Pineal adrenergic receptor numbers show circadian variations in both rat and Syrian hamster. In the rat pineal beta-adrenergic receptor density reaches peak values either late in the light phase or at middark; the differences in the circadian phase seem related to the light:dark cycle to which the animals are exposed. No circadian rhythm of pineal alpha-adrenergic receptors is documented in intact rats. In the Syrian hamster pineal beta-adrenergic receptor density is high throughout the light phase and drops to minimal values at the time of the nocturnal peak of melatonin production. The circadian rhythm of pineal alpha-adrenergic receptor numbers runs parallel to the beta-adrenergic receptor variation, but is less pronounced. In the rat, pineal melatonin production is rapidly induced by beta-adrenergic agonists at any time during a 24-hour period, even when the pinealocyte beta-adrenergic receptor number is lowest (early in the light phase). In contrast, the Syrian hamster pineal seems most responsive to beta-adrenergic agonists in the late night while being less responsive during the day when beta-adrenergic receptor density is high. Interestingly, the human pineal gland is also not especially responsive to adrenergic stimulation during the light phase, possibly making the Syrian hamster pineal a better model than the rat pineal for determining neural/pineal interactions in humans. Comparison of the circadian variations in pineal adrenergic receptors leads to the conclusion that the functional differences between rat and hamster pineal are probably not explicable in terms of the adrenergic receptors, but are caused most likely by (a) intracellular mechanism(s) beyond the adrenergic receptors.

The Harderian glands in Syrian hamsters exhibit a striking sexual dimorphism. Male Harderian glands show two cell types and low levels of porphyrins and melatonin. Of the enzymes involved in the synthesis of melatonin, N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) show high and low activity levels, respectively. Female Harderian glands show but one cell type and have high porphyrin and melatonin levels, low NAT activity, and high HIOMT activity. In castrated males, the Harderian glands exhibit a female pattern of morphology, porphyrin levels, and indoleamine metabolism. In an attempt to determine whether prolactin in involved in this sexually dimorphic response of the Harderian glands, intact and castrated male and intact female hamsters were injected daily with 500 micrograms of bromocriptine, a dopamine agonist. Bromocriptine led to reduced serum prolactin levels in all groups. It had no apparent effect on the Harderian glands of intact males. In contrast, in castrated males bromocriptine prevented the postcastrational rise in porphyrin levels but had no effect on NAT or HIOMT activities. In females, bromocriptine treatment had no effect on porphyrin concentrations or HIOMT activity; it led to a statistically significant increase in NAT activity. We propose that testosterone inhibits Harderian porphyrin synthesis while dopamine or prolactin stimulates it.

Scatchard analysis of saturation isotherms of [125I]-iodopindolol was used to characterize beta-adrenoceptor density (Bmax) and affinity constant (Kd) in female and male hamster Harderian glands. Single-point experiments were also completed in intact females, intact males, and castrated or superior cervical ganglionectomized males. Scatchard analysis described a single population of binding sites with a Bmax of 292.2 +/- 45.1 fmol/mg protein (X +/- SEM, n = 6) in females and 18.2 +/- 3.0 fmol/mg protein (n = 6, P less than .001) in males. The affinity also varied significantly (P less than .05) with a Kd of 1.08 +/- 0.18 versus 0.26 + 0.05 nM (n = 6) in the Harderian gland of females and males, respectively. Single-point [125I]-IPIN (400 pM) binding values in females were 67.3 +/- 4.0, in intact males were 12.8 +/- 3.2, and in castrated males were 31.2 +/- 4.2 fmol/mg protein (n = 7-9). Superior cervical ganglionectomy induced no significant changes in receptor binding. The results indicate pronounced sexual differences in the density and affinity of beta-adrenoceptors in the hamster Harderian gland, which may be sex hormone dependent.

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Radioreceptor assays with (-) [¹²⁵I]-iodopindolol (IPIN) were used to describe circadian variations of β-adrenoceptors in the pineal gland of male Syrian hamsters (Mesocricetus auratus). Single-point experiments (200 pM IPIN, 8–9 pineals pooled per time point) showed a significant circadian variation (p < 0.01) with maximal values (28.9 ± 1.8 fmol/mg protein, x ± SEM, n = 7) between 07:00 and 02:30 h, and minimal values at 04:00 (7.2 ± 2.9 fmol/mg protein, n = 3), 8 h after darkness onset. Either exposure of animals to light at night or treating dark exposed hamsters with a β-adrenergic receptor blocker, propranolol, prevented the nocturnal drop in the number of β-adrenoceptors. Scatchard analysis of saturation isotherms at 02:30, 04:00 and 08:00 h (30–600 pM IPIN, one saturation experiment with 25 pineals pooled per time point) confirmed the circadian variation.