Cholinergic gating of response to auditory stimuli in rat hippocampus

Knockout and knockin mice to investigate the role of nicotinic receptors in the central nervous system

The recent use of genetically engineered knockout (Ko) and knockin (Kin) animals for neurotransmitter receptor genes, in particular, nicotinic acetylcholine receptors (nAChRs) in the brain, has provided a powerful alternative to the classical pharmacological approach. These animal models are not only useful in order to reexamine and refine the results derived from pharmacological studies, but they do also provide a unique opportunity to determine the subunit composition of the nicotinic receptors which modulate various brain functions. Ultimately, this knowledge will be valuable in the process of designing new drugs that will mimic the effects of nicotine on several important pathologies or on smoking cessation therapies. In this review, we present recent data obtained from the studies of mutant animals that contributed to our understanding of the role and composition of nAChRs in the central nervous system (CNS). The advantages and pitfalls of Ko animal models will also be discussed.

Comparison of polymorphisms in the alpha7 nicotinic receptor gene and its partial duplication in schizophrenic and control subjects

The hypothesis that the 15q13-15 region of chromosome 15 contains a gene that contributes to the etiology of schizophrenia is supported by multiple genetic linkage studies. The alpha7 neuronal nicotinic acetylcholine receptor (CHRNA7) gene was selected as the best candidate gene in this region for molecular investigation, based on these linkage findings and biological evidence in both human and rodent models. CHRNA7 receptors are decreased in expression in postmortem brain of schizophrenic subjects. A dinucleotide marker, D15S1360, in intron two of the CHRNA7 gene is genetically linked to an auditory gating deficit found in schizophrenics and half of the first-degree relatives of patients. Single strand conformation polymorphism (SSCP) and sequence analyses of DNA from schizophrenic and control individuals identified 33 variants in the coding region and intron/exon borders of the CHRNA7 gene and its partial duplication, dupCHRNA7; common polymorphisms were mapped. Twenty-one variants were found in the exons, but non-synonymous changes were rare. Although the expression of CHRNA7 is decreased in schizophrenia, the general structure of the remaining receptors is likely to be normal.

The cholinergic hypothesis of cognitive impairment caused by traumatic brain injury

Cognitive impairments are among the most common neuropsychiatric sequelae of traumatic brain injury at all levels of severity. Cerebral cholinergic neurons and their ascending projections are particularly vulnerable to acute and chronic traumatically mediated dysfunction. In light of the important role of acetylcholine in arousal, attention, memory, and other aspects of cognition, cerebral cholinergic systems contribute to and may also be a target for pharmacologic remediation among individuals with post-traumatic cognitive impairments. This article will review the evidence in support of this hypothesis. Evidence of relatively selective damage to cholinergic injury, the development of persistent anticholinergic sensitivity, and the effects of cholinergic augmentation on memory performance are presented first. Thereafter, neuropathologic, electrophysiologic, and pharmacologic evidence of cholinergic dysfunction after traumatic brain injury in humans is reviewed. Finally, future directions for investigation of the cholinergic hypothesis and possible clinical applications of this information are discussed.

Micromolar brain levels of kynurenic acid are associated with a disruption of auditory sensory gating in the rat

Brain levels of kynurenic acid (KYNA), an endogenous antagonist of glycine(B)/NMDA and alpha-7 nicotinic acetylcholine receptors, are elevated in individuals with schizophrenia. Both receptors are broadly implicated in the pathophysiology of this disease, particularly in the deficits many patients show in filtering the sensorium. In the present study, we sought to determine whether elevated brain levels of KYNA disrupt auditory gating in anesthetized rats. A mid-latency evoked potential was recorded from the hippocampus in response to a pair of auditory tones. Gating was assessed by determining the ratio of the amplitude of test and conditioning responses (T/C ratio) in rats that had received KYNA's precursor L-kynurenine (KYN; 150 mg/kg, i.p.) together with probenecid (PBCD; 200 mg/kg, i.p.) 2 h prior to the start of the recording session. KYNA levels in the hippocampus of KYN+PBCD-treated rats were increased 500-fold, and accompanied by a significant increase in T/C ratio consistent with a disruption in sensory gating. PBCD alone increased hippocampal KYNA 12-fold, but did not significantly elevate T/C ratio. L-701,324 (3-30 mg/kg, i.v.), a centrally acting glycine(B) site antagonist, also failed to disrupt gating; however, large quantities of the competitive NMDA receptor antagonist DL-2-amino-5-phosphopentanoate (200 nmol, i.c.v.) markedly increased T/C ratio. Thus, while total blockade of NMDA receptors disrupts auditory gating, partial blockade achieved by antagonism of its glycine coagonist binding site does not. These observations indicate that the disruption in auditory processing in rats with greatly elevated KYNA levels is not attributable to the compound's antagonist actions at the glycine(B) receptor.

Propofol suppresses the sleep state-dependent P13 midlatency auditory evoked potential in the rat

Propofol (2,6-diisopropylphenol) is a widely used anesthetic agent, but its mechanisms of action are poorly understood. In this report, the effects of three dose levels of propofol (5, 7.5, and 10mg/kg) on the amplitude of the vertex-recorded, sleep state-dependent P13 midlatency evoked potential were investigated. The P13 potential is generated, at least in part, by the ascending cholinergic reticular activating system (RAS). The RAS is known to be affected by anesthetic agents. Intravenous injections of propofol were found to reduce the amplitude of the P13 potential in a dose- and time-dependent manner. At 2min post-injection, the mean P13 amplitude was suppressed to 40% of its pre-injection level by the lowest dose, but was suppressed to 10% of pre-injection levels by the two higher doses of propofol. The duration of the suppression of mean P13 potential amplitude was also dose-dependent such that complete recovery occurred by 5min using 5mg/kg, by 15min using 7.5mg/kg and by 30min using 10mg/kg of propofol. Using a paired stimulus paradigm, transient effects on habituation of the P13 potential were observed but only after the highest dose. Thus, one of the mechanisms of propofol may be to affect portions of the RAS which modulate the level of arousal. It may only transiently affect higher systems known to modulate the degree of habituation of responses by the RAS (i.e. processes which modulate habituation and may participate in sensory gating and distractibility).

Mouse genetic models for prepulse inhibition: an early review

Prepulse inhibition (PPI) is the phenomenon in which a weak prepulse stimulus attenuates the response to a subsequent startling stimulus. Patients with schizophrenia and some other neuropsychiatric disorders have impaired PPI. Impaired PPI in these patient populations is thought to reflect dysfunctional sensorimotor gating mechanisms. Recently, various inbred mouse strains and genetically modified mouse lines have been examined to investigate the potential genetic basis of sensorimotor gating. This review provides a synopsis of the use of mouse models to explore genetic and neurochemical influences on PPI. Studies describing the PPI responses of various inbred strains of mice, mice with genetic mutations, and mice treated with various drugs prior to July 2001 are reviewed. The continuous nature of the distribution of PPI responses among inbred strains of mice indicates that PPI is a polygenic trait. Findings from spontaneous and gene-targeted mutants suggest that mutant mice are important tools for dissecting and studying the role of single genes and their products, and chromosomal regions in regulating PPI. Pharmacological studies of PPI have typically confirmed effects in mice that are similar to those reported previously in rats, with some important exceptions. The use of mice to study PPI is increasing at a dramatic rate and is helping to increase our understanding of the biological basis for sensorimotor gating.

The genetics of sensory gating deficits in schizophrenia

Sensory gating abnormalities are an early clinical symptom of schizophrenia, and are characterized by a decrease in the brain's normal ability to inhibit the response to unimportant stimuli. Patients appear hypervigilant and have difficulty focusing their attention. A neurobiologic mechanism involved in these difficulties is nicotinic cholinergic modulation of inhibitory neuronal activity in the hippocampus. One measure of sensory gating abnormalities, diminished inhibition of the P50 evoked response to repeated auditory stimuli, has been linked to the chromosome 15q14 locus of the alpha-7-nicotinic receptor gene. This site is one of several that have shown evidence for linkage to schizophrenia, as well as to bipolar disorder, across several studies. Polymorphisms in the core promoter of the gene are associated with schizophrenia and also with diminished inhibition of the P50 response. These genetic data may identify a new pathophysiologic target for drug discovery.

The -2 bp deletion in exon 6 of the 'alpha 7-like' nicotinic receptor subunit gene is a risk factor for the P50 sensory gating deficit

Abnormality in the P50 auditory-evoked potential gating is an endophenotype associated with schizophrenia. Biochemical and genetic studies have suggested that the alpha 7 nicotinic acetylcholine receptor (nAChR) is involved in this sensory gating deficit. Two related alpha 7 genes (CHRNA7 and CHRNA7-like gene) resulting from a partial duplication (from exon 5 to exon 10) are present in the human genome. Two types of genetic variation, a large deletion and a -2 base-pair deletion in exon 6 resulting in a truncation of the open reading frame, affect specifically the CHRNA7-like gene. We developed a simple multiplex PCR assay on genomic DNA, allowing the quantification of the number of exons 6 and the distinction of all possible exon 6 genotypes. Genotyping of 70 schizophrenic patients and 77 controls showed that carrying at least one -2 bp deletion of exon 6 did not constitute a risk factor for schizophrenia. In contrast, the distribution of genotypes differed significantly between subjects with normal and abnormal P50 ratios, with an over-representation of genotypes carrying at least one -2 bp deletion of exon 6 among subjects exhibiting an abnormal P50 ratio. We thus conclude that the -2 bp deletion within the CHRNA7-like gene is a risk factor for P50 sensory gating deficit. Interestingly, most of the effect came from the non schizophrenic group, which may suggest that in schizophrenic patients other risk factors account for the large proportion of subjects exhibiting an abnormal P50 ratio.

Inhibitory neurophysiological deficit as a phenotype for genetic investigation of schizophrenia

Many investigators have proposed that biological endophenotypes might facilitate the genetic analysis of schizophrenia. A deficit in the inhibition of the P50 evoked response to repeated auditory stimuli has been characterized as a neurobiological deficit in schizophrenia. This deficit is linked to a candidate gene locus, the locus of the alpha7-nicotinic cholinergic receptor subunit gene on chromosome 15q14. Supportive evidence has been found by other investigators, including: 1) linkage of schizophrenia to the same locus; 2) linkage of bipolar disorder to the locus; and 3) replication of the existence of this neurobiological deficit and its relation to broader neuropsychological deficits in schizophrenia. It is certain that there are many genetic factors in schizophrenia and bipolar disorder; what is needed is a complete and precise description of the contribution of each individual factor to the pathophysiology of these illnesses.

Differential effects of cigarette smoking on performance of a smooth pursuit and a saccadic eye movement task in schizophrenia

Schizophrenic patients demonstrate a number of physiological defects including smooth pursuit eye movement dysfunction (SPEM), involuntary reflexive saccades to a prepotent stimulus during saccadic tasks, and increased response to the second of two identical auditory stimuli, the P50 evoked potential response. The P50 deficit appears to be mediated by the alpha7 nicotinic cholinergic receptor. This study compared the failure of saccadic inhibition demonstrated in two different eye movement tasks, to see if either deficit, like the P50 inhibitory deficit, was normalized by nicotine. Fifteen smoking schizophrenic patients and 15 smoking non-schizophrenic subjects were compared on the percentage of premature saccades in a memory-guided saccadic task, and the frequency of intrusive small and large anticipatory saccades during a SPEM task. No significant effects or interactions of smoking, group or time on premature or large anticipatory saccades were detected. However, leading saccades demonstrated a significant group x time x smoking interaction. Leading saccades may therefore be a measure of cholinergic inactivity and thus part of the alpha7 nicotinic receptor dysfunction observed in schizophrenia. However, premature saccades and large anticipatory saccades, although measures of inhibitory dysfunction in schizophrenia, appear to be unrelated to the nicotinic system.

The place of the hippocampus in fear conditioning

Pavlovian fear conditioning is a phenomenon amenable to laboratory analysis of the neurobiology of fear and the investigation of neural mechanisms of learning and memory. Investigators have made much progress in delineating the neurocircuitry and neurochemistry of fear conditioning. The place of the hippocampus in context fear remains a controversial issue. In this review, we examine the evidence that the hippocampus plays a role in fear conditioning. We then critically examine hypotheses concerning its exact role in learning and memory for cued and context fear conditioning.

Hippocampal alpha 7 and alpha 4 beta 2 nicotinic receptors and working memory

Nicotine and other nicotinic receptor agonists have been found in a variety of studies to improve memory, while nicotinic receptor blockade can impair memory. The critical neural mechanisms for nicotinic involvement with memory are still under investigation. Initial evidence supports the involvement of the ventral hippocampus. Lesions in this area block nicotine-induced memory improvement and mecamylamine-induced impairment. Local ventral hippocampal application of the nicotinic channel blocker mecamylamine impairs memory in the 8-arm radial maze. Both alpha 4 beta 2 and alpha 7 nicotinic receptors seem to be involved. Ventral hippocampal infusions of high doses of the alpha 4 beta 2 nicotinic antagonist dihydro-beta-erythrodine (DH beta E) and the alpha 7 nicotinic antagonist methyllycaconitine (MLA) impair memory performance on the 8-arm radial maze. However, high doses of these drugs may limit specificity and they cause preconvulsant effects, which in themselves may affect memory. The current study used the more challenging 16-arm radial maze to determine the effects of lower doses of these drugs on memory and to differentiate effects on working and reference memory. Adult female Sprague-Dawley rats were trained on a working and reference memory task in the 16-arm radial maze and then were implanted with bilateral chronic guide cannulae directed to the ventral hippocampus. After recovery from surgery, the rats received acute intrahippocampal infusions of dose combinations of DH beta E and MLA. In the first study, DH beta E (0 and 6.75 microg/side) and MLA (0, 6.75, 13.5 and 27 microg/side) were administered in a counter-balanced order. In the second study, lower doses of DH beta E (0, 1.6375, 3.275 and 6.75 microg/side) were administered alone or with MLA (0 and 6.75 microg/side) in a counter-balanced order. In the first study, DH beta E caused a significant increase in both working and reference memory errors. MLA at a dose of 27 microg/side caused a significant increase in working memory errors, but this dose had no significant effect on reference memory errors. Interestingly, no additive effects were seen with combined administration of DH beta E and MLA in this study, and at the doses used, no effects were seen on response latency. In the second study, lower doses of DH beta E did not cause a significant deficit in working memory performance. Co-administration of MLA with these subthreshold doses did precipitate a memory impairment. The current results confirm the specificity of the memory deficits caused by these drugs. These results support the involvement of alpha 4 beta 2 and alpha 7 nicotinic receptors in the ventral hippocampus as being critical for memory function.

A double blind placebo controlled trial of donepezil adjunctive treatment to risperidone for the cognitive impairment of schizophrenia

BACKGROUND

Despite the beneficial effects of atypical antipsychotics on cognition, these improvements will not return most schizophrenic patients to normative standards of cognitive functioning. Therefore, other treatments need to be considered. Subtle changes in cholinergic function in schizophrenic patients provide the rationale to test the effectiveness of cholinesterase inhibitors in treating cognitive impairment in schizophrenia.

METHODS

Given this, a 12-week, double-blind, placebo-controlled trial of donepezil (5 mg and 10 mg) as adjunctive treatment to risperidone was conducted in a total of 36 schizophrenic patients.

RESULTS

Neither the 5-mg nor 10-mg dose of donepezil produced significant improvements in any cognitive measure compared with placebo.

CONCLUSIONS

It is possible that nicotinic receptor desensitization produced by chronic tobacco use in these patients rendered their nicotinic receptors refractory to the effects of increased agonist activity produced by donepezil. An alternative treatment is the allosterically potentiating ligands, which enhance the activity of (sensitize) nicotinic receptors in the presence of acetylcholine.

Concomitant clozapine reduces smoking in patients treated with risperidone

This pilot study examines the smoking behaviors of patients treated with either risperidone alone or in combination with clozapine. Smoking behavior was evaluated using expired carbon monoxide (CO) measurements, the Fagerstrom Test for Nicotine Dependence (FTND), and a semi-structured interview. Our results indicate that patients co-prescribed clozapine with risperidone smoke significantly less than patients treated with risperidone alone (19.1+/-9.3 vs. 37.8+/-19.1 ppm CO, respectively, P=0.03). These data are consistent with previous studies showing that clozapine treatment is associated with significantly reduced smoking behavior relative to other antipsychotic agents.

Cholinergic modulation of pavlovian fear conditioning: effects of intrahippocampal scopolamine infusion

Cholinergic neurotransmission has been implicated in the acquisition of a variety of tasks, including Pavlovian fear conditioning. To more precisely define the role of cholinergic modulation in this process, the effect of site-specific cholinergic antagonism was assessed. Male Long-Evans rats were implanted with chronic, bilateral cannulae aimed at the dorsal hippocampus. Infusions of scopolamine hydrobromide (50 microg bilaterally) or phosphate-buffered saline (PBS) were made immediately prior to a signaled Pavlovian fear conditioning procedure. On consecutive days following training, all rats were given independent tests assessing freezing to both the training context and the tone conditional stimulus (CS). Relative to PBS infused controls, rats that received intrahippocampal infusions of scopolamine showed a significant attenuation of contextual freezing but comparable levels of freezing to the tone CS. Neither shock sensitivity nor general activity levels differed between rats infused with scopolamine or PBS. These findings suggest that fear conditioning to context, but not discrete CS, requires intact cholinergic neurotransmission in the hippocampus.

Alpha7-nicotinic receptor expression and the anatomical organization of hippocampal interneurons

C3H and DBA/2 mice differ in their hippocampal inhibitory function, as measured by the inhibitory gating of pyramidal neuron response to repeated auditory stimulation. This functional difference appears to be related to differences in expression of the alpha7 nicotinic cholinergic receptor, which may be generally expressed by interneurons. This study examines the relationship between genetic variation in alpha7 receptor subunit expression and GABAergic interneuron distribution in various regions and layers of the hippocampus in the two mouse strains. Subpopulations of hippocampal interneurons in both mouse strains were found to bind [(125)I]alpha-bungarotoxin. However, the distribution of the [(125)I]alpha-bungarotoxin-positive hippocampal interneurons was significantly different between C3H and DBA/2 mice. In region CA1, and to a lesser extent in region CA3, DBA/2 mice had increased numbers of [(125)I]alpha-bungarotoxin-positive neurons in stratum lacunosum-moleculare and decreased numbers in stratum oriens. Similar differences in GABAergic neuron distribution were observed in region CA1 in the two strains. C3H/DBA/2 F1 animals were backcrossed to the C3H parental strain for six generations, with selection for either the DBA/2 or C3H allelic variant of the alpha7 receptor gene. The distribution of [(125)I]alpha-bungarotoxin labeling closely resembled the DBA/2 parental phenotype in animals retaining the DBA/2 allele of the alpha7 gene. These data suggest that the alpha7 receptor gene locus may influence the anatomical organization of at least a subset of hippocampal interneurons by an as yet unidentified mechanism. This difference in interneuron anatomy may also contribute to functional differences in inhibitory sensory gating between the two strains.

Ventral hippocampal alpha 7 nicotinic receptor blockade and chronic nicotine effects on memory performance in the radial-arm maze

Chronic nicotine administration has been shown to significantly improve working memory. Nicotinic involvement in memory function critically involves the ventral hippocampus. Local ventral hippocampal infusions of the nicotinic antagonists mecamylamine, dihydro-beta-erythroidine (DH beta E) and methyllycaconitine (MLA) significantly impair working memory. The impairment caused by hippocampal infusion of the alpha 4 beta 2 antagonist DH beta E is reversed by chronic systemic nicotine. This study determined the interaction of chronic systemic nicotine with acute ventral hippocampal infusions of the alpha 7 antagonist MLA. Adult female Sprague-Dawley rats were trained on an 8-arm radial maze working memory task. Then they underwent ventral hippocampal cannulation and received sc implants of minipumps delivering nicotine (0 or 5 mg/kg/day for 28 days). Acute ventral hippocampal infusions of MLA (0, 4.88, 14.64 and 43.92 microg/side) were given during 3-4 weeks of chronic nicotine. MLA caused a significant dose-related memory impairment. In the rats not receiving nicotine, the 14.64 and 43.92 microg/side MLA doses caused significant memory impairment. Chronic systemic nicotine exposure did not block the MLA-induced memory impairment. Comparing the current results with MLA with previous results with DH beta E, equimolar ventral hippocampal DH beta E more effectively impaired memory than MLA, but the DH beta E-induced impairment was more effectively reversed by chronic systemic nicotine administration.

Smoking and mental illness

Patients with mental illness have a higher incidence of smoking than the general population and are the major consumers of tobacco products. This population includes subjects with schizophrenia, manic depression, depression, posttraumatic stress disorder (PTSD), attention-deficit disorder (ADD), and several other less common diseases. Smoking cessation treatment in this group of patients is difficult, often leading to profound depression. Several recent findings suggest that increased smoking in the mentally ill may have an underlying biological etiology. The mental illness schizophrenia has been most thoroughly studied in this regard. Nicotine administration normalizes several sensory-processing deficits seen in this disease. Animal models of sensory deficits have been used to identify specific nicotinic receptor subunits that are involved in these brain pathways, indicating that the alpha 7 nicotinic receptor subunit may play a role. Genetic linkage in schizophrenic families also supports a role for the alpha 7 subunit with linkage at the alpha 7 locus on chromosome 15. Bipolar disorder has some phenotypes in common with schizophrenia and also exhibits genetic linkage to the alpha 7 locus, suggesting that these two disorders may share a gene defect. The alpha 7 receptor is decreased in expression in schizophrenia. [(3)H]-Nicotine binding studies in postmortem brain indicate that high-affinity nicotinic receptors may also be affected in schizophrenia.

Intragastric DMXB-A, an alpha7 nicotinic agonist, improves deficient sensory inhibition in DBA/2 mice

BACKGROUND

Abnormal sensory inhibition is observed in the majority of schizophrenic patients. DBA/2 mice spontaneously exhibit a similar deficit in sensory inhibition and thus provide a model for drug development targeted to this physiologic abnormality. The impaired sensory inhibition is characterized by diminished response of the hippocampal evoked potential to the second of closely paired auditory stimuli (500-m/sec interstimulus interval). Subnormal levels of hippocampal alpha7 nicotinic cholinergic receptors are associated with the deficient sensory inhibition in both DBA/2 mice and people with schizophrenia.

METHODS

Our study examined the inhibition of the P20-N40 auditory evoked potential in DBA/2 mice after intragastric administration of DMXB-A (3-2,4-dimethoxybenzylidine anabaseine), an alpha7 nicotinic receptor partial agonist. After presentation of auditory stimuli, electroencephalographic responses were recorded and measured to monitor the effects of the DMXB-A, alone and in combination with selective nicotinic antagonists.

RESULTS

Gastric administration of DMXB-A (10 mg/kg) improved sensory inhibition in DBA/2 mice. This improvement was blocked by alpha-bungarotoxin, but not mecamylamine, indicating that DMXB-A exerts its effects through the alpha7 nicotinic receptor.

CONCLUSIONS

Intragastrically administered DMXB-A improves deficient sensory inhibition in DBA/2 mice through stimulation of alpha7 nicotinic receptors. These studies agree with results from previous studies with subcutaneously administered DMXB-A.

The role of hippocampal theta activity in sensory gating in the rat

Sensory gating is defined as a decreased reaction on the second click, measured as evoked potentials (EP) within a double click paradigm. Recently, it was established that gating in rats was decreased during REM sleep compared to wakefulness and non-REM sleep. REM sleep in the rat is characterized by hippocampal theta rhythm. Therefore, it was investigated whether sensory gating would also be diminished during other states with hippocampal theta. Twelve Wistar rats were implanted with hippocampal electrodes and exposed to double clicks during passive wakefulness, REM sleep, and activity (voluntary movements and walking on a moving belt). Gating was examined by use of the amplitudes of the EPs in reaction to the first conditioned amplitude (CAMP) and second click test amplitude (TAMP), as well as two gating parameters (C-T score and T/C ratio). Except passive wakefulness all behavioral conditions were accompanied by hippocampal theta. Normal gating was always found, except during REM sleep. The CAMP was than lower than during passive wakefulness. Gating was less disturbed during behavioral activity. Negative correlations were found between the percentage theta power on the one side and the CAMP, respectively, the C-T score, on the other. The correlation between the percentage theta power and the T/C ratio was also significant. It is concluded that the presence of hippocampal theta is not a sufficient condition to cause disturbances in auditory sensory gating. Behavioral states that accompany theta activity, however, tend to affect the CAMP. The decrease in gating found during REM sleep cannot be easily related to well-known neurochemical and pharmacological data.

The effect of state on sensory gating: comparison of waking, REM and non-REM sleep

OBJECTIVES

Auditory sensory gating is an electrophysiological assay that has been employed in clinical and basic research to clarify the neurobiological basis of perceptual and attentional impairments associated with schizophrenia and other diseases. In addition to genetically-linked characteristics, this measure also exhibits potentially confounding sensitivity to behavioral state, most notably acute stress. The goal of the present study is to determine if auditory sensory gating of evoked potential component P50 ('P1') could be measured during rapid eye movement (REM) sleep, as an alternative to the waking state.

METHODS

The suppression of vertex-recorded auditory evoked potential components, P30, P50 and N100, was measured as a function of stimulus redundancy using the paired-click paradigm during all-night sleep in 10 control subjects. Average evoked responses were computed separately for 30 min periods of waking, REM sleep, and non-REM (stage 2) sleep.

RESULTS

Evoked response component P50 exhibited suppression to the paired-click stimulus during REM sleep, not significantly different than waking. Suppression of wave N100 was significantly poorer during both sleep stages than waking. Component P30 was not suppressed in response to repetitive stimuli under any state of vigilance.

CONCLUSIONS

In addition to waking, response suppression of evoked potential component P50 can be measured during REM sleep, thus allowing the separation of trait- and state-dependent effects in future investigations of auditory sensory gating.

Chronic corticosterone treatment alters sensory gating in C3H mice

Two methods of evaluating inhibitory sensory processing are prepulse inhibition of acoustic startle (PPI) and gating of auditory evoked potentials. Studies using both methods suggest nicotinic acetylcholinergic receptor modulation of gating, specifically the alpha-bungarotoxin (alpha-BTX) binding site (alpha7 receptor subtype). However, recent assessment of alpha7 null mutant mice failed to demonstrate any effect of the loss of this receptor in either gating paradigm. An alternate approach to assessing the effects of the alpha7 receptor is to reduce its numbers in mature inbred mice, thus, avoiding the twin problems of background and developmental compensation inherent in null mutant mouse studies. Numerous studies have shown that chronic corticosterone (CCS) treatment selectively reduces alpha-BTX binding sites. C3H mice were adrenalectomized and implanted with corticosterone or cholesterol (control) pellets. After 8 days, they were tested in one of the gating paradigms. PPI and auditory gating were significantly diminished in corticosterone-treated mice concomitant with a reduction in alpha-BTX binding in several brain regions. Cholesterol-treated mice had no change in either paradigm. Nicotine treatment (1 mg/kg) produced significant improvement in both paradigms in corticosterone-treated mice. These data agree with previous pharmacological studies suggesting modulation of gating occurs through a nicotinic receptor.

The effect of nicotine and haloperidol co-treatment on nicotinic receptor levels in the rat brain

Genetic and biological data have suggested a role for the neuronal nicotinic acetylcholine receptors in the neuropathophysiology of schizophrenia. Studies in human postmortem brain demonstrate dose-dependent increases in nicotinic receptor binding in normal smokers. We found this upregulation to be reduced in schizophrenic smokers, many of whom had taken typical neuroleptics during their lifetime. The present study examined the hypothesis that typical antipsychotic drug treatment might modulate nicotinic receptor upregulation in a rat model. Nicotine, administered alone or in combination with haloperidol, increased both high and low affinity neuronal nicotinic receptors in a region specific manner. Haloperidol had no generalized effect on basal levels of nicotinic receptor binding or nicotine induced upregulation of nicotinic receptors. However, haloperidol attenuated high affinity nicotinic receptor upregulation in thalamus and low affinity receptor upregulation in hippocampus. These results suggest that haloperidol is not likely to affect nicotinic receptor regulation by smoking in most brain regions.

Neural correlates of sensory gating in the rat: decreased Fos induction in the lateral septum

In the P(50) gating or conditioning-testing paradigm in the rat, two identical click stimuli are presented with an inter-click interval of 500 ms. The reaction towards the second click, as measured with evoked potentials, is reduced in respect to that towards the first click; this phenomenon is called sensory gating. In the present experiments, the inter-click interval was varied systematically and auditory evoked potentials were measured. Sensory gating was found to occur only at intervals between 500 and 1000 ms, but not at longer intervals. Fos immunohistochemistry was then performed using two groups of rats exposed to double clicks: the inter-click interval was 500 ms in the experimental group and 2500 ms in the control group. Fos induction was analyzed in selected brain structures. In the auditory pathways, Fos-immunoreactive neurons were found in both groups of rats in the inferior colliculus and medial geniculate body. Fos-immunoreactive cells were also examined in the septum and hippocampus. In the ventral part of the lateral septal nucleus, the labeled neurons were significantly fewer in the experimental animals compared to the control group. Smaller and non-significant quantitative differences of Fos-positive neurons were documented in the medial septum and hippocampal CA1 region. These data point out a selective decrease in the lateral septum of Fos induced by auditory sensory gating, and suggest an involvement of this structure, and possibly of other parts of the septo-hippocampal system, in sensory gating mechanisms. The results might be relevant for theories on sensory gating deficits in schizophrenia.

The alpha7-nicotinic acetylcholine receptor and the pathology of hippocampal interneurons in schizophrenia

This paper is a review of a recent findings on the pathology of hippocampal interneurons in schizophrenia, with specific emphasis on a protein expressed by these cells, the alpha7-nicotinic acetylcholine receptor subunit. Convergent information indicates that interneurons in the hippocampus and other forebrain structures are decreased in number and function in subjects with schizophrenia. Among the neurochemical markers that are decreased in the hippocampus are synapsin I, cholecystokinin, somatostatin, glutamic acid decarboxylase, and nitric oxide synthase. GABA uptake sites and the GABA synthetic enzyme glutamic acid decarboxylase are also diminished. Included among these findings is decreased binding of alpha-bungarotoxin, which binds to low-affinity nicotinic acetylcholine receptors, such as the alpha7-nicotinic receptor. Co-labeling experiments in rodents indicate that these markers are expressed on overlapping populations of hippocampal interneurons. Thus, the finding of decreased neurochemical function of hippocampal interneurons is a widely replicated finding, with different groups reporting markedly similar findings using independent post mortem samples and different neurochemical strategies. Decreased alpha-bungarotoxin binding or decreased alpha7-nicotinic receptor immunoreactivity has also been found in the frontal cortex and in the nucleus reticularis thalami of schizophrenic subjects. The alpha7-nicotinic receptor subunit gene on chromosome 15q14 is a site of heritability for schizophrenia and bipolar affective disorder, and in, particular, for a deficit in inhibitory neuronal function associated with these illnesses. Thus, the post mortem data are further supported by psychophysiologic and genetic investigations that indicate a deficit in inhibitory interneuronal function, involving the alpha7-nicotinic receptor. The alpha7-receptor is a ligand-gated ion channel that admits calcium ions into cells, and it has been proposed to have various developmental roles. Its malfunction may be part of the developmental pathogenesis of schizophrenia.

Cholinergic modulation of the sleep state-dependent P13 midlatency auditory evoked potential in the rat

Injections into the pedunculopontine nucleus (PPN) of the cholinergic receptor agonist, carbachol (CAR), were found to reduce the amplitude of the vertex-recorded, sleep state-dependent P13 midlatency evoked potential in a dose- and time-dependent manner. This effect was blocked or reduced by pretreatment with the muscarinic receptor antagonist, scopolamine, injected into the PPN.

Different outcomes after acute and chronic treatment with nicotine in pre-pulse inhibition in Lister hooded rats

Excessive tobacco consumption by schizophrenic patients may be a form of self-medication, and nicotine in tobacco may alleviate deficits in information processing. We tested this hypothesis by determining whether nicotine (acute/chronic) would improve information processing in the rat using pre-pulse inhibition as a model. In study 1, rats were injected with nicotine 10 min prior to placement in startle chambers (0.001-0.1 or 0.03-0.3 mg/kg, s.c.). In study 2, rats were injected with either saline or nicotine (0.4 mg/kg, s.c.) for 21 consecutive days and assessed for locomotor activity, pre-pulse inhibition and changes in [3H]nicotine binding in whole brain. Acutely, nicotine had no effect on pre-pulse inhibition. By contrast, after chronic nicotine treatment, rats demonstrated a robust deficit in pre-pulse inhibition and significant increases in locomotor activity and [3H]nicotine binding. The deficit in pre-pulse inhibition after chronic treatment with nicotine may be the result of non-specific behavioural activation due to increased mesolimbic dopamine release or, possibly, nicotine may rapidly desensitize nicotinic receptors important for normal information processing.

Scopolamine impairs information processing in the hippocampus and performance of a learned eyeblink response in alert cats

The object of this work was to determine whether the changes in field activity and neuronal excitability recorded in the hippocampus during eyeblink classical conditioning are induced by the cholinergic input, and their relationships with the conditioned response performance. The pyramidal layer field activity, its response to fornix stimulation and eyelid responses were recorded during paired tone-air puff presentations, under scopolamine (25, 50 and 100 microg/kg) or saline administration, following well-established eyeblink conditioning in a trace paradigm in cats. Scopolamine impaired behavioral performance, and, in the hippocampus, disrupted conditioned stimulus-evoked field potential, high frequency shift in field activity, and paired presentation-induced hyperexcitability. These findings indicate that the cholinergic input participates in hippocampal information processing in a way that allows precise conditioned response performance and memory trace formation.

Inhibition of nitric oxide synthase prevents alpha 7 nicotinic receptor-mediated restoration of inhibitory auditory gating in rat hippocampus

The hippocampus rapidly inhibits its response to repetitive auditory stimulation, an example of an auditory sensory gating mechanism involved in human psychopathology. The neuronal basis of this inhibitory gating mechanism has been investigated in rats. Activation of the alpha 7 nicotinic receptor is required. alpha 7 nicotinic receptor activation also releases nitric oxide in the hippocampus and blockade of nitric oxide synthase reduces inhibitory gating of auditory response. There has not been a direct demonstration that blockade of nitric oxide synthase specifically prevents alpha 7 nicotinic receptor activation of the inhibition of auditory response. Therefore, the goal of the present study was to determine whether this functional effect of alpha 7 receptor activation requires release of nitric oxide. Lesions of the fimbria-fornix disrupt auditory gating by preventing cholinergic stimulation of the hippocampus. Following recovery from this surgery, rats were administered 3-(2,4-dimethoxybenzylidene) anabaseine (DMXB-A; 10 mg/kg, sc), an agonist at the alpha 7 receptor. DMXB-A restored auditory gating in the fimbria-fornix-lesioned rats, indicating that activation of the alpha 7 nicotinic receptor alone is sufficient to restore auditory gating following lesions of the fimbria-fornix. However, intracerebroventricular infusion of N(omega)-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase, blocked the DMXB-A-mediated restoration of auditory gating; infusion of the inactive D-enantiomer did not. Restoration of auditory gating by DMXB-A in the fimbria-fornix-lesioned rats was blocked by intracerebroventricular infusion of alpha-bungarotoxin, but not by mecamylamine or dihydro-beta-erythroidine. Together, these data support the hypothesis that nitric oxide mediates alpha 7 nicotinic receptor activation of gating of auditory response in rat hippocampus.

Neuronal nicotinic acetylcholine receptors: from the gene to the disease

The neuronal nicotinic acetylcholine receptors are excitatory ligand-gated channels. Widely expressed throughout the peripheral and central nervous system, their properties depend upon their subunit composition. Furthermore, genetic studies have revealed a high degree of variation at the genomic level and alternative splicing of the mRNAs coding for these integral membrane proteins. In particular, genes coding for alpha4 and alpha7 subunits harbour a high degree of polymorphisms. Although well characterised at their molecular and functional level, the role of these receptors in the central nervous system remains obscure. Despite accumulating evidence for the participation of nicotinic receptors in disorders of the central nervous system including nicotinic addiction, Parkinson's disease, Alzheimer's disease and Tourette's syndrome, the exact role of these receptors is still speculative. Because most of these phenotypes are complex and genetically heterogeneous, the investigation is difficult. However, in the past few years, significant progress has been made in understanding the contribution of nicotinic acetylcholine receptors to the origin of epilepsies and schizophrenia. By concentrating on the latest results gained for these diseases, we discuss in this review the possible relationships between neuronal nicotinic receptors and neurological and psychiatric disorders.

Role of EGR1 in hippocampal synaptic enhancement induced by tetanic stimulation and amputation

Hippocampal neurons fire spikes when an animal is at a particular location or performs certain behaviors in a particular place, providing a cellular basis for hippocampal involvement in spatial learning and memory. In a natural environment, spatial memory is often associated with potentially dangerous sensory experiences such as noxious or painful stimuli. The central sites for such pain-associated memory or plasticity have not been identified. Here we present evidence that excitatory glutamatergic synapses within the CA1 region of the hippocampus may play a role in storing pain-related information. Peripheral noxious stimulation induced excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal cells in anesthetized animals. Tissue/nerve injury caused a rapid increase in the level of the immediate-early gene product Egr1 (also called NGFI-A, Krox24, or zif/268) in hippocampal CA1 neurons. In parallel, synaptic potentiation induced by a single tetanic stimulation (100 Hz for 1 s) was enhanced after the injury. This enhancement of synaptic potentiation was absent in mice lacking Egr1. Our data suggest that Egr1 may act as an important regulator of pain-related synaptic plasticity within the hippocampus.

The P50 auditory event-evoked potential in adult attention-deficit disorder: comparison with schizophrenia

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) and schizophrenia are both conceptualized as disorders of attention. Failure to inhibit the P50 auditory event-evoked response, extensively studied in schizophrenia, could also occur in ADHD patients, if these two illnesses have common underlying neurobiological substrates.

METHODS

This study examined the inhibition of the P50 auditory event-evoked potential in 16 unmedicated adults with ADHD, 16 schizophrenic outpatients, and 16 normal control subjects. Auditory stimuli were presented in a paired stimulus, conditioning-testing paradigm.

RESULTS

The amplitude of initial or conditioning P50 response did not differ between the three groups; however, significant effects of psychiatric diagnosis on the amplitude of the test response and the ratio of the test to the conditioning response amplitudes were observed. Schizophrenic patients' P50 ratios and test amplitudes were higher than both the ADHD and normal groups.

CONCLUSIONS

Adults with ADHD do not have the inhibitory deficit seen in patients with schizophrenia, suggesting that the mechanism of attentional disturbance in the two illnesses may be fundamentally different.

Reduction of auditory P50 gating response in marihuana users: further supporting data

This report attempts to replicate our recent finding of a significantly reduced sensory gating response in medically and psychiatrically normal chronic marihuana users. After exclusions, 10 normal heavy marihuana users (> or = 3 times per week) and 10 normal non-user controls were tested with the paired auditory P50 sensory gating procedure. Sensory gating ratios were significantly higher (i.e., impaired suppression) for THC users as compared to controls. Using combined data from the current and previous report, the degree of sensory gating impairment among THC users was significantly correlated with the frequency of marihuana use per week. Suggestions for further research are offered.

Smoking and schizophrenia: abnormal nicotinic receptor expression

Biological and genetic evidence suggests a role for the neuronal nicotinic receptors in the neuropathophysiology of schizophrenia. Nicotine normalizes an auditory evoked potential deficit seen in subjects who suffer from the disease. Nicotinic receptors with both high and low affinity for nicotine are decreased in postmortem brain of schizophrenics compared to control subjects. The chromosomal locus of the human alpha-7 gene (15q14) is linked to the gating deficit with a lod of 5.3, and antagonists of the alpha-7 receptor (alpha-bungarotoxin and methyllycaconitine) induce a loss of gating in rodents. We have cloned the human alpha-7 gene and found it to be partially duplicated proximal to the full-length gene. The duplication is expressed in both the brain and in peripheral blood cells of normal subjects, but is missing in some schizophrenic subjects. The results of these studies suggest the presence of abnormal expression and function of the neuronal nicotinic receptor gene family in schizophrenia.

Serotonergic modulation of the P13 midlatency auditory evoked potential in the rat

The vertex-recorded, sleep state-dependent P13 midlatency auditory evoked potential in the rat may be generated, in part, by pedunculopontine nucleus (PPN) projections. Injections into the PPN of the 5-HT(1A) serotonin receptor agonist, 8-hydroxy-2-di-n-propylaminotetralin hydrobromide (DPAT), were found to reduce the amplitude of the P13 potential in a dose- and time-dependent manner. The suppressive effect of DPAT was blocked or reduced by pretreatment with the 5-HT(1A) serotonin receptor antagonist, Pindobind. These results show that the P13 potential can be modulated by known inhibitory serotonergic inputs to the PPN.

Nicotinic systems in central nervous systems disease: degenerative disorders and beyond

Advances in the understanding of the structure, function, and distribution of central nervous system (CNS) nicotinic receptors has provided the impetus for new studies examining the role(s) that these receptors and associated processes may play in CNS functions. Further motivation has come from the realization that such receptors are changed in degenerative neurologic diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Ongoing investigations of the molecular substructure of CNS nicotinic receptors and their pharmacology have begun to open up new possibilities for novel CNS therapeutics with nicotinic agents. Exploiting these possibilities will require understanding of the role(s) that these receptor systems play in human cognitive, behavioral, motor, and sensory functioning. Clues from careful studies of human cognition and behavior are beginning to emerge and will provide direction for studies of potentially therapeutic novel nicotinic agents. Modulation of these receptors with the ultimate goal of producing therapeutic benefits is the goal of these investigations and drug development. This paper will review studies from our laboratory and others that point to the importance of CNS nicotinic mechanisms in normal human cognitive and behavioral functioning as well as their role in disease states. In addition, this paper will examine potential clinical applications of nicotine and/or nicotinic agonists in a variety of CNS disorders with particular emphasis on structural brain disease including: movement disorders such as Parkinson's disease and Tourette's syndrome, cognitive/behavioral disorders such as Alzheimer's disease, attention deficit/hyperactivity disorder, and schizophrenia, and other more speculative applications. Important results from early therapeutic studies of nicotine and/or nicotinic agonists in these disease states are presented. For example, recent studies with nicotine and novel nicotinic agonists such as ABT-418 by our group in AD patients suggest that nicotinic stimulation can improve the acquisition and retention of verbal information and decrease errors. Preliminary results from a series of studies examining the acute and subchronic quantitative effects of nicotine on cognitive and motor functioning in Parkinson's disease suggest that acute nicotine administration and stimulation improves some aspects of cognitive and motor performance and may improve the processing speed of more complex tasks. The most likely near-term applications of novel nicotinic agonists in CNS disorders are likely to be in those disorders that are degenerative in nature, e.g. Parkinson's disease and Alzheimer's disease, or other movement disorders such as Tourette's syndrome. The most likely direct therapeutic role for nicotinic agonists is as augmentation therapy in combination with other agents rather than as monotherapy, except early in disease states or as a prophylactic or preventative treatment.

Familial transmission of risk factors in the first-degree relatives of schizophrenic people

Schizophrenia is a complex illness with multiple pathophysiologic factors that contribute to its psychopathology. One strategy to identify these factors is to observe them in isolation from each other, by characterizing their expression in the relatives of schizophrenic probands. By Mendel's second law, each genetic factor should be independently distributed in a sibship, so that each can be observed by itself, uncomplicated by the general problems of the illness. Such independently distributed phenotypes are obviously useful for genetic analyses; however, they can also be considered together, to model how various brain dysfunctions may combine to produce psychoses. In addition to a sensory gating deficit linked to the alpha 7-nicotinic acetylcholine receptor locus, schizophrenics and their families have a number of other deficits, including decreased hippocampal volume on magnetic resonance images and increased plasma levels of the dopamine metabolite homovanillic acid. Although such research is far from complete, a heuristic model combining a sensory gating deficit, decreased hippocampal neuron capacity, and increased dopaminergic neurotransmission is consonant with current understanding of the neuropsychology of schizophrenia.

Sensory gating in chronic posttraumatic stress disorder: reduced auditory P50 suppression in combat veterans

BACKGROUND

Posttraumatic stress disorder (PTSD) may be associated with a general impairment of cognitive function that extends beyond the processing of trauma-specific stimuli. Suppression of the auditory P50 response to repeated stimuli occurs in normal subjects and reflects the central nervous system's ability to screen out repetitive stimuli, a phenomenon referred to as sensory gating. This study examines P50 sensory gating to nonstartle auditory stimuli in PTSD subjects and normal controls.

METHODS

P50 generation and gating were studied using a conditioning/testing paradigm in 15 male subjects with PTSD and 12 male controls. P50 test/conditioning (T/C) ratios were estimated using the Singular Value Decomposition method.

RESULTS

The amplitude of the P50 response to the conditioning stimulus did not differ in subjects with PTSD compared to normal controls. The P50 T/C ratio is increased in PTSD subjects (mean = .408, SD = .275) as compared to the controls (mean = .213, SD = .126, two tailed t, p = .024).

CONCLUSIONS

This study provides evidence that PTSD is associated with impaired gating to nonstartle trauma-neutral auditory stimuli.

Selective alpha 7 nicotinic receptor stimulation normalizes chronic cocaine-induced loss of hippocampal sensory inhibition in C3H mice

BACKGROUND

A physiological alteration associated with schizophrenic and manic psychoses is diminished inhibition of the electrophysiological response to repeated auditory stimuli. This deficit also occurs in cocaine addicts. Studies in animals show that such inhibition is decreased by noradrenergic receptor stimulation and that the inhibition is enhanced by nicotinic cholinergic receptor stimulation.

METHODS

C3H mice were treated for 7 days with cocaine. They were then prepared for electrophysiological recording. After the effects of cocaine treatment were observed, they were treated with nicotine agonists.

RESULTS

Chronic cocaine administration markedly diminished inhibition of the hippocampal-evoked response to repeated auditory stimuli. The loss of inhibition was reversed by acute treatment with either nicotine or the selective alpha 7 nicotinic agonist 3-(2,4)-dimethoxybenzylidine anabaseine (DMXB; GTS21). The effects of nicotine showed tachyphylaxis, whereas those of DMXB did not.

CONCLUSIONS

This reversal of cocaine's effect by nicotinic agonists is consistent with previous pharmacological studies of the inhibition of auditory response. Additionally, the ability of nicotinic agonists to reverse a physiological defect associated with psychosis may have therapeutic implications for the neuropsychiatric sequelae of cocaine addiction in humans.

Human and animal studies of schizophrenia-related gating deficits

Prepulse Inhibition (PPI) of the startle response and the P50 auditory-evoked potential suppression are used to assess impairments in the regulation of the neural substrates and to determine the clinical significance of inhibitory deficits in schizophrenia. The study of gating deficits in schizophrenia and in related animal model studies have already advanced our understanding of the neural substrates of information processing abnormalities in patients with schizophrenia. Individuals with schizotypal personality disorder as well as clinically unaffected family members of patients with schizophrenia show PPI and P50 suppression deficits. These "schizophrenic spectrum" populations are not grossly psychotic, nor are they receiving antipsychotic medications. Therefore, the gating deficits are presumed to reflect core (eg, intermediate phenotypic) schizophrenia-linked information processing abnormalities. Several studies have reported that gating deficits are associated with clinical ratings of psychiatric symptoms, thought disorder, and neuropsychologic deficits in patients with schizophrenia. In addition, recent human pharmacologic studies have indicated that gating deficits can be reversed by rationally-selected compounds. Animal model studies have generally shown convergence with the human studies and may lead to improved identification of efficacious new antipsychotic medications for patients with schizophrenia.

Neurochemical modulation of the P13 midlatency auditory evoked potential in the rat

Previous studies have shown that the vertex-recorded P13 auditory evoked potential in the rat appears to be the rodent equivalent of the human P1 (or P50) potential. This sleep state-dependent potential appears to be generated, at least in part, by cholinergic pedunculopontine nucleus projections. The present studies used localized microinjections of neuroactive compounds into the region of the pedunculopontine nucleus in order to modulate the vertex-recorded P13 potential. Both the GABAergic agonist, muscimol, and the noradrenergic alpha2 receptor agonist, clonidine, were found to reduce the amplitude of the P13 potential in a dose-dependent manner. The suppressive effect of clonidine on P13 potential amplitude was blocked by pretreatment with the noradrenergic alpha2 receptor antagonist, yohimbine. In addition, habituation of the P13 potential, measured using a paired stimulus paradigm, was increased by micro-injection of a dose of muscimol or clonidine which did not change the amplitude of the P13 potential induced by the first stimulus of a pair. In contrast, microinjection of yohimbine decreased habituation of the P13 potential. These results show that the vertex-recorded P13 potential and its habituation can be modulated by activation of known inhibitory synapses, both GABAergic and noradrenergic, at the level of the pedunculopontine nucleus. This provides further evidence that the P13 potential is generated, at least in part, by pedunculopontine nucleus outputs.

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.

Reduced P50 auditory gating response in psychiatrically normal chronic marihuana users: a pilot study

BACKGROUND

Neurophysiological studies of marihuana (THC) often contain uncontrolled confounds [psychiatric diagnoses, polydrug use, central nervous system (CNS)-relevant injury, etc.] that can alter electrophysiological measures. This P50 sensory gating report is part of a larger neurophysiological and neurocognitive investigation of chronic THC exposure using rigorously screened medically and psychiatrically normal individuals without concurrent use of non-THC substances.

METHODS

Following medical and psychiatric screening, including serial urine drug screens, technically adequate P50 paired auditory recovery tests were obtained on 19 chronic THC users and 14 control subjects. Fifty pairs of 80-dB auditory clicks (1 pair per 10 sec, 500-msec interclick separation) were delivered through earphones. The sensory gating measure was the ratio between the P50 amplitudes at the vertex elicited by the conditioning (first) and test (second) click.

RESULTS

THC subjects had significantly higher sensory gating ratios (i.e., reduced suppression) than did control subjects. Among THC users, sensory gating ratios did not correlate with duration or frequency of THC use, although subjects with ratios above 40 had nearly twice the number of "joint-years" of THC exposure than did those with lower ratios.

CONCLUSIONS

Reduced P50 suppression in the sensory gating paradigm may be a possible neurophysiological CNS sequela of long-term cumulative exposure to THC.

No evidence for linkage between schizophrenia and markers at chromosome 15q13-14

Freedman et al. [1997: Proc Natl Acad Sci USA 94:587-592] reported linkage in nine multiplex schizophrenia families to markers on chromosome 15, using impaired neuronal inhibition to repeated auditory stimuli (P50), a neurophysiological deficit associated with schizophrenia, as the phenotype. The highest LOD score obtained (5.3 at theta = 0) was for marker D15S1360 mapped to chromosome 15q13-14, less than 120 kb from the alpha7-nicotinic receptor (CHRNA7) gene. The study also reported a small positive LOD score for D15S1360 when examined for linkage to the schizophrenia phenotype. Following these findings, we examined three polymorphic markers (D15S1360, L76630, and ACTC) on chromosome 15q13-14 near the CHRNA7 gene for linkage to schizophrenia, using 54 pedigrees from an independent study. Alleles for these three markers were genotyped and analyzed using parametric and nonparametric methods. No LOD score above 1.00 was obtained for any marker, and affected sib-pair analysis likewise showed no evidence for linkage. We conclude that in our families the region around the CHRNA7 locus does not contain a major locus for susceptibility to schizophrenia.

Sensory gating of the P13 midlatency auditory evoked potential and the startle response in the rat

The human P1/P50 midlatency auditory evoked potential and the startle response (SR) have been used as measures of sensory and sensorimotor gating, respectively. In the present study, both prepulse and paired stimulus paradigms were used in order to investigate the relationship between sensory gating mechanisms of the P13 potential, the putative rodent equivalent of the P1 potential, and those of the SR. In addition, these were compared to the properties of the N40 potential, another measure of sensory gating. Simultaneous recordings from the vertex (P13 potential and N40 potential) and neck musculature (SR) showed that (1) in a prepulse paradigm, increasing the intensity of the prepulse or decreasing the interstimulus interval resulted in increased inhibition of the P13 potential, N40 potential (to a lesser degree) and the SR (to a greater degree), (2) when using a low signal-to-noise ratio between the prepulse intensity and the background level, prepulse inhibition of the SR was reduced or absent while that of the P13 potential was present, (3) the amplitude of the 'prepulse evoked' P13 potential was significantly correlated with prepulse inhibition of the P13 potential, the N40 potential and the SR, (4) in a paired identical stimulus paradigm, decreasing the interstimulus interval resulted in increased habituation of the P13 potential, N40 potential (to a lesser degree) and the SR, and (5) increasing the intensity of the paired stimulation resulted in increased habituation of the P13 potential and the N40 potential (to a lesser degree), but not of the SR. These results demonstrate the presence of prepulse inhibition of the P13 potential, the N40 potential and the SR in a parallel manner, but show certain specific differences in their responses to parametric changes.

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.