The shape of synaptic ribbons in the rat pineal gland

Abstract

Under the transmission electron microscope, synaptic ribbons (SRs) of the mammalian pineal gland appear as rod-like organelles. Their three-dimensional structure is not precisely known. In the present study, pineal SRs were investigated using serial sections obtained from rats killed at noon and midnight. The shape of the SRs was reconstructed based on SR profile length and the number of sections in which the profiles were contained. The results obtained show that SRs are basically flat plate-like structures with polymorphic lateral edges. Reconstructions of SRs revealed that they had average dimensions of 300x150x35 nm and were 19.3% larger at night than at day; the difference in SR size points to perhaps major differences in synaptic function between day and night.

Analysis of synaptic bodies in the Sprague–Dawley rat pineal gland under extreme photoperiods

“…In general profiles are significantly longer at night. The increase was 23% in Spiwoks-Becker (1995) and 27.6% in Jastrow et al (1997a). Using serial sections the latter authors calculated surface areas and volume of SBs, which also increased (both by 19.3%).…”

“…5). Some of the plates also exhibit holes, as demonstrated in Jastrow et al (1997a). The only constant part of the organelles is their thickness that can be correctly determined only when it is oriented exactly at right angle to the section plane, i.e.…”

“…In rat pinealocytes SBs undergo changes in number and size during a 24 h period Mori, 1977, 1980;McNulty et al, 1985McNulty et al, , 1987Riemann et al, 1990;Jastrow et al, 1997a) and further show changes in different photoluminous ''seasons'' (Karasek et al, 1988a;Martínez-Soriano et al, 1992). Light mediates functional and ultrastructural changes in pinealocytes (Karasek, 1981;Martínez-Soriano et al, 2002) via the retina-hypothalamic-reticulo-superior cervical ganglia pathway, providing adrenergic innervation of the pineal gland (Wurtman et al, 1967).…”

Active zone proteins are dynamically associated with synaptic ribbons in rat pinealocytes

“…In the pinealocytes of mammals, SRs are basically flat plate-like structures. In the pineal organ of rats, they exhibit a thickness of 30–40 nm, a mean length of 200 nm, and a maximal dimension of 800 nm (Jastrow et al 1997 ). They are often, but not exclusively, localized in the immediate vicinity of the plasmalemma (Vollrath 1981 ).…”

“…As seen in rod photoreceptor cells, mammalian pineal SRs exhibit diurnal plasticity. Their profile number is higher at night than during the day (Vollrath 1973 ; Jastrow et al 1997 ); this may result from an increase in the size and/or number of SRs (Jastrow et al 1997 ). SRs of the mammalian pineal gland have been proposed to be non-functional, representing phylogenetic relics of the original neurosensory function of pinealocytes (Bhatnagar 1994 ).…”