Differential Involvement of Metabotropic and p75 Neurotrophin Receptors in Effects of Nerve Growth Factor and Neurotrophin‐3 on Cultured Purkinje Cell Survival

: We have examined the role of the p75 neurotrophin receptor in survival‐promoting effects of nerve growth factor (NGF) and neurotrophin‐3 (NT‐3) on cultured Purkinje cells. Previously, we showed that NGF promotes Purkinje cell survival in conjunction with (1S,3R)‐1‐aminocyclopentane‐1,3‐dicarboxylic acid (ACPD), an agonist of metabotropic excitatory amino acid receptors, whereas NT‐3 by itself increases cell number. We now present evidence that p75 plays different roles in Purkinje cell responses to the two neurotrophins. A metabotropic receptor of the mGluR1 subtype may interact with p75 function, so as to regulate Purkinje cell responsiveness to neurotrophins. When cerebellar cultures were grown for 6 days in the presence of ACPD and a mutant form of NGF that does not bind to p75, no increase in Purkinje cell number was observed. Moreover, the survival‐promoting effect of wild‐type NGF and ACPD could be inhibited by a neutralizing antiserum to p75 or by a pyrazoloquinazolinone inhibitor of neurotrophin binding to p75. In contrast, the response to NT‐3 was potentiated by anti‐p75 treatment and by the quinazolinone. These data indicate the mediation of p75 in the trophic response to NGF‐ACPD and a negative modulatory role of p75 in the action of NT‐3. To probe the role of ACPD in the p75‐dependent response to NGF, metabotropic receptor subtype‐specific ligands were tested. The pattern of agonist specificity implicated the mGluR1 subtype, a receptor that is expressed at high levels by Purkinje cells and linked to activation of protein kinase C (PKC). Down‐regulation or blockade of PKC abolished the response to NGF‐ACPD. Consistent with the opposite roles of p75 in effects of the two neurotrophins, blockade of mGluR1 or PKC potentiated the survival response elicited by NT‐3. In sum, our data suggest that afferent excitatory transmitters activate specific metabotropic receptors to elicit a p75‐mediated action of NGF. NT‐3 acts on Purkinje cells by a different mechanism that is not absolutely p75‐dependent and that is reduced by neurotrophin access to p75 and metabotropic receptor activity.