2.3 Neurotransmitters and Behaviour

What are neurotransmitters?

Chemical messengers used by neurons to communicate with each other.

How are Neurotransmitters sent? (send)

Released by axon terminals of sending neurons

How are the Neurotransmitters sent? (travel)

They travel across a small gap called a synaptic cleft.

How are Neurotransmitters sent? (received)

Bind onto the receptors of receiving neuron.

What is the nature of the nervous system process?

Partly electrical and partly chemical.

Action Potential Mechanism

Every neuron has threshold of excitation

if the sum excitation exceeds the threshold, the neuron "fires"

Generating a brief pulse called action potential

This travels along the axon to other neurons, passing excitation further

What is the Threshold of Excitation

The minimum level of electrical stimulation required to generate an action potential

Action potential

A rapid electrical impulse that travels along length of neuron, carrying information from one part of brain to another.

Different neurotransmitters?

More than 100 have been identified. All are broadly divided into two groups.

Excitatory neurotransmitters

Chemical messengers that bind to receptors and increase the likelihood of an action potential occurring.

Example of Excitatory Neurotransmitters

Dopamine - plays a key role in the brain’s reward and motivation system.

Inhibitory Neurotransmitters

Chemical messengers that bind to receptors and decrease the likelihood of an action potential occurring.

Example of Inhibitory Neurotransmitters

Serotonin - Responsible for regulating various physiological processes such as mood, appetite, sleep, and memory.

What are neurotransmitters affected by?

Agonists and Antagonists

What are Agonists?

Molecules that bind to activate receptors, mimicking the actions of the natural ligand.

How do Agonists activate receptors

When agonist binds to receptor, it activates the same signalling pathways as natural ligand, resulting in physiological effects that influence various bodily functions and processes.

What are full agonists?

Activate to the fullest extent possible.

What are partial agonists?

Activate it to a lesser degree, producing a weaker effect.

Antagonists

Bind to receptors without activating them and prevent the natural ligand from binding to activating the receptor.

What do Antagonists do?

Block the actions of agonists, reducing or preventing physiological response.

What are competitive antagonists?

Bind to the same site on receptor as natural ligand or agonist, but do not activate

What are non-competitive antagonists?

Bind to different site on the receptor, changing conformation of the receptor.

Passamonti et al. year

2012

Aim

To investigate effects of acute tryptophan (ATD) depletion on prefrontal-amygdala connectivity while viewing facial signals of aggression

Method

True Lab Experiment

Design

Repeated Measures; conditions administered on separate days (Double-blind)

Sampling Strategy

Self-selected with purposive; 30 healthy volunteers (17 women and 13 men)

IV

Whether they took Tryptophan Deleting drink or placebo

DV

Activity between Amygdala and PFC

Procedure: 1

Participants categorized the sex of angry, sad, and neutral faces

Findings: ATD

ATD = weaker connectivity between PFC and amygdala when ppts saw angry faces

Did not significantly affect connectivity when ppts saw sad oe neutral faces

Conclusion 1:

Low serotonin = waken emotional regulation in response to angry = increased reactive aggression

Conclusion 2:

Implication for understanding aggression and mental health disorders

ERQ: What study should be used with Passamonti for a ERQ on neurotransmitters

Crockett et al.

Crockett et al. Year

2010

Aim

To investigate whether Serotonin selectively influences moral judgement and behaviour through effects on harm aversion

Method

True lab experiment

Design

Repeated Measures (double-blind)

Sampling Strategy

Self-selected with purposive; 30 healthy subjects (13 males, 17 females)

Selection criteria

• any history of major organ or neurological/psychiatric disorders

• medication/drug use

• personal or family history of major depression or bipolar affective disorder

IV

Whether they took a SSRI, NRI, or placebo

DV

Moral Judgement Tasks e.g. Trolley Problem

Procedure 1: Start

Three session: one for each condition

Start of each session; ppts completed mood and trait questionnaires

Procedure 2: After 1.5

After 1.5 hours; ppts completed mood questionnaire, UG task, moral judgement, and executive function.

1.5 hours = coincide with peak compound effects

Procedure 3: Ultimatum Game Task (UG Task)

UG = two-player economic-decision making game

1. One player offers a way to split a sum of money; the other can accept or reject the offer

2. If rejected, nobody gets the money

Procedure 4: UG Task Data

• Proposer was pre-programmed

• 8 fair offers (40-50% of stake)

• 8 unfair offers (27-33% of stake)

• 8 extremely unfair offers (18-22% of stake)

• Ppts rated fairness 1 - 7

Procedure 5: Moral Judgement

• Judgements made judgements of a series of hypothetical scenarios

Procedure 6: Personal moral dilemmas (Moral Judgement type 1)

Direct physical harm to an individual to achieve greater good (e.g. pushing a person off a footbridge to stop a runaway trolley that would kill five people)

Procedure 7: Impersonal Moral Dilemmas (Moral Judgement type 2)

Harm but without direct physical activity (e.g. pulling a switch to divert a trolley onto one person instead of five)

Findings: UG

Higher serotonin - more likely to accept unfair compared to placebo

Serotonin = reduced emotional reactivity

Findings: Personal Moral Dilemma (Moral Judgement)

Citalopram = less likely to approve of personal moral dilemmas

Serotonin = influences moral decisions, especially personal harm

Findings: Impersonal Dilemma (Moral Judgement)

No significant differences

Conclusion

Serotonin = plays role in moral decision-making, particularly emotion reactions

Also affect social behaviour, increased accepting of unfairness