Functional Neuroanatomy in the CNS of Crayfish

 
 

Neuromodulators

Neurotransmitters are chemicals that are synthesized in the brain and are released from a cell across a synapse onto a target cell. Ones such as serotonin (5-HT), dopamine (DA) and GABA are also described as neuromodulators, because they change the properties of synapses and neurons. Crayfish use many types of escape when being attacked by predators. One of these escapes involves the abdomen tightening in such a way that it sends the crayfish upwards and forwards when feeling an attack from behind. This form of escape is called the Lateral Giant (LG) escape and is controlled by the LG neuron escape circuit. To clarify what is believed to transpire in LG, the individual functions of these neuromodulators must be explored.

Extensive research on serotonin in animals has shown 5-HT’s involvement in pain control systems, affective processes such as aggression and arousal, and in depression (Sapru, 2006). In previous crayfish studies 5-HT has been found to be a major facilitory modulator involved in the Lateral Giant (LG) escape circuit (Antonsen, 2002). DA and GABA, are thought to be involved in LG’s escape circuit also, though very little is known about their contributing functions. DA is a neurotransmitter that is understood to be involved in an animals’ ability to have motivation and desire, and also believed to be involved in the control of movements. There is even less known about GABA’s role in the LG circuit, but in general GABA is known to be one of the main inhibitors in the CNS (Edwards, 1999, Sapru, 2006) and it is understood that GABA deficiency can cause seizures. Previous studies suggest that GABA is involved in almost every inhibitory response in LG via binding to GABA receptors on Cl- channels (Edwards, 1999). A change in Cl- concentration causes an adjustment of the cell, for example when Cl- enters a cell inhibition and depolarization occurs.

Metamodulation

Varying studies have been done on different neuromodulators individually, yet how they interact with each other is enticingly unknown. Particularly, serotonin, dopamine and GABA have been seen to be expressed for some of the same behaviors, so would it not be advantageous to study their cooperative effects? One cannot simply assume that when different neuromodulators work together that their functions are additive. There has been some study done by Dr. Antonsen, and others, who have shown that in crayfish the metamodulation of serotonin and dopamine is quite interesting. When DA is combined with 5-HT there is an excitation of LG that is greater than the facilitation produced with 5-HT alone (Antonsen, 2003). This suggests that there is metamodulation between these two neurotransmitters.

Why Crayfish?

With the immense complexity of the human brain, it is incredibly difficult to study only one neurotransmitter, let alone three. With a crayfish's simpler nervous system and paralleling functions of these three neuromodulators, it is the perfect animal for study.