Dendritic spikes are enhanced by cooperative network activity in the intact hippocampus

Abstract

In vitro experiments suggest that dendritic fast action potentials may influence the efficacy of concurrently active synapses by enhancing Ca2+ influx into the dendrites. However, the exact circumstances leading to these effects in the intact brain are not known. We have addressed these issues by performing intracellular sharp electrode recordings from morphologically identified sites in the apical dendrites of CA1 pyramidal neurons in vivo while simultaneously monitoring extracellular population activity. The amplitude of spontaneous fast action potentials in dendrites decreased as a function of distance from the soma, suggesting that dendritic propagation of fast action potentials is strongly attenuated in vivo. Whereas the amplitude variability of somatic action potentials was very small, the amplitude of fast spikes varied substantially in distal dendrites. Large-amplitude fast spikes in dendrites occurred during population discharges of CA3-CA1 neurons concurrent with field sharp waves. The large-amplitude fast spikes were associated with bursts of smaller-amplitude action potentials and putative Ca2+ spikes. Both current pulse-evoked and spontaneously occurring Ca2+ spikes were always preceded by large-amplitude fast spikes. More spikes were observed in the dendrites during sharp waves than in the soma, suggesting that local dendritic spikes may be generated during this behaviorally relevant population pattern. Because not all dendritic spikes produce somatic action potentials, they may be functionally distinct from action potentials that signal via the axon.

Fig. 1.

Properties of evoked and spontaneous dendritic events. A, Photograph of the electrode track and the biocytin-filled neuron and its camera lucida reconstruction. Recording was made from the principal apical shaft at the border of strata radiatum (rad) and lacunosum-moleculare (l-m). The electrode was moved beyond the dendrite during the experiment. The electrode track is filled with red blood cells. pyr, Pyramidal layer. B, Current step-induced responses (0.3 and 1.0 nA). Note amplitude decrement of fast spikes (bottom trace). Note also large fast spike (arrow), slow spike (asterisk), and transient cessation of fast spikes (top trace).Inset, Temporal detail of the fast and slow spikes.C, Responses to commissural input stimulation (arrow). Traces were evoked using the same stimulus intensity. Note that the spike can precede or follow the peak depolarization (arrows). D, Spontaneous spike burst with decrementing amplitude spikes.

Fig. 2.

Properties of spontaneous dendritic action potentials in CA1 pyramidal cells in vivo. A, Amplitude of action potentials (measured from the inflection point to the peak) as a function of the distance from the cell body. Eachpoint is an average of at least 50 spontaneous single spikes. The average somatic spike amplitude is indicated by a single point (mean ± SD; n = 25).Insets, Examples of averaged spikes (n = 50). Filled triangles, Anatomically verified dendritic locations; open circles, dendritic recording sites are estimated from the distance from the cell body layer during the recording session. B, Spike amplitude, rate of rise, rate of decay, half-amplitude width (half A), fast spike afterpotential (SAP), and resting membrane potential (RMP) in the soma (pyr) and at dendritic sites (mean ± SD). The stratum radiatum was arbitrarily divided into three equal layers: proximal (prox), middle (mid), and distal (dist).

Fig. 3.

Sharp wave burst-induced amplitude enhancement of fast spikes. A, Reconstructed dendritic tree. The micropipette points to the anatomically verified penetrated dendrite.B, Responses to hyperpolarizing (−0.5 nA) and depolarizing (left, 0.5 nA; right, 0.6 nA) current steps. Arrow, Large-amplitude fast spike (LAS); asterisk, putative calcium spike.C, Responses to commissural (c) stimulation. Note large-amplitude-evoked fast spike (arrow) and absence of spike (bottom trace) with and without concurrent direct depolarization of the dendrite, respectively. The same stimulus intensity was used in both cases. D, E, Relationship between extracellularly recorded multiple unit activity (MUA) and field ripples from CA1 pyramidal layer and intradendritic activity (dendrite). There is a 45 sec gap betweentraces in D and E. Cross-correlogram (burst vs MUA) between intradendritic bursts as defined by repeating spikes at <10 msec, interspike intervals, and extracellular MUA activity illustrates that the incidence of intradendritic bursts was highest during ripple-related MUA. E, left inset, Large-amplitude fast spikes were present exclusively during MUA bursts. Note that the slow potential associated with the large fast spike is larger than the stimulation-evoked EPSP shown in C. Right inset, Cross-correlogram between large-amplitude (>30 mV) spikes and MUA activity (LAS vs MUA).Ordinate, Number of units per bin.

Fig. 4.

Recording from a first-order dendrite in the middle third of stratum radiatum. A, Reconstructed dendritic tree. The arrow (enlargement of theboxed area) shows a labeled astrocytic process at the site of electrode penetration. B, Relationship between extracellularly recorded multiple unit activity (MUA) and field ripples from CA1 pyramidal layer and intradendritic activity.C, D, Intradendritic (dendrite)-evoked potential in response to commissural (c) stimulation and extracellular response (extra) after the pipette was withdrawn from the dendrite. E, Current step-induced responses (0.4, 0.8, and 1.0 nA). Asterisks, Putative calcium spikes. Note that the commissurally evoked response (C) is smaller than the large depolarization associated with the sharp wave burst in B. Note also the similarity of the spontaneous and current-induced bursts.

Fig. 5.

Augmentation of fast spike amplitude by attenuating GABAergic inhibition. Amplitude histograms of action potentials in three dendrites (A–C) before (gray) and after (black) bicuculline administration (5 mg/kg, i.p.). Note bimodal distribution of dendritic spike amplitude in all three cells. Arrowsindicate large-amplitude spikes. Note also the increased incidence of large-amplitude spikes after drug administration.Insets, Action potential averages before (gray) and after (black) bicuculline in the indicated time window. The postdrug average is slightly shifted upward in B. A andC are midapical dendrites. C (also shown in Fig. 6) is recorded from a distal dendrite.

Fig. 6.

Augmentation of spike amplitude by attenuating GABAergic inhibition. Recordings from a distal dendrite 350 μm from the soma. A, Relationship between extracellularly recorded multiple unit activity (MUA) and field ripple from CA1 pyramidal layer and intradendritic activity 11 min after drug injection. B, Cross-correlation between large-amplitude fast spikes (LAS in A; >20 mV) and MUA.C, Large-amplitude fast spike-triggered average of intradendritic events (n = 8). D, Current step (0.5 nA)-induced response before drug administration.E, Two superimposed evoked potentials in response to commissural (c) stimulation at threshold intensity 110 min after drug administration. Only one of the stimuli evoked a spike. Bottom trace, Extracellular response after the pipette was withdrawn from the dendrite.Arrows, Large-amplitude spikes;asterisks, putative calcium spikes.

Fig. 7.

Fast spikes may be initiated at multiple locations. A1, Simultaneous recording of field ripple and intracellular response from soma. Note burst of fast spikes with similar amplitude. A2, Autocorrelogram of fast spikes. Note refractory period of >8 msec. Inset, Averaged somatic spike. B1, Simultaneous recording of field ripple and intracellular response from dendrite. Note the presence of small-amplitude (filled arrow) and large-amplitude (LAS, open arrow) fast spikes. B2, Autocorrelogram of fast large spikes (top) and all spikes (bottom). Note lack of a refractory period when all spikes were included.Inset, Averaged waveforms triggered by large amplitude fast spikes (LAS) and all spikes (all).