Sensory gating deficit assessed by P50/Pb middle latency event related potential in Alzheimer’s disease

Sensory gating is defined as the brain's ability to inhibit repetitive and irrelevant incoming sensory stimuli and is supposed to be related to cholinergic transmission. Indeed, Alzheimer's disease (AD) is characterized by a cholinergic deficit that is believed to be involved in cerebral cortex hyperexcitability and short latency afferent inhibition deficit. Therefore, a sensory gating deficit may be supposed present in AD within the frame of cortex hyperexcitability and loss of cortex modulation of sensory inputs. The authors investigated whether a sensory gating deficit may be present in AD and whether this deficit may be related to the presence of neuropsychiatric symptoms (NPS) and reversed by donepezil treatment. Sensory gating was evaluated using a paired-stimulus auditory P50 event-related potential paradigm. Eighteen drug-naïve probable AD patients (mean age 76.1 years; SD 5.6 years; 13 females and 5 males) and 15 healthy elderly controls (mean age 74.2 years; SD 5.4 years; 10 females and 5 males) were recruited. Sensory gating was evaluated in AD patients before starting therapy and after 1 and 3 months of donepezil treatment. Auditory P50 sensory gating was impaired in AD patients but no correlation was found between gating deficit and NPS. Moreover, AD patients displayed increased P50 amplitude when compared with healthy elderly subjects. Donepezil treatment did not improve P50 sensory gating in AD patients but decreased P50 amplitude. Patients with AD displayed an augmented P50 amplitude, in accordance with previous studies, suggesting increased cortex excitability. Donepezil does not affect P50 sensory gating but reduces P50 amplitude. Donepezil may induce P50 amplitude reduction by means of enhanced dopamine release. Indeed, it has been demonstrated that donepezil induces dopamine release "in vitro." The findings suggest that AD patients have a sensory gating impairment but the link with both NPS and the cholinergic deficit is doubtful.