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The effects of intracerebroventricular injection of colchicine or cytochalasin B were compared on radioimmunoassayable endorphin and enkephalin in several rat brain regions and pituitary gland. In addition, colchicine effects on brain immunoreactivity were also assessed by immunocytochemistry. Colchicine treatment increased endorphin levels in the hypothalamus without changing pituitary levels. Cytochalasin B did not change brain endorphin levels but increased pituitary content. These contrasting drug effects probably reflect the neuronal nature of the brain endorphin system and the endocrine nature of the pituitary system. Colchicine treatment also increased enkephalin radioimmunoreactivity in the hypothalamus with a corresponding decrease in the pituitary. This colchicine effect is compatible with reduced axonal transport of enkephalin in a hypothalamo-neurohypophysial pathway: immunocytochemistry after colchicine reveals stained hypothalamic neurons not seen normally. However, colchicine treatment did not alter enkephalin radioimmunoassay values in the other brain regions, even when these regions also revealed groups of cell bodies not seen in normal brain. This inconsistency between unchanged assay values and increased cell reactivity might be explained in part if most enkephalin neurons have short axons; thus, colchicine arrest of axonal transport might cause a local redistribution of enkephalin immunoreactive material that would not be detected by radioimmunoassay due to the limitations of tissue dissection. These results demonstrate the enhanced interpretive value of data obtained from both radioimmunoassay and immunocytochemistry.
Bayón et al. (1980) studied this question.