MN survival is promoted by BDNF, CNTF, and raised levels of intracellular cAMP. A, Slices were grown for 6 d with the indicated trophic factors GDNF (200 ng/ml), BDNF (500 ng/ml), and CNTF (500 ng/ml). BDNF and CNTF both promoted the survival of a proportion of the MNs. BDNF and CNTF in combination supported the survival of more MNs than either trophic factor alone and were able to match but not to exceed the effect of GDNF. B, The concomitant addition of BDNF or CNTF did not further increase the survival response of MNs to GDNF. C, Elevating intracellular levels of cAMP using the membrane-permeable analog CPT-cAMP also promoted the survival of a proportion of the MNS.D, Treatment of the slices with CPT-cAMP (200 μm) in addition to trophic factor did not further increase the survival response to BDNF or CNTF. CPT, CPT-cAMP. *p < 0.05; **p < 0.01; ***p < 0.001.