Thyroid hormone regulation of adrenergic receptors and beta-adrenergic responsiveness in the rat submandibular gland

The effects of altered thyroid function on the sensitivity of isoproterenol induced secretion of saliva and in the characteristics of adrenergic receptors from the rat submandibular gland were examined. Hyperthyroidism produced an increased sensitivity to beta-adrenergic stimulation of the gland, and this phenomenon was associated with an increase in the number of beta and alpha 1-adrenoceptors. On the other hand, surgical thyroidectomy produced a decrease sensitivity to isoproterenol stimulation of the submandibular gland and a diminished density of beta-adrenoceptors. In this case, no changes in alpha-adrenoceptors were observed. These results are discussed emphasizing the correlation between the functional control of saliva secretion and the adrenergic receptors in different thyroid states.     

Effects of adrenergic agonists and antagonists on the numbers of synaptic ribbons in the rat pineal gland

In the pineal gland numbers of synaptic ribbons (SR) undergo day/night changes which parallel the rhythm of melatonin synthesis. Since pineal biosynthetic activity is controlled by activation of adrenoreceptors, we investigated the effects of adrenergic agonists and antagonists on pineal synaptic ribbon numbers and N-acetyltransferase (NAT) activity, the key enzyme of melatonin synthesis in rats. In vivo application of the beta-adrenergic antagonist propranolol decreased melatonin synthesis when given during the dark phase but did not affect SR numbers. Treatment during daytime with the beta-adrenergic agonist isoproterenol increased pineal NAT activity whereas SR numbers did not change. Norepinephrine stimulated NAT activity in vitro in a dose-dependent manner, but did not elevate SR numbers. Incubation with an analog of the second messenger cyclic adenosine monophosphate increased both NAT activity and SR numbers. These results suggest that the beta-adrenergic system does not play a decisive role in the regulation of the nocturnal increase in SR numbers observed in the rat pineal gland.     

Monoamino-oxidase activity in rat pineal gland. Histochemical studies

Monoamine-oxidase (MAO) activity was detected in rat pineal gland with dopamine, 5-hydroxytryptamine (5-HT), norepinephrine and tryptamine as substrates, and nitroblue tetrazolium salt as electron acceptor. Pinealocytes stained deeply when 5-HT was the substrate. Dopamine and tryptamine substrates gave similar patterns, with moderate activity in the pinealocytes. Norepinephrine reactivity was detected in the nerve-endings.     

Antipunishment effects of acute and repeated administration of phencyclidine and NPC 12626 in rats.

The effects of phencyclidine (PCP) and NPC 12626 on punished responding were examined using a modified Geller-Seifter procedure in rats. Both drugs are known to antagonize N-methyl-D-aspartate (NMDA) receptor mediated neurotransmission, albeit at different sites on the NMDA receptor complex. Rats were trained to lever press for food reinforcement under a multiple schedule, with responding in one component reinforced under a fixed-interval 60-sec schedule, while each response in the other component resulted in both food and brief electric shock. Both PCP and NPC 12626 produced selective increases in punished responding, although the effects were not as large as those produced by chlordiazepoxide. Repeated daily administration of each of these drugs for 6 days resulted in increases in punished responding during different portions of the treatment. A 5 mg/kg dose of chlordiazepoxide produced increases over the last 2 days of administration. PCP (2 mg/kg) produced an increase only during the second session, whereas NPC 12626 (30 mg/kg) produced increases for all but the first and fifth days of the 6-day regimen. Both competitive and noncompetitive NMDA antagonists can have antipunishment effects in this model.     

Cholinergic signaling in the rat pineal gland

Summary 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 serotoninN-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.     

Opposite effects on feeding of suprachiasmatic nucleus neuropeptide Y administration in rats.

The effects of injecting or infusing neuropeptide Y (NPY) into the suprachiasmatic nucleus of rats on patterns of individual macronutrient and water intake were examined during the following 2 h and also across 12 and 24 h light/dark cycles. Increased total energy intake (218 and 170%) and energy intake from the dextrin/sucrose diet (499 and 247%) were observed in the 2 h following injection of 100 pmol NPY at early light and early dark, respectively, and in the following 24 h (total energy: 67%, dextrin/sucrose: 73%). Nocturnal casein energy intake was also increased (258%) following NPY injection. Continuous infusion of 10 pmol/h of NPY suppressed nocturnal total energy (36%) and dextrin/sucrose intake (36%) as well as 24 h energy intake from casein (43%). These results demonstrate divergent effects of NPY subsequent to different mode of administration.     

12- and 15-lipoxygenases in rat pineal gland

A whole organ or a homogenate of rat pineal gland was incubated with arachidonic Acid. Two predominant metabolites were identified by mass spectrometry to be 12-hydroxy-5,8,10,14-eicosatetraenoic acid and 10-hydroxy-11,,12-epoxy-5,8,14-eiconsatrienoic acid. 15-Hydroxy-5,8,11,13-eicosatetraenoic acid was also formed in a smaller amount. In addition, peroxy acids appeared rapidly only at the initial state of reaction. In various parts of rat brain the 12-lipoxygenase activity was by far the highest in pineal gland, and less than 5% of the activity was found in pituitary gland and hypothalamus.     

12- and 15-lipoxygenases in rat pineal gland

A whole organ or a homogenate of rat pineal gland was incubated with arachidonic Acid. Two predominant metabolites were identified by mass spectrometry to be 12-hydroxy-5,8,10,14-eicosatetraenoic acid and 10-hydroxy-11,12-epoxy-5,8,14-eicosatrienoic acid. 15-Hydroxy-5,8,11,13-eicosatetraenoic acid was also formed in a smaller amount. In addition, peroxy acids appeared rapidly only at the initial stage of reaction. In various parts of rat brain the 12-lipoxygenase activity was by far the highest in pineal gland, and less than 5% of the activity was found in pituitary gland and hypothalamus.     

Determination of rat pineal gland melatonin content by a radioimmunoassay.

Melatonin content in individual rat pineal glands was measured by radioimmunoassay (RIA). The RIA used can very reliably detect as little as 50 pg of melatonin. The various precursors, analogues, and the metabolite of melatonin (6-hydroxymelatonin) which were tested for cross-reactivity were not recognized by the antibody. The effects on melatonin levels in rat pineal glands following the administration of L-tryptophan, 5-hydroxy-L-tryptophan, serotonin, N-acetylserotonin, melatonin and pargyline are also presented.     

Rhythmic control of endocannabinoids in the rat pineal gland

Endocannabinoids modulate neuroendocrine networks by directly targeting cannabinoid receptors. The time-hormone melatonin synchronizes these networks with external light condition and guarantees time-sensitive and ecologically well-adapted behaviors. Here, the endocannabinoid arachidonoyl ethanolamide (AEA) showed rhythmic changes in rat pineal glands with higher levels during the light-period and reduced amounts at the onset of darkness. Norepinephrine, the essential stimulus for nocturnal melatonin biosynthesis, acutely down-regulated AEA and other endocannabinoids in cultured pineal glands. These temporal dynamics suggest that AEA exerts time-dependent autocrine and/or paracrine functions within the pineal. Moreover, endocananbinoids may be released from the pineal into the CSF or blood stream.     

Norepinephrine turnover in the rat pineal gland. Acceleration by estradiol and testosterone.

The administration of 0.5 mg of testosterone propionate (TP) to orchiectomized rats or of 2 ug of estradiol to oophorectomized rats resulted in significantly less ³H-norepinephrine remaining in the pineal gland 60 and 120 min after a pulse injection of the radioactive compound. This effect was not observed in animals administered with ³H-norepinephrine 45 or 180 min after a single hormone injection. Half-lives for ³H-norepinephrine disappearance in estradiol- and TP-treated rats were 62 and 60 min respectively whereas those of female and male vehicle-injected controls were 109 and 123 min respectively. Hormone treatment did not affect pinela norepinephrine content in any of the schedules used. Norepinephrine efflux expressed as pg. mg tissue⁻¹. min⁻¹ was 33 in estradiol-treated females, 17 in vehicle-treated females, 28 in TP-treated males and 12 in vehicle-treated males. These data indicate that the norepinephrine turnover in neurons innervating the pineal gland is increased by chronic administration of female and male sex hormones.     

DSIP affects adrenergic stimulation of rat pineal N-acetyltransferase in vivo and in vitro

The natural occurrence, sleep, and extra-sleep effects of delta sleep-inducing peptide (DSIP) have been shown by different laboratories. However, neither an in vitro assay system nor a probable mechanism of action of the peptide have been conclusively demonstrated so far. The recent finding that DSIP influences the nocturnal rise of N-acetyltransferase (NAT) activity in rat pineal led us to investigate a possible effect on pharmacologically induced NAT activity in vivo and in vitro. Stimulation of the enzyme with adrenergic drugs such as isoproterenol and phenylephrine was reduced by DSIP at doses of 150 and 300 μg/kg injected subcutaneously. In vitro, 6, 150 and 300 nM DSIP attenuated isoproterenol stimulation of the enzyme in cultured pineals, whereas 150 nM DSIP effectively reduced stimulation induced by a combination of the two drugs. The peptide alone did not influence NAT activity in vitro, but produced a slight stimulation in vivo. To our knowledge, these results represent the first report of a direct interaction of DSIP with adrenergic transmission. The in vitro system could prove useful for establishing possible mechanism(s) of action of the ‘sleep peptide.’