Characterization of allelic variants at chromosome 15q14 in schizophrenia.

Evidence of genetic linkage for schizophrenia at chromosome 15q14 has been reported in nine independent studies, but the molecular variants responsible for transmission of genetic risk are unknown. National Institute of Mental Health Schizophrenia Genetics Initiative families were genotyped for single nucleotide polymorphisms (SNPs) and dinucleotide repeat markers in the 15q14 linkage region and analyzed based on the presence of particular alleles of the dinucleotide repeat marker D15S165 in the 15q14 region. Two alleles showed both familial transmission disequilibrium and population-wide association with schizophrenia. The two groups identified by these two D15S165 alleles differ in age of onset, number of hospitalizations and intensity of nicotine abuse, as well as in predominant ethnicity. Variations in the frequency of SNPs in CHRNA7, the alpha-7-nicotinic acetylcholine receptor subunit gene at 15q14, were found in each group. Further sequencing in these two groups may yield more definitive identification of the molecular pathology.

Deficits in sensory gating in schizophrenic patients and their relatives. Evidence obtained with auditory evoked responses.

A deficit in inhibitory gating of auditory evoked responses was examined in 15 schizophrenic patients, their first-degree relatives, and normal subjects, using a conditioning-testing paradigm with the P50 wave of the auditory evoked response. This paradigm demonstrates inhibition by presenting paired stimuli to the subject; the P50 wave evoked by the second stimulus is reduced because of inhibitory mechanisms activated during the response to the first stimulus. In normal subjects, the mean amplitude of the second P50 response was reduced to less than 20%. In the schizophrenics, the mean amplitude of the second response was more than 85% of the first, a result that replicates our previous finding of a deficit in inhibition in schizophrenia. Approximately half the first-degree relatives, generally including at least one parent, had a similar deficit. Presence of this deficit in the parents was associated with a family history of schizophrenia. Family members with this deficit also had significantly higher scores on several scales of the Minnesota Multiphasic Personality Inventory than did family members without the deficit. Despite the deficit in inhibition, other characteristics of the P50 wave were normal in the relatives, in contrast to unmedicated schizophrenics, who showed additional abnormalities in wave latency and amplitude.

Inhibitory gating of an evoked response to repeated auditory stimuli in schizophrenic and normal subjects. Human recordings, computer simulation, and an animal model.

BACKGROUND: Altered sensory response is a prominent feature of schizophrenia. Inhibitory gatting mechanisms, shown by diminished P50 evoked responses to repeated auditory stimuli, seem to be deficient in schizophrenic persons. These inhibitory mechanisms usually are studied by averaging the electroencephalographic responses to many presentations of pairs of stimuli. Although averaging increases signal-to-noise ratio, it may obscure trial-to-trial differences. We compared differences between schizophrenic and normal persons in single trials and averages of P50 response. METHODS: Recordings from 10 schizophrenic patients and 10 normal subjects were analyzed using conventional averaging and single-trial measurements. A computer simulation of both methods examined their ability to extract evoked responses from background activity. Related single-neuron activity in the hippocampus in an animal model also was studied, because neuronal action potentials can be reliably identified in single trials. RESULTS: Averaged evoked potentials showed significant suppression of the P50 response to the second stimulus of the pair in normal patients, but not in schizophrenic patients. Single-trial analysis did not detect a response above background activity. Computer simulations gave similar results, suggesting that failure to detect suppression in single trials comes from inadequate differentiation of signal from noise. Recordings in animals confirmed almost complete suppression of the response of hippocampal pyramidal neurons to the second stimulus. CONCLUSIONS: The normal inhibition of response to repeated auditory stimuli seems to be compromised in schizophrenia. This loss of inhibitory gating could reflect a physiological deficit of hippocampal interneurons that is consonant with other evidence for interneuron pathologic defects in schizophrenia.

Transient impairment in P50 auditory sensory gating induced by a cold-pressor test.

Diminished gating of the auditory evoked response to repeated stimuli is a psychophysiological defect associated with schizophrenia and several other psychiatric illnesses. The P50 wave of the auditory evoked response to the second of paired stimuli is decreased in most normal subjects, whereas many psychotic subjects show significantly less decrement. The aim of this experiment was to test whether the cold-pressor test, which causes transient distress and pain accompanied by increased sympathetic activity, also causes a transient impairment in P50 auditory sensory gating in normal control subjects. Ten normal control subjects with normal gating of the P50 response immersed their hands in an ice water bath for 2 min. This cold-pressor test diminished P50 auditory gating in nine of these subjects, although the degree of impairment was highly variable among subjects. The impairment in gating was transient, with partial resolution by 30 min. The cold-pressor test was subjectively viewed as painful and also caused blood pressure to increase. Thus, a transient stressor can impair P50 auditory gating in some subjects.

Neurophysiological studies of sensory gating in rats: effects of amphetamine, phencyclidine, and haloperidol.

Central mechanisms of sensory gating were assessed in Sprague-Dawley rats by an evoked potential technique similar to one that we have previously used to show diminished sensory gating in psychotic patients. Middle latency (15-50 msec) auditory evoked potential responses were recorded at the skull in unanesthetized freely moving animals. Gating mechanisms were assessed in a conditioning-testing paradigm by measuring the suppression of response to a 74 dB click test stimulus following an earlier identical conditioning stimulus at 0.5-sec intervals. The rats demonstrated significant suppression of the N50 response to the second auditory stimulus. Amphetamine treatment significantly interfered with the suppression of the response to the second stimulus; haloperidol, injected after the amphetamine, returned the conditioning-testing ratio toward normal values. Phencyclidine caused a similar decrease in suppression and was similarly antagonized by haloperidol. During some periods of hyperarousal, animals showed spontaneous loss of suppression; this condition could be reversed by haloperidol treatment. These results with psychotomimetic drugs in an animal model parallel abnormalities in sensory gating previously observed in psychotic human subjects.

Alternative phenotypes for the complex genetics of schizophrenia.

The complexity of the genetics of schizophrenia has been described by many investigators. In the absence of simple Mendelian inheritance, genetic linkage has not achieved the definitive results found in other illnesses, where such methods have led to the identification of responsible genes. Alternative phenotypes for linkage analysis are proposed as one solution to this problem. These phenotypes, representative of discrete biological deficits in schizophrenia, may more closely reflect the effect of a single gene than the illness itself. The Mendelian inheritance of one alternative phenotypes, failure to inhibit the P50 auditory evoked response to repeated stimuli, has resulted in successful linkage of the deficit to the locus of a candidate gene, the alpha 7-nicotinic acetylcholine receptor on chromosome 15q14. Further support for this linkage has recently been found in families from the NIMH Schizophrenia Genetics Initiative, using schizophrenia as the phenotype. Alternative phenotypes based on discrete biological deficits in schizophrenia have enhanced power for linkage analysis. Such analyses can not only facilitate understanding of the genetic transmission of schizophrenia, but they also provide further support for neurobiological characterizations of the pathophysiology of schizophrenia; however, identification of responsible genetic mutations is necessary before definitive conclusions can be reached.

Neurophysiologic studies of sensory gating in schizophrenia: comparison of auditory and visual responses.

Gating of visual and auditory evoked responses was assessed in chronic schizophrenic patients treated with neuroleptic drugs. Middle latency components of the visual evoked response (N90-P130) were recorded at the occiput after flash stimulus. Possible inhibitory mechanisms of sensory gating were assessed in a conditioning-testing paradigm by measuring the change in amplitude of response to a second stimulus, relative to the response to the first stimulus. Simultaneous electrooculograms were recorded to detect contamination of recordings by eye movement. Neither schizophrenic patients nor normal control subjects demonstrated significant suppression of visual evoked responses in the conditioning-testing paradigm. These results differed markedly from similar measurements of a middle latency component of the auditory evoked response (P50) recorded using the same conditioning-testing paradigm in these subjects. Normal controls showed significant decrements of the P50 response to the second auditory stimulus (mean decrement over 80%), whereas schizophrenic patients failed to show a significant decrement (mean less than 40%). This finding for auditory evoked responses replicated previous studies of normal and schizophrenic subjects. Multiple conditioning stimuli were substituted for the single conditioning stimulus used previously in an attempt to enhance gating of auditory responses, but suppression of the P50 test response did not increase in either normal or schizophrenics.

Evidence in postmortem brain tissue for decreased numbers of hippocampal nicotinic receptors in schizophrenia.

This study tests the hypothesis that nicotinic cholinergic receptors, including those sensitive to the antagonist alpha-bungarotoxin, are decreased in the hippocampus of schizophrenics. The hypothesis is derived from the finding that alpha-bungarotoxin causes a defect in the inhibitory gating of auditory-evoked potentials in laboratory animals that resembles a defect in auditory sensory gating observed in schizophrenics. Nicotine transiently normalizes this psychophysiological deficit in schizophrenic patients. Postmortem brain tissue was obtained from eight schizophrenic and eight age-matched nonschizophrenic subjects. Sections of the hippocampus were labeled with [125I alpha-bungarotoxin and imagined by autoradiography. Binding of the nicotinic agonist [3H]-cytisine was determined in tissue homogenates. alpha-Bungarotoxin labeled a population of putative interneurons in the hippocampus, primarily in the dentate gyrus and the CA3 region of Ammon's horn. This labeling was significantly decreased in the tissue from the schizophrenic patients, with seven or eight patients below the range of the nonschizophrenic subjects. There was also a significant decrease in the binding of cytisine. The results were not related to generalized hippocampal cell loss, drug exposure at time of death, or smoking history. This initial study suggests that schizophrenic patients have fewer nicotinic receptors in the hippocampus, a condition which may lead to failure of cholinergic activation of inhibitory interneurons, manifest clinically as decreased gating of response to sensory stimulation.

Sensory gating in schizophrenics and normal controls: effects of changing stimulation interval.

Auditory evoked potentials were recorded using a paired click, conditioning-testing paradigm in 10 schizophrenics and in 10 normal subjects with no family history of psychotic disorder. The paradigm is used to demonstrate central nervous system gating of responsiveness to auditory stimuli by examining the extent to which the response to the conditioning stimulus diminishes the response to a test stimulus occurring a short time later. Recordings were made at conditioning-testing intervals of 500 msec, 150 msec, and 75 msec to determine subjects' gating of responsiveness to stimuli repeated at various intervals. The schizophrenics had conditioning-testing ratios indicative of poor gating of the auditory P50 wave at the 500-msec and 150-msec intervals, but most patients had good sensory gating at the 75-msec interval. Normal subjects showed good sensory gating at all three intervals. Results suggest that although sensory gating mechanisms responsible for changes in neuronal response at longer intervals are chronically defective in schizophrenics, other gating mechanisms functioning at shorter intervals appear to be intact and function well in most patients. The results may lead to increased specification of the neurobiological basis of sensory abnormalities in schizophrenia.

Modulation of the gating of auditory evoked potentials by norepinephrine: pharmacological evidence obtained using a selective neurotoxin.

Central mechanisms of sensory gating were assessed in Sprague-Dawley rats using an evoked potential technique similar to one that we have previously employed to show diminished sensory gating in psychotic patients. Gating mechanisms were examined using a conditioning-testing paradigm in which pairs of 74-dB clicks were delivered; the interval between the conditioning and test stimuli was 0.5 sec. A middle latency auditory evoked response (N50) recorded from the skull of unanesthetized, freely moving rats demonstrated significant suppression to the test click. Systemic administration of amphetamine (1 mg/kg, ip) significantly reduced the amount of suppression of the response to the test stimulus; haloperidol (1 mg/kg), injected after the amphetamine, returned the conditioning-testing suppression ratio toward normal values. Amphetamine also decreased the latency and amplitude of the conditioning response, an effect that was also reversed by haloperidol. Both decreased suppression of the test response and reduced amplitude and latency of the conditioning response have been observed in schizophrenia. To aid in determining the underlying mechanism of these effects, the animals were treated with two doses, given at a 1-week interval, of N-(2-chloroethyl-N-ethyl-2-bromobenzylamine) (DSP4; 50 mg/kg, ip), an agent that selectively depletes central norepinephrine. The extent and selectivity of the depletions were confirmed by chemical analysis. Following DSP4, the effects of amphetamine on the amplitude and latency of the conditioning response were largely unchanged. However, pretreatment with DSP4 significantly attenuated the reduction in conditioning-testing suppression observed following the administration of amphetamine. The data suggest a specific role for norepinephrine in the modulation of sensory processing.

Sensory gating in a computer model of the CA3 neural network of the hippocampus.

We have developed a unique computer model of the CA3 region of the hippocampus that simulates the P50 auditory evoked potential response to repeated stimuli in order to study the neuronal circuits involved in a sensory processing deficit associated with schizophrenia. Our computer model of the CA3 hippocampal network includes recurrent activation from within the CA3 region as well as input from the entorhinal cortex and the medial septal nucleus. We used the model to help us determine if the cortical and septal inputs to the CA3 hippocampus alone are responsible for the gating of auditory evoked activity, or if the strong recurrent activity within the CA3 region contributes to this phenomenon. The model suggests that the medial septal input is critical for normal gating; however, to a large extent the activity of the medial septal input can be replaced by simulated stimulation of the hippocampal neurons by a nicotinic agonist. The model is thus consistent with experimental data that show that nicotine restores gating of the N40 evoked potential in fimbria-fornix lesioned rats and of the P50 evoked potential in schizophrenic patients.

Elementary phenotypes in the neurobiological and genetic study of schizophrenia.

This review describes the strategy of using elementary phenotypes for neurobiological and genetic linkage studies of schizophrenia. The review concentrates on practical aspects of selecting the phenotype and then understanding the confounds in its measurement and interpretation. Examples from the authors' studies of deficits in P50 inhibition and smooth pursuit eye movement dysfunction are presented. These two phenotypes share considerable similarity in their neurobiology, including a similar response to nicotine. They also appear to co-segregate with the genetic risk for schizophrenia as autosomal co-dominant phenotypes. Although most schizophrenic patients inherit these abnormalities unilinealy, i.e., from one parent, apparent bilineal inheritance produces a more severe illness, observed clinically as childhood-onset schizophrenia. The initial study showing linkage of the P50 deficit to the chromosome 15q14 locus of the alpha 7-nicotinic acetylcholine receptor is an example of the potential usefulness of these phenotypes for combined genetic and neurobiological study of schizophrenia.

Neurophysiological studies of sensory gating in mania: comparison with schizophrenia.

The action of central nervous system mechanisms involved in sensory gating was assessed in acutely psychotic manic patients. An early positive component of the auditory average evoked response, recorded at the vertex 50 msec after a click stimulus, was studied. Stimuli were delivered at 10-sec intervals to establish a base-line response. Sensory gating mechanisms were then tested using a conditioning-testing paradigm to assess the change in response to a second stimulus following the first at either 0.5-, 1.0- or 2.0-sec intervals. A similar paradigm had been used previously to assess deficits in this function in acute and chronic schizophrenics. We found a deficit in sensory gating in acutely manic patients. similar to that found in schizophrenics, although the variability in response was more marked in the manic patients. We followed these patients during their treatment on lithium carbonate and found a return of these neuronal functions towards normal values which corresponded to their clinical improvement. A series of stable euthymic bipolar patients were found to have responses indistinguishable from normal controls. The data suggest that deficits in neuronal gating functions, similar to those found in schizophrenia, can be seen during acute mania but these deficits return to normal as the acute psychosis abates.

Neurophysiological evidence for a defect in neuronal mechanisms involved in sensory gating in schizophrenia.

The action of CNS inhibitory neuronal mechanisms was tested in acutely psychotic unmedicated schizophrenic patients and in normal controls. An early positive component of the auditory average evoked response recorded at the vertex 50 msec after a click stimulus was studied. Stimuli were delivered at 10-sec intervals to establish a base-line response. Inhibitory mechanisms were then tested using a conditioning-testing paradigm by assessing the change in response to a second stimulus following the first at either 0.5, 1.0, or 2.0-sec intervals. At the 0.5-sec interval, normal controls had over a 90% mean decrement in response, whereas schizophrenics showed less than a 15% mean decrement. At 2-sec intervals, responses from normals were still 30 to 50% diminished, but those from schizophrenics showed an increased response to the stimulus compared to base line. The data suggest that normally present inhibitory mechanisms are markedly reduced in schizophrenics. Failure of these inhibitory mechanisms may be responsible for the defects in sensory gating which are thought to be an important part of the pathophysiology of schizophrenia.

Neurophysiological evidence for a defect in inhibitory pathways in schizophrenia: comparison of medicated and drug-free patients.

Central nervous system inhibitory neuronal mechanisms were assessed in clinically stable chronic schizophrenic patients treated with neuroleptic drugs to provide data for comparison with those obtained previously from acutely psychotic, unmedicated patients. An early positive component of the auditory average evoked response recorded at the vertex 50 msec after a click stimulus (P50) was studied. After stimuli were delivered at 10-sec intervals to establish a baseline response, inhibitory mechanisms were assessed in a conditioning-testing paradigm, by measuring the change in response to a second stimulus following the first at either 0.5-, 1.0-, or 2.0-sec intervals. At the 0.5-sec interval, normal controls had over an 80% mean decrement in response, whereas schizophrenics showed a mean decrement of less than 10% but there was no significant difference in suppression between medicated and unmedicated patients. However, the amplitude of P50, which was smaller in unmedicated schizophrenics than in normal subjects, was significantly increased in the medicated patients. The data suggest that inhibitory mechanisms which are dysfunctional in acutely psychotic.

Anticipatory saccades during smooth pursuit eye movements and familial transmission of schizophrenia.

BACKGROUND: Smooth pursuit eye movement (SPEM) abnormalities are a putative marker of genetic risk for schizophrenia. Accurate SPEM performance requires the subject to activate neural systems responsible for smooth pursuit tracking, while simultaneously suppressing activity of neurons responsible for saccadic movements that would move the eye ahead of the target. This study examined whether specific aspects of SPEM dysfunction cosegregate with genetic risk in parents of schizophrenic probands. METHODS: Eighteen probands and their parents had SPEM recorded. Parents with an ancestral history of schizophrenia were hypothesized to be more likely than their spouses without such a history to carry a genetic risk for schizophrenia. RESULTS: Ten families had a single parent with a positive ancestral history for schizophrenia. The frequency of anticipatory saccades, which were mostly small, and the fraction of total eye movement that they represented were the only measures that differentiated the more likely genetic carrier parents in these families from their spouses and age-matched normals. CONCLUSIONS: Failure to suppress saccadic anticipation of target motion during smooth pursuit appears an aspect of SPEM dysfunction related to presumed genetic risk for schizophrenia.

Normalization by nicotine of deficient auditory sensory gating in the relatives of schizophrenics.

Diminished gating of the P50 auditory evoked response to repeated stimuli is a psychophysiological feature of schizophrenia, that is also present in many relatives of patients. Animal models of auditory sensory gating indicate that nicotinic cholinergic neurotransmission is a critical neuronal substrate. The aim of this experiment was to determine if the deficit in sensory gating could be reversed by nicotine administration. Nonsmoking relatives of schizophrenics with abnormal sensory gating were selected as subjects for this initial double-blind trial, to avoid effects of psychotropic medications that might complicate trials in schizophrenic patients themselves. Nicotine-containing gum increased P50 sensory gating to near normal levels within 30 min of administration. The effect was transient; the gating of P50 returned to baseline levels within 1 hr. There was no change observed after placebo administration. In one of the subjects, the anticholinesterase inhibitor physostigmine similarly normalized P50 gating. The results are consistent with the hypothesis that nicotinic cholinergic neurotransmission may mediate a familial psychophysiological deficit in schizophrenia.

The effects of age on a smooth pursuit tracking task in adults with schizophrenia and normal subjects.

BACKGROUND: Performance during a smooth pursuit eye movement (SPEM) task has been proposed as a marker of genetic risk for schizophrenia, although the precise component of SPEM tracking most associated with genetic risk remains undetermined. Normal adult aging is associated with deterioration on SPEM tasks; it remains unclear whether investigations of SPEM abnormalities will allow inclusion of older subjects in genetic studies. This study examines 1) the effect of normal aging on several components of SPEM performance; and 2) whether schizophrenic-normal differences found in young adults continue over a broad adult age span. METHODS: SPEM was recorded during a 16.7 degrees per sec constant velocity task in 64 normal adults, ages 18 to 79 years, and 58 schizophrenic subjects, ages 18 to 70 years. RESULTS: Smooth pursuit gain, the percent of total eye movements due to catch-up saccades, the frequency of large anticipatory saccades, and the frequency of leading saccades all deteriorate with increasing age. After correction for age, schizophrenic to control differences persist on most eye movement variables with the largest effect sizes for leading saccades (1.56) and smooth pursuit gain (1.17). CONCLUSIONS: The tendency to use saccades to anticipate target motion, even in small steps (leading saccades), deserves further attention as a potential marker useful in genetic analyses.

Gating of auditory P50 in schizophrenics: unique effects of clozapine.

Schizophrenic patients have a deficit in the ability to filter sensory stimuli, which can be demonstrated in several psychophysiological paradigms. For example, most unmedicated schizophrenic subjects fail to decrement the P50 auditory evoked response to the second of paired stimuli, when the interstimulus interval is 500 msec. This sensory gating deficit persists in schizophrenics treated with typical antipsychotics, even if they show significant clinical improvement. When the interstimulus interval is 100 msec, most schizophrenics exhibit impaired gating while acutely ill, but normalize with treatment. Clozapine, the prototypic atypical antipsychotic medication, is clinically more effective than conventional neuroleptics in a significant proportion of schizophrenics refractory to other drug treatment. Nine schizophrenic subjects who were refractory to conventional neuroleptic treatment were studied while being treated with typical neuroleptics and then restudied after 1 month's treatment with clozapine. In the six clozapine responders, there was significant improvement of P50 gating at the 500 msec interval. At the 100 msec interval there was an inverse relationship between sensory gating of P50 and clozapine dose, independent of clinical response. Thus, although this can only be considered preliminary data because of the small number of subjects, it appears that clozapine, compared to typical neuroleptics, has distinct effects on P50 gating.

Neurophysiological assessment of sensory gating in psychiatric inpatients: comparison between schizophrenia and other diagnoses.

Gating of auditory sensory responsiveness was examined in 75 psychiatric inpatients using a conditioning-testing paradigm with the P50 wave of the auditory evoked response, in which pairs of stimuli are presented to the subject. In previous studies, most schizophrenics did not decrement the second response to the extent seen in normals. Acutely ill patients, who were representative of patients admitted to a public university teaching service and a proprietary hospital, were used to examine the extent to which diminished sensory gating is found in diagnoses other than schizophrenia. About half of these patients showed diminished sensory gating that correlated with measures of severity of illness. The data, taken together with that from other studies using this paradigm, suggest that diminished sensory gating, like several other psychophysiological abnormalities, is a trait deficit in schizophrenia, but a state deficit in many other mental illnesses.