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This paper contributes the analysis of the persistence of innovation activities, as measured by different innovation indicators and explores its past and path dependent characteristics. The study provides new insights on the role of R&D investments in innovation persistence and analyses differentiated patterns of persistence across product and process innovation, by accounting for complementarity effects between the two types of innovative behaviour. The empirical analysis is based on a sample of 451 Italian manufacturing companies observed during the years 1998-2006, and exploits both descriptive techniques such as Transition Probability Matrix and econometric methods based on dynamic probit models. Results highlight the relevance of innovation persistence. The highest level of persistence is found for R&D-based innovation activities, witnessing the actual presence of significant entry and exit barriers. Moreover, we obtain more robust evidence of persistence for product innovation than for process innovation when complementarity effects between the two types of innovation are accounted for.

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This paper contributes the analysis of the persistence of innovation activities, as measured by total factor productivity (TFP), and explores its internal and external determinants stressing its path-dependent characteristics. The external conditions, namely the quality of local knowledge pools and the strength of the Schumpeterian rivalry, along with the internal conditions (the actual levels of dynamic capabilities, as proxied by wage levels and firm size) exert a specific and localised effect upon the persistent introduction of innovations. A Multiple Transition Probability Matrixes (MTPMs) approach has been implemented to capture the contingent effects of external effects on long-term innovation persistence. The empirical analysis of the dynamics of firm level TFP for a sample of approximately 7000 Italian manufacturing companies observed during the years 1996-2005 is based on both the comparison of different transition probability matrixes and on dynamic discrete choice panel data models. The evidence provided by the test of MTPMs in sub-periods suggests that innovation persistence is path-dependent, as opposed to past-dependent. JEL CLASSIFICATION: O31, C23, C25, L20

Low levels of central serotonin (5-HT) have been related to the state of depression, and 5-HT is the major target of the newer antidepressant drugs such as selective serotonin reuptake inhibitors (SSRIs). Neurons and platelets display structural and functional similarities, so that the latter have been proposed as a peripheral model of central functions. In particular, in blood more than 99% of 5-HT is contained in platelets, so that one could consider changes in 5-HT levels in platelets as a mirror of changes in central 5-HT. Here, this hypothesis has been studied via the analysis of the influence of: (1) the forced swimming test (FST, which has been proved to be of utility to predict the clinical efficacy of antidepressants in rodents) and (2) treatment with the SSRI fluoxetine upon 5-HT levels monitored in brain regions and in peripheral platelets by means of electrochemical in vivo and ex vivo measurements. The results obtained confirm that the FST increases immobility; furthermore they show a parallel and significant decrease in cerebral (brain homogenate) and peripheral (in platelet-rich plasma, PRP) voltammetric 5-HT levels following the FST in naive rats. In addition, subchronic treatment with fluoxetine was followed by a significant increase in 5-HT levels in PRP, while the same SSRI treatment performed within the FST resulted in a decrease in the 5-HT levels in PRP. However, this decrease was inferior to that observed without SSRI treatment. These data suggest that there is an inverse relationship between immobility and the levels of 5-HT in PRP and that these peripheral 5-HT levels are sensitive to: (1) the FST, (2) the treatment with fluoxetine and (3) the combination of both treatments, i.e. SSRI + FST. It has been reported that SSRI treatment at first inhibits the 5-HT transporter in brain, resulting in increased extracellular 5-HT, while following sustained SSRI treatments decreased intracellular levels of central 5-HT were observed. Accordingly, the present data show that the initial block of 5-HT reuptake is revealed by the selective increase in 5-HT levels (extracellular content) measured in PRP (not in insulated platelets, IPs) the 1st day of fluoxetine treatment. The initial action of this SSRI upon the 5-HT transporter in brain has also been confirmed by in vivo voltammetric data showing selective increase in the serotonergic signal following local injection of fluoxetine into the brain region studied. Successively, the major effect monitored is a decrease in 5-HT levels, which is more evident in IPs than in PRP. However, it is known that following 2 weeks treatment with an SSRI, 5-HT autoreceptors are desensitized and the serotonin synthesis is restored, together with the intracellular 5-HT levels. The present data showing that the levels of 5-HT in IPs tend to return to control values 12 days after the beginning of chronic fluoxetine treatment suggest that 5-HT levels in IPs (intracellular environment) mirror the influence of SSRI treatment upon the central 5-HT system. On the other hand, a...

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