PSY2061 Chapter Notes - Chapter 15: Dopaminergic Pathways, Sodium Bicarbonate, Tegmentum
5 Jun 2018
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PSY2061 – Readings – Week 9
~ drug administration, absorption and penetration of the central nervous
system
• oral ingestion
•
o drugs dissolve in the fluids of the stomach and are carried to the
intestine - where they are absorbed into the bloodstream
o some drugs pass readily through the stomach wall e.g. alcohol -
and these take effect sooner because they do not have to reach the
intestine to be absorbed
o advantages
o
▪ ease
▪ relative safety
o disadvantages
o
▪ unpredictability
▪
▪ absorption from the digestive tract into the
bloodstream can be greatly influenced by such
difficult-to-gauge factors as the amount and type of
food in the stomach
• injection
•
o common in medical practice because the effects of injected drugs
are strong, fast and predictable
o typically made subcutaneously SC into the fatty tissue just beneath
the skin, intramuscularly IM into the large muscles or
intravenously IV directly into the veins at points where they run
just beneath the skin
o IV - many drug addicted persons prefer the IV route because the
bloodstream delivers the drug directly to the brain
o
▪ after an intravenous injections - there is little or no
opportunity to counter the effects of an overdose, an
impurity or an allergic reaction
▪ many drug users develop scar tissue, infections and
collapsed veins at the view sites on their bodies where
there are large accessible veins
• inhalation
•
o some drugs can be absorbed into the bloodstream through the rich
network of capillaries in the lungs
o many aesthetics are administered by inhalation - as are tobacco
and marijuana
o disadvantages
o
▪ difficult to precisely regulate the dose of inhaled drugs
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▪ many substances damage the lungs if they are inhaled
chronically
• absorption through mucous membranes
•
o some drugs can be administered through the mucous membranes
of the nose, mouth and rectum
~ drug action, metabolism and elimination
• drug penetration of the central nervous system
•
o once a drug enters the bloodstream it is carried to the blood
vessels of the central nervous system
o a protective filter - the blood brain barrier makes it difficult for
many potentially dangerous blood borne chemicals to access from
the blood vessels of the CNS into the extracellular space around
CNS neurons and glia
• mechanisms of drug action
•
o some drugs - e.g. alcohol and general anaesthetics act diffusely on
neural membranes throughout the CNS
o others act in more specific ways - by bonding to a particular
synaptic receptor, by influencing the synthesis, transport, release
or deactivation of particular neurotransmitters or by influencing
the chain of chemical reactions elected in postsynaptic neurons by
the activation of their receptors
• drug metabolism and elimination
•
o the actions of most drugs are terminated by enzymes synthesised
by the liver
o these liver enzymes stimulate the conversion of active drugs to
nonactive forms - a process referred to as drug metabolism
o in many cases - drug metabolism eliminates a drug’s ability to pass
through lipid membranes of cells so that it can no longer penetrate
the blood brain barrier
o small amounts of some psychoactive drugs are passed from the
body in urine, sweat, faeces, breath and mother’s milk
~ drug tolerance, drug withdrawal and physical dependence
• drug tolerance
•
o decreased sensitivity to a drug that develops as a result of
exposure to it
o demonstrated in tow ways
o
▪ by showing that a given dose of the drug has less effect than
it had before drug exposure
▪ or showing it takes more of the drug to produce the same
effect
o drug tolerance is a shift in the dose response curve - a graph of the
magnitude of the effect of different doses of the drug
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o one drug can produce tolerance to other drugs that act by the same
mechanism - known as cross tolerance
o drug tolerance often develops to some effects of a drug but not to
others
o tolerance may develop to some effects of a drug while sensitivity
to other effects of the same drug increases - increasing sensitivity
is called drug sensitisation
o drug tolerance is not a unitary phenomenon - there is no single
mechanism that underlies all examples of it
o two categories of changes underlie drug tolerance
o
▪ metabolic tolerance
▪
▪ drug tolerance that results from changes that reduce
the amount of the drug getting to its sites of action
▪ functional tolerance
▪
▪ drug tolerance that results from changes that reduce
the reactivity of the sites of action to the drug
• drug withdrawal effects and physical dependence
•
o sudden elimination can trigger an adverse psychological reaction
called a withdrawal syndrome
o the effects of drug withdrawal are virtually always opposite to the
initial effects of the drug
o
▪ this suggests that withdrawal effects may be produced by
the same neural changes that produce drug tolerance
▪ according to this theory exposure to a drug produces
compensatory changes in the nervous system that offset the
drug’s effects and produce tolerance - then when the drug is
eliminated from the body these compensatory neural
changes - without the drug to offset them - manifest
themselves as withdrawal symptoms that are opposite to
the initial effects of the drug
o individuals who suffer withdrawal reactions when they stop taking
a drug are said to be physically dependent on that drug
o severity of symptoms depends on the drug, duration and degree of
the preceding drug exposure and on the speed with which the drug
is eliminated from the body
~ drug addiction
• habitual drug users who continue to use a drug despite its adverse effects
on their health and social life and despite their repeated efforts to stop
using it
~ role of learning in drug tolerance
• major role
• contingent drug tolerance
•
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