ELECTROPHYSIOLOGICAL AND MORPHOLOGICAL HETEROGENEITY OF NEURONS IN SLICES OF RAT SUPRACHIASMATIC NUCLEUS

1. Whole cell patch clamp recordings of neurons in slices of the supra chiasmatic nucleus (SCN) were made in order to assess their electrophy siological and morphological heterogeneity. This assessment was accomp lished by (i) quantification of intrinsic membrane properties recorded in current clamp mode, (ii) studying frequency distributions of these properties, (iii) grouping of cells based on visual inspection of dat a records, and (iv) use of cluster analysis methods. 2. Marked heterog eneity was found in the resting membrane potential, input resistance, time constant, rate of frequency adaptation, size of rebound depolariz ation (low-threshold Ca2+ potential) and regularity of firing. The fre quency distribution of these membrane properties deviated significantl y from a normal distribution. Other parameters, including spike amplit ude and width, amplitude and rising slope of the spike after-hyperpola rization (AHP) and amplitude of the spike train AHP, showed considerab le variability as well but generally obeyed a normal distribution. 3. Visual inspection of the data led to partitioning of cells into three clusters, viz. cluster I characterized by monophasic spike AHPs and ir regular firing in the frequency range from 1.5 to 5.0 Hz; cluster II w ith biphasic spike AHPs and regular firing in the same range; and clus ter III with large rebound depolarizations and biphasic spike AHPs. In a post hoc analysis, these clusters also appeared to differ in other membrane properties. This grouping was confirmed by hierarchical tree clustering and multidimensional scaling. 4. The light microscopic prop erties of recorded neurons were studied by biocytin labelling. Neurons had monopolar, bipolar or multipolar branching patterns and were ofte n varicose. Axons sometimes originated from distal dendritic segments and usually branched into multiple collaterals. Many cells with extra- SCN projections also possessed intranuclear axon collaterals. We found no morphological differences between clusters except that cluster III neurons possessed more axon collaterals than cluster I or II cells. 5 . These results suggest that SCN neurons are heterogeneous in some bas ic as well as active membrane properties and can be partitioned into a t least three clusters. Cluster I and II cells fire spontaneously in a regular and irregular mode, respectively, and sustain prolonged spike trains. In contrast, cluster III cells have low firing rates but may adopt a burst-like firing mode when receiving appropriate input. While all clusters transmit output to target cells within and outside SCN, cluster III cells in particular are suggested to affect excitability o f large numbers of SCN neurons by their extensive local network of axo n collaterals.