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 ph ysiological properties, and labeled the cells by intracellular injecti on of biocytin. Biocytin-labeled neurons were reconstructed and classi fied according to their somatic and dendritic morphology. 2. We identi fied 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 dendri tes with multiple branches that spread over large areas within the DNL L. The dendrites radiated equally in all directions. 3. Elongate cells were characterized by extended cell bodies with polar dendrites. In t he case of elongate I and II cells. the dendrites were preferentially oriented in the horizontal plane and the dendritic branches extended a cross 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 dorsov entrally along the margins of DNLL. This neuron was unique in our samp le 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 th e DNLL and showed no preferential orientation. 4. Round neurons had re latively small, round cell bodies and radial dendrites that extended o ver large areas within DNLL. These cells were quite common in our samp le and are almost certainly not the same as the infrequently encounter ed small round cells found in Nissl-stained sections. Some biocytin-la beled neurons were difficult to classify as either multipolar, elongat e I, II, or III, or round. These neurons had properties that most clos ely resembled elongate III cells, but they were treated separately her e to minimize heterogeneity within morphological categories. 5. The in trinsic physiological properties measured in this study were uncorrela ted 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 seri es of action potentials, the number of which was related to the magnit ude of current injection. The interspike intervals were regular althou gh some cells had a tendency toward an increase or decrease in the len gth of the interval with prolonged current injection. Injection of neg ative current produced a hyperpolarization that was proportional to th e current strength. With large current injections there was typically a sag in the membrane potential that emerged over the course of the in jection period. 6. We found two types of action potential afterhyperpo larization in DNLL neurons. Some neurons had a single undershoot and o thers had a double undershoot following the occurrence of a spike. The double undershoot was the most frequently encountered pattern. The ty pe of afterhyperpolarization, however, was unrelated to the morphologi cal cell type. 7. Recordings of postsynaptic responses indicated that all DNLL cell types received substantial convergence of synaptic activ ity from commissural and lemniscal sources. Many cells received both e xcitatory and inhibitory input or responded to stimulation of both ips ilateral 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 eviden ce among multipolar cells of inhibitory postsynaptic potentials evoked by stimulation of the commissure of Probst. Second there was no indic ation among elongate cells of excitatory postsynaptic potentials evoke d by commissural stimulation. These data suggest the possibility of se lective anatomic projections to anatomically distinct classes of neuro ns in DNLL.