H. Mack Brown

Ionic Basis of the Photoresponse of Aplysia Giant Neuron: K+ Permeability Increase

The giant neuron of the Aplysia abdominal ganglion hyperpolarizes during illumination. The light-initiated potential change is associated with an increase of membrane conductance. It reverses sign at the potassium equilibrium potential (about -83 millivolts), which was determined from direct measurements of internal potassium activity. The membrane hyperpolarization is produced entirely by a light-induced increase in potassium permeability.

Intracellular calcium changes in Balanus photoreceptor. A study with calcium ion-selective electrodes and Arsenazo III

Intracellular Ca2+ concentration (Cai) in the dark and during light stimulation, was measured in Balanus photoreceptors with Ca2+ ion-selective electrodes (Ca-ISE) and Arsenazo III absorbance changes (AIII). The average basal Cai of 17 photoreceptors in darkness was 300 +/- 160 nM determined with liquid ion-exchanger (t-HDOPP) Ca-ISE. Ca-ISE measurements indicated that light increased Cai by 700 nM (average), whereas AIII indicated an average change of 450 nM. The time course of AIII absorbance changes matched the time course of changes in the receptor potential more closely than did the Ca-ISE. Changes in Cai were graded with light intensity but the change in Cai was much greater for a decade change in intensity at high light intensity than at low intensity. The peak light induced conductance change of voltage clamped cells had a relationship to light intensity similar to that of the change in Cai. The peak Cai level measured with Ca-ISE was in good agreement with the free Ca2+ concentration of injected buffer solutions. Control Cai levels were usually restored within 5 min following injection of Ca2+ buffers. Injection of Ca2+ buffers with free Ca2+ of 0.6 microM produced a membrane depolarization. Larger increases in Cai (greater than microM) produced by injection of CaCl2 or release of Ca2+ from injected buffers by acidifying the cell, produced a pronounced membrane hyperpolarization. Increasing Cai with all of these techniques reduced the amplitude of the receptor potential. The time course of the receptor potential recovery was usually similar to that of Cai recovery.

Ionic Composition of Endolymph and Perilymph in the Inner Ear of the Oyster Toadfish, Opsanus tau

The concentrations of free Na+, K+, Ca2+, and Cl-in endolymph and perilymph from the inner ear of the oyster toadfish, Opsanus tau, were measured in vivo using double-barreled ion-selective electrodes. Perilymph concentrations were similar to those measured in other species, while endolymph concentrations were similar to those measured previously in elasmobranch fish, though significantly different from concentrations reported in mammals. Perilymph concentrations (mean ± std. dev.) were as follows: Na+, 129 mmol l-1 ± 20; K+, 4.96 mmol l-1 ± 2.67; Ca2+, 1.83 mmol l-1 ± 0.27; and Cl-, 171 mmol l-1 ± 20. Saccular endolymph concentrations were Na+, 166 mmol l-1 ± 22; K+, 51.4 mmol l-1 ± 16.7; Ca2+, 2.88 mmol l-1 ± 0.27; and Cl-, 170 mmol l-1 ± 12; and semicircular canal (utricular vestibule) endolymph concentrations were Na+, 122 mmol l-1 ± 15; K+, 47.7 mmol l-1 ± 13.2; Ca2+, 1.78 mmol l-1 ± 0.48; Cl-, 176 mmol l-1 ± 27. The relatively high concentrations of Ca2+ and Na+ in the endolymph may have significant implications for the physiological function of the mechanoelectrical transduction channels in the vestibular hair cells of fish compared to those of their mammalian counterparts.

Experimental procedures that modify light response frequency of regenerated planaria

Abstract Experimental variables that affect the response of planaria to light (a measure of planaria “learning”) were investigated. Planaria length and conditioned stimulus illuminance were found to have an effect on light response frequency. The origin of regenerated planaria along the central axis of the original worm also had an effect on response frequency. At a moderate level of CS illuminance (500 ft-c) the light response frequency of planaria regenerated from head segments was greater than that of tail segments. When the illuminance was 1000 ft-c, the performances of head and tail segments were indistinguishable. These findings are relevant to past studies which reported that planaria learning survives regeneration because they provide plausible alternative explanations for their behavior.

Dimethyl sulfoxide elevates intracellular Ca2+ and mimics effects of increased light intensity in a photoreceptor

A 1% (v/v) solution of dimethyl sulfoxide (DMSO) added to the saline bath of isolated Balanus eburneus photoreceptors increased receptor potential amplitude by 40–50% and shortened time to peak amplitude and latency by 20–25%. The light-sensitive membrane current of voltage-clamped cells was increased systematically as DMSO concentration was increased from 1% to 10%. The null potential of the light sensitive current was unaffected by DMSO with short pulses of light, indicating that DMSO has no direct effect on ion selectivity of the light-sensitive channel. Absorbance changes of cell injected with the calcium indicator arsenazo III show that DMSO elevates intracellular Ca2+ (Cai). Current-voltage relations in darkness reveal that DMSO induces a small sustained inward current (≈5 nA) which has a null potential similar to the lightinduced current. DMSO may activate the light-sensitive conductance via the increase in Cai However, the altered kinetics and increased amplitude of the receptor current are opposite to the desensitizing effects normally observed with increased Cai.

Sensitization in planaria

Abstract Planaria were trained under conditions that result in little or no learning in other animals (temporally separated light and shock) and with different levels of conditioned stimulus illuminance. Temporally separated light and shock resulted in increased responsiveness which resembled altered behavior of planaria trained with simultaneous light and shock. Increased illuminance also caused increased responsiveness. The data indicated that shock (a) sensitizes planaria to light and (b) causes spontaneous division of the animals, resulting in shorter more responsive worms. The combined effects of these variables resulted in increased response levels which others have attributed to learning.

Intracellular Changes of H+ and Ca2+ in Balanus Photoreceptors

Both H+ and Ca2+ have been under consideration as the intracellular mediator of the receptor potential or light adaptation in both vertebrate and invertebrate photoreceptors. Closer experimental scrutiny has qualified neither of them for an exclusive role in this regard. However, it remains that changes in these ions might represent important sequelae to certain events, such as biochemical changes, that are involved in important functional processes. Intracellular pH changes have been shown to occur with pH electrodes in an invertebrate photoreceptor (Brown and Meech, 1976; 1979). In vertebrate preparations, a suspension of rod disk membranes have been shown to acidify with illumination. This acidification has been shown to be caused by hydrolysis of cyclic GMP (Libman and Pugh, 1981).

Intracellular changes of h+ and ca2+ activities in Aplysia giant neurons as measured with ion selective microelectrodes

Abstract 1. 1. Intracellular pH (pH i ) and Ca 2+ activity (a i Ca ) were measured in giant cells (R 2 ) of Aplysia californica with recessed tip pH electrodes and diaryl-phosphate-PVC Ca electrodes. 2. 2. The intracellular pH with the cells bathed in Tris buffered saline (pH 0 7.65) was 7.25 ± 0.05 (SD) in four different cells. 3. 3. An acid load produced by 5% CO 2 13.25 mM HCO − 3 , reduced pH i to 6.95 ± 0.05 (SD) in approx 15 min. 4. 4. The buffer power of the cell (β) was approx 33 mM HCO − 3 /l.pH unit. 5. 5. Bright white illumination which produces a membrane hyperpolarization was without measureable effect on pH i or(a i Ca ). CO 2 -HCO 3 saline (5% CO 2 , 13.25 mM HCO 3 ) was also without effect on (a i Ca ). 6. 6. Sustained activation of the cell by outward current pulses from another microelectrode produced no measureable change in (a i Ca ). 7. 7. Pressure injection of Ca 2+ from a CaCl 2 -filled microelectrode into the cell cytoplasm raised (a i Ca ) to 1.4 × 10 −3 M. The time course of (a i Ca ) recovery was considerably longer than the membrane hyperpolarization.

Electrical characteristics f K+ and Cl− fet microelectrodes

Abstract A Cl − micro ion-selective field effect transistor (MISFET) is described. The device was prepared with Corning liquid ion exchanger. Electrical response time and transfer function measurements are presented for K + and Cl − MISFETs. These measurements reveal the dominant components of the passive electrical equivalent circuit of the devices investigated.

Effects of potassium-free solutions on membrane current—voltage relations of Aplysia giant neurons

Abstract 1. Cooling, ouabain and K + -free solutions are thought to produce inhibition of an electrogenic Na + -pump in Aplysia giant neurons. 2. Current-voltage relations of the membrane in ouabain and K + -free solution became increasingly dissimilar at membrane potentials more negative than the resting potential. 3. Ouabain produced a constant depolarization over the physiological range of membrane potentials. 4. K + -free solutions elicited a potential change that reversed sign at the normal potassium equilibrium potential, E k ⋍ −80 mV. 5. Measurements of external K + -activity (a o k ) next to the membrane indicate that a o k remains relatively constant even in K + -free solutions. 6. K + -free solutions produce membrane depolarization by K + -conductance decrease and ouabain inhibits the pump.