Optical recording of action potentials from vertebrate nerve terminals using potentiometric probes provides evidence for sodium and calcium components

Optical recording of electrical activity from parallel fibres and other cell types in skate cerebellar slices in vitro

1. A reliable and simple fish brain slice preparation was obtained from the cerebellum of the skate, and its properties were described. 2. A potentiometric oxonol dye, RH-482, and multiple site optical recording of transmembrane voltage (MSORTV) were used to reveal the electrophysiological properties of the parallel fibre action potential and to measure its conduction (0.13 m/s). The parallel fibre action potential was blocked in the presence of tetrodotoxin (TTX) and prolonged by tetraethylammonium (TEA), suggesting that the upstroke depends upon sodium entry and the repolarization upon potassium efflux. An after-hyperpolarization results from a calcium-dependent potassium conductance. 3. A second potentiometric dye, RH-155, differing only slightly from RH-482, exhibited a high affinity for glial cell membrane, and could be used to monitor changes in extracellular potassium concentration by detecting changes in glial membrane potential. 4. Calcium channel blockers such as cadmium ions blocked the optical signal that reflected the extracellular accumulation of potassium. 5. Interventions that modified the extracellular volume, and thereby affected the accumulation of potassium, produced large changes in the optical signal that monitored glial depolarization. Hypertonic and hypotonic bathing solutions resulted in decreases and increases, respectively, in the magnitude of the extrinsic absorption change that tracked potassium accumulation. 6. Blocking sodium-potassium pump activity by means of ouabain prolonged the time course of the optical signal that was related to potassium accumulation in the extracellular space. 7. Extracellular potassium accumulation was revealed to be critically dependent upon intracellular calcium ions.

Visualization of secretory activities in the Xenopus neurohypophysis by a high S/N video camera

The optical turbidity of neurohypophyses of the frog Xenopus laevis was observed with the help of a high signal to noise ratio video camera and a high speed image processor. Electrical stimulation of the pituitary stalk induced a decrease in turbidity of the neurohypophysis. This response was visualized on a monitor screen as a diffuse pattern consisting of many bright spots. The diameters of these spots were similar to those of individual nerve terminals, indicating that the optical response arises from a structural change in individual nerve terminals upon secretory activation.

Laser light scattering determination of size and dispersity of synaptosomes and synaptic vesicles isolated from squid (Loligo pealei) optic lobes

Quasi-elastic laser light scattering has been used to investigate the size and dispersity of synaptosomes and synaptic vesicles isolated from optic lobes of the squid Loligo pealei. Synaptosomal fractions were highly polydisperse (mu2/gamma -2 = 0.5) and the mean diameter (-d) ranged from 0.5-2.0 microns. Size distribution histograms yielded two major components - smaller particles (-d approximately 300-700 nm) and a larger group of particles (-d approximately 1,500-5,000 nm). The heterogeneity of the synaptosomal particles detected in solution is in agreement with published data obtained using electron microscopy. Purified synaptic vesicle fractions also yielded complex particle size distribution data. A component with a mean diameter in the range 150-250 nm was detected, though a smaller particle (-d approximately 40-110 nm) dominated the scattering signal. This smaller particle closely resembles in size the electron lucent vesicles seen in the majority of squid optic lobe nerve terminals when examined by electron microscopy. Osmotically-induced shrinkage and swelling of the synaptosomes was detected. Depolarization by veratridine (1.0 x 10(-4) M) did not result in a detectable change in the size of synaptosomal particles.

Odor-elicited activity monitored simultaneously from 124 regions of the salamander olfactory bulb using a voltage-sensitive dye

In response to controlled, odor pulse stimulation of the olfactory receptor mucosa, large fluorescence signals were recorded simultaneously from 124 contiguous anatomical regions of the salamander olfactory bulb using the potentiometric probe RH 414. The amplitudes and waveforms of the signals varied systematically across the bulbar surface in apparent correspondence with the laminae of the bulbar neurons. Qualitatively similar results were obtained using both intact and decorporate preparations, although fluorescence signals obtained from intact animals were distorted by optical noise generated by mechanical disturbances related to the functioning cardiovascular system. These results indicate that multiple site optical recording can be used to obtain information about spatio-temporal patterning of bulbar electrical activity evoked by physiological odor stimulation of the receptor mucosa. This is the first demonstration that activity elicited by a single, one second odor stimulus at physiological concentration and duration can be measured across many elements in the olfactory bulb. Information provided by this approach, in combination with complementary data derived from 2-deoxyglucose and single unit studies, may yield a better understanding of how the vertebrate central nervous system extracts quality and concentration information from olfactory afferent input.

Mapping of early development of electrical activity in the embryonic chick heart using multiple-site optical recording

1. Using a multiple-site optical recording method with a 100-element photodiode array and a voltage-sensitive merocyanine-rhodanine dye, we have been able to monitor, for the first time, spontaneous electrical activity in pre-fused cardiac primordia in the 6- and 7-somite chick embryos. 2. To study the regional development of spontaneous electrical activity in the early embryonic pre-contractile chick heart at the later 7-9-somite stages, we have also recorded optically action potentials simultaneously from the entire heart, and constructed maps of the early development of electrical activity. 3. The data show that during the 6-9-somite stages, the size of the active area gradually increases, and that the development of electrical activity was spatially non-uniform: two peaks of activity were found in the right and left sides of the cono-ventricular region at the 7-8-somite stages. As development proceeded to the 9-somite stage, several peak areas of activity appeared. 4. The results are discussed in relation to the spatial pattern of proliferation of electrically active cells in the early phases of cardiogenesis.

Single ion channel activity in peptidergic nerve terminals of the isolated rat neurohypophysis related to stimulation of neural stalk axons

Scanning electron micrographs revealed that the cut face of the sagittally bisected rat neural lobe was characterized by fine fibres bearing multiple rounded swellings (0.5-5 micron in diameter) presumed to be axonal swellings and peptidergic nerve terminals. Patch-clamp electrodes sealed (seal resistance greater than 10(10) omega) on to the cell membrane in such regions revealed (36 out of 43 patches) opening of channels conducting inward current following stimulation of the neural stalk at stimulus frequencies between 5 and 20 Hz. Channel opening did not occur immediately on stimulation (delay 5-15 s) and persisted 1-25 s after stimulation of the axons ceased, implying the possible existence of a depolarization-evoked intracellular messenger. Addition of TTX (5-20 microM) to the bath to block spike propagation abolished stalk stimulation-evoked channel opening but TTX in the patch electrode did not prevent opening. Opening was, however, prevented by addition of 4 mM Co2+ or 4 mM Mn2+. The slope conductance of this channel in medium with 10 mM Ca2+ was approximately 7 pS which was increased to 19 pS in medium containing 95 mM Ba2+. A second inward channel with a slope conductance of 56 pS in medium with 10 mM Ca2+ was also characterized and outward channels were noted in four patches. The electrical properties of nerve terminals are difficult to study because of their small size. The demonstration that ion channel activity can be recorded from mammalian peptidergic nerve terminals thus offers wide scope for future studies on the relation between stimulus and secretion.

Ca2+- and K+-dependent communication between central nervous system myelinated axons and oligodendrocytes revealed by voltage-sensitive dyes

The interactions between myelinated axons and surrounding glia cells, in rat optic nerve, were investigated by optical recording with voltage-sensitive dyes. Electrical stimulation of the nerve evoked an optical signal revealing two clearly distinct components: a fast propagating component, corresponding to the compound action potential, and a prominent slow component. Several lines of evidence suggest that part of the slow component originated from depolarization of the oligodendrocytes by potassium accumulation in the paranodal or internodal region. In addition, the experiments suggest that in this preparation axons also have voltage-dependent Ca2+ channels, and a Ca2+-dependent K+ conductance involved in the depolarization of oligodendrocytes. Thus, axons and oligodendrocytes communicate in an intimate, ionically-mediated fashion, and oligodendrocytes may play an important functional role beyond that of providing the myelin sheath.

Optical imaging of cell membrane potential changes induced by applied electric fields

We report the first imaging of the spatial distributions of transmembrane potential changes induced in nonexcitable cells by applied external electric fields. These changes are indicated by the fluorescence intensity of a charge-shift potentiometric dye incorporated in the cell plasma membrane and measured by digital intensified video microscopy.

Voltage-sensitive dyes measure potential changes in axons and glia of the frog optic nerve

Changes in dye absorption and fluorescence produced by electrical stimulation were measured in frog optic nerves stained with voltage-sensitive dyes. Following a single maximal stimulus applied through a suction electrode, the change in transmitted light intensity consisted of two components: one representing an axonal compound action potential and the second a slow depolarizing afterpotential which appeared to arise from the glial cells. The following results support this interpretation: during a train of stimuli the depolarizing potentials sum and can exceed 80% of the initial spike amplitude while the spike amplitude itself remains essentially constant. Thus, the axons cannot have undergone significant depolarization during the train. Optical recordings with simultaneous microelectrode recordings from the glial cells indicate that the change in glial membrane potential during the train has a time-course similar to that of the slow optical response. We conclude that voltage-sensitive dyes can monitor potential changes in both neurons and glia.

Luminal and basolateral surface membranes of secretory acinar cells: electrophysiological comparison of cationic sensitivities

Cation sensitivities (K+, Na+, and Ca2+) of luminal and basolateral membrane surfaces of secretory acinar cells were compared using a luminally perfused and externally superfused salivary gland from the aquatic snail, Helisoma trivolvis. Tight junctions delimiting the two membrane surfaces were observed near the acinar lumen suggesting that the total membrane area exposed to the superfusion solution exceeded that in contact with the luminal perfusion solution. The resting membrane potential of acinar cells was found to be dependent upon the K+ concentration in both the external superfusion and the luminal perfusion solutions. Unilateral K+ elevation at either membrane surface produced a rapid and sustained depolarization of the acinar cell. For a given K+ concentration, the level of depolarization produced by K+ elevation at the basolateral surface was significantly higher than at the luminal surface. The highest level of membrane depolarization was observed following simultaneous K+ elevation at both membrane surfaces. The ability of acinar cells to generate overshooting action potentials in response to electrical field stimulation was dependent upon both Na+ and Ca2+. Complete blockade invariably occurred following bilateral removal of either cation. The effects of unilateral removal of either Na+ or Ca2+ proved to be somewhat variable. In general, unilateral removal of Na+ was more effective in reducing the regenerative response than Ca2+ while removal of either cation from the basolateral surface was more effective in reducing the regenerative response than its removal from the luminal surface. Electrically evoked action potentials in acinar cells could also be blocked with unilateral application of the Ca2+ antagonist, cadmium (Cd2+), at either membrane surface. However, higher Cd2+ concentrations were required to achieve complete blockade when applied to the luminal than to the basolateral gland surface. This result fails to support a hypothesis of voltage-sensitive Ca2+ channels being spatially restricted to the luminal cell surface in this preparation.

Early events in development of electrical activity and contraction in embryonic rat heart assessed by optical recording

Spontaneous action potential and contraction in the early embryonic heart of the rat have been monitored optically using a voltage-sensitive merocyanine-rhodanine dye together with a multiple-element photodiode matrix array, and the onset of rhythmical action-potential activity in the early phases of rat cardiogenesis was conclusively determined for the first time. Spontaneous rhythmical action potentials were first generated in the central part of the embryonic heart at the middle period of the 3-somite stage of development, at 91/2 days after copulation. Subsequently, contractions coupled with the action potential also appeared at the end of the 3-somite stage. Usually, at the 3-somite stage, spontaneous action signals were synchronized among the different areas in the heart. From this result, it is evident that the paired right and left cardiac primordia are fused completely at the time of initiation of spontaneous electrical activity. In the 3-somite embryonic heart, excitatory waves were conducted radially over the heart, at a uniform rate (0.4-0.8 mm/s), from the pace-making area. However, the regional priority of pace-making activity is not rigid but is flexible.

Large and rapid changes in light scattering accompany secretion by nerve terminals in the mammalian neurohypophysis

Large changes in the opacity of the unstained mouse neurohypophysis follow membrane potential changes known to trigger the release of peptide hormones. These intrinsic optical signals, arising in neurosecretory terminals, reflect variations in light scattering and depend upon both the frequency of stimulation and [Ca2+]o. Their magnitude is decreased in the presence of Ca2+ antagonists and by the replacement of H2O in the medium by D2O. These observations suggest a correspondence between the intrinsic optical changes and secretory activity in these nerve terminals.

Activity dependence of action potential duration in rat supraoptic neurosecretory neurones recorded in vitro

Action potential durations, measured at one-third peak amplitude, were examined during intracellular recordings in 134 supraoptic nucleus neurones maintained in vitro in perfused hypothalamic explants. Spike durations ranged between 1.2 and 3.9 ms and were dependent on firing frequency. Shortest measurements (1.74 +/- 0.03 ms; mean +/- S.E. of mean) were obtained during relative quiescence, i.e. less than or equal to 0.5 Hz. A gradual increase in firing frequency through continuous injection of depolarizing current prolonged spike duration, with maximum levels (2.68 +/- 0.05 ms) achieved at 20 Hz. When interspike interval variability was eliminated and firing was more precisely regulated by brief 15-20 ms intracellular current pulses given at pre-determined frequencies, a proportional relationship between increasing spike duration and firing frequency was retained but the change in spike duration at frequencies between 2 and 10 Hz was less pronounced. Once action potentials had achieved the long duration configuration, their return to the shorter duration took place gradually during any succeeding silent interval with a time constant of 4.9 s. Action potential broadening occurred progressively and was most pronounced at the onset of spontaneous or current-induced bursts. In thirty-six phasically active neurones, spike broadening at the onset of a burst was concurrent with the presence of 5-10 consecutive short (less than or equal to 100 ms) interspike intervals; thereafter, despite a greater than 50% reduction in firing frequency, action potential durations remained prolonged throughout the burst. In all of nineteen cells tested, frequency-dependent changes in spike duration were reversibly decreased or blocked by Cd2+, Co2+ and Mn2+, or when CaCl2 was exchanged for equimolar amounts of EGTA in the perfusion medium. These observations indicate that a Ca2+ conductance contributes to frequency- and firing-pattern-dependent changes in spike duration in rat supraoptic nucleus neurones.

Calcium-dependent action potentials in rat supraoptic neurosecretory neurones recorded in vitro

Intracellular recordings obtained from thirty-nine supraoptic nucleus neurones in perfused hypothalamic explants displayed a mean resting membrane potential of -69 mV and spike amplitude of 79 mV. Voltage-current plots were linear in the hyperpolarizing direction and revealed a mean slope resistance of 197 M omega. After Na+ channel blockade with tetrodotoxin (TTX; 0.3-16 microM), the voltage-current relationship did not change significantly for hyperpolarizing pulses. An increase in spike threshold in TTX permitted visualization of a reduction in slope resistance to depolarizing current pulses. This rectification was reduced by the addition of the Ca2+ channel blockers Cd2+, Co2+ or Mn2+. High threshold TTX-resistant spikes with amplitudes ranging between 25 and 64 mV were evoked in an all-or-none manner by brief intracellular current pulses applied above 1.0 Hz. Current pulses presented at lower frequencies (less than or equal to 0.5 Hz) evoked graded responses. In seventeen of nineteen cells, prolonged depolarizing current pulses elicited repetitive firing of TTX-resistant spikes with a progressive increase in their amplitude, rise and fall times and after-hyperpolarizations. TTX-resistant spikes were reversibly abolished when CaCl2 was replaced by equimolar amounts of EGTA or by the addition of either Cd2+, Co2+ or Mn2+ to the perfusion medium. In artificial medium containing EGTA, both the shoulder on the repolarization phase of the spike and the subsequent after-hyperpolarization were reversibly abolished. Tetraethylammonium (TEA; 2-5 mM) induced prolongation of mean action potential durations from 1.9 to 12.3 ms (nineteen cells); in TTX, TEA also prolonged the duration and increased the over-all peak amplitude of the TTX-resistant (Ca2+) spike. While TEA also enhanced the amplitude of the Na+ spike, no comparable prolongation in spike duration was observed. These data indicate that somatic action potentials of supraoptic nucleus cells arise from the co-activation of a low threshold Na+ conductance and a higher threshold Ca2+ conductance; the latter is expressed as a shoulder on the repolarization phase of the action potential.

Spike broadening in magnocellular neuroendocrine cells of rat hypothalamic slices

Bursts of action potentials recorded from rat magnocellular neuroendocrine cells (MNCs) are known to be associated with enhanced release of oxytocin or vasopressin from their axon terminals in the neurohypophysis. Intracellular recordings from MNC somata in hypothalamic slices showed that spike broadening was characteristic of such bursts. Transitions from slow to fast firing caused spike broadening in all cells, whether they were silent, slow firing, phasic or fast-continuous. During phasic firing, broadening increased with the intraburst spike frequency. However, the parameters of maximal spike broadening recorded at the soma did not directly coincide with the previously described firing patterns evoking maximal hormone release from neurohypophysial terminals.

Active calcium responses recorded optically from nerve terminals of the frog neurohypophysis

Voltage-sensitive dyes were used to record by optical means membrane potential changes from nerve terminals in the isolated frog neurohypophysis. Following the block of voltage-sensitive Na+ channels by tetrodotoxin (TTX) and K+ channels by tetraethylammonium (TEA), direct electric field stimulation of the nerve terminals still evoked large active responses. These responses were reversibly blocked by the addition of 0.5 mM CdCl2. At both normal and low [Na+]o, the regenerative response appeared to increase with increasing [Ca++]o (0.1-10 mM). There was a marked decrease in the size of the response, as well as in its rate of rise, at low [Ca++]o (0.2 mM) when [Na+]o was reduced from 120 to 8 mM (replaced by sucrose), but little if any effect of this reduction of [Na+]o at normal [Ca++]o. In normal [Ca++]o, these local responses most probably arise from an inward Ca++ current associated with hormone release from these nerve terminals. At low [Ca++]o, Na+ appears to contribute to the TTX-insensitive inward current.

Effects of calcium on electrical propagation in early embryonic precontractile heart as revealed by multiple-site optical recording of action potentials

The effects of Ca2+ on electrical propagation in early embryonic precontractile chick hearts were studied optically using a voltage-sensitive merocyanine-rhodanine dye. Spontaneous optical signals, corresponding to action potentials, were recorded simultaneously from 25 separate regions of the eight-to-nine-somite embryonic primitive heart, using a square photodiode array. Electrical propagation was assessed by analyzing the timing of the signals obtained from different regions. Electrical propagation in the heart was suppressed by either lowering or raising extracellular Ca2+. Similar effects were produced by a Ca2+ ionophore (A23187). We have also found that electrical propagation across the primordial fusion line at the midline of the heart was enhanced by increasing, and depressed by lowering, external Ca2+. One possible interpretation is that intercellular communication in the embryonic precontractile heart is regulated by the level of the intracellular Ca2+ concentration, and it is suggested that intercellular communication across the primordial fusion line strongly depends on external Ca2+.

Optical recording of membrane potential responses from early embryonic chick ganglia using voltage-sensitive dyes

Changes in absorbance of voltage-sensitive merocyanine-rhodanine dyes were used to monitor electrical responses in the semilunar ganglion of 4-10-day-old developing chick embryos. The electrical responses were simultaneously recorded from many positions in the ganglion. Stimulation of the afferent nerve fibers (the ophthalmic division of cranial nerve V) with a suction electrode led to changes in light absorption of the stained ganglia. With both the depolarizing and hyperpolarizing pulses, the change was largest at 700 nm and was eliminated at a wavelength of 620 nm where the voltage-dependent absorption change of the dyes disappears. In the 4-10-day-old embryonic ganglia, two types of optical membrane potential responses, 'non-conducted' and 'conducted' responses, were identified. The non-conducted response varied with the intensity of the stimulus and had the nature of an electrotonic spread. Furthermore, this non-conducted response exhibited an 'initial upstroke-response' followed by the steady-state plateau evoked by larger depolarizing pulses. The conducted responses were blocked by tetrodotoxin (TTX) or by high external potassium concentration. The incidence of the conducted responses increased as development proceeded from the 5th to the 10th day of age. Thus, the TTX-sensitive action potential activity is probably generated initially in the semilunar ganglion during the 5-10-day-stage of development. These data represent the first demonstration of membrane potential responses in early embryonic intact nervous system. Furthermore, these studies demonstrate the usefulness of voltage-sensitive dyes in the analysis of the organizing process of embryonic neuronal functions during these early stages of development.

Intrinsic inhibition in magnocellular neuroendocrine cells of rat hypothalamus

Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with intracellular recordings in the rat hypothalamic slice preparation. Hyperpolarizing after-potentials (duration up to 125 ms) followed single action potentials and after-hyperpolarizations (a.h.p.s) lasting hundreds of milliseconds followed brief evoked spike trains. The amplitude and duration of the a.h.p. increased after spike trains of longer duration or higher frequency. The a.h.p. appears endogenous, rather than synaptically mediated from recurrent inhibition, because it persisted after pharmacological blockade of axonal conduction or of chemical synaptic transmission. The reversal potential of the a.h.p. was at least 20 mV more negative than that of inhibitory post-synaptic potentials. Cl- ionophoresis did not alter the a.h.p. Chelation of intracellular Ca2+ with EGTA injection eliminated the a.h.p. A Ca2+-activated K+ conductance, rather than recurrent synaptic inhibition, apparently causes the a.h.p. and is at least partly responsible for the inhibition after single spikes in magnocellular neurones. During hormone release, this endogenous mechanism may contribute to the post-burst silent period in putative oxytocinergic cells and to the interburst interval in phasic neurones, which are known to fire repetitive bursts associated with vasopressin release.

Real-time optical imaging of naturally evoked electrical activity in intact frog brain

A major obstacle to understanding the function and development of the vertebrate brain is the difficulty in monitoring dynamic patterns of electrical activity in the millesecond time domain; this has limited investigations of such phenomena as modular organization of functional units, seizure activities and spreading depression. The use of voltage-sensitive dyes and the recent development of the use of an array of photodiodes has provided a unique technique for monitoring the dynamic patterns of electrical activity in real time from a variety of invertebrate or vertebrate neuronal preparations including the rat cortex. In the present study, this technique has been used to investigate the intact optic tectum of the frog. We demonstrate that optical measurements can be used for real-time imaging of spatio-temporal patterns of neuronal responses and for identification of functional units evoked by natural visual stimuli. We report also the structure of the new voltage-sensitive probe that facilitates the in vivo applications of this technique.