Early development of the thalamic inhibitory feedback loop in the primary somatosensory system of the newborn mice.

Spontaneous neuronal activity plays an important role during the final development of the brain circuits and the formation of the primary sensory maps. In young rats, spindle bursts have been recorded in the primary somatosensory cortex. They are correlated with spontaneous muscle twitches and occur before active whisking. They bear similarities with the spindles recorded in adult brain that occur during early stages of sleep and rely on a thalamic feedback loop between the glutamatergic nucleus ventroposterior medialis (nVPM) and the GABAergic nucleus reticularis thalami (nRT). However, whether a functional nVPM-nRT loop exists in newborn rodents is unknown. We studied the reciprocal synaptic connections between nVPM and nRT in thalamic acute slices from mice from birth [postnatal day 0 (P0)] until P9. We first demonstrated that nVPM-to-nRT EPSCs could be distinguished from corticothalamic EPSCs by their inhibition by 5-HT attributable to the transient expression of functional presynaptic serotonin 1B receptors. The nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs were both detected the first day after birth; their amplitude near 2 nS was relatively stable until P5. At P6-P7, there was a rapid and simultaneous increase of both nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs that reached 8 and 9 nS, respectively. Our results show that the thalamic synapses implicated in spindle activity are functional shortly after birth, suggesting that they could already generate spindles during the first postnatal week. Our results also suggest an inhibitory action of 5-HT on the spindle bursts of the newborn mice.