Latent contextual inhibition of the cardiac component in the startle reaction to a sound stimulus in rats

Heart rate (HR), freezing score, and motor component were estimated during acoustic startle (ASR) habituation (two sessions with 24-hour interval, 10 trials in a session). It was shown that rats previously exposed to the experimental context (once for 5 min 24 h before training) demonstrated HR decrease in response to the first stimulus and tachycardia in response to the 4th-10th stimuli during the first session. The interstimulus HR declined from the 4th to the 6th trials. The same profile of cardiovascular response was observed in this group at the beginning of the second session with the following (to the 7th trial) habituation of tachycardia. These rats didn't demonstrate the intertrial HR decrease during the second session. Nonadapted rats responded by bradycardia to the 1st and 2nd trials of the first session. The response didn't change for tachycardia with continuation of stimuli presentation. Tachycardia appeared only in response to the 7th-10th trials of the second session and didn't habituate. The intertrial HR level decreased in this group only during the second session. The results are discussed in terms of contextual latent inhibition of the cardiac acoustic startle response.     

Startle responding and context conditioning. Naloxone pretreatment and stimulus intensity

The possibility that acoustic startle stimuli could support a conditional response (freezing) to contextual stimuli was investigated. Rats were exposed to three acoustic startle stimuli on the first day, and one on the second day. On day 1, 20 rats received naloxone pretreatment and another 20 received saline (placebo) pretreatment. Half of each group received a high-intensity acoustic stimulus, the other half a low-intensity acoustic stimulus. Both the higher stimulus intensity and the naloxone pretreatment led to greater freezing behavior during the 3-minute test period before the single startle stimulus on day 2. These findings support the notion that increased actual or perceived intensity of the acoustic startle stimulus increases conditioning to contextual stimuli as indexed by freezing behavior.     

Habituation and prepulse inhibition of acoustic startle in rodents

The acoustic startle response is a protective response, elicited by a sudden and intense acoustic stimulus. Facial and skeletal muscles are activated within a few milliseconds, leading to a whole body flinch in rodents(1). Although startle responses are reflexive responses that can be reliably elicited, they are not stereotypic. They can be modulated by emotions such as fear (fear potentiated startle) and joy (joy attenuated startle), by non-associative learning processes such as habituation and sensitization, and by other sensory stimuli through sensory gating processes (prepulse inhibition), turning startle responses into an excellent tool for assessing emotions, learning, and sensory gating, for review see( 2, 3). The primary pathway mediating startle responses is very short and well described, qualifying startle also as an excellent model for studying the underlying mechanisms for behavioural plasticity on a cellular/molecular level(3). We here describe a method for assessing short-term habituation, long-term habituation and prepulse inhibition of acoustic startle responses in rodents. Habituation describes the decrease of the startle response magnitude upon repeated presentation of the same stimulus. Habituation within a testing session is called short-term habituation (STH) and is reversible upon a period of several minutes without stimulation. Habituation between testing sessions is called long-term habituation (LTH)(4). Habituation is stimulus specific(5). Prepulse inhibition is the attenuation of a startle response by a preceding non-startling sensory stimulus(6). The interval between prepulse and startle stimulus can vary from 6 to up to 2000 ms. The prepulse can be any modality, however, acoustic prepulses are the most commonly used. Habituation is a form of non-associative learning. It can also be viewed as a form of sensory filtering, since it reduces the organisms' response to a non-threatening stimulus. Prepulse inhibition (PPI) was originally developed in human neuropsychiatric research as an operational measure for sensory gating(7). PPI deficits may represent the interface of "psychosis and cognition" as they seem to predict cognitive impairment(8-10). Both habituation and PPI are disrupted in patients suffering from schizophrenia(11), and PPI disruptions have shown to be, at least in some cases, amenable to treatment with mostly atypical antipsychotics(12, 13). However, other mental and neurodegenerative diseases are also accompanied by disruption in habituation and/or PPI, such as autism spectrum disorders (slower habituation), obsessive compulsive disorder, Tourette's syndrome, Huntington's disease, Parkinson's disease, and Alzheimer's Disease (PPI)(11, 14, 15) Dopamine induced PPI deficits are a commonly used animal model for the screening of antipsychotic drugs(16), but PPI deficits can also be induced by many other psychomimetic drugs, environmental modifications and surgical procedures.     

Involvement of neurotrophic factors in central mechanisms of behavior in adult animals]

The effect of antibodies against some neurotrophic factors (S-100b protein, CSL, etc.) on elaboration, storage, and retrieval of acoustic startle habituation and concomitant freezing behaviour, was found to be dose-dependent in adult rats. Thus, the 5-mcg dose impaired the consolidation and/or storage process, whereas 2 mcg only impaired the storage of long-term acoustic startle habituation, and 0.5 mcg exerted no effect at all. The habits retrieval displayed no dependence on the antibodies effect.     

Correlation between cataleptic freezing and prestimulation inhibition of the startle reflex in rats

In rats of GC strain bred for predisdposition to cataleptic freezing, a significant negative correlation between the duration of freezing and the level of prepulse inhibition (PPI) of the startle reflex, has been found. Besides, in a group of GC rats specific by their "nervousness" and jumpiness, there was also a negative correlation between the duration of freezing and the habituation to the startle reflex. None of this correlation have been found in Wistar rats. Since impairment of the PPI and habituation of the startle reflex is considered to be characteristic of schizophrenia, it is believed than cataleptic freezing in the GC rats may be used as a model of schizophrenic psychopathology.     

Short-term and long-term habituation of exploration in rats,hamsters and gerbils

Short-term (within-session) and long-term (between-session) habituation of exploratory behavior was studied in rats, hamsters and gerbils. Subjects were observed in an open field containing four different objects during three 15-min sessions with an 8–14 h interval between each session. Their exploratory activity was measured by the numbers of contacts they made with these objects. The three species differed from each other in both their long-term and short-term habituation of exploration. Rats showed disrupted between-session habituation because of an important initial burst of activity at the beginning of each session. In contrast, hamsters and gerbils displayed between-session habituation, but within-session habituation occured during the first session only. These results are discussed in relation both to the adaptive value of exploration, and to the natural habitat of each species.     

Analysis of possibility of genotypic correlation between fear and anxiety

Special features of anxious behavior in the elevated plus maze test and acoustic startle response were analyzed in 11 inbred mouse strains. A significant influence of the genotype both on the startle amplitude and behavior in the elevated plus maze was found. However, analysis of covariance did not reveal a genotype-related association between anxiety and startle amplitude. The data indicates that the fear-induced acoustic startle response and anxious behavior in the elevated plus maze (agoraphobia) are not genetically related.     

Interaction between acoustic startle and habituated neck postural responses in seated subjects.

Postural and startle responses rapidly habituate with repeated exposures to the same stimulus, and the first exposure to a seated forward acceleration elicits a startle response in the neck muscles. Our goal was to examine how the acoustic startle response is integrated with the habituated neck postural response elicited by forward accelerations of seated subjects. In experiment 1, 14 subjects underwent 11 sequential forward accelerations followed by 5 additional sled accelerations combined with a startling tone (124-dB sound pressure level) initiated 18 ms after sled acceleration onset. During the acceleration-only trials, changes consistent with habituation occurred in the root-mean-square amplitude of the neck muscles and in the peak amplitude of five head and torso kinematic variables. The subsequent addition of the startling tone restored the amplitude of the neck muscles and four of the five kinematic variables but shortened onset of muscle activity by 9-12 ms. These shortened onset times were further explored in experiment 2, wherein 16 subjects underwent 11 acceleration-only trials followed by 15 combined acceleration-tone trials with interstimulus delays of 0, 13, 18, 23, and 28 ms. Onset times shortened further for the 0- and 13-ms delays but did not lengthen for the 23- and 28-ms delays. These temporal and spatial changes in EMG can be explained by a summation of the excitatory drive converging at or before the neck muscle motoneurons. The present observations suggest that habituation to repeated sled accelerations involves extinguishing the startle response and tuning the postural response to the whole body disturbance.     

Difference in anxiety and sensitization of the acoustic startle response between the two inbred mouse strains BALB/cAN and DBA/2N

The inbred mouse strain BALB has been proposed to be an animal model for pathological anxiety. BALB exhibits a stronger acoustic startle response (ASR) than the 'less emotional' inbred strain DBA. Four experiments were conducted to determine whether this strong ASR is due to a higher anxiety level and/or to greater sensitization in BALB than in DBA, with the following results: (1) The ASR to the very first startle stimulus was found to be much stronger in BALB than in DBA, and freezing behavior evoked by startle stimuli was more pronounced in BALB than in DBA. These findings indicate a higher level of anxiety in this strain. (2) ASR amplitudes of BALB initially rose much higher during consecutive startle stimuli and remained at a high level much longer than in DBA. Thereafter, ASR amplitude dropped more slowly and to a lesser degree than in DBA. Startle amplitudes decreased similarly in both strains (strong exponential decrease) only when a low sound pressure level (SPL) was used which elicited approximately the same low ASR in both strains. These results can only be explained by increased sensitization in BALB. (3) The slope of the i/o-function, which represents the relation between sensory input and motor output, was steeper in BALB than in DBA. As it has been shown recently, sensitization increases the slope of the startle i/o-function indicating increased sensitization in BALB. It is discussed, however, whether anxiety also contributes to this effect. (4) Footshocks increased the ASR much less in BALB than in DBA, again showing increased sensitization in BALB. Both a higher level of anxiety and greater sensitization therefore determined the greater strength of the ASR in BALB than in DBA.     

High predisposition to catalepsy decreases intermale aggression and increases acoustic startle reflex amplitude

Reaction of freezing (a pronounced motor inhibition, catalepsy) is suggested to be associated with fear in response to predator appearance or attack of aggressive congener. In order to evaluate association between a kind of behavior such as freezing, aggressiveness and fear, the effects of high predisposition to catalepsy on intermale aggression, acoustic startle response and anxiety-related behavior in the light/dark test were studied. Mice of 14th and 15th generations of selective breeding for high predisposition to catalepsy were characterized by a significant decrease in aggressive behavior. The marked decrease in the percentage of aggressive mice in the catalepsy-prone strain is consistent with the notion that aggression and catalepsy represent two alternative kinds of behavior in intermale conflicts. A positive correlation was found between high predisposition to catalepsy and startle reflex amplitude (but not anxiety-related behavior).     

Renewal and reinstatement of the conditioned but not the unconditioned response following habituation of the unconditioned stimulus

Research on the inhibition of learned fear currently relies almost exclusively on one specific procedure, namely extinction of the conditioned stimulus (CS). Importantly, however, learned fear responses can be reduced by a number of other procedures, including habituation of the unconditioned stimulus (US). We recently demonstrated that reductions in learned fear following US habituation, like CS extinction, were subject to both renewal and reinstatement (Storsve et al., 2010). The present study further investigates the associative and non-associative processes shared between habituation and extinction. Given that habituation is typically context-independent (Mackintosh, 1987), in the present study we directly compared renewal and reinstatement of both a conditioned response (CR; freezing) and an unconditioned response (UR; startle) following habituation. It was found that the reduction in conditioned freezing resulting from habituation was context specific (i.e., a change in context led to a renewal of the conditioned fear response; Experiment 1) and was attenuated when a pre-test shock was given (i.e., reinstatement of conditioned fear was observed; Experiment 2). In contrast, habituation of an unconditioned response elicited by the US (i.e., a startle response) was unaffected by either a change in test context or administration of a pre-test shock. This dissociation in the effects of habituation on learned and unlearned responses is discussed in relation to theories of fear extinction.     

Effects of copper metabolism on neurological functions in Wistar and Wilson's disease model rats

Behavioral functions of Wistar and Long-Evans Cinnamon (LEC) rats, Wilson's disease animal model, were compared by measuring the open-field, acoustic startle reflex and prepulse inhibition (PPI), and shuttle-box avoidance learning tests with or without oral supplementation with copper or D-penicillamine, copper chelator. All of the LEC rats, irrespective of the treatment, exhibited higher locomotor activity, a decreased habituation to startle response or a lower PPI, compared with Wistar rats. The copper content of all brain regions examined, except for the medulla oblongata of LEC rats, was significantly lower than those in Wistar rats. Besides, in the region of the striatum and the nucleus accumbens of the LEC rats, lower content of norepinephrine, and higher content of dopamine and serotonin were observed compared with Wistar rats. Although copper supplementation did not affect the brain copper content, it reduced the PPI in both Wistar and LEC rats. In contrast, D-penicillamine supplementation decreased both the brain copper content and locomotor activity, and enhanced the startle amplitude only in Wistar rats. These findings suggest that an imbalance in copper homeostasis affects monoamine metabolism and behavioral functions.     

Neural cell adhesion molecule (NCAM-/-) null mice show impaired sensitization of the startle response

The neural cell adhesion molecule (NCAM) plays important roles in development of the nervous system and in synaptic plasticity and memory formation in the adult. The present study sought to further investigate the role of NCAM in learning by testing habituation and footshock sensitization learning of the startle response (SR) in NCAM null mutant (NCAM-/-) and wildtype littermate (NCAM+/+) mice. Whereas habituation is a form of non-associative learning, footshock sensitization is induced by rapid contextual fear conditioning. Habituation was tested by repetitive presentation of acoustic and tactile startle stimuli. Although NCAM-/- mice showed differences in sensitivity in both stimulus modalities, habituation learning was intact in NCAM-/- mice, suggesting that NCAM does not play a role in the mechanisms underlying synaptic plasticity in the startle pathway. Footshock sensitization was elicited by presentation of electric footshocks between two series of acoustic stimuli. In contrast to habituation, footshock sensitization learning was attenuated in NCAM-/- mice: the acoustic SR increase after the footshocks was lower in the mutant than in wildtype mice, indicating that NCAM plays an important role in the relevant brain areas, such as amygdala and/or the hippocampus.     

Decreased response or alternative defensive strategies in escape: two different types of long-term memory in the crab Chasmagnathus

An opaque screen moving overhead elicits an escape response in the crab Chasmagnathus that after a few presentations habituates for a long period (long-term habituation). Two types of long-term habituation were previously described: the (context-signal)-long-term habituation yielded by spaced training – context dependent, cycloheximide sensitive and long lasting; and the (signal)-long-term habituation yielded by massed training – context independent, cycloheximide insensitive and shorter lasting. Present research is focused on the defensive strategies crabs display during acquisition of both long-term habituations, using video analysis as the main method of study. Aside from the escape response, Chasmagnathus shows a rigid motionless display, an alternative defensive response we term freezing response. The escape response is predominantly exhibited at night and in summer months, while freezing occurs during day light hours and in winter months. During acquisition of (signal)-long-term habituation, the escape response vanishes without being replaced by freezing. During acquisition of (context-signal)-long-term habituation, the escape response vanishes and is replaced by a strong freezing that finally becomes the only defensive strategy. The former, but not the latter, meets the current concept of habituation. Accepted: 1 December 1998     

Fast habituation of the long-latency auditory evoked potential in the awake albino rat

Fast habituation of the long-latency, vertex-recorded auditory evoked potential (AEP) peaks in humans was first described by Callaway (1973) as a reduction in AEP amplitude that occurs to the second of a pair of acoustic stimuli when both stimuli are presented with an interstimulus interval (ISI) of no more than 10 sec. When acoustic stimuli are presented in pairs with an ISI of 2 sec and an interpair interval (IPI) of approximately 10 sec, reduction in amplitude to the second tone occurs by as much as 30–50%. Fast habituation may depend somewhat on a subject's anticipation of the stimulus and on other factors related to attention and orienting. Studies in our laboratory have demonstrated this amplitude decrement to the second tone of a pair in human infants, children and adults and have explored the implications of this finding with respect to attentional processes and the allocation of cerebral resources. In the present investigation we describe an animal model of fast habituation. Here, vertex-recorded AEPs were obtained to paired tone stimuli delivered to awake adult male Sprague-Dawley rats chronically implanted with skull electrodes. Findings showed: (a) an AEP wave form with 8 distinct peaks, (b) for one component there was a marked decrement in amplitude from tone 1 to tone 2 in recordings obtained from an electrode placed slightly to the right of midline, and (c) that there were no significant differences in peak latencies across tones. This methodology may further our understanding of fast habituation in humans and may prove useful for studies of attention, orienting, and resource allocation using techniques that are not possible for use with human subjects.     

Involvement of dopaminergic systems in the functional specialization of brain areas during formation of aggressive and submissive behavior in mice

In addition to its common activating action, the DA system is involved in the functional specialization of the brain areas which participate in the expression of discrete behavioral components. The evidence for different levels of activity of the mesocortical DA system in aggressive and submissive mice were obtained. In C57BL/6J mice, confrontations produced simultaneous increase in the extracellular DA content and its release from the nerve terminals in the nucleus accumbens and frontal cortex, while an elevation of the HVA concentration in these structures was found only in submissive mice. After 20 encounters, the habituation of animals to the repeated stress exposures and conditioning developed. Activation of the DA metabolism (increase in the DOPAC level and DOPAC/DA ratio) in the hippocampus was observed only in aggressive mice after 20 days of confrontation, when the extinction of the information novelty leading to aggression had been already accomplished. Our findings suggest the predominance of the role of the mesolimbic DA system, in particular, of its mesoaccumbens link, in the extinction of the information novelty in aggressors. A role of the mesocortical DA system in realization of the submissive behavior patterns, stress reactions, conditioned defensive behavior, anxiety-related behavior, and in modulation of the anxiety response to social stimuli is considered.     

More Meditation,Less Habituation? The Effect of Mindfulness Practice on the Acoustic Startle Reflex

BackgroundMindfulness as a mode of sustained and receptive attention promotes openness to each incoming stimulus, even if repetitive and/or aversive. Mindful attention has been shown to attenuate sensory habituation in expert meditators; however, others were not able to replicate this effect. The present study used acoustic startle reflex to investigate the effect of mindfulness practice intensity on sensory habituation.MethodsAuditory Startle Response (ASR) to 36 startling probes (12 trials x 3 block with 40ms inter-block intervals), was measured using electromyography (EMG) in three groups of participants (N = 12/group): meditation-naïve, moderate practice, and intensive practice.ResultsIntensive practice group showed attenuated startle habituation as evidenced by significantly less habituation over the entire experiment relative to the meditation-naïve and moderate practice groups. Furthermore, there was a significant linear effect showing between-block habituation in meditation-naïve and moderate practice groups, but not in the intensive practice group. However, the Block x Group interaction between the intensive practice and the meditation-naive groups was not significant. Moderate practice group was not significantly different from the meditation-naïve in the overall measure of habituation, but showed significantly stronger habituation than both meditation-naïve and intensive practice groups in Block 1. Greater practice intensity was significantly correlated with slower overall habituation and habituation rate in Blocks 2 and 3 in the intensive, but not in the moderate, practice group.ConclusionsThe study provides tentative evidence that intensive mindfulness practice attenuates acoustic startle habituation as measured by EMG, but the effect is modest. Moderate practice, on the other hand, appears to enhance habituation, suggesting the effect of mindfulness practice on startle habituation might be non-liner. Better understanding of the effect of mindful attention on startle habituation may shed new light on sensory information processing capacity of the human brain and its potential for de-automatisation of hard-wired processes.     

The sounds of silence: cessation of singing and song pausing are ultrasound-induced acoustic startle behaviors in the katydid Neoconocephalus ensiger (Orthoptera; Tettigoniidae)

Previous studies of acoustic startle in insects have dealt with behavioral and/or neural mechanisms employed in evading aerially hawking, echolocating bats; however, insects also face terrestrial predators. Here we describe an acoustic startle response of the nocturnal katydid, Neoconocephalus ensiger. Stridulating males disturbed in the field perform obvious anti-predatory behaviors – cessation of singing, freezing, jumping, and evasive flight. Under controlled laboratory conditions we found that cessation of singing and song pausing are ultrasound-specific behaviors: when stimulated with pulsed ultrasound (20–100 kHz), but not audio-sound (<20 kHz), males cease mate calling or insert pauses in their song. A second factor influencing acoustic startle is the phase of stimulation: an acoustic startle response occurs only when the pulse of ultrasound arrives during the window of silence between stridulatory syllables. The average startle threshold and response latency was 70 ± 5 dB SPL and 34.2 ± 6.0 ms, respectively. N. ensiger is particularly useful for examining acoustic startle responses of non-flying insects because (1) its calling song is broadband and contains ultrasound, thus the possibility exists of confusion over the biological meaning of ultrasound, and (2) this species shows the classic bat-avoidance response while flying, so a direct comparison between two types of acoustic startle is possible within the same species. Accepted: 6 November 1999     

Sensorimotor reactivity (acoustic startle response) in rats with experimental depressive syndrome

Alterations of the sensorimotor responses in Wistar rats with experimental dopamine deficit-dependent depressive syndrome induced by neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine were measured by acoustic startle. In rats with innate high level of anxiety the development of behavioral depression was accompanied by the decrease in startle amplitude. In rats with innate low level of anxiety the decrease in startle amplitude did not reach the statistical significance. Correlation between the anxiety-phobic level and the expression of behavioral depression was not revealed. Independently of the initial anxiety-phobic level, in rats with depressive syndrome at the stage of behavioral rehabilitation after the neurotoxin withdrawal the prepulse inhibition of the acoustic startle was decreased as compared to control animals. It is suggested that the decrease in startle amplitude and, to a greater extent, the decrease in prepulse inhibition may characterize the development of dopamine deficit-dependent states.     

Habituation of vasodilatation in the calf elicited by repeated sensory stimulation in man

In 28 subjects the cardiovascular response to repeated stimulation was monitored during six daily sessions. Calf blood flow was measured with mercury-in-silastic venous occlusion plethysmography, blood pressure with electronic sphygmomanometer. The stimuli used were: 1 kHz sound of 90 dB and 100 dB intensity and immersion of one foot for 60 s in water at 4 degrees C. Initially sounds induced large vasodilatation in the calf, immersion of one foot in the water induced in the contralateral calf vasodilatation in one group and vasoconstriction in another group of subjects. The blood pressure changes were less prominent and less consistent. After the first session of repeated stimulation the vascular response during the second session was significantly diminished. The reduction of the vasodilatation was the most rapid. During the remaining 5 days the responses were suppressed. It has been established that in the patients in the initial stage of hypertension the ability to habituate vascular response is impaired (Zbrozyna and Krebbel 1985). It is therefore suggested that the test of the ability for long-term vascular habituation could be used as a supplementary diagnostic test.