DEVELOPMENTAL-CHANGES IN ELECTROPHYSIOLOGICAL PROPERTIES OF LGND NEURONS DURING REORGANIZATION OF RETINOGENICULATE CONNECTIONS

Changes in electrophysiological properties of neurons in the ferret dorsal LGN (LGNd) were studied during early postnatal life, a critical developmental period when changes occur in morphology, connectivity, and response properties of LGNd neurons. Using the patch-clamp technique to obtain whole-cell recordings from cells maintained as in vitro slices of thalamus, several distinctive properties were observed in the immature LGNd. Relatively low resting membrane potentials were present that became more negative during the first 2 postnatal weeks. In addition, immature neurons exhibited high input resistances that decreased during early postnatal development. At all ages postnatally, neurons were capable of generating a train of Na+-dependent action potentials in response to intracellular injection of a depolarizing current pulse. Moreover, immature neurons resembled older cells in that little spike frequency adaptation was present during a train of action potentials. Action potential activity in immature neurons was nevertheless distinctive in several respects: (1) during the first 2-3 postnatal weeks action potentials became shorter in duration and larger in amplitude; (2) during the same period, thresholds for generation of action potentials changed in conjunction with the changes in resting membrane potential, becoming more negative; and (3) plots of frequency versus injected current revealed that thresholds for generation of trains of action potentials were reached with intracellular injection of lower current levels at earlier ages. These findings raise the possibility that relatively weak ionic currents generated at immature synapses have unexpectedly strong effects on the young LGNd neuron. Finally, immature cells possessed remarkably weak low-threshold calcium spikes and hyperpolariration-activated currents in comparison to those present in relay cells at the end of the third postnatal week. These results suggest that the intrinsic oscillatory behavior found in adult thalamus, which depends on activation of both types of current, may not be present at early ages. In conclusion, ferret LGNd neurons display substantial maturation in their electrophysiological properties during the first 2 postnatal weeks, a critical period for reorganization of retinogeniculate connectivity. The electrophysiological properties of immature relay cells may contribute to synaptic plasticity mechanisms by enhancing the excitatory responses of LGNd neurons to retinal and cortical input and limiting nonvisual intrinsic and intrathalamic oscillatory activity.