Chronic thyrotropin releasing hormone (TRH) administration on TRH receptors and muscarinic cholinergic receptors in CNS

The Thyrotropin-Releasing Hormone-Degrading Ectoenzyme, a Therapeutic Target?

Thyrotropin releasing hormone (TRH: Glp-His-Pro-NH2) is a peptide mainly produced by brain neurons. In mammals, hypophysiotropic TRH neurons of the paraventricular nucleus of the hypothalamus integrate metabolic information and drive the secretion of thyrotropin from the anterior pituitary, and thus the activity of the thyroid axis. Other hypothalamic or extrahypothalamic TRH neurons have less understood functions although pharmacological studies have shown that TRH has multiple central effects, such as promoting arousal, anorexia and anxiolysis, as well as controlling gastric, cardiac and respiratory autonomic functions. Two G-protein-coupled TRH receptors (TRH-R1 and TRH-R2) transduce TRH effects in some mammals although humans lack TRH-R2. TRH effects are of short duration, in part because the peptide is hydrolyzed in blood and extracellular space by a M1 family metallopeptidase, the TRH-degrading ectoenzyme (TRH-DE), also called pyroglutamyl peptidase II. TRH-DE is enriched in various brain regions but is also expressed in peripheral tissues including the anterior pituitary and the liver, which secretes a soluble form into blood. Among the M1 metallopeptidases, TRH-DE is the only member with a very narrow specificity; its best characterized biological substrate is TRH, making it a target for the specific manipulation of TRH activity. Two other substrates of TRH-DE, Glp-Phe-Pro-NH2 and Glp-Tyr-Pro-NH2, are also present in many tissues. Analogs of TRH resistant to hydrolysis by TRH-DE have prolonged central efficiency. Structure-activity studies allowed the identification of residues critical for activity and specificity. Research with specific inhibitors has confirmed that TRH-DE controls TRH actions. TRH-DE expression by β2-tanycytes of the median eminence of the hypothalamus allows the control of TRH flux into the hypothalamus-pituitary portal vessels and may regulate serum thyrotropin secretion. In this review we describe the critical evidences that suggest that modification of TRH-DE activity in tanycytes, and/or in other brain regions, may generate beneficial consequences in some central and metabolic disorders and identify potential drawbacks and missing information needed to test these hypotheses.

Lack of behavioral tolerance by repeated treatment with taltirelin hydrate, a thyrotropin-releasing hormone analog, in rats

In order to determine whether acute tolerance develops by taltirelin hydrate ((-)-N-[(S)-hexahydro-1-methyl-2,6-dioxo-4-pyrimidinylcarbonyl]-l-histidyl-l-prolinamide tetrahydrate; taltirelin), a thyrotropin-releasing hormone (TRH) analog, we examined the motor behavior, TRH receptors and dopamine D(2) receptors following 2 weeks treatment in rats. Taltirelin selectively bound to TRH receptors and increased the spontaneous motor activity by a single administration, suggesting that the motor effect of taltirelin is mediated by TRH receptors. Following repeated treatment with TRH, there was a significant reduction in the increment of spontaneous motor activity. In contrast, after repeated treatment with taltirelin at a dose that increased the motor activity to a similar extent to TRH by a single administration, there was no apparent change in its motor effect. In accord with the motor activity, we found a significant reduction in the [(3)H]methyl-TRH binding to TRH receptors in the brain following repeated treatment with TRH but not taltirelin. However, the [(3)H]spiperone binding to dopamine D(2) receptors in the corpus striatum did not change by repeated taltirelin and TRH treatments. Thus, the down-regulation of TRH receptors would be a main cause of the behavioral tolerance. These results suggest that taltirelin hardly develops the behavioral tolerance due to the lack of down-regulation of TRH receptors.

Abnormalities in muscarinic cholinergic receptors and their G-protein coupling systems in the cerebral frontal cortex in Alzheimer's disease

Receptor binding assays and in vitro macroautoradiography were used to analyze muscarinic cholinergic receptors (MCR) in the cerebral frontal cortex of Alzheimer's disease (AD), senile dementia of Alzheimer type (SDAT), and age-matched control brains at autopsy. Total MCR binding, detected by [(3)H]quiniclinidyl benzilate binding, did not differ significantly between the 3 groups. The concentrations of M1 subtype (M1-R), detected by [(3)H]pirenzepine binding, and high affinity state MCRs, however, were significantly lower in AD than in control and SDAT frontal cortices. No differences were detected in the affinity of these receptors for their ligands. The MCRs in AD frontal cortex were more sensitive to the agonist carbachol than were control MCRs. Autoradiography revealed a complete destruction of the laminar distribution of MCR and M1-R in AD and SDAT frontal cortices. Forskolin and phorbol ester binding sites, used to analyze second messenger systems, were significantly and markedly reduced in AD frontal cortex. In addition, coupling between MCR and second messenger systems was supersensitive in AD frontal cortex. Our findings that there are alterations in the structural distribution of MCR as well as reductions and abnormalities in second messenger systems in AD cerebral frontal cortex, suggest that drug therapy with acetylcholine precursors, choline esterase inhibitors and muscarinic agonists cannot eliminate symptoms in dementia patients. Furthermore, they point out the need for techniques to diagnose the disease prior to disintegration of the neuronal network, and the need for therapies to delay or prevent the progression of structural changes.

Early treatment with cyclosporin A ameliorates the reduction of muscarinic acetylcholine receptors in gerbil hippocampus after transient forebrain ischemia

Recent evidence has suggested that cyclosporin A (CsA), an immunosuppressive agent, has neuroprotective properties. However, its mechanisms associated with this activity remain unclear. We have previously shown that post-ischemic administration of CsA daily for 14 days prevented the decrease of muscarinic acetylcholine receptor binding in the hippocampus in the gerbil model of 5-min transient forebrain ischemia. In the present study, CsA (5 mg/kg, subcutaneously) was administered to each animal just after, 2 and 6 h after ischemia so as not to exert its immunosuppressive effect. Initial CsA treatment significantly restored the declined muscarinic acetylcholine receptor binding of the hippocampus 14 days after ischemia similar to the previous report. However, CsA did not alter reactive changes of astrocytes and microglia in the CA1 area of the hippocampus, which had been suppressed by daily administration. These results indicate that CsA could positively modulate the hippocampal acetylcholine neurotransmission system broken down through the ischemia-induced pyramidal cell death and its action mechanism may have no relation to the immunosuppressive properties.

Effect of Trasina, an ayurvedic herbal formulation, on experimental models of Alzheimer's disease and central cholinergic markers in rats

Trasina is a herbal formulation of some Indian medicinal plants classified in Ayurveda, the classic Indian system of medicine, as Medhyarasayanas or drugs reputed to improve memory and intellect. Earlier experimental and clinical investigations have indicated that the formulation has a memory-facilitating action. In this investigation, the effect of Trasina, after subchronic administration for 21 days, was assessed on two rodent models simulating some biochemical features known to be associated with Alzheimer's disease (AD). The models, in rats, included intracerebroventricularly (i.c.v.) administered colchicine (15 micrograms/rat) and lesioning of nucleus basalis magnocellularis (nbm) by ibotenic acid (10 micrograms/rat). Retention of an active avoidance response was used as the memory parameter. In addition, the effect of Trasina was evaluated on i.c.v. colchicine-induced depletion of acetylcholine (ACh) concentrations, reduction in choline acetyltransferase (ChAT) activity, and decrease in muscarinic cholinergic receptor (MCR) binding in rat brain frontal cortex and hippocampus. The behavioral and biochemical investigations were done 7, 14, and 21 days after colchicine or ibotenic acid lesioning. Trasina (200 and 500 mg/kg) was administered orally (p.o.) once daily for 21 days, the first drug administration being given just prior to lesioning. Colchicine and ibotenic acid induced marked retention deficit of active avoidance learning that was attenuated in a dose-dependent manner by Trasina after 14 and 21 days of treatment. Frontal cortical and hippocampal ACh concentrations, ChAT activity and MCR binding was significantly reduced after colchicine treatment. Trasina (200 and 500 mg/kg) reversed these deficits after 14 and 21 days of treatment. The findings indicate that the herbal formulation exerts a significant nootropic effect after subchronic treatment that may be due to reversal of perturbed cholinergic function.

Behavioral and electroencephalographic studies of beagles with an Eck's fistula: suitability as a model of hepatic encephalopathy

Behavioral manifestations, electroencephalograms (EEGs) and visually evoked potentials (VEPs) were studied in beagles with Eck's fistula (portacaval shunt [PCS]), an established model of hyperammonemia, to determine whether they developed CNS disorders characteristic of hepatic encephalopathy. After PCS, behavioral changes occurred in the form of listlessness, sluggishness (altered gait, snapping and transient catatonia-like symptoms) and apparent blindness, which appeared in that order and progressed to coma and death in some animals. The EEGs from the frontal cortex showed a gradual decrease in voltage and frequency. Development of snapping and catatonia-like symptoms coincided with the occurrence of high voltage fast waves in the EEGs from the occipital cortex. In comatose Eck's fistula dogs. flattening of the EEGs was recorded from the frontal cortex and a lowered voltage was noted in the EEGs from the occipital cortex. After PCS, the latencies and amplitudes of the components of VEP were increased. The snapping and catatonia-like symptoms were markedly ameliorated by carbamazepine and the coma by flumazenil and thyrotropin-releasing hormone. These findings indicate that Eck's fistula dogs provide a useful model of hepatic encephalopathy.

Chronic administration of Oren-gedoku-to (TJ15) inhibits ischemia-induced changes in brain indoleamine metabolism and muscarinic receptor binding in the Mongolian gerbil

We examined the effect of Oren-gedoku-to (TJ15), which is a traditional herbal Kampo prescription used as an anti-cerebral apoplexy agent on these changes. Chronic pre- and post-ischemia TJ15 oral administration almost completely abolished the ischemia-induced muscarinic receptor reduction and 5-hydroxyindoleacetic acid level increase. These results suggest that TJ15 prevents cholinergic synaptic dysfunction and serotonergic presynaptic hyperactivity induced by transient ischemia.

Preventive effects of bifemelane hydrochloride on decreased levels of muscarinic acetylcholine receptor and its mRNA in a rat model of chronic cerebral hypoperfusion

Changes in muscarinic acetylcholine receptor (mACh-R) binding and muscarinic cholinergic m1 receptor (m1-R) mRNA levels were determined in a rat model of cerebral hypoperfusion in which hypoperfusion was induced by permanent bilateral occlusion of the common carotid arteries. After 6 weeks of hypoperfusion, mACh-R binding activity was significantly reduced in the frontal cortex (79.0 percent, P <0.01), striatum (74.2 percent, P < 0.01) and hippocampus (78.6 percent, P < 0.01), and the m1-R mRNA levels in the frontal cortex (86.6 percent, P < 0.05) and striatum (89.4 percent, P < 0.05) compared with sham-operated control. Repeated administration of bifemelane hydrochloride (15 mg/kg/day, p.o., once a day from the day of operation for 6 weeks) prevented the hypoperfusion-induced loss of mACh-R binding and m1-R mRNA levels above described. Since the central cholinergic systems play an important role in learning and memory, these findings suggest that bifemelane hydrochloride is useful to treat and/or prevent vascular dementia which is closely related to cerebral hypoperfusion.

The effects of repeated amphetamine administration on the thyrotropin-releasing hormone level. Its release and receptors in the rat brain

The effects of single and repeated administration of amphetamine (5 mg/kg, i.p., twice a day for 14 days) on the thyrotropin-releasing hormone (TRH) level, release and receptors in the rat striatum and nucleus accumbens were evaluated. Both treatments decreased the TRH level in those structures at 2 h after the drug injection. These effects were accompanied with elevation of the basal release of TRH from the nucleus accumbens and striatal slices at the same time point, whereas the stimulated (K+, 56 mM) TRH release was attenuated following repeated amphetamine administration. Acute amphetamine had no effect on the density and affinity of TRH receptors. Repeated amphetamine increased the Bmax of TRH receptors in the striatum (by ca 49%) and nucleus accumbens (by ca 38%) at 2 h after the last drug injection. At 72 h after the last amphetamine administration, the Bmax of the TRH receptor in the striatum was still elevated (by ca 42%), whereas in the nucleus accumbens it returned to control level. No changes in the affinity of TRH receptors following repeated amphetamine were found. The obtained results indicate that repeated amphetamine evokes long- and short-term up-regulation of TRH receptors in the rat striatum and nucleus accumbens, respectively. Furthermore, it is suggested that these changes may be an adaptive response to the amphetamine-induced alterations in the TRH tissue level and release.

Acetylcholinesterase inhibitor ENA-713 protects against ischemia-induced decrease in pre- and postsynaptic cholinergic indices in the gerbil brain following transient ischemia

The effects of pre-treatment with ENA-713, an acetylcholinesterase (AChE) inhibitor, on changes in pre- and postsynaptic cholinergic indices in gerbil brain following transient ischemia were studied at 4 and 14 days after recirculation. In the ischemic group, hippocampal acetylcholine (ACh) level was significantly reduced (to 23% of sham-operated controls) at 4 days post-ischemia, but this reduction was completely prevented by ENA-713 treatment. Choline acetyltransferase (ChAT) and cholinesterase (ChE) activities were not significantly changed at 4 and 14 days post-ischemia. Although the maximum number (Bmax) of muscarinic ACh receptor (mACh-R) binding in the hippocampus was decreased (to 44%) without any change in affinity at 14 days post-ischemia, this decrease was also inhibited by ENA-713 treatment. In addition, histological experiment indicated that ENA-713 inhibited ischemia-induced pyramidal cell loss in the hippocampal CA1 regions. Thus, these findings suggest that ENA-713 has protective, neurotrophic and therapeutic effects on cerebrovascular type dementia due to cerebral ischemia.

Thyrotropin-releasing hormone release is enhanced in hippocampal slices after electroconvulsive shock

Hippocampal thyrotropin-releasing hormone (TRH) release was examined after seizures were induced by electroconvulsive shock (ECS). Rat hippocampal slices taken 12, 24, or 48 h after 3 days of alternate-day ECS treatment or sham-ECS treatment were stimulated with potassium with or without calcium in a superfusion system containing in-line charcoal adsorbent to concentrate TRH. Released TRH and tissue TRH were measured by radioimmunoassay. The TRH content of hippocampal slices was increased fivefold over sham-ECS levels 12, 24, and 48 h after ECS, but this was not associated with an increase in basal TRH release. Potassium-stimulated TRH release was significantly elevated over basal release 12, 24, and 48 h after ECS. Potassium-stimulated calcium-dependent TRH release increased linearly after ECS, reaching its highest level 48 h after seizure. Thus, although enhanced calcium-dependent TRH release was associated with elevated tissue levels, this relationship was not proportional in that tissue TRH was elevated to the same extent at all times after ECS, whereas potassium-evoked calcium-dependent TRH release increased gradually over time after seizure. These results suggest that postictal elevations in TRH are associated with an enhanced capacity for release that develops as a result of a time-dependent shift of TRH from a storage compartment ot a readily releasable pool. The observed elevation in stimulated TRH release may be relevant to seizure-induced modulation of TRH receptors in vivo.

Cholinergic deficits in aged rat brain are corrected with nicergoline

Choline acetyltransferase (CAT) and muscarinic cholinergic receptor (MCR) activities are markedly reduced in aged as compared with young-adult rat brain. Nicergoline was found to correct these reduced activities in most regions of the brain, especially in the cerebral cortex and hippocampus. Chronic administration of nicergoline had no effect on CAT activity or MCR binding in young-adult rat brain. Nicergoline thus appears to have a specific therapeutic effect on cholinergic functions in aged rat brain, where it acts both pre-synaptically and post-synaptically.

Effects of chronic codergocrine mesylate administration on the brain somatostatinergic system in aged rats

Codergocrine mesylate (dihydroergotoxine; DHET), which is an ergot derivative, has been reported to counteract some age-induced impairments in brain function, but the mechanism of these effects is not known. We examined the effect of chronic DHET administration on the somatostatinergic system in the brains of aged rats. Intraperitoneal injections of DHET (1 mg/kg per day) or of vehicle were given to aged rats for 14 days, and resulted in a significant increase in somatostatin (SOM) receptor binding in all six brain regions examined except the hindbrain. DHET had no effect on SOM receptor binding in the brains of young-adult rats. However, the SOM concentration in aged rats was nearly identical to that in young-adult rats and the SOM concentration in different brain areas did not change after chronic administration of DHET. Thus, the present results suggest that chronic administration of DHET can ameliorate at least one of the age-induced impairments of brain somatostatinergic function.

Effects of dihydroergotoxine on central cholinergic neuronal systems and discrimination learning test in aged rats

We evaluated changes in the cholinergic neuronal system and learning ability with aging. Choline acetyltransferase (ChAT) activity, a presynaptic index of the cholinergic system, was decreased in the cerebral cortex, hippocampus, striatum, and hypothalamus in the brain of aged rats compared with young adults. Muscarinic cholinergic binding sites (receptors, MCR), a postsynaptic index of the cholinergic system, were markedly decreased in all areas of the brain. However, intraperitoneal injection of 1 mg/kg of dihydroergotoxine (DHET) for 14 days normalized both ChAT and MCR in the cerebral cortex and hippocampus. In the striatum, ChAT was normalized, but MCR did not recover. Aged rats showed marked learning impairment in a 30-day operant type brightness discrimination learning test. Daily DHET administration restored the discrimination ability in the aged rats to nearly the young adult level. DHET had no effects on central cholinergic indices or learning test results in young adult rats. These findings suggest that learning is impaired in aged rats due to impairment in the central cholinergic neuronal system, and that DHET normalizes the decreased function in this system, restoring the learning ability.

Effect of thyrotropin releasing hormone (TRH) on acetylcholine release from different brain areas investigated by microdialysis

1. The effect of thyrotropin releasing hormone (TRH) administration upon acetylcholine (ACh) release in freely moving rats was investigated by means of transversal microdialysis coupled to h.p.l.c. TRH administered either s.c. or via local perfusion increased the ACh release from cortex and hippocampus but not from the striatum. The increase in ACh release was maintained after 7 days of s.c. administration of TRH. 2. After s.c. injection of the neuropeptide, the increase in ACh release was dose-dependent and reached a maximum at 40 min after administration. The maximal percentage increases were 18, 52, 66 and 89% at doses of 1, 2.5, 5 and 10 mg kg-1 and 35, 48 and 54% at doses of 2.5, 5 and 10 mg kg-1 in the cortex and hippocampus, respectively. The effect of TRH was dependent on neuronal activity since it was completely inhibited by perfusion with tetrodotoxin (TTX), 5 X 10(-7) M. 3. Perfusion with TRH, 2.5 micrograms microliters-1, caused 198% and 150% increase in ACh release 60 and 80 min after the beginning of the perfusion in the cortex and hippocampus, respectively. After this initial peak, a 100% increase in ACh release persisted throughout the perfusion. 4. Systemic TRH administration was followed by marked hyperactivity and stereotyped behaviour that showed a time course shorter than that of the increase in ACh release. 5. These findings demonstrate that TRH exerts a strong stimulant action on cortical and hippocampal cholinergic pathways.

Time course of changes in lipid peroxidation, pre- and postsynaptic cholinergic indices, NMDA receptor binding and neuronal death in the gerbil hippocampus following transient ischemia

Brief (5 min) bilateral carotid occlusion in the gerbil produces forebrain ischemia resulting, as previously reported, in almost complete neuronal loss in the CA1 region of the hippocampus; this neuronal destruction occurs between the 4th and 7th day post-ischemia. Various hippocampal biochemical indices were measured from just after such ischemia to 21 days of recirculation, and the temporal pattern of changes compared with that of cell loss. The level of thiobarbiturate reacting substances (TBARS), a measure of lipid peroxidation, was greatly elevated at 30 min after ischemia, rapidly returned to normal levels (by 60 min), but was again elevated on days 4-14. The beginning of this second period of elevation correlated closely with the onset of neuronal loss and the very abrupt and large (to about 32%) decrease in specific N-methyl-D-aspartate (NMDA) binding sites, measured with radioactive CPP. The number of muscarinic binding sites, measured with radioactive quinuclidinyl benzilate, showed an even greater decrease (to 13%) at 21 days post-ischemia, but the decrease was delayed (starting at day 7) and much more gradual than the loss in NMDA binding. In neither case was there any change in binding affinity at any time studied. Acetylcholine (ACh) concentrations were initially greatly decreased (to about 15% at 5 min), transiently increased (to about 130% at 30 min), and then decreased again (to about 15% at 60 min), after which gradual recovery occurred and was completed by day 14. Since no inhibition of choline acetyltransferase activity was observed at any time, the reversible depression in ACh must depend upon some factor other than loss of this key synthetic enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative efficacy and safety of intravenous and oral administration of a TRH analogue (RX77368) in motor neuron disease

Ten consecutive patients with motor neuron disease (MND) who had bulbar symptoms received one or two intravenous doses followed by increasing oral doses of a TRH analogue (RX77368). Similar improvements in speech, swallowing and in tongue and jaw movements were seen after iv and oral administration in nine, five and eight patients respectively. The initial time course of improvement correlated with increasing plasma levels of the drug, but most clinical effects persisted when the levels decreased and became undetectable after 24 hours. The oral solution was tasteless and had no, or minimal, side effects.

Repeated treatment with amitriptyline or electroconvulsive shock does not affect thyrotropin releasing hormone receptors in discrete rat brain structures

We studied the effect of repeated treatment with amitriptyline (10 mg/kg, p.o., twice daily for 14 days) or electroconvulsive shock (ECS) (once daily for 10 days) on the thyrotropin-releasing hormone (TRH) content and TRH receptors in the cerebral cortex, nucleus accumbens, striatum and septum of the rat. Repeated amitriptyline did not significantly affect the density or affinity of TRH receptors in the examined structures, but caused a marked increase in the TRH content in the striatum and nucleus accumbens. Long-term treatment with ECS reduced the density and affinity of TRH receptors in the septum only, but it increased the TRH concentration in the cerebral cortex and striatum. These results, together with the literature data, indicate that there is no simple relationship between the brain content (and release) of TRH and the functional sensitivity of TRH receptors on one hand, and the density of these receptors on the other.

Developmental and regional alteration of methionine enkephalin-like immunoreactivity in seizure-susceptible E1 mouse brain

Methionine enkephalin-like immunoreactivity (ME-LI) in the brain of El mice (seizure-susceptible strain) was measured by radioimmunoassay (RIA) to elucidate the relation between seizures and the opioid system. The lyophilized supernatants of tissue extracts were subjected to ME RIA. The concentration of ME-LI in 25-day-old El mice that had no seizures was significantly decreased in the hippocampus. At the age of 50 days when El mice displayed abortive seizures, the levels of ME-LI in both El(+) and nonstimulated El(o) mice were also significantly reduced in the hippocampus and septal area. It was further shown that the ME-LI concentrations in both 150-day-old adult El(+) during interictal periods and El(o) mice were markedly decreased in the cerebral cortex, septal area, and striatum, as compared with the corresponding regions in ddY mice (seizure-nonsusceptible strain; the mother strain of El). The decrease of ME-LI in the El mouse brain was generally compatible with our previous findings concerning the up-regulation of opioid delta receptors in this species. These results suggest that the reduction of ME-LI in the El mouse brain is not due to convulsions, but could be associated with the pathogenesis of seizure diathesis and seizure manifestations in the El mouse.

Effect of chronic intra-accumbens administration of the TRH analogue CG3509 on histamine-induced behaviour in the rat

1. The present study has investigated the effect of chronic intra-accumbens administration of the thyrotrophin-releasing hormone (TRH) analogue, CG3509, on CG3509- and histamine-induced spontaneous motor activity and brain TRH-like immunoreactive (TRH-LI) levels in the rat. 2. Chronic intra-accumbens administration of CG3509 (5 x 5 micrograms over 3 days) induced: (a) a significant (P less than 0.05) reduction in intra-accumbens CG3509 (0.5 micrograms)-induced hyperactivity, (b) reduced levels of TRH-LI in the nucleus accumbens but not other brain regions, (c) a marked increase (107%, P less than 0.01) in histamine-induced non-locomotor hyperactivity. 3. The present results demonstrate that alteration of central TRH function following treatment with a TRH analogue enhances the effect of intra-accumbens histamine on behavioural hyperactivity, possibly via changes in H1 receptors and suggest that the neuropeptide, TRH and histamine interact in behavioural arousal mechanisms in rat brain.

The effects of hemorrhagic shock on thyrotropin-releasing hormone and its receptors in discrete regions of rat brain

The effects of hemorrhagic shock on thyrotropin-releasing hormone (TRH) levels and its receptors were studied in different regions of the rat brain. Rats were bled for 30 min from the left femoral artery, and their mean arterial pressure was kept at 40 mmHg for the following hour. The rats were killed by decapitation. Rat brains were immediately removed and dissected into 7 regions. Hemorrhagic shock decreased TRH significantly in the frontal cortex, septum, hippocampus, and hindbrain but TRH was not changed in the striatum, hypothalamus, and midbrain. Hemorrhagic shock significantly decreased TRH receptor binding in the septum and hindbrain. Scatchard analysis of saturation isotherms of specific TRH binding showed that the decreased specific TRH binding in the hindbrain resulted not from an increase of the dissociation constant (Kd), but from a decrease in the maximum number of binding sites (Bmax). In the septum, the decrease in specific binding was due both to a decrease in Bmax and an increase in Kd. The findings indicate that TRH plays a role in the physiological response to hemorrhagic shock.

Reduction of rat striatal thyrotropin-releasing hormone receptors produced by repeated methamphetamine administration

It has been reported previously that repeated, but not continuous, administration of methamphetamine (MAP) to animals produces progressive and sustained enhancement of MAP-induced behavior (behavioral sensitization), which may be related to functional changes in central dopamine (DA) systems. To investigate the possible involvement of thyrotropin-releasing hormone (TRH), a neuromodulator of DA, both immunoreactive TRH (IR-TRH) levels and specific TRH binding were examined in rat brain regions after MAP administration either repeatedly (4 mg/kg intraperitoneally once a day for 14 consecutive days) or continuously (about 4 mg/kg/day for 13 consecutive days). Although no significant changes were observed in IR-TRH levels in any regions of the brain following repeated MAP injections, specific TRH binding in the striatum significantly decreased. Scatchard analysis revealed that the decrease was due to a reduction in the maximum number of binding sites (Bmax). Pretreatment with haloperidol prior to each MAP injection prevented this decrease. Continuous MAP administration had no effect on regional specific TRH binding. These results suggest that repeated MAP administration caused lasting dysfunction in the brain TRH system, which may be implicated in the behavioral sensitization.

Alteration of opioid receptors in seizure-susceptible El mouse brain

The distribution density of opioid receptors in the brain of El mice (seizure-susceptible strain) was examined to determine the relation between seizures and the opioid system. Saturation curves and Scatchard plots of [3H]2-D-alanine-5-D-leucine enkephalin binding revealed that the opioid delta receptor density in adult El mice during interictal periods was significantly increased in the cerebral cortex, hippocampus, and septal area. It was further shown that the concentration of such receptors in 25-day-old El mice that had no seizures was also significantly increased in the hippocampus and septal area, with no changes in apparent affinities, as compared with in the corresponding regions in ddY mice (seizure-nonsusceptible strain; the mother strain of El). Such up-regulation of opioid receptors in the El mouse brain could result from deficits in endogenous opioid peptides, which could be associated with the pathogenesis of seizure diathesis in the El mouse.

A method for parallel determination of choline acetyltransferase and muscarinic cholinergic receptors: application in aged-rat brain

We have devised a method for the parallel determination of choline acetyltransferase (CAT) and muscarinic cholinergic receptor (mCh-R) in the same brain tissue. The method for CAT activity determination is more rapid, simplified, stable, and economical than the usual Fonnum's method. With our method, age-associated changes in CAT activity and mCh-R levels were examined. Although CAT activity hardly changed with age except in a few areas, mCh-R binding of aged-rats was markedly reduced in all areas. These results suggest that the change in mCh-R represents an age-associated biochemical change in the brain and that determination of CAT activity is not sufficient for the study of age-associated changes in the brain cholinergic system.

Subacute administration of a TRH analogue (RX77368) in motorneuron disease: an open study

Sixteen patients with motor neuron disease received RX77368, a TRH analogue, IV, repeatedly over 1-12 weeks (median 2 weeks). Slight to moderate improvement in bulbar function, particularly speech, was reproduced or persisted with repeated infusions in 8 of 12 responders over a median of 18 days (range 14-90) during the period of study. Cramps (5/9) and spasticity (5/8) improved for a median of 14 days (range 7-35) and 7 days (range 2-14) respectively. The highest benefit/side effect ratio was seen with 0.2 mg/kg (0.15 mg/kg in those with severe bulbar palsy) every 3-4 days. Long term studies with this analogue in MND are indicated.

The TSH, T4, T3 and prolactin responses to consecutive infusions of a potent and stabilized thyrotrophin releasing hormone analogue, RX77368, in man

The endocrine manifestations of a stabilized thyrotrophin releasing hormone (TRH) analogue, RX77368, have been investigated in six male volunteers. Infusions were given on two occasions, with a 5-day interval between infusions. On the second exposure to RX77368, there was a significant reduction in the TSH response, despite normal basal T3 and T4 levels, while the response of prolactin to RX77368 was unaltered. Domperidone administered during the infusion of RX77368 caused a further elevation of prolactin levels, whilst TSH levels were unchanged. This study shows the differential regulation of thyrotrophs and lactotrophs in response to stimulation by a TRH analogue, and shows, for the first time, down-regulation of the TSH response in vivo, in the presence of normal peripheral thyroid hormone levels. The T3 response to infusions of RX77368 was less than to a bolus injection of TRH, despite a greater TSH response to the analogue, suggesting impaired biological activity of TSH released in response to an infusion of the analogue.

Spinal effects of chronic intrathecal administration of the thyrotrophin-releasing hormone analogue (CG 3509) in rats

The effect of repeated intrathecal administration of a thyrotrophin-releasing hormone (TRH) analogue (CG 3509; 2 micrograms twice daily for 5 days) on wet-dog shake (WDS) and forepaw-licking (FPL) behaviours and spinal cord TRH and indoleamine levels and choline acetyltransferase (ChAT) activity was examined in adult rats. A rapid behavioural tolerance developed to repeated intrathecal injections of CG 3509; WDS and FPL behaviours were reduced by 57% and 34%, respectively, following the fifth injection and remained reduced at the ninth injection. Repeated CG 3509 administration selectively elevated ChAT activity and the level of 5-hydroxytryptamine (5-HT) in the ventral but not in the dorsal horn of the spinal cord, while 5-hydroxyindoleacetic acid (5-HIAA) and TRH levels were unaltered in either region. As ventral horn ChAT activity is principally located within motoneurones this data implies that TRH exerts a trophic-like influence on mature rat motoneurones in vivo. The results also suggest that long-term intrathecal TRH administration may decrease the release of 5-HT from bulbospinal raphe neurones.

Chronic prolactin, gonadal and thyroid hormone treatments in vivo alter levels of TRH and muscarinic receptors in male and female rat tissues

Chronic subcutaneous administration of prolactin into female rats during proestrus led to a 20% (P less than 0.05) decrease in retinal and a 32% (n = 20; P less than 0.01) decrease in pituitary TRH receptors as compared to controls. In parallel experiments prolactin treatment during diestrus failed to influence TRH receptor levels in both tissues compared to vehicle-treated rats. Intraperitoneal injections of triiodothyronine for 8 weeks resulted in a selective 41% increase (P less than 0.02) in retinal TRH receptor levels without any changes in the pituitary and 4 other brain regions. Administration of 17-beta-estradiol for 2 weeks into male rats 1 month after castration resulted in a 68% increase (P less than 0.02) in pituitary TRH receptor content without any changes in the retina, amygdala or hypothalamus. Testosterone administration for 2 weeks into castrated male rats 30 days post-castration did not alter TRH receptor content in the latter 4 tissues but caused a 27% (n = 10; P less than 0.05) and a 40% increase (n = 5; P less than 0.05) in muscarinic cholinergic receptor levels in the superior cervical ganglia and anterior pituitary gland, respectively. In conclusion, these data have demonstrated that chronic administration of exogenous hormones selectively up- or down-regulates TRH and muscarinic receptors in a region-specific manner depending on the physiological state of the animal and the tissue under study, and provide further new evidence for feedback hormonal control of these neurotransmitter receptors.

Regional responses of rat brain muscarinic cholinergic receptors to immobilization stress

The effect of immobilization stress (IM-stress) on the muscarinic cholinergic (m-Ch) receptor binding was determined in 8 brain regions using [3H]quinuclidinyl benzilate (QNB). IM-stress produced an increase in specific QNB binding in the septum, striatum, hippocampus and pons + medulla oblongata. Scatchard analysis revealed that IM-stress produced an increase in the affinity of m-Ch receptors in the septum, hippocampus and pons + medulla oblongata, but did not alter the maximum number of binding sites (Bmax). In the striatum, an increase in specific QNB binding was due to both the increase in Bmax and reduction of the dissociation constant (Kd). The present study suggests that IM-stress induces supersensitivity of postsynaptic m-Ch receptors probably due to a decrease in presynaptic cholinergic activities in the septum, striatum, hippocampus and pons + medulla oblongata. As the m-Ch receptors in the striatum and pons + medulla oblongata are affected by IM-stress, further studies of the m-Ch neural system must be performed under stressful situations in these regions as well as in the septum, hippocampus and cerebral cortex.

Controlled acute trial of a thyrotrophin releasing hormone analogue (RX77368) in motor neuron disease

Twenty five patients with motor neuron disease completed a double blind randomised cross over trial of RX77368, a stabilised TRH analogue, iv over 2 hours against saline. Temporary improvement in bulbar symptoms including speech, respiratory parameters, tongue movements and swallowing were seen. Fasciculations increased and spasticity decreased. Change in muscle force with drug was different from placebo but both increase and decrease in force were seen and did not result in detectable changes in function. Side effects were clinically significant in 50% of the patients and cleared within 12 hours. Prolonged rise of thyroxine and an increase in plasma levels of prolactin, thyroid stimulating hormone and growth hormone were seen and followed characteristic patterns.

Analogs of thyrotropin-releasing hormone: hypotheses relating receptor binding to net excitation of spinal lower motor neurons

Experimentally and clinically, treatment with high-doses of TRH produces a net excitation of spinal lower motor neurons (LMNs) that is subsequently reduced or completely lost through continuous or repeated exposure to the peptide. This is operationally termed "autorefractoriness" (AR). We have performed biochemical and in vivo pharmacologic experiments to investigate the mechanism(s) of AR. Biochemically, we classified TRH and several analogs into three groups based on their binding by spinal-cord TRH-receptors (TRH-Rs): high-affinity, (low nanomolar range; MeTRH, TRH); intermediate-affinity (mid-nanomolar range; MK-771, RX77368) or low-affinity (micromolar range; DN-1417, PNP). When tested in vivo for LMN excitatory activity in cordotomized (T8) rats, TRH and MK-771 produced rapid-onset excitation followed AR. In contrast, sustained excitation with much less AR was produced by the low affinity analog DN-1417. Based on these results, we have formulated two receptor-based hypotheses to explain AR: a) rapid TRH-R desensitization (conversion to an inactive form) by high- but not low-affinity TRH-analogs; and b) a slower down-regulation (cellular internalization) of the agonist-receptor complex, most evident with high-affinity agonists. Thus, low-rather than high-affinity TRH-analogs may be superior to TRH for providing sustained LMN excitation (increase of strength) in motor neuron degenerative disorders.

Changes in the behavioural response to a TRH analogue following chronic amitriptyline treatment and repeated electroconvulsive shock in the rat

The arousal elicited in rats by injection into the nucleus accumbens of the thyrotrophin-releasing hormone analogue CG 3509 (orotyl-histidyl-prolineamide) was used to assess the responsiveness to thyrotrophin-releasing hormone following repeated treatment with amitriptyline or electroconvulsive shock. Fourteen day administration of amitriptyline (15 mg kg-1 i.p. twice daily) reduced the behavioural response to bilateral intra-accumbens injection of CG 3509 (2 X 2.5 micrograms). CG 3509-induced hyperactivity, recovery from pentobarbitone-induced anaesthesia and the reversal of both pentobarbitone-induced hypothermia and decreased respiration, were all significantly reduced compared to either the response of the animals prior to amitriptyline administration or that observed in rats following chronic saline administration. Repeated administration of electroconvulsive shock (5 shocks over 10 days) significantly increased CG 3509-induced hyperactivity and the degree of reversal of pentobarbitone-induced hypothermia and respiratory depression following CG 3509 administration. The results demonstrate that chronic antidepressant treatments alter the central functional responsiveness to thyrotrophin-releasing hormone. These changes are discussed with respect to the effects of antidepressant treatments on 5-hydroxytryptamine receptors and possible thyrotrophin-releasing hormone--aminergic interactions.

Ibotenic acid decreases thyrotropin-releasing hormone receptor binding in the rat amygdala

Thyrotropin-releasing hormone (TRH) receptor densities in the amygdala were examined with quantitative autoradiography in rats treated with the cellular neurotoxin ibotenic acid (IBO). Microinjections of IBO (10 micrograms) into the right basolateral amygdaloid nucleus (BsA) reduced the concentration of TRH receptors in this nucleus by over 50%, when compared to the contralateral BsA and to vehicle-injected control rats. IBO lesions left amygdaloid terminals intact, as shown by normal levels of presynaptic choline uptake sites. Our results strongly suggest that TRH receptors in the amygdala are predominantly located on cell bodies.

Involvement of thyrotropin-releasing hormone (TRH) neural system of the brain in pentylenetetrazol-induced seizures

In order to study the relationship between pentylenetetrazol (PTZ)-induced seizures and the thyrotropin-releasing hormone (TRH) neural system, immunoreactive TRH (IR-TRH) and TRH receptor binding activity were determined in discrete regions of the rat brain before as well as 40 s (immediately before seizures), 150 s (during seizures) and 24 h after an intraperitoneal injection of PTZ (75 mg/kg). IR-TRH markedly increased in the septum 40 and 150 s after the injection, and also in the hippocampus and the thalamus-midbrain region 40 and 150 s after the injection, respectively. However, no significant changes were observed in the TRH receptor binding before, during or after the seizures, suggesting that the increased IR-TRH was not released into the synaptic cleft. This speculation was supported by the dose-dependent inhibition of PTZ-induced generalized seizures by the pre-treatment with TRH or its analogue DN-1417 into the cerebral ventricle.

Seizures and thyrotropin-releasing hormone (TRH) neural system in the rat brain

In order to study the relationship between seizures and the thyrotropin-releasing hormone (TRH) neural system, immunoreactive TRH (IR-TRH) and TRH receptor binding activity were determined by pentylenetetrazol (PTZ)-induced seizures and amygdaloid (AM) kindling. IR-TRH markedly increased in the septum 40 and 150 seconds after the PTZ injection. A significant increase in the IR-TRH concentrations was also noted in the hippocampus and thalamus/midbrain 40 and 150 seconds after the PTZ injection, respectively. However, no significant changes were observed in the TRH receptor binding before, during or after the PTZ-induced seizures. In addition, a lasting change in the striatal TRH receptors after AM kindling as well as a transient IR-TRH increase in the limbic structures were seen 48 hours after Am-kindled convulsions. TRH and its analog (DN-1417) inhibited PTZ-induced generalized seizures dose-dependently. These findings indicate the involvement of the TRH neural system in seizure mechanisms, and suggest that endogenous TRH may be an antiepileptic substance in the brain.

Lack of interaction between thyrotropin releasing hormone and its analogs with 3H-quinuclidinyl benzilate recognition sites in the rat striatum

The effects of thyrotropin releasing hormone (TRH) on the binding of 3H-quinuclidinyl benzilate (QNB) to cholinergic muscarinic receptors of striatal region of rat brain were evaluated. In vitro studies indicate that neither TRH, its two analogs, MK-771 [L-N-(2-oxopiperidin-6-yl-carbonyl)-L-histidyl-L-thiazolidine-4-++ +carboximide] and DN-1417 (gamma-butyrolactone- gamma-carbonyl-L-histidyl-L-prolineamide) nor the metabolite histidyl-proline diketopiperazine [cyclo(His-Pro)] at concentrations ranging from 10(-9) to 10(-3) M affected 3H-QNB binding to striatal muscarinic receptors. On the other hand, p-Glu-His-Pro-OH (TRH-free acid), another metabolite of TRH caused significant inhibition (21%) at 10(-4) M concentration. Intraperitoneal administration of TRH (1 or 10 mg/kg) also failed to elicit any changes in the affinity (Kd) or density (Bmax) of muscarinic receptors in the striatum. The results suggest that TRH does not influence striatal muscarinic receptors and that the known effects of TRH and its analogs on central cholinergic system may be due to mechanisms other than those affecting postsynaptic muscarinic receptors directly.

Up-regulation of thyrotropin-releasing hormone (TRH) receptors in rat spinal cord after codepletion of serotonin and TRH

Immunohistochemical evidence indicates the coexistence of serotonin and TRH in many raphe neurons. We examined the biochemical changes in TRH receptors after destruction of the serotonergic pathways by 5,7-dihydroxytryptamine (5,7-DHT). 2 weeks after an intracerebroventricular injection of 5,7-DHT, rats were killed, and specific brain regions were dissected on ice. Serotonin levels in the CNS of lesioned rats was reduced by 50-85% in all regions, with the highest reduction in the spinal cord and hippocampus. Immunoreactive TRH was reduced in the spinal cord by 70%, but other brain regions contained normal levels of TRH. TRH receptor binding was increased by 40% in the spinal cord of lesioned rats, but appeared unchanged in rostral brain regions in which no decrease in TRH content was detected. Scatchard plots of TRH receptor binding in the spinal cord indicated that the increased binding after 5,7-DHT administration reflected an increased receptor number. These findings suggest that biochemical up-regulation of TRH receptors occurs in the spinal cord following depletion of TRH.

Treatment of rats with the TRH analog MK-771. Down-regulation of TRH receptors and behavioral tolerance

The regulation of receptors for thyrotropin-releasing hormone (TRH) in the central nervous system (CNS) was studied by administering the TRH analog, MK-771 to rats by three different schedules and then measuring changes in the binding of [3H](3MeHis2) TRH and behavioral responses to a challenge with MK-771. The behavioral responses monitored were wet-dog shakes, large motor movements, small motor movements and forepaw tremor. Temperature changes were also monitored. The first schedule consisted of intracerebroventricular (i.c.v.) administration of MK-771 for seven days (5 micrograms/microliter per hr) via a mini-osmotic pump. At the end of the treatment, rats showed no shaking or large motor movements typically induced by TRH, in response to a 5 mg/kg (i.p.) challenge of MK-771. Receptors were found to be 50% of control levels in the three areas of brain examined. The second schedule consisted of the administration of MK-771 (5 micrograms/2 microliters, i.c.v., once a day and 2 mg/kg, i.p., once a day). It was found that the number of receptors decreased on about the same time course as development of tolerance to wet-dog shakes and large motor movements. The third schedule consisted of the administration of MK-771 (5 micrograms/2 microliters, i.c.v.) once every 2 hr to a total of four doses. These animals eventually developed tolerance to the wet-dog shakes produced by the subsequent challenge with MK-771 and also showed a 50% decrease in receptor binding after the fourth exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium hopantenate on neurotransmitter receptors in the rat brain

Effects of calcium hopantenate (HOPA) on neurotransmitter and neuropeptide receptors in the central nervous system (CNS) were investigated. In the radioreceptor assay (RRA), HOPA inhibited the [3H]-gamma-aminobutylic acid (GABA) receptor binding in a dose-dependent manner with a cross-reactive potency of 0.2%. On the other hand, radiolabeled ligand binding to CNS receptors in the benzodiazepine (BDZ)-, muscarinic cholinergic (mACh)-, methionine-enkephalin (ENK)- and thyrotropin releasing hormone (TRH)-RRA systems was not inhibited even by the addition of HOPA up to 100 microM. Repeated injection of HOPA (250 mg/kg/day for 7 consecutive days) increased GABA receptor binding by 53% in the cerebral cortex, while GABA binding in the rest of the forebrain did not change. The increased GABA receptor binding in the cerebral cortex of HOPA treated rats was due to the increased affinity of the binding sites. BDZ-, mACh-, ENK- and TRH-receptor bindings were not affected in either the cerebral cortex or the rest of the forebrain by repeated injection of HOPA. These results suggest that at least a part of the therapeutic efficacy of HOPA is due to sensitization of the GABA receptor in the cerebral cortex.

Properties and distribution of vasoactive intestinal polypeptide receptors in the rat brain

Vasoactive intestinal polypeptide (VIP) interaction with the rat brain synaptic membrane was examined using 125I-labeled VIP as a tracer molecule. Ion, pH and incubation time significantly influenced VIP receptor binding. Scatchard analysis suggested that the rat brain membrane had a single binding site with an apparent dissociation constant (Kd) of 9.8 X 10(-9) M. The receptor activity was sensitive to trypsin and phospholipase A digestion, and heating at 100 degrees C for 5 min completely destroyed the binding activity. This indicates that protein and phospholipid moieties are essential for VIP binding. Thiol reagents reduced receptor binding activity, suggesting that an intrachain disulfide bond might be an important constituent of the VIP binding site. High levels of binding were found in the hippocampus, striatum and cerebral cortex, and binding was very low in the medulla/pons and cerebellum. The receptor density did not always parallel the brain distribution of immunoreactive VIP (IR-VIP) concentration. The cerebral cortex had the highest ratio of IR-VIP-to-receptor, and the striatum had the lowest ratio of IR-VIP-to receptor. Although intra-nigral or intra-striatal injection of 6-hydroxydopamine had no effect on striatal VIP-binding, an intra-striatal injection of kainic acid resulted in a substantial lowering of striatal VIP receptors. The neurotoxic effects of kainic acid have been shown to be dependent on the corticostriatal tract, and this suggests that the striatum receives the VIPergic innervation from the cerebral cortex. Our findings indicate that endogenous VIP and VIP receptors might act as a neurotransmission modulator of extrapyramidal function in the striatum.