No abstract

The anterior part of the piriform cortex (the APC) has been the focus of cortical-level studies of olfactory coding and associative processes and has attracted considerable attention as a result of a unique capacity to initiate generalized tonic-clonic seizures. Based on analysis of cytoarchitecture, connections, and immunocytochemical markers, a new subdivision of the APC and an associated deep nucleus are distinguished in the rat. As a result of its ventrorostral location in the APC, the new subdivision is termed the APC(VR). The deep nucleus is termed the pre-endopiriform nucleus (pEn) based on location and certain parallels to the endopiriform nucleus. The APC(VR) has unique features of interest for normal function: immunostaining suggests that it receives input from tufted cells in the olfactory bulb in addition to mitral cells, and it provides a heavy, rather selective projection from the piriform cortex to the ventrolateral orbital cortex (VLO), a prefrontal area where chemosensory, visual, and spatial information converges. The APC(VR) also has di- and tri-synaptic projections to the VLO via the pEn and the submedial thalamic nucleus. The pEn is of particular interest from a pathological standpoint because it corresponds in location to the physiologically defined "deep piriform cortex" ("area tempestas") from which convulsants initiate temporal lobe seizures, and blockade reduces ischemic damage to the hippocampus. Immunostaining revealed novel features of the pEn and APC(VR) that could alter excitability, including a near-absence of gamma-aminobutyric acid (GABA)ergic "cartridge" endings on axon initial segments, few cholecystokinin (CCK)-positive basket cells, and very low gamma-aminobutyric acid transporter-1 (GAT1)-like immunoreactivity. Normal functions of the APC(VR)-pEn may require a shaping of neuronal activity by inhibitory processes in a fashion that renders this region susceptible to pathological behavior.

Basket cells, defined by axons that preferentially contact cell bodies, were studied in rat piriform (olfactory) cortex with antisera to gamma-aminobutyric acid (GABA)ergic markers (GABA, glutamate decarboxylase) and to peptides and calcium binding proteins that are expressed by basket cells. Detailed visualization of dendritic and axonal arbors was obtained by silver-gold enhancement of staining for vasoactive intestinal peptide (VIP), cholecystokinin (CCK), parvalbumin, and calbindin. Neuronal features were placed into five categories: soma-dendritic and axonal morphologies, laminar distributions of dendritic and axonal processes, and molecular phenotype. Although comparatively few forms were distinguished within each category, a highly varied co-expression of features from different categories produced a "combinatorial explosion" in the characteristics of individual neurons. Findings of particular functional interest include: dendritic distributions suggesting that somatic inhibition is mediated by feedforward as well as feedback pathways, axonal variations suggesting a differential shaping of the temporal aspects of somatic inhibition from different basket cells, evidence that different principal cell populations receive input from different combinations of basket cells, and a close association between axonal morphology and molecular phenotype. A finding of practical importance is that light microscopic measurements of boutons were diagnostic for the molecular phenotype and certain morphological attributes of basket cells. It is argued that the diversity in basket cell form in the piriform cortex, as in other areas of the cerebral cortex, reflects requirements for large numbers of specifically tailored inhibitory processes for optimal operation that cannot be met by a small number of rigidly defined neuronal populations.

Effects of afferent stimulation on local synthesis of protein in CA1 pyramidal cell dendrites were studied using light microscope autoradiography. Tissue was fixed with paraformaldehyde immediately after 3 min exposure to 3H-leucine in order to trap 3H associated with macromolecules. The rate of 3H-leucine incorporation into dendrites of resting hippocampal slices was 10% the rate of incorporation into cell somata. Ninety percent of the incorporation into the somata was inhibited by cycloheximide (300 microM); none of the incorporation into dendrites was blocked by cycloheximide. Thus, there is no measurable extramitochondrial synthesis of protein in the dendrites of the resting slice. Slices were exposed to 50 microM carbachol and the Schaffer collateral afferents to the CA1 pyramidal cells were stimulated intermittently at 10 Hz over a 20 min period. In this case, 3H incorporation into dendrites was increased almost threefold over resting levels, with no effect on label over the cell somata. There was no associated increase in uptake of free 3H-leucine, and the increase in label was completely blocked by cycloheximide. Thus, associating carbachol and afferent stimulation appears to activate de novo protein synthesis in the dendrites. Neither the carbachol alone nor the Schaffer collateral stimulation alone increased synthesis. The activation of dendrite synthesis was completely blocked by 5 microM atropine, and also by 50 microM D-aminophosphonovalerate. It did not occur when carbachol was paired with steady stimulation of the Schaffer collaterals at 1 Hz for 20 min, rather than with the patterned high-frequency stimulation. Thus, associating a cholinergic agonist with a level of neural activity that occurs in CA3 and CA1 pyramidal cells during exploratory behavior (Muller et al., 1987) initiates local protein synthesis in target dendrites. This effect is dependent on muscarinic cholinergic receptors and NMDA-type glutamate receptors. The possible relationship of this phenomenon to mechanisms of learning is discussed.

Axonal markers injected into layers 5 and 6 of cortical areas 17, 18, or 19 labeled axons going to the lateral geniculate nucleus (LGN), the lateral part of the lateralis posterior nucleus (LPl), and pulvinar (P). Area 19 sends fine axons (type 1, Guillery [1966] J Comp Neurol 128:21-50) to LGN, LPl, and P, and thicker, type 2 axons to LPl and P. Areas 17 and 18 send type 1 axons to LGN, and a few type 1, but mainly type 2 axons to LPl and P. Type 1 and 2 axons from a single small cortical locus distribute to distinct, generally nonoverlapping parts of LP and P; type 1 axons have a broader distribution than type 2 axons. Type 2 axons, putative drivers of thalamic relay cells (Sherman and Guillery [1998] Proc Natl Acad Sci USA 95:7121-7126; Sherman and Guillery [2001] Exploring the thalamus. San Diego: Academic Press), supply small terminal arbors (100- to 200-microm diameter) in LPl and P, and then continue into the midbrain. Each thalamic type 2 arbor contains two terminal types. One, at the center of the arbor, is complex and multilobulated; the other, with a more peripheral distribution, is simpler and may contribute to adjacent arbors. Type 2 arbors from a single injection are scattered around and along "isocortical columns" in LPl, (i.e., columns that represent cells having connections to a common cortical locus). Evidence is presented that the connections and consequently the functional properties of cells in LP change along these isocortical columns. Type 2 driver afferents from a single cortical locus can, thus, be seen as representing functionally distinct, parallel pathways from cortex to thalamus.