Chocolate toxicity in dogs - theobromine

Theobromine is the compound in chocolate responsible for chocolate toxicity in dogs. Dogs metabolize theobromine very slowly in comparison to humans. The half life of theobromine is 17.5 hrs in dogs compared to 3 hrs in humans. Lethal toxicity in dogs is noted as 100-150 mg of theobromine per kg of body weight in dogs, though its toxic effects can be observed at much lower concentrations.

There are variables to take into account when dealing with chocolate/theobromine toxicity.

The weight of the dog.

Smaller dogs are many times more susceptible to toxicity. The ratio of mg/kg is much higher. Dogs are gorgers. Small dogs may have less body mass and weight, but can consume high amounts of chocolate if readily available.

The theobromine content in the chocolate eaten

The same amount of baker's chocolate is potentially more lethal than milk chocolate due to the content of cocoa solids. The more diluted the cocoa content in a chocolate product, the lesser the threat. This does not mean that it is ok to give dogs small amounts of chocolate. Even small amounts can lead to toxicity, even if it is not lethal. Dogs will suffer vomiting and diarreah after eating small amounts of chocolate as their body attempts to prevent further theobromine absorbtion. White chocolate is not dangerous since it does not contain theobromine. 

Beware of cocoa shell mulches used in landscaping. Cocoa shells naturally contain theobromine. Manufacturers often strip theobromine from the mulch, but not always. Check for content if your pet likes to eat the mulch.

So what happens when a dog consumes chocolate?

Theobromine stimulates the Central Nervous System (CNS). The dog shows signs of hyperactivity and restlessness, increased urine output, and irregular heartbeat. According to the Merck Veterinary Manual, Methylxanthines like theobromine increase intracellular calcium levels that lead to increased strength and contractility of skeletal and cardiac muscle. They compete for receptors in the CNS and inhibit esterases that result in increased cyclic AMP levels. They also increase circulating levels of epinephrine and norepinephrine. In severe cases, these chemical changes cause dogs to have tremors and seizures, hypertension, increased body temperature, and difficulty breathing. Coma and death may also result.

Other species are also susceptible to theobromine including cats and rabbits. However, other species are less prone to consume chocolate. Seek veterinary care immediately if you suspect your pet has ingested chocolate.

Chocolate and the Brain

Eating chocolate has long been associated with a feeling of well-being, alertness, and satisfaction. Researchers have identified some key components in chocolate that account for the emotions of eating chocolate.

Anandamide

Anandamide is a naturally-occurring neurotransmitter molecule synthesized in areas of the human brain important in enforcing and breaking short term neural connections. Its role is important in memory, higher thought processes, movement control, and forgetfulness. Anandamide binds to the so-called "bliss" receptor. Its effects include dulling pain and inducing an overall feeling of well-being. Unfortunately for humans, anandamide is a fragile molecule which breaks down easily.

Researchers have isolated at least three compounds in chocolate that strongly resemble anandamide. Those compounds are able to bind to the bliss receptor and produce effects similar to those of anandamide. Furthermore, they have also found in chocolate numerous other compounds that inhibit anandamide breakdown. According to the research of Daniele Piomelli and Emmanuelle diTomaso, chocolate has "pharmalogically-active substances that...may be responsible for certain drug-induced psychoses associated with chocolate craving."

Theobromine

According to xocoatl.org, the often-quoted caffeine content of chocolate is a myth. The stimulant effects often attributed to caffeine, are in fact, the effects of theobromine, which makes up about 1-2% of cocoa by weight.  Like caffeine, theobromine increases the activity of neurotransmitters responsible for increased attention and alertness. But unlike caffeine, theobromine is a much milder stimulant to the central nervous system and is not physiologically addictive. Theobromine also increases a sense of well-being as a mild anti-depressant. It is proven to be a cause of physical and mental relaxation. It stimulates the cardiovascular and  muscular systems. All in all, theobromine may be responsible for giving chocolate its aphrodisiac charactersitics.

Though chocolate contains trace amounts of cannabinoid and phenylethylamine chemicals, research has shown that there are no chemical properties that would cause a physical dependency. Craving chocolate and satisfying the crave has more to do with the sensory experience of consuming chocolate than meeting a  physiological need.

The following websites are good sources for more information on chocolate and its compounds:
http://antoine.frostburg.edu/chem/senese/101/features/anandamide.shtml
http://www.xocoatl.org/