Opioid antagonist (naltrexone) modulation of cerebellar development: histological and morphometric studies

Abstract

The role of endogenous opioid systems in preweaning cerebellar development was explored in rats utilizing naltrexone, a potent opioid antagonist. Sprague-Dawley rats were given daily subcutaneous injections of either 1 or 50 mg/kg naltrexone to invoke a temporary or complete blockade, respectively, of opioid receptors throughout the first 3 weeks of postnatal life; animals injected with sterile water served as controls. At weaning (day 21), macroscopic, morphometric, and histological determinations were conducted. In general, 50 mg/kg naltrexone had a stimulatory action on cerebellar development, whereas 1 mg/kg naltrexone had an inhibitory influence. Both sexes were affected comparably. Limits to naltrexone's ability to modulate cerebellar ontogeny were noted, with more latitude existing toward growth enhancement than impairment. The temporal course of ontogeny was generally unaltered in naltrexone-treated rats. Rather, only events that transpired over this period were dramatically affected. The most notable effects in 1 mg/kg naltrexone rats were marked decreases in cerebellar areal dimensions, the content of internal granule neurons, and cellular and tissue differentiation. Characteristics of the 50 mg/kg naltrexone group included increases in cerebellar areal dimensions, neural cell number, content, and size, and structural changes consistent with acceleration in growth and differentiation. Naltrexone influenced both neurons and glia, but only neural cells still being generated during the first 21 d after birth were altered in regard to quantity. Previous evidence has shown the presence of peak levels of endogenous opioids and opioid receptors in the cerebellum during the first weeks of life, as well as demonstrating the presence of enkephalin immunoreactivity on germinative cerebellar cells during postnatal neurogenesis but not in adult counterparts.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular localization of proenkephalin mRNA and enkephalin peptide products in cultured astrocytes

“…In the cerebellum, [ 3 H]-thymidine incorporation by neuroblasts of the external granular layer (EGL) in 6-day-old rats is inhibited by systemic administration of Met-enkephalin, while treatment with the opioid antagonist naltrexone, at dosages sufficient to completely block opioid receptors, is reported to increase [ 3 H]-thymidine incorporation compared to untreated controls 56 . These and other experiments suggest that endogenous opioids are normally available to cells in the developing CNS in sufficient quantities to tonically inhibit growth 17,18,41,[54][55][56] . In support of this hypothesis, opioid peptide levels and opioid binding are greatly increased in the rat cerebellum 48,49 28,32,43,52 .…”

“…Cellular differentiation 17,18 , cell number and packing density 54,55 , and cortical thickness 54,55 are modifiable by treatment with opioid antagonist drugs in vivo. In the cerebellum, [ 3 H]-thymidine incorporation by neuroblasts of the external granular layer (EGL) in 6-day-old rats is inhibited by systemic administration of Met-enkephalin, while treatment with the opioid antagonist naltrexone, at dosages sufficient to completely block opioid receptors, is reported to increase [ 3 H]-thymidine incorporation compared to untreated controls 56 .…”

“…Although the above findings suggest that endogenous opioids are present during development and tonically inhibit growth 17,18,[54][55][56][57] , few studies have attempted to identify the potential cellular sites of opioid synthesis in the developing central nervous system (CNS). Recent reports have demonstrated the presence of proenkephalin mRNA by Northern analysis of confluent glial cultures that are enriched in astrocytes (i.e., 95-97% astrocytes) 28,32,43,52 .…”

Morphine Inhibits Purkinje Cell Survival and Dendritic Differentiation in Organotypic Cultures of the Mouse Cerebellum

“…Endogenous opioid systems are present during development and can modify nervous system maturation (16,21,77,78). During development, endogenous opioid neuropeptides typically act by inhibiting the growth of the nervous system (21,77,78).…”

“…During development, endogenous opioid neuropeptides typically act by inhibiting the growth of the nervous system (21,77,78). Opiate drugs such as morphine also affect neural development (9,12,14,16,17,42,52,54,58,62,66,70,74).…”