3.3 FUNCTIONAL CHANGES
There are many functional changes that can occur with advancing age:
Central nervous system
Impaired neurotransmission results from the decreased production of neurotransmitters
and reduced breakdown of those that are produced.
Reduced serotonin levels increase sleeping time and may cause neudisorders and depression.
Depletion of noradrenaline in the brain is also associated with depression.
Hypoxia leads to short-term memory loss, but not a loss of long-term memory. Oxygen
supplementation can reverse this memory loss.
Signs of senility are frequently recognised in older animals and are probably associated
with ageing changes in the nervous system but the precise cause-effect relationships
have been poorly documented (see Table 3.3).
Regular exercise improves many bodily functions probably through its effects on the
neuroendocrine system by increasing the secretion of growth hormone and reducing
the secretion of adrenocorticotrophic hormone (ACTH) and glucocorticoids.
Peripheral nervous system
With advancing age reduced function may occur in both the sympathetic and parasympathetic
parts of the peripheral nervous system. Changes may be presynaptic, synaptic or postsynaptic
resulting in impaired transmission of impulses to and from the CNS. This may produce
abnormal neurological and neuromuscular function leading to sluggish reflexes, reduced
pain response, impaired proprioception and difficulty with locomotion. The animal
may be less able to respond to sudden stresses placed on it because of impaired ability
to maintain homeostasis through neuroendocrine control mechanisms.
Inexperienced clinicians may find it difficult to differentiate between proprioceptive
deficits and muscular weakness in older animals.
Recently two drugs - propentofylline (VMtonin, Hoechst) and nicergoline (Fitergol,
Rhone Meneux) - have been granted veterinary product licences based upon their ability
to improve the signs associated with ageing in dogs such as lethargy and dullness.
They both have numerous pharmaactions on the bod~ but their main mechanism of action
is thought to be an increase in b~od supply to the brain resulting in improved neufunctions.
Is indicated for the treatment of lethargy and dullness in old dogs.
12.5-100 mg b.i.d. depending upon body weight.
Is an µ-adrenoreceptor antagonist and is indicated
in the treatment of age-related lethargy and dullness in dogs.
0.25-0.5 mg/kg daily.