Two Key Studies of Excitatory & Inhibitory Synapses: Streeter et al. (2010) & Kraal et al. (2020) (HL IB Psychology)

Key Study 1 (Excitatory neurotransmitter): Kraal et al. (2020)

Aim: To investigate the role of dietary glutamate (in the form of monosodium glutamate (MSG)) in chronic pain and psychiatric disorders such as anxiety and schizophrenia.

Procedure: The researchers reviewed previously published research on the topic as part of their review article. Glutamate is a key excitatory neurotransmitter which, as a dietary source, can be found in the food flavouring additive MSG (which features particularly in cooking which uses a lot of soy sauce but also in parmesan cheese, meat, and many processed foods such as canned soup).

Results: The research reviewed suggests that an excess of dietary glutamate can be harmful to both physical and mental health. Too much dietary glutamate appears to be linked to ongoing chronic pain (pain which is continuous) and to a range of psychiatric disorders such as anxiety, PTSD, OCD, schizophrenia and depression.

Conclusion: Adopting a diet which is low in dietary glutamate such as MSG may lead to a decrease in chronic pain and better mental health.

Evaluation of Kraal et al. (2020)

   Strengths

• The findings of this review have good application as they can be used to inform preventative/intervention strategies for patients with chronic pain and/or mental illness

  • Conducting a review article means that the researcher is able to amass a lot of evidence to assess rather than relying on the results of one study alone

   Limitations

• A review article uses secondary data which means that the researcher has not been able to exert any control over the conditions in which it was obtained which reduces the reliability of the findings

  • It would be very difficult for policymakers such as governments or health committees to insist that people adopt a low-MSG diet which does limit the usefulness of the findings to some extent

Key terms:

• Excitatory neurotransmitter  

• Dietary glutamate  

• Monosodium glutamate

Key Study 2 (Inhibitory neurotransmitter): Streeter et al. (2010)

Aim: To investigate whether yoga is linked to improved mood, decreased anxiety and GABA (an inhibitory neurotransmitter) levels.

Participants: 34 healthy adults, 19 of whom were randomly allocated to the yoga group and 15 to the walking group.

Procedure: The participants were instructed to either practice yoga or take walking exercise for 60 minutes at a time, three times a week across the course of 12 weeks. Mood and anxiety scales were taken at weeks 0, 4, 8, 12. The participants also underwent magnetic resonance spectroscopy (MRS) scans at each of these time intervals as well (MRS uses the same machinery as MRI scanning but it measures metabolic changes instead of structure). 

Results: The participants who had engaged in yoga practice reported increased mood and a decrease in anxiety than the walking exercise group. There were positive correlations between improved mood and decreased anxiety and GABA levels in the thalamus (a structure in the brain involved in movement, sensory information, and alertness). The yoga group had positive correlations between changes in mood and changes in GABA levels i.e. as GABA levels increased so did mood.

Conclusion: Yoga may be linked to an increase in GABA which in turn may be linked to increased mood and decreased anxiety which could make it a good alternative to medication for patients with anxiety disorders.

Evaluation of Streeter et al. (2010)

   Strengths

• The procedure took place over 12 weeks, which means that any variation or changes in GABA levels and mood were easy to measure and compare

  • Using objective measures such as MRS increases the reliability of the findings  

   Limitations

• It is unclear as to whether the benefits experienced by the yoga group were short-term or long-term as no follow-up study was conducted

  • Individual differences could account for the improved mood of the yoga group: perhaps this group were naturally more positive and upbeat than the walking exercise group

Key terms:

• Inhibitory neurotransmitter  

• GABA  

• MRS