An in vitro slice preparation of rat cingulate cortex was used to analyze the responses of layer V neurons to electrical stimulation of the corpus callosum (CC). In addition, synaptic termination of callosal afferents with layer V neurons was evaluated electron microscopically to provide a structural basis for interpreting some of the observed response sequences. 2. Layer V neurons had a resting membrane potential (RMP) of 60 +/- 0.68 (SE) mV, an input resistance of 47 +/- 4.74 M omega, a membrane time constant of 4.37 +/- 0.51 ms, an electrotonic length constant of 1.38 +/- 0.25, and produced spontaneous action potentials that were 50 +/- 0.3 mV in amplitude. Intracellular depolarizing current pulses evoked spikes that were sometimes associated with low-amplitude (2-5 mV) depolarizing (5-10 ms in duration) and hyperpolarizing (10-20 ms in duration) afterpotentials. 3. A single stimulus of increasing intensities to the CC produced one of the following response sequences: a) antidromic spike and an excitatory postsynaptic potential (EPSP), which initiated one or more spikes; b) antidromic spike, EPSP-evoked action potentials, and a hyperpolarization, which may have represented an intrinsic cell property or inhibitory synaptic activity; c) EPSP and evoked spikes only; d) high-amplitude EPSP with an all-or-none burst of action potentials. 4. Antidromically activated (AA) neurons always produced EPSPs in response to CC stimulation. When compared with nonantidromically activated neurons, AA cells had a more negative RMP, greater electrotonic length constant (LN), higher ratio of dendritic to somatic conductance (rho), and formed shorter duration, callosal-evoked EPSPs. 5. Neurons in anterior cingulate cortex produced EPSPs of longer duration than did those in posterior cortex (50 +/- 3.57 versus 26 +/- 1.56 ms, respectively). EPSPs in anterior neurons also had a higher maximum amplitude (20.5 +/- 1.0 versus 11.5 +/- 0.79 mV) and longer time to peak (11.6 +/- 2.2 versus 8.2 +/- 0.8 ms). 6. Electron microscopy of Golgi-impregnated neurons following contralateral lesions demonstrated that both pyramidal and nonpyramidal neurons received direct callosal afferents. Synaptic termination of callosal axons with the apical dendritic trees of anterior pyramidal cells was 6 times greater than it was with posterior pyramidal neurons. 7. EPSP shape differences in anterior and posterior neurons may be partially accounted for by the density and distribution of callosal afferents to these two cortices.
