The acoustic startle reflex: neurons and connections

Habituation of the acoustic and the tactile startle responses in mice: two independent sensory processes

To test whether habituation is specific to the stimulus modality, the authors analyzed cross-habituation between the tactile startle response' (TSR) and the acoustic startle response (ASR). The acoustic artifacts of airpuffs used to elicit the TSR were reduced by using a silencer and were effectively masked by background noise of 90-100 dB sound-pressure level. ASR was elicited by 14-kHz tones. TSR and ASR habituated in DBA and BALB mice: both the TSR and ASR habituated to a greater extent in DBA mice than in BALB mice. In both strains, habituation of the TSR did not generalize to the ASR, and vice versa. From this, the authors concluded that habituation of startle is located in the sensory afferent branches of the pathway.

Can prepared responses be stored subcortically?

Quick voluntary responses to environmental stimuli are required of people on a daily basis. These movements have long been thought to be controlled via cortical loops involving processing of the stimulus and generation of a suitable response. Recent experiments have shown that in simple reaction time (RT) tasks, the appropriate response can be elicited much earlier (facilitated) when the "go" signal is replaced by a startling (124 dB) auditory stimulus. In the present experiment we combined a startling acoustic stimulus with an established RT paradigm that involved simple and choice RT. In a simple RT condition the prepared voluntary response was elicited at very short latencies following the startle. However, when cortical processing was required prior to responding (choice RT task), the startle did not facilitate the voluntary response, and gave rise to more movement production errors. Since movements requiring ongoing cortical processing following the stimulus are not facilitated by startle, it is unlikely that the startle facilitation is due to increased neural activation. In contrast, it appears more likely that the startle acts as an early trigger for subcortically stored prepared movements since movements that are prepared in advance can be initiated at such short latencies (<60 ms).

Regional dissociations within the hippocampus--memory and anxiety

The amnestic effects of hippocampal lesions are well documented, leading to numerous memory-based theories of hippocampal function. It is debatable, however, whether any one of these theories can satisfactorily account for all the consequences of hippocampal damage: Hippocampal lesions also result in behavioural disinhibition and reduced anxiety. A growing number of studies now suggest that these diverse behavioural effects may be associated with different hippocampal subregions. There is evidence for at least two distinct functional domains, although recent neuroanatomical studies suggest this may be an underestimate. Selective lesion studies show that the hippocampus is functionally subdivided along the septotemporal axis into dorsal and ventral regions, each associated with a distinct set of behaviours. Dorsal hippocampus has a preferential role in certain forms of learning and memory, notably spatial learning, but ventral hippocampus may have a preferential role in brain processes associated with anxiety-related behaviours. The latter's role in emotional processing is also distinct from that of the amygdala, which is associated specifically with fear. Gray and McNaughton's theory can in principle incorporate these apparently distinct hippocampal functions, and provides a plausible unitary account for the multiple facets of hippocampal function.

Kynurenate in the pontine reticular formation inhibits acoustic and trigeminal nucleus-evoked startle, but not vestibular nucleus-evoked startle

The startle reflex is elicited by acoustic, trigeminal or vestibular stimulation, or by combinations of these stimuli. Acoustic startle is mediated largely by ibotenate-sensitive neurons in the ventrocaudal pontine reticular formation (PnC). In these studies we tested whether startle elicited by stimulation of different modalities is affected by infusion of the non-selective glutamate antagonist, kynurenate, into the PnC. In awake rats, startle responses evoked by either acoustic or spinal trigeminal nucleus stimulation were inhibited by kynurenate, but not saline, infusions, with the most effective placements nearest PnC. In chloral hydrate-anesthetized rats, kynurenate in the PnC reduced trigeminal nucleus-evoked hindlimb EMG responses, but not vestibular nucleus-evoked startle. Kynurenate in the vestibular nucleus had no effect on trigeminal nucleus-evoked startle. These results indicate that trigeminal nucleus stimulation evokes startle largely through glutamate receptors in the PnC, similarly to acoustic startle, but vestibular nucleus-evoked startle is mediated through other pathways, such as the vestibulospinal tract.

Corticotropin-releasing factor receptors CRF1 and CRF2 exert both additive and opposing influences on defensive startle behavior

The corticotropin-releasing factor (CRF) receptors (CRF1 and CRF2) are crucial mediators of physiological and behavioral responses to stress. In animals, CRF1 appears to primarily mediate CRF-induced anxiety-like responses, but the role of CRF2 during stress is still unclear. Here we report the effects of CRF1 and CRF2 on the magnitude and plasticity of defensive startle responses in mice. Startle plasticity is measured by inhibition of startle by sensory stimuli, i.e., prepulse inhibition (PPI), and is disrupted in patients with panic or posttraumatic stress disorders in which CRF neurotransmission may be overactive. Pharmacological blockade of CRF1 reversed both CRF-induced increases in startle and CRF-induced deficits in PPI. CRF2 blockade attenuated high-dose but not low-dose CRF-induced increases in startle and reduced PPI. Conversely, activation of CRF2 enhanced PPI. CRF had no effect on startle and increased PPI in CRF1 knock-out mice. These data indicate that CRF receptors act in concert to increase the magnitude of defensive startle yet in opposition to regulate the flexibility of startle. These data support a new model of respective CRF receptor roles in stress-related behavior such that, although both receptors enhance the magnitude of defensive responses, CRF1 receptors contravene, whereas CRF2 receptors enhance, the impact of sensory information on defensive behavior. We hypothesize that excessive CRF1 activation combined with reduced CRF2 signaling may contribute to information processing deficits seen in panic and posttraumatic stress disorder patients and support CRF1-specific pharmacotherapy.

Enhancement of acoustic evoked potentials and impairment of startle reflex induced by reduction of GABAergic control of the neural substrates of aversion in the inferior colliculus

The neural network of the inferior colliculus (IC), implicated in the generation of defensive behavior to aversive acoustic stimuli, is under tonic GABAergic control. Dopamine also seems to have a modulatory role in these neural circuits. It is still unclear how such changes in transmission of acoustic information influence the motor expression of the defensive behavior. Startle reaction to a sudden noise has been used as an effective way to measure the motor reactivity of rats to fearful acoustic stimuli. In this work we examined the processing of sensorial information--assessed by the recording of auditory evoked potentials (AEP)--and the behavioral effects--evaluated by the freezing and startle responses--during the reduction of GABA levels caused by microinjections of semicarbazide (SMC, 6 microg/0.2 microl), a glutamic acid decarboxylase inhibitor, into the IC. These data were compared to the effects of the overall arousal elicited by apomorphine (APO, 0.5 mg/kg, i.p.). The results obtained show that IC microinjections of SMC induced freezing behavior, enhanced the AEP and impaired the startle reaction to a loud sound. On the other hand, APO changed neither the AEP nor the startle in the same experimental conditions. These results suggest that the release of GABAergic control of the neural substrates of aversion in the IC results in an increased processing of auditory information along with an inhibitory influence on the motor pathways responsible for the startle response.

Auditory startle response in cervical dystonia

The excitability of brainstem neurons is abnormally enhanced in patients with cervical dystonia (CD), but the extend of such abnormality is not known. We examined whether patients with CD showed abnormalities in the auditory startle response (ASR), a brainstem reflex elicited by an unexpected loud stimulus. Thirteen patients with CD were investigated 3 months after botulinum toxin treatment. Thirteen healthy volunteers served as controls. ASRs were elicited by binaural high-intensity auditory stimuli. Reflex electromyographic (EMG) activity was recorded simultaneously with surface electrodes bilaterally from masseter, orbicularis oculi, sternocleidomastoid, and biceps brachii muscles. We found that ASR onset latencies were similar for patients and controls. CD patients had significantly lower ASR probabilities than controls (P = 0.007). ASR area under the curve was significantly smaller in CD patients (P = 0.017). Similar to controls, patients showed a significant habituation of ASR (P < 0.001, each); however, CD patients showed a prolonged tonic or phasic EMG activity after the initial ASR that was not observed in controls. Normal latencies and recruitment pattern indicate a preserved organization of intrinsic neural pathways mediating ASR in CD. Reduced ASR probability and magnitude as well as prolonged EMG activity after the proper startle response corroborate and extend previous findings on brainstem dysfunction in CD.

Anteromedial temporal lobe damage blocks startle modulation by fear and disgust

The acoustic startle reflex (ASR) is potentiated during negative emotion, but attenuated during positive emotional experience. The modulation of the ASR by fear depends critically on the amygdala. The authors investigated ASR modulation to fearful, disgusting, pleasant, and neutral stimuli in 12 patients with unilateral damage to the anteromedial temporal lobe including the amygdala (6 left, 6 right), 1 patient with bilateral temporal lobe damage including the amygdala, and 12 comparison participants. Both groups with unilateral damage, as well as the subject with bilateral damage, showed a complete lack of ASR potentiation to both fear and disgust stimuli. The findings suggest that potentiation of the ASR by disgust and fear depends on the integrity of the anteromedial temporal lobe.

Neonatal thyroxine treatment: changes in the number of corticotropin-releasing-factor (CRF) and neuropeptide Y (NPY) containing neurons and density of tyrosine hydroxylase positive fibers (TH) in the amygdala correlate with anxiety-related behavior of wistar rats

Neonatal hyperthyroidism induces persisting alterations in the adult brain, e.g. in spatial learning and hippocampal morphology. In the present study, the relationship between anxiety-related behavior and amygdala morphology was investigated in the adult rat after transient neonatal hyperthyroidism (daily s.c. injections of 7.5 microg L-thyroxine in 0.5 ml 0.9% NaCl solution from postnatal day p1 to p12). The behavioral tests used to study anxiety-related behavior were the motility test, elevated plus-maze and fear-sensitized acoustic startle response. In the amygdala, the number of neurons containing the anxiogenic peptide corticotropin releasing factor (CRF-ir and CRF mRNA) and anxiolytic neuropeptide Y (NPY-ir), the total number of neurons and the density of tyrosine hydroxylase immunoreactive (TH-ir) fibers were quantified. Thyroxine-treated pups presented an accelerated development including opening of eyes and snout elongation as typical signs of hyperthyroidism. Thyroxine-treated adult animals displayed a reduced anxiety in the motility box and elevated plus maze, a reduction in the number of CRF-ir neurons in the central nucleus of the amygdala, as well as an increase in the number of NPY-ir neurons and density of TH-ir fibers in nuclei of the basolateral complex of the amygdala. Moreover, there was a reduction in the total number of neurons in all nuclei of the basolateral complex (despite the higher number of NPY-ir neurons), but not central nucleus of the amygdala. The number of CRF-ir neurons in the central nucleus correlated positively with anxiety-related behavior, and the number of NPY-ir neurons and the density of TH-ir fibers in the basolateral complex correlated inversely with anxiety-related behavior. The findings suggested a shift toward an anxiolytic rather than anxiogenic distribution of peptidergic neurons and fibers in the amygdala at adult age following transient neonatal hyperthyroidism.

High-resolution fragmentary decomposition—a model-based method of non-stationary electrophysiological signal analysis

Fragmentary decomposition (FD) is a recently developed method of non-stationary electrophysiological signal analysis addressed to mass potentials, such as electromyogram (EMG), event-related potential (ERP), evoked potential, electroencephalogram (EEG), electroretinogram, etc. Being supported by the generally accepted physiological notion that a peak is a functionally meaningful component of a mass potential, FD provides a way to avoid averaging and, instead, quantifies the component composition of complex electrophysiological signals directly from single-trials. The major computational procedures of FD include adaptive segmentation, the frequency domain component identification, and creation of the signal model as a linear aggregation of multiple components, with the generic mass potential (GMP) being the universal component template. This paper presents an improved, high-resolution FD technique which allows the resolution of overlapping sub-components and supports each identified component by an individual model. On the basis of this methodological innovation, we define two fundamental categories of multi-peak component waveforms: complex components (CC), comprised of multiple sub-components (GMPs), versus monolithic components (MC), involving a single GMP. We show that quantification of MCs and CCs from single-trial eyeblink EMG and single-trial ERP provides a more comprehensive analysis of these signals. Given single-trial eyeblink EMG, we find that the stimulus elicits strong though short-term (phasic) effects on MCs and moderate but long-lasting (tonic) effects on CCs. A new realm of single-trial ERP quantification is possible in that the MC appears as a marker of a single cognitive variable whereas the CC appears as a marker of a series of functionally related cognitive variables. The engagement of the brain in a specific cognitive task is accompanied by an increase of CCs in single-trial ERPs, which is especially informative with respect to the P3 cognitive potential. New methodology provides evidence for the three basic types of single-trial P3 sub-components: monolithic P3a, monolithic P3b, and a complex component, P3ab, which includes both P3a and P3b as sub-components.

Mutations in deadly seven/notch1a reveal developmental plasticity in the escape response circuit

The relatively simple neural circuit driving the escape response in zebrafish offers an excellent opportunity to study properties of neural circuit formation. The hindbrain Mauthner cell is an essential component of this circuit. Mutations in the zebrafish deadly seven/notch1a (des) gene result in supernumerary Mauthner cells. We addressed whether and how these extra cells are incorporated into the escape-response circuit. Calcium imaging revealed that all Mauthner cells in desb420 mutants were active during an elicited escape response. However, the kinematic performance of the escape response in mutant larvae was very similar to wild-type fish. Analysis of the relationship between Mauthner axon collaterals and spinal neurons revealed that there was a decrease in the number of axon collaterals per Mauthner axon in mutant larvae compared with wild-type larvae, indicative of a decrease in the number of synapses formed with target spinal neurons. Moreover, we show that Mauthner axons projecting on the same side of the nervous system have primarily nonoverlapping collaterals. These data support the hypothesis that excess Mauthner cells are incorporated into the escape-response circuit, but they divide their target territory to maintain a normal response, thus demonstrating plasticity in the formation of the escape-response circuit. Such plasticity may be key to the evolution of the startle responses in mammals, which use larger populations of neurons in circuits similar to those in the fish escape response.

Anxiety-related behavior and densities of glutamate, GABAA, acetylcholine and serotonin receptors in the amygdala of seven inbred mouse strains

The amygdala is a brain region involved in the regulation of anxiety-related behavior. The purpose of this study was to correlate anxiety-related behavior of inbred mouse strains (BA//c, BALB/cJ, C3H/HeJ, C57BL/6J, CPB-K, DBA/2J, NMRI) to receptor binding in the amygdala. Binding site densities of receptors (NMDA, AMPA, kainate, GABA(A), serotonin, muscarinergic M(1)-M(2)) were measured with quantitative receptor autoradiography using tritiated ligands. Measurements of fear-sensitized acoustic startle response (ASR; induced by footshocks), elevated plus maze (EPM) behavior and receptor binding studies showed differences between the strains except for AMPA and muscarinergic M(2) receptors. Factor analysis revealed a Startle Factor with positive loadings of the density of serotonin and kainate receptors, and the amplitudes of the baseline and fear-sensitized ASRs. A second Anxiety-related Factor only correlated with the fear-sensitized ASR and anxiety parameters on the EPM but not receptor densities. There were also two General Activity Factors defined by (negative) correlations with entries to closed arms of the EPM. Because the density of NMDA and muscarinergic M(1) receptors also correlated negatively with the two factors, these receptors had a positive effect on general activity. In contrast, correlations of GABA(A), serotonin, and kainate receptors had the opposite sign as compared to closed arm entries. It is concluded that hereditary variations in the amygdala, particularly in kainate and serotonin receptors, play a role for the baseline and fear-sensitized ASR, whereas the general activity is influenced by many neurotransmitter receptor systems.

Acoustic startle evokes bilaterally synchronous oscillatory EMG activity in the healthy human

Despite animal evidence that the reticulospinal system is of major importance to movement, this motor pathway has remained relatively inaccessible to experimentation in the human. Consequently, little is known about its function in health and disease. Here, we use the acoustic startle response to demonstrate that one type of reticulospinal activity in the human is associated with a characteristic pattern of bilateral synchronization between motor units. Surface electromyography (EMG) was recorded from upper limb muscles in 15 healthy subjects during the reflex startle to unexpected acoustic stimulation, voluntary movements mimicking the startle and during sustained voluntary tonic contraction. Frequency analysis demonstrated autospectral peaks at approximately 14 Hz in deltoid and biceps muscles only during the startle reflex. Similarly, coherence spectra of the EMG recorded between homologous proximal upper limb muscles demonstrated a peak centered approximately 12-16 Hz during reflex startles. Coherence in the 10- to 20-Hz band was significantly greater in the startle reflex than during voluntary sham startles or voluntary tonic contraction for deltoid, but not first dorsal interosseous, muscles. The coherence at 10-20 Hz between EMGs from homologous muscles represents a potential surrogate measure of reticulospinal activity that may be useful in determining the contribution of the reticulospinal system to different types of movement in health and disease.

Acoustic startle and prepulse inhibition in 40 inbred strains of mice

A high-throughput phenotype screening protocol was used to measure the acoustic startle response (ASR) and prepulse inhibition (PPI) in mice. ASRs were evoked by noise bursts; prepulses for PPI were 70 dB sound pressure level tones of 4, 12, and 20 kHz. Forty inbred strains of mice were tested (in most cases using 10 males and 10 females of each strain). The data on both the ASR and PPI had high internal and test-retest reliability and showed large differences among inbred strains, indicative of strong genetic influences. Previously obtained measures of hearing sensitivity in the same inbred strains were not significantly correlated with ASR or PPI measures.

Hippocampal modulation of sensorimotor processes

While the hippocampus makes unique contributions to memory, it has also long been associated with sensorimotor processes, i.e. innate processes involving control of motor responses to sensory stimuli. Moreover, hippocampal dysfunction has been implicated in neuropsychiatric diseases, such as schizophrenia and anxiety disorders, primarily characterized by non-mnemonic deficits in the processing of and responding to sensory information. This review is concerned with the hippocampal modulation of three sensorimotor processes in rats-locomotor activity, prepulse inhibition (PPI) of the startle reflex, and the startle reflex itself-whose alterations are related to human psychosis or anxiety disorders. Its main purpose is to present and discuss the picture emerging from studies examining the effects of pharmacological manipulations of the dorsal and ventral hippocampus by local drug microinfusions. While a role of the hippocampus in regulating locomotor activity, PPI, and startle reactivity has also been suggested based on the effects of hippocampal lesions, the microinfusion studies have revealed additional important details of this role and suggest modifications of notions based on lesion studies. In summary, the microinfusion studies corroborate that hippocampal mechanisms can directly influence locomotor activity, PPI, and startle reactivity, and that aberrant hippocampal function may contribute to neuropsychiatric diseases, in particular psychosis. The relation between different sensorimotor processes and hippocampal neurotransmission, the role of ventral and dorsal hippocampus, and the extrahippocampal mechanisms mediating the hippocampal modulation of different sensorimotor processes can partly be dissociated. Thus, the hippocampal modulation of these sensorimotor processes appears to reflect multiple operations, rather than one unitary operation.

Altered triggering of a prepared movement by a startling stimulus

An experiment is reported that investigated the effects of an auditory startling stimulus on a compound movement task. Previous findings have shown that, in a targeting task, a secondary movement can be initiated based on the proprioceptive information provided by a primary movement. Studies involving the presentation of a startling stimulus have shown that in reaction time (RT) tasks, prepared ballistic movements could be released early when participants are startled. In the present study we sought to determine whether the secondary component in an ongoing movement task, once prepared, could also be triggered by a startling stimulus. Participants performed a slow active elbow extension (22 degrees /s), opening their hand when the arm passed 55 degrees of extension from the starting point. An unexpected 124 dB startle stimulus was presented 5, 25, or 45 degrees into the movement. Findings showed that, when participants were startled, the secondary component was triggered despite incongruent kinesthetic information. However, this only occurred when the startle was presented late in the primary movement. This suggests that the secondary movement was not prepared prior to task initiation, but was "loaded" into lower brain structures at some point during the movement in preparation to be triggered by the CNS. This occurred late in the movement sequence, but >/=400 ms prior to reaching the target. These findings indicate that, in addition to ballistic RT tasks, a startle can be used to probe response preparation in ongoing compound movement tasks.

Cellular mechanisms of the trigeminally evoked startle response

The startle response is an important mammalian model for studying the cellular mechanisms of emotions and of learning. It consists of contractions of facial and skeletal muscles in response to sudden acoustic, tactile or vestibular stimuli. Whereas the acoustic startle pathway is well described, only a few recent studies have investigated the tactile startle pathway. It was proposed that there is a direct projection from the principal sensory nucleus to the central sensorimotor interface of the startle response, which is formed by the giant neurons in the caudal pontine reticular formation. We explored this projection in greater detail in vitro. Anterograde tracing in rat brain slices confirmed projections with large axon terminals from the ventral part of the principal sensory nucleus to the lateral caudal pontine reticular formation. Electrophysiological studies revealed a monosynaptic glutamatergic connection between principal sensory nucleus neurons and caudal pontine reticular formation giant neurons. The synapses displayed paired-pulse facilitation at high-frequency stimulation, and homosynaptic depression at 1 Hz stimulation. The latter form of plasticity is thought to underlie habituation of the startle response. Furthermore, postsynaptic currents in caudal pontine reticular formation giant neurons evoked by principal sensory nucleus neuron stimulation summed in a linear way with signals evoked by stimulation of auditory afferents. Synaptic plasticity and summation of synaptic currents correspond well with in vivo data previously published by other groups. We thus presume that these synapses mediate trigeminal input to the startle pathway.

Stimulus-induced drop episodes in Coffin-Lowry syndrome

OBJECTIVE

Coffin-Lowry syndrome (CLS) is a rare disorder characterized by moderate to severe mental retardation, facial dysmorphism, tapering digits, and skeletal deformity. Paroxysmal drop attacks occur in patients with CLS, characterized by sudden loss of muscle tone induced by unexpected tactile or auditory stimuli. Our objective is to characterize these attacks better using neurophysiologic studies.

METHODS

We report 2 teenage boys with CLS and stimulus-induced drop episodes (SIDEs). Simultaneous surface electromyogram (EMG) and video electroencephalogram were performed during SIDEs on our 2 patients.

RESULTS

Both patients had SIDEs stimulated by a loud noise, unexpected light touch stimulation, or visual threat that were characterized by abrupt episodes of complete or partial loss of lower extremity tone. These events were not associated with impairment of consciousness, and immediate recovery was noted. Simultaneous surface EMG and video electroencephalogram revealed no epileptiform discharges in either patient. In the first patient, after unexpected tactile or auditory stimulation, tonic EMG activity in paraspinal muscles was lost briefly, similar to that seen in cataplexy. In the second patient, at 6 years of age, sudden nonepileptic drop episodes were induced by an unexpected tactile, auditory, or visual stimulation. At 11 years of age, his episodes had changed to brief myoclonic jerk and tonic spasm that were triggered by unexpected tactile and auditory stimuli. An increase in tonic EMG activity occurred during the attacks, consistent with hyperekplexia.

CONCLUSIONS

Our data suggest that SIDEs in CLS are a heterogeneous group of nonepileptic events that may manifest features of both cataplexy and hyperekplexia, even in the same patient.

Acoustic startle responses and temperament in individuals who stutter

Fourteen individuals who stutter and 14 individuals who do not stutter were presented with 10 bursts of white noise to assess the magnitude of their eyeblink responses as a measure of temperament. Both the magnitude of the eyeblink response to the initial noise burst and the mean of the 10 responses were significantly greater for the stuttering group. The Taylor-Johnson Temperament Analysis (R. M. Taylor & L P. Morrison, 1996) did not distinguish between the two groups, but informal follow-up statistics indicated that the Nervous subscale showed a significant group difference. Scores on this subscale were also significantly positively correlated with the magnitude of the startle response. A discriminant analysis demonstrated that although both the startle response and the nervous trait differentiated the two groups, the startle response measures were more powerful in making this differentiation.

Neurons of the superior olivary complex do not excite startle-mediating neurons in the caudal pontine reticular formation

The acoustic startle response is an important mammalian model for studying the cellular mechanisms of emotions and learning. Lesions in the superior olivary complex have been shown to attenuate the amplitude of the acoustic startle response, thus a substantial contribution of these neurons to the startle response was proposed. We here explored the putative connection of olivary neurons to the startle-mediating giant neurons in the reticular formation in rat brain slices in vitro. Tracing studies and electrical stimulation in the superior olivary complex revealed a strong connection; however it was not possible to distinguish between olivary neurons and traversing auditory fibres. Glutamate uncaging in the olivary complex excited a subpopulation of olivary neurons but never PnC giant neurons, as shown by patch-clamp recordings. This clearly contradicts an excitatory connection from olivary neurons to PnC giant neurons and thus an involvement of the superior olivary complex in eliciting a startle response.

Effects of ovariectomy and estradiol on acoustic startle responses in rats

Long-term (3 months) ovariectomized (OVX) rats were used to model hormone withdrawal as occurring in menopause. We previously reported alterations in brain dopamine (DA), GABA and serotonin receptors following ovariectomy in this model. To assess the functional effect of these biochemical changes, we compared rats that were intact, OVX and OVX-treated with 17beta-estradiol (E(2); OVX+E(2)) for 2 weeks on measures of their acoustic startle responses (ASR) and prepulse inhibition (PPI) of acoustic startle. The effects of a mixed D(1)/D(2) dopaminergic agonist, apomorphine (APO; 0.25, 0.5 and 0.75 mg/kg sc) were tested on ASR and PPI of acoustic startle. Without APO, all groups of rats showed no difference in baseline ASR or PPI of acoustic startle. Following administration of APO (0.25, 0.5 and 0.75 mg/kg), ASR was significantly increased in OVX rats compared to intact rats and this was corrected with E(2) treatment. In all groups of animals, APO decreased PPI of acoustic startle. APO disrupted PPI to a lesser extent in OVX animals with or without E(2) treatment compared to intact rats. However, when group differences in APO-induced ASR were statistically controlled for, there were no longer any differences in APO disruption of PPI among the three treatment groups. These results indicate that long-term ovariectomy has persistent effects on the modulation of ASR, and these effects can be at least partly corrected with E(2) replacement therapy.

Synaptic plasticity in the acoustic startle pathway: the neuronal basis for short-term habituation?

The aim of the present study was to analyse the cellular mechanism underlying short-term habituation of the acoustic startle response (ASR). We explored distinct synapses of the neuronal startle pathway in rat brain slices by patch-clamp recordings of giant neurons in the caudal pontine reticular formation. Presynaptic stimulation of auditory afferents by repeated bursts at 0.1 and 1 Hz led to an exponential decay of EPSC magnitudes. This homosynaptic depression (HSD) was reversible and repeatedly inducible after recovery. Many parameters of HSD in vitro match those of ASR habituation in vivo. The mechanisms underlying HSD are distinct from classical short-term plasticity: paired-pulse as well as paired-burst stimulation revealed a facilitation of the second EPSC, occurring in a much smaller time window up to interstimulus intervals of 200 ms. Pharmacological experiments demonstrated that HSD could be completely blocked by the group II and III metabotropic glutamate receptor antagonist MPPG. Similar results were obtained by CPPG, another group II and III antagonist. In contrast, HSD was not affected by the group I and II antagonist MCPG. We conclude that we found a form of synaptic depression in synapses within the primary startle pathway which correlates in many respects with short-term habituation of the ASR and which is presumably mediated by group III metabotropic glutamate receptors.

Age-related impairment in the 250-millisecond delay eyeblink classical conditioning procedure in C57BL/6 mice

In this study we tested 4-, 9-, 12-, and 18-month-old C57BL/6 mice in the 250-msec delay eyeblink classical conditioning procedure to study age-related changes in a form of associative learning. The short life expectancy of mice, complete knowledge about the mouse genome, and the availability of transgenic and knock-out mouse models of age-related impairments make the mouse an excellent species for expanding knowledge on the neurobiologically and behaviorally well-characterized eyeblink classical conditioning paradigm. Based on previous research with delay eyeblink conditioning in rabbits and humans, we predicted that mice would be impaired on this cerebellar-dependent associative learning task in middle-age, at ~9 months. To fully examine age differences in behavior in mice, we used a battery of additional behavioral measures with which to compare young and older mice. These behaviors included the acoustic startle response, prepulse inhibition, rotorod, and the Morris water maze. Mice began to show impairment in cerebellar-dependent tasks such as rotorod and eyeblink conditioning at 9 to 12 months of age. Performance in hippocampally dependent tasks was not impaired in any group, including 18-month-old mice. These results in mice support results in other species, indicating that cerebellar-dependent tasks show age-related deficits earlier in adulthood than do hippocampally dependent tasks.

Auditory gating processes and binaural inhibition in the inferior colliculus

Physiological/behavioral/perceptual responses to an auditory stimulus can be inhibited by another leading auditory stimulus at certain stimulus intervals, and have been considered useful models of auditory gating processes. Two typical examples of auditory gating are prepulse inhibition of the startle reflex and the precedence effect (echo suppression). This review summarizes studies of these two auditory gating processes with regard to their biological significance, cognitive modulation, binaural properties, and underlying neural mechanisms. Both prepulse inhibition and the precedence effect have gating functions of reducing the disruptive influence of the lagging sound, but prepulse inhibition has a much longer temporal window than the precedence effect. Attentional processes can modulate prepulse inhibition, and the listener's previous experience can modulate the precedence effect. Compared to monaural hearing, binaural hearing reduces prepulse inhibition but enhances the precedence effect. The inferior colliculus, the major structure of the auditory midbrain, plays an important role in mediating these two auditory gating processes, and inhibitory neural transmissions within the inferior colliculus may account for binaural inhibition observed in prepulse inhibition and lag suppression recorded in the inferior colliculus. The neural mechanisms underlying binaural inhibition in the inferior colliculus are also discussed.

Differences between SHR and WKY following the airpuff startle stimulus in the number of Fos expressing, RVLM projecting neurons

The neurocircuitry responsible for excessive stress-induced cardiovascular responses in genetic hypertensive rats remains elusive. Prior studies detailed a differential cardiovascular response profile to airpuff startle stimuli between Spontaneously Hypertensive (SHR) and Wistar Kyoto (WKY) rats. We recently identified strain differential Fos expression in the rostroventrolateral medulla (RVLM) and several RVLM projecting sites following airpuff startle. The current study sought to define RVLM projecting neurons that also express Fos following placement in the test chamber and administration of the airpuff startle stimulus. Unilateral iontophoretic micro-injections of fluorogold were made into the RVLM of 9-10 week old SHR and WKY rats. Two to three weeks later, animals were subjected to a series of 60 airpuff startle stimuli. Brains were double labeled for Fos and fluorogold. Single fluorogold and single Fos cells, and double labeled cells were found in the nucleus tractus solitarius (NTS), caudal ventral lateral medulla (CVLM), Kölliker fuse (KF), ventral lateral, lateral, and dorsal central gray, lateral hypothalamus (LH), and paraventricular nucleus of the hypothalamus (PVN). These data are consistent with the notion that the RVLM receives differential excitatory and/or inhibitory input from higher brain centers, perhaps contributing to differential Fos expression in the RVLM, differential autonomic responding, or both.

Effects of ketanserin and haloperidol on prepulse inhibition of the acoustic startle (eyeblink) response and the N1/P2 auditory evoked response in man

Contraction of the orbicularis oculi muscle in response to a sudden loud sound (acoustic startle response) and the N1/P2 component of the auditory evoked potential are both attenuated when a brief low-intensity stimulus is presented 30-500 ms before the 'startle-eliciting' stimulus (prepulse inhibition). Here, we report the effects of the serotonin (5-HT)2 receptor antagonist ketanserin and the D2 dopamine receptor blocking antipsychotic drug haloperidol on these responses. Fifteen males (aged 18-35 years) participated in four sessions at 7-day intervals, in which they received ketanserin 20 mg, ketanserin 40 mg, haloperidol 3 mg and placebo, according to a balanced double-blind design. Electromyographic (EMG) responses of the orbicularis oculi muscle and N1/P2 auditory evoked potentials were recorded in a 20-min session, 3 h after ingestion of haloperidol or 1 h after ingestion of ketanserin. Subjects received 40 trials in which 1-kHz sounds were presented: (i) 40 ms, 115 dB ('pulse alone' trials), and (ii) 40 ms, 85 dB, followed after 120 ms by 40 ms, 115 dB ('prepulse/pulse' trials). Mean amplitudes of the EMG response and the N1/P2 potential were derived from the pulse-alone trials and, in each case, percentage prepulse inhibition was calculated. Serum prolactin was measured after each treatment, and autonomic (heart rate, blood pressure, salivation) and psychological (visual analogue self-ratings of mood and alertness, critical flicker fusion frequency) measures were taken before and after each treatment. Ketanserin 40 mg significantly reduced the amplitude of the EMG response and both doses of ketanserin significantly suppressed prepulse inhibition of the response; haloperidol had no effect on EMG response amplitude or prepulse inhibition. Neither drug affected N1/P2 amplitude or prepulse inhibition of this response. Ketanserin, but not haloperidol, reduced subjective alertness and critical flicker fusion frequency. Haloperidol, but not ketanserin, elevated serum prolactin level. These results confirm that prepulse inhibition of the startle response and of the N1/P2 complex have different pharmacological sensitivities. The ability of ketanserin to attenuate the startle response may reflect its sedative action, as other drugs with sedative properties have also been found to attenuate the startle response in man. The ability of ketanserin to suppress prepulse inhibition of the startle response is consistent with previous evidence for the involvement of 5-HTergic mechanisms in the regulation of prepulse inhibition in man.

Elevated plus maze behavior, auditory startle response, and shock sensitivity in predisease and in early stage autoimmune disease MRL/lpr mice

Behaviors indicative of anxiety have been suggested to emerge with the onset of autoimmune disease in MRL/MpJ-Fas(lpr) (MRL/lpr) mice. This study extends the behavioral characterization of MRL/lpr and congenic MRL/MpJ+/+ (MRL/+) mice using the elevated plus maze (EPM), acoustic startle response, and foot-shock sensitivity tasks. In the elevated plus maze, predisease MRL/lpr mice exhibited less anxiety while MRL/lpr mice in the early stage of autoimmunity did not differ from age-matched control MRL/+ mice. MRL/lpr mice exhibited lower startle responses compared to MRL/+ mice. Similarly, predisease MRL/lpr mice were less reactive to various foot-shock levels than MRL/+ mice. Both the MRL/lpr and the MRL/+ strains exhibited startle habituation deficits, implicating the background MRL strain in the impairment in this process. These data do not support the hypothesis that increased anxiety is apparent with the emergence of autoimmune disease in MRL/lpr mice; however, anxiety may appear as the disease advances.

Effects of ketamine and apomorphine on inferior colliculus and caudal pontine reticular nucleus evoked potentials during prepulse inhibition of the startle reflex in rats

Prepulse inhibition (PPI) of the startle reaction to a strong sound is the reduction of this reaction elicited by a weak stimulus, a tone for example, when it precedes the startling sound. Its pharmacological sensitivity has been used to characterize antipsychotic drugs. Not much is known about the level of action of such drugs in the neuronal network involved in PPI. In the present study, evoked potentials from two key structures, the inferior colliculus (IC) and the caudal pontine reticular nucleus (PnC), were obtained in freely moving rats during standard startle and PPI tests, under ketamine (5 mg/kg) or apomorphine (0.5 mg/kg). In the IC, the potential evoked by the noise did not vary whether tested in basic or PPI conditions. Only minor changes were elicited by the drugs. In the PnC, the noise elicited an evoked potential that was reduced under PPI conditions. This alteration of the evoked potential was reversed by ketamine. The results obtained with apomorphine were not homogeneous either when considering the behavioral or the electrophysiological results.

Contributions of the vestibular nucleus and vestibulospinal tract to the startle reflex

The startle reflex is elicited by strong and sudden acoustic, vestibular or trigeminal stimuli. The caudal pontine reticular nucleus, which mediates acoustic startle via the reticulospinal tract, receives further anatomical connections from vestibular and trigeminal nuclei, and can be activated by vestibular and tactile stimuli, suggesting that this pontine reticular structure could mediate vestibular and trigeminal startle. The vestibular nucleus, however, also projects to the spinal cord directly via the vestibulospinal tracts, and therefore may mediate vestibular startle via additional faster routes without a synaptic relay in the hindbrain. In the present study, the timing properties of the vestibular efferent pathways mediating startle-like responses were examined in rats using electrical stimulation techniques. Transient single- or twin-pulse electrical stimulation of the vestibular nucleus evoked bilateral, startle-like responses with short refractory periods. In chloral hydrate-anesthetized rats, hindlimb electromyogram latencies recorded from the anterior biceps femoris muscle were shorter than those for stimulation of the trigeminal nucleus, and similar to those for stimulation of the caudal pontine reticular nucleus or ventromedial medulla. In awake rats, combining vestibular nucleus stimulation with either acoustic stimulation or trigeminal nucleus stimulation enhanced the whole-body startle-like responses and led to strong cross-modal summation without collision effects. In both chloral hydrate-anesthetized and awake rats, combining vestibular nucleus stimulation with ventromedial medulla stimulation produced a symmetrical collision effect, i.e. a loss of summation at the same positive and negative stimulus intervals, indicating a continuous connection between the vestibular nucleus and ventromedial medulla in mediating vestibular startle. By contrast, combining trigeminal nucleus stimulation with ventromedial medulla stimulation resulted in an asymmetric collision effect when the trigeminal nucleus stimulation preceded ventromedial medulla stimulation by 0.5 ms, suggesting that a monosynaptic connection between the trigeminal nucleus and ventromedial medulla mediates trigeminal startle. We propose that the vestibulospinal tracts participate strongly in mediating startle produced by activation of the vestibular nucleus. The convergence of the vestibulospinal tracts with the reticulospinal tract within the spinal cord therefore provides the neural basis of cross-modal summation of startling stimuli.

Focal pathological startle following pontine infarction

A 36-year-old male developed an acute right-sided weakness due to left-sided pontine infarction. Two months later, he first noticed sudden right elbow flexion in response to a loud unexpected noise. Detailed electrophysiological assessment was performed. A large, short-latency (median 39 msec), synchronous electromyographic discharge occurred in the right biceps brachii electrodes following a 50-msec, 120-dB 1-kHz tone burst, with habituation only with very short (30-second) interstimulus intervals. Less synchronous activity at longer latencies was present both in a number of right-sided arm muscles at rest and on the clinically unaffected side during a tonic voluntary contraction. We discuss possible underlying mechanisms and our reasons for considering this a focally enhanced startle response. Our report broadens the range of expression of acquired startle disorders.

The human startle reflex and pons activation: a regional cerebral blood flow study

Using positron emission tomographic measurements of regional cerebral blood flow, we report activation of a medial pons area in humans during acoustic startle stimulation. Eight healthy volunteers were scanned during rest and when presented startle-eliciting stimuli. We performed a theory-driven directed search for activity in the nucleus reticularis pontis caudalis, situated in the pons. Because habituation of cerebellar activity during acoustic startle repetition has been reported [Timmann, D., Musso, C., Kolb, F.P., Rijntjes, M., Juptner, M., Muller, S.P., Diener, H.C. & Weiller, C. (1998) J. Neurol. Neurosurg. Psychiatry 65, 771-773], we also predicted habituation in the cerebellum and in the pons as a function of startle repetition. Measurements of eye electromyography validated the presence of a startle response and its habituation. Analysis of regional cerebral blood flow revealed higher neural activity during startle stimulation than at rest in a medial pons area consistent with the location of the pontine reticular nucleus. As a consequence of startle repetition, regional cerebral blood flow increased in the medial cerebellum, and habituated in the ventral cerebellum and in a ventral pons area separate from the pontine reticular nucleus. In the ventral pons, but not in the pontine reticular nucleus, regional cerebral blood flow and the startle reflex were positively correlated. In the cerebellum both positive and negative correlations with the startle reflex were observed. Thus we conclude that the neurofunctional correlates of the startle circuit and its habituation in humans are similar to that previously described in animals.

Centrally administered corticotropin-releasing hormone and peripheral injections of strychnine hydrochloride potentiate the acoustic startle response in preweanling rats

Attempts to condition fear potentiation of startle (FPS) in rats younger than 23 days of age have not been successful, regardless of the type of aversively conditioned stimulus used (P. S. Hunt, R. Richardson, & B. A. Campbell, 1994; R. Richardson, G. Paxinos, & J. Lee, 2000; R. Richardson & A. Vishney, 2000). In the present study, the authors report that peripheral injections of strychnine hydrochloride, a glycine receptor antagonist, and intracerebroventricular infusions of corticotropin releasing hormone (CRH) both potentiated the acoustic startle response (ASR) in 16-18-day-old rats. Because strychnine and CRH have distinct sites of activation in the primary startle pathway, it can be concluded that this pathway is functional and modifiable in rats younger than 23 days of age. This finding suggests that the failure to observe conditioned FPS in preweanling rats is due to an immaturity of the secondary fear circuit responsible for enhancing the ASR during a fear state.

A neural network approach to the acoustic startle reflex and prepulse inhibition

Prepulse inhibition (PPI) is the normal suppression of the startle reflex when an intense stimulus is preceded by a weak non-startling prestimulus. PPI is widely used as a model for sensorimotor gating processes and has been shown to be impaired in various neuropsychiatric disorders, including schizophrenia. We have reproduced startle-like behavior and basic PPI modifications with a neural network. The network design was constrained by the attempt (1) to use as few connections as possible and (2) to relate neuroanatomical structures to the simulated network. Performance of the network was evaluated by the behavior of the simulated motor neurons in response to prepulse and pulse stimuli presented with various lead intervals and prepulse intensities. A delayed inhibitory pathway via the pedunculopontine nucleus (PPTg) to the caudal pontine reticular nucleus was found to be a necessary but insufficient requirement to reproduce basic PPI output patterns. Additional requirements included (a) a low threshold at or below the caudal pontine reticular formation, (b) signal amplification in the inhibitory pathway and (c) prolongation of activity in the inhibitory pathway. On the grounds of the most appropriate output patterns of the simulations, we propose a mechanism of sustained activation in the PPTg due to recursive connections. Relations between stimuli, behavior (motor output) and the underlying architecture are discussed. Potentially, this modeling technique can be extended to investigate the impact of drugs and higher brain regions on PPI.

Startle response of human neck muscles sculpted by readiness to perform ballistic head movements

1. An acoustic startle stimulus delivered in place of a 'go' signal in a voluntary reaction time (RT) task has been shown previously to advance the onset latency of a prepared distal limb movement without affecting the amplitude of the muscle response or movement kinematics. The primary goal of this study was to use muscles with a larger startle response to investigate whether the startling stimulus only triggered the RT movement or whether some form of interaction occurred between a startle response and a temporally advanced RT movement. 2. Twenty healthy male or female subjects were instructed to react as quickly as possible to an acoustic 'go' stimulus by performing a ballistic head flexion or right axial rotation. The 'go' stimulus was periodically replaced by an acoustic stimulus capable of eliciting a startle reflex. Separate startle-inducing stimuli under relaxed conditions before and after the movement trials served as control trials (CT trials). Bilateral surface electromyography of the orbicularis oculi, masseter, sternocleidomastoid and cervical paraspinal muscles, and head-mounted transducers were used to measure the muscle response and movement kinematics. 3. Muscle activation times in startled movement trials (ST trials) were about half those observed in RT trials, and were not significantly different from those observed in the startle CT trials. The duration of head acceleration was longer in ST trials than in RT trials and the amplitude of both the neck muscle electromyogram (EMG) and head kinematics was larger during ST trials than during RT trials. The EMG amplitude of ST trials was biased upward rather than scaled upward compared with the EMG amplitude of RT trials. 4. Over the 14 ST trials used in this experiment, no habituation of the reflex response was observed in the muscles studied. This absence of habituation was attributed to a combination of motor readiness and sensory facilitation. 5. The results of this experiment indicated that the neck muscle response evoked by a startling acoustic stimulus in the presence of motor readiness could be described as a facilitated startle reflex superimposed on a temporally advanced, pre-programmed, voluntary RT movement. Parallel reticular pathways to the neck muscle motoneurones are proposed as a possible explanation for the apparent summation of the startle and voluntary movement responses.

Electrophysiological responses of cochlear root neurons

Cochlear root neurons (CRNs) are second-order neurons interspersed among the fibers of the cochlear nerve in certain rodents. They project, among other nuclei, mainly to the pontine reticular nucleus, and participate in the acoustic startle response (ASR), a short-latency motor reflex initiated by sudden intense sounds. The sound-evoked activity of CRNs has not previously been described. Here we describe extracellular responses of CRNs located in the infranuclear portion of the cochlear nerve root. CRNs exhibited secure responses to tone bursts, with first-spike latencies of approximately 2.2 ms. The characteristic frequencies of the recorded CRNs were about 30 kHz, and the best-characterized CRN had a threshold of 10 dB sound pressure level and sharpness of tuning similar to that of cochlear nerve fibers. The peristimulus time histograms were primary-like with notch. The observed response properties were consistent with the suggestion that CRNs provide the short-latency acoustic input to the reticular formation that leads to an ASR.

Biological markers and diagnostic accuracy in the genetics of posttraumatic stress disorder

Family and twin studies suggest a substantial genetic contribution to the etiology of posttraumatic stress disorder (PTSD). Identification of the nature of this genetic contribution should enhance understanding of the pathophysiology of PTSD and suggest improved therapeutic strategies for its treatment. However, a broadly defined phenotype, specific requirement for an environmental exposure and high frequency of comorbid psychiatric illness all complicate genetic studies of PTSD. It is likely that genetic heterogeneity, incomplete penetrance, pleiotropy and the involvement of more than one gene all constitute formidable obstacles to the genetic analysis of PTSD. One way to circumvent these problems is to perform genetic analysis of traits associated with PTSD, rather than PTSD itself, an approach that has been fruitful for other diseases with complex modes of inheritance. Hypothalamic-pituitary-adrenal axis hypofunction, physiologic markers of increased arousal, and increased acoustic startle response are all potential PTSD-associated traits that might be susceptible to genetic analysis. However, the capacity of these traits to distinguish PTSD from non-PTSD patients and their familial pattern must be better defined before they can be employed in genetic studies.

Exogenous cortisol exerts effects on the startle reflex independent of emotional modulation

Exogenous cortisol's modulation of the acoustic startle reflex (ASR) was tested alone and during exposure to affectively valenced photographs in healthy men and women. During nonmodulated startle, oral hydrocortisone had a biphasic dose effect, with 5 mg increasing and 20 mg decreasing, eyeblink reflex magnitude compared to placebo. During emotion modulation, 20 mg of hydrocortisone reduced reflex magnitude without affecting the usual pattern of modulation across positive, neutral, and negatively affective slides. Gender differences were not found in either relationship. These findings illustrate dose-dependent effects of cortisol on the startle pathway independent of emotional state and consistent across genders.

Fear-potentiated startle, but not prepulse inhibition of startle, is impaired in CREBalphadelta-/- mutant mice

Fear-potentiated startle was assessed in mice with a targeted disruption of the alpha and delta isoforms of the transcription factor cAMP response element binding protein (CREB) 24 hr after 5 tone + shock training trials. Whereas wild-type mice showed fear-potentiated startle that persisted up to 45 days after training, CREBalphadelta-/- mice failed to show fear-potentiated startle. However, CREBalphadelta-/- and wild-type mice had similar startle amplitudes and similar magnitudes of prepulse inhibition of startle, suggesting that CREBalphadelta-/- mice have no obvious sensory or motor deficits. These results add to the literature indicating that CREB-activated transcription plays a critical role in the formation of long-term memory and illustrate the utility of the fear-potentiated startle paradigm for assessing cognition in genetically altered mice.

Modulation of the startle response during human gait

While many studies have shown that there is a phase-dependent modulation of proprioceptive and exteroceptive reflexes during gait, little is known about such modulation for auditory reflexes. To examine how startle reactions are incorporated in an ongoing gait pattern, unexpected auditory stimuli were presented to eight healthy subjects in six phases of the step cycle during walking on a treadmill at 4 km/h. For both legs, electromyographic activity (EMG) was recorded in the biceps femoris (BF), the rectus femoris (RF), the tibialis anterior (TA), and the soleus (SO). In addition, stance and swing phases of both legs, along with knee angles of both legs and the left ankle angle, were measured. All subjects showed various response peaks. Responses with latencies of approximately 60 ms (F1), approximately 85 ms (F2), and approximately 145 ms (F3) were found. The amplitude of the reflex responses was dependent on the timing of the startle stimulus in the step cycle. Although the startle response habituated rapidly, the phase-dependent modulation pattern generally remained the same. The phase-dependent amplitude modulations were not strictly correlated with the modulation of the background activity. The TA even showed a transition from facilitatory F2 responses during stance to suppressive responses during midswing. Responses were observed in both flexors and extensors, often in coactivation, especially during stance. Furthermore the gait characteristics showed a shortening of the subsequent step cycle and a small decrease in the range of motion of ankle and knees. These results suggest that the responses are adapted to achieve extra stability dependent on the phase of the step cycle. However, even in the first trials, the changes in kinematics were small allowing a smooth progression of gait.

Neural transplants. effects On startle responses in neonatally MSG-treated rats

Neonatal monosodium glutamate (MSG) treatment has been associated with dysfunctions in stress responses. Therefore, the present study aimed at examining the acoustic startle response (ASR) in MSG-treated rats and the effects of fetal neural transplantation. Male and female rats were given MSG (4 mg/g) or saline on alternate days from days 2-10 after birth. To determine whether fetal transplants could reverse behavioral impairments observed in MSG-treated rats, at 12 days of age MSG-treated rats received either arcuate nucleus (AN), cortical fetal grafts, or sham surgery into the third ventricle. ASR amplitude was measured at 35-40 days of age, and again in adulthood. MSG produced the expected decrease in the density of hypothalamic neuropeptide Y (NPY) and tyrosine hydroxylase (TH) in the AN area. Corticotropin-releasing factor (CRF) neurons/fibers were not affected by MSG. Pituitary atrophy was observed in all MSG rats. We report a permanent increase in the amplitude and reduction in short-term habituation of ASR in all MSG-treated rats. No effect was observed on long-term habituation in male rats. Cortical, but not AN tissue significantly reduced the magnitude of ASR in MSG animals. The results are discussed in terms of the central pathways mediating ASR, in particular hypothalamo-amygdala connections. It is considered that nonspecific factors mediate recovery produced by cortical tissue grafts, as observed in other models of neural transplantation.

Fear conditioned potentiation of the acoustic blink reflex in patients with cerebellar lesions

OBJECTIVE

To investigate whether the human cerebellum takes part in fear conditioned potentiation of the acoustic blink reflex.

METHODS

A group of 10 cerebellar patients (eight patients with lesions involving the medial cerebellum, two patients with circumscribed lesions of the cerebellar hemispheres) was compared with a group of 16 age and sex matched healthy control subjects. The fear conditioned potentiation paradigm consisted of three phases. During the first, habituation phase subjects received 20 successive acoustic blink stimuli. In the subsequent fear conditioning phase, subjects passed through 20 paired presentations of the unconditioned fear stimulus (US; an electric shock) and the conditioned stimulus (CS; a light). Thereafter, subjects underwent the potentiation phase, which consisted of a pseudorandom order of 12 trials of the acoustic blink stimulus alone, 12 acoustic blink stimuli paired with the conditioned stimulus, and six conditioned stimuli paired with the unconditioned stimulus. The EMG of the acoustic blink reflex was recorded at the orbicularis oculi muscles. The potentiation effect was determined as the difference in normalised peak amplitude of the blink reflex evoked by pairs of CS and acoustic blink stimuli and evoked by the acoustic stimulus alone.

RESULTS

In the habituation phase, short term habituation of the acoustic blink reflex was preserved in all cerebellar patients. However, in the potentiation phase, the potentiation effect of the blink reflex was significantly reduced in patients with medial cerebellar lesions compared with the controls (mean (SD) potentiation effect (%), patients: -6.4 (15.3), controls: 21.6 (35.6)), but was within normal limits in the two patients with lateral lesions.

CONCLUSIONS

The present findings suggest that the human medial cerebellum is involved in associative learning of non-specific aversive reactions-that is, the fear conditioned potentiation of the acoustic blink reflex.

Prenatal heroin exposure. Effects on development, acoustic startle response, and locomotion in weanling rats

The purpose of this study was to investigate the effects of prenatal heroin exposure on the offspring in postnatal behavioral development. Pregnant Sprague-Dawley rats were injected daily (s.c.) with 10mg/kg of heroin from gestational day 8 to 20. The control dam received saline injections and the pair-fed dam received saline and was yoked to a weight-matched heroin-treated dam. Litters were culled to eight to ten pups and weighed at postnatal day (PND) 1, 8, 15, and 22. Acoustic prepulse inhibition and habituation were parameters used for evaluating the sensorimotor gating and simple form of learning respectively. Locomotor activity and rearing were assessed using the photobeam activity system. All behavioral tests were performed on the offspring at PND 21 to 23. Results showed that heroin treatment significantly reduced maternal food intake, water consumption, and weight gain. Both heroin-exposed and pair-fed groups showed a marked reduction in birth weight in both male and female pups when compared with controls; however the postnatal weight gain in heroin-exposed pups was significantly lower than the pair-fed group by 3 weeks postnatally, particularly in the female pups. These female pups also showed a significant increase in ambulation and rearing when compared to the pair-fed pups. The habituation rate in both types of behavioral tests was also decreased in these female pups as compared to control and pair-fed groups. The present study indicated that prenatal heroin exposure could result in a marked retardation of postnatal development and learning. These effects are sex related.

The superior olivary complex is necessary for the full expression of the acoustic but not tactile startle response in rats

The acoustic startle response (ASR) in rats is mediated by an oligosynaptic pathway from the cochlea via the brainstem to spinal and cranial motoneurons. The present study tested whether the superior olivary complex (SOC) plays a role in the mediation of the ASR. The SOC receives auditory information from the ventral cochlear nuclei and projects to the caudal pontine reticular nucleus (PnC), the sensorimotor interface of the ASR. Axon-sparing excitotoxic lesions of the SOC strongly reduced the ASR amplitude and slightly prolonged ASR onset and peak latencies. The integrity of PnC which is adjacent to the SOC was confirmed by testing the tactile startle response which was not affected by SOC lesions. We suggest that the SOC is necessary for a full expression of the ASR and discuss possible auditory input structures involved in the mediation of the ASR.

Nerve growth factor (NGF) augments cortical and hippocampal cholinergic functioning after p75NGF receptor-mediated deafferentation but impairs inhibitory avoidance and induces fear-related behaviors

Nerve growth factor (NGF) enhances cholinergic functioning in animals with a compromised cholinergic basal forebrain (CBF). Immunotoxic lesions targeting low-affinity NGF receptor (p75NGF receptor)-bearing CBF neurons provide a selective model for testing the effects of NGF on residual cholinergic neurons. Rats received PBS or the immunotoxin 192IgG-saporin (192Sap) intracerebroventricularly at two doses (1 or 2.7 microg) known to produce different degrees of cholinergic deficit. Seven weeks after lesioning, half of each group received either NGF or cytochrome c intracerebroventricularly for 7 weeks. The two doses of 192Sap produced 50 and 80% depletions of choline acetyltransferase (ChAT) activity in the neocortex and hippocampus. NGF produced the greatest increase in ChAT activity in controls, intermediate in low-lesioned, and smallest in highly lesioned animals. NGF-treated animals showed reduced weight gain, hyper-responsiveness to acoustic stimuli, and decreased inhibitory avoidance. Although general motor behavior was affected by neither 192Sap nor NGF in an open field task, highly lesioned rats took longer to reach the platform during water maze testing. Impaired spatial orientation in finding a hidden platform at the previously acquired position was mitigated by NGF. Hypertrophic changes of residual CBF neurons, Schwann cell hyperplasia, and aberrant axonal sprouting around the medulla were observed in NGF-treated animals only, independent of the preexisting lesion. Our results indicate that NGF has a limited capacity to enhance functioning of residual CBF neurons. More importantly, NGF augmented fear-related behaviors and adverse neuroproliferative changes that may restrict its therapeutic use.

Using intracranial electrical stimulation to study the timing of prepulse inhibition of the startle reflex

Due to the short latency and briefness of the startle reflex, event-related inhibition of startle has high temporal resolution and is useful for studying the hierarchical organization of sensorimotor gating and motive-motor gating. In this article, we describe methods for measuring the inhibitory effects of electrically stimulating each of the following four brain structures on startle in awake rats: the inferior colliculus (IC), the deeper layers of the superior colliculus (SC), the pedunculopontine tegmental nucleus (PPTg), and the ventral pallidum (VP). These four brain structures have been reported to be important in mediating sensorimotor or motive-motor gating. Startle responses are elicited by either intense noise bursts or electrical stimulation of the principal trigeminal nucleus. The time course of the IC-inhibited startle reflex is used as a standard for estimating timing of the neural transfer of startle-inhibitory information to motor outputs. We also discuss how these methods can be used in combination with neuropharmacology.

Azimuthal directional sensitivity of prepulse inhibition of the pinna startle reflex in decerebrate rats

Previous studies have indicated that the auditory midbrain, the inferior colliculus, is important for both sound localization and mediation of prepulse inhibition of the startle reflex. The present study investigated the azimuthal directional sensitivity of prepulse inhibition of the pinna startle reflex in decerebrate rats. The pinna startle reflex was measured by recording multi-unit action potentials from the cervicoauricular muscles. The startling noise burst (94 dB SPL) was produced by a stationary speaker at 0 degrees azimuth, and the non-startling prepulse noise burst (46 dB SPL) was produced by a movable speaker whose direction was changed in the frontal azimuthal plane. The interval between the onset of the prepulse sound and the onset of the startling sound was 100 ms. The pinna reflex to the startling sound was strongly inhibited by the prepulse sound, and the inhibited startle response exhibited a flat azimuthal directional curve. In addition to further confirming that the neural pathways mediating prepulse inhibition are located in the brainstem, the present results indicate that interaural disparities of binaural inputs used for sound localization are not capable of modulating prepulse inhibition of the startle reflex.

Comparison of the effects of diazepam on the fear-potentiated startle reflex and the fear-inhibited light reflex in man

It has been shown previously that the amplitude of the acoustic startle reflex is enhanced, and the amplitude of the light reflex reduced, when subjects anticipate an aversive event, compared to periods when subjects are resting ('fear-potentiated startle reflex' and 'fear-inhibited light reflex'). We examined whether the anxiolytic diazepam would reverse the effects of threat on the startle and pupillary reflexes. Twelve male volunteers participated in three weekly sessions in which they received oral treatment with placebo, diazepam 5 mg and diazepam 10 mg, according to a balanced crossover double-blind design. One hour after ingestion of the treatments, miotic responses to light pulses and electromyographic responses of the orbicularis oculi muscle to sound pulses were elicited during alternating periods in which the threat of an electric shock (electrodes attached to the subject's wrist) was present (THREAT) and absent (SAFE). The THREAT condition was associated with a significant increase in the amplitude of the electromyographic (EMG) response, a significant reduction of the miotic response amplitude, and an increase in self-rated anxiety. Diazepam attenuated all these effects of THREAT. Diazepam did not affect the amplitude of the miotic response under the SAFE condition, but did suppress the EMG response under this condition. These results confirm the validity of the fear-potentiated startle reflex and fear-inhibited light reflex as laboratory models of human anxiety, and reveal some differences between the effects of diazepam on the two reflexes.