Neurosecretory system of the American Dog Tick, Dermacentor variabilis (Acari: Ixodidae). ii. Distribution of secretory cell types, axonal pathways and putative neurohemal-neuroendocrine associations; comparative histological and anatomical implications

Isolation and electrophysiological recording of Ixodes ricinus synganglion neurons

The central nervous system of hard ticks (Ixodidae) consists of a concentrated merged nerve mass known as the synganglion. Although knowledge of tick neurobiology has dramatically improved over the last two decades, this is the first time that isolation and electrophysiological recordings have been carried out on tick neurons from the synganglion. Method: We developed a simple protocol for synganglion neuron isolation and used a whole-cell patch clamp to measure ionic currents induced by acetylcholine, nicotine and muscarine. Relatively large neurons (∼ 25 μm and ∼ 35 μm) were isolated and 1 mM acetylcholine was used to induce strong inward currents of -0.38 ± 0.1 nA and - 1.04 ± 0.1 nA, respectively, with the corresponding cell capacitances being at around 142 pF and 188 pF. In addition, successive application of 1 mM acetylcholine through ∼25 μm and ∼ 35 μm cells for increasing amounts of time resulted in a rapid reduction in current amplitudes. We also found that acetylcholine-evoked currents were associated with a reversible increase in intracellular calcium levels for each neuronal type. In contrast, 1 mM muscarine and nicotine induced a strong and non-reversible increase in intracellular calcium levels. This study serves as a proof of concept for the mechanical isolation of tick synganglion neurons followed by their electrophysiological recording. This approach will aid investigations into the pharmacological properties of tick neurons and provides the tools needed for the identification of drug-targeted sites and effective tick control measures.

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.

Identification of a complex peptidergic neuroendocrine network in the hard tick, Rhipicephalus appendiculatus

Neuropeptides are crucial regulators of development and various physiological functions but little is known about their identity, expression and function in vectors of pathogens causing serious diseases, such as ticks. Therefore, we have used antibodies against multiple insect and crustacean neuropeptides to reveal the presence of these bioactive molecules in peptidergic neurons and cells of the ixodid tick Rhipicephalus appendiculatus. These antibodies have detected 15 different immunoreactive compounds expressed in specific central and peripheral neurons associated with the synganglion. Most central neurons arborize in distinct areas of the neuropile or the putative neurohaemal periganglionic sheath of the synganglion. Several large identified neurons in the synganglion project multiple processes through peripheral nerves to form elaborate axonal arborizations on the surface of salivary glands or to terminate in the lateral segmental organs (LSO). Additional neuropeptide immunoreactivity has been observed in intrinsic secretory cells of the LSO. We have also identified two novel clusters of peripheral neurons embedded in the cheliceral and paraspiracular nerves. These neurons project branching axons into the synganglion and into the periphery. Our study has thus revealed a complex network of central and peripheral peptidergic neurons, putative neurohaemal and neuromodulatory structures and endocrine cells in the tick comparable with those found in insect and crustacean neuroendocrine systems. Strong specific staining with a large variety of antibodies also indicates that the tick nervous system and adjacent secretory organs are rich sources of diverse neuropeptides related to those identified in insects, crustaceans or even vertebrates.

Serotonin-like immunoreactivity in the central nervous system of two ixodid tick species

Immunocytochemistry was used to describe the distribution of serotonin-like immunoreactive (5HT-IR) neurons and neuronal processes in the central nervous system (CNS), the synganglion, of two ixodid tick species; the winter tick, Dermacentor albipictus and the lone star tick, Amblyomma americanum. 5HT-IR neurons were identified in the synganglion of both tick species. D. albipictus had a significantly higher number of 5HT-IR neurons than A. americanum. The labeling pattern and number of 5HT-IR neurons were significantly different between sexes in D. albipictus, but were not significantly different between sexes in A. americanum. 5HT-IR neurons that were located in the cortex of the synganglion projected processes into the neuropils, invading neuromeres in the supraesophageal ganglion including the protocerebrum, postero-dorsal, antero-dorsal and cheliceral neuromeres. In the subesophageal ganglion, dense 5HT-IR neuronal processes were found in the olfactory lobes, pedal, and opisthosomal neuromeres. Double-labeling with neurobiotin backfilled from the first leg damaged at the Haller's organ revealed serotoninergic neuronal processes surrounding the glomeruli in the olfactory lobes. The high number of the 5HT-IR neurons and the extensive neuronal processes present in various regions of the synganglion suggest that serotonin plays a significant role in tick physiology.

Immunocytochemical mapping of FMRFamide-like peptides in the argasid tick Ornithodoros parkeri and the ixodid tick Dermacentor variabilis

FMRFamide-like immunoreactivity was studied in the argasid tick Ornithodoros parkeri and the ixodid tick Dermacentor variabilis using immunocytochemistry based on the peroxidase-antiperoxidase method. FMRFamide-like immunoreactive cells are widely distributed in various regions of the tick synganglion including protocerebral, cheliceral, stomodeal, palpal, pedal I-IV, and opisthosomal regions in both species. However, there is one layer of immunoreactive cells located on the dorsal surface of the postoesophageal part of the synganglion that is found only in D. variabilis. Besides the immunoreactivity within the cell body and its axons, the neuropile and the neural lamella (the extracellular sheath of the synganglion) are rich in immunoreactive materials. Some coxal muscles are innervated by the FMRFamide-like immunoreactive processes of the nerve from the pedal ganglion.

Localization of insulin-like immunoreactivity in the synganglion of nymphal and adult Dermacentor variabilis (Acari: Ixodidae)

Immunocytochemical staining based on a peroxidase-antiperoxidase method showed neurosecretory cells (NSC) reactive to bovine insulin in five of 18 paraldehyde fuchsin-positive neurosecretory regions (NSR) in the synganglion of unfed adult Dermacentor variabilis. This is the first report of a neuropeptide in an ixodid tick. The insulin-specific immunoreactive cells included the posterior medial group of the protocerebral center, posterior group of dorsal opisthosomal center, anterior lateral group of the dorso-lateral cheliceral center, dorsal group of the frontal stomodeal center, and anterior group of the ventral palpal center. After feeding and mating, females no longer had immunoreactive cells in three of five NSR found in virgin, unfed females. However, two cells of the posterior group in dorsal opisthosomal center and anterior lateral group of the dorso-lateral cheliceral center remained immunoreactive throughout feeding. Fed, mated males continued to display immunoreactive cells in four of five NSR found in the virgin, unfed males. All developmental stages of nymphs examined had insulin-specific immunoreactive cells in two of the five NSR found in unfed adults, including two positively stained cells of the posterior group in dorsal opisthosomal center and anterior group of ventral palpal neurosecretory center.

Anatomy of synganglia, including their neurosecretory regions, in unfed, virgin female Ixodes scapularis say (Acari: Ixodidae)

The central nervous system of Ixodes scapularis is fused into a single compact synganglion. The esophagus runs through the synganglion and divides it into supraesophageal and subesophageal parts. The supraesophageal portion contains a single protocerebrum with four pairs of glomeruli, paired optic lobes and cheliceral ganglia, and a single stomodeal bridge. The subesophageal portion contains a centrally located network of commissures and connectives, a pair of palpal ganglia, paired olfactory lobes of the first pedal ganglia, four pairs of pedal ganglia, and a single opisthosomal ganglion. A retrocerebral organ complex (ROC) in close vicinity of the digestive tract, as described in some other tick species, apparently is lacking. Perhaps the function of the ROC is performed by the paired, large, ganglion-like bodies that lie anterolaterad to the cheliceral ganglia. The rind, which is formed from the neuronal somata and glial cells, surrounds the central fibrous core or neuropile. Neurosecretory cells (NSC) are distinct among rind cells due to their large size and concentration of cytoplasmic neurosecretions. NSC are present throughout the synganglion, although the subesophageal portion contains larger groups of these cells. Histological serial sections, stained with Meola's (Trans Am Microsc Soc 89:66-71, '70) paraldehyde fuchsin (PAF) procedure revealed 24 PAF-stained, putative neurosecretory regions in the synganglion of virgin, unfed females. All of these regions appear to be connected and associated with the nearest ganglion and are correspondingly named. Eighteen PAF-positive regions occur in the synganglion. In addition, PAF-negative (green-stained) cells occupy 6 distinct regions in the synganglion of unfed, unmated females.

Neurobiology of arthropod parasites

Many medically important diseases of man are caused by blood-sucking arthropods which serve as vectors for a wide range of viral, bacterial, protozoal and nematode infections (Table 1). Furthermore, serious economic losses are caused by the numerous arthropod parasites which infect domesticated animals (for examples, see Table 2). Among these the ixodid hard ticks are particularly important and it has been estimated that the global cost of hard tick infections is $7000 million per annum (F.A.O., 1984). Not surprisingly, there have been strenuous efforts to control infections caused by arthropods.

Synganglial and neurosecretory morphology of the chicken mite Dermanyssus gallinae (DeGeer) (Mesostigmata: Dermanyssidae)

Histological techniques and paraldehyde-fuchsin (PAF) staining were used to study the synganglion and to locate neurosecretory regions and neurosecretion within the synganglion of the chicken mite, Dermanyssus gallinae. The synganglion, which is formed internally by neuropilar ganglia, gives rise to a single esophageal and paired cheliceral, palpal, pedal (I-IV), and opisthosomal nerves. The neuropilar ganglia are interconnected by commissures and connectives within the synganglion. Twelve PAF-positive neurosecretory regions are present in unfed protonymphs, unfed deutonymphs, virgin males and females, and mated males. There are 11 PAF-positive neurosecretory regions in larvae, 24-72 hours post-fed deutonymphs and mated females. Neurosecretory regions in these developmental stadia are described in relation to their positions adjacent to individual neuropilar ganglia.

Ultrastructural identification of neurohaemal sites in a tick: evidence that the dorsal complex may be a true endocrine gland

A fine structural study has been made of the 'paraganglionic plates' associated with the perineurium and of the 'retrocerebral organ' associated with the periganglionic sheath of the tick Boophilus microplus; these structures have been postulated, from descriptions by light microscopy, to be tick neurohaemal organs. Neurosecretory terminals are observed frequently in the neural lamella/perineurial sheath, particularly in a dorso-lateral area which may correspond to the 'paraganglionic plates'. No evidence was found of a discrete peripheral neurohaemal organ such as the corpus cardiacum of insects. The 'retrocerebral organ' is comprised of periganglionic sheath cells, which appear to be glandular rather than neurohaemal, and peripheral ganglionic cells.

The subgenus Persicargas (Ixodoidea: Argasidae: Argas). 35. The lateral segmental organs and peritracheal gland in immature and adult A. (P.) arboreus

Lateral segmental organs and a peritracheal gland in adult and immature Argas (Persicargas) arboreus are described and compared with similar organs in other arthropods. Four pairs of lateral segmental organs containing neuronal cell bodies and innervated by branches of hemal nerves from pedal nerve roots are present in nymphal instars and in the adult; 3 pairs are present in the larva. In each postembryonic developmental stage, the peritracheal gland consists of cell masses with neurosecretory granular activity and is associated with the tracheal plexus in the region of the central nerve mass and adjacent hypodermis. The glandular cells show cyclic changes in size and activity related to the molting process in immature stages and probably to other events in the adult.

The subgenus Persicargas (Ixodoidea: Argasidae: Argas). 36. structure and postembryonic development of the neurohemal organ in A. (P.) arboreus

The structure and postembryonic development of the paraesophageal neurohemal organ lying posteriad to the central nerve mass of Argas (Persicargas) arboreus are described and compared with other arthropod neurohemal-endocrine organs. During postembryonic development, a few large cells differentiate in the larval esophageal epithelium and gradually multiply to form a compact cell mass. In the first nymphal instar, cell mass evagination forms the dorsal lobe of the neurohemal organ which develops in the second instar and attains its final lobular structure in the third instar. These observations provide evidence for homology to the insect corpora cardiaca and related organs in other arthropods.

Ultrastructural evidence for the endocrine nature of the lateral organs of the cattle tick Boophilus microplus

The lateral organs of the tick Boophilus microplus were previously thought to have a neurohaemal function, but the present study shows that they consist of glandular cells which contain a rich system of smooth endoplasmic reticulum (SER) and Golgi but no indication of neurosecretory production or release. There is acid phosphatase activity throughout the SER as well as in Golgi and a major function of the latter may be the production of lysosomal enzymes. It is suggested that the organs are endocrine glands and that, in engorged females, may secrete a hormone involved in the control of vitellogenesis. The organs are more active in feeding than in unfed males and a related function could be in control of the development of genital organs or spermatogenesis. Also present in the cells are coated vesicles, lipid droplets and microtubules. Coated vesicles close to Golgi are probably primary lysosomes whereas those near the periphery are shown by ferritin tracer to arise from coated pits. Pinocytosis could be involved in membrane retrieval but, in the absence of evidence for exocytosis, this seems unlikely. It is tentatively proposed that, by analogy with vertebrate and insect endocrine glands, the lateral organs may take up hormone precursor via coated vesicles for storage in lipid droplets and conversion to hormone in the SER. As in other SER-rich endocrine glands, the release mechanism for the hormone or other secretory product of the lateral organs is uncertain. Both the steroid, ecdysone and the terpenoid, juvenile hormone, are discussed as possible candidates for the lateral organ hormones.

Perineurial and glial cells in the tick Boophilus microplus (Acarina: Ixodidae): freeze-fracture and tracer studies

In the cattle tick Boophilus microplus, the cells of the perineurium are characterized by accumulations of glycogen which increase dramatically after feeding. Gap junctions couple both these perineurial cells which enshealth the C.N.S. and the underlying glial cells. No tight junctions have been found between perineurial cells and there is in consequence no blood-brain barrier. Using ionic lanthanum as a tracer the extensive gap junctions are shown to have no occluding effect and lanthanum penetrates through the perineurium and glial layers to the level of the axonal surfaces. By colloidal lanthanum impregnation and freeze-fracture studied, the gap junctions appear to be typical of arthropids in that their particles show a characteristic diameter (13 nm in freeze-fracture), are distributed relatively loosely within the junctional plaques and fracture onto the E face of the junctional membranes. Semi-ordered particle arrays are found on E face membranes of adjacent axons and glia which may represent axoglial junctions.