A possible role for cyclic guanosine monophosphate in the rat pineal gland

EF hand-mediated Ca- and cGMP-signaling in photoreceptor synaptic terminals

Photoreceptors, the light-sensitive receptor neurons of the retina, receive and transmit a plethora of visual informations from the surrounding world. Photoreceptors capture light and convert this energy into electrical signals that are conveyed to the inner retina. For synaptic communication with the inner retina, photoreceptors make large active zones that are marked by synaptic ribbons. These unique synapses support continuous vesicle exocytosis that is modulated by light-induced, graded changes of membrane potential. Synaptic transmission can be adjusted in an activity-dependent manner, and at the synaptic ribbons, Ca²⁺- and cGMP-dependent processes appear to play a central role. EF-hand-containing proteins mediate many of these Ca²⁺- and cGMP-dependent functions. Since continuous signaling of photoreceptors appears to be prone to malfunction, disturbances of Ca²⁺- and cGMP-mediated signaling in photoreceptors can lead to visual defects, retinal degeneration (rd), and even blindness. This review summarizes aspects of signal transmission at the photoreceptor presynaptic terminals that involve EF-hand-containing Ca²⁺-binding proteins.

Biphasic 24-Hour Variations in Cyclic GMP Accumulation in the Rat Pineal Gland Are Due to Corresponding Changes in the Activity of Cytosolic and Particulate Guanylate Cyclase

Various parameters of the rat pineal gland display a 24-h rhythm. However, nothing is known about possible 24-h variations in cyclic GMP (cGMP) metabolism. In the present study, 24-h variations in pineal gland cGMP accumulation were investigated by determining the increase in cGMP level with and without inhibitors of phosphodiesterase at different time points over a light/dark cycle (12/12 h). Furthermore, the activity of guanylate cyclase (GC) was determined under substrate-saturated conditions regarding the cytosolic and particulate forms of the enzyme. It has been found that cGMP accumulation and GC activity display biphasic 24-h variations with two peaks--one approximately 7 h after lights "on" and the other approximately 7 h after lights "off." The activity of cytosolic GC remains unchanged in the presence of the nitric oxide (NO) synthesis inhibitor N-monomethyl-L-arginine, indicating that 24-h variations in the activity do not reflect changes in the synthesis of the GC stimulator NO.

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.

Plasticity of synaptic ribbons of the rat pineal gland in vitro--minor effects of electrical stimulation

Synaptic ribbons (SRs) of mammalian pinealocytes exhibit day/night changes in number and size, changes that are apparently regulated by the suprachiasmatic nucleus via postganglionic sympathetic nerve fibres. Since the neural control of SR changes is far from clear and as pinealocytes produce action potentials, we undertook to investigate whether electrical stimulation affects SR changes. Isolated rat pineal glands removed during the daytime were kept in vitro for 0, 30, 60, 90 or 120 min, with or without continuous electrical stimulation (1 mA, 1 Hz), followed by the quantification of SR profiles (SRPs) by transmission electron microscopy. SRs were categorised as to whether they lay less than 100 nm away from the pinealocyte plasmalemma (SRPs(near)) or more distant from it (SRPs(dist)) and the lengths of the profiles were measured. Cultured pineal organs showed a significant numerical depression of SRPs(near), irrespective of whether the organs had been electrically stimulated or not. SRPs(near) length revealed a significant increase at 60 min in unstimulated control tissue and at 30 min in electrically stimulated glands. SRPs(dist) length decreased significantly at 30 min in control glands and after 60 min in electrically stimulated glands. Thus, action potentials inside the pineal gland appear to be minor factors regulating SR numbers. In future pineal studies, SRPs(near) and SRPs(dist) should be considered separately as they differ in plasticity.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Trigeminal innervation of the mammalian pineal gland

There is evidence that the trigeminal (Gasserian) ganglia innervate the mammalian pineal gland and serve in its regulation in addition to the sympathetic and cholinergic as well as further influences. By means of immunohistochemical methods, previous studies demonstrated fibers containing calcitonin gene-related peptide (CGRP-LI) or substance P (SP) in the superficial pineal of various mammalian species. In addition, SP and the related tachykinin, neurokinin A, were detected by radioimmunoassay and HPLC, respectively, in the rat and human pineal gland. In the present study, retrograde neuronal tracing upon injection of a tracer substance into the superficial pineal gland of rats was used in combination with immunohistochemistry to show that trigeminal ganglionic neurons innervate the gland, and that a considerable amount of these neurons are also CGRP- or SP-immunoreactive. These results reveal that afferent neuropeptidergic entering the superficial pineal gland originate from the trigeminal ganglia. The present paper reviews the evidence for a CGRP- and SP-ergic innervation of the mammalian pineal gland and discusses the possible role of these neuropeptides with regard to pineal function.

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.

Cholinergic signaling in the rat pineal gland

1. Innervation of the mammalian pineal gland is mainly sympathetic. Pineal synthesis of melatonin and its levels in the circulation are thought to be under strict adrenergic control of serotonin N-acetyltransferase (NAT). In addition, several putative pineal neurotransmitters modulate melatonin synthesis and secretion. 2. In this review, we summarize what is currently known on the pineal cholinergic system. Cholinergic signaling in the rat pineal gland is suggested based on the localization of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), as well as muscarinic and nicotinic ACh binding sites in the gland. 3. A functional role of ACh may be regulation of pineal synaptic ribbon numbers and modulation of melatonin secretion, events possibly mediated by phosphoinositide (PI) hydrolysis and activation of protein kinase C via muscarinic ACh receptors (mAChRs). 4. We also present previously unpublished data obtained using primary cultures of rat pinealocytes in an attempt to get more direct information on the effects of cholinergic stimulus on pinealocyte melatonin secretion. These studies revealed that the cholinergic effects on melatonin release are restricted mainly to intact pineal glands since they were not readily detected in primary pinealocyte cultures.

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)

Elevation of cyclic GMP levels in the rat pineal gland induced by nitric oxide

The present paper reports that nitric oxide (NO) released by sodium nitroprusside (SNP) is a potent activator of rat pineal cyclic GMP production without affecting cyclic AMP synthesis. Other drugs such as isoproterenol, vasoactive intestinal peptide, and peptide histidine isoleucine were ineffective in stimulating cyclic GMP production, but activated cyclic AMP production. However, L-arginine, the physiological precursor of NO, did not activate either cyclic GMP or NO synthesis. Because L-arginine failed to activate cyclic GMP production, results suggest that NO is not produced in the pineal gland, but behaves as a potent regulator of this cyclic nucleotide.

Intravitreal application of membrane-permeable analogs of cyclic GMP increases serotonin N-acetyltransferase (NAT) activity in retinas of light-exposed chicks: comparison with the effect of cyclic AMP analogs and darkness

Intravitreal administration of membrane-permeable analogs of cGMP and cAMP markedly enhanced serotonin N-acetyltransferase (NAT) activity in retinas of light-exposed chicks. The effects of cGMP analogs were usually weaker than the actions of cAMP analogs, however, they were more pronounced than the effects of non-cyclic derivative of GMP (i.e., 8-bromo-GMP). Combined treatment with dibutyryl-derivatives of cGMP and cAMP yielded NAT values that were similar to the values produced by the compounds administered separately, whereas NAT activity in chicks receiving a combination of db-cGMP and aminophylline was higher than the enzyme activity produced by the drugs individually. Neither db-cGMP nor aminophylline affected hydroxyindole-O-methyltransferase (HIOMT) activity in retinas of light-exposed chicks. It is hypothesized that cGMP analogs enhance the retinal NAT activity indirectly, via cAMP.

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.

Involvement of cyclic guanosine monophosphosphate (cGMP) and cytosolic guanylate cyclase in the regulation of synaptic ribbon numbers in rat pineal gland

In the rat pineal gland N-acetyltransferase (NAT) activity and synaptic ribbon (SR) numbers display a circadian rhythm. It is well-known that NAT activity is regulated by adrenergic mechanisms involving cyclic adenosine monophosphate (cAMP) as a second messenger. However, the mechanism involved in the regulation of SR numbers has not been established so far. In the present in vitro study, we have investigated the effects of 8-bromo-cyclic guanosine monophosphate (8-bromo-cGMP), a cyclic guanosine monophosphate (cGMP) analog, and stimulation of guanylate cyclase on SR numbers. Incubation with 8-bromo-cGMP increased SR numbers in a dose- and time-dependent manner. Further, stimulation of the cytosolic guanylate cyclase also resulted in increased SR numbers. Adrenergic agonists stimulated cGMP but did not alter SR numbers. These findings suggest that cGMP is involved as a second messenger in the regulation of SR numbers. Since the adrenergically stimulated increase in cGMP did not influence SR numbers, a non-adrenergic cGMP metabolic pathway seems to be involved in the regulation of SR numbers in the rat 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)