Intracerebroventricular Morphine Administration and Its Quantitative Impact on Peripheral Thyroid Hormone Regulation in Adult Male Rats

Abstract

Background: Opioid compounds exert profound effects on central nervous system function and are increasingly recognized as important modulators of neuroendocrine regulation. Among these, morphine has been shown to influence hypothalamic–pituitary axes; however, its direct central effects on peripheral thyroid hormone homeostasis remain insufficiently quantified. This study aimed to evaluate the quantitative impact of intracerebroventricular (ICV) morphine administration on circulating triiodothyronine (T3) and thyroxine (T4) levels in adult male rats.

Methods: Adult male rats underwent stereotaxic implantation of a guide cannula into the lateral ventricle using established intracerebroventricular techniques. Following recovery, animals received ICV injections of morphine at defined doses, while control groups received equivalent volumes of vehicle solution. Blood samples were collected at predetermined time points after injection, and serum concentrations of T3 and T4 were measured using validated immunoassay methods. Quantitative comparisons were performed to assess hormone alterations over time and between experimental groups.

Results: Central administration of morphine induced significant, dose-dependent alterations in peripheral thyroid hormone levels. Serum T3 concentrations exhibited a marked reduction following ICV morphine exposure, whereas T4 levels demonstrated a differential response characterized by transient suppression followed by partial normalization at later time points. These findings indicate that central opioid signaling can modulate peripheral thyroid hormone regulation independently of direct thyroid gland manipulation. The observed hormonal patterns are consistent with opioid-mediated inhibition of hypothalamic thyrotropin-releasing mechanisms reported in previous neuroendocrine studies.

Conclusion: The present findings provide quantitative evidence that intracerebroventricular morphine administration significantly alters peripheral thyroid hormone homeostasis in adult male rats. These results highlight the critical role of central opioid pathways in the regulation of the hypothalamic–pituitary–thyroid axis and suggest that central opioid exposure may contribute to endocrine dysregulation observed in chronic opioid use. Understanding these interactions may have implications for both experimental neuroendocrinology and clinical conditions associated with opioid administration.