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Prolonged changes in single-cell activity in lateral geniculate nucleus
CEREBROSPINAL
FLtTID
121
3. HASSI.ER, 0. 1966. Deep cerebral venous system in man. A microangiographic study on its areas of drainage and its anastomoses with the superficial cerebral veins. Neurology 16 : 505-511. 4. HEGEDUS, S. A., and R. T. SHACICELFORD. 1965. Comparative-anatomical study of the craniocervical venous s?-stems in mammals, with special reference to the dog: Relationship of anatomy to measurements of cerebral blood flow. :11l~zr. J. .J,raf. 116 : 375-386. 5. HOCHWALD, G. M., and hl. C. ~VALLENSTEIN. 1967. Exchange of albumin between blood, cerebrospinal fluid and brain in the cat. Lg~rze~. J. Pl~ysiol. 212 : 1199-1204. 6. HOCHWALD. G. M., A. SAHAR, -4. R. SADIE;, and J. RASSOHOFF. 1969. Cerebrospinal fluid production and histological observations in animals with experimental obstructive hydrocephalus. Exp. Newel. 25 : 190-199. 7. GERGENSEX. 51. I., and R. A. Ra~ox. 1959. Blood volume. Physiol. Rw. 39: 307-347. 8 GIBBS, E. L., and F. A. GIBBS. 1931. The cross section areas of the vessels that form the torcular and the manner in which flow is distributed to the right and to the left lateral sinus..-l+Iaf. Rec. 59 : 419-426. 9. IKGVAR. D. H., and U. S~DERBERG, 1956. A new method for measuring cerebral blood flow in relation to the electroencephalogram. ,EEG C/ill. ~Veur.ophysiol. 9: 403-412. 10. LASDAV. \I\:. M.. W. H. FREYGAP~C., JR.. L. P. ROLAXD, L. SOE;OI.OFF, and S. S. KETY. 1955. The local circulation of the living- brain; values in the unanesthetized and anesthetized cat. ?‘VCMS. .-l~rr~~~. lVelrro .-2ss. 60: 125-129. 11. PAPPEXHEIMER, J. R., S. R. HEISEY. and E. F. JORDAN. 1961. Active transport of Diodrast and phenolsulfonphthalein from cerebrospinal fluid to blood. A4wfcr. J. Ph~kol. 200 : l-10. 12. REIVICH. LI., J. JEHLE, L. SOICOLO~FF. and S. S. KETY. 1969. Measurement of regional cerebral blood flow with antipyrine-I’C in awake cats. J. . Ippl. PI~~siol. 27 : 296300. 13. REWICH. If., R. SLATER. and S. SAKO. 1969. Further studies on exponential models of cerebral clearance curves, pp. S-10. Ilc “Cerebral Blood Flow” M. Brock, C. Fieschi, D. H. Ingvar. N. A. Lassen and Ii. Schiirmann [ed.]. Springer-Verlag, Berlin. 14. SAHAR. A., G. M. HOCH~VALT). and J. RANSOHOFF. 1969. Alternate pathway for cerebrospinal fluid absorption in animals with experimental obstructive hydrocephalus. E.rp. Ncurol. 25 : 20@206. 15. SAFIAR. .\., G. M. HOCH\YALD, :I. R. SADIK, and J. RANSOHOFF. 1968. Cerebrospinal fluid absorption in animals with experimental obstructive hydrocephalus Arch. Newel. 21 : 638-644. 16. SAHAR, ;\., G. hi. HOCHWALU. and J. RANSOHOFF. 1970. Experimental hydrocephalus: Cerebrospinal fluid formation and ventricular size as a function of intraventricular pressure. J. A:cru-ol. Sci. 10 : 269-278. 17. SAHAR. ii., G. M. HOCHIVALD, and J. RANSOHOFF. 1970. Cerebrospinal fluid turnover in experimental hydrocephalic dogs. n’cllro[ogg In Press. 18. SCHLEZIXGFX, B. 1939. The venous drainage of the brain with special reference to the Galenic system. Btnir~ 62: 274-291. 19. SCHREISER. G. E. 1950. Determination of inulin by means of resorcinol. Pror. Sot. Esp. Biol. Mcti. 74: 117-120. 20. SHEKKIS, H. A., RI. H. HARMEL, and S. S. KETY. 1948. Dynamic anatomy of the cerebral circulation. Arch. Neural. Psychait. 60 : 240-252.
122 21.
SAHAR,
SKLnR,
ume: 24:
H., Values
I;.
79-82
HOCH
WALD,
AND
RANSOHOFF
E. F. BURKE, JR., and T. W. LANGFITT. 1965. Cerebral blood volobtained with Tr-labelled red cells and RISA. J. .4ppl. Pltysiol.
ESPERIJIEXT.+L
SEUROLOGY
Prolonged in
28, 123-131
(1970)
Changes in Single-Cell Activity Lateral Geniculate Nucleus
Prolonged changes in single-cell activity in the lateral geniculate nucleus are produced by sciatic nerve stimulation in anesthetized cats. The increases or decreases in activity may last for several minutes. Interval histograms show that the changes occur primarily in the short-interval distribution. Changes in singleand multi-unit records obtained simultaneously are usually, but not always, positively correlated. The possible mechanisms and implications of the prolonged changes are discussed. Introduction
Prolonged changes in the tonic electrical activity of the visual radiations and lateral geniculate nucleus (LGN) have been reported in earlier studies ( 14, 15). It was found that somatic stimulation. such as rubbing a paw, or electrical stimulation of the sciatic nerve, frequently produces marked, prolonged changes in mean-square ( MS) electrical activity recorded from discrete populations of neurons. The changes generally last 3-12 min, but sometimes persist for more than 30 min. Prolonged increases or decreases in tonic activity are produced, depending on the recording site and level of anesthesia (1-C. 15) The prolonged changes, characteristically observed when the cats are in a state of moderate anesthesia, are not seen in the awake, or deeply anesthetized animal. The changes are not specific to any particular anesthetic agent ; they are observed in animals anesthetized with either sodium pentobarbital or chloraloseurethane. Control experiments show that changes in EEG, blood pressure, and intraocular pressure are not the causal mechanisms of the prolonged activity changes. There is evidence (1.5 ). moreover, that the reticular formation may mediate the effects of the somatic input on visual system activity. Since the MS recording method samples populations of neurons 1 Supported by contracts SD-193 search Projects -4gency. \I’e thank technical assistance. Dr. Dubrol-sky Kniversit!
and D.4HC 1568-CO396 from J. A. Bridges and J. C. Vanagas is now at the Ilepartment of
1’3
the Advanced Kefor their excellent Psychiatry. McGill
124
DUBROVSKY
AND
MELZACK
(2), it is important to determine whether activity in individual units show similar long-term changes. The purpose of the present study was to examine the effects of somatic stimulation on single-cell activity in LGN, and to determine the correlation between mean-square and single-cell records. Methods
Experiments were performed on 20 cats anesthetized with chloralose urethane or sodium pentobarbital, and placed in a stereotaxic instrument. In all experiments, the pupils were maximally dilated with cyclogyl, and nicitating membrane movements were blocked by application of neosynephrine. The animals were then immobilized with Flaxedil and artificial respiration instituted. Craniotomy was performed (at A 6-8, L 8.5-10) to permit insertion of a bipolar electrode into LGN on one side and a microelectrode into LGN on the other side until cells driven by light were found, usually at a depth of about 11-12 mm. Previous studies (14, 15) showed that the prolonged changes in homologous regions, such as LGN on both sides, are in the same direction and have similar durations. The right sciatic nerve was exposed for stimulation. The stimulus parameters used were 3- to S-v square waves (0.05- to 1-msec duration, 2/set). Light flashes were produced by a Grass Photo-stimulator. The mean-square electrical recording method has been described elsewhere (14) in detail. Basically, a bipolar electrode led to an amplifier, Ballantine RMS voltmeter set in the mean-square mode, and d-c channel of a Grass Polygraph or Leeds-Northrup-Speedomax-G recorder. Singlecell records were obtained with tungsten microelectrodes ( 11) . The output led to a cathode follower and conventional amplifying equipment. The data were displayed on the oscilloscope screen and recorded on tape. The original spike discharges were transformed into standard pulses after being gated in a modified Schmidt trigger discriminator. The pulses were then fed into an electronic counter to obtain the absolute number of spikes. To determine the spike frequency, the pulses were fed into a rate meter (Fersh Electronics), and the interval histograms were obtained with an Enhancetron (Nuclear Data Corporation). The criteria proposed by Bishop, Burke, and Davis (4) and Hubel (12) were used to identify LGN cells: a complex positive-negative waveform with inflections in the rising phase of the positive deflection. In order to obtain stable recording conclitions, the procedure described by Kozak, Rodieck, and Bishop (13) was used: About 15 min after the microelectrode tip had reached the upper surface of LGN, the electrode was advanced a few microns at a time. Microelectrode recording sites were marked by passing d-c current through the electrode. The brains were fised in 10% form01 saline solution prior to sectioning on a freezing microtome. Serial sections were stained with cresyl violet and the probe tracks were reconstructed.
PROLOKGED
SINGLE-CELL
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125
Results Lifter the bipolar macroelectrode was lowered into LGN, stimulation of the sciatic nerve produced prolonged changes in the nlean-square tonic activity. Either increases (Fig. 1A ) or decreases (Fig. 1B) were observed. Sinlultaneous records of I~lood pressure and EEG confirmed earlier ohservations ( 1-I. 15) that the prolonged tonic activity changes frequently occurred in the absence of any changes in Mood pressure or EEG ( Fig. 1). TI’hen suc11 changes were observed, they were found to vary independently of the MS changes, indicating that they are not causally related. .After prolonged MS changes were recorded, the microelectrode was inserted into LGIY on the opposite side. Since the purpose of this stud? was to examine prolonged changes in tonic activity, only cells which showed the characteristic wavefornl of LGW units (Fig. 2)) a steady tnean firing development of the interval histograms rate (Fig. -!.I), and a uniform for 3 min (Fig. 3) were used to evaluate the effects of stimulation. The cells that met these criteria of stationarity, which were employed by Bishop and Levi& ( 5 ) in their stud\- of LGK units, showed ahnost without esception, a preponderance of short intervals in the interval histograms. liecordings were obtained from 50 cells. Eighteen showed a stable mean firing rate for at Ieast 3 min before stinxdation, anti met the estnl~lished criteria. Of these cells: 11 showed definite. prolonged changes to sciatic stimulaticn. Six cells showed an increase and five cells a decrease in tonic activit! after sciatic stiniulation. Successive stiniulus presentations had siinilar effects on the same unit. The absolute value and range of change for each
FIG. 1. X: Prolonged increase in mean-square activity evoked in LGN by electrical stimulation (0.05 msec. E/set. 8 \-) of the sciatic nerve of the ipsilateral hindpaw (IH). and hrief increase produced by light flashes (L). B.C,D : Simultaneous records
of LGN mean-square activity (B ). carotid blood pressure (CJ, and EEG from anterior ectosylvian gyrus (D ) before and after electrical stimulation (black bar) of IH sciatic ner\-e (0.1 msec. Z/set, 5 v). Note the prolonged decrease in LGN meansquare activity in the absence of any change in blood pressure or EEG. Sodium pentobarbital anesthesia was ~13x1 in these experiments.
126
FIG.
unit
DUBROVSKY
AND
2. Example of the type of LGN unit at fast sweep-speed. Positive up. Note
MELZACK
analyzed. inflections
A: Discharge on the initial
pattern. positive
B : Same phase.
individual unit is shown in Table 1. Figure 3 shows examples of an increase and a decrease in prolonged activity in LGN, obtained during two different experiments. In both cases the interval histograms of the cells show that it is in the short-interval distribution that most of the changes occur as a result of the stimulation. This observation is in agreement with results reported by Bindman and Boisacq-Schepens (3) and Evans and Robertson (8). In one case, a cell which in dim light diminished its firing frequency after sciatic stimulation (unit 9 in Table 1) stopped firing entirely for 7 min when tested in total darkness (Fig. 4A, B) . This testing sequence was repeated four times with similar results. It was clear, however, that only firing had been stopped since the unit could still be driven the spontaneous by light flashes. Changes in single- and multiunit activity patterns recorded simultaneously were usually positively correlated, although dissociation between the two was occasionally observed. Figure 4C shows a steady decrease in the MS activity when the background illumination was increased. The mean firing frequency of a cell which was recorded simultaneously (Fig. 4D) showed an increase. Similarly, unit 7 showed an increase in firing rate, but no change was observed in the MS activity recorded at the same time. The relationship between the two recording methods is further revealed by the observation that prolonged increases in MS activity produced by
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FIG. 3. Top: Mean square (MS) activity recorded from LGN, showing a prolonged decrease in activity after stimulation of the sciatic nerve (3 v, 0.2 msec, 2/set). The black bar indicates stimulus duration. The time units A-G represent l-ruin periods. The interval histogram and total number of spikes of simultaneously recorded single-cell activity is shown below the MS record for each 1-min period. Bottom: MS activity from LGK, showing a prolonged increase in activity after sciatic stimulation (3 v. 0.1 msec, Z/set). The interval histogram and total number of spikes of simultaneously recorded unit activity for each I-min sample, A-D, is shown below. Note the changes in the short-term interval distribution in both cases.
stimulation
were
sometinies
with activity in adjacent cells that 1E and F). was in spontaneously firing units, sotne
associated
had previously been silent (Fig. Although our major interest
12s
DUBROVSKY
AND
MELZACK
TABLE ABSOLUTE
FIRING VALUES STIMULATION
Time Cell IlO.
1
OF 11 CELLS IN LGN BEFORE AND AFTER BRIEF (S) OF THE SCIATIC NERVE. THE TIVE UNITS REPI~ESENT l-MIN PERIODS before
S
Time
(lo-20
SEC)
after
-~
1 2 3 4 5 6 7 8 9 10 11
-3
-2
-1
+1
+2
+3
1150 1300 1425 1480 1175 1362 1128 462 545 854 915
1143 1328 1448 1475 1130 1344 1068 4.58 5.53 883 907
1162 1312 1439 1468 1178 1375 1150 463 562 860 876
1200 2400 1775 1690 1349 2535 1300 407 480 435 670
900 2448 1698 1580 1420 2226 1269 415 509 461 713
850 2437 1680 1620 1270 2198 1241 412 537 518 743
/
E:
2ooflv I 20 mrec.
F#
,,m
,
FIG. 4. A: Change in firing frequency of a cell in LGN after stimulation of the sciatic nerve (4 v, 0.2 msec. Z/set) with the cat in dim light. Stimulus duration is indicated by the black bar. Ordinate : spike counts/set ; abscissa : time. Time-constant of the rate meter: 1.1 sec. B : Same cell as A, same stimulus parameters, with the cat in total darkness. C: Decrease in MS activity when background light is turned on (indicated by the arrow). D : Firing frequency of a cell, recorded at the same time as C, showing an increase in firing rate. Ordinate : spike counts/set; abscissa : time, same as C. E, F: Group of tonically firing cells recorded with a microelectrode in LGN before (E) and after (F) stimulation of the sciatic nerve, in which an increase in the MS activity was observed. Note the recruitment of new cells in F, showing that new units may contribute to the increase in MS activity.
PROLOKGED
Sl~K‘Gl,bl-CEI.I,
ACTIVITY
129
observations were made on cells that did not exhibit tonic I~ackgrouncl activity. Under these circumstances, when the cells were activated by light flashes, no change in the response of the cell could be detected after sciatic stimulation, although a change in multiunit activity \vas recorded. Discussion
The results show that the prolonged changes produced in LGK by brief somatic stimulation, hitherto seen only by multiunit recording methods. can also be observed in the firing patterns of units. Although the activity changes recorded with the two methods are usually positively correlated, the observed discrepancies between them argue against oversimplification in the interpretation of multiunit recording. Changes in individual units in response to sciatic stimulation may be accompanied by no change in simultaneously recorded MS activity or even a change in the opposite direction. The MS activity, then. can not be linearly correlated with the absolute firing of a defined set of neurons alone. Similar changes in the 11s activity can be brought about by changes in the frequency discharge of the observed set, or by changes in behavior of different neuronal grouljs surrounding the bipolar electrode for the MS recording. Data at the unit level indicate that sciatic stimulation can have selective effects on different neuronal groups in the same area of the LGIY. Cells which did not show tonic activity but could be activated hy phasic visual input showed no response change after sciatic stimulation, although simultaneously recorded MS activity showed long-term changes. Moreover, when tonic activity was stopped 1)~ sciatic stimulation. the phasic reqonse of the unit was nevertheless retained. These findings, taken together, suggest that only some spontaneously active central neurons are subject to prolonged changes. This conclusion is in agreenlent with observations by Bindman and Boisacq-Schepens ( 3 ) and Granit and Iienkin ( 10 ) which show that tonically firing cells are more easily modified than Ihnsically activated ones. The observations that prolonged changes are readily produced in the midbrain reticular formation and that reticular stimulation elicits prolonged changes similar to those produced by somatic stimulation ( 15 ) indicate that reticular mechanisms are at least partly involved in mediating the effects of somatic input on visual structures. There is convincing evidence that these effects are not clue to global, pervasive influences such as those that produce EEG arousal or blood pressure changes : Changes in EEG may or may not occur. and are usually observed in sensory projection areas that also show mean-square changes. They are rarely observed in association cortical areas. In general. there is no over-all correlation between prolonged tonic activity changes and changes in EI
130
DUBROVSKY
AND
MELZACK
pressure, indicating that the three types of change are not causally related (14, 15). Other possible causes of artifact, such as respiratory or intraocular pressure changes, pupillary dilatation, and poststimulus changes in somatic tissue or nerves have also been eliminated (14, 15). Indeed, the phenomenon is not generalized but occurs selectively in discrete brainstem and cortical areas. Moreover, if the changes were due to a generalized arousal, they should occur more easily as anesthesia wears off; instead, they become harder to elicit and are rarely seen at anesthetic levels in which pinch produces reflex withdrawal. The fact that prolonged changes in two distant regions may differ in both direction and duration, and are differentially affected by small doses of anesthesia (15), further indicates the selective, discrete properties of the prolonged changes. The observations that prolonged changes, such as those observed here, occur only when the animal is moderately anesthetized suggests two possible mechanisms : (i) that the anesthetic diminishes or suppresses the background bombardment that normally occludes activity in reverberatory circuits (7) ; or (ii) that anesthetic agents may selectively block inhibitory neurons at reticular or local levels (6, 9). In either case, the activity triggered by stimulation could be maintained for prolonged periods of time in recurrent collateral reverberatory circuits such as those described by Andersen, Eccles, and Sears (1) and Burke and Sefton (7). The interval histograms of the units before and after stimulation reveal that it is in the short interval that most of the changes occur. This implies either a modification of an intrinsic property of the cell or some activity in the network to which the cell belongs (8). If this is so, the prolonged changes, once initiated, can be self-sustaining in LGN itself. The prolonged removal of occlusive or inhibitory influences would have the effect, in a sensory system, of exerting a prolonged bias on synaptic transmission regions. The bias could result in a prolonged increase or decrease in information transmission depending on whether the projection system has excitatory or inhibitory functions. The prolonged changes in activity in single neurons in LGN after brief somatic stimulation indicates the existence of mechanisms that exert prolonged influences on information processing or modulation in the visual system. Furthermore, the observation ‘that prolonged changes in the optic tract and visual radiations may have different directions and durations (15) suggests that these influences may be exerted, by centrifugal projections, independently at several levels in the visual system. Activities such as these could play an important role in normal integrative behavior, such as prolonged attention, multimodal interaction and information selection, as well as in neural activities that may underlie prolonged pathological pain states (15) _
I’ROLOSGED
SI IZGLE-CELL
.‘.CTIVITY
131
References 1. ANDERSON, P., J. C. ECCLF:S, and T. A. SE.IRS. 1964. The ventrobasal complex of the thalamus: types of cells. their responses and their functional organization. J. Physiol. London 174: 370-399. 2. ARDUINI, i\., and L. R. PINNEO. 1962. A method for the cmant%cation of tonic activity in the nervous system. .-lrck. If&. Biof. 100: 315124. 3. BINDMAN. L. J.. and N. BOISACQ-SCIIEPEXS. 1966. Persistent changes in the rate of tiring of single, spontaneously active cortical cells in the rat produced by peripheral stimulation, J. Physiol. London 185 : l-l-17. 4. BISHOP, P. O., W. BURKE. and R. DAVIS. 1962. The identification of single units in central visual pathways. J. Physiol. Loudorc 162: 409431. 5. BISHOP, P. O., W. R. LEWX, and 1%‘. 0. WILLIAMS. 1961. Statistical analysis of the dark discharge of lateral geniculate neurons. J. Plzysiol. Lodo~z 170: 598-612. 6. BRAZIER, M. ,4. B. 1954. The action of anesthetics on the nervous system, pp. 163-193. Zrz “Brain Mechanisms and Consciousness,” J. I;. Delafresnaye (ed.). Blackwell, Oxford. 7. BURKE, W., and A, J. SEFTOS. 1966. Recovery of responsiveness of cells of lateral geniculate nucleus of rat. J. Phqsiol. Lortdou 187 : 213-229. 8. EVANS, C. R., and A. D. J. ROBERTSON. 1965. Prolonged excitation in the visual cortex of the cat. Science 150: 913-915. 9. FELDMAN, S., and I. H. WAGMAN. 1963. The effect of pentobarbital on evoked potentials in brain of AIncncn mulnttn. Elcct~ocrrcr~l~trlr~~~. Cli>z. Newophysiol. 5: 747-760. 10. GRANIT, R., and B. RENICIN. 1961. Net depolarisation and discharge rate of motoneurons as measured by recurrent inhibition. J. Physiol. Lo&m 158: 461475. 11. HUBEL. D. H. 1957. Tungsten microelectrode for recording from single units. Sciewe 125 : 549-550. 12. HUBEL, D. H. 1960. Single unit activity in lateral geniculate body and optic tract of unrestrained cats. J. Physiol. Lomfo~c 150 : 91-104. 13. KOZAIC. W., R. W. RODIECE;, and P. 0. BISHOP. 1965. Responses of single units in lateral geniculatc nucleus of cat to moving visual patterns. J. Ncrtrupkysiol. 28: 19-47. 14. ~~ELZACK. R., K. KONRAD. and B. DUBROVSI;~. 1968. Prolonged changes in visual system activity produced by somatic stimulation. Esp. ;l~cwol. 20: 443459. 15. h'kLZACK, R.. K. KONRAD. and 8. DCXROVSICY. 1969. Prolonged changes in central nervous system activity produced by somatic and reticular stimulation. Exp. Newel. 25 : 416-428.
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121
3. HASSI.ER, 0. 1966. Deep cerebral venous system in man. A microangiographic study on its areas of drainage and its anastomoses with the superficial cerebral veins. Neurology 16 : 505-511. 4. HEGEDUS, S. A., and R. T. SHACICELFORD. 1965. Comparative-anatomical study of the craniocervical venous s?-stems in mammals, with special reference to the dog: Relationship of anatomy to measurements of cerebral blood flow. :11l~zr. J. .J,raf. 116 : 375-386. 5. HOCHWALD, G. M., and hl. C. ~VALLENSTEIN. 1967. Exchange of albumin between blood, cerebrospinal fluid and brain in the cat. Lg~rze~. J. Pl~ysiol. 212 : 1199-1204. 6. HOCHWALD. G. M., A. SAHAR, -4. R. SADIE;, and J. RASSOHOFF. 1969. Cerebrospinal fluid production and histological observations in animals with experimental obstructive hydrocephalus. Exp. Newel. 25 : 190-199. 7. GERGENSEX. 51. I., and R. A. Ra~ox. 1959. Blood volume. Physiol. Rw. 39: 307-347. 8 GIBBS, E. L., and F. A. GIBBS. 1931. The cross section areas of the vessels that form the torcular and the manner in which flow is distributed to the right and to the left lateral sinus..-l+Iaf. Rec. 59 : 419-426. 9. IKGVAR. D. H., and U. S~DERBERG, 1956. A new method for measuring cerebral blood flow in relation to the electroencephalogram. ,EEG C/ill. ~Veur.ophysiol. 9: 403-412. 10. LASDAV. \I\:. M.. W. H. FREYGAP~C., JR.. L. P. ROLAXD, L. SOE;OI.OFF, and S. S. KETY. 1955. The local circulation of the living- brain; values in the unanesthetized and anesthetized cat. ?‘VCMS. .-l~rr~~~. lVelrro .-2ss. 60: 125-129. 11. PAPPEXHEIMER, J. R., S. R. HEISEY. and E. F. JORDAN. 1961. Active transport of Diodrast and phenolsulfonphthalein from cerebrospinal fluid to blood. A4wfcr. J. Ph~kol. 200 : l-10. 12. REIVICH. LI., J. JEHLE, L. SOICOLO~FF. and S. S. KETY. 1969. Measurement of regional cerebral blood flow with antipyrine-I’C in awake cats. J. . Ippl. PI~~siol. 27 : 296300. 13. REWICH. If., R. SLATER. and S. SAKO. 1969. Further studies on exponential models of cerebral clearance curves, pp. S-10. Ilc “Cerebral Blood Flow” M. Brock, C. Fieschi, D. H. Ingvar. N. A. Lassen and Ii. Schiirmann [ed.]. Springer-Verlag, Berlin. 14. SAHAR. A., G. M. HOCH~VALT). and J. RANSOHOFF. 1969. Alternate pathway for cerebrospinal fluid absorption in animals with experimental obstructive hydrocephalus. E.rp. Ncurol. 25 : 20@206. 15. SAFIAR. .\., G. M. HOCH\YALD, :I. R. SADIK, and J. RANSOHOFF. 1968. Cerebrospinal fluid absorption in animals with experimental obstructive hydrocephalus Arch. Newel. 21 : 638-644. 16. SAHAR, ;\., G. hi. HOCHWALU. and J. RANSOHOFF. 1970. Experimental hydrocephalus: Cerebrospinal fluid formation and ventricular size as a function of intraventricular pressure. J. A:cru-ol. Sci. 10 : 269-278. 17. SAHAR. ii., G. M. HOCHIVALD, and J. RANSOHOFF. 1970. Cerebrospinal fluid turnover in experimental hydrocephalic dogs. n’cllro[ogg In Press. 18. SCHLEZIXGFX, B. 1939. The venous drainage of the brain with special reference to the Galenic system. Btnir~ 62: 274-291. 19. SCHREISER. G. E. 1950. Determination of inulin by means of resorcinol. Pror. Sot. Esp. Biol. Mcti. 74: 117-120. 20. SHEKKIS, H. A., RI. H. HARMEL, and S. S. KETY. 1948. Dynamic anatomy of the cerebral circulation. Arch. Neural. Psychait. 60 : 240-252.
122 21.
SAHAR,
SKLnR,
ume: 24:
H., Values
I;.
79-82
HOCH
WALD,
AND
RANSOHOFF
E. F. BURKE, JR., and T. W. LANGFITT. 1965. Cerebral blood volobtained with Tr-labelled red cells and RISA. J. .4ppl. Pltysiol.
ESPERIJIEXT.+L
SEUROLOGY
Prolonged in
28, 123-131
(1970)
Changes in Single-Cell Activity Lateral Geniculate Nucleus
Prolonged changes in single-cell activity in the lateral geniculate nucleus are produced by sciatic nerve stimulation in anesthetized cats. The increases or decreases in activity may last for several minutes. Interval histograms show that the changes occur primarily in the short-interval distribution. Changes in singleand multi-unit records obtained simultaneously are usually, but not always, positively correlated. The possible mechanisms and implications of the prolonged changes are discussed. Introduction
Prolonged changes in the tonic electrical activity of the visual radiations and lateral geniculate nucleus (LGN) have been reported in earlier studies ( 14, 15). It was found that somatic stimulation. such as rubbing a paw, or electrical stimulation of the sciatic nerve, frequently produces marked, prolonged changes in mean-square ( MS) electrical activity recorded from discrete populations of neurons. The changes generally last 3-12 min, but sometimes persist for more than 30 min. Prolonged increases or decreases in tonic activity are produced, depending on the recording site and level of anesthesia (1-C. 15) The prolonged changes, characteristically observed when the cats are in a state of moderate anesthesia, are not seen in the awake, or deeply anesthetized animal. The changes are not specific to any particular anesthetic agent ; they are observed in animals anesthetized with either sodium pentobarbital or chloraloseurethane. Control experiments show that changes in EEG, blood pressure, and intraocular pressure are not the causal mechanisms of the prolonged activity changes. There is evidence (1.5 ). moreover, that the reticular formation may mediate the effects of the somatic input on visual system activity. Since the MS recording method samples populations of neurons 1 Supported by contracts SD-193 search Projects -4gency. \I’e thank technical assistance. Dr. Dubrol-sky Kniversit!
and D.4HC 1568-CO396 from J. A. Bridges and J. C. Vanagas is now at the Ilepartment of
1’3
the Advanced Kefor their excellent Psychiatry. McGill
124
DUBROVSKY
AND
MELZACK
(2), it is important to determine whether activity in individual units show similar long-term changes. The purpose of the present study was to examine the effects of somatic stimulation on single-cell activity in LGN, and to determine the correlation between mean-square and single-cell records. Methods
Experiments were performed on 20 cats anesthetized with chloralose urethane or sodium pentobarbital, and placed in a stereotaxic instrument. In all experiments, the pupils were maximally dilated with cyclogyl, and nicitating membrane movements were blocked by application of neosynephrine. The animals were then immobilized with Flaxedil and artificial respiration instituted. Craniotomy was performed (at A 6-8, L 8.5-10) to permit insertion of a bipolar electrode into LGN on one side and a microelectrode into LGN on the other side until cells driven by light were found, usually at a depth of about 11-12 mm. Previous studies (14, 15) showed that the prolonged changes in homologous regions, such as LGN on both sides, are in the same direction and have similar durations. The right sciatic nerve was exposed for stimulation. The stimulus parameters used were 3- to S-v square waves (0.05- to 1-msec duration, 2/set). Light flashes were produced by a Grass Photo-stimulator. The mean-square electrical recording method has been described elsewhere (14) in detail. Basically, a bipolar electrode led to an amplifier, Ballantine RMS voltmeter set in the mean-square mode, and d-c channel of a Grass Polygraph or Leeds-Northrup-Speedomax-G recorder. Singlecell records were obtained with tungsten microelectrodes ( 11) . The output led to a cathode follower and conventional amplifying equipment. The data were displayed on the oscilloscope screen and recorded on tape. The original spike discharges were transformed into standard pulses after being gated in a modified Schmidt trigger discriminator. The pulses were then fed into an electronic counter to obtain the absolute number of spikes. To determine the spike frequency, the pulses were fed into a rate meter (Fersh Electronics), and the interval histograms were obtained with an Enhancetron (Nuclear Data Corporation). The criteria proposed by Bishop, Burke, and Davis (4) and Hubel (12) were used to identify LGN cells: a complex positive-negative waveform with inflections in the rising phase of the positive deflection. In order to obtain stable recording conclitions, the procedure described by Kozak, Rodieck, and Bishop (13) was used: About 15 min after the microelectrode tip had reached the upper surface of LGN, the electrode was advanced a few microns at a time. Microelectrode recording sites were marked by passing d-c current through the electrode. The brains were fised in 10% form01 saline solution prior to sectioning on a freezing microtome. Serial sections were stained with cresyl violet and the probe tracks were reconstructed.
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Results Lifter the bipolar macroelectrode was lowered into LGN, stimulation of the sciatic nerve produced prolonged changes in the nlean-square tonic activity. Either increases (Fig. 1A ) or decreases (Fig. 1B) were observed. Sinlultaneous records of I~lood pressure and EEG confirmed earlier ohservations ( 1-I. 15) that the prolonged tonic activity changes frequently occurred in the absence of any changes in Mood pressure or EEG ( Fig. 1). TI’hen suc11 changes were observed, they were found to vary independently of the MS changes, indicating that they are not causally related. .After prolonged MS changes were recorded, the microelectrode was inserted into LGIY on the opposite side. Since the purpose of this stud? was to examine prolonged changes in tonic activity, only cells which showed the characteristic wavefornl of LGW units (Fig. 2)) a steady tnean firing development of the interval histograms rate (Fig. -!.I), and a uniform for 3 min (Fig. 3) were used to evaluate the effects of stimulation. The cells that met these criteria of stationarity, which were employed by Bishop and Levi& ( 5 ) in their stud\- of LGK units, showed ahnost without esception, a preponderance of short intervals in the interval histograms. liecordings were obtained from 50 cells. Eighteen showed a stable mean firing rate for at Ieast 3 min before stinxdation, anti met the estnl~lished criteria. Of these cells: 11 showed definite. prolonged changes to sciatic stimulaticn. Six cells showed an increase and five cells a decrease in tonic activit! after sciatic stiniulation. Successive stiniulus presentations had siinilar effects on the same unit. The absolute value and range of change for each
FIG. 1. X: Prolonged increase in mean-square activity evoked in LGN by electrical stimulation (0.05 msec. E/set. 8 \-) of the sciatic nerve of the ipsilateral hindpaw (IH). and hrief increase produced by light flashes (L). B.C,D : Simultaneous records
of LGN mean-square activity (B ). carotid blood pressure (CJ, and EEG from anterior ectosylvian gyrus (D ) before and after electrical stimulation (black bar) of IH sciatic ner\-e (0.1 msec. Z/set, 5 v). Note the prolonged decrease in LGN meansquare activity in the absence of any change in blood pressure or EEG. Sodium pentobarbital anesthesia was ~13x1 in these experiments.
126
FIG.
unit
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AND
2. Example of the type of LGN unit at fast sweep-speed. Positive up. Note
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analyzed. inflections
A: Discharge on the initial
pattern. positive
B : Same phase.
individual unit is shown in Table 1. Figure 3 shows examples of an increase and a decrease in prolonged activity in LGN, obtained during two different experiments. In both cases the interval histograms of the cells show that it is in the short-interval distribution that most of the changes occur as a result of the stimulation. This observation is in agreement with results reported by Bindman and Boisacq-Schepens (3) and Evans and Robertson (8). In one case, a cell which in dim light diminished its firing frequency after sciatic stimulation (unit 9 in Table 1) stopped firing entirely for 7 min when tested in total darkness (Fig. 4A, B) . This testing sequence was repeated four times with similar results. It was clear, however, that only firing had been stopped since the unit could still be driven the spontaneous by light flashes. Changes in single- and multiunit activity patterns recorded simultaneously were usually positively correlated, although dissociation between the two was occasionally observed. Figure 4C shows a steady decrease in the MS activity when the background illumination was increased. The mean firing frequency of a cell which was recorded simultaneously (Fig. 4D) showed an increase. Similarly, unit 7 showed an increase in firing rate, but no change was observed in the MS activity recorded at the same time. The relationship between the two recording methods is further revealed by the observation that prolonged increases in MS activity produced by
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FIG. 3. Top: Mean square (MS) activity recorded from LGN, showing a prolonged decrease in activity after stimulation of the sciatic nerve (3 v, 0.2 msec, 2/set). The black bar indicates stimulus duration. The time units A-G represent l-ruin periods. The interval histogram and total number of spikes of simultaneously recorded single-cell activity is shown below the MS record for each 1-min period. Bottom: MS activity from LGK, showing a prolonged increase in activity after sciatic stimulation (3 v. 0.1 msec, Z/set). The interval histogram and total number of spikes of simultaneously recorded unit activity for each I-min sample, A-D, is shown below. Note the changes in the short-term interval distribution in both cases.
stimulation
were
sometinies
with activity in adjacent cells that 1E and F). was in spontaneously firing units, sotne
associated
had previously been silent (Fig. Although our major interest
12s
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AND
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TABLE ABSOLUTE
FIRING VALUES STIMULATION
Time Cell IlO.
1
OF 11 CELLS IN LGN BEFORE AND AFTER BRIEF (S) OF THE SCIATIC NERVE. THE TIVE UNITS REPI~ESENT l-MIN PERIODS before
S
Time
(lo-20
SEC)
after
-~
1 2 3 4 5 6 7 8 9 10 11
-3
-2
-1
+1
+2
+3
1150 1300 1425 1480 1175 1362 1128 462 545 854 915
1143 1328 1448 1475 1130 1344 1068 4.58 5.53 883 907
1162 1312 1439 1468 1178 1375 1150 463 562 860 876
1200 2400 1775 1690 1349 2535 1300 407 480 435 670
900 2448 1698 1580 1420 2226 1269 415 509 461 713
850 2437 1680 1620 1270 2198 1241 412 537 518 743
/
E:
2ooflv I 20 mrec.
F#
,,m
,
FIG. 4. A: Change in firing frequency of a cell in LGN after stimulation of the sciatic nerve (4 v, 0.2 msec. Z/set) with the cat in dim light. Stimulus duration is indicated by the black bar. Ordinate : spike counts/set ; abscissa : time. Time-constant of the rate meter: 1.1 sec. B : Same cell as A, same stimulus parameters, with the cat in total darkness. C: Decrease in MS activity when background light is turned on (indicated by the arrow). D : Firing frequency of a cell, recorded at the same time as C, showing an increase in firing rate. Ordinate : spike counts/set; abscissa : time, same as C. E, F: Group of tonically firing cells recorded with a microelectrode in LGN before (E) and after (F) stimulation of the sciatic nerve, in which an increase in the MS activity was observed. Note the recruitment of new cells in F, showing that new units may contribute to the increase in MS activity.
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observations were made on cells that did not exhibit tonic I~ackgrouncl activity. Under these circumstances, when the cells were activated by light flashes, no change in the response of the cell could be detected after sciatic stimulation, although a change in multiunit activity \vas recorded. Discussion
The results show that the prolonged changes produced in LGK by brief somatic stimulation, hitherto seen only by multiunit recording methods. can also be observed in the firing patterns of units. Although the activity changes recorded with the two methods are usually positively correlated, the observed discrepancies between them argue against oversimplification in the interpretation of multiunit recording. Changes in individual units in response to sciatic stimulation may be accompanied by no change in simultaneously recorded MS activity or even a change in the opposite direction. The MS activity, then. can not be linearly correlated with the absolute firing of a defined set of neurons alone. Similar changes in the 11s activity can be brought about by changes in the frequency discharge of the observed set, or by changes in behavior of different neuronal grouljs surrounding the bipolar electrode for the MS recording. Data at the unit level indicate that sciatic stimulation can have selective effects on different neuronal groups in the same area of the LGIY. Cells which did not show tonic activity but could be activated hy phasic visual input showed no response change after sciatic stimulation, although simultaneously recorded MS activity showed long-term changes. Moreover, when tonic activity was stopped 1)~ sciatic stimulation. the phasic reqonse of the unit was nevertheless retained. These findings, taken together, suggest that only some spontaneously active central neurons are subject to prolonged changes. This conclusion is in agreenlent with observations by Bindman and Boisacq-Schepens ( 3 ) and Granit and Iienkin ( 10 ) which show that tonically firing cells are more easily modified than Ihnsically activated ones. The observations that prolonged changes are readily produced in the midbrain reticular formation and that reticular stimulation elicits prolonged changes similar to those produced by somatic stimulation ( 15 ) indicate that reticular mechanisms are at least partly involved in mediating the effects of somatic input on visual structures. There is convincing evidence that these effects are not clue to global, pervasive influences such as those that produce EEG arousal or blood pressure changes : Changes in EEG may or may not occur. and are usually observed in sensory projection areas that also show mean-square changes. They are rarely observed in association cortical areas. In general. there is no over-all correlation between prolonged tonic activity changes and changes in EI
130
DUBROVSKY
AND
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pressure, indicating that the three types of change are not causally related (14, 15). Other possible causes of artifact, such as respiratory or intraocular pressure changes, pupillary dilatation, and poststimulus changes in somatic tissue or nerves have also been eliminated (14, 15). Indeed, the phenomenon is not generalized but occurs selectively in discrete brainstem and cortical areas. Moreover, if the changes were due to a generalized arousal, they should occur more easily as anesthesia wears off; instead, they become harder to elicit and are rarely seen at anesthetic levels in which pinch produces reflex withdrawal. The fact that prolonged changes in two distant regions may differ in both direction and duration, and are differentially affected by small doses of anesthesia (15), further indicates the selective, discrete properties of the prolonged changes. The observations that prolonged changes, such as those observed here, occur only when the animal is moderately anesthetized suggests two possible mechanisms : (i) that the anesthetic diminishes or suppresses the background bombardment that normally occludes activity in reverberatory circuits (7) ; or (ii) that anesthetic agents may selectively block inhibitory neurons at reticular or local levels (6, 9). In either case, the activity triggered by stimulation could be maintained for prolonged periods of time in recurrent collateral reverberatory circuits such as those described by Andersen, Eccles, and Sears (1) and Burke and Sefton (7). The interval histograms of the units before and after stimulation reveal that it is in the short interval that most of the changes occur. This implies either a modification of an intrinsic property of the cell or some activity in the network to which the cell belongs (8). If this is so, the prolonged changes, once initiated, can be self-sustaining in LGN itself. The prolonged removal of occlusive or inhibitory influences would have the effect, in a sensory system, of exerting a prolonged bias on synaptic transmission regions. The bias could result in a prolonged increase or decrease in information transmission depending on whether the projection system has excitatory or inhibitory functions. The prolonged changes in activity in single neurons in LGN after brief somatic stimulation indicates the existence of mechanisms that exert prolonged influences on information processing or modulation in the visual system. Furthermore, the observation ‘that prolonged changes in the optic tract and visual radiations may have different directions and durations (15) suggests that these influences may be exerted, by centrifugal projections, independently at several levels in the visual system. Activities such as these could play an important role in normal integrative behavior, such as prolonged attention, multimodal interaction and information selection, as well as in neural activities that may underlie prolonged pathological pain states (15) _
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131
References 1. ANDERSON, P., J. C. ECCLF:S, and T. A. SE.IRS. 1964. The ventrobasal complex of the thalamus: types of cells. their responses and their functional organization. J. Physiol. London 174: 370-399. 2. ARDUINI, i\., and L. R. PINNEO. 1962. A method for the cmant%cation of tonic activity in the nervous system. .-lrck. If&. Biof. 100: 315124. 3. BINDMAN. L. J.. and N. BOISACQ-SCIIEPEXS. 1966. Persistent changes in the rate of tiring of single, spontaneously active cortical cells in the rat produced by peripheral stimulation, J. Physiol. London 185 : l-l-17. 4. BISHOP, P. O., W. BURKE. and R. DAVIS. 1962. The identification of single units in central visual pathways. J. Physiol. Loudorc 162: 409431. 5. BISHOP, P. O., W. R. LEWX, and 1%‘. 0. WILLIAMS. 1961. Statistical analysis of the dark discharge of lateral geniculate neurons. J. Plzysiol. Lodo~z 170: 598-612. 6. BRAZIER, M. ,4. B. 1954. The action of anesthetics on the nervous system, pp. 163-193. Zrz “Brain Mechanisms and Consciousness,” J. I;. Delafresnaye (ed.). Blackwell, Oxford. 7. BURKE, W., and A, J. SEFTOS. 1966. Recovery of responsiveness of cells of lateral geniculate nucleus of rat. J. Phqsiol. Lortdou 187 : 213-229. 8. EVANS, C. R., and A. D. J. ROBERTSON. 1965. Prolonged excitation in the visual cortex of the cat. Science 150: 913-915. 9. FELDMAN, S., and I. H. WAGMAN. 1963. The effect of pentobarbital on evoked potentials in brain of AIncncn mulnttn. Elcct~ocrrcr~l~trlr~~~. Cli>z. Newophysiol. 5: 747-760. 10. GRANIT, R., and B. RENICIN. 1961. Net depolarisation and discharge rate of motoneurons as measured by recurrent inhibition. J. Physiol. Lo&m 158: 461475. 11. HUBEL. D. H. 1957. Tungsten microelectrode for recording from single units. Sciewe 125 : 549-550. 12. HUBEL, D. H. 1960. Single unit activity in lateral geniculate body and optic tract of unrestrained cats. J. Physiol. Lomfo~c 150 : 91-104. 13. KOZAIC. W., R. W. RODIECE;, and P. 0. BISHOP. 1965. Responses of single units in lateral geniculatc nucleus of cat to moving visual patterns. J. Ncrtrupkysiol. 28: 19-47. 14. ~~ELZACK. R., K. KONRAD. and B. DUBROVSI;~. 1968. Prolonged changes in visual system activity produced by somatic stimulation. Esp. ;l~cwol. 20: 443459. 15. h'kLZACK, R.. K. KONRAD. and 8. DCXROVSICY. 1969. Prolonged changes in central nervous system activity produced by somatic and reticular stimulation. Exp. Newel. 25 : 416-428.