Nicotinic signs, acetylcholine accumulation

Nicotinic signs and symptoms result from the accumulation of acetylcholine at motor nerve endings in skeletal muscle and autonomic ganglia. Thus, there is fatigue, involuntary twitching, and muscular weakness, which may affect the muscles of respiration. Hypertension and hyperglycemia may also reflect the action of acetylcholine at sympathetic ganglia. 

The clinical picture of carbaryl intoxication results from inactivation of cholinesterase, resulting in the accumulation of acetylcholine at synapses in the nervous system, skeletal and smooth muscle, and secretory glands. Signs and symptoms of overexposure may include (1) muscarinic manifestations such as miosis, blurred vision, lacrimation, excessive nasal discharge or salivation, sweating, abdominal cramps, nausea, vomiting, and diarrhea (2) nicotinic manifestations including fasiculation of fine muscles and tachycardia and (3) central nervous system manifestations characterized by headache, dizziness, mental confusion, convulsions, coma, and depression of the respiratory center. 

The toxic effects can be divided into three types as the accumulation of acetylcholine leads to symptoms that mimic the muscarinic, nicotinic, and CNS actions of acetylcholine. Muscarinic receptors for acetylcholine are found in smooth muscles, the heart, and exocrine glands. Therefore, the signs and symptoms are tightness of the chest, wheezing due to bronchoconstriction, bradycardia, and constriction of the pupils (miosis). Salivation, lacrimation, and sweating are all increased, and peristalsis is increased, leading to nausea, vomiting, and diarrhea. 

Atropine is an antidotal treatment. It is used to reverse the muscarinic signs, but it will not reverse the nicotinic effects (muscular weakness, diaphragmatic weakness, etc.). Atropine blocks the effects of accumulated acetylcholine (ACh) at the synapse and should be continued until the nerve agent is metabohzed (Midthng et al, 1985). Over-atropinization can cause hyperthermia, tachycardia, agitation, mydriasis, and ileus, which can be life threatening in the horse (Meerstadt, 1982). 

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