Postnatal development of thalamic recipient neurons in the monkey striate cortex: III. Somatic inhibitory synapse acquisition by spiny stellate neurons of layer 4C.

The development of type 2 (Colonnier, '81) synapses on the cell bodies of thalamic recipient spiny stellate neurons in layers 4C alpha and 4C beta of primary visual cortex neurons was examined over the first 36 postnatal weeks and in the adult monkey. The type 2 synapses, known to be GABAergic (Ribak, '78) and therefore presumed to be inhibitory, developed faster on the alpha neurons than the beta neurons. Both neuron groups show a marked increase and then decline in the percentage of the somatic membrane covered by type 2 synaptic appositions during this 36-week time period. The time course of the type 2 synapses development is compared to that of the spine synapse development described in previous studies (Lund and Holbach, '91; Lund et al., '91), and it is clear that on both neuron groups this inhibitory synapse population is put in place and refined later than the spine synapses. These findings suggest that each cortical neural circuit has a unique time course for its early development within an overall time window (Rakic et al., '86), or sensitive period (Hubel and Wiesel, '70). Visual deprivation, although causing the alpha and beta neurons to adopt a more similar temporal and numerical developmental pattern than normal, did not prevent acquisition and loss phases of type 2 synapses or the assumption of a normal numerical loading by 36 weeks of age.