Anatomical imaging of the piriform cortex in epilepsy

Resection of the piriform cortex for temporal lobe epilepsy: a Novel approach on imaging segmentation and surgical application

BACKGROUND

The piriform cortex (PC) occupies both banks of the endorhinal sulcus and has an important role in the pathophysiology of temporal lobe epilepsy (TLE). A recent study showed that resection of more than 50% of PC increased the odds of becoming seizure free by a factor of 16.

OBJECTIVE

We report the feasibility of manual segmentation of PC and application of the Geodesic Information Flows (GIF) algorithm to automated segmentation, to guide resection.

METHODS

Manual segmentation of PC was performed by two blinded independent examiners in 60 patients with TLE (55% Left TLE, 52% female) with a median age of 35 years (IQR, 29-47 years) and 20 controls (60% Women) with a median age of 39.5 years (IQR, 31-49). The GIF algorithm was used to create an automated pipeline for parcellating PC which was used to guide excision as part of temporal lobe resection for TLE.

RESULTS

Right PC was larger in patients and controls. Parcellation of PC was used to guide anterior temporal lobe resection, with subsequent seizure freedom and no visual field or language deficit.

CONCLUSION

Reliable segmentation of PC is feasible and can be applied prospectively to guide neurosurgical resection that increases the chances of a good outcome from temporal lobe resection for TLE.

What do brain oscillations tell about the human sense of smell?

Brain activity may manifest itself as oscillations which are repetitive rhythms of neuronal firing. These local field potentials can be measured via intracranial electroencephalography (iEEG). This review focuses on iEEG used to map human brain structures involved in olfaction. After presenting the methodology of the review, a summary of the brain structures involved in olfaction is given, followed by a review of the literature on human olfactory oscillations in different contexts. A single case is provided as an illustration of the olfactory oscillations. Overall, the timing and sequence of oscillations found in the different structures of the olfactory system seem to play an important role for olfactory perception.

Unraveling the Neural Circuits: Techniques, Opportunities and Challenges in Epilepsy Research

Epilepsy, a prevalent neurological disorder characterized by high morbidity, frequent recurrence, and potential drug resistance, profoundly affects millions of people globally. Understanding the microscopic mechanisms underlying seizures is crucial for effective epilepsy treatment, and a thorough understanding of the intricate neural circuits underlying epilepsy is vital for the development of targeted therapies and the enhancement of clinical outcomes. This review begins with an exploration of the historical evolution of techniques used in studying neural circuits related to epilepsy. It then provides an extensive overview of diverse techniques employed in this domain, discussing their fundamental principles, strengths, limitations, as well as their application. Additionally, the synthesis of multiple techniques to unveil the complexity of neural circuits is summarized. Finally, this review also presents targeted drug therapies associated with epileptic neural circuits. By providing a critical assessment of methodologies used in the study of epileptic neural circuits, this review seeks to enhance the understanding of these techniques, stimulate innovative approaches for unraveling epilepsy's complexities, and ultimately facilitate improved treatment and clinical translation for epilepsy.

Practical microscopic neuroanatomy of the limbic system and basal forebrain identifiable on clinical 3T MRI

Microscopic neuroanatomy of limbic system and basal forebrain on MRI is complex and is a terra incognita for many radiologists, clinicians, and neuroscientists. Interestingly, most of the important structures/at least anatomical regions containing these structures demonstrable on cadaveric and surgical dissections can be identified on clinical MRI, with 3T being much better than 1.5T. This article teaches the practical MRI identification of these structures which will greatly help in evaluating complex ailments like temporal lobe epilepsy, Alzheimer dementia, and other neuropsychiatric disorders. This knowledge will also aid in accurate reporting of tumor spread along the white matter fasciculi in the temporal stem/basal forebrain region. Limbic system includes the mesial temporal structures and their connections, piriform cortex including "area tempestas," and the septal area comprising of subcallosal area and paraterminal gyrus. Basal forebrain includes structures like substantia innominata with basal nucleus of Meynert, diagonal gyrus/diagonal band of Broca, and nucleus accumbens lying in between the anterior perforated substance inferiorly and the anterior commissure superiorly.

Principal neurons in the olfactory cortex mediate bidirectional modulation of seizures

Although the piriform cortex (PC) has been previously implicated as a critical node for seizure generation and propagation, the underlying neural mechanism has remained unclear. Here, we found increased excitability in PC neurons during amygdala kindling acquisition. Optogenetic or chemogenetic activation of PC pyramidal neurons promoted kindling progression, whereas inhibition of these neurons retarded seizure activities induced by electrical kindling in the amygdala. Furthermore, chemogenetic inhibition of PC pyramidal neurons alleviated the severity of kainic acid-induced acute seizures. These results demonstrate that PC pyramidal neurons bidirectionally modulate seizures in temporal lobe epilepsy, providing evidence for the efficacy of PC pyramidal neurons as a potential therapeutic target for epileptogenesis. KEY POINTS: While the piriform cortex (PC) is an important olfactory centre critically involved in olfactory processing and plays a crucial role in epilepsy due to its close connection with the limbic system, how the PC regulates epileptogenesis is largely unknown. In this study, we evaluated the neuronal activity and the role of pyramidal neurons in the PC in the mouse amygdala kindling model of epilepsy. PC pyramidal neurons are hyperexcited during epileptogenesis. Optogenetic and chemogenetic activation of PC pyramidal neurons significantly promoted seizures in the amygdala kindling model, whereas selective inhibition of these neurons produced an anti-epileptic effect for both electrical kindling and kainic acid-induced acute seizures. The results of the present study indicate that PC pyramidal neurons bidirectionally modulate seizure activity.

Intracranial investigation of piriform cortex epilepsy during odor presentation

The piriform cortex (PC) is part of the olfactory system, principally receiving input from the lateral olfactory tract and projecting to downstream components of the olfactory network, including the amygdala. Based on preclinical studies, PC is vulnerable to injury and can be easily kindled as an onset site for seizures. While the role of PC in human epilepsy has been studied indirectly and the subject of speculation, cases of demonstrated PC seizure onset from direct intracranial recording are rare. We present a pediatric patient with drug-resistant focal reflex epilepsy and right mesial temporal sclerosis with habitual seizures triggered by coconut aroma. The patient underwent stereoelectroencephalography with implantation of olfactory cortices including PC, through which we identified PC seizure onset, mapped high-frequency activity associated with presentation of olfactory stimuli and performance on cognitive tasks, and reproduced habitual seizures via cortical stimulation of PC. Coconut odor did not trigger seizures in our work with the patient. Surgical workup resulted in resection of the patient's right amygdala, PC, and mesial temporal pole, following which she has been seizure free for 20 months without functional decline in cognition or smell. Histological findings from resected tissue showed astrogliosis and subpial gliosis.

Brain diffusion tensor imaging reveals altered connections and networks in epilepsy patients

Introduction

Accumulating evidence shows that epilepsy is a disease caused by brain network dysfunction. This study explored changes in brain network structure in epilepsy patients based on graph analysis of diffusion tensor imaging data.

Methods

The brain structure networks of 42 healthy control individuals and 26 epilepsy patients were constructed. Using graph theory analysis, global and local network topology parameters of the brain structure network were calculated, and changes in global and local characteristics of the brain network in epilepsy patients were quantitatively analyzed.

Results

Compared with the healthy control group, the epilepsy patient group showed lower global efficiency, local efficiency, clustering coefficient, and a longer shortest path length. Both healthy control individuals and epilepsy patients showed small-world attributes, with no significant difference between groups. The epilepsy patient group showed lower nodal local efficiency and nodal clustering coefficient in the right olfactory cortex and right rectus and lower nodal degree centrality in the right olfactory cortex and the left paracentral lobular compared with the healthy control group. In addition, the epilepsy patient group showed a smaller fiber number of edges in specific regions of the frontal lobe, temporal lobe, and default mode network, indicating reduced connection strength.

Discussion

Epilepsy patients exhibited lower global and local brain network properties as well as reduced white matter fiber connectivity in key brain regions. These findings further support the idea that epilepsy is a brain network disorder.

Interictal-period-activated neuronal ensemble in piriform cortex retards further seizure development

Epileptic networks are characterized as having two states, seizures or more prolonged interictal periods. However, cellular mechanisms underlying the contribution of interictal periods to ictal events remain unclear. Here, we use an activity-dependent labeling technique combined with genetically encoded effectors to characterize and manipulate neuronal ensembles recruited by focal seizures (FS-Ens) and interictal periods (IP-Ens) in piriform cortex, a region that plays a key role in seizure generation. Ca²⁺ activities and histological evidence reveal a disjointed correlation between the two ensembles during FS dynamics. Optogenetic activation of FS-Ens promotes further seizure development, while IP-Ens protects against it. Interestingly, both ensembles are functionally involved in generalized seizures (GS) due to circuit rearrangement. IP-Ens bidirectionally modulates FS but not GS by controlling coherence with hippocampus. This study indicates that the interictal state may represent a seizure-preventing environment, and the interictal-activated ensemble may serve as a potential therapeutic target for epilepsy.

Social defeat drives hyperexcitation of the piriform cortex to induce learning and memory impairment but not mood-related disorders in mice

Clinical studies have shown that social defeat is an important cause of mood-related disorders, accompanied by learning and memory impairment in humans. The mechanism of mood-related disorders has been widely studied. However, the specific neural network involved in learning and memory impairment caused by social defeat remains unclear. In this study, behavioral test results showed that the mice induced both learning and memory impairments and mood-related disorders after exposure to chronic social defeat stress (CSDS). c-Fos immunofluorescence and fiber photometry recording confirmed that CaMKIIα expressing neurons of the piriform cortex (PC) were selectively activated by exposure to CSDS. Next, chemogenetics and optogenetics were performed to activate PC CaMKIIα expressing neurons, which showed learning and memory impairment but not mood-related disorders. Furthermore, chemogenetic inhibition of PC CaMKIIα expressing neurons significantly alleviated learning and memory impairment induced by exposure to CSDS but did not relieve mood-related disorders. Therefore, our data suggest that the overactivation of PC CaMKIIα expressing neurons mediates CSDS-induced learning and memory impairment, but not mood-related disorders, and provides a potential therapeutic target for learning and memory impairment induced by social defeat.

Clinical and Magnetic Resonance Imaging (MRI) Features, Tumour Localisation, and Survival of Dogs with Presumptive Brain Gliomas

Brain gliomas are common tumours diagnosed in dogs. However, limited information is available on the clinical features and overall survival time (OS) in dogs receiving palliative treatment. The aim of this study was to evaluate possible associations between presenting complaint, tumour localisation, Magnetic Resonance Imaging (MRI) features, survival times, and reason for the death of dogs with suspected intracranial glioma treated palliatively. Sixty dogs from a single institution were retrospectively included (from September 2017 to December 2021). Dogs were included if a presumptive diagnosis of brain glioma was obtained based on an MRI scan and medical history. French Bulldogs were overrepresented (40/60); 46 out of 60 dogs (77%) presented due to epileptic seizures (ES) and in 25/60 dogs (42%), cluster seizures or status epilepticus were the first manifestation of the disease. Dogs with suspected gliomas located in the piriform lobe showed a higher probability of presenting due to epilepsy compared to dogs with glioma in other regions, and more frequently died or were euthanised because of increased ES. Magnetic Resonance Imaging (MRI) features differed between localisations. Fronto-olfactory tumours were more frequently, whereas piriform tumours were less frequently, classified as suspected high-grade glioma. The median survival time was 61 days. Dogs with contrast-enhancing suspected gliomas had significantly shorter OS. This study provides additional information on the clinical features and survival of dogs with suspected brain gliomas treated palliatively.

Piriform Cortex Ablation Volume Is Associated With Seizure Outcome in Mesial Temporal Lobe Epilepsy

BACKGROUND

Growing evidence suggests that piriform cortex resection during anterior temporal lobectomy is important for achieving good seizure outcome in mesial temporal lobe epilepsy (mTLE). However, the relationship between seizure outcome and piriform cortex ablation during MR-guided laser interstitial thermal therapy (MRgLITT) remains unclear.

OBJECTIVE

To determine whether ablation of piriform cortex was associated with seizure outcome in patients with mTLE undergoing MRgLITT.

METHODS

We performed preablation and postablation volumetric analyses of hippocampus, amygdala, piriform cortex, and ablation volumes in patients with mTLE who underwent MRgLITT at our institution from 2014 to 2019.

RESULTS

Thirty nine patients with mTLE were analyzed. In univariate logistic regression, percent piriform cortex ablation was associated with International League Against Epilepsy (ILAE) class 1 at 6 months (odds ratio [OR] 1.051, 95% CI [1.001-1.117], P = .045), whereas ablation volume, percent amygdala ablation, and percent hippocampus ablation were not (P > .05). At 1 year, ablation volume was associated with ILAE class 1 (OR 1.608, 95% CI [1.071-2.571], P = .021) while percent piriform cortex ablation became a trend (OR 1.050, 95% CI [0.994-1.109], P = .054), and both percent hippocampus ablation and percent amygdala ablation were not significantly associated with ILAE class 1 (P > .05). In multivariable logistic regression, only percent piriform cortex ablation was a significant predictor of seizure freedom at 6 months (OR 1.085, 95% CI [1.012-1.193], P = .019) and at 1 year (OR 1.074, 95% CI [1.003-1.178], P = .041).

CONCLUSION

Piriform cortex ablation volume is associated with seizure outcome in patients with mTLE undergoing MRgLITT. The piriform cortex should be considered a high yield ablation target to achieve good seizure outcome.

The role of the piriform cortex in temporal lobe epilepsy: A current literature review

Temporal lobe epilepsy is the most common form of focal epilepsy and can have various detrimental consequences within many neurologic domains. Recent evidence suggests that the piriform cortex may also be implicated in seizure physiology. The piriform cortex is a primary component of the olfactory network and is located at the junction of the frontal and temporal lobes, wrapping around the entorhinal sulcus. Similar to the hippocampus, it is a tri-layered allocortical structure, with connections to many adjacent regions including the orbitofrontal cortex, amygdala, peri- and entorhinal cortices, and insula. Both animal and human studies have implicated the piriform cortex as a critical node in the temporal lobe epilepsy network. It has additionally been shown that resection of greater than half of the piriform cortex may significantly increase the odds of achieving seizure freedom. Laser interstitial thermal therapy has also been shown to be an effective treatment strategy with recent evidence hinting that ablation of the piriform cortex may be important for seizure control as well. We propose that sampling piriform cortex in intracranial stereoelectroencephalography (sEEG) procedures with the use of a temporal pole or amygdalar electrode would be beneficial for further understanding the role of the piriform cortex in temporal lobe epilepsy.

Characteristics of olfactory dysfunction in patients with temporal lobe epilepsy

OBJECTIVE

To determine the characteristics of olfactory dysfunction in patients with temporal lobe epilepsy (TLE).

METHODS

Odor identification was assessed using the odor stick identification test for Japanese (OSIT-J, full score 12 points) in 65 patients with TLE and in 74 controls.

RESULTS

The mean OSIT-J score was significantly lower in patients with TLE (mean ± SD = 8.1 ± 2.8; median = 9) than in the control subjects (mean ± SD = 10.6 ± 1.1; median = 11) (P < 0.005). Olfactory dysfunction (hyposmia/anosmia) was associated with bilateral seizure foci and older age of onset in TLE. Patients who underwent temporal lobectomy for hippocampal sclerosis did not show significant decline after long-term recovery. The Indian ink part of OSIT-J was useful for the detection of olfactory deficits in patients with TLE (sensitivity = 47%, specificity = 93%). Patients with TLE tended to have preserved olfactory ability for stimulating odors and for familiar odors of daily life.

SIGNIFICANCE

We observed characteristic odor identification deficits for individual odors used in OSIT-J. Our study findings provide deeper insight into the underlying mechanism of olfactory function in patients with TLE and may be beneficial in the clinical management of these patients.

Differential Impact of Severity and Duration of Status Epilepticus, Medical Countermeasures, and a Disease-Modifier, Saracatinib, on Brain Regions in the Rat Diisopropylfluorophosphate Model

Acute organophosphate (OP) toxicity poses a significant threat to both military and civilian personnel as it can lead to a variety of cholinergic symptoms including the development of status epilepticus (SE). Depending on its severity, SE can lead to a spectrum of neurological changes including neuroinflammation and neurodegeneration. In this study, we determined the impact of SE severity and duration on disease promoting parameters such as gliosis and neurodegeneration and the efficacy of a disease modifier, saracatinib (AZD0530), a Src/Fyn tyrosine kinase inhibitor. Animals were exposed to 4 mg/kg diisopropylfluorophosphate (DFP, s.c.) followed by medical countermeasures. We had five experimental groups: controls (no DFP), animals with no continuous convulsive seizures (CS), animals with ∼20-min continuous CS, 31-60-min continuous CS, and > 60-min continuous CS. These groups were then assessed for astrogliosis, microgliosis, and neurodegeneration 8 days after DFP exposure. The 31-60-min and > 60-min groups, but not ∼20-min group, had significantly upregulated gliosis and neurodegeneration in the hippocampus compared to controls. In the piriform cortex and amygdala, however, all three continuous CS groups had significant upregulation in both gliosis and neurodegeneration. In a separate cohort of animals that had ∼20 and > 60-min of continuous CS, we administered saracatinib for 7 days beginning three hours after DFP. There was bodyweight loss and mortality irrespective of the initial SE severity and duration. However, in survived animals, saracatinib prevented spontaneous recurrent seizures (SRS) during the first week in both severity groups. In the ∼20-min CS group, compared to the vehicle, saracatinib significantly reduced neurodegeneration in the piriform cortex and amygdala. There were no significant differences in the measured parameters between the naïve control and saracatinib on its own (without DFP) groups. Overall, this study demonstrates the differential effects of the initial SE severity and duration on the localization of gliosis and neurodegeneration. We have also demonstrated the disease-modifying potential of saracatinib. However, its’ dosing regimen should be optimized based on initial severity and duration of CS during SE to maximize therapeutic effects and minimize toxicity in the DFP model as well as in other OP models such as soman.

Cell assembly formation and structure in a piriform cortex model

The piriform cortex is rich in recurrent excitatory synaptic connections between pyramidal neurons. We asked how such connections could shape cortical responses to olfactory lateral olfactory tract (LOT) inputs. For this, we constructed a computational network model of anterior piriform cortex with 2000 multicompartment, multiconductance neurons (500 semilunar, 1000 layer 2 and 500 layer 3 pyramids; 200 superficial interneurons of two types; 500 deep interneurons of three types; 500 LOT afferents), incorporating published and unpublished data. With a given distribution of LOT firing patterns, and increasing the strength of recurrent excitation, a small number of firing patterns were observed in pyramidal cell networks: first, sparse firings; then temporally and spatially concentrated epochs of action potentials, wherein each neuron fires one or two spikes; then more synchronized events, associated with bursts of action potentials in some pyramidal neurons. We suggest that one function of anterior piriform cortex is to transform ongoing streams of input spikes into temporally focused spike patterns, called here "cell assemblies", that are salient for downstream projection areas.

Stimulating Solutions for Intractable Epilepsy

Implantable devices for controlling medically intractable seizures nondestructively are rapidly advancing. These offer reversible, potentially, restorative options beyond traditional, surgical procedures, which rely, largely on resection or ablation of selected brain sites. Several lines of, investigation aimed at improving efficacy of these devices are discussed, ranging from identifying novel subcortical, white matter, or cell-type specific targets to engineering advances for adaptive techniques based- on continuous, dynamic system analysis.

Impaired olfactory neural circuit in patients with SLE at early stages

OBJECTIVE

To investigate the changes of olfactory function and odor-induced brain activation in patients with systemic lupus erythematosus (SLE) at early stages compared with healthy controls.

MATERIALS AND METHODS

Olfactory function and odor-induced brain activation in 12 SLE patients at early stages and 12 age, gender and education matched healthy controls were evaluated using olfactory behavior test and odor-induced task-functional magnetic resonance imaging (task-fMRI).

RESULTS

No significant differences in olfactory behavior scores (including olfactory threshold, olfactory identification, and olfactory memory) were found in the patients with SLE at early stages compared with the healthy controls, while significantly decreased odor-induced activations in olfactory-related brain regions were observed in the patients. In the SLE group, the patients with better performance in the olfactory threshold test had significantly lower levels of anti-dsDNA antibody.

CONCLUSION

The current study demonstrated that significant alterations in odor-induced brain activations occurred prior to measurable olfactory decline in SLE at early stages, which provided a new method for early diagnosis of olfactory dysfunction in SLE.

Changes in resting-state cerebral blood flow and its connectivity in patients with focal to bilateral tonic-clonic seizures

Arterial spin labeling (ASL) is an important tool for understanding cerebral perfusion in epilepsy patients. The aim of this study was to explore patterns of change in cerebral blood flow (CBF) and CBF connectivity in patients with focal to bilateral tonic-clonic seizures (FBTCS). High-resolution three-dimensional (3-D) T1-weighted and 3-D pseudo-continuous ASL magnetic resonance imaging (MRI) was collected from 32 patients with FBTCS and 16 healthy volunteers using a 3.0 T MRI scanner. Cerebral blood flow and its connectivity were compared between the FBTCS and control group. Correlation analysis was used to explore relationships of CBF and its connectivity changes with clinical parameters. Cerebral blood flow data of spatial standardization and normalization were used to improve statistical power. Patients with FBTCS exhibited increased CBF in the bilateral thalamus, caudate nucleus, olfactory cortex, and gyrus rectus, but decreased CBF in the bilateral supplementary motor areas (SMA) and middle cingulate cortex (MCC). Patients with FBTCS showed significant positive correlation between CBF and gray matter volume (GMV) in bilateral SMA and MCC. No significant correlations between CBF and clinical parameters were found among FBTCS patients. The anterior cingulate cortex (ACC) showed positive CBF connectivity with the bilateral SMA and MCC, and these CBF connectivity measures differed significantly between groups (cluster-level, FWE-corrected, P < 0.001). These findings suggest that patients with FBTCS have changes in cerebral CBF and CBF connectivity, which may relate to the underlying neuropathology of FBTCS.

Piriform cortex alterations in the Ts65Dn model for down syndrome

The piriform cortex is involved in olfactory information processing, that is altered in Down Syndrome. Moreover, piriform cortex has a crucial involvement in epilepsy generation and is one of the first regions affected in Alzheimer's Disease, both maladies being prevalent among Down Syndrome individuals. In this work, we studied the alterations in neuronal morphology, synaptology and structural plasticity in the piriform cortex of the Ts65Dn mouse model, which is the most used model for the study of this syndrome and mimics some of their alterations. We have observed that Ts65Dn piriform cortex displays: a reduction in dendritic arborisation, a higher density of inhibitory synapses (GAD67), a lower density of excitatory synapses (vGLUT1) and a higher density of inhibitory postsynaptic puncta (gephyrin). Under electron microscopy the excitatory presynaptic and postsynaptic elements were larger in trisomic mice than in controls. Similar results were obtained using confocal microscopy. There were less immature neurons in piriform cortex layer II in addition to a reduction in the expression of PSA-NCAM in the neuropil that subsequently can reflect impairment in structural plasticity. These data support the idea of an impaired environment with altered ratio of inhibition and excitation that involves a reduction in plasticity and dendritic atrophy, providing a possible substrate for the olfactory processing impairment observed in DS individuals.

Olfactory Testing in Temporal Lobe Epilepsy: a Systematic Review

PURPOSE OF REVIEW

Olfactory testing is a potentially safe, cost-effective, bedside evaluation tool for diagnosis, monitoring, and risk assessment for surgery in temporal lobe epilepsy (TLE) patients, but testing methods and relevant olfactory domains are not standardized. We conducted a systematic review to evaluate olfactory tests in TLE and summarize the results of the literature.

RECENT FINDINGS

Olfactory tests varied significantly in odorant administration tools and devices, target odorants, evaluation timing, and grading scales. The Smell Threshold Test and University of Pennsylvania Smell Identification Test were the most validated single-domain tests for odor detection and odor identification, respectively. For multi-domain tests, Odor Memory/Discrimination Test and the Sniffin' Sticks test were the most validated. Results of olfactory tests in TLE are presented by domain. Rigorous validation, standardization, and comparative analysis of existing olfactory tests by domain is urgently needed to establish the utility and efficacy of olfactory testing in TLE.

Epileptogenesis-induced changes of hippocampal-piriform connectivity

OBJECTIVE

Tissue remodeling has been described in brain circuits that are involved in the generation and propagation of epileptic seizures. Human and animal studies suggest that the anterior piriform cortex (aPC) is crucial for seizure expression in focal epilepsies. Here, we investigate the effect of kainic-acid (KA)-induced seizures on the effective connectivity of the aPC with bilateral hippocampal CA3 regions using cerebro-cerebral evoked potentials (CCEPs).

METHODS

Adult male Sprague-Dawley rats were implanted with a tripolar electrode in the left aPC for stimulation and recording, and with unipolar recording electrodes in bilateral CA3 regions. Single pulse stimulations were given to the aPC and CCEPs were averaged before KA injections and after the emergence of spontaneous recurrent seizures (SRS). Similar recordings at equivalent time intervals were obtained from animals that received saline injections instead of KA (controls).

RESULTS

In the experimental group, the percentage change of increased amplitude of the contralateral (but not ipsilateral) CA3 CCEPs between pre-KA injection and after the emergence of SRS was significantly greater than in controls. No significant single-pulse-induced spectral change responses were observed in either epileptic or control rats when comparing pre- and post-stimulus time intervals. Also, we found no correlation between seizure frequency and the extent of amplitude changes in the CCEPs.

CONCLUSIONS

In the KA model, epileptogenesis results in plastic changes that manifest as an amplification of evoked potential amplitudes recorded in the contralateral hippocampus in response to single-pulse stimulation of the aPC. These results suggest epileptogenesis-induced facilitation of interhemispheric connectivity between the aPC and the hippocampus. Since the amplitude increase of the contralateral CCEP is a possible in vivo biomarker of epilepsy, any intervention (e.g. neuromodulatory) that can reverse this phenomenon may hold a potential antiepileptic efficacy.

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.