Ecdysteroid receptors in the central nervous system of Manduca sexta: their changes in distribution and quantity during larval-pupal development

Whole-body and microscopic autoradiography to determine tissue distribution of biopharmaceuticals -- target discoveries with receptor micro-autoradiography engendered new concepts and therapies for vitamin D

Information about the distribution of biopharmaceuticals is basic for understanding their actions. Tissue and cellular localization is a key to function. Autoradiography with radiolabeled compounds has provided valuable information with both low resolution whole-body macro-autoradiography and high resolution microscopic autoradiography (micro-autoradiography). Whole-body macro-autoradiography is a uniform and expedient single method approach, providing convenient dose- and time-related overviews with data similar to those obtained with conventional bioassays - and therefore widely used. However, whole-body macro-autoradiography, like common bioassays, has limitations. High specificity-low capacity sites of binding and deposition frequently remain unrecognized. Lack of cellular resolution can cause false negatives and provide misleading results (e.g., false blood-brain barrier). For micro-autoradiography, different methods are advertised in the literature. Most of them are, however, unsuited for drug localization because of inadequate resolution and frequent artifacts. Most drugs interact with their receptors non-covalently by weak electrostatic forces. Therefore, translocation and loss can occur during tissue preparation. This has complicated the use of micro-autoradiography. Receptor micro-autoradiography has overcome these complications and is a method of choice. It has been validated through several diffusible compounds with known localization, extensively applied. It has contributed numerous discoveries, followed by new concepts and therapies. Pictorial evidence in this review indicates that cellular information is essential, a 'sine qua non' for meaningful drug distribution studies. High resolution cellular microscopic information obtained from autoradiography requires tissue dissection and the necessary precautions for preserving pristine in vivo drug deposition. Receptor micro-autoradiography fulfils these requirements. It reveals crucial information at the subcellular level that cannot currently be obtained with any other type of autoradiography or spectrometric imaging.

Drugs in the brain--cellular imaging with receptor microscopic autoradiography

For cell and tissue localization of drugs, receptor microscopic autoradiography is reviewed, including its development history, multiple testing, extensive applications and significant discoveries. This sensitive high-resolution imaging method is based on the use of radiolabeled compounds (esp. tagged with (3)H or (125)I), preservation through freezing of in vivo localization of tissue constituents, cutting thin frozen sections, and close contact with the recording nuclear emulsion. After extensive testing of the utility of this method, the distribution of radiolabeled compounds has been identified and characterized for estradiol, progestagens, adrenal steroids, thyroid hormone, ecdysteroids, vitamin D, retinoic acid, metabolic indicators glucose and 2-deoxyglucose, as well as extracellular space indicators. Target cells and associated tissues have been characterized with special stains, fluorescing compounds, or combined autoradiography-immunocytochemistry with antibodies to dopamine-beta-hydroxylase, GABA, enkephalin, specific receptor proteins, or other cellular products. Blood-brain barrier and brain entries via capillary endothelium, ependyma, or circumventricular recess organs have been visualized for (3)H-dexamethasone, (210)Pb lead, and (3)H-1,25(OH)(2) vitamin D(3). With this histopharmacologic approach, cellular details and tissue integrative overviews can be assessed in the same preparation. As a result, information has been gained that would have been difficult or impossible otherwise. Maps of brain drug distribution have been developed and relevant target circuits have been recognized. Examples include the stria terminalis that links septal-amygdaloid-thalamic-hypothalamic structures and telencephalic limbic system components which extend as the periventricular autonomic-neuroendocrine ABC (Allocortex-Brainstem-Circuitry) system into the mid- and hindbrain. Discoveries with radiolabeled substances challenged existing paradigms, engendering new concepts and providing seminal incentives for further research toward understanding drug actions. Most notable are discoveries made during the 1980s with vitamin D in the brain together with over 50 target tissues that challenged the century-old doctrine of vitamin D's main role as 'the calcitropic hormone', when the new data made it apparent that the main biological function of this multifunctional sunshine hormone rather is maintenance of life and adapting vital functions to the solar environment. In the brain, vitamin D, in close relation to sex and adrenal steroids, participates in the regulation of the secretion of neuro-endocrines, such as, serotonin, dopamine, nerve growth factor, acetyl choline, with importance in prophylaxis and therapy of neuro-psychiatric disorders. Histochemical imaging with high cellular-subcellular resolution is necessary for obtaining detailed information, as this review indicates. New spectrometric methods, like MALDI-MSI, are unlikely to furnish the same information as receptor microautoradiography does, but can provide important correlative molecular information.

Hormone-dependent expression of fasciclin II during ganglionic migration and fusion in the ventral nerve cord of the moth Manduca sexta

The ventral nerve cord of holometabolous insects is reorganized during metamorphosis. A prominent feature of this reorganization is the migration of subsets of thoracic and abdominal larval ganglia to form fused compound ganglia. Studies in the hawkmoth Manduca sexta revealed that pulses of the steroid hormone 20-hydroxyecdysone (20E) regulate ganglionic fusion, but little is known about the cellular mechanisms that make migration and fusion possible. To test the hypothesis that modulation of cell adhesion molecules is an essential component of ventral nerve cord reorganization, we used antibodies selective for either the transmembrane isoform of the cell adhesion receptor fasciclin II (TM-MFas II) or the glycosyl phosphatidylinositol-linked isoform (GPI-MFas II) to study cell adhesion during ganglionic migration and fusion. Our observations show that expression of TM-MFas II is regulated temporally and spatially. GPI-MFas II was expressed on the surface of the segmental ganglia and the transverse nerve, but no evidence was obtained for regulation of GPI-MFas II expression during metamorphosis of the ventral nerve cord. Manipulation of 20E titers revealed that TM-MFas II expression on neurons in migrating ganglia is regulated by hormonal events previously shown to choreograph ganglionic migration and fusion. Injections of actinomycin D (an RNA synthesis inhibitor) or cycloheximide (a protein synthesis inhibitor) blocked ganglionic movement and the concomitant increase in TM-MFas II, suggesting that 20E regulates transcription of TM-MFas II. The few neurons that showed TM-MFas II immunoreactivity independent of endocrine milieu were immunoreactive to an antiserum specific for eclosion hormone (EH), a neuropeptide regulator of molting.

Steroid hormone activation of wandering in the isolated nervous system of Manduca sexta

Steroid hormones modulate motor circuits in both vertebrates and invertebrates. The insect Manduca sexta, with its well-characterized developmental and endocrinological history, is a useful model system in which to study these effects. Wandering is a stage-specific locomotor behavior triggered by the steroid hormone 20-hydroxyecdysone (20E), consisting of crawling and burrowing movements as the animal searches for a pupation site. This study was undertaken to determine whether the wandering motor pattern is activated by direct action of 20E on the CNS. 20E acts on the isolated larval nervous system to induce a fictive motor pattern showing features of crawling and burrowing. The latency of the response to 20E is long, suggestive of a genomic mechanism of action. The abdominal motoneurons or segmental pattern generating circuits are unlikely to be the primary targets of 20E action in inducing fictive wandering. Exposure of the segmental ganglia alone to hormone did not evoke fictive wandering. Therefore, as suggested by an earlier study, the likely site of 20E action is within the brain.

Voltage-dependent ionic currents in the ventromedial eclosion hormone neurons of Manduca sexta

The ventromedial cells (VM cells) of the moth Manduca sexta belong to a peptide hormone signaling hierarchy that directs an episodic and stereotyped behavior pattern, ecdysis. The VM cells respond to declining ecdysteroid titers at the end of the final larval molt with a transcription-dependent decrease in spike threshold and the abrupt release of the previously stockpiled neuropeptide, eclosion hormone (EH). This report describes whole-cell patch-clamp recordings of acutely isolated VM cell somata made to identify membrane currents that may underlie the increase in VM cell excitability during EH release and that may contribute to abrupt peptide release. There were at least three voltage- and time-dependent conductances in the VM cells. The inward current was carried exclusively by a voltage-dependent inward Ca(2+) current (I(Ca)), and the outward currents were a combination of a Ca(2+)-dependent outward K(+) current (I(K(Ca))) and a transient, voltage-dependent outward K(+) current, the A current (I(A)). In current-clamp recordings, the currents present in the acutely isolated somata were sufficient to generate membrane properties similar to those observed in the VM cells in situ. This study represents the first step toward characterization of the mechanisms underlying the abrupt release of EH stores from the VM cells preceding ecdysis.

20-Hydroxyecdysone stimulates proliferation of glial cells in the developing brain of the moth Manduca sexta

The steroid hormone 20-hydroxyecdysone (20-HE) controls diverse aspects of neuronal differentiation during metamorphosis in the hawkmoth Manduca sexta. In the present study we have examined the effect of 20-HE on glial cells of the brain during the metamorphic period. The antennal (olfactory) lobe of Manduca provides an ideal system in which to study effects of hormones on glial cells, since three known classes of glial cells participate in its development, and at least one type is critically important for establishment of normal neuronal morphology. These glial cells, associated with the neuropil, form boundaries for developing olfactory glomeruli as a result of proliferation and migration. We determined whether glial cells proliferate in response to 20-HE by injecting a pulse of 20-HE into the hemolymph at different stages of development and monitoring proliferation of all three types of glial cells. Hormone injections at the beginning and end of metamorphic development, when hormone titers are normally low, did not stimulate proliferation of neuropil-associated glial cells. Injections during the period when hormone titers are normally rising produced significant increases in their proliferation. Injections when hormone titers are normally high were ineffective at enhancing their proliferation. One other class of glial cells, the perineurial cells, also proliferate in response to 20-HE. Thus, glial proliferation in the brain is under the control of steroid hormones during metamorphic development.

Ecdysteroid binding sites in gastrolith forming tissue and stomach during the molting cycle of crayfish Procambarus clarkii

The distribution of ecdysteroid binding sites in the stomach and gastrolith disc tissue of crayfish (Procambarus clarkii) was examined in relation to the molting stage by thaw-mount autoradiography. The radiolabeled hormone analogue ponasterone A (25-deoxy-20-hydroxyecdysone) was used. Ecdysteroid binding sites were demonstrated only in certain molting stages, the small gastrolith period and the aftermolt stage. In gastrolith epithelium, ponasterone A binding sites first appeared in the cytoplasm, and then in the nuclei and cytoplasm. In the stomach epithelium, many nuclear binding sites were detectable during the period of gastrolith secretion. These periodical changes in specific ponasterone A binding when correlated with the molting stages clearly show that ecdysteroids may function as an initiator for gastrolith formation and reabsorption. The findings also suggest that ecdysteroids control calcium transport in the stomach epithelium. The time-related and functional differences of cytoplasmic and nuclear concentration of ecdysteroid receptors indicate the presence of cytoplasmic and nuclear receptors associated with specific actions.