The striatum, a key structure of the basal ganglia system, is involved in implicit or ?habit? learning. This stimulus-response learning is acquired as the result of repeated pairing between stimulus and response. There is evidence suggesting that striatal activity present in early sessions of learning may be necessary for learning motor responses, whilst the automatic repetition of the same motor act may be independent from discharge of striatal neurons. According to this view, it has been recently demostrated that in young rats during the early stages of learning a sensorimotor paradigm, modifications of spontaneous excitability of a specific type of intrastriatal interneurons develop. This fact supports the view that habit learning, might induce consistent changes in neuronal discharge of striatal populations primarily involved in different aspects of behavioral learning.

Thus, we have started a study aimed at investigating whether reliable changes of the spontaneous firing pattern of different population of striatal neurons establish after that rats were overtrained (more than 8000 conditioned movements) to press a lever in response to a composite auditory/light conditioned stimulus to obtain a reward. The results obtained show that the frequency distribution of spontaneously active neurons in overtrained rats did not show the bias along the anteroposterior and mediolateral axis which characterizes the striatum of untrained rats. Furthermore, the distribution of the discharge rate shows that a differential change in specific neuronal population in the two groups of animals occurs when the striatal neurons are classified as either low-spiking (LS) neurons or fast-spiking (FS) neurons. The criteria to differentiate the neuronal types were based on the analysis of two variables: the coefficient of variance (CV), and the mean firing rate. The analysis of the firing rate of the two population, taken from a baseline of 2 min, showed a significant decrease of the spontaneous discharge rate of FS neurons in overtrained rats with respect to the same neuronal population recorded in untrained rats. This cluster of neurons was mainly located in the anteromedial region of the striatum, and exhibited a smaller CV than LS neurons, thus indicating that under overtraining their firing became more regular.

Overall, these preliminary data show: i) as a conditioned behavior becomes automatic, a change in neuronal striatal discharge develops. This modification consolidates to such a level to be recordable even in conditions of anesthesia under the influence of the same external sensory cues which usually drive behavior. ii) this conditioned change primarily affects the FS population in the anteromedial striatum, iii) following habituation, the sensory cues driving behavior might automatically activate intrastriatally stored informations for appropriate motor response. We are attempting to investigate whether among the different population of intrastriatal neuronal populations there is a particular class overexpressing specific neurochemical markers. The results of these studies may contribute to elucidate the role of striatal interneurons in cognitive deficits which characterize some neurodegenerative disorders.