Localization and role of serotonin in the adrenal gland of Podarcis sicula (Reptilia, Lacertidae)

The Adrenal Gland of Squamata (Reptilia): A Comparative Overview

The adrenal gland is a complex endocrine organ composed of two components: a steroidogenic tissue, which produces steroid hormones, and a chromaffin tissue, which mainly produces norepinephrine and epinephrine. Through evolution, their relationships with each other changed. They begin as isolated chromaffin and steroidogenic cell aggregates, typical of fish, and end with the advanced compact gland, typical of mammals, which consists of an external steroidogenic cortical zone and an internal chromaffin medullary zone. The adrenal gland of reptiles is unique because, with few exceptions, it is near the gonads and genital ducts, and the chromaffin and steroidogenic tissues are closely associated. However, the degree of mixing is variable. For example, in Squamata, the mixing degree of chromaffin and steroidogenic tissues, their reciprocal position in the gland, and the relative quantities of norepinephrine and epinephrine secreted by the chromaffin cells are extremely variable. This variability could be related to the phylogenetic history of the species. After a brief discussion of the adrenal gland and its main functions in vertebrates, this overview will examine the general characteristics of the adrenal gland of squamates, the differences in morphology of the gland, and the possible relationships with the phylogeny of the different species.

Transmission Electron Microscopy: A Method for Studying the Adrenal Chromaffin Cells

Transmission electron microscopy and the use of glutaraldehyde-osmium fixation allow to distinguish norepinephrine from epinephrine granules in the adrenochromaffin cells, a difficult distinction with histochemical methods if both types of granules are present in the same cell. Here we describe all the steps necessary to process the adrenochromaffin tissue for the transmission electron microscopy; this protocol is suitable for any kind of adrenal tissue, and personally we used it in mammals, reptiles, and amphibians.

The autonomic nervous system and chromaffin tissue: neuroendocrine regulation of catecholamine secretion in non-mammalian vertebrates

If severe enough, periods of acute stress in animals may be associated with the release of catecholamine hormones (noradrenaline and adrenaline) into the circulation; a response termed the acute humoral adrenergic stress response. The release of catecholamines from the sites of storage, the chromaffin cells, is under neuroendocrine control, the complexity of which appears to increase through phylogeny. In the agnathans, the earliest branching vertebrates, the chromaffin cells which are localized predominantly within the heart, lack neuronal innervation and thus catecholamine secretion in these animals is initiated solely by humoral mechanisms. In the more advanced teleost fish, the chromaffin cells are largely confined to the walls of the posterior cardinal vein at the level of the head kidney where they are intermingled with the steroidogenic interrenal cells. Catecholamine secretion from teleost chromaffin cells is regulated by a host of cholinergic and non-cholinergic pathways that ensure sufficient redundancy and flexibility in the secretion process to permit synchronized responses to a myriad of stressors. The complexity of catecholamine secretion control mechanisms continues through the amphibians, reptiles and birds although neural (cholinergic) regulation may become increasingly important in birds. Discrete adrenal glands are present in the non-mammalian tetrapods but unlike in mammals, there is no clear division of a steroidogenic cortex and a chromaffin cell enriched medulla. However, in all groups, there is an obvious intermingling of chromaffin and steroiodogenic cells. The association of the two cell types may be particularly important in the amphibians and birds because like in mammals, the enzyme catalysing the methylation of noradrenaline to adrenaline, PNMT, is under the control of the steroid cortisol.

Atrial natriuretic factor: localization in the adrenal gland of the lizard Podarcis sicula and effects on pituitary-adrenal axis activity

The occurrence of atrial natriuretic factor (ANF) immunoreactivity was investigated in the adrenal gland of the lizard Podarcis sicula by avidin-biotinylated peroxidase complex (ABC) immunocytochemical technique: ANF immunoreactivity was present in the chromaffin tissue, and was absent in the steroidogenic tissue. The role of ANF in the modulation of the pituitary-adrenal axis activity was investigated in vivo by intraperitoneal administration of ANF. The effects were evaluated by examination of the morphological and morphometrical features of the tissues, as well as the plasma levels of adrenocorticotropic hormone (ACTH), corticosterone, aldosterone, norepinephrine, and epinephrine. ANF (28 microg/100 g body wt) did not affect ACTH plasma levels, that remained almost unchanged; in contrast, corticosterone plasma levels increased from 6.45 +/- 0.070 ng/ml in carrier-injected lizards to 9.69 +/- 0.080 ng/ml 24 h after the injection; aldosterone levels decreased from 2.19 +/- 0.010 ng/ml in carrier-injected specimens to 0.58 +/- 0.003 ng/ml 24 h after the experimental treatment. In the chromaffin tissue, an increase in the number of epinephrine cells and a decrease in the number of norepinephrine cells were observed, decreasing the numeric norepinephrine/epinephrine cell ratio, from 1.4/1 of control specimens to 0.3/1 24 h after ANF administration. Moreover, norepinephrine plasma levels decreased from 998 +/- 4.600 pg/ml in carrier-injected specimens to 321 +/- 2.230 pg/ml 24 h after ANF administration; epinephrine plasma levels were elevated from 614 +/- 3.410 pg/ml in carrier-injected specimens to 1672 +/- 10.800 pg/ml 24 h after the experimental treatment. The presence of ANF in the adrenal gland suggests that, also in reptiles as in other vertebrates, this peptide, locally released from the chromaffin cells, may modulate the activity of the adrenal gland, probably in a paracrine manner. The effects of ANF on the adrenal gland suggest that this peptide may affect reptilian salt and fluid homeostasis.

Neuropeptide Y modulates pituitary-adrenal axis activity in the lizard, Podarcis sicula

The role of neuropeptide Y (NPY) in the modulation of the pituitary-adrenal axis activity in a lizard, Podarcis sicula, was investigated by in vivo NPY administration. The effects were evaluated by examination of the morphological and morphometrical features of the tissues as well as the plasma levels of ACTH, corticosterone, aldosterone, norepinephrine, and epinephrine. Intraperitoneally administered NPY (27 nmol /100g body wt) raised ACTH plasma levels (from 5.23+/-0.06 pg/ml in carrier injected specimens to 6.83+/-0.01 pg/ml, 24 h after the injection). In the steroidogenic cells a strong decrease of lipid amount was found; corticosterone plasma level increased from 6.28+/-0.02 ng/ml in carrier injected lizards to 7.96+/-0.01 ng/ml 24 h after the injection); aldosterone levels were raised from 1.88+/-0.02 ng/ml in carrier injected specimens to 6.38+/-0.05 ng/ml 24 h after the experimental treatment. In the chromaffin tissue, an increase in the number of epinephrine cells and a decrease in the number of norepinephrine cells were observed, decreasing the numeric norepinephrine/epinephrine (NE/E) cell ratio, from 1.4/1 of control specimens to 0.5/1 24 h after NPY administration. Moreover, norepinephrine plasma level were elevated from 922+/-4.30 pg/ml in carrier injected specimens to 3075+/-11.30 pg/ml 24 h after NPY administration; epinephrine plasma level increased from 502+/-2.40 pg/ml in carrier injected specimens to 2759+/-8.70 pg/ml 24 h after the experimental treatment. Consistent with these findings, morphological observations showed many chromaffin cells weakly stained and with a reduced content of secretory granules. These results suggest that, in P. sicula, NPY may play a role in the modulation of the pituitary-adrenal axis activity. Previous studies localized NPY in the epinephrine cells of P. sicula adrenal gland; taken together, these results suggest that this peptide might participate in the regulation of adrenal gland activity, enhancing corticosteroid and catecholamine secretion in a paracrine/autocrine manner. The mechanism of action of NPY is discussed.

Effects of dopamine on the adrenal gland of Podarcis sicula (Reptilia, Lacertidae)

The effects of dopamine administration on the adrenal gland of a lizard, Podarcis sicula, are described. Dopamine (0.7mg/100g body wt/day for 4 consecutive days) raised plasma ACTH and corticosterone levels (ACTH: from the basal level of 4.40+/-0.05-7.30+/-0.08pg/ml 24h after the fourth dopamine injection; corticosterone: from 3.59+/-0.03ng/ml in untreated lizards to 7.40+/-0.05ng/ml 24h after the fourth dopamine injection), showing a stimulatory effect on the pituitary-interrenal axis activity. In the chromaffin tissue dopamine apparently enhanced the activity of PNMT enzyme; in fact a strong raise in the number of adrenaline cells and a decrease in the number of noradrenaline cells were observed, decreasing the numeric NA/A cell ratio, from 1.4/1 of control specimens to 0.5/1 24h after the fourth dopamine injection. At EM level, chromaffin cells contained both NA and A granules, as well as very clear granules (CG); CG granules showed granular elements ranging between 340 and 347A in diameter. These cells might be the morphological expression of a process of catecholamine resynthesis, due to a possible increase in catecholamine release, following exposure to dopamine.

Immunocytochemical localization of substance P in the adrenal gland of Podarcis sicula (Reptilia, Lacertidae): evidence for its involvement in the modulation of adrenal activity

The occurrence of substance P (SP) immunoreactivity was investigated in the adrenal gland of the lizard Podarcis sicula by ABC immunocytochemical technique: SP-immunoreactivity was present in both adrenaline and noradrenaline cells, in ganglion cells and nerve fibers in the connective capsule surrounding the gland. The involvement of substance P in the modulation of pituitary-interrenal axis was studied in vivo by intraperitoneal injections of SP. The effects were estimated by means of the morphological and morphometrical features of the tissues, as well as the plasma levels of adrenocorticotropic hormone (ACTH), corticosterone and catecholamines, adrenaline and noradrenaline. Substance P (0.07 mg/100 g body wt) decreased ACTH plasma levels and raised corticosterone release from steroidogenic tissue, that showed clear signs of stimulation. In the chromaffin tissue, the decrease in the number of noradrenaline cells, and the increase in the number of adrenaline cells, lowered numeric noradrenaline/adrenaline cell ratio. Moreover, an increase in adrenaline plasma level and a decrease in noradrenaline plasma level were found. The results suggest that (1) also in Reptiles as in other Vertebrates, SP may affect pituitary-adrenal axis activity, and (2) the chromaffin cells may be involved in the paracrine control of steroidogenic activity.