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Regulation of the Pituitary Gland by Dopamine
Published in Nira Ben-Jonathan, Dopamine, 2020
Studies on the distribution of axon terminals within the IL during development and in adult rats found both synaptic and non-synaptic interactions between dopaminergic terminals and melanotrophs. Several studies have shown that DA has major inhibitory effects on the production and processing of POMC, as well as on the release of its derived peptides from the IL (for review [26]). For example, the regulation of POMC mRNA expression in rats was altered by injections of haloperidol, a DA antagonist, or bromocriptine, a DA agonist. In addition, the maintenance of high dopaminergic activity in rats by DA agonists prevented stress-released melanotrophic peptides, while a blockade of DA transmission augmented this response, especially in the presence of β-3 adrenergic agonists. These data suggested that reduced DA tone, coupled with enhanced β-3 adrenergic activity, is needed to produce the full effects of stress-induced release of melanotrophic peptides from the IL. Whereas a DA agonist, ergocryptine, inhibited basal α-MSH release from cultured fetal and early neonatal rat melanocytes, it had no effect on the release of POMC-derived peptides from the corticotrophs, indicating intrinsic differences in the responsiveness of the two POMC-producing cell types [28].
Adrenoceptor Antagonists
Published in Kenneth J. Broadley, Autonomic Pharmacology, 2017
The ergotoxin first extracted by Dale was in fact a mixture of ergocornine, ergocristine, α-ergocryptine and β-ergocryptine. Their structures are shown in Table 5.1, together with other ergot alkaloids including ergotamine and ergonovine (ergometrine). These alkaloids are amine derivatives of D-lysergic acid, to which they are converted when hydrolysed. The other products of hydrolysis are either amines, in the case of ergometrine (amine alkaloids), or amino acids, of which one is proline and the other is either phenylalanine, leucine, isoleucine or valine (amino acid alkaloids). The D-isomers are inactive. The amino acid alkaloids are active competitive α-adrenoceptor antagonists with a long duration of action. The amine alkaloids, which lack the amino acid side-chain, such as ergometrine, do not have a-adrenoceptor blocking properties. However, these compounds have a direct smooth muscle stimulant action on the uterus and blood vessels. This property is shared by the amino acid alkaloids and, although once thought to be independent of receptor-mediated events, it is now believed to involve activation of α- and 5-hydroxytryptamine (5-HT) receptors. The contractions of the uterus induced by ergometrine are antagonized by the 5-HT antagonist, cyproheptadine (Hashimoto et al. 1977).
Prolactin Receptor — Clinical Problems
Published in Nagasawa Hiroshi, Prolactin and Lesions in Breast, Uterus, and Prostate, 2020
Drugs known to effect PRL secretion have been administered to patients with prostatic tumors and although evaluating the prostatic tissue after such treatment is limited, certain clinical parameters will indicate prostatic tumor response. Estrogens have been used for nearly 60 years in the treatment of prostatic cancer, such that high plasma PRL concentrations have consequently been produced in thousands of patients in addition to the intended androgen suppression.18 Whether this has contributed to the refractory nature of the disease is debatable, but it is possible that it may enhance the uptake of small amounts of available androgen from adrenal and peripheral sources. Prostatic cancer treated by orchidectomy, where no concomitant rise in PRL concentrations is observed, still becomes refractory to endocrine therapy within a similar time period as estrogen treatment and would appear to indicate that high PRL levels have a negligible effect on the progression of the disease. Several groups have looked at the possible combination of estrogen therapy with a specific antiprolactin drug such as bromocryptine(2-bromo-a-ergocryptine), which depresses pituitary secretion of PRL. Jacobi et al.23 used a combination of diethylstilbestrol and bromocryptine and reported objective improvement in their patients. In a clinical trial which used bromocryptine administration alone for patients with prostatic cancer, no improvement was obtained.9 Subjective improvement in bone pain has been reported in patients given L-dopa as a palliative treatment.29 A trial of bromocryptine in patients with BPH on the relief of urinary symptoms and the reduction in prostate tumor size as assessed by ultrasound, carried out jointly at St. Woolos Hospital, Newport, and the Tenovus Institute, has yielded disappointing results. Therefore, the evidence is not overwhelming that altered PRL concentrations, either elevated or reduced significantly, affect established tumors of the prostate.
Comparative study on the metabolism of the ergot alkaloids ergocristine, ergocryptine, ergotamine, and ergovaline in equine and human S9 fractions and equine liver preparations
Published in Xenobiotica, 2019
Wiebke Rudolph, Daniela Remane, Dirk K. Wissenbach, Frank T. Peters
Finally, a dealkylation of the ergoline was observed in ergovaline, ergotamine, and ergocryptine. A loss of methyl (–14.0151 Da) was observed for the protonated molecules as well as the fragments m/z 223.1229 and m/z 268.1444 resulting in fragments at m/z 209.1075 as well as m/z 254.1286 as shown for ergocryptine in Figure 4(H). As described for LSD, the demethylation must occur at position N6 of the ergoline structure (Meyer & Maurer, 2011). In accordance with Maurer et al. (1982), the formation of a nor-bromocriptine was not observed in any of the incubation approaches in the present study. Maurer et al. attributed this observation to a steric hindrance by the large neighboring bromine atom (Maurer et al., 1982). However, in the present study ergocristine also showed no N-dealkyl metabolite, although there is no steric hindrance by a bromine atom. A systematic overview of the ergopeptine metabolites is given in Figure 5 exemplified for ergocryptine. The data for each compound under study are given in the supplementary data (Figures S1–S3).