CNS Stimulants

CNS Stimulants

The CNS stimulants are classified into three major categories

a.       Cortical stimulants

b.      Medullary stimulants

c.       Spinal stimulants

Cortical stimulants

These are further divided into three types

1.       Classical cortical stimulants – Cocaine, Xanthines, Ephedrine and Amphetamine

2.       Psychotomimetics – Lysergic acid

3.       Antidepressants (Thymoleptics) – Imipramine , tranylcypramine and Isoniazid.

Classical cortical stimulants

·         Cocaine, Amphetamine, Ephedrine and Xanthine are cortical stimulants.

·         These drugs affect mainly the sensory cortex but the motor cortex is also stimulated with high dose.

Cocaine

·         It is an alkaloid obtained from the South American shrub Erythroxylon coca.

·         They enhance mental activity, increase work performance and reduce fatigue.

·         Large doses produce delirium and clonic convulsions followed by depression.

·         Cocaine is a local anaesthetic but it is no longer used clinically because of its irritant action and side effects on the CNS and heart, causing a negative dromotrophic action.

Xanthines

·         The methylxanthines caffeine, theobromine, theophylline and aminophylline are a group of naturally occurring alkaloids present in certain beverages .Eg Tea (caffeine and theophylline), coffee (theophylline).

·         The effects of xanthenes are provided by the inhibition of enzyme phosphodiesterase which inactivates cAMP.

·         They increase mental activity , allay drowsiness and fatigue

·         Direct myocardial stimulation results in increased rate and force of contraction.

·         Relaxation of smooth muscles.

·         Diuretic action on kidney

·         Bronchodilator.

Clinical use

·         Aminophylline  - as bronchodilator and to treat congestive heart failure.

·         Theophylline  and theobromine were formally employed as diuretics in the treatment of edema.

Ephedrine and Amphetamine

·         These are potent CNS stimulants.

·         Ephedrine is present in the herb and used in Chinese medicine for many years.

·         Amphetamine and methylamphetamine are synthetic drugs possessing greater central stimulant actions than ephedrine.

·         The manifestations of CNS stimulation include increased alertness and wakefulness and a feeling of euphoria in man. In addition physical work capacity is improved and sleep is prevented.

·         Large doses produce sensations of dizziness, disorientation and apprehension.

·         The other agents belonging to psychomimetics and thymoleptics have some uses in man in the therapy of depression, but they are not used clinically in animals.

Medullary stimulants

·         The main clinical use of these drugs is to antagonize the effects of excessive dose of CNS depressants on the medullary centre.

·         Drugs used are picrotoxin, bemegride, leptazol, nikathemide and doxapram

·         Only doxapram is now used clinically to a significant extent in veterinary medicine.

·         In overdose they cause clonic convulsions

·         Doxapram is more efficacious and margin of safety to be convulsant and respiratory stimulant dose is relatively wide.

Spinal stimulants

·         Strychnine and brucine are alkaloids obtained from seeds of strychnos nuxvomica

·         Site of action is the spinalcord where they block the action of the inhibitory neurotransmitter glycine.

·         Reflex responses are enhanced with small dose. The sense of touch, hearing and smell may be rendered more acute. Larger dose produce extremely painful, tonic convulsions, alternating with periods of fatigue.