Immunocytochemical distribution of neurons containing a peptide derived from thyrotropin-releasing hormone precursor in the rat brain

Methimazole-induced hypothyroidism inhibits the panic-like behaviors produced by electrical stimulation of dorsal periaqueductal gray matter of rats

Conflicting clinical data on the relationship of panic disorder and thyroid diseases illustrate the need for a simpler approach using animal models. Defensive behaviors evoked by electrical or chemical stimulation of dorsal periaqueductal gray matter (DPAG) have been proposed as a model of panic attack. Therefore, the present study examined the effects of the anti-thyroid agent methimazole (MTZ) either on the panic-like behaviors induced by electrical stimulation of DPAG or the anxiety-like behaviors of rats exposed to the elevated plus-maze (EPM). Male Wistar rats bearing electrodes in the DPAG were stimulated with stepwise increased currents. Rats which displayed galloping at intensities below 60muA were retested following 5- and 10-day treatments with MTZ (0.6mg/kg/day, i.p.) or 10- and 15-day washout periods. MTZ effects on EPM performance were assessed in separate groups. MTZ-treated groups were compared to saline-treated controls. In other experiments, rats were similarly treated with MTZ and the blood was collected for hormone assays. The 10-day treatment with MTZ produced marked increases in the thresholds of exophthalmus (65%), immobility (75%), trotting (63%), galloping (56%), jumping (47%), defecation (114%) and micturition (85%). Effects outlasted the drug discontinuation. In contrast, MTZ had variable effects in the EPM, significantly increasing the open-arm exploration in 5-day treated and 10-day washout groups. Biochemical data revealed a small but significant decrease (13%) in free thyroxine in MTZ-treated groups. Although not significant, thyrotrophin levels showed a 111% increase following the 10-day treatment with MTZ. Selective attenuation by MTZ of DPAG-evoked defensive behaviors supports attenuation of panic attacks in hypothyroidism.

Dorsal periaqueductal gray matter-evoked panic-like behaviors are markedly inhibited by a low peripheral dose of thyrotropin releasing hormone

Stimulation of the dorsal periaqueductal gray matter (DPAG) produces defensive behaviors which are reminiscent of panic attacks. Recent evidence from our laboratory showed that DPAG-evoked defensive behaviors are markedly attenuated in short-term methimazole-induced hypothyroidism. It is not clear, however, whether these effects were due to an increase in thyrotropin releasing hormone (TRH), a decrease in thyroid hormones or to the overall effects of hypothyroidism. Accordingly, here we examined whether the peripheral injection of TRH has any effect either on the panic-like behaviors induced by electrical stimulation of DPAG or anxiety-like behaviors of rats exposed to the elevated plus-maze (EPM). Rats whose stimulation of DPAG produced flight responses (galloping or jumping) with intensities below 60 microA were injected with 1 microg/kg TRH (i.p.) and stimulated 10min after that. The day after, rats were treated with saline and subjected to the same stimulation procedure. Threshold curves were fitted through the logistic model and compared by likelihood-ratio chi(2) tests. TRH and saline effects on EPM performance were appraised in separate groups. Compared to saline-sessions, TRH-injected rats presented thresholds significantly higher for immobility (40%), trotting (33%), galloping (34%), jumping (39%) and exophthalmus (43%). In contrast, TRH had no effects on EPM arm exploration. TRH selective inhibition of DPAG-evoked defensive behaviors adds new evidence that panic attacks may be attenuated by increased levels of this hormone in hypothyroidism.

Prepro-TRH 178-199 inhibits histamine- or restraint stress-induced activation of corticotropin releasing hormone production in rat hypothalamus

Under restraint stress conditions, prepro-thyrotropin releasing hormone (TRH) 178-199 suppresses adrenocorticotropic hormone (ACTH) secretion from the rat pituitary, which indicates that prepro-TRH 178-199 is a candidate endogenous corticotropin releasing inhibitory factor (CRIF). Restraint stress also activates the release of hypothalamic neuronal histamine, which increases both the expression of CRH mRNA in the paraventricular nucleus (PVN) and plasma concentrations of ACTH. The aim of this study was to determine whether prepro-TRH 178-199 modulates histamine- or restraint stress-induced activation of corticotropin releasing hormone (CRH) in the rat hypothalamus. Infusion of prepro-TRH 178-199 into the third cerebroventricle (i3vt) at a dose of 6 μg/kg significantly decreased the amount of CRH in the PVN, as compared to vehicle-treated controls (p < 0.05), but did not affect the CRH amount in other hypothalamic regions. Restraint stress increased the amount of CRH in the PVN and ventromedial hypothalamic nucleus (VMH), as compared to non-restrained controls (p < 0.05); this was attenuated by pretreatment with i3vt infusion of prepro-TRH 178-199 (p < 0.05). I3vt infusion of histamine (270 nmol/rat) suppressed cumulative food consumption over 24 h, increased plasma ACTH concentrations, and increased the content of CRH in the PVN, as compared to vehicle-treated controls (p < 0.05 for each); these effects were attenuated by pretreatment with prepro-TRH 178-199 (p < 0.05). These results suggest that prepro-TRH 178-199 may regulate ACTH secretion by affecting basal and histamine- or stress-induced synthesis and/or secretion of CRH and ACTH by modulating histaminergic input to the PVN and VMH.:

Thyroid hormones selectively regulate the posttranslational processing of prothyrotropin-releasing hormone in the paraventricular nucleus of the hypothalamus

Over the last few years, our laboratory has demonstrated that different physiological conditions or stressors affect the posttranslational processing of hypophysiotropic and nonhypophysiotropic proTRH and, consequently, the output of TRH and other proTRH-derived peptides. These alterations in proTRH processing are generally associated with parallel changes in the levels of two members of the family of prohormone convertases 1/3 and 2 (PC1/3 and PC2). An important regulator of proTRH is thyroid hormone, which is the peripheral end product of the hypothalamic (TRH)-pituitary (TSH)-thyroid (T3/4) (HPT) axis. In this study we investigated the effect of thyroid status on the processing of proTRH inside and outside the HPT axis. Our data showed that high levels of thyroid hormone down-regulated PC1/3 and PC2 and TRH synthesis, which led to an accumulation of intermediate forms of proTRH processing. Conversely, low levels of thyroid hormone up-regulated proTRH synthesis and PC1/3 and PC2 levels. Control of the activity of PCs and proTRH processing occurred specifically in the paraventricular nucleus, whereas no change due to thyroid status was found in the lateral hypothalamus or preoptic area. The posttranslational regulation of proTRH processing in the paraventricular nucleus by thyroid status is a novel aspect of the regulation of the HPT axis, which may have important implications for the pathophysiology of hypo- and hyperthyroidism.

Phase-specific central regulatory systems of hibernation in Syrian hamsters

The central body temperature (T(b)) regulation system during hibernation was investigated in Syrian hamsters of either sex. Hibernation induced in Syrian hamsters by housing them in a cold room under short day-light/dark cycle was confirmed by marked reductions in the heart rate, T(b) and respiratory rate. The hibernation of hamsters was classified into (i) entrance, (ii) maintenance and (iii) arousal phases according to T(b) changes. In hibernating hamsters, T(b) elevations were phase-selectively elicited by intracerebroventricular (ICV) injection of 8-cyclopenthyltheophylline (CPT; a selective A1-adenosine receptor antagonist) and naloxone (a non-selective opioid receptor antagonist) during the entrance and maintenance phases, respectively. Moreover, a similar T(b) elevation tendency during the maintenance phase was also induced by ICV naloxonazine, (a selective mu1-opioid receptor antagonist), although such was not the case for naltrindole (a selective delta-opioid receptor antagonist) or nor-binaltorphimine (nor-BNI, a selective kappa-opioid receptor antagonist). Furthermore, T(b) elevations in hibernating hamsters were similarly induced with ICV thyrotropin-releasing hormone (TRH) during the entrance and maintenance phases. Furthermore, ICV injection of the anti-TRH antibody ameliorated the T(b) elevations induced by tactile stimulation. These results suggest that activation of the A1-receptor by adenosine is important for the generation of hypothermia in the entrance phase, and that activation of the mu1-opioid receptor by opioid peptides is required for perpetuation of hypothermia in the maintenance phase. In addition, TRH is a key endogenous substance involved in T(b) elevations during the arousal phase of hibernating hamsters.

The Mouse Defense Test Battery: pharmacological and behavioral assays for anxiety and panic

The Mouse Defense Test Battery was developed from tests of defensive behaviors in rats, reflecting earlier studies of both acute and chronic responses of laboratory and wild rodents to threatening stimuli and situations. It measures flight, freezing, defensive threat and attack, and risk assessment in response to an unconditioned predator stimulus, as well as pretest activity and postthreat (conditioned) defensiveness to the test context. Factor analyses of these indicate four factors relating to cognitive and emotional aspects of defense, flight, and defensiveness to the test context. In the Mouse Defense Test Battery, GABA(A)-benzodiazepine anxiolytics produce consistent reductions in defensive threat/attack and risk assessment, while panicolytic and panicogenic drugs selectively reduce and enhance, respectively, flight. Effects of GABA(A)-benzodiazepine, serotonin, and neuropeptide ligands in the Mouse Defense Test Battery are reviewed. This review suggests that the Mouse Defense Test Battery is a sensitive and appropriate tool for preclinical evaluation of drugs potentially effective against defense-related disorders such as anxiety and panic.

Prepro-thyrotropin releasing hormone 178-199 immunoreactivity is altered in the hypothalamus of the Wistar-Kyoto strain of rat

The rat prepro-thyrotropin releasing hormone (TRH) 178-199 is derived from prepro-TRH by the actions of the endopeptidases, prohormone convertase 1 (PC1) and PC2. PPTRH 178-199 attenuates the synthesis and secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary both in vitro and in vivo, suggesting an inhibitory action on hypothalamic-pituitary-adrenal (HPA) axis function. This peptide also acts centrally to increase activity and decrease anxiety related behaviors. To elucidate the involvement of this peptide in these functions, we have compared the expression of PPTRH 178-199, PPTRH mRNA, and PC1 and PC2 mRNAs in the Wistar-Kyoto (WKY) and Wistar strains of rat. WKY rats have been shown to possess neuroendocrine abnormalities (HPA hyper-activity) and hyper-emotional behavioral characteristics. Immunohistochemical analysis of PPTRH 178-199 demonstrated significant strain differences in the paraventricular nucleus (PVN) of the hypothalamus and the parastrial nucleus (PSN). WKY rats had significantly greater numbers of immunoreactive (IR) cell body profiles (P<0.0005) than Wistar rats in the PVN and a significantly lower fiber density (P<0.002) in the PSN. Levels of PPTRH, PC1, and PC2 mRNA were not different between strains in any brain region examined. These data suggest that altered levels of PPTRH 178-199 in WKY rats could cause, at least in part, the hyper-activity of the HPA axis and the hyper-emotional behavioral characteristics seen in this rat strain. Such data fit with the hypothesis that PPTRH 178-199 is involved in the regulation of the HPA axis and behavior.

Modulation of the growth hormone-releasing activity of thyrotropin-releasing hormone in the chicken by its gene-related peptide preproTRH((160-169))(Ps4): enhanced somatostatinergic tone?

Recent research demonstrated that endocrine actions of thyrotropin (TSH)-releasing hormone (TRH) are modulated by gene-related products within proTRH. In the present report we show that the growth hormone (GH) response to TRH is clearly inhibited after the preincubation of chicken pituitary glands with preproTRH((160-169))Ps4, whereas the TSH response is not impaired. Binding sites for(125)I-[Tyr(0)]-Ps4 were, however, not detected on chicken pituitary membranes, although (as a control) they were readily detectable on membranes from rat pituitary glands. An indirect action may therefore take place within the pituitary by modulating the action of somatostatin (SRIH), the inhibitor of GH release in the chicken. This hypothesis is strengthened by the observation that Ps4 increases the binding of(125)I-[Tyr(1)]-SRIH to chicken pituitary membranes in a dose-related way. Since Ps4 is also produced by pituitary tissue, this may reflect a local or paracrine action on the regulation of GH release.

A novel endogenous corticotropin release inhibiting factor

ACTH is the major regulator of the body's adaptive response to stress and the physiological stimulus for glucocorticoid secretion. A hypothalamic corticotropin release inhibiting factor (CRIF) that inhibits ACTH synthesis and secretion has long been postulated but was not characterized until recently. We have recently identified a 22 amino acid peptide, prepro-thyrotropin releasing hormone (TRH) 178-199 that inhibits basal and stimulated ACTH synthesis and secretion in vitro and stress-induced ACTH secretion in vivo. Prepro-TRH 178-199 is abundant in several brain regions, including the external zone of the median eminence, where its concentration changes in response to stress. We propose that this peptide is a physiological regulator of ACTH production: an endogenous CRIF. Because prepro-TRH 178-199 is encoded within the same precursor as TRH, its expression is likely to be negatively regulated by thyroid hormones leading to changes in endogenous glucocorticoid levels. Streptococcal cell wall (SCW)-induced inflammation, a model of rheumatoid arthritis (RA), was alleviated after long-term thyroxine treatment. Inversely, a hypothyroid milieu led to decreased basal hypothalamic-pituitary-adrenal activity, but increased expression of IL-1 beta and MIP-1 alpha, specific markers for RA in humans. These results suggest that this putative CRIF may be an important component in the development of RA and that regulation of prepro TRH may be highly relevant to the development of other autoimmune diseases that are also exacerbated by low endogenous glucocorticoid levels.

Inhibition of stress-induced neuroendocrine and behavioral responses in the rat by prepro-thyrotropin-releasing hormone 178-199

A corticotropin release-inhibiting factor (CRIF) in brain has been postulated for several decades, based on increased plasma levels of ACTH and corticosterone after hypothalamic-pituitary disconnection. Recent in vitro studies indicate that prepro-TRH178-199 may function as an endogenous CRIF, prompting us to examine stress-related neuroendocrine and behavioral responses after in vivo administration to the adult male rat. Animals that were administered prepro-TRH178-199 intravenously 5 min before restraint stress exhibited a significant attenuation of stress-induced elevations of ACTH, corticosterone, and prolactin, as compared with controls infused with vehicle, whereas thyroid-stimulating hormone (TSH) secretion was not changed. In behavioral studies of stress responsiveness, either the vehicle or prepro-TRH178-199 was administered intracerebroventricularly (ICV) 5 min before testing. In the open field, prepro-TRH178-199 significantly increased grooming, locomotor activity, rearing, and sniffing behaviors. In the light/dark box, it significantly increased the time animals spent in the light compartment and increased the number of crossings between the light/dark compartments. In the plus maze, the peptide significantly increased the amount of time animals spent in the open arms. The same dose of peptide, administered ICV, had no effect on peripheral hormone release in response to restraint stress. Overall, these results support a role for prepro-TRH178-199 in the inhibition of the neuroendocrine responses to stress at the level of the pituitary and indicate that it has central modulatory influences over stress-related behaviors.

Thyroxine administration prevents streptococcal cell wall-induced inflammatory responses

Administration of streptococcal cell wall (SCW) preparation induces an inflammatory response in susceptible animals that is a model frequently used for rheumatoid arthritis. The degree of inflammation produced by SCW is greatly enhanced by low endogenous levels of glucocorticoids due to diminished hypothalamic-pituitary-adrenal activity. Because decreased glucocorticoid production is known to occur in the hypothyroid state, we tested whether hypothyroidism would increase, and conversely, whether hyperthyroidism would decrease, the inflammatory responses to SCW. Adult female Sprague Dawley rats were fed a regular diet (control), L-T4 (T4; hyperthyroid), or 6-propyl-thiouracil (hypothyroid) in drinking water for 7 weeks. Hypothyroidism resulted in elevated plasma levels of TSH and hypothalamic preproTRH messenger RNA (mRNA) while reducing anterior pituitary POMC mRNA and plasma ACTH and corticosterone levels. In contrast, hyperthyroid rats produced opposite results: decreased measures of central thyroid function but increased pituitary-adrenal function. Three days after administration of SCW, macrophage inflammatory protein-1alpha and interleukin-1beta mRNA expression increased dramatically in controls and even further in hypothyroid animals, as measured by Northern blot analysis. In contrast, T4-treated rats showed significant inhibition of these inflammatory markers. Thus, the hyperthyroid state combined with increased endogenous glucocorticoid levels is protective against inflammatory challenges. The inverse relationship between preproTRH expression and pituitary-adrenal function suggests the possibility of a direct inhibitory link connecting the hypothalamic-pituitary-adrenal and thyroid axes, and suggests alternative sites of therapeutic intervention for rheumatoid arthritis and other inflammatory associated disorders.

Effects of acute ethanol administration and cold exposure on the hypothalamic-pituitary-thyroid axis

A single dose of ethanol increases cellular levels of the mRNA encoding thyrotropin-releasing hormone (TRH) in neurons of the paraventricular nucleus (PVN), but blocks the cold-induced increase in the levels of this mRNA. Because the thyrotropic response to cold is dependent upon TRH secretion, we hypothesized that ethanol uncouples the stimulus-induced regulation of TRH secretion from the stimulus-induced regulation of TRH expression. We employed two complementary strategies to test this hypothesis. The first was to determine whether ethanol alters pituitary sensitivity to TRH. Animals given a single intraperitoneal injection of ethanol (3 g/kg) that produced a blood alcohol concentration of nearly 300 mg/100 mL exhibited the same increase in circulating levels of TSH following an intravenous infusion of TRH. Thus, ethanol does not appear to alter pituitary sensitivity to TRH. Second, we tested whether ethanol blocks the cold induction of c-fos expression in TRH neurons of the PVN. Both cold exposure and ethanol induced the expression of c-fos in the PVN and in TRH neurons; the effects of cold and ethanol on c-fos expression were additive. Thus, ethanol clearly does not block the cold activation of TRH neurons.

Influence of thyroid status on TRH metabolism in rat olfactory bulb

The effect of thyroid hormones (TH) on the metabolism of thyrotropin-releasing hormone (TRH) in the olfactory bulb (OB) was compared with the hypothalamic response to TRH. Two methods were used to induce hypothyroidism: propylthiouracyl-methimazole (PTU-M) or 131I treatment. Hyperthyroidism was produced by 3,3',5-triiodo-L-thyronine (T3) injections to the hypothyroid animals. With PTU-M treatment, paraventricular TRH mRNA levels increased 57% and returned to the euthyroid level with T3 treatment. In OB, TRH mRNA was not altered. The TRH content was unaffected in the mediobasal hypothalamus of PTU-M-treated animals whereas it was reduced in OB (31%) with no further response upon T3 treatment. 131I-induced hypothyroidism did not modify the OB TRH content but it was decreased (31%) in hyperthyroids. In the median eminence, TRH increased 26% in hypothyroids, and the response was reversed with T3. Our results demonstrate that treatments that change thyroid status can alter TRH levels in the OB, probably at a translational or postranslational level, though the effects may be pharmacological.

Analysis of structural requirements for TRH-potentiating peptide receptor binding by analogue design

Previous studies established that the [125I-Tyr0]Ps4 derivative of TRH-potentiating peptide (Ps4), Ser-Phe-Pro-Trp-Met-Glu-Ser-Asp-Val-Thr, displays high affinity and selectivity for an orphan membrane receptor in rat anterior pituitary. To identify the sites in Ps4 that determine receptor binding affinity, we have synthesized and screened a panel of 15 single-point substituted analogues for their ability to displace bound [125I-Tyr0]Ps4. The affinity of [Tyr0]Ps4 for rat anterior pituitary membranes [inhibitory constant (Ki), approximately 5 nM] was drastically reduced by the substitution of either Tyr0 with Gly or Asp8 with Asn (Ki approximately 95 and approximately 51 nM, respectively). Deamination of [Tyr0]Ps4 also sharply reduced affinity (Ki approximately 1100 nM). In contrast, Ser1-->Ala, Pro3-->Ala and Thr10-->Val substitutions led to analogues showing a tenfold increase in binding affinity relative to the parent peptide. The change of Phe2-->Leu, Trp4-->Ala, Glu6-->Gln, Val9-->Thr and carboxamidation of Thr10 had no effect on binding affinity. The data suggest that substitutions of the amino-terminal group, Asp8 and Tyr0, have a marked effect on the ability of [Tyr0]Ps4 to compete with [125I-Tyr0]Ps4 for binding to TRH-potentiating peptide pituitary receptor.

Modulation of the biological activity of thyrotropin-releasing hormone by alternate processing of pro-TRH

Thyrotropin-releasing hormone prohormone contains multiple copies of TRH linked together by connecting sequences. Like other plurifunctional prohormone proteins, pro-TRH undergoes differential proteolytic processing in various tissues to generate, beside authentic TRH, several other novel peptides corresponding to C-terminally extended forms of TRH and connecting fragments. The pro-TRH connecting peptides are, together with TRH, predominant storage forms of TRH-precursor related peptides in the rat hypothalamus. Connecting peptides are co-localized with TRH in the median eminence nerve endings and co-released through a mechanism involving voltage-operated Ca2+ channels. The connecting peptide Ps4 is involved in potentiation of the action of TRH on thyrotropin hormone release by pituitary in vitro and in vivo through interactions with a specific pituitary cell receptor coupled to dihydropyridine and omega-connotoxin sensitive Ca2+ channels of the L-type. It also causes dose-dependent increases in the steady state levels of mRNAs of TSH and prolactin through stimulation of the respective gene promoter activities. These findings indicate that Ps4 and TRH, two peptides which originate from a single multifunctional biosynthetic precursor, can function on the same target tissues in a coordinate manner to promote hormonal secretion. This suggests that differential processing of the TRH prohormone may have the potential to modulate the biological activities of TRH.

Site-specific modulation of morphine and swim-induced antinociception following thyrotropin-releasing hormone in the rat periaqueductal gray

Central administration of thyrotropin-releasing hormone (TRH) produces a short-lived antinociceptive response in rats, and also modulates opioid and non-opioid forms of antinociception. Given the presence of TRH cells, fibers and receptors in the periaqueductal gray (PAG), the present study examined the effects of TRH administered into the PAG upon antinociception following either continuous cold-water swims (CCWS, 2 degrees C for 3.5 min) or morphine (0.1-2.5 micrograms) administered into the PAG on the tail-flick and jump tests, and measured changes in core body temperatures as well. Histological examination revealed two groups in which anterior PAG placements were found rostral to the dorsal raphe nucleus, and posterior PAG placements which were at the level of this nucleus. TRH produced brief (5-15 min) but significant increases in latencies and thresholds without altering body temperature in both anterior and posterior PAG placements. Whereas TRH in anterior PAG placements dose dependently (0.1-10 micrograms) decreased CCWS antinociception on both tests, TRH in posterior PAG placements significantly increased CCWS antinociception on the jump test. TRH in both placements reduced the magnitude of CCWS hypothermia. TRH significantly potentiated the magnitude and duration of both morphine antinociception and hyperthermia in both anterior and posterior PAG placements, and shifted mesencephalic morphine's antinociceptive dose-response curve significantly to the left. These data are discussed in terms of the role of the PAG in opioid and non-opioid forms of stress-induced antinociception as well as morphine antinociception, and in terms of the roles of TRH and anterior PAG placements as potential candidates for a collateral inhibition model of antinociceptive responses.

Isolation and amino acid sequence of the TRH-potentiating peptide from bovine hypothalamus

A neuropeptide termed TRH-potentiating peptide, which potentiates TRH-evoked thyrotropin secretion by antehypophysis in vitro, was isolated from an acetonic powder of bovine hypothalamus. The peptide was purified to homogeneity by a 3-step protocol involving molecular sieve filtration, ion-exchange chromatography and reverse phase high performance liquid chromatography. The complete amino acid sequence of the decapeptide was determined as Ser-Phe-Pro-Trp-Met-Glu-Ser-Asp-Val-Thr by automated Edman degradation with a solid-phase sequencer. Bovine TRH-potentiating peptide is structurally identical to Ps4, a decapeptide which was deduced from the cDNA encoding the rat TRH precursor. This study provides for the first time a direct chemical evidence for the existence of non-TRH peptides originating from posttranslational processing of the TRH precursor in vivo.

Neuroanatomical connections between corticotropin-releasing factor (CRF) and somatostatin (SRIF) nerve endings and thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of rat hypothalamus

In order to investigate the possible involvement of corticotropin-releasing factor (CRF) and somatostatin (SRIF) on thyrotropin-releasing hormone (TRH) neuronal cell activity in the rat hypothalamic paraventricular nucleus, we have proceeded to the simultaneous localization of CRF or SRIF and TRH. For this purpose, we used a dual immunostaining procedure that employed antibodies to CRF and SRIF and peroxidase-labeled goat anti-rabbit IgG as a first sequence, and antibodies to a cryptic fragment (Phe178-Glu199) of pro-TRH (to label TRH neurons) and alkaline phosphatase-labeled goat anti-rabbit IgG as the second sequence. A rich innervation of the paraventricular nucleus by immunoreactive CRF and SRIF fibers was observed. A large number of CRF and SRIF nerve endings were seen intimate anatomic proximity and often appeared to surround TRH-containing cell bodies. These results strongly suggest that TRH neurons might be regulated by both CRF and SRIF. These interactions might be the neuroanatomical basis for the already observed inhibitory effects of CRF and SRIF on TRH release.

TRH-related peptides in the rabbit prostate complex during development

The novel peptide, pyroglutamylglutamylprolineamide (pGlu-Glu-ProNH2), has recently been isolated and characterized from the rabbit prostate complex. The tripeptide is present in high concentrations in the prostate complex and semen, together with a 40-50 residue polypeptide which contains a TRH-immunoreactive fragment at its C-terminus. The present study investigates changes in the levels of these TRH-related peptides in rabbits aged 11 weeks, 4 months, 7 months, 13 months and 2 years. For each age group the peptides were extracted from the prostate complex, separated by gel exclusion chromatography, and located by TRH radioimmunoassay. The TRH-immunoreactive fragment was released from the polypeptide by trypsin digestion prior to radioimmunoassay. Very low concentrations of TRH-immunoreactive peptides were present at 11 weeks of age, but considerable levels of both peptides were found in all the other age groups. Anion exchange chromatography, under conditions which resolve TRH and pGlu-Glu-ProNH2, showed that the majority of the low molecular weight TRH immunoreactivity co-eluted with synthetic pGlu-Glu-ProNH2. The remaining TRH immunoreactivity, which had not bound to the anion resin, also failed to bind to a cation exchange column at pH 2.0, indicating that it was not authentic TRH. Dissection of the prostate complex into its four constitutive regions (vesicular gland, coagulating gland, prostate and bulbourethral gland) followed by extraction, chromatography and TRH radioimmunoassay of each region showed that the TRH-related peptides were located in the prostate.

Anatomical interactions of proopiomelanocortin (POMC)-related peptides, neuropeptide Y (NPY) and dopamine beta-hydroxylase (D beta H) fibers and thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of rat hypothalamus

In order to determine the nature of afferent fibres contacting thyrotropin-releasing hormone (TRH)-synthesizing neuronal cell bodies in the rat hypothalamic paraventricular nucleus, we used dual immunostaining procedures which employed antibodies to ACTH (to label proopiomelanocortin (POMC) neurons), neuropeptide Y (NPY) and dopamine-beta-hydroxylase (D beta H) and peroxidase-labeled goat anti-rabbit IgG as a first sequence and antibodies to a cryptic fragment (Phe178-Glu199) of pro-TRH (to label TRH neurons) and alkaline phosphatase-labeled goat anti-rabbit IgG as the second sequence. A rich innervation of the paraventricular nucleus by immunoreactive POMC, NPY and D beta H fibres was observed. Numerous NPY and POMC fibres were in intimate anatomic proximity and often appeared to surround in remarkable density TRH-containing cell bodies. Less frequent appositions between D beta H fibres and TRH cell bodies were detected. These results strongly suggest that TRH neurons might be regulated by POMC, NPY as well as adrenergic and/or noradrenergic systems. These interactions might be the neuroanatomical basis for the already observed effects of opiate peptides, NPY and catecholamines on TSH secretion.

Release of pro-thyrotropin-releasing hormone connecting peptides PS4 and PS5 from perifused rat hypothalamic slices

Thyrotropin-releasing hormone prohormone contains five copies of the thyrotropin-releasing hormone progenitor sequence Gln-His-Pro-Gly, each flanked by pairs of basic amino acids and separated by intervening sequences (connecting peptides). Using a perifusion system for rat hypothalamic slices, we have studied the ionic mechanisms underlying the release of two connecting peptides originating from the thyrotropin-releasing hormone precursor: prepro-thyrotropin-releasing hormone-(160-169) (Ps4) and prepro-thyrotropin-releasing hormone-(178-199) (Ps5). Quantification of these two peptides in the effluent fluid was performed using sensitive and highly specific radioimmunoassay procedures. Reverse phase high performance liquid chromatography analysis of the effluent perifusate showed that released peptides co-eluted with synthetic Ps4 and Ps5. The secretion of Ps4 and Ps5 was stimulated by depolarizing agents such as (i) high potassium concentrations, (ii) ouabain, an Na+/K(+)-ATPase inhibitor, and (iii) veratridine, a stimulator of voltage-operated Na+ channels. The response to potassium (70 mM) was not affected by the specific Na+ channel blocker tetrodotoxin. The K+ channel blocker tetraethylammonium did not modify K(+)-evoked release of Ps4 and Ps5. These data suggest that voltage-operated Na+ channels are not involved in the stimulatory effect of high K+ on the release of Ps4 and Ps5. The lack of effect of picrotoxin, a Cl- channel blocker, on the secretion of the connecting peptides indicates that chloride ions play a minor role in the release process. In contrast, deprivation of Ca2+ in the perifusion medium suppressed K(+)-evoked release of the two peptides, indicating that voltage-operated Ca2+ channels are implicated in the release process. Taken together, the present results show that non-thyrotropin-releasing hormone peptides originating from the thyrotropin-releasing hormone precursor are secreted by mediobasal hypothalamic fragments. The release of these peptides is stimulated by depolarization through a calcium-dependent process. These data indicate that Ps4 and Ps5 may be released at the level of the median eminence into the portal circulation, suggesting that these peptides may play a role in the control of anterior pituitary cells.

TRH-extended peptides in the olfactory lobe are formed by incomplete cleavage at pairs of arginine residues in the TRH prohormone

High concentrations of thyrotrophin-releasing hormone (TRH) are known to be present in the olfactory lobe, and the processing of the TRH prohormone in this region of the brain has been examined in this study. TRH-extended peptides have been detected in the rat olfactory lobe: these peptides accounted for approximately 11% of the total TRH immunoreactivity present in the tissue and contained the sequence pGlu-His-Pro-Gly-Arg exclusively at their N-termini. Extended peptides containing pGlu-His-Pro-Gly-Lys at their N-termini were not detected suggesting that incomplete cleavage occurs only at Arg-Arg residues in the TRH-prohormone. In view of the highly specific processing of the prohormone, it is likely that the TRH-extended peptides play important physiological roles.

Immunoelectron microscopic localization of thyrotropin-releasing hormone (TRH) precursor in the rat raphe nuclei

Using antisera to two pro-thyrotropin-releasing hormone (pro-TRH)-derived cryptic peptides, we have studies by immunocytochemistry the ultrastructural localization of pro-TRH in the rat raphe nuclei. The same results were obtained with both antisera. Immunostaining was found in cell bodies, dendrites and endings. In cell bodies, the reaction product was restricted to Golgi saccules and dense core vesicles which were very few in number. In dendrites, the staining was rather diffuse without any association with specific organelles. These results suggest that the Golgi apparatus might be involved in pro-TRH processing.

Pro-TRH-connecting peptides in the rat pancreas during ontogenesis

Rat thyrotropin-releasing hormone prohormone (pro-TRH) is a protein containing five copies of TRH, separated by connecting peptides. We have recently developed radioimmunoassays to synthetic peptides corresponding to prepro-TRH(160-169) and prepro-TRH(178-199). In the present study we have used these assays to investigate the ontogenesis of pro-TRH-derived peptides in the rat pancreas. Reverse-phase HPLC analysis of pancreatic extracts from 2-day-old rats showed the presence of two major immunoreactive peptides exhibiting the same retention time as synthetic prepro-TRH(160-169) and prepro-TRH(178-199), respectively. The concentrations of TRH and pro-TRH cryptic peptides in the rat pancreas rose rapidly after birth, reached a maximum at day 2-4 and decreased gradually afterwards. Streptozotocin treatment of newborn rats induced a marked decrease of TRH (96%), prepro-TRH(160-169) (97%) and prepro-TRH(178-199) content (94%) in pancreatic extracts. These results indicate that the evolution of TRH and pro-TRH-derived peptides follows the same pattern during the postnatal period. Our results also suggest that beta-cells are the only source of pro-TRH-derived peptides in the rat pancreas.

Brain neuropeptides: actions on central cardiovascular control mechanisms

The many peptides we have not considered (e.g. gastrin, motilin, FMRFamide, carnosine, litorin, dermorphin, casomorphin, eledoisin, prolactin, growth hormone, neuromedin U, proctolin, etc.) were omitted due to lack of information as far as any putative central cardiovascular effects are concerned. However, even for some of these peptide pariahs intriguing snippets of information are available now (e.g. ref. 85), although as we write, the list of possible candidates for investigation grows longer. On an optimistic note, it is becoming clear that many brain neuropeptides may have important effects on cardiovascular regulation. It seems feasible that 'chemically coded' pathways in the brain might be the neuroanatomical correlate of a 'viscerotopic' organization of cardiovascular control mechanisms, whereby the activity of the heart and flows through vascular beds are individually controlled, but in an integrated fashion, utilizing particular combinations of neurotransmitters and neuropeptides within the brain. Such possibilities can only be investigated, properly, by measurement of changes in cardiac output and regional haemodynamics in response to appropriate interventions, in conscious, unrestrained animals.