The acoustic startle response in inbred Roman high- and low-avoidance rats

To investigate the emotional reactions of two rat strains selectively bred for good and poor two-way avoidance acquisition (RHA/Verh and RLA/Verh), male animals of both strains were tested in an acoustic startle response test. They received 40 acoustic stimuli followed by 10 electric foot shocks and another 30 acoustic stimuli. RLA/Verh rats showed a significantly higher startle response compared to RHA/Verh animals, indicating a stronger emotional reaction to acoustic stimuli. In addition, the former showed a stronger response to foot shocks. Combined with earlier findings, we conclude that selection for two-way avoidance learning does not result in cognitive defects in the RLA/Verh strain but, rather, in stronger emotional reactions to fearful stimuli.     

4-OH amphetamine enhances retention of an active avoidance response in rats and decreases regional brain concentrations of norepinephrine and dopamine.

In Exp I, an .82 mg/kg dose of 4-OH amphetamine hydrobromide (AMP) administered ip immediately following training in a 1-way active avoidance task enhanced retention performance of male ARS Sprague-Dawley rats measured 24 hrs later. In contrast, AMP in a dose range of .41–2.64 mg/kg, ip, did not affect retention of a swim escape task (Exp II). The behaviorally active dose of .82 mg/kg decreased dopamine concentrations in the amygdala and hippocampus. A dose of 8.2 mg/kg administered ip to naive untrained Ss (Exp III) decreased concentrations of norepinephrine measured in the amygdala, cortex, hippocampus, hypothalamus, and midbrain; decreased concentrations of dopamine in the amygdala, cortex, hippocampus, and striatum; and significantly reduced concentrations of norepinephrine and epinephrine in the adrenal medulla. In addition, because the integrity of the adrenal medulla is necessary for the enhancing action of AMP and because AMP reduces concentrations of catecholamines in the brain and adrenal medulla, it is possible that this drug affects retention performance by a dual action on the brain and the adrenal medulla. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of posttraining injection of cholinergic agonists and antagonists into the amygdala on retention of passive avoidance training in rats.

144 adult male Long-Evans rats were given a single footshock while licking a water tube and tested 24 hrs later for retention of the footshock experience. A single bilateral injection of a subseizure dose of physostigmine into the amygdala applied immediately but not 18 hrs after the footshock impaired retention. This effect appeared to be somewhat localized, as physostigmine injected into the hippocampus or lateral ventricles did not disrupt retention. Conversely, a subseizure dose of atropine sulfate into the amygdala, given immediately or 18 hrs after the footshock did not impair retention. Atropine injected concurrently with physostigmine into the same amygdaloid loci counteracted a potential physostigmine-induced retention deficit. Injection of carbachol into the amygdala also impaired retention; however, carbachol precipitated seizures and possibly exerted proactive consequences on performance. The time-dependent nature of the deficit following physostigmine is consistent with the view that injection of cholinergic agonists into the amygdala disrupts memory for the footshock experience. (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Microinjection of thyrotropin-releasing hormone analogue into the central nucleus of the amygdala stimulates gastric contractility in rats

The effect on gastric contractility following bilateral microinjection of thyrotropin-releasing hormone (TRH) analog, RX 77368, into the central nucleus of the amygdala was examined in fasted, urethane-anesthetized rats. Extraluminal force transducers were used to measure gastric corpus contractility. Bilateral microinjection of RX 77368 (0.5 microgram, 1.0 microgram, n = 6 each) stimulated gastric contractility for up to 120 min post-injection, P < 0.05. Gastric contractility was not significantly stimulated by microinjection of 0.1 microgram RX 77368, 0.1% bovine serum albumin (BSA) into the central nucleus or RX 77368 (0.5 microgram, 1.0 microgram) into sites adjacent to the central nucleus. Peak responses (1.0 microgram) occurred 40 min post-injection and represented a 16-26-fold increase over basal values. The frequency of gastric contraction waves was attenuated for 0-90 min in rats receiving central amygdaloid microinjection of RX 77368 (0.1, 0.5 or 1.0 microgram) versus rats microinjected with the vehicle or RX 77368 into sites adjacent to the central nuclei. The stimulatory effect of RX 77368 (1.0 microgram) on gastric contractility was abolished by subdiaphragmatic vagotomy. These results indicate that the TRH analog, RX 77368, acts within the central amygdala to vagally stimulate gastric contractility.     

Valence dependent asymmetric release of norepinephrine in the basolateral amygdala.

Emerging evidence from literature on humans suggests the valence of emotionally laden environmental stimuli may dictate whether amygdala activation is greater in one hemisphere relative to the contralateral side. However, only a paucity of animal studies attempt to unravel the mechanisms underlying the selective, valence-dependent initiation of activity in the amygdala of opposing hemispheres. The present studies assessed whether exposure to positive or negative appetitive conditions in an operant learning task differentially impacts norepinephrine activation of the left versus right amygdala, respectively. Dialysate samples of norepinephrine were collected from the basolateral nucleus of male Sprague–Dawley rats. Fluctuations in norepinephrine activity were sampled during training conditions involving an abrupt shift (i.e., increase or decrease) in the magnitude of food rewards normally provided for bar press responses. In a subsequent study, dialysate samples of norepinephrine were collected before, during, and after presentation of a negatively valenced stimulus involving footshock delivery during Pavlovian fear conditioning. Norepinephrine concentrations in the left but not right basolateral amygdala were elevated in groups presented with a positive experience of an unexpected increase in food reward after bar pressing (p     

Electrophysiological activity in the central nucleus of the amygdala, emotionality and stress ulcers in rats.

Multiple-unit activity in the central nucleus of the amygdala was continuously recorded during 4 hr of restraint stress in rats. Five different activity profiles were found. Two types were associated with stress ulceration: one with increased stomach pathology, and the other with decreased stomach pathology. The same unit profiles were also differentially related to the emotionality characteristics of Wistar-derived rats, as well as to those of the genetically selected lines of Roman high- and low-avoidance rats. The type of profile that had been associated with increased pathology was generally seen in the Roman low-avoidance rats and in the Wistar rats that had been judged to be more emotional, that is, defecated before five "rearings" had occurred in an open-field test. The other unit profile was significantly more frequent in the Roman high-avoidance animals and the less emotional Wistar rats. Low-level electrical stimulation of both types of units produced stomach erosions in all cases. It was concluded that the unit activity in the central nucleus of the amygdala reflects certain emotionality characteristics of rats and also their susceptibility to stress ulcers. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An oxytocin antagonist infused into the central nucleus of the amygdala increases maternal aggressive behavior.

Decreased oxytocin levels in the amygdalas of rat dams following chronic gestational cocaine exposure have been correlated with heightened maternal aggressive behavior. In this experiment, drug-naive dams were implanted with bilateral cannulas into the central nucleus of the amygdala (CNA) or control area and infused with 1,000 or 500 ng of an oxytocin antagonist (OTA) or buffer, 4 hr before testing. Behavior was compared among dams infused with OTA into target areas just outside the CNA and cocaine-treated dams (infused with buffer). Dams infused with 1,000 ng OTA attacked intruders significantly more often than buffer-infused dams. OTA did not affect other behaviors, suggesting that disruption of oxytocin activity in the CNA may be sufficient to selectively alter maternal aggressive behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inhibitory learning tests of conditioned stimulus associability in rats with lesions of the amygdala central nucleus.

Normal rats showed faster inhibitory learning about a light conditioned stimulus (CS) if it had previously been an inconsistent predictor of a tone CS than if it had been a consistent predictor of the tone. In contrast, the inhibitory learning of rats with ibotenic acid lesions of the amygdala central nucleus (CN) was unaffected by the prior predictive value of the light. These results support claims that the CN is critical to surprise-induced enhancement of attentional processing of CSs. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Chronic central leptin infusion enhances insulin-stimulated glucose metabolism and favors the expression of uncoupling proteins

Continuous (4 days) intracerebroventricular leptin infusion (12 microg/day) was performed in lean rats, and its hormonometabolic effects were determined. Intracerebroventricular leptin administration did not result in leakage of the hormone into the peripheral circulation. Thus, its effects were elicited by its presence within the central nervous system. Intracerebroventricular leptin infusion produced marked decreases in food intake and body weight gain relative to vehicle-infused fed ad libitum rats. Because decreases in food intake alter hormonometabolic homeostasis, additional control rats pair-fed to the amount of food consumed by leptin-infused ones were included in the study. Intracerebroventricular leptin-infused and vehicle-infused pair-fed rats were characterized, relative to vehicle-infused ad libitum-fed animals, by decreases in body weight and insulinemia and by increases in insulin-stimulated overall glucose utilization and muscle and brown adipose tissue glucose utilization index. Brown adipose tissue uncoupling protein (UCP)1, UCP2, and UCP3 mRNA levels were markedly decreased in pair-fed animals relative to those of fed ad libitum control animals, as were liver and white adipose tissue UCP2 and muscle UCP3 mRNA levels. In marked contrast, intracerebroventricular leptin administration was accompanied by the maintenance of high UCP1, UCP2, and UCP3 expression in all these tissues. Thus, despite analogies between leptin's effects and those of pair-feeding with regard to glucose handling, their respective underlying mechanisms differ. While leptin maintains or favors energy-dissipating mechanisms (UCP1, UCP2, and UCP3), the latter are markedly depressed in pair-fed rats. This effect of leptin may prevent subsequent excessive storage processes, thereby maintaining normal body homeostasis.     

Conditioned and unconditioned fear organized in the inferior colliculus are differentially sensitive to injections of muscimol into the basolateral nucleus of the amygdala.

Chemical stimulation of the inferior colliculus (IC) with semicarbazide--an inhibitor of the gamma aminobutyric acid synthesizing enzyme--functions as an unconditioned stimulus in the conditioned place aversion test (CPA), and electrolytic lesions of the basolateral amygdala (BLA) enhance the aversiveness of the IC stimulation. This study examined the effects of inactivation of the BLA with muscimol on the conditioned and unconditioned fear using semicarbazide injections into the IC of rats subjected to conditioned (CPA) or unconditioned (open field) fear tests. In both tests, the rats were injected with semicarbazide or saline into the IC and muscimol or saline into the BLA. Muscimol decreased the CPA and increased the unconditioned fear elicited by IC injections of semicarbazide. These findings indicate that distinct modulatory mechanisms in the BLA are recruited during the conditioned and unconditioned fear triggered by IC activation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Posttraining paradoxical sleep in rats is increased after spatial learning in the Morris water maze.

The role of posttraining paradoxical sleep (PS) in spatial or nonspatial learning in the Morris water maze was evaluated. Sprague-Dawley rats were given a 12-trial training session in either the hidden or the visible platform versions of the task. Subgroups then underwent paradoxical sleep deprivation (PSD) beginning at different times after training. Rats with PSD imposed from 14 hr after spatial training had poorer retest scores than any other group. Other rats, implanted with electrodes to permit continuous recording of sleep electroencephalography, were found to undergo a prolonged period of elevated PS after spatial training. By contrast, rats trained in the nonspatial version of the water maze task did not show retention deficits after PSD or elevated PS after training. These results support a role for PS in spatial, but not nonspatial, learning in the Morris water maze. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

CS modality transfer of two-way avoidance in rats with central and basolateral amygdala lesions

Post-lesion acquisition of two-way avoidance and subsequent transfer to two warning signals (conditioned stimulus, CS) of different modality were investigated in 60 rats. In Experiment I the animals were originally trained with less salient (darkness) CS, then transferred to more salient compound (darkness and white noise), and finally to white noise CS. The opposite arrangement of the conditioned stimuli (CSi) during the subsequent stages was employed in Experiment II. In control animals, avoidance acquisition was faster and the intertrial responding (ITR) rate lower with the auditory than with the visual CS. Lesioned rats learned avoidance responses more slowly, independently of CS modality. The transfer to other CSi revealed dramatic between-group difference in the level and consistency of avoidance response, shuttle-box latencies and ITR rate. In control animals, transfer to more salient CSi enhanced avoidance performance, whereas change to less salient CS decreased it. Rather small changes in shuttle-box performance and consistency of avoidance response due to CS modality were seen in rats with the basolateral lesions. In contrast, central nucleus injury caused a strong deterioration in the avoidance transfer, especially when the visual CS followed the acoustic one. The results indicate differential involvement of the basolateral and central amygdala nuclei in stimulus-processing mechanisms of instrumental defensive behavior.     

Pre- and posttraining infusion of N-methyl-D-aspartate receptor antagonists into the amygdala impair memory in an inhibitory avoidance task

Involvement of amygdaloid N-methyl-D-aspartate (NMDA) receptors in memory processes was investigated. Rats with cannulas implanted in the basolateral amygdala were trained on a 1 trial step-through inhibitory avoidance task and tested for 24-hr retention. Pretraining infusion of 2-amino-5-phosphonovaleric acid (APV) into the amygdala, but not striatum or hippocampus, produced a dose-dependent retention deficit, which was attenuated by immediate posttraining intra-amygdala infusion of NMDA. Posttraining APV infusion also caused a dose- and time-dependent retention deficit. Pretest APV infusion had no effect on performance in the retention test. Further, pre- or posttraining infusion of 5.0 micrograms APV failed to affect acquisition and retention in the Morris water maze task. These findings suggest that amygdala NMDA receptors are normally activated by aversive training and play a critical role in memory formation for affective experience.     

Disconnection of the amygdala central nucleus and substantia innominata/nucleus basalis disrupts increments in conditioned stimulus processing in rats.

Rats with a neurotoxic lesion of the amygdala central nucleus (CN) in one hemisphere and a 192 immunoglobulin G (192IgG)-saporin lesion of cholinergic neurons in the contralateral substantia innominata/nucleus basalis (SI/nBM) failed to show the enhanced attentional processing of a conditioned stimulus (CS) observed in sham-operated rats when that CS's predictive value was altered. Performance of these asymmetrically lesioned rats was poorer than that of rats with a unilateral lesion of either structure or with a symmetrical lesion of both structures in the same hemisphere. These results implicate connections between the CN and SI/nBM in the incremental attentional processing of CSs, extending previous research that has shown similar effects of bilateral lesions of either the CN or the SI/nBM. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Operant and Pavlovian control of visual stimulus orienting and food-related behaviors in rats with lesions of the amygdala central nucleus.

The effects of superimposing operant reward and omission contingencies on 2 Pavlovian conditioned responses evoked by a visual conditioned stimulus paired with food were examined in rats with lesions of the amygdala central nucleus (CN). In sham-lesioned rats, the frequency of an orienting response, rearing, was increased by reward contingencies and decreased by omission contingencies, compared with yoked Pavlovian controls. In contrast, in CN-lesioned rats, rearing was not affected by either operant contingency and occurred at lower levels with Pavlovian procedures alone than in sham-lesioned rats. Nevertheless, CN-lesioned and sham-lesioned rats showed similar increases in the frequency of conditioned food-cup behavior with reward contingencies, similar decreases with omission contingencies, and similar levels of that response with Pavlovian procedures. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Corticosterone and avoidance in rats with basolateral amygdala lesions.

Produced deficient acquisition of 2-way active and passive avoidance in 15 male Moll-Wistar rats after bilateral electrolytic lesions restricted to the dorsal part of the basolateral nuclei. Other deficits also suggest a general reduction in fear or arousal: less immobility in the open field and during active-avoidance intertrial intervals, and slower escape latencies and less pituitary-adrenal activation during the initial active-avoidance session. Anatomical analysis of the areas producing the greatest deficit suggests that differential involvement of the insula may explain phylogenetic differences between these data from the rat and previous data from the cat, which show only active-avoidance deficiency after basolateral lesions. (26 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Posttraining lesions of the amygdala interfere with fear-potentiated startle to both visual and auditory conditioned stimuli in C57BL/6J mice.

The fear-potentiated startle paradigm has been used with great success to examine conditioned fear in both rats and humans. The purpose of the present experiment was to extend the authors' previous findings and further validate the fear-potentiated startle paradigm in mice. In Experiments 1 and 2, C57BL/6J mice were given Pavlovian fear conditioning with either an auditory or a visual conditioned stimulus. Similar to data collected with rats, fear-potentiated startle was observed for both stimulus modalities. In Experiment 3, posttraining lesions of the amygdala disrupted fear-potentiated startle in both conditioned stimulus modalities. These data are consistent with amygdala lesion studies in rats and suggest that fear-potentiated startle in mice requires an intact amygdala. Together, these results extend the authors' previous results and provide the basis for using this well-understood behavioral paradigm for examining the molecular mechanisms of conditioned fear in transgenic and knockout mice. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of benzodiazepine microinjection into the amygdala or periaqueductal gray on the expression of conditioned fear and hypoalgesia in rats.

Four experiments studied the effects of an intracranial microinjection of a benzodiazepine (midazolam) on the expression of conditioned fear (measured as passive avoidance) and conditioned hypoalgesia in rats. Unilateral microinjection of midazolam into the basolateral amygdala reduced both hypoalgesic and avoidance responses, whereas unilateral microinjection of midazolam into the ventrolateral region of the periaqueductal gray (vPAG) reduced the hypoalgesic response but not the avoidance response. The results are discussed in terms of gamma-aminobutyric acid–ergic inhibition of antinociceptive mechanisms in the vPAG and of the activation of these mechanisms by amygdala-based fear processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of a microinjection of morphine into the amygdala on the acquisition and expression of conditioned fear and hypoalgesia in rats.

A unilateral microinjection of morphine into the amygdala impaired the acquisition of fear and hypoalgesic responses in rats exposed to a heated floor in a hot-plate apparatus. This impairment was dose dependent, receptor specific, and not observed in rats microinjected with morphine into the caudal basolateral amygdala. A microinjection of morphine into the amygdala reduced the expression of fear responses and of naloxone-sensitive hypoalgesic responses, but did not reduce the expression of naloxone-insensitive hypoalgesic responses. The results document an involvement of opioidergic mechanisms in the amygdala in learned danger and of the amygdala in the control of opioid hypoalgesic responses. They also suggest that learned danger can activate antinociceptive mechanisms independently of the amygdala. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of the central nucleus of the amygdala in olfactory heart rate conditioning.

The role of the central nucleus of the amygdala on olfactory heart rate conditioning in the infant rat was investigated. The conditioned stimulus (CS) consisted of a 10-s presentation of grape juice odor that was immediately followed by a 0.5-s, 0.35-mA subcutaneous shock. A sensitization control group was also run. Three days prior to testing, Ss received either bilateral electrolytic lesions of the central nucleus of the amygdala, sham lesions, or were left unperturbed. Results show that damage to the central nucleus of the amygdala severely impaired olfactory heart rate conditioning but that it had no deleterious effect on the heart rate orienting response to that stimulus or on the heart rate unconditioned response (UCR) to shock. Results are analogous to those in previous research on auditory heart rate conditioning and suggest that the central nucleus of the amygdala may constitute a necessary stage in the transduction of the CS into a cardiac conditioned response (CR) regardless of sensory modality. (PsycINFO Database Record (c) 2010 APA, all rights reserved)