Dendritic synapses and reversal potentials: theoretical implications of the view from the soma

When complex neuronal structures may not matter

Much work has explored animal-to-animal variability and compensation in ion channel expression. Yet, little is known regarding the physiological consequences of morphological variability. We quantify animal-to-animal variability in cable lengths (CV = 0.4) and branching patterns in the Gastric Mill (GM) neuron, an identified neuron type with highly-conserved physiological properties in the crustacean stomatogastric ganglion (STG) of Cancer borealis. We examined passive GM electrotonic structure by measuring the amplitudes and apparent reversal potentials (E_revs) of inhibitory responses evoked with focal glutamate photo-uncaging in the presence of TTX. Apparent E_revs were relatively invariant across sites (mean CV ± SD = 0.04 ± 0.01; 7–20 sites in each of 10 neurons), which ranged between 100–800 µm from the somatic recording site. Thus, GM neurons are remarkably electrotonically compact (estimated λ > 1.5 mm). Electrotonically compact structures, in consort with graded transmission, provide an elegant solution to observed morphological variability in the STG.

DOI:http://dx.doi.org/10.7554/eLife.23508.001

Measurements of dendritic conductance changes to GABA in granule cells of the rat dentate gyrus

The magnitude of dendritic conductance changes occurring distantly from the somatic site of recording can be difficult to measure. We have used measurements of the neuronal time constant, tau 0, instead of the neuronal input resistance, RN, to estimate the resistance decrease that accompanies the depolarizing response of the dendrites of granule cells when GABA is applied. From the changes in tau 0, we estimated the reversal potential of the response and found that the conductance change accompanying a given GABA-mediated voltage response as measured at the cell body was the same regardless of where in the dendritic tree the drug was applied. On the other hand, RN changes underestimated the increase in conductance of the GABA responses in the distal dendrites and were not accurate for determining the reversal potential.

Excitatory synaptic interactions between CA3 neurones in the guinea-pig hippocampus

Excitatory synaptic interactions between CA3 neurones in slices from guinea-pig hippocampus were examined. Recurrent excitatory post-synaptic potentials (e.p.s.p.s) were evoked by action potentials in a single presynaptic neurone or by the antidromic activation of part of the CA3 pyramidal cell population. The peak amplitude of unitary e.p.s.p.s was 1-2 mV at potentials between -64 and -70 mV. Their time to peak was 7-12 ms and the initial phase of their decay was slower than that of a somatically injected voltage pulse. Recurrent e.p.s.p.s were often followed by a small (0.3 mV) hyperpolarization, or undershoot. Recurrent e.p.s.p.s were compared with e.p.s.p.s evoked by stimulating mossy fibres, which terminate proximally on apical dendrites of CA3 pyramidal cells. They were of slower time course and reversed at a more positive potential than mossy fibre e.p.s.p.s. Some synaptic terminals made by recurrent axon collaterals apparently terminate at distant locations on apical dendrites. The decay of both recurrent e.p.s.p.s and dendritic voltage pulses was prolonged by membrane depolarization within a 10-15 mV subthreshold potential range. Voltage-dependent inward currents activated by the synaptic depolarization may contribute to the slow initial decay of these synaptic events. The undershoot did not occur when transmission of a unitary e.p.s.p. failed and was of slower time course than the hyperpolarization due to an inhibitory post-synaptic potential (i.p.s.p.). It was suppressed by intracellular application of K+ channel blockers and probably reflects an intrinsic outward current activated as a consequence of the synaptic depolarization. Considerable temporal summation of synaptic potentials occurred when recurrent synapses were activated twice at an interval of 5-10 ms, typical of the spontaneous burst firing pattern of CA3 neurones. The mean facilitation of a second e.p.s.p. at this interval was about 0.6. The efficacy of a third and subsequent e.p.s.p.s at similar interval was reduced. Presynaptic bursts of three to five action potentials evoked summed e.p.s.p.s of amplitude 2-4 mV, with time to peak 20-40 ms and decaying phase of similar duration. Their rising phase was relatively smooth and summed events were succeeded by an undershoot. Presynaptic bursts could cause a post-synaptic neurone to discharge.

Post-synaptic inhibition of bulbar inspiratory neurones in the cat

Stable intracellular recordings from thirty-six bulbar inspiratory neurones revealed three centrally originating, rhythmic patterns of synaptic inhibition (i.p.s.p.s). (i) A declining pattern of i.p.s.p.s accompanying the early stages of inspiration (early inspiratory inhibition) was identified in a total of twenty neurones representing examples of each of the functional classes of bulbar neurones examined, i.e. six R alpha- and two R beta-neurones of the dorsal respiratory group and twelve R alpha-neurones of the ventral respiratory group. (ii) A transient pattern of i.p.s.p.s just preceding or coinciding with the cessation of inspiration (late inspiratory inhibition) was present in the remaining sixteen neurones which were tested, representing six R alpha-neurones and three R beta-neurones of the dorsal respiratory group and seven R alpha-neurones of the ventral respiratory group. (iii) An augmenting pattern of expiratory i.p.s.p.s was present in all thirty-six neurones. Late inspiratory and expiratory i.p.s.p.s in the same neurones showed a similar time course of reversal when chloride was injected or allowed to diffuse into the cells and were associated with similar increases in input conductance. Both patterns of i.p.s.p.s appear to arise at or close to the cell soma. Early inspiratory i.p.s.p.s required a relatively longer period of chloride injection for reversal to be accomplished. Input conductance changes were either absent or smaller than those associated with late inspiratory or expiratory inhibition. These i.p.s.p.s appear to arise at more distal dendritic sites. These patterns of i.p.s.p.s are discussed in relation to the mechanisms shaping the growth of central inspiratory activity, bringing this activity to an end, and suppressing its redevelopment during expiration.

Excitation of hippocampal pyramidal cells by glutamate in the guinea-pig and rat

1. The mechanism by which L-glutamic acid depolarizes hippocampal CA1 pyramidal neurones was investigated by using the in vitro slice and ionophoretic techniques. 2. Two types of responses were seen. One (in 85% of cells) consisted of spike discharges that outlasted the glutamate-induced depolarization. In the other (the rest of the cells), spikes were produced only during the rising phase of the depolarization. 3. The effect was highly localized; it disappeared when the ionophoretic electrode was moved vertically by as little as 20 micrometers. 4. The effect of glutamate persisted after synaptic transmission was blocked; this probably was due to a direct effect of glutamate on the cell membrane. 5. Small doses of glutamate produced either no change or an apparent increase in input resistance. With larger doses, the input resistance invariably decreased. The apparent increase in input resistance was not seen in cells treated with Mn2+ and TTX and is believed to be an effect of the depolarization rather than a direct effect of glutamate. 6. By extrapolation, the reversal potential for the glutamate response (EGlu) was found to -3.6 mV. 7. Following intracellular injection of Cs+, neurones could be depolarized to a range of +20 to +50 mV. The glutamate response could then be reversed. EGlu in these cells was -1.5 mV. 8. Using the Cs+-injection technique, it was also possible to reverse the e.p.s.p. E.e.p.s.p. was similar to EGlu. 9. When the external sodium concentration was reduced, the size of the glutamate response decreased, and EGlu became more negative.

Morphological and electrophysiological characteristics of rat cerebellar slices maintained in vitro

1. The morphological and electrophysiological characteristics of sagittal cerebellar slices of adult rat cerebellum maintained in vitro were studied. 2. The ultrastructural preservation of the different neuronal cell types in many areas of these slices after 2-3 h incubation was very similar to that observed in material fixed in situ. A limited degree of glial swelling was observed in some regions. 3. The conduction velocity of parallel fibres was within the normal in vivo range and the fibres retained their ability to activate Purkinje cells and inhibitory interneurones. 4. Purkinje cells, recorded intrasomatically, responded to white matter stimulation with characteristic antidromic activation and climbing fibre responses, and typical parallel fibre responses were evoked following parallel fibre stimulation. 5. Climbing fibre excitatory post-synaptic potentials (e.p.s.p.s) were very similar whether recorded in the dendrites or somata of Purkinje cells. By contrast, marked differences in the associated spike potentials were evident, the initial fast, low-threshold somatic spike appearing in the dendrites as a slow, high-threshold spike. The secondary spikes, both in the soma and dendrites, were of the latter type. 6. The initial somatic spike was readily inactivated by cell depolarization but resisted moderate hyperpolarization, whereas the converse was true for the slow, high-threshold spikes recorded in the dendrites. These differences suggest that these responses are generated in the soma and in the dendrites respectively. 7. Climbing fibre and parallel fibre e.p.s.p.s recorded in Purkinje cell somata were reversed under depolarizing current injected through the recording micro-electrode. As in vivo, the parallel fibre e.p.s.p.s was more sensitive to injected current than the climbing fibre e.p.s.p. in several instances, despite the more proximal location of the synapses involved.

Reduced inhibition during epileptiform activity in the in vitro hippocampal slice

1. Intracellular recordings were made from CA1 pyramidal cells in the hippocampal slice in vitro. The responses to orthodromic and antidromic activation and to ionophoretically applied GABA were studied. 2. The epileptogenic agent sodium benzyl penicillin reduced the recurrent i.p.s.p. evoked by subthreshold antidromic stimulation. Reversal potential studies of the i.p.s.p. and resistance measurements showed that this reduction was mainly due to a decrease in i.p.s.p. conductance. 3. Penicillin also reduced the conductance and associated membrane potential changes induced by ejecting GABA near the soma or into the apical dendritic region. 4. The mixed e.p.s.p.-i.p.s.p. evoked by orthodromic stimulation was converted to a pure depolarizing potential as the i.p.s.p. was blocked. Concurrently the probability of discharge to a constant orthodromic stimulus was increased. Similar changes were seen in a low chloride solution. 5. The time course of the reduction of inhibition was similar to that of the enhanced orthodromic response seen after penicillin treatment. 6. We conclude that reduction of postsynaptic inhibition is partly responsible for the increased probability of orthodromic discharge caused by penicillin. The longer latency all-or-nothing burst seen in some cells, however, seems to require an additional mechanism, although reduced inhibition may facilitate the triggering of this burst.

Time integral of synaptic conductance

1. Inhibitory post-synaptic currents (i.p.s.c.s) were recorded under voltage clamp from neurones of Aplysia buccal ganglia. 2. The synaptic charge, Q, transferred by each i.p.s.c. was calculated as the time integral of the synaptic current, approximated by numerical integration. For typical i.p.s.c.s recorded at or near resting potential, Q = -100 to -500 pC. The majority of the charge is transferred during the interval between the peak and a time one time constant later. 3. In order to characterize an alternative measurement of synaptic efficacy, the slope of the Q vs. membrane potential curve was calculated and defined as the time integral of conductance, b. Values of b ranged from 2.6 to 51 pC/mV, averaging 14 pC/mV. For i.p.s.c.s recorded in thirty-one cells at room temperature, b was well correlated with Gpeak, the peak synaptic conductance (r = 0.86). 4. In most synapses, the time integral of conductance, which incorporates both amplitude and duration, may be a more revealing measure of synaptic efficacy than peak conductance. 5. Size of synaptic response was determined as a function of temperature, T. While Gpeak decreases with decreasing temperature over the range 9-22 degrees C, b peaks at 12-18 degrees C and decreases at higher and lower values of T. The data permit the speculation that lengthening average channel lifetime, and therefore time constant of decay, with decreasing temperature, may have adaptive significance in maintaining synaptic efficacy.

Dual excitatory inputs to caudate spiny neurons from substantia nigra stimulation

In the present set of experiments of two component excitatory postsynaptic potential (EPSP) (fast and slow) was recorded from caudate (Cd) neurons after substantia nigra (SN) stimulation in the alpha-chloralose anesthetized cat. Several unique properties of the fast response are demonstrated and used to determine certain functional properties of Cd neurons. The following conclusions have been drawn from these experiments. (1) A dual excitatory innervation pattern (fast and slow) exists to the Cd from SN stimulation. (2) The fast system is independent of the Cd efferent system. (3) Neurons receiving this dual input are spine laden neurons. (4) Dendritic inhibition is operating on these neurons and often has little or no direct influence on the observed soma membrane potential. The dendritic inhibition reduced test EPSP amplitude for several hundred milliseconds for all Cd afferents except for the fast SN input. Such a decoupling of the fast input from the inhibition may play an important role in Cd signal processing.

Facilitation of monosynaptic excitatory synaptic potentials in spinal motoneurones evoked by internuncial impulses

1. Descending tracts and primary afferent fibres were chronically degenerated in the lumbosacral cord of the cat, and attempts were made to evoke monosynaptic e.p.s.p.s in motoneurones by stimulation of interneurones with a pair of fine electrodes inserted into the cord.2. The reversal potential of monosynaptic e.p.s.p.s so produced was more negative than that measured for monosynaptic e.p.s.p.s produced by afferent impulses.3. Monosynaptic e.p.s.p.s evoked in motoneurones by internuncial impulses showed a significantly greater facilitation than those produced by afferent impulses.4. Monosynaptic e.p.s.p.s in a motoneurone produced by supramaximal intraspinal stimuli often revealed a fluctuation in amplitude. In such cases, when two successive stimuli were applied at a short interval, the mean amplitude of the second e.p.s.p.s was greater than that of the first e.p.s.p.s. This facilitation was associated with a decrease in the coefficient of variation of the e.p.s.p. amplitude fluctuation.5. The degree of facilitation of monosynaptic e.p.s.p.s evoked by internuncial impulses was not related to the amount of transmitter released by the preceding impulses.6. It is concluded that facilitation of monosynaptic e.p.s.p.s evoked by both afferent and internuncial impulses is based on the same mechanism and that the degree of facilitation of e.p.s.p.s is entirely determined by the nature of presynaptic elements.