Morphological and functional study of dwarf neurons in the rat striatum

Combination of morphological and electrophysiological techniques provided d ata, suggesting existence in the young rat striatum of a peculiar class of neurons, the neurogliaform or dwarf neurons. Striatal neurons (n = 92), int racellularly recorded from rat brain slices, were filled (one in each slice ) with the intracellular marker biocytin, to compare physiological and morp hological properties in the same cell. Moreover, some neurons (n = 7) were filled with biocytin plus the fluorescent calcium indicator fura-2, identif ying cells during electrophysiological recording. Electrophysiological recordings showed that striatal neurons had different firing patterns, suggestive in most cases (n = 80) of spiny neuron class an d in others (n = 12) of interneuron class. Fura-2 injection clearly identif ied the body of six medium-sized cells and of one distinctive tiny cell. Th is small cell, however, showed a resting membrane potential and spontaneous and evoked firing pattern characteristic of striatal interneurons, Moreove r, the fura-2 injected in such small neuron also completely filled the cell body of a near large neuron; the fura-2 fluorescence changed synchronously in the two paired neurons after electrical stimulation of the impaled smal l one. Accordingly, the biocytin staining identified the morphology of the small recorded neuron as a neurogliaform-like cell apposed to a dendrite of an aspiny neuron, suggesting that the dye injected in one neuron had diffu sed to the other of a different type. Furthermore, such heterologous dye co upling unexpectedly involved seven pairs of cells detected with biocytin st aining (7.6% of the recorded neurons), invariably represented by a medium o r large neuron on one side, and on the other side by a small (5.44 +/- 0.15 x 9.14 +/- 0.7 mu m, mean +/- SD; n = 7) neurogliaform cell, roundish in s hape with few slender and short processes, usually apposed to a dendrite of the companion neurons (six out of seven). In the other cases, the biocytin staining revealed in each slice either the morphology of single spiny or a spiny neurons (80.4% of recorded neurons), or of two-three medium-sized spi ny neurons detected near to each other, suggesting that dye coupling had oc curred typically between similar neurons (11.9% of the recorded neurons). These data suggest that some neurogliaform cells in the striatum of young r at can be identified as dwarf interneurons, that may be dye-coupled with ne urons of different classes.