Hyperresponsiveness to phencyclidine in animals lesioned in the amygdala on day 7 of life. Implications for an animal model of schizophrenia

Phencyclidine (PCP) has been described to exacerbate psychotic symptoms in patients suffering from schizophrenia. In rats, PCP, dose-dependently, induces hyperactivity, stereotyped behaviour and social isolation, postulated to represent the positive (hyperactivity, stereotypy) and negative (social isolation) symptoms of schizophrenia. Based on previous studies, ibotenic acid lesions in the amygdala on day 7 of life have been proposed as an animal model of psychiatric neurodevelopmental disorders like schizophrenia. The purpose of the present study was to determine whether the responsiveness to PCP on locomotor activity in animals lesioned in the amygdala on day 7 of life is different from the response to this drug in sham-operated animals. The effect of graded doses of PCP on behaviour was assessed in a small open field. Animals lesioned in the amygdala on day 7 of life appeared to be hyperresponsive to PCP compared to sham-operated animals. The hyperresponsiveness to PCP in rats lesioned in the amygdala on day 7 of life further contributes to the validation of this putative animal model of schizophrenia.

Influence of sex hormone antagonists on the anticonvulsant action of conventional antiepileptic drugs against amygdala-kindled seizures in male and female rats

The effects of three gonadal steroid antihormones, tamoxifen (TXF, an estrogen antagonist), cyproterone acetate (CYP, an antiandrogen) and mifepristone (MIF, a progesterone antagonist) alone or combined with conventional antiepileptics were evaluated in amygdala-kindled seizures in male and female rats. None of the three antihormones used in this study affected any seizure parameter. TXF (50 mg/kg) and CYP (50 mg/kg), when combined with carbamazepine, or phenobarbital applied at their subprotective doses of 15 mg/kg, resulted in significant reductions of the seizure and afterdischarge durations, both in male and female rats. Additionally, the combination of carbamazepine and CYP markedly increased the afterdischarge threshold in fully-kindled rats of both genders. The interaction between antihormones and carbamazepine, or phenobarbital, was not reversed by respective sex steroid hormones (estradiol, testosterone). However, the TXF- and CYP-induced anticonvulsant effects in combinations with carbamazepine were attenuated by bicuculline, N-methyl-D-aspartate (NMDA) and aminophylline. Kainic acid and strychnine remained ineffective in this respect. The effect of a combination of TXF with phenobarbital was reversed by bicuculline and NMDA and that of CYP with phenobarbital-by bicuculline and aminophylline. Neither TXF nor CYP altered the free plasma concentrations of carbamazepine or phenobarbital, so a pharmacokinetic interaction is not probable. The combined treatment of the two antihormones with antiepileptic drugs did not affect motor performance, and did not result in significant long-term memory deficits. Our data confirm the hypothesis that sex hormone antagonist-mediated events may play some role in seizure processes in the central nervous system and can modulate the protective activity of some conventional antiepileptic drugs against kindled seizures.

Attenuated stress responsiveness in an animal model for neurodevelopmental psychopathological disorders

Day 7 amygdala-lesioned (D7 AMX) rats have been proposed as a model for neurodevelopmental psychopathological disorders such as schizophrenia. Patients with schizophrenia are sensitive to stress and show an impaired hypothalamic-pituitary-adrenal response to certain stressful stimuli. Therefore, we investigated neuroendocrine and behavioral stress responses in the D7 AMX lesion model. Plasma concentrations of ACTH, corticosterone, and catecholamines were measured in response to foot shock and novelty in D7 and D21 lesioned (AMX) and non-lesioned (SHAM) animals. Behavior was recorded and analyzed afterwards. D7 AMX rats, unlike other rats, had a reduced ACTH response to foot shock and showed less active behavior in response to novelty. Neurodevelopmental dysfunction of target structures of the amygdala is associated with disturbed endocrine and behavioral responses to stress. These data accord with the notion that the D7 amygdala-lesioned rat can function as a neurodevelopmental model with relevance to schizophrenia.

The amygdala is part of the behavioural reinforcement system modulating long-term potentiation in rat hippocampus

Long-term potentiation (LTP) in the dentate gyrus can be modulated and prolonged by emotional/motivational influences when concurrently activated. A similar effect on LTP can be obtained by stimulating the amygdala, suggesting that this limbic structure might be part of the neural system involved in behavioural reinforcement. To confirm this we have performed a series of experiments in which the basolateral amygdala was either temporary inactivated by injection of lidocaine or permanently lesioned electrolytically. Both manipulations completely blocked the reinforcing effect of a motivational stimulus (drinking after 24-h deprivation) on LTP at the perforant pathway-dentate gyrus synapses, whilst leaving intact the non-reinforced potentiation. These results demonstrate that the basolateral amygdala is a key structure within the system involved in the modulatory interaction between the affective status of the animal and the mechanisms of functional plasticity.

Effects of the central amygdaloid nucleus lesions on the gustatory responses in the parabrachial nucleus in rats

To access the role of the central nucleus of the amygdala (CeA) in the gustatory activity in the pontine parabrachial nucleus (PBN), the responses to four prototypical taste stimuli (NaCl, HCl, QH2SO4 and sucrose) in the PBN were observed before and after bilateral electrolytic lesion of the CeA in the urethane-anesthetized rat. Of 29 neurons, 14 were classified as NaCl-best, 9 as HCl-best, 3 as QH2SO4-best and 3 sucrose-best. After CeA lesions, the response rates to HCl and QH2SO4 were statistically higher across all PBN neurons (P<0.01). According to the best-stimulus category, the effects on the responses to HCl and QH2SO4 were similarly subjected to these modulations in NaCl-best, HCl-best and QH2SO4-best neurons. Correlation analysis indicated that the CeA lesion depressed the effect on the chemical selection between NaCl and QH2SO4. These findings suggest that the CeA plays an important role in the taste coding at the pontine level and it may be involved in mediating the feeding behavior via modulating the gustatory responses.

Double dissociation of the effects of lesions of basolateral and central amygdala on conditioned stimulus-potentiated feeding and Pavlovian-instrumental transfer

Pavlovian conditioned stimuli (CSs) for food can enhance both the performance of instrumental responses that earn food (Pavlovian-instrumental transfer; PIT) and the consumption of food itself (CS-potentiated feeding). After a single phase of Pavlovian training, each rat was tested in both PIT and potentiated feeding tasks. Rats with lesions of the central nucleus of the amygdala failed to exhibit PIT but showed normal CS-potentiated feeding. By contrast, rats with lesions of the basolateral amygdala showed normal PIT but failed to display CS-potentiated feeding. Performances in a variety of comparison conditions suggested that both lesion effects reflected impairment of acquired motivational functions, rather than with attentional processes or the display of specific learned responses. Implications of the double dissociation of these two aspects of Pavlovian conditioned incentive motivation for amygdala function in associative learning are considered.

A treatment program for individuals with deficient evaluative processing and consequent impaired social and risk judgement

The current study evaluated the effectiveness of a rehabilitation program developed to assist individuals such as AM who have impaired social and risk judgement. AM's difficulties developed after a severe traumatic brain injury that resulted in bilateral frontal and temporal lobe damage including damage to the amygdala. Previous work (Park et al., 2001) established that AM had impaired automatic processing of negative, but not positive evaluative information, and relatively spared processing of both types of evaluative information when using controlled or strategic processing. In the Strategic Evaluation of Alternatives (SEA) treatment program, AM was trained to compensate for his impairments by explicitly retrieving positive and negative attributes associated with potential actions prior to performing them. The SEA treatment focused specifically on improving AM's ability to obtain financial compensation for his work-related activities. Results showed improved performance on work-related activities and evidence of generalization. Analyses suggested that the process underlying improved performance was compensatory rather than restorative in nature. We discuss the implications of these results for the development of rehabilitation treatment for patients with impaired social and risk judgement.

The basolateral complex of the amygdala is necessary for acquisition but not expression of CS motivational value in appetitive Pavlovian second-order conditioning

The basolateral complex of the amygdala (ABL) is involved in processing information about stimulus motivational value. However, it is not clear whether the ABL is critical for acquisition, maintenance, or expression of this information. Our previous work has shown that ABL lesions made prior to training, block acquisition of an appetitive Pavlovian second-order conditioning task, in which performance is thought to depend on the acquisition of motivational (conditioned reinforcement) value by the first-order conditioned stimulus (CS). The present experiments examined the effects of ABL lesions made after first-order conditioning, when the CS acquires motivational value, but before second-order conditioning, the test for acquired value of that CS. Rats received pairings of a visual CS with a food reinforcer. They then received bilateral sham or excitotoxic lesions of the ABL. After recovery, they received pairings of a second-order auditory CS with the previously conditioned visual CS. In two experiments, both sham and lesioned rats acquired normal second-order conditioned behaviours. Some of the same rats were then given another round of second-order conditioning with novel CSs. In this case, when first-order training occurred after surgery, some second-order conditioned behaviours were impaired in lesioned rats. Tests of the associative underpinnings of second-order conditioned behaviours showed that those behaviours impaired by ABL lesions were based on stimulus-response associations. The results show that although the ABL is critical for second-order conditioning, this role is limited to acquisition of information about the motivational value of the first-order CS, and it is not critical for maintenance of this information or for its use in forming second-order associations.

D-amphetamine-induced behavioral sensitization: effect of lesioning dopaminergic terminals in the medial prefrontal cortex, the amygdala and the entorhinal cortex

The behavioral sensitization produced by the repeated administration of D-amphetamine is known to involve dopaminergic neurons in the mesoaccumbens pathway. Induction of this process is dependent on action of the drug in the ventral tegmental area while its expression involves action in the nucleus accumbens. We studied here the putative involvement of dopaminergic projections other than the mesoaccumbens in this phenomenon. We examined the influence of dopaminergic lesion of the medial prefrontal cortex, the amygdala and the entorhinal cortex in the behavioral sensitization produced by repeated injections of amphetamine either peripherally or directly into the ventral tegmental area of the brain. The repeated administration of amphetamine induced a behavioral sensitization, with the ventral tegmental area a critical site for induction of the process. This sensitization to amphetamine cross-reacted with morphine and was still observed 2 weeks after cessation of the treatment. Bilateral 6-hydroxydopamine lesion of dopaminergic terminals in either the medial prefrontal cortex or the amygdala, but not in the entorhinal cortex, prevented the development of behavioral sensitization to amphetamine and the cross-sensitization with morphine, whether the amphetamine pretreatment was administered peripherally or directly into the ventral tegmental area. In conclusion, these results indicated that behavioral sensitization to amphetamine, which involves dopaminergic neurons of the ventral tegmental area, is also dependent on dopaminergic neurotransmission of the medial prefrontal cortex and amygdala but not of the entorhinal cortex.

Basolateral amygdala lesions block the memory-enhancing effect of 8-Br-cAMP infused into the entorhinal cortex of rats after training

There is extensive evidence suggesting that the basolateral nucleus of the amygdala plays a critical role in modulating memory consolidation processes in other brain regions. The present experiments examined interactions between the basolateral amygdala and the entorhinal cortex in modulating memory consolidation for inhibitory avoidance training. Several studies have reported that activation of the second messenger system adenosine 3',5'-cyclic monophosphate (cAMP) in several brain regions enhances memory and induces long-term plasticity. In the present experiments, a unilateral infusion of the cAMP analogue, 8-Br-cAMP (0.25 or 1.25 microg in 0.5 microL), administered into the entorhinal cortex of male Sprague-Dawley rats immediately after training, enhanced 48-h retention. An N-methyl-d-aspartate-induced lesion of the ipsilateral basolateral amygdala did not impair retention, but blocked the memory-enhancing effect of 8-Br-cAMP (infused into the entorhinal cortex) post-training. A lesion of the contralateral basolateral amygdala did not block the 8-Br-cAMP-induced retention enhancement. These findings indicate that an intact basolateral amygdala is essential for modulation of memory consolidation involving the entorhinal cortex, and are consistent with evidence that the basolateral amygdala regulates memory consolidation mediated by other brain regions.

Lesions of the nucleus basalis magnocellularis induced by 192 IgG-saporin block memory enhancement with posttraining norepinephrine in the basolateral amygdala

Extensive evidence indicates that drugs and stress hormones act in the basolateral amygdala (BLA) to modulate memory consolidation. The BLA projects to the nucleus basalis magnocellularis (NBM), which sends broad cholinergic projections to the neocortex. NBM-cortex projections have been implicated in learning, memory storage, and plasticity. The current study investigated whether the cholinergic NBM-cortex projections are involved in BLA-mediated modulation of memory consolidation. Bilateral cholinergic cell lesions of the NBM were induced in rats with infusions of 192 IgG-saporin (0.1 microg/0.5 microl per side). Additionally, cannulae were implanted bilaterally in the BLA. One week after surgery, the rats were trained in an inhibitory avoidance task and, immediately after training, norepinephrine (0.3 microg, 1.0 microg, or 3.0 microg in 0.2 microl) or vehicle (PBS) was infused bilaterally into the BLA. Norepinephrine infusions produced a dose-dependent enhancement of 48-h retention (0.3 microg and 1.0 microg doses enhanced) in nonlesioned rats but did not affect retention in NBM-lesioned rats. Choline acetyltransferase assays of frontal and occipital cortices confirmed the NBM lesions. These findings indicate that cholinergic NBM-cortex projections are required for BLA-mediated modulation of memory consolidation.

Increased social fear and decreased fear of objects in monkeys with neonatal amygdala lesions

The amygdala has been implicated in the mediation of emotional and species-specific social behavior (Kling et al., 1970; Kling and Brothers, 1992; Kluver and Bucy, 1939; Rosvold et al., 1954). Humans with bilateral amygdala damage are impaired in judging negative emotion in facial expressions and making accurate judgements of trustworthiness (Adolphs et al., 1998, 1994). Amygdala dysfunction has also been implicated in human disorders ranging from social anxiety (Birbaumer et al., 1998) to depression (Drevets, 2000) to autism (Bachevalier, 1994; Baron-Cohen et al., 2000; Bauman and Kemper, 1993). We produced selective amygdala lesions in 2-week-old macaque monkeys who were returned to their mothers for rearing. At 6-8 months of age, the lesioned animals demonstrated less fear of novel objects such as rubber snakes than age-matched controls. However, they displayed substantially more fear behavior than controls during dyadic social interactions. These results suggest that neonatal amygdala lesions dissociate a system that mediates social fear from one that mediates fear of inanimate objects. Furthermore, much of the age-appropriate repertoire of social behavior was present in amygdala-lesioned infants indicating that these lesions do not produce autistic-like behavior in monkeys. Finally, amygdala lesions early in development have different effects on social behavior than lesions produced in adulthood.

Adaptation and habituation to an open field and responses to various stressful events in animals with neonatal lesions in the amygdala or ventral hippocampus

A rat model of neurodevelopmental psychopathological disorders, designed to determine neurodevelopmental deficits following damage to the brain early in life, was used to investigate behavioural changes in adaptation and habituation to an open field and responses to different kinds of stressful events. Animals with bilateral ibotenic acid lesions in the amygdala or ventral hippocampus on day 7 or 21 of life were compared to sham-operated animals. According to the model it was assumed that behavioural changes in animals lesioned on day 7, but not in animals lesioned on day 21 of life, were caused by maldevelopment of one or more structures connected to the damaged area. Animals lesioned in the amygdala or ventral hippocampus on day 7, but not animals lesioned in these structures on day 21 of life, displayed decreased (within-session) adaptation and (between-session) habituation to the open field and a decrease in immobility in the forced swim test, whereas only animals lesioned in the amygdala displayed enhanced general activity. These results were indicative of neurodevelopmental deficits. No changes in stress-induced hyperthermia were found, while animals lesioned in the amygdala both on day 7 or 21 of life exhibited decreased conditioned ultrasonic vocalizations. These latter results suggest that the amygdala is implicated in the conditioned stress-induced response. The contribution of the present findings to the animal model of neurodevelopmental disorders like schizophrenia and possible brain structures and neurotransmitter systems involved in the neurodevelopmental deficits are discussed.

Verbal and nonverbal emotional memory following unilateral amygdala damage

The amygdala is involved in the normal facilitation of memory by emotion, but the separate contributions of the left and right amygdala to memory for verbal or nonverbal emotional material have not been investigated. Fourteen patients with damage to the medial temporal lobe including the amygdala (seven left, seven right), 18 brain-damaged, and 36 normal controls were exposed to emotional and neutral pictures accompanied by verbal narratives. Memory for both narratives and pictures was assessed with a free recall test 24 h later. Subjects with left amygdala damage failed to show the normally robust enhancement of memory for verbal and nonverbal emotional stimuli. The group with right amygdala damage showed the normal pattern of facilitation of memory by emotion for both verbal and nonverbal stimuli despite an overall reduction in memory performance. Furthermore, subjects with left amygdala damage were disproportionately impaired on memory for emotional narratives as compared with memory for emotional pictures. The latter finding offers partial support for a lateralized and material-specific pattern of the amygdala's contribution to emotional memory.

Obesity-inducing lesions of the central nervous system alter leptin uptake by the blood-brain barrier

Leptin regulates body adiposity by decreasing feeding and increasing thermogenesis. Obese humans and some obese rodents are resistant to peripherally administered leptin, suggesting a defect in the transport of leptin across the blood-brain barrier (BBB). Defective transport of exogenous leptin occurs in some models of obesity, but in other models transport is normal. This shows that factors other than obesity are associated with impairment of leptin transport across the BBB. In order to further investigate these factors, we determined leptin transport in rats made obese by lesioning of the ventromedial hypothalamus (VMH), paraventricular nucleus (PVN), or posterodorsal amygdala (PDA). These regions all contain leptin receptors and lesions there induce obesity and hyperleptinemia and alter the levels of many feeding hormones which might participate in leptin transporter regulation. We measured the uptake of radioactively labeled leptin by the BBB by multiple-time regression analysis which divides uptake into a reversible phase (Vi, e.g., receptor/transporter binding to the brain endothelial cell) and an irreversible phase (Ki, complete transport across the BBB). Leptin uptake was not affected in rats with VMH lesions. No significant change occurred in the entry rate (Ki) for any group, although Ki declined by over 35% in rats with PVN lesions. Decreased uptake was observed in rats with PVN lesions and with PDA lesions. This was primarily due to a reduced Vi (about 21% for the PDA). This decreased uptake is most likely explained by decreased binding of leptin to the brain endothelial cell, which could be because of decreased binding by either receptors or transporters. This suggests that some of the feeding hormones controlled by the PVN and PDA may participate in regulating leptin uptake by the BBB.

Amygdala lesions block conditioned enhancement of the early component of the rat eyeblink reflex

A tone conditioned stimulus (CS) previously paired with a grid shock unconditioned stimulus (US) can greatly enhance the early electromyographic (EMG) component (R1) of the rat eyeblink reflex. The hypothesis that the central nucleus of the amygdala (ACe) is an essential part of the circuitry mediating conditioned R1 enhancement was tested. After bilateral ACe lesions (L) or a sham operation (S), rats received paired presentations of the CS and US (P) or explicitly unpaired CS and US presentations (U), resulting in 4 groups: P/S, P/L, U/S, and U/L. ACe lesions completely prevented conditioned R1 enhancement, which was only exhibited in Group P/S. In the latter group, the "preextinction" conditioned enhancement effect was roughly a 2-fold increase in the R1 magnitude. Circuit-level mechanisms are discussed, and some advantages of the eyeblink EMG response in this general conditioning paradigm are considered.

Lesions of the human amygdala impair enhanced perception of emotionally salient events

Commensurate with the importance of rapidly and efficiently evaluating motivationally significant stimuli, humans are probably endowed with distinct faculties and maintain specialized neural structures to enhance their detection. Here we consider that a critical function of the human amygdala is to enhance the perception of stimuli that have emotional significance. Under conditions of limited attention for normal perceptual awareness-that is, the attentional blink-we show that healthy observers demonstrate robust benefits for the perception of verbal stimuli of aversive content compared with stimuli of neutral content. In contrast, a patient with bilateral amygdala damage has no enhanced perception for such aversive stimulus events. Examination of patients with either left or right amygdala resections shows that the enhanced perception of aversive words depends specifically on the left amygdala. All patients comprehend normally the affective meaning of the stimulus events, despite the lack of evidence for enhanced perceptual encoding of these events in patients with left amygdala lesions. Our results reveal a neural substrate for affective influences on perception, indicating that similar neural mechanisms may underlie the affective modulation of both recollective and perceptual experience.

Lesions of the posterior basolateral amygdala block feeding induced by systemic 8-OH-DPAT

We have recently reported that bilateral electrolytic lesions of the posterodorsal amygdala (PDA) in female rats which induce protracted overeating and weight gain also attenuate short-term feeding stimulated by intraraphe infusions of the serotonin (5-HT) 1A agonist, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin, (8-OH-DPAT). Bilateral lesions of the posterior basolateral amygdala (pBLA) in male rats have also been reported to enhance feeding and weight gain, but much less so than PDA lesions do in female rats. The present study was performed to determine if pBLA lesions in female rats might attenuate 8-OH-DPAT feeding and what, if any, relationship exists between 8-OH-DPAT-induced feeding and lesion-induced weight gain. Lesioned rats showed reliable increases in 24-h food intake and weight gain relative to shams during the days between surgery and acute drug-induced feeding tests. 8-OH-DPAT (0, 60, 120 or 240 microg/kg in saline) increased feeding of shams in a dose-dependent manner over 2 h. Feeding at the most effective dose (120 microg/kg) was reduced to vehicle levels in lesioned rats. The feeding induced by this dose correlated inversely (r=-.59, P<.01) with the magnitude of weight gained following lesions. Feeding at the highest dose (240 microg/kg) showed a biphasic effect of feeding inhibition over the first vs. second hour that was unaffected by lesions. These findings imply that either fibers of passage and/or cellular elements in both the PDA and pBLA normally inhibit overeating and weight gain via intact serotonergic mechanisms.

Automatic activation of positive but not negative attitudes after traumatic brain injury

This study investigated social judgment problems of an individual (AM) with bilateral frontal and temporal lobe damage including damage to the amygdala. We hypothesized that AM could automatically process positive, but not negative evaluative information and could process both types of evaluative information using controlled processing. In Phase 1 of Experiment 1 AM and controls were shown a series of words one at a time and were required to make good/bad judgments as quickly as possible. Results showed that AM was more likely than controls to rate words as good, and was significantly slower to make good/bad judgments of negatively, but not positively, evaluated words. In Phase 2 AM was shown a prime (positive or negative) then target (positive or negative) and instructed to evaluate whether the target word was good or bad. Results showed that AM responded more quickly when prime and target were both positive, but not when prime and target were both negative, whereas controls showed both types of priming. Experiment 2 determined whether AM's impaired processing of negative evaluative information could be abolished under controlled processing. AM was explicitly instructed to generate positive and negative connotations of a series of single words and given essentially unlimited time. Under these conditions, AM and controls did not differ significantly in their ability to generate positive versus negative connotations of words. In Experiment 3 AM and controls both showed normal semantic priming effects. The results are interpreted within the component process model of memory.

Peripheral blood natural killer cell cytotoxicity after damage to the limbic system in the rat

The present work was aimed at examining the possible involvement of different parts of the septal area (dorsal, medial, lateral, and septohypothalamic nucleus), the basolateral amygdala, and the bed nucleus of the stria terminalis (BNST) in the regulation of the cytotoxic activity of NK cells (NKCC). The experimental approach included performing electrolytic (or sham) lesions in the tested brain areas and to measuring the peripheral blood NKCC (chromium-51 release assay), the number of leukocytes and lymphocytes, and the plasma corticosterone levels both before and at different time points after the lesion. Lesions were also induced in the three extralimbic structures: the paraventricular hypothalamic nucleus (PVN), the dorsal caudate-putamen, and the cerebellum. To test for a possible effect on NKCC of stress associated with blood collection, anesthesia, cranial surgery, and passing electric current through the brain the proper control experiments were also performed. Lesions of the medial septum and BNST caused gradual depression of NKCC, which peaked on the 10th day after the lesion, followed by a recovery to the baseline on days 21 (medial septum) and 42 (BNST) postinjury. In the respective sham-lesioned groups, mere insertion of electrodes into the medial septum and BNST evoked transient enhancement of NKCC (on the 3rd postlesion day), probably resulting from mechanical stimulation of the nervous tissue. Destruction of the other limbic and extralimbic structures appeared ineffective. After PVN lesions NKCC remained unchanged, despite an approximately 60% decrease in the basal corticosterone level. No adverse effects of the experimental and surgical procedures on NKCC, leukocyte and lymphocyte number, and corticosterone level were found, indicating that electrolytic lesions and other stereotaxic techniques can be safely used to study the brain-immune system interactions. The results obtained raise the question about the interrelationship between the medial septum and the hippocampal formation, BNST, the medial amygdala, and the hypothalamus (both medial and lateral) as a possible circuit involved in the regulation of cellular immune functions.

Early amygdala damage in the rat as a model for neurodevelopmental psychopathological disorders

Neurodevelopmental disorders in medial temporal lobe structures may underlie psychopathological diseases such as schizophrenia and autism. To construct an animal model for these developmental disorders, social and non-social behavioural responses were assessed in rats with ibotenic acid lesions of the (baso-)lateral and central amygdala or ventral hippocampus, induced early in life. Lesioning the amygdala on day 7 after birth resulted in a variety of behavioural disturbances later in life, whereas after similar lesions on day 21 after birth no disturbances developed, except for deficits in social behaviours. Lesioning the hippocampus led to much less disturbances. The results show that amygdala and hippocampus damage at a specific point early in life results in enduring behavioural disturbances that become more manifest after puberty. In particular, lesions of the amygdala on day 7 of life may serve as a rat model with face and construct validity for neurodevelopmental disorders in studying psychopathology.

Posterodorsal amygdala lesions reduce feeding stimulated by 8-OH-DPAT

Injections of the serotonin (5-HT)(1A) agonist, 8-hydroxy-2(di-n-propylamino)tetralin, (8-OH-DPAT), either systemically or into the midbrain raphe nuclei, elicit food intake in otherwise satiated rats. Lesions of the paraventricular nucleus of the hypothalamus are well known for producing long-term overeating, but past research has excluded this site as a potential locus for short-term 8-OH-DPAT feeding effects. More recent work shows that small lesions of the posterodorsal amygdala (PDA) elicit overeating in their own right. Since this and related regions of the amygdala receive 5-HT innervations from the dorsal raphe nucleus (DRN), we determined if PDA lesions might alter feeding after injecting 8-OH-DPAT into this midbrain region. Adult female rats received either bilateral electrolytic lesions of the PDA or sham lesions. After recording weight gains for over 1 month, all rats were implanted with DRN cannulae, then randomly tested every 3-4 days for 1 h intake of standard lab chow after 0, 0.4, 0.8 or 1.6 nmol injections of 8-OH-DPAT. Additional 90 min measures of intake were also made after 0 vs. 250 microg (760 nmol) 8-OH-DPAT s.c. At the two highest DRN doses tested, lesioned rats showed 50% less intake compared to shams. A similar profile emerged after the single s.c. dose. These results suggest that the PDA may be an important locus at which reduced release of endogenous 5-HT stimulates feeding. Alternatively, the PDA may represent part of a larger brain circuit whose integrity is necessary for eliciting intake in response to a variety of feeding stimuli.

Effects of amygdala or hippocampus lesion on hypergravity-induced motion sickness in rats

We examined the effects of amygdala lesion (AL) or hippocampal lesion (HL) on hypergravity-induced motion sickness in rats. Rats do not vomit, but the behavior known as pica, the eating of non-nutritive substances such as kaolin, can be used as an index of motion sickness. In the present study, hypergravity-induced kaolin intake and apomorphine-induced kaolin intake were measured before and after brain lesions. After AL, hypergravity-induced kaolin intake and the ratio of the hypergravity- to apomorphine-induced kaolin intakes were decreased. These results indicate that AL suppressed motion sickness more than pica behavior itself, suggesting that the amygdala plays an important role in the development of motion sickness in rats. Conversely, after HL, hypergravity-induced kaolin intake was increased, as was the ratio of the hypergravity- to apomorphine-induced kaolin intakes. These results indicate that HL aggravates motion sickness induced by hypergravity in rats, suggesting that the hippocampus counteracts motion sickness.

Amygdala, hippocampus and discriminative fear conditioning to context

Various measures of fear have been shown to condition to a fearful context with different acquisition rates (Antoniadis EA, McDonald RJ. Fear conditioning to context expressed by multiple measures of fear in the rat, Behav Brain Res 1999;101(1):1-14). Freezing, locomotion, urination and preference are 'fast' measures of fear in that they discriminatively condition to context after a single training session, while ultrasonic vocalizations and defecation are 'slow' measures of fear given that they condition following three training sessions. In the present experiment we sought to assess the contribution of the amygdala and the hippocampus in this form of learning. Existing views differ on the degree of involvement of each memory structure. This discord probably emerges from the common use of non-discriminative paradigms and the assessment of a single measure of fear. With the use of a discriminative paradigm and the assessment of multiple measures of fear, results indicate that the amygdala is a memory structure that selectively mediates the conditioning of heart rate, and the hippocampus selectively mediates the conditioning of defecation and body temperature. The conditioning of preference, locomotion, freezing and ultrasonic vocalizations, necessitate the participation of both memory structures while the conditioning of urination does not seem to require the participation of either the hippocampus or the amygdala. The proposed view ascribes an equal role in fear conditioning to both the amygdala and the hippocampus.

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.

Effects of aspiration versus neurotoxic lesions of the amygdala on emotional responses in monkeys

All previous reports describing alterations in emotional reactivity after amygdala damage in monkeys were based on aspiration or radiofrequency lesions which likely disrupted fibres of passage coursing to and from adjacent ventral and medial temporal cortical areas. To determine whether this associated indirect damage was responsible for some or all of the changes described earlier, we compared the changes induced by aspiration of the amygdala with those induced by fibre-sparing neurotoxic lesions. Four different stimuli, two with and two without a social component, were used to evaluate the expression of defence, aggression, submission and approach responses. In unoperated controls, defence and approach behaviours were elicited by all four stimuli, 'social' and inanimate alike, whereas aggression and submission responses occurred only in the presence of the two 'social' stimuli. Furthermore, all defence reactions were reduced with an attractive inanimate item, while freezing was selectively increased with an aversive one. Relative to controls, monkeys with neurotoxic amygdala lesions showed the same array of behavioural changes as those with aspiration lesions, i.e. reduced fear and aggression, increased submission, and excessive manual and oral exploration. Even partial neurotoxic lesions involving less than two-thirds of the amygdala significantly altered fear and manual exploration. These findings convincingly demonstrate that the amygdala is crucial for the normal regulation of emotions in monkeys. Nevertheless, because some of the symptoms observed after neurotoxic lesions were less marked than those seen after aspiration lesions, the emotional disorders described earlier after amygdalectomy in monkeys were likely exacerbated by the attendant fibre damage.

Long-term effects of neonatal damage to the hippocampal formation and amygdaloid complex on object discrimination and object recognition in rhesus monkeys (Macaca mulatta)

Rhesus monkeys with neonatal aspiration lesions of the hippocampal formation or the amygdaloid complex were tested on concurrent discrimination learning (24-hr intertrial interval [ITI]) at 3 months, on object recognition memory (delayed nonmatching-to-sample [DNMS]) at 10 months, and retested on both tasks at 6-7 years of age. Neonatal amygdaloid damage mildly impaired acquisition at the 24-hr ITI and the performance test of DNMS at both ages. In contrast, early hippocampal lesions impaired performance only on the longest lists of 10 items in DNMS in adult monkeys. Thus, early amygdala lesions appeared to have resulted in a greater object memory loss than early hippocampal lesions. However, in light of recent findings from lesion studies in adult monkeys, the object memory impairment after early amygdaloid lesions is better accounted for by damage to the entorhinal and perirhinal cortex than by damage to the amygdaloid nuclei.

Intact recognition of emotional prosody following amygdala damage

Bilateral damage to the amygdala in a variety of animal species can impair emotional reactions to stimuli in several sensory modalities. Such damage in humans impairs visual recognition of emotion in facial expressions, but possible impairments in modalities other than vision have not been sufficiently explored. We examined two subjects with complete bilateral amygdala damage, and seven with unilateral amygdala damage, on a standardized task of emotional prosody recognition. The data were compared to those from 15 brain-damaged and from 14 normal control subjects. One of the bilateral amygdala subjects, whose lesions were restricted to the amygdala, was entirely normal in recognizing emotion in prosody on all tasks, the other, whose damage included substantial lesions also in extra-amygdalar structures, especially in right hemisphere, was normal on most, albeit not all, measures of emotional prosody recognition. We suggest that the human amygdala's role in recognizing emotion in prosody may not be as critical as it is for facial expressions, and that extra-amygdalar structures in right hemisphere may be more important for recognizing emotional prosody. It remains possible that recognition of emotion in classes of auditory stimuli other than prosody will require the amygdala.

Increased sweet taste preference following the lesion of basolateral nucleus of amygdala (BLA) in rat

A study of gustatory preference was carried out in Wistar strain albino rats by electrolytically lesioning the basolateral nucleus of amygdala. The intake of sweet tasting saccharin (Sn-0.1% soln w/v), NaCl solutions (1% Soln w/v) and tap water, were tested in single bottle, 2 bottle choice and 3 bottle choice situations. The consumption of fluids both before and after the stereotaxic surgery was recorded and statistically analysed. Lesion of BLA increased the intake of all fluids in the single bottle tests (P < 0.01). But the increase in the group provided with saccharin was more than that in other two groups. When taste preference was tested using 2-bottle and 3-bottle choice situations, the lesioned rats were seeking Sn solution. In our previous study, we found that the lesion of BLA increased fluid intake in rats. The intake, in the present study was more in those animals provided with Sn solution. When choice was given, the rats shifted their preference from water to the Sn following the lesion. Thus this study confirms that BLA is involved in the preference for sweet tasting solution over the NaCl or plain water.

Effects of lesions to the glutamatergic afferents to the nucleus accumbens in the modulation of reactivity to spatial and non-spatial novelty in mice

The purpose of this study was to compare the effects of selective lesions of the three main sources of limbic afferents to the nucleus accumbens-fornix, prelimbic cortex and amygdala-with those induced by N-methyl-D-aspartate receptor blockage in this structure, in a non-associative task designed to estimate the ability of rodents to encode spatial and non-spatial relationships between discrete stimuli. The task consists of placing mice in an open field containing five objects and, after three sessions of habituation, examining their reactivity to object displacement (spatial novelty) and object substitution (object novelty). Focal administrations of the competitive N-methyl-D-aspartate antagonist DL-2-amino-5-phosphonopentanoic acid (0.1 microg/side) induced a selective impairment in the ability of mice to react to the spatial change. Lesions to the different structures affect the response of mice to spatial and non-spatial novelty in different ways. In particular, while fornix lesions induced a decrease in re-exploration of the displaced objects, prelimbic cortex lesions enhanced the exploration of both displaced and non-displaced objects. Finally, the basolateral amygdala lesions did not induce any impairment in the detection of the displaced objects but decreased the latencies to approach novel objects. It is concluded that N-methyl-D-aspartate receptor blockage in the nucleus accumbens subsumes the effects of the three lesions. Some hypotheses on the role of glutamatergic transmission in the accumbens on information processing are briefly discussed.

Recognition of facial emotion in nine individuals with bilateral amygdala damage

Findings from several case studies have shown that bilateral amygdala damage impairs recognition of emotions in facial expressions, especially fear. However, one study did not find such an impairment, and, in general, comparison across studies has been made difficult because of the different stimuli and tasks employed. In a collaborative study to facilitate such comparisons, we report here the recognition of emotional facial expressions in nine subjects with bilateral amygdala damage, using a sensitive and quantitative assessment. Compared to controls, the subjects as a group were significantly impaired in recognizing fear, although individual performances ranged from severely impaired to essentially normal. Most subjects were impaired on several negative emotions in addition to fear, but no subject was impaired in recognizing happy expressions. An analysis of response consistency showed that impaired recognition of fear could not be attributed simply to mistaking fear for another emotion. While it remains unclear why some subjects with amygdala damage included here are not impaired on our task, the results overall are consistent with the idea that the amygdala plays an important role in triggering knowledge related to threat and danger signaled by facial expressions.

Normal recognition of emotional similarity between facial expressions following bilateral amygdala damage

Bilateral damage to the amygdala in humans has been previously linked to two deficits in recognizing emotion in facial expressions: recognition of individual basic emotions, especially fear, and recognition of similarity among emotional expressions. Although several studies have examined recognition of individual emotions following amygdala damage, only one subject has been examined on recognition of similarity. To assess the extent to which deficits in recognizing similarity among facial expressions might be a general consequence of amygdala damage, we examined this ability in two subjects with complete bilateral amygdala damage. Both subjects had previously demonstrated entirely normal recognition of individual facial emotions. Here we report that these two patients also are intact in their ability to recognize similarity between emotional expressions. These results indicate that, like the recognition of individual basic emotions in facial expressions, the recognition of similarity among emotional expressions does not have an absolute dependence on the amygdala.

Ibotenic acid lesions of the basolateral, but not the central, amygdala interfere with conditioned taste aversion: evidence from a combined behavioral and anatomical tract-tracing investigation

Rats (Rattus norvegicus) with almost complete ibotenic acid lesions (at least 90%) of the basolateral amygdaloid complex (BLA) failed to learn a conditioned taste aversion (CTA; Experiment 1A). In these same BLA rats, the bidirectional parabrachial-insular pathway that courses through the central nucleus of the amygdala (Ce) was shown to be spared (Experiment 1B), indicating that the BLA per se is critical for CTA learning. In contrast to the deleterious effect of BLA lesions on CTA, ibotenic acid lesions of the Ce did not block CTA learning (Experiment 2). Nonreinforced preexposure to the gustatory stimulus attenuated CTA acquisition in normal rats, and, under these conditions, rats with BLA lesions were no longer impaired (Experiment 3). Thus, ibotenic acid lesions centered over the Ce, sparing a considerable extent of the BLA, together with the testing procedure used in previous experiments (e.g., L. T. Dunn & B. J. Everitt, 1988), led to the belief that the CTA deficits reported after electrolytic lesions of the amygdala were the result of incidental damage to fibers of passage.

Evidence of contextual fear after lesions of the hippocampus: a disruption of freezing but not fear-potentiated startle

The roles of the dorsal hippocampus and the central nucleus of the amygdala in the expression of contextual fear were assessed using two measures of conditioned fear: freezing and fear-potentiated startle. A discriminable context conditioning paradigm was developed that demonstrated both conditioned freezing and fear-potentiated startle in a context paired previously with foot shock, relative to a context in which foot shock had never been presented. Post-training lesions of the central nucleus of the amygdala completely blocked both contextual freezing and fear-potentiated startle. Post-training lesions of the dorsal hippocampus attenuated contextual freezing, consistent with previous reports in the literature; however, these same lesions had no effect on fear-potentiated startle, suggesting preserved contextual fear. These results suggest that lesions of the hippocampus disrupt the freezing response but not contextual fear itself.

Medial geniculate lesions block amygdalar and cingulothalamic learning-related neuronal activity

This study assessed the role of the thalamic medial geniculate (MG) nucleus in discriminative avoidance learning, wherein rabbits acquire a locomotory response to a tone [conditioned stimulus (CS)+] to avoid a foot shock, and they learn to ignore a different tone (CS-) not predictive of foot shock. Limbic (anterior and medial dorsal) thalamic, cingulate cortical, or amygdalar lesions severely impair acquisition, and neurons in these areas develop training-induced activity (TIA): more firing to the CS+ than to the CS-. MG neurons exhibit TIA during learning and project to the amygdala. The MG neurons may supply afferents essential for amygdalar and cingulothalamic TIA and for avoidance learning. To test this hypothesis, bilateral electrolytic or excitotoxic ibotenic acid MG nuclear lesions were induced, and multiunit recording electrodes were chronically implanted into the anterior and posterior cingulate cortex, the anterior-ventral and medial-dorsal thalamic nuclei, and the basolateral nucleus of the amygdala before training. Learning was severely impaired and TIA was abolished in all areas in rabbits with lesions. Thus learning and TIA require the integrity of the MG nucleus. Only damage in the medial MG division was significantly correlated with the learning deficit. The lesions abolished the sensory response of amygdalar neurons, and they attenuated (but did not eliminate) the sensory response of cingulothalamic neurons, suggesting the existence of extra geniculate sources of auditory transmission to the cingulothalamic areas.

Impaired declarative memory for emotional material following bilateral amygdala damage in humans

Everyday experience suggests that highly emotional events are often the most memorable, an observation supported by psychological and pharmacological studies in humans. Although studies in animals have shown that nondeclarative emotional memory (behaviors associated with emotional situations) may be impaired by lesions of the amygdala, little is known about the neural underpinnings of emotional memory in humans, especially in regard to declarative memory (memory for facts that can be assessed verbally). We investigated the declarative memory of two rare patients with selective bilateral amygdala damage. Both subjects showed impairments in long-term declarative memory for emotionally arousing material. The data support the hypothesis that the human amygdala normally enhances acquisition of declarative knowledge regarding emotionally arousing stimuli.

[The comparative effect of melatonin and diazepam on shifts in the anxiety-phobia status of rats evoked by damage to the amygdala]

Disorders develop in the emotional sphere in the form of increased anxiety and fear in amygdalectomized rats. The pineal hormone melatonin and diazepam (in an equal dose of 0.1 mg/kg) alleviate the state of anxiety and fear in the animals. Despite some differences in behavioral activity, the effect of the drugs appears to be similar. This suggests that melatonin possesses anxiolytic properties which may be related to its effect on the function of the limbic structures.

[A comparative evaluation of the effect of removal of the epiphysis and damage to the amygdala on the behavioral reactions of rats]

Pinealectomy and lesions of the amygdala variously changed aggression of rats after social isolation and behavior in a conflict situation. It is suggested that the pineal gland by means of secreted melatonin suppresses the function of the limbic structures. It may regulate the pineal-amygdaloid relationships and play an important role in the organization of emotional behaviour.

Lesions of the ventral striatum mimic the effect of olfactory bulbectomy to prevent short photoperiod-induced testicular regression in golden hamsters

Bilateral olfactory bulbectomy (BX) or bilateral transection of the rostral lateral olfactory tract (LOT) at the level of the anterior olfactory nucleus markedly increases gonadotropin secretion and prevents the testicular regression associated with maintenance on short photoperiod in golden hamsters. In an effort to further elucidate the neural tracts involved in this influence on gonadotropin secretion, lesions were placed in several potential pathways. Hamsters underwent sham surgery (SH), bilateral BX, or electrolytic or radiofrequency lesions of the: medial nucleus of the amygdala (MeX) caudal LOT just rostral to the medial nucleus of the amygdala (LOTX); or ventral striatum (VSX). Lesions were either bilateral or unilateral with contralateral olfactory bulbectomy. All animals were then placed on short photoperiod (LD 10:14) for 10 weeks and testicular size and body weight were assessed at weekly intervals. Lesion placement was assessed in brain sections stained with cresyl violet and animals with misplaced lesions were excluded. The following represent the number of animals in each group undergoing testicular regression in response to short photoperiod: SH: 32/35; BX: 8/31 (P < 0.01 vs. SH); MeX: 5/5; caudal LOTX: 8/9 and VSX: 3/8 (P < 0.05 vs. SH). Serum LH, FSH and testosterone at the end of the study correlated with the testicular regression data. These results suggest that the tonic inhibitory effect of the olfactory bulbs on gonadotropin secretion is mediated by fibers that exit the LOT rostral to the amygdala and project medially, either passing through or synapsing in the ventral striatum.

Involvement of the central nucleus and basolateral complex of the amygdala in fear conditioning measured with fear-potentiated startle in rats trained concurrently with auditory and visual conditioned stimuli

The goal of this work was to test the involvement of the central nucleus and basolateral complex of the amygdala in fear conditioning, using auditory and visual conditioned stimuli (CSs). The acoustic startle reflex in rats was used as the behavioral index of conditioning because startle is reliably enhanced in the presence of a conditioned stimulus (CS) previously paired with a footshock. Initially, differential conditioning procedures indicated reliable discrimination between a noise CS and a visual CS. Subsequently, the effects of amygdala lesions were evaluated when both modalities were paired with shocks in the same rats. Electrolytic or ibotenic acid lesions of the central nucleus of the amygdala blocked fear-potentiated startle to both auditory and visual CSs, consistent with the idea that the central nucleus serves as a response independent, final common relay for fear conditioning. Similarly, pre- or post-training electrolytic or NMDA-induced lesions of the basolateral complex of the amygdala, which damaged the lateral nucleus, and most of the basolateral nucleus, disrupted fear-potentiated startle to both CS modalities. This finding is consistent with the suggestion that, in fear conditioning, the basolateral complex of the amygdala serves as an obligatory relay of sensory information from subcortical and cortical sensory areas to the central nucleus of the amygdala.