Trh in the rat cerebellum: II. Uptake by cerebellar slices

GPCRs in Intracellular Compartments: New Targets for Drug Discovery

The architecture of eukaryotic cells is defined by extensive membrane-delimited compartments, which entails separate metabolic processes that would otherwise interfere with each other, leading to functional differences between cells. G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors, and their signal transduction is traditionally viewed as a chain of events initiated from the plasma membrane. Furthermore, their intracellular trafficking, internalization, and recycling were considered only to regulate receptor desensitization and cell surface expression. On the contrary, accumulating data strongly suggest that GPCRs also signal from intracellular compartments. GPCRs localize in the membranes of endosomes, nucleus, Golgi and endoplasmic reticulum apparatuses, mitochondria, and cell division compartments. Importantly, from these sites they have shown to orchestrate multiple signals that regulate different cell pathways. In this review, we summarize the current knowledge of this fascinating phenomenon, explaining how GPCRs reach the intracellular sites, are stimulated by the endogenous ligands, and their potential physiological/pathophysiological roles. Finally, we illustrate several mechanisms involved in the modulation of the compartmentalized GPCR signaling by drugs and endogenous ligands. Understanding how GPCR signaling compartmentalization is regulated will provide a unique opportunity to develop novel pharmaceutical approaches to target GPCRs and potentially lead the way towards new therapeutic approaches.

Amygdala kindling differentially regulates the expression of the elements involved in TRH transmission

Subthreshold electrical stimulation of the amygdala (kindling) activates neuronal pathways increasing the expression of several neuropeptides including thyrotropin releasing-hormone (TRH). Partial kindling enhances TRH expression and the activity or its inactivating ectoenzyme; once kindling is established (stage V), TRH and its mRNA levels are further increased but TRH-binding and pyroglutamyl aminopeptidase II (PPII) activity decreased in epileptogenic areas. To determine whether variations in TRH receptor binding or PPII activity are due to regulation of their synthesis, mRNA levels of TRH receptors (R1, R2) and PPII were semi-quantified by RT-PCR in amygdala, frontal cortex and hippocampus of kindled rats sacrificed at stage II or V. Increased mRNA levels of PPII were found at stage II in amygdala and frontal cortex, and of pro-TRH and TRH-R2, in amygdala and hippocampus. At stage V, pro-TRH mRNA levels increased and those of PPII, decreased in the three regions; TRH-R2 mRNA levels diminished in amygdala and frontal cortex and of TRH-R1 only in amygdala. In situ hybridization analyses revealed, at stage II, enhanced TRH-R1 mRNA levels in dentate gyrus and amygdala while decreased in piriform cortex; those of TRH-R2 increased in amygdala, CA2, dentate gyrus, piriform cortex, thalamus and subiculum and of PPII, in CAs and piriform cortex. In contrast, at stage V decreased expression of TRH-R1 occurred in amygdala, CA2/3, dentate gyrus and piriform cortex; of TRH-R2 in CA2, thalamus and piriform cortex, and of PPII in CA2, and amygdala. The magnitude of changes differed between ipsi and contralateral side. These results support a trans-synaptic modulation of all elements involved in TRH transmission in conditions that stimulate the activity of TRHergic neurons. They show that reported changes in PPII activity or TRH-binding caused by kindling relate to regulation of the expression of TRH receptors and degrading enzyme.

Expression of PEPT2 peptide transporter mRNA and protein in glial cells of rat dorsal root ganglia

The peptide-transporter PEPT2 mediates electrogenic uphill transport of di- and tripeptides, selected peptidomimetics and delta aminolevulinic acid. The transporter was cloned from rat central nervous tissue recently and its mRNA was localized to astrocytes. In the present studies the expression of PEPT2-protein and -mRNA in rat dorsal root ganglia was investigated. Immunohistochemistry revealed PEPT2-immunoreactivity in satellite glial cells surrounding the ganglionic neurons. There was no expression in neuronal cells. In-situ-hybridization studies colocalized the expression of PEPT2-mRNA to satellite cells. This is the first report on the expression of PEPT2-protein in the peripheral nervous system where PEPT2 may serve as uptake system for products of protein degradation, for removal of biologically active short-chain peptides and non-peptides such as delta aminolevulinic acid.

Degradation of Dip-allatostatins by hemolymph from the cockroach, Diploptera punctata

Incubation of Dip-AST 7 (APSGAQRLYGFGLa) or Dip-AST 9 (GDGRLYAFGLa) (5 microM) with hemolymph for 30 min results in cleavage by a putative endopeptidase, yielding the C-terminal hexapeptide. This metabolic product is subsequently cleaved by an amastatin-sensitive aminopeptidase to yield the the C-terminal pentapeptide, as treatment with the competitive aminoexopeptidase inhibitor, amastatin, results in a significant accumulation of the C-terminal hexapeptide. Interestingly, Dip-AST 5 (DRLYSFGLa) (6 microM), which in common with Dip-AST 7 and 9 possesses Arg-Leu-Tyr, is not rapidly cleaved. However, [3H-Tyr]Dip-AST 5 at physiological concentrations (4 nM), appears to be cleaved by the same enzymes that cleave Dip-AST 7 and 9, albeit at a reduced rate. Incubation of other members of the Dip-allatostatin family with hemolymph also results in cleavage of the peptides, suggesting that there are a variety of endo- and/or exopeptidases present in the hemolymph of D. punctata.

How peptidergic neurons cope with variation in physiological stimulation

A general scheme for neuropeptide metabolism is outlined and the potential sites of regulation are discussed. Two major sites of regulation are distinguished: transcription which ultimately limits the rate of translation to form the prepropeptide, and post-translational processing steps. The consequences of up-regulation of these steps in response to increased metabolic demand are discussed. An alternative strategy for peptidergic neurons, reliance on a large pool of neuropeptide, is proposed. Data on the response of enkephalin-containing cells to increased levels of stimulation are reviewed. It is concluded that there is good evidence for genomic up-regulation, perhaps in association with regulation of processing. Evidence based on studies on enkephalin-containing amacrine cells in the chicken retina is also reviewed. It is suggested that these cells rely on a large pool of neuropeptide to cope with changes in demand.

Presence of a membrane bound pyroglutamyl amino peptidase degrading thyrotropin releasing hormone in rat brain

In the present work we studied the pattern of degradation of [3H-Pro]-TRH by soluble and membrane fractions from rat brain. Demonstration of the membrane bound or soluble nature of the activities was obtained by comparing their distribution to that of lactate dehydrogenase and by looking at the effect of NaCl washes on the membrane fractions. We observed that the pyroglutamyl amino peptidase activity detected in brain homogenates is a result of two different enzymes. One of them is a soluble enzyme previously characterized, that needs DTT and EDTA for its expression, is inhibited by SH-blocking agents such as iodoacetamide and utilizes p-glu-beta-naphtylamide as a substrate. The other one, a membrane enzyme, is inhibited by chelating agents such as EDTA and DTT, is not affected by iodoacetamide and does not degrade p-glu-beta-naphtylamide. The later presents some specificity towards TRH as shown by competition experiments with TRH analogs. We were able to corroborate that the post proline cleaving enzyme acting on TRH is a soluble enzyme. In membranes we demonstrated also the presence of a post-proline dipeptidyl aminopeptidase. The membrane bound pyroglutamidase activity is a potential new source of L-his-L-pro-diketopiperazine in brain. The presence of a TRH degrading enzyme in membrane fractions is of particular importance in searching an inactivation mechanism of this peptide once it is released into the synaptic cleft.

Accumulation of thyrotropin releasing hormone by rat hypothalamic slices

It has been postulated that thyrotropin releasing hormone (TRH) may play an active role in synaptic transmission. If such is the case, an inactivation mechanism must exist, in analogy to other neuroactive substances. In these studies we have considered the possibility that TRH may be taken up by rat hypothalamic slices. We observed that in the presence of bacitracin TRH was stable in the medium up to 90 min. We detected intact [3H]Pro-TRH associated with the slices as evidenced by TLC and paper electrophoresis; the association was time-dependent up to 60 min, and the maximum tissue-to-medium ratio was 1.3 at this time. At 5 min incubation, 30-50% of the TRH was not extracellular, and the plot of TRH-associated tissue versus the total amount of tissue was linear up to two hypothalami per flask. The association was saturable (Km 1.07 microM) and temperature-dependent, and the saturable part of the accumulation was inhibited by ouabain, dinitrophenol, and the absence of glucose. These results suggest that an uptake mechanism for TRH exists in the hypothalamus; its physiological relevance remains to be elucidated.

Enzymic inactivation of hypothalamic regulatory hormones

The considerable expansion in studies on the enzymic inactivation of thyrotrophin-releasing hormone, luteinizing hormone-releasing hormone and somatostatin (growth hormone release-inhibiting hormone) has necessitated a re-evaluation of the peptidase enzymes responsible. Through the use of new methods such as high-performance liquid chromatography and the development of artificial enzyme substrates, it has been possible to clarify the mechanisms of enzyme cleavage of these hypothalamic regulatory hormones and to attempt purification of the peptidases. This has brought about a renewed interest in the physiological significance of the enzymes, as well as their role in biotransformation of the hypothalamic hormones. From such studies, the information gained may be used in the design of agonist and antagonist analogues, as well as providing details of the mechanisms of action of such analogues through their increased stability to enzymic degradation. The characterization of corticotrophin-releasing factor and growth hormone-releasing factor will provide a new field for the application of peptidase inactivation to analogue design. Similarly, future examination of the peptidases inactivating the hypothalamic hormones in certain clinical conditions may give new insight into the significance of the enzymes in pathological conditions. Identification of these enzymes, investigation of their localization, properties and functions and assessment of their contribution to the control of hormone action may yield valuable insight into the physiology and pathology of the hypothalamic regulatory hormones.

Interactions of thyrotropin releasing hormone, its metabolites and analogues with endogenous and exogenous opiates

The interactions of thyrotropin releasing hormone, its metabolites and synthetic analogues with acute and chronic effects of endogenous and exogenous opiates have been described. The endogenous and exogenous opiates are represented by beta-endorphin and morphine, respectively. The pharmacological effects of opiates include analgesia, temperature effects, respiratory depression, catalepsy, locomotor activity, opiate receptor binding, tolerance, and physical dependence. Thyrotropin releasing hormone and related compounds appear to (a) antagonize hypothermia, respiratory depression, locomotor depression and catalepsy but not the analgesia induced by opiates, (b) inhibit the development of tolerance to the analgesic effect but not to the hypothermic effect of opiates, (c) inhibit the development of physical dependence on opiates as evidenced by the inhibition of development of certain withdrawal symptoms, and (d) suppress the abstinence syndrome in opiate dependent rodents. Thyrotropin releasing hormone does not interact with the opiate receptors in the brain. Potential therapeutic applications of thyrotropin releasing hormone and its synthetic analogues in counteracting some of the undesirable effects of opiates are discussed.

TRH in the rat cerebellum: I. Distribution and concentration

We determined the regional distribution and concentration of endogenous TRH in the rat cerebellum. Radioimmunoassay of endogenous TRH extracted and purified from five different regions of the rat cerebellum and whole hypothalamus showed that the cerebellar vermis contained 24 pg/mg, the hemispheres 74 pg/mg, the deep cerebellar nuclei 148 pg/mg, and the flocculo-nodular region 559 pg/mg of TRH. The highest concentration of TRH was in the cerebellar paraflocculi, which contained 786 pg/mg. The hypothalamic concentration of TRH was 465 pg/mg. Assay of the non-purified tissue fractions (crude extracts) resulted in lower TRH values in accordance with data previously reported by other authors. Bioassay analysis of TRH in purified fractions resulted in values similar to those obtained by radioimmunoassay. On the basis of these findings we hypothesize a functional role for TRH in the cerebellum.