PHYSIOLOGY AND ANATOMY OF SYNAPTIC CONNECTIONS BETWEEN THICK TUFTED PYRAMIDAL NEURONS IN THE DEVELOPING RAT NEOCORTEX

1. Dual voltage recordings were made from pairs of adjacent, synaptica lly connected thick tufted layer 5 pyramidal neurones in brain slices of young rat (14-16 days) somatosensory cortex to examine the physiolo gical properties of unitary EPSPs. Pre- and postsynaptic neurones were filled with biocytin and examined in the light and electron microscop e to quantify the morphology of axonal and dendritic arbors and the nu mber and location of synaptic contacts on the target neurone. 2. In 13 8 synaptic connections between pairs of pyramidal neurones 96 (70%) we re unidirectional and 42 (30%) were bidirectional. The probability of finding a synaptic connection in dual recordings was 0.1. Unitary EPSP s evoked by a single presynaptic action potential (AP) had a mean peak amplitude ranging from 0.15 to 5.5 mV in different connections with a mean of 1.3 +/- 1.1 mV, a latency of 1.7 +/- 0.9 ms, a 20-80% rise ti me of 2.9 +/- 2.3 ms and a decay time constant of 40 +/- 18 ms at 32-3 4 degrees C and -60 +/- 2 mV membrane potential.3. Peak amplitudes of unitary EPSPs fluctuated randomly from trial to trial. The coefficient of variation (c.v.) of the unitary EPSP amplitudes ranged from 0.13 t o 2.8 in different synaptic connections (mean, 0.52; median, 0.41). Th e percentage of failures of single APs to evoke a unitary EPSP ranged from 0 to 73% (mean, 14%; median, 7%). Both c.v. and percentage of fai lures decreased with increasing mean EPSP amplitude. 4. Postsynaptic g lutamate receptors which mediate unitary EPSPs at -60 mV were predomin antly of the pha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor type. Receptors of the N-methyl-D-aspartate (NMDA) type cont ributed only a small fraction (< 20%) to the voltage-time integral of the unitary EPXP at -60 mV, but their contribution increased at more p ositive membrane potentials. 5. Branching patterns of dendrites and ax on collaterals of forty five synaptically connected neurones, when exa mined in the light microscope, indicated that the axonal and dendritic anatomy of both projecting and target neurones and of uni- and bidire ctionally connected neurones was uniform. 6. The number of potential s ynaptic contacts formed by a presynaptic neurone on a target neurone v aried between four and eight (mean, 5.5 +/- 1.1 contacts; n = 19 conne ctions). Synaptic contacts were preferentially located on basal dendri tes (63%, 82 +/- 35 mu m from the soma, n = 67) and apical oblique den drites (27%, 145 +/- 59 mu m, n = 29), and 35% of all contacts were lo cated on tertiary basal dendritic branches. The mean geometric distanc es (from the soma) of the contacts of a connection varied between 80 a nd 585 mu m (mean, 147 mu m; median, 105 mu m). The correlation betwee n EPSP amplitude and the number of morphologically determined synaptic contacts or the mean geometric distances from the soma was only weak (correlation coefficients were 0.2 and 0.26, respectively). 7. Compart mental models constructed from camera lucida drawings of eight target neurones showed that synaptic contacts were located at mean electroton ic distances between 0.07 and 0.33 from the soma(mean, 0.13). Simulati ons of unitary EPSPs, assuming quantal conductance changes with fast r ise time and short duration, indicated that amplitudes of quantal EPSP s at the soma were attenuated, on average, to < 10% of dendritic EPSPs and varied in amplitude up to 10-fold depending on the dendritic loca tion of synaptic contacts. The inferred quantal peak conductance incre ase varied between 1.5 and 5.5 nS (mean, 3 nS). 8. The combined physio logical and morphological measurements in conjunction with EPSP simula tions indicated that the 20-fold range in efficacy of the synaptic con nections between thick tufted pyramidal neurones, which have their syn aptic contacts preferentially located on basal and apical oblique dend rites, was due to differences in transmitter release probability of th e projecting neurones and, to a lesser extent, to differences in the n umber of release sites per bouton or quantal size. 9. The continuum of efficacies in their synaptic connections implies that layer 5 pyramid al neurones can be recruited to ensemble electrical activity via their axon collaterals if as few as five of the strongly and reliably conne cted neighbouring neurones are active synchronously, whereas coinciden t APs of as many as 100 of the weakly connected pyramidal neurones are necessary.