Ipsilateral Piriform Cortex Research Articles

Identification of hepatosensitive region and their neural connections in the hippocampus of rats.

Visceral function localisation of the brain is very complex. For many years, people have been actively exploring the neural mechanism regulating visceral and substance metabolism, clarifying the complex relationship between the brain and peripheral nervous system related to the regulation of visceral activity, and analysing its complex neural pathways. The brain is the advanced centre of visceral function regulation. As an advanced centre for substance metabolism and visceral regulation, the hippocampus is crucial for regulating visceral function. The liver is the core organ of material metabolism, and its afferent signals are mainly projected to the nucleus of the solitary tract (NTS) through vagus nerve, and then they are projected to the hypothalamus and limbic system. We placed a stereotaxic instrument on the head of each rat and performed craniotomy to open a window above the left hippocampus. We used gold-plated tungsten electrodes to monitor hippocampal neuronal discharges. Grounding was achieved using screws and silver wire. We electrically stimulated the liver branch of the vagus nerve and observed changes in hippocampal neuron discharges using a biological method; in this way, we identified hepatosensitive hippocampal region. We injected FluoroGold into this region and related brain areas. After 3 days, the rats were sacrificed and perfused; the hippocampi were fixed, dehydated, frozen, sectioned, and subjected to fluorescence microscopy. Nerve discharge frequency and amplitude significantly increased in the hippocampal CA3 region (AP: -4.9, ML: -5.1, DV: -5.0 mm). After FluoroGold was injected into the left hepatosensitive region in the hippocampus, labelled cells were found in the contralateral hippocampus, ipsilateral piriform cortex (PC), locus coeruleus (LC) and bilateral lateral hypothalamus (LHA); fluorescence in the ipsilateral hypothalamus was stronger than that of the contralateral hypothalamus. FluoroGold was injected into the LHA, PC, and LC; no labelled cells were found in the hippocampal CA3 region or in the control group. The hippocampal CA3 area of rats may contain a hepatosensitive region that plays important roles in the regulation of liver and other organ function. This region may receive input from the LHA, PC, and LC.

Centrifugal Innervation of the Olfactory Bulb: A Reappraisal.

The inter-regional connectivity of sensory structures in the brain allows for the modulation of sensory processing in manners important for perception. In the olfactory system, odor representations in the olfactory bulb (OB) are modulated by feedback centrifugal innervation from several olfactory cortices, including the piriform cortex (PCX) and anterior olfactory nucleus (AON). Previous studies reported that an additional olfactory cortex, the olfactory tubercle (OT), also centrifugally innervates the OB and may even shape the activity of OB output neurons. In an attempt to identify the cell types of this centrifugal innervation, we performed retrograde tracing experiments in mice utilizing three unique strategies, including retrobeads, retrograde adeno-associated virus (AAV) driving a fluorescent reporter, and retrograde AAV driving Cre-expression in the Ai9-floxed transgenic reporter line. Our results replicated the standing literature and uncovered robustly labeled neurons in the ipsilateral PCX, AON, and numerous other structures known to innervate the OB. Surprisingly, consistent throughout all of our approaches, no labeled soma were observed in the OT. These findings indicate that the OT is unique among other olfactory cortices in that it does not innervate the OB, which refines our understanding of the centrifugal modulation of the OB.

Somatotopic Organization and Temporal Characteristics of Cerebrocortical Excitation in Response to Nasal Mucosa Stimulation With and Without an Odor in the Rat: An Optical Imaging Study

Nasal mucosa has roles in warming and humidifying inspired air and is highly sensitive to mechanical stimuli. Moreover, the upper part of the nasal mucosa expresses olfactory receptors processing olfactory information. Although the somatosensory map of the face in the primary (S1) and secondary (S2) somatosensory cortices is clearly documented, the map of the nasal mucosa and the effect of odors on their activities are largely unknown. This study aimed to identify the cortical regions in S1 and their temporal features in response to somatosensory stimulation of the nasal mucosa using an optical imaging technique in urethane-anesthetized rats. An air puff application response to nasal mucosa first occurred in a part of contralateral S1 and subsequently, spread toward the rostrally and ventrally adjacent sites. Upper pharynx stimulation initially activated this rostrally expanded site and the excitatory propagation from the initially activated region toward ventral region likely represented S2. Signal intensity and activated area increased dependent on air pressure. Nasal tip stimulation initially excited S1 region caudally adjacent to that of nasal mucosa. Moreover, the amplitude of S1 excitation was similar between air puff stimulation with and without an odor, amyl acetate. In contrast to contralateral S1, air puff stimulation with the odor showed a faint optical signal increase in the ipsilateral piriform cortex. These results suggest that somatosensory information from the nasal mucosa and skin, and upper pharynx are processed in spatially continuous regions of S1, and interaction between somatosensory and olfactory systems is relatively small in contralateral S1.

Abnormal Brain Areas Common to the Focal Epilepsies: Multivariate Pattern Analysis of fMRI.

Individuals with focal epilepsy have heterogeneous sites of seizure origin. However, there may be brain regions that are common to most cases of intractable focal epilepsy. In this study, we aim to identify these using multivariate analysis of task-free functional MRI. Fourteen subjects with extratemporal focal epilepsy and 14 healthy controls were included in the study. Task-free functional MRI data were used to calculate voxel-wise regional connectivity with regional homogeneity (ReHo) and weighted degree centrality (DCw), in addition to regional activity using fraction of amplitude of low-frequency fluctuations (fALFF). Multivariate pattern analysis was applied to each of these metrics to discriminate brain areas that differed between focal epilepsy subjects and healthy controls. ReHo and DCw classified focal epilepsy subjects from healthy controls with high accuracy (89.3% and 75%, respectively). However, fALFF did not significantly classify patients from controls. Increased regional network activity in epilepsy subjects was seen in the ipsilateral piriform cortex, insula, and thalamus, in addition to the dorsal anterior cingulate cortex and lateral frontal cortices. Decreased regional connectivity was observed in the ventromedial prefrontal cortex, as well as lateral temporal cortices. Patients with extratemporal focal epilepsy have common areas of abnormality (ReHo and DCw measures), including the ipsilateral piriform cortex, temporal neocortex, and ventromedial prefrontal cortex. ReHo shows additional increase in the "salience network" that includes anterior insula and anterior cingulate cortex. DCw showed additional effects in the ipsilateral thalamus and striatum. These brain areas may represent key regional network properties underlying focal epilepsy.

Neuroimaging of drug resistance in epilepsy

Drug resistance is an important clinical problem: it is associated with higher rates of somatic and psychiatric comorbidities and cognitive/memory decline, with seizures being just the 'tip of the iceberg'. This review summarizes recent developments in imaging research, focusing specifically on the functional consequence of chronic epilepsies and mechanisms of drug resistance, restricted to work published in 2013. Functional imaging approaches reliably identify underlying specific networks in patients with different epileptic syndromes, show specific responses to certain antiepileptic drugs and differentiate between responder and nonresponder. Functional MRI (fMRI) and the intracarotid amobarbital test (IAT) are generally congruent, but fMRI may be more sensitive than IAT to right hemisphere language processing. In addition, memory fMRI supports the functional adequacy of ipsilateral structures rather than functional reserve of the contralateral hemisphere. There is further evidence from group analysis of fMRI data for a node within the ipsilateral piriform cortex to be important for seizure modulation in focal refractory epilepsies of different cortical origin. Molecular imaging with verapamil-PET identifies P-glycprotein overexpression as a mechanism contributing to drug resistance in individual patients. Neuroimaging in epilepsy has progressed from correlations with demographic, semiologic, neuropsychological and other observational data primarily in patients undergoing presurgical investigations to imaging network connectivity changes in epilepsy syndromes, and testing specific mechanisms underlying drug-resistant epilepsy.

EEG–fMRI in focal epilepsy: Local activation and regional networks

ObjectiveTo identify features of BOLD signal change associated with interictal epileptiform discharges (IEDs) in a heterogeneous group of focal epilepsy patients. MethodsEEG/fMRI studies in 27 focal epilepsy patients were reviewed with attention given to the extent and location of the IED and the resulting pattern of BOLD signal change. Second order group analysis was used to identify common features. ResultsfMRI results provided novel clinical information for individual patients. We identified a significant common node within the ipsilateral piriform cortex as well as patterns involving distant cortical or subcortical areas. ConclusionDespite the heterogeneity of IEDs in focal epilepsy, there are important common features underpining IEDs with a highly significant fMRI node in the ipsilateral piriform cortex. SignificanceThere are important common features in the networks involved in IEDs in patients with a heterogeneous range of epileptogenic foci. We confirm that the piriform cortex is a common node underlying IEDs in patients with focal epilepsy and so provides a target for further study and potential therapy. We describe important features of BOLD signal change that accompany focal and diffuse IEDs that will help researchers and clinicians navigate the sometimes complex findings revealed by these studies.

Cytoarchitectonics and afferent/efferent reorganization of neurons in layers II and III of the lateral entorhinal cortex in the mouse pilocarpine model of temporal lobe epilepsy

With the mouse pilocarpine model of temporal lobe epilepsy (TLE), we showed a progressive loss of both principal cells and calbindin (CB)-, calretinin (CR)-, and parvalbumin (PV)-immunopositive interneurons in layers II-III of lateral entorhinal cortex (LEnt) from 2 months to 1 year after pilocarpine-induced status epilepticus (PISE). In the efferent pathway of LEnt, more Phaseolus vulgaris leucoagglutinin (PHA-L)-labelled en passant and terminal boutons with larger diameters were shown in the hippocampus and subiculum; in the prefrontal, piriform, and perirhinal cortices; and in the amygdaloid complex in experimental mice at the two time points compared with the control after iontophoretical injection of an anterograde tracer PHA-L into the LEnt. Furthermore, the numbers of CB- or CR-immunopositive neurons contacted by PHA-L-labelled en passant and terminal boutons decreased in most of these areas at 2 months or 1 year after PISE. In the afferent pathway of LEnt, the numbers of retrogradely labelled neurons were reduced significantly in the ipsilateral piriform cortex and endopiriform nucleus at 2 months and 1 year and in the reuniens thalamic nucleus only at 1 year after injection of a retrograde tracer cholera toxin B subunit (CTB) into the LEnt. The percentages of the number of CTB and CB or CR double-labelled neurons of all the retrogradely labelled neurons were also decreased in the reunions thalamic nucleus at 1 year after PISE. It is concluded that both cytoarchitectonic change and reorganization of afferent and efferent pathways in LEnt may be involved in the occurrence of TLE.

MRNA expression of activity-regulated cytoskeleton-associated protein ( arc) in the amygdala-kindled rats

Activity-regulated cytoskeleton-associated protein ( arc) is an immediate early gene (IEG) whose mRNA is targeted to dendrites of activated synapses. The present study investigated the expression of three IEGs, arc and two transcription factors (NGFI-B and Nurr 1), in regions of the brain of rats that were subjected to a single afterdischarge-inducing electrical stimulation of the left amygdala and in the amygdala-kindled rats that developed consecutive generalized seizures. One hour after a single stimulus was administered to non-kindled rats, mRNA levels of arc and Nurr1 increased significantly in the ipsilateral piriform cortex and medial amygdaloid nucleus. mRNA levels of NGFI-B increased significantly in the left piriform cortex, basolateral amygdaloid nucleus and medial amygdaloid nucleus. In the amygdala-kindled rats, mRNAs of the three IEGs increased significantly in bilateral brain regions including the piriform cortex and dentate gyrus at 0.5–1 h after the last generalized kindled seizure, and returned near to basal levels by 6 h after. Photographic emulsion autoradiograms showed that the arc mRNA signals induced by the last generalized kindled seizures were associated with the cell bodies of the dentate granule cell layer and extended beyond the granule cell layer into the molecular layer, which contains the dendrites of granule cells. These results support that arc serves as an effector IEG whose mRNA is associated with synaptic reorganization in kindling.

Olfactory system modulation of hippocampal cell death

The hippocampal dentate gyrus is a major recipient of olfactory input in rodents, via connections from the olfactory (piriform) cortex and the olfactory bulb to the entorhinal cortex. Given this connectivity and the known role of activity in dentate gyrus granule cell survival, the present experiment examined the immediate effects of loss of olfactory input to the hippocampus on apoptosis. Adults rats underwent unilateral or bilateral olfactory bulb ablations (OBX), and allowed to recover 24-72 h before the piriform cortex and hippocampal dentate gyrus were processed for terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling [TUNEL] of apoptotic cells. OBX transiently increased TUNEL-positive cells in the ipsilateral piriform cortex and dentate gyrus. Increased TUNEL-labeling was apparent within 24h in both structures, but was more extensive and prolonged in piriform cortex. The results suggest a trans-synaptic regulation of cell survival through at least two synapses.

A Circadian Clock in the Olfactory Bulb Controls Olfactory Responsivity

Recently, it has been shown that multiple mammalian cell types express daily rhythms in vitro. Although the suprachiasmatic nucleus (SCN) of the hypothalamus is known to regulate a wide range of circadian behaviors, the role for intrinsic rhythmicity in other tissues is unknown. We tested whether the main olfactory bulb (OB) of mice mediates daily changes in olfaction. We found circadian rhythms in cedar oil-induced c-Fos, a protein marker of cellular excitation, in the mitral and granular layers of the OB and in the piriform cortex (PC). These oscillations persisted in constant darkness with a fourfold change in amplitude and a peak approximately 4 h after the onset of daily locomotor activity. Electrolytic lesions of the SCN abolished circadian locomotor rhythms, but not odor-induced c-Fos rhythms in the OB or PC. Furthermore, removal of the OB abolished spontaneous circadian cycling of c-Fos in the PC, shortened the free-running period of locomotor rhythms, and accelerated re-entrainment after a 6 h advance and slowed re-entrainment after a 6 h delay in the light schedule. OB ablation or odorant altered the amplitude of c-Fos rhythms in the SCN and ablation of one OB abolished c-Fos rhythms in the ipsilateral PC, but not in the contralateral OB and PC. We conclude that the OB comprises a master circadian pacemaker, which enhances olfactory responsivity each night, drives rhythms in the PC, and interacts with the SCN to coordinate other daily behaviors.

Transient and Widespread Astroglial Activation in the Brain after a Striatal 6-Ohda-Induced Partial Lesion of the Nigrostriatal System

The authors have previously described astroglial activation in the ipsilateral nigrostriatal system and ventral tegmental area following small doses of 6-hydroxydopamine (6-OHDA) injected unilaterally in the striatum. This article further evaluated astroglial reactivity in several brain regions after striatal 6-OHDA-induced punctate lesion in the nigrostriatal pathway. Adult male Wistar rats received a unilatera1 stereotaxical injection of the 6-OHDA (8 μg/4 μl) in the neostriatum and sacrificed 1 or 3 weeks later. Control animals received only solvent. Immunohistochemistry was employed for visualization of the tyrosine hydroxylase (TH), marker for dopamine cells, and glial fibrillary acidic protein (GFAP), marker for astrocytes. TH immunoreactive terminals disappeared in the striatum close to the injection site and a disappearance of a small number of a defined population of dopamine cell bodies was observed in the ipsilateral pars compacta of the substantia nigra (SNc). No dopamine lesion was detected in the contralateral nigrostriatal pathway. Astroglial reaction was seen close to the lesion in the neostriatum and in the ipsilateral SNc of the 1 week 6-OHDA lesioned rats. Specific stereological tools employing point intercepts and rotator, revealed an increased presence of reactive astrocytes in many forebrain regions like frontal, parietal and piriform cortex, septum, neostriatum and SNc, bilaterally, and also corpus callosum after 1 week of 6-OHDA injection. The astroglial activation was characterized by increases in the size of the cell body and/or processes. Astrocytic reaction was found only in the ipsilateral nigrostriatal pathway by 3 weeks of 6-OHDA, a slight activation also remaining in the ipsilateral septum and piriform cortex. Astrocytic reaction was seen in the solvent-injected rats only in the neostriatum close to the needle track. The transient widespread astroglial reaction observed in many brain regions following a striatal injection of 6-OHDA may represent a global paracrine trophic response in the brain.

Trans-neuronal regulation of cortical apoptosis in the adult rat olfactory system

Previous work has identified a population of neurons within the anterior piriform cortex that undergo rapid apoptosis following de-afferentation by olfactory bulbectomy in adult rats. The specific initiation signal for apoptosis in this paradigm is unknown, but may include an activity-dependent trans-neuronal cascade. The present report examined the effect of adult-onset unilateral naris occlusion, which reduces olfactory bulb afferent excitation of piriform cortex, on apoptosis (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling [TUNEL]) in the rat anterior piriform cortex. Adult Long-Evans hooded rats received unilateral naris occlusion or a control manipulation and were sacrificed after 1, 5, 7, 10 or 20 days later. For comparison, a second group of rats received a unilateral bulbectomy and were sacrificed 24 h later. Counts of TUNEL-stained cell profiles were performed for layers I/II and layer III of the anterior piriform cortex ipsilateral and contralateral to the manipulation. The results confirmed that unilateral bulbectomy produced a dramatic increase in TUNEL labeling in layers I/II of the ipsilateral piriform cortex 24 h after bulbectomy. Unilateral naris closure also produced enhanced TUNEL labeling, although the magnitude of the effect was less than that produced by bulbectomy, and enhanced TUNEL labeling was apparent both ipsilateral and contralateral to the sealed naris compared to controls. Deprivation-induced TUNEL labeling was detectable by 24 h post-closure, peaked at 5 days and was no different from controls by 20 days post-closure. Neither bulbectomy nor naris closure affected TUNEL labeling in layer III. Together, these results suggest that there is a population of superficial cells in piriform cortex whose survival is tightly regulated by sensory input.

Secondary generalization of hippocampal kindled seizures in rats: examining the role of the piriform cortex

A primary feature of epilepsy is the potential for focal seizures to recruit distant structures and generalize into convulsions. Key to understanding generalization is to identify critical structures facilitating the transition from focal to generalized seizures. In kindling, development of a primary site leads progressively to secondarily generalized convulsions. In addition, subsequent kindling of a secondary site results in rapid kindling from that site, presumably because of its facilitated access to the primary kindled network. Here, we investigated the role of the piriform cortex in convulsive generalization from a secondary site kindled in the hippocampus after primary site amygdala kindling. In a necessarily complicated design, rats initially experienced forebrain commissurotomy to lateralize the experiment to one hemisphere. Then the amygdala was kindled and, 3 weeks later, it was electrically-triggered into status epilepticus, which destroyed the ipsilateral piriform cortex. This experience occurred several days before secondary site kindling of the dorsal hippocampus. In rats with complete piriform cortex loss, there was no disruption in kindling or convulsive seizure expression from the hippocampus. However, when damage also involved parts of the perirhinal, insular and entorhinal cortices, convulsive expression was blocked. Although other evidence suggests that piriform lesions affect generalization of primary site kindling, the present study shows that they do not alter secondary site kindling in the dorsal hippocampus. The additional involvement of parahippocampal cortical areas in convulsive expression suggests an important functional association between these cortical regions and the hippocampus in seizure propagation and clinical expression.

Learning and memory of cue-reward association meaning by modifications of synaptic efficacy in dentate gyrus and piriform cortex.

This article begins with a review of recent experiments investigating the synaptic efficacy changes occurring in rat dentate gyrus and piriform cortex during an associative olfactory task. In all these experiments, animals were trained to discriminate among an artificial cue, a patterned electrical stimulation distributed to the lateral olfactory tract associated with a water reward, and a natural odor associated with a flash of light. Monosynaptic field potential responses evoked by single electrical stimuli to the lateral olfactory tract were recorded in the ipsilateral piriform cortex before and just after each training session. Monosynaptic field and polysynaptic field potentials evoked by single electrical stimuli applied respectively to the lateral perforant pathway and lateral olfactory tract were also recorded in ipsilateral dentate gyrus. The results showed an increase in synaptic efficacy subsequent to the first training session in the dentate gyrus network when compared with piriform cortex at the later stage of the learning. The early increase of monosynaptic response in the dentate gyrus was observed immediately after the first learning session but disappeared 24 h later. Inversely, a synaptic depression developed across sessions, becoming significant at the onset of the last (fifth) session. The polysynaptic potential recorded in this structure increased substantially when rats began to discriminate the leaming cues, usually after the second or third learning session. Then, from the third to the fifth session, an LTP like-phenomenon appeared in piriform cortex when rats perfectly mastered the associations. Experiments using high-frequency stimulation to prevent changes in gyrus dentatus indicated that the onset of the observed depression was necessary for the learning of the olfactory associations. The fact that hippocampal and cortical neuronal networks exhibited different timing in synaptic efficacy changes could physiologically explain learning and memory processes.

Expression of an embryonic intermediate filament protein in amygdaloid kindled rats

Amygdaloid kindling is well known as an experimental model of temporal lobe epilepsy. However, the mechanism of kindling epileptogenesis remains unclear. To examine the remodelling process in kindling, we performed immunohistochemistry of nestin, an embryonic intermediate neurofilament protein, in amygdaloid kindled rats. In rats expressing focal seizures (kindling stage C3), nestin immunoreactive cells (NIC) were detected at ipsilateral piriform cortex (PC) and ipsilateral perirhinal cortex (PRh), and at PC bilaterally in fully kindled rats expressing secondary generalized seizures (kindling stage C5). Double staining with glial fibrillary acidic protein revealed that almost all reactive astrocytes at PC express nestin immunoreactivity. These results suggest that glial NIC may participate in the remodelling process at the PC and PRh areas. This is the first report of nestin expression in kindling and suggests that glial nestin at PC and PRh may play a significant role in permanent epileptogenesis in kindling.

Regional expression and role of cyclooxygenase-2 following experimental traumatic brain injury.

Prostaglandins, potent mediators of inflammation, are generated from arachidonic acid (AA) via the action of cyclooxygenase-1 and -2 (COX-1 and COX-2). In this study, we report that lateral cortical impact injury in rats significantly increases COX-2 protein levels both in the cortex surrounding the injury site and the ipsilateral hippocampus. COX-2 protein level was elevated as early as 3 h postinjury and persisted for up to 3 days. Increases in immunoreactivity were detected not only in the adjacent cortex and hippocampus, but were also observed in the contralateral cortex and hippocampus, the ipsilateral piriform cortex and the ipsilateral amygdaloid complex. COX-2 immunoreactive cells appear morphologically normal and do not present any of the characteristic features of apoptosis. Double immunostaining experiments using either a neuron-specific or an astroglial-specific marker show that the expression of COX-2 is localized almost exclusively in neuronal cells. Administration of the COX-2 inhibitor 4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfona mide (celecoxib, marketed as Celebrex) worsens motor, but not cognitive, performance, suggesting that COX-2 induction following traumatic brain injury may play a protective role.

Increased striatal proenkephalin mRNA subsequent to production of spreading depression in rat cerebral cortex: activation of corticostriatal pathways?

Cortical Spreading Depression (CSD) is a slowly propagating wave of depolarization and negative interstitial DC potential, that when induced in the rat brain extends across the entire homolateral hemisphere. Despite evidence that CSD does not penetrate into subcortical regions, neurochemical changes in areas anatomically connected to cortex have been reported. In this study in situ hybridization histochemistry was used to examine the levels of cholecystokinin (CCK), proenkephalin (ENK) and prodynorphin (DYN) mRNA in cortex and forebrain basal ganglia following KCl-induced CSD. Unilateral CSD was induced by topical application of 3 M KCl (∼10 μl) onto the right parietal cortex for 10 min and rats were then killed 1–6 h and 1–28 days later. CCK mRNA levels were increased ( P<0.01) in the ipsilateral neocortex 3 h after CSD (13% above levels in contralateral side), reached a peak at 2 days (∼70%) and were still elevated at 7 (30%) but not, 14 or 28 days later. Unilateral CSD also produced a rapid and sustained increase ( P<0.05) in ENK mRNA in ipsilateral piriform cortex (from 3 h to 2 days; 70–250% above contralateral), and a delayed increase in caudate putamen and olfactory tubercle at 1 and 2 days (∼25% in both regions), but levels were again equivalent to control at 7 days and beyond. In contrast, no marked changes in neocortical ENK mRNA, or DYN mRNA in both cortex and basal ganglia, were observed under these conditions. These findings demonstrate that CSD has specific, rapid and long-lasting effects on neuropeptide expression in neocortex and subcortical areas. CSD-induced changes in mesostriatal ENK mRNA are proposed to reflect synaptic activation of local neurons via cortical afferent projections.

Amygdala-kindling induces a lasting reduction of GABA-immunoreactive neurons in a discrete area of the ipsilateral piriform cortex.

Several lines of evidence indicate a critical role of the piriform cortex (PC) in the kindling model of temporal lobe epilepsy, suggesting that the PC is part of an epileptic network that is pivotal in the genesis of kindling, facilitating, and intensifying the spread of seizures from a focus in amygdala, hippocampus, or other limbic brain regions to cortical and subcortical regions. Kindling of the amygdala has been shown to induce long-lasting changes in synaptic efficacy in the ipsilateral PC comparable to abnormalities seen in epileptic foci, but the neurochemical alterations possibly underlying these functional changes are not known. The possibility that the enhanced excitability of the PC in response to kindling is related to a reduction of GABAergic neurotransmission prompted us to examine if a lasting reduction in GABA-immunoreactive PC neurons is detectable after kindling of the basolateral amygdala (BLA) in rats. Furthermore, GABA immunoreactivity was determined in the BLA in order to investigate whether GABAergic neurons decrease in focal tissue, as previously suggested by neurochemical and immunocytochemical studies in amygdala-kindled rats. Three groups of age-matched rats were used: (1) a group of rats that was kindled via electrical stimulation by a bipolar electrode implanted in the right BLA, (2) a group of BLA-implanted but nonstimulated rats, and (3) a group of non-implanted, naive control rats. The kindled rats were sacrificed 40 days after the last fully kindled seizure. The two other groups of rats were sacrificed together with the kindled rats on the same days, and tissues from kindled and control rats were treated concurrently throughout the immunohistochemical analysis. GABA neurons were stained by a monoclonal antibody to GABA. Kindling of the BLA led to a pronounced decrease in the number of GABA immunoreactive neurons in the ipsi- and contralateral BLA at all section levels examined. In the PC, no significant differences between groups were seen in the contralateral hemisphere, while a significant reduction in GABA immunoreactive cells was observed in the transition zone between anterior and posterior PC in the hemisphere ipsilateral to the BLA electrode. The present findings add to the accumulating evidence that the PC is critically involved in kindling-induced epileptogenesis. The data furthermore substantiate that the PC is not a homogeneous structure but that there are differences along the anterior-posterior axis of this region in neurochemical (and most certainly also functional) consequences in response to kindling stimulation from other limbic brain regions.

Zinc transport in the rat olfactory system

To study zinc (Zn) mobility in the rat olfactory tract, brain distribution of 65Zn after injection into the olfactory bulb or amygdaloid nuclei was analyzed by autoradiography. Twenty-four hours after 65Zn injection into the olfactory bulb, 65Zn was distributed in the ipsilateral piriform cortex, amygdaloid nuclei and the anterior commissure. Moreover, in the case of injection of a higher level of 65Zn into the olfactory bulb, 65Zn was distributed in the ipsilateral entorhinal cortex in addition to the above regions. Twenty-four hours after 65Zn injection into the amygdaloid nuclei, 65Zn was distributed in the ipsilateral piriform and entorhinal cortex. These results suggest that Zn is intraneuronally transported along the olfactory tract. Zn may be taken up by the piriform neurons after release from the secondary olfactory neuron terminals and transported to the entorhinal area.

THE ROLE OF THE PIRIFORM CORTEX IN KINDLING

In epilepsy research, there is growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures, i.e. the most common type of seizures in human epilepsy. The PC (“primary olfactory cortex”) is the largest area of the mammalian olfactory cortex and receives direct projections from the olfactory bulb via the lateral olfactory tract (LOT). Beside the obvious involvement in olfactory perception and discrimination, the PC, because of its unique intrinsic associative fiber system and its various connections to and from other limbic nuclei, has been implicated in the study of memory processing, spread of excitatory waves, and in the study of brain disorders such as epilepsy with particular emphasis on the kindling model of temporal lobe epilepsy with complex partial seizures. The interest in the kindling model is based primarily on the following observations. (1) the PC contains the most susceptible neural circuits of all forebrain regions for electrical (or chemical) induction of limbic seizures. (2) During electrical stimulation of other limbic brain regions, broad and large afterdischarges can be observed in the ipsilateral PC, indicating that the PC is activated early during the kindling process. (3) The interictal discharge, which many consider to be the hallmark of epilepsy, originates in the PC, independent of which structure serves as the kindled focus. (4) Autoradiographic studies of cerebral metabolism in rat amygdala kindling show that, during focal seizures, the area which exhibits the most consistent increase in glucose utilization is the ipsilateral paleocortex, particularly the PC. (5) During the commonly short initial afterdischarges induced by stimulation of the amygdala at the early stages of kindling, the PC is the first region that exhibits induction of immediate-early genes, such as c-fos. (6) The PC is the most sensitive brain structure to brain damage by continuous or frequent stimulation of the amygdala or hippocampus. (7) Amygdala kindling leads to a circumscribed loss of GABAergic neurons in the ipsilateral PC, which is likely to explain the increase in excitability of PC pyramidal neurons during kindling. (8) Kindling of the amygdala or hippocampus induces astrogliosis in the PC, indicating neuronal death in this brain region. Furthermore, activation of microglia is seen in the PC after amygdala kindling. (9) Complete bilateral lesions of the PC block the generalization of seizures upon kindling from the hippocampus or olfactory bulb. Incomplete or unilateral lesions are less effective in this regard, but large unilateral lesions of the PC and adjacent endopiriform nucleus markedly increase the threshold for induction of focal seizures from stimulation of the basolateral amygdala (BLA) prior to and after kindling, indicating that the PC critically contributes to regulation of excitability in the amygdala. (10) Potentiation of GABAergic neurotransmission in the PC markedly increases the threshold for induction of kindled seizures via stimulation of the BLA, again indicating a critical role of the PC in regulation of seizure susceptibility of the amygdala. Microinjections of NMDA antagonists or sodium channel blockers into the PC block seizure generalization during kindling development. (11) Neurophysiological studies on the amygdala-PC slice preparation from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility to evoked burst responses. The epileptiform potentials in PC slice preparations from kindled rats seem to originate in neurons at the deep boundary of PC. Spontaneous firing and enhanced excitability of PC neurons in response to kindling from other sites is also seen in vivo, substantiating the fact that kindling induces long-lasting changes in the PC comparable to abnormalities seen in primary foci. Taken together, these observations indicate that the PC might be part of an epileptic network which is pivotal in the genesis of kindling, facilitating and intensifying the spread of seizures from a focus in amygdala or hippocampus to cortical and subcortical regions along pathways that also are utilized in normal movements. Although direct evidence implicating the PC in the pathogenesis of human epilepsy is not yet available, the experimental data reviewed in this paper should initiate clinical studies on the potential role of this brain structure as a pacemaker or secondary focus in TLE and other types of epilepsy. Copyright © 1996 Elsevier Science Ltd.