**2. Central diabetes insipidus: an overview**

#### **2.1. General characteristics of human central diabetes insipidus**

CDI is a neurobiological syndrome characterized by the excretion of copious amounts of diluted urine, i.e., containing a reduced concentration of electrolytes (mainly Na+ ). The excretory response in CDI is associated with a rise in serum osmolality (hyperosmolality) and sodium concentration (hypernatremia) and with the intake of large amounts of water (polydipsia) (see **Table 1**) [2, 5, 6].

#### **2.2. Neuroendocrine control of antidiuresis and electrolyte excretion**

Urine and electrolyte excretions by the kidneys are related to the hormones AVP and oxytocin (OT), among other endocrine mechanisms [7, 8]. AVP and OT have similar chemical structures and are both nonapeptides, differing in amino acids at positions 3 and 8 (**Figure 1**). They are synthesized with their corresponding carriers, neurophysins I and II, respectively,


**Table 1.** General characteristics of patients with CDI in comparison to a healthy population.

#### (A) Arginine Vasopressin

excretion of abundant diluted, "tasteless" urine and was first described by Willis in the seventeenth century. The incidence of this disorder is around 1:25,000 cases, with no statistically

In general, two main DI types can be distinguished: one that is related to deficient secretion of the antidiuretic hormone arginine vasopressin (AVP), which is better documented and referred to as central, neurogenic, neurohypophyseal, or hypothalamic DI, and another characterized by renal insensitivity to the antidiuretic effect of AVP, designated nephrogenic

This chapter starts with a description of the general characteristics of central diabetes insipidus (CDI), including its symptoms, the brain systems involved, and the etiologies of acquired and hereditary CDI. We then review the main animal models of acquired (neurohypophysectomy, pituitary stalk compression (PSC), hypophysectomy, and hypothalamic mediobasal lesions) and hereditary (Brattleboro rats) CDI. Finally, data from animal and human studies are discussed in relation to the therapeutic usefulness of oxytocin and a low-

CDI is a neurobiological syndrome characterized by the excretion of copious amounts of

tory response in CDI is associated with a rise in serum osmolality (hyperosmolality) and sodium concentration (hypernatremia) and with the intake of large amounts of water (poly-

Urine and electrolyte excretions by the kidneys are related to the hormones AVP and oxytocin (OT), among other endocrine mechanisms [7, 8]. AVP and OT have similar chemical structures and are both nonapeptides, differing in amino acids at positions 3 and 8 (**Figure 1**). They are synthesized with their corresponding carriers, neurophysins I and II, respectively,

**Diabetes insipidus Normal**

). The excre-

diluted urine, i.e., containing a reduced concentration of electrolytes (mainly Na+

significant differences between males and females.

70 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

**2. Central diabetes insipidus: an overview**

**2.1. General characteristics of human central diabetes insipidus**

**2.2. Neuroendocrine control of antidiuresis and electrolyte excretion**

Urine volume (liters/day) Up to 20 1–1.5 Urine osmolality (mOsm/L) <300 300–1400 Serum osmolality (mOsm/kg) >300 280–300 Serum sodium (mEq/L) >145 135–145

**Table 1.** General characteristics of patients with CDI in comparison to a healthy population.

DI [1–5].

sodium diet.

dipsia) (see **Table 1**) [2, 5, 6].

**Figure 1.** Chemical structure of (A) AVP and (B) OT. AVP is formed by amino acids cysteine-tyrosine-phenylalanineglutamine-asparagine-cysteine-proline-arginine-glycine-NH2 and OT by cysteine-tyrosine-isoleucine-glutamine-asparaginecysteine-proline-leucine-glycine-NH2 . In both cases, a ring of amino acids 1–6 is formed by a disulfide bond.

and human genes for OT-neurophysin I and AVP-neurophysin II are both on chromosome 20, separated by only 12 kb of intergenic sequences [9].

Both hormones are synthesized in the perikarya of the magnocellular neurons of hypothalamic paraventricular (PVN) and supraoptic nuclei (SON). There is a predominance of AVPproducing cells in the SON, while oxytocinergic cells are confined to the rostral and dorsal nucleus region. For its part, the magnocellular PVN possesses a similar number of oxytocinergic and vasopressinergic neurons; its anteromedial portion contains mostly OT neurons, whereas its anterolateral portion contains an inner part with AVP neurons, surrounded by a ring of OT neurons. Less densely packed groups of AVP and OT neurons are found in posterior PVN. The axons of this complex (magnocellular PVN and SON) pass through the inner part of the median eminence, forming the neurohypophyseal stalk, and terminate in the neurohypophysis. Axonal swellings have been identified near fenestrated capillaries in both the median eminence and neurohypophysis, permitting access of these neurohormones to the bloodstream [10].

Hyperosmolality, hypernatremia, and hypovolemia (isotonic loss of fluid and electrolytes) are the main triggers of AVP and OT secretion [7, 11, 12]. After AVP is released into the blood, it acts by binding to AVP receptor 2 (AVPR2) on the basal surface of renal collecting tubule cells, triggering an intracellular signaling cascade. This concludes with activation of a cyclic adenosine monophosphate kinase pathway, increasing the production and insertion of aquaporin-II (AQP2) channels into the cell membrane. This in turn leads to passive water resorption from the lumen of the nephron into the cells of the collecting duct along an osmotic gradient [12–15]. The consequent excretion of concentrated urine is a survival mechanism for prolonged starvation periods. Conversely, diluted urine is excreted in the absence of AVP [8]. The neurohormone OT is especially involved in the excretion of body sodium or natriuresis [11, 16–20]. This secretion appears to be stimulated by increases in the glomerular filtration rate [21] and reductions in tubular sodium reabsorption [22]. These effects are mediated by actions on OT receptors present in the kidney and also, indirectly, by cardiac secretion of atrial natriuretic peptide (ANP) [11]. OT may also act on AVPR2 to exert antidiuretic effects (**Figure 2**) [23, 24].

CDI manifestations can be transient, permanent, or triphasic. Transient CDI starts with an abrupt onset of polyuria and polydipsia within 24–48 h of surgery/trauma and gradually resolves over a 3- to 5-day period [13, 30]. In permanent CDI, polyuric and polydipsic symptoms arise immediately and remain chronic in the absence of treatment [2, 33]. Triphasic CDI (**Figure 3**) was first described by Fisher and Ingram [35]. The first phase, clinically identical to transient CDI, starts with polyuria and polydipsia within 24 h of surgery, followed by an interphase (oliguric phase), with a reduction in urine excretion volume and water intake, and

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The regulatory and behavioral symptoms of CDI may be explained by the arrangement of hypothalamic-neurohypophyseal fibers. In this way, CDI with only transient polydipsia may be produced by lesions located ventrally to the median eminence, possibly attributable to secretion into the median eminence area of the hormonal contents of magnocellular PVN and SON neurons. Distinct effects are observed after damage to the whole hypothalamic-neurohypophyseal tract, which results in a consistently increased water intake from the onset (permanent CDI). Finally, the presence of intact/preserved axons or the release of AVP by degenerating terminals is considered responsible for the oliguric interphase in triphasic CDI [28, 37, 38].

Hereditary CDI (hCDI) accounts for only 1–2% of all cases [5, 39]. It is frequently related to mutations in the AVP-neurophysin II gene, located distally at the short arm of chromosome 20

**Figure 3.** Water intake and urine volume in triphasic CDI (adapted from Ref. [34] with permission from Elsevier).


(20p13) and containing three exons. The signal peptide AVP and the NH<sup>2</sup>

a final persistent phase of polyuria and polydipsia [13, 28, 36].

*2.3.2. Hereditary CDI*

Both AVP and OT have been proposed to have synergic effects, with the natriuretic effect of their combined administration having a greater intensity and longer duration than the sum of the effects of each neurohormone [25, 26].

It has been reported that other hormones are involved in hydromineral regulatory processes, such as ANP co-localized with OT in hypothalamic magnocellular neurons (see Bundzikova et al. [27] for a review).

#### **2.3. Etiological bases of human CDI**

### *2.3.1. Acquired CDI*

The most common CDI follows brain injury or surgery in the region of the pituitary and hypothalamus with damage to mediobasal hypothalamus (MBH), hypophyseal stalk, infundibulum, or the pituitary gland itself [28–32].

**Figure 2.** *Upper*: Sagittal section of the brain (*left*) and kidney (*right*). *Lower* (*magnified insets*): AVP and OT neurosecretory system (*left*) and simplified structure of a nephron (*right*).

CDI manifestations can be transient, permanent, or triphasic. Transient CDI starts with an abrupt onset of polyuria and polydipsia within 24–48 h of surgery/trauma and gradually resolves over a 3- to 5-day period [13, 30]. In permanent CDI, polyuric and polydipsic symptoms arise immediately and remain chronic in the absence of treatment [2, 33]. Triphasic CDI (**Figure 3**) was first described by Fisher and Ingram [35]. The first phase, clinically identical to transient CDI, starts with polyuria and polydipsia within 24 h of surgery, followed by an interphase (oliguric phase), with a reduction in urine excretion volume and water intake, and a final persistent phase of polyuria and polydipsia [13, 28, 36].

The regulatory and behavioral symptoms of CDI may be explained by the arrangement of hypothalamic-neurohypophyseal fibers. In this way, CDI with only transient polydipsia may be produced by lesions located ventrally to the median eminence, possibly attributable to secretion into the median eminence area of the hormonal contents of magnocellular PVN and SON neurons. Distinct effects are observed after damage to the whole hypothalamic-neurohypophyseal tract, which results in a consistently increased water intake from the onset (permanent CDI). Finally, the presence of intact/preserved axons or the release of AVP by degenerating terminals is considered responsible for the oliguric interphase in triphasic CDI [28, 37, 38].

#### *2.3.2. Hereditary CDI*

atrial natriuretic peptide (ANP) [11]. OT may also act on AVPR2 to exert antidiuretic effects

Both AVP and OT have been proposed to have synergic effects, with the natriuretic effect of their combined administration having a greater intensity and longer duration than the sum of

It has been reported that other hormones are involved in hydromineral regulatory processes, such as ANP co-localized with OT in hypothalamic magnocellular neurons (see Bundzikova

The most common CDI follows brain injury or surgery in the region of the pituitary and hypothalamus with damage to mediobasal hypothalamus (MBH), hypophyseal stalk, infun-

**Figure 2.** *Upper*: Sagittal section of the brain (*left*) and kidney (*right*). *Lower* (*magnified insets*): AVP and OT neurosecretory

(**Figure 2**) [23, 24].

et al. [27] for a review).

*2.3.1. Acquired CDI*

the effects of each neurohormone [25, 26].

72 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

**2.3. Etiological bases of human CDI**

dibulum, or the pituitary gland itself [28–32].

system (*left*) and simplified structure of a nephron (*right*).

Hereditary CDI (hCDI) accounts for only 1–2% of all cases [5, 39]. It is frequently related to mutations in the AVP-neurophysin II gene, located distally at the short arm of chromosome 20 (20p13) and containing three exons. The signal peptide AVP and the NH<sup>2</sup> -terminal region of

**Figure 3.** Water intake and urine volume in triphasic CDI (adapted from Ref. [34] with permission from Elsevier).

neurophysin II are encoded by exon A of the AVP-neurophysin II gene, and the central region of neurophysin II is encoded by exon B, while the COOH-terminal region of neurophysin II and glycopeptide are encoded by exon C.

range enhanced the natriuretic response to AVP, reversing the renal sodium excretion deficit in neurohypox rats. Moreover, the combined use of OT and AVP at low doses that do not separately promote significant body sodium excretion revealed a marked synergic natriuretic effect superior to the sum of their individual effects. Consequently, the reduced natriuretic capacity of

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In anesthetized rats, the pituitary gland is exposed by parapharyngeal approach, and a dental drill is used to penetrate the base of the skull. The whole pituitary (anterior and posterior

Hypophysectomy (hypox) interrupts the hormonal secretion of both AVP and OT, depresses sodium excretion, and increases serum sodium concentration and water intake [45]. However, the peak sodium excretion of hypox animals treated with both neurohormones (AVP and OT) remains below that of intact rats, despite the synergic natriuretic effects of the combined treatment. Consequently, it has been proposed that the deficit produced in some adenohypophyseal hormones, such as prolactin (with antidiuretic effects in animals lacking AVP [46], may

Besides their hormone deficit, hypox animals often show marked disorders in their body sodium regulation. Thus, hypox rats consumed larger amounts of NaCl solutions than intact rats [47], an unexpected behavior considering that these animals are hypernatremic. However, hypox animals maintain an adequate hydromineral regulation capacity under homeostatic challenges. Thus, hypox animals significantly reduced their urinary sodium outputs on lowsodium diets [47], while hypertonic NaCl injection into the third ventricle stimulated their sodium excretion and antidiuresis [48, 49]. Given that these capacities could not have been activated by the secretion of AVP or OT, these and other data have been considered to demonstrate the functional integrity of the renin-angiotensin-aldosterone system in hypox rats [47].

Anesthetized rats are mounted on a stereotaxic frame to bilaterally apply an anodic current (1.5 mA) for 15 s through a stainless steel electrode insulated except at the tip. In Wistar rats, stereotaxic coordinates are 6.44 mm anterior to interaural line, 0.4 mm lateral to midline, and 0.2 mm dorsal to interaural line; the extent of lesions is usually estimated by using the rat brain atlas of Paxinos and Watson [50]. Generally, all lesioned animals show extensive MBH lesions in the rostrocaudal dimension with complete damage of the median eminence region. However, partial injury of the arcuate (Arc), ventromedial (VM), and dorsomedial hypotha-

MBH lesions in animals generate hypernatremia with triphasic polyuric and polydipsic CDI. A distinctive characteristic of MBH-lesioned animals is hyperphagia [51–53], which is not observed in hypox rats [54]. This greater food intake is likely attributable to damage to the Arc and/or VM hypothalamic nuclei [55]. From a neuroendocrine perspective, MBH lesions

lamic nuclei has also been observed in most rats (**Figure 4**) [51–53].

interrupt AVP, OT, and ANP secretions [49, 56, 57].

neurohypox animals appears to be attributable to a lack of both AVP and OT.

*3.1.2. Hypophysectomy*

lobes) is then removed by suction.

be relevant in this type of CDI.

*3.1.3. Mediobasal hypothalamic (MBH) lesion*

More than 50 AVP mutations segregating with autosomal dominant or autosomal recessive CDI have been described (see http://omim.org/entry/125700). Limited capacity to secrete AVP during severe dehydration is maintained by patients with autosomal dominant CDI, whose polyuric-polydipsic symptoms generally appear after the age of 1 year, when adults are more likely to comprehend the child's requests for water. There have also been reports of three families with autosomal recessive neurohypophyseal DI in which the patients were homozygous or compound heterozygotes for AVP mutations. Two of these families were phenotypically characterized by severe polyuria, polydipsia, dehydration, and early onset in the first 3 months of life.
