Afferents to Visually Responsive Grafts of Embryonic Occipital Neocortex Tissue Implanted into V1 (Oc1) Cortical Area of Adult Rats

Abstract

The aim of this study is to determine, as precisely as possible, the topography and the density of host afferents to visually responsive grafts of occipital embryonic cells implanted in block form into the occipital neocortex of adult rats. The presence of visual activity in the grafts was assessed through field potential and single unit electrophysiological recordings. Field potentials appeared triphasic in shape, had low peak-to-peak amplitude (= 100 μV), and had normal time latencies (~30 msec). Polarity reversal was never observed. Single unit recordings showed that graft neurons exhibited normal (desynchronized) spontaneous activity, had discrete receptive fields (~20° in dia.), and responded to small stationary light flashes. A topic projection of the visual field in the grafts was also observed. Injections of cholera toxin sabunit B (CTB) into these responsive grafts induced retrograde labeling in almost all the brain regions normally projecting to the occipital cortical areas. The visual related cortical (Oc 1, Oc2) and thalamic (LP, LD, LGB) regions of the host provide the largest contingent (70-75%) of afferents to the graft. Finally, one of the major findings of this study is that 93-97% of the labeled cortical cells were found in cortical layers V and VI with a net preference for layer VI. A noticeable proportion of these layer VI labeled neurons (15-20%) was systematically observed in sublayer VIb, very close to, or even within, the white matter. We suggest thus that grafts inserted into the occipital cortex of adult rats receive functional visual inputs through various neuronal circuits. Visual inputs could be conveyed to graft eells by: (i) regenerating axons of geniculate neurons previously innervating the injured cortical site; (ii) formation of collateral branches from thalamic axons ending normally in the host cortex close to the graft boundary; and (iii) development of neuronal processes from cells located in the host cortex, mainly in layers V and VI of the occipital areas. Depending on multiple factors, yet unknown but very likely related to the host-graft integration, the cortical circuits might be either the principal afferent inflow to the graft or only a complement to the thalamic input.