Characterization of an unusual catecholamine-containing cell type in the toad hypothalamus. A correlated ultrastructural and fluorescence histochemical study

Ultrastructural radioautographic analysis of neurogenesis in the hypothalamus of the adult frog, Rana temporaria, with special reference to physiological regeneration of the preoptic nucleus. II. Types of neuronal cells produced

Light- and electron-microscopic radioautography was used to identify the newly formed neuronal cells in the hypothalamic preoptic area of the frog. Adult Rana temporaria that had been caught in May/June received repeated 3H-thymidine injections and were sacrificed 30 days later. Heavily labeled cells were found in 1-micron plastic coronal sections of the preoptic area and then analysed in electron-microscopic radioautographs of neighbouring thin sections. The cells were identified as newly generated by the presence of 3H-thymidine label over the nucleus. All frogs showed considerable numbers of new peptidergic neurosecretory cells, small conventional neurons, and glia in the preoptic area. Some new ependymally located cells contacting the cerebrospinal fluid displaying ultrastructural characteristics of monoaminergic cells were also revealed. We conclude that prominent ventricular neurogenesis normally exists in the intact adult frog hypothalamus. The birth of small hypothalamic neurons seems to represent a case of sustained growth leading to a net increase in neuron numbers without loss. Conversely, the birth of large peptidergic neurosecretory cells, in which the increased secretory activity often leads to natural death of some cells, is considered as a neuronal replacement phenomenon, referred to as physiological regeneration of the magnocellular preoptic nucleus. The possible significance of this phenomenon in adult Anamnia is discussed.

Epinephrine in mammalian brain

1. Epinephrine is widely distributed in brains of various species throughout phylogeny but maintains its localization to hypothalamus and brainstem/medulla in all species studied. 2. A general decrease in brain epinephrine content is observed phylogenetically beyond fishes with wide variation within species. 3. The cellular localization of epinephrine forming enzyme is dissociated from epinephrine stores in hypothalamus where epinephrine appears to be primarily a hormone. 4. Three proposed functional pools of epinephrine are described. Synthesis of a hormonal pool and a second, perhaps nonfunctional, pool co-stored in noradrenergic terminals in the forebrain occurs extraneuronally and is probably inhibited acutely in the presence of high corticosteroids due to inhibition of uptake 2. Synthesis of epinephrine in the neuronal pool found primarily in the medulla may be enhanced due to increased PNMT activity in the presence of elevated corticosteroids. 5. Phylogenetic and pharmacological data suggest that epinephrine may play an important role in tonic regulation of the level of arousal, reward and sensitivity to environmental stimuli in mammals.

Changes in formaldehyde-induced fluorescence of the hypothalamus and pars intermedia in the frog, Rana temporaria, following background adaptation

Adaptation of the frog, Rana temporaria, to a white background for 12 hr has resulted in an intense formaldehyde-induced fluorescence (FIF) in the neurons of the preoptic recess organ (PRO), paraventricular organ (PVO), nucleus infundibularis dorsalis (NID) and their basal processes permitting visualization of the PRO- and PVO-hypophysial tracts that extend into the median eminence (ME) and pars intermedia (PI); the FIF is reduced in all the structures by 3 days. In frogs adapted to a black background, for 12 hr and 3 days, there was a general reduction in the FIF of the PRO neurons and PRO-hypophysial tract. By 12 hr black background adaptation, the PVO/NID neurons and only their adjacent basal processes show FIF which was sharply reduced by 3 days, making the PVO-hypophysial tract undetectable. In the PI fibers the fluorescence was more intense in black-adapted frogs than in white-adapted ones at both the intervals studied. The simultaneous changes in the FIF of the hypothalamic nuclei, tracts and PI suggest that the PRO and PVO/NID neurons participate in PI control through release of neurotransmitter(s) at the axonal ends.

Extrahypothalamic peptidergic neurosecretion. II. Neurosecretion in the subfornical organ of Rana esculenta L

In the subfornical organ of Rana esculenta, three basic structural elements can be demonstrated by light microscopic and immunohistological techniques used for the demonstration of products of the neurosecretory system. These elements are: (i) neurones and their processes, which the constituents of the subfornical organ proper, (ii) afferent axons of the preoptic nucleus, and (iii) subependymal cells with coarse processes. The vesicular inclusions of the two former structures correspond to the neurophysin vesicles with respect to their size, structure and reactivity. The vesicles of the subependymal cells belong to the same size class, possess a somewhat granular internal structure and react atypically after the application of the ultrahistochemical technique for the identification of neurophysin vesicles. Presumably, their content is a glycoprotein with a high proportion of cystine. The peptidergic axons of the preoptic nucleus projecting to the subfornical organ form neuroneuronal synapses.

Scanning and transmission electron microscopy of the subfornical organ of the grass frog (Rana pipiens)

The ventricular surface of the subfornical organ of the frog is made up of ependymal cells with numerous apical microvilli, occasional cytoplasmic protrusions and many vacuoles projecting into the lumen of the third ventricle. Between these cells dendrites of cerebrospinal fluid-contacting neurons reach the ventricle to terminate in bulbous enlargements. In addition, flask-shaped encephalo-chromaffin cells, containing granulated vesicles and aggregates of filaments in their cytoplasm, project into the cerebrospinal fluid. Surrounding the centrally located capillaries are enlarged dendrites and axons of heterogeneous morphology, some of which appear to originate within the subfornical organ, intermingled with dendrites and axons of normal structure. The glial cells in this region, especially the microglial cells, often contain large lipofuscin inclusions, suggestive of degeneration and subsequent phagocytosis of some of the enlarged dendrites and axons. The normally scarce neurosecretory peptidergic axons become more evident and form typical Herring bodies in stalk-transected animals. Neuronal perikarya of varying morphology are predominantly located peripheral to the region of enlarged dendrites and axons. Supraependymal macrophages are particularly numerous on the subfornical organ.

The development of monamine-containing neurons in the brain and spinal cord of the salamander, Ambystoma mexicanum

The distribution of monoamine-containing neurons in the CNS of the developing and adult axolotl, Ambystoma mexicanum, has been investigated using the histochemical fluorescence technique of Falck and Hillarp combined with microspectrofluorimetry. The earliest catecholamine-containing neurons to be detected are located in the ventral ependymal zone of the spinal cord at the time of hatching (Stage 41). Between stages 43 and 46, catecholamine fluorescence can be detected in neurons in the following regions: nucleus preopticus, the hypothalamic-infundibular region, and the brain stem reticular formation. 5-HT-containing neurons are only observed in the midbrain raphe region and are first detected at stage 44. In contrast to these early monoamine fluorescing groups, catecholamine-containing neurons are not routinely detectable in the nucleus interpeduncularis until six months of age. All monoamine-containing neuronal groups detected in developing axolotls are also present in both sexes of the adult. However, the fluorescence intensity is less in monoamine-containing neurons observed in adults than in early developing subjects. All catecholamine-containing neuronal groups, with the exception of those located in the midbrain region (nucleus interpeduncularis, reticular zone) have fluorescent processes that contact the cerebrospinal fluid (CSF). The presence of CSF-contacting processes in the hypothalamic and spinal cord regions suggest that the CSF may act as a medium through which bioactive substances are transported from one brain region to another. Intense catecholamine fluorescence is observed in cells of the notochord prior to the detection of the monoamine-containing neurons in the CNS. A possible involvement of catecholamines in the inductive effects of the notochord during development is discussed.

Cytological evidence for different types of cerebrospinal fluid-contacting subependymal cells in the preoptic and infundibular recesses of the frog

Blue-green fluorescent subependymal cells with intraventricular processes were shown by the fluorescent histochemical method to be distributed from the preoptic recess to the infundibular recess of the frog hypothalamus. Electron microscopy revealed at least two types of CSF-contacting subependymal cells, type 1 containing large dense granules (about 100-200 nm in diameter) and type 2 containing small dense core vesicles (about 60-100 nm in diameter). Subsequent to fixation in permanganate solution, the small dense core vesicles in type 2 cells reacted with the fixative and consistently showed a dense content. However, the large granules in type 1 cells were mostly pale or less dense after this fixation. Two hours after intraventricular injection of 3H-dopamine, a large number of silver grains appeared only in the cytoplasm of intraventricular processes possessing dense core vesicles (type 2 cells). A few grains were also found in the perikarya. It is concluded that type 2 cells are catecholamine-storing cells. It is suggested that type 1 cells in the infundibular recess are peptidergic neurons which may secrete some hypothalamic regulating hormones of the anterior pituitary. Most of these cells in the preoptic recess belong to the neurosecretory cells of the preoptic nucleus, while some cells probably function similarly to those in the infundibular recess.

Hormonal regulation of gap junction differentiation

Thin-section, tracer, and freeze-cleave experiments on hypophysectomized Rana pipiens larvae reveal that gap junctions form between differentiating ependymoglial cells in response to thyroid hormone. These junctions assemble in large particle-free areas of the plasma membrane known as formation plaques. Between 20 and 40 h after hormone application, formation plaque area increases approximately 26-fold while gap junction area rises about 20-fold. The differentiation of these junctions requires the synthesis of new protein and probably RNA as well. On the basis of inhibitor experiments, it can be reported that formation plaques develop at about 16-20 h after hormone treatment and stages in the construction of gap junctions appear 4-8 h later. These studies suggest that gap junction subunits are synthesized and inserted into formation plaque membrane during the differentiation of the anuran ependymoglial cells.

The lateral recess of the third ventricle in teleosts: an electron microscopic and Golgi study

The ependyma lining the lateral recess of the third ventricle of the teleost inferior lobe has been studied by light and electron microscopy, including Golgi impregnation methods. As many as five different cell types appear to line the ventricle, but some of these may be similar cells in different stages of activity. One cell type contains small dense-cored vesicles and appears to have processes extending into deeper portions of the lobe. Golgi preparations reveal subependymal cells with apical processes extending to the ventricle and basal extensions which may reach the pial surface. The present observations are discussed in relation to similar studies in other fishes, amphibians and mammals. Possible functions for the various cells observed are suggested.

The monoaminergic innervation of the telencephalon of the frog, Rana pipiens

A large number of serotonin (5-hydroxytryptamine, 5-HT)-type axon terminals has been visualized in the so-called 'striatum' (pars ventralis and dorsalis) of Rana pipiens by means of the Falck-Hillarp histochemical method. The frog striatum contains, however, a much smaller number of catecholamine (CA) type axon terminals and differs strikingly in that aspect from the neostriatum of 'higher' vertebrates which is known to receive a massive CA innervation. In the telencephalon of Rana pipiens the highest density of CA axon terminals occurs at the level of the medial wall, which appears highly hypertrophied in comparison to the thin lateral wall. The CA terminals are mainly concentrated within nucleus accumbens septi and in the ventrolateral portion of nucleus lateralis septi where they surround intimately the non-fluorescent neuronal somata of both septal nuclei. The three pallial fields (medial, dorsal and lateral) of Rana pipiens are nearly devoid of CA axon terminals. A moderate amount of 5-HT terminals is present, however, within the caudal half of both the medial and dorsal pallial fields. Finally, a few CA-type neuronal somata are found lying amongst the mitral cells of the olfactory bulb. As a whole, the telecenphalon of Rana pipiens appears more profusely innervated by axons of the 5-HT-type than by those of the CA-type. This finding has been related to the fact that in the frog brain stem the CA neuronal somata are scarce in comparison to the highly developed 5-HT neuronal systems.

The structure of the inferior lobe of the teleost hypothalamus

Electron microscopic and Golgi studies on the inferior lobes of sunfish and goldfish are described. The inferior lobe consists primarily of a nucleus ventricularis of densely packed cells surrounding the lateral recess of the third ventricle, and a peripherally situated nucleus diffusus consisting mostly of scattered neurons. A cell-sparse zone of dense neuropil is located between the two cellular areas. Neurons of both nuclei have spiny dendrites and axons which originate from basal dendrites. In some cases axons are found to send a collateral into the cell-sparse zone. Neurons of the nucleus diffusus possess collaterals that extend a considerable distance within the nucleus itself. The ultrastructure of cells of both nuclei reveals cytoplasmic organelles typical of most neurons. Synapses containing dense-cored and clear vesicles are present on the spines and shafts of the dendrites of both neuronal types. In only rare cases synapses were observed on the soma of neurons of the nucleus ventricularis. Possible anatomical substrates involved in the control of feeding and aggression in teleosts are considered in light of the present findings. Morphological similarities of the inferior lobes and related areas in various fishes and amphibians are discussed and their possible significance for the understanding of the evolution of hypothalamic mechanisms is considered.