Quantitative and ultrastructural study of ascending projections to the medial mammillary nucleus in the rat

Summary We analyzed the termination pattern of axons from the superior central nucleus and the ventral tegmental nucleus of Gudden within the medial mammillary nucleus (MM) in the rat. The neuropil of the MM consists of two classes of terminals, that is, terminals containing round synaptic vesicles and forming asymmetric synaptic contact, and terminals containing pleomorphic synaptic vesicles and forming symmetric synaptic contact. The number of axodendritic terminals with round vesicles is almost equal to that of terminals with pleomorphic vesicles. Almost all axosomatic terminals contain pleomorphic vesicles with symmetric synaptic contact. Injection of WGA-HRP into the central part of the superior central nucleus permitted ultrastructural recognition of many anterogradely labeled terminals within the median region of MM. The labeled terminals contacted mainly intermediate (1–2 m diameter) and proximal dendrites (more than 2 m diameter) as well as the neuronal somata. Serial ultrathin sections of neurons of the median region of the MM revealed that 37% of the axosomatic terminals were labeled anterogradely. The pars compacta of the superior central nucleus had reciprocal connections with the median region of MM. The axon terminals from this nucleus occupied 53% of axosomatic terminals, and contacted mainly intermediate dendrites. Following injection of WGA-HRP into the ventral tegmental nucleus, many labeled terminals were found in the medial and lateral regions of MM. They contacted mainly intermediate dendrites as well as neuronal somata. In the medial region, 78% of axosomatic terminals contacting retrogradely labeled neurons were labeled anterogradely. All labeled terminals from these nuclei contained pleomorphic vesicles, and made symmetric synaptic contact.     

Ultrastructural study of ascending projections to the lateral mammillary nucleus of the rat

Summary We examined the synaptic organization of ascending projections from the pars ventralis of the dorsal tegmental nucleus of Gudden (TDV) and the laterodorsal tegmental nucleus to the lateral mammillary nucleus (LM). The LM neuropil consists of terminals containing pleomorphic synaptic vesicles and forming symmetric synaptic contact, and terminals containing round synaptic vesicles and forming asymmetric synaptic contact. They make up 63% and 37%, respectively, of all axodendritic terminals. All axosomatic terminals contain pleomorphic vesicles and make symmetric contact. Following injection of WGA-HRP into the TDV, many anterogradely labeled terminals and retrogradely labeled cells are found in the LM. Labeled terminals contact mainly proximal (more than 2 m diameter) and intermediate (1–2 m diameter) dendrites. Serial ultrathin sections of the LM show that 55% of axosomatic terminals are labeled anterogradely. Following injection of WGA-HRP into the laterodorsal tegmental nucleus, many anterogradely labeled terminals are found in the LM, but no retrogradely labeled cells are present. Labeled terminals contact mainly distal (less than 1 m diameter) and intermediate dendrites as well as somata. In the LM neurons, 46% of axosomatic terminals are labeled anterogradely. All labeled terminals from these nuclei contain pleomorphic vesicles and make symmetric synaptic contact. These results indicate that almost all axosomatic terminals come from the TDV and the laterodorsal tegmental nucleus, which send inhibitory inputs to the lateral mammillary nucleus.     

On certain fluorescent axon terminals containing granular synaptic vesicles in the cerebellar nucleus lateralis

Summary Thin, unmyelinated, green fluorescent fibers bearing fine beads or varicosities have been found in the neuropil and near blood vessels of the nucleus lateralis. In electron micrographs these fibers are identifiable as a class of axons, the CAT fibers, which contain large and small granular synaptic vesicles and agranular vesicles in their varicosities. There are two types of CAT fiber. 1) The CAT₁ terminals contain many large and elongated vesicles, 700–1700 Å in size, with dark, homogeneously dense centers; a few small granular vesicles each with an intensely osmiophilic particle, and small agranular vesicles. These terminals have not been seen in synaptic contact with other elements of the neuropil. 2) The CAT₂ terminals have a very thin unmyelinated connecting thread between small varicosities. The varicosities contain small agranular synaptic vesicles and small granular ones containing either a single dense particle, or an elliptical, intensely osmiophilic droplet flanked by lighter semicircular particles. Large granular vesicles, 750–950 Å each, with a variably dense center, are also found. These terminals form conventional axodendritic synapses with Gray's type 1 synaptic junctions and the subsynaptic specialization of Taxi, as well as synapses on thorns of spiny neurons.It is suggested that the CAT₁ and CAT₂ fibers may be the electron microscope equivalents of norepinephrine- and 5-hydroxytryptamine-containing fluorescent axons. These fibers probably have extrinsic origins since no fluorescent cells or perikarya with small or large granular vesicles have been found in the lateral nucleus. Their origins, however, are unknown. The proximity of these fluorescent fibers to blood vessels is discussed, and their functions are the subject of some speculation.Supported in part by U.S. Public Health Service grants NS10536, NS03659, Training grant NS05591 from the National Institute of Neurological Diseases and Stroke and a William F, Milton Fund Award from Harvard University.     

Choline acetyltransferase immunoreactivity is localized to four types of synapses in the rat interpeduncular nucleus

Summary The cholinergic synapses of the rat interpeduncular nucleus (IPN) were demonstrated by immunostaining that utilized a monoclonal antibody directed against choline acetyltransferase. The rostral, central, intermediate and lateral subnuclei of the IPN each contained a single type of immunoreactive terminal. Immunoreactivity was localized to synaptic vesicle membranes, inner and outer mitochondrial membranes, the cytoplasmic surface of plasma membranes (especially at the contact zones), and longitudinal microtubules in preterminal portions of axons. Terminals were identified by comparison to previous studies of the synaptic organization of the IPN. In the rostral subnucleus, the immunoreactive terminals were characterized by their content of spherical vesicles, 45 nm in diameter, intermixed with moderate numbers of dense-cored vesicles, 75–100 nm in diameter. These terminals formed asymmetrical contacts. They correspond to the more numerous of the two types of axodendritic terminals described in this subnucleus, i.e. those which degenerate after lesions of the habenula. The moderate number of immunoreactive terminals in the lateral subnucleus contained pleomorphic vesicles, 30–45 nm in diameter. Up to three of these formed symmetrical contacts with individual dendrites, which ranged in diameter from 0.35 to 0.55m. The other types of axodendritic terminal in this subnucleus, which often contacted the same dendrites, were unstained. These latter terminals have been interpreted as being those which contain substance P. The immunoreactive terminals in the central subnucleus consisted of moderate numbers of S terminals. These contained spherical vesicles, 40–60 nm in diameter, and formed markedly asymmetricalen passant contacts with small dendritic processes or spines. The immunoreactive terminals in the intermediate subnucleus had the same presynaptic and contact morphology. Many were clearly crest synapses. The remainder appeared to be such, but seen only partially within the plane of section. In the intermediate subnucleus there were up to several hundred immunostained terminals per grid square in some sections.These findings are consistent with the existence of a dense cholinergic projection to the IPN. The crest and S synapses, both of which degenerate after lesions of the habenular region, are shown to be cholinergic as previously suggested. Additionally, demonstration of the cholinergic innervation of other subnuclei of the IPN increases understanding of the relation of cholinergic to other transmitters localized to various portions of this nucleus.On leave of absence from Semelweis University, Hungary.     

Correlation between noradrenaline content of the brain and the number of granular vesicles in rat hypothalamus during nialamid administration

Summary The fine structure of ventromedial hypothalamic nucleus was studied in the rat with the electron microscope under circumstances of elevated brain monoamine level following treatment with a monoamine oxidase inhibitor Nialamid. The number of granular vesicles (size in diameter 450–1100 Å) in synaptic terminals increased after Nialamid treatment significantly, while their size did not change; the number of agranular vesicles remained unchanged. The time courses of the increase of granular vesicles and elevation of brain noradrenaline content were approximately parallel. It is inferred that the granular vesicles of size 450 to 1100 Å may possibly be the storage sites of noradrenaline.     

An immunohistochemical study of somatostatin and neurotensin positive neurons in the septal nuclei of the rat brain

Summary Antibodies to the neuropeptides somatostatin (SOM) and neurotensin were used to study the distribution of the two peptides within the septum of the rat brain. In colchicine treated rats, numerous somatostatin-positive cell bodies were found in the dorsal and ventral subdivisions of the alteral septum, along the border of the nucleus accumbens, in the ventral tip of the horizontal limb of the diagonal band of Broca as well as in the anterior hippocampal rudiment, infralimbic area and several other structures of the basal forebrain (e.g., nucleus accumbens, olfactory tubercle and substantia innominata). Cell bodies containing immunoreactivity for neurotensin were situated in the intermediate and ventral subdivisions of the lateral septum, the medial septal nucleus, the diagonal band of Broca, the rostro-medial continuation of the substantia innominata and the olfactory tubercle.In untreated rats, somatostatin positive processes formed terminal plexuses in the medial septal nucleus and along an area close to the ventricular wall of the lateral ceptal nucleus. Other septal nuclei, such as the diagonal band of Broca contained a sparse innervation by somatostatin positive fibers. In contrast, the nucleus accumbens olfactory tubercle, and the substantia innominata contained a rich innervation by somatostatin positive axons and terminals. Within these structures the density of SOM positive processes show great variations with patches of densely packed terminals separated by areas of sparser or no innervation. The neurotensin positive terminals were situated predominantly within the intermediate part of the lateral septum and the medial septal nucleus. Both of these regions contained numerous pericellular baskets of neurotensin positive terminals around septal neurons. In addition to the septal innervation, several of the basal forebrain structures were rich in neurotensin positive processes with the densest innervation found in the nucleus accumbens and substantia innominata. Like the SOM-immunoreactivity distinct islands of dense neurotensin innervation separated by less or no innervation occur throughout the basal forebrain. Taken together, these findings suggest that somatostatin and neurotensin occur in separate neuronal populations and that each may influence important physiological functions within the individual septal nuclei.     

Organization of the projections from the subiculum to the ventral striatum in the rat. A study using anterograde transport of Phaseolus vulgaris leucoagglutinin

The projections of the subiculum, as the main output structure of the hippocampal formation, to the striatum were studied in the rat using the anterograde tracer Phaseolus vulgaris leucoagglutinin. It appears that not only the entire nucleus accumbens, part of the so-called ventral striatum, receives fibres from the subiculum, but that the hippocampal projection area in the striatum includes also the most medial, ventral, rostral and caudal parts of the caudate-putamen complex. Moreover, a relatively small number of fibres and terminals are present in the striatal elements of the medial part of the olfactory tubercle. The projections to the ventral and caudal parts of the caudate-putamen are predominantly derived from the ventral subiculum, whereas the projections to the rostral part of the caudate-putamen are derived from the dorsal subiculum. Furthermore, with respect to the subiculum-accumbens pathway a topographical organization could be established. Thus, the ventral or temporal part of the subiculum projects predominantly to the caudomedial part of the nucleus accumbens, and to a lesser degree to its rostromedial portion, whereas progressively more dorsal or septal parts of the subiculum send fibres to successively more lateral and rostral portions of the nucleus accumbens. Very sparse projections are found to the contralateral nucleus accumbens, arranged in a topographical manner similar to the ipsilateral projections. An important observation with respect to the structure of the nucleus accumbens is that the subicular terminations are inhomogeneously distributed, although a relation with earlier described mosaic patterns in the connectivity and neurochemical composition of the nucleus is not yet clear. Subicular fibres have their densest terminations in relatively cell-poor regions of the nucleus accumbens, and in particular tend to avoid small cell clusters.     

Neuron morphology and synaptic architecture in the medial superior olivary nucleus

Summary Dendritic arborization pattern, spatial and synaptic relations of various neuron types and the terminal distribution of afferent axons of various origin were studied in the medial superior olivary nucleus of the cat using Golgi, degeneration, electron microscope and horseradish peroxidase techniques. Three types of neurons clearly different in morphological features, distribution, neighbourhood relations, input and output characteristics were distinguished: (1) fusiform cells having specific dendritic orientations and arborization patterns and synaptic relations to various types of terminal axon arborizations (2) multipolar neurons with wavy dendrites bearing spine-like appendages, receiving relatively few synaptic contacts and having a locally arborizing axon, and (3) elongated marginal cells, largely restricted to the fibrous capsule of the nucleus. The fusiform and marginal neurons were identified by retrograde peroxidase labeling as the olivo-collicular projection cells.Ultrastructural analysis of normal and experimental material revealed the presence of four distinct kinds of axon terminals differing in size, synaptic vesicles type, relation to postsynaptic targets and in origin: (i) large terminals with multiple extended asymmetric synaptic membrane specializations and containing round, clear vesicles arise from the spherical cells of the ipsilateral anteroventral cochlear nucleus, (ii) most of the small axon terminal profiles — engaged in asymmetric synaptic contacts — originated from the trapezoid nucleus, (iii) terminal boutons containing pleomorphic vesicles belong to fibers descending from the ipsilateral multipolar neurons in the central nucleus of the inferior colliculus and from the nuclei of the lateral lemniscus while (iv) boutons containing exclusively ovoid vesicles and remaining intact after complete deafferentation of the nucleus were considered to be of local origin.     

Anterograde tracer study on the nucleus geniculatus dorsalis and its internal synaptic structure in chick brain

In the present study the terminals of retinal fibres and those of internal layer cells in ventral geniculate nucleus of chicks were labelled with the anterograde tracer biotinylated dextran amine. The tracer showed the connections from the internal cell layers of ventral geniculate nucleus to the medial part of the dorsal lateral geniculate nucleus. The labelled retinal terminals were located exactly in the lateral part of nucleus. The labelled terminals in the two parts of the nucleus were analysed with the electron microscope and showed a different synaptic organisation in the two parts of the dorsal lateral geniculate nucleus. In the lateral part, two kinds of synaptic glomeruli were found mostly in the vicinity of large dendrites, which are proximal dendrites of projection neurons. One type is a simple glomerulus containing a large dendrite, a large optic terminal and a large and/or series of asymmetrical synapses surrounded by glial processes. The other type is a complex synaptic unit with several pre- and postsynaptic components, among them synapses of GABA-positive axon terminals and/or dendraxons. No glomeruli were found in the medial part of the nucleus. In the medial part of the lateral geniculate nucleus, the terminals of internal layer cell axons established asymmetrical synapses with dendrites. Often, a large terminals and large dendritic profiles established serial asymmetrical synapses. GABA-positive myelinated fibres entered and ramified in both parts of the dorsal lateral geniculate nucleus, and GABA-positive terminals were seen to form synapses on the same dendrite near to the asymmetrical contacts. To our knowledge, this is the first report of the connection from ventral geniculate internal layer cells to the dorsal lateral geniculate nucleus in the chick.     

Ultrastructure of the anterior ventral and anterior medial nuclei of the cat thalamus

Summary Electron microscopical studies of the thalamic AV-AM nuclei substantiated the presence of two main types of neurons, i.e. principal (or relay) cells and Golgi type II interneurons. Characteristic synaptic islands are found in abundance in the AV-AM. Four different types of synaptic terminals have been identified in these islands: RL-boutons = large axonal terminals with round synaptic vesicles; RS-boutons = small axonal terminals with round synaptic vesicles; F₁-boutons = small axonal profiles containing flattened synaptic vesicles, and F₂-profiles interpreted as presynaptic dendrite appendages, bearing pleomorphic vesicles, both belonging to the Golgi type II interneurons. — The synaptic relations were studied in normal preparations and after lesions in the mamillary body, limbic cortex and hippocampus. The specific afferents (RL-boutons) — originating from the medial mamillary nucleus — are presynaptic to both relay cell dendrites and presynaptic dendrite profiles of Golgi type II interneurons, which in turn are presynaptic to the same relay dendrites (synaptic triads). RS-boutons originate mainly from limbic cortex and hippocampus.     

Direct projections from Ammon's horn to the septum in the cat

Summary Direct projections from Ammon's horn to the septum were studied in the cat by the anterograde tracing method after injecting WGA-HRP (wheat germ agglutinin-horseradish peroxidase conjugate) into Ammon's horn. The results were further confirmed by the retrograde WGA-HRP method after injecting WGA-HRP into the septum. Pyramidal neurons in fields CA1, CA2 and CA3 were observed to send their axons ipsilaterally to the lateral septal nucleus; the septal parts of the hippocampus sent projection fibers to the dorsomedial portions of the lateral septal nucleus via the medial aspects of the subcallosal fornix, while the hippocampal regions successively more proximal to the temporal pole sent projection fibers to progressively more ventrolateral portions of the lateral septal nucleus via more lateral aspects of the subcallosal fornix. It was also found that the septal parts of fields CA1, CA2 and CA3 sent projection fibers bilaterally to the dorsomedial aspects of the lateral septal nucleus. Field CA4 appeared to send projection fibers only sparsely, if at all, to the medial septal nucleus. The rudimentary parts of the hippocampal formation, taenia tecta and indusium griseum, were found to have reciprocal ipsilateral connections with the dorsal portions of the lateral septal nucleus.     

Electron microscopic investigation of the vestibular projection to the cat trochlear nuclei

Ultrastructural degeneration studies were carried out on the cat trochlear nucleus following lesion of the vestibulo-trochlear pathway in order to characterize the location and type of presynaptic endings involved in this pathway. Four types of boutons are found in the normal trochlear nucleus. Types I and II are large and demonstrate typical en passant profiles with small diameter synaptic vesicles (35 and 40 nm). These terminals are characterized by the absence of neurofilaments in the Type II endings. Types III and IV are smaller boutons, located more axondendritically, and contain larger diameter synaptic vesicles (45 nm). Type V terminals contain large, granulated vesicles and occur only rarely. Following the interruption of the ascending projection from the ipsilateral superior and medial vestibular nuclei by parasagittal medullary lesions, degeneration of Type II boutons was commonly encountered in the ipsilateral trochlear nucleus. Predominantly Type III degeneration was found in the contralateral trochlear nucleus. Electrical stimulation of the vestibular nerve showed that these lesions resulted in (1) a complete loss of inhibition in the ipsilateral trochlear nucleus and (2) a significant (75-90%) reduction in the contralateral excitatory pathway to the trochlear nucleus. Midline sagittal lesions in the floor of the fourth ventricle interrupting the decussating fiber projection from the bilateral medial vestibular nuclei resulted in selective degeneration of only Type III boutons in both trochlear nuclei. We conclude that inhibitory vestibular neurons eminating from the superior vestibular nucleus terminate on trochlear motoneurons with Type II boutons and excitatory vestibular neurons from the contralateral medial vestibular nucleus end on trochlear motoneurons with Type III boutons.     

Ultrastructure and synaptic organization of the spinal accessory nucleus of the rat

The accessory nucleus is composed of neurons in the medial column that innervate the sternocleidomastoid muscle, and neurons in the lateral column that innervate the trapezius muscle. We retrogradely labeled these neurons by injection of cholera toxin conjugated horseradish peroxidase into the sternomastoid (SM) or the clavotrapezius (CT) muscles, and investigated fine structure and synaptology of these neurons. Almost all SM and CT motoneurons had the appearance of alpha-motoneurons, i.e., large, oval or polygonal cells containing well-developed organelles, Nissl bodies, and a prominent spherical nucleus. More than 60% of the somatic membrane was covered with terminals. The SM motoneurons (34.4 x 52.2 microm, 1,363.1 microm(2) in a section) were slightly larger than the CT motoneurons (32.8 x 54.2 microm, 1,180.8 microm(2)). The average number of axosomatic terminals in a section was 52.2 for the SM, and 54.2 for the CT motoneurons. More than half of them (58.0%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II) with the SM motoneurons, while 57.9% of them contained round vesicles and made asymmetric synaptic contacts (Gray's type I) with the CT motoneurons. A few C-terminals were present on the SM (3.5) and the CT (3.7) motoneurons. About 60% of the axodendritic terminals were Gray's type I in both the SM and the CT motoneurons. A few labeled small motoneurons were also found among the SM and the CT motoneurons. They were small (19.2 x 26.2 microm, 367.0 microm(2)), round cells containing poorly developed organelles with a few axosomatic terminals (9.3). Only 20% of the somatic membrane was covered with the terminals. Thus, these neurons were presumed to be gamma-motoneurons. These results indicate that the motoneurons in the medial and the lateral column of the accessory nucleus have different ultrastructural characteristics.     

Ontogenetic development of septal nuclei in the rat

Summary Prenatal development of septal cell groups was studied in the rat on samples taken daily from the 14th day of gestation until birth. Coronal serial sections of brains were prepared in which the topography coordinates of septal nuclei were determined, their section profiles measured and their volumes calculated. The rat septum begins to develop on embryonic days 14–15. First the individual neurons start to differentiate, then cell groups characteristic for the adult are formed between days 14 and 17, which is followed by the delineation of nuclei. The only exception is the anterior subdivision of the lateral septal nucleus where the formation of the nucleus precedes the differentiation of its constituent cells. The individual nuclei start to develop at different times defined by a medio-lateral gradient of cell migration. By embryonic day 20 the formation of the nuclei can be considered as complete: all septal nuclei and their subdivisions are to be recognized and distinguished from each other.Abbreviations CA Anterior commissure - CC corpus callosum - CH hippocampal commissure - dm dorsal septal nucleus, magnocellular part - dp dorsal septal nucleus, parvocellular part - f fimbrial septal nucleus - FI fimbria septi - FX fornix - H hippocampal rudiment - i intermediate septal nucleus - l lateral septal nucleus - la lateral septal nucleus, anterior part - ld lateral septal nucleus, dorsal part - lv lateral septal nucleus, ventral part - md medial septal nucleus, dorsal part - mv medial septal nucleus, ventral part - NE neuroepithelium - ni bed nucleus of the commissura fornicis - po median preoptic nucleus - sfo subfornical organ - t triangular septal nucleus - td nucleus of the diagonal tract (Broca) - VL lateral ventricle     

Fine structural survey of the intermediate subnucleus of the nucleus tractus solitarii and its glossopharyngeal afferent terminals

The intermediate subnucleus of the nucleus tractus solitarii (imNTS) receives somatosensory inputs from the soft palate and pharynx, and projects onto the nucleus ambiguus, thus serving as a relay nucleus for swallowing. The ultrastructure and synaptology of the rat imNTS, and its glossopharyngeal afferent terminals, have been examined with cholera toxin-conjugated horseradish peroxidase (CT-HRP) as an anterograde tracer. The imNTS contained oval or ellipsoid-shaped, small to medium-sized neurons (18.2×11.4 μm) with little cytoplasm, few cell organelles and an irregularly shaped nucleus. The cytoplasm often contained one or two nucleolus-like stigmoid bodies. The average number of axosomatic terminals was 1.8 per profile. About 83% of them contained round vesicles and formed asymmetric synaptic contacts (Gray’s type I), while about 17% contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray’s type II). The neuropil contained small or large axodendritic terminals, and about 92% of them were Gray’s type I. When CT-HRP was injected into the nodose ganglion, many labeled terminals were found in the imNTS. All anterogradely labeled terminals contacted dendrites but not somata. The labeled terminals were usually large (2.69±0.09 μm) and exclusively of Gray’s type I. They often contacted more than two dendrites, were covered with glial processes, and formed synaptic glomeruli. A small unlabeled terminal occasionally made an asymmetric synaptic contact with a large labeled terminal. The large glossopharyngeal afferent terminals and the neurons containing stigmoid bodies characterized the imNTS neurons that received pharyngeal afferents.     

The ultrastructure of prosternal sensory hair afferents within the locust central nervous system

The sensory neurones innervating long prosternai hairs of Locusta migratorioides were backfilled with horseradish peroxidase through their dendrites. The neurones' central projections in and around the medial ventral tract were examined with electron microscopy. Most synapses occur on axon collaterals which ramify through the neuropile around the tract where both input and output synapses were observed. Serial sectioning methods were used to determine the relative distribution of inputs and outputs which often lie in close proximity to one another on the axon terminals. The prosternai hair terminals contain agranular synaptic vesicles approximately 37 nm in diameter. Surrounding unidentified neuropilar profiles contain vesicles which are either statistically indistinguishable in size, or are larger, 45 nm diameter agranular vesicles. Neurones which are pre- or postsynaptic to labelled terminals generally contain vesicles of the second type.Input synapses onto the central terminals of primary afferent neurones can be recognised as a widespread phenomenon in the nervous systems of both invertebrates and vertebrates which will allow a fine degree of control of sensory inflow into the central nervous system.     

Restricted origin and distribution of projections from the lateral to the medial septal complex in rat and guinea pig.

The origin and distribution of projections from the lateral to the medial septal complex were studied at the light- and electron-microscopical level in the rat and the guinea pig, with the use of sensitive anterograde tracing techniques. Injections in the lateral septal complex resulted in only weak to moderate labeling in the medial septal nucleus. In contrast, injections in all but the ventral subdivision of the lateral septal complex labeled restricted terminal arborizations in the angular portions of the vertical diagonal band. Analyses at the electron-microscopical level indicated that these fibers form both symmetrical and asymmetrical synapses with dendrites and somata, but not with axons. These findings are discussed in the light of a presumed functional circuit from the hippocampal formation, via the lateral septal complex, to cells in the medial septal complex that originate projections to the hippocampal formation.     

The entorhino-septo-supramammillary nucleus connection in the rat: morphological basis of a feedback mechanism regulating hippocampal theta rhythm.

Recent electrophysiological observations suggest that, in addition to the medial septal area pacemaker system, several alternative or additional mechanisms are involved in the generation/regulation of hippocampal theta activity. Discharging neurons phase-locked to hippocampal theta waves have been observed in the dorsal raphe, nucleus reticularis pontis oralis and especially in the supramammillary region of rats. Since these areas are reciprocally interconnected with the hippocampal formation, including the entorhinal cortex, it would aid our understanding of limbic function to elucidate the location and neurochemical content of the entorhino-septal and septo-supramammillary projection neurons, as well as that of their postsynaptic targets. Light and electron microscopic immunostaining for calretinin, in combination with antero- and retrograde tracer techniques, postembedding immunostaining for GABA and the transmitter specific [3H]D-aspartate retrograde radiolabeling, as well as a co-localization experiment for calretinin and glutamate decarboxylase in rat supramammillary and septal neurons, demonstrated that: (i) a large population of entorhinal cells that forms asymmetric synaptic contacts on calretinin-containing neurons located at the border between the medial and lateral septal areas contains calretinin and are aspartate/glutamatergic; (ii) the overwhelming majority of calretinin-immunoreactive cells located at the border between the lateral and medial septal area are GABAergic; (iii) these neurons can be retrogradely labeled from the supramammillary area; (iv) anterogradely labeled axons originating in the border between the medial and lateral septum are GABAergic and (v) terminate on supramammillary area non-GABAergic, calretinin-containing neurons, which are known to project to the septal complex and hippocampus. These observations indicate that a large population of cells participating in the hippocampal feedback regulation of theta regulation/generation contain the same calcium-binding protein. Furthermore, entorhinal excitatory transmitter-containing neurons can depress the activity of supramammillary theta generating/regulating cells via septal inhibitory neurons.     

Efferent projections of the periaqueductal gray in the rabbit.

The efferent projections of the periaqueductal gray in the rabbit have been described by anterograde tract-tracing techniques following deposits of tritiated leucine, or horseradish peroxidase, into circumscribed sites within dorsal, lateral or ventral periaqueductal gray. No attempts were made to place labels in the fourth, extremely narrow (medial), region immediately surrounding the aqueduct whose size and disposition did not lend itself to confined placements of label within it. These anatomically distinct regions, defined in Nissl-stained sections, corresponded to the same regions into which deposits of horseradish peroxidase were made in order for us to describe afferent projections to the periaqueductal gray. In this present study distinct ascending and descending fibre projections were found throughout the brain. Terminal labelling was detected in more than 80 sites, depending somewhat upon which of the three regions of the periaqueductal gray received the deposit. Therefore, differential projections with respect to both afferent and efferent connections of these three regions of the periaqueductal gray have now been established. Ventral deposits disclosed a more impressive system of ramifying, efferent fibres than did dorsal or lateral placements of labels. With ventral deposits, ascending fibres were found to follow two major pathways from periaqueductal gray. The periventricular bundle bifurcates at the level of the posterior commissure to form hypothalamic and thalamic components which distribute to the anterior pretectal region, lateral habenulae, and nuclei of the posterior commissure, the majority of the intralaminar and midline thalamic nuclei, and to almost all of the hypothalamus. The other major ascending pathway from the periaqueductal gray takes a ventrolateral course from the deposit site through the reticular formation or, alternatively, through the deep and middle layers of the superior colliculus, to accumulate just medial to the medial geniculate body. This contingent of fibres travels more rostrally above the cerebral peduncle, distributing terminals to the substantia nigra, ventral tegmental area and parabigeminal nucleus before fanning out and turning rostrally to contribute terminals to ventral thalamus, subthalamus and zona incerta, then continuing on to supply amygdala, substantia innominata, lateral preoptic nucleus, the diagonal band of Broca and the lateral septal nucleus. Caudally directed fibres were also observed to follow two major routes. They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus. These ventrally directed fibres merge just dorsal to the pons on the ventral surface of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electrophysiological analysis of the hippocampal-septal projections: I. Response and topographical characteristics

Summary The hippocampal input to the septal region was studied, in rats, by electrophysiological methods. The major observations were: 1. the hippocampal fibers are excitatory with respect to their target cells in the lateral septal nucleus (LSN); 2. the distribution of the subcallosal fornix is restricted to the dorsomedial regions of the LSN; 3. the fimbria contains fibers terminating throughout the dorsal aspect of the ipsilateral and contralateral septum; 4. there is an extensive overlapping in the distribution of the ipsi-and contra-lateral fimbria components with convergence upon single septal cells frequently seen; 5. the posterior part of hippocampal region CA1 contributes fibers to the fimbria as well as to the fornix.Abbreviations ACB Bed nucleus of the anterior commissure - AP Ref 0 The anterior-posterior and midline reference zero - CA Anterior commissure - CC Corpus callosum - CCA 3–4 Contralateral hippocampal fields CA-3–4 - Cd Pt Caudate putamen complex - CFim Contralateral fimbria - CMA Corticomedial maygdaloid division - GP Globus pallidus - Hipp Hippocampus - ICA1 (a) Anterior part of the ipsilateral hippocampal field CA1 - ICA1 (p) Posterior part of the ipsilateral hippocampal field CA1 - ICA3–4 Ipsilateral hippocampal field CA3–4 - IFim Ipsilateral fimbria - IFx Ipsilateral fornix - LSN Lateral septal nucleus - ME Microelectrode - MSN Medial septal nucleus - POA Pre-optic area This study was supported by U.S. Public Health Service Grants RR 5384 and NB 00405.