Current generators and properties of late components evoked in rat olfactory cortex.

Following main olfactory bulb (MOB) stimulation at frequencies of 0.1-0.3 Hz, in addition to early field potentials, a frequency-sensitive, surface negative late N2 wave (latency range: 63-96 msec) followed occasionally by a late N3 transient, was evoked in the piriform cortex and endopiriform nucleus of the rat. The N2 wave inverted polarity at the Ib-II cortical layer interface (P2 wave) and was associated with late unit discharges 200 to 1200 microns deep to the turnover point. Response probability, peak latency, recovery curve and frequency-sensitivity of the P2 wave were not significantly different in animals under urethane or pentobarbital. Current-source-density (CSD) analysis revealed that the N2 wave generators were localized to the Ib-II layer interface. Since inhibitory activity does not contribute substantially to the second derivative curve, CSD analysis strengthens the assumption that late components (LCs) are excitatory events (compound EPSPs) presumably generated on the proximal apical dendritic segments of pyramidal cells by association axons. The early "b" wave in a test response was facilitated, rather than occluded, when a LC was present in the conditioning response, or when the priming volley was delivered to the mediodorsal thalamic nucleus. Clustering of unit and field activity in two distinct periods of the evoked response separated by a prolonged interval of cell silence suggests that cortical coding of olfactory cues might be more efficiently achieved by temporal modulation of the neuronal response rather than by spatial distribution of firing patterns.

A computer program for automatic plotting of isopotential contours in CNS.

A computer program was developed in a Basic (Applesoft) version for generating up to five isopotential curves from field potentials recorded in nervous structures. Voltages are fed according to a cartesian coordinate system, and an area is delineated each four points in which a certain number of intermediate voltage points are calculated, according to the required resolution. The calculated values are compared to those prefixed for each curve and, if similar, their coordinates are stored in corresponding bidimensional matrixes. A special subroutine was designed for constructing an isometric tridimensional perspective of the isopotential curve ensemble. The reliability of this program was tested in the localization of sensory representation areas on the neocortex of the South American armadillo (Chaetophractus vellerosus) studied by evoked potential mapping following visual, auditory and somatosensory stimuli. The isopotential curves traced permitted a quantitative evaluation of the cortical activated areas, and from their topographical distribution, relative unresponsive zones could be inferred where only inconspicuous responses were obtained. It is concluded that the present program provides a reliable and fast method for studying the evoked potential's spatial distribution over the entire neocortex. In addition, it can be extended to the study of curves or contours which connect equivalent values pertaining to biophysical magnitudes other than voltage data.

Reciprocal functional connections of the olfactory bulbs and other olfactory related areas with the prefrontal cortex.

Reciprocal putative connections of the prefrontal cortex (PFC) (agranular insular, ventral and lateral orbital region) with the ipsi and contralateral main olfactory bulb (IOB; COB), the mediodorsal thalamic nucleus (MD), the basolateral amygdaloid nucleus (BLA) and the piriform cortex (PC) were investigated with electrophysiological techniques. Evoked field responses and orthodromic unit driving, generated in PFC following electrical stimulation of the above mentioned structures, were abolished following topical application of KCl, except for COB evoked mass potentials. Thus, locally generated activity was elicited in agranular insular cortex following IOB activation, the same region where recently, the taste cortex in the rat was localized. Since gustatory-visceral afferent information reaches insular cortex via 2-3 synaptic relays, autonomic, olfactory and gustatory inputs may interact at this level, and, as suggested previously for the mouse, play a key integrative role in flavor perception. Antidromically invaded neurons, 47% of which were identified by the collision-extinction technique, were also found in PFC areas which overlapped to a considerable extent with those from which orthodromic unit responses were obtained. In particular, closely spaced neurons in ventrolateral orbital (VLO) and lateral orbital (LO) regions were antidromically invaded following IOB and PC shocks; some neurons antidromically discharged by IOB were also transsynaptically activated following PC stimulation. These findings are in agreement with recent neuroanatomical studies which demonstrate axonal projections from PFC neurons to the IOB and COB in the rat and South American armadillo. In addition, stimulation of PFC regions dorsal to the rhinal fissure mostly inhibited spontaneous unit discharges recorded at the mitral cell layer of the IOB, suggesting that this effect may be partially mediated by excitatory inputs of prefrontal axons onto granule cells. The conduction properties, antidromic thresholds and activity-dependent variations in conduction velocity (CV) of bulbopetal neurons in prefrontal cortex were found to be similar to those exhibited by cells projecting to the IOB from olfactory peduncle regions, but not to those present in bulbopetal neurons of the horizontal limb of diagonal band, indicating that the OB may be subjected to centrifugal control by at least two cell groups differing in both histochemical and electrophysiological properties.