Effects of various inhibitors of phenylethanolamine N-methyltransferase on pulsatile release of LH in ovariectomized rats

Cell-Effective Transition-State Analogue of Phenylethanolamine N-Methyltransferase

Phenylethanolamine N-methyltransferase (PNMT) catalyzes the S-adenosyl-l-methionine (SAM)-dependent methylation of norepinephrine to form epinephrine. Epinephrine is implicated in the regulation of blood pressure, respiration, Alzheimer's disease, and post-traumatic stress disorder (PTSD). Transition-state (TS) analogues bind their target enzymes orders of magnitude more tightly than their substrates. A synthetic strategy for first-generation TS analogues of human PNMT (hPNMT) permitted structural analysis of hPNMT and revealed potential for second-generation inhibitors [Mahmoodi, N.; J. Am. Chem. Soc. 2020, 142, 14222-14233]. A second-generation TS analogue inhibitor of PNMT was designed, synthesized, and characterized to yield a Ki value of 1.2 nM. PNMT isothermal titration calorimetry (ITC) measurements of inhibitor 4 indicated a negative cooperative binding mechanism driven by large favorable entropic contributions and smaller enthalpic contributions. Cell-based assays with HEK293T cells expressing PNMT revealed a cell permeable, intracellular PNMT inhibitor with an IC50 value of 81 nM. Structural analysis demonstrated inhibitor 4 filling catalytic site regions to recapitulate both norepinephrine and SAM interactions. Conformation of the second-generation inhibitor in the catalytic site of PNMT improves contacts relative to those from the first-generation inhibitors. Inhibitor 4 demonstrates up to 51,000-fold specificity for PNMT relative to DNA and protein methyltransferases. Inhibitor 4 also exhibits a 12,000-fold specificity for PNMT over the α2-adrenoceptor.

Determination of serum neopterin levels in patients with autoimmune thyroid diseases

An elevated serum level of neopterin indicates the activation of the cellular immune system. Aim: The objective was to find a correlation in autoimmune thyroid patients between neopterin levels and the clinical stage of the disease and to examine whether neopterin can predict the relapse of the disease. Methods: Serum neopterin, thyroid stimulating hormone, free thyroxine, free triiodothyronine, anti-thyroglobulin and anti-thyroid peroxidase antibody levels were determined in 137 patients with Graves’ disease (in different stages), 25 with Hashimoto’s thyroiditis and 14 with toxic adenoma. Results: The neopterin levels were significantly higher in patients with Graves’ disease (hyper-, eu-, hypothyroidism and relapsed hyperthyroidism) and Hashimoto’s thyroiditis. Positive correlation was found between neopterin and anti-thyroglobulin and anti-tyhroid peroxidase antibody levels, but no correlation was detected between neopterin levels and thyroid hormones, thyroid stimulating hormone values and antibodies against thyroid stimulating hormone receptors. Conclusions: Higher level of serum neopterin reflects an underlying autoimmune process, and does not correlate with changes in thyroid hormone levels. Determination of neopterin level can be an important indicator in the exacerbation of autoimmune processes. Orv. Hetil., 2012, 153, 1127–1131.

Comparative study of the sources of neuronal projections to the site of gonadotrophin-releasing hormone perikarya and to the anteroventral periventricular nucleus in female rats

The rat ovulatory cycle is dependent on the preoptic region encompassing the gonadotrophin-releasing hormone (GnRH) perikarya and the anteroventral periventricular nucleus (AVPV). Retrograde tract tracing was used to identify and compare the sources of inputs to these sites in female rats. Within the telencephalon and diencephalon, the incidence of retrograde labelling from both sites was moderate to abundant in the ventral lateral septum, posteromedial bed nucleus of the stria terminalis, amygdalohippocampal area and the periventricular, medial preoptic, anterodorsal preoptic, dorsomedial suprachiasmatic, arcuate, and posterior ventrolateral ventromedial hypothalamic nuclei. In these regions, the incidence of retrograde labelling was either greater from the AVPV than from the GnRH perikarya site or similar from both sites. In the medial amygdaloid, parastrial, striohypothalamic, and ventral premammillary nuclei, the retrograde labelling from the AVPV greatly exceeded the sparse incidence from the GnRH perikarya site. In contrast, retrograde labelling from the GnRH perikarya site predominated in the median preoptic, lateroanterior and dorsomedial hypothalamic nuclei, subparaventricular zone, and retrochiasmatic area; it was abundant in the AVPV. Caudal to the diencephalon, retrograde labelling from either site was sparse, except in the lateral parabrachial nucleus, which displayed a particularly high incidence from the GnRH perikarya site. Other mesencephalic regions labelled from either site included the periaqueductal gray and dorsal and median raphe nuclei. The most caudal labelling was found in the ventrolateral medulla and region of the solitary tract nucleus; this was almost exclusively from the GnRH perikarya site. These findings further elucidate the neuroanatomical connections underlying the control of the ovulatory cycle.

Participation of both adrenergic and opioidergic systems in the negative feedback of adrenal progesterone on LH secretion

It has been shown that adrenal progesterone plays an important role in regulating the negative feedback of oestrogen on luteinizing hormone (LH) release in ovariectomized and oestrogen-treated rats. The purpose of the present study was to determine whether adrenal progesterone modulation of LH secretion is mediated by adrenergic and opioidergic systems in ovariectomized and oestrogen-treated rats. Oestradiol benzoate (20 micrograms/rat) was given s.c. to ovariectomized rats on day 0. Control animals were injected with the vehicle alone. The specific adrenoceptor antagonists prazosin (10 mg/kg), idazoxan (100 micrograms/kg), metoprolol (10 mg/kg) or ICI 118,551 (200 micrograms/kg) were injected at 12.00 and 20.00 h on day 2 and at 08.00 h on day 3 to oestrogen-primed rats treated or not with RU486. Control animals were injected with saline. RU486 (10 mg/kg) was administered s.c. at 08.00 h on day 3 to oestradiol-treated animals receiving adrenoceptor antagonists or saline. Naloxone (2 mg/kg) was administered i.p. 30 min before blood-sampling to oestrogen-primed rats treated or not with RU486. All groups were blood-sampled at 13.00 and 18.00 h on day 3, and LH concentration was measured by radioimmunoassay. The administration of oestradiol to ovariectomized rats decreased serum LH levels at 13.00 and 18.00 h on day 3. Prazosin or idazoxan partially prevented the effect of oestradiol at 13.00 h, while metoprolol, ICI 118,551 or naloxone totally blocked the inhibitory effect of oestradiol on LH secretion; both adrenoceptor and opioid receptor antagonists also prevented the effect of oestrogen on LH concentration at 18.00 h. RU486 increased serum LH concentration at 18.00 h in oestrogen-primed rats to values higher than in ovariectomized control rats, with no effect at noon. The administration of prazosin to ovariectomized and oestrogen-primed rats treated with RU486 prevented this increase while the other adrenoceptor antagonists or naloxone increased serum LH concentrations at 18.00 h. The present study shows that RU486 switches the feedback of oestradiol on LH secretion from negative to positive in ovariectomized and oestradiol-primed rats, activating a stimulatory alpha 1-adrenergic pathway during the afternoon, and gives strong evidence about the participation of adrenal progesterone modulating neurotransmitter systems involved in the secretion of LH. It also supports the participation of endogenous opioid peptides in the negative feedback of oestradiol, suggesting that the inhibitory tone of endogenous opioid peptide is active regardless the action of adrenal progesterone.

Sodium pentobarbitone and the suppression of luteinizing hormone pulses in the female rat: the role of hypothermia

The role of hypothermia in the suppression of pulsatile luteinizing hormone (LH) release by the barbiturate sodium pentobarbitone has been investigated in ovariectomized rats. Each animal was fitted with an intraperitoneal miniature radio transmitter to monitor core temperature and with an indwelling intravenous and intraperitoneal catheter. During the 6-h sampling period the animal's core temperature was recorded automatically every 5 min and a 25 microliters blood sample was obtained concurrently using an automated system. After the initial 3 h of sampling either the drug or the vehicle was administered via the intraperitoneal cannula from outside the cage, thus ensuring minimal disturbance to the animal. Administration of sodium pentobarbitone (40 mg/kg) at an ambient temperature of 21 degrees C resulted in a significant hypothermia throughout the 3-h post-injection period. During this period there was a significant reduction in mean LH concentration, and in the frequency and amplitude of the LH pulses. When the drug was administered at an ambient temperature of 35 degrees C there was no reduction in core temperature and no significant change in the LH pulse parameters. Vehicle treatment was without significant effect on core temperature or on the LH pulse parameters when administered at an ambient temperature of either 21 degrees C or 35 degrees C. These results indicate that the effects of this barbiturate on the pulsatile release of LH are secondary to the induced hypothermia and suggest that hypothermia per se may be able to disrupt LH pulses. It is therefore imperative to reassess the significance of previous studies that have implicated particular neurotransmitter systems in the control of LH pulses; unrecognized hypothermic effects of the treatments may have been the primary cause of the pulse suppression, rather than a direct involvement of the neurotransmitter in question in the regulation of LH releasing hormone. The neurophysiological process by which the hypothermic state may inhibit LH pulses remains to be determined.

Control of fertility by metabolic cues

In female mammals, reproduction is extremely sensitive to the availability of oxidizable metabolic fuels. When food intake is limited or when an inordinate fraction of the available energy is diverted to other uses such as exercise or fattening, reproductive attempts are suspended in favor of processes necessary for individual survival. Both reproductive physiology and sexual behaviors are influenced by food availability. Nutritional effects on reproductive physiology are mediated by changes in the activity of gonadotropin-releasing hormone (GnRH) neurons in the forebrain, whereas the suppression of sexual behaviors appears to be due, at least in part, to decreases in estrogen receptor in the ventromedial hypothalamus. Work using pharmacological inhibitors of glucose and fatty acid oxidation indicates that reproductive physiology and behavior respond to short-term (minute-to-minute or hour-to-hour) changes in metabolic fuel oxidation, rather than to any aspect of body size or composition (e.g., body fat content or fat-to-lean ratio). These metabolic cues seem to be detected in the viscera (most likely in the liver) and in the caudal hindbrain (probably in the area postrema). This metabolic information is then transmitted to the GnRH-secreting or estradiol-binding effector neurons in the forebrain. There is no evidence to date for direct detection of metabolic cues by these forebrain effector neurons. This metabolic fuels hypothesis is consistent with a large body of evidence and seems to account for the infertility that is seen in a number of situations, including famine, eating disorders, excessive exercise, cold exposure, lactation, some types of obesity, and poorly controlled diabetes mellitus.

Immunocytochemical characterization of afferents to estrogen receptor-containing neurons in the medial preoptic area of the rat

Double-label immunocytochemistry has been employed to elucidate the chemical nature of the afferent neuronal projections to the estrogen receptor-containing neurons located in the medial preoptic area of the rat brain. To ensure a clear separation of the immunolabelled afferent profiles from the estrogen receptors, the former were visualized first and the diaminobenzidine reaction product was silver-gold intensified. Using a monoclonal antibody raised against purified human estrogen receptors, we observed an intense nuclear immunoreactivity in Vibratome, semithin and ultrathin sections. Neuropeptide-Y, serotonin-, phenylethanolamine N-methyltransferase- and adrenocorticotrophin-immunoreactive axons and varicosities were observed in close apposition to the estrogen receptor-positive cells. At the ultrastructural level, neuropeptide-Y-immunoreactive boutons were seen in synaptic contact with cells showing estrogen receptor immunoreactivity in their nucleus. These results indicate that neurons located in the medial preoptic area, one of the principal sites for the control of female reproductive function, may be influenced by both estrogen and neurotransmitters/neuropeptides via, respectively, nuclear receptors and synaptic contacts.

Soluble and particulate phenylethanolamine N-methyltransferase in hypothalamus of diabetic rats

Experimental diabetes increases total phenylethanolamine N-methyltransferase (PNMT) activity in the medulla-pons but not in the hypothalamus. In this study diabetes was induced with streptozotocin (65 mg/kg) in male Sprague-Dawley rats. Twenty-eight days after treatment there were no differences in soluble PNMT activity in the hypothalamus of diabetics and controls, but PNMT activity in a membrane-associated (particulate) fraction of hypothalamus was evaluated approximately twofold in tissues of diabetic animals compared with controls. A specific PNMT inhibitor, incubated with tissue extracts of control rats, abolished greater than 90% of particulate PNMT activity in the hypothalamus but reduced soluble PNMT activity in the hypothalamus by only 47%. These findings indicate that membrane-associated PNMT activity in rat hypothalamus differs from soluble hypothalamic PNMT in the in vitro response to an inhibitor and the in vivo response to diabetes and suggest the importance of separating subcellular hypothalamic fractions prior to assay of PNMT.

Brainstem projections to the medial preoptic region containing the luteinizing hormone-releasing hormone perikarya in the rat. An immunohistochemical and retrograde transport study

The afferent projections to the anterior medial preoptic area (MPA) from the brainstem have been studied, in female Wistar rats, by retrograde tracing with horseradish peroxidase (HRP). The HRP was injected by iontophoresis into the preoptic region containing the luteinizing hormone-releasing hormone (LHRH) perikarya. The brain sections including the MPA were reacted with diaminobenzidine (DAB) to reveal the injection site; the LHRH cells were then immunohistochemically identified using DAB with ammonium nickel sulphate. When the injection site incorporated the LHRH cells, the brainstem sections were reacted with the DAB nickel solution to detect lysosomal HRP and then immunohistochemically processed to locate the adrenaline-synthesizing cells using DAB alone. The results confirm the brainstem projections to the MPA from the central grey matter, ventral tegmental area, subcoeruleus area, the dorsal raphe nucleus, the lateral parabrachial nucleus, the raphe pontis nucleus, the raphe obscurus nucleus, the region of the paragigantocellular nucleus and the nucleus of the solitary tract. Given the considerable evidence implicating the ascending adrenergic systems in the regulation of LHRH, we focused our attention on the afferents from the locus coeruleus, area postrema and the adrenaline-synthesizing cell groups (C1-3). The only cells which were retrogradely labelled and immunopositive for phenylethanolamine N-methyltransferase were found in C3.

The role of neopterin as a monitor of cellular immune activation in transplantation, inflammatory, infectious, and malignant diseases

The accumulated knowledge about the organization and function of the human immune system contributes to a better understanding of the pathogenesis of most diverse disorders and is opening new avenues for therapeutic regimens. To gain further insight into the complex interactions within the components of the immune system, it has become increasingly necessary to develop rapid and simple methods to monitor the status of the immune system in patients. The determination of neopterin concentrations in human body fluids allows to investigate sensitively the cell-mediated immune status to be investigated with considerable sensitivity. In recent years it was shown that production and release of neopterin is inducible in human monocytes/macrophages by interferon gamma. Increased neopterin levels indicate endogenous formation of gamma interferon, and monitoring of neopterin levels therefore permits the activation status of the cell-mediated immune system to be examined. Neopterin concentrations in serum and in urine increase in parallel to the clinical course of infections with viruses, intracellular bacteria, and parasites. In patients with human immunodeficiency virus infection neopterin concentration in serum and urine is a significant predictor of disease progression, the statistical power being similar to CD4+ T-cell numbers. In patients with autoimmune disorders, neopterin levels correlate with the extent and the activity of the disease. Neopterin concentrations are also sensitive indicators of immunological complications in allograft recipients. In certain malignant diseases neopterin concentrations correlate with the stage of the disease and bear prognostic information. Results of neopterin measurements agree with the important role that the cellular immune system plays in these disorders.

Effects of depleting central and peripheral adrenaline stores on blood pressure in stroke-prone spontaneously hypertensive rats

The potential role of adrenaline, both circulating and in the central nervous system, in the maintenance of high blood pressure was examined in stroke-prone spontaneously hypertensive rats (SHRSP). alpha-Monofluoromethyldopa, a long-lasting inhibitor of dopa decarboxylase, was used to induce rapid depletion of central and peripheral catecholamine stores. Subsequent inhibition of phenylethanolamine-N-methyltransferase (PNMT) allowed the gradual restoration of dopamine and noradrenaline but not adrenaline, resulting in a greater relative depletion of adrenaline. Adrenaline was almost totally depleted in the circulation and peripheral tissues. The resting level of blood pressure, however, was unaffected, excepting after administration of a vasopressin (AVP) antagonist. Moreover, there was no reduction in the magnitude of acute pressor responses to electrical stimulation of the rostral ventrolateral medulla oblongata (C1 area), despite extensive loss of adrenaline from the brainstem and spinal cord. The results suggest that adrenaline contributes to the resting level of blood pressure but that its loss can be offset by the pressor activity of AVP. Thus neither central nor peripheral adrenaline stores appear to be essential for the maintenance of hypertension or for centrally-evoked vasoconstriction in adult SHRSP.

Catecholamine synthesis inhibitors increase pineal adrenaline content by stimulating adrenal medullary activity

Estimation of noradrenaline and adrenaline utilization in the pineal gland of female rats was attempted using inhibitors of the enzymes that catalyse the catecholamine biosynthetic pathway. Treatment with FLA63, an inhibitor of dopamine beta-hydroxylase (10 mg/kg, 2 h before killing), induced depletion of noradrenaline and adrenaline in the preoptic area and median eminence (sites, respectively, inside and outside the blood-brain barrier) but, paradoxically, resulted in a significant increase (+77%) in the pineal content of adrenaline without affecting that of noradrenaline. Treatment with LY134046, an inhibitor of phenylethanolamine N-methyltransferase (40 mg/kg, 5 and 2 h before killing), induced depletion of adrenaline in the preoptic area and median eminence but, again, resulted in a paradoxical and large increase in pineal adrenaline (+224%); this increase was prevented by prior adrenalectomy. Blood samples taken from free-moving rats fitted with intravenous and intraperitoneal cannulae revealed a marked increase in plasma levels of adrenaline after each injection of LY134046. These results suggest that the adrenal medulla is the primary source for the increase in pineal adrenaline seen after administration of the enzyme inhibitors. The precise site of uptake and the biological implications of this phenomenon remain to be elucidated. Nevertheless, interpretation of in vivo experiments involving these catecholamine synthesis inhibitors should take this adrenal response into account.

Interactions between opioid peptides and adrenaline-containing neurons modulate luteinizing hormone secretion in male rats

Abstract There is increasing evidence that the opioid inhibition of luteinizing hormone (LH) secretion is mediated, at least in part, by catecholaminergic mechanisms. This study determined the effects of selective manipulation of noradrenergic and adrenergic systems on the ability of opiate receptor blockade to induce the release of LH in adult male rats. Selective depletion of hypothalamic noradrenaline levels by 80% following 6-hydroxydopamine infusions into the central tegmental tract did not alter the 2- to 3-fold increase in serum LH levels following opiate receptor blockade with naloxone (2.5 mg/kg). In contrast, both selective depletion of hypothalamic adrenaline by prior treatment with the phenylethanolamine N-methyltransferase inhibitor, LY134046 (2 x 50 mg/kg) and non-selective depletion of all three catecholamines with alpha-methyl-p-tyrosine (250 mg/kg), abolished the naloxone-induced increase in LH. These results suggest that the inhibition of LH secretion by endogenous opioid peptides is influenced by catecholaminergic neurotransmission and further support the view that adrenaline rather than noradrenaline or dopamine is of importance in this context.

Adrenergic and noradrenergic regulation of pulsatile luteinizing hormone release

Abstract The object of this study was to further define the roles of both norepinephrine (NE) and epinephrine (EPIN) in regulating pulsatile luteinizing hormone (LH) release in 4-day ovariectomized rats, in particular to examine the effect of decreasing NE synthesis on pulsatile LH secretion in animals with already greatly depleted levels of brain EPIN. Rats were injected ip with vehicle or drug at -27, -20, -5 and - 3 h relative to the onset of a 3-h blood sampling period. Hypothalamic-preoptic area (HPOA) levels of NE and EPIN were determined by high-performance liquid chromatography. Compared to controls, FLA-63 (25 mg/kg, a dopamine-ss- hydroxylase inhibitor), given at - 3 h, produced 50% and 22% declines in HPOA-NE and EPIN, respectively, and reductions in pulse amplitude and frequency. LY134046 (50 mg/kg, a phenylethanolamine N-methyltransferase inhibitor), given at - 27, - 20 and - 5 h, or -27, -20, -5 and -3 h, produced no change in NE, 88% and 86% declines in EPIN, respectively, and reductions in pulse frequency only. Each LY134046 treatment protocol produced the same decline in EPIN and pulse frequency. Thus, EPIN levels were maximally decreased by three LY134046 injections. When rats were given LY134046 at -27, -20 and -5 h, and FLA-63 at -3 h, compared to rats treated with LY134046 alone, there was no further decrease in HPOA-EPIN (82% decline), a 46% decline in NE, a further reduction in pulse frequency and a reduction in pulse amplitude. This further suppression of LH release must be due to a reduction in HPOA-NE levels since no further decrease in EPIN levels occurred. These data demonstrate within the same animal that NE and EPIN are both stimulatory to pulsatile LH release. NE stimulates the amplitude and frequency, and EPIN stimulates the frequency of pulsatile LH secretion.