Afferents to visually responsive grafts of embryonic occipital neocortex tissue implanted into V1 (Oc1) cortical area of adult rats

The aim of this study is to determine, as precisely as possible, the topogr aphy and the density of host afferents to visually responsive grafts of occ ipital 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. Fi eld potentials appeared triphasic in shape, had low peak-to-peak amplitude (less than or equal to 100 mu V), and had normal time latencies (approximat e to 30 msec). Polarity reversal was never observed. Single unit recordings showed that graft neurons exhibited normal (desynchronized) spontaneous ac tivity, had discrete receptive fields (approximate to 20 degrees in dia.), and responded to small stationary light Bashes. A topic projection of the v isual field in the grafts was also observed. Injections of cholera toxin su bunit B (CTB) into these responsive grafts induced retrograde labeling in a lmost all the brain regions normally projecting to the occipital cortical a reas. The visual related cortical (Oc1, Oc2) and thalamic (LP, LD, LGB) reg ions 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 - 9 7 % of the labeled cortical cells were found in cortical layers V and VI wi th a net preference for layer VI. A noticeable proportion of these layer VI labeled neurons (15 - 20 %) was systematically observed in sublayer VIb, v ery 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 cells by: (i) regenerating axons of geniculate neurons previously innervating the injured cortical site; (ii) formation of collateral branche s from thalamic axons ending normally in the host cortex close to the graft boundary; and (iii) development of neuronal processes from cells located i n the host cortex, mainly in layers V and VI of the occipital areas. Depend ing on multiple factors, yet unknown but very likely related to the host-gr aft integration, the cortical circuits might be either the principal affere nt inflow to the graft or only a complement to the thalamic input.