Effects of pimozide on the ultrastructure of the pars distalis in the rat

The effects of pimozide, a dopamine receptor-blocking agent, were studied in the pars distalis of the rat. The animals received 100 micrograms/100 g pimozide daily for 5, 10, 15, and 20 days. Pimozide induces striking ultrastructural changes after 5 days of treatment. The number of luteotroph (LTH) cells is signficantly increased; they display characteristics of stimulation. The extrusion of granules into the intercellular space via exocytosis is frequently observed. The intercellular spaces are highly dilated, forming a lacunar system filled with an amorphous material, erythrocytes and involuted LTH cells. Transitional stages in the process of involution are observed in LTH cells. Luteotroph cells also form a syncytium. Twenty days after treatment the above-described changes decrease in magnitude. The present findings suggest that pimozide stimulates the mechanism of synthesis and release in the luteotroph cells, an effect that is less evident with longer treatment.