IN-VITRO BRAIN SLICE STUDIES OF THE RATS DORSAL NUCLEUS OF THE LATERAL LEMNISCUS .2. PHYSIOLOGICAL-PROPERTIES OF BIOCYTIN-LABELED NEURONS

1. We made intracellular recordings from neurons in rat dorsal nucleus of the lateral lemniscus (DNLL), determined intrinsic and synaptic physiological properties, and labeled the cells by intracellular injection of biocytin. Biocytin-labeled neurons were reconstructed and classified according to their somatic and dendritic morphology. 2. We identified a diversity of morphological cell types in DNLL. Five main groups of neurons were recognized: multipolar: elongate I, II, and III; and round. The multipolar cells were characterized by several large dendrites with multiple branches that spread over large areas within the DNLL. The dendrites radiated equally in all directions. 3. Elongate cells were characterized by extended cell bodies with polar dendrites. In the case of elongate I and II cells. the dendrites were preferentially oriented in the horizontal plane and the dendritic branches extended across most of the cytoarchitectonic breadth of DNLL from the medial to lateral borders. The classification of elongate II was reserved for a single neuron with profuse dendritic branching that fanned out dorsoventrally along the margins of DNLL. This neuron was unique in our sample and was distinguished from the more common elongate I cells. which had less profuse dorsoventral dendritic branching. Elongate III cells had extended cell bodies, but their dendrites did not extend across the DNLL and showed no preferential orientation. 4. Round neurons had relatively small, round cell bodies and radial dendrites that extended over large areas within DNLL. These cells were quite common in our sample and are almost certainly not the same as the infrequently encountered small round cells found in Nissl-stained sections. Some biocytin-labeled neurons were difficult to classify as either multipolar, elongate I, II, or III, or round. These neurons had properties that most closely resembled elongate III cells, but they were treated separately here to minimize heterogeneity within morphological categories. 5. The intrinsic physiological properties measured in this study were uncorrelated with the morphological class of DNLL neurons. All DNLL neurons had similar current-voltage curves regardless of their anatomic category. Intracellular injection of positive current produced a sustained series of action potentials, the number of which was related to the magnitude of current injection. The interspike intervals were regular although some cells had a tendency toward an increase or decrease in the length of the interval with prolonged current injection. Injection of negative current produced a hyperpolarization that was proportional to the current strength. With large current injections there was typically a sag in the membrane potential that emerged over the course of the injection period. 6. We found two types of action potential afterhyperpolarization in DNLL neurons. Some neurons had a single undershoot and others had a double undershoot following the occurrence of a spike. The double undershoot was the most frequently encountered pattern. The type of afterhyperpolarization, however, was unrelated to the morphological cell type. 7. Recordings of postsynaptic responses indicated that all DNLL cell types received substantial convergence of synaptic activity from commissural and lemniscal sources. Many cells received both excitatory and inhibitory input or responded to stimulation of both ipsilateral and contralateral pathways (lateral lemniscus and commissure of Probst). Two possible correlations with anatomic cell type emerged from recordings of postsynaptic potentials. First, there was no evidence among multipolar cells of inhibitory postsynaptic potentials evoked by stimulation of the commissure of Probst. Second there was no indication among elongate cells of excitatory postsynaptic potentials evoked by commissural stimulation. These data suggest the possibility of selective anatomic projections to anatomically distinct classes of neurons in DNLL.