Excitatory & Inhibitory Drugs

Excitatory Drugs

Nicotine

Nicotine is a tobacco product that acts on the cholinergic synapses of the body and the brain to cause a calming effect. After it’s received by the receptors it’s broken down by acetylcholinesterase but the enzyme can’t break down the nicotine molecules, which bind to the same receptors. This excites the postsynaptic neuron and it begins to fire, release a molecule called dopamine.

Dopamine gives the feeling of pleasure, a molecule of the ‘reward pathway’ of our brains.

Cocaine

Dopamine transporters are responsible for removing dopamine molecules from the synaptic cleft after they’ve done their job. Cocaine blocks these transporters, leaving dopamine trapped in the synaptic cleft. This causes dopamine to bind again and again to the receptors over stimulating the cell.

Cocaine concentrates in the reward pathway. It also actives the part of the brain controlling voluntary movements – this is the reason cocaine abusers are unable to stay still.

Amphetamine

Ampetamine stimulates transmission at adrenergic synapses and gives increased energy and alertness. It acts by passing directly into the nerve cells which carry dopamine and noraderenaline. It moves directly into the cesicles of the presynaptic neurone and causes them to release into the synaptic cleft.

These neurotransmitters would normally be broken down by enzymes into the synapse, but amphetamines interfere with the breakdown. This causes a high concentration to build in the synapse which causes euphoria. High concentrations of noraderanline may be responsible for alertness and the high energy effect of amphetamines.

Inhibitory Drugs

Benzodiazepine

Benzodiazepine reduces anxiety and can also be used against epileptic seizures. Its effect is to modulate the activity of GABA, which is the main inhibitory transmitter. When GABA binds to the postsynaptic membrane, it causes chloride ions to enter the neuron. This hyperpolarizes the neuron and prevents action potential.

Benzadiazepine increases the binding of FABA to the receptor and causes the post synaptic neuron to become more hyperpolarized.

Alcohol

The inhibitory neurotransmitter GABA is active throughout the brain. These transmitters act to control neural activity along brain pathways. When GABA binds to its receptors, the cell is less likely to fire. However, in other areas of the brain, the neurotransmitter glutamate acts as the brain’s general-purpose excitatory neurotransmitter.

When alcohol enters the brain it delivers a double sedative punch. First, it interacts with GABA receptors to make them even more inhibitory. Second, it binds to glutamate receptors, preventing the glutamate from exciting the cell.

Alcohol particularly effects areas of the brain involved in memory formation, decision making and impulse control.

THC

THC is the main psychoactive chemical in marijuana. Before marijuana enters the system, inhibitory neurotransmitters are active in the synapse. These neurotransmitters inhibit dopamine from being released. When activated by the body’s own native cannabinoid (called anandamide) cannabinoid receptors turn off the release of inhibitory transmitters. Without inhibition, dopamine can be released.

THC mimics anandamide and binds to cannabinoid receptors; inhibition is turned off and dopamine is allowed to move into the synapse.

Anandamide is known to be involved in removing unnecessary short term memories. It’s also involved for slowing down movement, making the user feel relaxed and calm. Unlike THS, anandamide breaks down very quickly in the body. This explains why anandamide doesn’t produce a perpetual natural ‘high’. 

THC & Cocaine

THC (Tetrahydrocannabinol) / Marijuana

Marijuana is usually smoked as a cigarette or in a pipe. When someone smokes marijuana, THC rapidly passes from the lungs into the bloodstream, which carries the chemical to the brain and other organs in the body.

THC acts on specific sites in the brain called cannabinoid receptor. They start off a series of cellular reactions that lead to the “high” that users experience when they use the drug. Some brain areas have many cannabinoid receptors while others have few or none. The highest density of cannabinoid receptors are found in parts of the brain that influence pleasure, memory, thinking, concentrating, sensory and time perception and coordinated movement. Marijuana intoxication can caused distorted perceptions impaired coordination, difficulty with thinking and problem solving and problems with learning and memory.

http://www.drugabuse.gov/infofacts/marijuana.html

Cocaine

Cocaine is a powerfully addictive central nervous system stimulant. This drug usually makes the user feel euphoric and energetic, but also increases body temperature, blood pressure and heart rate. Users risk heart attacks, respiratory failure, strokes, seizures, abdominal pain and nausea.

Cocaine also causes dopamine release. Cocaine blocks removal of dopamine from the synapse so that it builds up. This leads to over stimulation of the postsynaptic neurone. The synaptic effect of cocaine results from its ability to sustain the level of dopamine in the synapse. Since dopamine is the neurotransmitter in the ‘reward pathway’, the longer it stays in the synapse the better you feel.

http://www.nida.nih.gov/drugpages/cocaine.html

Synaptic Transmission

E1.

a)     Explain how pre-synaptic neurons can affect post-synaptic transmission of impulses (7)

Typically a presynaptic neurone excites a post synaptic neurone – the impulse is transmitted across the synapse. This is called an excitatory synapse. When the action potential reaches the area of the terminal buttons of the pre-synaptic neuron, it causes calcium ions to diffuse into the terminal buttons. Vesicles containing neurotransmitters fuse with the plasma membrane and release them into the synaptic cleft. The neurotransmitters bind with a receptor protein on the postsynaptic neurone membrane, this binding results in an ion channel opening and sodium ions diffusing through this channel. This initiates the action potential to begin moving down the post-synaptic neurone because it has been depolarized (made more positive).

However, some synapses are inhibitory synapses; the release of neurotransmitters into the cleft inhibits an action potential being generated in the post-synaptic neurone. At an inhibitory synapse the release of neurotransmitters into the synaptic cleft triggers the opening of ion channels, which allows Cl- ions to enter the neurone and K+ to leave. This makes the interior of the post-synpatic neurone more negative (hyperpolarised) and therefore less likely to initiate an action potential.


Mark Scheme: 
a. Pre-synaptic neurons can be excitatory or inhibitory
b. Chlorinegic  neurons released acetylcholine (standard wide use neurotransmitters)
c. Found in neuro-muscular junctions/ in autonomic nervous system/ most junction in voluntary nervous system
d. Adrengic nuerons release noradrenaline 
e. Found in sympathetic pathways 
f. Both types of neuron can be excitatory 
g. Neurotransmitters bind to receptors on post-synaptic membrane
h. Triggers opening of sodium channels / sodium moves across membrane
i. Causes depolarization 
j. Neurotransmitters are degraded/destroyed : acetyly choline esterase breaks down acetyl choline 
k. Other inhibitory neurotransmitters, e.g. GABA, dopamine 
l. Inhibitory neurotransmitters are less permeable to sodium / cause chloride ions to diffuse in
m. Causes hyperpolarisation
n. Potassium ions diffuse out 

b)     Explain the process of synaptic transmission (7)

At the far end of axons are swollen membranous areas called terminal buttons. Within these terminal buttons are many vesicles filled with neurotransmitters.

When an action potential reaches the area of the terminal buttons, it causes calcium ions to diffuse into the terminal buttons. Vesicles containing neurotransmitters fuse with the plasma membrane and releases the neurotransmitters into the synaptic cleft. Neurotransmitters diffuse across the synaptic cleft from the presynaptic neurone to the postsynaptic neurone.

Neurotransmitters bind with a receptor protein on the postsynaptic neurone membrane. This binding results in an ion channel opening and sodium ions diffusing in through this channel. This initiates the action potential to begin moving down the postsynaptic neurone because it’s been depolarized.

The neurotransmitter is degraded and broken into two or more fragments by specific enzymes. They’re then released from the receptor protein. The ion channel closes the sodium ions. The neurotransmitter fragments diffuse back across the synaptic gap to be reassembled into the terminal buttons of the presynaptic neurone. 


Mark Scheme: 
a. Presynaptic neurons pass the stimulus to post-synaptic neurons 
b. Presynaptic releases neurotransmitters into the synaptic cleft 
c. Process involves exocytosis
d. Exocytosis triggered by calcium ions entering into the bulb of the neuron
e. Neurotransmitter binds with the protein receptors on the post-synaptic cleft
f. Neurotransmitter binding causes ion channels to open
g. Ions diffuse into the cell
h. Causes depolarisation/hyperpolarisation of the pre-synaptic neuron
i. Outcome of the transmission depends on the type of receptor / channel opened
j. Specific example: sodium ions going into the post-synaptic neurone causes depolarisation
k. Specific example: chlorine ions going into the post-synaptic neurone causes hyperpolarisation
l. Neurotransmitters are destroyed by enzymes

Database Questions: Garter Snakes

DBQ Pg. 303-304

1.     (a) For each of the three populations calculate the percetages that were in the <5 and >5 groups. (3)

Lassen County Total = 567 + 114 = 681

Lassen County <5 % = (567 / 681) x 100 = 83.3%

Lassen County >5 % = (114 / 681) x 100 = 16.7%

Humboldt County Total = 149 + 314 = 463

Humboldt County <5 % = (149 / 463) x 100 = 32.2%

Humboldt County >5 % = (314 / 463) x 100 = 67.8%

Santa Cruz Total = 30 + 165 = 195

Santa Cruz <5% = (30 / 195) x 100 = 15.4%

Santa Cruz >5% = (165 / 195) x 100 = 84.6%

                 

(b)  Explain the differences in the behaviour of inland and coastal populations, in terms of natural selection. (4)

Coastal populations show a greater percentage of the population eating slugs more than five days while the inland population shows a greater number of the population eating slugs less than five days.

In coastal environments in California slugs are abundant so that coastal populations become more adapted to eating slugs through natural selection. Coastal snakes are adapted to eat slugs have an advantage and produce more healthy offspring. 


Slugs within inland areas are scarce so the inland populations of the garter snake are more adapted through natural selection to not eat slugs. Natural selection happens as inland snakes reliant on eating slugs struggle living within inland areas and their numbers begin to diminish, leaving the population of inland snakes not reliant on eating slugs to grow in the area. Inland snakes have a selection pressure to feed on other prey and these individuals are more successful. 

2.     Predict, with reasons, the result if garter snakes were moved from Lassen County to Santa Cruz. (3)

Garter snakes moved from Lassen County to Santa Cruz will face a greater abundance of slugs than they're used to. The Lassen County snakes will be less likely to survive in this new environment because they're more adapted to feed on other prey whereas in Santa Cruz there are more slugs. 

3.    Discuss the evidence from the bar charts for:

a.     The slug-eating response being inherited (2)

There’s a greater frequency of F1 hybrids accepted slugs during the first couple of days of the experiment. This is a similar characteristic of the Lassen County garter snakes that have the slug-eating response. The Lassen County and F1 hybrids have the same feeding pattern. 

b.     The alleles for slug eating being recessive (2)

In the F1 hybrid the behavioral pattern to not eat slugs seems to be prominent. This suggests that the gene to not eat slugs is dominant and the gene for eating slugs is recessive.

4. Suggest a type of receptor for the slug-eating snakes to detect slugs (1)

Chemoreceptors - they smell the slugs. They find the trail of the slug and follow it.