Andrology (1) EN hormones1

Anatomy of the Canine Penis

Understanding Erection: The canine penis undergoes specific morphological changes during erection and copulation. It is explained through a schematic interpretation of the glans penis:
- A: Resting State. The penis is in a soft, non-erect position.
- B: Erection: During this phase, blood fills the cavernous spaces of the bulbus glandis and pars longa glandis, causing the penis to become erect.
- C: Copulation: In this state, both the male's ischiourethral muscle and the female's constrictor vestibulae contract, leading to venous occlusion and engorgement of the penis. This results in intromission and formation of the corona glandis, displayed when the penis is situated in the fornix of the vagina, aligning the urethra's opening closer to the cervix.

Ejaculation Mechanism: The ejaculation reflex is primarily controlled by the parasympathetic system, initiated by sensory nerves within the glans penis. These nerves relay signals to the spinal cord via the dorsal nerve of the penis (a branch of the pudendal nerve).
Semen Expulsion: The propulsion of semen from the urethra is facilitated by rhythmic contractions of the bulbospongiosus muscle surrounding the corpus spongiosum penis and the smooth muscles of the urethra.

Hormonal Changes: Daniel M. Kelly and Hugh Jones identify that obesity can reduce libido due to high aromatase activity in adipocytes, which converts testosterone to estradiol. Elevated levels of estradiol and adipocytokines such as TNFα, IL6, and leptin (caused by leptin resistance inherent in obesity) can inhibit the hypothalamic-pituitary-testicular axis response to lowered androgen levels.

Exercise Influence: Increased physical activity, such as the case in marathon runners, can lead to a phenomenon known as the "marathon syndrome," characterized by lower libido due to stress-induced reductions in GnRH, which alters the hypothalamic and pituitary hormone balance affecting testosterone production.

Potential Conditions: Certain diseases and disorders can diminish reproductive functions including:
- Hypothyroidism
- Cushing’s disease
- Diabetes mellitus
- Lesions in the testicles
- Lesions in the pituitary gland

HPT Axis Dysfunction: Hypothyroidism can lead to reduced reproductive function through disruptions in the HPT axis. A lower level of TRH from the hypothalamus results in diminished levels of TSH from the pituitary, subsequently decreasing thyroid hormones (T4 and T3). This negative feedback loop impacts the HPG axis, reducing GnRH, LH, and FSH production in the gonads, which ultimately can lead to decreased testosterone levels, decreased libido, erectile dysfunction, mood disorders, weight gain, and delayed ejaculation.

Hormonal Imbalance: Cushing's disease can also lower libido through reduced GnRH secretion triggered by stress and glucocorticoids, affecting the hypothalamus and leading to imbalances in peripheral sex hormones.

Pathophysiology: Type 1 and Type 2 diabetes can impact libido due to diabetic neuropathy and secondary testicular failure. Prolonged hyperglycemia results in oxidative stress affecting insulin action and damaging spermatozoa:
- Consequences: This leads to erectile dysfunction, ejaculatory dysfunction, reduced libido, and nuclear and mitochondrial DNA damage.
Testicular Health: Diabetes can cause degeneration of testicular tissue, reduced levels of testosterone, and a decrease in Sertoli cell function, significantly impairing sperm motility.

Testicular Lesions: Factors like degeneration or trauma to testicles can be diagnostics conditions to evaluate, typically revealing elevated LH and requiring chorionic gonadotropin (HCG) testing to assess testosterone production capability.
Secondary Diseases: Hypophysis disorders such as trauma or neoplasia can also lead to low libido.
- Diagnostics: Low levels of FSH or LH may be indicative, and treatment may include HCG or testosterone therapy.