Distribution of thyrotropin-releasing hormone (TRH) in the hippocampal formation as determined by radioimmunoassay

Function of Cathepsin K in the Central Nervous System of Male Mice is Independent of Its Role in the Thyroid Gland

Cathepsin K deficiency in male mice (Ctsk^-/-) results in decreased numbers of hippocampal astrocytes and altered neuronal patterning as well as learning and memory deficits. Additionally, cathepsin K carries essential roles in the thyroid gland where it contributes to the liberation of thyroid hormones (TH). Because TH are essential for brain development, in particular for the cerebellum, we investigated whether cathepsin K's function in the thyroid is directly linked to the brain phenotype of Ctsk^-/- mice. Serum levels of thyroid stimulating hormone, brain concentrations of free TH, and deiodinase 2 (Dio2) activity in brain parenchyma as well as cerebellar development were comparable in Ctsk^-/- and WT animals, suggesting regular thyroid states and TH metabolism. Despite unaltered transcript levels, protein expression of two TH transporters was enhanced in specific brain regions in Ctsk^-/- mice, suggesting altered TH supply to these regions. Thyrotropin releasing hormone (Trh) mRNA levels were enhanced threefold in the hippocampus of Ctsk^-/- mice. In the striatum of Ctsk^-/- mice the mRNA for Dio2 and hairless were approximately 1.3-fold enhanced, while mRNA levels for monocarboxylate transporter 8 and Trh were reduced to 60% and 40%, respectively, pointing to altered striatal physiology. We conclude that the role of cathepsin K in the thyroid gland is not directly associated with its function in the central nervous system (CNS) of mice. Future studies will show whether the brain region-specific alterations in Trh mRNA may eventually result in altered neuroprotection that could explain the neurobehavioral defects of Ctsk^-/- mice.

TRH modulates glutamatergic synaptic inputs on CA1 neurons of the mouse hippocampus in a biphasic manner

Thyrotropin Releasing Hormone (TRH) is a tripeptide that induces the release of Thyroid Stimulating Hormone (TSH) in the blood. Besides its role in the thyroid system, TRH has been shown to regulate several neuronal systems in the brain however its role in hippocampus remains controversial. Using electrophysiological recordings in acute mouse brain slices, we show that TRH depresses glutamate responses at the CA3-CA1 synapse through an action on NMDA receptors, which, as a consequence, decreases the ability of the synapse to establish a long term potentiation (LTP). TRH also induces a late increase in AMPA/kainate responses. Together, these results suggest that TRH plays an important role in the modulation of hippocampal neuronal activities, and they contribute to a better understanding of the mechanisms by which TRH impacts synaptic function underlying emotional states, learning and memory processes.

Identification of thyrotropin-releasing hormone as hippocampal glutaminyl cyclase substrate in neurons and reactive astrocytes

Recently, Aβ peptide variants with an N-terminal truncation and pyroglutamate modification were identified and shown to be highly neurotoxic and prone to aggregation. This modification of Aβ is catalyzed by glutaminyl cyclase (QC) and pharmacological inhibition of QC diminishes Aβ deposition and accompanying gliosis and ameliorates memory impairment in transgenic mouse models of Alzheimer's disease (AD). QC expression was initially described in the hypothalamus, where thyrotropin-releasing hormone (TRH) is one of its physiological substrates. In addition to its hormonal role, a novel neuroprotective function of TRH following excitotoxicity and Aβ-mediated neurotoxicity has been reported in the hippocampus. Functionally matching this finding, we recently demonstrated QC expression by hippocampal interneurons in mouse brain. Here, we detected neuronal co-expression of QC and TRH in the hippocampus of young adult wild type mice using double immunofluorescence labeling. This provides evidence for TRH being a physiological QC substrate in hippocampus. Additionally, in neocortex of aged but not of young mice transgenic for amyloid precursor protein an increase of QC mRNA levels was found compared to wild type littermates. This phenomenon was not observed in hippocampus, which is later affected by Aβ pathology. However, in hippocampus of transgenic - but not of wild type mice - a correlation between QC and TRH mRNA levels was revealed. This co-regulation of the enzyme QC and its substrate TRH was reflected by a co-induction of both proteins in reactive astrocytes in proximity of Aβ deposits. Also, in primary mouse astrocytes a co-induction of QC and TRH was demonstrated upon Aβ stimulation.

Rapid modulation of TRH and TRH-like peptide release in rat brain and peripheral tissues by prazosin

Hyperresponsiveness to norepinephrine contributes to post-traumatic stress disorder (PTSD). Prazosin, a brain-active blocker of α(1)-adrenoceptors, originally used for the treatment of hypertension, has been reported to alleviate trauma nightmares, sleep disturbance and improve global clinical status in war veterans with PTSD. Thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH(2)) may play a role in the pathophysiology and treatment of neuropsychiatric disorders such as major depression, and PTSD (an anxiety disorder). To investigate whether TRH or TRH-like peptides (pGlu-X-Pro-NH(2), where "X" can be any amino acid residue) participate in the therapeutic effects of prazosin, male rats were injected with prazosin and these peptides then measured in brain and endocrine tissues. Prazosin stimulated TRH and TRH-like peptide release in those tissues with high α(1)-adrenoceptor levels suggesting that these peptides may play a role in the therapeutic effects of prazosin.

Thyrotropin-releasing hormone increases GABA release in rat hippocampus

Thyrotropin-releasing hormone (TRH) is a tripeptide that is widely distributed in the brain including the hippocampus where TRH receptors are also expressed. TRH has anti-epileptic effects and regulates arousal, sleep, cognition, locomotion and mood. However, the cellular mechanisms underlying such effects remain to be determined. We examined the effects of TRH on GABAergic transmission in the hippocampus and found that TRH increased the frequency of GABAA receptor-mediated spontaneous IPSCs in each region of the hippocampus but had no effects on miniature IPSCs or evoked IPSCs. TRH increased the action potential firing frequency recorded from GABAergic interneurons in CA1 stratum radiatum and induced membrane depolarization suggesting that TRH increases the excitability of interneurons to facilitate GABA release. TRH-induced inward current had a reversal potential close to the K+ reversal potential suggesting that TRH inhibits resting K+ channels. The involved K+ channels were sensitive to Ba2+ but resistant to other classical K+ channel blockers, suggesting that TRH inhibits the two-pore domain K+ channels. Because the effects of TRH were mediated via Galphaq/11, but were independent of its known downstream effectors, a direct coupling may exist between Galphaq/11 and K+ channels. Inhibition of the function of dynamin slowed the desensitization of TRH responses. TRH inhibited seizure activity induced by Mg2+ deprivation, but not that generated by picrotoxin, suggesting that TRH-mediated increase in GABA release contributes to its anti-epileptic effects. Our results demonstrate a novel mechanism to explain some of the hippocampal actions of TRH.

Effects of the neuropeptide thyrotropin-releasing hormone on GABAergic synaptic transmission of CA1 neurons of the rat hippocampal slice during hypoxia

Because thyrotropin-releasing hormone (TRH) has been suggested to improve recovery of brain neurons from hypoxia, which strongly impairs GABAergic synaptic transmission, the present electrophysiological study used intracellular recording from CA1 neurons of the rat hippocampal slice to examine the cellular mechanisms underlying this phenomenon. Hypoxia induced by superfusion with a medium devoid of oxygen evoked typical membrane hyperpolarization, fall in input resistance, and strong depression of monosynaptic, GABAA receptor-mediated fast inhibitory postsynaptic potentials (IPSPs). The depression of fast IPSPs during hypoxia was found to be due to a combination of factors such as shift in the IPSP reversal potential and membrane hyperpolarization. GABAB receptor-mediated slow IPSPs were comparatively less sensitive to hypoxia. TRH (10 microM), applied 1 min prior to hypoxia, selectively accelerated recovery of membrane potential and delayed return of fast IPSPs to control amplitude without changing the mechanisms responsible for depression of GABAergic transmission. In conclusion, despite a slower recovery of IPSPs, TRH facilitated earlier return of neuronal excitability after the hypoxic period.

Changes in thyrotropin-releasing hormone levels in hippocampal subregions induced by a model of human temporal lobe epilepsy: effect of partial and complete kindling

Endogenous thyrotropin-releasing hormone has been hypothesized to modulate seizure activity, possibly by subserving an anticonvulsant function in limbic brain. A specific and sensitive radioimmunoassay was utilized to quantitate thyrotropin-releasing hormone levels in dorsoventrally dissected hippocampal subregions after partially (an experimental paradigm of complex partial epilepsy) or fully kindled (repeated generalized) seizures, to define specific seizure-related limbic pathways that may contain thyrotropin-releasing hormone. Samples were taken from electrode controls and 1, 6, 24, 48 and 144 h after a fully kindled seizure or 24 h after the first occurrence of a stage 3-4 (partially kindled) seizure in rats. Thyrotropin-releasing hormone levels were below controls in all subregions taken 1 h after a fully kindled seizure. They resembled control values 6 h after seizure, were substantially elevated at 24 and 48 h, and then returned to control levels by 144h. Low thyrotropin-releasing hormone levels seen shortly after the seizure presumably indicate peptide depletion during the ictus. The higher levels seen at later times occurred during a postictal period coinciding with refraction to additional seizure-generating stimulation. These values probably reflect enhanced synthesis since the largest increases were seen in subregions (dentate gyrus, hilus/CA4, CA3) that contain perforant path terminals, and where previously observed intrinsic hippocampal thyrotropin-releasing hormone messenger RNA increases were seen. The thyrotropin-releasing hormone response was less robust in ventral hilus/CA4 and CA3 areas, leading to speculation that this smaller response could, in part, explain why the ventral (temporal) hippocampus may be more susceptible to seizure-induced damage. No changes in thyrotropin-releasing hormone were detected after partially kindled seizures, suggesting that thyrotropin-releasing hormone is not involved in epileptogenesis or its stereotypic motor behavior. The time-course and distribution of thyrotropin-releasing hormone elevations seen after a fully kindled (repeated generalized) seizure, and the lack of effect of partial kindling (complex partial seizure) are consistent with previous observations concerning postictal thyrotropin-releasing hormone messenger RNA expression. These neurochemical results support the hypothesis that endogenous thyrotropin-releasing hormone can serve an anticonvulsant neuromodulatory function in specific limbic pathways relevant to temporal lobe epilepsy.

Effects of thyrotropin-releasing hormone on GABAergic synaptic transmission of the rat hippocampus

The effect of thyrotropin-releasing hormone (TRH), a neuropeptide physiologically present in the mammalian hippocampus, on spontaneous, miniature and evoked GABAergic postsynaptic currents was investigated using whole-cell patch-clamp recording from pyramidal cells and interneurons of the rat hippocampal thin slice preparation. Bath application of 10 microM TRH induced an increase in the frequency of spontaneous postsynaptic currents from 1.07 +/- 0.68 to 3.16 +/- 0.73 Hz in pyramidal neurons and interneurons of the stratum lacunosum-moleculare (SL-M). In tetrodotoxin solution TRH did not change miniature postsynaptic currents. Application of TRH to minislices containing only the CA1 region still produced an increase in the spontaneous postsynaptic current frequency, indicative of an action by TRH upon a GABAergic circuitry. Paired recordings from one pyramidal cell and one stratum lacunosum moleculare interneuron displayed synchronous events whose frequency rose after TRH application, suggestive of a common, TRH-sensitive input. In a small subset of cells TRH induced the appearance of highly rhythmic large postsynaptic currents at a frequency of approximately 2 Hz, as confirmed by autocorrelation analysis. Postsynaptic currents electrically evoked by focal stimulation of stratum lacunosum-moleculare were depressed from 90 +/- 27 to 44 +/- 15 pA after application of TRH. This phenomenon was solely due to an increase in the number of synaptic failures. It is proposed that the effect of TRH on the GABAergic system was primarily exerted on a subset of interneurons controlling the activity of pyramidal cells as well as stratum lacunosum-moleuclare interneurons.

Electrophysiological interactions between 5-hydroxytryptamine and thyrotropin releasing hormone on rat hippocampal CA1 neurons

Intracellular recording from CA1 neurons of the rat hippocampal slice preparation was used to examine the possibility of functional interactions between 5-hydroxytryptamine (5-HT) and thyrotropin releasing hormone (TRH), which act as cotransmitters in other areas of the central nervous system. 5-HT (30 microM) elicited complex effects consisting of biphasic changes in membrane potential and a strong depression of the afterhyperpolarization (AHP) following a spike burst. TRH (10 microM) did not alter membrane potential or input conductance but it produced a partial block of the AHP. Under single-electrode voltage clamp, 5-HT and TRH both reduced the amplitude of voltage-activated total K+ currents. When the two substances were co-applied, their actions were occluded. The voltage-activated K+ current remaining in Ca(2+)-free solution lost its sensitivity to 5-HT and TRH, suggesting that the K+ current modulated by TRH and 5-HT was Ca(2+)-dependent, although TRH itself did not depress high-threshold voltage-activated Ca2+ currents. When a relatively small concentration (5 microM) of 5-HT was co-applied with an equimolar amount of TRH, the degree of block of the spike AHP was the sum of the two individual effects of these drugs. It is suggested that in hippocampal pyramidal cells 5-HT and TRH influenced neuronal excitability by depressing a Ca(2+)-dependent K+ current, a phenomenon perhaps mediated through a common intracellular second messenger pathway.

Potassium currents operated by thyrotrophin-releasing hormone in dissociated CA1 pyramidal neurones of rat hippocampus

1. Membrane currents activated by thyrotrophin-releasing hormone (TRH) were investigated in the dissociated rat hippocampal CA1 pyramidal neurone using the nystatin perforated patch recording configuration. 2. Under current-clamp condition, TRH caused a transient hyperpolarization accompanied by a decrease of firing activity and a successive long-lasting depolarization. The latter induced a blockade of firing. 3. When neurones were held at a holding potential (VH) of -40 mV under voltage clamp, TRH elicited a transient outward current with an increase in the membrane conductance, which was followed by a sustained inward current with a decrease in membrane conductance. The inactive TRH metabolite, TRH free acid, did not induce any currents. 4. The reversal potential of TRH-induced outward current (ETRH) was close to the K+ equilibrium potential (EK). The change in ETRH for a 10-fold change in extracellular K+ concentration was 56.4 mV, indicating that the membrane behaves like a K+ electrode in the presence of TRH. On the other hand, the TRH-induced inward current was due to suppression of a slow inward current relaxation during hyperpolarizing voltage commands to -50 mV from a VH of -40 mV, indicating the suppression of the voltage- and time-dependent component of the K+ current (M-current). 5. The TRH-induced outward current (ITRH) increased in a concentration-dependent manner over the concentration range 10(-8)-10(-6) M. The half-maximum concentration was 7.4 x 10(-8) M and the Hill coefficient was 1.5. 6. The TRH-induced outward current (ITRH) was antagonized by K+ channel blockers such as tetraethylammonium (TEA), 4-aminopyridine (4-AP) and Ba2+ in a concentration-dependent manner. ITRH was insensitive to both apamin and iberiotoxin. 7. The first application of TRH to neurones perfused with Ca(2+)-free external solution containing 2 mM EGTA could induce ITRH but the TRH response diminished dramatically with successive applications. Intracellular perfusion with a Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), also diminished the TRH response. 8. The depletion of Ca2+ from the intracellular Ca2+ store by thapsigargin blocked the TRH response without affecting the caffeine response. Pretreatment with Li+ significantly enhanced ITRH, suggesting that ITRH is involved in the elevation of intracellular free Ca2+ released from the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store site but not from the caffeine-sensitive one. 9. Staurosporine, a protein kinase C (PKC) inhibitor, suppressed ITRH in a concentration-dependent manner (the half-maximum inhibitory concentration (IC50), was 2.45 x 10(-8) M).(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of thyrotropin-releasing hormone binding sites: autoradiographic study in infant and adult human hippocampal formation

The rostrocaudal distribution of thyrotropin-releasing hormone (TRH) binding sites was studied in the human hippocampus. Cryostat sections of the right and left hippocampi from 6 infants (2 h to 5 months of age) and 11 adults (24 to 92 years) were subjected to in vitro quantitative autoradiography using [3H]MeTRH as a ligand. A single class of high affinity [3H]MeTRH binding sites with an apparent dissociation constant in the nanomolar range has been shown both in the infant and the adult. The maximal number of these sites was higher in the infant. No significant difference was observed between the general patterns of the right and the left hippocampi when taking postmortem delay and age as parameters. The highest concentrations of [3H]MeTRH binding sites were localized in the uncinate gyrus, the uncal subiculum and in the whole length of the molecular layer of the dentate gyrus. The lowest densities were present in the ventral subiculum. The major difference observed between the infant and the adult appeared in the molecular layer of the dentate gyrus where the densities were two-fold higher in infants (189 +/- 6 versus 88 +/- 2 fmol/mg of tissue). The only marked difference in the distribution was localized in the caudal part of the body where no specific labeling was found in the presubiculum of the infant.

Thyrotropin-releasing hormone gene expression and receptors are differentially modified in limbic foci by seizures

Previous studies using two seizure paradigms, electroconvulsive shock and kindling, suggested potential sites of endogenous thyrotropin-releasing hormone (TRH) action in specific epileptogenic areas. We studied TRH gene expression and TRH receptors in rat limbic areas using the kindling model of epilepsy. Immunoassayable TRH increased 4- to 20-fold over control levels in specific subregions of the hippocampus 24 hours after a single stage 5 seizure. Concurrently, TRH receptor binding was significantly reduced in hippocampal (23-39%) and amygdaloid (21-22%) membranes. Dramatic temporal and spatial changes in prepro-TRH messenger RNA were visualized by in situ hybridization histochemistry in the hippocampal dentate gyrus, the piriform cortex, and the amygdala. Peak hybridization occurred 6 and 12 hours postictally in these loci and returned toward basal levels by 24 hours. These results are consistent with the hypothesis that TRH may have an important role in the pathophysiology epilepsy by modulating excitatory processes.

Some regional anatomical relationships of TRH to 5-HT in rat limbic forebrain

It is now a recognized principle that various neuropeptides are neuronally co-localized with biogenic amine or aminoacid neurotransmitters. In the rat CNS it has previously been shown that TRH is co-localized with 5-HT (and also with substance P) in cell bodies of the posterior raphe that project to the spinal cord. Although TRH cell bodies are known to be widely distributed throughout the forebrain there is no other known co-localization with 5-HT. In this study we further specify the forebrain there is no other known co-localization with 5-HT. In this study we further specify the anatomical relationship of TRH with 5-HT by use of surgical and neurotoxic lesioning with reference to limbic forebrain regions wherein TRH is greatly increased following seizures. In groups of rats, the fimbria-fornix was lesioned alone, or combined with a lesion of the dorsal perforant path or the ventral perforant path. There was a sham lesioned control group. Additional groups were lesioned with 5,7 dihydroxytryptamine, 100 micrograms i.v.t., 45 min. after i.p. desipramine, 25 mg/kg. All rats were sacrificed three weeks after lesions. Indoleamines were determined by HPLC in left anterior cortex, left pyriform/olfactory cortex, left dorsal hippocampus and left ventral hippocampus. TRH was determined by specific RIA in the corresponding right brain regions. The modal n was 7 rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thyrotropin-releasing hormone and a related analog, CNK-602A, on long-term potentiation in the mossy fiber-CA3 pathway of guinea pig hippocampal slices

The effects of thyrotropin-releasing hormone (TRH) and a related analog, CNK-602A, that induces the release of catecholamines, on long-term potentiation (LTP) of the population spike in mossy fiber-CA3 pathways were investigated in guinea pig hippocampal slices. TRH augmented LTP of the population spike at concentrations of 10(-6)-10(-5) M. CNK-602A also augmented LTP at concentrations of 10(-6)-10(-5) M in a dose-dependent manner. LTP in slices of the hippocampus obtained from animals given 6-hydroxydopamine (6-OHDA) intraventricularly was significantly lower than that in non-treated animals. However, both TRH and CNK-602A (10(-6) M) augmented LTP in slices from 6-OHDA-treated animals. These findings suggest that TRH and CNK-602A augment LTP in the mossy fiber-CA3 pathway without activating noradrenergic and/or dopaminergic fibers.