Notes

Some Major Circuits in Aversion Processing: Focus on Amygdala People with lesioned amygdala face problems in feeling emotions. The amygdala is deeply involved in learning of aversive stimuli. The amygdala is made of several nuclei and the most important ones involved in learning aversive stimuli are: Basolateral Complex (BLA) and Central Nucleus (CEA). The amygdala is mainly composed of Glutamate and GABAergic inhibitory neurons.

Pavlovian, Classical or Stimulus-Stimulus Conditioning: Tone-Electroshock Conditioned Freezing

We know the importance of amygdala from these experiments: we have the animal in a chamber where a tone is played and a shock is exerted. If we present a neutral stimulus such that the end of the tone coincides with the shock, then we will quickly see not just an unconditioned response, but also a conditioned one. The conditioned stimulus takes on the emotional properties of the unconditioned one. At the first trial, the mice won't have any conditioned response, but already from the second trial the mice will learn (with a certain probability) and show a conditioned response. By increasing the salience of the unconditioned stimuli, the learning drastically increase as well. (Approx. from 50% after first trial to approx. 80% after first trial).

Amygdala Microcircuitry Cortex-like structure comprising glutamate long-range projections neurons (80%) and GABA interneurons (e.g., parvalbumin, somatostatin) (20%).

It is an example of Hebbian Associative Learning, when pavlovian conditioning is performed both the auditory system and the somatosensory system are involved, both projecting to thalamus, then cortices and then to through glutamate neurons in amygdala. As a result of some trials, the auditory neurons stimulated by the conditioned stimulus are capable of exciting the lateral amygdala neurons that were before only stimulated by the unconditioned one. However, the amygdala is not efficient in detecting and predicting unconditioned stimuli when a time delay occurs between the tone and the electroshock, in such cases the Hypothalamus plays a major role in predicting the unconditioned stimulus. So, the stimulus arrives to the central amygdala which in turn excites the lateral Hypothalamus (LH) that controls blood pressure, the Para-Ventricular-Nucleus (PVN) where Hormones are causing release of Cortisol are synthesized, which will then go back into the Amygdala to fixate emotional memory.

Optogenetic Investigation of Neural Circuits Underlying Emotional Behaviour AAV transduction-expression of light-sensitive proteins (opsins) in a specific regions/cell populations allows for the experimental regulation of the firing activity of those regions/cells. Optogenetic has been widely utilized to increase understanding of Pavlovian Aversion Learning.

Pavlovian Aversion Learning: Importance of Amygdala In Mouse, Optogenetic Inhibition (halorhodospin) of Basolateral amygdala (BLA) Glutamate neurons projecting to Central Nucleus (CeM) reduces Pavlovian (CS-US) aversion learning. Hence, they used an optic light to induce optogenetic inhibition when the unconditioned stimulus was presented. The inhibition of such pathway reduced learning, demonstrating that this pathway is important in the regulation of pavlovian aversive learning conditioning.