Pinealocyte synaptic ribbons and neuroendocrine function

An ultrastructural study of the deep pineal gland of the Sprague Dawley rat using transmission and serial block face scanning electron microscopy: cell types, barriers, and innervation

The morphology of the deep pineal gland of the Sprague Dawley rat was investigated by serial block face scanning electron microscopy. Cells were three-dimensionally (3-D) reconstructed using the software Fiji TrackEM. The deep pineal gland consisted of 2-5 layers of electron-lucent pinealocytes, with a euchromatic nucleus, endowed with one or two processes. Laterally, the deep pineal merged with the habenula and the stria medullaris thalami, via an intermediate area containing cells with more electron-dense cytoplasm and an indented nucleus with heterochromatin. Neither nerve terminals nor capillaries were observed in the deep pineal itself but present in the intermediate parts of the gland. The deep pineal was in contact with the third ventricle via the pineal and suprahabenular recesses. The ependymal lining in these recesses was an epithelium connected by tight junctions between their lateral cell membranes. Several intraventricular nerve terminals were in contact with the ependyma. 3-D reconstructions showed the ependymal cells endowed with long slender process penetrating the underlying pineal parenchyma. Few "tanocyte-like" ependymal cells, endowed with a process, reaching the subarachnoid space on the inferior surface of the deep pineal were observed. In addition, pinealocyte and astrocyte processes, often connected by gap junctions, bordered the inferior surface. In summary, the rat deep pineal gland is a neuroendocrine structure connected to the habenula. We here report specialized ependymal cells that might transmit signals from the cerebrospinal fluid to the deep pineal parenchyma and a "trans-pineal tanocyte-like cell" that connects the ventricular system with the subarachnoid space.

The pineal gland of the shrew (Blarina brevicauda and Blarina carolinensis): a light and electron microscopic study of pinealocytes

The pineal gland structure and ultrastructure in the Northern (Blarina brevicauda) and Southern short-tailed shrew (Blarina carolinensis) are described by light and electron microscopy. Results observed were similar to other mammals of Insectivora described previously, specifically, the hedgehog (Erinaceus europaeus) and the Old World mole (Talpa europea). Two different types of pinealocytes were noticed by electron microscopy, in addition to relatively few glial cells. Granular vesicles were not noticed in abundance. The granular endoplasmic reticulum was observed and studded with vesicles. The golgi apparatus was well developed and appeared often. Synaptic ribbons were observed in several different formations consisting of ribbons and/or rods. The ciliary derivative, the rudimentary photoreceptor structures found in the pinealocytes of population I, was noticed in a 9 + 0 tubular pattern. Within these semifossorial shrews, the relationship between specific intracellular organelles and their function was discussed.

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

Synaptic ribbons (SRs) are prominent organelles that are abundant in the ribbon synapses of sensory neurons where they represent a specialization of the cytomatrix at the active zone (CAZ). SRs occur not only in neurons, but also in neuroendocrine pinealocytes where their function is still obscure. In this study, we report that pinealocyte SRs are associated with CAZ proteins such as Bassoon, Piccolo, CtBP1, Munc13–1, and the motorprotein KIF3A and, therefore, consist of a protein complex that resembles the ribbon complex of retinal and other sensory ribbon synapses. The pinealocyte ribbon complex is biochemically dynamic. Its protein composition changes in favor of Bassoon, Piccolo, and Munc13–1 at night and in favor of KIF3A during the day, whereas CtBP1 is equally present during the night and day. The diurnal dynamics of the ribbon complex persist under constant darkness and decrease after stimulus deprivation of the pineal gland by constant light. Our findings indicate that neuroendocrine pinealocytes possess a protein complex that resembles the CAZ of ribbon synapses in sensory organs and whose dynamics are under circadian regulation.

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.

Frequency selectivity of synaptic exocytosis in frog saccular hair cells

The ability to respond selectively to particular frequency components of sensory inputs is fundamental to signal processing in the ear. The frog (Rana pipiens) sacculus, which is used for social communication and escape behaviors, is an exquisitely sensitive detector of sounds and ground-borne vibrations in the 5- to 200-Hz range, with most afferent axons having best frequencies between 40 and 60 Hz. We monitored the synaptic output of saccular sensory receptors (hair cells) by measuring the increase in membrane capacitance (deltaC(m)) that occurs when synaptic vesicles fuse with the plasmalemma. Strong stepwise depolarization evoked an exocytic burst that lasted 10 ms and corresponded to the predicted capacitance of all docked vesicles at synapses, followed by a 20-ms delay before additional vesicle fusion. Experiments using weak stimuli, within the normal physiological range for these cells, revealed a sensitivity to the temporal pattern of membrane potential changes. Interrupting a weak depolarization with a properly timed hyperpolarization increased deltaC(m). Small sinusoidal voltage oscillations (+/-5 mV centered at -60 mV) evoked a deltaC(m) that corresponded to 95 vesicles per s at each synapse at 50 Hz but only 26 vesicles per s at 5 Hz and 27 vesicles per s at 200 Hz (perforated patch recordings). This frequency selectivity was absent for larger sinusoidal oscillations (+/-10 mV centered at -55 mV) and was largest for hair cells with the smallest sinusoidal-stimuli-evoked Ca2+ currents. We conclude that frog saccular hair cells possess an intrinsic synaptic frequency selectivity that is saturated by strong stimuli.

Bizarre alterations of the morphology of pineal synaptic bodies under constant light and an evaluation of suitable 3D-reconstruction software

Three dimensional (3D) reconstruction and modelling software was evaluated to find a procedure suitable for visualization of small subcellular structures in transmission electron microscope images. The method applied in this study demonstrates bizarre alterations of the structure of synaptic bodies (SBs) in pinealocytes of the guinea-pig pineal gland caused by constant illumination. It can, in general, be used for any 3D reconstruction from serial sections. Pineal glands of five guinea-pigs (two kept under a LD cycle of 12:12 h; three kept in constant light, for 4 months) were investigated. SBs consist of an electron-dense centre with attached vesicles. Under normal lighting conditions most SBs are flat plates (about 35 nm in thickness), which eventually may be bent. The proteins comprising the molecular basis of SBs, mainly RIBEYE A and B are polymerised in a regular manner in these plates. This is not the case in other SBs, which appear as spheres or irregular lumps. SBs lie in groups in which usually some of the plates are arranged in parallel arrays Constant illumination caused different changes in morphology: many of the SBs lie in 'paired fields', i.e. appear in groups attached to the cell membranes of two pinealocytes directly opposite to each other. Some of the SBs in such groups are strongly bent, showing blebs and irregular thickened areas, others seem to aggregate and show inclusions of cytoplasm. Further goblet-like, shield-like and other bizarre forms of SBs occurred and the relative number of spheroid and lump-like SBs increased. Protrusions on larger SBs suggest detachment or fusion of SB material to a greater extent than in the control animals. There is a reduction of areas in which the polymerisation of the SB proteins remains well ordered, i.e. where the typical thickness of 35 nm is maintained. It remains unclear why this polymerisation pattern is only partly affected by constant light.

Evidence for implication of tryptophan hydroxylase in the regulation of melatonin synthesis in ovine pinealocytes in culture

1. Tryptophan hydroxylase (TPOH) is the first enzyme in the melatonin synthesis pathway and the rate-limiting enzyme in serotonin synthesis. We established in this study an in vitro model of ovine pinealocytes to investigate the role of TPOH in melatonin production. 2. We demonstrated that TPOH is highly expressed both in vivo and in vitro at the protein and mRNA levels. In vitro pinealocytes show ultrastructural features similar to those previously described in vivo. 3. Moreover, our in vitro model allowed us to study the regulation mechanisms for melatonin synthesis in sheep pinealocytes and to demonstrate that both transcriptional and posttranscriptional mechanisms are involved. 4. In particular, our results suggest that TPOH plays an essential role in the regulation of melatonin synthesis.

Structure suggests function: the case for synaptic ribbons as exocytotic nanomachines

Synaptic ribbons, the organelles identified in electron micrographs of the sensory synapses involved in vision, hearing, and balance, have long been hypothesized to play an important role in regulating presynaptic function because they associate with synaptic vesicles at the active zone. Their physiology and molecular composition have, however, remained largely unknown. Recently, a series of elegant studies spurred by technical innovation have finally begun to shed light on the ultrastructure and function of ribbon synapses. Electrical capacitance measurements have provided sub-millisecond resolution of exocytosis, evanescent-wave microscopy has filmed the fusion of single 30 nm synaptic vesicles, electron tomography has revealed the 3D architecture of the synapse, and molecular cloning has begun to identify the proteins that make up ribbons. These results are consistent with the ribbon serving as a vesicle "conveyor belt" to resupply the active zone, and with the suggestion that ribbon and conventional chemical synapses have much in common.

Synaptic-like microvesicles in mammalian pinealocytes

The recent deciphering of the protein composition of the synaptic vesicle membrane has led to the unexpected identification of a compartment of electron-lucent microvesicles in neuroendocrine cells which resemble neuronal synaptic vesicles in terms of molecular structure and function. These vesicles are generally referred to as synaptic-like microvesicles (SLMVs) and have been most intensively studied in pancreatic beta-cells, chromaffin cells of the adrenal medulla, and pinealocytes of the pineal gland. This chapter focuses on the present knowledge of SLMVs as now well-established constituents of mammalian pinealocytes. I review the results of morphological, immunocytochemical, and biochemical studies that were important for the characterization of this novel population of secretory vesicles in the pineal organ. The emerging concept that SLMVs serve as a device for intercellular communication within the pineal gland is outlined, and unanswered questions such as those pertaining to the physiological function and regulation of pineal SLMVs are discussed.

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.

Adaptation of the disector method to rare small organelles in TEM sections exemplified by counting synaptic bodies in the rat pineal gland

The disector is the only objective method for quantifying particles of variable size in a given volume. With this method, cell organelles are identified on adjacent sections, but only those present in one section are counted. When counting extremely rare structures in transmission electron microscope sections (physical disector), the usual procedure of counting on electron micrographs is limited for economic reasons (e.g. micrographs highly outnumbering the investigated structures). Hence, to apply this unbiased stereological method, a modification of the physical disector concerning 3 aspects has been developed. (1) The prerequisite of screening large corresponding tissue areas (here approximately 65000 microm2) was fulfilled by examining tissue areas along the edges of ultrathin sections. (2) The size of the counting frame was determined by measuring the lengths of the section margins (minus a guard area) by means of a Morphomat. This value was multiplied by the width of the investigated tissue zone, corresponding to the diameter of the electron microscope viewing screen. (3) Disector counting was carried out simultaneously on both sections (bidirectional disector) to improve efficiency. In the present study tiny synaptic bodies (SBs) were quantitated by disector in a rat pineal gland, yielding approximately 30 SBs/1000 microm3. By contrast, single section profile counts of SBs amounted to 90 SBs/20000 microm2. Since the presently described adaptation of the disector is time-consuming, it is proposed to determine a proportion factor allowing to estimate number of structures per volume based on single section profile counts. This would decrease the evaluation time by more than 50%.

Plasticity of retinal ribbon synapses

Ribbon synapses differ from conventional chemical synapses in that they contain, within the cloud of synaptic vesicles (SV's), a specialized synaptic body, most often termed synaptic ribbon (SR). This body assumes various forms. Reconstructions reveal that what appear as rod- or ribbon-like profiles in sections are in fact rectangular or horseshoe-shaped plates. Moreover, spherical, T-shaped, table-shaped, and highly pleomorphic bodies may be present. In mammals, ribbon synapses are present in afferent synapses of photoreceptors, bipolar nerve cells, and hair cells of both the organ of Corti and the vestibular organ. Synaptic ribbons (SR's) are also found in the intrinsic cells of the third eye, the pineal gland, and in the lateral line system. The precise function of SR's is enigmatic. The prevailing concept is that SR's function as conveyor belts to channel SV's to the presynaptic membrane for neurotransmitter release by means of exocytosis. The present article reviews the evidence that speaks for a plasticity of these organelles in the retina and the third eye, as reflected in changes in number, size, shape, location, and grouping pattern. SR plasticity is especially pronounced in the mammalian and submammalian pineal gland and in cones and bipolar cells of teleost fishes. Here, SR number and size wax and wane according to the environmental lighting conditions. In the pineal SR numbers increase at night and decrease during the day. In teleost cones, SR's are in their prime during daytime and decrease or disappear at night, when transmitter release is enhanced. In addition to numerical changes, SR's may also exhibit changes in size, shape, grouping pattern, and location. In the mammalian retina of adults, in contrast to the developing retina, the reported signs of SR plasticity are subtle and not always consistent. They may reflect changes in function or may represent signs of degradation. To distinguish between the-two, more detailed studies under selected experimental conditions are required. Probably the strongest evidence for SR plasticity in the mammalian retina is that in hibernating squirrels SR's leave the synaptic site and accumulate in areas as far as 5 microns from the synapse. Changes in shape include the occurrence of club-shaped SR's and round SR's or synaptic spheres (SS's). SS's may represent a special type of synaptic body, yet belonging to the family of SR's, or may be related to the catabolism of SR's. SR number is regulated by Ca2+ in teleost cones, whereas in the mammalian pineal gland cGMP is involved. An interesting biochemical feature of ribbon synapses is that they lack synapsins. The presently reviewed results suggest to us that SR's do not primarily function as conveyor belts, but are devices to immobilize SV's in inactive ribbon synapses.

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.

Pineal 'synaptic' ribbon numbers and melatonin synthesis of rat are resistant to guanethidine sympathectomy

Chemical sympathectomy is widely used to study the impact of the noradrenergic system on neuronal and neuroendocrine circuits. We tested the effects of intraperitoneal injections of guanethidine, an adrenergic neuron blocking agent, on selected functional parameters of the rat pineal gland which are known to be under sympathetic influence. The reliability of the method was demonstrated by the clear enophthalmus developed by experimental animals. However, neither the numbers of 'synaptic' ribbons nor melatonin synthesis differed between treated and control rats, both parameters exhibiting the nocturnal increase seen in intact animals. These results are in striking contrast to those obtained upon chemical sympathectomy with 6-hydroxydopamine or surgical superior cervical ganglionectomy. We conclude that guanethidine is not capable of sufficiently removing noradrenergic influence from the rat pineal gland, and that this substance is thus inferior to other experimental methods of sympathectomy.

p-Chlorophenylalanine treatment depresses the number of synaptic ribbon profiles in the rat pineal gland, but does not abolish their day-night rhythm

It is largely unknown as to how the day/night rhythm of pineal synaptic ribbon number is regulated. Transcriptional events related to the nocturnal formation of new synaptic ribbons occur early in the morning, when pineal serotonin levels begin to increase. The present study was carried out in order to elucidate as to how altered serotonin levels affect the day/night changes in the number of synaptic ribbon profiles. To this end, male Sprague-Dawley rats received a single dose of p-chlorophenylalanine (pCPA, 300 mg/kg body weight, 72 hours before sacrifice), which depresses tryptophan hydroxylase activity and hence pineal serotonin levels. Control animals received saline injections. Experimental and control animals were killed in the middle of the light period and in the middle of the dark period, respectively. The pineal glands were removed and bisected. One half was used for electron microscopic quantification of synaptic ribbon profile numbers, and the other half for determination of several biochemical parameters (see below). pCPA decreased pineal serotonin levels to c. 30% of the controls and synaptic ribbon profile numbers by c. 30%, both at day and night. The day/night rhythm of the two parameters did not appear to be affected. The rate-limiting enzyme of melatonin synthesis, serotonin N-acetyltransferase, which is only measurable at night, and pineal melatonin levels were not affected at night. The intermediary product of melatonin formation, N-acetylserotonin, was significantly depressed at night. It is concluded that serotonin plays a more important modulatory role on pineal synaptic ribbons than does melatonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate immunoreactivity is enriched over pinealocytes of the gerbil pineal gland

Mammalian pinealocytes have been shown to contain synaptic-like microvesicles with putative secretory functions. As a first step to elucidate the possibility that pinealocyte microvesicles store messenger molecules, such as neuroactive amino acids, we have studied the distributional pattern of glutamate immunoreactivity in the pineal gland of the Mongolian gerbil (Meriones unguiculatus) at both light- and electron-microscopic levels. In semithin sections of plastic-embedded pineals, strong glutamate immunoreactivity could be detected in pinealocytes throughout the pineal gland. The density of glutamate immunolabeling in pinealocytes varied among individual cells and was mostly paralleled by the density of immunostaining for synaptophysin, a major integral membrane protein of synaptic and synaptic-like vesicles. Postembedding immunogold staining of ultrathin pineal sections revealed that gold particles were enriched over pinealocytes. In particular, a high degree of immunoreactivity was associated with accumulations of microvesicles that filled dilated process terminals of pinealocytes. A positive correlation between the number of gold particles and the packing density of microvesicles was found in three out of four process terminals analyzed. However, the level of glutamate immunoreactivity in pinealocyte process endings was lower than in presumed glutamatergic nerve terminals of the cerebellum and posterior pituitary. The present results provide some evidence for a microvesicular compartmentation of glutamate in pinealocytes. Our findings thus lend support to the hypothesis that glutamate serves as an intrapineal signal molecule of physiological relevance to the neuroendocrine functions of the gland.

Current methodologies for the study of pineal morphophysiology

Light and electron microscopes, with or without the use of immunohistochemical techniques, have been the instruments of choice for study of the pineal complex even up to recent times. Other morphological technologies have become available during the past decade that, if applied to current questions concerning pineal morphophysiology, could add considerably to our understanding of this complex system. Those technologies discussed include confocal scanning laser microscopy (in conjunction with other techniques including immunohistochemistry and three-dimensional reconstruction), tissue culture methodologies, carbocyanine dyes (i.e., DiI), in situ hybridization, and application of microinjection methodologies. It is suggested that these technologies will be necessary for morphophysiologists to not only collaborate with molecular biologists and biochemists who study the pineal complex, but to corroborate the molecular and biochemical results of our colleagues.

Lack of effect of oxytocin on the numbers of "synaptic" ribbons, cyclic guanosine monophosphate and serotonin N-acetyltransferase activity in organ-cultured pineals of three strains of rats

In addition to the stimulating influence of the sympathetic system on the function of the mammalian pineal gland, neuropeptides such as neuropeptide Y, vasoactive intestinal polypeptide and arginine-vasopressin (AVP) are thought to function as modulators. Since AVP has been shown to influence pineal melatonin synthesis, the aim of the present study was to investigate the possible effects of the second hypothalamic nonapeptide oxytocin (OT), which likewise has been detected in the pineal gland. We therefore studied "synaptic" ribbon (SR) numbers, N-acetyltransferase (NAT) activity and the intracellular concentration of cyclic guanosine monophosphate (cGMP) following in vitro incubation of rat pineals in media containing OT (10(-5) M), noradrenaline (NA, 10(-5) M) or both NA and OT. Pineal glands were derived from rats of three different strains (Sprague-Dawley, Long-Evans and the AVP-deficient strain Brattleboro). Neither morphological nor biochemical analyses showed a difference between control and OT-incubated organs in any of the strains tested. In Brattleboro rats, but not in the other strains, noradrenaline slightly increased the number of SR which was not observed when NA and OT were combined. The addition of NA resulted in distinct augmentation of NAT activity and cGMP content, which were not affected by additional OT application. These results suggest that oxytocin is not crucially involved in the regulation of pineal gland function.