for dementia (10 mg daily intramuscularly for 3 months) reported only three TRH-treated patients of 107 (2.8%) dropped out because of adverse side effects; this was not significantly different than the placebo group [14]. No morbidity or mortality has been reported with TRH use, and only one report has appeared in which a patient with preexisting epilepsy had a seizure subsequent to diagnostic TRH injection [18]. Thus TRH and its analogs seem to be safe when used in a wide age group ranging from prenatal exposure to elderly patients. No definitive data are available concerning the best route of TRH administration, because TRH, like other neuropeptides, has poor blood– brain barrier penetration [13,19]. This most likely is attributable to TRH being nonlipophilic and being rapidly metabolized in several tissues, including blood and the central nervous system. A cascade of both specific and nonspecific serum, membrane-bound, and cytosolic enzymes have been found to rapidly degrade TRH to pyroglutamic acid, histidine, proline, and [cyclo-(his-pro)] [20-25]. Several metabolically stable and selective TRH analogs have better transport through the blood– brain barrier with enhanced central nervous system bioavailability [20-23]. Collectively, these reports provide convincing evidence to merit continued TRH investigation as an additional treatment in severe medically intractable epilepsy.

Kubek and Garg: Thyrotropin-Releasing Hormone in Epilepsy 11

Basic Studies

TRH in Kindled Seizure Models

Established animal models representing the two major classes of epilepsy have been used to identify potential sites of endogenous TRH activity. Most investigations have focused on various kindling paradigms as a focal/ partial seizure model. Generalized seizures have been studied after electrically induced seizures (electroconvulsive shock) and chemically induced seizures and in genetically epilepsy-prone rat and mice models [26-28].

Several studies have now demonstrated TRH and its analogs to inhibit amygdala-kindled seizures in rats and cats, and in cortically kindled baboons [6,47–52], and to inhibit tonic and absence-like seizures in spontaneously epileptic rats and mice [38 – 40]. However, studies on the antiepileptic effect of TRH in neonate and juvenile animals are lacking. Although TRH is known to increase seizure threshold [47,53], the neuroanatomic, neurochemical, and electrophysiologic basis for this antiepileptic effect is poorly understood. In earlier reports, we proposed that the elevated postictal TRH pool is not an epiphenomenon, and may represent an endogenous compensatory mechanism for reducing the susceptibility to subsequent seizures and protection against neuronal damage [4,54]. For example, in the hippocampus, generalized kindled seizures induce significant increases of TRH in the dentate gyrus, hilus/CA4, CA3, CA1, and subiculum over time, with the largest increases occurring in the dentate gyrus, hilus, and CA3 subfields between 24 and 48 hours postictally [54,55]. These three subregions contain predominantly perforant path or mossy fiber pathways, which form a major component of the excitatory (glutamatergic) trisynaptic loop [31,36,44,45,55]. Such postictal increases are preceded temporally by a significant TRH decrease in these same subfields just after a fully kindled seizure, suggesting seizure-induced TRH release from intracellular stores and its subsequent metabolism [55]. Immunocytochemistry and in situ hybridization studies of TRH, TRH mRNA, and synthetic enzymes (prohormone convertases; PC1, PC2) indicate that the postictal increases are attributable to de novo synthesis, especially in dentate granule cells, suggesting that TRH is colocalized and most likely coreleased with glutamate from these same neurons [3,31– 33,36,55–57]. TRH receptors and preproTRH receptor mRNA are also localized in dentate granule cells and CA3 pyramidal cells of the hippocampus and in selected cells of the amygdala, piriform, and entorhinal cortex [58 – 65]. In contrast with the postkindling TRH alterations, neither TRH mRNA nor its translational product TRH, is substantially altered in the hippocampus during kindling development [29,32,55–57]. It is significant that TRH mRNA was not induced by brief afterdischarges (less than 30 seconds) during kindling, but both TRH mRNA and TRH were upregulated only when repeated afterdischarges of more than 60 seconds occurred (full kindling) [55,56]. We suggest that TRH gene induction is activity dependent and related to afterdischarge duration. Interestingly, Shin et al. [66] found c-fos mRNA induction in limbic structures to be activity dependent, with c-fos induction occurring with afterdischarges lasting more than 30 seconds. Members of the Fos family of immediate early genes participate in transcription regulation of specific target genes that express the AP-1 binding domain [67,68]. A putative AP-1 binding site is located on the TRH gene promoter [69]. Rosen et al. [70] reported immunocyto-

TRH in Generalized Seizure Models Initial neuroanatomic and neurochemical studies linking TRH to seizures first appeared with electroconvulsive shock models [4]. Briefly, electroconvulsive shock-induced seizures and chemically induced seizures produce substantial and prolonged selective increases in TRH and TRH intermediates (TRH-Gly and prepro-TRH160 –169) in central nervous system regions strongly associated with epileptic foci; (i.e., the amygdala, hippocampus, piriform, cingulate, frontal, and other cortical regions) [4,29-36]. For example, in the hippocampus, electroconvulsive shock induced significant increases of TRH in the dentate gyrus, hilus/CA4, CA3, CA1, and subiculum over time, with the largest increases occurring in the dentate gyrus, hilus, and CA3 subfields between 24 and 48 hours postictally [36]. Similarly, TRH responses have been reported in genetic epilepsy models [37]. Recent pharmacologic studies have demonstrated antiepileptic and neuroprotective effects of TRH in these generalized seizure models [38-40] suggesting a common neuroprotective mechanism. Kindled Seizure Models. Kindling is a well-established model of focal epilepsy, especially that of temporal lobe origin. Kindling occurs in response to repeated focal subconvulsive (electrical or chemical) stimulation of specific brain structures, most notably the basolateral amygdala [27,28,41]. A fully kindled animal is one in whom a subconvulsive stimulus results in stepwise stage 1-4 behavioral stereotypy with a subsequent generalized (stage 5) seizure on 3-5 consecutive days [42]. Full kindling is permanent and includes behavioral (arrest, chewing, head nodding, clonus), electrophysiologic (synchronous high amplitude spike and wave on the EEG), neuroanatomic (limbic and cortical), and neurochemical (glutamate receptors) characteristics similar to those associated with partial seizures that secondarily evolve into generalized seizures [28,43-45]. Kindling requires elicitation of afterdischarges and can be induced in several forebrain structures, most notably the amygdala, hippocampus, piriform cortex, and other cortical areas [28,42,46]. Continued subconvulsive stimulations beyond full kindling can result in the development of spontaneous seizures refractory to antiepileptic drugs. This effect has been described as a paradigm for intractable partial seizures [41].

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chemical and in situ hybridization results indicating a close correlation between Fos protein and TRH mRNA colocalization in dentate granule cells (more than 98%) and pyramidal cells of piriform (62%), entorhinal (70%), and perirhinal (79%) cortices 5 hours after fully kindled seizures. Thus it seems possible that the TRH gene is induced as a result of increasing neuronal excitatory activity, in part, by Fos or Fos-related proteins postictally [56,70]. Because granule cell TRH is not upregulated by short afterdischarges during kindling, we believe that TRH is not involved in granule cell sprouting associated with synaptic reorganization or kindling development [44,55,56,71–78]. In fully kindled animals, TRH is markedly and selectively upregulated, particularly in granule cells postictally, suggesting a key functional role for TRH in this subregion wherein massive perforant path excitatory input occurs [31,55,56,79]. Also, a significant proportion of the elevated postictal TRH pool in the hippocampus is available for calcium- and time-dependent synaptic release from hippocampal slices [80]. This correlates well with reports indicating significant and long-term (4 –24 hour) downregulation of the TRH receptor in hippocampal dentate gyrus (molecular layer), amygdala, and entorhinal cortex after kindled seizures [31,32,79]. Homologous downregulation of the TRH receptor has been demonstrated (see Gershengorn and Osman for review [81]). However, the significance of this response in the central nervous system is yet to be elucidated. In summary, endogenous TRH is not substantially upregulated during kindling development. This suggests that endogenous TRH is insufficient to influence epileptogenesis, nor is it involved in granule cell sprouting. On the other hand, site-specific delivery of sufficient TRH to a seizure focus can significantly delay the development of kindling [6] and inhibit fully kindled seizures [82]. We suggest that, in the postictal state, newly synthesized and transported TRH could participate in events associated with reorganized granule cell terminals in an activity- and time-dependent fashion wherein TRH/receptor interactions could participate in the well-known period of postictal inhibition/neuroprotection in the hippocampus and other cortical areas [31,55,56,83]. Mechanism(s) of TRH Action. The apparent prolonged duration of the antiepileptic effect of TRH observed clinically suggests a novel mechanism of action. Presently, two high-affinity TRH receptors have been cloned (TRHR1 and TRH-R2). TRH-R1 is highly expressed in limbic structures, whereas TRH-R2 is expressed primarily in sensory processing regions [65]. Both are members of the G-protein coupled receptor superfamily that uses the G␣q/11 subunit for effector coupling to phospholipase C, resulting in diacylglycerol and inositol tris-phosphate (IP3) as second messengers that are known to influence ion channels, intracellular calcium, and several downstream cellular events through protein kinase C mediation of protein phosphorylation [61,62,65,81,84]. TRH and its

analogs are also known to modulate the central actions of classical neurotransmitters, such as dopamine [85,86], norepinephrine [87,88], serotonin [88,89], acetylcholine [90,91], ␥-aminobutyric acid [92], and glutamate [93–95], by poorly defined mechanisms. Potentially, one or several of these neurotransmitter interactions, as well as others, could be involved in the antiepileptic effects of TRH. In a study involving seizure-prone El mice with concomitant monoamine abnormalities, it was found that hippocampal TRH may play an inhibitory role with regard to seizures, whereas striatal TRH may be important in seizure susceptibility [37]. Some investigations have suggested a relationship between catecholamine and/or prolactin levels in the antiepileptic response to TRH in intractable epilepsy [8,9], but evidence for this has not been substantiated [9,11]. This neuromodulatory role may extend to drug interactions. Nemeroff et al. [96] reported that TRH potentiated the antiepileptic effects of phenobarbital (phenobarbitone), suggesting the first link between TRH and seizure control [4]. More recently, endogenous TRH was found to play a significant functional role in the antiepileptic effects of carbamazepine and its tolerance, because the effects of the drug on amygdala kindled seizures may require an increase in TRH levels [79,83]. Additionally, carbamazepine therapy was reported to increase TRH levels in the cerebrospinal fluid of patients with affective illness [97]. Thus TRH may increase sensitivity to certain antiepileptic drugs. Because these antiepileptic drugs alone are ineffective in infantile spasms and LennoxGastaut syndrome, it may be worthwhile to consider combination therapy with TRH. Early studies by Renaud et al. [98,99] reported a selective inhibition of glutamatestimulated neuronal activity by TRH. In addition, Koenig et al. [100] reported that TRH was capable of inhibiting glutamate-stimulated Ca2⫹ uptake in cultured cortical cells. Our own ongoing research in vitro suggests that TRH is a potent, selective, and long-lasting modulator of glutamate release [95]. Of interest is a more recent report by Takeuchi et al. [11], who found elevated cerebrospinal fluid kynurenine in only those patients who responded positively to TRH therapy for treatment of infantile spasms and Lennox-Gastaut syndrome. Kynurenine, a metabolite of tryptophan, is the precursor of kynurenic acid and quinolinic acid in the central nervous system [101]. Kynurenic acid functions as an anticonvulsant by antagonism of N-methyl-d-aspartate and other glutamate receptors, whereas quinolinic acid functions as a proconvulsant and is an N-methyl-d-aspartate agonist [101]. Moreover, reduced levels of kynurenic acid have been reported in the cerebrospinal fluid of patients with infantile spasms and Lennox-Gastaut syndrome, suggesting that seizures may be related to reduced production of kynurenic acid in these children [102,103]. Taken together, it is reasonable to conclude that TRH may attenuate excessive glutamate activity within the central nervous system. TRH may also share a functional relationship with adrenocorticotropic hormone and glucocorticoids in the

Kubek and Garg: Thyrotropin-Releasing Hormone in Epilepsy 13

treatment of infantile spasms and Lennox-Gastaut syndrome. It has been well documented that adrenocorticotropic hormone/glucocorticoids provide a negative feedback function at the level of the hypothalamus and pituitary [104,105]. Thus glucocorticoids affect the biosynthesis and release of corticotropin-releasing hormone in the hypothalamic paraventricular nucleus. Recently, Kakucska et al. [106] reported that excess glucocorticoids led to a reduction of both corticotropin-releasing hormone and proTRH in the paraventricular nucleus, but this negative feedback effect did not occur in the lateral hypothalamus (outside the hypophysiotropic area regulating the pituitary-thyroid axis [106]. Similarly, it has been found that thyroid status has no effect on the elevated proTRH response to kindled seizures in limbic structures [107]. Likewise, feedback responses may be developmentally regulated. Jackson and others [108,109] have reported that glucocorticoids enhance, rather than inhibit, TRH gene transcription in fetal rat diencephalic neurons in monolayer culture [3,110,111]. Taken together, the data suggest that the feedback effects of certain hormones are cell- and locus-specific, as well as age-dependent [106 – 109,112]. Brunson et al. [113], Hollirigel et al., [114], and Baram et al. [115–119] have provided strong evidence that corticotropin-releasing hormone, when administered to infant rats, is a potent convulsant that is inversely related to age. These observations led to the corticotropin-releasing hormone-excess hypothesis for both the etiologic pathophysiology and the therapeutic mechanism of adrenocorticotropic hormone/glucocorticoids in infantile spasms. Furthermore, it is suggested that the deranged corticotropin-releasing hormone responsiveness is reduced by glucocorticoid suppression of excess corticotropinreleasing hormone synthesis when given to infants with infantile spasms [117]. Given that TRH is an endogenous antiepileptic, and that kindled seizures upregulate both TRH and corticotropin-releasing hormone in the same limbic areas [4,120], the notion that TRH is associated with glucocorticoid therapy should be considered, especially inasmuch as it has been revealed that corticosterone and dexamethasone can exert either stimulatory and/or inhibitory effects on TRH biosynthesis [106,108]. Thus one could hypothesize that, in some cases, adrenocorticotropic hormone/glucocorticoid therapy may inhibit corticotropin-releasing hormone synthesis and stimulate TRH synthesis, whereas in others adrenocorticotropic hormone/ glucocorticoid therapy may inhibit both corticotropinreleasing hormone and TRH synthesis and release. Alternatively, a third possibility is the ineffectiveness of adrenocorticotropic hormone/glucocorticoid on both corticotropin-releasing hormone/ TRH systems, suggesting an unrelated mechanism. Conclusions Clinical and basic studies indicate that TRH and its analogs may represent a new class of antiepileptic, or

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perhaps even an antiepileptogenic neuropeptide. Side effects and risks associated with adrenocorticotropic hormone/glucocorticoid therapy in West syndrome and pharmacological refractoriness in Lennox-Gastaut syndrome underscore the need for new and safer treatments [121– 124]. Because TRH seems to be efficacious in these syndromes and has relatively few side effects, continued clinical investigation of TRH seems to be a fertile area of investigation. The role of TRH therapy in other intractable focal and generalized seizure disorders needs additional study. Continued research with such neuropeptides could lead to rewarding new treatments in intractable partial and generalized epilepsies. Development of new methods to enhance TRH bioavailability should lead to increased efficacy for this and other neuropeptides. Finally, discerning the antiepileptic mechanism(s) of action of TRH and related neuropeptides will enhance our understanding of the pathophysiology of epilepsy. The authors acknowledge the expert review and comments on this manuscript by Drs. Jorge J. Asconape´ and Willie T. Anderson, III, Department of Neurology, Indiana University School of Medicine. This research was supported in part by grants from National Institute of Neurological Disorders and Stroke (NINDS), the Bi-national Science Foundation and an Indiana University and Purdue University at Indianapolis (IUPUI) Research Venture Award.

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