Localization of acetylcholinesterase in the main and accessory olfactory bulbs of the mouse by light and electron microscopic histochemistry

The Accessory Olfactory System: Innately Specialized or Microcosm of Mammalian Circuitry?

In mammals, the accessory olfactory system is a distinct circuit that has received attention for its role in detecting and responding to pheromones. While the neuroscientific investigation of this system is comparatively new, recent advances and its compact size have made it an attractive model for developing an end-to-end understanding of such questions as regulation of essential behaviors, plasticity, and individual recognition. Recent discoveries have indicated a need to reevaluate our conception of this system, suggesting that ( a) physical principles—rather than biological necessity—play an underappreciated role in its raison d'être and that ( b) the anatomy of downstream projections is not dominated by unique specializations but instead consists of an abbreviated cortical/basal ganglia motif reminiscent of other sensorimotor systems. These observations suggest that the accessory olfactory system distinguishes itself primarily by the physicochemical properties of its ligands, but its architecture is otherwise a microcosm of mammalian neurocircuitry.

Dichotomous Distribution of Putative Cholinergic Interneurons in Mouse Accessory Olfactory Bulb

Sensory information processing in the olfactory bulb (OB) relies on diverse populations of bulbar interneurons. In rodents, the accessory OB (AOB) is divided into two bulbar regions, the anterior (aAOB) and posterior (pAOB), which differ substantially in their circuitry connections and associated behaviors. We previously identified and characterized a large number of morphologically diverse cholinergic interneurons in the main OB (MOB) using transgenic mice to visualize the cell bodies of choline acetyltransferase (ChAT-expressing neurons and immunolabeling (Krosnowski et al., 2012)). However, whether there are cholinergic neurons in the AOB is controversial and there is no detailed characterization of such neurons. Using the same line of ChAT^{(bacterial artificial chromosome, BAC)}-enhanced green fluorescent protein (eGFP) transgenic mice, we investigated cholinergic neurons in the AOB. We found significant differences in the number and location of GFP-expressing (GFP+), putative cholinergic interneurons between the aAOB and pAOB. The highest numbers of GFP+ interneurons were found in the aAOB glomerular layer (aGL) and pAOB mitral/tufted cell layer (pMCL). We also noted a high density of GFP+ interneurons encircling the border region of the pMCL. Interestingly, a small subset of glomeruli in the middle of the GL receives strong MCL GFP+ nerve processes. These local putative cholinergic-innervated glomeruli are situated just outside the aGL, setting the boundary between the pGL and aGL. Many but not all GFP+ neurons in the AOB were weakly labeled with antibodies against ChAT and vesicular acetylcholine transporter (VAChT). We further determined if these GFP+ interneurons differ from other previously characterized interneuron populations in the AOB and found that AOB GFP+ interneurons express neither GABAergic nor dopaminergic markers and most also do not express the glutamatergic marker. Similar to the cholinergic interneurons of the MOB, some AOB GFP+ interneurons express the calcium binding protein, calbindin-D28K. Moreover, exposure to either a male intruder or soiled bedding from a mating cage leads to an increase in the number of c-Fos-expressing MCL GFP+ neurons. Taken together, our data reveal a population of largely unidentified putative cholinergic neurons in the AOB. Their dichotomous distribution in the aAOB and pAOB suggests region-specific cholinergic involvement in olfactory information processing.

Structural basis for cholinergic regulation of neural circuits in the mouse olfactory bulb

Odor information is regulated by olfactory inputs, bulbar interneurons, and centrifugal inputs in the olfactory bulb (OB). Cholinergic neurons projecting from the nucleus of the horizontal limb of the diagonal band of Broca and the magnocellular preoptic nucleus are one of the primary centrifugal inputs to the OB. In this study, we focused on cholinergic regulation of the OB and analyzed neural morphology with a particular emphasis on the projection pathways of cholinergic neurons. Single-cell imaging of a specific neuron within dense fibers is critical to evaluate the structure and function of the neural circuits. We labeled cholinergic neurons by infection with virus vector and then reconstructed them three-dimensionally. We also examined the ultramicrostructure of synapses by electron microscopy tomography. To further clarify the function of cholinergic neurons, we performed confocal laser scanning microscopy to investigate whether other neurotransmitters are present within cholinergic axons in the OB. Our results showed the first visualization of complete cholinergic neurons, including axons projecting to the OB, and also revealed frequent axonal branching within the OB where it innervated multiple glomeruli in different areas. Furthermore, electron tomography demonstrated that cholinergic axons formed asymmetrical synapses with a morphological variety of thicknesses of the postsynaptic density. Although we have not yet detected the presence of other neurotransmitters, the range of synaptic morphology suggests multiple modes of transmission. The present study elucidates the ways that cholinergic neurons could contribute to the elaborate mechanisms involved in olfactory processing in the OB. J. Comp. Neurol. 525:574-591, 2017. © 2016 Wiley Periodicals, Inc.

Synaptic connectivity of the cholinergic axons in the olfactory bulb of the cynomolgus monkey

The olfactory bulb (OB) of mammals receives cholinergic afferents from the horizontal limb of the diagonal band of Broca (HDB). At present, the synaptic connectivity of the cholinergic axons on the circuits of the OB has only been investigated in the rat. In this report, we analyze the synaptic connectivity of the cholinergic axons in the OB of the cynomolgus monkey (Macaca fascicularis). Our aim is to investigate whether the cholinergic innervation of the bulbar circuits is phylogenetically conserved between macrosmatic and microsmatic mammals. Our results demonstrate that the cholinergic axons form synaptic contacts on interneurons. In the glomerular layer, their main targets are the periglomerular cells, which receive axo-somatic and axo-dendritic synapses. In the inframitral region, their main targets are the granule cells, which receive synaptic contacts on their dendritic shafts and spines. Although the cholinergic boutons were frequently found in close vicinity of the dendrites of principal cells, we have not found synaptic contacts on them. From a comparative perspective, our data indicate that the synaptic connectivity of the cholinergic circuits is highly preserved in the OB of macrosmatic and microsmatic mammals.

Muscarinic receptor type 1 (M1) stimulation, probably through KCNQ/Kv7 channel closure, increases spontaneous GABA release at the dendrodendritic synapse in the mouse accessory olfactory bulb

Cholinergic modulation of spontaneous GABAergic currents (mIPSC) was studied using whole-cell patch methods in mouse accessory olfactory bulb slices. Carbachol (above 100 microM) administration produced an increase in the mIPSC frequency in mitral cells, but did not affect the responses of mitral cells to GABA. The carbachol effect persisted in the presence of combined ionotropic and metabotropic glutamatergic receptor antagonists. The carbachol effect was reduced by the muscarinic receptor type-1 and -4 (M1 and M4) antagonist pirenzepine (10 microM), but not by the M2 and M4 antagonist himbacine (10 microM). The KCNQ/Kv7 potassium channel openers retigabine (80 microM) and diclofenac (300 microM) blocked the carbachol action, while the KCNQ potassium channel blocker XE-911 (20 microM) increased the mIPSC frequency. XE-911's action persisted in the presence of glutamate receptor blockers. In the presence of carbachol, mIPSCs were abolished by Ni (200 microM), while being insensitive to the calcium channel blocker nimodipine (30 microM), suggesting a role for R-type calcium channels in the GABA release. These results suggest that carbachol closed KCNQ channels by stimulating M1 receptors on granule cell dendrites, and the resulting depolarized and unstable membrane promoted calcium influx, thus increasing the GABA release. The possible role of acetylcholine in facilitating formation of a pheromone memory in mice is also discussed.

Cholinergic innervation of the zebrafish olfactory bulb

A number of fish species receive forebrain cholinergic input but two recent reports failed to find evidence of cholinergic cell bodies or fibers in the olfactory bulbs (OBs) of zebrafish. In the current study we sought to confirm these findings by examining the OBs of adult zebrafish for choline acetyltransferase (ChAT) immunoreactivity. We observed a diffuse network of varicose ChAT-positive fibers associated with the nervus terminalis ganglion innervating the mitral cell/glomerular layer (MC/GL). The highest density of these fibers occurred in the anterior region of the bulb. The cellular targets of this cholinergic input were identified by exposing isolated OBs to acetylcholine receptor (AChR) agonists in the presence of agmatine (AGB), a cationic probe that permeates some active ion channels. Nicotine (50 microM) significantly increased the activity-dependent labeling of mitral cells and juxtaglomerular cells but not of tyrosine hydroxlase-positive dopaminergic neurons (TH(+) cells) compared to control preparations. The nAChR antagonist mecamylamine, an alpha7-nAChR subunit-specific antagonist, calcium-free artificial cerebrospinal fluid, or a cocktail of ionotropic glutamate receptor (iGluR) antagonists each blocked nicotine-stimulated labeling, suggesting that AGB does not enter the labeled neurons through activated nAChRs but rather through activated iGluRs following ACh-stimulated glutamate release. Deafferentation of OBs did not eliminate nicotine-stimulated labeling, suggesting that cholinergic input is primarily acting on bulbar neurons. These findings confirm the presence of a functioning cholinergic system in the zebrafish OB.

Chemical organization of the macaque monkey olfactory bulb: III. Distribution of cholinergic markers

The distribution patterns of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) were studied in the olfactory bulb (OB) of three species of macaque. AChE was detected by a histochemical method and ChAT immunoreactivity by immunocytochemistry. Similar results were observed in all species analyzed. With the exception of the olfactory nerve layer, all layers of the macaque monkey OB demonstrated a dense innervation of AChE- and ChAT-positive fibers. The distribution patterns of AChE- and ChAT-labeled fibers were similar for both cholinergic markers, although the number of AChE-labeled fibers was clearly higher than the number of ChAT-immunoreactive fibers. The highest density of AChE and ChAT-stained fibers was observed in the interface between the glomerular layer and the external plexiform layer and in the internal plexiform layer. Dense bundles of labeled fibers were observed in the caudal OB, coursing from the olfactory peduncle. All ChAT-immunopositive elements were identified as centrifugal fibers, derived from neurons caudal to the OB. Neither olfactory fibers nor intrinsic neurons were observed after ChAT immunocytochemistry. However, a few AChE-positive cells were observed in the glomerular layer and in both external and internal plexiform layers. These neurons were presumably identified as periglomerular cells, superficial short-axon cells, and/or external tufted cells and deep short-axon cells. Contrary to other neurotransmitters and neuroactive substances, the distribution patterns of ChAT and AChE activities in the macaque monkey OB closely resembled the patterns described in macrosmatic mammals and showed laminar differences with the distribution pattern observed in humans.

Acetylcholine and olfactory perceptual learning

Olfactory perceptual learning is a relatively long-term, learned increase in perceptual acuity, and has been described in both humans and animals. Data from recent electrophysiological studies have indicated that olfactory perceptual learning may be correlated with changes in odorant receptive fields of neurons in the olfactory bulb and piriform cortex. These changes include enhanced representation of the molecular features of familiar odors by mitral cells in the olfactory bulb, and synthetic coding of multiple coincident odorant features into odor objects by cortical neurons. In this paper, data are reviewed that show the critical role of acetylcholine (Ach) in olfactory system function and plasticity, and cholinergic modulation of olfactory perceptual learning at both the behavioral and cortical level.

Retrograde tracing of zinc-enriched (ZEN) neuronal somata projecting to the olfactory bulb

Zinc ions seem to be important to several neurological functions and have been connected to the pathophysiology of epilepsy, neuronal cell death after seizure or stroke, and Alzheimer's disease. Both epilepsy and Alzheimer's disease are clinical conditions believed to involve the olfactory bulb. The mammalian olfactory bulb is densely innervated by zinc-enriched (ZEN) neurons, and the distribution of the ZEN terminals in the mouse olfactory bulb has previously been described. The aim of this study was to describe the origins of ZEN terminals projecting into the main olfactory bulb of the rat. Selective labeling of ZEN terminals was accomplished by intracerebral infusion of sodium selenide, whereby zinc selenium clusters are created in the ZEN terminals. Some of these clusters move by retrograde axonal transport to the somata where they can be silver-enhanced by autometallography (AMG). After infusion of sodium selenide into the main olfactory bulb, retrogradely labeled ZEN somata were found (1) ipsilaterally in all anterior olfactory nuclei, taenia tecta, piriform cortex and lateral entorhinal cortex, and (2) contralaterally in anterior olfactory nuclei except the external division. The ipsilateral anterior olfactory nucleus had the densest population of ZEN somata, and it was found that these somata originated mainly from pyramidal neurons in layers II and III of each area. The olfactory-related centrifugal afferents to the main olfactory bulb are discussed.

Distribution and co-localization of choline acetyltransferase and p75 neurotrophin receptors in the sheep basal forebrain: implications for the use of a specific cholinergic immunotoxin

The basal forebrain cholinergic system is involved in different forms of memory. To study its role in social memory in sheep, an immunotoxin, ME20.4 immunoglobulin G (IgG)-saporin, was developed that is specific to basal forebrain cholinergic neurons bearing the p75 neurotrophin receptor. The distribution of sheep cholinergic neurons was mapped with an antibody against choline acetyltransferase. To assess the localization of the p75 receptor on basal forebrain cholinergic neurons, the distribution of p75 receptor-immunoreactive neurons with ME20.4 IgG was examined, and a double-labeling study with antibodies against choline acetyltransferase and p75 receptor was undertaken. The loss of basal forebrain cholinergic neurons and acetylcholinesterase fibers in basal forebrain projection areas was assessed in ewes that had received intracerebroventricular injections of the immunotoxin (50, 100 or 150 microg) alone, as well as, in some of the ewes treated with the highest dose, with bilateral immunotoxin injections in the nucleus basalis (11 microg/side). Results indicated that choline acetyltransferase- and p75 receptor-immunoreactive cells had similar distributions in the medial septum, the vertical and horizontal limbs of the band of Broca, and the nucleus basalis. The double-labeling procedure revealed that 100% of the cholinergic neurons are also p75 receptor positive in the medial septum and in the vertical and horizontal limbs of the band of Broca, and 82% in the nucleus basalis. Moreover, 100% of the p75 receptor-immunoreactive cells of these four nuclei were cholinergic. Combined immunotoxin injections into ventricles and the nucleus basalis produced a near complete loss (80-95%) of basal forebrain cholinergic neurons and acetylcholinesterase-positive fibers in the hippocampus, olfactory bulb and entorhinal cortex. This study provides the first anatomical data concerning the basal forebrain cholinergic system in ungulates. The availability of a selective cholinergic immunotoxin effective in sheep provides a new tool to probe the involvement of basal forebrain cholinergic neurons in cognitive processes in this species.

Zinc-enriched (ZEN) terminals in mouse olfactory bulb

The present study was designed to localize zinc-enriched (ZEN) terminals in mouse olfactory bulb by means of ZnT3 immunocytochemistry (ICC) and zinc autometallography (AMG). The immunocytochemical staining of ZnT3 was closely correlated with the AMG pattern. ZEN terminals were defined as terminals showing both ZnT3 immunoreactivities and AMG granules. At the light microscopic level, dense staining patterns for ZnT3 immunoreactivity were seen in the granule cell layer and the olfactory glomerular layer. At the ultrastructural level, ZEN terminals were restricted to presynaptic terminals with single or multiple postsynaptic thickenings. The postsynaptic profiles contacting ZEN terminals appeared to be dendrites or somata of granule cells in the granule cell layer and periglomerular cells and mitral/tufted (M/T) cells in the olfactory glomerular layer. This suggests that two main sources of ZEN terminals are present in mouse olfactory bulb: (1) centrifugal fibres making asymmetrical synapses with granule cells and periglomerular cells, and (2) olfactory receptor terminals contacting dendritic profiles of M/T cells or periglomerular cells. The close correlation between ZEN terminals and the glutamatergic system is discussed.

Distribution of acetylcholinesterase and choline acetyltransferase in the main and accessory olfactory bulbs of the hedgehog (Erinaceus europaeus)

The distribution of cholinergic markers was studied in the main olfactory bulb (MOB) and accessory olfactory bulb (AOB) of the western European hedgehog (Erinaceus europaeus) by using choline acetyltransferase (ChAT) immunocytochemistry and acetylcholinesterase (AChE) histochemistry. A dense network of AChE-containing and ChAT-immunoreactive fibers was observed innervating all layers of the MOB except the olfactory nerve layer, where neither AChE- nor ChAT-labeled elements were found. The highest density of AChE- and ChAT-positive axons was found in the glomerular layer (GL)/external plexiform layer (EPL) boundary, and in the internal plexiform layer. This general distribution pattern of ChAT- and AChE-stained axons resembled the distribution pattern found in rodents. Nevertheless, some interspecies differences, such as the lack of atypical glomeruli in the hedgehog, were also found. In addition to fibers, a population of noncholinergic and presumably cholinoceptive AChE-active neurons was observed in the hedgehog. All mitral and tufted cells of the hedgehog MOB showed a dark AChE staining unlike previous observations in the mitral and tufted cells of rodents. As in other species previously reported, subpopulations of external tufted cells and short-axon cells were also AChE-active. Finally, a population of small AChE-containing cells was observed in the EPL of the hedgehog MOB. The size, shape, and location of these cells coincided with those of satellite and perinidal cells, two neuronal types described previously in the EPL of the hedgehog and not present in the rodent MOB. The AOB of the hedgehog showed a distribution of AChE- and ChAT-positive fibers similar to the rodent AOB. Nevertheless, a heterogeneous innervation of vomeronasal glomeruli by bundles of AChE- and ChAT-labeled axons found in the hedgehog has not been previously found in any other species. As in the MOB, all mitral cells in the AOB showed a strong AChE activity. These results demonstrate some similarities but also important differences between the distribution of ChAT and AChE in the MOB and AOB of rodents and this primitive mammalian. These variations may indicate a different organization of the cholinergic modulation of the olfactory information in the insectivores.

Partial co-existence of NADPH-diaphorase and acetylcholinesterase in the hypothalamic magnocellular secretory nuclei of the rat

Co-localization of NADPH-diaphorase (ND) and acetylcholinesterase (AChE) activities were explored in the magnocellular secretory nuclei of the rat hypothalamus by means of a double histochemical staining of the same sections. Partial co-existence was found in all the nuclei studied (paraventricular, supraoptic, fornicals and circular nuclei). No particular location of the neurons expressing both markers was found, although in the paraventricular nucleus all of them (ND +, AChE + and neurons expressing both markers) were preferentially located in the magnocellular subdivisions whereas in the parvicellular ones only some neurons belonging to all three types were detected, mainly located in the periventricular and medial subdivisions. The lowest degree of co-existence was found at the level of the main magnocellular nuclei (supraoptic and paraventricular) when compared with the accessory magnocellular nuclei, especially the posterior fornical and the circular nuclei. These results extend previous data on the chemical nature of the neurons producing nitric oxide in the neurosecretory nuclei and the possible functional role of this atypical messenger in the hypothalamus.

Calcium-binding proteins in the periglomerular region of typical and typical olfactory glomeruli

The distribution of chemically identified neuronal populations was studied in the glomerular layer of the rat olfactory bulb using calcium-binding protein immunocytochemistry combined with acetylcholinesterase histochemistry. Four calcium-binding proteins (calbindin D-28k, parvalbumin, calretinin, and neurocalcin) were analyzed in the periglomerular region of two different glomerular subsets; typical and atypical glomeruli. Atypical glomeruli were clearly distinguishable from typical ones by their dense network of acetylcholinesterase-positive centrifugal fibers. Each calcium-binding protein studied showed a specific distribution pattern in the rat olfactory bulb. Calbindin D-28k-, calretinin-, and neurocalcin-immunoreactive neurons were specially abundant in the glomerular layer. These three calcium-binding proteins had their main expressions in neuronal subpopulations directly involved in the glomerular circuitries of the rat olfactory bulb. Specific populations of periglomerular cells were stained for calbindin D-28k, parvalbumin, calretinin, or neurocalcin, whereas external tufted cells were only immunoreactive to neurocalcin. Both neuronal types, periglomerular cells and external tufted cells, were found in the periglomerular region of both glomerular subsets. Nevertheless, a homogeneous distribution of calbindin D-28k- or calretinin-immunopositive periglomerular cells were found between typical and atypical glomeruli, whereas the neurocalcin-immunostained external tufted cells were statistically more abundant in typical glomeruli than atypical ones (P < 0.001). These data suggest that some neuronal subpopulations are related with general properties of the glomerular physiology, and they have a homogeneous distribution in different subsets of glomeruli, whereas other chemically identified populations are related with a finer tuning of the olfactory processing, and they are segregately distributed in relation to particular glomerular subsets. In addition, this work adds new differences in the cellular composition of typical and atypical glomeruli.

Cholinoceptive neurons without acetylcholinesterase activity and enzyme-positive neurons without cholinergic synaptic innervation are present in the main olfactory bulb of adult rat

Light and electron microscopic histochemistry revealed acetylcholinesterase-positive and acetylcholinesterase-negative neurons in the main olfactory bulb of adult rat. Their distribution patterns on various neuron types have been analysed in detail. (1) No acetylcholinesterase staining could be demonstrated in the granule cells which receive a large number of the cholinergic synapses. (2) In contrast, enzyme activity was present in the soma and dendrites in most of the non-cholinergic and non-cholinoceptive relay cells (mitral cells and tufted cells) and in a subset of short-axon interneurons, where cholinergic synapses could not be detected. (3) Within the neuropil of glomeruli, two compartments were present, one of which was free of acetylcholinesterase-positive structures, while many enzyme-positive neuronal elements were seen in the other. (4) Characteristically, cholinergic and non-cholinergic neuronal structures showed triadic arrangements. (5) The axonal release of acetylcholinesterase from cholinergic axons is probable. It is suggested that, in the olfactory bulb, acetylcholinesterase is release by cholinergic afferent axons, and it is the cholinergic synapses that determine which postsynaptic neurons are cholinoceptive rather than the intraneuronal presence of acetylcholinesterase. In the main olfactory bulb, the acetylcholinesterase present in the relay cells therefore appears to have functions other than the hydrolysis of acetylcholine.

Colocalization of NADPH-diaphorase and acetylcholinesterase in the rat olfactory bulb

The colocalization of NADPH-diaphorase and acetylcholinesterase activities in the rat main and accessory olfactory bulbs has been studied by successive double histochemical staining of the same sections. In the main olfactory bulb, three patterns of glomerular labeling were found: typical/NADPH-diaphorase-positive, typical/NADPH-diaphorase-negative, and atypical/NADPH-diaphorase-negative glomeruli. Although both enzymatic activities were present in periglomerular cells and superficial short-axon cells, colocalization of NADPH-diaphorase and acetylcholinesterase was not observed in these neuronal types. By contrast, both enzymes were colocalized in a small subpopulation (less than 3% of NADPH-diaphorase- or acetylcholinesterase-positive cells) of short-axon cells located in the external plexiform layer, internal plexiform layer, granule cell layer, and white matter. In the accessory olfactory bulb, deep short-axon cells were the only neurons where both enzymes were present, and colocalization of both markers was observed in some of these cells located in the granule cell layer.

Comparative laminar distribution of various autoradiographic cholinergic markers in adult rat main olfactory bulb

To provide anatomical information on the complex effects of acetylcholine (ACh) in the olfactory bulb (OB), the distribution of different cholinergic muscarinic and nicotinic receptor sub-types was studied by quantitative in vitro autoradiography. The muscarinic M1-like and M2-like sub-types, as well as the nicotinic bungarotoxin-insensitive (alpha 4 beta 2-like) and bungarotoxin-sensitive (alpha 7-like) receptors were visualized using [3H]pirenzepine, [3H]AF-DX 384, [3H]cytisine and [125I] alpha-bungarotoxin (BTX), respectively. In parallel, labelling patterns of [3H]vesamicol (vesicular acetylcholine transport sites) and [3H]hemicholinium-3 (high-affinity choline uptake sites), two putative markers of cholinergic nerve terminals, were investigated. Specific labelling for each cholinergic radioligand is distributed according to a characteristic laminar and regional pattern within the OB revealing the lack of a clear overlap between cholinergic afferents and receptors. The presynaptic markers, [3H]vesamicol and [3H]hemicholinium-3, demonstrated similar laminar pattern of distribution with two strongly labelled bands corresponding to the glomerular layer and the area around the mitral cell layer. Muscarinic M1-like and M2-like receptor sub-types exhibited unique distribution with their highest levels seen in the external plexiform layer (EPL). Intermediate M1-like and M2-like binding densities were found throughout the deeper bulbar layers. In the glomerular layer, the levels of muscarinic receptor subtypes were low, the level of M2-like sites being higher than M1. Both types of nicotinic receptor sub-types displayed distinct distribution pattern. Whereas [125I] alpha-BTX binding sites were mostly concentrated in the superficial bulbar layers, [3H]cytisine binding was found in the glomerular layers, as well as the mitral cell layer and the underlying laminae. An interesting feature of the present study is the visualization of two distinct cholinoceptive glomerular subsets in the posterior OB. The first one exhibited high levels of both [3H]vesamicol and [3H]hemicholinium-3 sites. It corresponds to the previously identified atypical glomeruli and apparently failed to express any of the cholinergic receptors under study. In contrast, the second subset of glomeruli is not enriched with cholinergic nerve terminal markers but displayed high amounts of [3H]cytisine/nicotinic binding sites. Taken together, these results suggest that although muscarinic receptors have been hypothesized to be mostly involved in cholinergic olfactory processing and short-term memory in the OB, nicotinic receptors, especially of the cytisine/ alpha 4 beta 2 sub-type, may have important roles in mediating olfactory transmission of efferent neurons as well as in a subset of olfactory glomeruli.

Acetylcholinesterase-containing intrinsic neurons in the rat main olfactory bulb: cytological and neurochemical features

Acetylcholinesterase (AChE) histochemistry in light and electron microscopy was used to identify cholinoceptive neurons in the olfactory bulb of adult and 15-day-old rats. Double-labelling experiments using AChE histochemistry and either tyrosine hydroxylase or GABA immunocytochemistry with light microscopy were also performed in order to specify the chemical nature of cholinoceptive neurons. Superficial short-axon cells and several morphological subtypes of deep short-axon cells (second-order interneurons) are the most numerous AChE-containing intrinsic neurons in the olfactory bulb. Short-axon interneurons seem to be the only neurons expressing AChE in the deep olfactory bulb since the numerous granule cells (first-order interneurons) were never found to be AChE-positive, even in electron microscopy. In the superficial olfactory bulb, cholinoceptive cells belong to several neuronal categories. In addition to the intensely labelled superficial short-axon cells, a few periglomerular cells (first-order interneurons) display weak but significant AChE expression, clearly visible in electron microscopy. Both ultrastructural and double-labelling observations support the hypothesis that a subset of superficial tufted cells is also cholinoceptive. The coexistence of AChE and tyrosine hydroxylase in large neurons located in the glomerular and superficial external plexiform layers indicates that some, if not all, cholinoceptive tufted cells belong to the dopaminergic population previously observed in this area. These observations indicate that several types of intrinsic neurons express AChE and can be tentatively considered as cholinoceptive. Our results provide an anatomical substrate for hypotheses concerning the complex effects of acetylcholine in the processing of sensory information in the olfactory bulb.

Effects of chronic ethanol administration on acetylcholinesterase activity in the somatosensory cortex and basal forebrain of the rat

A chronic diet of ethanol has detrimental effects on the cholinergic system in adult humans and rats. This study examined the effects of chronic exposure to dietary ethanol on the anatomical organization of true acetylcholinesterase (AChE) active elements in rat cerebral cortex. We focused on the somatosensory cortex because of its highly organized chemical and cellular structure. Following 42 days of exposure to an ethanol diet (6.7% v/v), there were marked changes in the cortical plexus of AChE-positive fibers. The AChE-positive plexus in ethanol-treated rats was reduced in all cortical layers, in comparison to age-matched pair-fed control and chow-fed rats. The most marked reduction was evident in layers II/III, IV, and VIa. Moreover, the density of AChE-positive cell bodies was significantly reduced in the cortices of ethanol-fed rats, particularly in the deep laminae. These alterations in the chemoarchitecture of somatosensory cortex occurred in the absence of changes in the cytoarchitectonic organization of neocortex. There was no detectable ethanol-induced change in the density of Cresyl violet-stained neurons either in the horizontal limb of the diagonal band of Broca or in the nucleus basalis. The density of AChE-positive neurons in the nucleus basalis, however, was significantly lower in ethanol-fed rats than in controls. Thus, it appears that a mere 6 weeks of ethanol exposure is sufficient to alter the cholinergic innervation of the cerebral cortex. These cortical alterations occur despite the lack of an ethanol-induced death of neurons in the basal forebrain. Such changes may contribute to the memory loss associated with alcohol dementia.

Cholinergic innervation of olfactory glomeruli in the rat: an ultrastructural immunocytochemical study

The ultrastructural organization of cholinergic afferents to the rat olfactory bulb (OB) was studied with the aid of choline acetyltransferase (ChAT) immunocytochemistry in electron microscopy. Particular attention has been paid to a subset of glomeruli characterized by a remarkably high density of cholinergic afferents. Numerous cholinergic terminals making symmetric or asymmetric synaptic contacts were observed in the periglomerular area. ChAT-labelled terminals have a diameter ranging from 0.3 to 1.5 micron and contain numerous clear agranular vesicles. Axo-somatic and axo-dendritic contacts were both observed in contact with several types of target neurons. Three types of cholinoceptive, noncholinergic neurons could be identified: periglomerular cells, superficial short-axon cells, and external tufted cells. Our results provide an anatomical substrate for the hypotheses concerning the complex effects of acetylcholine in the processing of sensory information in the olfactory bulb.

Olfactory bulb organization and development in Monodelphis domestica (grey short-tailed opossum)

The olfactory bulbs of adult and developing Monodelphis domestica were examined with a number of techniques. Golgi, Nissl, and Timm stains as well as acetylcholinesterase histochemistry revealed a high degree of order within the adult bulb. All major cell classes characteristic of most mammalian species were observed. Tufted cells appeared to be restricted to the superficial portion of the external plexiform layer. Developing Monodelphis pups were examined with Nissl-stained semithin sections and with immunocytochemistry for tyrosine hydroxylase, microtubule-associated protein 2, vimentin, and glial fibrillary acidic protein. Newborn pups are extremely immature, with few postmitotic cells present in the forebrain. Considerable maturation occurs over the first four postnatal weeks, and by postnatal day 30, the bulb assumes an adult-like organization. The extreme immaturity of the bulb at birth, coupled with its strict organization, suggest that Monodelphis is a particularly appropriate species for experimental examinations of olfactory system development.

Small cholinergic neurons within fields of cholinergic axons characterize olfactory-related regions of rat telencephalon

Small immunoreactive cholinergic neurons were detected in the main and accessory olfactory bulbs of the rat with choline acetyltransferase immunocytochemistry. Such cells were also found in additional forebrain regions that received direct efferent innervation from the main olfactory bulb, such as the anterior olfactory nucleus, two subdivisions of the olfactory amygdala (nucleus of the lateral olfactory tract and anterior cortical nucleus), and the cortical-amygdaloid transition zone. Cholinergic neurons located in these olfactory-related regions were similar to each other morphologically and to those previously described by other investigators in the cerebral cortex, the hippocampus, and the basolateral amygdala. Somal measurements indicated that choline acetyltransferase-positive cells in olfactory-related regions were all essentially the same size, measuring 13-14 by 8-9 microns in major and minor diameters, respectively. In addition, these small cells were commonly bipolar in form with thin, smooth dendrites, and such characteristics have generally been associated with intrinsic, local circuit neurons in the forebrain. Depending on their location, however, these small cholinergic neurons differed from each other with regard to their frequency and dendritic orientation within planar sections. Choline acetyltransferase-immunoreactive cells in most cortical regions were relatively numerous and usually exhibited long, planar dendrites oriented perpendicularly to the pial surface. In contrast, dendrites of cholinergic neurons found in "cortical-like" regions (e.g. olfactory bulbs or nucleus of the lateral olfactory tract) were relatively sparse in number and appeared to be distinctly non-planar and randomly oriented. Despite these differences, the small choline acetyltransferase-positive cells had many features in common, including their distribution within forebrain regions that contained substantial terminal networks of choline acetyltransferase-positive axons thought to be derived primarily from the basal forebrain complex. In the rat, at least, the presence of small cholinergic interneurons within forebrain regions innervated by the large cholinergic projection neurons of the basal forebrain seems to be developing as a general principle of telencephalic organization. However, differences in both the size and the distribution of the terminal fields derived from each source imply a functional diversity between the intrinsic and extrinsic cholinergic systems of the forebrain.

Postnatal development of cholinergic markers in the rat olfactory bulb: a histochemical and immunocytochemical study

The present study has defined the developmental time course and the distribution patterns of neuronal fibers and cell bodies displaying acetylcholinesterase (AChE) activity or choline acetyltransferase (ChAT) immunoreactivity in the rat olfactory bulb. The results indicate that the deployment of centrifugal AChE-containing fibers is essentially postnatal. The subset of atypical glomeruli, including the modified glomerular complex, is innervated as early as the first postnatal day while the normal ones are not reached by this type of afferent before postnatal day 6. The comparison of AChE labelling with ChAT immunoreactivity strongly supports the assumption that AChE-containing fibers represent mainly, if not exclusively, the cholinergic bulbopetal innervation emanating from the basal forebrain. A quantitative study has confirmed that the density of labelled fibers increases gradually in the postnatal period and spreads heterogeneously among the bulbar layers. The selective precocious innervation of atypical glomeruli is in favor of their involvement in the early processing of olfactory information in the olfactory bulb. Acetylcholinesterase is also expressed within a subset of ChAT-negative interneurons of the developing olfactory bulb. The number of neurons expressing AChE increases from birth to postnatal day 15 and then decreases to reach the adult value on about postnatal day 30. This neuronal population could constitute a cholinoceptive subset mediating the effects of cholinergic afferents on the bulbar neuronal network.

The effect of acetylcholine on rat olfactory bulb unit activity

The olfactory bulb (OB) of the rat receives an extrinsic innervation from the most anterior part of the basal forebrain cholinergic complex. The effect of microiontophoretically applied acetylcholine (ACh) on OB unit activity was studied in 16 adult male rats. A total of 80 units was recorded and in 50% of the cases the cell layer where the recording was done was clearly identified. The results provide evidence for a particularly high level of sensitivity to ACh in the outer glomerular layer (73%). Both inhibitory and excitatory responses were found. These results are in agreement with histological studies indicating that this layer presents the highest density of cholinergic terminals. As a whole, present knowledge suggests the existence of a strong cholinergic control of the olfactory input at the level of the first synapse in the system.

Cholinergic innervation of the main and the accessory olfactory bulbs of the rat as revealed by a monoclonal antibody against choline acetyltransferase

The main and accessory olfactory bulbs (MOB and AOB) of the rat were immunohistochemically stained with a monoclonal antibody against choline acetyltransferase (ChAT) in order to know the difference in the distribution patterns of cholinergic fibers between these two structures. A few ChAT-immunoreactive cell bodies were found in the superficial and middle parts of the external plexiform layer (EPL) of the MOB, in the granule cell layer (GCL) of the MOB, and in the GCL of the AOB. The frequency in appearance of these cells was 0.9 cells/section in the MOB and 0.3 cells/section in the AOB. While the glomerular layer (GL) and the superficial part of the EPL were most densely innervated in the MOB, the internal plexiform layer received the richest innervation in the AOB. There were no immunoreactive structures in the olfactory nerve layer of the MOB and in the vomeronasal nerve layer and glomerular layer of the AOB. In addition to a relatively homogenous distribution of cholinergic fibers in the MOB and AOB, there were several foci of very dense network of immunoreactive fibers at the posterior level of the OB. These foci formed a part of the modified glomerular complex that was recently identified using 2-deoxyglucose method and was presumed to be related to suckling behaviour in the neonatal rat.

Topography of centrifugal acetylcholinesterase-positive fibres in the olfactory bulb of the rat: evidence for original projections in atypical glomeruli

An original pathway of centrifugal acetylcholinesterase-positive fibres is described in the olfactory bulb of the rat. A dense network of positive fibres spreads out superficially at the boundaries of the lateral olfactory tract and the glomerular layer. These labelled fibres converge towards atypical glomerular structures lying close to the classical olfactory glomeruli. The atypical glomeruli are located dorsally at the medial border of the accessory olfactory bulb, in the area previously described as the "modified glomerular complex", and in the ventrolateral bulbar area. They structurally differ from typical glomeruli, as suggested by observations on semithin sections. The ultrastructural distribution of acetylcholinesterases into axonal and dendritic profiles, around and inside atypical glomeruli, is consistent with the hypothesis of centrifugal modulatory influences at this level. This study illustrates several new aspects of morphofunctional heterogeneity in the olfactory system. The glomerular layer of the main olfactory bulb can no longer be considered as morphologically and functionally uniform. Atypical glomeruli located in the mediodorsal and the ventrolateral boundaries of the glomerular layer are characterized by both structural features and an uncommonly high convergence of acetylcholinesterase-positive centrifugal fibres. Such areas might be involved in the processing of specific olfactory signals as demonstrated elsewhere for the "modified glomerular complex".

Effect of amitriptyline on laminar differentiation of neonatal rat olfactory bulb

Amitriptyline (AT) was injected i.p. (0.01 g/kg) into newborn rats for 12 days. Stereometric measurements of coronal sections of the olfactory bulbs showed that the area of the external plexiform layer (EPL) in the AT-treated group was significantly smaller than that of control rats. Rats treated with AT also showed a significantly lower increase in body weight compared with the saline-injected control group.

Transneuronal transport of peroxidase-conjugated wheat germ agglutinin (WGA-HRP) from the olfactory epithelium to the brain of the adult rat

The sensory neurons of the olfactory epithelium, as a consequence of their odor detection function, contact both the external environment and the central nervous system. The possibility that substances applied to the epithelium might reach the central nervous system was investigated by the intranasal application of peroxidase-conjugated wheat germ agglutinin (WGA-HRP). WGA-HRP was transported through olfactory receptor axons to the glomerulus of the olfactory bulb. Reaction product was localized electron microscopically to tubulovesicular profiles and dense bodies in sensory axons. Evidence of transneuronal transport was indicated by reaction product localized in dense bodies in dendrites postsynaptic to receptor cell axons. Periglomerular, tufted and mitral cells in the olfactory bulb also were transneuronally labeled. Anterograde transneuronal labeling occurred in the olfactory tubercle, piriform cortex and surrounding the lateral olfactory tract. Retrograde transneuronal label was found in neurons of the basal forebrain with the largest number of perikarya in the lateral nucleus of the horizontal limb of the diagonal band, a major source of cholinergic afferents to the olfactory bulb. These data suggest that substances, specifically those which bind to receptors, are transported from the olfactory receptor neurons in the nasal epithelium to the brain. Thus, the olfactory system may provide a route of entry for exogenous substances to the basal forebrain.

Olfaction and Alzheimer's disease

Disturbances in the sense of smell may be important both clinically and theoretically in Alzheimer's disease. Initial evidence of poor olfactory recognition performance in Alzheimer's and parkinsonian dementias was followed by two reports which corroborated olfactory dysfunction in Alzheimer's disease. The neuroanatomical and neurochemical bases for this disturbance are discussed. Despite an abundance of preclinical and clinical data linking olfaction with acetylcholine, a preliminary study failed to show any effect of scopolamine on olfactory thresholds. Two of the olfaction-Alzheimer's studies found significantly better performance in other demented groups (alcoholics and patients with vascular dementia), suggesting possible utility in the differential diagnostic workup. The effect of aging per se on olfactory performance cannot be assessed without rigorous control for cognitive dysfunction in sampled populations.

Acetylcholinesterase activity in the developing olfactory bulb: a biochemical study on normal maturation and the influence of peripheral and central connections

In the newborn rat olfactory bulb (OB), the specific activity of acetylcholinesterase (AChE) is about 20% of the adult value. During postnatal development, the specific activity remains unchanged until day 10; a growth spurt of 5X is observed between days 10 and 25, when adult values (90 nmoles ACh/min/mg protein; 6 nmol/min/mg wet wt.) are reached. However, total activity shows continuous increase slowly at first and rapidly between days 10 to 30, reaching a plateau by day 60. Between birth to day 60 total activity increases 65X. To determine the influence of peripheral and central connections on the development of AChE activity in the OB, rats were subjected to unilateral olfactory denervation and/or transection of olfactory peduncle, carried out either neonatally or at day 30; the bulbs were assayed a month later (days 30 and 60 respectively). It was found that in both neonatal and 30-day-old rats, denervation caused a 15% decrease in total activity, while transection led to more than 60% reduction. In the older rats, the reduction due to transection represented the degenerative loss of activity, but in the neonatally transected bulbs the growth of some cholinergic elements continued although very slowly. Between birth to day 30, total AChE activity increased only 12X in completely isolated OB, 25X in transected OB and 40X in denervated OB, compared to 45-50X in control OB. In the transected and isolated bulbs specific activity of AChE was also reduced significantly (25-50% depending on age and operation). These results suggest that while centrifugal fibers are the main source of cholinergic activity in the mature as well as the developing OB, the olfactory nerve and some intrabulbar sources such as cholinergic cells or cholinoceptive membranes also contribute to AChE activity in the OB. These intrinsic sources of AChE activity can persist and even show some growth in the developing olfactory bulb in the absence of the centrifugal fibers and/or the olfactory afferents.

Localization of acetylcholinesterase-positive neurons projecting to the mouse main olfactory bulb

In this study a technique for staining brain tissue for both acetylcholinesterase (AChE) and axonally transported horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) has been used to identify and quantitate the AChE-synthesizing neurons that project to the main olfactory bulb (MOB) of the mouse. The primary sites of double stained neurons included the nucleus of the horizontal limb of the diagonal band (including the nucleus basalis and the magnocellular preoptic nucleus), locus coeruleus and the lateral hypothalamic area. The AChE activity in the neurons of the locus coeruleus and lateral hypothalamic area is suggested to be related to possible cholinoceptive function or some other role, but not to cholinergic projections to the MOB. Therefore, the cholinergic projection to the mouse MOB is suggested to originate in the nucleus of the horizontal limb of the diagonal band, an area that, for the purposes of this study, includes the mouse equivalent of the nucleus basalis of Meynert and what has previously been known as the magnocellular preoptic nucleus.

Quantitative localization of neurons projecting to the mouse main olfactory bulb

Horseradish peroxidase conjugated to wheat germ agglutinin was used as the tracer to demonstrate the cells of origin of the main olfactory bulb (MOB) afferent fibers in the mouse. The neurons projecting to the olfactory bulb were counted by area to show the contribution of each brain area to the system of MOB afferents. The majority of neurons projecting to the MOB were found in the nucleus olfactorius anterior and the cortex cerebri, area pyriformis, being respectively 53.8% and 35.7% of the total number of labeled neurons counted. The average total number of neurons per mouse brain projecting to the MOB was about 146,600.

Ultrastructural identification of substance P immunoreactive neurons in the main olfactory bulb of the hamster

The neurons containing substance P immunoreactivity in the main olfactory bulb of the hamster are located in the glomerular layer. Their cell bodies lie in the periglomerular region and contain spherical or ovoid nuclei which lack invaginations of the nuclear membrane and tend to be positioned eccentrically in the cell body. Dendrites of these neurons extend throughout the periglomerular region and project into the glomerular neuropil. Within the glomerular neuropil, processes with substance P immunoreactivity contain agranular, spherical synaptic vesicles. Primary olfactory axons, and processes of uncertain origin which contain pleomorphic synaptic vesicles, form synaptic contacts with substance P immunoreactive processes. These ultrastructural findings confirm that the substance P immunoreactive neurons are external tufted cells. Their likely physiological properties are considered in relation to the synaptic organization in the glomerular layer of the main olfactory bulb and to the other putative neurotransmitters or neuromodulators located in this layer.

Distribution and morphological characteristics of acetylcholinesterase-containing neurons in the basal forebrain of the cat

The topographical distribution and morphological characteristics of neurons containing acetylcholinesterase (AChE, EC 3.1.1.7) in the basal forebrain of the cat were studied by means of the Butcher's pharmaco-histochemical technique involving staining for AChE at various times after the administration of di-isopropylfluorophosphate (DFP). This method allows a clear visualization of numerous AChE-producing neurons in olfactory tubercle, medial septal-diagonal band area, hypothalamus and basal ganglia. In olfactory tubercle the AChE neurons abound in the deep polymorph layer where they form several types of cell clusters topographically related to the islands of Calleja. The AChE neurons in medial septal nucleus appear morphologically similar to those in nucleus of the diagonal band. As a whole, the medial septal-diagonal band area stands out as one of the major AChE cell collections in basal forebrain. In hypothalamus, neurons staining intensely for AChE occur particularly in supraoptic and paraventricular nuclei and in supramammillary nucleus. The neurons in lateral hypothalamic area stain only moderately for the enzyme. On the other hand, numerous large-sized AChE cells are scattered throughout the neostriatum. However, the AChE cells in putamen appear morphologically different, more numerous per mm2, and larger than those in caudate nucleus suggesting that the feline neostriatum is not a homogeneous structure. The globus pallidus contains both small and large cells that stain lightly and intensely for AChE, respectively. The small pallidal cells are similar to entopeduncular cells and it is proposed that they form the typical pallidal elements. In contrast, the large pallidal AChE cells appear similar to AChE neurons lying in adjacent substantia innominata, and are continuous with the AChE cells in medial septal-diagonal band area. It is suggested that these magnocellular AChE neurons form a population of "limbic" elements within the feline basal ganglia.

Cholinergic and catecholaminergic afferents to the olfactory bulb in the hamster: a neuroanatomical, biochemical, and histochemical investigation

A series of neuroanatomical, biochemical, and histochemical studies have been conducted to determine the sources of cholinergic afferents to the main olfactory bulb (MOB) in the hamster. Following horseradish peroxidase (HRP) injections that are restricted to the MOB, retrograde neuronal labeling is observed bilaterally in the anterior olfactory nucleus, locus coeruleus, and raphe nuclei, and ipsilaterally in the ventral hippocampal rudiment, dorsal peduncular cortex, piriform cortex, nucleus of the lateral olfactory tract, anterior pole of the medial septal area and vertical limb of the diagonal band, nucleus of the horizontal limb of the diagonal band (HDB), and hypothalamus. Spread of HRP into the accessory olfactory bulb results in additional neuronal labeling ipsilaterally in the bed nucleus of the accessory olfactory tract, medial amygdaloid nucleus, and bed nucleus of the stria terminalis, and bilaterally in the posteromedial cortical amygdaloid nucleus. Retrograde tracing studies also have been conducted in cases with lesions in the basal forebrain or hypothalamus to assess the extent to which such lesions interrupt fibers of passage from other sources of centrifugal afferents, and the effects of such lesions on choline acetyltransferase (CAT) activity and catecholamine content in the MOB and on acetylcholinesterase (AChE) activity in the forebrain have been evaluated. Lesions in the basal forebrain reduce or eliminate CAT and AChE activity in the MOB in direct relationship to the extent of damage to the HDB. Norepinephrine (NE) content in the MOB also is reduced by basal forebrain lesions, but in relationship to damage of the medial forebrain bundle (MFB). The hypothalamic lesions have no effect on AChE activity in the forebrain or on CAT activity in the MOB, but they eliminate retrograde labeling in the locus coeruleus and raphe nuclei and reduce the NE content of the MOB to undetectable levels. The dopamine content of the MOB is not reduced by any of the lesions. Anterograde tracing studies have been conducted to compare the rostral projection patterns of the HDB with the distribution of AChE activity. Most of the rostrally directed axons travel in association with the MFB. A small component of axons travels in association with the lateral olfactory tract. Within the MOB, the axons terminate predominantly in the glomerular layer and in the vicinity of the internal plexiform layer. The projection and termination patterns of the HDB correspond well with the distribution of AChE activity. These various results indicate that the HDB is the major source of cholinergic afferents to the MOB.