THE ELECTRICAL GEOMETRY, ELECTRICAL-PROPERTIES AND SYNAPTIC CONNECTIONS ONTO RAT V-MOTONEURONS INVITRO

1. We have developed a tissue slice preparation which allows the study of the actions of single presynaptic neurones onto single trigeminal motoneurones in the immature rat. Our aim in this first stage of the work has been to assess the validity of this preparation as a model for responses obtained in vivo from trigeminal motoneurones in adult rats. We have quantified the integrative properties of the motoneurones and also the variability in transmission at synapses of single presynaptic neurones onto the motoneurones. This data has then been compared to similar published data obtained from adult (rat) trigeminal motoneurones in vivo. 2. Quantitative reconstructions were made of the morphology of three motoneurones which had been labelled with biocytin by intracellular injection. The neurones gave off six to nine dendrites, of mean length 522 muM (S.D. = 160; n = 22), which branched on average 10.5 times to produce 11.45 end-terminations per dendrite (S.D. = 8.57; n = 22). The mean surface area of the dendrites was 0.92 x 10(4) mum2 (S.D. = 0.67; n = 22), and, for individual cells, the ratio of the combined dendritic surface area to the total neuronal surface area ranged from 98.3 to 99.2 % (n = 3). At dendritic branch points the ratio of the summed diameters of the daughter dendrites to the 3/2 power against the parent dendrite to the 3/2 power was 1.09 (S.D. = 0.21; n = 217), allowing branch points to be collapsed into a single cylinder. The equivalent cylinder diameter of the combined dendritic tree remained approximately constant over the proximal 25-40 % of the equivalent electrical length of the dendritic tree and then showed tapering. The tapering could be ascribed to termination of dendrites at different electrical distances from the soma. 3. Electrical properties were determined for a total of eighty-seven motoneurones, all with membrane potentials more negative than 60 mV (mean = 66.0 mV; S.D. = 5.2) and spikes which overshot zero (mean spike amplitude = 77 mV; S.D. = 10.5; n = 87). The spikes were followed by after-hyperpolarizations (AHPs) of mean amplitude 2.2 mV (S.D. = 1.7; n = 47), and mean duration 54-1 MS (S.D. = 9-5; n = 47). The mean input resistance of the neurones was 7.5 MOMEGA (s.D. = 2.5; n = 69), the mean membrane time constant was 3-5 ms (S.D. = 2.2; n = 35), and the mean rheobase was 1.6 nA (s.D. = 1.1; n = 56). The frequency-current relationship of seven out of eleven motoneurones examined showed a typical primary and secondary range in firing during the first interspike interval. The mean slope of firing in the primary range was 44 impulses s-1 nA-1 (S.D. = 22; n = 7), and 120 impulses s-1 nA-1 (S.D. = 74; n = 7) in the secondary range. 4. Six EPSPs (mean amplitude = 106 muV; S.D. = 114.8) and six IPSPs (mean amplitude = 28 muV; S.D. = 18.2) were obtained from examining the connections of 100 presynaptic neurones onto 182 motoneurones. The latencies of the EPSPs ranged from 0 to 1.7 ms (mean = 1.0 ms; n = 6), and from 0.15 to 0.95 ms (mean = 0.68 ms; n = 5) for the IPSPs. We assessed the variability in transmission at excitatory synapses by determining both the incidence of EPSPs and the averaged EPSP amplitude in sequential blocks of 100 sweeps collected for pairs of neurones which yielded EPSPs. Four of the five EPSPs examined in this manner showed pronounced changes in amplitude of the averaged EPSP obtained in different blocks, with three showing changes of approximately 100 muV and the fourth a change of 27 muV. For three of these four EPSPs, there was a significant linear relationship between the incidence of EPSPs in a block and the amplitude of the averaged EPSP in the block (P less-than-or-equal-to 0-02). Thus, at least for some connections, variations in averaged EPSP amplitude can be ascribed to changes in the incidence of EPSPs (i.e. in failures of transmission). 5. We compare these data to data previously reported for adult trigeminal motoneurones and draw two main conclusions. Firstly, the electrical properties of presumed trigeminal motoneurones in vitro are compatible with those of immature motoneurones, and thus that the preparation of the tissue slices may not result in a significant modification of neuronal properties. Secondly, that the mechanisms, or pathways, responsible for modulations in the incidence of failures of transmission, seen both in vivo and in vitro, are either intrinsic, or local, to the trigeminal motor nucleus. We can largely exclude an involvement of N-methyl-D-aspartate receptors as variations in the incidence of failures were observed in slices bathed in 2.5 x 10(-5) m D-2-amino-5-phosphonovaleric acid. We suggest that the variations in incidence of failures are likely to involve presynaptic mechanisms, and that the likely candidates may involve the operation of autoreceptors, or gamma-aminobutyric acid acting via axoaxonic contacts.