QUANTITATIVE MORPHOLOGY OF PHYSIOLOGICALLY IDENTIFIED AND INTRACELLULARLY LABELED NEURONS FROM THE GUINEA-PIG LATERODORSAL TEGMENTAL NUCLEUS IN-VITRO

Mesopontine cholinergic neurons have been implicated in the initiation and maintenance of rapid eye movement sleep via their efferent connec tions to the thalamus and the medial pontine reticular formation. As a first step toward understanding how these modulatory neurons integrat e synaptic input, we have investigated the dendritic architecture of l aterodorsal tegmental nucleus neurons. The principal cells of the guin ea-pig laterodorsal tegmental nucleus were identified electrophysiolog ically in a brain slice preparation, then were intracellularly injecte d with biocytin and reconstructed using a computer-aided tracing syste m. The somata were large (27 +/- 3 mu m; n = 11) and gave rise to an a verage of 4.8 primary dendrites which, in most cases, emerged from the soma in a pattern that was radially symmetric in the plane of the sli ce. Primary dendrites had an average of 3.7 endings. A single axon aro se from either the soma or a proximal dendrite and exited the nucleus with a medial and/or lateral trajectory. Some axons also gave rise to a local terminal plexus composed of fine fibers bearing numerous punct ate swellings that ramified profusely within the neuron's dendritic fi eld. Total dendritic area averaged about 10(5) mu m(2), and therefore the average contribution of the soma to the total surface area (20%) w as significantly larger than the values reported for many other cell t ypes. Dendritic diameters were non-uniform in three respects. Some pro cesses were sparsely spiny. Most processes were varicose, with the deg ree of varicosity increasing substantially in secondary and tertiary d endritic segments. There was also a large degree of taper in dendritic processes; those processes with a non-negative taper had an average d iameter decrease of 40 +/- 25%. Dendritic processes deviated from the criteria necessary for a Rall equivalent cylinder approximation due to non-uniformity in morphotonic path length, failure to conform to the Rall 3/2 branching rule and non-uniformity of dendritic diameter. An a nalysis was done to assess the impact of dendritic varicosities on the extraction of cable parameters for these cells. Voltage traces were s imulated by solving the cable equation for a varicose dendrite and the n membrane parameters were recovered using an equivalent cylinder mode l. Errors in the extracted values of specific membrane conductance and specific membrane capacitance were quite small (less than or equal to 5%), while larger errors were seen for electrotonic length (less than or equal to 21%) and intracellular resistivity (less than or equal to 50%). These data indicate that the principal cells of the laterodorsa l tegmental nucleus, while possessing a relatively simple dendritic st ructure in terms of number and branchiness of dendrites, display a het erogeneity of dendritic process types. Processes range from smooth to markedly varicose, and can be aspiny or sparsely spiny. The possibilit y that the dendritic varicosities function as sites of either electric al or chemical compartmentalization is discussed. The degree of error resulting from a Rall equivalent cylinder approximation in light of th ese varicosities indicated that a generalized cable model approach may prove more effective in estimating their cable parameters. Copyright (C) 1996 IBRO. Published by Elsevier Science Ltd.