Wide-field neurons in somatosensory thalamus of domestic cats under barbiturate anesthesia

EXPERIMENTAL

68, 27-49 (1980)

NEUROLOGY

Wide-Field Neurons in Somatosensory Thalamus Domestic Cats under Barbiturate Anesthesia F. A.

Received

May

14.

1979:

of

HARRIS’

rc>tision

received

No\~euzher

13.

1979

A sample of 392 somatosensory thalamic neurons from barbiturate-anesthetized cats, all responsive to stimulation of the contralateral forepaw (CFP). was compared with a previously described sample from chloralose-anesthetized animals with respect to (i) the functional properties of CFP-responsive neurons isolated within nucleus ventralis posterolateralis (VPL), (ii) the spatial distributions of distinguishable neuronal subsets within VPL, and (iii) the distribution of CFPevoked single-unit discharge along the mediolateral dimension of the nucleus and over time. Eighty-two percent of the barbiturate neurons were responsive only to CFP; their receptive fields were small, usually confined to one or two digits or an area on the paw dorsum. Members of this sa subset were predominantly excited by light touch or deflection of hairs. The other 18% responded to stimulation of at least one “off-focus” limb in addition to CFP. These -.sa neurons possessed larger receptive fields (though not as large as those observed under chloralosel. which were bilaterally disposed in 32% of this subset. Fifty-eight percent of the -sa neurons were excited by light touch or hair deflection. The proportion of the barbiturate sample subject to corticofugal influences (62%) was smaller than that under chloralose (74%). with inhibitory influences predominating on both sa and -sa neurons in contrast to an excitatory effect on -sa neurons under chloralose. Compared with chloralose data, impulse discharge reached peak intensity earlier and overall duration of response was reduced for all subsets. Although the spike Abbreviations: VPL-nucleus ventralis posterolateralis; CFP, CHP-contralateral forepaw, hind paw; IFP. IHP-ipsilateral forepaw, hind paw. ’ This work was supported by U.S. Public Health Service training grant GM260-07 from the National Institute of General Medical Science, research grants NS00396 and NS05136 (to Dr. Arnold L. Towel from the National Institute of Neurological and Communicative Disorders and Stroke, and Initiative 171 Funds, State of Washington. The author thanks Dr. Arnold Towe for his counsel and use of his laboratory facilities, Mr. Gary Harding for data processing, and Ms. Hanna H. Atkins for preparation of illustrations and typing the manuscript. 27 0014-4886/80/040027-23$02.00/O Copyright C, 1980 by Academic Press. Inc. All rights of reproduction in any form reserved

28

F. A. HARRIS density distribution for -.sa well represented among VPL they are seen in barbiturate neurons are not an artifact

neurons was particularly abbreviated, this subset was neurons responding to stimulation ofthe forepaw. As preparations also, wide-field somatosensory thalamic of chloralose anesthesia.

INTRODUCTION The paradoxical appearance during chloralose anesthesia of manifestations of “hyperexcitability” (i.e., jerks or twitches of the appendages that occur spontaneously and/or are elicited by abrupt peripheral stimulation) (2, 8, 17-20, 32, 47) continues to cloud interpretation of results from investigations utilizing preparations anesthetized with this drug (21-23,29, 40,43,44). Advocates of strict topographic organization in somatosensory systems attribute the findings that certain neurons in the somatosensory thalamus (10, 11, 14, 16,22,23,25) and sensorimotor (i.e., pericruciate and midsigmoid) cortex (21, 35, 40, 43. 44) of the cat possess wide, often bilaterally disposed receptive fields, to artifacts of chloralose anesthesia. Such purported artifacts (1, 3, 4, 12, 29, 32) were described as due to “a facilitating spread which makes the cortex act as a unit” (38) (in contrast to the “preferentially directed spread over the cortex following definite pathways” that is believed to prevail during barbiturate anesthesia) or an “abnormal diffusion of activity ]that] exaggerates the degree of sensory overlap in the somesthetic association projection system” (8). Bava et al. (lo), most succinctly, simply refer to chloralose as “peculiar.” To determine if the properties of somatosensory thalamic neurons elucidated through experiments involving cats anesthetized with chloralose2 (22,23) might have been influenced or determined by that anesthetic agent (12, 29, 32), a parallel series of experiments utilizing animals anesthetized with pentobarbital was carried out (24). The barbiturate experiments were identical to those utilizing chloralose in every respect except for the anesthetic and paralyzing agents used. In this report, data from preparations under barbiturate anesthesia are presented alongside chloralose data to facilitate comparison. Wide-field neurons, many influenced by ipsilateral as well as contralateral inputs, were encountered within the ventralis posterolateralis (VPL) of cats anesthetized with pentobarbital, and in the same (i.e., caudolateral) region of the nucleus (10, 23) as they had been in cats anesthetized with cw-chloralose (24). Their response pattern was weaker under pentobarbital, however. Findings from the present study are compared with those of other investigators pertaining to properties of thalamic and cortical neurons in un2 Hereafter barbital will

referred be referred

to as “chloralose to as “barbiturate

cats.” cats.”

Similarly

those

anesthetized

with

pento-

WIDE-FIELD

NEURONS

IN VPL

29

anesthetized cats (6,7,25) and also with those from studies of sensorimotor cortical neurons in one and the same animal under barbiturate anesthesia, under chloralose anesthesia, and in the awake, unanesthetized state (21, 40). Neither of the drugs yields data comparable in quantitative detail to that obtained from unanesthetized animals. METHODS Forty-four mongrel cats, selected for uniformity in body weight, head size, and head shape were used. All were anesthetized with sodium pentobarbital (Nembutal)” at an initial intraperitoneal dose of 30 to 36 mg/kg, with supplemental intravenous doses of 10 to 15 mg whenever withdrawal reactions were elicited by pinching a forelimb toe (a test that was made between doses of paralyzing agent). Barbiturate experiments were interspersed with others involving animals anesthetized with a-chloralose (22,23). Cats anesthetized with pentobarbital were paralyzed with d-turbocurarine chloride (1 to 3 mg/h, intravenously), and decamethonium bromide or gallamine triethiodide was used in experiments with chloralose.” The animals were artificially respired with a stroke volume of 35 cc at a rate of 17 to 20 cycles per minute, with bilateral pneumothorax to reduce brain movements associated with respiration. Rectal temperature was maintained at 37.5-38.o”C by a feedback-controlled DC heating pad. Frontal cortex including the S I forepaw projection area and a parietal area overlying the somatosensory thalamus were exposed and bathed with warm mineral oil to protect the tissue from cooling and drying. A Trent Wells microdrive and Pfeiffer stereotaxic instrument were used to drive 2.5 M NaCl-filled glass microelectrodes in vertically oriented penetrations, through overlying tissue, into the thalamus on the left side. The electrodes were prepared with a long, gradual taper to 1.5 to 2.0 pm tip diameter for strength and rigidity, and lubricated with mineral oil to facilitate penetration; their DC resistances were on the order of 3 MR. Penetrations were made in a 1 x l-mm grid pattern superimposed on a dorsal projection of the VPL constructed from the atlas for feline diencephalon by Jiminez-Castellanos (27). The stereotaxic coordinates at which each neuron was isolated were recorded. The relationship between the grid, which overlapped the nucleus [see Fig. 1A in Harris (22)], and the physiologically defined forepaw area of the VPL was checked for each animal by mapping evoked potentials to stimulation of the contralateral :I Brand of sodium pentobarbital, Abbott Laboratories. North Chicago, Illinois. 4 Experience has shown that these particular combinations provide optimal stability the animals require the minimum number of supplemental administrations of paralyzing during an experiment).

(i.e., agent

30

F. A. HARRIS

forepaw. Twenty penetrations were made at each of 16 grid loci (i.e., the same number as in the chloralose experiments). Only one penetration was made at a given locus in each animal, with the order more or less randomized.Successive penetrations were at least 2 mm apart to minimize possible effects of tissue damage on data collected subsequently. While the recording electrode was advanced, the contralateral forepaw (CFP) was stimulated once per second” with electrical pulses of 0.1 ms duration and supramaximal intensity (25 mA), applied via bipolar needle electrodes with 5 mm tip separation inserted into the central footpad to a depth of 10 mm. Each neuron isolated using the CFP “hunting” stimulus was tested for response to electrical stimuli applied to the central pad of the other paws as well. The threshold to electrical stimulation of the CFP and maximal rate of repetitive stimulation to which the neuron responded in a one-to-one fashion were measured for each unit. The extent of the receptive field and the natural stimulus modality were determined using physiologic stimuli including (i) moving hairs with puffs of air or with forceps, (ii) touching, pressing, or tapping the skin with a small probe, (iii) moving each claw separately, and (iv) manipulating the limbs so as to change joint angles. Each neuron isolated was tested for antidromic activation and for corticofugal influences by applying 0.05ms pulses to precoronal, postdimple, and postcruciate cortex through bipolar ball electrodes. The criteria for antidromic activation were response to cortical stimulation with an invariant latency less than 1 ms and frequency-following at a stimulus repetition rate of at least 100/s. Condition-test interactions for the purpose of studying corticofugal influences were conducted with the peripheral stimulus reduced to threshold intensity. Details of the instrumentation and methods for single-neuron recording and data reduction were identical to those described previously for chloralose preparations (22, 23). RESULTS Confirmation of Electrode Placement through Evoked Potential Mapping. In 10 animals, evoked potential recordings utilizing a glass micro-

electrode with its tip enlarged to 1 pm were made at OS-mm intervals in depth at all loci on the penetration grid, either before or after the day’s unit recordings. In all other experiments, several penetrations were made at and near the location corresponding to the presumptive forepaw “focus,” and this location was verified or corrected according to the direction of 5 Single-unit responses (i.e., there is no buildup

to continuous or habituation

stimulation at this rate remain of response).

stable for several

hours

WIDE-FIELD

L8

L7

NEURONS

IN VPL

L6

31 L5

FIG. 1. Gross potential map constructed for the thalamic nucleus ventralis posterolateralis in the left hemisphere of a barbiturate-anesthetized cat, based on thalamic “localizing” responses to electrical stimulation of the contralateral forepaw (CFP). Recordings were made using glass electrode with 5-to IO-pm tip: positivity is downward. Each trace represents the largest evoked potential observed during the course of a track at the corresponding position on the penetration grid Idescribed in (2211. Horsley-Clarke coordinates are indicated at the top and right margin, and depth at which the particular response was recorded is noted above each trace. Largest (peak-to-peak) response was at A 9, L 6 in this animal, which corresponds to the average location of the maximal response to CFP for the group of 44 animals studied.

potential field gradients. The average coordinates thus obtained for the position at which the maximal amplitude response to the CFP was recorded in barbiturate cats were A 9, L 6, 1 mm posterior to the location of the amplitude maximum in chloralose preparations at A 10, L 6. The overall distribution of significantly large CFP-evoked responses appeared to be shifted posterolaterally in barbiturate cats as well (Fig. 1). Evoked potentials to electrical stimulation of the CFP in barbiturate cats were similar in configuration to those observed in chloralose animals; an early, sharp, biphasic positive-negative complex [corresponding to Mountcastle’s “thalamic localizing potential” (34)] was usually followed by a later, rounded, positive-negative wave described for chloralose preparations (22). The late wave covaried in amplitude with the early component of the

32

F. A. HARRIS

response, and was often observed with superimposed indicating local origin. FUNCTIONAL

PROPERTIES

single-unit

activity,

OF NEURONS

Neuronal Subsets. A total of 392 VPL neurons responsive to CFP stimulation was isolated and studied in 44 barbiturate cats, compared with 640 such neurons in 46 chloralose preparations. The average yield per penetration in barbiturate experiments was 1.2 units, compared with two units per track for chloralose cats. Forty-three percent of those penetrations at grid points corresponding to tracks that should have passed through the nucleus yielded no units, 35% yielded one or two, 20% yielded three to five, and only 2% yielded as many as six units. Only 35% of the penetrations in chloralose cats yielded no units, and 10% of the penetrations in those animals yielded 6 to 13 units responsive to stimulation of the CFP. Because all other factors affecting unit yield were identical, and the same number of penetrations were made in both experimental series, the above data indicate that barbiturate anesthesia reduced the number of thalamic neurons responding to peripheral stimulation to slightly less than two-thirds of those responding under chloralose. The population sample was partitioned into functional neuronal subsets according to convergence of peripheral input and adequate stimuli (Table 1). Those responding only to the CFP (sa neurons) comprised 82% of the barbiturate sample, compared with 68% under chloralose. Those responding to the CFP and at least one other limb (-sa neurons) comprised 18% of the barbiturate sample, compared with 29% under chloralose. Approximately 32% of the -sa neurons responded to bilateral inputs, compared with 60% in chloralose cats; 61% of the -sa neurons were of the SCvariety [responding to the CFP and contralateral hind paw (CHP)], 11% were m [responding to all paws] and 3% were sb neurons [responding to both forepaws]. The remainder had odd combinations of inputs from two or three appendages, including the CFP. Under pentobarbital. neurons excited by gentle mechanical stimulation (i.e., either hair or touch stimuli, or both) constituted 61% of thesa and 58% of the --sa subsets, compared with 75% ofsa and 48% of the --sa neurons, respectively, under chloralose. Hair sensitivity was most prevalent among both sa and -sa neurons in barbiturate animals, as also was the case in chloralose cats. Likewise, many barbiturate neurons were responsive to stimuli of more than one submodality type (e.g., combinations of hair and touch, hair and claw, or touch and claw).

WIDE-FIELD

NEURONS TABLE

Nembutal

Population

Sample

I Partitioned Number

Total

33

IN VPL

into Subsets” Percentage

sample

Experiments performed Units isolated s a neurons --.\a neurons CFP at least’

44 392 310 72 0

(46)” (640) (437) (186) (17)

82 (691 18 (29) 0 (2)

120 59, 23 21 3 39

(226) (79) (20) (1% (1) (23)

38 (52) 16 (18) 7 15) 7 (3) 1 (0) I? (51

19 (0) 43 (73) 320 (437)

6 (0) 13 (I71 100 (100)

sa neurons Hair Touch Hair and touch Noxious MTJ” Claw Miscellaneous combinations of the above” Mute, x) Total --sa neurons Hair Touch Hair and touch Noxious MTJ” Claw Miscellaneous combinations of the above Mute, xi Total

29 8 5 9 0 0

(54) (25) (9) (26) (I) (1)

I (0) 20 (70) 72 ( 186)

40 II 7 I3 0 0

(30) (13) (5) (13) (0) (0)

I (0) 28 (39) 100 ( 100)

” Classification of neurons according to extent of convergence of peripheral input determined via electrical stimulation, with partitioning into additional subsets according to natural stimulus sensitivities. ’ Chloralose data in parentheses for comparison here and in other tables. c Includes neurons lost immediately after isolation with contralateral forepaw hunting stimulus. rl Neurons responsive to passive movement of the limbs. ” Includes multimodal neurons with approximately equal sensitivities to each of the effective inputs. ’ Includes neurons unresponsive to all forms of natural stimulation tested and those lost before natural stimulation could be tested.

34

F. A. HARRIS TABLE

2

Dynamic Response Properties of Thalamic Neurons Responsive to Contralateral Forepaw (CFP) Stimulation” Response to CFP L (ms)

T OPY

S/D

FF (SC’1

s a’ -.sa

7.9 (7.9) 10.7 (10.3)

4.6 (6.0) 3.3 (5.5)

2.6 (2.6) 4.9 (5.4)

31.3 (42) 12.6 (29)

m (group within --sa subset) Response to CFP stimulation Response to CHP” Response to IFP Response to IHP

10.7 13.6 15.0 20.8

Type of neuron

(12.5) (20.9) (18.2) (28.5)

n Summary of average response properties for thalamic neuronal subsets. Mean values for first-spike latency (L,), threshold (T), spikes per discharge (S/D), and frequency-following (FF), all with respect to electrical stimulation of CFP as input, are indicated for sa neurons, --sa neurons, and them group within the --sa subset of neurons. Mean first-spike latency for response to electrical stimulation of each of the off-focus paws is indicated for m neurons. Chloralose data in parentheses. * Relative potentiometric unit, 6.0 = maximum (corresponds to 25 mA). I’ N for each type, sa = 307 (420); --sa = 69 (178); m = 8 (48). ” Abbreviations: CHP-contralateral hind paw, IFP, IHP-ipsilateral forepaw, hind paw.

About 7% of the sa and 13% of the -sa neurons in barbiturate cats required intense mechanical stimuli (e.g., sharp “raps” with a metal probe) for their excitation, values similar to those under chloralose. Thus, it cannot be said that -sa neurons in barbiturate cats exclusively represent nociceptive inputs any more than such a claim could be substantiated on the basis of chloralose data. Only three barbiturate neurons (all sa) discharged during passive movement of the limbs, such as would be expected to affect joint and/or muscle stretch receptors; this is comparable to the finding of only two “MTJ” units in chloralose cats (one of these a --sa neuron). Dynamic Response Properties of sa and --sa Neurons in Barbiturate Preparations. Mean first-spike latencies for CFP responses of both sa and

-sa neurons were comparable to chloralose values (Table 2). Mean firstspike latency for m neurons in chloralose cats increased regularly in the order: CFP (12.5), ipsilateral forepaw (IFP) (18.2), CHP (20.9), and ipsilateral hind paw (IHP) (28.5 ms). Off-focus m neuron responses in

WIDE-FIELD

NEURONS TABLE

3

Receptive Field Size and Location on Contralateral

Subset

Total number

Digits only

Paw and digits

sa --s a

259 (347) 29 (110)

171 (140) 7 (IO)

36 (83) 9 (28)

35

IN VPL

Forelimb”

Wrist

Lower arm, paw, and digits

Upper arm, elbow, and shoulder

Whole arm and paw

37 (33) 4 (3)

6 (65) 3 (45)

8 (13) I (4)

I (13) 5 (20)

o Tabulation of numbers of sa and -sa neurons having receptive fields or field portions on various areas of the contralateral forelimb, determined with physiologic stimuli. Only the portions of the --.~a fields on the contralateral forelimb are considered here. Chloralose data in parentheses.

barbiturate cats were similarly of longer latency than those to the CFP, but all values were reduced compared with those in chloralose cats, and the order of IFP and CHP latencies was reversed (i.e., responses to ipsilateral inputs had longest latencies). Thresholds for excitation by electrical stimuli to the CFP were reduced for both sa and --sa neurons compared with values obtained under chloralose; the threshold for -sa neurons was lower than that for sa neurons under both anesthetic agents. In both preparations, --sa neurons discharged longer bursts of impulses to the CFP than did sa neurons, although the difference between mean spikes per discharge for the two subsets was smaller in barbiturate cats. Under pentobarbital, -sa neurons had poorer frequency-following ability (on the average) than sa neurons; both values were lower than those for chloralose cats. Receptive Field Size and Location. All sa neurons had receptive fields limited to the contralateral forelimb. A larger proportion of barbiturate sa neurons (66%, compared with 40% under chloralose) had very small receptive fields limited to one or two of the digits, however. As shown in Table 3, only 15 sa neurons had fields extending beyond or located above the wrist. The portions of the receptive field on the contralateral forelimb for barbiturate --sa neurons also were smaller than those of their counterparts under chloralose; 69% were limited to the digits, digits and paw, or wrist area. Under chloralose, 62% of the -sa neurons had contralateral forelimb field portions encompassing large areas, such as a “stocking” from the elbow to the tips of the digits. Differential Responses of -sa Neurons to Stimulation of Individual Pa,z,s. Although spike discharge reached peak intensity about 7 ms after

CFP stimulation

for both the barbiturate

and chloralose samples (Fig. 2A),

36

F. A. HARRIS 250 A

Whole

Population

250 B. All

2 200 .% 8 150 6 .qii 100 5 l 50 0 * .* 3

0 ms 5

IO

15

100

‘0

50

9

0

20

25 C All

0

5

IO

15

sa

20

25

Neurons

30 -5a

35

40

45

40

45

Neurons

30

35

FIG. 2. A-poststimulus spike discharge histogram showing unpartitioned sample response to single-shock electrical stimulation of the contralateral forepaw (CFP) at intensity 6.0; time (r) in ms following stimulus delivery at zero. B.C-poststimulus spike discharge histograms comparing responses of sa and -sa neuronal subsets to stimulation of the CFP (barbiturate data solid bars; chloralose, hatched).

the total duration of discharge under pentobarbital was reduced by almost 40% compared with that under chloralose. Both sa (Fig. 2B) and -sa (Fig. 2C) subsets followed this trend; -sa discharge to the CFP was markedly curtailed (this was even more the case for ipsilateral inputs). The time course of -sa neuron activity in barbiturate cats differed depending on which limb was stimulated, as had been the case for chloralose animals. Discharge to stimulation of the CFP began at 4 ms and peaked at 10 ms, whereas that to the CHP began at 6 ms and peaked at 12 ms. Ipsilateral forepaw-initiated activity began at 8 ms and ended at 28 ms, and IHP-initiated activity began at 14 ms and ended at 32 ms. Ipsilateral hind paw activity was more intense overall than that resulting from IFP stim-

WIDE-FIELD

NEURONS

37

IN VPL

ulation, although the average discharge rate never exceeded 1 S impulses per millisecond for either ipsilateral input. Cerebral InJluertces 011 Somatosensory Thalamus. Of 358 barbiturate neurons for which data on cortical influences were obtained, only 14 could be activated antidromically (five from precoronal, five from postdimple, and four from the postcruciate region), less than half the number in chloralose cats; only one of these was a -sa neuron (Table 4A). Combining excitation, inhibition, and mixed effects, a smaller percentage of thalamic neurons was subject to synaptically mediated effects from the cortex in barbiturate cats (62%) than in chloralose preparations (74%). Of 222 barbiturate neurons in this group, 18% were influenced only from the precoronal cortex, 14% only from postdimple, and 15% only from postcruciate cortex (Table 4B). Thus, the precoronal site was most potent for production of well localized, synaptically mediated effects in barbiturate animals. It was the most potent site for both antidromic and synaptically mediated influences under chloralose. More thalamic neurons in both sa and -sa subsets were inhibited (In) than were excited (Ex) from cortex (3.75 In: 1.O Ex for -sa, and 2.0 In: 1 .O Ex for sa neurons) in barbiturate cats (Table 4A). Under chloralose, more -sa neurons tended to be excited than inhibited (1.8 Ex: 1 .O In) and the proportions for sa neurons were about equal (1.1 In: 1.O Ex). Under pentobarbital, the number inhibited exceeded those excited from the cortex for both hair-sensitive (2.1 1n:l.O Ex) and touch-sensitive subsets (2.5 In: 1.O Ex). whereas under chloralose, the numbers excited and inhibited were approximately equal for both of these subsets (Table 4C). SPATIAL

AND TEMPORAL DISTRIBUTION NEURONAL ACTIVITY

OF

Contralateral Forepa\r> Focus in Terms of Unit Density. The spatial distribution of CFP-responsive units isolated in chloralose-anesthetized cats showed a pronounced density peak at A 10, L 5-6) on the penetration grid. The focus for CFP-driven unit activity under pentobarbital was shifted posterolaterally (Fig. 3A); response was weak in the region corresponding to the high density representation under chloralose, leaving a block of four positions (A 8, L 6; A 8, L 7; A 9, L 6; and A9, L 7)-with approximately the same number of CFP units per penetration at these loci under chloralose-to stand out as a new (relatively) high density region for barbiturate preparations. The depth distributions of CFP-responsive neurons under the two anesthetic agents were roughly similar

F. A. HARRIS

38

TABLE 4 Cortical Influences on Nembutal Population Sample and Its Subsets” A: Antidromic Effect Antidromic activation Cortical Ex Cortical In Mixed” NCI’ Total

and Corticofugal Synaptic Effects on Convergence Subsets Number Percentage Number Percentage Number total total sa sa --sa

14 (651 62 (1831 139 (164) 21 (901 122 (92) 358 (5941

4 (11) 17 (31) 39 (28) 6 (15) 34 (151 100 (100)

13 (45) 54 (117) 109 (127) 7 (681 115 (65) 298 (422)

1 (20) 8 (66) 30 (371 14 (221 7 (271 60 (172)

4 (11) 18 (28) 37 (30)

2 (16) 39 (15) 100 (100)

Percentage --sa

2 (12) 13 (37) 50 (221 23 (13) 12 (16) 100 (100)

B: Sites from which Pure Antidromic or Pure Synaptic Effect Occurred N = 222 (N = 437) Stimulation site Effect Antidromic activation only Cortical Ex only Cortical In only Number, cortical Ex or cortical In Percentage of total, cortical Ex or cortical In

Precoronal

Postdimple

5 (441 18 (1091 23 (138)

5 (41 12 (28) 20 (211

4 (171 11 (17) 22 (191

41 (2471

32 (49)

33 (36)

18 (56)

14 (11)

15 (8)

C: Corticofugal Synaptic Effects on Modality Subsets Hair-sensitive Effect Cortical Ex Cortical In Mixed* NC1 Total

Postcruciate

Touch-sensitive

Number

Percent

Number

30 63 6 0 99

30 (361 64 (39) 6 (171 0 (8) 100 (100)

20 (271 3 (311 0 (6) 31 (82)

(82) (871 (391 (171 (2251

8 (28)

Percent 26 (34) 65 (33) 9 (26) 0 (71 100 (100)

” Summary of antidromic and synaptically mediated effects of cortical stimulation. Corticofugal excitatory (Cortical Exl and inhibitory (Cortical In) effects were measured against threshold responses of neurons to stimulation of the contralateral forepaw. A-antidromic and synaptic effects on unpartitioned sample, sa and --sa subsets. B-cortical areas from which pure antidromic and pure excitatory or inhibitory synaptic effects were elicited. C-synaptic effects on hair- and touch-sensitive subsets. Chloralose data in parentheses. h Refers to neurons showing combinations of excitatory and inhibitory effects to various cortical inputs. I’ Refers to neurons that were not influenced by cortical stimulation.

WIDE-FIELD

A. Grid LB

NEURONS

IN VPL

39

Distribution L7

L6

L5

LB

L7

L6

L5

Chloralose

Nembutal

6. Depth Distribution 29r z 9 9) $

H3 H2 HI

v HO -3 0 H(4)

Bti(-2) z3 0 IO 20 30 40 50 60 70 80 90 100 110 Number

of Units

Isolated

FIG. 3. Location of contralateral forepaw (CFP) focus within the thalamic nucleus ventralis posterolateralis VPL, defined in terms of evoked unit density. A-each square corresponds to a penetration position. and the numbers within are the average number of forepaw-evoked neurons isolated per penetration at that position, for a total of 20 penetrations. (Barbiturate data on left, chloralose data on right for this and other similar illustrations. High-density areas indicated by hatching.) B-distribution of CFP-excited neurons in depth. (Barbiturate data solid bars, chloralose hatched in this and other similar illustrations.)

in profile, except that the distribution for pentobarbital peaked 0.5 mm higher than that for chloralose (Fig. 3B). Distribution of Neuronnl Subsets. There were greater numbers of sa neurons at the A 9, L 6; A 8, L 5; and A 8, L 6 positions under pentobarbital than were isolated at these loci under chloralose (Fig. 4A). Although comparable numbers of sa neurons were encountered at A 9. L 7 with both agents, the number of sa neurons in the barbiturate grid distribution was reduced below values for the chloralose distribution at all other loci, especially at A 9, L 5; A 10, L 5; and AlO, L 6. The overall result was a posterolateral shift in the sa focus under pentobarbital. The overall shape

FIGS. squares data on isolated

20

30 40 50 60 70 Number of Units Isolated

80

;

B.

HO

IO

Chlorolose

3 s ‘9 91 $

L5

IO

20

30 40 Number of

A6

80

Chloralose

L7

50 60 70 Units Isolated

~6

L6

90

100

L5

4 (left) AND 5 (right). Grid and depth distributions of sa (specific input) and --sa (convergent input) neurons. In 4A and 5A. numbers in represent total number of neurons of each type isolated in 20 penetrations made at each of the corresponding grid positions (barbiturate left. chloralose on right). In 48 and 5B, depth distributions of sa and -sa neurons are compared in terms of numbers of neurons within each 0.Smm interval in depth from dorsal to ventral limits of the volume enclosing the nucleus ventralis posterolateralis.

Q

s H(-2) T a 0

zH(-21 z E D

HO

HI

H2

H3

Depth Distribution

Nembutal

3 Hkl)

HI

H2

L6

A9

L7

A9

LB

AI0

L5

AI0

L6

Neurons

Al I

L7

All -sa

Al I

LB

H3

0

L5

A. Grid Distribution

B H(-1,

$ P 9, p G a

2g

L6

Distribution

Nembutal

L7

Distribution

B. Depth

LB

A. Grid

All so Neurons

WIDE-FIELD

NEURONS

IN VPL

41

of the depth distribution for sa neurons in barbiturate animals (Fig. 4B) was the same as that for the total (unpartitioned) sample (Fig. 3B), and the peak was 0.5 mm higher than that for chloralose sa neurons. The density maximum for -sa neurons was at A 8, L 8 under pentobarbital compared with A 8-A 9, L 7 under chloralose (Fig. 5A); the number of -sa neurons per penetration at the former position was greater than was observed at the same location under chloralose. The number of -sa neurons was reduced at all other positions under pentobarbital, particularly at A8,L7;A9,L5-L8;AlO,L8;All.L5,andAll,L8.Thus,thefocus of --sa neurons was more distinct under pentobarbital than with chloralose, and the distribution of --sa neurons was shifted posterolaterally overall. The depth distributions for --sa neurons under both anesthetic agents were fairly uniform, except for proportionately fewer cells at depths below H - 1 under pentobarbital (Fig. 5B). Indicative of a trend toward predominance of inhibitory effects, the number of cortically excited neurons was reduced overall in barbiturate animals, although more so at A 8-9, L 7; A 10, L 5-6; and A 10, L 8 than at other loci (Fig. 6). Cortically inhibited neurons were present in large numbers throughout approximately the same regions under both anesthetic agents, however: the number of cortically inhibited neurons was actually greater under pentobarbital than under chloralose at the A 8, L 8: A 9, L 7; and A 9, L 8 positions. Spike Density Distributions. Discharge of sa neurons under barbiturate anesthesia was confined primarily to planes L 5 through L 7; sa activity in the L 8 plane was limited in duration to 20 ms and in depth to levels above H 0. In both L 5 and L 6 planes, duration of sa discharge was reduced by about 5 ms compared with that observed under chloralose. Spike density foci occurred in the L 5. L 6, and L 7 planes at the same depths as they had under chloralose, but 2.0 ms earlier. The maximum (1.0) spike density focus for sa neurons occurred in the L 6 plane under both anesthetic agents (Fig. 7A), although the overall intensity of activity in this plane was lower under pentobarbital. There was a tendency toward stratification of sa discharge, with prolongation of activity at H 2.0, H 0, and H -2.5 levels. Only in the L 7 plane was the duration of impulse discharge comparable with that under chloralose. At levels above H 0, sa neuron discharge under pentobarbital was curtailed in all planes. Discharge of -sa neurons was quite weak under pentobarbital; spike density plots generally gave the impression of widely dispersed “nodes” (brief bursts of activity) or “strata” (discharge continuing as long as 15 ms) at particular depths. There were a few early, short-lived nodes of -sa activity in the L 5 plane; somewhat more intense --sa activity was distributed sparsely throughout L 6. In both the L 5 and L 6 planes, -sa discharge

42

F. A. HARRIS Cortically L8

Excited L7

L6

Neurons L5

L8

Nembutal

Cortically L8

Inhibited L7

L6

L7

L6

L5

Chlorolose

Neurons L5

L8

L7

L6

L5

Al I AI0 A9 A8 Nembutal

Chloralose

FIG. 6. Distributions of corticofugal excitatory and inhibitory influences within the nucleus. Each number indicates the total cells isolated at the corresponding grid position, for 20 penetrations, that were influenced from cortex in the manner indicated (see text for particulars concerning the cortical areas stimulated). The peripheral stimulus was reduced to threshold intensity during condition-test interactions for determination of these effects. Excitation was manifested as a reduction in latency and/or an increase in the number of spikes or probability of discharge; inhibition, as an increase in latency and/or a reduction in number of spikes or probability of discharge to the test stimulus.

terminated from 6 to 15 ms earlier than under chloralose. Discharge of --sa neurons was most intense and extensive in the L 7 plane, the maximal density focus occurring here at H 0.5, 6 ms after CFP stimulation (Fig. 7B). Although activity in the L 8 plane was minimal under pentobarbital, two maximal density foci occurred here under chloralose-one at H 0.5 and 13 ms latency, and the other at H -0.5 and 11 ms latency (the latter presumably an extension of the focus at approximately the same depth in the L 7 plane). Discharge of hair-sensitive neurons in barbiturate cats terminated 4 to 13 ms earlier, depending on the sagittal plane in question, than under

WIDE-FIELD

NEURONS

IN VPL

43

chloralose. The maximal spike density focus among this subset occurred in the L 7 plane, at about the same depth as the hair cell discharge focus in the L 6 plane under chloralose, but 2 ms earlier. Discharge of hairsensitive neurons in the L 5 and L 6 planes tended toward a pattern of nodes and strata, and the L 8 plane showed extremely sparse hair-cell activity, ending by 11 ms. Touch-sensitive elements showed maximal activity in the L 5 plane under pentobarbital. Overall duration of touch-cell activity here was approximately 4 ms shorter under pentobarbital than under chloralose. The distribution of touch-cell discharge in the L 6 and L 7 planes showed nodes and strata, and was comparable in duration to that under chloralose only near H 0. Touch-cell discharge in the L 8 plane, limited to three lowdensity nodes, terminated 10 ms earlier than under chloralose. DISCUSSION The number of wide-field, bilaterally excited neurons in the VPL of barbiturate-anesthetized cats recorded in this study is sufficient that they warrant serious consideration. Clearly, the earlier descriptions of widefield neurons in the VPL of chloralose-anesthetized cats, reported by Harris (22,23), Jabburet al. (25), Bavaet cif. (IO), and Bowsher (11) cannot be discounted as having resulted from an artifact of anesthesia unique to the latter agent. Baker (6) demonstrated that many somatosensory thalamic neurons in unanesthetized cats had larger receptive fields than those described for barbiturate preparations by Poggio and Mountcastle (36); none was responsive to ipsilateral inputs, however. Thus, it might even be argued that both pentobarbital and chloralose alter excitability of certain thalamic neurons to the point at which they become responsive to relatively weak ipsilateral inputs. On the other hand, Slimp and Towe (40) found that receptive fields for cortical somatosensory neurons were larger under chloralose and smaller under pentobarbital than they were in the same cat when unanesthetized. Less than 2% of the cortical neurons responded to bilateral inputs in animals under barbiturate anesthesia, whereas 3% of unanesthetized and 27% of chloralose neurons were bilaterally responsive (21). Their results indicate that barbiturates depress excitability of cortical --sa neurons, while chloralose increases their excitability compared with data from unanesthetized animals. Pharmacological studies showed that the degree of synaptic depression produced by barbiturates is proportionately related to the anesthetic dose (9, 31, 37, 41, 42, 45). Therefore, it is possible that the studies which led to advocacy of strict somatotopy as an organizational principle for soma-

44

F. A. HARRIS

sa Neurons

5

9

13 17 21 TIME (msec)

A

---sa

-31 5 B

c

9

(L6 plane)

Neurons

8

8

25

29

(L7 plane)

’

’

1

1

13 17 21 TIME (msec)

’

’

25

’

1’

29

FIG. 7. Spike density plots depicting barbiturateand chloralose-population sa and --sa neuronal subset responses to stimulation of the contralateral forepaw (CFP). For purposes is represented by of clarity. each response set indicated (A. sa neurons; B, --sa neurons) a single plot for the sagittal plane in which maximal activity among that set of neurons occurred. For purposes of constructing density plots, all neurons isolated in tracks made in a given sagittal plane are treated as if they were found in a single “compressed” penetration at the corresponding distance from the midline, regardless of their A-P positions.

WIDE-FIELD

NEURONS

IN VPL

45

tosensory thalamus (29, 30, 34, 36) ]or more recently, to the postulation of a trend toward strict somatotopy as an evolutionary principle (46)] utilized animals that had received a dosage of barbiturate sufficiently high to render thalamic and cortical --sa neurons unresponsive. Although chloralose may give an impression of “looser” somatotopic representation, pentobarbital may have given an impression of stricter somatotopic representation than prevails in unanesthetized preparations. (It seems preferable to err on the side of assessing the maximal degree of organizational complexity, because in the process information might be gained about mechanisms that modulate excitability of various neuronal subsets). Differences in slow potential activity observed under the two anesthetic conditions may provide additional clues. Evoked potentials to stimulation of the off-focus paws are reduced in amplitude or fail entirely under barbiturate anesthesia (Harris, unpublished observations), an indication that barbiturates interefere with impulse discharge by reducing the amplitude of synaptic potentials. Such an effect should have an impact at all levels of the neuraxis; local thalamic and cortical effects might therefore be involved in barbiturate anesthesia in addition to the well-known depressant effect on the polysynaptic substrate of the brain stem reticular formation (5, 13. 15, 28, 33, 391. Haimann, rt crl. (16) found that intravenous doses of cu-chloralose administered to locally anesthetized cats led to alterations in the response properties of forepaw-specific neurons within VPL. Although “aspecific” properties such as those described by Harris for the -sa subset of VPL neurons in cats initially anesthetized with and maintained on chloralose (22, 23) did not appear, 8 of 20 cells they examined displayed continuous or discontinuous enlargements of their peripheral receptive fields after administration of chloralose. Fadigart u/. (14) postulated the “unmasking” by chloralose of additional, convergent specific input to forepaw neurons, normally blocked by afferent or cortical inhibition, to explain this effect. Pertinent to the latter point, Balis and Monroe (8) suggested that chloralose exerts a depressant effect on the cortex, thereby eliminating tonically active descending influences on thalamic nuclei and reticular formation. The demonstration in the present study of a preponderance of inhibitory corticofugal influence on VPL neurons in barbiturate-anesthetized cats (whereas excitatory influences predominate in chloralose preparations) is The contour lines enclose regions in which density of spike discharge to single-shock electrical stimulation of the CFP was at 1.O(maximal). 0.75.0.50.0.25. and 0.0 density levels. Stimulus presentation was at time 0. though plots begin at 5 ms. A bin size of 0.5mm depth x 2-ms time was used for construction of these plots. Solid contour lines and italicized numerals represent barbiturate data: dotted lines and roman numerals represent chloralose data.

46

F. A. HARRIS

consistent with the above suggestions. Such an inhibitory influence might contribute toward the lower incidence (rather than total absence) of widefield neurons in barbiturate preparations, and also to the unusually long latency of m neuron responses to stimulation of the ipsilateral paws in particular. (That is, if this inhibitory influence is at all selective, ipsilateral responses may be preferentially antagonized.) If loss of a facilitatory influence exerted on the cortex by the reticular activating system does in fact account for the almost complete elimination of cortical m neuron activity in barbiturate animals (8, 21), then those thalamic wide-field neurons that remain active under barbiturate anesthesia probably are not activated via a reticulothalamic route. (Reticular facilitation of other pathways might influence the response of nonspecific thalamic neurons, however: their diminished number in barbiturate preparations could be due to a reduction in rather than total cessation of reticular facilitation of other paths, particularly those including ipsilateral projections.) A remote possibility is that barbiturates and chloralose might base their anesthetic effects on the shared ability to depress neurons with deep tissue (i.e., muscle, tendon, and joint) (40) sensitivities. Because conscious perception of the body might reasonably be expected to depend in large degree on neuronal activity initiated from slowly adapting receptors in deep tissue, rather than from rapidly adapting cutaneous receptors, perhaps suppression of those neurons whose activity reflects mechanical events in deep tissues contributes to loss of consciousness. The observation of so few movement-sensitive neurons within the VPL of both barbiturate- and chloralose-anesthetized cats (22, 24) indicates either that the responses of such neurons are severely curtailed, or those neurons are situated elsewhere. [Jasper and Bertrand (26) reported that movementsensitive neurons tend to cluster within a zone rostra1 to the VPL in human thalamus; an analagous zone in cat thalamus was included in the volume of tissue explored in the present study, however.] Winters and Spooner (47) suggested that what appear to be paradoxical aspects of chloralose anesthesia may actually represent two sequential phases as the animal succumbs to progressively higher local concentrations of the drug (i.e., at low effective concentrations, excitability changes result in hypersynchronous discharge and jerks; at higher effective concentrations, there is diminution or elimination of jerks with periods of intermittent electroencephalogram silence as the animal passes into unconsciousness which these investigators compare to the postictal stage of an epileptic seizure). They suggest that chloralose may block arousal at the brain stem level through disruption of the “rhythm” of information flow necessary for consciousness. If hyperexcitation precedes the phase

WIDE-FIELD

NEURONS

IN VPL

47

of deep anesthesia, rather than being a concomitant, this would necessitate revision of attitudes based on the assumption that an abnormally high level of central nervous system excitability continues at all times while slow wave and/or single-unit data are collected from animals anesthetized with chloralose. REFERENCES 1. ADRIAN, E. D. 1941. Afferent discharges to the cerebral cortex from peripheral sense organs. J. Physiol. (London) 100: 159- 191. 2. ALVORD, E. C., JR., AND G. F. FOURTES. 1954. A comparison of generalized reflex myoclonic reactions elicitable in cats under chloralose anesthesia and under strychnine. Am. J. Physiol. 176: 253-261. 3. AMASSIAN, V. E. 1952. Organization of somatosensory systems. Frd. Proc. 11: 5. 4. AMASSIAN. V. E. 1954. Studies on organization of a somesthetic association area, including a single unit analysis. J. Nrrrroph.vsiol. 17: 39-58. 5. ARDUINI. A., AND M. G. ARDUINI. 1954. Effect of drugs and metabolic alterations on brain stem arousal mechanism. J. Phurmcrcol. Erp. 7’17rr. 110: 76-85. 6. BAKER, M. A. 1971. Spontaneous and evoked activity of neurons in the somatosensory thalamus of the waking cat. J. Physiol. ~London) 217: 359-379. 7. BAKER, M. A., C. F. TYNER. AND A. L. TOWE. 1971. Observations on single neurons recorded in the sigmoid gyri of awake, nonparalyzed cats. Cp. Nrurol. 32: 388-403. 8. BALI% G. U.. AND R. R. MONROE. 1964. The pharmacology of chloralose (a review). Ps~~~hophurnlac‘ologia 6: I-30. 9. BARKER, J. L.. AND H. GAINER. 1973. Pentobarbital: selective depression of excitatory postsynaptic potentials. Science 182: 720-722. 10. BAVA, A., F. FADIGA, AND T. MANZONI. 1968. Extralemniscal reactivity and commissural linkages in the VPL nucleus of cats with chronic cortical lesions. Arch. Ifal. Bid. 106: 204-226. 1 I. BOWSHER, D. 1970. Properties of ventrobasal thalamic neurons in cat following interruption of specific afferent pathways. Arch. Ital. Bid. 109: 59-74. 12. BROWN, R. V., AND J. G. HILTON. 1955. The effectiveness of baroreceptor reflexes under different anesthetics. 1. Phurmucol. Erp. Ther. 118: 198-203. 13. DELL, P.. M. BONVALLET. AND A. HUGELIN. 1960. Mechanisms of reticular deactivation. Pages 86-102 in G. E. W. WOLSTENHOLME AND M. O’CONNOR. Eds., CIBA Fortndution Symposium. The Nature of Sleep. Little. Brown, Boston. 14. FADIGA. E., C. HAIMANN, M. MARGNELLI, AND M. L. SOTGIU. 1978. Variability of peripheral representation in ventrobasal thalamic nuclei of the cat: effects of acute reversible blockade of the dorsal column nuclei. E.xp. Nrurol. 60: 484-498. IS. FELDBERG. W. S. 1958. Catatonia, anesthesia and sleep-like conditions produced by the injection of drugs into the cerebral ventricles of the cat. Pages 277-294 in M. RINKEL, H. C. DENBER, MCDOWELL, AND OBLENSKY. Eds., ChemiruI Concepts in P.s.vchosis. Astor-Honor. New York. 16. HAIMANN, C., M. MARGNELLI. AND M. L. SOSCIU. 1978. Variability of peripheral representation in ventrobasal thalamic nuclei of the cat: effects of chloralose treatment. E.x~. Nrurol. 60: 469-483. 17. HANRIOT. M., AND A. GAUTIER. 1896. Sur les chloraloses. C. R. Acad. Sci. (Paris) 22: 1127- 1129. 18. HANRIOT, M., AND C. RICHET. 1893. De l’action physiologiques du chloralose. C. R. Sm. Bid. (Paris) 5: 1-7.

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