I. CELLULAR COMUNICATIONS

a. Body functions regulated by cell-to-cell communication

1. Homeostasis maintains internal Equilibrium. Many biological factors are maintained at or near certain
    levels.

2. There are two broad classes of chemical messengers

a. Hormones are chemicals released from specialized groups of secretory cells

b. Neurotransmitters are chemicals released from neurons (nerve cells)

b. Receptors enable target cells to detect the presence of the signal cells

a. Neurons act as secretory cells, target cells, and conductors of electrical signals. Each neuron releases one or more  specialized signal chemicals, called neurotransmitters. Each neurotransmitters has a unique three-dimensional structure. Some parts of the molecule are electrically more positive or negative than other parts. The location of these "polar" groups also make each neurotransmitter unique.

b. We are mainly concerned with the following neurotransmitters:

acetylcholine
norepinepherine
dopamine
serotonin
gamma amino butyric acid (GABA)
endorphins

c. Receptors are part of the "target-cell" membrane, their specific molecular structure allows ONLY certain neurotransmitters to bind to it at a synapse (a minute gap upon which nerve impulses can pass from one neuron to the next). This is referred to as affinity and is dependent upon molecular structure and polarity (specifically "charged" groups on the neurotrnsmitter and the receptor cell).

d. In its simplest form, when a neurotransmitter binds to a receptor, a slight conformational change occurs in the receptor structure, triggering "voltage-gated" channels in the "receptor" neuron to open. This allows an electrical current to flow across the membrane, further sending the electrical stimulus.

Because neurons conduct electrical signals along their membranes, the effect of a particular type of receptor is either to excitethis electrical activity or to inhibit it. Be familiar with the basic structure of the neuron.

Whether the effect of a neurotransmitter is excitatory or inhibitory depends on the type of receptor.

e. Effects of neurotransmitters at the chemical synapse (i.e., possible interaction by drug molecules):

1. neurotransmitter synthesis can be stimulated or inhibited.

2. neurotransmitter release can be blocked or enhanced.

3. neurotransmitter removal can be stimulated or inhibited

4. receptor site can be blocked or activated.

An agonist is an agent (drug chemical) that enhances synaptic transmission or mimics the effect of a natural neurotransmitter.

An antagonist an agent that blocks the action of a neurotransmitter.

f. Life Cycle of a Neurotransmitter

   Precursors circulate in the blood (usually amino acids) and taken up into the reuron by a process known as active
  "uptake." The cell expends energy in this step.

   Once in the neuron, the reurotransmitter is synthesized (formed) with the help of an enzyme.The
   neurotransmitter is now ready to act at a synapse.

   Removal from the synape occurs by 1) a process known as reuptake, in which the reurotransmitter is recognized
   and recaptured by the neuron that released it, expending energy in the process, or it is 2) metabolized by
   enzymes.

a. Central nervous system (CNS) consists of the brain and the spinal cord.

b. Somatic system carries sensory information into the CNS and carries motor information out and is responsible for
    voluntary actions. The neurotrnsmitter at the neuromuscular junctions (NMJ) in the somatic system is
    acetylcholine, which acts on receptors that excite muscle.

c. Autonomic nervous system (ANS) controls involuntary functions and divides into sympathetic (fight or flight) and
    parasympathetic (slow down) branches. The body's internal environment is monitored and controlled by the ANS.
    Amphetamines and other diet pills (chemical structure resembles norepinephrine) reduce stomach motility (part of
    the fight-or-flight mechanism)

The brain has a rich supply of blood (from 4 major arteries) so that drugs circulating n the blood have rapid access to the brain. However, the capillaries in the brain are different from those in the rest of the body - the cells are tightly joined together so that some molecules cannot pass freely into the brain. This specialization is part of the "blood-brain barrier," which keeps many drugs from reaching the brain. For a drug to be pyschoactive, it's molecules must be capable of passing through the blood-brain barrier.

a. Cerebral cortex processes viewed and auditory information, and higher mental processes take place in the cerebral
   cortex

b. Basal ganglia maintain proper muscle tone - damaged in Parkinson's disease

c. Hypothalamus is located at the base of the brain and above the pituitary gland and is involved in feeling, drinking,
   temperature regulation, and sexual behavior

d. Limbic system affects emotion, memory for location, and physical activity

e. Medial forebrain bundle is considered the pleasure system in the brain

f. Brainstem controls vomiting and regulates rate of breathing

a. Drugs are carried to the brain via blood

b. Blood-brain barrier prevents some drugs from entering the brain

c. Neurotransmitter molecules send messages from one neuron to another

a. Some drugs influence neuron functioning, and thus behavior, by affecting all neurons

b. Other drugs work by mimicking or blocking one kind of neurotransmitter

c. Presynaptic neurons can be affected in that the synthesis, storage, uptake, or deactivation of neurotransmitters can
   be altered by drugs

d. Drugs become deactivated by either being removed from the body or by being chemically changed before being
    excreted by the body

Drugs that affect existing biochemical processes in the brain often affect behavior, and this has led to many attempts to explain normal (not drug-induced) variations in behavior in terms of changes in brain chemistry.

a. Hippocrates believed that behavior rested on balances of four humors - blood, phlegm, yellow bile and black bile.

b. Chinese explained behavior through the Yin Yang

c. Searches for differences in the amounts of norepinephrine, dopamine, serotonin and other neurotransmitters have
   NOT found evidence to relate the levels of these substances to personality differences, pyschopathology, or mood
   swings. Again, as mentioned in a previous lecture, NO SINGLE biochemical theory of addiction has yet obtained
   sufficient experimental support to be considered an explanation.

d. Catecholamine theory of mood states that drugs interfere with the catecholamine neurotransmitters which influence
   mood (norepinephrine and dopamine). Too little activity in theswe systems can cause depression and too much can
   cause an excited or manic state.

e. Techniques that help in the study of brain activity are positron emission tomography (PET) - use of radiolabeled
   glucose - and nuclear magnetic resonance (NMR) scanning - use of magnetic fields