External plexiform layer

Immediately deep to the glomeruli is a layer with a relatively low cell density but a very dense neuropil, the external plexiform layer (EPL). Golgi-stained sections reveal that the predominant neural elements in this layer are the dendrites of mitral/tufted and granule cells. The principal neuron types in EPL are external, middle (Fig. 12B) and deep tufted cells, named according to their relative depth in EPL, and the Van Ge-huchten cells. 

Candidate transmitters of tufted cells and Van Gehuchten cells 

The axons of the external tufted cells project mainly to other sites in the same olfactory bulb (Schoenfeld et al. 1985). Middle and deep tufted cells also have local collaterals in the ipsilateral bulb but most of them appear to project out of the olfactory bulb to the anterior olfactory nucleus and other rostral olfactory cortical structures (Schoenfeld et al. 1985 Scott, 1986). The intrabulbar collaterals of the superficial tufted cells form 

A second class of EPL neurons are the Van Gehuchten cells. These cells are characterized by two or more thick primary dendrites that remain in the EPL. Axons from these cells terminate around mitral and tufted cells. Many of these Van Gehuchten cells stain positively for vasoactive intestinal polypeptide (Sanides Kohlrausch and Wahle, 1990b) calcium binding protein (Brinon et al. 1992 Alonso et al. 1993) and parvalbumin (Celio, 1990) (Fig. 9B). 

Many of the middle tufted cells are reported to contain vasoactive intestinal polypeptide (VIP) in the rat (Fig. 11 A) (Gall et al. 1986), but in the cat, this peptide appears to be in van Gehuchten cells not tufted cells (Sanides Kohlrausch and Wahle, 1990a). In the hamster, a few middle tufted cells are NADPH diaphorase positive (Davis, 1991). 

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Mu receptors are almost always located proximally, on the presynaptic side of the synapse. The periaqueductal gray is the region containing the most mu receptors, but they are also found in the superficial dorsal hom of the spinal cord, the external plexiform layer of the olfactory bulb, the nucleus accumbens (an area deeply implicated in the process of addiction), in some parts of the cerebral cortex, and in some of the nuclei of the amygdala. Mu receptors avidly bind enkephalins and beta-endorphin, but they have a low affinity for dynorphins (primarily a kappa receptor agonist).6... 

In the olfactory bulb, D2 receptor immunoreactivity was detected in the glomerular and external plexiform layers the olfactory nerve also exhibited immunopositivity (Levey et ah, 1993). 

EPIA external plexiform layer of the accessory olfactory bulb 3... 

Ezeh PI, Wellis DP, Scott JW. 1993. Organization of inhibition in the rat olfactory bulb external plexiform layer. J Neurophysiol 70 263-274. 

Hamilton KA, Heinbockel T, Ennis M, Szabo G, Erdelyi E, et al. 2005. Properties of external plexiform layer inter-neurons in mouse olfactory bulb slices. Neuroscience 133 819-829. 

Jackowski A, Parnavelas JG, Lieberman AR. 1978. The reciprocal synapse in the external plexiform layer of the mammalian olfactory bulb. Brain Res 159 17-28. 

Orona E, Scott JW, Rainer EC, 1983, Different granule cell populations innervate superficial and deep regions of the external plexiform layer in rat olfactory bulb. J Comp Neurol 217 227-237. 

Toida K, Kosaka K, Heizmann CW, Kosaka T. 1996. Electron microscopic serial-sectioning/reconstruction study of parvalbumin-containing neurons in the external plexiform layer of the rat olfactory bulb. Neuroscience 72 449-466. 

Glutaminase-immunoreactive neuronal cell bodies were observed in the mitral cell layer, in the outer part of the external plexiform layer, and periglomerular regions of the olfactory bulb (Figs. 2a,b and 3b Kaneko and Mizuno, 1992b). The neuropil in the external plexiform layer and that in the glomeruli were intensely immunolabeled for glutaminase. 

Abbreviations Al = primary auditory area ac = anterior commissure Acc = accumbens nucleus AON = anterior olfactory nucleus BF = barrel field BLA = basolateral nucleus of the amygdala CAl = cornu ammonis 1 CA3 = cornu ammonis 3 cc = corpus callosum Cg = cingulate area CPu = caudate-putamen DCb = deep cerebellar nuclei DCo = dorsal cochlear nucleus DG = dentate gyrus DMV = dorsal motor nucleus of the vagus nerve ECu = external cuneate nucleus EP = external plexiform layer ER = entorhinal cortex f = fornix Fa = facial nucleus fa = facial nerve fr = fasciculus retroflexus G1 = glomerular layer GPe = (external segment of the)... 

Fig. J. Glutaminase-immunoreactive neurons in layer V of the neocortex (ti), mitral cell layer of the olfactory bulb (b), lateral geniculate nucleus (c) and pontine nuclei (d). The method for immunostaining is described in the legend of Fig. I. EP = external plexiform layer IG = internal granular layer M = mitral cell layer. Modified from Kaneko (1991).

Abbreviations GL, glomerular layer EPL, external plexiform layer MCL, mitral cell layer IPL, internal plexiform layer GCL, granule cell layer EZ, ependymal zone. 

Deep to the external plexiform layer is the mitral cell layer (Fig. 2). This is a thin layer that contains the somata of mitral cells (25- 35 pm diameter) arranged in almost a monolayer. These cells are the principal output cells of the bulb and, with some minor species differences (cf. Scott, 1986), have one apical dendrite that enters a single glomerulus, where it branches extensively and is synaptically contacted by olfactory axons (Shepherd, 1972a) (Figs. 4C,E, 5 and 12). 

HT,a receptor is found mainly in the external plexiform layer. On the other hand, the mRNA for the S-HTja receptor has been shown by in situ hybridization to be in the mitral cell and external plexiform layers (Pompeiano et al. 1994) and more precisely, in mitral and tufted cells (McLean et al. 1994). This leads one to speculate that the dendrites or cell bodies of olfactory bulb output cells receive serotonergic input via... 

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