The invagination complex in nerve endings on adrenomedullary adrenaline cells: quantitative ultrastructural description, and analysis of changes with time-of-day and their modification by sham-surgery and pinealectomy.

This work describes a special ultrastructural feature, the invagination complex (IC), involving the plasma membrane of nerve endings on adrenomedullary adrenaline cells. Quantitative characteristics of the IC and their changes were studied in 122 male albino (Holtzman strain) rats in 3 surgical groups: normal (non-operated, NO) sham-operated (SO) and pinealectomized (PX), all maintained in a standardized daily photoperiod (light : dark 12 : 12 h). Animals were decapitated 14 days postsurgery and at 8 specific times during the light : dark cycle. Left adrenal glands were removed, dn strain) rats in 3 surgical groups: normal (non-operated, NO) sham-operated (SO) and pinealectomized (PX), all maintained in a standardized daily photoperiod (light : dark 12 : 12 h). Animals were decapitated 14 days postsurgery and at 8 specific times during the light : dark cycle. Left adrenal glands were removed, dn strain) rats in 3 surgical groups: normal (non-operated, NO) sham-operated (SO) and pinealectomized (PX), all maintained in a standardized daily photoperiod (light : dark 12 : 12 h). Animals were decapitated 14 days postsurgery and at 8 specific times during the light : dark cycle. Left adrenal glands were removed, dissected and prepared for electron microscopy. In section profiles the diameter of each IC was usually 0.12-0.40 micrometers, and the depth 0.2-1.0 micrometers. They were frequently seen to be located near the synaptic complex (or the active zone). Coated pits, about 50 nm wide and 60 nm deep, often opened near the bottom of the invaginations of the IC. In NO animals, relative number and depth of the ICs showed daily rhythmic changes with minimal values about 1 h after onset and maximal (acrophase) values 3--5 h later (P less than 0.02 to less than 0.005, depending on index or measure). These changes occurred 3--5 h earlier, but less apparently, in SO animals, and appeared to be more greatly modified and dampened in PX animals. Two-way analysis of variance (ANOVA) and t-tests applied to 11 kinds of indices derived from counts and measurements of the ICs, support the differences between surgical groups, at least in many instances (P less than 0.05 to less than 0.001). It is concluded that the IC is a characteristic and dynamic feature of the nerve terminals and that it may possibly have a role in such phenomena as recycling of synaptic vesicles or related membrane constituents. It is also concluded that significant time-of-day and neuroendocrine effects are demonstrable in these structures, and that the time-of-day effects shown in these chronic studies have importance in the design of acute experiments designed to further test the functional relations and importance of ICs.