Resumo

Our aim was to categorize layer I neurons of the rat visual cortex according to morphological and electrophysiological parameters and to evaluate whether these characteristics are functionally related. We believe the assessment of these properties will shed new light on the question of how these cells respond to stimuli and their ultimate physiological role in the control of cortical function. We used whole cell current-clamp mode associated with biocytin injections in layer I neurons of rats (14-21 days post-natal). A total of 244 cells were recorded, of which 106 were successfully labeled. Active neuronal properties were evaluated by evoking action potentials with pulses of depolarizing current (+10 to +100 pA). Passive properties were measured by injecting small hyperpolarizing current pulses (-10 to -100 pA). Two-dimensional digital reconstructions of biocytin labeled neurons were made and Sholl analysis, polar histograms and dendritic length measurements were used for the assesment of possible correlations between morphometric and electrophysiological data. Statistical significance was established by G-tests and Kruskal-Walis ANOVA (α = 0.05). Based on polar histogram analysis, layer I cells were divided in four major morphotypes: horizontal cells (26.4%), polymorphic cells (38.7%), ascendant cells (20.8%) and descendent cells (12.3%). The majority (over 70%) of horizontal cell’s dendrites extended directly unilaterally (25%) or bilaterally (75%) from the soma. Polymorphic cell’s dendrites were distributed without a statistically predominant orientation. Ascendant cells had the majority (55%) of their dendrites projecting directly upwards. Conversely, descendent cells projected most of their dendrites (50%) directly downwards. Sholl distributions were found to be different between the horizontal and ascendant cells and between the horizontal and the descendent cells (p < 0.05). Descendent cells had lower values of total dendritic length than horizontal cells and a lower number of intersections per five micrometers than horizontal and polymorphic neurons (p < 0.05). Horizontal cells had a larger maximum Sholl radius than all other subtypes (p < 0.05). Note that, with the exception of polymorphic cells, all morphotypes differ from each other in at least one morphometric parameter. Polymorphic cells, therefore, can only be distinguished by polar histogram analysis. All recorded cells were capable of repetitive firing and 84% of them showed little or no frequency adaptation. All neurons were identified as fast-spiking cells, of which 13.24% were late-spiking, 15.98% were early-spiking and 4.57% had quiescent periods. In spite of the relative diversity of firing patterns, cells were homogeneous regarding their passive membrane properties (p > 0.05). Contrary to our expectations, no significant difference was found in the distribution of firing patterns, as well as in the passive and active membrane properties, between the distinguished morphotypes (p > 0.05). Our data reveal that, although morphologically layer I interneurons are relatively heterogeneous, this does not correlate with a differential distribution of active and passive electrophysiological properties among distinct morphotypes. No significant relationship was found between the electrophysiological parameters and the dendritic morphology of rat layer I neurons.