Effects of a new thyrotropin releasing hormone analogue, YM-14673, on the in vivo release of acetylcholine as measured by intracerebral dialysis in rats

Evidence for interplay between thyrotropin-releasing hormone (TRH) and its structural analogue pGlu-Glu-Pro-NH2 ([Glu2]TRH) in the brain: an in vivo microdialysis study

Local perfusion of pGlu-Glu-Pro-NH2, an endogenous peptide structurally related to thyrotropine-releasing hormone (TRH), via in vivo microdialysis into the rat hippocampus did not change the basal level of extracellular acetylcholine. However, co-perfusion of pGlu-Glu-Pro-NH2 with TRH in equimolar concentrations yielded a significant attenuation of TRH-induced acetylcholine release. The results have supported the study's hypothesis that pGlu-Glu-Pro-NH2 opposes the cholinergic effect of TRH in the mammalian central nervous system. The enantiomer pGlu-d-Glu-Pro-NH2 affected neither basal extracellular nor TRH-induced increase of acetylcholine levels.

Taltirelin improves motor ataxia independently of monoamine levels in rolling mouse nagoya, a model of spinocerebellar atrophy

To examine the relationship between motor ataxia and monoamine levels in the central nervous system, the contents and concentrations of noradrenaline (NA), dopamine (DA) and serotonin (5-HT) in the cerebellum, brain stem and spinal cord were measured in rolling mouse Nagoya (RMN), a murine model of spinocerebellar atrophy. The tissue weight of the cerebellum and spinal cord, but not that of the brain stem was significantly lower in RMN than in the control group. In RMN, the NA content of the brain stem and spinal cord, but not the cerebellum were decreased relative to the control, and the concentration of NA in the spinal cord was also lower, but not significant. The DA and 5-HT contents in each tissue did not differ from those of the control, but the concentrations of monoamines, except for DA, were elevated in the brain stem and spinal cord in RMN. In particular, the concentrations of NA, DA and 5-HT in the cerebellum were significantly increased in RMN. Repeated administration of tartilerin hydrate, an analog of thyrotropin-releasing hormone, improved the ataxia of RMN, and elicited no obvious changes in either monoamine content or concentration of cerebellum, brain stem and spinal cord. These results indicate that the concentration of DA, as well as NA and 5-HT, increased in the RMN cerebellum, and that tartilerin improves the motor function of these mice via mechanisms other than changes in the levels of NA, DA and 5-HT in the central nervous system.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

The effect of methamphetamine on the release of acetylcholine in the rat striatum

We examined the effect of methamphetamine on the release of acetylcholine in the striatum of freely moving rats, using an in vivo microdialysis method. The basal level of acetylcholine was 3.67+/-0.47 pmol/30 microl per 15 min in the presence of neostigmine (10 microM). Tetrodotoxin (1 microM), a selective blocker of voltage-dependent Na+ channels, markedly inhibited the release of acetylcholine in the striatal perfusates. Apomorphine (1.0 mg/kg, i.p.), a dopamine receptor agonist, also significantly attenuated acetylcholine release. Methamphetamine (0.1 and 0.5 mg/kg, i.p.) did not immediately affect acetylcholine release in the striatum, but a dose of 1.0 mg/kg (i.p.) induced an increase of acetylcholine release in the striatum at 15-60 min. Striatal infusion of methamphetamine (5 and 10 microM) did not influence acetylcholine release. The increase following intraperitoneal administration of methamphetamine was slightly diminished by haloperidol (0.5 mg/kg). After microinjection of the neurotoxin, 6-hydroxydopamine (6 microg/3 microl), in the substantia nigra 7 days before, the increase of acetylcholine induced by the administration of methamphetamine (1.0 mg/kg) was slightly attenuated, whereas the administration of reserpine (2 mg/kg, i.p.) 24 h before, combined with alpha-methyl-p-tyrosine (300 mg/kg, i.p.) 2.5 h before, completely blocked the increase in release of acetylcholine. These findings suggest that methamphetamine exerts an excitatory influence on striatal acetylcholine release in freely moving rats, and that this excitatory effect involves the dopaminergic system and the catecholaminergic system.

An update on the CNS actions of TRH and its analogs

This brief review will discuss the recent literature on several of the central actions of TRH and its analogs. The most prominent of these actions include: (1) the arousal or analeptic effect in drug narcotized animals or in concussion models; (2) the reversal of cognitive deficits produced by various drugs or procedures, and (3) the improvement of several neurological deficits produced in animal models of spinal and/or cerebellar injury. The mediation of these TRH effects by neurotransmitters is discussed. While little has been published on the human neuropsychopharmacology of TRH, and especially of its analogs, the future holds considerable therapeutic promise for these interesting drugs.

TRH mimetics: differentiation of antiamnesic potency from antidepressant effect

For the purpose of rational modification of the TRH molecule, we were pursuing an approach that consists of two steps: (1) 'obligatory' replacement of histidine with glutamine in TRH and (2) the application of conformational constraints for putative bioactive conformation I stabilized by an intramolecular hydrogen bond between C-terminal carboxamide proton and alpha-carbonyl of histidyl (glutaminyl), and conformation II formed by an intramolecular hydrogen bond between alpha-carbonyl of pyroglutamyl and prolinamide proton. Significant antiamnesic potency was discovered in the passive avoidance test (ECS and Scopolamine induced amnesia) for conformation II mimic (8S,10aS)-8-carbamoyl-1,2,3,6,7,8,9,10a- octahydro-5H,10H-pyrrolo[1,2-a][1,4]diazocin-5,10-dione (2) at doses of 0.1 and 1.0 mg/kg. EEG analysis indicates a mild activating effect of compound 2 on EEG, which is similar to that of piracetam and differs from hard amphetamine activation. Conformation I mimic 3-(2-carbamoylethyl)-2,3,6,7,8,8a-hexahydro-1H,4H-pyrrolo[1,2-a] pyrazin-1,4-dione (1) exhibited an antidepressant effect at a dose of 1 mg/kg. The transition from two putative quasi-cyclic bioactive conformations of TRH and its obligatory similar analogue [Gln2]-TRH to their cyclic mimics led to differentiation of antiamnesic and antidepressant activity of TRH.

Effects of thyrotrophin-releasing hormone tartrate and its sustained release formulation on cerebral glucose metabolism in aged rats

The effects of a sustained release formulation of thyrotrophin-releasing hormone (TRH) over two weeks (TRH-SR, 10 or 50 mg kg-1, equivalent to 0.56 or 2.80 mg kg-1 free TRH, respectively) and repeated treatment with TRH tartrate (TRH-T, 0.3, 1.0 or 3.0 mg kg-1, equivalent to 0.2, 0.7 or 2.0 mg kg-1 free TRH, respectively) on the rate of local cerebral glucose utilization (LCGU) were investigated using the quantitative autoradiographic 2-deoxy-[14C]D-glucose method in various brain regions of aged rats. In aged rats (28 months old), the LCGU was significantly reduced as compared with young adult rats (3 months old), while treatment with TRH-SR ameliorated the reduction of the LCGU in a dose-dependent manner. The brain regions ameliorated by TRH-SR were the auditory cortex, septal nucleus, substantia nigra, cerebellar cortex and cerebellar nucleus. In contrast, once-daily repeated treatment over one week with TRH-T at a dose of 0.3 mg kg-1 (equivalent to 50 mg kg-1 of TRH-SR) had no effect on the reduced LCGU in various brain regions in aged rats (27 months old), whereas treatment with a higher dose of TRH-T (0.7 or 2.0 mg kg-1 free TRH) significantly ameliorated the reduction. The comparison of the ameliorating potencies between TRH-T and TRH-SR indicated that TRH-SR had a potency about 7 times greater than TRH-T.

Effect of neuropeptides on cognitive function

Recent evidence indicates that, in addition to the involvement of cholinergic and other neurotransmitter systems, various neuropeptides that occur in cortical and subcortical brain regions have a role in cognitive behavior. This evidence results largely from behavioral studies in rodents and other animals, following peptide administration and only in a very few cases from similar studies in human subjects. Several neuropeptides studied appear to enhance or produce changes conducive to improvement in cognitive performance and these include vasopressin, corticotrophin-releasing hormone (CRH), somatostatin, substance P, neuropeptide Y, and thyrotrophin-releasing hormone (TRH), while one peptide, galanin, has been reported to inhibit cognitive processes. Of those neuropeptides that improve performance, only TRH has been shown recently to attenuate the memory impairment of human subjects and Alzheimer patients treated with an anticholinergic drug, and this review describes a series of complimentary studies in adult and aged rodents that contribute to our understanding of the possible mechanisms involved in the role of TRH in cognition.

Minimal effects of dextroamphetamine on scopolamine-induced cognitive impairments in humans

The central anticholinergic drug scopolamine has been used to model aspects of the memory impairment that occurs in Alzheimer's disease and in aging. To determine whether nonspecific stimulant effects can attenuate the cognitive impairment induced by scopolamine, we studied the effects of scopolamine and the stimulant dextroamphetamine in 17 young normal volunteers. After a baseline day of cognitive testing, subjects participated in two study days, in which they received dextroamphetamine (d-AMP) (0.25 mg/kg p.o.) + scopolamine (0.5 mg i.v.) and placebo + scopolamine, in randomized order under double-blind conditions. There were no statistically significant differences in cognitive test performance between the two drug conditions with the exception of one of the category retrieval tasks. Stimulant effects were documented to occur by other measures. We conclude that d-AMP at the dose used does not attenuate the memory impairment induced by scopolamine.

Alterations in local cerebral glucose metabolism and endogenous thyrotropin-releasing hormone levels in rolling mouse Nagoya and effect of thyrotropin-releasing hormone tartrate

To identify the brain region(s) responsible for the expression of ataxic gaits in an ataxic mutant mouse model, Rolling mouse Nagoya (RMN), changes in local cerebral glucose metabolism in various brain regions and the effect of thyrotropin-releasing hormone tartrate (TRH-T), together with alterations in endogenous thyrotropin-releasing hormone (TRH) levels in the brains of RMN, were investigated. Ataxic mice [RMN (rol/rol)] showed significant decreases in glucose metabolism in regions of the diencephalon: thalamic dorsomedial nucleus, lateral geniculate body and superior colliculus; brain stem: substantia nigra, raphe nucleus and vestibular nucleus; and cerebellar nucleus as compared with normal controls [RMN (+/+)]. When RMN (rol/rol) was treated with TRH-T (10 mg/kg, equivalent to 7 mg/kg free TRH), glucose metabolism was significantly increased in these regions. These results suggest that these regions may be responsible for ataxia. We also found that TRH levels in the cerebellum and brain stem of RMN (rol/rol) were significantly higher than those of RMN (+/+). These results suggest that ataxic symptoms in RMN (rol/rol) may relate to the abnormal metabolism of TRH and energy metabolism in the cerebellum and/or brain stem and that exogenously given TRH normalizes them.

Effect of TA-0910, a novel thyrotropin-releasing hormone analog, on in vivo acetylcholine release and turnover in rat brain

To examine the action of a novel thyrotropin-releasing hormone (TRH) analog, TA-0910 ((-)-N-[(S)-hexahydro-1-methyl-2,6-dioxo-4-pyrimidinylcarbonyl]-L- histidyl-L-prolinamide tetrahydrate), on the cerebral cholinergic systems, the release of acetylcholine (ACh) and choline in freely-moving rats and ACh accumulation in gamma-butyrolactone (GBL, a nerve impulse flow blocker)- and physostigmine-treated rats were examined. TA-0910 (0.1-1 mg/kg, i.p.) caused a marked dose-dependent increase in extracellular ACh levels and a decrease in choline levels in the hippocampus of freely moving rats. These effects were significantly stronger and longer-lasting than similar effects of TRH. TA-0910 (1, 3 mg/kg, i.p.) depressed the ACh accumulation in the cerebral cortex and hippocampus of GBL (1000 mg/kg, i.p.)-treated rats. Moreover, this analog (1, 3 mg/kg, i.p.) increased the accumulation rate of ACh in these regions in physostigmine (1 mg/kg, i.p.)-treated rats. TRH (30 mg/kg, i.p.) affected the ACh accumulation only in the hippocampus of the GBL-treated rats. These results suggest that TA-0910 not only enhances the release of ACh, but also accelerates the ACh turnover, i.e., ACh release and synthesis, at the cholinergic neuronal terminals in normal rats.

NS-3(CG3703), a TRH analog, ameliorates scopolamine-induced memory disruption in rats

The effects of a metabolically stable TRH analog, N-[[(3R, 6R)-6-methyl-5-oxo-3-thiomorpholinyl]carbonyl]-L-histidyl-L- prolinamide tetrahydrate (NS-3, CG3703) on the scopolamine-induced memory disruption in maze performance tests were investigated in rats. a) In the delayed nonmatching-to-sample (DNMS) task using a T-maze, NS-3 (0.3 mg/kg) produced a significant reversal of the marginal disruption of choice accuracy induced by scopolamine (0.3 mg/kg) at the short (5 s) and long (120, 480 s) interval delays. Physostigmine (0.5 mg/kg) produced a significant reversal only at a 5-s interval delay. b) In the eight-arm radial maze task, NS-3 (0.3 mg/kg) significantly reversed the deficit of choice accuracy induced by scopolamine (0.3 mg/kg), whereas neither TRH (3-30 mg/kg) nor physostigmine (0.1-1 mg/kg) had any effect. The consistent reversal of these maze-learning performances by NS-3, but not by TRH or physostigmine, may be due to its potent enhancement of cholinergic and noradrenergic neuronal activities.

NS-3 (CG3703), an analog of thyrotropin-releasing hormone, ameliorates cognitive impairment in rats

The effects of thyrotropin-releasing hormone (TRH) and its analog, N-[[(3R,6R)-6-methyl-5-oxo-thiomorpholinyl] carbonyl]-L-histidyl-L- prolinamide tetrahydrate (NS-3, CG3703) on disturbance of memory of a passive avoidance response (PAR) and an escape response in rats were investigated. NS-3 improved amnesia caused by scopolamine, electroconvulsive shock (ECS), and cycloheximide (CXM), but TRH improved only the ECS-induced amnesia. NS-3 reversed learning deficits caused by hypercapnia, but TRH had no effect. These differences in the effect between NS-3 and TRH may be due to their biological half-life in rat plasma. These results suggest that NS-3 possesses more potent antiamnestic effects than TRH in rats.

Effects of a sustained release formulation of thyrotropin-releasing hormone on behavioral abnormalities in senescence-accelerated mice

Effects of a sustained release formulation of thyrotropin-releasing hormone (TRH-SR) on reduced anxiety-like behavior and learning impairment in senescence-accelerated mice (SAM) were examined. SAMP8/Ta (SAMP8) mice showing age-related emotional changes as well as learning and memory impairments, and SAMR1TA (SAMR1) mice exhibiting normal aging were used at 8 months of age. Subcutaneous injection of TRH-SR (2.8 mg/kg as free TRH) produced a sustained increase in immunoreactive plasma TRH levels up to about 4 weeks after dosing in SAMP8. TRH-SR antagonized the reduced neophobia to novel food in SAMP8 in a dose-dependent manner when tested 10 days but not 3 days after the injection. In the elevated plus-maze test, the SAMP8 control group treated with vehicle had significant increases in the number of entries into open arms and the time spent in open arms in comparison to SAMR1 mice. TRH-SR showed dose-dependent decreases in the number of entries into open arms, and reduced the time spent in open arms in SAMP8 mice. Furthermore, TRH-SR significantly improved the impairment of water maze learning in SAMP8 mice. In contrast, bolus administration of TRH had no significant effects on behavioral abnormalities in SAMP8 even at high doses, implying that long-term and continuous infusion of TRH may be important for amelioration of the behavioral abnormalities. These results suggest that TRH-SR may be useful for treatment of age-related emotional disorders and memory disturbance in dementia.

Dopamine depletion attenuates amphetamine-induced increases of cortical acetylcholine release

The extent to which the d-amphetamine (2.0 mg/kg)-induced increase in cortical acetylcholine release is mediated by dopamine and/or noradrenaline was assessed using in vivo microdialysis in freely moving rats. Unilateral 6-hydroxydopamine lesions of the mesotelencephalic dopaminergic system, which depleted forebrain dopamine by 99% on the lesioned side, significantly attenuated the effect of d-amphetamine on cortical acetylcholine release compared to a surgical control group (160% baseline vs. 270%), suggesting that dopamine at least in part mediates this effect of d-amphetamine. In contrast, bilateral 6-hydroxydopamine lesions of the dorsal noradrenergic bundle which depleted forebrain noradrenaline by at least 95% had no effect on d-amphetamine-stimulated cortical acetylcholine release. These results point to an important role for forebrain dopamine in the regulation of cortically projecting cholinergic neurons and fail to support the hypothesis that the ascending noradrenergic projections of the locus coeruleus are significantly involved.

Possible involvement of central cholinergic system in ameliorating effects of indeloxazine, a cerebral activator, on disturbance of learning behavior in rats

Effects of indeloxazine, a cerebral activator, on passive avoidance learning by disruption of cholinergic transmission were studied in rats. The shortened latency was prolonged when indeloxazine was administered before training in rats subjected to scopolamine-injection, ethylcholine aziridinium ion (AF64A)-treatment and nucleus basalis magnocellularis (NBM) lesion. Indeloxazine administered immediately after training also showed ameliorating effects on passive avoidance in NBM-lesioned rats. Indeloxazine increased extracellular acetylcholine concentration in the frontal cortex of rats. Indeloxazine possesses facilitatory effects on cerebral function in part due to activation of the central cholinergic system.

NS-3, a TRH analog, reverses repeated ECS-induced deficits in water maze performance in the rat

Rats given five consecutive daily electroconvulsive shock (ECS) treatments and trained to run in the Morris water maze, starting three days posttreatment, showed deficits in learning and memory functions. Treatment before each training session with the thyrotropin-releasing hormone (TRH) analog NS-3 [(CG-3703), (3R),(6R)-6-methyl-5-oxo-3-thiomorphorinyl-l-histidyl-l-prolinamid e tetrahydrate] reversed these behavioral deficits. The possible use of TRH and its analogs as therapeutic treatment for the cognitive dysfunctions resulting from electroconvulsive shock treatment for depression and the possible involvement of central cholinergic systems in the cognitive dysfunctions are discussed.

Behavioural effects of scopolamine and the TRH analogue RX77368 on radial arm maze performance in the rat

Effects of repeated intracerebroventricular administration of the thyrotrophin-releasing hormone (TRH) analogue, RX77368 (3,3'-dimethyl-TRH, 2 μg, once daily), on a scopolamine-induced performance deficit in an eight-arm radial maze were evaluated in adult rats. Scopolamine (0.3 mg/kg i.p.-30 min) pre-treatment produced a significant deficit in the number of unrepeated arm entries and total arm entries and increased the percentage of incorrect arm entries and the total time on the maze, compared with saline-treated controls. Prior treatment with RX77368 (40 min before maze testing) produced a partial but significant attenuation of the scopolamine-induced performance deficit on the maze during the first five trials but RX77368 also enhanced maze performance during the same period when given alone. These results suggest that the observed scopolamine-induced performance deficit on the radial arm maze partly results from a reduction in locomotion and maze exploration rather than solely impairment of memory, and that RX77368 treatment may improve radial maze performance by increasing arousal and exploratory behaviour in rats rather than directly enhancing cognition.

Effects of sustained release formulation of thyrotropin-releasing hormone on learning impairments caused by scopolamine and AF64A in rodents

The effects of a sustained-release formulation of thyrotropin-releasing hormone (TRH-SR) on learning impairments induced by scopolamine and a cholinergic neurotoxin, ethylcholine aziridinium ion (AF64A), were examined in rodents. Subcutaneous injection of TRH-SR (2.8 mg/kg as free TRH) produced a sustained increase in immunoreactive plasma TRH levels up to about 2 weeks after dosing in rats. TRH-SR (0.56 and 2.8 mg/kg) given subcutaneously 7 days before the acquisition trial markedly ameliorated scopolamine-induced amnesia in mice, as evaluated with a passive avoidance task. Repeated administration of TRH for 7 days at doses of 0.2-5 mg/kg s.c. elicited a dose-dependent recovery from amnesia induced by scopolamine, whereas only the group treated with 5 mg/kg/day showed a significant improvement. The rats with bilateral intracerebroventricular injection of AF64A (3.75 nmol/brain) showed a significant impairment in the water maze task 2 weeks after surgery. TRH-SR (0.56 and 2.8 mg/kg) also exhibited a dose-dependent ameliorating action on the deficit. These findings indicate that TRH-SR ameliorates learning impairments produced by scopolamine and AF64A, and suggest that continuous infusion of TRH may have a potent learning and memory improving action at low doses.

Effects of indeloxazine hydrochloride, a cerebral activator, on passive avoidance learning impaired by disruption of cholinergic transmission in rats

The effect of indeloxazine [(+/-)-2-[(inden-7-yloxy)methyl]morpholine hydrochloride, YM-08054], a cerebral activator, on passive avoidance learning by disruption of cholinergic transmission was investigated in rats. Indeloxazine prolonged the latency for stepping out of an illuminated compartment into a dark compartment, in both mature and aged rats. Disruption of cholinergic transmission was induced by injection of scopolamine, ethylcholine, treatment with aridinium ion (AF64A) and by lesioning the nucleus basalis magnocellularis. The shortened latency in these models was prolonged when indeloxazine was administered before training in doses which did not affect spontaneous movement or the response to pain in mature rats and administration of indeloxazine, immediately after training, also had an ameliorating effect on passive avoidance in the lesioned rats. In biochemical studies, indeloxazine increased the extracellular concentration of acetylcholine in the frontal cortex of mature rats. These results suggest that indeloxazine possesses facilitatory effects on cerebral function, in part due to activation of the central cholinergic system.

Effects of a novel thyrotropin-releasing hormone analogue, JTP-2942, on extracellular acetylcholine and choline levels in the rat frontal cortex and hippocampus

The effects of a novel thyrotropin-releasing hormone (TRH) analogue, N alpha-[(1S,2R)-2-methyl-4-oxocyclopentanecarbonyl]-L-histidyl-L-pr olinamide (JTP-2942) on acetylcholine (ACh) release and on the extracellular choline level were investigated in rat frontal cortex and hippocampus by microdialysis, and were compared with effects of TRH. JTP-2942 (0.3 mg/kg i.p.) produced a marked (> 300%) and persistent increase of ACh release in both the frontal cortex and hippocampus, while TRH (3 mg/kg i.p.) caused a significant but transient increase of ACh to about 200% in the frontal cortex. Both drugs significantly decreased the choline levels in both brain regions. Investigation of the effects of JTP-2942 (0.001-1 mM) and TRH (1 and 10 mM) on ACh release and choline levels when perfused through the dialysis probe revealed that JTP-2942 had a greater effect than TRH in both the frontal cortex and the hippocampus. The action of JTP-2942 was about 1000-fold more potent than that of TRH in both brain regions. Oral administration of JTP-2942 at a dose of 10 mg/kg markedly and persistently increased the release of ACh and at doses of 1-10 mg/kg decreased the extracellular choline level in the frontal cortex and hippocampus. These results also suggest that JTP-2942 has some selectivity for the hippocampus compared to the frontal cortex after both systemic administration and local injection. The increase of ACh release caused by JTP-2942 was completely antagonized by perfusion with tetrodotoxin (TTX, 1 microM), suggesting that the action of JTP-2942 on cholinergic neurons was mediated via neuronal activity.

Dopaminergic regulation of cortical acetylcholine release

The extent to which the activity of basal forebrain cholinergic neurons is influenced by dopamine (DA) was investigated using in vivo microdialysis of cortical acetylcholine (ACh). Systemic administration of the DA receptor agonist apomorphine significantly increased dialysate concentrations of ACh. Systemic, but not local, administration of d-amphetamine produced similar effects. Both D1 (SCH 23390) and D2 (haloperidol, raclopride) DA receptor antagonists attenuated the amphetamine-induced increase in cortical ACh release; however, only the D1 antagonist significantly reduced basal output of cortical ACh. These findings suggest that the activity of cortically projecting cholinergic neurons in the nucleus basalis is regulated in an excitatory manner by central dopaminergic neurons and that both D1 and D2 receptors are involved.

The effects of thyrotropin-releasing hormone and scopolamine in Alzheimer's disease and normal volunteers

Thyrotropin-releasing hormone (TRH), a neuromodulator and possibly a neurotransmitter in the central nervous system, was shown in a prior study of young normal volunteers to attenuate the memory impairment induced by the anticholinergic drug scopolamine. In the present study, the cognitive, behavioral and physiologic effects of high dose TRH (0.5 mg/kg), both alone and following administration of scopolamine, were examined in 10 Alzheimer's disease (AD) patients (mean age±SD=63.5 years) and 12 older normal volunteers (mean age=64.9±8.8 years). On the day AD subjects received TRH alone, modest but statistically significant improvement from baseline performance was documented on some tests of learning and memory, especially in those with mild dementia severity. In comparing cognitive test performance between the scopolamine alone and scopolamine+TRH conditions, only two test scores were significantly higher in the latter condition. In the group of older volunteers, TRH did not attenuate scopolamine-induced cognitive impairment, contrary to prior findings in a group of younger controls. In fact, older subjects performed worse after receiving scopolamine followed by TRH than after receiving scopolamine alone. In addition, no change from baseline cognitive performance was detected after subjects received TRH alone. These findings raise several questions and speculations on possible age-related changes in the cholinergic system, as well as on the mechanism of the interaction of TRH with the cholinergic system.

Thyrotropin-releasing hormone selectively reverses lorazepam-induced sedation but not slowing of saccadic eye movements

To investigate preliminary reports that benzodiazepine-induced sedation may be reversed by thyrotropin-releasing hormone (TRH), we examined the effect of TRH or saline placebo on two variables which are sensitive to benzodiazepine agonists: changes in sedation and saccadic eye movements. Lorazepam 10 micrograms/kg i.v. increased self-ratings of sedation and reduced self-ratings of alertness and these changes were almost completely reversed by TRH. In contrast the slowing of saccadic eye movements by lorazepam was not reversed by TRH. The effects of TRH do not appear to be due to a direct antagonism at the benzodiazepine receptor, since flumazenil reverses changes in both variables. Moreover ligand binding studies reveal that TRH has very low affinity at this receptor. These clinical data provide the first demonstration that it is possible to distinguish between the effects of benzodiazepines on saccadic eye movements and psychological self-ratings.

TRH protection against memory retrieval deficits is independent of endocrine effects

An electrobrainshock (EBS)-induced memory retrieval deficit was produced in normal and hypophysectomized mice. In normal mice, thyrotropin-releasing hormone (TRH) (0.1 to 30 mg/kg) protected against this EBS disruption of memory after intraperitoneal but not oral (1.0 to 100 mg/kg) administration. In hypophysectomized mice, TRH (0.3 and 3.0 mg/kg) also protected against the retrieval deficit induced by EBS. The memory protection afforded by TRH was unrelated to its ability to elevate plasma levels of triiodothyronine (T3) and thyroxine (T4), nor was TRH's memory protection mediated through an anticonvulsive mechanism. These results support the notion that TRH may play an important role in memory modulation and may have therapeutic value in certain disease states in humans.