Role of anterior piriform cortex in the acquisition of conditioned flavour preference

Olfactory Evaluation in Alzheimer’s Disease Model Mice

Olfactory dysfunction is considered a pre-cognitive biomarker of Alzheimer’s disease (AD). Because the olfactory system is highly conserved across species, mouse models corresponding to various AD etiologies have been bred and used in numerous studies on olfactory disorders. The olfactory behavior test is a method required for early olfactory dysfunction detection in AD model mice. Here, we review the olfactory evaluation of AD model mice, focusing on traditional olfactory detection methods, olfactory behavior involving the olfactory cortex, and the results of olfactory behavior in AD model mice, aiming to provide some inspiration for further development of olfactory detection methods in AD model mice.

Role of Projections between Piriform Cortex and Orbitofrontal Cortex in Relapse to Fentanyl Seeking after Palatable Food Choice-Induced Voluntary Abstinence

We recently developed a rat model of relapse to drug seeking after food choice-induced voluntary abstinence. Here, we used this model to study the role of the orbitofrontal cortex (OFC) and its afferent projections in relapse to fentanyl seeking. We trained male and female rats to self-administer palatable food pellets for 6 d (6 h/d) and intravenous fentanyl (2.5 μg/kg/infusion) for 12 d (6 h/d). We assessed relapse to fentanyl seeking after 13-14 voluntary abstinence days, achieved through a discrete choice procedure between fentanyl infusions and palatable food (20 trials/d). In both sexes, relapse after food choice-induced abstinence was associated with increased expression of the activity marker Fos in the OFC. Pharmacological inactivation of the OFC with muscimol plus baclofen (50 + 50 ng/side) decreased relapse to fentanyl seeking. We then determined projection-specific activation of OFC afferents during the relapse test by using Fos plus the retrograde tracer cholera toxin B (injected into the OFC). Relapse to fentanyl seeking was associated with increased Fos expression in the piriform cortex (Pir) neurons projecting to the OFC, but not in projections from the basolateral amygdala and thalamus. Pharmacological inactivation of the Pir with muscimol plus baclofen decreased relapse to fentanyl seeking after voluntary abstinence. Next, we used an anatomical disconnection procedure to determine whether projections between the Pir and OFC are critical for relapse to fentanyl seeking. Unilateral muscimol plus baclofen injections into the Pir in one hemisphere plus unilateral muscimol plus baclofen injections into the OFC in the contralateral, but not ipsilateral, hemisphere decreased relapse. Our results identify Pir-OFC projections as a new motivation-related pathway critical to relapse to opioid seeking after voluntary abstinence.SIGNIFICANCE STATEMENT There are few preclinical studies of fentanyl relapse, and these studies have used experimenter-imposed extinction or forced abstinence procedures. In humans, however, abstinence is often voluntary, with drug available in the drug environment but forgone in favor of nondrug alternative reinforcers. We recently developed a rat model of drug relapse after palatable food choice-induced voluntary abstinence. Here, we used classical pharmacology, immunohistochemistry, and retrograde tracing to demonstrate a critical role of the piriform and orbitofrontal cortices in relapse to opioid seeking after voluntary abstinence.

CB1 Receptors in the Anterior Piriform Cortex Control Odor Preference Memory

The retrieval of odor-related memories shapes animal behavior. The anterior piriform cortex (aPC) is the largest part of the olfactory cortex, and it plays important roles in olfactory processing and memory. However, it is still unclear whether specific cellular mechanisms in the aPC control olfactory memory, depending on the appetitive or aversive nature of the stimuli involved. Cannabinoid-type 1 (CB1) receptors are present in the aPC (aPC-CB1), but their potential impact on olfactory memory was never explored. Here, we used a combination of behavioral, genetic, anatomical, and electrophysiological approaches to characterize the functions of aPC-CB1 receptors in the regulation of appetitive and aversive olfactory memory. Pharmacological blockade or genetic deletion of aPC-CB1 receptors specifically impaired the retrieval of conditioned odor preference (COP). Interestingly, expression of conditioned odor aversion (COA) was unaffected by local CB1 receptor blockade, indicating that the role of aPC endocannabinoid signaling is selective for retrieval of appetitive memory. Anatomical investigations revealed that CB1 receptors are highly expressed on aPC GABAergic interneurons, and ex vivo electrophysiological recordings showed that their pharmacological activation reduces miniature inhibitory post-synaptic currents (mIPSCs) onto aPC semilunar (SL), but not pyramidal principal neurons. COP retrieval, but not COA, was associated with a specific CB1-receptor-dependent decrease of mIPSCs in SL cells. Altogether, these data indicate that aPC-CB1 receptor-dependent mechanisms physiologically control the retrieval of olfactory memory, depending on odor valence and engaging modulation of local inhibitory transmission.

Parallel odor processing by mitral and middle tufted cells in the olfactory bulb

The olfactory bulb (OB) transforms sensory input into spatially and temporally organized patterns of activity in principal mitral (MC) and middle tufted (mTC) cells. Thus far, the mechanisms underlying odor representations in the OB have been mainly investigated in MCs. However, experimental findings suggest that MC and mTC may encode parallel and complementary odor representations. We have analyzed the functional roles of these pathways by using a morphologically and physiologically realistic three-dimensional model to explore the MC and mTC microcircuits in the glomerular layer and deeper plexiform layer. The model makes several predictions. MCs and mTCs are controlled by similar computations in the glomerular layer but are differentially modulated in deeper layers. The intrinsic properties of mTCs promote their synchronization through a common granule cell input. Finally, the MC and mTC pathways can be coordinated through the deep short-axon cells in providing input to the olfactory cortex. The results suggest how these mechanisms can dynamically select the functional network connectivity to create the overall output of the OB and promote the dynamic synchronization of glomerular units for any given odor stimulus.

Flavor preferences conditioned by nutritive and non-nutritive sweeteners in mice

Recent studies suggest that preferences are conditioned by nutritive (sucrose) but not by non-nutritive (sucralose) sweeteners in mice. Here we compared the effectiveness of nutritive and non-nutritive sweeteners to condition flavor preferences in three mouse strains. Isopreferred sucrose and sucralose solutions both conditioned flavor preferences in C57BL/6J (B6) mice but sucrose was more effective, consistent with its post-oral appetition action. Subsequent experiments compared flavor conditioning by fructose, which has no post-oral appetition effect in B6 mice, and a sucralose+saccharin mixture (SS) which is highly preferred to fructose in 24-h choice tests. Both sweeteners conditioned flavor preferences but fructose induced stronger preferences than SS. Training B6 mice to drink a flavored SS solution paired with intragastric fructose infusions did not enhance the SS-conditioned preference. Thus, the post-oral nutritive actions of fructose do not explain the sugar's stronger preference conditioning effect. Training B6 mice to drink a flavored fructose solution containing SS did not reduce the sugar-conditioned preference, indicating that SS does not have an off-taste that attenuates conditioning. Although B6 mice strongly preferred flavored SS to flavored fructose in a direct choice test, they preferred the fructose-paired flavor to the SS-paired flavor when these were presented in water. Fructose conditioned a stronger flavor preference than an isopreferred saccharin solution, indicating that sucralose is not responsible for the limited SS conditioning actions. SS is highly preferred by FVB/NJ and CAST/EiJ inbred mice, yet conditioned only weak flavor preferences. It is unclear why highly or equally preferred non-nutritive sweeteners condition weaker preferences than fructose, when all stimulate the same T1r2/T1r3 sweet receptor. Recent findings support the existence of non-T1r2/T1r3 glucose taste sensors; however, there is no evidence for receptors that respond to fructose but not to non-nutritive sweeteners.