PATTERNS OF SYNAPTIC ACTIVITY IN FORWARD AND FEEDBACK PATHWAYS WITHIN RAT VISUAL CORTEX. Domenici, Luciano, Gary W. Harding and Andreas Burkhalter. Department of Anatomy and Neurobiology, Department of Neurosurgery, Washington University School of Medicine.
APStracts 2:0225N, 1995.
SUMMARY AND CONCLUSIONS
(1) The laminar and temporal distribution of synaptic activity supplied by forward and feedback connections between different areas of rat visual cortex, was determined using current source density (CSD) analysis in in vitro slices. In forward connections, synaptic potentials were evoked by electrically stimulating area 17 and recording in the extrastriate area LM (lateromedial), that ranks at the second hierarchical level, one step above primary visual cortex. For activating feedback connections, the location of stimulating and recording electrodes was reversed. (2) The synaptic interactions in reciprocal intracortical circuits are excitatory and they are mediated through glutamate receptors that are blocked by kynurenic acid. (3) Forward connections from area 17 to area LM provide input to all layers including a strong input to layer 4. In contrast, feedback input to layer 4 is weak and is mainly directed to superficial and deep layers. This laminar distribution closely resembles that seen in anatomical studies (Coogan and Burkhalter 1993). (4) Both forward and feedback connections evoke distinct temporal patterns of synaptic activation in different layers. Although onset and peak latencies are slightly shorter in the forward than in the feedback pathway, the difference is not statistically significant. (5) The spatio-temporal distribution of synaptic activation by forward connections resembles the pattern evoked by geniculocortical inputs. Feedback connections show greater similarities to long-range connections within area 17, although they are not identical. Our results support the notion derived from anatomical and in vivo physiological studies that forward and feedback pathways belong to functionally distinct cortical circuits. 

Received 7 December 1994; accepted in final form 27 July 1995.
APS Manuscript Number J766-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.