Effects of octopamine on fluid secretion by isolated salivary glands of a feeding ixodid tick

Octopamine elicited a dose-related secretory response by salivary glands isolated from the feeding female tick Amblyomma americanum. Half-maximal stimulation occurred at about 60 μM. Phentolamine (10 μM) failed to inhibit the octopamine-mediated response; however, thioridazine (50 μM) inhibited both octopamine (1,000 μM) and dopamine-stimulated (0.1 μM) secretion. Maximal stimulation by dopamine (1.0 μM) showed no further increase in the rate of secretion after adding octopamine (1,000 or 0.1 μM). Glands responded to octopamine (100 μM) with rates significantly lower than controls following exposure to amphetamine (1,000 μM). Octopamine receptors do not appear to mediate the secretory response, and octopamine may stimulate secretion by releasing catecholamines from presynaptic neurons. These results support the hypothesis that dopamine is the natural transmitter mediating fluid secretion in the feeding tick salivary gland.     

Involvement of calcium and cyclic AMP in controlling ixodid tick salivary fluid secretion.

Catecholamine-stimulated salivary fluid secretion (in vitro) by ixodid ticks is reduced by deletion or lowering the concentration of exogenous bathing medium Ca++. The Ca++ antagonist, verapamil, reversibly inhibits dopamine-stimulated secretion. Ionophore A-23187 is unable to induce glands to secrete. Studies in which labeled and unlabeled Ca++ flux were measured indicate that catecholamines induce release of calcium from intracellular stores during secretion. Cyclic AMP/theophylline-stimulated secretion is inhibited by verapamil, and the exclusion of calcium from the support medium. It is concluded that the primary catecholamine stimulus induces cyclic AMP formation and mobilization of Ca++ (intra- and extracellular). Cyclic AMP and calcium are thought to interact to control secretion within the fluid transporting cells of types II and III alveoli.     

Neural control of salivary glands in ixodid ticks

Studies of tick salivary glands (SGs) and their components have produced a number of interesting discoveries over the last four decades. However, the precise neural and physiological mechanisms controlling SG secretion remain enigmatic. Major studies of SG control have identified and characterized many pharmacological and biological compounds that activate salivary secretion, including dopamine (DA), octopamine, γ-aminobutyric acid (GABA), ergot alkaloids, pilocarpine (PC), and their pharmacological relatives. Specifically, DA has shown the most robust activities in various tick species, and its effect on downstream actions in the SGs has been extensively studied. Our recent work on a SG dopamine receptor has aided new interpretations of previous pharmacological studies and provided new concepts for SG control mechanisms. Furthermore, our recent studies have suggested that multiple neuropeptides are involved in SG control. Myoinhibitory peptide (MIP) and SIFamide have been identified in the neural projections reaching the basal cells of acini types II and III. Pigment-dispersing factor (PDF)-immunoreactive neural projections reach type II acini, and RFamide- and tachykinin-immunoreactive projections reach the SG ducts, but the chemical nature of the latter three immunoreactive substances are unidentified yet. Here, we briefly review previous pharmacological studies and provide a revised summary of SG control mechanisms in ticks.     

Control of fluid secretion by isolated salivary glands of the lone star tick.

Adrenaline, noradrenaline, cyclic-AMP, and theophylline stimulated salivary fluid secretion in glands isolated from rapidly engorging females of the lone star tick, Amblyomma americanum. Cyclic-AMP and theophylline effectively initiated secretion only after the glands were pre-incubated and pre-stimulated to secrete with adrenaline. Pilocarpine nitrate, 5-hydroxytryptamine (5-HT), and glutamate did not stimulate glandular secretion, 2,4-Dinitrophenol (DNP) slowed adrenaline's ability to stimulate secretion. Results obtained after additions of harmaline and ouabain point to the possibility of a (Na⁺+K⁺)-ATPase system for helping to move fluid across the glandular epithelium. The significance of these findings to the mechanisms and control of tick salivary glandular function is discussed.     

The effect of avermectins on feeding,salivary fluid secretion,and fecundity in some ixodid ticks

We tested the effects of the potent acaricides, avermectin B₁a (AVM) and 22,23-dihydroavermectin B₁ (ivermectin; IVM) when injected directly into partially fed and fully engorged female ticks. When injected into small ticks (Amblyomma hebraeum Koch), neither drug (up to 100 g/kg b.w.) inhibited subsequent engorgement nor affected oviposition latency, weight of total egg mass laid nor viability of laid eggs. At higher concentrations (1000 and 5000 g/kg b.w.), AVM and IVM were markedly toxic. When injected into engorged ticks, both drugs increased oviposition latency, and reduced fecundity at about 75–100 g/kg b.w. Vitellogenesis, as assessed by a spectrophotometric assay of the ovaries, was not inhibited. Also at 50–100 g/kg b.w., AVM and IVM caused paralysis of the abdominal dorso-ventral muscles and the leg muscles. Both drugs, at 7 days post-injection, proved detrimental to salivary gland function in both small and large ticks, but had little effect on salivary gland weight. At concentrations which did not inhibit oviposition (20–50 g/kg b.w.) many of the eggs dried out even though they were kept at high RH. We then demonstrated inAmblyomma americanum, Dermacentor andersoni andD. albipictus that removal of egg wax (by extraction with hexane) induced a marked increase in water permeability. IVM neither increased water permeability ofD. andersoni eggs nor diminished the amount of egg wax deposited on the surface of the eggs, when injected posteriorly through the alloscutum. However, injection of IVM, dimethylsulphoxide (vehicle for IVM) or distilled water through the articulation between the capitulum and scutum (anterior injection), did markedly reduce the wax coating and increased egg permeability. We suggest that anterior injection damages Gené's organ and thus causes the latter effects.     

Possible role in uptake of water vapour by ixodid tick salivary glands

The mouth is confirmed as the site of water vapor uptake in the lone star tick, Amblyomma americanum. It was shown that the level of chloride (³⁶Cl) increased in the mouthparts of desiccated ticks. The highest levels of ³⁶Cl were found in the mouthparts, salivary glands, and gut tissue during rehydration. It is suggested that ions are secreted by the salivary glands into the mouth where water is picked up hygroscopically by the secretion. It is further suggested that the water and ions are then swallowed and absorbed from the lumen of the gut.     

Effects of the high-molecular-weight cryoprotectant dextran on fluid secretion by Calliphora salivary glands

The rate of fluid secretion by isolated salivary glands of Calliphora was inhibited as a linear function of dextran and poly vinyl pyrrolidone concentrations in the range 15–35% w/w. This inhibition was not overcome by supramaximal concentrations of 5-hydroxytryptamine, nor was it caused by a decreased availability of K⁺ from the medium. Although the polymers caused large decreases of freezing point (and vapor pressure) of the incubation medium, the glands did not respond to this by secreting a more K ⁺-rich saliva. When dextran and polyvinyl pyrrolidone were added as powders to salt solutions, the total freezing-point depression of the mixture was equal to the sum of that exerted by the pure salt solution and that expected for the polymer concentration. The activities of K⁺ and Cl^?, as measured by ion-selective electrodes, were not increased in solutions by the addition of dextran. Dextran was demonstrated by electron microscopy to penetrate into the basal clefts and intercellular spaces of the isolated glands. These results demonstrate that addition of dextran (and probably of polyvinyl pyrrolidone) does not decrease the solvent activity of water in physiological salt solutions. The inhibition of fluid secretion by isolated salivary glands of Calliphora seems therefore due only to the altered physical characteristics of the medium.     

Amblyomma americanum (L.): protein kinase C-independent fluid secretion by isolated salivary glands.

Protein kinase C activity was partially purified from tick salivary glands by fast protein liquid chromatography anion-exchange chromatography. Enzyme activity was stimulated by Ca2+, phosphatidylserine, and diacylglycerol with the highest activity observed in the presence of all three modulators. Enzyme activity was inhibited by a synthetic pseudosubstrate peptide with an amino acid sequence resembling the protein kinase C substrate phosphorylation site. The protein kinase C activator, 1-oleoyl-2-acetyl-sn-glycerol (OAG), when added to whole in vitro salivary glands previously prelabeled with 32P, stimulated the phosphorylation of salivary gland proteins. Activators of protein kinase C (phorbol ester or OAG) did not stimulate fluid secretion by isolated tick salivary glands. OAG and phorbol ester had only minimal affects on the ability of dopamine to stimulate secretion by isolated salivary glands and dopamine's ability to increase salivary gland cyclic AMP.     

Potentiation of salivary fluid secretion in ixodid ticks: a new receptor system for gamma-aminobutyric acid

gamma-Aminobutyric acid (GABA), having minimal intrinsic activity, potentiates dopamine-induced fluid secretion in salivary glands of female ixodid ticks. Because the effect of GABA was similar to that of spiperone, we tested whether these two drugs act at a common recognition site. Potentiation was not augmented when salivary glands were exposed to supramaximal concentrations of spiperone (1 microM) plus GABA (100 microM). (+/-)-Sulpiride (100 microM), a spiperone antagonist in this system, also blocked GABA-induced potentiation. Picrotoxin (100 microM) and (-)-bicuculline (100 microM), two GABA antagonists, blocked GABA-induced and spiperone-induced potentiation. Inhibition of GABA by picrotoxin and (-)-bicuculline was noncompetitive. Muscimol (an agonist at GABAA receptors) also potentiated dopamine-induced secretion. Baclofen (an agonist at GABAB receptors) did not elicit potentiation. We suggest that GABA may function as a neuromodulator for dopamine-induced fluid secretion in tick salivary glands.     

Autonomic effects on protein secretion by rat submandibular salivary glands

1. Following treatment with cholinergic and beta-adrenergic drugs, the beta-type protein, associated with cAMP, was secreted regardless of the doses used. 2. Following treatment with alpha 1-adrenergic drugs, both the beta-type and alpha-type proteins were secreted depending on the doses used and the alpha-type protein was completely converted to the beta-type with alpha-blockers. 3. Following treatment with alpha 2-adrenergic drugs, the gamma-type protein, associated with cGMP, was secreted independent of the doses used.     

Salivary glands in ixodid ticks: control and mechanism of secretion

The salivary glands are vital to the biological success of ixodid ticks and the major route for pathogen transmission. Important functions include the absorption of water vapor from unsaturated air by free-living ticks, excretion of excess fluid for blood meal concentration, and the secretion of bioactive protein and lipid compounds during tick feeding. Fluid secretion is controlled by nerves. Dopamine is the neurotransmitter at the neuroeffector junction regulating secretion via adenylate cyclase and an increase in cellular cAMP. Dopamine also affects the release of arachidonic acid which is subsequently converted to prostaglandins. Prostaglandin E(2) (PGE(2)) is secreted at extremely high levels into tick saliva for export to the host where it impacts the host physiology. Additionally, PGE(2) has an autocrine or paracrine role within the salivary gland itself where it interacts with a PGE(2) receptor to induce secretion (exocytosis) of bioactive saliva proteins via a phosphoinositide signalling pathway and an increase in cellular Ca(2+). Regulation of fluid secretion has been extensively studied, but little is known about the mechanism of fluid secretion. Continuing advances in tick salivary gland physiology will be made as key regulatory and secretory gland proteins are purified and/or their genes cloned and sequenced.     

Increased secretion of salivary glands produced by facial vibrotactile stimulation

Patients with low-back pain can be evaluated immediately by means of an electrical tool that produces bony vibration to the lumbar spinal processes (Yrjama M, Vanharanta H. Bony vibrotactile stimulation: A new, non-invasive method for examining intradiscal pain. European Spine Journal 1994;3:233–235). In the rehabilitation of masticatory disturbance and dysphagia, an electric toothbrush is commonly used as an oral motor exercise tool for the facilitation of blood flow and metabolism in the orofacial region in Japanese hospitals. However, subjects receiving vibration in the facial regions reported increased salivary secretion. We attempted to develop an oral motor exercise apparatus modified by a headphone headset that was fixed and could be used for extended periods. The vibration apparatus of the heating conductor is protected by the polyethyle methacrylate (dental mucosa protective material), and electric motors for vibration control of the PWM circuit. We examined the amount of salivation during vibration stimuli on the bilateral masseter muscle belly, using a cotton roll positioned at the opening of the secretory duct for 3 min. Although the quantity of salivation in each subject showed various and large fluctuations in the right and left sides of the parotid and submandibular and sublingual glands, one or more of the salivary glands were effectively stimulated by 89 Hz vibration. The reported apparatus will be useful as an additional method in orofacial rehabilitation.     

The effects of m-octopamine on salivary flow rates and protein secretion by rat submandibular glands

1. m-Octopamine given i.v. or i.p. was a potent sialogogue for rat salivary glands.2. Salivation in response to i.v. m-octopamine was completely abolished by prazosin and phenoxybenzamine.3. The α-type of proteins were secreted in response to all doses of i.v. and i.p. m-octopamine and these were converted into the β-type with prazosin, but not with yohimbine.4. m-Octopamine stimulated both α- and β-adrenoceptors and was a much more selective α₁-agonist than was the p-isomer.