Effects of haloperidol and SCH 23390 on acoustic startle and prepulse inhibition under basal and stimulated conditions

A Computational Model for the Simulation of Prepulse Inhibition and Its Modulation by Cortical and Subcortical Units

The sensorimotor gating is a nervous system function that modulates the acoustic startle response (ASR). Prepulse inhibition (PPI) phenomenon is an operational measure of sensorimotor gating, defined as the reduction of ASR when a high intensity sound (pulse) is preceded in milliseconds by a weaker stimulus (prepulse). Brainstem nuclei are associated with the mediation of ASR and PPI, whereas cortical and subcortical regions are associated with their modulation. However, it is still unclear how the modulatory units can influence PPI. In the present work, we developed a computational model of a neural circuit involved in the mediation (brainstem units) and modulation (cortical and subcortical units) of ASR and PPI. The activities of all units were modeled by the leaky-integrator formalism for neural population. The model reproduces basic features of PPI observed in experiments, such as the effects of changes in interstimulus interval, prepulse intensity, and habituation of ASR. The simulation of GABAergic and dopaminergic drugs impaired PPI by their effects over subcortical units activity. The results show that subcortical units constitute a central hub for PPI modulation. The presented computational model offers a valuable tool to investigate the neurobiology associated with disorder-related impairments in PPI.

Dose-Dependent Changes in Auditory Sensory Gating in the Prefrontal Cortex of the Cynomolgus Monkey

BACKGROUND Sensory gating, often described as the ability to filter out irrelevant information that is repeated in close temporal proximity, is essential for the selection, processing, and storage of more salient information. This study aimed to test the effect of sensory gating under anesthesia in the prefrontal cortex (PFC) of monkeys following injection of bromocriptine, haloperidol, and phencyclidine (PCP). MATERIAL AND METHODS We used an auditory evoked potential that can be elicited by sound to examine sensory gating during treatment with haloperidol, bromocriptine, and PCP in the PFC in the cynomolgus monkey. Scalp electrodes were located in the bilateral PFC and bilateral temporal, bilateral parietal, and occipital lobes. Administration of bromocriptine (0.313 mg/kg, 0.625 mg/kg, and 1.25 mg/kg), haloperidol (0.001 mg/kg, 0.01 mg/kg, and 0.05 mg/kg), and the N-methyl-D-aspartic acid receptor antagonist PCP (0.3 mg/kg) influenced sensory gating. RESULTS We demonstrated the following: (1) Administration of mid-dose bromocriptine disrupted sensory gating (N100) in the right temporal lobe, while neither low-dose nor high-dose bromocriptine impaired gating. (2) Low-dose haloperidol impaired gating in the right prefrontal cortex. Mid-dose haloperidol disrupted sensory gating in left occipital lobe. High-dose haloperidol had no obvious effect on sensory gating. (3) Gating was impaired by PCP in the left parietal lobe. CONCLUSIONS Our studies showed that information processing was regulated by the dopaminergic system, which might play an important role in the PFC. The dopaminergic system influenced sensory gating in a dose- and region-dependent pattern, which might modulate the different stages that receive further processing due to novel information.

Probing the molecular basis for an inherited sensitivity to the startle-gating disruptive effects of apomorphine in rats

BACKGROUND

Prepulse inhibition of acoustic startle (PPI) is deficient in several heritable brain disorders. In rats, the dopamine agonist, apomorphine (APO), reduces PPI and expression of the early gene, c-fos, within the nucleus accumbens (NAC) core. Both of these effects are greater in Sprague-Dawley (SD) vs. Long Evans (LE) rats, and this PPI strain pattern is inherited. Here, we examined phosphorylation of cyclic-AMP response element-binding protein (CREB), a putative intermediary step between dopamine receptor stimulation and Fos expression, in SD and LE rats.

METHODS

The effects of APO (vehicle vs. 0.5 mg/kg) on PPI were tested in SD and LE rats in a within-subject design. Seven days later, under conditions mimicking PPI testing, half of the rats from each strain received either vehicle or APO (0.5 mg/kg) 20 min before euthanasia. NAC CREB and phospho-CREB levels were quantified from tissue sections reacted immunohistochemically.

RESULTS

APO reduced PPI in both strains, with a significantly greater effect in SD vs. LE rats. APO also significantly reduced NAC core phospho-CREB levels in both strains, with a significantly greater effect in SD vs. LE rats. Among SD rats receiving APO, the reduction in NAC core CREB phosphorylation correlated significantly with the APO-induced reduction in PPI (R = 0.49).

CONCLUSIONS

A dose of APO that disrupts PPI of acoustic startle causes a profound suppression of CREB phosphorylation in the NAC; both dopamine-sensitive behavioral and molecular phenotypes are more robust in SD vs. LE rats, and within SD rats, they are significantly correlated.

PPI deficit induced by amphetamine is attenuated by the histamine H1 antagonist pyrilamine, but is exacerbated by the serotonin 5-HT2 antagonist ketanserin

RATIONALE

Prepulse inhibition (PPI) of the startle response is a classic model of sensorimotor gating. Robust PPI impairments can be induced by dopamine agonists such as the indirect agonist amphetamine. The antipsychotic clozapine can attenuate PPI impairment induced by dopamine agonists. Clozapine is a complex drug with antagonistic effects on a variety of receptors, including serotonin and histamine. The relative contribution of its component actions to its efficacy is still unclear.

OBJECTIVES

To better characterize the role of histamine and serotonin receptors in the modulation of PPI in rats, we studied the effects of the H(1) histamine antagonist pyrilamine (10, 20, and 40 mg/kg) on amphetamine-induced (1 mg/kg) PPI deficits (Experiment 1); and the interaction of pyrilamine (20 mg/kg) with the 5-HT(2) antagonist ketanserin (1 and 2 mg/kg) on the amphetamine-induced PPI disruption (Experiment 2).

METHODS

Tactile startle stimuli consisted of 30 PSI air-puffs. Three acoustic prepulse intensity levels were used: 68, 71, and 77 dB, presented on a 65-dB background noise. In both experiments, all animals received all drug doses and combinations with different counterbalanced orders.

RESULTS

Pyrilamine (20 mg/kg) was effective in counteracting the PPI impairment caused by amphetamine administration, whereas ketanserin exacerbated the amphetamine-induced PPI deficit.

CONCLUSIONS

Based on its ability to reverse amphetamine-induced PPI deficits, blockade of histamine H(1) receptors seems to contribute to the therapeutic effect of the antipsychotic clozapine. Serotonin 5-HT(2)-receptor blockade, though, does not appear to contribute to this effect, and may in fact detract from it.

Dopamine is necessary for cue-dependent fear conditioning

Dopamine (DA) is implicated in many behaviors, including motor function, cognition, and reward processing; however, the role of DA in fear processing remains equivocal. To examine the role of DA in fear-related learning, dopamine-deficient (DD) mice were tested in a fear-potentiated startle paradigm. DA synthesis can be restored in DD mice through administration of 3, 4-dihydroxy-l-phenylalanine (l-Dopa), thereby permitting the assessment of fear processing in either a DA-depleted or -replete state. Fear-potentiated startle was absent in DD mice but could be restored by l-Dopa administration immediately after fear conditioning. Selective viral-mediated restoration of DA synthesis within the ventral tegmental area fully restored fear learning in DD mice, and restoration of DA synthesis to DA neurons projecting to the basolateral amygdala restored short-term memory but not long-term memory or shock sensitization. We also demonstrate that the DA D(1) receptor (D(1)R) and D(2)-like receptors are necessary for cue-dependent fear learning. These findings indicate that DA acting on multiple receptor subtypes within multiple target regions facilitates the stabilization of fear memory.

Abnormal social behavior, hyperactivity, impaired remote spatial memory, and increased D1-mediated dopaminergic signaling in neuronal nitric oxide synthase knockout mice

Background

Neuronal nitric oxide synthase (nNOS) is involved in the regulation of a diverse population of intracellular messenger systems in the brain. In humans, abnormal NOS/nitric oxide metabolism is suggested to contribute to the pathogenesis and pathophysiology of some neuropsychiatric disorders, such as schizophrenia and bipolar disorder. Mice with targeted disruption of the nNOS gene exhibit abnormal behaviors. Here, we subjected nNOS knockout (KO) mice to a battery of behavioral tests to further investigate the role of nNOS in neuropsychiatric functions. We also examined the role of nNOS in dopamine/DARPP-32 signaling in striatal slices from nNOS KO mice and the effects of the administration of a dopamine D1 receptor agonist on behavior in nNOS KO mice.

Results

nNOS KO mice showed hyperlocomotor activity in a novel environment, increased social interaction in their home cage, decreased depression-related behavior, and impaired spatial memory retention. In striatal slices from nNOS KO mice, the effects of a dopamine D1 receptor agonist, SKF81297, on the phosphorylation of DARPP-32 and AMPA receptor subunit GluR1 at protein kinase A sites were enhanced. Consistent with the biochemical results, intraperitoneal injection of a low dose of SKF81297 significantly decreased prepulse inhibition in nNOS KO mice, but not in wild-type mice.

Conclusion

These findings indicate that nNOS KO upregulates dopamine D1 receptor signaling, and induces abnormal social behavior, hyperactivity and impaired remote spatial memory. nNOS KO mice may serve as a unique animal model of psychiatric disorders.

Attenuation of auditory startle and prepulse inhibition by unexpected changes in ambient illumination through dopaminergic mechanisms

We investigated the role of dopaminergic mechanisms in the attenuation of the acoustic startle response and prepulse inhibition (PPI) in rats by the introduction of unexpected changes in environment illumination. Experiment 1 showed that Dark-to-Light transitions robustly reduce startle responses and PPI. Experiment 2 showed that this phenomenon habituates across repeated testing sessions and reappears after an interval without testing. Experiment 3 demonstrated that haloperidol blocks the startle and PPI-reducing effect of the Dark-to-Light transition. We show how a computational model of acoustic startle response and prepulse inhibition can be extended to incorporate the empirical effects demonstrated in this study. We conclude that sensory gating as measured by prepulse inhibition is markedly attenuated in situations where novel stimuli are introduced during a test session and that dopaminergic systems may be involved in the dynamic changes evoked by the onset of illumination.

Actions of novel antipsychotic agents on apomorphine-induced PPI disruption: influence of combined serotonin 5-HT1A receptor activation and dopamine D2 receptor blockade

The dopamine D1/D2 agonist apomorphine (0.63 mg/kg) disrupted prepulse inhibition (PPI) of acoustic startle in rats, a model of sensorimotor gating deficits observed in schizophrenia. All current antipsychotics, which antagonize D2 receptors, prevent this apomorphine-induced deficit. A novel class of antipsychotics possesses, in addition to D2 antagonist property, various levels of 5-HT1A agonist activity. Considering that the latter itself produces PPI deficits, it appeared necessary to assess the potential of this novel class of antipsychotics to reverse apomorphine-PPI deficits. Potent D2 antagonists, like haloperidol (0.63-2.5 mg/kg), risperidone (0.63-10 mg/kg), and olanzapine (0.63-40 mg/kg) prevented apomorphine PPI disruption. The atypical antipsychotics, clozapine (40 mg/kg), nemonapride (0.01-2.5 mg/kg), ziprasidone (10 mg/kg), and aripiprazole (0.01 and 10 mg/kg), which all exhibit 5-HT1A agonist properties, reversed PPI deficits at some doses only, whereas the anti-dyskinetic agent sarizotan (0.16-10 mg/kg), an efficacious 5-HT1A agonist, did not. New generation antipsychotics with marked 5-HT1A agonist properties, such as SLV313 and SSR181507 (0.0025-10 mg/kg and 0.16-10 mg/kg, respectively) did not reverse these deficits whereas bifeprunox (0.04-2.5 mg/kg) did. To reveal the contribution of 5-HT1A agonist properties in the lack of effects of SLV313 and SSR181507, we pretreated rats with the 5-HT1A antagonist WAY100635 (0.63 mg/kg). Under these conditions, significant reversal of PPI deficit was observed, indicating that D2 antagonist properties of SLV313 and SSR181507 are now sufficient to overcome the disruptive effects of apomorphine. To summarize, antipsychotics possessing agonist efficacy at 5-HT1A receptors exhibit diverse profiles against apomorphine-induced PPI deficits, depending on the balance between D2 and 5-HT1A activities, suggesting that they may display distinct activity on some aspects of gating deficits in schizophrenic patients.

Enhanced effect of dopaminergic stimulation on prepulse inhibition in mice deficient in the alpha subunit of G(z)

RATIONALE

G(z) is a member of the G(i) G protein family associated with dopamine D2-like receptors; however, its functions remain relatively unknown. The aim of the present study was to investigate prepulse inhibition (PPI) of acoustic startle, locomotor hyperactivity and dopamine D2 receptor binding in mice deficient in the alpha subunit of G(z).

METHODS

We used automated startle boxes to assess startle and PPI after treatment with saline, amphetamine, apomorphine or MK-801. We used photocell cages to quantitate locomotor activity after amphetamine treatment. Dopamine D2 receptor density was determined by autoradiography.

RESULTS

Startle responses and baseline PPI were not different between the Galpha(z) knockout mice and wild-type controls (average PPI 46+/-4 vs 49+/-3%, respectively). Amphetamine treatment caused a marked disruption of PPI in Galpha(z) knockouts (average PPI 22+/-2%), but less so in controls (average PPI 42+/-3%). Similar genotype-dependent responses were seen after apomorphine treatment (average PPI 23+/-3% vs 40+/-3%), but not after MK-801 treatment (average PPI 29+/-5 vs 33+/-2%). Amphetamine-induced locomotor hyperactivity was greater in Galpha(z) knockouts than in controls. There was no difference in the density of dopamine D2 receptors in nucleus accumbens.

CONCLUSIONS

Mice deficient in the alpha subunit of G(z) show enhanced sensitivity to the disruption of PPI and locomotor hyperactivity caused by dopaminergic stimulation. These results suggest a possible role for G(z) in neuropsychiatric illnesses with presumed dopaminergic hyperactivity, such as schizophrenia.

Reduced startle gating after D1 blockade: effects of concurrent D2 blockade

BACKGROUND

Prefrontal D1 systems have been implicated in the regulation of working memory and in the pathophysiology of schizophrenia. D1 hypofunction might contribute to reduced sensorimotor gating in schizophrenia patients since D1 activity in the medial prefrontal cortex (MPFC) regulates prepulse inhibition of startle (PPI) in animal models. We studied the neurochemical basis for the D1 regulation of PPI in rats.

METHODS

PPI to weak (1-5 dB over background) prepulses was measured after systemic or intra-MPFC administration of the D1 antagonist, SCH 23390, in rats pretreated systemically with the D2 antagonist, haloperidol (vehicle or 0.1 mg/kg).

RESULTS

After vehicle pretreatment, systemic and intra-MPFC SCH 23390 disrupted PPI produced by weak prepulses. This effect was not significantly opposed by pretreatment with haloperidol (0.1 mg/kg). In contrast, the PPI-disruptive effects of the DA agonist amphetamine were significantly opposed by this dose of haloperidol.

CONCLUSIONS

D1 blockade reduces PPI, but this effect does not appear to be mediated entirely via increased dopamine transmission at D2 receptors.

Effects of amisulpride, risperidone and chlorpromazine on auditory and visual latent inhibition, prepulse inhibition, executive function and eye movements in healthy volunteers

In view of the evidence that cognitive deficits in schizophrenia are critically important for long-term outcome, it is essential to establish the effects that the various antipsychotic compounds have on cognition, particularly second-generation drugs. This parallel group, placebo-controlled study aimed to compare the effects in healthy volunteers (n = 128) of acute doses of the atypical antipsychotics amisulpride (300 mg) and risperidone (3 mg) to those of chlorpromazine (100 mg) on tests thought relevant to the schizophrenic process: auditory and visual latent inhibition, prepulse inhibition of the acoustic startle response, executive function and eye movements. The drugs tested were not found to affect auditory latent inhibition, prepulse inhibition or executive functioning as measured by the Cambridge Neuropsychological Test Battery and the FAS test of verbal fluency. However, risperidone disrupted and amisulpride showed a trend to disrupt visual latent inhibition. Although amisulpride did not affect eye movements, both risperidone and chlorpromazine decreased peak saccadic velocity and increased antisaccade error rates, which, in the risperidone group, correlated with drug-induced akathisia. It was concluded that single doses of these drugs appear to have little effect on cognition, but may affect eye movement parameters in accordance with the amount of sedation and akathisia they produce. The effect risperidone had on latent inhibition is likely to relate to its serotonergic properties. Furthermore, as the trend for disrupted visual latent inhibition following amisulpride was similar in nature to that which would be expected with amphetamine, it was concluded that its behaviour in this model is consistent with its preferential presynaptic dopamine antagonistic activity in low dose and its efficacy in the negative symptoms of schizophrenia.

Dopamine D1 rather than D2 receptor agonists disrupt prepulse inhibition of startle in mice

Although substantial literature describes the modulation of prepulse inhibition (PPI) by dopamine (DA) in rats, few reports address the effects of dopaminergic manipulations on PPI in mice. We characterized the effects of subtype-specific DA agonists in the PPI paradigm to further delineate the specific influences of each DA receptor subtype on sensorimotor gating in mice. The mixed D1/D2 agonist apomorphine and the preferential D1-family agonists SKF82958 and dihydrexidine significantly disrupted PPI, with differing or no effects on startle. In contrast to findings in rats, the D2/D3 agonist quinpirole reduced startle but had no effect on PPI. Pergolide, which has affinity for D2/D3 and D1-like receptors, reduced both startle and PPI, but only at the higher, nonspecific doses. In addition, the D1-family receptor antagonist SCH23390 blocked the PPI-disruptive effects of apomorphine on PPI, but the D2-family receptor antagonist raclopride failed to alter the disruptive effect of apomorphine. These studies reveal potential species differences in the DA receptor modulation of PPI between rats and mice, where D1-family receptors may play a more prominent and independent role in the modulation of PPI in mice than in rats. Nevertheless, due to the limited selectivity of DA receptor agonists, further studies using specific receptor knockout mice are warranted to clarify the respective roles of specific DA receptor subtypes in modulating PPI in mice.

Differential effects of direct and indirect dopamine agonists on prepulse inhibition: a study in D1 and D2 receptor knock-out mice

Stimulation of the dopamine (DA) system disrupts prepulse inhibition (PPI) of the acoustic startle response. On the basis of rat studies, it appeared that DA D2 receptors (D2Rs) rather than D1 receptors (D1Rs) regulate PPI, albeit possibly in synergism with D1Rs. To characterize the DA receptor modulation of PPI in another species, we tested DA D1R and D2R mutant mice with direct and indirect DA agonists and with the glutamate receptor antagonist, dizocilpine (MK-801). Neither the mixed D1/D2 agonist apomorphine (5 mg/kg) nor the more selective D1-like agonist SKF82958 (0.3 mg/kg) altered PPI in D1R knock-out mice, although both compounds disrupted PPI in D2R mutant and wild-type mice, suggesting that the D1R alone might modulate PPI in mice. However, amphetamine (10 mg/kg) significantly lowered PPI in each genotype of D1R mice, suggesting that the D1R is not necessary for the PPI-disruptive effect of the indirect agonist in mice. As reported previously, amphetamine (10 mg/kg) failed to disrupt PPI in D2R knock-out mice, supporting a unique role of the D2R in the modulation of PPI. Dizocilpine (0.3 mg/kg) induced similar PPI deficits in D1R and D2R mutant mice, confirming that the influences of the NMDA receptor on PPI are independent of D1Rs and D2Rs in rodents. Thus, both D1Rs and D2Rs modulate aspects of PPI in mice in a manner that differs from dopaminergic modulation in rats. These findings emphasize that further cross-species comparisons of the pharmacology of PPI are essential to understand the relevance of rodent PPI studies to the deficits in PPI observed in patients with schizophrenia.

Dopamine D1 receptors and adenosine A1 receptors in the rat nucleus accumbens regulate motor activity but not prepulse inhibition

Locomotor activity and sensorimotor gating (measured as prepulse inhibition of startle) are regulated by mesoaccumbal dopamine. Recent evidence indicated antagonistic interactions between adenosine A(1) receptors and dopamine D(1) receptors, as well as between adenosine A(2) receptors and dopamine D(2) receptors in the nucleus accumbens. Therefore, it is conceivable that accumbal dopamine and adenosine are both involved in the regulation of prepulse inhibition and locomotion. We tested whether accumbal adenosine A(1) and dopamine D(1) receptors control locomotor activity and prepulse inhibition using the following four treatments. (1) Injections of the selective adenosine A(1) receptor agonist N(6)-cyclopentanyladenosine (CPA 1.5 and 3 microg/microl per side) into the nucleus accumbens. (2) Stimulation of the ventral tegmental area by local infusion of the GABA(A) receptor antagonist picrotoxin (25-100 ng/0.5 microl bilaterally). (3) Picrotoxin injections into the ventral tegmental area (100 ng/0.5 microl) and simultaneous bilateral injections of CPA (3 microg/microl per side) into the nucleus accumbens. (4) Injections of the selective dopamine D(1) receptor antagonist SCH 23390 (3 microg/0.5 microl per side) into the nucleus accumbens and ventral tegmental area stimulation by picrotoxin. Intra-accumbal CPA infusion reduced locomotor activity but had no effect on prepulse inhibition. Picrotoxin stimulation of the ventral tegmental area increased locomotor activity which was antagonized by co-administration of CPA or SCH 23390 into the nucleus accumbens. An enhancement of prepulse inhibition was observed after stimulation of the ventral tegmental area and co-administration of SCH 23390 into the nucleus accumbens. These findings demonstrate that adenosine A(1) and dopamine D(1) receptors are involved in the regulation of locomotor activity mediated by the mesoaccumbal dopamine system. The finding that locomotor effects induced by stimulation of the mesoaccumbal dopamine system were not accompanied by a prepulse inhibition-deficit suggests a dissociation of the neuronal substrates involved in the control of locomotion and the regulation of sensorimotor gating.

Hippocampal and cortical sensory gating in rats: effects of quinpirole microinjections in nucleus accumbens core and shell

Sensory processing disturbances, as measured in the P50/sensory gating paradigm, have been linked to aberrant auditory information processing and sensory overload in schizophrenic patients. In this paradigm, the response to the second of paired-click stimuli is attenuated by an inhibitory effect of the first stimulus. Sensory gating has been observed in most healthy human subjects and normal laboratory rats. Because mesolimbic dopamine has been implicated in other filtering disturbances such as prepulse inhibition of the acoustic startle response and given the fact that amphetamine and apomorphine have been shown to disrupt gating, this study was performed to investigate the role of mesolimbic dopamine in sensory gating. The dopamine D2 receptor agonist quinpirole (10 microg/0.5 microl) was injected bilaterally in nucleus accumbens core and shell and effects on cortical and hippocampal sensory gating were investigated. Also, effects of the dopamine D2 receptor antagonist haloperidol (0.1 mg/kg, subcutaneously) as pretreatment were studied. First, quinpirole significantly reduced both the amplitude to the first click and gating as measured in the cortex and in the hippocampus. There was a tendency for the quinpirole effects on hippocampal gating to be more pronounced in rats injected in the shell. Secondly, haloperidol did not antagonize effects of quinpirole on hippocampal parameters, whereas haloperidol pretreatment fully antagonized quinpirole effects on cortical parameters. In conclusion, gating can be significantly reduced when a dopamine agonist is specifically targeted at mesolimbic dopamine D2 receptors. However, an important consideration is that the dopaminergic effects in the present study on gating are predominantly mediated by the effects on the amplitude to the first click. This has also been suggested for systemic amphetamine injections in rats and schizophrenic patients. This casts doubt on whether dopamine receptor activation affects the putative inhibitory process between the first and the second stimulus.

Neonatal dexamethasone on day 7 in rats causes behavioral alterations reflective of hippocampal, but not cerebellar, deficits

Developmental glucocorticoid treatment in rats has been shown to cause body and brain weight decrements concurrent with behavioral alterations. Here, Sprague-Dawley rats were treated with the synthetic glucocorticoid, dexamethasone (DEX), on postnatal day (PND) 7 (1.5 mg/kg, sc, injected in the morning and afternoon). Behavioral assessments of negative geotaxis, locomotor activity (open field, maze exploration, residential running wheel, residential figure 8 maze), open-field activity response to amphetamine, acoustic startle, prepulse inhibition (PPI) of acoustic startle, juvenile play behavior, anxiety (emergence tests), motor coordination (rotarod performance), spatial learning (Morris water maze and food-reinforced complex maze), and operant performance (time estimation and response inhibition) were assessed in male rats. Body weight was decreased beginning at PND 43 until sacrifice on PND 127. Whole and regional brain weights were less, especially hippocampus, cerebellum, brainstem, and cortical remnant. Indications of delayed development were apparent; specifically, DEX-treated rats took significantly longer to turn on PND 8, but not PND 9, in the negative geotaxis test. DEX treatment induced deficits in the Morris water maze that were similar to hippocampal deficits. Open-field activity changes were inconsistent; however, DEX-treated rats were hyperactive during the dark period in running wheel tests. There were no indications of changes in reactivity or emotionality.

Circadian time does not modify the prepulse inhibition response or its attenuation by apomorphine

The present study investigated the influence of circadian time (experimental testing during the light or dark phase of the light:dark cycle) on the acoustic startle response (ASR), prepulse inhibition (PPI), and apomorphine-induced PPI deficits in Wistar rats housed under a reversed light:dark cycle (lights off at 0700 h and on at 1900 h). There was no significant difference in the startle response amplitude or PPI response of animals tested during the light phase compared with those tested during the dark phase. Similarly, the response to apomorphine (0.01-0.05 mg/kg subcutaneously) was not modulated by circadian time. Thus, under the conditions adopted in the present study, ASR, PPI, and apomorphine-induced PPI deficits remained stable across the circadian cycle. Such findings may be of importance for other investigators using the PPI paradigm to study brain plasticity mechanisms and pharmacological manipulations of apomorphine-induced PPI deficits in rats housed under normal or reversed light:dark cycle conditions.

The neurobiology of startle

Startle is a fast response to sudden, intense stimuli and probably protects the organism from injury by a predator or by a blow. The acoustic startle response (ASR) of mammals is mediated by a relatively simple neuronal circuit located in the lower brainstem. Neurons of the caudal pontine reticular nucleus (PnC) are key elements of this primary ASR pathway. The ASR in humans and animals has a non-zero baseline, that is, the response magnitude can be increased or decreased by a variety of pathological conditions and experimental manipulations. Therefore, the ASR has been used as a behavioral tool to assess the neuronal basis of behavioral plasticity and to model neuropathological dysfunctions of sensorimotor information processing. Cross-species examples for the increase of the ASR magnitude are sensitization, fear-potentiation and drug-induced enhancement. Examples for the reduction of the ASR magnitude are habituation, prepulse inhibition, drug-induced inhibition and the attenuation by positive affect. This review describes the neuronal basis underlying the mediation of the ASR, as well as the neuronal and neurochemical substrates of different phenomena of enhancement and attenuation of the ASR. It also attempts to elucidate the biological background of these forms of behavioral plasticity. Special emphasis is put on the potential relevance of ASR modulations for the understanding of human psychiatric and neurological diseases.

The disruption of prepulse inhibition by social isolation in the Wistar rat: how robust is the effect?

Postweaning isolation rearing in rats is shown to have consequences for the expression of numerous behaviors. The present studies investigated isolation-induced disruptions of the prepulse inhibition (PPI) response in the Wistar rat strain, as a function of exposure of the animals to locomotor activity testing. Further, repeated testing of PPI was investigated to examine the robustness of the isolation-induced disruptions. The results indicate that experimentally naive isolation-reared animals exhibit disruptions in the PPI response that are retained in a second test 7 days later. The disruptions obtained are shown to be consistent across all pulse frequencies examined and independent of effects on startle. Exposure to activity testing, however, either before or after the measurement of PPI, abolished the isolation-induced disruption of PPI in a subsequent test. In contrast, locomotor activity testing consistently revealed a hyperactivity response in isolation-reared animals that was not influenced by the temporal occurrence of the testing. The findings are discussed relative to the interpretation of data emerging from studies where both activity testing and PPI are performed in the same animals, and in the relation to the use of PPI in isolation-reared animals as representing a nonpharmacological animal model of schizophrenia.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.

Prepulse inhibition and latent inhibition: the role of dopamine in the medial prefrontal cortex

The prefrontal cortex has often been implicated in the pathophysiology of schizophrenia. Schizophrenic patients are known to suffer from certain information processing deficits, which can be detected, among others, in the prepulse inhibition and the latent inhibition paradigm. The present study was designed to investigate the role of dopamine receptors in the medial prefrontal cortex in prepulse inhibition and latent inhibition. The results show that the local application of the selective antagonist of the dopamine D1-like receptor family, SCH 39166, into the medial prefrontal cortex dose-dependently reduced prepulse inhibition. Likewise, the selective antagonist of the dopamine D2-like receptor family, sulpiride, injected into the medial prefrontal cortex dose-dependently reduced prepulse inhibition. Neither of these antagonists, however, influenced latent inhibition as measured with the conditioned taste aversion paradigm. These data further indicate that the neuronal substrates of latent inhibition and prepulse inhibition are clearly different. Since the prefrontal cortex is intimately related to subcortical dopamine, the possible differential involvement of subcortical dopaminergic terminal fields in prepulse inhibition and latent inhibition is discussed.

Effects of the PCP analog dizocilpine on sensory gating: potential relevance to clinical subtypes of schizophrenia

Prepulse inhibition (PPI) of the acoustic startle reflex, a measure of sensory gating, is reduced in schizophrenic patients. Dopamine agonists and NMDA receptor antagonists such as phencyclidine (PCP) can disrupt PPI in animals, consistent with both the dopamine and glutamate hypotheses of schizophrenia. In this study, we sought to further characterize the effects of the NMDA antagonist dizocilpine on acoustic startle modulation. Fischer 344 rats were tested after one of three doses of dizocilpine (0.05, 0.2, and 0.5 mg/kg) and assessed for PPI as well as for alterations in baseline startle and prepulse facilitation (PPF). Results showed complete disruption of PPI for each inhibitory trial type after 0.2 and 0.5 mg/kg of dizocilpine. Baseline startle and PPF were enhanced by the low dose but decreased with the moderate and high doses of dizocilpine. Although dizocilpine caused alterations in prepulse modulation of startle similar to dopamine agonists, some effects differ. Unique effects of dizocilpine on sensory gating are discussed in terms of their potential for discriminating subtypes of schizophrenic illness with different underlying pathophysiology.

Effects of haloperidol and SCH 23390 on acoustic startle in animals depleted of dopamine as neonates: implications for neuropsychiatric syndromes

Animals depleted of dopamine (DA) in the neonatal period and tested in adulthood exhibit some similarities to patients with schizophrenia, including increased sensitivity to DA agonists, altered sensitivity to DA receptor antagonists, and abnormalities of the acoustic startle response (ASR). In this study, we examined the contributions of D1-like and D2-like DA receptors to ASR measures in animals depleted of DA as neonates. Male rat pups received intracerebroventricular injections of 6-hydroxydopamine (DA depleted) or its vehicle (controls) at 3 days of age. Animals underwent startle testing as adults (60-75 days of age) after administration of DA antagonists (haloperidol: 0.1 or 0.3 mg/kg, SCH 23390:0.01 or 0.05 mg/kg) with and without DA agonist administration (apomorphine 0.5 mg/kg). ASR amplitude and prepulse inhibition (PPI: percentage decrease in startle amplitude due to a low intensity prepulse) were measured. DA depleted animals showed increased ASR amplitude and reduced PPI compared to controls. Administration of D1-like or D2-like DA antagonists significantly reduced overall ASR and increased PPI in both control and DA depleted animals, with DA depleted animals showing a relatively greater sensitivity to the D1-like antagonist SCH 23390. Findings are discussed in terms of the role of residual DA in mediating ASR phenomena in depleted animals, differences between D1/D2 DA receptor mediation of ASR compared to other behaviors in DA depleted animals, and potential implications for neuropsychiatric syndromes such as schizophrenia.

Prepulse inhibition of acoustic startle, a measure of sensorimotor gating: effects of antipsychotics and other agents in rats

Schizophrenic patients are deficient in various neurologic measures reflecting information processing. One such measure in prepulse inhibition (PPI) of acoustic startle, in which schizophrenics display less inhibition than normal subjects. PPI is also diminished in rats treated with psychotomimetic drugs such as amphetamine and phencyclidine. PPI has been suggested as a model relevant for studying the pathophysiology of schizophrenia. We studied the effect of a variety of antipsychotics and putative antipsychotics and some key reference compounds on the acoustic startle response (ASR) and PPI. Some, but not all, antipsychotics tested (mainly selective dopamine D2 antagonists) enhanced PPI. Remoxipride and clozapine, both of which are antipsychotics, and the very potent and highly selective D2 antagonist, NCQ-298, did not. It is concluded that enhanced PPI in otherwise untreated rats does not reflect antipsychotic efficacy. We further noted that the effect on PPI was independent of the effect on ASR.

Effects of single and repeated exposure to apomorphine on the acoustic startle reflex and its inhibition by a visual prepulse

The acoustic startle reflex (ASR) is inhibited by startle-irrelevant stimuli that briefly precede reflex elicitation. This effect, prepulse inhibition (PPI), is reduced in strength for animals that have received dopamine agonists, such as apomorphine (APO). Reduction in PPI is most evident for weak masked noise prepulses, thus suggesting that APO disrupts the reception of stimuli to the extent that they present a low signal-to-noise ratio. Here we examine the effect of APO on PPI produced by non-masked visual prepulses. Light flashes were given at two intensities, 40, 70, 110, or 220 ms before ASR elicitation. In phase 1 (5 weeks in duration) half of the animals received one weekly injection of APO (0.5 mg/kg, IP) and one of vehicle (VEH), while the other half received two injections of VEH. Within these groups, half were tested 30 min after the injections, the other half kept test naive (four groups total). In phase 2, following a 4-week rest, all groups were tested after a low dose of APO (0.1 mg/kg) and VEH, 1 week apart. APO eliminated PPI for a dim flash and reduced PPI for a brighter flash to a level normally obtained with the dim flash, while increasing both ASR control values and activity. The bright light was maximally effective at a lead time of 70 ms and APO did not alter this value. Because in general the time of maximal inhibition varies with prepulse intensity for visual stimuli, the finding that the time of the peak remained constant reveals that APO has its effect on inhibition rather than on effective stimulus intensity. In phase 2, APO reduced PPI with no sign of sensitization from past drug exposure. However, APO increased the ASR only in groups previously exposed to APO, indicating behavioral sensitization. The differential effects of repeated exposure on these response measures suggest that neural substrates for the several behavioral effects of APO function at least in part independently.

Deficient sensorimotor gating after 6-hydroxydopamine lesion of the rat medial prefrontal cortex is reversed by haloperidol

The present study sought to test the hypothesis that dopamine in the prefrontal cortex exerts an inhibitory influence on subcortical dopamsine systems and that depletion of prefrontal dopamine may affect behaviour via an increase in dopamine release in the basal ganglia. We used prepulse inhibition of the acoustic startle response, i.e. the inhibition of the acoustic startle response by a preceding non-startling stimulus, as the behavioural test, because this phenomenon of sensorimotor gating is modified in opposite directions by dopamine in the prefrontal cortex and in the basal ganglia. Rats were tested for prepulse inhibition before and after injections of the neurotoxin 6-hydroxydopamine into the medial prefrontal cortex. We attempted to differentiate the contributions of prefrontal dopamine and noradrenaline by pretreating the animals with desipramine (6-OHDAMI rats) or bupropion (6-OHDABUP rats), selective inhibitors of noradrenaline and dopamine reuptake respectively. 6-Hydroxydopamine lesion reduced prefrontal dopamine by 90% and noradrenaline by 80% in 6-OHDADMI rats, while prefrontal dopamine was reduced by 54% and noradrenaline by 95% in 6-OHDABUP rats. The ability of an acoustic prepulse (75 dB, 10 kHz) to inhibit the response to a startle pulse (100 dB noise burst) was maintained in sham-lesioned rats and in 6-OHDABUP rats. However, there was a marked reduction of prepulse inhibition (by 26%) in the 6-OHDADMI rats. Systemic administration of the dopamine antagonist haloperidol (0.05 mg/kg), which did not affect prepulse inhibition in sham-lesioned and in 6-OHDABUP rats, antagonized the lesion-induced deficit in prepulse inhibition in 6-OHDADMI rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Concurrent assessment of acoustic startle and auditory P50 evoked potential measures of sensory inhibition

The acoustic startle response (ASR) and midlatency auditory evoked potentials (AEP) have been utilized in the measurement of sensory inhibition. Using these different paradigms, abnormalities suggesting a lack of normal inhibition have been noted in a number of psychiatric syndromes. To date, the most commonly used sensory inhibition paradigms have not been studied in the same individuals, making generalizations across studies tenuous. In this report, reduction of ASR over multiple trials (habituation), prepulse inhibition (PPI) of ASR (decrease in ASR caused by low intensity prepulses) and P50 suppression (P50 AEP amplitude reduction in a paired-click paradigm) were measured in the same individuals. Relationships between these measures of acoustic startle and AEP inhibition were then assessed. Twenty subjects with no personal history of psychiatric disorder were tested and exhibited significant habituation and PPI of ASR as well as P50 suppression. Habituation of ASR was significantly and positively correlated with P50 suppression early, but not late, in AEP testing. Only a modest trend for a positive association between PPI and P50 suppression was noted. Habituation and PPI of startle were both highly correlated (positively) with P50 AEP amplitude. Habituation of startle remained significantly predictive of P50 suppression after controlling for P50 amplitude, whereas the modest association between PPI and P50 suppression was removed when P50 amplitude was factored out. Results indicate that habituation of acoustic startle, but not PPI, is highly associated with P50 suppression in control subjects. An unexpected finding was a robust positive correlation between P50 amplitude and both measures of startle inhibition. These findings and methodologic issues are discussed in terms of possible neural substrates involved in different measures of sensory inhibition.