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PubMed1999

Amygdala enlargement in dysthymia--a volumetric study of patients with temporal lobe epilepsy.

BACKGROUND: Previous studies indicated an important role of the amygdala for emotional information processing. We investigated a possible relationship between amygdala volumes, aggressive behavior, and dysthymia, in patients with temporal lobe epilepsy (TLE). METHODS: Patients with TLE with and without aggression or dysthymia and healthy volunteers were assessed using quantitative MRI. Amygdala volumes were measured in a blinded fashion and corrected for total brain volumes. RESULTS: There was a highly significant enlargement of left and right amygdala volumes in patients with dysthymia (right side, p < .000; left side, p = .001). We found a significant positive correlation between left amygdala volumes (p = .02) and a trend towards positive correlation between right amygdala volumes and depression (p = .06), as measured with the Beck Depression Inventory. Amygdala volumes of females were significantly larger than those of males (left side: p = .005; right side: p = .06). CONCLUSIONS: This is the second report of a relationship between amygdala volumes and depressed mood, confirming an earlier finding in patients with bipolar disease, and the first study reporting a correlation between amygdala volumes and depression. Increased processing of emotional information might increase amygdala blood flow and subsequently, result in amygdala enlargement.

Adolescent↗
PubMed1996

Opioid antagonists in the periaqueductal gray inhibit morphine and beta-endorphin analgesia elicited from the amygdala of rats.

In addition to brainstem sites of action, analgesia can be elicited following amygdala microinjections of morphine and mu-selective opioid agonists. The present study examined whether opioid analgesia elicited by either morphine or beta-endorphin in the amygdala could be altered by either the general opioid antagonist, naltrexone, the mu-selective antagonist, beta-funaltrexamine (BFNA) or the delta 2 antagonist, naltrindole isothiocyanate (Ntii) in the periaqueductal gray (PAG). Both morphine (2.5-5 micrograms) and beta-endorphin (2.5-5 micrograms) microinjected into either the baso-lateral or central nuclei of the amygdala significantly increased tail-flick latencies and jump thresholds in rats. The increases were far more pronounced on the jump test than on the tail-flick test. Placements dorsal and medial to the amygdala were ineffective. Naltrexone (1-5 micrograms) in the PAG significantly reduced both morphine (tail-flick: 70-75%; jump: 60-81%) and beta-endorphin (tail-flick: 100%; jump: 93%) analgesia elicited from the amygdala, indicating that an opioid synapse in the PAG was integral for the full expression of analgesia elicited from the amygdala by both agonists. Both BFNA (68%) and Ntii (100%) in the PAG significantly reduced morphine, but not beta-endorphin analgesia in the amygdala on the tail-flick test. Ntii in the PAG was more effective in reducing morphine (60%) and beta-endorphin (79%) analgesia in the amygdala on the jump test than BFNA (15-24%). Opioid agonist-induced analgesia in the amygdala was unaffected by opioid antagonists administered into control misplacements in the lateral mesencephalon, and the small hyperalgesia elicited by opioid antagonists in the PAG could not account for the reductions in opioid agonist effects in the amygdala. These data indicate that PAG delta 2, and to a lesser degree, mu opioid receptors are necessary for the full expression of morphine and beta-endorphin analgesia elicited from the amygdala.

Amygdala↗
PubMed1983

Response of ventral pallidal neurons to amygdala stimulation and its modulation by dopamine projections to nucleus accumbens.

It has been shown that the nucleus accumbens receives input from the amygdala and that mesolimbic dopaminergic projection from the ventral tegmental area (VTA) modulates the response of accumbens neurons to amygdala input. Since the nucleus accumbens projects to the ventral pallidum, the purpose of this study was to investigate, using electrophysiological techniques, whether or not the nucleus accumbens relays the projection from the amygdala to the ventral pallidum and whether or not the mesolimbic dopamine projection interacts with this pathway. Extracellular single-unit recordings were obtained from the ventral pallidum of urethan-anesthetized rats, and the responses of these neurons to electrical stimulation of the amygdala were investigated. Of 392 neurons tested, 36% were inhibited and 11% were excited following amygdala stimulation. Latency of onset of inhibitory responses showed a bimodal distribution with peaks in the ranges of 4-6 ms and 16-18 ms, respectively. Fifty-four percent of inhibitory responses with latencies greater than 12 ms were attenuated by 1) injection of procaine hydrochloride into the nucleus accumbens, or 2) injection of d-amphetamine into the nucleus accumbens, or 3) stimulation of VTA with a train of 10 pulses (10 Hz) prior to stimulation of amygdala. Acute administration of haloperidol intraperitoneally or injection of 6-hydroxydopamine into the ipsilateral VTA, 2 days prior to the recording experiment, reduced the attenuating effects of intra-accumbens injection of d-amphetamine and VTA conditioning stimulations on the inhibitory response of ventral pallidal neurons to amygdala stimulation. These results support the hypothesis that the nucleus accumbens provides a link between the amygdala and the ventral pallidum. Since the amygdala is a limbic structure and the ventral pallidum has possible connections with the extrapyramidal motor system, it is suggested that the amygdala to nucleus accumbens to ventral pallidum projection may be a bridge between the limbic and motor systems. We also suggest that this relay of output from the amygdala to the ventral pallidum via the nucleus accumbens is under the modulating influence of the mesolimbic dopamine projection from the ventral tegmental area.

Amygdala↗
PubMed2006

Association between amygdala volume and anxiety level: magnetic resonance imaging (MRI) study in autistic children.

Our objective was to evaluate brain-behavior relationships between amygdala volume and anxious/depressed scores on the Child Behavior Checklist in a well-characterized population of autistic children. Volumes for the amygdala, hippocampus, and whole brain were obtained from three-dimensional magnetic resonance images (MRIs) captured from 42 children who met the criteria for autistic disorder. Anxious/depressed symptoms were assessed in these children by the Anxious/Depressed subscale of the Child Behavior Checklist. To investigate the association between anxious/depressed scores on the Child Behavior Checklist and amygdala volume, data were analyzed using linear regression methods with Pearson correlation coefficients. A multivariate model was used to adjust for potential covariates associated with amygdala volume, including age at MRI and total brain size. We found that anxious/depressed symptoms were significantly correlated with increased total amygdala volume (r = .386, P = .012) and right amygdala volume (r = .469, P = .002). The correlation between anxious/depressed symptoms and left amygdala volume did not reach statistical significance (r = .249, P = .112). Child Behavior Checklist anxious/depressed scores were found to be a significant predictor of amygdala total (P = .014) and right amygdala (P = .002) volumes. In conclusion, we have identified a significant brain-behavior relationship between amygdala volume and anxious/depressed scores on the Child Behavior Checklist in our autistic cohort. This specific relationship has not been reported in autism. However, the existing literature on human psychiatry and behavior supports our reported evidence for a neurobiologic relationship between symptoms of anxiety and depression with amygdala structure and function. Our results highlight the importance of characterizing comorbid psychiatric symptomatology in autism. The abundance of inconsistent findings in the published literature on autism might reflect differences between study populations regarding age at MRI, level of impairment within autistic subjects, and underlying anxiety level in the selected study groups.

Adolescent↗
PubMed2006

The role of the amygdala and olfaction in unconditioned fear in developing rats.

Early in ontogeny, young rats must be able to detect dangerous stimuli and to exhibit appropriate defensive behaviors. Different nuclei of the amygdala mediate unconditioned and conditioned fear responses to threat in adult rats. The aim of this study was to determine the role of the amygdala in unlearned fear behavior in young rats. When exposed to an unfamiliar adult male rat, preweaning rat pups freeze, with peak levels on postnatal day 14 and declining levels on day 18. Pups were made anosmic to block olfactory input to the amygdala, and amygdala activation was assessed by quantifying the neuronal marker c-fos. Anosmic pups did not freeze in the presence of the male rat and had decreased c-fos expression in the medial amygdala on day 14 and in the medial and lateral amygdala on day 18. However, the decrease in freezing between days 14 and 18 was not associated with a decrease in c-fos expression in the medial amygdala. The medial and lateral amygdala were then inactivated by local muscimol infusion on day 14. Muscimol infusion into the medial amygdala decreased freezing to the male rat but not to a loud noise, whereas infusion into the lateral amygdala blocked freezing to a loud noise but not to the male. These findings indicate that different nuclei of the amygdala process sensory information of different modalities, mediate unconditioned freezing, and may be involved in developmental changes in the fear response in young rats.

Amygdala↗
PubMed1996

The amygdala: corticotropin-releasing factor, steroids, and stress.

The possible function of corticotropin-releasing factor (CRF), adrenal steroids, and gonadal steroids in amygdala-mediated responses to anxiogenic or stressful stimuli is reviewed. The amygdala is part of an endogenous CRF circuitry within the brain that mediates neuroendocrine, autonomic, and behavioral changes in response to stress. The amygdala contains CRF-expressing neurons that communicate with widespread regions of the neural axis. High densities of CRF, CRF-binding protein, and CRF receptors are located in the amygdala. Direct injections of CRF into the amygdala produce anxiety-like behaviors. Release of endogenous CRF can be measured in the amygdala during stress. Potent anxiolytic actions are observed when CRF receptor antagonists are administered into the amygdala. CRF-containing neurons of the amygdala can be directly modulated by alterations in circulating glucocorticoids through glucocorticoid receptors, which are expressed in amygdaloid CRF-containing neurons. Gonadal steroid hormone receptors are found in the amygdala. They are not located in CRF immunoreactive neurons, but they are located adjacent to CRF-expressing neurons and in amygdaloid neurons that are likely to participate in central autonomic and neuroendocrine circuitry. Differences are noted between the steroid influences in the amygdala of male and female animals. Also, evidence is reviewed suggesting a modulatory role in the amygdala for gonadal and adrenal steroids in behavioral, autonomic, and neuroendocrine responses to anxiogenic stimuli.

Amygdala↗
PubMed2002

Hippocampal hyperexcitability facilitates amygdala kindling in rats.

BACKGROUND & OBJECTIVES: The amygdala and hippocampus are recognized as the two important structures in the brain involved in the development and control of kindled seizures. The study on the precise interconnection between these two regions can provide important insights into the functional anatomy of complex partial seizures. In this study the effect of an experimentally increased excitability in hippocampal neurons, via hippocampal kindling, on the amygdala kindling rate was investigated in rats. METHODS: Animals were divided into four groups. Tripolar electrodes were implanted in the amygdala and CA1 region of the dorsal hippocampus of animals of Groups 1, 3 and 4. In Group 2 animals, tripolar electrodes were only implanted in the amygdala. In Group 1, one week after surgery, the rats were kindled first from the hippocampus and the next day kindled by amygdala stimulation. In Groups 2 and 3, one week after surgery, rats were kindled from the amygdala. Group 4 animals had a recovery period of one week plus 32 days, which was the mean of the hippocampal kindling rate in Group 1, and then were kindled from the amygdala. RESULTS: In Group 1, the amygdala kindling rate (n; number of days for which animals were stimulated before a stage 5 motor convulsion is triggered) and seizure stage at day n/2 were significantly facilitated and increased respectively. There was also a significant positive correlation between hippocampal and amygdala kindling rates. INTERPRETATION & CONCLUSION: Results obtained show that an increase in hippocampal excitability can facilitate kindling from the amygdala. Thus, it is suggested that the hippocampus has an important role in the development and propagation of seizures from the amygdala.

Amygdala↗
PubMed1993

Involvement of amygdala N-methyl-D-asparate receptors in long-term retention of an inhibitory avoidance response in rats.

This study examined the involvement of amygdala N-methyl-D-aspartate (NMDA) receptors in long-term retention of an inhibitory avoidance response. Rats bearing chronic cannulae implanted into the basolateral amygdala were trained on a one-trial inhibitory avoidance task and tested for retention 21 days later. They received intra-amygdala injections of vehicle (Veh) or 0.25, 1.25 or 5.0 micrograms of a competitive NMDA antagonist-2-amino-5-phosphonopentoic acid (AP5) either 5 min before training, immediately after training or 5 min before testing. Results indicated that pretraining intra-amygdala injections of AP5 at all doses impaired retention performance profoundly. Intra-amygdala injections of AP5 immediately after training caused a dose-dependent retention deficit: 0.25 microgram induced no deficit and 5.0 micrograms induced a great deficit. Immediate posttraining intra-amygdala injections of a non-competitive NMDA antagonist MK-801, also impaired retention but MK-801 given 2 hrs after training had no significant effect. In contrast to the marked amnestic effect produced by pre- or posttraining intra-amygdala injections of AP5, intra-amygdala injections of AP5 given just before retention tests had no discernible effect on retention performance. The retention deficit induced by pretraining intra-amygdala injections of 1.25 micrograms AP5 was ameliorated completely by N-methyl-DL-aspartate (0.25 microgram), but also partially by norepinephrine (0.2 microgram) infused into the amygdala immediately after training. However, posttraining infusion 0.2 microgram norepinephrine failed to attenuate significantly the amnestic effect induced by 5.0 micrograms AP5. These findings, taken together, suggest that NMDA receptors in the amygdala are normally involved in memory formation processing of affective experience.

2-Amino-5-phosphonovalerate↗
PubMedDate not supplied

[The neuronal organization of the amygdala-visceral reflex arch].

The paper summarizes the histomorphic, histochemical, autoradiographic, immune-histochemical and electrophysiological data on the structural-functional organization of the amygdalar complexes. While discussing the problem of neuron organization of the afferent and efferent groups in the amygdala-visceral arch the authors present their data of microelectrophysiological studies of the specific amygdala neuron reactions in response to stimulation of the vagus, ventric, and sciatic nerves, and of the second somatic sensor cortex zone. It was observed that interoceptive impulsation reaching the polysensor neurons of the central, corticomedial, and basolateral groups of the amygdala complex nuclei have no significant qualitative differences. A series of experiments demonstrated that the polysensor convergent amygdala neurons are also polyeffectoral. Comparative studies of the sympathetic discharges in the white connecting branches produced by stimulation of the above nuclear amygdala groups, confirmed absence of a clear topographic localization of the sympatho-activating structures within an amygdala. Comparative exploration of the parasympathetic discharges in the pelvis nerve discovered a discharge of complex configuration in response to simulation of the pelvis nerve nuclei. There was no notable topographic localization of the amygdala parasympathoactivating structures. The system's arterial pressure was studied in response to stimulation of various amygdala structures. Low-frequency stimulation (5 Hertz) of all amygdala structures produces depressor reactions, high-frequency stimulation (100 Hertz) produces pressor reactions. Microelectrophysiological data are provided on the specific reactions of the inspiration and expiration neurons of the bulbar respiratory centre under frequency stimulation of various amygdala structures. The authors have concluded that the bulbar-spinal mechanisms regulating the vegetative functions are implemented by polysensoral-polyeffectoral integrative amygdala neurons of the convergent-divergent type. Consequently, there are no separate populations of the sympatho-activating and parasympathoactivation neurons and of the selective structures regulating the respiratory and the cardiovascular functions.

Amygdala↗
PubMed1981

Direct and indirect pathways from the amygdala to the frontal lobe in rhesus monkeys.

To elucidate the anatomical relationships between the frontal association cortex and the limbic system in primates, projections from the amygdala to frontal cortex were studied in the rhesus monkey using retrograde and anterograde tracing methods. Following injections of horseradish peroxidase (HRP) into the orbital prefrontal cortex, the gyrus rectus, the superior frontal gyrus, and the anterior cingulate gyrus of the frontal lobe, labeled neurons were found in the basolateral, basomedial, or basal accessory nuclei of the amygdala. None of these nuclei contained labeled neurons following HRP injections into the principal sulcus or the lateral inferior convexity of the frontal lobe. This selective distribution of amygdala connections was confirmed by injection tritiated amino acids into the amygdala. Silver grains were present only over the orbital cortex and gyrus rectus on the ventral surface of the frontal lobe and over the superior prefrontal gyrus and anterior cingulate gyrus on the medial wall of the hemisphere, while the dorsolateral prefrontal cortex was free of radioactivity. The isotope injection of the amygdala also revealed a projection to the magnocellular moiety of the mediodorsal nucleus (MDmc) which is known to innervate the same ventromedial regions of the frontal lobe that receive direct connections from the amygdala. Although MDmc and amygdala project to the same cortical regions, their terminal fields are different. The direct amygdala input terminates in layer 1 in orbital cortex and gyrus rectus and layer 2 in the dorsomedial cortex and cingulate gyrus, while the thalamic input is primarily to layer 3 and, in some areas, also the superficial half of layer 1. These findings indicate that the frontal lobe of rhesus monkeys can be subdivided into two separable cortical regions: 1) A ventromedial region including the anterior cingulate gyrus which receives both direct (amygdalo-cortical) and indirect (amygdalo-thalamo-cortical) input from the amygdala; and 2) a dorsolateral frontal region which is essentially devoid of either direct or indirect amygdalofugal axons. On the basis of its selective relationship with the amygdala, the ventromedial region may be considered the "limbic" portion of the frontal association cortex.

Amygdala↗
PubMed1996

Parabrachial nucleus projection to the amygdala in the rat: electrophysiological and anatomical observations.

The amygdala, an important limbic forebrain centre, is the recipient of projections from a number of autonomic brainstem nuclei including the pontine parabrachial nucleus. This study examined the influence of electrical stimulation of the parabrachial nucleus on the excitability of amygdala neurons and their response to two cardiovascular stimuli, namely baroreceptor activation and the administration of systemic angiotensin II. We also defined the chemical identity of some amygdala neurons that receive parabrachial nucleus projections by combining the transport of the anterograde tracer Phaseolus vulgaris leucoagglutinin injected into the parabrachial nucleus with immunocytochemical labelling of neurotensin and galanin profiles within the amygdala. In urethane-anesthetized rats, stimulation of parabrachial nucleus evoked four basic types of synaptic responses in amygdala cells: (1) a short duration (< 100 ms) excitation in 75 of 167 neurons, (2) a longer duration (> 100 ms) excitatory response in 36 neurons, (3) an inhibitory response in 32 cells, and (4) more complex responses consisting of excitation-inhibition or inhibition-excitation sequences in the remainder of the cells. Thirty-seven of 72 amygdala neurons activated synaptically by parabrachial nucleus stimulation also responded to baroreceptor activation or intravenous angiotensin II. Anatomical data revealed the presence of Phaseolus vulgaris leucoagglutinin labelled terminals predominantly within the lateral, medial, and capsular subdivisions of the central nucleus of amygdala. Phaseolus vulgaris leucoagglutinin varicosities and boutons were observed apposed to the neurotensin and galanin neuronal perikarya within the central nucleus of amygdala. The electrophysiological results provide a framework whereby parabrachial nucleus efferents influence the activity of amygdala neurons that are responsive to cardiovascular stimuli. Furthermore, the anatomical data indicate that a portion of the parabrachial nucleus input is directed toward galanin and neurotensin neurons within the central nucleus of amygdala.

Amygdala↗
PubMed2004

Lateralization of amygdala activation: a systematic review of functional neuroimaging studies.

Functional neuroimaging studies of emotion processing consistently report amygdala activation. Most of these studies observed lateralized amygdala activity, indicative of a clear hemisphere-specific processing difference between the left and right amygdalae. Because individual studies use varying paradigms and are limited by statistical power and sensitivity, it has remained unclear whether the left or the right amygdala is more consistently involved in emotional processing. By combining results across 54 fMRI and PET studies in a metaanalysis, we sought to establish if a common pattern of lateralized amygdala activation exists. Our findings indicate that across studies, the left amygdala is more often activated than the right amygdala, suggesting different roles for the left and right amygdalae in emotional processing. Further analysis showed that this predominant left amygdala activation is not significantly related to stimulus type, task instructions, differential habituation rates of the left and right amygdalae or elaborate processing. The results are discussed in relation to methodological and theoretical issues regarding functional brain asymmetry.

Amygdala↗
PubMed2005

Amygdala kindling differentially regulates the expression of the elements involved in TRH transmission.

Subthreshold electrical stimulation of the amygdala (kindling) activates neuronal pathways increasing the expression of several neuropeptides including thyrotropin releasing-hormone (TRH). Partial kindling enhances TRH expression and the activity or its inactivating ectoenzyme; once kindling is established (stage V), TRH and its mRNA levels are further increased but TRH-binding and pyroglutamyl aminopeptidase II (PPII) activity decreased in epileptogenic areas. To determine whether variations in TRH receptor binding or PPII activity are due to regulation of their synthesis, mRNA levels of TRH receptors (R1, R2) and PPII were semi-quantified by RT-PCR in amygdala, frontal cortex and hippocampus of kindled rats sacrificed at stage II or V. Increased mRNA levels of PPII were found at stage II in amygdala and frontal cortex, and of pro-TRH and TRH-R2, in amygdala and hippocampus. At stage V, pro-TRH mRNA levels increased and those of PPII, decreased in the three regions; TRH-R2 mRNA levels diminished in amygdala and frontal cortex and of TRH-R1 only in amygdala. In situ hybridization analyses revealed, at stage II, enhanced TRH-R1 mRNA levels in dentate gyrus and amygdala while decreased in piriform cortex; those of TRH-R2 increased in amygdala, CA2, dentate gyrus, piriform cortex, thalamus and subiculum and of PPII, in CAs and piriform cortex. In contrast, at stage V decreased expression of TRH-R1 occurred in amygdala, CA2/3, dentate gyrus and piriform cortex; of TRH-R2 in CA2, thalamus and piriform cortex, and of PPII in CA2, and amygdala. The magnitude of changes differed between ipsi and contralateral side. These results support a trans-synaptic modulation of all elements involved in TRH transmission in conditions that stimulate the activity of TRHergic neurons. They show that reported changes in PPII activity or TRH-binding caused by kindling relate to regulation of the expression of TRH receptors and degrading enzyme.

Amygdala↗
PubMed2005

Differential amygdala habituation to neutral faces in young and elderly adults.

Habituation is a highly adaptive property of the nervous system, which allows for the allocation of attention and other cognitive resources to more imperative environmental events. The amygdala is an important site of habituation in humans, but no studies to date have examined the effects of aging on amygdala habituation. Given the amygdala's role in evaluating the salience of a stimulus and initiating behavioral responses, the potential importance of amygdala habituation in aging may be far-reaching. Therefore, we assessed for differences in habituation in the amygdalae of healthy young and elderly adults during repeated presentations of neutral human faces using functional magnetic resonance imaging (fMRI). In addition, we evaluated the relationship between amygdala volume and habituation, to examine the effects of atrophy. Eighteen healthy young controls and 18 healthy elderly subjects were scanned with fMRI during viewing of repeatedly presented neutral human face stimuli. Significant fMRI signal decrement was observed across all subjects for early versus late face presentations. Analysis of group, condition, and hemisphere revealed a significant three-way interaction, with right greater than left habituation in the young, but left greater than right amygdala habituation in the elderly. Volumetric and correlational analyses demonstrated that amygdala volume is associated with habituation in the right, but not left, hemisphere. We conclude that, in healthy elderly adults, the amygdala retains its adaptive habituation response, but speculate that intrinsic changes in amygdala anatomy during aging may modulate its laterality.

Adult↗
PubMed2005

Pathways for fear perception: modulation of amygdala activity by thalamo-cortical systems.

Effective perception of fear signals is crucial for human survival and the importance of the amygdala in this process is well documented. Animal, lesion and neuroimaging studies indicate that incoming sensory signals of fear travel from thalamus to amygdala via two neural pathways: a direct subcortical route and an indirect pathway via the sensory cortex. Other lines of research have demonstrated prefrontal modulation of the amygdala. However, no study to date has examined the prefrontal modulation of the thalamo-cortico-amygdala pathways in vivo. We used psychophysiological and physiophysiological interactions to examine the functional connectivity within thalamus, amygdala and sensory (inferior occipital, fusiform) cortices, and the modulation of these networks by the anterior cingulate cortex (ACC). Functional magnetic resonance imaging (fMRI) data were acquired for 28 healthy control subjects during a fear perception task, with neutral as the 'baseline' control condition. Main effect analysis, using a region of interest (ROI) approach, confirmed that these regions are part of a distributed neural system for fear perception. Psychophysiological interactions revealed an inverse functional connectivity between occipito-temporal visual regions and the left amygdala, but a positive connectivity between these visual region and the right amygdala, suggesting that there is a hemispheric specialization in the transfer of fear signals from sensory cortices to amygdala. Physiophysiological interactions revealed a dorsal-ventral division in ACC modulation of the thalamus-sensory cortex pathway. While the dorsal ACC showed a positive modulation of this pathway, the ventral ACC exhibited an inverse relationship. In addition, both the dorsal and ventral ACC showed an inverse interaction with the direct thalamus-amygdala pathway. These findings suggest that thalamo-amygdala and cortical regions are involved in a dynamic interplay, with functional differentiation in both lateralized and ventral/dorsal gradients. Breakdowns in these interactions may give rise to affect-related symptoms seen in a range of neuropsychiatric disorders.

Adult↗
PubMed2006

The 'amygdala theory of autism' revisited: linking structure to behavior.

The 'amygdala theory of autism' suggests a crucial role for the amygdala in the neurobiological basis of autism spectrum disorders. However, to date evidence is lacking of a direct relationship between amygdala measures and behavioral manifestations of autism in affected individuals. In 17 adult individuals with Asperger syndrome (AS) and 17 well-matched controls we therefore assessed associations between MRI-derived amygdala volume and behavioral variables of emotion recognition and social cognition, as well as with core AS symptomatology. Results revealed that individuals with AS exhibited impairments in emotion recognition and social cognition compared to controls and also showed atypical relationships between amygdala volumes and overall head size. We found positive associations between emotional and social understanding and amygdala volume in the control group, but not in the AS group. In the AS group however, amygdala size was negatively related to diagnostic parameters, with smaller amygdala volumes involving higher levels of restricted-repetitive behavior domains. Our data seem to indicate that in AS the amygdala is not crucially involved in social and emotional understanding. It may, however, be a mediator for narrow interest patterns and the imposition of routines and rituals. Our data, in conjunction with current literature, seem to argue for a modification of the 'amygdala theory of autism'.

Adult↗
PubMed2004

Extinction deficit and fear reinstatement after electrical stimulation of the amygdala: implications for kindling-associated fear and anxiety.

Generalized seizures produced by electrical kindling of the amygdala in laboratory rats are a widely used animal model of temporal lobe epilepsy. In addition to seizure evolution amygdala kindling enhances emotionality. The relative roles of electrical stimulation and seizure induction in fear responding are unclear. Here we investigate this issue using extinction and reinstatement of fear-potentiated startle. After classical conditioning (light+footshock pairings) laboratory rats were fear extinguished with each light presentation followed by nonepileptogenic amygdala stimulation. In contrast to the normal extinction learning of control subjects, amygdala stimulated animals exhibited conditioned fear after 120 presentations of the nonreinforced conditioned stimulus (CS). In a second experiment electrical stimulation of the amygdala restored extinguished fear responding and the fear reinstatement was specific to extinction context. The reinstatement effect did not involve sensitized fear to the CS produced by amygdala stimulation. The possibility that electrical activation of the amygdala produces unconditioned fear was considered. Animals uniformly failed to demonstrate fear-potentiated startle using electrical stimulation of the amygdala as the unconditioned stimulus. This was the case with a subthreshold afterdischarge stimulus and a stimulation schedule that produced kindled seizures. The extinction deficit and fear reinstatement results were interpreted to suggest that amygdala stimulation activates acquired excitatory stimulus-affect neural connections formed during Pavlovian fear conditioning. Our data supports a model in which excitation of an amygdala-based memory-retrieval system reinforces the expression of learned fear behaviors.

Acoustic Stimulation↗
PubMed2004

Volume reduction of the amygdala in patients with schizophrenia: a magnetic resonance imaging study.

The amygdala is known to be involved in the pathology of schizophrenia. While only a limited number of studies in schizophrenia have measured the amygdala as a single structure. The aim of this study was to examine the hypothesis that patients with schizophrenia would show reduced volumes in the amygdala compared with normal controls. We investigated amygdala volume in 40 patients with schizophrenia (20 males, 20 females) and 40 age- and gender-matched normal controls using three-dimensional magnetic resonance imaging (MRI). Whole volumes of both the amygdala and the temporal lobe were measured on consecutive coronal 1-mm slices. The amygdala volume was significantly smaller in schizophrenia patients than in controls. Considering gender differences, male patients had significantly smaller volumes in the bilateral amygdala than male controls; female patients had a significantly reduced right amygdala compared with female controls. Furthermore, a significant left-smaller-than-right volumetric asymmetry of the amygdala was detected in male patients with schizophrenia. The results may be important for understanding the role of the amygdala in the pathophysiology of schizophrenia and the anatomical substrates of gender difference in the expressions of the illness.

Adult↗