Neuropharmacology: Neurotransmitters in Action

We’ve explored how neurons transmit electrical spikes, but chemical signalling is just as important. In this experiment, you’ll use crickets to see first-hand how compounds such as MSG and nicotine alter neural activity.

Devices and Accessories2

About the Experiment

What Will You Learn?

How neurotransmitters can increase or decrease neural firing.
How to record spikes from a cricket’s cercal system.
Why the same chemical can have different effects depending on the receptor.

Background

In the late 1800s, scientists debated whether neurons were individual cells (Neuron Doctrine) or a continuous mesh (Reticular Theory). Electron microscopy confirmed that neurons are separate, meeting at tiny gaps called synapses. Chemicals—neurotransmitters—cross these ~20 nm gaps to pass messages.

We’ll record spikes from the cricket’s cercal system, two posterior appendages that detect wind and vibration. By applying MSG or nicotine we can watch the nervous system react in real time and see how evolution reuses the same molecules in very different ways.

Experiment

Preparation, Baseline & Chemical Tests
Preparation & Baseline
1. Collect a healthy cricket (pet shops sell them as feeders).
2. Anesthetize it in ice water for a few minutes.
3. Secure the cricket to cork or balsa, leaving the cerci exposed.
4. Place a ground electrode near the midline and a recording electrode close to the cerci.
5. Blow gently on the cerci and record the baseline spikes.
Experiment 1 – MSG Injection
1. Dissolve monosodium glutamate (MSG) in water to make a saturated solution.
2. Fill an insulin syringe with ~0.1 cc of solution.
3. Inject a small amount near the cerci, wait a few seconds, then record.
4. Blow on the cerci again and compare with the baseline. Many observers find that MSG reduces or even silences firing in crickets.
Experiment 2 – Nicotine Injection
1. Steep shredded tobacco in water for 1–2 days to produce a nicotine solution.
2. Inject a few drops (~0.1 cc) near the cerci.
3. Observe: firing rates usually rise sharply and the cricket may twitch.
4. Blow on the cerci once more and note the heightened response. Nicotine overstimulates insect neurons via nicotinic acetylcholine receptors.

Discussion & Next Steps

Observations

MSG often inhibits the cricket’s cercal system, reducing responsiveness to wind.
Nicotine excites the nervous system, raising baseline firing and response.

Think Further

Glutamate excites most human brain neurons and insect muscles—so why did MSG quiet the cricket’s cercal response? (Hint: insects also have glutamate receptors that inhibit neurons, and your MSG solution was very salty.)
Farm insecticides called neonicotinoids target the same receptor as nicotine—why might that make them deadly to insects?
What would caffeine, antihistamines or other household chemicals do?

This experiment shows how the same molecule can produce opposite outcomes depending on receptor type and species. Share your data or questions with hello@backyardbrains.com and keep exploring!

Draw the pathway from a wind-sensing hair on a cercus, along its sensory neuron, to the synapse in the cricket’s nerve cord where the message is passed on. Mark where your recording electrode sat and where you think MSG and nicotine act.

NGSS alignment

Performance expectations

Chemical synaptic signaling; drugs alter information processing.

MS-LS1-8 Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.
HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.

Science and engineering practices

2 Developing and Using Models

Draw the pathway from a cercal hair through its sensory neuron to the synapse in the nerve cord, and mark where you recorded and where you think the chemicals act.
3 Planning and Carrying Out Investigations

Record a baseline from the cricket's cerci, then inject MSG and nicotine and record again.
4 Analyzing and Interpreting Data

Compare firing before and after each injection.

Crosscutting concepts

Cause and Effect

Chemicals alter firing.
Structure and Function

The receptor a chemical binds decides the direction of its effect.

Disciplinary core ideas

LS1.D Information Processing

Chemical transmission.
LS1.A Structure and Function

Membrane receptors and channels.

Performance expectations, crosscutting concepts and core ideas from Hands-On Neuroscience Curriculum Overview, Unit 1, module 1.4, whose core lab this is. The practices are the ones this page asks students to do.