Effect of Photoperiod on Developmental Morphology and Enolase Isoenzyme Immunohistochemistry in Rat and Djungarian Hamster Superficial Pineal Glands

Abstract

The best understood functional activity of the pineal gland is its diurnal production of melatonin in response to environmental lighting cues. Several enzymes of the melatonin pathway respond to daily photoperiod changes, for example hydroxyindole-O-methyltransferase (HIOMT) and serotonin N-acetyltransferase (SNAT). Increased levels of the glycolytic enzyme neuron-specific enolase (NSE) are thought to reflect increased physiological demands placed on neurons and neuroendocrine tissues. Homodimer non-neuronal enolase isoenzyme (NNE) is immunolocalized to cells, and the hybrid enolase (consisting of subunits from NSE and NNE) has been seen in cerebellar stellate and basket cells. Although not rate limiting, concentrations of both NSE and NNE in the rat pineal gland rival that of the forebrain. The rat pinealocytes demonstrates a mosaic pattern of immunostaining for the NSE antigen. Furthermore, an increasing proximal to distal immunostaining gradient has been reported in the gland. In this study, it was hypothesized that structural and immunohistochemical differences represent pinealocyte heterogenous functional activity. This was tested in two rodent populations (rats and Djungarian hamsters) by means of evaluating the effect of photoperiod manipulation on morphological development and immunohistochemical localization of enolase isoenzymes. Djungarian hamsters respond well to photoperiod changes, while laboratory rats are thought to be insensitive to environmental lighting changes. The effect of photoperiod in the hamster was confirmed by significantly different (p \u3c 0.01) body and testes weights for animals housed under the two different photoperiods (long day, 16L:8D; short day 6L:18D). In addition, rats exposed to the long day photoperiod (12L:12D) had significantly greater body and testes weights (p \u3c 0.05) than animals raised under the short day photoperiod (6L:18D). Photoperiod also influenced pineal gland soluble protein, protein concentration, and NSE concentration in both species. Generally, the levels were greatest in extracts from the glands of animals subjected to the short day environment. Gel electrophoresis revealed a protein or group of proteins that were particularly responsive to changes in environmental lighting; the estimated molecular weight was 80 kD. In both species, NNE exhibited the same heterogenous immunostaining as NSE, and was localized to the same pinealocytes in adjacent sections. The preceding strongly suggests that the hybrid enolase is found in the rat and hamster pineal gland. If so, this may reflect a balance between simultaneous expression of the NSE and NNE genes. The NSE increasing proximal to distal immunostaining gradient was repeated by NNE in the rat gland, but it was in the opposite direction for the hamster. Morphological features, however, may have contributed to the gradient nature of enolase isoenzyme immunostaining, in that regional nuclear and blood vessel volume fractions were influenced by photoperiod