The effect of tail pinch on the acoustic startle response in rats

A series of 3 experiments was carried out to evaluate the effect of tail pinch (TP) on the amplitude of the acoustic startle response (ASR) in rats. There is a consistent group of pharmacological findings which support the view that the amplitude of the ASR is facilitated by transmission in both dopaminergic and noradrenergic neural systems. It has recently been reported that TP increases cortical norepinephrine release and pars compacta unit activity. It might therefore be expected that TP facilitate the amplitude of the ASR. This hypothesis was tested in the first experiment. Surprisingly, it was found that TP significantly depressed startle amplitude. In the second experiment if was found that this TP-induced depression in startle amplitude was reduced by damage to the nucleus accumbens and that the amount of reduction correlated with the extent of damage. In the third experiment the generality of the effect of TP on sensorimotor reactivity was evaluated by testing its effect on footshock threshold and airpuff-elicited startle response. TP also depressed responsiveness in these tests. These results are consistent with other observations that the nucleus accumbens plays a role in the modulation of sensorimotor reactivity.     

Footshock-induced sensitization of the acoustic startle response in two strains of mice

It has been shown before that unconditioned footshocks can augment the acoustic startle response in rats. In the present study, male mice of two strains, C57Bl/6N and BALB/c, were compared with regard to footshock-induced sensitization of the acoustic startle response. Presentation of footshocks did not affect the acoustic startle response in C57Bl/6N mice, while in contrast, footshock-induced sensitization was apparent in the BALB/c strain. Shocked C57Bl/6N mice, but not BALB/c mice, displayed robust conditioning to the startle context when re-tested the next day. These findings indicate that mice may exhibit footshock-induced sensitization of the acoustic startle response, but that the effects of footshocks on the acoustic startle are strain- and time-dependent.     

Electromyographical differentiation between the acoustic blink and startle reflex. Implications for studies investigating startle behavior

Recent evidence suggests that electromyographic activity in the orbicularis oculi muscle occurring in response to sudden acoustic stimuli consists of two overlapping components: the blink and the startle reflex. The aim of the present study was to identify these two components in acoustically elicited eyeblink responses and to analyze their differential modulation by weak acoustic prepulses. The prevalence, latency and amplitude characteristics of double EMG peaks in pulse-alone and prepulse-pulse trials (PP) with 30 ms and 100 ms interstimulus intervals were assessed in 16 healthy volunteers. EMG responses with two peaks were registered in 42.6 % of the pulse-alone trials and in 56.2 % of the PP30 and 48.7 % of the PP100 trials, respectively. Prepulse inhibition of the amplitude was greater for the second peak (14.2 % (P2) vs. -11.5 % (P1) in PP30 trials; 62.6 % (P2) vs. 32.3 % (P1) in PP100 trials), resulting also in higher P1/P2 amplitude ratios in prepulse-pulse trials (P1/P2: 62.9 % in pulse-alone, 92.6 % in PP30 and 100.1 % in PP100 trials). In conclusion, double peaks are a common phenomenon in human studies of acoustically elicited blink responses. It is postulated that the first peak represents the auditory blink reflex, whereas the second peak corresponds to the startle reflex, which may be more susceptible to prepulse inhibition. This complexity should be taken into account in clinical studies of the modulation of the startle reflex. Received: 15 November 2001 / Accepted: 14 June 2002     

Interaction between acoustic and electric sensitization of the acoustic startle response in rats.

Sensitization is the general increase of responsiveness observed after aversive stimulation. Usually footshocks are used as aversive stimuli. According to the 'Dual Process Theory' by Groves and Thompson. Psychol. Rev. 1970;77:419-450, not only additional aversive stimuli but also the response-eliciting stimuli themselves have a sensitizing effect, the degree of sensitization depending upon the stimulus intensity. We tested this suggestion in the footshock sensitization paradigm of the acoustic startle response (ASR): (1) High SPL (sound pressure level) acoustic stimuli (119 dB SPL) presented instead of footshocks also elicited strong sensitization. (2) While footshocks presented after startle stimuli with low SPL (95 dB) were able to produce a strong further sensitization of the ASR, footshocks presented after startle stimuli with high SPL (110 dB) only caused a minor sensitization of the ASR. (3) Diazepam (3 mg/kg i.p.) decreased ASR to high SPL (115 dB) stimuli. In this case footshocks elicited significant sensitization of the ASR despite intense startle stimuli. The present results support the 'Dual Process Theory'. Furthermore we could show that acoustic and footshock sensitization interact. We therefore suggest that both, acoustic and footshock sensitization, are mediated partly via the same neural circuitry.     

Long-term habituation of the startle response in mice evoked by acoustic and tactile stimuli

The present study shows that repetitive presentation of tactile and acoustic stimuli evoke long-term habituation (LTH) of the startle response in C57BL/6J mice. This was indicated by a decrease in response strength over several days. For the LTH of the acoustic startle response two controls were included: first, developing hearing loss during the time of testing did not account for the startle decrease--only 7 days of acoustic stimulation but not 7 days of adaptation led to a decrease in the startle. Second, repetitive presentation of loud acoustic startle stimuli did not raise the auditory threshold, which might otherwise have accounted for the startle decrease: prepulse inhibition (used here as a hearing test) was identical after both 7 days of acoustic startle stimulation and 7 days without stimulation. This proves that LTH to tactile and acoustic stimuli is present and fully functional in mice.     

Factors governing prepulse inhibition and prepulse facilitation of the acoustic startle response in mice

The influence of prepulses on the acoustic startle response (ASR) was measured in three inbred mouse strains, C57BL/6J, 129/SvHsd, and AKR/OlaHsd, and one hybrid strain produced by crossing wild mice and NMRI mice. Prepulse inhibition (PPI), i.e. reduction of ASR by prepulses, was maximal when the interval between prepulses and startle stimuli was in the range of 37.5-100 ms. Prepulse facilitation (PPF), i.e. increase of ASR by prepulses, was maximal when the prepulse preceded the startle stimulus by 12.5 ms. PPI increased with increasing prepulse SPL, PPF first increased then decreased when prepulse SPL was increased. Percent PPI was independent from startle stimulus SPL. All strains showed a long-term increase of PPI when tested for several days; one strain (129) also showed an increase of PPF over days. The present results clearly show that PPI and PPF are independent processes, which add to yield the final response change. PPF and the observed long-term changes of PPI and PPF are stronger expressed in mice than have been observed in rats under similar conditions. Since there were significant differences between the strains of mice with respect to PPI and PPF, genetically different strains of mice are a promising tool to study these two processes.     

The influence of stimulus repetition on the acoustic startle response and on responses of inferior colliculus neurons of mice

The acoustic startle response (ASR) and discharges of single units in the inferior colliculus (IC) were evaluated as a function of the repetition rate and trial (stimulus 1–10 at each rate) of intense noise bursts in mice. The amplitude of the ASR was inversely proportional to both rate and trial. The mean number of discharges for the sample population of units of the central nucleus (ICC) and the external nucleus (ICX) were also inversely proportional to stimulus rate. However, only ICX units were inversely proportional to trial (i.e. as was the ASR). When response properties of individual units were examined, a variety of profiles was observed with regard to rate and trial sensitivity: units were either not sensitive to stimulus rate, inversely sensitive, directly sensitive, or complexly sensitive. Superimposed on these rate classifications were trial sensitivity, with some units being insensitive to trial, directly sensitive, or inversely sensitive. The incidence of response types differed in some respects between ICC and ICX and these differences were, in general, consistent with the mean responses of the neuronal populations.     

Rat strain-dependent effects of repeated stress on the acoustic startle response

Amplitude and habituation of the acoustic startle response were assessed in four recombinant inbred (RI) rat strains. One group from each strain underwent repeated restraint stress, the last session of which was 24h before startle testing while, a second group from each strain was not stressed prior to testing. Additionally, prepulse inhibition of the acoustic startle response, and anxiety behavior in the elevated plus-maze were assessed in separate, non-stressed groups of each strain. In the non-stressed condition, these RI strains differed significantly from each other on all behaviors measured. In the two RI strains that showed the greatest habituation of the startle response, repeated stress resulted in significantly lower acoustic startle amplitude than that seen in non-stressed controls of those strains. In the strains showing low levels of habituation, repeated stressed increased the level. Neither genotype-dependent levels of startle amplitude, prepulse inhibition of the startle response, nor anxiety in the plus-maze were closely related to the effect of stress on either startle amplitude or habituation. The results suggest that genotype-dependent habituation of the startle response may be important in determining whether stress will alter startle amplitude.     

Opiate-dopamine interactions in the neural substrates of acoustic startle gating in the rat

1. The acoustic startle reflex (ASR) was measured in adult male Dawley rats using startling acoustic stimuli presented either alone or 60-500 msec after a weak acoustic prepulse. 2. The inhibition of the ASR by the prepulse, termed "prepulse inhibition" (PPI), was blocked in animals treated either with the indirect dopamine (DA) agonist d-amphetamine (AMPH) or with the direct DA receptor agonist apomorphine (APO). 3. Pretreatment with the opiate receptor antagonist naloxone (NAL) prevented the AMPH-induced loss of PPI, but did not diminish the APO-induced loss of PPI. 4. The opiate heroin had no significant effect on PPI. 5. Dopaminergic mechanisms that modulate PPI in the rat may be regulated by opiate systems that act presynaptic to the DA receptor; brain opiate receptors may not have direct effects on startle gating independent of this opiate-DA interaction.     

The role of spinal cord cyclic AMP in the acoustic startle response in rats

Drugs thought to increase intracellular levels of cAMP were infused intrathecally into the subarachnoid space of the lumbar spinal cord, and the effects on the acoustic startle response in rats were measured. Intrathecal infusions of the cAMP analogs dibutyryl cAMP or 8-bromo cAMP (12.5-100 micrograms) produced marked, dose-dependent increases in startle amplitude compared to the infusion of artificial cerebrospinal fluid (CSF). Local infusions of dibutyryl cAMP at more rostral levels of the spinal cord or brain failed to mimic the excitatory effect seen following lumbar intrathecal infusion. No excitation of startle was seen following intrathecal infusion of cAMP itself, ATP, 5'-AMP, or dibutyryl cGMP. A weak excitation of startle was seen following intrathecal, but not intraventricular, infusion of the water-soluble adenylate cyclase activator forskolin 7-deacetyl-7-O-hemisuccinic acid (forskolin-DHA; 5.0-100 micrograms, in artificial CSF), whereas forskolin itself [0.01-200 micrograms, in dimethyl sulfoxide (DMSO)] was without consistent effect. Finally, intrathecal infusion of the selective phosphodiesterase inhibitor Rolipram (12.5-200 micrograms) produced a marked excitation of startle similar in magnitude to the effects produced by cAMP analogs. The excitatory effects of intrathecally infused dibutyryl cAMP, 8-bromo cAMP, forskolin-DHA, or Rolipram support a functional link between spinal cord cAMP and the acoustic startle reflex. Possible sites of cAMP action on startle are discussed.     

The effect of thyroid hormone and an antithyroid drug on the startle response of the rat

In two experiments, male albino rats were made hypothyroid by injecting methimazole or hyperthyroid by injecting triiodothyronine. Control Ss were injected with saline or were uninjected. In Experiment 1 (18 Ss/group), 2 startle tests were given following a pre-treatment test used for matching Ss. Experiment 2 (17 Ss/group) replicated Experiment 1, except that treatments were begun before the initial test. The effectiveness of the treatments was indicated by the increased heart rate (HR) of the hyperthyroid Ss and the lower HR of the hypothyroids. The higher startle response of hyperthyroid Ss in both studies was attributed to a potentiating effect on endogenous adrenalin. Since hypothyroid Ss exhibited heightened startle only in Experiment 1, it was hypothesized that methimazole produced state-dependent habituation of startle.     

Effects of experimental acute tryptophan depletion on acoustic startle response in females

Previous studies suggest an important role for serotonergic (5-HT) modulation of the acoustic startle reflex (ASR) and prepulse inhibition (PPI). Acute challenge of brain serotonin by means of tryptophan depletion test (TDT) represents an established human challenge tool for temporary reduction of tryptophan (−TRP) levels and central nervous serotonin. Under these experimental conditions, PPI was found attenuated in males, but greater biochemical effects of TDT in the central nervous system of females are known. Therefore, in order to explore influence of 5-HT on various standard startle parameters in females, 16 young healthy females participated in a double-blind, cross-over TDT study. Acoustic stimuli were presented in 15 pulse-alone trials (100 dB, 40 ms) randomly followed by 25 pulse-alone or prepulse (70 dB, 30 ms; 120 ms interval) trials alongside electromyographic eyeblink recordings and mood state assessments. During 81% depletion of free plasma TRP, mean ASR magnitudes were significantly reduced compared to control (+TRP) condition while there were no differences in habituation or PPI nor did startle parameters correlate with mood states. Changes of plasma TRP and mood states correlated in tendency negatively in (−TRP) for depression and positively in (+TRP) for fatigue. In conclusion, this first study of startle parameters after TDT in a homogenous female population demonstrates that depletion of brain 5-HT in women only influences ASR.     

Light-potentiation of acoustic startle response (ASR) and monoamine efflux related to fearfulness in Fyn-deficient mice

Fyn tyrosine kinase deficient mice are known to show increased fearfulness. We investigated the fear response of these mice using the light-potentiation of the acoustic startle response (ASR) and examined its neurochemical correlates using in vivo microdialysis. Female homozygous Fyn-deficient mice showed an enhancement of the startle amplitude under a bright light while heterozygotes and wild-types did not show such a change. Along with these behavioral findings, the homozygous Fyn-deficient mice showed an increase in extracellular serotonin (5-HT) and dopamine (DA) in the prefrontal cortex and 5-HT in the hippocampus when they were exposed to bright light, while heterozygous and wild-type mice did not show such changes. These results suggest that the increased fearfulness of Fyn-deficient mice is related to enhanced serotonergic and dopaminergic activity in the prefrontal cortex and limbic system.     

Increased sensitization of acoustic startle response in spasmodic mice with a mutation of the glycine receptor alpha1-subunit gene

The spontaneous mutant mouse spasmodic (spd) carries a missense mutation affecting the glycine receptor alpha1-subunit gene. This results in a decreased binding affinity to glycine. Spd mutants show exaggerated acoustic startle responses (ASR). The present study sought to elucidate whether this increased ASR is due to a changed auditory processing or to stronger motor output resulting from a disinhibited motor system or, alternatively, to changes in modulatory influences on the startle pathway, namely in the mechanisms underlying habituation and sensitization. We found that in homozygous spd/spd mutants the startle threshold was lower, and the recorded slope of input/output (i/o) function, which reflects the relation between sensory input and motor output, was steeper. During repetitive presentation of high sound pressure level (SPL) startle stimuli (25 dB above startle threshold), ASR amplitudes did not decrease in spd/spd mutants as they do in the wildtype. In contrast, ASR amplitudes decreased when low SPL startle stimuli were presented. Footshocks presented after high SPL startle stimuli did not cause a further increase in ASR amplitudes of spd/spd mutants as in the wildtype. In heterozygous spd/+ mutants all these parameters were between those of spd/spd mutants and wildtype mice but closer to those of the wildtype. The steeper slope of i/o function in spd/spd mutants may be caused by both an increased sensory input and an increased motor output. The altered course of ASR amplitudes during repetitive stimulation and the deficit in additional footshock sensitization, however, can only be explained by an increased sensitization level in the spd/spd mutants. In accordance with the "dual process theory" strong sensitization evoked by high SPL startle stimuli supposedly counteracts habituation, leading to a constant high ASR amplitude. Furthermore, additional footshock sensitization is prevented. The increased sensitization level may be due to a change in auditory processing leading to a stronger sensitizing effect of the startle stimuli with high SPL. Alternatively, glycinergic tonic inhibition of sensitizing structures (e.g. the amygdala) in the wildtype may be diminished in spd/spd mutants, thus leading to a high sensitization level.     

Impaired prepulse inhibition of acoustic and tactile startle response in patients with Huntington''s disease.

The corpus striatum serves a critical function in inhibiting involuntary, intrusive movements. Striatal degeneration in Huntington's disease results in a loss of motor inhibition, manifested by abnormal involuntary choreiform movements. Sensorimotor inhibition, or "gating", can be measured in humans using the startle reflex: the startle reflex is normally inhibited when the startling stimulus is preceded 30-500 ms earlier by a weak prepulse. In the present study, prepulse inhibition (PPI) was measured in patients with Huntington's disease to quantify and characterise sensorimotor gating. Compared with age matched controls, patients with Huntington's disease exhibit less PPI. Startle gating deficits are evident in patients with Huntington's disease when startle is elicited by either acoustic or tactile stimuli. Even with stimuli that elicit maximal PPI in normal subjects, patients with Huntington's disease exhibit little or no PPI, and their pattern of startle gating does not show the normal modulatory effects usually elicited by changing the prepulse interval or intensity. Startle amplitude and habituation and latency facilitation are largely intact in these patients, although reflex latency is significantly slowed. In patients with Huntington's disease, startle reflex slowing correlates with cognitive impairment measured by the dementia rating scale, and with the performance disruptive effects of interference measured by the Stroop test. These findings document a profound disruption of sensorimotor gating in patients with Huntington's disease and are consistent with preclinical findings that identify the striatum and striatopallidal GABAergic efferent circuitry as critical substrates for sensorimotor gating of the startle reflex.     

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.     

Lesions of the central gray block the sensitization of the acoustic startle response in rats

The amplitude of the acoustic startle response (ASR) in rats is increased after administration of footshocks, a phenomenon termed sensitization. The neural circuitry underlying this kind of modulation of the ASR is only partly understood. It has been shown that the central nucleus of the amygdala (cA) and its efferent pathway to the caudal pontine reticular nucleus (PnC), an essential part of the primary startle circuit, is important for the sensitization of the ASR [23]. It was unclear, however, whether the amygdaloreticular pathway directly transfers the effects of footshocks onto the PnC, or whether there exists a relay nucleus within this pathway. The present study tested the hypothesis that the midbrain central gray (CG) is important for the sensitization of the ASR. Neuroanatomical tracing experiments indicate that a descending projection from the medial part of the cA might form synapses in the region of the midbrain CG, where a descending projection to the PnC takes its origin. We lesioned the dorsal and lateral part of the CG with the neurotoxin quinolinic acid and measured the effects of this lesion on the sensitization of the ASR by footshocks. Lesions confined to the dorsal and lateral parts of the CG totally blocked the sensitization of the ASR, without affecting the ASR amplitude in the absence of sensitizing stimuli. These findings suggest a crucial role of the CG for the sensitization of the ASR. The present data are reconciled with other findings from our laboratory and from the literature and we discuss possible mechanisms underlying the mediation of the sensitization of the ASR in rats.     

5-HT1B receptor knockout, but not 5-HT1A receptor knockout mice, show reduced startle reactivity and footshock-induced sensitization, as measured with the acoustic startle response

To investigate whether the hyperreactivity to mild environmental and novel stimuli in 5-HT1B receptor knockout (1BKO) mice, as suggested by measures of exploratory, aggressive, and impulsive behaviors, can be extended to phasic stimuli, 1BKO and wildtype mice were tested in acoustic startle reactivity and plasticity paradigms, including habituation, prepulse inhibition, and footshock-induced sensitization of the startle response. Furthermore, we compared 5-HT1A receptor knockout (1AKO) and 1BKO mice to further test the suggested opposite behavioral profiles in these two genotypes. Results show that startle reactivity and footshock-induced sensitization was reduced in 1BKO mice, with no changes in habituation or PPI. In contrast, 1AKO mice did not differ from WT mice in any of the measures. These results indicate that an absence of 5-HT1B receptors, but not of 5-HT1A receptors, affects the modulation of startle reactivity and footshock-induced sensitization, without influencing startle plasticity. Moreover, this study suggests that 1AKO mice display a distinct, but not opposite behavioral profile from 1BKO mice. Furthermore, it is concluded that the hyperreactivity in 1BKO mice cannot be generalized to all stimuli, including the startling stimuli used in this study, but is probably restricted to mild environmental stimuli only.     

The effect of the non-steroidal antiandrogen flutamide on neural receptor binding of testosterone and intermale aggressive behavior in mice

(1) The effect of the potent non-steroidal antiandrogen, flutamide, on neural receptor binding of testosterone and intermale aggressive behavior was studied. (2) The main findings were: (i) flutamide exhibited a high affinity in vivo for androgen binding macromolecules within the hypothalamus and preoptic-septal region of the brain. This finding was in contrast with the negligible testosterone receptor antagonist activity of flutamide in vitro; (ii) flutamide, given in conjunction with testosterone to castrated male mice, had no suppressive effect on the restoration of intermale aggressive behavior. (3) The relative effectiveness of flutamide in suppressing [³H]-testosterone binding in vivo and in vitro is interpreted to mean that flutamide exerts its antiandrogenic effect within the central nervous system, at least in part, by means of its conversion in vivo to an active metabolite. (4) The results of the study on the effects of flutamide on aggressive behavior are interpreted in support of the aromatization hypothesis for the mechanism of action of testosterone on the central nervous system.