Information about Sensitization

Sensitization is an example of non-associative learning in which the progressive amplification of a response follows repeated administrations of a stimulus (Bell et al., 1995). An everyday example of this mechanism is the repeated tonic stimulation of peripheral nerves that will occur if a person rubs his arm continuously. After a while, this stimulation will create a warm sensation that will eventually turn painful. The pain is the result of the progressively amplified synaptic response of the peripheral nerves warning the person that the stimulation is harmful. Sensitization is thought to underlie both adaptive as well as maladaptive learning processes in the organism.

Types of sensitization

For example, electrical or chemical stimulation of the rat hippocampus causes strengthening of synaptic signals, a process known as long-term potentiation or LTP (Collingridge, Isaac & Wang, 2005). LTP is thought to underlie memory and learning in the human brain.

A different type of sensitization is that of kindling, where repeated stimulation of hippocampal or amygdaloid neurons in the limbic system eventually leads to seizures in laboratory animals. Having been sensitized, very little stimulation is required to produce the seizures. Thus, kindling has been suggested as a model for temporal lobe epilepsy in humans, where stimulation of a repetitive type (flickering lights for instance) can cause epileptic seizures (Morimoto, Fahnestock & Racine, 2004). Often, people suffering from temporal lobe epilepsy report symptoms of negative affect such as anxiety and depression that might result from limbic dysfunction (Teicher et al., 1993).

A third type is central sensitization, where nociceptive neurons in the dorsal horns of the spinal cord become sensitized by peripheral tissue damage or inflammation (Ji et al., 2003). This type of sensitization has been suggested as a possible causal mechanism for chronic pain conditions. These various types indicate that sensitization may underlie both pathological and adaptive functions in the organism.

A common mechanism for the three mentioned types of sensitization is the activation of AMPA receptors on the post-synaptic membrane. Repeated stimulation of the pre-synaptic neuron will cause glutamate to be released into the synaptic cleft. The increased release of glutamate will activate the AMPA receptors. AMPA receptors will allow for additional Na+ to enter the post-synaptic neuron, thus increasing its depolarization. This will cause the post-synaptic neuron to fire continuously, thereby creating a prolonged response. It is possible that the intensity of the stimulation is what distinguishes the different types of sensitization, in that kindling may require more intense stimulation than LTP. Another possibility are alterations in the function of inhibiting GABAergic neurons. This, however, has not been established (McEarchern & Shaw, 1999).

Sensitization is a term referred to in psychology as how your body reacts to a drug.

Etiology

Sensitization has been implied as a causal or maintaining mechanism in a wide range of apparently unrelated pathologies including substance abuse and dependence, allergies, asthma, and some medically unexplained syndromes such as fibromyalgia and multiple chemical sensitivity. Sensitization has also been suggested in relation to psychological disorders such as post-traumatic stress disorder, panic anxiety and mood disorders (Rosen & Schulkin, 1998; Antelman, 1988; Post, 1992).

Experimental basis

Eric Kandel was one of the first to describe sensitization based on his experiments with the seasnail Aplysia in the 1960's and 1970's. Kandel and his colleagues gave Aplysia electric shocks to the head, which caused it to retract its siphon and gills (Kandel, 2004). Eventually, very little stimulation was needed to provoke the response, indicating that Aplysia had been sensitized. When tested several days after the initial trials, this response was still manifest. In 2000, Eric Kandel was awarded the Nobel Prize in Physiology or Medicine for his research in neuronal learning processes.

See also

• Long-term potentiation
• Synaptic plasticity
• Neuroplasticity

References

• Antelman, S. M.: Time-dependent sensitization as the cornerstone for a new approach to pharmacotherapy: drugs as foreign/stressful stimuli. Drug Development Research. 1988: 14; 1-30.
• Bell, I.R., Hardin, E.E., Baldwin, C.M. & Schwartz, G.E.: Increased limbic system symptomatology and sensitizability of young adults with chemical and noise sensitivities. Environmental Research. 1995; 70: 84-97.
• Collingridge, G.L., Isaac, J.T.R. & Wang, Y.T.: Receptor trafficking and synaptic plasticity. Nature Reviews. 2004; 5: 952-962.
• Ji, R., Kohno, T., Moore, K.A. & Woolf, C.J.: Central sensitization and LTP: do pain and memory share similar mechanisms? Trends in Neurosciences. 2003; 26(12): 696-705.
• Kandel, E.R.: The molecular biology of memory storage: a dialog between genes and synapses. Bioscience Reports. 2004; 24(4-5): 477-522.
• McEachern, J.C. & Shaw, C.A.: The plasticity-pathology continuum: defining a role for the LTP-phenomenon. Journal of Neuroscience Research. 1999; 58: 42-61.
• Morimoto, K., Fahnestock, M. & Racine, R.J.: Kindling and status epilepticus models of epilepsy: rewiring the brain. Progress in Neurobiology. 2004; 73: 1-60.
• Post, R.M.: Transduction of psychosocial stress into the neurobiology of recurrent affective disorder. American Journal of Psychiatry. 1992; 149(8): 999-1010.
• Rosen, J.B. & Schulkin, J.: From normal fear to pathological anxiety. Psychological Review. 1998: 105(2); 325-350.
• Teicher MH, Glod CA, Surrey J, Swett C Jr.: Early childhood abuse and limbic system ratings in adult psychiatric outpatients. J Neuropsychiatry Clin Neurosci. 1993 Summer;5(3):301-6.