Cell piriform cortex

Intracerebral injections of ibotenic acid produce cell loss in several cerebral areas, including the striatum, the hippocampus, substantia nigra, and piriform cortex (Schwarcz et al. 1979). This degeneration is limited to the site of injection and does not affect axons, passage, or synaptic terminals originating in other areas. 

The long length fibres link the ventral tegmental and substantia nigra dopamine-containing cells with the neostriatum (mainly the caudate and the putamen), the limbic cortex (the medial prefrontal, cingulate and entorhinal areas) and with limbic structures such as the septum, nucleus accumbens, amygdaloid complex and piriform cortex. These projections are... 

Major connections of the main olfactory system. Axons of MOB mitral/tufted cells (circles in the EPL and MCL, respectively) project as the LOT to synapse in a number of structures collectively referred to as primary olfactory cortex (POC). Centrifugal inputs to MOB include feedback projections from POC as well as inputs from subcortical forebrain and brainstem neuromodulatory cell groups. Abbreviations AON, anterior olfactory nucleus DP, dorsal peduncular cortex Ent, entorhinal cortex IG-AHC, indusium griseum-anterior hippocampal continuation LC, locus coeruleus NDB, nucleus of the diagonal band PeCo, periamygdaloid cortex PC, piriform cortex RN, raphe nuclei (dorsal and median raphe) TT, taenia tecta Tu, olfactory tubercle... 

Bouret S, Sara SJ. 2002. Locus coeruleus activation modulates firing rate and temporal organization of odour-induced single-cell responses in rat piriform cortex. Eur J Neurosci 16 2371-2382. 

Ekstrand JJ, Domroese ME, Feig SL, Illig KR, Haberly LB. 2001a. Immunocytochemical analysis of basket cells in rat piriform cortex. J Comp Neurol 434 308-328. 

Gellman RL, Aghajanian GK. 1993. Pyramidal cells in piriform cortex receive a convergence of inputs from monoamine-activated GABAergic intemeurons. Brain Res 600 63-73. 

Haberly LB, Presto S. 1986. Ultrastructural analysis of synaptic relationships of intracellularly stained pyramidal cell axons in piriform cortex. J Comp Neurol 248 464-474. 

Johnson DM, Illig KR, Behan M, Haberly LB. 2000. New features of connectivity in piriform cortex visualized by intracellular injection of pyramidal cells suggest that primary olfactory cortex functions like association cortex in other sensory systems. J Neurosci 20 6974-6982. 

Kapur A, Pearce RA, Lytton WW, Haberly LB. 1997. GABAA-mediated IPSCs in piriform cortex have fast and slow components with different properties and locations on pyramidal cells. J Neurophysiol 78 2531-2545. 

Sheldon PW, Aghajanian GK. 1990. Serotonin (5-HT) induces IPSPs in pyramidal layer cells of rat piriform cortex Evidence for the involvement of a 5-HT2-activated interneuron. Brain Res 506 62-69. 

Sheldon PW, Aghajanian GK. 1991. Excitatory responses to serotonin (5-HT) in neurons of the rat piriform cortex Evidence for mediation by 5-HTic receptors in pyramidal cells and 5-HT2 receptors in intemeurons. Synapse 9 208-218. 

Zimmer LA, Ennis M, Shipley MT. 1999. Diagonal band stimulation increases piriform cortex pyramidal cell excitability in vivo. Neuroreport 10 2101-2105. 

Fig. 4. (A-J) Distribution of kainate receptor subunit mRNAs in the adult rat brain (X-ray film autoradiographs, coronal sections). Arrowheads in E and F mark neocortical layer III cells expressing the GluR7 gene. AV, anteroventral thalamic nucleus BST, bed nucleus stria terminalis CC, corpus callosum white matter tract Cg. cingulate cortex Cpu, caudate putamen DG, denate granule cells DM, dorsomedial hypothalamic nucleus GP, globus pallidus MPA, medial preoptic area Pir, piriform cortex Rt, reticular thalamic nucleus SCh, suprachiasmatic nucleus. Scale bar, 3.2 mm (Wisden and Seeburg, 1993a).

Blakely et al. 1987 Ffrench-Mullen et al. 1985). NAG has been observed in mitral cells using immunocytochemistry (Blakely et al. 1987). However, a recent neurophysiological study cast doubt on a transmitter role for NAG (Whittemore and Koerner, 1989) in mitral cells (see below, 2.5.4. Transmitter(s) mediating MOB to PC monosynaptic excitation). A few, unusually small, mitral cells appear to contain aspartate and project to the piriform cortex (Fuller and Price, 1988). Many mitral cells, as well as tufted cells in the EPL, have been reported to express glutamate immunoreactivity (Liu et al. 1989). 

Recently, the neuropeptide, corticotropin releasing factor (CRF), has been demonstrated in mitral and some tufted cells using both immunocytochemistry (Fig. 1 IB) and in situ hybridization in the rat (Imaki et al. 1989). CRF fibers were also observed in the molecular layer of the piriform cortex. This finding is consistent with CRF being a releasable neural peptide in mitral cells since mitral cells synaptically terminate in the molecular layer of the piriform cortex. A similar localization of CRF has been reported in the squirrel monkey suggesting that this peptide may be a conserved transmitter/ modulator in the mitral/tufted cells of many mammals (Bassett et al. 1992). Finally, calretinin, a calcium binding protein, has been shown by immunohistochemistry to be localized in mitral cells (Jacobowitz and Winsky, 1991). Transmitter candidates for mitral and tufted cells are discussed further in 2.5.3., Projections to olfactory cortex. 

The cytoarchitecture of NLOT (Fig. 17C) has been studied extensively by McDonald (1983). It is considered an anterior part of the amygdala. NLOT can be subdivided into 3 layers on the basis of Nissl preparations a superficial plexiform layer I which contains a few small and medium-sized cells, a layer II which contains many tightly packed cells, and layer III located dorsal to layer II and containing fairly large, loosely packed cells. Most cells of NLOT are medium-sized pyramidal shaped with extensive spines on secondary and distal dendrites. According to McDonald (1983), layers I and II appear similar in connections to the piriform cortex while layer III seems to be a closely related subcortical area. Many neurons of layers II and fewer neurons of layer III project to the olfactory bulb (de Olmos et al. 1978 Shipley and Adamek, 1984). In addition to olfactory bulb projections, many axons of NLOT neurons make up the stria terminalis and cross to the contralateral piriform cortex, olfactory tubercle, lateral nucleus of the amygdala, and bed nucleus of the stria terminalis (de Olmos, 1972). Afferent connections to NLOT arise mainly from olfactory related areas and the basolateral nucleus of the amygdala. 

PC also contains numerous interneurons. The distribution of GABAergic interneurons has been described for the opossum and appears to be quite similar in the rat (Haberly, personal communication). These cells are found in all layers of piriform cortex (Fig. 25), including layer I, where they may function as a feedforward inhibitory system. There have been numerous reports of neuropeptide containing neurons in olfactory cortex many of these have a morphology consistent with an interneuron, but little is known about their connections or functions. 

The main olfactory bulb sends a projection to the entire extent of piriform, peri-amygdaloid and lateral entorhinal cortex (see above. Outputs of MOB). This projection terminates in the superficial half of layer I, layer la. Both mitral and tufted cells project to the rostral parts of AON and piriform cortex while the projection to more caudal parts of olfactory cortex becomes progressively dominated by mitral cells (Schoenfeld and Macrides, 1984). 

Piriform cortex, lateral entorhinal cortex and the transitional cortical areas project heavily back to the olfactory bulb (Figs. 13,14, 18,19). The projections are heavier from the rostral than the caudal parts of primary olfactory cortex in rat and mouse (Shipley and Adamek, 1984). A few cells in the posterolateral and medial cortical amygdaloid areas may project to the MOB (Shipley and Adamek, 1984). These feedback projections to the olfactory bulb arise mainly from pyramidal neurons in layers II and III in primary olfactory cortex. 

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