An autoradiographic study of complementary laminar patterns of termination of afferent fibers to the olfactory cortex

Cortical areas are revealed by distribution patterns of proteoglycan components and parvalbumin in the Mongolian gerbil and rat

Cortical areas in rodents have been basically characterized by its cytoarchitecture, connectivity or by physiological parameters. In this study we show that they are revealed by distribution patterns of proteoglycans and parvalbumin-immunoreactivity. Brains of young adult Mongolian gerbils (Meriones unguiculatus) and Wistar rats were cut into series of transversal sections. Proteoglycan components were detected using the N-acetylgalactosamine binding Wisteria floribunda agglutinin (WFA) and antibodies against chondroitin sulphate proteoglycan (CSPG). Differences between cortical areas were found to exist with regard to the occurrence and the density of perineuronal nets, but were also expressed in varying staining intensities for WFA and CSPG of the neuropil. Primary neocortical areas (somatosensory, auditory, visual cortex) were characterized by an intense neuropil staining in layer IV and the upper part of layer VI. Using the same methods strong labelling was also typical of the neuropil in the retrosplenial cortex, of layer Ia in the prepiriform cortex and the hippocampal CA3 field. In tangential sections cut from gerbil cortical hemispheres, some of the heavily lectin-stained cortical areas were sharply delineated from adjacent faintly labelled regions, others showed more diffuse borders. In the rat, the area-specific staining for WFA was less clearly expressed than in the gerbil. Immunocytochemistry of the calcium-binding protein parvalbumin in alternate sections showed labelling patterns of neuropil which resembled those of WFA-binding and CSPG-immunoreactivity in the entire neocortex and hippocampus. From these results it can be concluded that functional peculiarities of cortical fields may not only be determined by neuronal network parameters but also by the spatial arrangement of extracellular matrix proteoglycans.

Central olfactory connections in the macaque monkey

The connections between the olfactory bulb, primary olfactory cortex, and olfactory related areas of the orbital cortex were defined in macaque monkeys with a combination of anterograde and retrograde axonal tracers and electrophysiological recording. Anterograde tracers placed into the olfactory bulb labeled axons in eight primary olfactory cortical areas: the anterior olfactory nucleus, piriform cortex, ventral tenia tecta, olfactory tubercle, anterior cortical nucleus of the amygdala, periamygdaloid cortex, and olfactory division of the entorhinal cortex. The bulbar axons terminate in the outer part of layer I throughout these areas and are most dense in areas that are close to the lateral olfactory tract. Labeled axons also were found in the superficial part of nucleus of the horizontal diagonal band. Retrograde tracers injected into the olfactory bulb labeled cells in the nucleus of the diagonal band and in all of the primary olfactory cortical areas except the olfactory tubercle. Electrical stimulation of the olfactory bulb evoked short-latency unit responses and a characteristic field wave in the primary olfactory cortex. Multiunit activity in layer II tended to be of shorter latency than that in layer III and the endopiriform nucleus. Associational connections within the primary olfactory cortex were demonstrated with anterograde tracer injections into the piriform cortex and the entorhinal cortex. Injections into the piriform cortex near the lateral olfactory tract labeled axons in the deep part of layer I of many primary olfactory areas, but especially in areas near the tract. An injection into the rostral entorhinal cortex, distant to the lateral olfactory tract, labeled a complementary distribution of axons in deep layer I of olfactory areas medial and caudoventral to the tract. This organization resembles that reported in the primary olfactory cortex of the rat [Luskin and Price (1983) J. Comp. Neurol. 216:264-291]. The anterograde tracer injections into the piriform cortex and retrograde tracer injections into the orbital and medial prefrontal cortex and rostral insula label connections from the primary olfactory cortex to nine areas in the caudal orbital cortex, including the agranular insula areas Iam, Iai, Ial, Iapm, and Iapl and areas 14c, 25, 13a, and 13m. The piriform cortex projects most heavily to layer I of these areas. Only Iam, Iapm, and 13a receive a substantial projection to the deeper layers. Areas Iam, Iapm, and 13a were also the only areas that responded with multiunit action potentials to olfactory bulb stimulation in anesthetized animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Development and early postnatal maturation of the primary olfactory cortex

Tritiated thymidine autoradiography was used to study the origin and distribution of neurons in the primary olfactory cortex of the rat. The principal interest was devoted to animals injected at embryonic day 12 (E12) and sacrificed at different pre- and postnatal ages. The first generated neurons appearing at E12 were studied from E15 to P63. Animals sacrificed at E15 show a group of heavily labeled cells occupying a large area of the ventro lateral region of the telencephalic vesicle. At E16 this group differentiates into the principal cells of the accessory olfactory bulb and cells of the prospective primary olfactory cortex (POC). At E18-E20 the ventral tip of the cortical plate apparently divides this group into a superficial part corresponding to layer I and a deep part, corresponding to cells located in the adult in layer III. Labeled cells in layer I were found flanking the lateral olfactory tract (TOL), but rarely in the adult suggesting that they disappear or transform postnatally. Golgi observations were carried out from E15 to postnatal day 8. The morphology of different cells were studied. Layer I contains polymorphic cells resembling Cajal-Retzius cells. Among other cell types, layer II includes kinds of pyramidal cells lacking basal dendrites known as semilunar cells and intrinsic neurons. Layer III contains pyramidal cells having more than one apical dendrite ascending to the surface.

Signal propagation from piriform cortex to the endopiriform nucleus in vitro revealed by optical imaging

Optical signals were recorded from the posterior piriform cortex slices of guinea pigs stained with a voltage-sensitive dye to analyse spatio-temporal spread of neural activity evoked by electrical stimulation of afferent fibers. After propagation of activity along layers II and III, an isolated island of activity appeared deep to the layer III and moved caudally. Histological inspection revealed that the area where the island appeared corresponded well to the endopiriform nucleus. The present results provided an evidence for one of the main outflows of olfactory information from the posterior piriform cortex.

Mapping patterns of neuronal activity and seizure propagation by imaging intrinsic optical signals in the isolated whole brain of the guinea-pig

Image analysis techniques were used to examine changes in the intrinsic optical properties in the isolated brain of the guinea-pig in order to map normal neuronal activity patterns and seizure propagation in the olfactory cortex. Electrical stimulation of the lateral olfactory tract decreased light reflectance in distant cortical areas where fibres of the tract are known to project. These areas included the amygdalar, anterior and posterior piriform, and entorhinal cortices, as well as the olfactory tubercle. Stimulation of the lateral entorhinal cortex decreased reflectance in a more circumscribed area in the lateral and medial entorhinal cortex. By imaging intrinsic signals in real-time, we also demonstrated that seizure activity elicited in the entorhinal cortex/hippocampus preferentially propagated to the posteromedial cortical amygdaloid nucleus. The magnitudes of the intrinsic optical signals were correlated with the amplitudes of field potentials recorded in laminae II or III of the olfactory cortex of the same preparations. These signals had onset times of approximately 3 s during 5 Hz stimulation, consistently recovered and were graded with stimulation frequency. The generation of the intrinsic signals required postsynaptic activation, since attenuating synaptic transmission with kynurenic acid (an excitatory amino acid antagonist) eliminated the signals. The intrinsic signals exhibited maxima at 425-450, 550 and 600 nm, suggesting that they arose from changes in light absorption by cytochromes. Intrinsic signals of relatively constant magnitude were also present at 400, 475-500 and 575 nm, and at wavelengths greater than 600 nm. This suggested that an additional component of the intrinsic signal arose from changes in light scattering, possibly due to cellular swelling.

Optical imaging of the in vitro guinea pig piriform cortex activity using a voltage-sensitive dye

The spatio-temporal patterns of signal processing in guinea pig piriform cortex (PC) slices were analyzed by optical imaging using a voltage-sensitive dye. Slices (400 microns thick) were cut in a plane parallel to the lateral olfactory tract and perpendicular to the cortical surface. In all the anterior PC and the majority of the posterior PC preparations, neural activity elicited by electrical stimulation of layer Ia propagated along the same layer, then it invaded into layers II and III and propagated along them. In addition to the above pattern, invasion of activity into the deeper area than layer III was observed in some posterior PC preparations. Real-time imaging of an active zone evoked by Ia shocks and its spatio-temporal behavior will contribute to resolving olfactory information processing.

Organization of cortical afferent input to orbitofrontal areas in the rhesus monkey

Odorant signal processing takes place in a diverse group of primary olfactory areas which receive direct input from the olfactory bulb. Orbitofrontal cortices participate in olfactory functions, but the pathways through which they receive olfactory or other input have not been clearly defined. The retrograde tracers horseradish peroxidase and fluorescent dyes were injected in orbital cortices to study their afferent cortical connections. Labeled neurons in primary olfactory areas (prepiriform cortex, anterior olfactory nucleus and olfactory tubercle) were directed mainly to a posterior orbitofrontal region and to a lesser extent the neighboring caudal part of area 13. There was no evidence of direct projections from primary olfactory areas to the rostral parts of area 13, or to areas 12 or 11. Most labeled neurons in primary olfactory areas were directed to agranular cortices, fewer projected to dysgranular areas, and there was no evidence that any reached granular cortices. The areas which received the most robust olfactory projections showed the lowest degree of laminar organization among prefrontal cortices. Early processing in the olfactory system thus takes place in areas which differ sharply on structural grounds from "early" eulaminate post-Rolandic sensory cortices. In addition to olfactory cortical projections, numerous labeled neurons in transitional (limbic) cortices were directed to orbital areas, and fewer but still substantial numbers of afferent neurons were found in eulaminate cortices. Unlike post-Rolandic unimodal sensory areas, which seems to be committed to the processing of input from one sensory modality via sequential and/or parallel pathways, caudal orbital areas received highly distributed input from primary olfactory areas, and in addition, from gustatory, visual, auditory and somatosensory areas. The structural and connectional features of olfactory recipient orbital cortices thus differ markedly from those observed in other sensory association areas and suggest a mode of processing adapted early in cortical evolution.

Sexual dimorphism in the vomeronasal pathway and sex differences in reproductive behaviors

Several years ago we hypothesized that the vomeronasal system (VNS), a complex neural network involved in the control of reproductive behavior, might be sexually dimorphic. This hypothesis sprung from several facts; (a) the existence of steroid receptors in the VNS; (b) sexual dimorphism was already described in some structures that receive vomeronasal input, such as the medial preoptic area, the ventromedial hypothalamic nucleus, the ventral region of the premammillary nucleus and the medial amygdaloid nucleus; and (c) the vomeronasal organ, which is the receptor organ of the VNS, was also sexually dimorphic. After that point, the accessory olfactory bulb (AOB), the bed nucleus of the accessory olfactory tract (BAOT) and the bed nucleus of the stria terminalis were found to be sexually dimorphic. The aim of the present review is to show the experimental facts that confirm our earlier hypothesis and, consequently, to present the existence of a sexually dimorphic multisynaptic pathway for the first time in mammals. Sexual dimorphism in the VNS might provide a comprehensive approach to understanding the neural bases of sexually dimorphic reproductive behavior and it is suggested here that the greater number of neurons which male rats present in relation to females in most VNS structures might contribute to the inhibition of the expression of feminine copulatory behavior (lordosis) and maternal behavior in males. In addition, the mechanisms that control the development of sexual dimorphism in the VNS are discussed. The discussion takes into account the two patterns of sexual dimorphism found in the rat brain. Estrogens seem to promote the development of sexual dimorphism in both male and female rats. However, an inhibitory role of androgens might be necessary to hypothesize when males or females present a lower number of neurons and/or volume than the opposite sex. There are experimental data supporting this hypothesis in the female, since dihydrotestosterone seems to facilitate neuronal death in VNS structures, such as the AOB and the BAOT, in which females present a lower number of neurons and volume than male rats. Finally, since the lateral division of the bed nucleus of the stria terminalis, which belongs to the main olfactory system (MOS), is sexually dimorphic and presents anatomical relationships with some VNS structures the MOS might be sexually dimorphic.

Distribution of calbindin D-28K and parvalbumin immunoreactivities in the nucleus olfactorius anterior of the rat

The distributions of calbindin D-28K (CaBP) and parvalbumin (PV) in the rat nucleus olfactorius anterior (NOA) were described using monoclonal antibodies and the avidin-biotin-peroxidase method. The NOA showed a high immunoreactivity for CaBP, with a rostrocaudal increase in the positive neurons and fibres. Pars externa (NOAe) was the only subdivision which showed a low CaBP immunostaining. PV-positive elements were less abundant than those CaBP immunostained. The main difference in the distributions for both proteins was observed in the pars medialis which was practically PV negative. PV- and CaBP-stained neurons showed similar morphologies in the subdivisions where they were present. In NOAe, we observed a characteristic PV- and CaBP-positive neuronal type, with an oriented dendritic pattern. Transition areas were clearly observable in both CaBP- and PV-labelled sections.

Connections of the posterior nucleus of the amygdala

The connections of a relatively homogeneous band of neurons in the caudal amygdala have been examined with anterograde and retrograde axonal tracing methods in the rat. This region, called here the posterior nucleus of the amygdala (PA), corresponds in part to an area that has been referred to as the cortico-amygdaloid transition area, posterior part of the medial nucleus of the amygdala, amygdalo-hippocampal transition area, and posteromedial basal nucleus. Experiments with fluorogold and phaseolus vulgaris leucoagglutinin (PHAL) indicate that the major neuronal input to the PA arises in the ventral premammillary nucleus, and that substantial projections also arise in olfactory-related areas such as the medial nucleus of the amygdala, bed nucleus of the accessory olfactory tract, and posterior cortical nucleus of the amygdala, as well as in the ventral subiculum and adjacent parts of hippocampal field CA1. Other seemingly minor inputs, including cholinergic fibers from the substantia innominata, dopaminergic fibers from the ventral tegmental area, and serotoninergic fibers from the dorsal nucleus of the raphe, were also identified. The efferent projections of the PA as determined with the PHAL method appear to follow five major routes: 1) a relatively small group of laterally directed fibers innervates the dorsal endopiriform nucleus, and a few of these fibers reach cortical area TR and the lateral entorhinal area; 2) another small group of fibers courses medially to innervate the ventral subiculum and adjacent parts of field CA1; 3) many fibers course ventrally to innervate the outer molecular layer of the medial part of the posterior cortical nucleus of the amygdala; 4) a moderate group of fibers courses rostrally to innervate primarily the posterodorsal part of the medial nucleus of the amygdala, although some fibers continue on to end less densely in rostral parts of the medial nucleus of the amygdala before leaving the amygdala through the ansa peduncularis; and 5) the major output of the PA courses through the stria terminalis. One branch of this pathway massively innervates the principal nucleus of the bed nuclei of the stria terminalis before entering the medial hypothalamus, where it ends massively in the anteroventral periventricular and medial preoptic nuclei, ventrolateral part of the ventromedial nucleus and adjacent parts of the basal lateral hypothalamic area, and ventral premammillary nucleus. The other branch sends fibers to the ventral lateral septal nucleus, nucleus accumbens, olfactory tubercle, and infralimbic area of the prefrontal cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization of the thalamocortical connections of the mediodorsal thalamic nucleus in the rat, related to the ventral forebrain-prefrontal cortex topography

The medial and central segments of the mediodorsal nucleus of the thalamus (MD) receive afferents from the ventral forebrain, including the piriform cortex, the ventral pallidum, and the amygdaloid complex. Because MD is reciprocally interconnected with prefrontal and agranular insular cortical areas, it provides a relay of ventral forebrain activity to these cortical areas. However, there are also direct projections from the piriform cortex and the amygdala to the prefrontal and agranular insular cortices. This study addresses whether this system has a "triangular" organization, such that structures in the ventral forebrain project to interconnected areas in MD and the prefrontal/insular cortex. The thalamocortical projections of MD have been studied in experiments with injections of retrograde tracers into prefrontal or agranular insular cortical areas. In many of the same experiments, projections from the ventral forebrain to MD and to the prefrontal/insular cortex have been demonstrated with anterograde axonal tracers. The connections of the piriform cortex (PC) with MD and the prefrontal/insular cortex form an organized triangular system. The PC projections to the central and medial segments of MD and to the lateral orbital cortex (LO) and the ventral and posterior agranular insular cortices (AIv and AIp) are topographically organized, such that more caudal parts of PC tend to project more medially in MD and more caudally within the orbital/insular cortex. The central and medial portions of MD also send matching, topographically organized projections to LO, AIv and AIp, with more medial parts of MD projecting further caudally. The anterior cortical nucleus of the amygdala (COa) also projects to the dorsal part of the medial segment of MD and to its cortical targets, the medial orbital area (MO) and AIp. The projections of the basal/accessory basal amygdaloid nuclei to MD and to prefrontal cortex, and from MD to amygdaloceptive parts of prefrontal cortex, are not as tightly organized. Amygdalothalamic afferents in MD are concentrated in the dorsal half of the medial segment. Cells in this part of the nucleus project to the amygdaloceptive prelimbic area (PL) and AIp. However, other amygdaloceptive prefrontal areas are connected to parts of MD that do not receive fibers from the amygdala. Ventral pallidal afferents are distributed to all parts of the central and medial segments of MD, overlapping with the fibers from the amygdala and piriform cortex. Fibers from other parts of the pallidum, or related areas such as the substantia nigra, pars reticulata, terminate in the lateral and ventral parts of MD, where they overlap with inputs from the superior colliculus and other brainstem structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.

Anatomical and immunohistological demonstration of the primary neural connections of the vomeronasal organ in the dog

Macro- and microdissection methods together with conventional histology and lectin immunohistochemistry have been used to identify the course of the vomeronasal nerves and their site of termination (accessory olfactory bulb; AOB) in the dog. The AOB in this species is small and variable in size, situated on the medial surface of the main olfactory bulb, and has an anatomical structure unlike that described for other mammals. The vomeronasal nerves and their terminal glomeruli in the AOB are easily identifiable by selective immunohistochemical staining using Ulex europeus agglutinin I.

Localization of corticotropin-releasing factor-like immunoreactivity in monkey olfactory bulb and secondary olfactory areas

Electrophysiological and anatomical observations suggest that terminals of olfactory bulb mitral cells ending in rat primary olfactory cortex exert certain postsynaptic effects via an excitatory amino acid neurotransmitter. Recent anatomical studies have shown that several peptides, most notably corticotropin-releasing factor (CRF) (Imaki et al., '89) Brain Res., 496: 35-44), are also localized within rat olfactory bulb projection neurons, thus raising the possibility that there is a peptide cotransmitter in this system. In contrast to the availability of data for rodents, very little is known about the distribution of peptides and other putative transmitters in the olfactory systems of primate species. In the present study, sections through the olfactory bulb and its target areas were obtained from two monkey species (Saimiri sciureus and Macaca fascicularis) and processed for immunohistochemistry with a well-characterized polyclonal antiserum directed against the human form of CRF. Virtually identical results were obtained in the two species. Within the olfactory bulb, nearly all mitral and many tufted cells contained CRF-like immunoreactivity. CRF-positive fibers were seen within the olfactory tract and olfactory stria, which contain the axons of mitral and tufted cells. Within the anterior olfactory nucleus and layer Ia of the olfactory tubercle and piriform cortex, immunoreactivity was seen within fine processes, as well as in coarse, varicose fibers and isolated puncta. CRF-positive cells were seen within layer III of the olfactory tubercle and piriform cortex. Immunoreactive fibers and varicosities were also seen within olfactory-recipient regions of the amygdala and entorhinal cortex. These observations suggest that CRF may act as a transmitter and/or neuromodulator in primate olfactory system.

Olfactory pathways and the sense of smell

Rats were trained using operant conditioning to detect isoamyl acetate vapor generated by an olfactometer. They received lesions of olfactory pathways and were tested for retention of the odor detection task and trained on two-odor tasks. Deficits in odor detection and two-odor discrimination were related to the extent to which lesions disconnected the olfactory bulb from the forebrain. Transection of only the lateral olfactory tract, only the anterior limb of the anterior commissure, or lesions of the olfactory tubercle had little effect but combined lesions of these structures produced severe deficits in both odor detection and discrimination. Only rats with almost complete transection of the olfactory peduncle or cortex were anosmic; those with transections that spared a small segment of tissue between the olfactory bulb and olfactory cortex had detectable olfactory function. The results are discussed with regard to efferent connections of the olfactory bulb.

Calretinin in rat brain: an immunohistochemical study

Calretinin is a calcium-binding protein related to calbindin-D28k; both are present in different though overlapping sets of neurons in brains of birds and mammals. We describe in detail the pattern of calretinin immunoreactivity in the rat brain. As in chick brain, calretinin immunoreactivity is abundant in various sensory pathways (particularly certain cells and fibres of the cochlear nuclei and olfactory bulb), in the heterogeneous parts of the brainstem and in parts of the hypothalamus. Many primary sensory fibres are strongly positive. Major groups of calretinin-positive neurons also include the thalamic reticular nucleus, triangular septal nucleus, lateral mammillary nucleus and substantia nigra pars compacta. Many other calretinin-positive cells are recognizable as local inhibitory neurons. Calretinin is absent from all but a few cells in the cerebral cortex, and is never found in motor neurons. There are also some distinctive positive structures whose identity is uncertain, notably irregular "shells" of cells and fibres around the thalamus and in the amygdala and an unnamed cell type in the vestibulocerebellum.

Higher olfactory processes: perceptual learning and memory

The past year has seen several important findings emerge from studies of higher olfactory processes. The identification of synaptic long-term potentiation in the olfactory cortex, induced via repetitive burst stimulation at the theta rhythm, and physiological activity patterns associated with learning, some of which mimic long-term potentiation induction patterns, have suggested relationships between rhythmic activity, behavioral learning and synaptic plasticity. In addition, the construction of computational models of the olfactory bulb and cortex have generated testable behavioral and physiological predictions which have been supported by experimental evidence.

The Projections of Mitral Cells from Small Local Regions of the Olfactory Bulb: An Anterograde Tracing Study Using PHA-L (Phaseolus vulgaris Leucoagglutinin)

Numerous anatomical and electrophysiological studies have demonstrated a lack of simple point-to-point topographical relationships between the olfactory bulb and primary olfactory projection areas. They reveal instead, a complex pattern of divergence and convergence. Furthermore, several authors reported that a single mitral cell could project onto different widely spaced cortical regions of the olfactory cortex. In the present study, we attempted to label the projections of a few mitral cells so close together so that they might be assumed to be connected to the same glomerulus, and to determine if these cells had similar patterns of axonal projections. For this purpose small Phaseolus vulgaris leucoagglutinin (PHA-L) injections were performed in the olfactory bulb of adult rats. We found that labelling two to five mitral cells, lying close together in the mitral cell layer, resulted in well-delineated patches of labelled fibres in the cortex. The number of patches was not related to the number of labelled mitral cells but the fibre density in each patch increased with the number of PHA-L filled somata in the olfactory bulb. We conclude that mitral cells lying close together in the mitral cell layer have similar patterns of axonal projections. Functional implications of such an organization in olfactory coding is discussed.

Selective suppression of afferent but not intrinsic fiber synaptic transmission by 2-amino-4-phosphonobutyric acid (AP4) in piriform cortex

Differences in the glutaminergic modulation of afferent and intrinsic fiber synaptic transmission in piriform (olfactory) cortex were investigated using extracellular and intracellular recording techniques in a transverse slice preparation. 2-Amino-4-phosphonobutyric acid (AP4) strongly suppressed synaptic potentials evoked by afferent fiber stimulation in layer 1a, while having a much weaker effect on synaptic potentials evoked by intrinsic fiber stimulation in layer 1b. Both the racemic mixture and L-(+)-enantiomer of AP4 showed this differential effect. Suppression of afferent fiber synaptic potentials was accompanied by an increase in paired pulse facilitation, suggesting a pre-synaptic mechanism, while intrinsic fiber synaptic potentials showed little change in facilitation. Previous work has shown that cholinergic modulation in piriform cortex appears selective for intrinsic fiber synapses. The present data describes a pre-synaptic glutaminergic modulation complementary to the cholinergic modulation.

Age-related remodeling of glutamic-acid decarboxylase-labeled elements in deafferented piriform cortex of rats

Olfactory bulb (OB) removal has been shown to result in plasticity in the piriform cortex (PC) that is age dependent. We are studying this phenomenon using immunoelectron microscopy of glutamic acid decarboxylase immunoreactivity (GAD, the enzymatic precursor for GABA) at selected postnatal ages and in adults with emphasis on short survival times of 4-7 days after OB ablation. Normally GAD-labeled synaptic terminals form type II symmetric contacts onto unlabeled dendrites and GAD-labeled dendrites receive type I, asymmetric contacts from unlabeled terminals (Westenbroek, et al., 1988a). The OB lesion results in degenerating terminals with type I contacts onto unlabeled and onto GAD-labeled dendrites. Type I postsynaptic sites may be seen partially contacted by or entirely devoid of degenerating terminals and occasionally may be apposed to variable degrees by normal unlabeled or by GAD-positive terminals. Subsequently, some GAD-labeled terminals may form asymmetric type I contacts usually with unlabeled dendrites and rarely with GAD-labeled dendrites. The findings are most common in the youngest subjects and essentially absent in the adult subjects. A sequence of reinnervation of deafferented type I sites by GAD-labeled terminals is suggested for the formation of this "atypical" synapse and the sequelae of this reorganization are discussed.

Olfactory projections to the hypothalamus

Electrophysiological recording, together with anterograde and retrograde axonal tracers, was used to provide a comprehensive description of the origin and distribution of the olfactory input to the lateral hypothalamus. This input was much more substantial to the caudal part of the hypothalamus than to the rostral part and originates from several different areas of the olfactory cortex. Positive responses to electrical stimulation of the olfactory bulb were found consistently in the postero-lateral hypothalamus, but only occasionally at more rostral levels. In agreement with this, injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) in the posterior half of the lateral hypothalamus labeled cells in four cortical areas that receive input from the olfactory bulb: the anterior olfactory nucleus, the piriform cortex (in the deepest layer or ventral endopiriform nucleus), the olfactory tubercle (in the deep polymorphic layer), and the anterior cortical nucleus of the amygdala. Injections of WGA-HRP in the anterolateral hypothalamus labeled cells only in the anterior cortical nucleus of the amygdala. Anterograde axonal tracing confirmed these projections. Injections of 3H-leucine in the anterior olfactory nucleus, the piriform cortex, and the olfactory tubercle produced axonal label that was light and confined to the medial forebrain bundle in the rostral hypothalamus but was more substantial and extended throughout the lateral hypothalamic area caudally. Injections in the anterior cortical amygdaloid nucleus labeled axons in the anterior hypothalamus and in the premammillary nuclei as well as in the posterolateral hypothalamic area. In addition, a projection was demonstrated to the nuclei gemini from the polymorphic zone deep to the olfactory tubercle. Injections of two fluorescent retrograde tracers into the mediodorsal nucleus of the thalamus and the posterolateral hypothalamus showed that cells projecting to both diencephalic sites were intermingled in all of the olfactory cortical areas except the anterior olfactory nucleus, where cells were labeled only from the hypothalamus. In the deep layer of the piriform cortex and in the anterior cortical amygdaloid nucleus cells were also double labeled, indicating that they send collateral axons to both parts of the diencephalon.

Differential projections of the main and accessory olfactory bulb in the frog

The central projections of the main olfactory bulb and the accessory olfactory bulb of the adult leopard frog (Rana pipiens) were reexamined, by using a horseradish peroxidase anterograde tracing method that fills axons with a continuous deposit of reaction product. The fine morphology preserved by this method allowed the terminal fields of the projection tracts to be delineated reliably, and for the first time. Herrick's amygdala has been newly subdivided into cortical and medial nuclei on the basis of cytoarchitecture, dendritic morphology, and the differential projections of the main and accessory olfactory tracts. The main olfactory bulb projects through the medial and lateral olfactory tracts to the postolfactory eminence, the rostral end of the medial cortex, the rostral end of the medial septal nucleus, the cortical amygdaloid nucleus, the nucleus of the hemispheric sulcus, and both the dorsal and ventral divisions of the lateral cortex, including its retrobulbar fringe. The lateral olfactory tract overlaps the dorsal edge of the striatal plate along the ventral border of the lateral cortex, but it is not certain whether any striatal cells are postsynaptic to the tract fibers. The lateral cortex is the largest of these territories, and receives the terminals of the main olfactory projection throughout its extent. It extends from the olfactory bulb to the posterior pole, and from the striatum to the summit of the hemisphere, where it borders the dorsal cortex. The medial and lateral olfactory tracts combine in the region of the amygdala to form a part of the stria medullaris thalami. These fibers cross in the habenular commissure and terminate in the contralateral cortical amygdaloid nucleus and periamygdaloid part of the lateral cortex. Cells projecting to the main olfactory bulb are found in the diagonal band and adjacent cell groups, but there is no evidence of an interbulbar projection arising from either the olfactory bulb proper or a putative anterior olfactory nucleus. The accessory olfactory bulb projects through the accessory olfactory tract to the medial and cortical amygdaloid nuclei. A fascicle of the tract crosses in the anterior commissure to terminate in the contralateral amygdala. While the main and accessory olfactory projections may converge in the cortical amygdaloid nucleus, the medial amygdaloid nucleus is connected exclusively with the accessory olfactory bulb.

Characterization and anatomical distribution of selective long-term potentiation in the olfactory forebrain

High-frequency stimulation of the granule cell layer of the olfactory bulb (OB) has previously been shown to result in a form of long-term potentiation in the piriform cortex (PC) that is selective to late components of the potential evoked in the PC58. This phenomenon was explored in male Long-Evans rats with chronically implanted electrodes by recording potentials evoked in the OB and in various sites in the ipsilateral and contralateral PC before and after repeated high-frequency stimulation of the OB. Recordings at all sites exhibited a gradually developing potentiation that was selective to late components of the evoked potential. In the OB and ipsilateral PC this potentiation had an overt long-term component that lasted for days, and all sites exhibited a latent potentiation that enabled the reestablishment of substantial levels of potentiation by mild patterns of stimulation that had no effect in control animals. No potentiation of the population EPSP representing input from the lateral olfactory tract to the PC was seen. Available evidence concerning the neuronal elements activated by the stimulation and the neuronal events likely to underlie the potentiated components of the evoked potentials suggests that this potentiation may represent an enhancement of inhibitory interactions within the PC and between the PC and OB.

Fetal olfactory bulb transplants send projections to host olfactory cortex in the rat

We are using the rat olfactory system to study developmental details of neurotransplantation. Tritiated [3H]thymidine-labeled fetal olfactory bulbs (OBs), were transplanted immediately into sites from which the neonatal host OB was removed. Subsequently, a small lesion was placed in the region of the transplanted OB and the tissue studied, using degeneration methods and autoradiography. Only OB's with extensive [3H]-label and precise lesions confined to the labeled areas were used. Degeneration was found mainly in the ipsilateral piriform cortex with lesser amounts at other nearby sites. The results demonstrate successfully transplanted donor OBs that send axons to specific and appropriate target areas of the host brain.

Development of the anterior olfactory nucleus in normal and unilaterally odor deprived rats

The development of a second order structure in the olfactory pathway, the anterior olfactory nucleus, was examined in both normal rat pups and in subjects which underwent unilateral naris closure on postnatal day 1 (P1). Naris occlusion in neonatal rats produces a constellation of changes within the first relay in the pathway, the olfactory bulb, including a 25% reduction in total volume. Such large changes suggest that higher order structures might also be affected. Anterior olfactory nucleus development was quantified in several ways. Laminar volumes were computed by using serial section planimetry. In control animals differential development was observed, with regions extending most rostrally (e.g., pars externa and pars lateralis) exhibiting the least growth. The anterior olfactory nucleus on the "deprived" side of subjects with a single naris occluded was identical in size to that observed in controls, development within the pars lateralis was examined in control animals at P10, P20, P30, and adults. Developmental increases in numbers of both branches per cell and spines were noted, but mean branch length remained relatively constant. Finally, the effects of naris occlusion on histological patterns of succinate dehydrogenase (SDH) staining and 2-deoxyglucose uptake within pars lateralis were examined at P20 to test for more subtle effects of naris occlusion. SDH staining was quite similar in deprived and control rats at P20. However, 3H-2-DG uptake was decreased in rostral areas of the anterior olfactory nucleus ipsilateral to the deprived olfactory bulb, suggesting that naris closure does affect the structure.

VIP- and PHI-immunoreactivity in olfactory centers of the adult cat

The purpose of the study was to determine the morphology and distribution of vasoactive intestinal polypeptide- and peptide histidine isoleucine-immunoreactive (VIP- and PHI-ir) neurons and innervation patterns in the main and accessory olfactory bulb, anterior olfactory nucleus, and piriform cortex of the adult cat. In these centers, VIP- and PHI-immunoreactive material are present in the same neuronal types, respectively, therefore summarized as VIP/PHI-ir neurons. In the main olfactory bulb, the majority of VIP/PHI-ir neurons are localized in the external plexiform layer. These neurons give rise to two or more locally branching axons. They form boutons on mitral and external tufted cell bodies. According to the morphology and location, we have classified these neurons as Van Gehuchten cells. Some VIP/PHI-ir neurons are present in the glomerular layer. They have small somata and give rise to dendrites branching exclusively into glomeruli. We have classified these neurons as periglomerular cells. In the granule cell layer, neurons with long apical dendrites and one locally projecting axon are present. In the accessory olfactory bulb, VIP/PHI-ir neurons are localized in the mixed external/mitral/internal plexiform layer. They represent Van Gehuchten cells. In the anterior olfactory nucleus and piriform cortex, VIP/PHI-ir bipolar basket neurons are present. They are localized mainly in layers II/III. These neurons are characterized by a bipolar dendritic pattern and by locally projecting axons forming basket terminals on large immunonegative cell somata. Because of their common morphological features, we summarize them as the retrobulbar VIP/PHI-ir interneuron population. The PHI-ir neurons display the same morphology as the VIP-ir cells. However, they are significantly lower in number with a ratio of VIP-ir to PHI-ir cells about 2:1 in the main and accessory olfactory bulb and in the anterior olfactory nucleus. By contrast, in the piriform cortex the ratio is about 1:1.

Studies on unmyelinated axons and varicosities in the olfactory cortex

The main afferent input to the olfactory cortex from the olfactory bulbs is via the lateral olfactory tract (LOT). The axons within the lateral olfactory tract are myelinated. On leaving the LOT, they lose their myelination as they fan out over the layer immediately beneath the pial surface to make en passant synaptic connections with dendrites from neurones within the olfactory cortex. Using the guinea-pig, a semiquantitative electron micrographical study was made of the density and dimensions of these unmyelinated axons and the varicosities they create. The unmyelinated axons were very fine (0.17 +/- 0.004 micron in diameter) and punctuated at 2 microns intervals by varicosities containing a single type of vesicle. The electrophysiological consequences of this close varicosity spacing is that axonal and varicosity membranes behave electrically as single units.

Simulation of paleocortex performs hierarchical clustering

Simulations were performed of layers I and II of olfactory paleocortex, as connected to its primary input structure, olfactory bulb. Induction of synaptic long-term potentiation by means of repetitive sampling of inputs caused the simulation to organize encodings of learned cues into a hierarchical memory that uncovered statistical relationships in the cue environment, corresponding to the performance of hierarchical clustering by the biological network. Simplification led to characterization of those parts of the network responsible for the mechanism, resulting in a novel, efficient algorithm for hierarchical clustering. The hypothesis is put forward that these corticobulbar networks and circuitry of similar design in other brain regions contain computational elements sufficient to construct perceptual hierarchies for use in recognizing environmental cues.

Region-specific consequences of PCD gene expression in the olfactory system

These studies investigated the response of olfactory bulb juxtaglomerular dopamine neurons to the loss of mitral cells in 6-7-month-old Purkinje cell degeneration (PCD) mice. Previous studies in normal mice, with tyrosine hydroxylase (TH) enzyme as a marker, demonstrated that following peripheral olfactory afferent denervation the juxtaglomerular dopamine neurons exhibited a large reduction in TH activity and immunoreactivity. These intrinsic dopamine neurons also receive afferent input via dendrodendritic contacts with mitral cells. In contrast to the deficits produced by peripheral denervation, following mitral cell degeneration in homozygous recessive PCD mice, TH activity and immunoreactivity were unaltered as compared to normal heterozygous littermates. Moreover, TH activity in the substantia nigra also was unchanged, thus suggesting that the dopamine phenotype is resistant to the influences of the pcd gene. Despite the absence of a well-defined effect of the pcd gene on neurons bearing the TH phenotype, the expression of this mutation within the olfactory system is not limited to mitral cell degeneration. The current studies also demonstrate the absence of the anterior commissure, especially pars anterior, in homozygous recessive PCD mice at 6-7 months postnatal. Whether or not the loss of the anterior commissure is a primary effect or one that is secondary to mitral cell degeneration, this structural alteration provides evidence that the pcd gene exerts more widespread effects within the olfactory system that previously appreciated. The neuronal specificity of those effects remains apparent as indicated by the lack of change in TH expression.

Disynaptic olfactory input to the hippocampus mediated by stellate cells in the entorhinal cortex

Electrophysiological and anatomical studies indicate functional relationships between the olfactory bulb and the hippocampus, mediated by the lateral olfactory tract and perforant path. Fibres from the lateral olfactory bulb terminate in the molecular layer of the lateral entorhinal cortex, which contains stellate and pyramidal cells that project to the hippocampus. Therefore this study was performed to analyze whether a trineuronal, disynaptic chain connects the olfactory bulb and the hippocampus. In adult rats, Fast Blue was unilaterally injected into the septal hippocampus to label cells of origin of the entorhinohippocampal pathway. Lesions of the ipsilateral olfactory bulb induced anterograde terminal degeneration in the entorhinal cortex of the same animals. Fast Blue labelled, and thus hippocampally projecting entorhinal neurones in fixed vibratome slices of the operated brains were injected with Lucifer Yellow. Most of these neurones were stellate layer II and pyramidal layer III cells; in addition there were some sparsely spinous multipolar cells in layers II and III and sparsely spinous horizontal cells at the layer I/II border. Injected cells were photoconverted and processed for electron microscopy. Olfactory bulb lesions resulted in electron-dense degeneration of abundant terminal boutons in the outer zone of entorhinal layer I. The relative frequency of degenerating boutons decreased towards deeper zones of the layer. In the outer zone, degenerated terminals predominantly contacted dendritic spines. These contacts could be seen on injected stellate cells but not on pyramidal cells. This study shows that the area dentata of the rat is reached by disynaptic afferent input from the olfactory bulb and thus is likely to process olfactory information. Oligosynaptic pathways might provide the hippocampus also with visual and auditory inputs; such fast transmitted polysensory information could be essential for the proposed participation of the hippocampus in attention-related mechanisms.

Neurons of the lateral entorhinal cortex of the rhesus monkey: a Golgi, histochemical, and immunocytochemical characterization

This study identifies the neuronal types of the rhesus monkey lateral entorhinal cortex (LEC) and discusses the importance of these data in the context of the connectional patterns of the LEC and the possible role of these cells in neurodegenerative diseases. These neuronal types were characterized with the aid of Golgi impregnation techniques. These characterizations were based upon their spine densities, dendritic arrays, and, where possible, axonal arborizations. The cells could be segregated into only spinous and sparsely spinous types. The most numerous spinous types were pyramidal neurons. Other spinous types included multipolar, vertical bipolar and bitufted, and vertical tripolar neurons. The sparsely spinous neuronal types consisted of multipolar, horizontal bipolar and bitufted, and neurogliaform cells. These cells were further classified with the aid of histochemical stains and immunocytochemical markers. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry stained multipolar, bipolar, and bitufted neurons. Stain for cytochrome oxidase (CO) was found in pyramidal and nonpyramidal cell types. Immunocytochemical techniques revealed several nonpyramidal neurons that contain somatostatin (Som) or substance P (SP). This study complements previous analyses of the neuronal components described in the LEC and adds further information about the distribution of selected neurochemicals within this cortex.

Screening of aromatase-containing neurons in rat forebrain: an immunohistochemical study with antibody against human placental antigen X-P2 (hPAX-P2)

Aromatase-containing neurons were immunohistochemically examined in rat brains by using a polyclonal antibody against human placental antigen. The antibody recognizes cytochrome P-450 portion of aromatase, an enzyme converting androgen to estrogen. A large group of strongly immunoreactive cells was identified in the ventral pallidum, which extends caudally from the area surrounding the islands of Calleja. Other strongly or moderately stained cell groups were observed in the cerebral cortex, the amygdaloid area, the nucleus of the diagonal band, and the area anterior to the posterior commissure. Only a few stained cells were present in the medial preoptic region. These findings cast doubt upon the previous assumption, based on biochemical analysis of tissue samples, that the center of the aromatizing system is in the medial preoptic region. They indicate instead that most aromatase-containing neurons of rats lie within the ventral pallidum ventromedially adjacent to the preoptic area.

Metabolic anatomy of brain: a comparison of regional capillary density, glucose metabolism, and enzyme activities

Regional variations in capillary density, glucose utilization rate, and activities of the glycolytic enzyme lactate dehydrogenase and the mitochondrial enzyme cytochrome oxidase were compared in the rat brain. The distributions of capillaries and enzymes were studied by means of histochemical staining techniques, and glucose metabolism was measured by means of [14C]2-deoxyglucose autoradiography. Analysis of 18 gray and five white matter regions revealed a positive correlation between capillary density and glucose utilization rate. A negative correlation was found between capillary density and lactate dehydrogenase among gray matter structures. Analysis of capillaries and enzymes was also performed within laminated histological fields: hippocampus, olfactory bulb, and olfactory cortex. In general, this revealed reciprocal patterns of staining for lactate dehydrogenase and cytochrome oxidase. Capillary density paralleled cytochrome oxidase activity. The zones of intense staining for lactate dehydrogenase and cytochrome oxidase corresponded to the synaptic terminal fields of different input pathways. These findings demonstrate distinct distributions of a glycolytic and an oxidative enzyme within the brain which are at least partly associated with pathway specificity.

Olfactory cortex: model circuit for study of associative memory?

The piriform (olfactory) cortex is a phylogenetically old type of cerebral cortex with parallels in its organization to the architecture of certain 'neural network' models for distributed pattern recognition and association. These features, in combination with unique structural characteristics that facilitate experimental study, make the piriform cortex a potentially good model for analysis of associative (content-addressable) memory processes.

Terminal field of cholecystokinin-8-like immunoreactive projection neurons of the rat main olfactory bulb

The terminal field of cholecystokinin-8 (CCK)-like immunoreactive (CCK-IR) tufted cells in the rat main olfactory bulb was examined by means of immunohistochemistry combined with either an anterograde tracer or a degeneration method. CCK immunostaining was carried out in animals in which Phaseolus vulgaris agglutinin (PHA) had been injected into the main olfactory bulb. Pairs of adjacent sections were processed for CCK and PHA immunostaining, respectively. Dense CCK-IR terminallike staining was noted in layer Ia of the anterior olfactory nucleus and lateral part of the olfactory tubercle; weaker staining was also observed in the transitional area between the anterior olfactory nucleus and the piriform cortex, in the medial part of the olfactory tubercle, and in the cortical amygdaloid nucleus. The CCK-IR staining was limited to the area containing PHA-labeled terminals and was diminished in these sites after unilateral olfactory bulbectomy. Immuno-electron microscopic analysis showed that CCK-IR profiles in such regions made asymmetric synaptic contacts, mainly with dendritic spines. These results suggest that CCK-IR tufted cells project mainly to the anterior olfactory nucleus and lateral part of the olfactory tubercle, and act mainly via axospinous synapses.

Aggregations of granule cells in the basal forebrain (islands of Calleja): Golgi and cytoarchitectonic study in different mammals, including man

The granule cell islands in the olfactory tubercle (islands of Calleja) and the insula magna of Calleja are present in all species examined in this study: cat, rat, mouse, rabbit, hedgehog, monkey, man, and dolphin, displaying the same basic morphology. They appear as rather undifferentiated neurons with a poorly developed dendritic tree and a short unramified axon that does not leave the island. The larger islands and the insula magna are associated with medium-sized neurons often lying in cell-sparse core regions; they probably represent the efferent component of the islands. The distribution of granule cell islands in the olfactory tubercle varies from species to species: in the cat, they are restricted to the superficial cap regions; in the hedgehog and rabbit, they lie in cap regions and in the deep polymorph layer. In the rat, they are confined mainly to the deep polymorph layer, whereas in the mouse they extend through the three layers. In most species, the lateral islands form part of the cap regions, and they may receive fibers from the lateral olfactory tract. However, the consistent relationship between dwarf cells in the cap regions and granule cells seems to be a merely topographical one. The variable location of granule cell islands indicates that they are not related to specific cell types or cell groups in the olfactory tubercle, except to the large neurons in the hilus zones, which send their dendrites into the islands. Another close and constant relationship exists between granule islands and fibers of the medial forebrain bundle. The medial islands and the insula magna are the largest and most constant aggregations of granule cells. They are present even in the dolphin, which lacks lateral islands. Medial islands and insula magna are continuous in the hedgehog and the newborn kitten and seem to belong to a medial system of granule cells that is independent from the olfactory tubercle and from olfactory fibers. Aggregations of granule cells occur also outside the olfactory tubercle and the insula magna: in the hedgehog and the rabbit, clusters lie scattered in the n. accumbens. Distribution of granule cells outside the olfactory tubercle is related to ontogenetic development: in newborn kittens, granule cells extend from the subependymal layer of the lateral ventricle, where they probably originate, to the medioventral border of the hemisphere, and also distribute throughout the n. accumbens and the ventral pallidum. Thus, the granule cell territory is initially wider, and the original distribution is maintained in some species.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonneuronal localization for steroid converting enzyme: 3 alpha-hydroxysteroid oxidoreductase in olfactory tubercle of rat brain

3 alpha-Hydroxysteroid oxidoreductase (EC 1.1.1.50) was localized in the rat brain by cryostat sectioning, microassay, and neurochemical lesions. Single 16-microns sections were cut, homogenized, and assayed. In the olfactory tubercle 3 alpha-hydroxysteroid oxidoreductase activity is high in the piaglial layer at the surface, 20-fold lower at a depth of 50 microns, and 50-fold lower at a depth of 200 microns. A similar pattern of activity was seen in the olfactory bulb, the interpeduncular nucleus, the frontal pole of the cortex, and the frontoparietal cortex. When kainic acid, a toxin that destroys neurons but leaves glia and axons of passage intact, was injected into the olfactory tubercle, 3 alpha-hydroxysteroid oxidoreductase activity was undiminished whereas glutamic acid decarboxylase activity was reduced by 80%. This laminar distribution and insensitivity to kainic acid are consistent with a nonneuronal localization. The high concentration of astrocytes in the piaglial layer, where 3 alpha-hydroxysteroid oxidoreductase activity is highest, lead us to suggest that this enzyme is localized to astrocytes. The presence of particular enzymes in some brain regions and not in others determines which products are synthesized and which are inactivated in those regions. Thus, the location of 3 alpha-hydroxysteroid oxidoreductase and other steroid converting enzymes can affect the activity of neuronal circuits and the behaviours regulated by those circuits.

Structure of the nucleus olfactorius anterior of the hedgehog (Erinaceus europaeus)

The cytoarchitecture, topography, and cellular structure of the nucleus olfactorius anterior (NOA) in the hedgehog have been studied in Nissl-stained and Golgi preparations. The NOA is an important receptive allocortical formation for olfactory fibers and the major source of association fibers relating the main olfactory bulb with the rest of the olfactory brain. It was divided into a bulbar part; four subdivisions named lateral, dorsal, medial, and ventral; an external part; and a posterior part. Except for the external and posterior subdivisions, the NOA is relatively homogeneous and, in spite of the apparent lack of sublamination in Niss-stained material, four clearly defined cellular laminae were distinguished by the Golgi method. These layers were found to be strikingly similar to those in the piriform cortex. Layer I contains the terminal ramifications of apical dendrites of pyramidal cells and the collaterals of the lateral olfactory tract. The superficial part of layer II contains extraverted pyramidal cells with two or three apical dendrites ramifying in layer I. Most pyramidal cells in the deep part of layer II and layer III are typical pyramidal cells with axons entering the commissura anterior. Some pyramidal cell axons bifurcate into two branches running in opposite directions in the commissura anterior. The interstitial zone below layer III contains deep pyramidal cells and polymorphic cells with ascending branches. Cells with intrinsic axons were classified into four main categories according to the distribution of their axonal ramifications: 1) cells with very restricted axons, 2) cells with axons oriented tangentially in the superficial part of layer II, 3) cells with ascending axons located in the deep part, and 4) chandelierlike cells. Finally, some functional considerations are discussed.

Forebrain projections from cholecystokininlike-immunoreactive neurons in the rat midbrain

The purpose of the present study was to analyze the distribution of cholecystokininlike-immunoreactive (CCK-I) neurons within the rat ventral mesencephalon which project to several forebrain areas. The peroxidase-antiperoxidase immunocytochemical technique was used to examine the anatomical localization of CCK-I within the ventral midbrain and in the following forebrain regions: caudate-putamen, nucleus accumbens, olfactory tubercle, bed nucleus of the stria terminalis, septum, amygdala, and prefrontal, anterior cingulate, and piriform cortices. CCK-I perikarya were distributed throughout the substantia nigra, ventral tegmental area, and several midline raphe nuclei to a greater extent than previously reported, particularly in the substantia nigra pars compacta. Terminallike immunoreactivity for CCK was observed in all of the above forebrain sites. In addition, infrequent CCK-I cell bodies were localized in the caudate-putamen, nucleus accumbens, olfactory tubercle, septum, and bed nucleus of the stria terminalis. To analyze forebrain projections of the ventral midbrain CCK-I neurons, indirect immunofluorescence was combined with fluorescence retrograde tracing. CCK-I neurons of the substantia nigra and/or ventral tegmental area were found to project, to varying extents, to all of the above CCK-I forebrain terminal fields. The nucleus accumbens, olfactory tubercle, and septal and prefrontal cortical projections arose primarily from CCK-I perikarya in the ventral tegmental area whereas the projections to the caudate-putamen and anterior cingulate cortex arose predominantly from immunoreactive neurons in the substantia nigra pars compacta. The amygdala received innervation mainly from CCK-I cell bodies located in the substantia nigra pars lateralis. CCK-I afferents to the bed nucleus of the stria terminalis and piriform cortex originated from perikarya distributed approximately equally across the ventral tegmental area and substantia nigra pars compacta. The general topography of CCK-I forebrain innervation observed in this study is similar to that previously reported for the ascending dopaminergic projections from ventral mesencephalic neurons. CCK-I neurons of the midline raphe nuclei were found to provide relatively minor afferents to the caudate-putamen, bed nucleus of the stria terminalis, septum, and prefrontal cortex and more substantial projections to the amygdala. The results of this study demonstrate that CCK-I neurons of the ventral midbrain supply a much broader innervation of forebrain regions than previously appreciated.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization of the projection from the cerebral cortex to the striatum in the rat

The detailed organization of the corticostriate projection has been investigated in the brain of the rat using the technique of retrograde transport of horseradish peroxidase following the placement of small, iontophoretic injections of horseradish peroxidase conjugated to lectin throughout all major regions of the striatum (caudate-putamen, nucleus accumbens and olfactory tubercle). The results demonstrate that all major regions of the cerebral cortex project to the striatum on both sides of the brain with an ipsilateral predominance. The cells of origin of both the ipsilateral and contralateral corticostriate projections lie mainly in lamina V (especially lamina Va) with very small numbers in lamina III of the neocortex and mesocortex, and in the deep laminae of the allocortex. The results show that each striatal locus receives inputs from several cortical regions, i.e. there is extensive overlap in the corticostriate projection, and that, in general terms, each cortical region projects onto a longitudinally oriented region of the striatum. In particular, the major subdivisions of the cerebral cortex--the neocortex, mesocortex and allocortex--project onto defined but partially overlapping regions of the striatum: the neocortex projects to the caudate-putamen; the mesocortex projects mainly to the medial and ventral regions of the caudate-putamen but also to the ventral striatum (nucleus accumens and olfactory tubercle); and the allocortex projects mainly to the ventral striatum but also to the medial and ventral parts of the caudate-putamen. Within each of these major projection systems there is a further organization, with the constituent parts of each major cortical region projecting to smaller longitudinal components of the major projection fields. Each neocortical area projects to a longitudinal region of the dorsal striatum (caudate-putamen): the sensory and motor areas project topographically onto the dorsolateral striatum such that the rostral sensorimotor cortex (head areas) projects to central and ventral regions and the more caudal sensorimotor cortex (limb areas) projects to dorsal regions of the dorsolateral striatum; the visual area projects to the dorsomedial striatum; and the auditory area projects to the medial striatum. Each mesocortical area projects to a longitudinal area of the striatum: the most posteromedial mesocortex (the retrosplenial area) projects to the dorsomedial striatum; more anterior and lateral parts of the mesocortex project to more ventral parts of the striatum: and the most lateral mesocortex (the agranular insular and perirhinal areas) project to the ventrolateral striatum.(ABSTRACT TRUNCATED AT 400 WORDS)

New high-resolution 2-deoxyglucose method featuring double labeling and automated data collection

A new approach to high-resolution 2-deoxy-D-glucose (2DG) emulsion-autoradiography which combines improved retention of 2DG labeling, staining with immunohistochemical and other specific markers, and automated data collection and analysis of local silver grain and stain densities is described. The Durham et al. (J. Neurosci. 1:519-526, '81) procedure for fixation of 2DG with periodate-lysine-paraformaldehyde (PLP, McLean and Nakane: J. Histochem. Cytochem. 22:1077-1083, '74) was adapted to increase retained label roughly tenfold. Phenobarbital anesthesia is induced 45 minutes after 2DG injection. Barbiturate anesthesia increases brain glycogen (Nelson et al.: J. Neurochem. 15:1271-1279, '68) and presumably increases the incorporation of intracellular 2DG from 2DG-6P into brain glycogen and other molecules (Nelson et al.: J. Neurochem. 43:949-956, '84; Pentreath et al.: Neuroscience 7:759-767, '82). Iodoacetate is added to cold fixative to prevent glycogen breakdown (Cammermeyer and Fenton: Histochemistry 76:339-356, '82). This high-resolution 2DG protocol is directly compatible with many other neuroanatomical techniques. We demonstrate 2DG emulsion autoradiography combined with cytochrome oxidase (CO) histochemistry, markers for axonal pathway tracing, plastic embedding for semithin sections, and immunohistochemical staining for glutamate decarboxylase (GAD). The method should be compatible with antibodies for other antigens and with other neuroanatomical stains. To collect the data directly from microscope slides, a computer-controlled microscope was integrated with image-processing software to eliminate the need for manual counting and scoring of autoradiograms. Regions of interest are scanned automatically at high resolution to map regional labeling and/or stain density. There is excellent correspondence between computer-enhanced two-dimensional maps of the data and the original autoradiograms. Automated counts for five specimens were compared to counts of labeled cells by trained observer. The correlation between the two sets of measurements is high (r = .93). Automated data collection has been generalized to measure regional stain densities on the autoradiographed sections for direct comparison with silver grain density. The method is extremely flexible, especially since new image-processing strategies can be developed in software to extract the desired information from materials labeled by other methods (e.g., HRP).(ABSTRACT TRUNCATED AT 400 WORDS)

Piriform cortex brain slices: techniques for isolation of synaptic inputs

Methods are described for preparation of 3 different slices of piriform cortex which allow convenient study of pyramidal neurons and segregation of synaptic inputs. In slices cut parallel to the pyramidal neurons (perpendicular to the brain surface) one can study chemosensitivity of the various parts of the dendritic tree and the soma. By selected division of this slice the population postsynaptic response to activation of the lateral olfactory tract can be studied without action potential generation. Alternatively the superficial lateral olfactory tract can be removed. Stimulation of deeper regions of the slice under these circumstances elicits a pharmacologically different excitation which appears to be that of association fibers.

Effects of intracerebroventricular angiotensin II and olfactory stimuli on multiple unit activity in preoptic and anterior hypothalamic areas: medial-lateral comparison

Under urethane anesthesia multiple unit activity (MUA) recordings were taken from medial and lateral preoptic and anterior hypothalamic sites in 21 rats during multiple dose intracerebroventricular (i.vt.) injections of angiotensin II (AII), using artificial CSF as control. Olfactory stimuli were also presented. Whilst lateral sites on average were significantly less responsive to AII than were medial sites, some of the former were very responsive. None of the 14 lateral sites that yielded an MUA response to AII failed to yield an MUA response to olfactory stimulation. On the other hand, 11 of 12 medial sites that yielded an MUA response to AII failed to yield an MUA response to olfactory stimulation. On the basis of these data it is suggested that the medial and lateral regions of the basal forebrain serve different functions, the former more related to internal sensing and the latter more related to integration of internal and external sensing. The findings are discussed in relation to the dual olfactory system and to theories of motivation.

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.

Ultrastructure of synaptic remodeling in piriform cortex of adult rats after neonatal olfactory bulb removal: an immunocytochemical study

The purpose of this investigation was to study possible remodeling in synaptic structures of the piriform cortex (PC) of adult rats following neonatal deafferentation by removal of the olfactory bulb (OB) at birth. Emphasis was placed on possible qualitative changes in the ultrastructure and immunocytochemical localization of cholecystokinin (CCK, a possible excitatory neurotransmitter or modulator) and glutamic acid decarboxylase (GAD, precursor enzyme to the inhibitory transmitter GABA) in axons, terminals, and synaptic complexes. Light microscopic results in normal adult material show that GAD-positive terminals form a dense band subjacent to the lateral olfactory tract (LOT), become less dense in deeper Ib, and are rare in layer II. Following deafferentation, GAD-positive terminals appear denser and more homogeneously distributed throughout layer I and are also more prevalent in layer II. Ultrastructural results of normals and controls indicate GAD-positive terminals normally contain pleomorphic or flattened vesicles and form symmetric contacts onto dendritic shafts and branches throughout layer I. In deafferented layer I not only do there appear to be greater numbers of symmetric GAD-positive contacts, but in contrast to normals, asymmetric contacts mainly onto spines are now present. Light microscopic results from deafferented material also show an apparent proliferation with spread or sprouting of CCK-positive fibers or axonlike structures mainly into layer Ia, whereas these fibers are normally observed only in the LOT and are generally few in number. Also in normals the few CCK-positive terminals in the area subjacent to the LOT contain flattened or pleomorphic vesicles and form symmetric contacts. Deafferentation results in CCK-positive terminals throughout layer I with a greater frequency of synaptic contacts which now also include a few asymmetric contacts onto spines. The findings clearly show modifications in synaptic patterns of immunocytochemical-labeled terminals that might be compatible with the process of atypical reinnervation of deafferented postsynaptic sites and possible ingrowth of new axons.

Electron microscopy of plasticity in rat olfactory cortex

Electron microscopy (EM) is being used to study the ultrastructural basis for the age-dependent reorganization of afferents in the olfactory cortex (OC) of rat after deafferentation of the area by removal of the ipsilateral olfactory bulb (OB). The double-lesion technique was used with a primary lesion of the OB at various postnatal (PN) ages between PN 0 and 30 and in the adult (PN 100). After appropriate survival times to remove initial lesion-degenerated terminals from the OB lesion, a second lesion was placed in the ipsilateral OC. One to 3 days later the tissue is prepared for EM with emphasis on a study of changes in the superficial and deep dendritic layer (Ia and Ib respectively) rostral to the lesion. In control litter mates with both OBs intact, but with a single OC lesion only, degenerating synaptic terminals occur onto dendritic spines and branches only in deeper Ib. However, in adults with OB lesions at PN 0-9, OC lesions produce degenerating terminals throughout Ia and Ib including immediately subjacent to the pia. In Ia degenerating terminals are greatly reduced in the PN 13 group and rare to absent in experiments with OB lesions at older ages (PN 30-100). Electron-dense debris within glia occurs throughout layer I in each double-lesion group but is greatest in experiments with OB lesions at older ages. Some transsynaptic alterations are seen throughout, especially in the PN 30-100 group even at a distance from the OC lesion. The results support earlier light microscopic (LM) findings, suggesting PN 9-13 as critical ages for developmental plasticity and prove that at least in the younger ages, synapses are involved in the phenomenon. This may be explained by either reinnervation of deafferented sites or persistence of synapses that would otherwise have been eliminated by afferents from the OB. In addition, some of the LM degeneration particles probably are engulfed masses of debris and not synaptic structures, especially in cases which were operated at older ages and survived for 3 days. The various afferent pathways involved in the events as well as factors that limit the phenomenon in older ages are discussed.