Maturation of the hypothalamo-pituitary-gonadal axis of male Syrian hamsters.

Light-microscope immunocytochemistry (ICC) was used to investigate postnatal changes in the morphology of LHRH neurons in the brains of male Syrian hamsters and to relate these changes to more overt maturational developments within the hypothalamo-pituitary-gonadal axis. The animals were maintained under long-day photoperiods (14L:10D), and groups of 6-7 were killed at 10-day intervals from Day 15 to Day 65. Their brains were fixed with 4% paraformaldehyde, sectioned sagittally with a vibratome (75 microns), and processed for ICC using monoclonal LHRH antibody HU4H. Throughout the study period, the hamsters showed a progressive increase in plasma gonadotropin levels, closely followed by an increase in testicular weight and plasma testosterone levels. Histology of the testes revealed that spermatogenesis was already qualitatively completed by Day 35 and quantitative aspects were established by Day 45. Within the brain, LHRH neuronal perikarya were distributed primarily in the medial septal-preoptic area and the diagonal band of Broca; morphologically, these immunopositive neurons were either monopolar or bipolar. The total number of LHRH neurons detected in the areas examined was approximately 440 throughout the developmental period, and the relative proportions of monopolar and bipolar subtypes (86% and 14%, respectively) remained unchanged. In contrast, the area of the perikarya, as determined by autoimage analysis, showed a highly significant age-related increase, both for the monopolar and bipolar neurons. It is suggested that these developmental changes in the LHRH neurons reflect an increase in LHRH synthesis and may, therefore, provide a neuroendocrine trigger for the onset of puberty.     

An elementary information processing component in the circuitry of the retina generating the on-responses

A pattern of neural connections that is a compulsory feature of photoreceptor terminals and is referred to as the synaptic ribbon complex was analyzed, and by combination of the structural information and information gained by intracellular recordings from photoreceptors, horizontal cells, and bipolar cells, it is possible to explain how the hyperpolarizing effect of light stimulating the photoreceptors is changed to a depolarization of depolarizing bipolar cells. The sign reversal is accomplished by the hyperpolarizing action of the horizontal cells on the photoreceptors, which blocks the transmission between the photoreceptors and the bipolar cells. This blocking action is controlled by the photoreceptor and it functions like a gate that is opened only when the photoreceptor is stimulated by light. The synaptic ribbon complex offers an example of an elementary information processing component with three input channels to the bipolar cells with each channel contributing a different piece of information and with the processing occurring presynaptically. Additional processing of information occurs within the dendritic tree through interactions of the responses of the individual dendritic endings to different types of input. This interaction can involve partial blocking of the conduction within the dendritic tree, making the interaction considerably more complex than simple summation. The responses recorded intracellularly from neurons reveal only the end result of the processing of information at the level of that neuron.     

Pineal neurons projecting to the brain of the rainbow trout,Salmo gairdneri Richardson (Teleostei)

Summary The morphology of intrapineal neurons that give rise to the pineal tract and project to the brain in the rainbow trout was visualized by the use of neuronal backfilling with horseradish peroxidase (HRP). The tracing was performed on excised pineal organs under in-vitro conditions at 4° C, with filling times ranging from 6 to 24 h. Large multipolar, bipolar and unipolar neurons were visualized in the rostral tip of the pineal organ (pineal ganglion). These neurons possessed extended dendritic trees participating in the formation of a circumscribed neuropil-like area. Throughout the pineal organ small bipolar elements were the most ubiquitous type of neuron, however, with markedly smaller numbers in the proximal portion of the pineal end-vesicle. In the pineal stalk, some bipolar neurons were observed to contact the pineal lumen, which is continuous with the third ventricle, via dendritic processes of various types. It could not be established whether any of these CSF-contacting processes were identical with photoreceptor outer segments. The basal processes of the bipolar neurons sometimes possessed distally projecting collaterals. In conclusion, it has been shown that (i) different types of neurons displaying varied patterns of regional distribution contribute to the pineal tract, and (ii) certain CSF-contacting neurons in the pineal organ send axonal processes directly toward the brain.Supported by Research grant Ko 758/2-4 from the Deutsche ForschungsgemeinschaftFellow of the Alexander von Humboldt Foundation, Bonn, Federal Republic of Germany     

B-wave of the electroretinogram. A reflection of ON bipolar cell activity

Light-evoked intraretinal field potentials (electroretinogram, ERG) have been measured simultaneously with extracellular potassium fluxes in the amphibian retina. The application of highly selective pharmacologic agents permitted us to functionally isolate various classes of retinal neurons. It was found that: (a) application of APB (2-amino-4-phosphonobutyrate), which has previously been shown to selectively abolish the light responsiveness of ON bipolar cells, causes a concomitant loss of the ERG b-wave and ON potassium flux. (b) Conversely, PDA (cis 2,3-piperidine-dicarboxylic acid) or KYN (kynurenic acid), which have been reported to suppress the light responses of OFF bipolar, horizontal, and third-order retinal neurons, causes a loss of the ERG d-wave as well as OFF potassium fluxes. The b-wave and ON potassium fluxes, however, remain undiminished. (c) NMA (N-methyl-DL-aspartate) or GLY (glycine), which have been reported to suppress the responses of third-order neurons, do not diminish the b- or d-waves, nor the potassium fluxes at ON or OFF. This leads to the conclusion that the b-wave of the ERG is a result of the light-evoked depolarization of the ON bipolar neurons. This experimental approach has resulted in two further conclusions: (a) that the d-wave is an expression of OFF bipolar and/or horizontal cell depolarization at the termination of illumination and (b) that light-induced increases in extracellular potassium concentration in both the inner (proximal) and outer (distal) retina are the result of ON bipolar cell depolarization.     

Double-labeling techniques demonstrate that rod bipolar cells are under GABAergic control in the inner plexiform layer of the rat retina

The synaptic connectivity between rod bipolar cells and GABAergic neurons in the inner plexiform layer (IPL) of the rat retina was studied using two immunocytochemical markers. Rod bipolar cells were stained with an antibody specific for protein kinase C (PKC, α isoenzyme), and GABAergic neurons were stained with an antiserum specific for glutamic-acid decarboxylase (GAD). Some amacrine cells were also labeled with the anti-PKC antiserum. All PKC-labeled amacrine cells examined showed GABA immunoreactivity, indicating that PKC-labeled amacrine cells constitute a subpopulation of GABAergic amacrine cells in the rat retina. A total of 150 ribbon synapses established by rod bipolar cells were observed in the IPL. One member of the postsynaptic dyads was always an unlabeled AII amacrine cell process, and the other belonged to an amacrine-cell process showing GAD immunoreactivity. The majority (n=92) (61.3%) of these processes made reciprocal synapses back to the axon terminals of rod bipolar cells. In addition, 78 conventional synapses onto rod bipolar axons were observed, and among them 52 (66.7%) were GAD-immunoreactive. Thus GABA provides the major inhibitory input to rod bipolar cells.     

Parallel processing in two transmitter microenvironments at the cone photoreceptor synapse

A cone photoreceptor releases glutamate at ribbons located atop narrow membrane invaginations that empty onto a terminal base. The unique shape of the cone terminal suggests that there are two transmitter microenvironments: within invaginations, where concentrations are high and exposures are brief; and at the base, where concentrations are low and exposure is smoothed by diffusion. Using multicell voltage-clamp recording, we show that different subtypes of Off bipolar cells sample transmitter in two microenvironments. The dendrites of an AMPA receptor-containing cell insert into invaginations and sense rapid fluctuations in glutamate concentration that can lead to transient responses. The dendrites of kainate receptor-containing cells make basal contacts and respond to a smoothed flow of glutamate that produces sustained responses. Signaling at the cone to Off bipolar cell synapse illustrates how transmitter spillover and synapse architecture can combine to produce distinct signals in postsynaptic neurons.     

The maturation of the somatostatin systems in the rat visual cortex

The development of somatostatin-immunoreactive (SS) neurons and processes in the rat visual cortex (VC) was studied in animals from embryonic day 20 (E20) to postnatal day 21 (D21). Three distinct patterns of immunoreactivity were seen. From E20 to birth (D0), VC was characterized by a small number of mainly bipolar SS neurons throughout the cortical plate. In the perinatal period, from D1 to D6, there were large numbers of immature immunoreactive neurons which were confined to layer VI and the subplate zone, a few bipolar neurons in the cortical plate and an extremely dense plexus of SS processes throughout the neuropil. The third phase, from D8 to weaning, was characterized by the absence of immature SS neurons, an increase in the number of multipolar SS neurons and a decrease in the density of SS fibers. By D15, the time of eye-opening, the number and distribution of SS neurons and processes was close to that seen in the adult. These results indicate that the SS system of neurons and fibers is among the earliest of the transmitter systems to be established in VC and suggests a role for the peptide in cortical organization as well as visual processing.     

Auditory hair cell explant co-cultures promote the differentiation of stem cells into bipolar neurons

Auditory neurons, the target neurons of the cochlear implant, degenerate following a sensorineural hearing loss. The goal of this research is to direct the differentiation of embryonic stem cells (SCs) into bipolar auditory neurons that can be used to replace degenerating neurons in the deafened mammalian cochlea. Successful replacement of auditory neurons is likely to result in improved clinical outcomes for cochlear implant recipients. We examined two post-natal auditory co-culture models with and without neurotrophic support, for their potential to direct the differentiation of mouse embryonic SCs into characteristic, bipolar, auditory neurons. The differentiation of SCs into neuron-like cells was facilitated by co-culture with auditory neurons or hair cell explants, isolated from post-natal day five rats. The most successful combination was the co-culture of hair cell explants with whole embryoid bodies, which resulted in significantly greater numbers of neurofilament-positive, neuron-like cells. While further characterization of these differentiated cells will be essential before transplantation studies commence, these data illustrate the effectiveness of post-natal hair cell explant co-culture, at providing valuable molecular cues for directed differentiation of SCs towards an auditory neuron lineage.     

Light and electron microscopic observations on the normal structure of the vomeronasal organ of garter snakes

The vomeronasal epithelium of adult garter snakes (Thamnophis sirtalis and T. radix) was studied by light and electron microscopy. The sensory epithelium is extraordinarily thick, consisting of a supporting cell layer, a bipolar cell layer, and an undifferentiated cell layer. The supporting cell layer is situated along the luminal surface and includes supporting cells and the peripheral processes (dendrites) of bipolar neurons. The luminal surfaces of both supporting cells and bipolar neurons are covered with microvilli. Specializations of membrane junctions are always observed between adjacent cells in the subluminal region. Below the supporting cell layer, the epithelium is characterized by a columnar organization. Each column contains a population of bipolar neurons and undifferentiated cells. These cells are isolated from the underlying vascular and pigmented connective tissue by the presence of a thin sheath of satellite cells and a basal lamina. Heterogeneity of cell morphology occurs within each cell column. Generative and undifferentiated cells occupy the basal regions and mature neurons occupy the apical regions. Transitional changes in cell morphology occur within the depth of each cell column. These observations suggest that the vomeronasal cell column is the structural unit of the organ and may represent the dynamic unit for cell replacement as well. A sequential process of cell proliferation, neuronal differentiation, and maturation appears to occur in the epithelium despite the adult state of the animal.     

Use of a monoclonal antibody to classify neurons isolated from the head region of Hydra

A mouse monoclonal antibody (JD1) to Hydra attenuata using the peroxidase-antiperoxidase (PAP) method revealed unipolar, bipolar, and multipolar sensory and ganglion cells in the head region of H. littoralis. Neurons isolated from macerated hypostomes and tentacles were classified according to the number of their cytoplasmic processes and the position of the cilium, when present, relative to the perikaryon. PAP-stained sensory cells had an apical ciliary cone, whereas ganglion cells did not. Neurons with cytoplasmic processes longer than 50 microns stained faintly, whereas those with processes shorter than 50 microns in length stained mainly dense brown. Unipolar neurons had an oval, crescent, round, or elliptic perikaryon with a single short axon. The perikaryal shape of bipolar neurons varied from round to tall triangular, short triangular, crescent, oval, or elliptic with two oppositely directed symmetric or asymmetric processes. Asymmetric processes were present in a bipolar sensory cell with a long apical cilium typical of gastrodermal sensory cells. One type of bipolar ganglion cell had a short perikaryal cilium. Another type had neurites longer than 50 microns. We found seven morphological variations of multipolar neurons, including one with an apical knob, two with a short perikaryal cilium, two with cytoplasmic loops near the perikaryon, one with perpendicular processes projecting from the major neurites, and one with a branched process longer than 50 microns opposite a tangled mass of neurites.     

Electron microscopy of the indoleamine-accumulating neurons in the retina of the rabbit

Summary The recently discovered indoleamine-accumulating retinal neurons were studied electron microscopically after destruction of the dopaminergic retinal neurons and subsequent labeling with 5,6-dihydroxytryptamine. These observations confirm earlier fluorescence microscopical studies on the distribution of the indoleamine-accumulating neurons in the rabbit retina. Their perikarya are known to be located in the inner nuclear layer (INL) among the amacrine cell bodies. Their processes are found only in the inner plexiform layer (IPL), most of them in the innermost third part of that layer. The indoleamine-accumulating terminals are pre- and postsynaptic to bipolar neurons in the innermost sublayer of the IPL. Reciprocal synapses are probably the rule. The synaptic vesicles of indoleamine-accumulating synapses onto bipolar cells are arranged in globular clusters around a central electron dense, round body. A number of synapses formed by unlabeled amacrine neurons with postsynaptic indoleamine-accumulating elements were also detected. These synapses were mainly found in the outermost third of the IPL. Synaptic contacts between presynaptic indoleamine-accumulating neurons and postsynaptic unlabeled processes of amacrine cells are very rare.     

Fine structure of the ventral organ of the house fly larva,Musca domestica L.

Summary The ventral organs of the cephalic lobes of the house fly larvae, Musca domestica L., were studied by scanning and transmission electron microscopy. Four sensilla were found. Three of them are each innervated by a single dendrite whose ending possesses a tubular body and communicates to the exterior through an opening. These sensilla are assumed to be mechanoreceptors. The 4th sensillum is supplied by 2 bipolar neurons with the unbranched dendritic tips (without tubular bodies) exposed to the exterior through a single opening and is probably a contact chemoreceptor.This research was supported in part by the Office of Naval Research and NIH Training Grant ES-00069. Paper no. 3724 of the North Carolina State University Agricultural Experiment Station journal series.     

Tyrosine hydroxylase- and corticotropin releasing factor-immunoreactivity in the olfactory bulb of the opossum (Didelphis virgianina)

The nerve fiber layer of the opossum olfactory bulb, formed by axons originating from bipolar neurons in the olfactory epithelium, and glomeruli are intensely immunoreactive for olfactory marker protein. The surrounding extra-glomerular neuropil contains numerous periglomerular neurons immunoreactive for either tyrosine hydroxylase or corticotropin releasing factor. Dendrites of both types of immunoreactive neurons extend into the intraglomerular neuropil. CRF-immunoreactive neurons are fewer in number than TH-immunoreactive neurons and are observed primarily in the periglomerular region. Occasional, scattered TH-immunoreactive neurons are seen in the deeper layers of the olfactory bulb.     

Interstitial cells of Cajal in the striated musculature of the mouse esophagus

A key feature of signal processing in the mammalian retina is parallel processing, where the segregation of visual information, e.g., brightness, darkness, and color, starts at the first synapse in the retina, the photoreceptor synapse. These various aspects are transmitted in parallel from the input neurons of the retina, the photoreceptor cells, through the interconnecting bipolar cells, to the output neurons, the ganglion cells. The photoreceptors and bipolar cells release a single excitatory neurotransmitter, glutamate, at their synapses. This parsimony is contrasted by the expression of a plethora of glutamate receptors, receptor subunits, and isoforms. The detailed knowledge of the synaptic distribution of glutamate receptors thus is of major importance in understanding the mechanisms of retinal signal processing. This review intends to highlight recent studies on the distribution of glutamate receptors at the photoreceptor synapses of the mammalian retina.     

Postnatal growth inhibition in the rat retina after actinomycin D adminstration]

The effects of actinomycin D on the development of the rats retina were observed. At the day of birth the inner neurons and the inner cells of the bipolar layer are vulnerable. The pale degeneration of these neurons accompanied by a dilatation of the endoplasmatic reticulum and the dark degeneration accompanied by a pycnosis and a shrinkage of the cytoplasm persist during the first 11 days after birth. The same alterations are to be seen in bipolar cells on day 11 after birth. The transient disorganisation of the inner layers could effect the ramification because the stratum reticulare internum is smaller as in untreated animals.     

Distribution of immunoreactive cholecystokinin in the human hippocampus

The distribution of cholecystokinin immunoreactive (CCK-IR) nerve cell bodies and processes is reported in the human hippocampus by using the peroxidase-antiperoxidase technique of Sternberger. The CCK-immunoreactivity occurs in three major classes of interneurons: small (10-20 microns) horizontal multipolar neurons of the alveus and stratum oriens; small vertically oriented bipolar or multi-polar neurons in the stratum oriens and stratum pyramidale of Ammon's horn, layers II and III of the subicular system and the entorhinal area; large (20-35 microns) bipolar neurons in the hilus. Each region of the hippocampus is distinct in its CCK-IR nerve fibers content. Those fibers are particularly abundant around pyramidal cells of the CA2 and CA3 subfields of the Ammon's horn and around granular cells suggesting synaptic interaction between the CCK nerve terminals and glutamate neurons of these two regions. No CCK-IR fiber is detected in the fimbria and only a few number of CCK-IR beaded fibers are seen in the angular bundle. These anatomical data suggest that CCK interacts in the functional circuitry of the human hippocampus.     

Development of catecholaminergic neurons in the human medulla oblongata

Distribution and maturation of catecholaminergic (CA) neurons have been studied by tyrosine hydroxylase immunohistochemistry in the medulla oblongata of human fetuses aged 14.5-25 weeks of gestation. Already at 14.5 weeks, CA neurons were observed in two longitudinally oriented cell clusters, one located ventrolaterally in the area of the lateral reticular and ambiguous nuclei, the other one dorsomedially forming 4 groups related to the dorsal vagal nucleus, the commissural nucleus of the vagus, the nucleus of the tractus solitarius and the area postrema. CA neurons in the area postrema were often found close to blood vessels. Scattered intermediate CA neurons were seen between these two larger clusters. CA neurons still appeared immature exhibiting bipolar morphology with only one or two short stout processes, which hardly branched. At 21 weeks, CA neurons occupied essentially the same location, but had a more mature morphology. Though still bipolar in shape, they had thinner and much longer processes which frequently branched. Both in the ventrolateral and the dorsomedial cell clusters, these processes were frequently lying close to blood vessels. At 25 weeks, CA cells had matured into multipolar neurons with long thin processes forming fine fiber networks in the ventrolateral medulla as well as around and within the dorsal vagal and solitarius nuclei. Only at this stage, a distinct CA fiber tract was seen located in the region of the tractus solitarius. Our results indicate that CA neurons in the human medulla, which are presumably involved in the control of ventilation and blood pressure, though generated rather early during development, mature relatively late.     

A Golgi study of the parvocellular neurons in the paraventricular nucleus of the domestic fowl

The present study was focussed on the typology of small and medium-sized neurons in the hypothalamic paraventricular nucleus (PVN) of the domestic fowl as revealed by means of Golgi impregnation. This region is provided with different systems of neurons that can be distinguished on the basis of their location and dendritic morphology. Intraependymal neurons and CSF-contacting nerve cells are found in the periventricular layer together with bipolar elements endowed with processes extending parallel to the surface of the third ventricle. The short axons of these neurons may contact the magnocellular elements. Numerous isodendritic neurons are scattered throughout the entire PVN; these nerve cells possessing short and branched axons may be considered as local-circuit neurons. The complex intrinsic organization of the PVN of the domestic fowl might provide the structural basis for local interactions among the neuronal elements of this hypothalamic region.     

Labial tip sensilla of Blissus leucopterus leucopterus (Hemiptera: Blissidae): Ultrastructure and behavior

The cuticular sensory receptors that are found on the apex of the labium of hemipterans play an important role in their feeding behavior. In this study we describe the ultrastructure, number, and distribution of sensilla on the labium apex of the chinch bug, Blissus leucopterus leucopterus. Each apical field of sensilla on the labium contains 11 uniporous peg sensilla and one sensillum chaeticum. The uniporous peg sensilla are innervated by 4–5 bipolar neurons that send dendrites in the lumen of each peg. Three neurons are associated with each sensillum chaeticum, two neurons have dendrites in the lumen of the sensillum, and the third dendrite ends in a tubular body at the base of the sensillum. Behavioral tests that involve chemical blockage of the sensory receptors show the importance of the labial sensilla in feeding behavior. Both morphological and behavioral evidence indicate that the labial sensilla have a chemosensitive function.     

The characteristics of the electrovomeronasogram: its loss following vomeronasal axotomy in the garter snake

Electrovomeronasogram (EVG) recordings were made from adult garter snakes, Thamnophis sirtalis. Stimulation of vomeronasal epithelium with a stimulus prepared from prey, earthworm electric shock secretion (ESS), evoked EVG response in a dose-dependent manner. The magnitude of the EVG response to ESS was remarkably larger than n-amyl acetate and glutamate, which elicited insignificant responses, supporting the idea that the vomeronasal system is differentially sensitive to liquid delivery of biologically significant chemical stimuli. Fourteen days following vomeronasal axotomy, the magnitudes of the EVG responses of animals which received bilateral axotomy without cauterization or with cauterization was -0.19+/-0.07 mV or -0.05+/-0.02 mV respectively, compared with the normal EVG response of -0.41+/-0.10 mV. The epithelia of animals which received bilateral axotomy without cauterization exhibited remarkable degeneration of the bipolar neurons. Maximal depletion of bipolar neurons occurred in the epithelia denervated with cauterization, though the difference between cell densities in vomeronasal neuron layers in these epithelia was not statistically significant. The present results clearly indicate that the fewer neurons the epithelium contains, the smaller EVG response it generates, suggesting that the receptor neurons are the primary origin of EVG responses.