Spinal modulation of acoustic startle: opposite effects of clonidine and d-amphetamine

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.

Effects of melatonin on prepulse inhibition, habituation and sensitization of the human startle reflex in healthy volunteers

Prepulse inhibition of the startle reflex (PPI) is an operational measure of sensorimotor gating, which is demonstrated to be impaired in patients with schizophrenia. In addition, a disruption of the circadian rhythm together with blunted melatonin secretion is regularly found in patients with schizophrenia and it is theorized that these may contribute to their attentional deficits. The aim of this study was to assess the effects of acute melatonin on healthy human sensorimotor gating. Twenty-one healthy male volunteers were administered melatonin or placebo after which their levels of PPI were assessed. Melatonin significantly reduced startle magnitude and ratings of alertness, but did not influence PPI, nor sensitization and habituation. However, when taking baseline scores in consideration, melatonin significantly increased PPI in low scoring individuals while significantly decreasing it in high scoring individuals in low intensity prepulse trialtypes only. In addition, subjective ratings of alertness correlated with PPI. The results suggest that melatonin has only minor influences on sensorimotor gating, habituation and sensitization of the startle reflex of healthy males. The data do indicate a relationship between alertness and PPI. Further research examining the effects of melatonin on these processes in patients with schizophrenia is warranted.

Yohimbine disrupts prepulse inhibition in rats via action at 5-HT1A receptors, not alpha2-adrenoceptors

RATIONALE

Prepulse inhibition (PPI) of the acoustic startle response is an operational measure of sensorimotor gating that can be assessed in both humans and animals. The noradrenergic system appears to play a role in PPI as the alpha1 agonist cirazoline disrupts PPI and the alpha1 antagonist prazosin blocks the disruptions in PPI produced by phencyclidine.

OBJECTIVES

To better understand the role of adrenergic receptors in the modulation of PPI, we assessed the effects of the alpha2 adrenergic antagonist yohimbine (2.5, 5.0, and 7.5 mg/kg) on PPI.

RESULTS

Yohimbine reduced PPI at the 5.0 and 7.5 mg/kg doses, without significantly affecting startle magnitude. In separate experiments, we examined whether adrenergic or serotonergic compounds blocked this disruption in PPI produced by yohimbine. There was a trend for the alpha2 agonist clonidine (0.01, 0.02 mg/kg) to attenuate the PPI disruption produced by yohimbine. However, other alpha2 agonists (guanfacine, medetomidine) and an alpha1 antagonist (prazosin) failed to prevent the disruption. The alpha2 antagonist atipamezole weakly decreased PPI in a narrow dose range (0.3-1.0 mg/kg). The 5-HT1A antagonist WAY100,635 (0.1, 0.3 mg/kg) significantly prevented the yohimbine-induced disruption of PPI.

CONCLUSIONS

These findings indicate that (1) yohimbine disrupts PPI in rats and (2) the yohimbine-induced disruption of PPI is largely due to the 5-HT1A partial agonist properties of yohimbine.

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.

Elevated startle during withdrawal from acute morphine: a model of opiate withdrawal and anxiety

RATIONALE

An elevated startle response has been observed in humans and animals during withdrawal from multiple substances of abuse, a phenomenon thought to reflect the anxiogenic effects of withdrawal. Although anxiety is a common symptom of opiate withdrawal, few studies have examined the effects of morphine withdrawal on acoustic startle.

OBJECTIVE

To develop a procedure for assessing opiate dependence through measurement of the startle reflex in rats.

METHODS

The effects of opiate withdrawal on startle were evaluated using both spontaneous and naloxone-precipitated withdrawal from an acute dose of morphine. The ability of the treatment drugs clonidine and chlordiazepoxide to block withdrawal-induced increases in startle was also tested.

RESULTS

Spontaneous withdrawal from an injection of morphine sulfate produced a significant increase in acoustic startle 2 h (3.2 mg/kg) or 4 h (10 mg/kg) after drug administration. Morphine withdrawal (10 mg/kg morphine sulfate) precipitated by the opiate antagonist naloxone (2.5 mg/kg) also produced a significant increase in startle magnitude. This elevation of startle was blocked by both clonidine (35 microg/kg) and chlordiazepoxide (10 mg/kg).

CONCLUSIONS

These data demonstrate that both spontaneous and precipitated withdrawal from an acutely administered opiate produce anxiety-like effects on acoustic startle. This paradigm may be useful in the study of anxiety and the early mechanisms of drug dependence.

Intraventricular insulin suppresses the acoustic startle response in rats

We and others have previously reported that the hormone insulin alters brain noradrenergic function at the synaptic and molecular levels. In the present study, we examined the in vivo effect of insulin (administered chronically via osmotic minipumps at a dose of 5 mU/day into the third cerebral ventricle) on the acoustic startle response. Rats receiving chronic intraventricular insulin had a significantly reduced startle response relative to vehicle-treated controls (i.e., 47 +/- 21% of baseline control startle response). Because our previous findings suggest that on an acute basis, insulin may enhance endogenous noradrenergic activity by inhibiting norepinephrine reuptake, we speculate here that the chronic effect of insulin is similar to that of the noradrenergic reuptake blocker, desipramine, which has been reported to decrease baseline startle performance.

Effects of TRH on acoustic startle, conditioned fear and active avoidance in rats

The effects of intracerebroventricular injection of thyrotropine-releasing hormone (TRH) on acoustic startle, conditioned fear and active avoidance were examined in rats. Acoustic startle was significantly depressed by 12.5 microg TRH, while increasing motor activity. In a fear-potentiated startle paradigm, 12.5 microg TRH reduced the overall startle response amplitude, but did not decrease the amount of fear-potentiated startle. When TRH was administered 15 min before contextual fear conditioning, neither fear-related freezing in acquisition nor in a retention test was affected. In contrast, when TRH was administered 15 min before the retention test, TRH significantly reduced mean percentage of time spent freezing. TRH had no effect on active avoidance. The results demonstrate that TRH decreased acoustic startle and freezing responses, but had little effect on fear conditioning and active avoidance. It is suggested that the results may be due to TRH's effects on motor activity and arousal, independent of its effects on fear.

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.

Involvement of cyclic AMP at the level of the nucleus reticularis pontis caudalis in the acoustic startle response

Rats were implanted with cannulas in the nucleus reticularis pontis caudalis (PnC), an obligatory part of the neural pathway that mediates the acoustic startle reflex. Following at least 1 week of recovery, rats were tested for acoustic startle amplitude before or after infusion of compounds known to alter the second messenger, adenosine cyclic 3', 5'-monophosphate (cAMP). Local infusion into the PnC of the cAMP analog, 8-bromo cAMP (0.125-1.0 micrograms), increased the amplitude of the acoustic startle response in a dose-dependent manner. In addition, local infusion of a phosphodiesterase inhibitor, rolipram (10 micrograms) or the water soluble adenylate cyclase activator, forskolin-DHA (2.5 micrograms), produced a significant enhancement of startle amplitude. These effects probably resulted from intracellular actions because cAMP itself, which does not readily penetrate lipid membranes, had no effect. Moreover, the effects seemed somewhat specific because the precursor of cAMP, ATP or 8-bromo cGMP, also failed to alter startle at doses where 8 bromo-cAMP did. The fact that a phosphodiesterase inhibitor elevated startle suggests that cAMP serves to tonically elevate startle at this level of the pathway. Hence, treatments that either increase (fear, sensitization) or decrease (habituation, pre-pulse inhibition) startle at the level of the PnC may do so via release of neurotransmitters either positively or negatively coupled to cAMP, which in turn may alter either sound evoked transmitter release, excitability of PnC neurons or both.

Baseline and fear-potentiated startle in panic disorder patients

The present study investigated whether patients with panic disorder had an increase in the startle response and whether this effect, if present, was specific to anticipatory anxiety. The eyeblink component of the acoustic startle reflex was measured in a paradigm involving the anticipation of electric shocks (fear-potentiated startle) in 34 patients with panic disorder and 49 healthy controls. Startle was also recorded in the absence of specific threat at the beginning and at the end of the testing. The testing consisted of three phases: adaptation, fear-potentiated startle, and recovery. In the adaptation and recovery phases, startle stimuli were delivered in the absence of threat. In the fear-potentiated startle phase, startle stimuli were delivered in threat conditions, when subjects anticipated shocks, and in safe conditions that predicted the absence of shocks. Startle was larger in the younger patients (age < 40 years old) compared to the younger controls throughout the testing. The difference reached significance only during the fear-potentiated startle phase, however. Startle was nonsignificantly reduced in the older patients (age > or = 39 years old), compared to the older controls. The results are discussed in terms of the contextual effects of the experimental setting.

Yohimbine potentiates active defensive responses to threatening stimuli in Swiss-Webster mice

Yohimbine HCl, an antagonist at alpha 2-noradrenaline receptors with putative panicogenic effects in human subjects, was administered to Swiss-Webster mice at doses of 0.5, 1.0, and 2.0 mg/kg. Animals were then tested in two defense test batteries. Yohimbine produced increases in flight from an approaching/contacting human and potentiated animals' reactions to dorsal contact. During a 5-min exposure to a cat (separated from the mouse by a wire-mesh screen) and the 15-min period thereafter, yohimbine produced a dose-dependent pattern of changes in defensive behaviors that included increases in locomotion, transits from one segment of the test chamber to another, fore- and hindpaw wall climbing, screen climbing and hanging, and roof pushing. Crouching (relative immobility while in a hunched-back posture) was notably decreased at all doses. During the postcat period, two different response patterns, "high-escape" and "low-escape," characterized in part by high and low frequencies of wall climbing, were observed in cat-exposed groups. In yohimbine-injected mice, the low-escape behavior pattern also included a tendency to avoid the segment of the test chamber closest to the cat compartment. Both patterns differed from the crouching and immobility generally exhibited by vehicle-injected, cat-exposed controls. It was suggested that yohimbine effected these behavioral changes by either potentiating neural mechanisms mediating flight or inhibiting mechanisms mediating freezing. This model may have some utility for the investigation of panicogenic and antipanic compounds and may contribute insights into the etiology of panic disorder.

Differences in fear motivated behaviors among inbred mouse strains

The behavioral performance of inbred mouse strains was examined in animal models of anxiety to evaluate the potential contribution of genetic factors to fear-motivated behaviors. The preference that randomly bred mice and rats exhibit for the enclosed as opposed to the open arms of an elevated maze has been considered a fear-motivated behavior. Pronounced differences were observed in this measure among 16 inbred mouse strains. An estimate of the proportion of the variance attributable to between-strain differences, eta 2, revealed that 78% and 69% of the variance in time and number of entries in the open arms of an elevated maze, respectively, can be attributed to genetic factors. In contrast, only 27% and 42% of the variance could be attributed to between-strain differences in ambulatory activity in the open field and elevated maze, respectively. Furthermore, performance in the elevated maze was predictive of behavior in other animal models of anxiety. Thus, significant negative correlations were observed among inbred mouse strains between the percent time spent in the open arms of the elevated maze and amplitude of an acoustic startle response (rs = -0.88m P < 0.01) or latency to initiate chow consumption in a hyponeophagia paradigm (rs = -0.71, P < 0.05). These results indicate that genetic factors substantially contribute to fear motivated behaviors in these animal models of anxiety. The use of such inbred mouse strains may provide a novel approach to investigate the biochemical and genetic bases of fear.

Effects of yohimbine and idazoxan on motor behaviors in male rats

Yohimbine, an alpha 2 adrenergic antagonist, facilitates copulatory behaviors in male rats. This facilitation may reflect nonspecific activation of behavior rather than a more selective activation of copulatory behaviors. The present experiments assessed the effects of yohimbine on locomotor behaviors at a dose (2.0 mg/kg) known to facilitate sexual behaviors. Experiment 1 used a computer pattern-recognition system to classify motor behaviors into specific acts and act groups. Male albino rats were tested in three conspecific conditions: estrous female, anestrous female, or no conspecific. Yohimbine decreased locomotor activity in all three conspecific conditions. Experiment 2 examined the effects of yohimbine (2.0 mg/kg) and amphetamine (1.0 mg/kg) on locomotor behavior in a photocell-equipped activity measurement system. Amphetamine increased and yohimbine decreased locomotor activity. Experiment 3 used the computer pattern-recognition system to compare the effects of yohimbine and idazoxan, another alpha 2 adrenergic antagonist, on motor behaviors. Yohimbine and idazoxan both decreased activity but produced different patterns of behavioral change. The facilitatory effects of yohimbine on copulatory behaviors at a dose of 2.0 mg/kg are apparently not mediated by nonspecific activation of behavior.

Blockade of only spinal alpha 1 adrenoceptors is insufficient to attenuate DDT-induced alterations in motor function

Male Fischer 344N rats were chronically implanted with an intrathecal cannula and gavaged with p,p'-DDT (1,1,1-trichloro-2,2-bis[p-chlorophenyl]ethane; 30 or 45 mg/kg) or corn oil. Seven hours later, subjects were intrathecally infused with vehicle, 15, 30, 60, or 120 micrograms of prazosin (an alpha 1-adrenergic antagonist). Spectral analysis of bodily movements was performed 7.5, 8, and 10 hr after DDT administration. In control rats, 15 micrograms of prazosin reduced the spectral profiles of spontaneous movements. A 30-micrograms dose produced motor impairments, without significantly changing the spectral profiles. Tremulous movements induced by DDT were unaffected by 15 or 30 micrograms, whereas 60 or 120 micrograms of intrathecal prazosin significantly reduced the spectral profiles of rats pretreated with 45 mg/kg of DDT. Other subjects were administered vehicle or DDT (45 mg/kg), intrathecally infused with 15 or 60 micrograms of prazosin (7 hr), and sacrificed (7.5 hr). Noncannulated rats were gavaged with 60 mg/kg of DDT, injected subcutaneously (sc) with 0.5 mg/kg of prazosin (5.5 hr), and sacrificed (8 hr). Cortical and spinal tissues were used in ex vivo binding assay utilizing [3H]prazosin. Fifteen or sixty micrograms of intrathecal prazosin occupied similar percentages of spinal [3H]prazosin binding sites, but produced a dose-related increase in cortical prazosin equivalents. Sixty micrograms of intrathecal or 0.5 mg/kg of sc prazosin resulted in similar concentrations of cortical prazosin equivalents. Together, these data indicate that while intrathecal prazosin will attenuate DDT-induced motor dysfunction, this effect requires blockade of alpha 1 adrenoceptors in regions other than solely the spinal cord.

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.

Behavioural effects of the alpha 2-adrenoceptor antagonists idazoxan and yohimbine in rats: comparisons with amphetamine

Although yohimbine has long been known to increase arousal, reactivity and anxiety in animals and humans, little is known about the behavioural effects of more selective alpha 2-adrenoceptor antagonists such as idazoxan. In a recent experiment, however, it was found that in rats both yohimbine and idazoxan increased low rates of lever pressing, an effect also produced by amphetamine. The purpose of the present study was to investigate further the effects of yohimbine and idazoxan in comparison with those of d-amphetamine on the operant behaviour of rats. In rats trained to press a lever on a FI 60s schedule to obtain food both yohimbine and idazoxan increased response rates, although the effect of yohimbine was considerably greater than that of idazoxan. Lower doses of d-amphetamine had no consistent effect on overall rates of responding whereas a higher dose suppressed responding. Characteristically, d-amphetamine increased responding during early portions of the intervals and decreased responding during the final portions. Idazoxan and yohimbine tended to increase responding throughout the intervals except immediately after reinforcement. When idazoxan was administered in combination with prazosin FI response rates were markedly decreased. Administration of DSP4 did not alter the response rate-increasing effects of either yohimbine or idazoxan. In rats trained to discriminate d-amphetamine from saline both idazoxan and yohimbine gave rise to responding on the saline associated lever. Combination of idazoxan with d-amphetamine did not antagonise the amphetamine cue but produced substantial reductions in response rates, probably due to toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of p,p'-DDT-induced tremor by catecholaminergic agents

p,p'-DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane; 75 mg/kg) or corn oil was administered po to male Fischer 344N rats. Tremor was quantified 8 hr later by spectral analysis of whole body movements. The effect of sc injection of pharmacological challenges on the spectral profile of body movements was determined. The alpha antagonist phenoxybenzamine decreased the intensity of movements over most of the power spectra in animals exposed to DDT, but decreased spectral power only at lower frequencies in control subjects. The alpha-1 antagonist prazosin had similar effects in animals given DDT. The alpha-2 antagonist yohimbine and the beta antagonist propranolol produced lethality and increased the intensity of movements in animals administered DDT, without significantly affecting control animals. The alpha-2 agonist clonidine decreased the spectral profile over a wide range of frequencies in animals exposed to DDT, while depressing the spectral power of control animals only at higher frequencies. The dopamine antagonist haloperidol increased the intensity of movements in DDT-treated animals, without altering the spectral profile of controls. The dopamine agonist apomorphine induced stereotypy in control animals, but failed to significantly alter the power spectra in subjects given DDT. These data suggest a facilitatory and inhibitory role, respectively, for alpha-1 and alpha-2 receptors in the modulation of DDT-induced tremor. Dopamine and beta receptors may be involved in the tonic inhibition of tremor produced by DDT.

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.

Behavioral effects of intrahippocampal injections of clonidine, yohimbine and salbutamol in the rat

Intrahippocampal injections of adrenergic drugs, clonidine (an alpha 2-agonist), yohimbine (an alpha 2-antagonist), and salbutamol (a beta 2-agonist) were performed in the awake rat. The injection of a high dose of clonidine caused a depression in locomotion in the open-field. Yohimbine partially antagonized the clonidine-induced hypomotility. The intrahippocampal injection of salbutamol had no effect on ambulatory behavior of the rat. These results suggest that the role played by the anterodorsal hippocampus in modifying behavior in novel situations is dependent on the specific sub-population of adrenoceptors that is stimulated.

Pharmacological modification of tremor and enhanced acoustic startle by chlordecone and p,p'-DDT

Pretreatment of rats with phenoxybenzamine (5 mg/kg; SC), an alpha adrenergic antagonist, decreased the peak tremor power and startle magnitude of rats subsequently given DDT (75 mg/kg; PO) or chlordecone (60 mg/kg; IP), without having a significant effect on control animals. Pretreatment with an intracerebroventricular injection of calcium (3.75 microM in 5 microliters NaCl) decreased the peak tremor power due to subsequently administered DDT, while increasing the tremor response in rats later dosed with chlordecone. The effects of phenoxybenzamine are postulated to be due to a blockade of an excitatory influence of the adrenergic system. Calcium may decrease DDT-induced tremor by acting as a neuronal stabilizer. Potentiation of the tremorigenic effect of chlordecone by calcium may be due to increased levels of intracellular calcium, resulting in augmented release of neurotransmitters in chlordecone-exposed animals.

Noradrenergic influence on the prepulse inhibition of acoustic startle

Oral administration of p,p'-1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), a chemical believed to increase neuronal membrane excitability increased the acoustic startle responsiveness of rats. Inhibition of the acoustic startle response with a brief, prepulse white-noise stimulus was evident at 12.5 to 25 mg/kg of p,p'-DDT, but not at 50 mg/kg. Pretreatment of rats with phenoxybenzamine, an adrenergic receptor antagonist, attenuated the effects of 12.5 mg/kg of p,p'-DDT on the acoustic startle reflex, and decreased the maximum magnitude reduction produced by the prepulse stimulus in DDT-exposed rats. These data extend previous work showing that p,p'-DDT augments startle reactivity in rats and is in accord with the existence of an excitatory norepinephrine(NE)-containing pathway modulating motor outflow in the acoustic startle reflex arc.

Neuropharmacological investigations in the stiff-man syndrome

In a patient with the stiff-man syndrome, abnormal exteroceptive reflexes in hand and arm muscles were used as a tool for investigating the effects of various centrally acting adrenergic drugs by means of acute testing. Clonidine, tizanidine and methamphetamine induced reflex suppression whereas reserpine had a biphasic effect: transient reflex suppression and delayed reflex enhancement. The drug actions on the reflexes paralleled to some degree those on the muscle stiffness. It is concluded that alpha-adrenergic stimulation leads to reflex suppression and muscular relaxation, whereas alpha-adrenergic blockade has the opposite effect. Correspondingly, long-term oral treatment with the alpha-stimulator, tizanidine, proved successful. The hypothesis is put forward that a noradrenergic system which exerts a net inhibitory influence on muscle tone and on exteroceptive reflexes plays an important role in the pathogenesis of the stiff-man syndrome.

Corticotropin-releasing factor potentiates acoustic startle in rats: blockade by chlordiazepoxide

A series of experiments was performed to investigate the effects of corticotropin-releasing factor (CRF) on the amplitude of the acoustic startle response (ASR) in rats. Intracerebroventricular (ICV) administration of 1 microgram rat CRF significantly potentiated acoustic startle amplitude; these effects were reversed in a dose-dependent manner by pretreatment with the benzodiazepine chlordiazepoxide (CDP). Doses of CDP that antagonized CRF-potentiated ASR did not lower startle baseline or antagonize amphetamine- or strychnine-potentiated ASR. These results suggest that CRF has "anxiogenic" properties and may serve as a neuroendocrine modulator of stress-enhanced behaviors.

Pharmacology of spinal adrenergic systems which modulate spinal nociceptive processing

Spinopetal pathways may be activated by a variety of brainstem manipulations including microinjections of morphine which are known to modulate spinal nociceptive processing. Based on the ability of these manipulations to release spinal noradrenalin; the ability to reverse the antinociceptive effects by intrathecal adrenergic antagonists and the fact that intrathecal injections of noradrenalin mimic the antinociceptive effect, it appears that the descending modulation may be mediated by descending noradrenergic systems. Examination of the spinal receptor systems with intrathecally administered agents indicates that spinal alpha, but not beta adrenergic receptor agonists produce a powerful analgesia as measured on a variety of reflex and operant measures in mouse, rat, cat, primate and man. On the basis of agonist and antagonist structure-activity relationships it appears that a significant effect can be produced in the absence of any detectable effect on motor function by the occupation of spinal alpha 2 receptors. Distinguishable alpha 1 receptors also appear "analgetically-coupled," but their effects are uniformly contaminated by signs of cutaneous hyperreflexia at doses required to produce analgesia. The ordering of potency with which intrathecal adrenergic antagonists reverse the effects of intrathecal noradrenalin is indistinguishable from that of the reversal by these intrathecal agents of the antinociceptive effects evoked by brainstem morphine. This suggests that the population of spinal receptors acted upon by exogenously administered adrenergic agonists and endogenously released noradrenaline have indistinguishable characteristics.

Central noradrenergic involvement in yohimbine excitation of acoustic startle: effects of DSP4 and 6-OHDA

It was previously shown that i.p. administration of the alpha 2-adrenergic antagonist yohimbine increased the magnitude of the acoustic startle response in rats. The purpose of the present study was to determine possible central noradrenergic involvement in yohimbine's effect on startle. Pretreatment with N-(2-chloroethyl)-N-ethyl-2-bromo-benzylamine (DSP4; 50 mg/kg, i.p.; 1-2 days before testing) completely blocked the excitatory effect of yohimbine on startle. DSP4 reduced forebrain and spinal cord NE levels by 47% and 56%, respectively, without affecting forebrain or spinal serotonin (5-HT), or forebrain dopamine (DA). Pretreatment with the NE reuptake blocker desmethylimipramine (DMI; 20 mg/kg, i.p.; 30 min before DSP4) prevented the ability of DSP4 to block the yohimbine effect. DMI partially reversed the NE-depleting effects of DSP4. Neither bilateral adrenalectomy nor intravenously administered 6-hydroxydopamine (6-OHDA; 20 mg/kg; 1-2 days before testing) altered the excitatory effect of yohimbine, indicating that peripheral NE is not involved. 6-OHDA (2 X 200 micrograms) injected into the lateral ventricles blocked yohimbine's effect, and depleted NE by 95% (spinal cord) and 86% (forebrain), without affecting 5-HT in either region. 6-OHDA also depleted forebrain DA levels by 49%. Finally, intrathecal administration of 6-OHDA (20 micrograms; 14 days before testing) into the subarachnoid space of the lumbar spinal cord blocked the excitatory effect of yohimbine, and produced an extensive (94%) depletion of spinal cord NE. Intrathecal 6-OHDA did not alter spinal levels of 5-HT or forebrain levels of NE, 5-HT or DA. In summary, these data indicate that central descending NE neurons are necessary for yohimbine's excitatory effect on startle.

Effects of alpha-adrenoceptor agonists and antagonists in a maze-exploration model of 'fear'-motivated behaviour

An elevated X-maze with alternating open and enclosed arms was investigated as a model for the study of fear-induced behaviour. As predicted, the anxiolytics diazepam and amylobarbitone increased, and the putative anxiogenics ACTH and picrotoxin decreased the proportion of open arm entries. The alpha 1-adrenoceptor agonists phenylephrine and ST587, and the alpha 2-adrenoceptor antagonists idazoxan, piperoxane, RS-21361 and yohimbine decreased relative open-arm entries, thus resembling the putative anxiogenics. On the other hand, azepexole, clonidine and guanabenz, agonists at alpha 2-adrenoceptors, and the alpha 1-adrenoceptor antagonists prazosin and thymoxamine, enhanced the proportion of open arm entries at low doses, suggesting anxiolytic-like properties. A paradoxical fall in open arm entries occurred with these agents at higher doses. These results provide further evidence for the involvement of noradrenergic systems in 'fear'-motivated behaviour.

Spinal monoaminergic modulation of masculine copulatory behavior in the rat

The sexual behavior of male rats receiving infusions of 5-HT, dopamine and noradrenaline intrathecally in the spinal cord of intracerebroventricularly (i.c.v.) in the lateral ventricle was observed. Intrathecal infusions of 5-HT and noradrenaline inhibited penile insertions and ejaculation, noradrenaline being the more potent of the amines. Dopamine was without effect. I.c.v. amine infusions impaired to various extents the masculine mating pattern, primarily by interfering with the males' tendency to approach the females. The facilitation of mating seen after intrathecal administration of the aminotetraline, 8-OH-DPAT, was more pronounced than that observed following its i.c.v. infusion.

Diethyldithiocarbamate depresses the acoustic startle response in rats

The relationship between norepinephrine (NE) content in cortex and spinal cord and acoustic startle amplitude was investigated in two experiments. Administration of diethyldithiocarbamate (DDC) depressed startle amplitude at the same time and dose that it most severely depleted NE content. These results support the conclusion that NE facilitates the normal elaboration of the acoustic startle reflex and also support evidence that NE activity in the spinal cord may be of particular importance in the maintenance of normal startle amplitude.

Spinal clonidine inhibits neural firing in locus coeruleus

Intrathecally-administered clonidine inhibited the spontaneous firing of single neurons in the pontine nucleus locus coeruleus of rats. Such inhibition of neuron firing was not observed when the non-lipophilic alpha 2 adrenoceptor agonist (oxymetazoline) was administered intrathecally. It is concluded that lipophilic drugs like clonidine, when administered intrathecally, can have profound supraspinal actions and thus caution should be exercised in interpreting the sites of action of such drugs.

Behavior and binding: desensitization to alpha 1-adrenergic stimulation of acoustic startle is associated with a decrease in alpha 1-adrenoceptor binding sites

Desensitization of the excitatory effects of alpha 1-adrenergic agonists on acoustic startle occurred 6 h after intrathecal administration of the alpha 1-adrenergic agonist, phenylephrine. This desensitization was associated with a decrease in alpha 1-adrenoceptor sites in the lumbar spinal cord.

Behavior and binding: correlations between alpha 1-adrenergic stimulation of acoustic startle and alpha 1-adrenoceptor occupancy and number in rat lumbar spinal cord

The relationship between alterations in alpha 1-adrenoceptors and behavioral effects of alpha 1-adrenergic agonists were investigated in a localized region of the rat central nervous system. Direct infusion of the alpha 1-adrenergic agonists, D-amphetamine or phenylephrine. into the subarachnoid space of the lumbar cord (intrathecal administration) increased the amplitude of the acoustic startle reflex, The magnitude of this behavioral facilitation correlated highly with the degree of alpha 1-adrenoceptor occupation measured by [3H]prazosin binding in lumbar spinal tissue. Using an in vitro estimate of receptor occupation, maximal potentiation of startle occurred following approximately 30% occupation of the receptors, using either D-amphetamine or phenylephrine. Intrathecal administration of 6-OHDA produced a 95% decrease in spinal norepinephrine and markedly enhanced the behavioral response to intrathecal phenylephrine as well as the number of alpha 1-adrenoceptors. The correlation between the time course of the behavioral and binding changes was 0.99. No change in receptor affinity (KD) or receptor occupation by phenylephrine was found after 6-OHDA. The data indicate that receptor binding parameters do have predictive value for behavior, especially if localized regions of the nervous system, critical to the behavior, are analyzed.

Opposite effects of N,N-dimethyltryptamine (DMT) and 5-methoxy-n,n-dimethyltryptamine (5-MeODMT) on acoustic startle: spinal vs brain sites of action

The present studies examined the role of the spinal cord and the brain in mediating the effects of the hallucinogens N,N-dimethyltryptamine (DMT) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) on the acoustic startle response in the rat. Systemic administration of these agents, which distributes to both the brain and spinal cord, produced opposite effects, as DMT depressed and 5-MeODMT increased acoustic startle. However, when administered directly into the lateral ventricle in the forebrain (intraventricular administration) 5-hydroxytryptamine (5-HT), DMT and 5-MeODMT all depressed acoustic startle, DMT and 5-MeODMT being about equipotent in this regard. In contrast, when administered directly into the spinal cord subarachnoid space (intrathecal administration), 5-HT and 5-MeODMT increased startle, whereas DMT was without effect. In another series of studies, the effects of systemically-administered DMT and 5-MeODMT on the "startle" elicited by electrical stimulation of the nucleus reticularis pontis caudalis (RPC) were determined. Since the RPC is the last nucleus of the primary startle circuit before the spinal cord, agents which act downstream from the RPC (i.e., in the lower brainstem and spinal cord) would be expected to alter RPC-elicited "startle," while agents which act upstream from the RPC would be without effect. Given systemically, 5-MeODMT markedly increased RPC-elicited "startle" while DMT was without effect. These data indicate that DMT and 5-MeODMT are equipotent in depressing startle through actions in the brain. In contrast, the difference in the effects of DMT and 5-MeODMT on acoustic startle is related to the spinal excitatory effects of 5-MeODMT which DMT does not possess. From the present results it is suggested that the relative potencies of DMT and 5-MeODMT in other behavioral measures may relate to the role of brain (equipotent) or spinal (5-MeODMT more potent than DMT) sites of action for the various behaviors.

Tricyclic antidepressants vary in decreasing alpha 2-adrenoceptor sensitivity with chronic treatment: assessment with clonidine inhibition of acoustic startle

1 Clonidine inhibition of the acoustic startle reflex in the rat was used as a behavioural measure of alpha 2-adrenoceptor sensitivity following acute or chronic administration of tricyclic antidepressants. 2 Chronic (14 day) administration of desipramine (10 mg/kg, i.p.) attenuated the depressant effect of clonidine (20 or 40 microgram/kg) on the startle reflex. 3 No change in response to clonidine was obtained after chronic treatment with two other tricyclic antidepressants, amitriptyline (10 mg/kg) or iprindole (5 mg/kg). 4 Acute administration of these tricyclics (1 h) did not modify the effect of clonidine on startle. 5 It is suggested that the development of alpha 2-adrenoceptor subsensitivity produced by chronic tricyclics may be unique to those compounds, such as desipramine, which are active in blocking the uptake of noradrenaline.