Effects of clonidine on habituation and sensitization of acoustic startle in normal, decerebrate and locus coeruleus lesioned rats

Startling stimuli increase maximal motor unit discharge rate and rate of force development in humans

Maximal rate of force development in adult humans is determined by the maximal motor unit discharge rate, however the origin of the underlying synaptic inputs remains unclear. Here, we tested a hypothesis that the maximal motor unit discharge rate will increase in response to a startling cue, a stimulus that purportedly activates the pontomedullary reticular formation neurons that make mono- and disynaptic connections to motoneurons via fast-conducting axons. Twenty-two men were required to produce isometric knee extensor forces "as fast and as hard" as possible from rest to 75% of maximal voluntary force, in response to visual (VC), visual-auditory (VAC; 80 dB), or visual-startling cue (VSC; 110 dB). Motoneuron activity was estimated via decomposition of high-density surface electromyogram recordings over the vastus lateralis and medialis muscles. Reaction time was significantly shorter in response to VSC compared to VAC and VC. The VSC further elicited faster neuromechanical responses including a greater number of discharges per motor unit per second and greater maximal rate of force development, with no differences between VAC and VC. We provide evidence, for the first time, that the synaptic input to motoneurons increases in response to a startling cue, suggesting a contribution of subcortical pathways to maximal motoneuron output in humans.

Phasic activation of the locus coeruleus attenuates the acoustic startle response by increasing cortical arousal

An alerting sound elicits the Acoustic Startle Response (ASR) that is dependent on the sound volume and organisms’ state, which is regulated by neuromodulatory centers. The locus coeruleus (LC) neurons respond to salient stimuli and noradrenaline release affects sensory processing, including auditory. The LC hyperactivity is detrimental for sensorimotor gating. We report here that priming microstimulation of the LC (100-ms at 20, 50, and 100 Hz) attenuated the ASR in rats. The ASR reduction scaled with frequency and 100 Hz-stimulation mimicked pre-exposure to a non-startling tone (prepulse). A rapid (~ 40 ms) EEG desynchronization following the LC stimulation suggested that the ASR reduction was due to elevated cortical arousal. The effects of LC stimulation on the ASR and EEG were consistent with systematic relationships between the ASR, awake/sleep state, and the cortical arousal level; for that matter, a lower ASR amplitude corresponded to a higher arousal level. Thus, the LC appears to modulate the ASR circuit via its diffuse ascending projections to the forebrain saliency network. The LC modulation directly in the brainstem and/or spinal cord may also play a role. Our findings suggest the LC as a part of the brain circuitry regulating the ASR, while underlying neurophysiological mechanisms require further investigation.

Alpha2-adrenergic receptor agonists prevent emotional hyperthermia

Emotionally significant stimuli, including potential threats from the external environment, trigger an increase in body temperature, a response known as emotional hyperthermia. Sympathetically-mediated brown adipose tissue (BAT) thermogenesis contributes substantially to this hyperthermic response. The systemic administration of α₂-adrenergic agonists is known to inhibit both febrile and shivering responses. In the present study, we investigated whether systemic administration of clonidine, a α₂-adrenoceptor agonist, attenuates the emotional hyperthermia evoked in conscious unrestrained rats suddenly confronted with a second (intruder) rat, itself confined to a small cage. Pre-implanted thermistors were used to measure BAT and body temperature in conscious, freely moving, male Sprague-Dawley rats. The rats were pre-treated with intraperitoneally administered vehicle (Ringer solution) or clonidine (1, 10 and 100 µg/kg). Clonidine, in a dose-dependent manner, reduced the intruder-elicited increases in BAT (log-dose linear regression F(1,16) = 9.52, R² = 0.37, P < 0.01) and body temperature (F(1,16) = 6.48, R² = 0.29, P < 0.05). We also investigated, in anesthetized rats, whether systemic clonidine administration inhibits BAT sympathetic nerve discharge evoked via activation of neurons in the lateral habenula (LHb) - a nucleus involved in the regulation of emotional hyperthermia. In anesthetized rats, clonidine abolished the BAT sympathetic nerve discharges elicited via bicuculline-mediated disinhibition of the LHb. These results suggest that activation of central α₂-adrenergic receptors attenuates the process of emotional hyperthermia by reduction of BAT thermogenesis.

Locus coeruleus noradrenergic innervation of the amygdala facilitates alerting-induced constriction of the rat tail artery

The amygdala, innervated by the noradrenergic locus coeruleus, processes salient environmental events. α2-adrenoceptor-stimulating drugs (clonidine-like agents) suppress the behavioral and physiological components of the response to salient events. Activation of sympathetic outflow to the cutaneous vascular bed is part of the physiological response to salience-mediated activation of the amygdala. We have determined whether acute systemic and intra-amygdala administration of clonidine, and chronic immunotoxin-mediated destruction of the noradrenergic innervation of the amygdala, impairs salience-related vasoconstrictor episodes in the tail artery of conscious freely moving Sprague-Dawley rats. After acute intraperitoneal injection of clonidine (10, 50, and 100 μg/kg), there was a dose-related decrease in the reduction in tail blood flow elicited by alerting stimuli, an effect prevented by prior administration of the α2-adrenergic blocking drug idazoxan (1 mg/kg ip or 75 nmol bilateral intra-amygdala). A dose-related decrease in alerting-induced tail artery vasoconstriction was also observed after bilateral intra-amygdala injection of clonidine (5, 10, and 20 nmol in 200 nl), an effect substantially prevented by prior bilateral intra-amygdala injection of idazoxan. Intra-amygdala injection of idazoxan by itself did not alter tail artery vasoconstriction elicited by alerting stimuli. Intra-amygdala injection of saporin coupled to antibodies to dopamine-β-hydroxylase (immunotoxin) destroyed the noradrenergic innervation of the amygdala and the parent noradrenergic neurons in the locus coeruleus. The reduction in tail blood flow elicited by standardized alerting stimuli was substantially reduced in immunotoxin-treated rats. Thus, inhibiting the release of noradrenaline within the amygdala reduces activation of the sympathetic outflow to the vascular beds elicited by salient events.

The noradrenergic projection from the locus coeruleus to the cochlear root neurons in rats

The cochlear root neurons (CRNs) are key components of the primary acoustic startle circuit; mediating auditory alert and escape behaviors in rats. They receive a great variety of inputs which serve to elicit and modulate the acoustic startle reflex (ASR). Recently, our group has suggested that CRNs receive inputs from the locus coeruleus (LC), a noradrenergic nucleus which participates in attention and alertness. Here, we map the efferent projection patterns of LC neurons and confirm the existence of the LC-CRN projection using both anterograde and retrograde tract tracers. Our results show that each LC projects to the CRNs of both sides with a clear ipsilateral predominance. The LC axons terminate as small endings distributed preferentially on the cell body and primary dendrites of CRNs. Using light and confocal microscopy, we show a strong immunoreactivity for tyrosine hydroxylase and dopamine β-hydroxylase in these terminals, indicating noradrenaline release. We further studied the noradrenergic system using gene expression analysis (RT-qPCR) and immunohistochemistry to detect specific noradrenergic receptor subunits in the cochlear nerve root. Our results indicate that CRNs contain a noradrenergic receptor profile sufficient to modulate the ASR, and also show important gender-specific differences in their gene expression. 3D reconstruction analysis confirms the presence of sexual dimorphism in the density and distribution of LC neurons. Our study describes a coerulean noradrenergic projection to the CRNs that might contribute to neural processes underlying sensory gating of the ASR, and also provides an explanation for the gender differences observed in the behavioral paradigm.

Sensation seeking: A comparative approach to a human trait

A comparative method of studying the biological bases of personality compares human trait dimensions with likely animal models in terms of genetic determination and common biological correlates. The approach is applied to the trait of sensation seeking, which is defined on the human level by a questionnaire, reports of experience, and observations of behavior, and on the animal level by general activity, behavior in novel situations, and certain types of naturalistic behavior in animal colonies. Moderately high genetic determination has been found for human sensation seeking, and marked strain differences in rodents have been found in open-field behavior that may be related to basic differences in brain neurochemistry. Agonistic and sociable behaviors in both animals and humans and the trait measure of sensation seeking in humans have been related to certain common biological correlates such as gonadal hormones, monoamine oxidase (MAO), and augmenting of the cortical evoked potential.

The monoamine systems in the rodent brain are involved in general activity, exploratory behavior, emotionality, socialization, dominance, sexual and consummately behaviors, and intracranial self-stimulation. Preliminary studies have related norepinephrine and enzymes involved in its production and degradation to human sensation seeking. A model is suggested that relates mood, behavioral activity, sociability, and clinical states to activity of the central catecholamine neurotransmitters and to neuroregulators and other transmitters that act in opposite ways on behavior or stabilize activity in the arousal systems. Stimulation and behavioral activity act on the catecholamine systems in a brain–behavior feedback loop. At optimal levels of catecholamine systems activity (CSA) mood is positive and activity and sociability are adaptive. At very low or very high levels of CSA mood is dysphoric, activity is restricted or stereotyped, and the organism is unsocial or aggressively antisocial. Novelty, in the absence of threat, may be rewarding through activation of noradrenergic neurons.

Neurochemistry of the afferents to the rat cochlear root nucleus: possible synaptic modulation of the acoustic startle

Afferents to the primary startle circuit are essential for the elicitation and modulation of the acoustic startle reflex (ASR). In the rat, cochlear root neurons (CRNs) comprise the first component of the acoustic startle circuit and play a crucial role in mediating the ASR. Nevertheless, the neurochemical pattern of their afferents remains unclear. To determine the distribution of excitatory and inhibitory inputs, we used confocal microscopy to analyze the immunostaining for vesicular glutamate and GABA transporter proteins (VGLUT1 and VGAT) on retrogradely labeled CRNs. We also used reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry to detect and localize specific neurotransmitter receptor subunits in the cochlear root. Our results show differential distributions of VGLUT1- and VGAT-immunoreactive endings around cell bodies and dendrites. The RT-PCR data showed a positive band for several ionotropic glutamate receptor subunits, M1-M5 muscarinic receptor subtypes, the glycine receptor alpha1 subunit (GlyRalpha1), GABAA, GABAB, and subunits of alpha2 and beta-noradrenergic receptors. By immunohistochemistry, we confirmed that CRN cell bodies exhibit positive immunoreaction for the glutamate receptor (GluR) 3 and NR1 GluR subunits. Cell bodies and dendrites were also positive for M2 and M4, and GlyRalpha1. Other subunits, such as GluR1 and GluR4 of the AMPA GluRs, were observed in glial cells neighboring unlabeled CRN cell bodies. We further confirmed the existence of noradrenergic afferents onto CRNs from the locus coeruleus by combining tyrosine hydroxylase immunohistochemistry and tract-tracing experiments. Our results provide valuable information toward understanding how CRNs might integrate excitatory and inhibitory inputs, and hence how they could elicit and modulate the ASR.

Undertaking positive control studies as part of developmental neurotoxicity testing: a report from the ILSI Research Foundation/Risk Science Institute expert working group on neurodevelopmental endpoints

Developmental neurotoxicity testing involves functional and neurohistological assessments in offspring during and following maternal and/or neonatal exposure. Data from positive control studies are an integral component in developmental neurotoxicity risk assessments. Positive control data are crucial for evaluating a laboratory's capability to detect chemical-induced changes in measured endpoints. Positive control data are also valuable in a weight-of-evidence approach to help determine the biological significance of results and provide confidence in negative results from developmental neurotoxicity (DNT) studies. This review is a practical guide for the selection and use of positive control agents in developmental neurotoxicology. The advantages and disadvantages of various positive control agents are discussed for the endpoints in developmental neurotoxicity studies. Design issues specific to positive control studies in developmental neurotoxicity are considered and recommendations on how to interpret and report positive control data are made. Positive control studies should be conducted as an integral component of the incorporation and use of developmental neurotoxicity testing methods in laboratories that generate data used in risk decisions.

Effects of clonidine injections into the bed nucleus of the stria terminalis on fear and anxiety behavior in rats

Emotions such as fear and anxiety are mediated by a neural network containing nuclei like the amygdala, the bed nucleus of the stria terminalis and the periaqueductal gray. Noradrenaline is a neurotransmitter closely connected with the processing of stimuli eliciting these emotions. The bed nucleus of the stria terminalis contains the highest density of noradrenaline within the brain. In the present study, we investigated effects of injections of the noradrenergic alpha2-adrenoceptor agonist clonidine into the bed nucleus of the stria terminalis on learned and unlearned fear (anxiety) in rats on different animal models of fear and anxiety: acquisition and expression of fear-potentiated startle, sensitization of the acoustic startle response by foot shocks and light-enhanced startle. Clonidine injections disrupted acquisition and expression of fear-potentiated startle, as well as light-enhanced startle, whereas sensitization was not affected. These results indicate that noradrenaline within the bed nucleus of the stria terminalis mediates both fear and anxiety. We suggest that there is rather a neurochemical than a neuroanatomical dissociation between learned fear and anxiety as hypothesized by Walker and Davis (Walker, D.L. and M. Davis, 1997b, Double dissociation between the involvement of the bed nucleus of the stria terminalis and the central nucleus of the amygdala in startle increases produced by conditioned versus unconditioned fear, J. Neurosci. 17, 9375-9383.).

Alpha2A-adrenoceptors are important modulators of the effects of D-amphetamine on startle reactivity and brain monoamines

Amphetamines are commonly used to treat attention-deficit hyperactivity disorder, but are also widely abused. They are employed in schizophrenia-related animal models as they disrupt the prepulse inhibition (PPI) of the acoustic startle response. The behavioral effects of amphetamines have mainly been attributed to changes in dopamine transmission, but they also involve increases in the synaptic concentrations of norepinephrine (NE). alpha2-Adrenoceptors (alpha2-ARs) regulate the excitability and transmitter release of brain monoaminergic neurons mainly as inhibitory presynaptic auto- and heteroreceptors. Modulation of acoustic startle and its PPI by the alpha2A-AR subtype was investigated with mice lacking the alpha2A-AR (alpha2A-KO) and their wild-type (WT) controls, without drugs and after administration of the alpha2-AR agonist dexmedetomidine or the antagonist atipamezole. The interaction of D-amphetamine (D-amph) and the alpha2-AR-noradrenergic neuronal system in modulating startle reactivity and in regulating brain monoamine metabolism was assessed as the behavioral and neurochemical responses to D-amph alone, or to the combination of D-amph and dexmedetomidine or atipamezole. alpha2A-KO mice were supersensitive to both neurochemical and behavioral effects of D-amph. Brain NE stores of alpha2A-KO mice were depleted by D-amph, revealing the alpha2A-AR as essential in modulating the actions of D-amph. Also, increased startle responses and more pronounced disruption of PPI were noted in D-amph-treated alpha2A-KO mice. alpha2A-AR also appeared to be responsible for the startle-modulating effects of alpha2-AR drugs, since the startle attenuation after the alpha2-AR agonist dexmedetomidine was absent in alpha2A-KO mice, and the alpha2-AR antagonist atipamezole had opposite effects on the startle reflex in alpha2A-KO and WT mice.

Validation of an auditory startle response system using chemicals or parametric modulation as positive controls

Neurotoxicity regulatory guidelines mandate that automated test systems be validated using chemicals. However, in some cases, chemicals may not necessarily be needed to prove test system validity. To examine this issue, two independent experiments were conducted to validate an automated auditory startle response (ASR) system. In Experiment 1, we used adult (PND 63) and weanling (PND 22) Sprague-Dawley rats (10/sex/dose) to determine the effect of either d-amphetamine (4.0 or 8.0 mg/kg) or clonidine (0.4 or 0.8 mg/kg) on the ASR peak amplitude (ASR PA). The startle response of each rat to a short burst of white noise (120 dB SPL) was recorded over 50 consecutive trials. The ASR PA was significantly decreased (by clonidine) and increased (by d-amphetamine) compared to controls in PND 63 rats. In PND 22 rats, the response to clonidine was similar to adults, but d-amphetamine effects were not significant. Neither drug affected the rate of the decrease in ASR PA over time (habituation). In Experiment 2, PND 31 Sprague-Dawley rats (8/sex) were presented with 150 trials consisting of either white noise bursts of variable intensity (70-120 dB SPL in 10 dB increments, presented in random order) or null (0 dB SPL) trials. Statistically significant sex- and intensity-dependent differences were detected in the ASR PA. These results suggest that in some cases, parametric modulation may be an alternative to using chemicals for test system validation.

Clonidine injections into the lateral nucleus of the amygdala block acquisition and expression of fear-potentiated startle

Numerous studies of aversive learning with different animal models have shown that the noradrenergic system has an important role in the acquisition, consolidation and expression of aversive learning. We used intracerebral clonidine injections to investigate the role of the noradrenergic amygdaloid system in the fear-potentiated startle paradigm. Clonidine is a noradrenergic alpha2-receptor agonist which can decrease noradrenergic transmission by stimulating presynaptic alpha2-receptors. Rats received injections of 0, 2.5, 5 and 10 nmol clonidine into the lateral amygdala (i) before fear-conditioning, (ii) immediately after fear-conditioning, (iii) before testing and (iv) before both fear-conditioning and the testing of conditioned fear. Clonidine injections blocked the acquisition and expression of conditioned fear. The effect on acquisition was not caused by state-dependency or possible side-effects of clonidine on consolidation. Given that clonidine decreases amygdaloid noradrenaline release, these results show a crucial role of noradrenergic transmission within the amygdala in classical fear-conditioning. Surprisingly, both the acquisition and the expression of conditioned fear were blocked after amygdaloid injections of clonidine, suggesting that amygdaloid noradrenaline is necessary to induce both unconditioned and conditioned fear.

Effects of clonidine and diazepam on prepulse inhibition of the acoustic startle response and the N1/P2 auditory evoked potential 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). We examined the effects of two sedative/anxiolytic drugs, diazepam and clonidine, on prepulse inhibition of these two responses in healthy volunteers. Fifteen males (aged 18-35 years) participated in three sessions in which they received oral doses of placebo, diazepam 10 mg and clonidine 0.2 mg according to a balanced double-blind protocol. Thirty-minute simultaneous recordings of the electromyographic (EMG) responses of the orbicularis oculi muscle of the right eye and the vertex auditory evoked potentials took place 120 min after ingestion of clonidine and 60 min after ingestion of diazepam. Sound stimuli (1 kHz) were presented in 60 trials separated by variable intervals (mean 25 s): (i) 40-ms 115-dB ('pulse alone', 20 trials); (ii) 40-ms 85-dB (20 trials); (iii) 40-ms 85-dB, followed after 120 ms by 40-ms 115-dB ('prepulse/pulse', 20 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. The amplitude of the EMG response was significantly reduced both by diazepam and by clonidine; neither drug significantly altered prepulse inhibition of the EMG response. Diazepam, but not clonidine, significantly reduced the amplitude of the N1/P2 potential; neither drug significantly affected prepulse inhibition of the N1/P2 potential. Both drugs reduced self-rated alertness and anxiety, and systolic blood pressure; clonidine, but not diazepam reduced diastolic blood pressure and salivation. The results confirm previous findings that sedative drugs can suppress the startle response without affecting prepulse inhibition of this response, and provide new information on the effects of these drugs on the N1/P2 potential and its inhibition by prepulses.

Effects of bromocriptine and haloperidol on prepulse inhibition of the acoustic startle response in man

Experiments with animals have shown that D2 dopamine (DA) receptors are involved in prepulse inhibition of the acoustic startle reflex (suppression of the reflex response evoked by a loud sound by prior presentation of a low-intensity stimulus). The present experiment attempted to extend this observation to man. Twelve healthy males (18-30 years), screened for normal hearing thresholds, participated in four sessions in which they received oral doses of placebo, bromocriptine 1.25 mg (a D2 receptor agonist), haloperidol 3 mg (a D2 receptor antagonist) and combined treatment with bromocriptine 1.25 mg+haloperidol 3 mg, according to a balanced double-blind protocol. Thirty-minute electromyographic recordings from the orbicularis oculi muscle of the right eye were carried out 120 min after ingestion of haloperidol and/or 90 min after ingestion of bromocriptine. Subjects received 36 40-msec sound pulses (115 dB), separated by variable intervals (mean 25 sec); in 24 of the trials the pulse was preceded by a 40-msec prepulse (75 dB in 12 trials and 85 dB in 12 trials; prepulse-pulse interval, 120 msec). The amplitude of the startle response was not significantly altered by any of the active treatments. Under the placebo condition, both 75- and 85-dB prepulses inhibited the startle response. Bromocriptine significantly attenuated this prepulse inhibition; haloperidol also produced a small but statistically significant attenuation of prepulse inhibition. Haloperidol significantly antagonized the attenuation of prepulse inhibition produced by bromocriptine. Neither drug altered self-rated alertness, physiological finger tremor, systolic or diastolic blood pressure or salivation. Bromocriptine significantly suppressed and haloperidol significantly elevated serum prolactin levels, these changes being absent when the two drugs were given in combination. The results provide evidence for the involvement of D2 DA receptors in prepulse inhibition of the startle reflex in man.

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.

The neuroanatomical and neurochemical basis of conditioned fear

After a few pairings of a threatening stimulus with a formerly neutral cue, animals and humans will experience a state of conditioned fear when only the cue is present. Conditioned fear provides a critical survival-related function in the face of threat by activating a range of protective behaviors. The present review summarizes and compares the results of different laboratories investigating the neuroanatomical and neurochemical basis of conditioned fear, focusing primarily on the behavioral models of freezing and fear-potentiated startle in rats. On the basis of these studies, we describe the pathways mediating and modulating fear. We identify several key unanswered questions and discuss possible implications for the understanding of human anxiety disorders.

Effects of clonidine and diazepam on the acoustic startle response and on its inhibition by 'prepulses' in man

This experiment examined the effects of two sedative/anxiolytic drugs, diazepam and clonidine, on the eyeblink component of the acoustic startle response in healthy volunteers. Twelve males (18-30 years), screened for normal hearing thresholds, participated in three sessions in which they received oral doses of placebo, diazepam 10mg and clonidine 200 microg according to a balanced double-blind protocol. Thirty-minute electromyographic recordings from the orbicularis oculi muscle of the right eye were carried out 120 min after ingestion of clonidine and 60 min after ingestion of diazepam. Subjects received 36 40-msec sound pulses (115 dB), separated by variable intervals (mean 25 sec); in 24 of the trials the pulse was preceded by a 40-msec prepulse (75 dB in 12 trials and 85 dB in 12 trials; prepulse-pulse interval, 120 msec). The amplitude of the startle response was significantly reduced both by diazepam (mean+/-SEM: -43.9+/-7.4%) and by clonidine (-75.7+/-4.7%). Under the placebo condition, the 75 and 85dB prepulses inhibited the startle response by 38.6+/-6.5 and 70.3+/-2.9%, respectively. Neither drug significantly altered the degree of prepulse inhibition. Both drugs reduced self-rated alertness; clonidine reduced systolic blood pressure and salivation. The results confirm the suppressant effect of clonidine on the startle response and show a qualitatively similar effect of diazepam; the results also demonstrate the insensitivity of prepulse inhibition of the startle response to doses of sedative drugs that are sufficient to attenuate the startle response itself.

Effect of clonidine on the human acoustic startle reflex

The present study investigated in healthy human volunteers whether clonidine reduced the amplitude of the acoustic startle reflex and whether this effect, if found, was due to an accelerated rate of habituation. Subjects were presented with startle-eliciting noise-bursts after intravenous (iv) infusion of clonidine (1.5 microgram/kg) and saline on separate days. Clonidine significantly reduced the amplitude of the acoustic startle reflex (as indexed by the eyeblink component) relative to the saline treated condition. This effect was neither due to an accelerated rate of habituation of the startle reflex nor to the sedative effect of clonidine. These findings complement an earlier report (Morgan et al. 1993) that yohimbine augments the amplitude of the startle reflex in man. Taken together, the two reports indicate a new model for the clinical investigation of central alpha2 adrenoceptor function in humans.

Flinders resistant hypocholinergic rats exhibit startle sensitization and reduced startle thresholds

Based on the hypothesis that depression involves a cholinergic-adrenergic neurotransmitter imbalance, a putative genetic animal model of depression has been developed by selectively breeding rats to exhibit hypocholinergia (Flinders Resistant Line--FRL), or hypercholinergia (Flinders Sensitive Line--FSL). The present experiments were designed to test the behavioral reactivity of these rats to external stimuli by measuring acoustic startle responses. The FRL rats exhibited lower startle thresholds compared to both FSL and control rats, while the FSL rats' startle thresholds were between those of controls and FRL rats. Despite the differences in thresholds, the three groups demonstrated similar levels of maximal startle reactivity to a high-intensity acoustic stimulus. With repeated stimulus presentations, FRL rats developed startle sensitization, a rarely observed phenomenon, while FSL and control rats exhibited habituation. There were no differences between the three groups in prepulse inhibition of startle. These results indicated that FRL rats exhibited interesting startle phenomena that are characteristic of certain psychiatric disorders, such as schizophrenia, post-traumatic stress disorder, and, potentially, depression.

Giant somatosensory evoked potentials and pathophysiology of hyperekplexia. Neurophysiological study of one patient

EEG during wakefulness and sleep and somatosensory evoked potentials from the median nerve were recorded in a 3 year-old boy with hyperekplexia and his close relatives (parents and two sisters). Centro-temporal spikes during sleep were found in the patient and in the older sister, while somatosensory evoked potentials, in the patient, showed abnormally high amplitude over the centroparietal regions. Pathophysiological mechanisms of hyperekplexia are discussed and the existence of symptomatic forms is suggested.

Amygdaloid noradrenaline is involved in the sensitization of the acoustic startle response in rats

The present study examined the role of noradrenaline (NA) in the central nucleus of the amygdala (cA) in the sensitization of the acoustic startle response (ASR) in rats. In the first experiment, local microinjections of 0, 0.5, 1, 2 nmol of the alpha 2-adrenergic antagonist yohimbine into the cA increased the magnitude of the ASR in a dose-dependent way. In the second experiment, foot shocks were applied to increase the ASR amplitude (sensitization). Local microinjections of 0, 4, 8, 16 nmol of the alpha 2-adrenergic agonist ST-91 into the cA dose dependently decreased the sensitizing effects of foot shocks on the amplitude of the ASR. It is conjectured that yohimbine increases and ST-91 decreases local NA release by acting at presynaptic autoreceptors. The present data suggest that the release of NA in the cA is involved in the mediation of the sensitizing effects of foot shocks on the ASR.

Comparison of the effects of yohimbine and clonidine on the behaviour of female mice during social encounters in an "approach-avoidance" situation

Effects of yohimbine (2 and 5 mg/kg, i.p.) and clonidine (10 and 50 micrograms/kg, i.p.) on the behaviour of adult female CD1 mice during 5 min encounters in a neutral cage with unfamiliar male partners have been examined by ethological procedures at 30 min after injection. Yohimbine induced dose-related increases in the frequency, bout length and duration of the immobile postures, "sit" and "social crouch", while decreasing the frequency of "explore", "scan", "attend" and "investigate", and increasing their bout lengths in a dose-related manner. These results suggest that yohimbine decreased the rate of switching from one behavioural act to another. Pausing between acts was increased by yohimbine to a similar extent at both of the tested dose levels. The act "wash" was increased in duration by yohimbine, whereas the strenuous activity of "digging" showed a dose-related decrease in frequency, duration and bout length. It is proposed that these effects are induced by the known interactions of yohimbine with receptors for dopamine as well as with alpha 2-adrenoceptors. Clonidine reduced motor activity, evident as a dose-related increase in the frequency and duration of "sitting" coupled with decreased frequency and increased bout length of the act, "explore" (significant at 50 micrograms/kg). Clonidine also dose-dependently reduced the frequency and duration of substrate "sniffing". Clonidine decreased occurrence of the specific social acts, "attend" and "investigate", as well as reducing frequency although not duration of overall social investigation. These findings have parallels with reported clinical effects of clonidine, such as sedation and impairments of attention, which must limit its clinical usefulness.

Startle disease, or hyperekplexia: Response to clonazepam and assignment of the gene (STHE) to chromosome 5q by linkage analysis

Familial startle disease (also known as hyperekplexia and congenital "stiff-man" syndrome) is an autosomal dominant disorder characterized by an exaggerated startle reaction of sudden, unexpected auditory or tactile stimuli; affected neonates also have severe and occasionally fatal hypertonia. We recently encountered a large, five-generation family with startle disease, and treated 16 patients (including 1 neonate) with clonazepam; all experienced dramatic and sustained improvement. We performed systematic linkage analysis in this family, and found tight linkage between the disease locus and a polymorphic genetic marker locus (colony-stimulating factor receptor, or CSF1R) that has been physically mapped to chromosome 5q33-q35. The maximum odds ratio favoring linkage over nonlinkage is greater than 10,000,000:1 (lod score, 7.10) at 3% recombination. Several genes encoding neurotransmitter receptor components have been physically mapped to the subtelomeric region of chromosome 5q, and are thus candidates for the startle disease gene. The availability of additional large pedigrees with startle disease should facilitate identification and characterization of the gene for this disorder.

Effects of anxiety drugs on the modification of the acoustic startle reflex by noise gaps

The acoustic startle reflex was modified by presenting gaps (0-30 ms) in a continuous noise (70 dB) before the startle eliciting stimulus. The effects of midazolam (0.4-1.8 mg/kg, IP), DMCM (0.1-0.4 mg/kg, IP), buspirone (5-20 mg/kg, PO), 8-OH-DPAT (0.5-8 mg/kg, IP), and clonidine (0.009-0.08 mg/kg, IP) on startle amplitudes (reflecting sensorimotor reactivity) on the one hand, and on gap inhibition of the startle reflex (a measure of temporal acuity) on the other hand, were investigated. The results showed that midazolam and clonidine attenuated sensorimotor reactivity dose-dependently. DMCM had no systematic effect on sensorimotor reactivity. Buspirone and 8-OH-DPAT increased sensorimotor reactivity dose dependently. Furthermore, midazolam and clonidine did not affect the neuronal systems underlying inhibition of the startle reflex. In contrast, DMCM and buspirone increased inhibition dose-dependently, and 8-OH-DPAT first increased and then decreased inhibition of the startle reflex. The possibility that drug-induced behavior affected the startle reflex was discussed.

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

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

Estimation of trial-to-trial variation in evoked potential signals by smoothing across trials

Averaging single trial evoked potential data to produce an estimate of the underlying signal obscures trial-to-trial variation in the response. We describe a method for estimating slow changes in the evoked potential signal by smoothing the data over trials. We discuss the crucial issue of deciding how much to smooth and suggest that an appropriate smoothing parameter is one that minimizes the estimated mean average square error of the smoothed data. Equations to estimate the mean average square error for a one-dimensional local linear regression smoother are presented. Performance of the method is assessed using simulated evoked potential data with several different models of a changing signal and different values of the signal-to-noise ratio. We find that the method rarely imputes trial-to-trial variation to data sets that have an unchanging signal, while it almost always produces less error than averaging when estimating a varying signal. The ability of the method to reveal signal heterogeneity is hampered by very low signal-to-noise ratios. When applied to real auditory evoked potential data from a sample of elderly subjects, the method indicated a changing signal in 35% of all subjects and in 56% of subjects with signal-to-noise ratios above 0.6. Consistent patterns of variation in the auditory evoked potential were present in this sample.

Clonidine produces mydriasis in conscious mice by activating central alpha 2-adrenoceptors

Intraperitoneal (i.p.) injection of the alpha 2-adrenoceptor agonist clonidine (1-3000 micrograms/kg) produced dose-dependent pupil dilatation in conscious C57/Bl/6 mice with an ED50 of 54 micrograms/kg (95% confidence limits 40-74 micrograms/kg). This response was rapid in onset and of approximately 30 min duration. The alpha 2-adrenoceptor antagonists idazoxan (1 or 3 mg/kg i.p.) and yohimbine (1 or 3 mg/kg i.p.) both produced dose-related miosis, but the alpha 1- and beta-adrenoceptor antagonists prazosin (1 or 3 mg/kg i.p.) and pindolol (1 or 3 mg/kg i.p.) were without effect. These doses of idazoxan and yohimbine potently reversed the mydriasis induced by clonidine (100 micrograms/kg i.p.), while prazosin and pindolol were again ineffective. Clonidine-induced mydriasis was also unaltered by the 5-HT antagonists, methysergide (2.5 mg/kg i.p.) and ketanserin (0.1 mg/kg i.p.) or 0.1 mg/kg i.p. of the dopamine antagonists, haloperidol, SCH 23390 and BRL 34778. A dose of 0.25 microgram clonidine, which was ineffective when administered i.p., produced marked mydriasis after intracerebroventricular (i.c.v.) injection. In addition, the mydriasis produced by i.p. injection of clonidine (100 micrograms/kg) was abolished by i.c.v. dosing of 2.5 micrograms idazoxan or yohimbine, but again not by prazosin or pindolol. Together, these data provide strong evidence to indicate that clonidine-induced mydriasis is exclusively mediated via central alpha 2-adrenoceptors and that this response provides a useful model for studying the function of these receptors.

Spinal vs. supraspinal sites of action of the alpha 2-adrenergic agonists clonidine and ST-91 on the acoustic startle reflex

Previous work has shown that stimulation of alpha 2-adrenergic receptors depresses the startle responses in rats. The present study suggests that this depressant effect involves supraspinal rather than spinal alpha 2-adrenergic receptors because intraventricular but not intrathecal infusion of the hydrophilic alpha 2-adrenergic agonist ST-91 depressed the acoustic startle reflex. To determine the point in the acoustic startle pathway where alpha 2-adrenergic receptor activation might ultimately alter neural transmission, startle responses were elicited electrically from different points along the acoustic startle pathway after systemic administration of clonidine. Clonidine depressed acoustically-elicited startle and startle elicited by electrical stimulation of the ventral cochlear nucleus to a comparable magnitude and over a similar time course. It also partially depressed startle elicited by electrical stimulation of the nucleus reticularis pontis caudalis (RPC). Taken together, these data suggest that alpha 2-adrenergic stimulation depresses startle by acting on supraspinal receptors, but that this effect is ultimately expressed, at least in part, by actions at both spinal and brainstem levels of the acoustic startle response pathway. The results are compared to other drugs known to affect the startle reflex.

Behavioral performance effects of antihypertensive drugs: human and animal studies

Antihypertensive drug treatments have been reported in clinical investigations to produce adverse effects to a degree that causes hypertensive patients to discontinue medication. Many of the debilitating effects reported by patients appear to be of central nervous system origin, such as sedation, fatigue, memory loss and sensorimotor disturbances. Human and animal laboratory studies in the past two decades have been characterizing the psychotropic effects of antihypertensive medications with use of a wide range of behavioral techniques. Antihypertensive drug classes covered in this review are beta-adrenergic blocking agents, alpha-adrenergic agonists, diuretics, angiotensin-converting enzyme inhibitors and calcium channel blockers. While findings in animal studies show generally greater behavioral impairments after administration of alpha-adrenergic agonists in comparison with other drug classes, the few laboratory studies conducted with hypertensive subjects present a confusing picture. A need for further laboratory research with hypertensive subjects and, study of antihypertensive drug combinations is discussed.

Effect of clonidine on consummatory negative contrast and on novelty-induced stress

In Experiments 1 and 1a rats were shifted from 32% to 4% sucrose solutions. The resultant negative contrast effect in consummatory behavior was not alleviated by clonidine (3.12, 6.25, 12.5, 25.0 and 50.0 micrograms/kg). The lower dose of the drug had no effect on behavior, the higher doses reduced consumption in shifted and unshifted rats in a dose dependent fashion. In Experiment 2 clonidine (6.25, 12.5 micrograms/kg) raised plasma glucose levels in a dose dependent fashion when the animals were exposed to a novel environment. These results are at variance with those obtained with chlordiazepoxide (and other anxiolytics in the case of contrast effects) and suggest limits on the degree to which clonidine can be considered to function as an anxiolytic.

High-dose clonidine motor syndrome: relationship to serotonin syndrome

Reciprocal forepaw treading, hindlimb abduction, and Straub tail are some of the abnormal motor behaviors of the classical 'serotonin syndrome,' which results from activation of serotonin (5-HT) receptors. However, we also observed them in the syndrome evoked by the alpha-adrenergic agonist clonidine, at high doses (5-40 mg/kg). Other features of the clonidine syndrome (scored from videotapes) were body and head tremor, forelimb hyperextension, ataxia, vertical jumping, tactile hyperreactivity, and autonomic signs (piloerection, pupillary dilatation, salivation, proptosis). The clonidine syndrome persisted for several hours and was not lethal. Clonidine suppressed locomotor activity (photocell recording) and induced episodes of catalepsy and 5-HT-independent impairment of motor habituation. Single high doses of drugs active at several different neurotransmitter receptors significantly reduced total behavioral score through effects primarily on tremor and autonomic signs, but none prevented the clonidine syndrome. Lesions of monoaminergic neurons [intracisternal 5,7-dihydroxytryptamine (DHT) or 6-hydroxydopamine] or monoamine depletion by intraperitoneal reserpine all failed to prevent this motor syndrome. Co-administration of 5-HTP and clonidine did not exacerbate the clonidine syndrome in naive rats and did not prevent the onset of the serotonergic syndrome in rats with DHT lesions. These data suggest that neither catecholamines nor 5-HT have a major role in the serotonin-like behavioral responses to high doses of clonidine.

Pertussis toxin or 8-bromo-cAMP block inhibition of the acoustic startle response by the alpha 2-adrenergic agonist ST-91

Stimulation of supraspinal alpha 2-adrenergic receptors by intraventricular infusion of the alpha 2-adrenergic agonist ST-91 depresses a simple vertebrate behavior, the acoustic startle response. Intraventricular pretreatment with pertussis toxin, an agent known to inactivate the inhibitory guanine nucleotide binding protein (Gi) which can inhibit adenylate cyclase, completely prevented the depressant behavioral effect of ST-91. In contrast, pertussis toxin did not alter the depressant effect of intraventricular infusion of the 5-HT 1B agonist 1-m-chlorophenylpiperazine (mCPP). Intraventricular infusion of the cyclic adenosine monophosphate (cAMP) analog 8-bromo-cAMP also reversed the depressant effect of ST-91 without altering the effect of mCPP. These data suggest that inhibition of adenylate cyclase may be involved in the effect of activation of central alpha 2-adrenergic receptors.

Clonidine-induced sedation is not modified by single or combined neurochemical lesions of the locus coeruleus, the median and dorsal raphe nuclei

Single or combined neurochemical lesions of the locus coeruleus, the dorsal and the median raphe nuclei were performed on different groups of rats. Starting 10 days after the lesion, the locomotor activity of all rats was measured for 5 min every day in an open-field. For the first 21 days all lesioned rats, independently of the lesion site, were significantly less active than controls, but from the 11th to the 16th day the locomotor activity of lesioned animals increased progressively and, thus on days 15 and 16, the mean activity of all lesioned groups was not significantly different from that of the controls. From the 17th day onwards the sedative effect of small doses of clonidine (5-100 micrograms/kg) was measured. Neither single nor combined lesions modified the response to clonidine and the linear decrease of activity produced by increasing doses of clonidine was the same in all groups, lesioned or not. Biochemical assays showed a marked loss of corresponding amines as a result of the lesions in cortex, hippocampus and the brainstem. These results suggest that the alpha 2-receptors involved in clonidine-induced sedation are located neither on noradrenergic fibers coming from the locus coeruleus, nor on serotoninergic fibers originating in the median and dorsal raphe nuclei.

Persisting subsensitivity of the striatal dopamine system after fetal exposure to beta-endorphin

Fetal exposure of rats to beta-endorphin during the third trimester, either alone or with alpha-MSH, resulted in mild developmental delay and significant decreases in striatal dopamine receptor density (subsensitivity) persisting through maturity. The apparent paradoxical down-regulation of dopamine receptors in the presence of beta-endorphin was consistent with fetal exposure to dopamine receptor antagonists and synthesis inhibitors. These findings suggest biophysical properties of receptors which are unique to fetal development including loss of plasticity after exposure to antagonists. Permanent, down-regulation of the striatal dopamine system may be one mechanism underlying delayed development after fetal exposure to beta-endorphin which may accompany hypoxia. Even though there were no statistically significant differences between males and females in density of the dopamine receptor, the behavioral profile after peptide treatment was sexually demorphic. Behaviorally, female rats appeared sensitized to perinatal alpha-MSH and males to alpha-endorphin.

The effects of clonidine on the partial reinforcement extinction effect (PREE)

Clonidine has been reported to exert anti-anxiety effects in animals and man similar to those of benzodiazepines. The present experiment examined the effects of clonidine administration on the partial reinforcement extinction effect (PREE) which is known to be sensitive to benzodiazepine action. Two groups of rats were trained to run in a straight alley. The continuously reinforced (CRF) group received food reward on every trial. The partially reinforced (PRF) group was rewarded on a quasi-random 50% schedule. All animals were then tested in extinction. Clonidine 50 micrograms/kg was administered in a 2 X 2 design, i.e., drug-no drug in acquisition and drug-no drug in extinction. The PREE, i.e., increased resistance to extinction exhibited by PRF animals as compared to CRF animals, was obtained in animals that received saline in acquisition, independently of drug treatment in extinction, as well as in animals that received clonidine in both acquisition and extinction, but not in animals that received clonidine in acquisition alone. The administration of clonidine in extinction alone increased resistance to extinction in both the CRF and PRF animals. The increase in resistance to extinction, typically obtained with benzodiazepine treatment, indicates that clonidine exerts anxiolytic effects, supporting the involvement of the noradrenergic system in anxiety. However, clonidine did not fully reproduce the effects of benzodiazepines on the PREE, suggesting that the two classes of drugs may act via different noradrenergic mechanisms.