Host Serotonin Axons Innervate Intrastriatal Ventral Mesencephalic Grafts After Implantation in Newborn Rats (original) (raw)

1994, European Journal of Neuroscience

This study investigated the potential of immature and adult serotonin (5HT) neurons for axonal growth into intrastriatal grafts of ventral mesencephalic tissue. Implantation of dissociated fetal (embryonic days 14 -15) ventral mesencephalic tissue was carried out in immature [postnatal days (P) 5-141 and adult rat neostriatum. The brains were processed 2 -6 months later for dopamine and 5-HT immunocytochemistry. A few grafts implanted into adult and P7 recipients contained small numbers of cotransplanted 5HT cell bodies. These also displayed a rich network of 5-HT axons, even in adult rats prelesioned with 5,7dihydroxytryptamine, indicating the graft origin of these axons. All other grafts were totally devoid of 5-HT cell bodies. After implantation in adults, such grafts contained rare 5-HT axons. In contrast, in P5 -P7 recipients, they displayed many 5HT fibres, which were uniformly distributed. Such was no longer the case after implantation in P14 recipients, which showed minimal 5HT innervation, as in adult recipients. Processing of naive rat brain at different ages for 5HT immunocytochemistry showed that 5-HT axons were still clearly less numerous in the neostriatum at P21 than in adults, whereas in the substantia nigra the 5-HT innervation developed more rapidly and was comparable, at P21, to that of adults. It was concluded that 5-HT axons are able to grow into ventral mesencephalic grafts, but mainly at the fetal stage and with decreasing capacity after birth. Since 5-HT axons are still normally growing in the developing striatum beyond P21, it appears that their inability to innervate mesencephalic grafts after P14 is not the result of a developmental decrease in growth capacity. Rather, the better temporal correlation with the maturation of the 5-HT innervation in the substantia nigra suggests that this inability reflects an ontogenic change in the affinity of these axons for nigral tissue.

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