Effects of isoproterenol on synaptic ribbons in pinealocytes of the rat and C57BL/6J mouse

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

Synaptic bodies (SBs) are small, prominent organelles in pinealocytes, most probably involved in signal transduction processes. To check the influence of the photoperiod on their shape plasticity and number we chose two extreme lighting conditions, i.e. 20h of illumination followed by 4h of darkness (LD 20:4) versus (LD 4:20). Pineal glands were assessed at 0, 4 and 13h after dark onset. Under both conditions reconstructed SBs were plates or ribbons but never spheres and there were no obvious differences in morphology. Photoperiodic changes in SB profile size and number were investigated: application of the established method for SB quantification based on single section profile counts (SSPC) of areas showed a significant increase of SB profiles under LD 20:4. However, it has to be noted that SSPC depend on both, number and size of the structures. In contrast to this, modification of the disector counting method, also applied for unbiased quantification of whole SBs, revealed that rat pinealocytes show insignificantly more SBs under LD 20:4 than under 4:20 conditions. The lengths of the SB profiles, which were first measured under different conditions in this study, depend on SB size. They increased significantly under LD 20:4. In conclusion, we detected only an increase in SB size but not in their number. We further prove that, at least for SBs, it is of no value to calculate disector levels from SSPCs.

Pineal rhythms are synchronized to light-dark cycles in congenitally anophthalmic mutant rats

Genetically mutant anophthalmic rats lacking a complete visual system due to the absence of eyeballs and optic nerves up to the optic chiasma were used as a model to study photo-regulated physiological activities. The photoreception in these mutant rats was determined by measuring the neuroendocrine response of the pineal gland-melatonin levels in the serum, and synaptic ribbon complexes (SRCs) in the pinealocytes. These parameters were studied in both normal and anophthalmic rats maintained under light-dark (LD 12:12), continuous dark (DD) and light (LL) conditions. Both normal and mutant anophthalmic animals showed nocturnal increases in serum melatonin levels and in the number and diameter of SRC and their vesicles in the pinealocytes in LD. The daily rhythms persisted even upon transfer to DD both in normal and mutant rats, whereas in LL, the nocturnal elevation of both the parameters disappeared. These observations suggested that congenitally blind rats can perceive light. The studies of these parameters in both normal and mutant rats in reversed-LD conditions confirmed that pineal rhythms can be entrained by light-dark cycles in congenitally anophthalmic mutant rats through a nonvisual system for light perception.

The response to isoproterenol of synaptic ribbon numbers in the rat pineal gland changes during postnatal development

In the mammalian pineal gland synaptic ribbons (SRs) are dynamic organelles of pinealocytes undergoing a day/night rhythm, with small numbers during daytime and significantly higher numbers at night, similar to the formation of the pineal hormone melatonin. Whereas the day/night rhythm of melatonin synthesis is adrenergically regulated, data on the adrenergic regulation of SR numbers in the rat pineal gland are at variance. While some authors have demonstrated that isoproterenol (ISO) stimulates SR numbers, others could not find any effect. To clarify the issue, we carried out identical experiments in two age groups. It was found that in male Sprague-Dawley rats, administration of the beta-adrenergic agonist ISO (2.5 or 10 mg/kg body weight) increased SR profile numbers in one age group (3 weeks), but not in the other (8 weeks). These findings resolve the apparent discrepancy reported in the literature, by showing that the response to ISO of SR profile numbers is apparently lost during postnatal differentiation of the pineal gland.

In vitro effects of putative neurotransmitters on synaptic ribbon numbers and N-acetyltransferase activity in the rat pineal gland

The pineal contains a large number of classical transmitters and neuropeptides. Some of these neurochemicals are involved in the regulation of serotonin N-acetyltransferase (NAT) activity and hence in melatonin synthesis. Synaptic ribbons present in the pineal gland also exhibit a numerical day/night rhythm parallel to that of NAT activity. There is scarcity of information regarding the regulation of synaptic ribbon (SR) numbers. In the present study, we have investigated in vitro effects of a number of classical neurotransmitters and neuropeptides. NAT activity was used to monitor melatonin synthesis under the experimental conditions used. Norepinephrine (NE), Delta sleep-inducing peptide (DSIP), vasoactive intestinal polypeptide (VIP), adenosine and N-acetyl-asp-glu (NAAG) significantly increased NAT activity in rat pineal. DSIP and VIP also increase the stimulatory effect of NE on NAT activity. These neurochemicals had no effect on SR numbers. Gamma aminobutyric acid (GABA), serotonin and taurine affected neither NAT activity nor SR. Somatostatin increased SR numbers significantly, without having any effect on NAT activity. The effect of somatostatin is regarded to be pharmacologic, since rather high dosages (10(-4) M) were required to obtain a significant effect. Although somatostatin is present in the pineal and may change rhythmically, the inconsistency of the day/night rhythmicity and the lack of such a rhythm in female rats and male gerbils speaks against an important physiological role of somatostatin in regulating SR numbers.

Pinealocyte synaptic ribbons and neuroendocrine function

A comparative study of pinealocyte synaptic ribbons (SR) revealed two predominant populations exhibiting either a rod/ribbon shape (SRr) or a spherical/punctate shape (SRsp). Species-specific differences were found in the abundance of SR, the ratio of SRr/SRsp, and the occurrence of SR in ribbon fields. The close topographical relationship of SR to the plasma membrane and the numerical changes that occurred with changes in metabolism of the pinealocytes suggest that SR have important vesicle-mediated interactions with the cell membrane. Experiments designed to clarify the relationship between SR and pineal neuroendocrine function revealed a positive correlation between SR numbers and indole intermediates during pineal development in the rat, and increased SR frequency after denervation of the rat pineal gland or administration of the beta-adrenergic agonist, isoproterenol. These data are consistent with the hypothesis that SR function is linked to receptor mechanisms regulating indoleamine production in the pineal gland.

Acetylcholine and muscarinic agonists increase synaptic ribbon numbers in the rat pineal

Mammalian pinealocytes possess synaptic ribbons (SR) which are commonly present in photoreceptor cells at synaptic junctions. Pineal SR numbers undergo a diurnal rhythm parallel to that of pineal N-acetyltransferase (NAT) activity and melatonin levels. Recent findings suggest that SR numbers, unlike NAT activity and melatonin synthesis and release, do not seem to be regulated by adrenergic mechanisms or neuropeptides in adult rats. Since the pineal gland also receives cholinergic nerve fibres, we have investigated in vitro effects of acetylcholine (ACh) and carbamyl-beta-methylcholine (CBMC; a specific muscarinic agonist) in the presence and absence of pirenzipine (a specific inhibitor of muscarinic M1 receptors). ACh and CBMC increased SR numbers significantly. Pirenzipine inhibited the CBMC-induced increase in SR numbers. On the basis of these findings, it is suggested that cholinergic agonists increase pineal SR numbers by acting through muscarinic M1 receptors. Hence muscarinic mechanisms may have a functional role in pineal physiology.

Effects of LHRH, progesterone, estradiol-17 beta and dexamethasone in vitro on pineal synaptic ribbons and serotonin N-acetyltransferase activity in diestrous rats

Pineal glands of regularly cycling Sprague Dawley rats (180-220 g) killed on the diestrous morning (between 0900-1000 h) were incubated in appropriate media for six hours with LHRH (8.5 microM), progesterone (3.2 microM), estradiol-17 beta (370 nM) or dexamethasone (250 nM). Pineals incubated in hormone-free medium and unincubated glands served as controls. Six rats were used in each group. After incubation the glands were divided into two parts. One part was used to estimate serotonin N-acetyltransferase (NAT) activity. The other part was processed for electron microscopy to quantify synaptic ribbons (SR). The SR numbers were computed to 20,000 microns 2 area of pineal tissue. The number and distribution pattern of SR were identical in incubated as well as in the unincubated controls. In both these groups the SR located close to the cell membrane were more (23 +/- 1) than those that lay away from it (9 +/- 2). LHRH had no effect on the number of SR located close, to or distant from, the cell membrane. Incubation of pineals with progesterone significantly (p less than or equal to 0.05) depressed the number of SR present close to membranes (23 +/- 1 in controls vs 11 +/- 2 in treated group), total SR (34 +/- 3 in controls vs 21 +/- 2 in treated group) and synaptic fields (26 +/- 2 in controls vs 17 +/- 2 in treated group). Likewise, in the estradiol-17 beta group also membrane-associated SR decreased significantly. The effect of progesterone was more severe than estrogen on the SR possibly due to the differences in the doses used.(ABSTRACT TRUNCATED AT 250 WORDS)

A circannual cycle in pinealocyte synaptic ribbons in the hibernating and seasonally reproductive 13-lined ground squirrel (Spermophilus tridecemlineatus)

Pinealocyte synaptic ribbons (SR) in the 13-lined ground squirrel were quantified at monthly intervals over a one-year period spanning the hibernating and reproductive cycles of these animals. SR numbers were high (70-90/20,000 microns2) during periods of activity and reproductive quiescence (May-October). With the onset of hibernation (November-December) there was a rapid 6 to 7-fold reduction in SR frequency. During arousal (January-February) and sexual maturation (March-April) there was a gradual increase in SR frequency. The winter decline in SR frequency is consistent with the decline in pineal melatonin during hibernation and supports the hypothesis that pinealocyte SR play an important role in the neurotransduction of melatonin biosynthesis.