Sensory-evoked activity in somatosensory cortex as a model to probe cortical plasticity in a mouse model of Rett syndrome

Abstract

Rett syndrome (RTT), a severe neurodevelopmental disorder, affects females resulting from loss-of-function mutations in the X-linked transcription factor methyl-CpG-binding protein 2 (MECP2). RTT patients show severe verbal, motor, respiratory, and intellectual impairments. We studied two forms of activity-dependent plasticity in Mecp2 mutant mice to better understand the loss of MECP2 function in neuronal circuit and sensory processing. Sensory deprivation was applied by trimming one whisker to 3 mm to study long-term cortical plasticity in Mecp2-/y mice. Intrinsic optical signaling (IOS) imaging showed the neuronal response to wiggling a non-trimmed was consistent from day 0 to 14 but reduced for the trimmed whisker by 49.0 ± 4.3% in wild type (WT) and 22.7 ± 4.6% (p=0.0135) in RTT mice. Primary hindlimb (HL) somatosensory cortical responses to vibratory stimulation were assessed by IOS and intracortical local field potential (LFP). Responses were assessed before, during and, after 1 hour of repeated HL vibratory stimulation (100Hz,1sec, ISI 6 sec) in symptomatic male (4-6 week), female (10-12 month) and pre-symptomatic young female (4 week) RTT model mice. After 1-hour, cortical responses to test vibrations were reduced by approximately 40% in RTT and WT mice as assessed by both methods. Recovery of the IOS responses (1 sec vibration at 100Hz) and LFP (300µm below pia, 7 stimuli, 100mse ISI) were tested at 15 min intervals for 1 hour after ceasing the repeated stimulation. Reduced responses persisted for at least 60 min in WT but recovered to 90-100% of normal within 15-30 min in RTT. Analysis of the LFP responses within the test train indicated that the reduced cortical sensitivity during and after continuous stimulation resulted primarily from an increase in adaptation during the 7-stimulus test train rather than a reduction in the response to a single vibratory stimulus in all groups. Retention of this increased STA is the primary cause of the persistently reduced tactile response in young WT female mice, while in RTT mice the rapid recovery of tactile sensitivity was due to the return of STA to lower, baseline levels. Male RTT mice exhibited a marked increased excitability to the first stimulus in the test train resulting in hypersensitivity to a single vibration by 45 minutes. Old females exhibited the same pattern of adaptation and recovery but retention of adaptation was less pronounced in both WT and RTT compared to younger animals suggesting an age-dependent reduction in neural plasticity may mask deficits specific to RTT. Recording sciatic nerve sensory afferent activity did not reveal any STA, persistent adaptation or sensitization of peripheral afferent endings in any groups. I propose persistent sensory adaptation mediated by increased short-term adaptation may reflect enhanced feedback by inhibitory elements of circuits within the sensory pathway. The rapid recovery of responsiveness in young female RTT mice may therefore reflect a deficit in the capacity for activity dependent plasticity to consolidate and thus could provide a platform to understand the causes of learning and cognitive deficits in RTT patients.