drugs

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  • Created by: Charlie
  • Created on: 06-05-15 00:12

Opiates

  • Opiates are a type of narcotic drug that act as depressants in the central nervous system (CNS). These drugs operate by affecting neurons in the CNS, which leads to symptoms such as drowsiness, relaxation, decreased inhibition, anesthesia, sleep, coma, and even death.Opiates come from from opium, which can be produced naturally from poppy plants or derived from semi-synthetic alkaloids. Some of the most common opiates include morphine, codeine, heroin, hydrocodone and oxycodone.

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heroin reward pathway

  • When a person injects heroin , the drug travels quickly to the brain through the bloodstream. Once in the brain, the heroin is converted to morphine by enzymes; the morphine binds to opiate receptors in certain areas of the brain. These include the cerebral cortex, the VTA, nucleus accumbens and the Amygdala.
  • the morphine binds to opiate receptors that are concentrated in areas within the reward pathway this includes the VTA, nucleus accumbens, and cortex). Opiates binding to opiate receptors in the nucleus cause increased dopamine realse which creates a euphoric effect. Three types of neurons participate in opiate action: one that releases dopamine , a neighboring terminal  that contains a different neurotransmitter, and the post-synaptic cell that contains dopamine receptor. 
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heroin reward pathway 2

opiates bind to opiate receptors  on the neighboring terminal and this sends a signal to the dopamine terminal to release more dopamine. It has been theorixed that opiate receptor activation decreases GABA release, which normally inhibits dopamine release, so dopamine release is increased.Heroin phyically causes changes in the body such as reduced size of pupils,  profuse sweating , severe depression of breathin, reduced levels of sex hormones and even death.

  • Morphine also binds to areas involved in the pain pathway (including the thalamus, brainstem, and spinal cord). Binding of morphine to areas in the pain pathway leads to analgesia (loss of pain).

 

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Heroin addiction

  • Heroin is so addictive because it activates many regions of the brain particularly the regions that are responsible for producing both the sensation of “reward” and physical dependence.  Together, these actions account for the user’s loss of control and the drug’s habit-forming action.
  • When drugs such as heroin are used repeatedly over time, tolerance may develop. Tolerance occurs when the person no longer responds to the drug in the way that person initially responded. Stated another way, it takes a higher dose of the drug to achieve the same level of response achieved initially. For example, in the case of heroin, tolerance develops rapidly to the analgesic effects of the drug.

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Heroin addiction 2

  • when morphine binds to opiate receptors, it triggers the inhibition of an enzyme (adenylate cyclase) that orchestrates several chemicals in the cell to maintain the firing of impulses. After repeated activation of the opiate receptor by morphine, the enzyme adapts so that the morphine can no longer cause changes in cell firing. Thus, the effect of a given dose of morphine or heroin is diminished. The development of tolerance to the analgesic effects of morphine involves different areas of the brain separate from those in the reward pathway. the two areas involved here, the thalamus, and the spinal cord. Both of these areas are important in sending pain messages and are responsible for the analgesic effects of morphine.
  • With repeated use of heroin, dependence also occurs. Dependence develops when the neurons adapt to the repeated drug exposure and only function normally in the presence of the drug. When the drug is withdrawn, several physiologic reactions occur.  This is known as the withdrawal syndrome. In the case of heroin, withdrawal can be very serious and the abuser will use the drug again to avoid the withdrawal syndrome.Many of the withdrawal symptoms from heroin or morphine are generated when the opiate receptors in the thalamus and brainstem are deprived of morphine.
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Psychomotor stimulants

psychomotor stimulant drugs are defined as those which produce increased spontaneous motor activity. In contract to  Opirates such as Heroin which have a depressive, calming effect on the indivdual. an example of a psychomotor stimulant cocaine. Cocaine acts as an inhibtor for dopamine (agonist) as it prevents the reuptake dopmine by blocking doamine reuptake transporter. Whereas Heroin falcitates this.

cocaine, reaches all areas of the brain, but it binds to sites in some very specific areas. These are h the VTA, the nucleus accumbens, and the caudate nucleus (the largest structure). cocaine binds to sites in areas of the brain that are rich in dopamine synapses such as the VTA and the nucleus accumbens.pamine (inside the terminal) that is released into the synaptic space. The dopamine binds to dopamine receptors and then is taken up by uptake pumps back into the terminal.Cocaine binds to the uptake pumps and prevents them from transporting dopamine back into the neuron terminal. So more dopamine builds up in the synaptic space and it is free to activate more dopamine receptors. is the same effect with morphine, where morphine increased dopamine release from the terminal to produce more dopamine in the synaptic space.  The reinforcing (rewarding) effects of psychomotor stimulants (cocaine and amphetamine) depend on the mesocorticolimbic dopamine system innervating the nucleus accumben.

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Cocaine reward pathway

cocaine binds within the reward pathway (the VTA and the nucleus accumbens). As a result of cocaine's actions in the nucleus accumbens (point to the dots of cocaine in the VTA and nucleus accumbens), there are increased impulses leaving the nucleus accumbens to activate the reward system. This pathway can be activated even in the absence of cocaine (i.e., during craving). Indicate that with repeated use of cocaine, the body relies on this drug to maintain rewarding feelings. The person is no longer able to feel the positive reinforcement or pleasurable feelings of natural rewards (i.e. food, water, sex)--the person is only able to feel pleasure from the cocaine. Thus the user becomes dependent and when the cocaine is no longer present, anhedonia (inability to feel pleasure) and depression emerge as part of a withdrawal syndrome. To avoid this, the user goes back to the cocaine.

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Rats and Cocanie Tomsen et al 2011

Scientists have measured increased dopamine levels in the synapses of the reward pathway in rats self-administering cocaine. Just as they did for heroin, rats will press a bar to receive injections of cocaine directly into areas of the reward pathway such as the nucleus accumbens and the VTA. Again, if the injection needle is placed near these regions (but not in them), the rat will not press the bar to receive the cocaine. The ability of rats to self-administer cocaine is an excellent predictor of the addictive potential of this drug.

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Conclusion

the reward pathway is shown along with several drugs that have addictive potential. Just as heroin or morphine and cocaine activate the reward pathway in the VTA and nucleus accumbens, other drugs such as nicotine and alcohol activate this pathway as well, although sometimes indirectly (point to the globus pallidus, an area activated by alcohol that connects to the reward pathway).

Although each drug has a different mechanism of action, each drug increases the activity of the reward pathway by increasing dopamine transmission. Because of the way our brains are designed, and because these drugs activate this particular brain pathway for reward, they have the ability to be abused. Thus, addiction is truly a disease of the brain. As scientists learn more about this disease, they may help to find an effective treatment strategy for the recovering addict.

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