Olfactory relationships of the telencephalon and diencephalon in the rabbit. I. An autoradiographic study of the efferent connections of the main and accessory olfactory bulbs.

The efferent connections of the main and accessory olfactory bulbs in the female albino rabbit have been studied using the autoradiographic method for tracing axonal pathways. Following unilateral injections of 3H-proline or 3H-leucine into the main olfactory bulb, radioactively labeled material transported intraaxonally by axoplasmic flow in an anterograde direction from soma to axon terminal is present ipsilaterally in the superficial half of the plexiform layer (IA) of: the entire circumference of the olfactory peduncle, the tenia tecta, the full mediolateral extent of the olfactory tubercle, the entire length of the prepyriform cortex, a transition area between the prepyriform cortex and the horizontal limb of the nucleus of the diagonal band, the nucleus of the lateral olfactory tract, the anterior cortical and posterolateral cortical amygdaloid nuclei (periamygdaloid areas 1, rostral half of 2, 5 of Rose, '31), and the ventrolateral entorhinal cortex (entorhinal areas 1, 2, 4, 5, 7 of Rose, '31). No subcortical or contralateral projection of main bulb efferents was found. After a unilateral injection of 3H-leucine into the accessory olfactory bulb, transported material could be followed caudally along the dorsal surface of the ipsilateral lateral olfactory tract. This heavily labeled projection is distinct from the unlabeled lateral olfactory tract and has been termed the accessory olfactory tract. Beginning at the level of the caudal third of the olfactory tubercle and extending caudally to the nucleus of the lateral olfactory tract is a group of small neurons intimately associated with the accessory olfactory tract. This cell group is referred to as the bed nucleus of the accessory olfactory tract. Projection sites of the accessory bulb include the bed nucleus of the accessory olfactory tract and layer IA of the medial nucleus and the posteromedial cortical nucleus of the amygdala (periamygdaloid areas 3, 4, PAM, caudal half of 2, 6 of Rose, '31). An additional accessory bulb efferent projection was found to enter the stria terminalis at the level of the medial amygdaloid nucleus and could be traced to a posterior segment of the bed nucleus of the stria terminalis. The autoradiographic findings indicate that the accessory olfactory bulb connects with portions of the amygdala that do not receive afferent input from the main olfactory bulb and provide evidence for the existence of two distinct and separate olfactory systems.     

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...     

An autoradiographic study of the efferent connections of the ventromedial nucleus of the hypothalamus

Efferent projections from the ventromedial nucleus of the hypothalamus (VMN) were traced using tritiated amino acid autoradiography in albino rats. Ascending fibers passed through the anterior hypothalamus. Labelled fibers and terminal fields were seen in the preoptic area, bed nucleus of the stria terminalis, substantia innominata, the anterior amygdaloid area, diagonal bands of Broca and lateral septum. Fibers also projected laterally from VMN and entered the supraoptic commissures and zona incerta. These lateral projections were responsible for the fibers observed in the cerebral peduncle, the amygdala, the thalamus and the reticular formation. Fibers descending in a medial position projected through the posterior hypothalamus and then swept dorsally to terminate in the mesencephalic and pontine central grey. A projection from VMN into the median eminence was noted. The overall patterns of projection from different parts of VMN were similar; differences that existed were primarily in the relative strengths of the different projections. The efferent projections from VMN are extensive, well organized, and would appear capable of supporting significant physiological actions on extra-hypothalamic structures.     

Sources of olfactory inputs to opossum mediodorsal nucleus identified by horseradish peroxidase and autoradiographic methods

Some sources of olfactory input to the opossum mediodorsal thalamic nucleus (MD) were identified by retrograde horserdish peroxidase and anterograde autoradiographic methods. One major source originated from the olfactorytubercle and a narrow strip of piriform cortex bordering the tubercle. The tubercle-MD projection exhibited a definite spatial organization and included all except the most medial part of MD. The fact that the projection reached the most lateral and ventral extent of MD abutting the intralaminar complex suggests that the entire opossum MD may correspond to only the medial, magnocellular division in the primate and that the equivalents of both the parvocellular and paralamellar divisions may be absent.     

Afferent and efferent connections of the parabigeminal nucleus in cat revealed by retrograde axonal transport of horseradish peroxidase

Afferent and efferent connections of the parabigeminal nucleus (PBG) of the cat have been demonstrated by means of horseradish peroxidase (HRP) tracing technique. Following HRP injection in the PBG, labelled cells were observed mainly in the deep layers of the ipsilateral superior colliculus (SC). The other labelled structures were the prepositus hypoglossi complex (PH), the ventral nucleus of the lateral geniculate body (LGV), the locus coeruleus, the cuneiform nucleus, the periaqueductal gray and the dorsomedial hypothalamic area. Efferent projections of the PBG were investigated by HRP injection in SC, LGV, PH, hypothalamus and in some acoustic relays, i.e. medial geniculate body and inferior colliculus. Only the PBG-SC projection appeared to be well systematized. The positive labelling of the PBG following injection of LGV and hypothalamus is discussed in terms of the specificity of the injection. The absence of afferent and efferent connections of the PGB with any acoustic relay tends to exclude this nucleus from the auditory system in contrast to previous suggestions. On the basis of the close reciprocal PBG-SC connections a possible role of the PBG within visuomotor tectal function is proposed.     

An autoradiographic study of the efferent connections of the ventral lateral geniculate nucleus in the albino rat and the cat

The efferent connections of the ventral lateral geniculate nucleus (LGN_v) of the albino rat and the cat have been studied using the autoradiographic method for tracing axonal pathways. Following the injection of ³H-proline or ³H-leucine into the LGN_v of the rat, label transported in the rapid phase of axonal flow was found bilaterally in the olivary pretectal nuclei, the lateral terminal nuclei of the accessory optic system, and the ventral portion of the suprachiasmatic nuclei of the hypothalamus, and ipsilaterally in the rostrolateral portion of the superior colliculus. Since these regions are known to receive a direct projection from the retina, comparisons have been made of the distribution of silver grains in autoradiographs of each region following injections of ³H-proline into the eye and into the LGN_v; in every nuclear region except the superior colliculus the grain distributions were found to overlap precisely and, in the suprachiasmatic nuclei there also appears to be a similarity in the relative intensity of the input to the nuclei on the two sides. In the superior colliculus, the retinal fibers end mainly within the more superficial laminae, whereas those from the LGN_v are distributed mainly to the deeper layers where they overlap the projection from the striate and peristriate cortex. The LGN_v has also been found to project to the zona incerta on the same side and to the contralateral LGN_v, In the cat a similar set of projections to the lateral terminal nuclei of the accessory optic tract, the suprachiasmatic nuclei, and the pretectal areas of both sides has been found, together with a projection to the ipsilateral superior colliculus and the zona incerta of both sides. No evidence could be found in either species for a projection from the LGN_v to the visual cortex.     

An autoradiographic study of the efferent connections of the lateral hypothalamic area in the rat.

The efferent connections of the lateral hypothalamic area (LHA) have been analyzed in a series of 30 rat brains with injections of 3H-amino acids into different parts of the area and the surrounding regions. Our findings indicate that all parts of the LHA contribute ascending and descending fibers to the medial forebrain bundle, and also project medially to certain of the adjoining hypothalamic nuclei. All levels of the LHA appear to send some fibers to a continuous group of structures that extends from the medial septal-diagonal band complex rostrally, through the lateral preoptic and lateral hypothalamic areas to the mammillary complex and the ventral tegmental area caudally. In addition, it is evident that cells at different levels within the LHA may have differential projections. Thus, the anterior and lateral parts of the LHA also appear to project substantially to the anterior hypothalamic area, the ventromedial and dorsomedial hypothalamic nuclei, the parataenial and paraventricular nuclei of the thalamus, and the medial part of the lateral habenular nucleus. Similarly, cells in the tuberal and posterior parts of the LHA project to the central gray, the longest projections from the posterior region reaching as far caudally as the central tegmental field, the parabrachial nucleus, the locus coeruleus, and the superior central and dorsal nuclei of the raphe. Viewed as a whole, the LHA is therefore well-suited to integrate inputs from the limbic system and brainstem and to relay them on the one hand to the medial zone of the hypothalamus and on the other to virtually every structure closely associated with the medial forebrain bundle and to the nuclei of origin of the major ascending monoaminergic systems.     

An autoradiographic study of the efferent connections of the entorhinal cortex in the rat

The longer connections of the entorhinal cortex have been studied autoradiographically in a series of rats, each of which received a small injection of 3H-amino acids in one of the various cytoarchitectonic subfields of the entorhinal cortex. The major findings can be summarized as follows. Whereas the projection of the lateral entorhinal area (LEA) to the dentate gyrus is broad in its longitudinal extent, the medial entorhinal area (MEA), and especially the ventral portion of this zone, projects in a more lamellar fashion. In the transverse plane the LEA preferentially projects to the inner (dorsal) blade of the dentate gyrus, while the MEA innervates both blades equally. Within the radial dimension, the entorhinal cortex projects to the dentate gyrus according to a medial to lateral gradient, with lateral portions of the LEA projecting along the pial surface and successively more medial portions of the entorhinal projecting closer to the granule cells. The commissural entorhinal to dentate projections are similar to the ipsilateral projections in location; however, they are considerably reduced in septotemporal extent and do not arise from cells in the ventral half of either LEA or the intermediate entorhinal area (IEA). The projection of the entorhinal cortex to Ammon's horn reflects the same longitudinal characteristics as the dentate projections. An alvear input which extends only to the pyramidal cells at the CA1-subicular junction was most noticeable at ventral hippocampal levels. Finally the extrahippocampal projections have been analyzed. These arise predominantly from cells in the LEA and project forward along the angular bundle to the piriform and periamygdaloid cortices, as well as the endopiriform nucleus, the lateral, basolateral, and cortical amygdaloid nuclei, the nucleus of the lateral olfactory tract, the olfactory tubercle, the anterior olfactory nucleus, the taenia tecta, and the indusium griseum. These extrinsic projections are to a large extent reciprocal to the major extrinsic inputs to the LEA.     

An autoradiographic study of the efferent connections of the superior colliculus in the cat

The differential projections of the three main cellular strata of the superior colliculus have been examined in the cat by the autoradiographic method. The stratum griseum superficiale projects caudally to the parabigeminal nucleus and rostrally to several known visual centers: the nucleus of the optic tract and the olivary pretectal nucleus in the pretectum; the deepest C laminae of the dorsal lateral geniculate nucleus; the large-celled part of the ventral lateral geniculate nucleus; the posteromedial, large-celled part of the lateral posterior nucleus of the thalamus. Several of these projections are topographically organized. The stratum griseum profundum gives rise to most of the descending projections of the superior colliculus. Ipsilateral projections pass to both the dorsolateral and lateral divisions of the pontine nuclei, the cuneiform nucleus, and the raphe nuclei, and to extensive parts of the brainstem reticular formation: the tegmental reticular nucleus, and the paralemniscal, lateral, magnocellular, and gigantocellular tegmental fields. Contralateral projections descending in the predorsal bundle pass to the medial parts of the tegmental reticular nucleus and of some of the tegmental fields, the dorsal part of the medial accessory nucleus of the inferior olivary complex, and to the ventral horn of the cervical spinal cord. Ascending projections of the stratum griseum profundum terminate in several nuclei of the pretectum, the magnocellular nucleus of the medial geniculate complex and several intralaminar nuclei of the thalamus, and in the fields of Forel and zona incerta in the subthalamus. The strata grisea profundum and intermediale each have projections to homotopic areas of the contralateral superior colliculus, to the pretectum, and to the central lateral and suprageniculate nuclei of the thalamus. However, the stratum griseum intermediale has few or no descending projections.     

Evidence of sub-collicular auditory projections to the medial geniculate nucleus in the cat: An autoradiographic and horseradish peroxidase study

Connections of a posteromedial region of the ventral nucleus of the lateral lemniscus were examined in the cat using the autoradiographic tracing method. This sub-collicular region previously had been shown, using retrograde transport of horseradish peroxidase, to send axons to the superior colliculus¹⁰. The autoradiographic findings revealed that many axons from the posteromedial region of the ventral nucleus of the lateral lemniscus that entered the superior colliculus continued into the midbrain reticular formation. Moreover, other axons traced rostral to the inferior colliculus into the thalamus ended in the medial geniculate nucleus, bilaterally. Experiments in which horseradish peroxidase was placed in the medial geniculate nucleus retrogradely labeled the large neurons in the posteromedial region supporting the autoradiographic observations. Other sub-collicular regions also contained labeled cells in these cases, including the main body of the ventral nucleus of the lateral lemnicus and scattered cell groups around the superior olivary complex.     

The anterior ectosylvian sulcal auditory field in the cat: II. A horseradish peroxidase study of its thalamic and cortical connections.

The thalamic and cortical projections to acoustically responsive regions of the anterior ectosylvian sulcus were determined by identifying retrogradely labelled cells after physiologically guided iontophoretic injections of horseradish peroxidase. The medial division of the medial geniculate nucleus, the intermediate division of the posterior nuclear group, the principal division of the ventromedial nucleus, and the lateroposterior complex were consistently labelled after these injections, although each animal showed slightly different patterns of labelling. The suprageniculate nucleus and the lateral and medial divisions of the posterior nuclear group were also labelled in most experiments. The cortex of the suprasylvian sulcus was the most consistently and densely labelled cortical region; each experiment showed a slightly different pattern of labelling throughout the suprasylvian sulcus, with an overall tendency for greater labelling in the ventral (lateral) bank of the middle region of the sulcus. Other cortical regions labelled less consistently included the anterior ectosylvian sulcus itself, the insular cortex of the anterior sylvian gyrus, and the posterior rhinal sulcus. In three experiments the contralateral cortex was examined and a small number of labelled cells was located in the anterior ectosylvian and suprasylvian sulci. Input from extralemniscal auditory thalamus is compatible with previously described auditory response properties of anterior ectosylvian sulcus neurons. The results also confirm the presence of input from visual and multimodal regions of thalamus and cortex, and therefore support claims of overlap of modalities within the sulcus. This overlap, as well as input from motor regions, suggests that the anterior ectosylvian sulcal field serves a sensorimotor role.     

An autoradiographic study of the organization of the efferent connections of the hippocampal formation in the rat.

The efferent connections of the hippocampal formation of the rat have been re-examined autoradiographically following the injection of small quantities of 3H-amino acids (usually 3H-proline) into different parts of Ammon's horn and the adjoining structures. The findings indicate quite clearly that each component of the hippocampal formation has a distinctive pattern of efferent connections and that each component of the fornix system arises from a specific subdivision of the hippocampus or the adjoining cortical fields. Thus, the precommissural fornix has been found to originate solely in fields CA1-3 of the hippocampus proper and from the subiculum; the projection to the anterior nuclear complex of the thalamus arises more posteriorly in the pre- and/or parasubiculum and the postsubicular area; the projection to the mammillary complex which comprises a major part of the descending columns of the fornix has its origin in the dorsal subiculum and the pre- and/or parasubiculum; and finally, the medial cortico-hypothalamic tract arises from the ventral subiculum. The lateral septal nuclei (and the adjoining parts of the posterior septal complex) constitute the only subcortical projection field of the pyramidal cells in fields CA1-3 of Ammon's horn. There is a rostral extension of the pre-commissural fornix to the bed nucleus of the stria terminalis, the nucleus accumbens, the medial and posterior parts of the anterior olfactory nucleus, the taenia tecta, and the infralimbic area, which appears to arise from the temporal part of field CA1 or the adjacent part of the ventral subiculum. The projection of Ammon's horn upon the lateral septal complex shows a high degree of topographic organization (such that different parts of fields CA1 and CA3 project in an ordered manner to different zones within the lateral septal nucleus). The septal projection of "CA2" and field CA3 is bilateral, while that of field CA1 is strictly unilateral. In addition to its subcortical projections, the hippocampus has been found to give rise to a surprisingly extensive series of intracortical association connections. For example, all parts of fields CA1, CA2 and CA3 project to the subiculum, and at least some parts of these fields send fibers to the pre- and parasubiculum, and to the entorhinal perirhinal, retrosplenial and cingulate areas. From the region of the pre- and parasubiculum there is a projection to the entorhinal cortex and the parasubiculum of both sides. That part of the postsubiculum (= dorsal part of the presubiculum) which we have examined has been found to project to the cingulate and retrosplenial areas ipsilaterally, and to the entorhinal cortex and parasubiculum bilaterally.     

Time of neuron origin in the anterior olfactory nucleus and nucleus of the lateral olfactory tract of the mouse: an autoradiographic study

Time of neuron origin in the anterior olfactory nucleus (AO) and nucleus of the lateral olfactory tract (NTOL) was determined autoradiographically in adult mice which received tritiated thymidine once during development. Each subdivision of AO has an individualized period and pattern of neuron origin. All subdivisions contain neurons which arise on E11; but the medial, dorsal, and external subdivisions complete their complement by E18 whereas posterior and lateral neurons continue to arise through P1. No gradients along the caudorostral axis were discerned. Neurons of NTOL arise early (E10-14), typical of large neurons and similar to those of the surrounding anterior amygdaloid area.     

Afferent connections of the subthalamic nucleus: a combined retrograde and anterograde horseradish peroxidase study in the rat

A comprehensive characterization of the afferent connections of the subthalamic nucleus of Luys (STN) is a necessary step in the unraveling of extrapyramidal mechanisms. In the present study, the STN afferents in the rat were systematically investigated with the aid of retrograde and anterograde horseradish peroxidase tracer techniques. The results indicate that, besides a massive input from the dorsal pallidum, the STN receives substantial projections from several districts of the cerebral cortex (the medial division of the prefrontal cortex, the first motor and primary somatosensory areas, and the granular insular territory), the ventral pallidum, the parafascicular nucleus of the thalamus and the pedunculopontine tegmental nucleus, as well as a modest innervation from the dorsal raphe nucleus. In spite of the fact that many additional structures were found to contain retrogradely labeled neurons after tracer injections in the STN, no other projection to the latter nucleus could be effectively established in our anterograde experimental series.     

Efferent connections from the anterior pretectal nucleus to the diencephalon and mesencephalon in the rat

The anterior pretectal nucleus has been described as part of the visual pretectal complex. However, several electrophysiological and behavioural studies showed that this area is involved in somatosensory modulation, more specifically, antinociception. The efferents of the anterior pretectal nucleus have not been identified taking into account the different function of this nucleus in relation to the rest of the pretectal complex. In the study herein described, a sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin was used to trace the mesencephalic and diencephalic efferents of the anterior pretectal nucleus in the rat. The majority of the connections were ipsilateral. Fibres with varicosities were observed in discrete areas of the thalamus (central lateral, posterior complex), hypothalamus (lateral, posterior and ventromedial), zona incerta, parvocellular red nucleus, intermediate and deep layers of the superior colliculus, central grey, deep mesencephalon, pontine parabrachial region, and pontine nuclei. Fibres en passant were detected in the medial lemniscus, from the level of the injection site to rostral medullary levels. Some labelled axons were seen coursing to the contralateral side through the posterior commissure and the decussation of the superior cerebellar peduncle. These results show that the anterior pretectal nucleus projects principally to areas involved in somatosensory and motor control in a manner that permits sensory modulation at higher and lower levels of the brain. These connections may explain the antinociceptive and antiaversive effects of stimulating the anterior pretectal nucleus in freely moving animals.     

Ascending and descending projections from nucleus reticularis magnocellularis and nucleus reticularis gigantocellularis: an autoradiographic and horseradish peroxidase study in the rat

The projections of the rostal medulla were studied using retrograde and orthograde transport techniques in the rat. The present horseradish peroxidase (HRP) studies indicate that the ventral portion of nucleus reticularis (NGC) and nucleus reticularis magnocellularis (NMC) project to both rostral and caudal levels of the spinal cord, while dorsal NGC projects only to the rostral cord. A differential density distribution of labeled cells was observed, with the greatest density of NGC-spinal neurons located rostral to the level of the inferior olive; and the greatest density of NMC-spinal neurons located caudally.This differential density distribution, when coupled with microiontophoretic application of [³H]amino acids allowed relatively independent labeling of the adjacent NGC- and NMC-spinal systems. On the basis of the HRP and autoradiographic studies 3 separate regions were delineated: dorsal NGC, ventral NGC and NMC. Descending projections from NGC were observed to the lateral vestibular nucleus, facial nucleus, hypoglossal nucleus and cuneatus. At cervical levels NGC fibers projected through the ventral and ventrolateral columns. Terminal fields were observed in laminae VII, VIII and to a lesser extent in IX. Labeled NGC fibers became difficult to follow by thoracic levels, which is consistent with the present HRP results. A continuum of descending NGC projections was observed with dorsally located NGC neurons projecting bilaterally through the ventral columns, and ventrally located NGC cells projecting through the ipsilateral ventrolateral columns. Ascending projections from NGC to the motor nucleus of V, trochlear nucleus, oculomotor nucleus, Edinger-Westphal nucleus, the ventral aspect of the periaqueductal gray, the deep and intermediate layers of the superior colliculus, nucleus parafasicularis and centromedianus, the Fields of Forel and the dorsal and lateral hypothalamic nuclei were observed. Descending projections from NMC to the dorsal nucleus of the vagus, hypoglossal nucleus, nucleus commissuralis and intercalatus were observed. At cervical levels, fibers project through the ipsilateral lateral columns, particularly its dorsal aspect. Terminal fields are located ipsilaterally in lamonae IV, V and VI, and bilaterally in VII, VIII and X. NMC projections continue through cadual levels of the spinal cord including a projection to the ipsilateral intermediolateral columns. Ascending NMC projections are limited to the ventral pontine reticular formation.The differing projections and cytoarchitecture of the rostral medulla of the rat observed in the present study are compared to that of the cat and opossum, with implications for the subdivision of this region discussed. The possible involvement of NMC and NGC projections in the modulation of pain is reviewed.     

Electrophysiological studies of centrifugal and centripetal connections of the anterior olfactory nucleus

In order to investigate the role of anterior olfactory nuclei in the interbulbar connections, systematic exploration of all possible pathways interconnecting both olfactory bulbs, anterior olfactory nuclei (A.O.N.) and the anterior limb of the anterior commissure was undertaken. Electrical stimulation and field potential recording were extensively used. Olfactory bulb stimulation results in a positive-negative diphasic response in the A.O.N. lasting 80–200 msec.It is demonstrated that: (a) axonal fibers of mitral as well as tufted cells send collaterals in the ipsilateral A.O.N., and (b) the synaptic relay of the interbulbar fibers lies in the ventro-lateral part of the A.O.N.     

Intrinsic and efferent connections of the endopiriform nucleus in rat.

The endopiriform nucleus is a large group of multipolar cells located deep to the piriform cortex. The function of this nucleus is unknown, but studies with animal models suggest that it plays an important role in temporal lobe epileptogenesis. To address questions concerning mechanisms of epileptogenesis and to gain insights into its normal function, efferent axons from the endopiriform nucleus were labeled by anterograde transport from small extracellular injections of Phaseolus vulgaris leucoagglutinin. Several principles of organization were derived: (1) heavy local and long intrinsic connections are present throughout the endopiriform nucleus; (2) endopiriform efferents target cortical rather than nuclear structures; (3) extensive projections from the endopiriform nucleus extend to most basal forebrain areas including the piriform cortex, entorhinal cortex, insular cortex, orbital cortex, and all cortical amygdaloid areas. The perirhinal cortex, olfactory tubercle, and most subdivisions of the hippocampal formation receive light projections; (4) projections are highly distributed spatially within all target areas; (5) efferent axons from the endopiriform nucleus are unmyelinated and give rise to boutons along their entire course rather than arborizing locally; and (6) the endopiriform nucleus and piriform cortex share target areas, but efferents from the endopiriform nucleus lack the precise laminar order of those from the piriform cortex, and provide a heavy caudal to rostral pathway that is lacking in the cortex. The significance of these findings for the triggering of generalized seizures from the deep piriform region are discussed. An hypothesis for a role of the endopiriform nucleus in memory storage is presented.