Characteristics of GABAergic neurons and their synaptic relationships withintrinsic axons in the cat motor cortex

The intrinsic circuitry of the motor cortex comprises a complex network of connections whose synaptic relationships are poorly understood. This study was designed to determine the characteristics of subsets of GABAergic neuro ns containing the calcium-binding proteins parvalbumin (PV) and calbindin ( CB), and their relationships with intrinsic axons in motor cortex. Immunohi stochemically identified PV-containing neuronal profiles were more evenly d istributed across cortical laminae (38% in II-III, 32% in V, 30% in VI) and more numerous (2.1/1) than CB-containing neuronal profiles (71% in II-III, 17% in V, 12% in VI). Relationships between neurons and axons intrinsic to motor cortex were visualized with fluorescent markers using the laser scan ning confocal microscope. Similar percentages of PV (43%) and CB-immunoreac tive (IR) (40%) neurons formed sparsely distributed appositions (1-5/neuron ) with anterogradely labeled axons. The mean distances of such appositions from the somata were significantly different for the two groups (PV, mean 2 2 mu m, range = 1.6-93 mu m; CB, mean = 32 mu m, range = 6.2-132 mu m). PV- IR neurons had a lower ratio of axosomatic/ axodendritic appositions (1/99) compared with CB-IR neurons (14/86). Ultrastructural studies confirmed the se findings. Fifty-seven percent of CB-IR neurons and 38% of PV-IR neurons formed synapses with intrinsic axons. Both populations received sparse inpu t (1-6 synapses/neuron). Nearly all appositions between labeled terminals a nd postsynaptic profiles formed one synapse. Postsynaptic dendrites of PV-I R neurons (mean = 1.4 mu m diameter) were larger than those of CB-IR neuron s (mean = 1.1 mu m), indicating more proximal synapses. Distinct input patt erns of intrinsic axons to the two populations of neurons suggest unique ro les in cortical processing.