Poststimulatory endocytosis, microvesicle repopulation and changes in smooth endoplasmic reticulum in nerve endings of the median eminence superfused in vitro

The caveolae membrane system

The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

Tyrosine kinase receptors concentrated in caveolae-like domains from neuronal plasma membrane

Recent evidence suggests that tyrosine kinases are highly organized in caveolae of tissue culture cells. We now report the isolation of a membrane domain from neuronal plasma membranes that has the biochemical characteristics of caveolae. A low density membrane (LDM) fraction with the same density as caveolae was highly enriched in tyrosine kinases such as insulin receptors, neurotrophin receptors, Eph family receptors, and Fyn. Grb2, Ras, heterotrimeric GTP-binding proteins, and Erk2 were also concentrated in the LDM. Incubation of the LDM fraction at 37 degrees C stimulated the phosphorylation on tyrosine of multiple, resident proteins, whereas the bulk membrane fraction was devoid of tyrosine kinase activity. The LDM, which makes up approximately 5-10% of the plasma membrane protein, appears to be organized for signal transduction.

Afferent synaptic changes in auditory hair cells during noise-induced temporary threshold shift

This study presents evidence in support of the hypothesis that one of the sites of failure during noise-induced temporary threshold shift (TTS) is the afferent synapse between auditory hair cells and auditory nerve fibers. Our results show clear evidence indicating changes in the quantity of afferent synapses and the morphology of presynaptic structures in the alligator lizard auditory hair cells during TTS. In TTS hair cells there are statistically significant decreases in: 1) the number of afferent synapses, 2) the number of synaptic vesicles at the afferent synapses, 3) the size of synaptic bodies, and 4) the packing density of synaptic vesicles around the synaptic body. These results suggest that the presynaptic components of the afferent synapse reflect the functional state of the synapse, and that the reduction of these synapses, both in number and component size, contributes to TTS.

What do the synaptic vesicles contain?

The intersynaptic membranes of the rat brain cortex were found to remain firmly attached to one another after perfusion of strongly anisotonic solutions. Brains perfused with depolarizing and excitotoxic agents showed abundant, apparent intermingling of mitochondria and synaptic vesicles. The results suggest (i) that the intersynaptic membranes are not separated from one another by an essentially fluid intersynaptic medium as it is commonly assumed, but rather firmly attached to one another by a layer of faintly osmiophilic yet remarkably stable, water-insoluble material; and (ii) that the synaptic vesicles may be involved in adenosine triphosphate carriage. Well established multidisciplinary data are presented which appear to be in line with both possibilities.

Changes in size and shape of auditory hair cells in vivo during noise-induced temporary threshold shift

In this study we describe changes in the size and shape of auditory hair cells of the alligator lizard in vivo during noise-induced temporary threshold shift. These changes consist of a decrease in cell volume, a decrease in cell length and an increase in cell width. We speculate that these changes are due to relaxation of cytoskeletal contractile elements and osmotic loss of intracellular water. We also describe a decrease in the surface area of the hair cell plasmalemma, and speculate that it is related to the endocytosis and intracellular accumulation of cell membrane during synaptic vesicle recycling. Finally we describe an increase in the endolymphatic surface area of the hair cell, and speculate that this could alter the micromechanics of the stereociliary tuft to attenuate the effective stimulus.

A novel hypothalamic-dispersed pituitary co-perifusion model for the study of growth hormone secretion

This study presents a novel, in vitro, hypothalamic-dispersed pituitary co-perifusion system (HPPS) developed to examine the influence of the hypothalamus on pituitary growth hormone (GH) secretion in a controlled environment. In this perifusion system, dispersed rat pituitary cells were loaded onto Biogel P-2 (P-2) beads in a 0.5-ml plexiglas chamber and were submerged in a 37 degrees C water bath. After stabilization, two hypothalami were placed into each chamber on a thin layer of P-2 beads and the chamber was re-equilibrated. To test the system, pituitary cells were stimulated either directly with growth hormone-releasing factor (GRF) or indirectly via the hypothalamus, with clonidine, an alpha 2-adrenergic (alpha 2) receptor agonist. Perifusion of HPPS or pituitary cells with GRF (40 ng/ml) induced a substantial endogenous GH surge. Clonidine (2 x 10(-8) M) treatment stimulated a GH surge in HPPS chambers, but not in chambers containing only pituitary cells. Thus, somatotrophs respond to hypothalamic factors released in response to clonidine and not directly to alpha 2 stimulation. To determine if the components involved in GH feedback are present in the perifusion system, HPPS chambers were sequentially perifused with hGH, clonidine, and GRF. hGH pretreatment suppressed the clonidine but not the GRF-induced GH surge(s) observed in chambers perifused with clonidine and GRF only. In chambers only containing pituitary cells, GH was only increased in response to GRF when sequentially perifused with all three substances. This study demonstrates the dynamic interaction between the hypothalamus and pituitary in the regulation of GH secretion in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the synaptic region of auditory hair cells during noise-induced temporary threshold shift

During noise-induced temporary threshold shift cytoplasmic vacuoles increase in the synaptic region of auditory hair cells. This structural change in the presynaptic region suggests that afferent synapses between hair cells and auditory nerve fibers fail during the period of threshold elevation, and that synaptic fatigue may play a major role.

Identification of gonadotropin releasing hormone (GnRH) neurons projecting to the median eminence from third ventricular preoptic area grafts in hypogonadal mice

Functional gonadotropin releasing hormone (GnRH) neurosecretory activity can be restored in genetically hypogonadal (hpg) adult mice with grafts of GnRH-containing fetal or neonatal septal-preoptic area (S/POA) tissue. Neurons implanted into the third ventricle of the host brain survive and send out axons which innervate one of the normal targets of these neurons, the median eminence (ME). Fibers terminate near primary portal vessels where GnRH is available for release into the vasculature, and this axonal outgrowth is essential for the stimulation of gonadotropin secretion, gonadal and accessory sex structure growth, gametogenesis, and fertility. Although it is known that GnRH axons reach their target, it is not known if all such neurons in a graft contribute to the projection. Taking advantage of the fact that axons in the ME, the sole host target, are outside the blood-brain barrier (BBB), long-term grafted animals were injected intraperitoneally with a retrograde tracer, Fluorogold (FG). Normal male mice were injected for comparison. Animals were sacrificed 5 days after injection and brain sections in the area of the graft were stained immunocytochemically for GnRH. In the normal male mice, two-thirds of the GnRH neurons were double-labeled with FG. In grafted individuals which showed increased gonadal growth, the percentage of labeled cells ranged from 17 to 75%. The results indicate that despite tissue injury, ectopic location, and a vastly reduced population, transplanted fetal GnRH neurons recapitulate a pattern seen in normal intact mice where some but not all neurons were capable of capturing a peripherally delivered tracer.

Corticotropin-releasing factor release from in vitro superfused and incubated rat hypothalamus. Effect of potassium, norepinephrine, and dopamine

We have compared the release of CRF induced by potassium depolarization, noradrenaline or dopamine as monitored either during superfusion of mediobasal hypothalamus or during incubation of whole hypothalamus. The superfusion device was improved in order to prevent gas leakage and to keep constant pO2 and pCO2 in the superfusion chamber. Basal CRF secretion as well as KCl- and norepinephrine-induced CRF release were comparable in superfusion and incubation experiments. Pharmacological investigations suggest that the stimulatory effect of norepinephrine on CRF release is mediated mainly through alpha 1 and alpha 2 adrenergic receptors, and partially through beta receptors.

Alignment and intracytoplasmic disintegration of synaptic vesicles in the brain cortex

Perfusion fixation with highly concentrated aldehydes suggests that the synaptic vesicles undergo disintegration within the presynaptic ending upon touching the presynaptic membrane rather than being released by exocytosis into the intersynaptic cleft. Three factors have been explored in order to inquire further into the possible significance of the findings: (a) fixative concentration; (b) physiological activity; (c) cell depolarization. The transformation of the vesicles into amorphous, electron-dense material was observed in all experiments in all synapses, including those fixed with the lowest concentration of aldehydes. Besides, after acute ischemia and perfusion of excitatory and depolarizing pharmacological agents, the synaptic vesicles were seen to conflue upon the intersynaptic cleft in well-aligned rows. It was also found that the vesicles flow post mortem towards the intersynaptic cleft with absolute specificity.

Gonadotropin-releasing hormone immunoreactive neurons with access to fenestrated capillaries in mouse brain

Gonadotropin-releasing hormone (GnRH) producing neurons which have access to fenestrated capillaries were identified through a combination of indirect immunofluorescence for GnRH with a fluorescein-taged second antibody and histochemical demonstration of the localization of blood borne and retrogradely transported horseradish peroxidase. In the mouse, GnRH positive neurons were present in the septum, which includes neurons originating from the nervus terminalis, the nucleus medialis and triangularis septi, and the nucleus of the diagonal band. Also, GnRH immunoreactive neurons could be seen in the lateral anterior hypothalamus, the nucleus preopticus medianus, the rostral nucleus periventricularis hypothalami and, to a lesser extent, the nucleus preopticus medialis. Single GnRH positive neurons were found in the nucleus supraopticus, the bed nucleus of the stria terminalis and the cingulate cortex. GnRH neurons which showed uptake of horseradish peroxidase were located in all of these regions and intermingled with unlabeled GnRH neurons. No preferential topographical concentration of GnRH neurons with access to the fenestrated vasculature was apparent. In animals in which GnRH secretion was stimulated by castration for 2 weeks, 65% of all GnRH neuronal perikarya contained horseradish peroxidase. This was reduced to 35% after a 2-week treatment of ovariectomized animals with 10 micrograms/day estradiol while the total number of immunoreactive GnRH cells remained unchanged. No differences in the number of GnRH-horseradish peroxidase positive cells was seen when the dose of horseradish peroxidase of the survival time were increased. While the presence of certain GnRH neurons with dual actions via collaterals cannot be excluded, the results suggest that there are two populations of GnRH neurons, one with access to fenestrated capillaries which is probably related to neurosecretory endocrine regulation of anterior pituitary gonadotropin secretion, and one without access to fenestrated capillaries which is probably related to intracerebral neurotransmission only.

Effects of polyunsaturated fatty acids and hormones on synaptogenesis in serum-free medium cultures of mouse fetal hypothalamic cells

The effects of soluble factors on synaptogenesis by mouse fetal hypothalamic cells cultured in chemically defined conditions have been examined using transmission electron microscopy. Hypothalami taken on the 16th day of gestation were mechanically dissociated and cells were seeded in a minimum serum-free medium supplemented or not with the following components: triiodothyronine, corticosterone and a mixture of polyunsaturated fatty acids (arachidonic acid plus docosahexaenoic acid bound to defatted bovine serum albumin). In the minimum serum free medium synapses were found after 10 days in culture. However, the development of synaptic vesicles was very limited, whereas that of the presynaptic and postsynaptic densities was apparently normal. Supplementation of the minimum serum-free medium with triiodothyronine, corticosterone and polyunsaturated fatty acids added simultaneously, permitted a full development of synapses as attested to by the increase in number and the regular shape and diameter of synaptic vesicles as well as by the complexity and diversity of synapse configurations. Among those three factors, polyunsaturated fatty acids clearly played a key role. The ability of synapses formed in culture to respond to potassium evoked depolarization was examined on cultures grown for 12 days in the simultaneous presence of the three above mentioned supplements. Exposure for 3 min to 60 mM potassium chloride induced in synaptic boutons vesicular depletion, apposition of vesicle clusters onto the presynaptic grid, appearance of a rich filamentous network and of some coated vesicles. Return to 3mM potassium chloride induced in 3 min a massive restoration of the population of vesicles which slightly differed from synaptic vesicles in control cultures. These results show that: (1) the formation of synaptic vesicles in this system is regulated by soluble factors among which polyunsaturated fatty acids play a major role, and (2) synapses formed de novo in chemically defined conditions of culture display the same ability to respond to and to recover from potassium evoked depolarization as adult axon terminals. Thus, they offer a suitable model for analysis of the mechanisms involved in membrane traffic in central neurons.