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

Memory Integration as a Challenge to the Consolidation/Reconsolidation Hypothesis: Similarities, Differences and Perspectives

We recently proposed that retrograde amnesia does not result from a disruption of the consolidation/reconsolidation processes but rather to the integration of the internal state induced by the amnesic treatment within the initial memory. Accordingly, the performance disruption induced by an amnesic agent does not result from a disruption of the memory fixation process, but from a difference in the internal state present during the learning phase (or reactivation) and at the later retention test: a case of state-dependency. In the present article, we will review similarities and differences these two competing views may have on memory processing. We will also consider the consequences the integration concept may have on the way memory is built, maintained and retrieved, as well as future research perspectives that such a new view may generate.

Differential modifications of synaptic weights during odor rule learning: dynamics of interaction between the piriform cortex with lower and higher brain areas

Learning of a complex olfactory discrimination (OD) task results in acquisition of rule learning after prolonged training. Previously, we demonstrated enhanced synaptic connectivity between the piriform cortex (PC) and its ascending and descending inputs from the olfactory bulb (OB) and orbitofrontal cortex (OFC) following OD rule learning. Here, using recordings of evoked field postsynaptic potentials in behaving animals, we examined the dynamics by which these synaptic pathways are modified during rule acquisition. We show profound differences in synaptic connectivity modulation between the 2 input sources. During rule acquisition, the ascending synaptic connectivity from the OB to the anterior and posterior PC is simultaneously enhanced. Furthermore, post-training stimulation of the OB enhanced learning rate dramatically. In sharp contrast, the synaptic input in the descending pathway from the OFC was significantly reduced until training completion. Once rule learning was established, the strength of synaptic connectivity in the 2 pathways resumed its pretraining values. We suggest that acquisition of olfactory rule learning requires a transient enhancement of ascending inputs to the PC, synchronized with a parallel decrease in the descending inputs. This combined short-lived modulation enables the PC network to reorganize in a manner that enables it to first acquire and then maintain the rule.

Non-topographical contrast enhancement in the olfactory bulb

Background

Contrast enhancement within primary stimulus representations is a common feature of sensory systems that regulates the discrimination of similar stimuli. Whereas most sensory stimulus features can be mapped onto one or two dimensions of quality or location (e.g., frequency or retinotopy), the analogous similarities among odor stimuli are distributed high-dimensionally, necessarily yielding a chemotopically fragmented map upon the surface of the olfactory bulb. While olfactory contrast enhancement has been attributed to decremental lateral inhibitory processes among olfactory bulb projection neurons modeled after those in the retina, the two-dimensional topology of this mechanism is intrinsically incapable of mediating effective contrast enhancement on such fragmented maps. Consequently, current theories are unable to explain the existence of olfactory contrast enhancement.

Results

We describe a novel neural circuit mechanism, non-topographical contrast enhancement (NTCE), which enables contrast enhancement among high-dimensional odor representations exhibiting unpredictable patterns of similarity. The NTCE algorithm relies solely on local intraglomerular computations and broad feedback inhibition, and is consistent with known properties of the olfactory bulb input layer. Unlike mechanisms based upon lateral projections, NTCE does not require a built-in foreknowledge of the similarities in molecular receptive ranges expressed by different olfactory bulb glomeruli, and is independent of the physical location of glomeruli within the olfactory bulb.

Conclusion

Non-topographical contrast enhancement demonstrates how intrinsically high-dimensional sensory data can be represented and processed within a physically two-dimensional neural cortex while retaining the capacity to represent stimulus similarity. In a biophysically constrained computational model of the olfactory bulb, NTCE successfully mediates contrast enhancement among odorant representations in the natural, high-dimensional similarity space defined by the olfactory receptor complement and underlies the concentration-independence of odor quality representations.

Cue valence representation studied by Fos immunocytochemistry after acquisition of a discrimination learning task

The piriform cortex (PCx) and related structures such as hippocampus and frontal cortex could play an important role in olfactory memory. We investigated their involvement in learning the biological value of an odor cue, i.e. predicting reward or non-reward in a two-odor discrimination task. Rats were sacrificed after stimulation by either rewarded or non-rewarded odor and Fos immunocytochemistry was performed. The different experimental groups of rats did not show strongly differentiated Fos expression pattern in either the PCx or the hippocampus. A few differences were noted in frontal areas. In the ventro-lateral orbital cortex, rats, ramdomly rewarded during the conditionning had a higher Fos level in comparison with other groups. In infralimbic cortex, rats, which learned the reward value of the olfactory cue and were water-reinforced the day of sacrifice, showed a higher Fos expression. Data are discussed in view of the olfactory learning paradigm and of the accuracy of the control groups used in the present experimental design. The behavioural conditions leading to Fos expression are further discussed since Fos is a marker of learning-induced plasticity as well as a general activity marker which can be activated by a wide range of stimuli not directly linked to memory.

Noradrenergic Induction of Selective Plasticity in the Frequency Tuning of Auditory Cortex Neurons

Neuromodulators have long been viewed as permissive factors in experience-induced cortical plasticity, both during development and in adulthood. Experiments performed over the last two decades have reported the potency of acetylcholine to promote changes in functional properties of cortical cells in the auditory, visual, and somatosensory modality. In contrast, very few attempts were made with the monoaminergic systems. The present study evaluates how repeated presentation of brief pulses of noradrenaline (NA) concomitant with presentation of a particular tone frequency changes the frequency tuning curves of auditory cortex neurons determined at 20 dB above threshold. After 100 trials of NA-tone pairing, 28% of the cells (19/67) exhibited selective tuning modifications for the frequency paired with NA. All the selective effects were obtained when the paired frequency was within 1/4 of an octave from the initial best frequency. For these cells, selective decreases were prominent (15/19 cases), and these effects lasted ≥15 min after pairing. No selective effects were observed under various control conditions: tone alone ( n = 10 cells), NA alone ( n = 11 cells), pairing with ascorbic acid ( n = 6 cells), or with GABA ( n = 20 cells). Selective effects were observed when the NA-tone pairing was performed in the presence of propranolol (4/10 cells) but not when it was performed in the presence phentolamine (0/13 cells), suggesting that the effects were mediated by alpha receptors. These results indicate that brief increases in noradrenaline concentration can trigger selective modifications in the tuning curves of cortical neurons that, in most of the cases, go in opposite direction compared with those usually reported with acetylcholine.

Early polysynaptic potentiation recorded in the dentate gyrus during an associative learning task

In this report, we investigated the electrophysiological dynamics of the neuronal circuit including the dentate gyrus during an associative task. A group of rats was trained to discriminate between a patterned electrical stimulation of the lateral olfactory tract, used as an artificial cue associated with a water reward, and a natural odor associated with a light flash. Polysynaptic field potential responses, evoked by a single electrical stimulation of the same lateral olfactory tract electrode, were recorded in the molecular layer of the ipsilateral dentate gyrus prior to and just after each training session. An increase in this response was observed when a significant discrimination of the two cues began. A positive correlation was found between the change in the polysynaptic potentiation and behavioral performances. The onset latency of the potentiated polysynaptic response was 35-45 ms. When a group of naive animals was pseudoconditioned, no change in field potential was observed. These results are consistent with the hypothesized dynamic activation of the dentate gyrus early in the making of association, allowing gradual storage of associative information in a defined set of synapses. Moreover, the onset latency of the potentiated response suggests the existence of reactivating hippocampal loops during the processing of associative information.

Olfactory learning abilities in staggerer mutant mice

Staggerer mutant mice were compared to non-mutant mice in two olfactory learning tasks. It was found that, in spite of a delayed acquisition compared to non-mutants, staggerer mice were able to learn an olfactory habituation task. On the other hand, staggerer presented deficits in an associative olfactory task and, contrary to non-mutants, did not learn this task. Perturbations in olfactory bulbs of staggerer mice could explain their olfactory learning deficits.

Facilitation of olfactory recognition by lateral entorhinal cortex lesion in rats

An original olfactory recognition task was developed in order to examine the effect of lateral entorhinal cortex (LEC) lesion on olfactory mnesic processes. The task was based on the spontaneous exploratory behavior of rats toward odor sources. It consisted of a learning phase during which an odor was presented twice and in a recognition test, during which the same odor plus a new one was presented. The time rats spent sniffing the odor sources was measured. Olfactory recognition was identified by a short investigatory duration for the familiar odor as compared to a normal investigatory duration for the new odor during the test. The first three experiments aimed to validate the procedure. Experiment 1 was designed to show the decay of investigatory behavior caused by repeated exposure of the rats to one odor. Experiment 2 showed that normal rats display recognition when a short (5 or 40 min) pre-test delay was used, but not when a long pre-test delay (120 min) was used. Experiment 3 showed that FG7142, a well-known promnesic drug, enhanced the performance of the rats in this test as it allowed recognition at longer pre-test delays. The last experiment aimed at testing the effects of aspirative lesion of the LEC. Therefore, LEC-lesioned and sham-lesioned rats were submitted to variable pre-test delays. The experiment showed that an entorhinal lesion did not produce an impairment, but on the contrary facilitated olfactory recognition, as lesioned rats displayed recognition for delays at which sham-operated rats did not. These results show that LEC lesion apparently prolongs the duration of the olfactory mnesic trace. This effect might result from a modification of the functioning of structures innervated by the LEC. In this regard, it is noteworthy that LEC lesion produced a sprouting of septo-hippocampal fibers in the dentate gyrus of the hippocampus as assessed by acetylcholinesterase staining. Although the functional significance of this regrowth is not fully understood, the possible role of this sprouting should be considered.

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.

Synaptic mechanisms of olfactory recognition memory

The complexity and inaccessibility of the mammalian brain prevent the localization and description of memory traces and the definition of the processes that produce memories. The model reviewed here is the olfactory recognition memory formed to male pheromones by a female mouse at mating. The memory trace has been localized to the reciprocal dendrodendritic synapse between mitral cells and granule cells in the accessory olfactory bulb. An increase in noradrenaline after mating reduces inhibitory transmission of gamma-aminobutyric acid (GABA) from the granule cells to mitral cells and induces an olfactory memory of pheromones present at mating. Recent work has shown that the activation of mGluR2, a metabotropic glutamate receptor, localized at granule cell dendrites suppresses the GABA inhibition of the mitral cells and permits the formation of a specific olfactory memory that faithfully reflects the memory formed at mating. This simple olfactory memory may provide an excellent model system with which to investigate the molecular mechanisms of the synaptic plasticity involved in learning and memory.

Role of olfactory bulb norepinephrine in the identification and recognition of chemical cues

In this report the role of olfactory bulb (OB) norepinephrine (NE) in the identification and recognition of urinary chemical cues was examined. In Experiment 1, sexually naive adult male Sprague-Dawley rats were treated with either the noradrenergic neurotoxin, DSP-4, or the water vehicle, and tested for their ability to identify and recognize urinary chemical cues using a habituation-dishabituation paradigm. Treatment with DSP-4 produced an overall decrease in the amount of investigation directed to urine stimuli, with greatest reductions to urine from Zucker females. Overall, DSP-4 treatment did not alter habituation-dishabituation responses. Animals treated with DSP-4 showed a significant reduction in OB-NE, but not dopamine, concentrations. In Experiment 2, hypothalamic catecholamine concentrations and serum samples assayed for testosterone were determined from identically treated animals. Although the NE and dopamine content in MBH was significantly lower in the DSP-4 group, no significant differences in testosterone concentrations were obtained between DSP-4 and controls. These results demonstrate that DSP-4 produces significant reductions in OB-NE and in the amount of investigation directed to urinary chemical cues from females without altering serum testosterone levels. Treatment with DSP-4 treatment does not impair the male rats' ability to demonstrate a habituation-dishabituation response.