Asla Pitkänen

Organization of intra-amygdaloid circuitries in the rat : an emerging framework for understanding functions of the amygdala

The amygdala is located in the medial aspects of the temporal lobe. In spite of the fact that the amygdala has been implicated in a variety of functions, ranging from attention to memory to emotion, it has not attracted neuroscientists to the same extent as its laminated neighbours, in particular the hippocampus and surrounding cortex. However, recently, principles of information processing within the amygdala, particularly in the rat, have begun to emerge from anatomical, physiological and behavioral studies. These findings suggest that after the stimulus enters the amygdala, the highly organized intra-amygdaloid circuitries provide a pathway by which the representation of a stimulus becomes distributed in parallel to various amygdaloid nuclei. As a consequence, the stimulus representation may become modulated by different functional systems, such as those mediating memories from past experience or knowledge about ongoing homeostatic states. The amygdaloid output nuclei, especially the central nucleus, receive convergent information from several other amygdaloid regions and generate behavioral responses that presumably reflect the sum of neuronal activity produced by different amygdaloid nuclei.

Reciprocal connections between the amygdala and the hippocampal formation, perirhinal cortex, and postrhinal cortex in rat. A review.

Abstract: Recent anterograde and retrograde studies in the rat have provided detailed information on the origin and termination of the interconnections between the amygdaloid complex and the hippocampal formation and parahippocampal areas (including areas 35 and 36 of the perirhinal cortex and the postrhinal cortex). The most substantial inputs to the amygdala originate in the rostral half of the entorhinal cortex, the temporal end of the CA1 subfield and subiculum, and areas 35 and 36 of the perirhinal cortex. The amygdaloid nuclei receiving the heaviest inputs are the lateral, basal, accessory basal, and central nuclei as well as the amygdalohippocampal area. The heaviest projections from the amygdala to the hippocampal formation and the parahippocampal areas originate in the lateral, basal, accessory basal, and posterior cortical nuclei. These pathways terminate in the rostral half of the entorhinal cortex, the temporal end of the CA3 and CA1 subfields or the subiculum, the parasubiculum, areas 35 and 36 of the perirhinal cortex, and the postrhinal cortex. The connectional data are summarized and the underlying principles of organization of these projections are discussed.

Is epilepsy a progressive disorder? Prospects for new therapeutic approaches in temporal-lobe epilepsy

During the past decade, it has become apparent that neural circuits undergo activity-dependent reorganisation. In pathological disorders with recurring episodes of excessive neural activity, such as temporal-lobe epilepsy, brain circuits can undergo continual remodelling. For clinical practice, seizure-induced remodelling implies that after a diagnosis of epilepsy, recurring seizures can cause continuing neural reorganisation and potentially contribute to progressive severity of the epilepsy and to cognitive and behavioural consequences. The alterations induced by seizures include neuronal death and birth, axonal and dendritic sprouting, gliosis, molecular reorganisation of membrane and extracellular-matrix proteins, and intermediates involved in cellular homoeostasis. These changes are influenced by genetic background and seizure type, thus identification of genetic risk factors should be a priority. Therapeutic modification of seizure-induced molecular and cellular responses offers new opportunities for intervention beyond seizure suppression.

INTRINSIC CONNECTIONS OF THE RAT AMYGDALOID COMPLEX : PROJECTIONS ORIGINATING IN THE ACCESSORY BASAL NUCLEUS

The amygdaloid complex plays an important role in the detection of emotional stimuli, the generation of emotional responses, the formation of emotional memories, and perhaps other complex associational processes. These functions depend upon the flow of information through intricate and poorly understood circuitries within the amygdala. As part of an ongoing project aimed at further elucidating these circuits, we examined the intra‐amygdaloid connections of the acessory basal nucleus in the rat. In addition, we examined connections of the anterior cortical nucleus and amygdalahippocampal area to determine whether portions of these nuclei should be included in the accessory basal nucleus (as some earlier studies suggest). Phaseolus vulgaris leucoagglutinin was injected into different rostrocaudal levels of the accessory basal nucleus (n = 12) or into the anterior cortical nucleus (n = 3) or amygdalahippocampal area (n = 2). The major intra‐amygdaloid projections from the accessory basal nucleus were directed to the medial and capsular divisions of the central nucleus, the medial division of the amygdalohippocampal area, the medial division of the lateral nucleus, the central division of the medial nucleus, and the posterior cortical nucleus. The projections originating in the anterior cortical nucleus and the lateral division of the amygdalohippocampal area differed from those originating in the accessory basal nucleus, which suggests that these areas are not part of the deep amygdaloid nuclei have different intra‐amygdaloid connections. The pattern of these various connections suggests that information entering the amygdala from different sources can be integrated only in certain amygdaloid regions.

PROJECTIONS FROM THE LATERAL, BASAL, AND ACCESSORY BASAL NUCLEI OF THE AMYGDALA TO THE HIPPOCAMPAL FORMATION IN RAT

The amygdaloid complex and hippocampal formation mediate functions involving emotion and memory. To investigate the connections that regulate the interactions between these regions, we injected the anterograde tracer Phaseolus vulgaris‐leucoagglutinin into various divisions of the lateral, basal, and accessory basal nuclei of the rat amygdala. The heaviest projection to the entorhinal cortex originates in the medial division of the lateral nucleus which innervates layer III of the ventral intermediate and dorsal intermediate subfields. In the basal nucleus, the heaviest projection arises in the parvicellular division and terminates in layer III of the amygdalo‐entorhinal transitional subfield. In the accessory basal nucleus, the parvicellular division heavily innervates layer V of the ventral intermediate subfield. The most substantial projection to the hippocampus originates in the basal nucleus. The caudomedial portion of the parvicellular division projects heavily to the stratum oriens and stratum radiatum of CA3 and CA1. The accessory basal nucleus projects to the stratum lacunosum‐moleculare of CA1. The subiculum receives a substantial input from the caudomedial parvicellular division. The parasubiculum receives dense projections from the caudal portion of the medial division of the lateral nucleus, the caudomedial parvicellular division of the basal nucleus, and the parvicellular division of the accessory basal nucleus. Our data show that select nuclear divisions of the amygdala project to the entorhinal cortex, hippocampus, subiculum, and parasubiculum in segregated rather than overlapping terminal fields. These data suggest that the amygdaloid complex is in a position to modulate different stages of information processing within the hippocampal formation. J. Comp. Neurol. 403:229–260, 1999.

Amyloid β-Induced Neuronal Hyperexcitability Triggers Progressive Epilepsy

Alzheimers disease is associated with an increased risk of unprovoked seizures. However, the underlying mechanisms of seizure induction remain elusive. Here, we performed video-EEG recordings in mice carrying mutant human APPswe and PS1dE9 genes (APdE9 mice) and their wild-type littermates to determine the prevalence of unprovoked seizures. In two recording episodes at the onset of amyloid β (Aβ) pathogenesis (3 and 4.5 months of age), at least one unprovoked seizure was detected in 65% of APdE9 mice, of which 46% had multiple seizures and 38% had a generalized seizure. None of the wild-type mice had seizures. In a subset of APdE9 mice, seizure phenotype was associated with a loss of calbindin-D28k immunoreactivity in dentate granular cells and ectopic expression of neuropeptide Y in mossy fibers. In APdE9 mice, persistently decreased resting membrane potential in neocortical layer 2/3 pyramidal cells and dentate granule cells underpinned increased network excitability as identified by patch-clamp electrophysiology. At stimulus strengths evoking single-component EPSPs in wild-type littermates, APdE9 mice exhibited decreased action potential threshold and burst firing of pyramidal cells. Bath application (1 h) of Aβ1–42 or Aβ25–35 (proto-)fibrils but not oligomers induced significant membrane depolarization of pyramidal cells and increased the activity of excitatory cell populations as measured by extracellular field recordings in the juvenile rodent brain, confirming the pathogenic significance of bath-applied Aβ (proto-)fibrils. Overall, these data identify fibrillar Aβ as a pathogenic entity powerfully altering neuronal membrane properties such that hyperexcitability of pyramidal cells culminates in epileptiform activity.

A new model of chronic temporal lobe epilepsy induced by electrical stimulation of the amygdala in rat

Spontaneous seizures are the hallmark of human epilepsy but they do not occur in most of the epilepsy models that are used to investigate the mechanisms of epilepsy or to test new antiepileptic compounds. This study was designed to develop a new focal epilepsy model that mimics different aspects of human temporal lobe epilepsy (TLE), including the occurrence of spontaneous seizures. Self-sustained status epilepticus (SSSE) lasting for 6-20 h was induced by a 20-30 min stimulation of the lateral nucleus of the amygdala (100 ms train of 1 ms, 60 Hz bipolar pulses, 400 microA, every 0.5 s). Stimulated rats (n = 16) were monitored with a video-EEG recording system every other day (24 h/day) for 6 months, and every other video-EEG recording was analyzed. Spontaneous epileptic seizures (total number 3698) were detected in 13 of the 15 animals (88%) after a latency period of 6 to 85 days (median 33 days). Four animals (31%) had frequent (697-1317) seizures and 9 animals (69%) had occasional seizures (1-107) during the 6-months follow-up period. Fifty-seven percent of the seizures occurred during daytime (lights on 07:00-19:00 h). At the end of the follow-up period, epileptic animals demonstrated impaired spatial memory in the Morris water-maze. Histologic analysis indicated neuronal loss in the amygdala, hippocampus, and surrounding cortical areas, and mossy fiber sprouting in the dentate gyrus. The present data indicate that focal stimulation of the amygdala initiates a cascade of events that lead to the development of spontaneous seizures in rats. This model provides a new tool to better mimic different aspects of human TLE for investigation of the pathogenesis of TLE or the effects of new antiepileptic compounds on status epilepticus, epileptogenesis, and spontaneous seizures.

A model of posttraumatic epilepsy induced by lateral fluid-percussion brain injury in rats

Although traumatic brain injury is a major cause of symptomatic epilepsy, the mechanism by which it leads to recurrent seizures is unknown. An animal model of posttraumatic epilepsy that reliably reproduces the clinical sequelae of human traumatic brain injury is essential to identify the molecular and cellular substrates of posttraumatic epileptogenesis, and perform preclinical screening of new antiepileptogenic compounds. We studied the electrophysiologic, behavioral, and structural features of posttraumatic epilepsy induced by severe, non-penetrating lateral fluid-percussion brain injury in rats. Data from two independent experiments indicated that 43% to 50% of injured animals developed epilepsy, with a latency period between 7 weeks to 1 year. Mean seizure frequency was 0.3+/-0.2 seizures per day and mean seizure duration was 113+/-46 s. Behavioral seizure severity increased over time in the majority of animals. Secondarily-generalized seizures comprised an average of 66+/-37% of all seizures. Mossy fiber sprouting was increased in the ipsilateral hippocampus of animals with posttraumatic epilepsy compared with those subjected to traumatic brain injury without epilepsy. Stereologic cell counts indicated a loss of dentate hilar neurons ipsilaterally following traumatic brain injury. Our data suggest that posttraumatic epilepsy occurs with a frequency of 40% to 50% after severe non-penetrating fluid-percussion brain injury in rats, and that the lateral fluid percussion model can serve as a clinically-relevant tool for pathophysiologic and preclinical studies.

Amygdala damage in experimental and human temporal lobe epilepsy

The amygdala complex is one component of the temporal lobe that may be damaged unilaterally or bilaterally in children and adults with temporal lobe epilepsy (TLE) or following status epilepticus. Most MR (magnetic resonance) imaging studies of epileptic patients have shown that volume reduction of the amygdala ranges from 10-30%. In the human amygdala, neuronal loss and gliosis have been reported in the lateral and basal nuclei. Studies in rats have more specifically identified the amygdaloid regions that are sensitive to status epilepticus-induced neuronal damage. These areas include the medial division of the lateral nucleus, the parvicellular division of the basal nucleus, the accessory basal nucleus, the posterior cortical nucleus, and portions of the anterior cortical and medial nuclei. Otherwise, other amygdala nuclei, such as the magnocellular and intermediate divisions of the basal nucleus and the central nucleus, remain relatively well preserved. Amygdala kindling studies in rats have shown that the density of a subpopulation of GABAergic inhibitory neurons that also contain somatostatin may be reduced even after a low number of generalized seizures. While analyses of histological sections and MR images indicate that in approximately 10% of TLE patients, seizure-induced damage is isolated to the amygdala, more often amygdala damage is combined with damage to the hippocampus and/or other brain areas. Moreover, recent data from rodents and nonhuman primates suggest that structural and functional alterations caused by seizure activity originating in the amygdala are not limited to the amygdala itself, but may also affect other temporal lobe structures. The information gathered so far on damage to the amygdala in epilepsy or after status epilepticus suggests that local alterations in inhibitory circuitries may contribute to a lowered seizure threshold and greater excitability within the amygdala. Furthermore, damage to select nuclei in the amygdala may predict impairment of performance in behavioral tasks that depend on the integrity of the amygdaloid circuits.

Distribution of parvalbumin, calretinin, and calbindin-D28k immunoreactivity in the rat amygdaloid complex and colocalization with ?-aminobutyric acid

To understand the organization of inhibitory circuitries in the rat amygdala, the distribution of parvalbumin, calretinin, and calbindin immunoreactivity was investigated in the rat amygdaloid complex. Colocalization of various calcium‐binding proteins with the inhibitory transmitter γ‐aminobutyric acid (GABA) was studied by using the mirror technique. Parvalbumin‐immunoreactive (‐ir) elements were located mostly in the deep amygdaloid nuclei, whereas the calretinin‐ir and calbindin‐ir staining were most intense in the cortical nuclei as well as in the central nucleus and the amygdalohippocampal area . Second, the distribution of immunopositive neurons largely parallelled the distribution of terminal and neuropil labeling. Third, immunostained neurons could be divided into four major morphologic types (types 1–4) based on the characteristics of the somata and the dendritic trees. The fourth lightly stained neuronal type that had a pyramidal GABA‐negative soma was observed only in calretinin and calbindin preparations. Fourth, parvalbumin‐ir terminals formed basket‐like plexus and cartridges, which suggests that parvalbumin labels GABAergic inhibitory basket cells and axo‐axonic chandelier cells, respectively. Colocalization studies indicated that 521 of 553 (94%) of parvalbumin‐ir, 419 of 557 (75%) of calbindin‐ir, and 158 of 657 (24%) of calretinin‐ir neurons were GABA‐positive in the deep amygdaloid nuclei. A high density of large GABA‐negative calbindin‐ir neurons was observed caudally in the medial division of the lateral nucleus and GABA‐negative calretinin‐ir neurons were observed in the magnocellular division of the accessory basal nucleus as well as in the intermediate and parvicellular divisions of the basal nucleus . These data suggest that in various amygdaloid areas, neuronal excitability is controlled by GABAergic neurons that contain different calcium‐binding proteins. The appearance of basket‐like plexus and cartridges in the parvalbumin preparations, but not in calretinin preparations, suggests that like in the hippocampus, the distribution of inhibitory terminals in the dendritic and perisomatic regions of postsynaptic neurons in the rat amygdala is organized in a topographic manner. J. Comp. Neurol. 426:441–467, 2000.