Amygdaloid GABA, not glutamate neurotransmission or mRNA transcription controls footshock-associated fear arousal in the acoustic startle paradigm.

In Pavlovian conditioning the fear-evoking properties of the aversive unconditioned stimulus are represented by the conditioned stimulus. A major challenge for theories of classical fear conditioning has been to understand how associations are formed between a conditioned stimulus and unconditioned stimulus. Although the cellular mechanisms in the amygdala that underlie fear learning have received considerable attention relatively little is known about the neural substrates underlying unconditioned stimulus-associated fear. In the present study we examined the role of GABA(A), N-methyl-D-aspartic acid and non-N-methyl-D-aspartic acid receptors, and protein synthesis inhibition on the immediate fear arousal produced by footshock as measured by the shock sensitization of acoustic startle. Laboratory rats showed shock-enhanced startle after infusion into the basolateral amygdala of the N-methyl-D-aspartic acid receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (5.0 microg), the non-N-methyl-D-aspartic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione disodium (5.0 microg) and the protein synthesis inhibitor anisomycin (80.0 microg). We concluded that fear arousal provoked by footshock is not mediated by glutamate neurotransmission in the amygdala and does not involve de novo protein synthesis. Bilateral infusion into the basolateral amygdala of the GABA(A) receptor agonist muscimol in doses ranging from 0.001-0.5 microg reliably blocked the shock sensitization of acoustic startle responding. None of the muscimol doses altered shock reactivity amplitudes indicating the normal perception of footshock. The muscimol results were interpreted to suggest that decreased GABA neurotransmission in the amygdala may be essential for the neural causation of fear that is acquired and expressed by conditioned stimuli.

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.

The involvement of ventral tegmental area cholinergic muscarinic receptors in classically conditioned fear expression as measured with fear-potentiated startle.

Accumulating evidence suggests that dopamine (DA) neurons in the ventral tegmental area (VTA) contribute to the complex amygdala-based neurocircuitry that mediates fear-motivated behaviors. Because of acetylcholine's (ACh) role in DA neuronal activation, the involvement of VTA cholinergic muscarinic receptors in Pavlovian conditioned fear responding was evaluated in the present study. Fear-potentiated startle was used to assess the effects of intraVTA infused methylscopolamine on conditioned fear performance in laboratory rats. Application of this nonspecific muscarinic receptor antagonist to VTA neurons was observed to inhibit the ability of a conditioned stimulus (CS) previously paired with footshock to enhance the amplitude of the acoustic startle reflex. Doses of methylscopolamine that blocked conditioned fear expression did not alter baseline sensorimotor responding. These results identify ACh neurotransmission in the VTA as a potential excitatory mechanism underlying the fear-arousing properties of threatening environmental stimuli.

The effects of cocaine, amphetamine, and the dopamine D1 receptor agonist SKF 38393 on fear extinction as measured with potentiated startle: implications for psychomotor stimulant psychosis.

Using Pavlovian conditioned increases in the amplitude of the acoustic startle reflex as a behavioral indicator of fear motivation, the authors previously showed a resistance to extinction after repeated associations of cocaine with the fear-evoking conditioned stimulus (CS). In Experiment 1, acute administration of cocaine, amphetamine, and the dopamine (DA) D1 receptor agonist SKF 38393 produced a similar fear enhancement. In Experiment 2, a noncontingent injection of cocaine and SKF 38393 provoked a CS potentiation of acoustic startle in fear-extinguished laboratory rats. Potential behavioral, neurochemical, and neuroendocrine explanations for the effects of psychomotor stimulants on conditional fear were discussed. It was suggested that DA agonist drugs increase fear expression possibly by activating mesoamygdaloid associative neurocircuitry involved in excitatory conditioned fear reactions.

Infusion of the dopamine D1 receptor antagonist SCH 23390 into the amygdala blocks fear expression in a potentiated startle paradigm.

Dopamine (DA) D1 receptors are distributed in the nucleus accumbens and the amygdala, two regions of the mesocorticolimbic DA system known to be activated by aversive environmental stimuli. The objective of the present study was to determine the contribution of D1 receptors in these brain regions to the expression of a fear-motivated behavior, notably, potentiated startle in rats. Injection of the DA D1 receptor antagonist SCH 23390 into the amygdala blocked the ability of a conditioned light stimulus previously paired with footshock to enhance acoustic startle amplitudes. Bilateral intracerebral administration of SCH 23390 into the nucleus accumbens had no effect on fear-potentiated startle. The observed opposing effects of amygdaloid DA D1 receptor antagonism on fear expression, along with earlier research demonstrating the involvement of ventral tegmental area (VTA) DA neurons on fear-potentiated startle, suggest a role for mesoamygdaloid activity in conditioned excitatory fear reactions.

Contribution of ventral tegmental area dopamine neurons to expression of conditional fear: effects of electrical stimulation, excitotoxin lesions, and quinpirole infusion on potentiated startle in rats.

Potentiated startle was used in this study to determine the fear-motivational functions of the ventral tegmental area (VTA). In Experiment 1, electrical stimulation of the VTA increased acoustic startle amplitudes. In subsequent experiments fear-potentiated startle was assessed following axon-sparing N-methyl-D-aspartic acid (NMDA) lesions of the VTA and after bilateral intra-VTA infusion of the dopamine (DA) D2/3 receptor agonist quinpirole. The NMDA lesions produced substantial cell loss in the medial ventral tegmentum and suppressed fear expression. Similarly, inhibition of DA neuronal activity associated with locally administered quinpirole blocked fear-potentiated startle. It was suggested that VTA neurons and their forebrain DA projections regulate levels of aversive emotional arousal within the amygdala-based fear system.

Cocaine enhances the expression of fear-potentiated startle: evaluation of state-dependent extinction and the shock-sensitization of acoustic startle.

Cocaine's effects on fear extinction and on the shock-sensitization of acoustic startle were examined. Following fear acquisition, rats exposed to the nonreinforced conditioned stimulus (CS) after cocaine administration demonstrated significant levels of fear-potentiated startle when evaluated in the drug-free state. The CS also increased startle amplitudes in subjects extinguished and tested with cocaine, indicating that mechanisms other than state-dependent learning are involved in the extinction deficit. The presentation of 10 footshocks augmented acoustic startle, and the shock enhancement was unaffected by cocaine preexposure. These data indicate that the aversive consequences of footshock relevant to the acquisition of conditional fear are not sensitized by the drug. It was suggested that cocaine reinforces fear responding to a threatening stimulus.

Cocaine preexposure sensitizes conditioned fear in a potentiated acoustic startle paradigm.

The consequences of chronic cocaine administration on fear-potentiated startle were evaluated in two experiments. Cocaine treatment (40 mg/kg) for 7 days prior to fear acquisition (light + shock pairings) had an attenuating influence on the ability of the conditioned stimulus (CS) to increase acoustic startle. When cocaine was administered in the context of the CS, following fear conditioning, a marked enhancement of potentiated startle was observed. In contrast, an extinction of the fear response was seen in saline and procaine animals repeatedly exposed to the nonreinforced CS. The results from control subjects injected with cocaine either in the shock chambers (contextual cues) or in their home cage environment, suggest that the systemic effects of this stimulant served to intensify the fear-eliciting properties acquired by the CS during fear conditioning. These findings demonstrate a cocaine sensitization of conditioned fear, and were related to the emotional and psychological disturbances associated with long-term cocaine use.

Behavioral evidence implicating dopamine in sensorimotor arousal and norepinephrine in the sedative effects of antidepressant drugs.

The effects of acute and chronic antidepressant treatment on acoustic startle were evaluated in three experiments. Administration of 2.5-10.0 mg/kg desipramine, amitriptyline, and nortriptyline depressed acoustic startle responding after repeated sensory stimulation. In contrast to the tricyclic drugs, the serotonin reuptake inhibitor zimelidine increased acoustic startle, and inhibition of dopamine reuptake following acute nomifensine and bupropion administration did not influence startle reactivity in the doses examined. The response reducing effects of desipramine and amitriptyline persisted following chronic exposure to these drugs, and these findings were discussed in relation to the inhibitory actions of the tricyclics on locus coeruleus neurons. A second major finding in this study was that animals challenged with d-amphetamine during desipramine and amitriptyline withdrawal showed a facilitated startle response. Enhanced startle reactivity to amphetamine was also observed following long-term exposure to iprindole, and a withdrawal hyperactivity of acoustic startle was evident after chronic treatment with amoxapine, bupropion, and nomifensine. These results agree with evidence that repeated administration of antidepressants increases dopamine neurotransmission which modulates sensorimotor arousal.

Amphetamine withdrawal: a behavioral evaluation.

The effects of withdrawal from long-term amphetamine treatment of intracranial self-stimulation, forced swim-induced immobility, shuttle escape performance, acoustic startle and locomotor activity were evaluated. Mice implanted with stimulating electrodes in the lateral hypothalamus demonstrated stable and reliable rates of self-stimulation responding. After exposure to a chronic schedule of amphetamine treatment response rates were severely depressed. In addition to modifying intracranial self-stimulation responding, amphetamine withdrawal increased the duration of immobility in a forced-swim situation. Although chronic amphetamine exposure induced pronounced behavioral changes in the intracranial self-stimulation and forced swim tasks, drug withdrawal had little effect on shuttle escape performance, acoustic startle and locomotor activity. Based on these findings it was suggested that the development of post-amphetamine depression in the self-stimulation and forced swim paradigms was not related to variations in motoric or arousal mechanisms resulting from amphetamine withdrawal, but rather involved drug-induced changes in motivational processes.

Effects of chronic intermittent and continuous amphetamine administration on acoustic startle.

Acoustic startle was evaluated after mice were exposed to two different schedules of long-term amphetamine treatment. Under one schedule, mice received two daily subcutaneous injections of d-amphetamine for 7 consecutive days, whereas the other consisted of continuous administration of amphetamine via a subcutaneously implanted minipump. The enhanced acoustic startle observed after a test dose of d-amphetamine (3.0 mg/kg) was further facilitated when animals were exposed to long-term intermittent amphetamine administration. In contrast, the enhanced startle response to amphetamine was attenuated when mice received chronic continuous exposure to amphetamine. Possible behavioral and neurochemical mechanisms that may be involved in the development of tolerance after continuous amphetamine administration, and reverse tolerance after intermittent amphetamine treatment were discussed.

Stress-induced facilitation of acoustic startle after d-amphetamine administration.

Administration of d-amphetamine enhanced the startle response to an auditory stimulus. In contrast to saline treated mice, startle activity after amphetamine administration did not wane with repeated exposure to the auditory stimulus. Rather, the effects of amphetamine on startle activity increased as a function of stimulus presentation. Whereas exposure to isolation stress or inescapable shock had no effect on startle activity, both types of stress potentiated the effects of amphetamine on startle arousal. The observation that stress sensitized animals to later amphetamine administration is consistent with the effects of stress on other amphetamine behaviors, e.g., stereotypy. Results were related to the development of dopamine post-synaptic receptor supersensitivity after exposure to stress and were discussed in terms of the role played by stress in the expression of behavioral arousal, in the etiology of schizophrenia.

Potentiation of d-amphetamine and L-dopa-induced acoustic startle activity after long-term exposure to amphetamine.

The startle response to an auditory stimulus was potentiated by treatment with d-amphetamine sulfate. Administration of L-dopa after pretreatment with the extracerebral decarboxylase inhibitor MK-486 also increased startle activity. After long-term exposure to amphetamine the startle response to L-dopa and d-amphetamine was enhanced. These findings are consistent with the consequences of long-term amphetamine administration on other amphetamine-induced behaviors (e.g. stereotype), and are discussed in terms of the effects of long-term amphetamine treatment on pre- and postsynaptic dopamine receptors and serotonin.