CHARACTERIZATION OF SINGLE VOLTAGE-GATED NA+ AND CA2+ CHANNELS IN APICAL DENDRITES OF RAT CA1 PYRAMIDAL NEURONS

1. We have used dendrite-attached patch-clamp techniques to record sin gle Na+ and Ca2+ channel activity from the apical dendrites (up to 350 mu m away from soma) of CA1. pyramidal neurons in rat hippocampal sli ces (ages: 2-8 weeks). 2. Na+ channels were found in every patch exami ned (range: 2 to >20 channels per patch). Channel openings, which had a slope conductance of 15 +/- 0.3 pS (mean +/- S.E.M.), began with tes t commands to around -50 mV and consisted of both early transient chan nel activity and also later occurring prolonged openings of 5-15 ms. A ll Na+ channel activity was suppressed by inclusion of TTX (1 mu M) in the recording pipette. 3. Ca2+ channel activity was recorded in about 80% of the patches examined (range: 1 to >10 channels per patch). Sev eral types of channel behaviour were observed in these patches. Single channel recordings in 110 mM BaCl2, revealed an approximately 10 pS c hannel of small unitary current amplitude (-0.5 pA at -20 mV). These c hannels began activating at relatively hyperpolarized potentials (-50 mV) and ensemble averages of this low voltage-activated (LVA) channel activity showed rapid inactivation. 4. A somewhat heterogeneous popula tion of high voltage-activated, moderate conductance (HVA(m); approxim ately 17 pS), Ca2+ channel activity was also encountered. These channe ls exhibited a relatively large unitary amplitude (-0.8 pA at 0 mV) an d ensemble averages demonstrated moderate inactivation. The HVA, popul ation of channels could be tentatively subdivided into two separate gr oups based upon mean channel open times. 5. Less frequently, HVA, larg e conductance (27 pS) Ca2+ channel activity (HVA,) was also observed. This large unitary amplitude (-1.5 pA at 0 mV) channel activity began with steps to approximately 0 mV and ensemble averages did not show an y time-dependent inactivation. The dihydropyridine Ca2+ channel agonis t Bay K 8644 (0.5 or 1 mu M) was found to characteristically prolong t hese channel openings. omega-Conotoxin MVIIC (10 mu M), did not signif icantly reduce the amount of channel activity recorded from the LVA, H VA, or HVA, channel types in dendritic patches. In patches from somata , 6. w-conotoxin MVIIC was effective in eliminating a significant amou nt of HVA(m) Ca2+ channel activity. Inclusion of 50 or 100 mu M NiCl2 to the recording solution significantly reduced the amount of channel activity recorded from LVA and HVA, channel types in dendritic patches . A subpopulation of HVA, channels was, however, found to be Ni2+ inse nsitive. Dendritic HVA, channel activity was unaffected by these low c oncentrations of Ni2+. Amiloride (1 mM) substantially reduced channel activity recorded from LVA and HVA, channels located in dendritic patc hes. 7. A fairly constant density of about nine Na+ and five Ca2+ chan nels was found in dendritic patches from adult animals. The basic prof ile for dendritic Ca2+ channel distribution was as follows: HVA, chann els were relatively restricted to the proximal 50 mu m, while LVA and HVA, channels were found with similar frequency along the length of de ndrite studied. 8. A single type of TTX-sensitive Na+ channel and at l east three distinct types of Ca2+ channel were found at substantial de nsities in the apical dendrites of CA1 pyramidal neurons. The three ma in types of dendritic Ca2+ channel presented characteristics most simi lar to T-, R- and L-type Ca2+ channels. A fourth type of Ca2+ channel, with characteristics similar to the N-type, was also found at a lower density.