Isolation, primary structure, and effects on alpha-melanocyte-stimulating hormone release of frog neurotensin

Neuroendocrine Regulation of Frog Adrenocortical Cells by Neurotensin

We previously characterized the primary structure of neurotensin (NT) from an extract of the intestine of the frog Rana esculenta. In this study, we provide evidence for the involvement of NT in the neurocrine regulation of the secretory activity of frog adrenocortical cells. Immunohistochemical studies revealed that the adrenal gland of R. esculenta is innervated by a dense network of NT-immunoreactive fibers. Graded concentrations of frog NT induced a dose-dependent stimulation of corticosterone and aldosterone secretion by frog adrenocortical explants through activation of two receptors with pEC(50) of 9.8 and 6.9. These data support the view that NT, released by nerve fibers within the frog adrenal gland, acts locally to control corticosteroid secretion.

Detection of new amino acid sequences of alamethicins F30 by nonaqueous capillary electrophoresis–mass spectrometry

The microheterogeneous alamethicin F30 (ALM F30) isolated from the fermentation of Trichoderma viride strain NRRL 3199 was analyzed by nonaqueous capillary electrophoresis coupled to electrospray ion-trap mass spectrometry (ESI-IT-MS) and electrospray time-of-flight mass spectrometry (ESI-TOF-MS). Tandem ESI-IT-MS was used for elucidation of the amino acid sequence based on the fragmentation pattern of selected parent ions. The MS/MS spectra using the [M + 3H](3+) or [M + 2H](2+) ions as precursor ions displayed the respective b- and the y-type fragments resulting from cleavage of the particularly labile Aib-Pro bond. The MS(3) of these fragments generated the b acylium ion series, as well as internal fragment ion series. Eleven amino acid sequences were identified, characterized by the exchange of Ala to Aib in position 6, Gln to Glu in positions 7 or 19 as well as the loss of the C-terminal amino alcohol. In addition, two truncated pyroglutamyl peptaibols were found. Overall, seven new sequences are reported compared to earlier LC-MS studies. The composition of the components was confirmed by on-line ESI-TOF-MS detection. Mass accuracy well below 5 ppm was observed. Quantification of the individual components was achieved by a combination of UV and TOF-MS detection.

Signal transduction in Rana melanotrope cells: mechanism of action of neurotensin on secretory and electrical activities

The pars intermedia of the frog (Rana esculenta) pituitary, which is composed of a single population of endocrine cells, the melanotrophs, is a very suitable model to study the mode of action of hypophysiotropic neuropeptides. We have recently characterized neurotensin (NT) in Rana esculenta and found that synthetic frog NT (fNT) stimulates the electrical and secretory activities of melanotrophs. By combining biochemical, pharmacological, microfluorimetric, and electrophysiological approaches, we observed that NT stimulates inositol-trisphosphate production that provokes Ca(2+) release from intracellular stores. The resulting increase in cytosolic Ca(2+) concentration ([Ca(2+)](c)) activates the secretion of alpha-melanocyte-stimulating hormone (alpha-MSH), stimulates Ca(2+)-dependent protein kinase (PKC) activity, and provokes a depolarizing chloride efflux. PKC reduces the amplitude, whereas membrane depolarization increases the frequency of L- and N-type Ca(2+) currents underlain by the action potential discharge. The complex regulatory processes exerted by NT on Ca(2+) signaling likely generate discrete variations in the [Ca(2+)](c) at various distances from secretory vesicles, contributing to the fine-tuning of alpha-MSH secretion.

Neurotensin stimulates both calcium mobilization from inositol trisphosphate-sensitive intracellular stores and calcium influx through membrane channels in frog pituitary melanotrophs

Neurotensin (NT) is a potent stimulator of electrical and secretory activities in frog pituitary melanotrophs. The aim of the present study was to characterize the transduction pathways associated with activation of NT receptors in frog melanotrophs. Application of synthetic frog NT (fNT) increased the cytosolic calcium concentration ([Ca2+]c) and stimulated the formation of inositol trisphosphate (IP3). The phospholipase C inhibitor U-73122 blocked the electrophysiological and secretory effects of fNT. Intracellular application of the IP3 receptor antagonist heparin abolished fNT-induced electrical activity. Suppression of Ca2+ in the incubation medium markedly reduced the effect of NT on [Ca2+]c, firing rate, and alpha-melanocyte-stimulating hormone (alphaMSH) secretion. Similarly, the inhibitor of IP3-induced Ca2+ release and store-operated Ca2+ channels, 2-Aminoethoxydiphenylborane, and the nonselective Ca2+ channel blockers GdCl3 and NiCl2, attenuated the [Ca2+]c increase and the electrical and secretory responses evoked by fNT. Coapplication of the L- and N-type Ca2+ channel blockers nifedipine and omega-CgTx GVIA reduced the effects of fNT on action potential discharge, [Ca2+]c increase, and alphaMSH release. The protein kinase C (PKC) inhibitors, PKC-(19-31) and chelerythrine, reduced the electrophysiological and secretory responses induced by iterative applications of fNT. Collectively, these results demonstrate that, in frog melanotrophs, NT stimulates the phospholipase C/PKC pathway and increases [Ca2+]c. Both Ca2+ release from intracellular stores and Ca2+ influx through L- and N-type Ca2+ channels are involved in fNT-induced alphaMSH secretion. In addition, the present data indicate that PKC plays a crucial role in maintenance of the responsiveness of melanotrophs to NT.

Neurotensin modulates the amplitude and frequency of voltage-activated Ca2+ currents in frog pituitary melanotrophs: implication of the inositol triphosphate/protein kinase C pathway

Many excitatory neurotransmitters and neuropeptides regulate the activity of neuronal and endocrine cells by modulating voltage-operated Ca2+ channels. Paradoxically, however, excitatory neuromediators that provoke mobilization of intracellular calcium from inositol trisphosphate (IP3)-sensitive stores usually inhibit voltage-gated Ca2+ currents. We have recently demonstrated that neurotensin (NT) stimulates the electrical and secretory activities of frog pituitary melanotrophs, and increases intracellular calcium concentration in these cells. In the present study, we have investigated the effects of NT on Ca2+ currents in cultured frog melanotrophs by using the perforated patch-clamp technique. Frog neurotensin (f NT) reduced the amplitude and facilitated the inactivation of both L- and N-type Ca2+ currents. Application of the membrane-permeant Ca2+ chelator BAPTA-AM, the sarcoendoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, or the IP3 receptor antagonist 2-APB suppressed the reduction of Ca2+ currents induced by f NT. Incubation of melanotrophs with the diacylglycerol analogue PMA, which causes desensitization of protein kinase C (PKC), or with the PKC inhibitors chelerythrine and calphostin C, reduced the inhibitory effect of f NT. The NT-induced action potential waveforms, applied as voltage-clamp commands, decreased the amplitude of Ca2+ currents, and enhanced Ca2+ influx by increasing the Ca2+ spike frequency. Altogether, these data indicate that the inhibitory effect of f NT on Ca2+ currents results from activation of the IP3/PKC pathway. The observation that NT controls Ca2+ signalling through both amplitude and frequency modulations of Ca2+ currents suggests that NT might induce spacial and temporal changes of intracellular Ca2+ concentration leading to stimulation of exocytosis.

Immunohistochemical localization, biochemical characterization, and biological activity of neurotensin in the frog adrenal gland

The primary structure of neurotensin has been recently determined for the frog Rana ridibunda (Endocrinology 139: 4140-4146, 1998). In the present study, we have investigated the distribution and biochemical characterization of neurotensin-like immunoreactivity in the frog adrenal gland, using an antiserum directed against the conserved C-terminal region of the peptide. Neurotensin-like immunoreactivity was detected in two populations of nerve fibers: numerous varicose fibers coursing between adrenal cells, and a few processes located in the walls of blood vessels irrigating the gland. Reversed-phase HPLC analysis of frog adrenal gland extracts revealed the existence of a major peak of neurotensin-like immunoreactivity that exhibited the same retention time as synthetic frog neurotensin. The possible involvement of neurotensin in the regulation of steroid secretion was studied in vitro using perifused frog adrenal slices. For concentrations ranging from 10(-10) to 10(-5) M, synthetic frog neurotensin increased corticosterone and aldosterone production in a dose-dependent manner (EC50 = 1.2 x 10(-9) M and 5.8 x 10(-10) M, respectively). Repeated administration of neurotensin induced a reproducible stimulation of steroid output without any tachyphylaxis. Prolonged administration (3 h) of frog neurotensin caused a transient increase in corticosterone and aldosterone secretion followed by a decline of corticosteroid secretion. Neurotensin also produced a significant stimulation of corticosteroid secretion from dispersed frog adrenal cells. This study demonstrates that neurotensin is located in nerve processes innervating the adrenal gland of amphibians. The results also show that synthetic frog neurotensin exerts a direct stimulatory effect on corticosteroid output. Taken together, these data support the view that neurotensin, released by nerve fibers, may act as a local regulator of corticosteroid secretion.

The pituitary-skin connection in amphibians. Reciprocal regulation of melanotrope cells and dermal melanocytes

In amphibians, alpha-MSH secreted by the pars intermedia of the pituitary plays a pivotal role in the process of skin color adaptation. Reciprocally, the skin of amphibians contains a number of regulatory peptides, some of which have been found to regulate the activity of pituitary melanotrope cells. In particular, the skin of certain species of amphibians harbours considerable amounts of thyrotropin-releasing hormone, a highly potent stimulator of alpha-MSH release. Recently, we have isolated and sequenced from the skin of the frog Phyllomedusa bicolor--a novel peptide named skin peptide tyrosine tyrosine (SPYY), which exhibits 94% similarity with PYY from the frog Rana ridibunda. For concentrations ranging from 5 x 10(-10) to 10(-7) M, SPYY induces a dose-related inhibition of alpha-MSH secretion. At a dose of 10(-7) M, SPYY totally abolished alpha-MSH release. These data strongly suggest the existence of a regulatory loop between the pars intermedia of the pituitary and the skin in amphibians.

Purification, characterization, and spasmogenic activity of neurotensin from the toad Bufo marinus

Neurotensin (NT) was isolated from an extract of the intestine of the cane toad, Bufo marinus and its primary structure established as: pGlu-Ala-Ile-Val-Ser-Lys-Ala-Arg-Arg-Pro-Tyr-Ile-Leu. This amino acid sequence shows five substitutions (Leu2 --> Ala, Tyr3 --> Ile, Glu4 --> Val, Asn5 --> Ser, and Pro7 --> Ala) compared with bovine NT. Synthetic Bufo NT (pD2 = 8.05 +/- 0.28) was equipotent and equally effective as bovine NT (pD2 = 8.24 +/- 0.38) in producing spasmogenic contraction of isolated segments of toad small intestine. However, the maximum response produced by Bufo NT was only 35 +/- 2% of that produced by substance P. The potencies, but not the maximum responses, to Bufo and bovine NT were significantly (p < 0.05) attenuated by pre-treatment with atropine but neither parameter was significantly diminished by tetrodotoxin and indomethacin. The data suggest that the action of NT involves interaction with receptors on toad intestinal smooth muscle that recognize the C-terminal region of NT (residues 8-13) that has been fully conserved during evolution of tetrapods. Contractile activity is mediated, at least in part, by release of acetylcholine.