**5.6 Thyroid hormones**

The thyroid mainly secretes triyodo tyrosine (T3), tiroxine (T4) and calcitonin. T3 and T4 play several roles in regulating all the mammalian organs and systems.

Rats with hypothyroidism present high progesterone and low testosterone levels, without apparent changes in basal estradiol levels (Hatsuta et al., 2004). Acute experiments with murine Leydig cells, T3 induced StAR expression and progesterone production (Manna et al., 1999). In contrast, chronically stimulating mice Leydig tumor cells with T3 inhibits StAR expression and progesterone production, mainly by decreasing the delivery of cholesterol to the inner mitochondrial membrane (Manna et al., 2001). In cultivated Leydig cells, T3 treatment increased testosterone and estradiol secretions in a dose dependent manner (Maran et al., 2000).

Hormonal and Neural Mechanisms Regulating Hormone Steroids Secretion 13

diffuse into zone glomerulosa cells and modulate the synthesis of aldosterone in a paracrine way. The enzymes that may terminate the effect of ATP are present in the nerve endings, suggesting that ATP and its metabolites influence the production of aldosterone. Thus, catecholamines and ATP play a paracrine non-synaptic modulator role of in the regulation

Stimulation of beta-adrenoreceptors, VIP receptors or the forskolin-induced activation of cAMP formation of 2-day-old rat ovaries increases the steady state levels of the mRNAs encoding P-450aromatase and FSH receptors. Based on these results it was suggested that ovarian nerves, acting via neurotransmitters coupled to the cAMP generating system, contribute to the differentiation process by which newly formed primary follicles acquire FSH receptors and responsiveness to FSH. Follicles that grow in more densely innervated ovarian regions may have a selective advantage over those not exposed to neurotransmitteractivated, cAMP-dependent signals; and thus may become more rapidly subjected to

The intra-cerebro-ventricular injection (icv-i) of isoproterenol (beta-adrenergic agonist) to rats in diestrus 2 lowers progesterone levels in the ovarian vein blood. However, no apparent effects are observed when both superior ovarian nerves (SON) are sectioned before the icv-i treatment. Blocking the beta-adrenergic receptors with propranolol icv-i increased progesterone levels, an effect that was not observed when both SONs were sectioned (De Bortoli et al., 1998, 2002). According to De Bortoli et al. (2000), the neural signals arriving to the ovary through the SON antagonize the ovarian LH regulation of progesterone and

The adrenal gland and the ovaries receive innervation from several nerve fibers of extrinsic and intrinsic origin. Most of the extrinsic innervations in the adrenal derive from the sympathetic nervous system, including cholinergic fibers containing nitric oxide synthase (Holgert et al., 1995), thyrosine hydroxylase- and neuropeptide Y-positive postganglionic sympathetic fibers (Holgert et al., 1998; Kondo, 1985). Encephalin was exclusively found in choline-acethyl-transferase positive fibers among adrenaline chromaffin cells (Holgert et al., 1995). Intrinsic innervation originates from two different types of medullary ganglion cells: Type I and Type II cells. Type I cells are NPY-positive noradrenergic, while type II ganglion cells synthesize VIP and nitric oxide synthase (Holgertet al., 1998; Ulrich-Lai et. al., 2006). The adrenals have efferent fibers connecting to the dorsal motor nucleus of the vagus nerve, while other fibers of vagal origin reach the gland via the celiac or suprarenal ganglion (Berthoud and Powley, 1993; Coupland et al., 1989). In the rat, the motor and sensory vagal innervations of the adrenal gland originate from bilaterally situated cell bodies that have slight ipsi-lateral predominance (Coupland et al., 1989). Nerve fibers that go to and from the adrenal gland also possess afferent viscero-sensory fibers. According to Tóth et al. (2007), the steroid feedback mechanism affects the cerebral structures that send descending input to

The bilateral sectioning of the thoracic splanchnic nerve resulted in lower corticosterone plasma levels measured in the afternoon seven days after treatment, without apparent changes in ACTH levels; results that suggest that the splanchnic adrenal innervation modulates the response to ACTH. The effects are related to functional changes in the adrenal medulla and do

the sympathetic preganglionic neurons innervating the adrenal gland.

not depend on the sensitive of the afferent fibers (Ulrich-Lai et al., 2006).

of adrenocortical steroid secretion (Szalay et al., 1998).

gonadotropin control (Mayerhofer et al., 1997).

**7. Ovarian and adrenal innervations** 

androstenedione.
