Calcitonin gene-related peptide stimulates intracellular cAMP via a protein kinase C-controlled mechanism in human ocular ciliary epithelial cells

Aqueous Humor Dynamics Associated with the Phorbol Ester-Induced Decrease in Intraocular Pressure in the Rabbit

PURPOSE

To determine the effects of injection of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) into the anterior chamber of the eye on intraocular pressure (IOP) and aqueous humor dynamics.

METHODS

IOP was measured for 24 h after intracameral injection of PMA (3 to 50 pmol) in unanesthetized rabbits. Aqueous humor dynamics (aqueous flow, total outflow facility, and uveoscleral outflow) were determined approximately 6 h after injection of 50 pmol of PMA in animals pretreated with indomethacin.

RESULTS

Intracameral injection of 50 pmol of PMA induced a biphasic effect on IOP, consisting of a transient increase apparent at 0.5 and 1 h and a sustained decrease apparent after 2 h. This effect of PMA was dose dependent. Whereas pretreatment with indomethacin attenuated the PMA-induced increase in IOP, the sustained decrease in IOP remained apparent in the pretreated rabbits. Intracameral injection of 4alpha-PMA, an inactive PMA analog, had no effect on IOP. PMA also significantly increased uveoscleral outflow, but it had no effect on aqueous flow or total outflow facility.

CONCLUSION

Intracameral injection of PMA reduced IOP in the rabbits by increasing the rate of uveoscleral outflow. This IOP-lowering effect of PMA may be mediated by activation of PKC.

Calcitonin gene-related peptide receptor activation produces PKA- and PKC-dependent mechanical hyperalgesia and central sensitization

Calcitonin gene-related peptide (CGRP), acting through CGRP receptors, produces behavioral signs of mechanical hyperalgesia in rats and sensitization of wide dynamic range (WDR) neurons in the spinal cord dorsal horn. Although involvement of CGRP receptors in central sensitization has been confirmed, the second-messenger systems activated by CGRP receptor stimulation and involved in pain transmission are not clear. This study tested whether the hyperalgesia and sensitizing effects of CGRP receptor activation on WDR neurons are mediated by protein kinase A or C (PKA or PKC) signaling. Intrathecal injection of CGRP in rats produced mechanical hyperalgesia, as shown by paw withdrawal threshold tests. CGRP-induced hyperalgesia was attenuated significantly by the CGRP1 receptor antagonist, CGRP8-37. The effect was also attenuated significantly by a PKA inhibitor (H89) or a PKC inhibitor (chelerythrine chloride). Electrophysiological experiments demonstrated that superfusion of the spinal cord with CGRP-induced sensitization of spinal dorsal horn neurons. The CGRP effect could be blocked by CGRP8-37. Either a PKA or PKC inhibitor (H89 or chelerythrine) also attenuated this effect of CGRP. These results are consistent with the hypothesis that CGRP produces hyperalgesia by a direct action on CGRP1 receptors in the spinal cord dorsal horn and suggest that the effects of CGRP are mediated by both PKA and PKC second-messenger pathways.

Calcitonin gene-related peptide relaxes rabbit iris dilator smooth muscle via cyclic AMP-dependent mechanisms: cross-talk between the sensory and sympathetic nervous systems

PURPOSE

The purpose of these studies is to determine whether or not cyclic AMP is involved in the relaxant action of calcitonin gene-related peptide (CGRP) in rabbit iris dilator muscle.

METHODS

Iris dilator muscle isolated from rabbit was used. Accumulation of cAMP and cGMP in the tissue extracts was measured by radioimmunoassay (RIA), IP3 production was measured by ion-exchange chromatography, and changes in tension were recorded isometrically.

RESULTS

CGRP, vasoactive intestinal peptide, prostaglandin E2, isoproterenol and forskolin (1 microM of each) increased cAMP accumulation by 136, 256, 78, 141 and 315%, respectively. CGRP dose-dependently increased cAMP accumulation (EC50 = 5.25 nM), inhibited IP3 production (EC50 = 5.4 nM) and induced relaxation (EC50 = 10 nM) in muscle precontracted with norepinephrine (NE) (10 microM). Prostaglandin E2, isoproterenol and forskolin also induced relaxation. CGRP stimulated cAMP formation either in the presence or absence of 3-isobutyl-1-methylxanthine (IBMX), a cAMP phosphodiesterase inhibitor, in a time- and concentration-dependent manner. The neuropeptide had no effect on cGMP accumulation. CGRP (8-37), a CGRP receptor antagonist, reversed the relaxant action of the neuropeptide and inhibited CGRP-induced cAMP accumulation in a concentration-dependent manner (IC50 = 12.5 nM). 2',5'-dideoxyadenosine (DDA), a specific adenylate cyclase inhibitor, significantly reduced the inhibitory actions of CGRP on NE-induced contraction and IP3 production and inhibited CGRP-induced cAMP accumulation in a concentration-dependent manner (IC50 = 6.9 nM).

CONCLUSIONS

These results strongly suggest that cAMP mediates the relaxant action of CGRP in rabbit iris dilator. The mechanism of cAMP inhibition of NE-induced IP3 production and contraction is unclear. Modulation of alpha 1-adrenergic function in the iris dilator by CGRP-induced cAMP formation is yet another example of cross-talk between the cAMP and IP3-Ca2+ second messenger systems, it demonstrates a cross-talk between the sympathetic and sensory nervous systems. CGRP-containing sensory nerve fibers could play an important role in regulation of smooth muscle function in the iris-ciliary body.

Acetylcholine receptors in innervated muscles of dystrophic mdx mice degrade as after denervation

Acetylcholine receptors (AChRs) are present at the top of the postsynaptic membrane of the neuromuscular junction (NMJ) at very high density, possibly anchored to cytoskeletal elements. The present study investigated whether AChR degradation is affected in animals lacking dystrophin, a protein that is an integral part of the cytoskeletal complex and is missing in Duchenne muscular dystrophy. The animal model for Duchenne muscular dystrophy, the mutant mdx mouse, was used to determine whether disruption of the cytoskeleton, caused by the absence of dystrophin, affects AChR degradation. Of the two populations of junctional AChRs, Rs (expressed in innervated adult muscles) and Rr (expressed in embryonic or denervated muscles), only Rs are affected in mdx animals. In innervated mdx soleus, diaphragm, and sternomastoid muscles, the AChRs have an accelerated degradation rate (t1/2 of approximately 3-5 d), similar to that acquired by Rs in control muscles after denervation. The Rs in mdx NMJs do not accelerate further when the muscles are denervated. The absence of dystrophin does not affect the degradation rate of the Rr AChRs (t1/2 of 1 d), which are expressed after denervation in mdx as in control muscles. These results suggest that dystrophin or an intact cytoskeletal complex may be required for neuronal stabilization of Rs receptors at the adult neuromuscular junctions.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Innervation of melanocytes in human skin

Communication between the nervous system and epidermal melanocytes has been suspected on the basis of their common embryologic origin and apparent parallel involvement in several disease processes, but never proven. In this study, confocal microscopic analysis of human skin sections stained with antibodies specific for melanocytes and nerve fibers showed intraepidermal nerve endings in contact with melanocytes. This intimate contact was confirmed by electron microscopy, which further demonstrated thickening of apposing plasma membranes between melanocytes and nerve fibers, similar to synaptic contacts seen in nervous tissue. Since many intraepidermal nerve fibers are afferent nerves that act in a "neurosecretory" fashion through their terminals, cultured human melanocytes were stimulated with calcitonin gene-related peptide (CGRP), substance P, or vasoactive intestinal peptide, neuropeptides known to be present in cutaneous nerves, to examine their possible functions in the epidermal melanin unit. CGRP increased DNA synthesis rate of melanocytes in a concentration- and time-dependent manner. Cell yields after 5 d were increased 25% compared with controls maintained in an otherwise optimized medium. Furthermore, stimulation by CGRP induced rapid and dose-dependent accumulation of intracellular cAMP, suggesting that the mitogenic effect is mediated by the cAMP pathway. These studies confirm and expand a single earlier report in an animal model of physical contact between melanocytes and cutaneous nerves and for the first time strongly suggest that the nervous system may exert a tonic effect on melanocytes in normal or diseased human skin.

Calcitonin gene-related peptide induced relaxation of the rabbit iris dilator muscle

Calcitonin gene-related peptide (CGRP) and Substance P (SP) -immunoreactive nerves have been found in the anterior uvea of various mammalian species. Although SP is known to play a major role in control of pupil motility in rabbits, little is known about the effect of CGRP on the iris smooth muscles. We isolated iris sphincter and dilator muscles from rabbit eyes and investigated the mechanical responses and intracellular cyclic AMP (cAMP) levels in these muscles. CGRP (up to 0.1 microM) had no effect on either the resting muscle tone or the amplitude of contraction evoked by field stimulation of the sphincter. On the other hand, CGRP (0.1 microM) relaxed dilator muscle which had been pre-contracted by phenylephrine and reduced the amplitude of contraction evoked by field stimulation. These responses were antagonized by CGRP (8-37), a CGRP antagonist. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), dose-dependently inhibited the contraction evoked by field stimulation. However, 3 microM IBMX had no effect on CGRP inhibition of twitch contraction in this preparation. CGRP had little effect on cAMP production in dilator muscle either with or without IBMX. In conclusion, the miosis which occurs during an ocular inflammatory response, when both CGRP and SP are thought to be released from terminals of sensory neurons, results from CGRP relaxation of the dilator and from the strong contractile effect of SP on the sphincter. Adenylate cyclase activation does not seem to be involved in the relaxant effect of CGRP on the dilator.

Specific induction of cAMP in Langerhans cells by calcitonin gene-related peptide: relevance to functional effects

Epidermal Langerhans cells (LC) are associated anatomically with epidermal nerves, and a product of these nerves, calcitonin gene-related peptide (CGRP), inhibits the antigen-presenting capacity of LC and macrophages. As the CGRP receptor appears to be coupled to Gs alpha protein, which in turn activates adenylate cyclase, the ability of CGRP to induce cAMP in LC was examined and correlated with functional effects. LC were isolated from murine epidermal cells using antibodies on magnetic microspheres. Exposure to CGRP induced a significant increase in cAMP content, which could be inhibited by coculture with a truncated form of CGRP [CGRP-(8-37)] that is a specific competitive inhibitor of CGRP. Substance P and calcitonin failed to induce cAMP in LC. Although culture in CGRP reduced the ability of murine epidermal cells enriched for LC content to present pigeon cytochrome c to a responsive clone or to present antigen for elicitation of delayed-type hypersensitivity in immune mice, culture in forskolin had little or no effect on antigen presentation despite increased cAMP content of LC as much or more than that induced by CGRP. The effect of CGRP on antigen presentation in these systems could be blocked with CGRP-(8-37). CGRP inhibited the induction of B7-2 by lipopolysaccharide on peritoneal macrophages and a LC line, whereas calcitonin did not. CGRP induces specific accumulation of cAMP in LC and inhibits LC antigen-presenting function by a receptor-mediated event. However, the induction of cAMP by itself does not account for inhibition of antigen presentation. Suppression of the expression of B7-2 may be one mechanism by which CGRP inhibits antigen presentation.

Regulation of cyclic AMP production in adult human ciliary processes

Cyclic AMP production in intact ciliary processes from elderly human donors is subject to stimulatory and inhibitory control by various agents. Stimulation of cAMP production is observed with forskolin, vasoactive intestinal peptide, or the beta-adrenergic agonist isoproterenol. Inhibition of forskolin-stimulated cAMP production is observed with endothelin-2 or PAC. The inhibitory effect of PAC is blocked by the specific alpha 2-adrenergic antagonist, yohimbine. Endothelin-2 has no effect on basal cAMP production. These data document the positive and negative regulation of cAMP responses in adult human ciliary processes and support the idea that cAMP is a key intermediate in the regulation of aqueous humor formation.

Calcitonin gene-related peptide receptors in human gastrointestinal epithelia

1. The secretory responses to calcitonin gene-related peptide (CGRP) receptor agonists have been characterized in two human adenocarcinoma cell lines, namely HCA-7 and Colony-29 (Col-29) epithelia. These cells form polarized epithelial layers when grown on permeable supports and allow changes in electrogenic ion transport in response to agonists to be monitored continuously. 2. alpha-CGRP (rat and human sequences), rat beta-CGRP and human [Tyr0]CGRP applied to the basolateral surface were found to be full agonists, causing prolonged increases in short-circuit current. Concentration-response curves exhibited EC50 values of 0.6-1.5 nM in HCA-7 cells. The same agonists were less effective in Col-29 epithelia, the EC50 values ranging from 1 to 10 nM in these cells. [Cys(ACM)2,7]CGRP was effective in both cell lines and was more potent in HCA-7 cells. 3. CGRP receptors were preferentially located on the basolateral surface in both cell types. Addition of r alpha-CGRP to the apical domain produced significantly smaller secretory responses (8.1% in HCA-7 and 29.2% in Col-29) compared with those produced following basolateral application (100%). 4. In both cell lines r alpha-CGRP-elevated short-circuit current was inhibited by the loop diuretic piretanide (200 microM) and by somatostatin (100 nM). Pretreating epithelia with the cyclo-oxygenase inhibitor, piroxicam (5 microM) had no significant effect upon CGRP responses in either cell line. 5. Rat alpha-CGRP (0.2 nM) responses in HCA-7 epithelia were inhibited by the C-terminal fragment CGRP(8-37) (1 microM). Pretreatment of Col-29 cells with CGRP(8-37) did not, however, alter the size or profile of responses to r alpha-CGRP (1 nM).6. We conclude that high-affinity CGRP receptors exist on the basolateral surface of both cell lines,however they differ in their sensitivity to CGRP(8-37) and agonist orders of potency. Thus different CGRP receptor subtypes appear to predominate in these two epithelial cell types.

High affinity vasoactive intestinal peptide receptors on fetal human nonpigmented ciliary epithelial cells

The effect of vasoactive intestinal peptide (VIP) on stimulation of adenylyl cyclase in fetal human nonpigmented ciliary epithelial (NPE) and pigmented ciliary epithelial (CPE) cells was studied. 1 microM VIP elicited a 5-10 fold increase in intracellular cAMP in NPE cells from three fetal donors, but caused little or no response in CPE from two fetal donors and other ocular cell types employed as controls. Appearance of cAMP in the extracellular medium was stimulated in NPE but not in CPE in response to VIP. Both NPE and CPE gave similar cAMP responses (8-13 fold) to the beta-adrenergic agonist, isoproterenol. Binding studies of [125I]VIP to intact NPE and CPE revealed that VIP bound to NPE cells at a high affinity site (KD = .33 nM and a low affinity site (KD = 16 nM), whereas VIP bound to CPE cells only at the low affinity site (KD = 18 nM). In NPE cells, VIP stimulated cAMP formation with an EC50 of approximately 0.6-1 nM, similar to the high affinity binding site KD, with maximal stimulation at 10 nM. Four peptides with various degrees of sequence homology to VIP were also studied. Of these, PHM and PHI stimulated cAMP with EC50s of 50 and 300 nM, respectively, while secretin and glucagon stimulated only at concentrations above 0.1 microM. These results suggest that in fetal human ciliary epithelium, as in rabbit ciliary epithelium (Mittag et al., J Pharm Exp Ther 241: 230, [1987]), VIP stimulation of adenylyl cyclase is a characteristic of NPE but not CPE cells.