A polyclonal antibody against mammalian FOS can be used as a cytoplasmic neuronal activity marker in a teleost fish.

A polyclonal antibody raised against a conserved region of a mammalian FOS sequence was tested for its use as an activity marker in the rainbow trout. The FOS-like expression in the trout is entirely cytoplasmic and appears in a Nissl-like pattern. The reaction is specifically induced by both orthodromic and antidromic electrical stimuli and during motor responses evoked by natural stimulation, although some positive neurons are found at locations that are not obviously related to the presented stimuli. Following spinal nerve stimulation, antidromically activated motoneurons were found to be positive in the ipsilateral spinal cord. Orthodromic driving of spinal moto- and interneurons by stimulation of the medial longitudinal fasciculus (MLF) in the hindbrain evoked FOS-like immunoreactivity throughout the motor column in the spinal cord, but not in regions lying caudal to a lesion of the MLF-axons. Evoking about 25 startle responses by natural auditory stimulation gives FOS-like immunoreactivity in the Mauthner cell, which initiates the response, whereas positive Mauthner cells were never observed in control fish. The stimulation protocols that were used strongly activated the stimulated cells and so the observed FOS-like immunoreactivity might be related to an increase protein synthesis needed to restore their depleted transmitter levels.