Pineal gland free amino acids and indoles during postnatal development of the rat: correlations in individual glands

Interstitial glial cells of the gerbil pineal gland display immunoreactivity for the metabotropic glutamate receptors mGluR2/3 and mGluR5

Recent studies have strengthened the hypothesis that neuroactive amino acids such as L-glutamate play an important role in the physiology of the mammalian pineal gland. In particular, there is now considerable evidence that L-glutamate is liberated from electron-lucent microvesicles of pinealocytes for a paracrine modulation of melatonin synthesis and release which may at least partially be mediated by the metabotropic glutamate receptor mGluR3. In order to expand our incomplete knowledge of possible pineal target cells and signal transduction mechanisms which are involved in glutamate-dependent intercellular communication, we have performed an immunohistochemical study of the gerbil pineal gland with antibodies directed against the metabotropic glutamate receptors mGluR2/3 and mGluR5. Using microwave irradiation of cryostat sections prior to immunostaining, strong immunoreactivity for both receptor subtypes was constantly observed in a subpopulation of pineal cells. Interestingly, these mGluR-positive cells could be identified as interstitial glial cells since they were labeled by antibodies against the intermediate filament protein vimentin in double immunofluorescence histochemistry. This indicates that interstitial glial cells in the gerbil possess the capacity to express at least two metabotropic glutamate receptors coupled to different intracellular signal transduction pathways. Therefore, it can be concluded that the glutamatergic communication system of the pineal gland may not only enable paracrine crosstalk among pinealocytes but probably also relies on interactions between pinealocytes and interstitial cells analogous to neuronal-glial signaling.

Ontogeny of pineal melatonin rhythm in rats under 12:12-hr and 14:14-hr light:dark conditions

The aim of the study was to determine whether a discrepancy between the genetically determined endogenous circadian period and an abnormally long Zeitgeber period disturbs the development of melatonin synthesis. Breeding pairs of rats were kept under 12:12- or 14:14-hr light:dark (LD) conditions. Pineal melatonin contents in the offspring were measured by radioimmunoassay. At 2 weeks of age high melatonin contents were found from lights-off to lights-on in both conditions suggesting dominance of the photic regulation. At 3 weeks of age the signs of the circadian regulation in the melatonin profiles were evident: a lag period after the light offset in control conditions and a significant decline before the light onset in both conditions. However, in 14:14-hr LD conditions the melatonin content did not decrease to daytime levels until the lights were on. This could suggest incomplete maturation of the circadian system. The phase relationships between the melatonin peak and LD cycle were different in the two conditions. A statistically significant LD difference was first found at the age of 8-10 days in male pups and at 14 days in female pups under both lighting. The results suggest that the abnormally long LD cycle did not cause any major disorders in the development of photic or circadian regulation of the melatonin synthesis.

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.

Characterization of a [3H]glutamate binding site in rat pineal gland: enhanced affinity following superior cervical ganglionectomy

Glutamate, an excitatory neurotransmitter/neuromodulator involved in cell-to-cell communication within the central nervous system, is now believed to play a role in neuroendocrine function. In this study we describe a single, saturable, stereospecific, and temperature-, time-, and pH-dependent binding site for glutamate in the pineal gland of the rat (Kd = 612 +/- 23 nM, Bmax = 3.17 +/- 0.33 pmol/mg protein). After removal of the sympathetic innervation to the pineal gland, [3H]glutamate binding displayed a higher apparent affinity (Kd = 412 +/- 28 nM) (P < 0.05) without a change in binding site number (Bmax = 3.60 +/- 0.24 pmol/mg protein). No difference in [3H]glutamate binding site number was observed in pineal glands obtained from animals sacrificed during the middle of the light and dark periods. These data suggest a possible modulatory role for a glutamate binding site in pineal gland function.

Immunocytochemical and circadian biochemical analysis of neuroactive amino acids in the pineal gland of the rat: effect of superior cervical ganglionectomy

Semiquantitative immunocytochemistry by immuno-gold techniques revealed differences in the spatial distribution of glutamate, glutamine, and taurine within the pineal gland, with greatest labeling over pinealocytes, glia, and endothelia, respectively. At the subcellular level, glutamate labeling tended to be highest over pinealocyte synaptic ribbons and mitochondria, and lowest over lipid inclusions. Pineal levels of glutamate, glutamine and taurine, as measured by high performance liquid chromatography, did not vary over a light: dark cycle. Superior cervical sympathetic denervation, which abolishes pineal melatonin synthesis, resulted in a nearly 50% reduction in pineal glutamate levels, but had no effect on levels of glutamine and taurine. Other amino acids (alanine, arginine, aspartate, serine) were reduced by 23%-33% following sympathectomy. These data suggest an important role for glutamate in pinealocyte function(s) possibly related to the noradrenergic innervation of the gland.