Paradoxical firing of thalamic neurons under neuropathic pain state in rats.

OBJECTIVES: A novel evaluative approach was used to determine single unit activities of non-bursting intralaminar thalamic nuclei under neuropathic pain state following dorsal rhizotomy.

SETTINGS AND DESIGN: Extensive dorsal rhizotomy at cervicothoracic level in rats was used as a model of central pain. After rhizotomy, rats were divided into two groups: rats without any signs of self-mutilation, and those presenting self-mutilation. Spontaneous single unit activities of neurons of intralaminar thalamic nuclei were recorded and interspike intervals (ISIs) of non-bursting cells were counted for both groups and compared with that of non-rhizotomized control rats. Chaodynamic methods were applied for the evaluation of the ISIs.

RESULTS: In control rats Lyapunov exponents, Shannon entropy and mutual information average values were significantly higher than those of rhizotomized rats without any signs of self-mutilation. Paradoxically, in animals presenting self-mutilation following rhizotomy the evaluated parameters were similar to those of controls. Further, Lyapunov exponents were positive values in all animals indicating chaotic pattern of the neuronal firing.

MAIN FINDINGS: 1. Neurons behave in chaotic way in all animals, 2. The most regular firing was found in non-mutilating rhizotomized animals, 3. Patterns of the firing in selfmutilating rats were similar to those in controls.

CONCLUSIONS: It is concluded that pain feeling is not executed neither by changes of chaotic dynamics of non-bursting intralaminar thalamic neurons. On the other hand, the paradoxical firing of the neurons under pathological brain matrix might participate in modification pain feeling.