Melatonin formation in different parts of the guinea-pig pineal complex as assessed over 24 hours

Abstract

There is morphological evidence that the pineal gland is not a uniformly built organ but rather a complex. In the guinea-pig the gland is 6--7 mm in length and dumbbell-shaped, the proximal part coming into intimate contact with central commissural fibres. The aim of the present 24-hour study was to examine in male guinea-pigs whether the proximal, intermediate and distal areas of the gland are involved in melatonin formation and to compare their rhythmicities. Levels of melatonin in serum and the whole pineal gland as assessed by RIA show day/night ratios of 1 : 1.25 and 1 : 3.3, respectively. Serotonin N-acetyltransferase activity was found to exhibit ratios of 1 : 1.75 (Experiment I) and 1 : 4.4 (Experiment II). All three pineal regions are involved in melatonin formation, and to the same extent. Whether the rhythms in the different regions are identical could not be clarified as the curves obtained exhibited striking oscillations and the day/night differences were rather small. As the extrapineal portion of the habenular commissure was found to contain melatonin there is a possibility that melatonin may be taken up when the commissural fibres pass through the pineal parenchyma.

Response of pineal serotonin N-acetyltransferase activity in male guinea pigs exposed to light pulses at night

“…In accordance with previous studies (Rudeen et al, 1975;Welker and Vollrath, 1985) guinea pigs revealed the well-known moderate day/night increase of pineal NAT activity (Fig. 1).…”

“…respect to whether or not light is effective at night. Light depresses NAT activity in species with small (1.5-fold, guinea pig: Rudeen et al, 1975;Welker and Vollrath, 1985, this study; 3-fold, sheep: Namboodiri et al, 1985) and large differences (50-70-fold, rat: Weller, 1970, Illnerovfi andVan66ek, 1979), and non-suppression is observed in animals with large (34-fold, Eastern chipmunk: Reiter et al, 1982 a, b) and with relatively small increases (aproximately 5.5-fold, 13-lined ground squirrel, Reiter et al, 1983b) (see Table 1, for further details).…”

Quantitative analysis of ?synaptic? ribbon profiles in the pineal complex of male and female Pirbright-White guinea pigs

“…In accordance with this, no significant numerical differences were noted in the present study. Secretion-related parameters, such as pineal melatonin content and serotonin N-acetyltransferase activity likewise failed to exhibit clear-cut regional differences in male guinea-pigs (Welker and Vollrath, 1985). More detailed studies carried out in female Hartley-strain guinea pigs revealed that melatonin, 5-methoxy-indoleacetic acid, 5-methoxytryptophol and cAMP levels did not differ between regions; likewise, estradiol, testosterone and progesterone affected pineal cAMP levels, pineal indoleamine content and noradrenaline release identically in the three regions (Cardinali et al, 1986).…”

Melatonin Production by the Guinea Pig Pineal Gland in vitro

“…It has been assumed that production is equivalent to synthesis and release [2,29], and the rapidity and consistency of the changes that followed the application of treatments support this. However, occasional differences in serum/gland or gland/perfusate ratios have been seen [e.g., 14,30,31 ], and a variable clearance from the gland has been sug gested [14], The early release from glands confined within a suction electrode was found to be less than from glands not so constrained, suggesting that a physical barrier such as the wall of a suction electrode may interfere with the exchange between the isolated gland and the surrounding medium.…”

“…However, recent studies indi cate that the rise in the levels of cAMP and cGMP and the in creased activity of NAT that are produced by activation of |ii-adrenoceptors are much enhanced by the activation of «-adrenoceptors [1,8], This pattern, with some variations, appears to obtain in other species. It seems that (¡-adrenoceptors are always inReceived: September 12, 1989 Accepted after revision: January 30, 1990 observed intracellularly in this species when the pineal gland is volved, since a blocker such as propranolol inhibits the noc turnal increase in pineal MT synthesis [see 9(. Demonstration that MT production can be stimulated by substances such as isoproterenol had seemed more difficult -in humans circul ating MT was unaffected [ 10], and a similar lack of response was seen both in vivo and in vitro in Syrian hamsters [II,12], al though isoproterenol increases pineal MT in the Djungarian hamster in vivo [11], and the guinea pig pineal in vitro responds to orciprenaline, a [5-adrenoceptor stimulant [ 13].…”