Postnatal development of thalamic synaptic events underlying evoked recruiting responses and electrocortical activation

Abstract

Postsynaptic potentials were intracellularly recorded from thalamic neurons in kittens of different ages during stimulation of medial thalamic (MTh) regions at low and high frequencies. In neonatal and young kittens (< 10 days old) low-frequency (3.3/sec) MTh stimulation elicited IPSPs in thalamic neurons with a mean duration of 50 msec. EPSPs at this age were weak and variable. In older kittens (2–3 weeks old) MTh-evoked IPSPs exhibited a mean duration of 125 msec and EPSPs were more prominent in EPSP-IPSP sequences. High-frequency (80/sec) MTh stimulation in young kittens produced sustained summation of IPSPs and suppression of cell discharge. IPSPs were also elicited by high-frequency MTh stimulation in 2–3-week-old kittens but these IPSPs were rapidly terminated by powerful and sustained EPSPs. The relationship of these observations to previous studies of evoked synchronizing and desynchronizing processes in thalamic neuronal organizations in adult animals indicates that: (1) the development of evoked thalamic neuronal synchronization is associated with the functional maturation of interneuronal pathways involved in the production of EPSP-IPSP sequences in thalamic neurons; and (2) the development of electrocortical activation subsequent to high-frequency MTh stimulation occurs pari passu with an increasing capacity of excitatory synaptic inputs to elicit powerful and sustained EPSPs in thalamic neurons. Thalamic synaptic mechanisms underlying evoked synchronization and desynchronization of electrocortical activity attain functional maturation by the end of the third postnatal week in the kitten.