Martine Cattarelli

Serotonergic and non-serotonergic projections from the raphe nuclei to the piriform cortex in the rat: a cholera toxin B subunit (CTb) and 5-HT immunohistochemical study

Retrograde axonal transport of the cholera toxin B subunit (CTb) was combined with 5-HT immunohistochemistry to determine the origin of the serotonergic innervation of the piriform cortex (PC) in the rat. After iontophoretic CTb injections in the PC, a substantial number of retrogradely labeled cells were found in the middle and medio-ventral part of the dorsal raphe nucleus (RD). A few retrogradely labeled cells were also observed in the median raphe nucleus (MnR) and the B9 serotonergic cell groups. Following CTb and 5-HT immunohistochemistry on the same sections, double-labeled cells were observed in the RD, MnR and B9 groups. In the RD, 30% of CTb stained cells were immunoreactive to 5-HT. After colchicine or nialamide (a monoamine oxidase inhibitor) pretreatment the percentage of these double-labeled cells reached 70%. These results indicate that both 5-HT and non-5-HT neurons in the RD innervate the PC and that the percentage of double-labeled cells is influenced by drug pretreatment. To determine the terminal fields of the RD efferent fibers in the PC, injections of the anterograde tracer PHA-L were also performed. Analysis of the fiber distribution in the PC further revealed some medio-lateral and antero-posterior differences.

Reciprocal and topographic connections between the piriform and prefrontal cortices in the rat : A tracing study using the B subunit of the cholera toxin

In the present study, the reciprocal connections between the piriform cortex and the prefrontal areas are described on the basis of experiments using the anterograde and the retrograde transport of the cholera toxin B subunit (CTb). Following CTb injections placed in the anterior part of the piriform cortex, retrogradely labeled cells and anterogradely labeled fibers were mainly found in the ventrolateral and lateral orbital areas as well as in the anterior part of the agranular insular cortex. Following injections placed in the posterior part of the piriform cortex, the CTb labeling was primarily observed in the infralimbic area and the posterior part of the agranular insular cortex. Thus, we described a topographical organization of the direct reciprocal connections between the anterior and the posterior parts of the piriform cortex parts and some prefrontal areas. This could support a differential modulation of the olfactory processing along the rostrocaudal dimension of the piriform cortex.

Influence of some biologically meaningful odorants on the vigilance states of the rat

In freely moving rats, mitral cells multiunit activity and vigilance states (EEG and neck muscles EMG) were recorded during stimulations by some odorous stimuli known from previous experiments to elicit distinct emotional behaviors. An awaking influence may be pointed out; the greatest awaking influence is noted for the opposite most biologically meaningful stimuli: odor of fox (predator of the rat) and odor of grouped rats. Mitral cells responsiveness is not only modulated by the biological meaning of the olfactory stimulus but is also under the control of the arousal level of the animal; it decreases from wakefulness (W) to slow wave sleep (SWS).

Metabolic mapping of functional activity in the rat olfactory system after a bilateral transection of the lateral olfactory tract.

The effects of the bilateral transection of the LOT on the patterns of 2-deoxyglucose uptake within the olfactory bulb and the olfactory projections were studied in adult rats. Animals were exposed to a pure neutral odor, ethyl acetoacetate (EAA) or to a biological alarming odor, fox odor. In intact animals, the patterns of glomerular activity elicited by EAA and fox odor appeared complex. An average number of 30-40 foci of 2-DG uptake was noted in each bulb and the optical density of the foci spread out according to a continuous gradient. These patterns were largely overlapping on the lateral aspect and the third-to-posterior part of the medial aspect of the bulb. Nevertheless, they were somewhat different and the spatial distribution of the darkest foci seemed particularly relevant for the pattern recognition. In lesioned animals, the same spatial distribution of the foci as in intact rats, was observed in the lateral and the medial aspects of the bulb. Nevertheless, lesioned animals presented some quantitative changes in their patterns of glomerular labeling. Moreover, these patterns appeared different according to the biological meaning of the odor tested. The bilateral transection of the LOT brought about a very strong decrease of the optical density in the direct olfactory projections. No evident change of 2-DG uptake was noted in the different tertiary olfactory projections. These 2-DG results confirm the anatomical data relating to the LOT projections.

Intrinsic association fiber system of the piriform cortex: A quantitative study based on a cholera toxin B subunit tracing in the rat

By using retrograde and anterograde transport of the B subunit of cholera toxin (CTb), we examined quantitatively the association fiber systems, i.e., the collaterals of pyramidal cell axons, that reciprocally connect both the rostral and the caudal parts of the piriform cortex (PC). Well‐defined CTb injections were obtained in layers Ib or II‐III of the rostral and the caudal parts of the PC. Using precision counting, we determined the proportion of cellular profiles in layers II and III that gave rise to association fibers and thus demonstrated a predominance of rostrocaudal fibers over the caudorostral ones. Our data also support a precise laminar organization of the PC in which the rostrocaudal fibers originated mainly from layer II and the caudorostral fibers primarily from layer III. Cholera toxin injections into layer Ib produced a peak of labeled profiles 2 mm from the site, indicating that a large proportion of the association fibers from layer II travel for at least 2 mm and then synapse in layer Ib. At either end of the PC, the association projections are concentrated laterally. The functional significance of these anatomical features is discussed with respect to olfactory processing, propagation of the activity within the PC, and the possible role of intrinsic fibers in olfactory memory.

The role of the medial olfactory pathways in olfaction: Behavioral and electrophysiological data

The role of the olfactory inputs transmitted by the medial olfactory pathways --i.e. anterior limb of the anterior commissure and medial forebrain bundle-was studied in rats submitted either to a bilateral lesion of the anterior commissure (bAC group) or to a bilateral transection of the lateral olfactory tract followed by an anterior commissure bilateral lesion (bCL group). In the bCL group, the characteristic emotional reactions usually observed in sham-operated rats to biologically meaningful odorants (odor of a predator or of conspecifics) were no longer observed; in this group, the modulation of mitral cell electrical responses according to the biological meaning of the stimuli disappeared and no habituation was noted when the stimulations were repeated: few responses appeared at random; lastly, all the stimuli tested, the odor of conspecifics excepted, did not exert an awaking influence on the sleeping rats (slow wave sleep). On the contrary, emotional reactions were always observed in the bAC group, a slight increase of reactivity to the odorants was even noted. In this group of animals, the anterior commissure bilateral lesion induced a large increase of the bulbar responses to all the odorants; the habituation of the electrical responses disappeared, a large percentage of responses being always noted even for the tenth series of stimulation; these results point out the great inhibitory influence that the anterior commissure exerts on the olfactory bulb activity. In these bAC rats almost usual awaking influences were noted, however, the odor of conspecifics became as efficient as fox odor to awake the animals. Results are discussed in relation to the functional role of the medial and lateral olfactory pathways, the integrity of both the pathways being necessary for an accurate olfactory process.

Origin of the in vivo rat piriform cortex activity recorded with voltage-sensitive dyes: comparison of the optical signals and the field potentials☆

The comparison of optical recordings and evoked field potentials recorded on the rat piriform cortex pointed out that both signals were strongly correlated. As the field potentials, the two waves of the optical signals originated from the mono- (direct olfactory bulb afferents) and polysynaptic (intrinsic association fibers) excitatory postsynaptic potentials. Such optical recordings will be used for studying spatiotemporal distribution of the piriform cortex activity.

Transmission and integration of biologically meaningful olfactory information after bilateral transection of the lateral olfactory tract in the rat

The contribution of the olfactory inputs conveyed by the lateral olfactory tract (LOT) to the central olfactory areas in the onset of behavioral and electrophysiological responses i.e. multiunit activity of mitral cells was studied in rats which were submitted to a bilateral transection of LOT. The characteristic emotional reactions elicited in sham-operated rats by biologically meaningful odorants--odors of predator or of conspecific--were no longer observed after such lesion. At the olfactory bulb level, the differential habituation of mitral cell electrical responses according to the biological meaning of the stimuli disappeared in the rats with a bilateral section of LOT. However the awaking influence of the odorants when stimulations occurred during slow wave sleep remained unchanged: all the rats were more often awakened by the odor of predator than by the other stimuli. On the other hand, an inhibitory centrifugal influence of LOT on mitral cell electrical responses was noted in rats with a unilateral transection of LOT. The results are discussed in relation to the two olfactory pathways (lateral and medial). During wakefulness, LOT seems essential to the olfactory information processing; in slow wave sleep, the medial olfactory pathways appear to mediate olfactory discrimination and to elicit adapted arousal.

Piriform cortex late activity revealed functional spatial heterogeneity.

Optical signals were recorded in the in vivo rat piriform cortex (PC) in response to olfactory bulb electrical stimulation. Sometimes the early response was followed by a longer latency component with an occurrence probability of 0.25. In order to compare the early and late activity, the ratio between early (disynaptic) and late wave amplitudes was measured at each recording site on the whole PC. Its spatial distribution revealed that the relative importance of the late activity was larger in the most posterior part of the PC whereas the late wave was rarely observed in the anterior PC. Such a result gave new information on the functional heterogeneity of the PC.

Multi-site optical recording of the rat piriform cortex activity

Optical signals were recorded in the in vivo rat piriform cortex (PC) in response to olfactory bulb (OB) electrical stimulations delivered at 4 different sites. Afferent activity had a relatively wide (26.6% of the recorded area) but nonhomogeneous distribution on the PC surface. The different patterns of afferent activity observed in response to the 4 OB stimulations were intermixed with an overlap of only 38.5%. This activity was redistributed to the whole PC by intrinsic association fibres. The increase in the delay (from 4 ms to 12 ms) between afferent and redistributed activities along the antero-posterior axis indicated that the rostral to caudal association fibre system originating in the anterior PC was mainly responsible for the redistribution.