Cellular physiology of the turtle visual cortex: synaptic properties and intrinsic circuitry

Area of Science:

• Neuroscience
• Synaptic Physiology
• Comparative Neuroanatomy

Background:

• The dorsal cortex of the turtle, Pseudemys scripta elegans, serves as a model for investigating fundamental cortical circuitry.
• Understanding neuronal communication, including excitatory and inhibitory postsynaptic potentials, is crucial for deciphering brain function.

Purpose of the Study:

• To elucidate the synaptic physiology and intrinsic neuronal circuitry of the isolated turtle dorsal cortex.
• To identify the roles of different neuron types (pyramidal and stellate cells) and neurotransmitters (GABA) in cortical processing.

Main Methods:

• Electrophysiological recordings were performed on isolated turtle dorsal cortex.
• Stimulation of afferent pathways and focal application of neurotransmitters (GABA) and antagonists (bicuculline methiodide) were used.
• Analysis of excitatory postsynaptic potentials (EPSPs), inhibitory postsynaptic potentials (IPSPs), and action potentials in response to stimuli.

Main Results:

• Stellate cells exhibited long-lasting EPSPs and fired action potentials, while pyramidal cells showed short excitation followed by prolonged IPSPs.
• Two distinct types of IPSPs were identified in pyramidal cells, one chloride-dependent and GABA-sensitive, the other chloride-insensitive.
• GABA application mimicked the short-latency IPSP, and bicuculline blocked this IPSP and GABA responses, inducing epileptiform activity.

Conclusions:

• Stellate cells intrinsically generate GABAergic inhibition of pyramidal cells, forming feedforward and feedback inhibitory pathways.
• Pyramidal cells appear to be mutually excitatory, contributing to network dynamics.
• The intrinsic cortical circuitry in turtles shares fundamental similarities with mammalian telencephalic cortices.