Morphology of neuropeptide Y-immunoreactive neurons in the cat olfactory bulb and olfactory peduncle: postnatal development and species comparison

The distribution and morphology of Neuropeptide Y-immunoreactive (NPY-ir) neurons in the olfactory bulb and the olfactory peduncle was studied in the adult cat and rat, and the common marmoset Callithrix jacchus. Significant species differences were not observed. In all three species, the population of NPY-ir neurons is localized in the white matter extending from the main olfactory bulb to the border of the striatum. The neurons are characterized by a conspicuously looping axonal ramification pattern with some major collaterals running toward the olfactory bulb and others running toward the internal olfactory tract. The former, ipsilateral projection terminates in the granule cell layer of the main and accessory olfactory bulb and in layer II/III of the anterior olfactory nucleus. Reconstruction of the latter projection has revealed that the fibers are continuous with the olfactory limb of the anterior commissure and the anterior commissure proper suggesting a commissural contralateral projection. The analysis of the postnatal development of the cat NPY neuron system supports this assumption in a very clear-cut way. In young animals growing fibers are observed to cross the brachium of the commissure. The NPY neuron system develops postnatally. The maximum cell number is reached during the third postnatal week. The appearance of more and more NPY-ir neurons slightly precedes the formation of the terminal fields and of the fiber projection in the internal olfactory tract. The density of this early fiber projection by far exceeds the fiber density observed in the adult. Later in development the fiber density in the olfactory limb and the anterior commissure becomes considerably reduced. In contrast, the plexus density in the anterior olfactory nucleus and the granule cell layer of the main and accessory olfactory bulb undergoes only a slight reduction, and the NPY-ir cell number remains roughly constant. These observations suggest that the ipsilateral NPY-ir projection remains largely unchanged, in contrast to the contralateral projection, which exists to a large extent only for the first four postnatal months. The observation that the NPY neuron system gives rise to a contralateral projection does not support a classification of NPY neurons as short axon cells.

A Golgi study on the accessory olfactory bulb in the snake, Elaphe quadrivirgata

The intrinsic organization of the accessory olfactory bulb (AOB) in the snake was studied using the rapid Golgi method. A distinct laminar organization was observed in the snake AOB. Beginning with the most superficial surface, the following layers were distinguished: the layer of the vomeronasal fibers, the olfactory glomeruli, the mitral cells, the deep fiber plexus, the granule cells and the ependymal cells. While the general organizational pattern of the snake AOB resembles that of the main olfactory bulb (MOB) and the AOB reported in various vertebrate species, the present study shows that: (1) the external and internal plexiform layers cannot be identified as independent layers and are considered to be included in the mitral cell layer; (2) the afferent and efferent paths, which are disseminated in the granule cell layer in the mammalian MOB, accumulate external to the granule cell layer to form the layer of the deep fiber plexus: and (3) as a result of accumulation of the afferent and efferent paths in the layer of the deep fiber plexus, the granule cell layer is very fiber-sparse. These structural patterns are quite similar to those of the snake MOB.

A Golgi study on the main olfactory bulb in the snake Elaphe quadrivirgata

The intrinsic organization of the main olfactory bulb in the snake was studied using the rapid Golgi method. A distinct laminar structure was recognized. From the periphery inward, the following layers were distinguished: the layer of the olfactory fibers, the olfactory glomeruli, the mitral cells, the deep fiber plexus, the granule cells and the ependymal cells. Olfactory fibers derived from the nasal cavity reached the entire surface of the bulb, forming a dense fiber plexus, then swung deeply and terminated in the olfactory glomeruli which were arranged in 2-4 rows. The mitral cell layer occupied a wide zone and was composed of scattered mitral cells. The mitral cells had 2-9 primary dendrites proceeding externally to terminate in the olfactory glomeruli and 2-4 secondary dendrites extending tangentially in the mitral cell layer to be distributed therein. The axons of the mitral cells travelled deeply and entered the layer of the deep fiber plexus. The deep fiber plexus was the path for the bulbar efferent and afferent fibers and could be traced caudally as the main olfactory tract, up to the anterior olfactory nucleus and vicinity. The granule cell layer was composed of small cells, the granule cells, packed closely with no special arrangement. The granule cells had long processes which extended superficially to be distributed mainly in the mitral cell layer. The ependymal cells were located at the deepest layer forming the wall of the olfactory ventricle and generated a long process which extended towards the surface to terminate in the peripheral portion of the bulb. In the snake bulb, the well-documented external and internal plexiform layers were considered to be included in the wide mitral cell layer. Thus, while several specific structures were observed, the fundamental organization of the main olfactory bulb in the snake seemed to be identical to that of the main olfactory bulb in various other vertebrate species.

Persistence of the pars externa system of the anterior olfactory nucleus in a microsmatic primate, Callithrix jacchus

The pars externa (PE) system of the anterior olfactory nucleus (AON) in a primate, Callithrix jacchus, was defined by its architecture and by its connection patterns with the main olfactory bulb (MOB) as revealed by tracing techniques. Focal, unilateral injections of wheat germ agglutinin-conjugated horseradish peroxidase into the MOB yielded ipsilaterally labelled afferent neurons in all subdivisions of the AON, with the exception of a clearly circumscribed area in the ventrolateral retrobulbar field of the basocaudal frontal lobe; in the contralateral hemisphere, this same area contained intensely stained neurons forming a horizontal flat plate of small neurons. This unique commissural connection pattern parallels the organization of the PE to MOB connection in sub-primates (Schoenfeld and Macrides, 1984, J. Comp. Neurol., 227: 121-135). Thus, despite earlier controversy (Crosby and Humphrey, 1939, J. Comp. Neurol., 71: 121-213), there appears to be a PE system in a microsmatic primate whose organization is quite similar to that in sub-primates.

Interbulbar axonal collateralization and morphology of anterior olfactory nucleus neurons in the rat

The organizational patterns of the bilateral projections of the anterior olfactory nucleus (AON) to the main olfactory bulb (MOB) were defined in the rat with Golgi staining, HRP tracing-methods and fluorescent dyes. Three issues were addressed: (1) description of the morphology of the AON-neurons projecting to the MOB, (2) quantitative analysis of the bilateral pathways arising in different AON subdivisions and (3) ultrastructural identification of AON to MOB channels. The cytoarchitectural features of the AON as recognized in Golgi preparations were correlated with its neural architecture as revealed by retrograde HRP-tracing from the MOB. The following cell types were determined: (1) pyramidal like neurons typified by a lack of basal dendrites and a sparse covering with long spines (pars externa), (2) fusiform shaped cells with bipolar dendritic arborisations (pars medialis) and (3) densely spined fusiform, pyramidal, and polygonal neurons (pars ventroposterior, lateralis and dorsalis) with a tendency of radial orientation of their apical dendrites. In addition, in the more caudal parts of the pars ventroposterior there were neurons with tertiary dendritic processes oriented nearly parallel to the molecular layer. Quantitative analysis of AON neurons projecting to the MOB showed that the pars externa neurons project exclusively to the contralateral MOB while pars medialis neurons project almost exclusively to the ipsilateral MOB. All subdivisions of the AON which establish specific termination patterns within the MOB, participated in about equal portion in the ipsilateral projections to the MOB. The highest proportion of the bilaterally projecting neurons were found in the dorsal subdivision, followed by the lateral and ventroposterior subdivisions. The postsynaptic targets of the AON to MOB channel are the spinous processes and varicosities of the proximal and distal-most dendrites of granule cells. The boutons derived from AON projection neurons contained clear spherical vesicles and established exclusively asymmetric synaptic junctions.

Olfactory bulb interconnections in soft shell turtle

Unilateral injections of horseradish peroxidase and radioactive amino acids in the olfactory bulbs of soft shell turtles revealed unique pathways for interbulbar communication. The two olfactory bulbs exchange dendrites and axons directly through their fused external plexiform and internal plexiform laminae.

A forebrain atlas and stereotaxic technique for the lizard, Anolis carolinensis

A forebrain atlas and stereotaxic neurosurgical techniques were developed for use in anatomical and behavioral experiments on the green anolis lizard (Anolis carolinensis). Green anoles are convenient and robust experimental subjects with a rich behavioral repertoire, the social components of which are partly under hormonal control. The technique and atlas were devised to conduct neuroethological investigations of the effect of lesions on species-typical display behavior. The atlas consists of 12 transverse sections from an average size adult male. The figures (4-15) are based on Nissl material and supplemented with fiber-stained material from adjacent sections. They appear at the end of the article. Limitations on the accuracy of stereotaxic coordinates are discussed and tables of correlative nomenclature for principal telencephalic and diencephalic nuclei are provided.

Neurobehavioral evidence for the involvement of the vomeronasal system in mammalian reproduction

Jacobson's organ of the vomeronasal system is found in every order of mammals with the possible exception of Cetacea. The equivocal evidence claiming a vestigial or absent organ in humans is reviewed. Based upon anatomical considerations, the sensory epithelium of Jacobson's organ is one of five possible sensory components within the nasal cavity. Many methods designed to test the role of olfaction (sensu strictu) in physiology and behavior do not discriminate among the possible systems. Therefore, erroneous conclusions may have been drawn from the results of intervention experiments. The central neuroanatomical projections of the vomeronasal and olfactory systems are different and relatively independent of each other. The vomeronasal system reciprocally communicates with central areas concerned with reproductive events. On the other hand, the olfactory system may subserve individual maintenance tasks (e.g., feeding). As a periscope from the diencephalon, the vomeronasal system may monitor exogenous hormones, "pheromones".

Forebrain projections of the pigeon olfactory bulb

The olfactory system of the pigeon (Columba livia) was examined. Our electrophysiological and experimental neuroanatomical (Fink-Heimer technique) data showed that axons from the olfactory bulb terminated in both sides of the forebrain. The cortex prepiriformis (olfactory cortex), the hyperstriatum ventrale and the lobus parolfactorius comprised the uncrossed terminal field. The crossed field included the paleostriatum primitivum and the caudal portion of the lobus parolfactorius, areas which were reached through the anterior commissure. In this report the relationships between areas that receive olfactory information and the possible roles that olfaction plays in the birds' behavior are discussed.

The afferent connections of the main and the accessory olfactory bulb formations in the rat: an experimental HRP-study

The afferent connections of the main and accessory olfactory bulbs in the rat were examined by injecting horseradish peroxidase (HRP) into one or the other of these structures either by microelectrophoresis or by hydraulic pressure. Alternate sections were stained with newly developed HRP-procedures using either benzidine dihydrochloride (de Olmos and Heimer, '77) or tetramethyl-benzidine. Eighteen to twenty-four hours after unilateral HRP injections confined to the main olfactory bulb, a large number of HRP-labeled perikaria appeared in the following telencephalic structures on the ipsilateral side: All portions of the anterior olfactory nucleus (AON) except its external part, the lateral transitional field (LT) between AON and the paleocortex, the whole extent of the primary olfactory cortex (POC); the medial forebrain bundle area deep to the olfactory tubercle, the nucleus of the horizontal limb of the diagonal band (NHDB) and the nucleus of the lateral olfactory tract (NLOT). A moderate to small number of labeled cells, furthermore, were seen in the dorsal (DT) and medial (MT) transition fields, the ventral praecommissural hippocampus (tt2), the ventral superficial part of the nucleus of the vertical limb of the diagonal band (NVDB), the sublenticular part of the substantia innominata (SI), the anterior amygdaloid area, the posterolateral cortical amygdaloid nucleus (C2) and the transition region (28 L') between the olfactory cortex and the lateral entorhinal area proper. On the contralateral side a large number of labeled cells were found in all parts of the AON, with especially heavy labeling in its external part. A moderate number of labeled cells could also be detected in the lateral transition field (LT) and the NLOT. In the diencephalon and the brain stem a moderate number of HRP-labeled perikaria were observed in the dorsal, perifornical, and lateral hypothalamus, as well as in locus coeruleus and the dorsal and medial raphae nuclei. Following large HRP injections in the main olfactory bulb a moderate to small number of labeled cells were seen also in the posterior and premammillary hypothalamus and in field CA1 of the retrocommissural hippocampus on the ipsilateral side, as well as in POC on the contralateral side. It is possible, however, that the uptake of label took place in an undetected pool of HRP in the very rostal part of AON rather than in the olfactory bulb. HRP injections in the accessory olfactory bulb resulted in labeled neurons in the posterior ventro-lateral part of the bed nucleus of the stria terminalis, the nucleus of the accessory olfactory tract, the rostrodorsal portions of the medial amygdaloid nucleus, and the whole extent of the posteromedial cortical amygdaloid nucleus (C3) on the ipsilateral side. A few lightly labeled cells were seen also in the contralateral C3.

The locus and cytoarchitecture of the projection areas of the olfactory bulb in Macaca mulatta

A study was made of the normal and experimental anatomy of the olfactory system of the young adult male rhesus monkey. The cytoarchitecture of the central olfactory areas was studied with cell and fiber stains, while the extent and pattern of the projections of the olfactory bulb were determined by the Fink-Heimer and autoradiographic methods. The brain of one animal that had sustained damage to the olfactory bulb two days prior to sacrifice, and of one that had a transection of the olfactory tract ten days prior to sacrifice, were processed with the Fink-Heimer technique. The first of these and four others received injections of 3H-proline or 3H-leucine into the olfactory bulb, and following a survival period of 18 hours, or 2, 4, 12, or 20 days, their brains were processed with the autoradiographic technique. The results were the same for both experimental methods and for all survival periods. The projections of the olfactory bulb in this microsmatic animal are entirely ipsilateral. All of the structures that receive direct olfactory afferents have a laminar organization except for the anterior olfactory nucleus, which is laminated only in its anterior, peduncular, portion. While the olfactory bulb projects to the entire extent and depth of the anterior olfactory nucleus, the olfactory afferents of all other structures are confined to layer IA of the plexiform layer. These structures are: all divisions of the olfactory tubercle; the frontal and temporal prepiriform cortices; the oral, medial, and dorsal divisions of the superficial amygdaloid nucleus; and polar and anterior entorhinal cortex. The rhesus monkey does not have a recognizable accessory olfactory bulb, and no projections were seen to one of its targets, the nucleus of the stria terminalis. Also, no projections were seen to the taenia tecta or the ventral division of the superficial amygdaloid nucleus. With these exceptions, the projections of the olfactory bulb in the rhesus monkey are similar to those in macrosmatic species.

Olfactory relationships of the telencephalon and diencephalon in the rabbit. III. The ipsilateral centrifugal fibers to the olfactory bulbar and retrobulbar formations

The axoplasmic retrograde transport of horseradish peroxidase (HRP) from axon terminals to their parent cell bodies and histochemical fluorescence microscopy have been used to study the ipsilateral centrifugal fibers to the olfactory bulbs and anterior olfactory nucleus in the rabbit. Focal injections of peroxidase were placed unilaterally into the main or accessory olfactory bulb or into the anterior olfactory nucleus. In animals with injected HRP confined within the main bulb, perikarya retrogradely labeled with the protein in the ipsilateral forebrain were observed in the anterior prepyriform cortex horizontal limb of the nucleus of the diagonal band, and far lateral preoptic and rostral lateral hypothalamic areas. Brain stem cell groups that contained HRP-positive somata include the locus coeruleus and midbrain dorsal raphe nucleus. Except for the prepyriform cortex, the basal forebrain structures with labeled perikarya correlate well with locations of cell bodies containing acetylcholinesterase and choline acetyltransferase. These somata may represent a cholinergic afferent system to the main olfactory bulb. Peroxidase-labeled cell bodies in the locus coeruleus and midbrain raphe are indicative of noradrenergic and serotonergic innervations respectively of the olfactory bulb. In rabbits in which peroxidase was injected or diffused into the accessory olfactory bulb and anterior alfactory nucleus, HRP-positive somata were identified in the prepyriform cortex bilaterally, the horizontal limb of the diagonal band nucleus, lateral hypothalamic region, nucleus of the lateral olfactory tract, corticomedial complex of the amygdala, mitral and tufted cell layers of the ipsilateral main olfactory bulb, locus coeruleus, and the midbrain raphe. Evidence for centrifugal fibers to the accessory olfactory bulb from the corticomedial complex of the amygdala, locus coeruleus, and possibly the nucleus of the lateral olfactory tract and midbrain raphe is discussed. A similar distribution of labeled perikarya in the forebrain and brain stem was seen in rats in which peroxidase injected into the main olfactory bulb had spread into the accessory bulb and anterior olfactory nucleus. Histochemical fluorescence microscopy of the main and accessory olfactory bulbs in the rabbit and rat revealed fine caliber, green fluorescent fibers and varicosities predominantly in the granule cell layer and less so among cells in the glomerular layer. In sections through the root of the main olfactory bulb, a similar fluorescence was seen in the deep half of the plexiform layer of the pars externa of the anterior alfactory nucleus. These fluorescent fibers likely represent the noradrenergic innervation of the olfactory bulbar and retrobulbar formations. A fluorescent yellow hue was observed in the glomerular layer of the main bulb and may signify a serotonergic innervation of this lamina...

Olfactory relationships of the telencephalon and diencephalon in the rabbit. II. An autoradiographic and horseradish peroxidase study of the efferent connections of the anterior olfactory nucleus

The efferent connections of the anterior olfactory nucleus in the female albino rabbit have been studied using the autoradiographic and horseradish peroxidase methods for tracing axonal pathways. Following a unilateral injection of 3H-leucine into the olfactory peduncle, radioactively labeled efferent projections from the anterior olfactory nucleus were traced into all layers of the ipsilateral main olfactory bulb beneath the olfactory nerve layer and through the ipsilateral anterior limb of the anterior commissure and plexiform layer of the medial side of the cerebral hemisphere to the deep half of the plexiform (IB) and pyramidal cell (II) layers of the prepyriform cortex, the tenia tecta, and the entire surface of the olfactory tubercle. Labeled projections crossing the midline within the anterior commissure were followed to the layers IB and II of the contralateral anterior prepyriform cortex and pars externa, pars lateralis, and pars dorsalis of the anterior olfactory nucleus, and through the periventricular layer of the olfactory peduncle to all layers of the main olfactory bulb beneath the olfactory nerve layer. No well-defined labeled projection was traced to the contralateral accessory olfactory bulb. Evidence for possible anterior olfactory nucleus and/or prepyriform cortical projections to the ipsilateral paleocortical half of the claustrum, horizontal limb of the nucleus of the diagonal band, the posterior lateral hypothalamus at the level of the mammillary complex, and to the bed nucleus of the stria terminalis is discussed. Intra-axonal retrograde transport of horseradish peroxidase from axon terminals to parent cell bodies after unilateral injection of the protein into the main olfactory bulb or anterior olfactory nucleus revealed that anterior olfactory nucleus projections to the olfactory bulbs and the contralateral anterior olfactory nucleus arise predominately from the pars externa. The autoradiographic data indicate that the anterior olfactory nucleus projects to olfactory cortical structures which also receive afferent input from the olfactory bulb and that the termination of these projections is complementary to those from the olfactory bulb.