Jeffrey B. Rosen

From normal fear to pathological anxiety.

In this article the authors address how pathological anxiety may develop from adaptive fear states. Fear responses (e.g., freezing, startle, heart rate and blood pressure changes, and increased vigilance) are functionally adaptive behavioral and perceptual responses elicited during danger to facilitate appropriate defensive responses that can reduce danger or injury (e.g., escape and avoidance). Fear is a central motive state of action tendencies subserved by fear circuits, with the amygdala playing a central role. Pathological anxiety is conceptualized as an exaggerated fear state in which hyperexcitability of fear circuits that include the amygdala and extended amygdala (i.e., bed nucleus of the stria terminalis) is expressed as hypervigilance and increased behavioral responsivity to fearful stimuli. Reduced thresholds for activation and hyperexcitability in fear circuits develop through sensitization- or kindling-like processes that involve neuropeptides, hormones, and other proteins. Hyperexcitability in fear circuits is expressed as pathological anxiety that is manifested in the various anxiety disorders.

A direct projection from the central nucleus of the amygdala to the acoustic startle pathway : anterograde and retrograde tracing studies

Previous work has shown that lesions of the central nucleus of the amygdala block fear-potentiated acoustic startle and that electrical simulation of the central nucleus enhances acoustic startle in rats. In the present study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was used to identify and delineate the course of a direct projection from the central nucleus of the amygdala to the nucleus reticularis pontis caudalis, a nucleus in the acoustic startle circuit. Experiments using the retrograde tracer Fluoro-Gold confirmed this and indicated that the rostral part of the medial subdivision of the central nucleus of the amygdala contains the cells that project to the startle circuit. With this information, lesion studies (see companion article Hitchcock & Davis, 1991) may be used to determine whether this projection plays a role in fear-potentiated startle.

Induction of the c-fos proto-oncogene in rat amygdala during unconditioned and conditioned fear

Induction of the nuclear proto-oncogene c-fos in rat amygdala was investigated 30-40 min following the presentation of mild footshocks (unconditioned fear) or of contextual cues associated with similar footshocks 24 h earlier (conditioned fear). Initially, it was found that handling rats for the first time elevated c-fos mRNA levels, but this response could be blocked completely by repeated handling. Unconditioned and conditioned fear both elevated amygdala c-fos mRNA dramatically above control levels.

Quenching: inhibition of development and expression of amygdala kindled seizures with low frequency stimulation.

Using low frequency (quenching) stimulation parameters (1 Hz for 15 min), similar to those that induce long-term depression (LTD) in vitro, we attempted to alter amygdala kindling in vivo in rats. Quenching completely blocked the development and progression of after-discharges and seizures in seven of eight animals. In fully kindled animals, once-daily quenching stimulation for one week (without concurrent kindling) suppressed the seizures when kindling stimulation was resumed. These effects of quenching probably resulted from the marked and long-lasting increases in the afterdischarge and seizure thresholds that were observed in these animals. These data indicate that quenching with low frequency electrical stimulation (which does not disrupt ongoing behavior) can have profound and long-lasting effects on seizure development, expression, and thresholds. The ultimate clinical applicability of low frequency stimulation in the treatment of seizures and related neuropsychiatric disorders remains to be explored.

Enhancement of acoustic startle by electrical stimulation of the amygdala

The present study demonstrated that electrical stimulation of the amygdala enhanced the acoustic startle response. A 25-ms train of 0.1-ms pulses initiated 5 ms before the onset of a 20-ms noise burst significantly increased startle at currents from 40 to 400 microA. Electrode placements just medial to the amygdala (in the pathway connecting the amygdala to the brain stem) increased startle with the lowest currents. Startle was also increased in all animals with stimulation in the central, medial, and intercalated nuclei of the amygdala. Stimulation in areas surrounding the amygdaloid complex was ineffective. In a second experiment, paired pulses with interpulse intervals between 0.1 and 20.0 ms delivered to the amygdala demonstrated that the stimulated axons had a distribution of refractory periods between 0.6 and 1.0 ms. This suggests that the population of neurons which subserves the enhancement of acoustic startle is fairly homogeneous and has small, myelinated axons.

Anxiety and the Amygdala: Pharmacological and Anatomical Analysis of the Fear-Potentiated Startle Paradigm

Publisher Summary This chapter describes the fear-potentiated startle paradigm and the advantages it provides for the pharmacological and neuroanatomical analyses of fear conditioning. It also discusses the pharmacological treatments that block fear-potentiated startle, the neural pathways involved in the startle reflex, and the role of the amygdala in fear-potentiated startle and its possible connections to the startle pathway and critical visual structures that carry information about the conditioned stimulus. The importance of the central nucleus of the amygdala and its efferent projections to several brainstem target areas for fear and anxiety is outlined in the chapter. Drugs can potentially act on afferent or efferent systems of the amygdala or in the amygdala itself; different classes of anxiolytic drugs may block fear or anxiety by acting at different points along the neural pathways involved in fear. While some drugs act by blocking the transmission of a conditioned stimulus to the amygdala, others might block transmission along its efferent pathways.

Hyperexcitability: exaggerated fear-potentiated startle produced by partial amygdala kindling.

The present study asked whether partial amygdala kindling would affect the expression of conditioned fear-potentiated startle. Rats were conditioned to be fearful of a light. They were then stimulated bilaterally in the amygdala or hippocampus on 2 consecutive days (partial kindling). Rats were tested 24 hr later for fear-potentiated startle. Amygdala-kindled rats had exaggerated fear-potentiated startle compared to sham-kindled rats. Hippocampus-kindled rats also displayed fear-potentiated startle, but no greater than that of sham-kindled rats. Partial amygdala kindling induced c-fos messenger RNA (mRNA) expression, a marker for neuronal activation, throughout the limbic and neocortices. In contrast, partial hippocampus kindling induced c-fos mRNA in the hippocampus only. The data suggest that kindled-induced hyperexcitability of the amygdala and limbic cortices produced exaggerated conditioned fear-potentiated startle.

Enhancement of electrically elicited startle by amygdaloid stimulation

Previous experiments showed that acoustic startle amplitude can be enhanced by electrical stimulation of the amygdala. Because the acoustic startle pathway is organized in a serial fashion, startle can be elicited electrically with progressively shorter latencies by stimulating different points along this pathway [i.e., ventral cochlear nucleus (VCN), paralemniscal zone (PLZ), nucleus reticularis pontis caudalis (RPC) or medial longitudinal fasciculus (MLF)]. The present study evaluated the temporal characteristics of the facilitatory effect of amygdaloid stimulation on startle elicited electrically from different points along the acoustic startle pathway. A single 0.1-msec pulse was delivered to the central nucleus of the amygdala at various times before the onset of a 1-msec pulse in various sites in the startle pathway. The shortest amygdaloid stimulation-startle onset interval to significantly enhance startle was 0 msec for the VCN, 2 msec for the PLZ, 3 msec for the RPC and 7 msec for the MLF. These results indicate that amygdaloid stimulation enhances electrically elicited startle in a temporal manner that is complementary to facilitation of acoustic startle. The similarity of amygdala-stimulated enhancement and fear potentiation of electrically elicited startle is also discussed.

Temporal characteristics of enhancement of startle by stimulation of the amygdala

A previous study demonstrated that electrical stimulation of the amygdala facilitated the acoustic startle reflex. The purpose of the present study was to determine the temporal relationship between activation of the amygdala and its enhancement of the acoustic startle reflex. A single 0.1 msec pulse was delivered to the central nucleus of the amygdala at various times before or after the onset of 20 msec noise burst or a 0.1 msec click. Startle was enhanced when the stimulation occurred within 5 msec of the startle stimulus onset. Electromyographic recordings from the neck muscles demonstrated that the short 6 msec latency startle response was facilitated even when amygdala stimulation was presented 1.25 msec after the onset of the startle stimulus. This indicates that the time for the effects of amygdala stimulation to reach the brain stem startle circuit is less than 5 msec, suggesting a very direct pathway from the amygdala to the startle circuit. The similarity of this effect to fear-potentiated startle is also discussed.

Differential regional and time course increases in thyrotropin-releasing hormone, neuropeptide Y and enkephalin mRNAs following an amygdala kindled seizure.

Previous studies have shown that neuropeptide mRNA expression is altered in the dentate gyrus, and pyriform, entorhinal and perirhinal cortices following amygdala kindling. However, because rats were kindled every day and some mRNA alterations last longer than 24 h, a true measure of the alterations induced by a single seizure was confounded by the previous days seizure. To circumvent this problem, rats were fully kindled, had six days without stimulation, and then were given one more seizure. Rats were sacrificed either 4 h, 24 h or 4 days after this last seizure. The levels of mRNAs for TRH, NPY and ENK were measured in the dentate gyrus and limbic cortices. Four hours after a seizure, TRH and NPY mRNAs were maximally increased in the dentate gyrus granule layer, but returned to baseline levels by 24 h. In contrast, 4 h after a seizure, TRH and NPY mRNAs were not, or only slightly, increased in the pyriform, entorhinal and perirhinal cortices, but significantly elevated 24 h after a seizure. ENK mRNA was increased both 4 and 24 h after a seizure in the pyriform, entorhinal and perirhinal cortices but showed no increases in the dentate gyrus at any time. By 4 days, peptide mRNA levels returned to baseline, except for ENK mRNA in the pyriform cortex. These results demonstrate a non-uniform and complex pattern of peptide mRNA expression following an amygdala kindled seizure. They further suggest that regional and time course differences in gene transcription and expression may be important factors in understanding both the transient, adaptive anticonvulsant and longer lasting proconvulsant effects of these neuropeptides.