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

The 6-Hydroxydopamine model of parkinson’s disease

The neurotoxin 6-hydroxydopamine (6-OHDA) continues to constitute a valuable topical tool used chiefly in modeling Parkinson's disease in the rat. The classical method of intracerebral infusion of 6-OHDA involving a massive destruction of nigrostriatal dopaminergic neurons, is largely used to investigate motor and biochemical dysfunctions in Parkinson's disease. Subsequently, more subtle models of partial dopaminergic degeneration have been developed with the aim of revealing finer motor deficits. The present review will examine the main features of 6-OHDA models, namely the mechanisms of neurotoxin-induced neurodegeneration as well as several behavioural deficits and motor dysfunctions, including the priming model, modeled by this means. An overview of the most recent morphological and biochemical findings obtained with the 6-OHDA model will also be provided, particular attention being focused on the newly investigated intracellular mechanisms at the striatal level (e.g., A(2A) and NMDA receptors, PKA, CaMKII, ERK kinases, as well as immediate early genes, GAD67 and peptides). Thanks to studies performed in the 6-OHDA model, all these mechanisms have now been hypothesised to represent the site of pathological dysfunction at cellular level in Parkinson's disease.

Prepulse inhibition and P50 suppression are both deficient but not correlated in schizophrenia patients

BACKGROUND

Inhibitory measures such as prepulse inhibition of the acoustic startle reflex (PPI) and event related potential P50 suppression have been widely reported to show deficits in schizophrenia patients. The relationship between PPI and P50 suppression in schizophrenia patients has remained unclear.

METHODS

One hundred fifty-six schizophrenia patients and 104 normal comparison subjects (NCS) were tested on PPI and P50 suppression.

RESULTS

Eighty-one patients and 70 NCS had valid and scorable data on both PPI and P50 suppression paradigms. As in the larger groups, these cohorts had deficits on both PPI (p < .05) and P50 suppression (p < .05). Analyses revealed a weak, but significant correlation between PPI and P50 suppression in the NCS group (r = .33, p < .05) but not in the patient group (r = .03, ns).

CONCLUSIONS

Although PPI and P50 suppression were both reduced in the patients, they were not correlated. This divergence suggests that these gating functions are complementary or redundant levels of "protection" against processes that may lead to cognitive fragmentation.

Perinatal AZT exposure alters the acoustic and tactile startle response to 8-OH-DPAT and apomorphine in adult rats

The present study was designed to assess the dopaminergic and serotonergic contributions of the acoustic startle response (ASR) and the tactile startle response (TSR) in adult rats that had been perinatally exposed to AZT (azidothymidine, zidovudine; an antiretroviral agent). Each dam was randomly assigned to a treatment group: non-treated, AZT0, 100 or 150 mg/kg. Once daily gastric intubation began prenatally on gestational day (G) 19 and continued to G22 and then the pups were intubated between postnatal day (PND) 2-20. On PND60, animals were tested for responses to both acoustic and tactile stimuli following a challenge of vehicle, 0.25 or 0.5 mg/kg 8-OH-DPAT, a 5-HT(1A) agonist, or 0.75 or 2.0 mg/kg apomorphine (APO, a dopaminergic agonist) IP. Both DPAT and APO increased startle magnitude as expected. Additionally, perinatal AZT exposure enhanced startle responses following both DPAT and APO, an effect not due to perinatal handling or intubation. Similarly, perinatal AZT increased tactile responses following drug challenge in a gender-specific manner. Perinatal AZT also prolonged startle latencies, a change which may indicate that perinatal AZT alters conduction velocity. Therefore, the administration of AZT during the perinatal period results in long-term functional alterations within the startle reflex pathways.

HZE radiation and dopaminergic modification of startle and prepulse inhibition in mice

C57BL/6 mice were exposed to 5 Gy (28)Si or (56)Fe particle radiation in order to explore the immediate or short-latency effect of exposure to high energy (HZE) particle radiation on dopaminergic modification of acoustic startle and prepulse inhibition. The radiation is representative of the type which would be encountered as galactic cosmic rays during long-duration space flight. The acoustic startle response was elicited with 120 dB white noise and prepulse inhibition of the startle response was produced with 79 dB and 86 dB stimuli presented with a 125 ms onset asynchrony. Startle reactivity was inhibited by (56)Fe radiation but not by (28)Si particles. Apomorphine (3 mg/kg) produced a general inhibition of startle reactivity while haloperidol (1 mg/kg) facilitated it. Apomorphine disrupted prepulse inhibition, but only in animals which were not exposed to radiation. Both (56)Fe and (28)Si radiation exposure attenuated the disruption of prepulse inhibition induced by apomorphine. In contrast, the facilitation of prepulse inhibition induced by haloperidol was not modified by radiation. These data are consistent with a short-latency disruption of dopaminergic systems by HZE particle radiation. We speculate that this disruption may occur as a restriction in the capacity of the dopaminergic system.

The neonate-6-hydroxydopamine-lesioned rat: a model for clinical neuroscience and neurobiological principles

In 1973, a technique of administering 6-hydroxydopamine (2,4,5-trihydroxyphenylethylamine) intracisternally to neonate rats was introduced to selectively reduce brain dopamine (neonate-lesioned rat). This neonate treatment proved unique when compared to rats lesioned as adults with 6-hydroxydopamine--prompting the discovery of differing functional characteristics resulting from the age at which brain dopamine is reduced. A realization was that neonate-lesioned rats modeled the loss of central dopamine and the increased susceptibility for self-injury in Lesch-Nyhan disease, which allowed identification of drugs useful in treating self-injury in mentally retarded patients. The neonate-lesioned rat has also been proposed to model the hyperactivity observed in attention-deficit hyperactivity disorder. Because the neonate-lesioned rat exhibits enhanced sensitization to repeated NMDA receptor antagonist administration and has functional changes characteristic of schizophrenia, the neonate lesioning is believed to emulate the hypothesized NMDA hypofunction in this psychiatric disorder. Besides modeling features of neurological and psychiatric disorders, important neurobiological concepts emerged from pharmacological studies in the neonate-lesioned rats. One was the discovery of coupling of D1/D2-dopamine receptor function. Another was the progressive increase in responsiveness to repeated D1-dopamine agonist administration referred to as "priming" of D1-dopamine receptor function. Additionally, a unique profile of signaling protein expression related to neonate reduction of dopamine has been identified. Thus, from modeling characteristics of disease to defining adaptive mechanisms related to neonatal loss of dopamine, the neonate-lesioned rat has had a persisting influence on neuroscience. Despite an extraordinary legacy from studies of the neurobiology of this treatment, a host of unknowns remain that will inspire future investigations.

Effect of acute and chronic olanzapine treatment on phencyclidine-induced behavioral sensitization in rats with neonatal dopamine loss

In agreement with previous work, adult rats given selective lesions to dopamine (DA)-containing neurons as neonates exhibited a greater behavioral sensitization to repeated phencyclidine (PCP) treatment in comparison to sham-lesioned controls. Acute administration of olanzapine (1-5 mg/kg ip) or clozapine (15 mg/kg ip) decreased sensitized PCP-induced activity in both lesioned and control animals. Acute haloperidol (0.5 mg/kg ip) had no impact on PCP responsiveness in lesioned animals, but significantly antagonized PCP effects in sham-lesioned controls. Ketanserin, a selective 5-HT(2A)/5-HT(2C)-receptor antagonist, significantly reduced PCP activation in both lesioned and control rats, suggesting that the efficacy of atypical antipsychotics against PCP-induced sensitized responses may be mediated by one of the 5-HT(2)-receptor subtypes. A 6-week chronic regimen of orally administered olanzapine, clozapine, or haloperidol failed to block the sensitization induced by repeated PCP exposure. However, a 10-month oral olanzapine treatment significantly blunted the behavioral sensitization to repeated PCP exposure in lesioned animals, even after withdrawal from chronic olanzapine for more than 3 weeks. A 10-month oral haloperidol treatment had no effect on the sensitization induced by repeated PCP dosing. The persistent effect of chronic olanzapine administration on PCP sensitization may be relevant to the chronic therapeutic efficacy of atypical antipsychotics treating schizophrenia-a clinical syndrome linked to enhanced sensitivity to N-methyl-d-aspartate (NMDA)-receptor antagonists.

Pathological gambling: a comprehensive review of biobehavioral findings

In this review, findings of biobehavioral research into pathological gambling (PG) are discussed, focusing on neuropsychological, psychophysiological, neuroimaging, neurochemical and genetic studies. Neuropsychological studies indicate deficiencies in certain executive functions. Psychophysiological studies indicate that arousal in PG is of importance when reward is present. Neuroimaging studies point to abnormalities in brain functioning. Recent research into the neurochemistry of PG indicates that abnormalities exist in different neurotransmitter systems. Finally, genetic studies indicate the existence of abnormal dopamine receptor genes in PG. Methodological and theoretical factors that may explain discrepancies between studies include differences in screening and assessment, heterogeneity of gambling problems and different underlying cognitive or motivational mechanisms. Results from the PG studies fit in with recent theoretical models of addiction and PG, which stress the involvement of brain reward pathways, neurotransmitter abnormalities, the frontal cortex and the psychophysiological stress system. A framework for future studies is suggested, indicating the need for studies that integrate knowledge from different research areas, and that employ stricter diagnostic screening methods and inclusion of clinical control groups.

Disrupted sensory gating in pathological gambling

BACKGROUND

Some neurochemical evidence as well as recent studies on molecular genetics suggest that pathologic gambling may be related to dysregulated dopamine neurotransmission.

METHODS

The current study examined sensory (motor) gating in pathologic gamblers as a putative measure of endogenous brain dopamine activity with prepulse inhibition of the acoustic startle eye-blink response and the auditory P300 event-related potential. Seventeen pathologic gamblers and 21 age- and gender-matched healthy control subjects were assessed. Both prepulse inhibition measures were recorded under passive listening and two-tone prepulse discrimination conditions.

RESULTS

Compared to the control group, pathologic gamblers exhibited disrupted sensory (motor) gating on all measures of prepulse inhibition. Sensory motor gating deficits of eye-blink responses were most profound at 120-millisecond prepulse lead intervals in the passive listening task and at 240-millisecond prepulse lead intervals in the two-tone prepulse discrimination task. Sensory gating of P300 was also impaired in pathologic gamblers, particularly at 500-millisecond lead intervals, when performing the discrimination task on the prepulse.

CONCLUSIONS

In the context of preclinical studies on the disruptive effects of dopamine agonists on prepulse inhibition, our findings suggest increased endogenous brain dopamine activity in pathologic gambling in line with previous neurobiological findings.

Neonatal lesions in the amygdala or ventral hippocampus disrupt prepulse inhibition of the acoustic startle response; implications for an animal model of neurodevelopmental disorders like schizophrenia

Prepulse inhibition of the acoustic startle response is a behavioural tool applied to assess sensorimotor gating processes in humans and rats. Schizophrenic patients show deficits in prepulse inhibition of the acoustic startle response. The animal model of neurodevelopmental disorders such as schizophrenia, as purported in earlier reports and the present study, is based on the assumption that damage to brain structures early in life (on day 7) disrupts brain maturation of structures connected to the damaged areas, measurable by behavioural changes, whereas similar damage later in life (on day 21) does not result in these behavioural changes. Locomotor activity, the acoustic startle response and its prepulse inhibition were investigated in adult rats lesioned in the amygdala or ventral hippocampus on day 7 or 21 of life. The acoustic startle response was increased in animals lesioned in the amygdala on day 7 or 21 of life, but not in animals lesioned in the ventral hippocampus. Prepulse inhibition was impaired and locomotor activity enhanced in animals lesioned in the amygdala or ventral hippocampus on day 7, but not in animals lesioned in these structures on day 21 of life. The results on the acoustic startle response are suggestive of amygdaloid influences on modulation of the acoustic startle response. The effects of early postnatal lesions on prepulse inhibition and locomotor activity are in support of the animal model of neurodevelopmental disorders like schizophrenia.

Prepulse inhibition of acoustic startle in aromatase knock-out mice: effects of age and gender

Estrogen has been suggested to play a neuromodulatory and neuroprotective role on the brain dopamine system. We used aromatase knockout (ArKO) mice that lack a functional aromatase enzyme and are unable to convert testosterone into estrogen, and assessed prepulse inhibition of acoustic startle, locomotor hyperactivity to amphetamine treatment and rotarod performance. Mice were tested at either 1 month, 4-5 months or 12-18 months of age. In male, but not female ArKO mice, there was an age-related reduction of prepulse inhibition. The 12-18 months old male ArKO mice also showed significantly greater amphetamine-induced hyperactivity. Mice heterozygous for the mutation showed no deficits or were in-between wildtype mice and ArKO mice. We postulate that these data indicate a neuroprotective role of estrogen, particularly in male mice, on ageing of brain mechanisms involved in pre-pulse inhibition and locomotor activity regulation. It is likely that these brain mechanisms are or include dopaminergic activity.

The effects of some antidepressant drugs on prepulse inhibition of the acoustic startle (eyeblink) response and the N1/P2 auditory evoked response in man

Both the acoustic startle (eyeblink) response and the N1/P2 complex of the auditory evoked potential can be suppressed by presentation of a brief low-intensity stimulus 30-500 ms before the 'startle-eliciting' stimulus ('prepulse inhibition', PPI). We examined the effects of three antidepressants on PPI of these two responses. Fifteen males (19-30 years) participated in four weekly sessions, in which they received placebo, amitriptyline (100 mg), fluvoxamine (100 mg), and reboxetine (4 mg) (p.o.), according to a balanced double-blind design. Twenty minute simultaneous recordings of electromyographic (EMG) responses of the orbicularis oculi muscle of the right eye and vertex auditory evoked potentials were carried out 195 min after drug ingestion. Sound stimuli (1 kHz) were presented in 40 trials separated by variable intervals (mean 25 s): (1) 40 ms, 115 dB ('pulse alone', 20 trials) and (2) 40 ms, 85 dB, followed after 120 ms by 40 ms, 115 dB ('prepulse/pulse', 20 trials). Under the placebo condition, both the EMG response and the N1/P2 complex showed >50% PPI. Fluvoxamine and reboxetine did not significantly alter the amplitude or PPI of either response. Amitriptyline significantly reduced the amplitudes of both responses; it had no effect on PPI of the EMG response, but significantly attenuated PPI of the N1/P2 complex. Amitriptyline also reduced arousal, as indicated by an increase in power of low-frequency electroencephalographic waves. The results confirm the susceptibility of the N1/P2 complex to PPI. The reduction of the amplitudes of the EMG response and N1/P2 complex by amitriptyline may be related to its sedative action. The differential effect of amitriptyline on PPI of the N1/P2 complex supports the suggestion that different mechanisms may be involved in PPI of this response and PPI of the N1/P2 complex.

The acoustic startle response in rats--circuits mediating evocation, inhibition and potentiation

This review describes the neuronal mechanisms underlying the mediation and modulation of the acoustic startle response (ASR) in rats. The combination of anatomical, physiological and behavioral methods has identified pathways which mediate and modulate the ASR. The ASR is mediated by a relatively simple, oligosynaptic pathway located in the lower brainstem which activates spinal and cranial motor neurons. An important element of the pathway which mediates the ASR is the caudal nucleus of the pontine reticular formation (PnC). Interestingly, this nucleus is also the target of input from various brain nuclei which are involved in the modulation (e.g. fear-potentiation, sensitization, habituation, prepulse inhibition and pleasure-attenuation) of the ASR. Hence, the PnC can be described as a sensorimotor interface, where the transition of sensory input into the motor output can be directly influenced by excitatory or inhibitory afferents. On the basis of these facts we conclude that the ASR may be a valuable model for the study of general principles of sensorimotor-motivational information processing at the behavioral and neurophysiological level in mammals.