The pineal gland of the Indian palm squirrel, Funambulus pennanti (Wroughton)

In the adult palm squirrel, F. pennanti the pineal is a club shaped, elongated structure with a connective tissue capsule. It consists of various types of pinealocytes, glial cells, neurons, nerve fibres, blood vessels and connective tissue. Two types of pinealocytes could be identified by light microscopy. They are large rounded with centrally placed nucleus, and small rounded pinealocytes. They have medium sized processes stainable with Alcian blue, periodic acid Schiff and Nissl methods. The pinealocytes are not stainable with bromophenol blue. However, they are moderately stainable with PAS, Sudan black and Baker's acid hematin. Neurons are seen either singly or in groups with axonal processes. Cystic cavities often lined by cells are a normal feature of adult squirrel pineal, and the lining cells are both pinealocytes and glial cells. Often neuronal endings are seen terminating on these lining cells. PAS positive globules were also seen inside the cysts. In some squirrel pineals, fibrous cysts with an inner core of cells are also seen. Occasionally groups of lymphocytes were also encountered in the pineal. In the fetal pineal, the cells are both larger and smaller ones and arranged in a cortex and medulla pattern and no cystic cavities are seen. The third ventricle enters the base of the pineal as pineal recess.     

Cytology of the hypophysis, pars anterior of the Indian five-striped palm squirrel, Funambulus pennanti (Wroughton)

By employing various staining techniques the pituitary cells of the squirrels, Funambulus pennanti, can broadly be divided into 'acidophilic', 'basophilic' and 'amphophilic' types. The acidophilic cell types include somatotrophs and lactotrophs. The basophilic cell types include thyrotrophs, folliculotrophs and interstitiotrophs while the amphophilic cells are recognized as corticotrophs. The identification of the pituitary cells and their histochemical characteristics are described at the light microscopical level. Attempt has been made to interpret the cells identified in the present study on a comparative and tinctorial basis.     

Gonado-modulatory effects of high humidity is pineal dependent in Indian palm squirrel Funambulus pennanti (Wroughton).

F. pennanti presented a clear biphasic pineal mediated seasonal sexual cycle. This sexual cycle was essentially characterised by a very short period of sexual quiescence with an arrest of spermatogenesis during October-November. A small but clear decrease in sexual activity was also observed during March-April. This decrease in sexual activity, however, had no quantifiable effect on spermatogenesis. Sexual recrudescence was observed from December-January. The testes remained sexually active from January till September. Almost an inverse relationship was observed between pineal and testicular weight. Pinealectomy, however, prevented naturally induced gonadal regression during both the periods, i.e. September-November and February-April. Exposure of animals to high RH (80 +/- 4%) during sexually active phase induced a steep regression in testicular weight of sham-operated animals even in the presence of gonad stimulatory long photoperiod (16L:8D) and high temperature (40 degrees +/- 5 degrees C) while exposure of animals to moderate RH (65 +/- 5%) during sexual regression phase partially prevented testicular regression even in presence of inhibitory short photoperiod (11L:13D) and normal environmental temperature (30 degrees +/- 5 degrees C). Pinealectomized animals, neither exhibited testicular regression in February-March nor had involuted testes in September-October, thus, suggesting that the effect of humidity is mediated via the pineal gland.     

Effect of metopirone on pituitary gland of male squirrel, Funambulus pennanti (Wroughton)

1. The ACTH cell type has been demonstrated for the first time in the hypophysis of the squirrels. These are large cells with homogenous cytoplasm and are more abundant in the anteromedian zone and in the peripheral portion than elsewhere. 2. A great hypertrophy and hyperplasia of the pituitary corticotrophs were observed in the experimental group at all the intervals. 3. Metopirone treated animals showed a moderate hypertrophy and degranulation of pituitary thyrotrophs. 4. By effect of metopirone, a marked regression of pituitary interstitiotrophs and total degranulation took place by 30 days treatment. The folliculotrophs did not show alterations.     

Antigonadotropic activity of pineal gland of the Indian palm squirrel Funambulus pennanti

Bioassay of the pineal extract of F. pennanti was performed in immature female mice which was previously sensitized with human chorionic gonadotrophin. Reduction of ovarian and uterine weights indicated an antigonadotropic nature of the pineal gland of this animal.     

Relationship between the annual rhythms in melatonin and immune system status in the tropical palm squirrel, Funambulus pennanti

Melatonin (MEL) regulation of seasonal variation in immunity has been studied extensively in temperate mammals. This report is the first on a tropical mammal, the Indian palm squirrel, F. pennanti. In response to the annual environmental cycle, we studied the rhythms of plasma MEL and the immune parameters of total blood leucocytes, absolute blood lymphocytes and blastogenic responses of blood, thymus and spleen lymphocytes. We found that in parallel with MEL all the immune parameters increased during the month of April onward, when natural day length, temperature, humidity and rainfall were increasing. Maximum values occurred during November (reproductively inactive phase) when the values of all the physical factors were comparatively low. Lowest values occurred during January-March (reproductively active phase) when the values of the physical factors were lowest. In order to establish a clear interrelationship between the pineal MEL and the immune system function, we manipulated these squirrels with exogenous MEL (25mg/100g B wt/day) at 1730 h during their pineal inactive phase (March) while another group was pinealectomized (Px) during November when their pineal was active. The MEL injection significantly increased all the immune parameters, while Px decreased them significantly. Hence, we suggest that MEL is immuno-enhancing for this tropical squirrel, and plays an important role in the maintenance of its immunity in accordance with the seasonal changes in environmental factors and gonadal status. (Chronobiology International, 18(1), 61-69, 2001)     

Seasonal reproductive periodicity and thyroid fluctuations in female Indian palm squirrel, Funambulus pennanti.

Seasonal reproductive rhythmicity and concomitant thyroid fluctuations have been studied in the female Indian palm squirrel, Funambulus pennanti. The majority of the squirrels enter estrus during March and April when ovarian follicles attain maximum diameter. Ovulation starts in April and continues till June. A large number of degenerating follicles, indicating decline in ovarian functions, is a characteristic feature of the ovary in August. Minimum percent thyroid 131I uptake is recorded during May and June, i.e. at the onset of mating season. In July, which also marked the gestation period in the majority of the squirrels dissected, thyroid activity significantly increases. Gonadal regression begins in August and this is accompanied by corresponding fall in the thyroid 131I uptake. Alterations in thyroid cytoarchitecture during different months of the year are in accordance with the results obtained from radioiodine uptake studies. Fluctuations in thyroid functions during gonadal regression, recrudescence and peak gonadal activity are discussed in the present communication.     

Melatonin blocks dexamethasone-induced immunosuppression in a seasonally breeding rodent Indian palm squirrel, Funambulus pennanti

In vivo effect of dexamethasone and melatonin on immunomodulation has been investigated by studying the lymphocyte proliferation to the mitogen Con A from various lymphoid tissues including bone marrow cells of a seasonally breeding rodent adult male F. pennanti during reproductively inactive phase (October to December). During this phase, animal faces the maximum challenges of the nature (hypothermic stress, scarcity of food and shelter). Dexamethasone treatment (60 microg/day/squirrel) for 60 consecutive days significantly decreased the thymus and spleen activity. The lymphoid tissues mass, total leukocyte, lymphocyte count of peripheral blood, bone marrow and T-cell mediated immune function was also significantly suppressed following the dexamethasone treatment but treatment of melatonin (25 microg/squirrel/day) along with dexamethasone significantly restored the suppressed immune status in squirrels. Further, histological study of the thymus showed profound changes in the cellularity with a depletion of thymocytes in the cortex region of thymic lobules and increased in connective tissues and spindle cells. Melatonin treatment alone increased thymocytes density in thymic cortex, clearly suggesting that melatonin counteracted the experimentally induced immune stress by dexamethasone. Therefore, in nature during reproductively inactive phase of the squirrel a high level of melatonin was noted, that is required to combat nature's stress, which might have increased the internal level of corticoids.     

Reproductive phase depedent circadian variation in the pineal biochemical constituents of Indian palm squirrel, Funambulus pennanti

In mammals, pineal gland is intimately concerned with the co-ordination of rhythm physiology. Biochemical characteristics of pineal gland in man and other mammals may provide strong, yet sometimes elusive support for the belief in functional individuality and probable importance of this tiny gland. In seasonal breeding animals, pineal gland function is very much dependent on the reproductive status. Therefore, the aim of this experiment is to note the circadian rhythmicity of different biochemical constituents of pineal gland during active and inactive phases of reproductive cycle of a seasonally breeding rodent, F. pennanti. In the present study, pineal biochemical constituents i.e. protein and cholesterol showed higher values during daytime (1400 h). The plasma melatonin level presented two peaks during active (April; at 1800 h and 0200 h) and inactive (December; at 1400 h and 0200 h) phases of reproductive cycle. The pineal protein, cholesterol and plasma melatonin values in term of basal and peak levels were higher during the reproductive inactive/pineal active phase. Therefore, pineal--also known to have antigonadotropic properties and cholesterol which appears conjugated with pineal serotonin, presented circadian rhythmicity along with the plasma level of melatonin. This rhythmicity noted in present study was dependent on the reproductive and pineal activity status, and might be regulated by the sex steroid receptor present on the pineal gland.     

Maternal transfer of melatonin alters the growth and sexual maturation of young Indian palm squirrel Funambulus pennanti

To date, the phenomenon of maternal transfer of hormones to the young is an enigma. The present study explains for the first time the maternal transfer of melatonin (MEL) to the young, affecting neonatal growth and sexual maturation. The suckling pups of MEL-treated mothers exhibited significant decreases in body, testicular, vas deferens (male pups), ovarian and uterine (female pups) weights and increases in pineal gland activity along with high plasma MEL levels. The plasma level of testosterone decreased significantly in male pups, while estradiol increased and progesterone decreased in female pups of MEL-treated mothers. These results clearly suggest that MEL could be transported from the mothers to their young postnatally via the milk in order to influence neonatal growth and sexual maturation. Our results support the earlier concept and show for the first time that MEL can be transported from the mother to the young either prenatally through the placenta or postnatally via the milk. Therefore, maternal MEL can act as a biological signal for neonatal growth and sexual maturation.     

Studies of calcitonin cells and parathyroid glands of the Indian palm squirrel, Funambulus pennanti in response to experimental hypercalcaemia.

Long term hypercalcaemia was induced in F. pennanti by alternate day intramuscular injections of 50,000 IU of vitamin D2 and by giving them 1% CaCl2 solution prepared in tap water to drink. The controls were not injected with vitamin D2 and were given tap water. The serum calcium levels at various stages of the experiment (1-29 days) show increased values as compared with those of control animals. The calcitonin cells in the treated animals generally exhibit an increase in their number up to the 15th day. Mitotic figures are also encountered between the 7th and the 15th day of treatment. This exhibits the increase in the number of C cells. Constant calcium challenge results in increased quantities of secretory granules among these cells up to the 15th day and in degranulation from the 17th day onwards. It also causes degenerative changes in a certain number of C cells. The parathyroids exhibit atrophic changes (25 days onwards) due to chronic hypercalcaemia. For short term hypercalcaemia, animals were injected intravenously with 1 ml of 10% solution of calcium gluconate. The calcitonin cells do not exhibit any change during the first half hour but thereafter they exhibit progressive degranulation, resulting in marked degranulation after 5 hours of the injection. The parathyroids remain unaffected throughout the experiment and show no histological change.     

Reproductive phase dependent circadian variations of plasma melatonin, testosterone, thyroxine and corticosterone in Indian palm squirrel, Funambulus pennanti

To explain the complex mechanism of environmental influence along with internal hormonal (factors) milieu on daily variations in the circulating levels of melatonin, testosterone, thyroxine and corticosterone were analyzed with the help of inferential statistics (Cosinor rhythmometry) in a seasonally breeding tropical rodent, F. pennanti during the reproductively active (RAP) and inactive phases (RIP). Plasma melatonin, thyroxine and corticosterone levels exhibited a significant circadian oscillation during both the active and inactive phases of the annual reproductive cycle. Melatonin showed higher amplitude during RIP in the circulating plasma. Testosterone presented a peak level during evening hours (16:00 - 18:00 h) during RAP only. The phase of thyroxine was noted ∼09:76 h and ∼10.35 h during active and inactive phases, respectively. Corticosterone showed a peak level at ∼12.00 h during both phases of the reproductive cycle. Further, in this tropical rodent, the minimum difference in photoperiod (∼3 - 4 hours) and maximum variation in temperature (max. 18°C - min. 10°C during RIP and max. 45°C - min. 32°C during RAP) and humidity (85% during RIP and 35% during RAP) regulated the diurnal rhythm of circulating melatonin circadian rhythm by ∼1 hour phase advance during RIP. In conclusion, the studied hormonal rhythms may be part of an integrative system to coordinate reproduction and physiological processes successfully with environmental factors.