Adrenergic and VIP stimulation of cyclic AMP accumulation in ovine pineals

Somatostatin and somatostatin receptors in the pig pineal gland during postnatal development: an immunocytochemical study

An immunohistochemical study of the pineal gland of the domestic pig was carried out using rabbit antisera raised against synthetic peptide fragments corresponding to different amino acid sequences of the prosomatostatin, the somatostatin-14, and the somatostatin-28 molecule. The study was supplemented by immunohistochemical staining with rabbit antisera raised against five subtypes of somatostatin receptors. The pineal glands were taken from the newborn, 21-day-old and 7-month-old pigs. Immunoreactive nerve fibers and cells were observed in the pineal gland with all the antisera against somatostatin and prosomatostatin. The nerve fibers were located throughout the pineal gland-in the capsule, connective septa, and parenchyma-with the highest density in proximo-ventral part of the gland. The somatostatin positive fibers were also found in the habenular and posterior commissurae areas. Somatostatin-immunoreactive cell bodies were observed mostly in the central part of the gland. These results point to the existence of two somatostatin sources in the pig pineal gland: 1) nerve fibers, probably of central origin; and 2) cells that may represent intrapineal neurons or specialised pinealocytes. A clear difference in the immunoreactivity between newborn, 21-day-old, and 7-month-old pigs was found. Generally, the density of nerve fibers was lower in adult than young animals. The number of the cells also decreased with age. By using the antisera against the five somatostatin receptors, only sst3 - receptor immunoreactivity could be detected. The receptor-immunoreactivity was confined to varicose and smooth fibers and some cells. The sst(3)-receptor positive structures were localised in all parts of the gland and their number was higher in younger pigs.

Neuropeptide Y in the mammalian pineal gland

The present review describes the anatomy of the neuropeptide (NPY)ergic innervation of the mammalian pineal gland with emphasis on the rat. The proNPY-molecule is post-translationally processed by a single cleavage to neuropeptide Y (NPY) and a C-terminal peptide of NPY (CPON). NPY is C-terminally amidated, and the amidation is essential for binding of NPY to its corresponding receptor(s). Since no proNPY has been detected in rat pineal extracts, it is considered that proNPY is immediately processed to its final products in the gland. In the rat, numerous NPY- and CPON-immunoreactive (ir) nerve fibers are present in the capsule of the superficial pineal gland and in the pineal parenchyma, mostly related to the connective tissue spaces and the vasculature of the gland, but also present between the pinealocytes. Furthermore, a substantial number of fibers was observed in the deep pineal gland, the pineal stalk, and the underlying epithalamus. Occasionally, NPY- or CPON-immunoreactive fibers were found adjacent to the stria medullaris and in the posterior commissure, which could be followed to the adjacent deep pineal gland. At the ultrastructural level, the NPY-immunoreactivity was confined in boutons containing large granular vesicles (100-200 nm) as well as small (40-60 nm) granular vesicles. Some terminals were located in very close apposition to the pinealocyte cell membrane. Terminals were identified in perivascular spaces, but synaptic contacts between the immunoreactive terminals and pinealocytes were never observed. These data show that NPY is highly concentrated in nerve fibers throughout the rat pineal complex. Double-fluorescence histochemistry using tyrosine hydroxylase as marker for catecholaminergic fibers and NPY revealed that nearly all NPYergic fibers co-stored tyrosine hydroxylase in the superficial pineal gland. A minor portion of both immunoreactivities was not colocalized. In accordance, about 65% of the neurons in the superior cervical ganglion contained both CPON and tyrosine hydroxylase. In bilateral superior cervical ganglionectomized rats, a few NPY-ir nerve fibers remained mostly in the pineal capsule, but few fibers were also found in the superficial pineal parenchyma. Contrarily, only a moderate decrease was observed in the number of immunoreactive fibers in the deep pineal gland, and no reduction was observed in the adjacent epithalamus. In the ganglionectomised rats, co-localisation of tyrosine hydroxylase and NPY in intrapineal nerve fibers was not observed either in the superficial pineal gland, nor in the deep pineal gland. These results together with the available literature show that NPY is a sympathetic transmitter, and its actions in the pineal gland are, therefore, associated with the well-documented roles of noradrenaline. Possible roles of NPY in pineal biochemistry and physiology are discussed.

Vipergic innervation of the mammalian pineal gland

The present article reviews the literature relative to VIP- and PHI-containing nerve fibers in the pineal gland of mammals. The article summarizes data on the presence and distribution of the two peptides in the brain of mammals, their role in neuronal metabolism, and the significance and origin of VIPergic and PHIergic cerebrovascular nerve fibers. Special emphasis is placed on VIP- and PHI-containing nerves in the pineal gland. The morphology of the fibers, the nature of the innervation, and the distribution of immunoreactive nerves within the pineal gland are examined. The review discusses the nature of the classical and "central" innervation of the pineal gland. The possible site of origin of pinealopetal VIPergic and PHIergic fibers is investigated, with special reference to ganglia of the head, and particularly to the pterygopalatine, otic, and trigeminal ganglia. The nature of VIP (and PHI) receptors is examined with reference to the most recent acquisitions in the field. Based on the data, a role for VIP (and PHI) in pineal metabolism is discussed.

Pharmacological characterisation of oxytocin binding sites in the ovine pineal gland

Both oxytocin (OT) and [Arg8]vasopressin (AVP) are found within the ovine pineal gland and may function to modulate melatonin secretion. However, the receptors which mediate the actions of these peptides have yet to be characterised. Preliminary studies of ovine pineal microsomal cell membranes showed binding of [3H]OT (79+/-9 fmol/mg) 10 times greater than binding of [3H]AVP (8+/-3 fmol/mg). Saturation studies using either [3H]OT or the selective OT receptor ligand [125I]d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin (OTA) revealed high affinity, single site kinetics (Kd = 1.72+/-0.32 nM; Bmax = 68+/-18 fmol/mg). Binding of [3H]AVP was more effectively displaced by OT than AVP, suggesting that binding may be due to cross-reaction with the OT binding site. Displacement of [3H]OT using a range of selective agonists and antagonist analogues revealed pharmacological characteristics similar to [3H]OT binding sites in the ovine and rat uterus. These data show that the ovine pineal expresses a high density of OT binding sites which may participate in the regulation of melatonin secretion.

Density of peptide histidine-isoleucine- and vasoactive intestinal peptide-immunoreactive nerve fibers in the sheep pineal gland is not affected by superior cervical ganglionectomy

Peptide histidine-isoleucine (PHI) is a regulatory peptide, synthesized as part of the same propeptide that includes also vasoactive intestinal peptide (VIP). The present study describes the distribution of PHI-immunoreactive nerve fibers in the sheep pineal organ and compares their location with the distribution of VIP-immunoreactive fibers in both normal and superior cervical ganglionectomized sheep in order to elucidate the origin of the PHI/VIP immunoreactive nerve fibers. Several PHI-immunoreactive nerve fibers were present in the meninges and in the pineal capsule. Numerous positive nerve fibers entered the pineal gland and travelled within connective tissue spaces. Individual PHI-positive nerve fibers were either smooth, without specialization, or varicose. Generally VIP- and PHI-immunoreactive fibers were located close to connective septa and blood vessels. However, many PHIergic and VIPergic fibers possessing varicosities of variable sizes were also dispersed between pinealocytes. The distribution, density, and morphology of PHI- and VIP-immunoreactive fibers in the sheep pineal gland were similar. In superior cervical ganglionectomized animals, intrapineal VIP- and PHI-immunoreactive nerve fibers were present with the same density as in control animals. In agreement, the concentration of immunoreactive VIP and PHI did not change after ganglionectomy. No VIP- and PHI-immunoreactive cell bodies were observed in the superior cervical ganglia. Thus this study shows that the intrapineal VIP- and PHI-immunoreactive nerve fibers do not originate from the sympathetic superior cervical ganglion.

The presence of nerve fibers immunoreactive for vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and preproVIP(111-122) in the mouse pineal gland

A low to moderate number of vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI)-immunoreactive nerve fibers with prominent boutons-en-passage were demonstrated in the pineal gland of the mouse. The two peptides, which are parts of the same precursor molecule, were distributed identically in the gland. Positive fibers were present in the connective tissue septae in the gland, in the pineal capsule, and in the pineal parenchyma. No VIP-PHI-immunoreactive elements were found in the deep pineal gland, in the pineal stalk, or in habenular and posterior commissures. This morphological distribution of immunoreactive nerve fibers, which is similar to the distribution in other mammals, indicates that the VIP/PHI fibers of the mouse pineal gland originate exclusively from perikarya in a peripheral ganglion, presumably one of the cholinergic ganglia of the head. No evidence for a VIPergic central innervation was found. VIP and PHI are connected via a bridging peptide equivalent to amino acids 111-122 of the precursor (preproVIP(111-122)). In order to demonstrate the possible existence of this peptide in intra-pineal nerve fibers, antisera directed against a synthetic sequence identical to preproVIP(111-122) and immunohistochemistry were applied. PreproVIP(111-122)-immunoreactive nerve fibers were observed in the mouse pineal gland, with the same distribution pattern and morphology as those immunoreactive for VIP and PHI. To quantify the peptide-immunoreactivities, 50 mice pineals were pooled, extracted, and the concentrations were measured radioimmunologically. The concentrations of the VIP and preproVIP(111-122) immunoreactivities were 1.7 and 2.0 pmol/g, respectively, whereas the concentration of PHI was 0.9 pmol/g.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and localization of delta opioid binding sites in the bovine pineal gland

Opioid binding sites in the bovine pineal were characterized using the highly selective delta opioid agonist 3H-[D-Pen2,pCl-Phe4,D-Pen5] enkephalin (DPDP(C1)E). Pineal membranes possess a single class of high affinity binding sites for this delta ligand (Kd = 0.26 nM; Bmax = 250 fmol/mg protein). The specific opioid antagonist naloxone dose dependently inhibited 3H-DPDP(C1)E binding, confirming that this ligand is indeed binding to opioid receptors. The delta selective ligands deltorphin and [D-Pen2,5]enkephalin (DPDPE) were much more potent than the mu selective compounds dermorphin and [D-Ala2,MePhe4,Gly5- ol]enkephalin (DAMGO) in inhibiting 3H-DPDP(C1)E binding. These results demonstrate that in bovine pineal membranes, DPDP(C1)E binds to delta opioid sites. Autoradiographic studies showed a uniform distribution of 3H-DPDP(C1)E binding over the bovine pineal in the sections we analyzed. This distribution suggests that delta opioid binding sites are associated with pinealocytes which account for the majority of cell types in the pineal. However, it is not possible to rule out that these receptors may also be associated with other cell types which are present in the bovine pineal. The density and widespread distribution of delta opioid receptors supports the hypothesis that endogenous opioid peptides directly modulate pineal function.

Vasoactive intestinal peptide stimulates type II thyroxine 5'-deiodinase and N-acetyltransferase activities in dispersed pineal cells of euthyroid and hypothyroid rats

The effect of vasoactive intestinal peptide (VIP) on thyroxine type II 5'-deiodinase (5'-D) and N-acetyltransferase (NAT) activities were studied using pineal cells of euthyroid and hypothyroid rats. VIP activated 5'-D activity in a dose-dependent manner in both euthyroid and hypothyroid animals. However, basal and VIP stimulated activity was higher in pinealocytes from hypothyroid than in cells from euthyroid rats. VIP was also able to stimulate NAT activity but hypothyroidism did not induce modifications in its activity. Both 5'-D and NAT activities were stimulated not only by VIP, but also by isoproterenol, a beta-adrenergic receptor agonist, and forskolin, a potent activator of adenylate cyclase activity. The results suggest that VIP may be involved in the physiological regulation of pineal 5'-D activity.

Autoradiographic localization of dopaminergic and noradrenergic receptors in the bovine pineal gland

Dopamine and norepinephrine are involved in regulation of melatonin synthesis in the pineal gland. In bovine pineal gland, D1- and D2-dopaminergic and alpha 1-adrenergic receptors have been characterized pharmacologically in several laboratories, while beta 1-adrenergic receptors have been studied using physiological technique. The current study presents a quantitative autoradiographic analysis of these four dopaminergic and noradrenergic receptors in bovine pineal gland. The density order of the receptors is D1 greater than alpha 1 greater than D2 greater than or equal to beta 1. The Bmax of dopamine D1 receptors is about 5 to 6 times higher than the Bmax for alpha 1-adrenergic receptors and about 20 times higher than the Bmax values for beta 1-adrenergic and D2-dopaminergic receptors. Dopamine D1 receptors are significantly denser in the pineal cortex than in the medulla. Both dopamine receptors are more concentrated in the distal area than in the proximal area (close to the habenula), whereas both noradrenergic receptors are homogeneously distributed along the longitudinal axis. Only D1-dopaminergic receptors display a heterogeneous distribution between the superior and the inferior areas, being denser in the inferior area. The observation of a much higher concentration of D1-dopaminergic receptors relative to the other receptors suggests an important role for dopamine in the regulation of bovine pineal physiology.

Tyrosine hydroxylase- and neuropeptide Y-immunoreactive nerve fibers in the pineal complex of untreated rats and rats following removal of the superior cervical ganglia

The distribution of tyrosine hydroxylase (TH)- and neuropeptide Y (NPY)-immunoreactive(IR) nerve fibers in the pineal complex was investigated in untreated rats and rats following bilateral removal of the superior cervical ganglia. In normal animals, a large number of TH- and NPY-IR nerve fibers were present in the pineal capsule, the perivascular spaces, and intraparenchymally between the pinealocytes throughout the superficial pineal and deep pineal gland. A small number of TH-IR and NPY-IR nerve fibers were found in the posterior and habenular commissures, a few fibers penetrating from the commissures into the deep pineal gland. To elucidate the origin of these fibers, the superior cervical ganglion was removed bilaterally in 10 animals, and the pineal complex was examined immunohistochemically. Two weeks after the ganglionectomy, the TH-IR and NPY-IR nerve fibers in the superficial pineal gland had almost completely disappeared. On the other hand, in the deep pineal and the pineal stalk, the TH-IR and NPY-IR fibers were still present after ganglionectomy. These data show that the deep pineal gland and the pineal stalk possess an extrasympathetic innervation by TH-IR and NPY-IR fibers. It is suggested that the extrasympathetic TH-IR and NPY-IR nerve fibers innervating the deep pineal and the pineal stalk originate from the brain.

Beta-adrenergic stimulation increases cAMP and melatonin production in ovine pinealocyte cultures

An ovine pinealocyte suspension culture method, using glands from young sheep as a source of cells, has been developed to study mechanisms of pineal regulation in this species. Cell suspensions are obtained by enzymatic digestion (preliminary trypsinization followed by collagenase/pronase treatment) of pineal glands and these cells may be usefully retained in culture for up to 48 hr. Initial characterization of cyclic adenosine monophosphate and melatonin production responses to adrenergic stimuli (norepinephrine, phenylephrine, isoproterenol) in the absence and presence of antagonists (propranolol, prazosin) by pinealocytes in suspension culture indicate that a beta-adrenergic receptor-mediated mechanism is primarily involved in transduction of the adrenergic signal to melatonin synthesis. This is in agreement with data from earlier work on the sheep using short-term tissue incubations but contrasts with in vivo evidence suggesting a predominant alpha 1-adrenergic receptor-mediated mechanism.

Innervation of the mink pineal gland with neuropeptide Y (NPY)-containing nerve fibers. An experimental immunohistochemical study

An immunohistochemical investigation of the mink pineal gland was performed by use of antibodies raised in rabbits against neuropeptide Y (NPY) and Cys-NPY (32-36)-amide recognizing neuropeptide Y with an amidation at position 36 (NPYamide). NPY-immunoreactive nerve fibers were located predominantly in the rostral part of the pineal gland and in the pineal stalk. Immunoreactive nerve fibers were found throughout the pineal gland, but the number of fibers in the caudal part of the gland was low. The fibers were present both in the perivascular spaces and between the pinealocytes. Many NPY-immunoreactive fibers were also located in the posterior and habenular commissures; some of these fibers were connected with the fibers in the rostral part of the mink pineal gland, indicating that at least some of the NPY-immunoreactive nerve fibers are of central origin. The nerve fibers immunoreactive to amidated NPY were distributed in a similar manner. However, the number of fibers immunoreactive to NPYamide was lower than the number of fibers immunoreactive to NPY itself. After removal of the superior cervical ganglia bilaterally 22 days or 12 months before sacrifice, NPY-immunoreactive nerve fibers remained in the gland. This immunohistochemical study of the mink pineal gland therefore shows that the NPY/NPYamide-immunoreactive nerve fibers innervating the pineal gland in this species are a component of the central innervation or originate from extracerebral parasympathetic ganglia.

Vasoactive intestinal peptide stimulates melatonin release from perifused pineal glands of rats

The rat pineal gland is known to release melatonin in response to noradrenergic stimulation. The effect of vasoactive intestinal peptide (VIP), one of the neuropeptides present in the pineal, was examined on perifused rat pineal glands. VIP stimulated melatonin release with a dose-dependent effect above 10(-7) M. In regard of kinetic characteristics, the pattern of melatonin release after VIP stimulation was similar to that after isoproterenol stimulation. 10(-6) M VIP-stimulated melatonin release was not altered when the pineal glands were treated with 10(-5) M propranolol (a beta-adrenergic antagonist) or 10(-5) M prazosin (an alpha 1-adrenergic antagonist). Thus VIP has a noradrenergic-independent effect on melatonin secretion. Conversely, this VIP effect is greatly inhibited by the specific action of a VIPergic antagonist. This suggests that VIP acts on melatonin synthesis through its own binding sites. This study demonstrates that melatonin secretion from rat pineal glands may be elicited through a VIPergic system which is independent of the well-known noradrenergic system.

Melatonin biosynthesis in the mammalian pineal gland

Rhythmic production of melatonin by the mammalian pineal occurs in response to noradrenergic stimulation which produces a cascade of biochemical events within the pinealocyte. In the rat, massive changes in NAT activity result from an increase in intracellular c-AMP levels produced by a synergistic interaction whereby an alpha 1 activation amplifies beta-adrenergic stimulation. The intracellular events mediating this effect are described. A major aspect of the temporal control of melatonin production is the programmed down-regulation of responses to noradrenergic stimulation once the initial surge of c-AMP is produced. Noradrenergic activation of the gland also influences other enzymic functions, including tryptophan hydroxylase and HIOMT activities, and produces a dramatic increase in intracellular c-GMP levels. Other neurotransmitters and neuropeptides, e.g. VIP, may also influence pineal function and comparisons are made between the rat, the subject of the bulk of experimental studies, and other species.

Kinetic study of melatonin release from rat pineal glands using a perifusion technique

In previous studies, noradrenaline was found to elicit a rise of melatonin secretion through activation of typical beta-adrenergic receptors. In the present study, a perifusion system was developed to characterize the kinetics of melatonin release from rat pineal glands. Isolated pineal glands from adult male rats were continuously perifused for 15 h in a Krebs-Ringer solution, and the concentration of melatonin in the effluent perifusate was monitored using a specific radioimmunoassay. The rate of release of melatonin declined during the first 3-4 h of perifusion and then remained fairly stable for at least 11 h. The spontaneous release of melatonin was around 20 pg per min and per gland. When pineal glands were stimulated with isoproterenol, melatonin release output linearly increased for at least 2 h after the stimulation. The increase in melatonin release depended on the isoproterenol concentration and on the duration of the stimulation. The analysis of the pattern of melatonin secretion by a single rat pineal gland showed that the secretion was irregular but did not present a clear feature of pulsatile or oscillatory release over a 11 h-long study. The perifusion system was found useful in order to follow the characteristics of melatonin release from pineal glands and should allow investigations of neuronal or hormonal control of pineal gland activities.

Vasoactive intestinal peptide-like immunoreactivity in rodent taste cells

The neuropeptide vasoactive intestinal peptide was localized to taste buds of the posterior tongue regions of hamsters and rats by immunocytochemical techniques. Tissue sections, taken from foliate and circumvallate papillae, generally revealed taste buds in which all cells were immunoreactive; however, occasionally some taste buds were found to contain highly reactive individual cells adjacent to non-reactive cells. Additionally, some non-reactive taste buds were observed. Taste buds that displayed vasoactive intestinal peptide-like immunoreactivity usually had a tendency for much darker staining at the apical ends of the cells than the basal ends, suggesting a polar cytoplasmic distribution of the peptide. The multi-functional roles of vasoactive intestinal peptide in other physiological systems combined with both its cytoplasmic localization in taste cells and the known histochemistry/ultrastructure of taste cells raises interesting speculations of this peptide's function in gustation that include secretion, stimulation of a second messenger system, and neuromodulation.

Evidence for dual adrenergic receptor regulation of ovine pineal function

Both alpha 1- and beta-adrenergic receptors are present on ovine pineals. In the rat these two receptors interact so that activation of the alpha 1-receptor potentiates the beta-receptor-mediated changes in cyclic AMP and the correlated changes in pineal N-acetyltransferase (EC 2.3.1.87). Here we investigate possible interactions between alpha 1- and beta-receptors through changes in cyclic AMP and assess the importance of each receptor to the melatonin response in ovine pineal punches/slices in vitro. The adrenergic agonists isoproterenol (ISO), noradrenaline (NA), and phenylephrine (PHE) stimulated dose-dependent changes in cyclic AMP with the order of potency of ISO greater than NA greater than PHE, consistent with their relative binding affinities for the beta-receptor. The beta-receptor antagonist, propranolol, showed dose-dependent inhibition of the ISO effect, whereas the alpha 1-selective antagonist, prazosin, had no effect. The S-shape of the stimulation and inhibition curves for ISO reflects cyclic AMP changes mediated by the beta-receptor only without interaction through the alpha 1-receptor. Each of the adrenergic agonists stimulated indistinguishable dose-dependent increases in melatonin release. The ability of PHE to stimulate changes in melatonin release in the absence of concomitant changes in cyclic AMP indicates an important role for the alpha 1-receptor. Prazosin inhibits this response, substantiating this conclusion. However, as propranolol is also inhibitory, it seems that the alpha 1-receptor response is absolutely dependent upon a small level of beta-receptor stimulation, thus providing evidence for adrenergic receptor interaction at a step other than cyclic AMP.