Effects of the GABA mimetic drug, sodium valproate, on event-related potentials and its relation to the law of initial value

Effect of GAD1 genotype status on auditory attention and acute nicotine administration in healthy volunteers

OBJECTIVE

The effects of GABA modulating drugs and nicotine, the prototypical nicotinic cholinergic agonist, on attention have been investigated using subcomponents of the P300 event-related potentials (ERP), which index involuntary (P3a) and voluntary attention (P3b). However, investigations into how such pharmacologic effects interact with genetic features in the GABA system remain unclear. This study examined the moderating effects of a single nucleotide polymorphism (rs7557793) in the glutamic acid decarboxylase 67 (GAD1) gene, which is implicated in the conversion of glutamate to GABA, on P300-indices of auditory attentional processing; the influence of nicotine administration was also assessed.

METHODS

The effects of GAD1 genotype (TT/CC/CT) were examined on the P3a/b in response to an auditory selective attention task in healthy, nonsmoking male volunteers (N = 126; 18-40 years). Participants responded to rare target stimuli (P3b-eliciting) and ignored frequent nontarget stimuli as well as rare distractor stimuli (P3a-eliciting). In a subsample (N = 59), P3a/b profiles to acute nicotine (vs. placebo) administration were examined as a function of GAD1 genotype. As a secondary aim, earlier sensory processes were assessed with N200 ERP subcomponents elicited by novel (N2a) and target (N2b) auditory stimuli.

RESULTS

GAD1 allelic variation moderated early sensory processes, enhancing N2a amplitudes in CT versus TT carriers. Further, TT homozygotes exhibited larger P3b amplitudes than CC homozygotes in the placebo versus nicotine condition. Regardless of genotype, nicotine versus placebo moderated the N200 ERP.

CONCLUSION

These findings expand our knowledge regarding the attentional effects of GAD1 genetic variants in relation to nicotine.

Age and stage dependency of P300 latency alterations in non-demented Parkinson's disease patients without therapy

Acoustic P300 was recorded from Fz, Cz and Pz by means of an 'odd-ball' paradigm in 44 non-demented de novo Parkinson's disease patients (PD) or PD patients under treatment withdrawal, and in 31 age-matched normal subjects, to evaluate whether a P300 latency increase was present in PD patients. The influence of age and disease stage on latency was successively verified by subgrouping PD patients according to different age ('young' and 'old') and disease stage ('early' or 'advanced'). PD patient data were compared to data of normal subjects subgrouped into 'young' and 'old' or, to eliminate the age-dependent shift of latency, this latter was adjusted to 60 years in all the examined subjects. A significant increase of latency has been found in Fz and Cz in the 'old' group of PD patients (n = 23) but not in the 'young' group (n = 21) utilising both methods. Moreover, a significant latency increase was also present in Fz and Cz in the group of 'advanced' PD patients (n = 8), but not in the group of 'early' PD patients (n = 36) utilising age-adjusted measurements. When the 'early' PD patient group was divided into 'young' (n = 20) and 'old' (n = 16), the 'early old' group displayed significantly increased latencies in Fz compared with normal subjects. Abnormal P300 latencies were observed, at least in one electrode, by analysing the raw data, in 5.0% of the 'early young', 43.7% of the 'early old' and up to 62.7% of the 'advanced' patients. Fz represented the site in which abnormal P300 latencies were most often observed. Moreover, in the total group of PD patients, the P300 delay was significant only on the frontal (Fz) site when compared with normal subjects. The reported findings were interpreted as if PD produces a sort of 'accelerated effect of age' on the cognitive functions, presumably produced by a mechanism different from that producing motor impairment since no clear correlation could be detected between P300 latency and motor score. The frontal impairment of P300 is in line with previous neuropsychological findings obtained in these patients. Considering that about 30% of PD patients develop dementia during their disease progression, a border-line or abnormal P300 latency observed at disease onset may represent a predictive marker of this evolution.