was a milestone in the study of growth factors and in stimulating the search for other neurotrophic factors. It is now known that

NGF

is only one of a series of secreted factors with the ability to promote neuronal survival. The most studied class of trophic factors is the neurotrophins (

neurotrophins

). Four major neurotrophic factors have been isolated from mammals:

NGF

, brain-derived neurotrophic factor (

brain

derived

neurotrophic

factor,

BDNF

), neurotrophic factor

3

(

NT-3

) and neurotrophic factor

4/5

(

NT-4/5

). In addition,

there may

be

neurotrophic factor

6

(

NT-6

).

NGF

is widely present in humans and many species of animals. If antiserum to

NGF

is injected into a newborn animal, it can cause damage to almost all sympathetic ganglia, while injecting antiserum into a female rat can result in sensory neurologic deficits in the fetal rat. It is evident that

NGF

is necessary for the growth and survival of sympathetic and sensory neurons. It has also been shown that

NGF

increases

cAMP

levels of cholinergic neurons in the basal forebrain and striatum, increases choline acetyltransferase activity, and is important for the growth and survival of these neurons. Various

NT

have different roles in protecting neurons and preventing apoptosis, but there is some crossover and overlap. Knockout

NT-3

genes cause a marked absence of cutaneous touch

-

pressure receptors;

DBNF

-deficient mice have a reduced number of peripheral sensory neurons and severe degeneration of the vestibular ganglia.