Dendritic and axonal organization of mitral and tufted cells in the rat olfactory bulb

The output cells of the main olfactory bulb, the mitral and tufted cells, can be categorized into subclasses on the basis of their intrabulbar dendritic and axonal characteristics. Their form was studied in rats following labeling by iontophoretic injection of horseradish peroxidase into the external plexiform layer (EPL). The fact that these extracellular injections labeled small numbers of neurons permitted reconstruction of individual cells. The injection depth within the EPL determined the type of cells labeled. Secondary dendrites of each cell type lay in one of three partially overlapping zones in the EPL. The deepest zone contained the secondary dendrites of one group of mitral cells (Type I), which had the deepest and longest dendrites of the output cells. An intermediate zone of the EPL contained the secondary dendrites of middle tufted and a second class of mitral cells (Type II). The superficial zone, adjacent to the glomerular layer, contained the relatively short, asymmetric dendritic fields of external tufted cells. The few labeled internal tufted cells had secondary dendrites in either the intermediate or deep zones. Every cell type, except the Type I mitral cells, had axon collaterals in the internal plexiform and upper granule cell layers. No cell types had axons re-entering the EPL. These results for output cells combined with our previous observations on granule cells point to a functional sublaminar organization of the EPL that has not previously been proposed.     

Topographic organization of tufted cell axonal projections in the hamster main olfactory bulb: an intrabulbar associational system

The organization of intrinsic axonal projections of principal neurons in the main olfactory bulb (MOB) was studied in hamsters by using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and fluorescent dyes. Punctate injections of either WGA-HRP or fast blue (FB) that are restricted to small sectors on one side of the MOB produce comparably restricted fields of retrograde labeling on the opposite side. Label is found predominantly in superficially situated (middle and external) tufted cells that lie near and at the border between the external plexiform and glomerular layers. Few of the deeper middle tufted, internal tufted, or mitral cells and no external tufted cells that lie in the superficial two-thirds of the glomerular layer are labeled in regions remote to the injection site. Anterograde transport of WGA-HRP from the injection site labels axons that travel dorsally and ventrally in restricted bands through the internal plexiform layer and then terminate within this layer in the punctate sector on the opposite side that contains retrogradely labeled neurons. Such reciprocal projections between opposing regions of the medial and lateral sides of the MOB are found at all rostrocaudal and dorsoventral levels. When punctate injections of FB into the MOB are paired with restricted injections of a second fluorescent tracer (nuclear yellow or diamidino yellow dihydrochloride) into the appropriate sector of pars externa (pE) of the anterior olfactory nucleus, the punctate region of remote retrogradely labeled principal neurons is embedded within a topographically restricted longitudinal wedge of retrogradely labeled mitral and tufted cells that project extrinsically to or through pE. However, extremely few of these neurons are double-retrogradely labeled. The results reveal the existence of an intrabulbar associational system in which principal neurons engage in point-to-point, reciprocal projections between opposing regions of the medial and lateral MOB. Moreover, the results indicate that this associational system largely arises from superficially situated tufted cells distinct from those that support bulbofugal projections into the topographically organized interbulbar commissural system via pE.     

Axonal projection of anterior olfactory nuclear neurons to the olfactory bulb bilaterally.

The axonal projection of anterior olfactory nuclear (AON) neurons to the ipsilateral and contralateral olfactory bulbs and to the prepiriform cortex was analyzed electrophysiologically in the rabbit. Of 117 AON neurons which sent their axons to the anterior commissure, 46 cells (39%) and 55 cells (47%) were activated antidromically by ipsilateral and contralateral olfactory bulb stimulation, respectively, and 22 AON neurons (19%) were activated antidromically from both. The mean axonal conduction velocity of the AON neurons was 2.8 m/s in the AON—anterior commissure axonal segment, 1.6 m/s in the AON—contralateral offactory bulb segment, and 1.0 m/s in the AON—ipsilateral bulb segment. These results and the collision tests between the antidromically evoked spikes indicate that a number of AON neurons send their axons to the contralateral olfactory bulb via the anterior commissure and that the same neurons send thin axon collaterals to the ipsilateral bulb. These axonal projections are significant in relation to the synaptic influences of these axons upon olfactory bulb neurons.     

Mapping of an olfactory receptor population that projects to a specific region in the rat olfactory bulb

An anatomically distinct group of glomeruli, termed the modified glomerular complex (MGC), is present in the posterior dorsomedial portion of the main olfactory bulb. This region has been strongly implicated as part of the pathway that processes odor cues for suckling in neonatal rat pups. We studied the distribution pattern of olfactory receptor neurons that project to the MGC region after ionophoretic injections of WGA-HRP into the olfactory bulbs of 12-day-old rat pups. HRP label was confined to an identifiable localized region in the MGC of the main olfactory bulb. Label extended over 2-7% of the glomerular sheet of the main olfactory bulb, including the MGC. Olfactory receptor neurons within the olfactory epithelium of the nasal cavity were labeled with HRP ipsilateral to the injected side. Maps constructed of the olfactory epithelium revealed that the labeled neurons occurred within topographically defined regions. Anteriorly, labeled olfactory neurons were confined to a narrow strip medial to the dorsal recess, and, more posteriorly, this strip widened medially along the septal wall and laterally onto a limited area on the nasal turbinates. Only a portion of the receptor population within a region was labeled. The boundaries between labeled and unlabeled regions were sharp. These findings support the concept that the olfactory epithelium is an anatomical mosaic in which receptors with different glomerular projections sites are intermingled. In conjunction with previous evidence on the functional specificity of the MGC, and staining of receptor neuron subgroups with monoclonal antibodies, these findings further suggest that olfactory receptor neurons form a functional mosaic within the olfactory epithelium.     

Connections of the olfactory bulb and nucleus olfactorius anterior in the hedgehog (Erinaceus europaeus): fluorescent tracers and HRP study

The projections of the main olfactory bulbs (MOBs) and the dorsal part of the anterior olfactory nucleus (NOA) in the hedgehog (Erinaceus europaeus) have been studied by fluorescent tracers and the horseradish peroxidase method (HRP), respectively, to reveal the pattern of labeling from these structures. After different dye injections in both MOBs, labeled cells were present in the following structures: tenia tecta, vertical limb of the diagonal band of Broca, and medial septal nucleus in the ipsilateral injection site; and the NOA, piriform cortex, nucleus of the lateral olfactory tract, horizontal limb of the diagonal band of Broca, posterolateral cortical amygdaloid nucleus, anterior amygdaloid area, and dorsal raphe nucleus in both hemispheres. Structures showing double-labeled cells were the NOA, horizontal limb of the diagonal band of Broca, nucleus of the lateral olfactory tract, anterior amygdaloid area, and posterolateral cortical amygdaloid nucleus. After HRP injections in the dorsal part of the NOA, labeled cells were distributed in the NOA, nucleus of the lateral olfactory tract, posterolateral cortical amygdaloid nucleus, piriform cortex, horizontal and vertical limbs of the diagonal band of Broca, mitral cell layer of the MOB, tenia tecta, anterior amygdaloid area, and the contralateral NOA. We suggest that the contralateral projection nuclei to the MOB of the hedgehog, unusual in other mammals, and the large number of cells with axonal collaterals projecting to both hemispheres, may be a strategy in these animals to bilaterally integrate brain functions at the expense of its reduced corpus callosum.     

Labelling of olfactory bulb glomeruli following horseradish peroxidase lavage of the nasal cavity

The transport of horseradish peroxidase (HRP) from the nasal cavity to the olfactory bulb was examined in rat. HRP was present primarily in the olfactory nerve and glomerular layer. In some animals the glomeruli were densely filled with product while in others there was considerable interglomerular variation in density. Examination of the decalcified noses revealed a restricted distribution of HRP in those rats with partially labelled olfactory bulbs. The presence of small groups of densely labelled glomeruli was also noted using the 2-deoxyglucose method to examine odor-induced metabolic activity.     

Differences in chemo- and cytoarchitectural features within pars principalis of the rat anterior olfactory nucleus suggest functional specialization

The anterior olfactory nucleus (AON) lies between the olfactory bulb and piriform cortex and is the first bilaterally innervated structure in the olfactory system. It is typically divided into two subregions: pars externa and pars principalis. We examined the cytoarchitecture of pars principalis, the largest cellular area of the region, to determine whether it is homogeneously organized. Quantitative Nissl studies indicated that large cells (cell body area >2 standard deviations (SD) larger than the mean cell size) are densest in lateral and dorsolateral regions, while small cells (>1 SD smaller than the mean) are more numerous in medial and ventral areas. Further evidence for regional differences in the organization of the AON were obtained with immunohistochemistry for calbindin (CALB), parvalbumin (PARV), glutamic acid decarboxylase (GAD), and choline transporter (CHT). Cells immunopositive for CALB (CALB+) were denser in the deep portion of Layer II, although homogeneously dispersed throughout the circumference of the AON. PARV+ cells were located in the superficial half of Layer II and were sparse in ventral and medial regions. CHT+ and GAD+ fibers were denser in lateral versus medial regions. No regional differences were found in GAD+ somata, or in norepinephrine transporter or serotonin transporter immunoreactivity. The observed regional differences in cyto- and chemoarchitectural features may reflect functional heterogeneity within the AON.     

A quantitative study of the effects of early unilateral olfactory deprivation on the number and distribution of mitral and tufted cells and of glomeruli in the rat olfactory bulb

Anatomic effects of early unilateral olfactory deprivation on the developing olfactory bulb were investigated in the albino rat. Unilateral anosmia was experimentally induced by neonatal cauterization of the left or right nare; regenerating tissue permanently blocked the nostril. The anosmic olfactory bulb (ipsilateral to the blocked nare) and its contralateral counterpart, serving as the normal control, were compared for the following quantitative anatomic parameters: total number and distribution of mitral and tufted cells and olfactory glomeruli; average diameters of mitral cells and glomeruli; relative dimensions of the bulb and its layers. The effects on mitral cells and glomeruli were studied at the ages of 25 and 60 days and at 2 years; other studies were carried out in the 25-day-old animals only. In the normal mature bulb, the number of mitral cells, tufted cells and glomeruli was found to be about 70,000, 160,000 and 3000, respectively. It was found that the absence of early olfactory stimulation was invariably correlated with a significant and permanent loss of mitral cells, amounting to more than a quarter of the total number. This loss apparently occurred rapidly during the first 3 week, as it was already evident by day 25 and did not increase appreciably with prolongation of deprivation. Tufted cells were apparently even more susceptible, because their number decreased by about 45%. As evident from their distribution profiles, the loss of mitral and tufted cells occurred uniformly throughout the bulb. It is shown that these differences were due neither to inherent interbulbar differences, nor to a hyperplasia in the normal bulb. Early anosmia had no significant effects on the number or average diameter of the glomeruli. Morphometric studies revealed that the dimensions and thickness of layers (internal and external plexiform and granular) of the anosmic bulb were significantly reduced. It is suggested that early olfactory stimulation is necessary for survival of the developing mitral and tufted cells; thus the first 2–3 postnatal weeks, covering the final developmental stages of these cells, would constitute a vulnerable period in the development of the rat olfactory system.     

Neuronal morphology and efferent projections of the dorsal nucleus of the lateral lemniscus in the rat

The dorsal nucleus of the lateral leminsucs (DLL) is the main source of inhibitory influence in the auditory brainstem of mammals. The cytoarchitecture and connectional properties of DLL were established in the cat in contrast to the rat. The goal of the present study was to establish to what extent the anatomical properties of the rat DLL compare to those of the cat, thus providing a basis of interpretation for future functional studies in the rat, an animal model used more and more in the auditory system. DLL of the rat contains four well-differentiated neuronal types, as seen in Nissl-stained material. Type I neurons are large and multipolar with abundant cytoplasm and darkly stained Nissl substance. Type II neurons are large, bipolar and darkly stained in Nissl material. Type III neurons are medium in size and their soma is round or ovoid. Type IV neurons are small and round with scant cytoplasm; they seem to be also the least common neuronal type of the DLL. After Phaseolus vulgaris-leucoagglutinin or biocytin injections in the DLL, fibers and terminals labeled by orthograde transport were observed in the corresponding region of the contralateral DLL and in the inferior colliculus, bilaterally. A few labeled fibers and terminal fields were seen in the deep layers of the superior colliculus bilaterally, as well as in the medial division of the medial geniculate body and, even more rostrally, in the posterior nucleus of the thalamus. Descending projections from DLL terminated in the periolivary regions of the ipsilateral superior olivary complex. Retrograde tracing based on injections of horseradish peroxidase in the various targets of the DLL confirmed the connections established with orthograde labeling. © 1993 Wiley-Liss, Inc.     

Contralateral projections of the rat anterior olfactory nucleus

The anterior olfactory nucleus (AON) is a central olfactory cortical structure that has heavy reciprocal connections with both the olfactory bulb (OB) and piriform cortex. While it has been firmly established that the AON is a primary source of bilateral projections in the olfactory system through extensive connections with both the ipsilateral and contralateral OB, AON, and piriform cortex, few studies have examined this circuitry in detail. In the present study we used small injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde tracer FluoroGold in specific subregions of the AON to explore the topography of the interconnections between the left and right AONs. Labeled fibers were found in the contralateral AON following injections in all areas. However, detailed quantitative analyses revealed that different regions of the AON have distinct patterns of interhemispheric innervation; contralateral fibers were most heavily targeted to dorsal and lateral AON subregions, while the medial and ventral areas received relatively light projections. These results demonstrate important features of the interhemispheric circuitry of the AON and suggest separate functional roles for subregions of the AON in olfactory information processing.     

Selective retrograde transport of tritiated d-aspartate from the olfactory bulb to the anterior olfactory nucleus, pyriform cortex and nucleus of the lateral olfactory tract in the rat

After an injection of [³H]d-aspartate into the olfactory bulb of the rat, retrogradely labeled cells were detected bilaterally in the anterior olfactory nucleus (AON), and ipsilaterally in the pyriform cortex (PC) and nucleus of the lateral olfactory tract (NLOT). These results suggest a certain selective retrograde transport of this amino acid, and are discussed in relation to transmitter candidates in the olfactory bulb.     

Time frame of mitral cell development in the mice olfactory bulb

Along with tufted cells, mitral cells are the principal projection neurons in the olfactory bulb (OB). During the development of the OB, mitral cells migrate from the ventricular zone to the intermediate zone, where they begin to send axons along the lateral olfactory tract (LOT) to the cortical olfactory zones. Subsequently, they lose their tangential orientation, enabling them to make contact with the axons of the olfactory sensory neurons (OSN) that innervate the whole OB. Here, we investigated the distinct morphological features displayed by developing mitral cells and analyzed the relationship between the changes undertaken by these neurons and the arrival of the OSN axons. Immunostaining for specific markers of developing axons and dendrites, coupled with the use of fluorescent tracers, revealed the morphological changes, the continuous reorientation, and the final refinement that these cells undergo. We found that some of these changes are dependent on the arrival of the OSN axons. Indeed, we identified three main chronological events: 1) newly generated neurons become established in the intermediate zone and project to the LOT; 2) the cells reorient and spread their dendrites at the same time as OSN axons penetrate the OB (this is a sensitive period between embryonic day (E)15-16, in which the arrival of afferents establishes a spatial and temporal gradient that facilitates protoglomerulus and glomerulus formation); and 3) final refinement of the radially orientated cells to adopt a mature morphology. These results suggest that both afferent inputs and intrinsic factors participate to produce the well-defined sensory system.     

Peripheral contributions to olfactory bulb cell populations (migrations towards the olfactory bulb)

The olfactory system represents one of the most suitable models to study interactions between the peripheral and central nervous systems. The developing olfactory epithelium (olfactory placode and pit) gives rise to several cell populations that migrate towards the telencephalic vesicle. One of these cell populations, called the Migratory Mass (MM), accompanies the first emerging olfactory axons from the olfactory placode, but the fate of these cells and their contribution to the Olfactory Bulb (OB) populations has not been properly addressed. To asses this issue we performed ultrasound-guided in utero retroviral injections at embryonic day (E) 11 revealing the MM as an early source of Olfactory Ensheathing Cells in later postnatal stages. Employing a wide number of antibodies to identify the nature of the infected cells we described that those cells generated within the MM at E11 belong to different cell populations both in the mesenchyma, where they envelop olfactory axons and express the most common glial markers, and in the olfactory bulb, where they are restricted to the Olfactory Nerve and Glomerular layers. Thus, the data reveal the existence of a novel progenitor class within the MM, potentially derived from the olfactory placode which gives rise to different neural cell population including some CNS neurons, glia and olfactory ensheathing cells.     

The connections between the olfactory bulb and the brain in the goldfish

We have investigated olfactory bulb connections in the goldfish by using both horseradish peroxidase applied to olfactory tract lesions and wheat germ agglutinin-conjugated peroxidase administered to the olfactory bulb. The projections from the bulb pass to the brain in two major bundles: the lateral and the medial olfactory tracts. The lateral tract innervates the posterior terminal field in the area dorsalis. The medial tract divides into two rami. The dorsolateral ramus is the most substantial olfactory bundle in the brain and innervates several targets in both the area dorsalis and the area ventralis. The ventromedial ramus appears to innervate targets in the area ventralis exclusively. In addition, fibers of the medial olfactory tract (both rami) innervate the preoptic nucleus as well as targets in the diencephalon and, possibly, in the mesencephalon as well. Olfactory fibers from the dorsolateral and ventro-medial rami cross the midline in the dorsal and ventral olfactory decussations, respectively. Between these two decussations is a dense olfactory plexus which has not previously been reported, and which may serve as a nexus allowing interchange of fibers between the two olfactory rami. The terminal nerve in the goldfish has two parts. The major part of the nerve projects to the ventral nucleus (Vv) and is far more extensive than has previously been reported. A much less substantial component of the terminal nerve projects to the two retinae. There are four large groups of cells in the telencephalon which project to the olfactory bulb. Of these two, the dorsal and the anterolateral groups have not been described in previous studies of the goldfish. We also report weakly labeled bulbopetal cells in the nucleus preglomerulosus and in the locus ceruleus.     

Subregions of the periaqueductal gray topographically innervate the rostral ventral medulla in the rat.

Previous anatomical and physiological studies have revealed a substantial projection from the periaqueductal gray (PAG) to the nucleus paragigantocellularis (PGi). In addition, physiological studies have indicated that the PAG is composed of functionally distinct subregions. However, projections from PAG subregions to PGi have not been comprehensively examined. In the present study, we sought to examine possible topographic specificity for projections from subregions of the PAG to PGi. Pressure or iontophoretic injections of wheat germ agglutinin-conjugated horseradish peroxidase, or of Fluoro-Gold, placed into the PGi of the rat retrogradely labeled a substantial number of neurons in the PAG from the level of the Edinger-Westphal nucleus to the caudal midbrain. Retrogradely labeled neurons were preferentially aggregated in distinct subregions of the PAG. Rostrally, at the level of the oculomotor nucleus, labeled neurons were i) compactly aggregated in the ventromedial portion of the PAG corresponding closely to the supraoculomotor nucleus of the central gray, ii) in the lateral and ventrolateral PAG, and iii) in medial dorsal PAG. More caudally, retrogradely labeled neurons became less numerous in the dorsomedial PAG but were more widely scattered throughout the lateral and ventrolateral parts of the PAG. Only few retrogradely labeled neurons were found in the ventromedial part of the PAG at caudal levels. Injections of retrograde tracers restricted to subregions of the PGi suggested topography for afferents from the PAG. Injections into the lateral portion of the PGi yielded the greatest number of labeled neurons within the rostral ventromedial PAG. Medially placed injections yielded numerous retrogradely labeled neurons in the lateral and ventrolateral PAG. Injections placed in the rostral pole of the PGi (medial to the facial nucleus) produced the greatest number of retrogradely labeled neurons in the dorsal PAG. To examine the pathways taken by fibers projecting from PAG neurons to the medulla, and to further specify the topography for the terminations of these afferents in the PGi, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoretically deposited into subregions of the PAG that contained retrogradely labeled neurons in the above experiments. These results revealed distinct fiber pathways to the rostral medulla that arise from the dorsal, lateral/ventrolateral, and ventromedial parts of the PAG. These injections also showed that there are differential but overlapping innervation patterns within the PGi. Consistent with the retrograde tracing results, injections into the rostral ventromedial PAG near the supraoculomotor nucleus yielded anterograde labeling immediately ventral to the nucleus ambiguus in the ventrolateral medulla, within the retrofacial portion of the PGi.(ABSTRACT TRUNCATED AT 400 WORDS)     

Different granule cell populations innervate superficial and deep regions of the external plexiform layer in rat olfactory bulb

Studies on the morphological organization of the main olfactory bulb have indicated that there are subpopulations of granule cells with different dendritic patterns in the external plexiform layer (EPL). Small, extracellular injections of horseradish peroxidase (HRP) were made iontophoretically into superficial and deep parts of the EPL and the granule cell layer (GCL) in adult rats. Superficial EPL injections principally labeled superficial granule cell somata, whereas deep EPL injections labeled both superficial and deep granule cell somata. Injections in the superficial GCL labeled granule cell dendritic processes extending across the entire EPL. However, deep GCL injections labeled few granule cell dendrites in the superficial EPL, but labeled many such processes in the deep EPL. These results were the same in material processed with the Hanker-Yates procedure, where the morphology of individual neurons could be studied, and in the more sensitive tetramethyl benzidineprocedure. Serial reconstructions of individual granule cells were made from both HRP and Golgi-Kopsch material. The distal dendrites of deep granule cells reached only as far as the deep EPL, where they branched extensively and had many dendritic spines. The dendrites of superficial granule cells, however, reached the most superficial part of the EPL where they ramified most extensively. The superficial granule cells typically had a higher spine density in the superficial part of the EPL than in the deep part. On the basis of these results, we conclude that the superficial granule cells predominantly innervate the superficial EPL and that the deep granule cells exclusively innervate the deep EPL. Granule cells are believed to exert inhibitory influences on the bulbar output neurons, the mitral and tufted cells, through reciprocal dendrodendritic synapses. Since the secondary dendrites of the tufted cells ramify in the superficial EPL and the dendrites of most mitral cells ramify in deep EPL, the superficial and deep granule cells may preferentially modulate the responses of tufted and mitral cells, respectively.     

Efferent connections of the nucleus of the lateral olfactory tract in the rat

The efferent connections of the nucleus of the lateral olfactory tract (LOT) were examined in the rat with the Phaseolus vulgaris leucoagglutinin (PHA-L) technique. Our observations reveal that layers II and III of LOT have largely segregated outputs. Layer II projects chiefly ipsilaterally to the olfactory bulb and anterior olfactory nucleus, bilaterally to the anterior piriform cortex, dwarf cell cap regions of the olfactory tubercle and lateral shell of the accumbens, and contralaterally to the lateral part of the interstitial nucleus of the posterior limb of the anterior commissure. Layer III sends strong bilateral projections to the rostral basolateral amygdaloid complex, which are topographically organized, and provides bilateral inputs to the core of the accumbens, caudate-putamen, and agranular insular cortex (dorsal and posterior divisions). Layer II projects also to itself and to layers I and II of the contralateral LOT, whereas layer III projects to itself, to ipsilateral layer II, and to contralateral layer III of LOT. In double retrograde labeling experiments using Fluorogold and cholera toxin subunit b tracers, LOT neurons from layers II and III were found to provide collateral projections to homonymous structures on both sides of the brain. Unlike other parts of the olfactory amygdala, LOT neither projects directly to the extended amygdala nor to the hypothalamus. Thus, LOT seemingly influences nonpheromonal olfactory-guided behaviors, especially feeding, by acting on the olfactory bulb and on ventral striatal and basolateral amygdaloid districts that are tightly linked to lateral prefrontal cortical operations.     

Volumetric changes in the anterior olfactory nucleus of the rat after neonatal olfactory deprivation

The effect of olfactory deprivation in the postnatal development of the anterior olfactory nucleus (AON) was studied in 60-day-old rats which underwent unilateral naris closure after birth (postnatal day 1). Volumetric and morphometric analyses of the AON ipsilateral and contralateral to the closed naris were performed and data were statistically compared among them and with those of control animals. The volumes of the AONs and those of their subdivisions were calculated by the Cavalieri method and the area of the subdivisions was measured at seven established rostrocaudal levels. Whereas no statistically significant differences were detected between the ipsilateral and the contralateral AONs, comparison of these with controls revealed significant reductions in the volumes and dimensions of most AON subdivisions. The reduction was larger in the ipsilateral than in the contralateral AON and more pronounced in the rostralmost subdivisions (external and lateral) than in the caudal ones, the dorsal subdivision not being affected. These data demonstrate that the disruption of the normal afferent activity to one olfactory bulb has effects on the postnatal development of both the ipsilateral and the contralateral AONs. In addition, the most affected subdivisions were those that develop later and that receive the bulk of projections from the olfactory bulb, suggesting that the degree of maturity is an important factor in susceptibility to changes induced by reduced afferent activity. Finally, the results indicate that, contrary to the olfactory bulb, the contralateral AON cannot be used as a control structure in deprivation studies.     

Corticomotoneuronal connections in the rat: evidence from double-labeling of motoneurons and corticospinal axon arborizations.

In order to investigate the possibility of direct corticomotoneuronal (CM) connections in the rat, an anterograde-retrograde double-labeling method was developed. Phaseolus vulgaris-leucoagglutinin (PHA-L) anterograde tracing of corticospinal axons was combined with retrograde labeling of spinal motoneurons either by a conjugate of choleragen subunit B with horseradish peroxidase (CB-HRP) or by wheat germ agglutinin (WGA). The location of PHA-L injection unilaterally in the forelimb area of sensorimotor cortex and the CB-HRP or WGA injections in corresponding contralateral wrist or digit extensors or flexors were determined and matched on the basis of movement responses elicited by intracortical microstimulation. Light microscopic observation showed, in addition to the main contralateral dorsal corticospinal tract (CST), the presence of four other CST minor components in the contralateral lateral, ipsilateral ventral, and ipsilateral dorsal funiculi of the cervical spinal white matter and at the base of contralateral dorsal horn of the gray matter, respectively. PHA-L-labeled CST axonal arbors were observed from Rexed's lamina I through lamina X of contralateral spinal gray matter, most extensively in laminae VI and VII; some CST axons reached the zone of motoneuronal somata in lamina IX and a few of them also entered the lateral and occasionally the ventral funiculi, ramifying in the white matter. Between the zones of PHA-L-labeled CST axonal arbors on the one hand and CB-HRP/WGA labeled spinal motoneuronal somata with their extensive dendritic trees on the other, there was a large overlap, covering partly both the gray and the white matter. PHA-L-labeled axonal boutons (en passant or terminaux) were seen to contact the dendrites or even the somata of motoneurons in the gray matter, according to light-microscopic criteria for identification of synaptic contacts. Axodendritic CM contacts were occasionally observed in the lateral funiculus of the white matter as well. In general, only a single contact was observed between an individual PHA-L-labeled CST axon and a given retrogradely labeled motoneuron. In contrast to the common notion that direct CM connections are a specialty of primates, the present morphological data support the presence of direct CM connections also in some other mammals, such as the rat.