The temperature dependence of intrinsic membrane conductances and synaptic potentials in guinea pig hippocampal CA1 pyramidal neurons were examined in vitro as they were cooled from 37°C to between 33 and 27%. Cooling reversibly increased resting input resistance in a voltage-independent manner (Go = 0.58 to 0.75). The amplitude and duration of orthodromically evoked action potentials were increased by cooling (Cl0 = 0.87 and 0.52 to 0.53, respectively), whereas the maximum rates of rise and fall were reduced (Q,,, =

In order to study the electrical properties of dendritic membranes independent of the effects of somatic potentials, intracellular recordings in guinea pig hippocampal slices were obtained from the dendrites of CA1 pyramidal neurons (HPCs) which had been isolated from their somata by cuts made through the proximal stratum radiatum. Spikes and subthreshold membrane responses to intracellular current pulses were compared in intact and isolated dendrites and in the residual portions of neurons whose apical dendrites had been severed ("isolated somata"). Isolated dendrites generated both fast, QX-314-sensitive, sodium-mediated spikes, and slow higher threshold spikes which were QX-314-resistant and presumably mediated by Ca2+. Depolarization of "isolated somata" ordinarily evoked only fast (Na+) spikes, but presumed Ca2+ spikes could be elicited after exposure to QX-314 (a local anesthetic). Anomalous inward rectification was depressed by QX-314 in somata but not in dendrites, suggesting that the ionic basis for subthreshold as well as regenerative conductances was different at different sites on the neuron. The dendritic membrane in CA2 HPCs thus generates both Na+- and CA2+-mediated spike potentials and a subthreshold response which probably is mediated primarily by CA2+. Attempts to describe the integrative functions of these neurons must take into account the variety of conductances which are activated nonuniformly in somata and dendrites by changes in membrane potential.

The apical dendrites of CA1 pyramidal cells were isolated from their cell bodies by making cuts through proximal stratum radiatum of transverse hippocampal slices from the guinea pig. This lesion separated the distal apical dendritic elements from the somata, basal dendrites, and 50 to 100 microns of the proximal apical dendritic tree. Orthodromic stimuli in stratum radiatum evoked excitatory synaptic responses in isolated dendrites, but no phasic inhibitory components could be detected. In spite of this surgically produced disinhibition, orthodromic stimuli did not elicit burst activity at the resting membrane potential. However, isolated dendrites and intact dendrites could generate multiple slow spike activity when directly stimulated with depolarizing current pulses. When isolated dendrites were depolarized by DC current, excitatory postsynaptic potentials could evoke subthreshold intrinsic slow depolarizations, or repetitive slow spikes, similar to responses elicited by depolarizing current pulses alone. After exposure to bicuculline (5 microns), both intact and isolated dendrites generated bursts of activity following synaptic activation. A possible mechanism for this action of bicuculline is blockade of a residual GABA-mediated inhibition which was not expressed as a postsynaptic hyperpolarization in isolated dendrites. This bicuculline-sensitive event was capable of depressing dendritic excitability in the absence of the recurrent inhibitory synaptic input and was very effective in controlling burst activity. Our results indicate that the dendritic electrical behavior is dependent on a complex interaction between synaptic and voltage-sensitive events.