Electrophysiological elucidation of pathways of intrinsic horizontal connections in rat visual cortex

Cortical neurons receive synaptic inputs through both vertical and horizontal pathways. We made a systematic survey of the synaptic strength and intracortical pathways of intrinsic horizontal connections in rat visual cortex using intracellular recordings from alice preparations. Excitatory postsynaptic potentials were recorded from pyramidal neurons of layers 2/3 and layers 5/6 in response to electrical shocks applied to these layers at a lateral distance of 1.0 mm from the impaled neuron or to the underlying white matter. When the threshold intensity of stimulation to activate monosynaptic excitatory postsynaptic potentials was compared, the vertical input had a lower threshold than the horizontal inputs. The threshold intensity of the horizontal inputs was lower, and the amplitude of the responses was larger in sagittally sectioned slices than in coronal slices, suggesting that the horizontal synaptic connection in the rat visual cortex was stronger in the rostrocaudal than in the mediolateral direction. Tetrodotoxin puffs focally applied to gray matter between the stimulation and the recording sites caused a transient depression of excitatory postsynaptic potentials, which was selective to the input conveyed through the puffed area. This pharmacological dissection revealed that routes parallel to the cortical Iaminae in the same layer as the stimulation site mediated the largest part of excitation conduction of intrinsic connections. Obliquely ascending routes mediated almost half of all the detected inputs originating from a deep layer to the neurons in either layers 5/6 or layers 2/3, whereas the contribution of obliquely descending routes from layers 2/3 to layers 5/6 was small (25%). Our results present semi-quantitative data on the connection diagram of the intrinsic neuronal circuits in the rat visual cortex, which will provide the basis for further investigations of the roles of the intrinsic connections in information processing in rodent cerebral cortex.