A specific olfactory cortico-thalamic pathway contributing to sampling performance during odor reversal learning

Cortico-thalamocortical interactions for learning, memory and decision-making

The thalamus and cortex are interconnected both functionally and anatomically and share a common developmental trajectory. Interactions between the mediodorsal thalamus (MD) and different parts of the prefrontal cortex are essential in cognitive processes, such as learning and adaptive decision-making. Cortico-thalamocortical interactions involving other dorsal thalamic nuclei, including the anterior thalamus and pulvinar, also influence these cognitive processes. Our work, and that of others, indicates a crucial influence of these interdependent cortico-thalamocortical neural networks that contributes actively to the processing of information within the cortex. Each of these thalamic nuclei also receives potent subcortical inputs that are likely to provide additional influences on their regulation of cortical activity. Here, we highlight our current neuroscientific research aimed at establishing when cortico-MD thalamocortical neural network communication is vital within the context of a rapid learning and memory discrimination task. We are collecting evidence of MD-prefrontal cortex neural network communication in awake, behaving male rhesus macaques. Given the prevailing evidence, further studies are needed to identify both broad and specific mechanisms that govern how the MD, anterior thalamus and pulvinar cortico-thalamocortical interactions support learning, memory and decision-making. Current evidence shows that the MD (and the anterior thalamus) are crucial for frontotemporal communication, and the pulvinar is crucial for frontoparietal communication. Such work is crucial to advance our understanding of the neuroanatomical and physiological bases of these brain functions in humans. In turn, this might offer avenues to develop effective treatment strategies to improve the cognitive deficits often observed in many debilitating neurological disorders and diseases and in neurodegeneration.

Neural Representation of Intraoral Olfactory and Gustatory Signals by the Mediodorsal Thalamus in Alert Rats

The mediodorsal thalamus is a multimodal region involved in a variety of cognitive behaviors, including olfactory attention, odor discrimination, and the hedonic perception of flavors. Although the mediodorsal thalamus forms connections with principal regions of the olfactory and gustatory networks, its role in processing olfactory and gustatory signals originating from the mouth remains unclear. Here, we recorded single-unit activity in the mediodorsal thalamus of behaving female rats during the intraoral delivery of individual odors, individual tastes, and odor-taste mixtures. Our results are the first to demonstrate that neurons in the mediodorsal thalamus dynamically encode chemosensory signals originating from the mouth. This chemoselective population is broadly tuned, exhibits excited and suppressed responses, and responds to odor-taste mixtures differently than an odor or taste alone. Furthermore, a subset of chemoselective neurons encodes the palatability-related features of tastes and may represent associations between previously experienced odor-taste pairs. Our results further demonstrate the multidimensionality of the mediodorsal thalamus and provide additional evidence of its involvement in processing chemosensory information important for ingestive behaviors.SIGNIFICANCE STATEMENT The perception of food relies on the concurrent processing of olfactory and gustatory signals originating from the mouth. The mediodorsal thalamus is a higher-order thalamic nucleus involved in a variety of chemosensory-dependent behaviors and connects the olfactory and gustatory cortices with the prefrontal cortex. However, it is unknown how neurons in the mediodorsal thalamus process intraoral chemosensory signals. Using tetrode recordings in alert rats, our results are the first to show that neurons in the mediodorsal thalamus dynamically represent olfactory and gustatory signals from the mouth. Our findings are consistent with the mediodorsal thalamus being a key node between sensory and prefrontal cortical areas for processing chemosensory information underlying ingestive behavior.

Thalamic activations in rat brain by fMRI during tactile (forepaw, whisker) and non-tactile (visual, olfactory) sensory stimulations

The thalamus is a crucial subcortical hub that impacts cortical activity. Tracing experiments in animals and post-mortem humans suggest rich morphological specificity of the thalamus. Very few studies reported rodent thalamic activations by functional MRI (fMRI) as compared to cortical activations for different sensory stimuli. Here, we show different portions of the rat thalamus in response to tactile (forepaw, whisker) and non-tactile (visual, olfactory) sensory stimuli with high field fMRI (11.7T) using a custom-build quadrature surface coil to capture high sensitivity signals from superficial and deep brain regions simultaneously. Results demonstrate reproducible thalamic activations during both tactile and non-tactile stimuli. Forepaw and whisker stimuli activated broader regions within the thalamus: ventral posterior lateral (VPL), ventral posterior medial (VPM), lateral posterior mediorostral (LPMR) and posterior medial (POm) thalamic nuclei. Visual stimuli activated dorsal lateral geniculate nucleus (DLG) of the thalamus but also parts of the superior/inferior colliculus, whereas olfactory stimuli activated specifically the mediodorsal nucleus of the thalamus (MDT). BOLD activations in LGN and MDT were much stronger than in VPL, VPM, LPMR and POm. These fMRI-based thalamic activations suggest that forepaw and whisker (i.e., tactile) stimuli engage VPL, VPM, LPMR and POm whereas visual and olfactory (i.e., non-tactile) stimuli, respectively, recruit DLG and MDT exclusively.

Mediodorsal Thalamus Is Critical for Updating during Extradimensional Shifts But Not Reversals in the Attentional Set-Shifting Task

Cognitive flexibility, attributed to frontal cortex, is vital for navigating the complexities of everyday life. The mediodorsal thalamus (MD), interconnected to frontal cortex, may influence cognitive flexibility. Here, male rats performed an attentional set-shifting task measuring intradimensional (ID) and extradimensional (ED) shifts in sensory discriminations. MD lesion rats needed more trials to learn the rewarded sensory dimension. However, once the choice response strategy was established, learning further two-choice discriminations in the same sensory dimension, and reversals of the reward contingencies in the same dimension, were unimpaired. Critically though, MD lesion rats were impaired during the ED shift, when they must rapidly update the optimal choice response strategy. Behavioral analyses showed MD lesion rats had significantly reduced correct within-trial second choice responses. This evidence shows that transfer of information via the MD is critical when rapid within-trial updates in established choice response strategies are required after a rule change.

Basolateral amygdala to posterior piriform cortex connectivity ensures precision in learned odor threat

Odor perception can both evoke emotional states and be shaped by emotional or hedonic states. The amygdala complex plays an important role in recognition of, and response to, hedonically valenced stimuli, and has strong, reciprocal connectivity with the primary olfactory (piriform) cortex. Here, we used differential odor-threat conditioning in rats to test the role of basolateral amygdala (BLA) input to the piriform cortex in acquisition and expression of learned olfactory threat responses. Using local field potential recordings, we demonstrated that functional connectivity (high gamma band coherence) between the BLA and posterior piriform cortex (pPCX) is enhanced after differential threat conditioning. Optogenetic suppression of activity within the BLA prevents learned threat acquisition, as do lesions of the pPCX prior to threat conditioning (without inducing anosmia), suggesting that both regions are critical for acquisition of learned odor threat responses. However, optogenetic BLA suppression during testing did not impair threat response to the CS+ , but did induce generalization to the CS-. A similar loss of stimulus control and threat generalization was induced by selective optogenetic suppression of BLA input to pPCX. These results suggest an important role for amygdala-sensory cortical connectivity in shaping responses to threatening stimuli.

Thalamocortical interactions in cognition and disease: the mediodorsal and anterior thalamic nuclei

The mediodorsal thalamus (MD) and anterior thalamic nuclei (ATN) are two adjacent brain nodes that support our ability to make decisions, learn, update information, form and retrieve memories, and find our way around. The MD and PFC work in partnerships to support cognitive processes linked to successful learning and decision-making, while the ATN and extended hippocampal system together coordinate the encoding and retrieval of memories and successful spatial navigation. Yet, while these distinctions may appear to be segregated, both the MD and ATN together support our higher cognitive functions as they regulate and are influenced by interconnected fronto-temporal neural networks and subcortical inputs. Our review focuses on recent studies in animal models and in humans. This evidence is re-shaping our understanding of the importance of MD and ATN cortico-thalamocortical pathways in influencing complex cognitive functions. Given the evidence from clinical settings and neuroscience research labs, the MD and ATN should be considered targets for effective treatments in neuropsychiatric diseases and disorders and neurodegeneration.

Validation of Olfactory Network Based on Brain Structural Connectivity and Its Association With Olfactory Test Scores

Olfactory perception is a complicated process involving multiple cortical and subcortical regions, of which the underlying brain dynamics are still not adequately mapped. Even in the definition of the olfactory primary cortex, there is a large degree of variation in parcellation templates used for investigating olfaction in neuroimaging studies. This complicates comparison between human olfactory neuroimaging studies. The present study aims to validate an olfactory parcellation template derived from both functional and anatomical data that applies structural connectivity (SC) to ensure robust connectivity to key secondary olfactory regions. Furthermore, exploratory analyses investigate if different olfactory parameters are associated with differences in the strength of connectivity of this structural olfactory fingerprint. By combining diffusion data with an anatomical atlas and advanced probabilistic tractography, we found that the olfactory parcellation had a robust SC network to key secondary olfactory regions. Furthermore, the study indicates that higher ratings of olfactory significance were associated with increased intra- and inter-hemispheric SC of the primary olfactory cortex. Taken together, these results suggest that the patterns of SC between the primary olfactory cortex and key secondary olfactory regions has potential to be used for investigating the nature of olfactory significance, hence strengthening the theory that individual differences in olfactory behaviour are encoded in the structural network fingerprint of the olfactory cortex.

Relationship Between Olfactory Disturbance After Acute Ischemic Stroke and Latent Thalamic Hypoperfusion

Odor detection, recognition, and identification were assessed in 19 acute ischemic stroke patients who had no magnetic resonance imaging-detectable thalamic lesions but in whom technetium-99m ethyl cysteinate dimer single photon emission tomography revealed thalamic hypoperfusion. Although these patients were unaware of reduced olfactory function, they exhibited significantly lower scores in tests for odor identification and recognition threshold as compared with 9 ischemic stroke controls that had normal thalamic hypoperfusion. However, absolute odor detection thresholds were similar in the 2 groups. These results demonstrate the usefulness of cerebral perfusion scintigraphy in assessing sensory loss after ischemic stroke and provide further evidence for the role of the thalamus in olfaction.

Cell-Type-Specific Whole-Brain Direct Inputs to the Anterior and Posterior Piriform Cortex

The piriform cortex (PC) is a key brain area involved in both processing and coding of olfactory information. It is implicated in various brain disorders, such as epilepsy, Alzheimer’s disease, and autism. The PC consists of the anterior (APC) and posterior (PPC) parts, which are different anatomically and functionally. However, the direct input networks to specific neuronal populations within the APC and PPC remain poorly understood. Here, we mapped the whole-brain direct inputs to the two major neuronal populations, the excitatory glutamatergic principal neurons and inhibitory γ-aminobutyric acid (GABA)-ergic interneurons within the APC and PPC using the rabies virus (RV)-mediated retrograde trans-synaptic tracing system. We found that for both types of neurons, APC and PPC share some similarities in input networks, with dominant inputs originating from the olfactory region (OLF), followed by the cortical subplate (CTXsp), isocortex, cerebral nuclei (CNU), hippocampal formation (HPF) and interbrain (IB), whereas the midbrain (MB) and hindbrain (HB) were rarely labeled. However, APC and PPC also show distinct features in their input distribution patterns. For both types of neurons, the input proportion from the OLF to the APC was higher than that to the PPC; while the PPC received higher proportions of inputs from the HPF and CNU than the APC did. Overall, our results revealed the direct input networks of both excitatory and inhibitory neuronal populations of different PC subareas, providing a structural basis to analyze the diverse PC functions.

A thalamocortical circuit for updating action-outcome associations

The ability to flexibly use knowledge is one cardinal feature of goal-directed behaviors. We recently showed that thalamocortical and corticothalamic pathways connecting the medial prefrontal cortex and the mediodorsal thalamus (MD) contribute to adaptive decision-making (Alcaraz et al., 2018). In this study, we examined the impact of disconnecting the MD from its other main cortical target, the orbitofrontal cortex (OFC) in a task assessing outcome devaluation after initial instrumental training and after reversal of action-outcome contingencies. Crossed MD and OFC lesions did not impair instrumental performance. Using the same approach, we found however that disconnecting the OFC from its other main thalamic afferent, the submedius nucleus, produced a specific impairment in adaptive responding following action-outcome reversal. Altogether, this suggests that multiple thalamocortical circuits may act synergistically to achieve behaviorally relevant functions.