Regulation of discrete sub-populations of transmitter-identified neurones after inhibition of electrical activity in cultures of mouse spinal cord.

The effects of blockade of electrical activity by tetrodotoxin in cultures of mouse spinal cord and dorsal root ganglion on immunohistochemically-identified neuronal sub-populations have been investigated. Some spinal cord neuronal types, such as those storing methionine-enkephalin, substance P or calcitonin gene-related peptide were almost totally depleted after inhibition of electrical activity for 4 days. By contrast, putative substance P- and calcitonin gene-related peptide-immunoreactive dorsal root ganglion neurones were not significantly affected by such treatment. Several other neuronal types were reduced by about 30-40% after exposure to tetrodotoxin. The decrement in methionine-enkephalin-, substance P- and calcitonin gene-related peptide-immunoreactive neurones caused by tetrodotoxin was reversible, and, in the case of methionine-enkephalin, could not be elicited after day 30 in culture. Radioimmunoassay of levels of methionine-enkephalin in cultures confirmed the immunohistochemical data. It is concluded, therefore, that exposure to tetrodotoxin selectively reduces peptide immunoreactivity in specific neuronal sub-populations, but that the selectivity is not based on a single known neuronal characteristic such as transmitter phenotype, or a particular structural protein. The action of tetrodotoxin on those cells most severely attenuated is an alteration in transmitter expression rather than a lethal effect. The diminution with time of the ability of tetrodotoxin to attenuate methionine-enkephalin levels may reflect a reduction in the activity-dependent regulation of peptide expression relative to other competing trophic influences.