An in vitro electrophysiological and Co2+-uptake study on the effect of infraorbital nerve transection on the cortical and thalamic neuronal activity

Early recovery of neuronal functioning in the sensory cortex after nerve reconstruction surgery

BACKGROUND

Nerve reconstructive surgery induces a transient loss and a prolonged and a gradual return of sensory inputs to the brain. It is unknown whether, following this massive peripheral denervation, the brain will experience a prolonged period of severe, intrinsic dysfunction.

OBJECTIVE

We aim to investigate the mechanisms of return of processing function in cortical neurons.

METHODS

We used the whisker model in rats to evaluate the functional recovery in the somatosensory cortex after a nerve reconstruction surgery. Multi-unit recording in the barrel cortex was performed in lightly anesthetized rats while their whiskers were stimulated by a whisker stimulator.

RESULTS

We observed a loss of neuronal responses to whisker stimulation 1 week after surgery, which started to recover 2 weeks after surgery. Following the surgery, only 11.8% of units had principle whiskers (PWs) returned to their original status while 17.7% had PWs different from their original status, indicating the effect of aberrant reinnervation on the whisker response map.

CONCLUSIONS

Robust neuronal responses to sensory stimulation even when only sparse sensory inputs are available in the early recovery phase. During this phase, aberrant reinnervation induces disorganized whisker tuning, a finding that might be account for the hypoesthesia and paresthesia during early recovery after nerve reconstruction.

Enhancement of synaptic strength in the somatosensory cortex following nerve injury does not parallel behavioural alterations

Following infraorbital nerve transection, underlying mechanisms of the altered synaptic strength were studied in rat barrel cortex slice experiments. In addition to the in vitro electrophysiological studies, open-field tests were run to detect possible behavioural changes associated with cortical oversensitization. Enhanced NMDA receptor-mediated component of the evoked field response appeared in the barrel cortex after nerve injury. The alteration was transient, very distinct on the first day following injury, and almost returned to normal level by the end of the second week. Behavioural changes had not followed this time-course since long-lasting alterations were detected in the open-field test. These observations are in agreement with findings that showed biphasic regenerative processes following nerve injuries in other cortical areas.

Transneuronal induction of the highly sialylated isoform of the neural cell adhesion molecule following nerve injury

The polysialylated, embryonic form of the neuronal cell adhesion molecule (PSA-NCAM) is known to participate in a whole series of synaptic rearrangements even in adult animals. The possible role of this molecule in neuroplastic changes of the adult rat somatosensory cortex induced by unilateral transection of the infraorbital branch of the trigeminal nerve was studied with PSA-NCAM immunostaining at light microscopic level. Two- and three-month-old CFY albino rats were sacrificied on days 1, 4, 6, 14 and 21 following operation and PSA-NCAM immunoreaction was examined at three levels of the vibrissa-cortex neuraxis, namely, in the principal nucleus of the trigeminal nerve, in the ventral posteromedial nucleus of the thalamus and in the somatosensory cortex. The lower levels of the neuraxis remained free of PSA-NCAM labeling, similarly to control, intact animals. However, a large number of scattered small neurons became PSA-NCAM immunoreactive in layers IV-VI on both ipsi- and contralateral sides of the somatosensory cortex from day 6 onwards, suggesting a possible transynaptic regulation of NCAM sialylation state.