Entorhinal cortex stimulation modulates amygdala and piriform cortex responses to olfactory bulb inputs in the rat.

The rodent olfactory bulb sends direct projections to the piriform cortex and to two structures intimately implicated in memory processes, the entorhinal cortex and the amygdala. The piriform cortex has monosynaptic projections with the amygdala and the piriform cortex and is therefore in a position to modulate olfactory input either directly in the piriform cortex, or via the amygdala. In order to investigate this hypothesis, field potential signals induced in anesthetized rats by electrical stimulation of the olfactory bulb or the entorhinal cortex were recorded simultaneously in the piriform cortex (anterior part and posterior part) and the amygdala (basolateral nucleus and cortical nucleus). Single-site paired-pulse stimulation was used to assess the time courses of short-term inhibition and facilitation in each recording site in response to electrical stimulation of the olfactory bulb and entorhinal cortex. Paired-pulse stimulation of the olfactory bulb induced homosynaptic inhibition for short interpulse interpulse intervals (20-30 ms) in all the recording sites, with a significantly lower degree of inhibition in the anterior piriform cortex than in the other structures. At longer intervals (40-80 ms), paired-pulse facilitation was observed in all the structures. Paired-pulse stimulation of the entorhinal cortex mainly resulted in inhibition for the shortest interval duration (20 ms) in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. Double-site paired-pulse stimulation was then applied to determine if stimulation of the entorhinal cortex can modulate responses to olfactory bulb stimulation. For short interpulse intervals (20 ms) heterosynaptic inhibition was observed in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. The level of inhibition was greater in the basolateral nucleus than in the other structures. Taken together these data suggest that the entorhinal cortex exerts a main inhibitory effect on the olfactory input via the amygdala basolateral nucleus and to a lesser extent the piriform cortex. The potential role of these effects on the processing of olfactory information is discussed.

Olfactory learning induces differential long-lasting changes in rat central olfactory pathways.

In the present work, we investigated lasting changes induced by olfactory learning at different levels of the olfactory pathways. For this, evoked field potentials induced by electrical stimulation of the olfactory bulb were recorded simultaneously in the anterior piriform cortex, the posterior piriform cortex, the lateral entorhinal cortex and the dentate gyrus. The amplitude of the evoked field potential's main component was measured in each site before, immediately after, and 20 days after completion of associative learning. Evoked field potential recordings were carried out under two experimental conditions in the same animals: awake and anesthetized. In the learning task, rats were trained to associate electrical stimulation of one olfactory bulb electrode with the delivery of sucrose (positive reward), and stimulation of a second olfactory bulb electrode with the delivery of quinine (negative reward). In this way, stimulation of the same olfactory bulb electrodes used for inducing field potentials served as a discriminative cue in the learning paradigm. The data showed that positively reinforced learning resulted in a lasting increase in evoked field potential amplitude restricted to posterior piriform cortex and lateral entorhinal cortex. In contrast, negatively reinforced learning was mainly accompanied by a decrease in evoked field potential amplitude in the dentate gyrus. Moreover, the expression of these learning-related changes occurred to be modulated by the animals arousal state. Indeed, the comparison between anesthetized versus awake animals showed that although globally similar, the changes were expressed earlier with respect to learning, under anesthesia than in the awake state. From these data we suggest that associative olfactory learning involves different neural circuits depending on the acquired value of the stimulus. Furthermore, they show the existence of a functional dissociation between anterior and posterior piriform cortex in mnesic processes, and stress the importance of the animal's arousal state on the expression of learning-induced plasticity.

Exposure to behaviourally relevant odour reveals differential characteristics in rat central olfactory pathways as studied through oscillatory activities.

This study investigated how changes in nutritional motivation modulate odour-related oscillatory activities at several levels of the olfactory pathway in non-trained rats. Local field potential recordings were obtained in freely moving animals in the olfactory bulb (OB), anterior and posterior parts of the piriform cortex (APC and PPC respectively) and lateral entorhinal cortex (EC). Dynamic signal analysis detected changes in power during odour presentation for several frequency bands The results showed that in most cases odour presentation was associated with changes in a wide 15-90 Hz frequency band of activity in each olfactory structure. However, nutritional state modulated initial responses to food odour (FO) in the OB and EC selectively in the 15-30 Hz frequency band. Changes in nutritional state also modulated responses to repeated FO stimuli. Habituation was expressed differentially across structures with a clear dissociation between the two parts of the piriform cortex. Finally, systemic injections of scopolamine (0.125 mg/kg) selectively blocked expression of the nutritional modulation in the OB found in the beta band. These results suggest that internal state can differentially modulate odour processing among different olfactory areas and point to a cholinergic-sensitive beta band oscillation during presentation of a behaviourally meaningful odorant.

Spatiotemporal distribution of a late synchronized activity in olfactory pathways following stimulation of the olfactory bulb in rats.

The evoked potential recorded in the rat piriform cortex in response to electrical stimulation of the olfactory bulb is composed of an early component occasionally followed by a late component (60-70 ms). We previously showed that the late component occurrence was enhanced following an olfactory learning. In the present study carried out in naive rats, we investigated the precise conditions of induction of this late component, and its spatiotemporal distribution along the olfactory pathways. In the anaesthetized rat, a stimulating electrode was implanted in the olfactory bulb. Four recording electrodes were positioned, respectively, in the olfactory bulb, the anterior and posterior parts of the piriform cortex, and the entorhinal cortex. Simultaneous recording of signals evoked in the four sampled structures in response to stimulation of the olfactory bulb revealed that the late component was detected in anterior and posterior piriform cortex as well as in entorhinal cortex, but not in the olfactory bulb. The late component occurred reliably for a narrow range of low intensities of stimulation delivered at frequencies not exceeding 1 Hz. Comparison of late component amplitude and latency across the different recorded sites showed that this component appeared first and with the greatest amplitude in the posterior piriform cortex. In addition to showing a functional dissociation between anterior and posterior parts of the piriform cortex, these data suggest that the posterior piriform cortex could be the locus of generation of this late high amplitude synchronized activity, which would then propagate to the neighbouring regions.

Learning-induced changes in rat piriform cortex activity mapped using multisite recording with voltage sensitive dye.

The piriform cortex (PCx) has a potential role in storage and recall of olfactory information. This study is a first extensive investigation of the spatiotemporal distribution of activity in the PCx induced by learned sensory inputs following conditioning. In a conditioned group, rats chronically implanted with four electrodes in the olfactory bulb were trained to associate the electrical stimulation of a given bulbar electrode with a positive reinforcement, while stimulation of a different electrode predicted a negative reinforcement. In a familiarized group, rats received the same protocol of daily electrical stimulation with no associated reinforcement. At the end of the conditioning or familiarization episode, activity evoked in the PCx was optically mapped using a 144 photodiode array. In the anaesthetized rats, PCx maps were recorded in response to stimulation of each of the four bulbar electrodes using either high (0.5-1 mA) or low (0.1 mA) test current intensities. Low intensity stimulation revealed that conditioning selectively enhanced the probability of occurrence of a signal composed of a single late (56-73 ms) component which occurred almost simultaneously on a large PCx area. In the conditioned group, high intensity stimulation through either of the four electrodes revealed a potentiation of the early (17-30 ms) disynaptic component of the PCx response in the most posterior part of the PCx as well as a homogeneous increase of the late (39-52 ms) component spread over the PCx areas. These data suggest that learning induces synaptic changes at different nodes of the PCx circuitry.

A study of the effects of noradrenaline in the rat olfactory bulb using evoked field potential response.

In the rat, the main olfactory bulb receives a strong noradrenergic (NA) input from the locus coeruleus which is critical for different types of olfactory learning. However, the resulting effect of NA modulation on on the olfactory bulb electrical activity and its pharmacology are not well understood. In this study, we investigated the action of NA on the bulbar neuronal population using evoked field potentials (EFP) elicited antidromically in the olfactory bulb of anesthetized rats, by stimulation of the lateral olfactory tract (LOT). EFPs in response to single and paired-pulse stimulation of the LOT were collected before, during and until 2 h after a 10 min perfusion of pharmacological agents through a push-pull cannula. Four concentrations of NA were tested ranging from 10(-5) M to 10(-2) M. NA induced a reversible dose-dependent effect. The major effect was observed at 10(-3) M. It consisted of an increase in Component 2 amplitude (depolarization of granules cell dendrites) and a decrease in Component 3 amplitude (depolarization of granule cell bodies). In parallel, paired-pulse inhibition of mitral cells by granule cells was increased. The alpha 1 agonist phenylephrine (10(-3) M) mimicked most of the effects of NA whereas the alpha 1 antagonist prazosin (10(-3) M) blocked its main action. Isoproterenol (beta agonist, 10(-3) M) and clonidine (alpha 2 agonist, 10(-3) M) could not reproduce the effects of NA. Thus mainly through the activation of alpha 1 receptors, NA enhances synaptic activation of granule cells and increases feed-back inhibition of mitral cells. Consequences of such effects in the context of learning and memory are discussed.

Involvement of the olfactory bulb in consolidation processes associated with long-term memory in rats.

In a daily training paradigm, rats were trained to discriminate between spatially distinct electrical stimulations delivered to one olfactory bulb. Xylocaine injections were used to disrupt the olfactory bulb functioning in the region close to the electrode tips for 1 hr after training session. The treatment started either just after the session or 2 hr later. When compared with the performance of saline-injected rats, the performance of Xylocaine-injected rats was unimpaired except when the treatment started just after the daily session. In that case, acquisition of the task was slightly altered, and retention over a 5-day period was dramatically impaired. We therefore concluded that, within about 1 hr following training, the olfactory bulb is engaged in consolidation processes critical for long-term retention of learned olfactory cues.

Evidence for the Involvement of Rat Olfactory Bulb in Processes Supporting Long-Term Olfactory Memory.

Current advances in the neurobiology of learning and memory suggest the existence of experience-induced plasticity in sensorial pathways conveying relevant information to higher integrative brain structures. For instance, olfactory learning is known to induce long-lasting modifications of neural activity at the level of the first relay structure of the olfactory system, the olfactory bulb. The observed forms of plasticity depend on the action exerted during learning by ascending neuromodulatory systems, such as the noradrenergic (NA) system originating from the locus ceruleus. This study was aimed at investigating the importance of olfactory bulb plasticity in learning and retention of an olfactory task. In a daily training schedule animals had to learn to use multi-site electrical stimulation patterns of the olfactory bulb as discriminative cues for choosing between a palatable and a nonpalatable solution. We first examined the effects of a continuous intrabulbar infusion of propranolol (a beta-NA receptor antagonist) carried out during the learning period. We found that this treatment neither impaired the retention of a previously learned task nor the learning of a new task. However, the animals presented a severe deficit in long-term retention (>5 days) of the task learned under perfusion. Unexpectedly, this effect cannot be ascribed to a selective blockade of beta-NA receptors since infusion of the drug vehicle (saline-ascorbate) produced exactly the same deficit while a saline solution remained without effect. A final experiment showed that the selective deficit in long-term retention was not observed when the infusion of the saline-ascorbate solution started on the day following completion of learning. Taken together, these results suggest that ascorbate-sensitive neural processes occurring within the olfactory bulb during learning are of functional importance for long-term storage of olfactory information.

An investigation of some temporal aspects of olfactory coding with the model of multi-site electrical stimulation of the olfactory bulb in the rat.

Electrical stimulation of the olfactory bulb was used to investigate some temporal aspects of olfactory coding, with reference to respiration. Food-deprived rats implanted with permanent electrodes were trained to use bulbar multi-site stimulation patterns as discriminative stimuli for predicting the nature of an incoming reinforcement. Electrical pulse trains (100 Hz) were periodically delivered in phase with precisely defined moments of the respiratory cycle (during inspiration or expiration). Temporal aspects of olfactory coding were first considered through the measurement of the minimum duration of a stimulus necessary to identify this stimulus. The results showed that a bulbar stimulation lasting for 30 ms (3 pulses), and delivered during inspiration, was clearly identified by the rats. Stimulus identification induced a discriminative respiratory response which could manifest itself as early as the first cycle concomitant with the beginning of stimulation. It was then shown that a bulbar electrical stimulation pattern was identified with the same latency whether it occurred during expiration or during inspiration. Moreover, the perceptive events induced in those two conditions of stimulation were not different enough to be discriminated by the animals. The findings are discussed within the framework of olfactory information processing.

Perceptive properties of the multi-site electrical microstimulation of the olfactory bulb in the rat.

Electrical microstimulation of the olfactory bulb in different locations has been shown to provide water-deprived rats with discriminative cues for selecting a palatable solution without tasting it in a two-choice test. Some perceptive properties of bulbar electrical stimulation were investigated. It was shown that the perceptive effect evoked by stimulating a given site could be recognized when this site was stimulated together with several others. The animals' perception of multi-site stimulation patterns seems therefore to be analytical rather than synthetic. Discrimination of stimulation patterns did not require presentation of concurrent patterns inside a short time interval. Identification of a multi-site pattern was possible when this pattern was presented alone in a test session. Individual characteristics of bulbar microstimulation appear to be perceived absolutely rather than differentially. A good retention of the discrimination learning of specific stimulation patterns was observed. Animals could identify stimulation patterns after complete interruption of the training for 17 days. The results are discussed with reference to the properties of the natural stimulation of the olfactory system.

On the ability of rats to discriminate between microstimulations of the olfactory bulb in different locations.

An investigation was made into the ability of rats to discriminate between electrical stimulations applied to the mitral cell layer of the olfactory bulb in different locations. Water-deprived rats implanted with permanent electrodes were trained to use single- or multi-site microstimulations as discriminative stimuli for selecting a palatable solution without tasting it in a two-choice test. Spontaneous reactions of the animals to stimulation with sinusoidal currents higher than 3 microA per electrode resembled sensory arousal. All rats were found to discriminate between the effects of concurrent microstimulations applied to bulbar sites separated by 500 micron. Changing the current intensity in the range 4-20 microA had no detectable effect on the discrimination. Discrimination was still possible, with a few exceptions, when electrodes were separated by 250 micron and even when they were closely adjacent. Spatial resolution of discrimination seemed not to vary in different regions along the rostrocaudal axis of the bulb. The discrimination of patterns of simultaneous stimulation at several sites was also investigated. Different multi-site patterns were easily distinguished, even when their respective components were closely adjacent or when some components occupied the same area. The findings are discussed with reference to the concept of spatial coding of odours in the olfactory bulb.

[Multipolar electrical stimulation of the olfactory bulb as a model for studying spatial basis of olfactory coding]. / La stimulation électrique multipolaire du bulbe olfactif, modèle d'étude du codage spatial dans le système olfactif.

Low-intensity electrical stimulation of the olfactory bulb through a set of spatially distributed electrodes was used as a model in order to study the spatial basis of the olfactory coding, in rats. The results indicate that this multipolar stimulation can serve as a conditional stimulus in a learned aversion paradigm; in addition, water-deprived animals displayed high ability to discriminate and memorize various combinations of electrode positions paired with differently palatable solutions.