A preliminary investigation of the effects of vasoactive intestinal peptide on secretion from the lingual salt glands of Crocodylus porosus

Autonomic control of glands and secretion: a comparative view

The autonomic nervous system together with circulating and local hormones control secretion from glands. This article summarizes histochemical and functional studies on the autonomic innervation and control of secretory glands in non-mammalian vertebrates, including secretion of saliva in the mouth and gastric acid in the stomach, secretion of enzymes and bicarbonate from the pancreas and gut wall, secretion of mucus in the gut epithelium and onto the skin, and salt secretion from salt glands and rectal glands. Cholinergic and adrenergic nerves, directly or indirectly, in combination with different types of peptidergic and other nerves appear to innervate gland tissues and affect secretion in all investigated species.

The effects of saltwater acclimation on neurotransmitters in the lingual salt glands of the estuarine crocodile, Crocodylus porosus

INTRODUCTION

Most avian and reptilian salt glands display marked phenotypic plasticity when animals are exposed to hyperosmotic conditions. In addition, the activity of most salt glands is under considerable control by the nervous system and nerves containing cholinergic, adrenergic and peptidergic neurotransmitters have been identified in avian and reptilian salt gland tissues. The present study sought to determine whether the salt glands of the estuarine crocodile, Crocodylus porosus contain the peptidergic neurotransmitters SP, CGRP, VIP, and PACAP and the gaseous neurotransmitter, NO. In addition, we sought to determine whether there was any evidence for the adaptation of the C. porosus salt gland nervous system to hyperosmotic conditions.

METHODS

Salt glands from freshwater- and saltwater-acclimated C. porosus hatchlings were sectioned and examined immunohistochemically for neurotransmitters within the tissue.

RESULTS

Neurons containing SP, CGRP, VIP, PACAP and NO synthase were identified within C. porosus salt glands. There was no difference in the overall number (density) of neurons within SW-acclimated tissues when compared with FW-acclimated animals. However, there was a significant reduction in density of neurons containing SP and PACAP in SW-acclimated animals.

CONCLUSION

C. porosus salt glands display phenotypic plasticity following exposure to hyperosmotic conditions. In addition to cholinergic and adrenergic neurons, they contain a variety of peptidergic neurotransmitters and the gaseous neurotransmitter NO. Additionally, there appears to be some evidence of acclimation of the nervous system of C. porosus to hypersaline conditions, although the functional significance of these changes remains to be determined.

Cholinergic and adrenergic innervation of lingual salt glands of the estuarine crocodile, Crocodylus porosus

Many marine reptiles and birds possess extrarenal salt glands that facilitate the excretion of excess sodium and chloride ions accumulated as a consequence of living in saline environments. Control of the secretory activity of avian salt glands is under neural control, but little information is available on the control of reptilian salt glands. Innervation of the lingual salt glands of the salt water crocodile, Crocodylus porosus, was examined in salt water-acclimated animals using histological methods. Extensive networks of both cholinergic and adrenergic nerve fibres were identified close to salt-secreting lobules and vasculature. The identification of both catecholamine-containing and cholinergic neurons in the salt gland epithelium and close to major blood vessels in the tissue suggests the action of the neurotransmitters on the salt-secreting epithelium itself and the rich vascular network of the lingual salt glands.

Salt and water regulation by the leatherback sea turtleDermochelys coriacea

We measured the salt and water balance of hatchling leatherback sea turtles, Dermochelys coriacea, during their first few days of life to investigate how they maintain homeostasis under the osmoregulatory challenge of a highly desiccating terrestrial environment and then a hyperosmotic marine environment. Hatchlings desiccated rapidly when denied access to sea water,with their hematocrit increasing significantly from 30.32±0.54 % to 38.51±1.35 % and plasma Na+ concentration increasing significantly from 138.2±3.3 to 166.2±11.2 mmoll-1 in 12 h. When hatchlings were subsequently put into sea water, hematocrit decreased and plasma Na+ concentration was unchanged but both were significantly elevated above pretreatment values. In other hatchlings kept in sea water for 48 h, body mass and plasma Na+ concentration increased significantly, but hematocrit did not increase. These data show that hatchlings were able to osmoregulate effectively and gain mass by drinking sea water. We stimulated hatchlings to secrete salt from the salt glands by injecting a salt load of 27 mmol kg-1. The time taken for secretion to begin in newly hatched turtles was longer than that in 4-day-old hatchlings, but the secretory response was identical at 4.15±0.40 and 4.13±0.59 mmol Na+ kg-1 h-1respectively. Adrenaline and methacholine were both potent inhibitors of salt gland secretion in a dose-dependent manner, although methacholine administered simultaneously with a subthreshold salt load elicited a transient secretory response. The results showed that hatchling leatherbacks are able to tolerate significant changes in internal composition and efficiently use their salt glands to establish internal ionic and water balance when in sea water.