Interhemispheric links in brain stimulation reward

Behavioural Brain Research 68 (1995) 117-137

ELSEVIER

Review

BEHAVIOURAL BRAIN RESEARCH

article 1

Interhemispheric links in brain stimulation reward Judith Malette

and Eleftherios Miliaressis*

School of Psychology, Universityof Ottawa, 275 Nicholas, Ottawa. Ont., K1N 6N5, Canada

Received 18 July 1994; revised 7 November 1994; accepted 7 November 1994

Abstract

The MFB substrate of self-stimulation (SS) has generally been viewed as a unilateral system. We re-examined this belief with pairs of moveable SS electrodes placed bilaterally in the MFB. Rats barpressed for trains of single or twin cathodal pulses of fixed intensity and width and of variable frequency. The first (C) and second (T) pulse of each pair was delivered through the left and right electrode or inversely. C - T intervals ranging from 0.2 to 5.0 ms were tested. The frequency of C pulses required for criterial bar-pressing was used to plot the stimulation efficacy (SE), as a function of the C - T interval and pulse presentation order. The electrodes were subsequently moved and the same procedure repeated for more ventral sites. With some pairs of contralateral hypothalamic (H) sites, the SE was independent of the C - T interval. However, with other pairs of contralateral H sites, the SE increased with C - T interval in a manner resembling a collision effect, with the important exception that no conduction time (CT) was apparent in the data. The absence of CT excludes the presence of a genuine collision effect. When one pulse was sent to the H and another to the contralateral ventral tegmentum (VT), the H - V T curve rose always earlier than the V T - H curve, thus resembling a transynaptic collision effect. However, the C - T interval at which the V T - H curve began rising (always 1.0 ms or less) fails to support the contention that the electrodes activated fibers separated by a synapse. Finally, a typical collision effect was noted with ipsilateral H - V T electrode placements, confirming the presence of direct linkage between ipsilateral MFB sites. Computer-generated data based on two parsimonious assumptions were found to match the empirical results. These assumptions were that each electrode activated a different branch of the same reward neuron and that conduction failure occurred at the branchpoint. The model, which posits that a large number of MFB reward neurons send branches to the other hemisphere, is testable and makes clear-cut predictions about the effects of lesions. In a preliminary test, we recorded the H and contralateral VT threshold frequencies before and after lesioning the H. The H threshold increased more when using small pulse current and remained constant throughout the 4-week testing period. The VT threshold was elevated more for intermediate pulse current and kept increasing with time. The VT changes suggest that the H lesion induced retrograde degeneration which affected the output of fibers coursing through a short, identifiable distance from the VT electrode. A model based on decussating branches predicts that lesions of the MFB should have variable effects on the SS of distal ipsilateral sites, a prediction matched by a substantial body of contradictory data. Key words: Self-stimulation; Brain stimulation reward; Collision test; Medial forebrain bundle; Lateral hypothalamus; Ventral tegmental

area; Interhemispheric link

1. Introduction F r o m the time of its discovery and with very few exceptions, the neural substrate of self-stimulation has been viewed as a unilateral system. A practical consequence of this belief has been the frequent use o f the contralateral hemisphere as a control in dissociating performance effects from reward-specific changes in self-stimulation [4,7,8,13,

* Corresponding author. Fax: (1) (613) 237-4943. Invited review paper, 0166-4328/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 1 6 6 - 4 3 2 8 ( 9 4 ) 0 0 1 7 0 - 7

14,21,25,29,33,35,36,39,40,58-61]. Evidence for uninterrupted linkage between distant ipsilateral M F B selfstimulation sites has been obtained using the collision test [2,3,12,18,47]. However, a neural model constraining the reward neurons exclusively within the same hemisphere predicts large and consistent effects of lesions on ipsilateral self-stimulation, a prediction that has rarely been met. A review of the lesion literature falls outside the scope of the present work but some recent studies using threshold measurements deserve a mention. Stellar et al. [51 ] ablated telencephalic regions and noted little or no effect on threshold current for ipsilateral M F B self-stimulation.

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J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

Sprick et al. [49] found that lateral hypothalamic (LH) self-stimulation persits after injection of excitotoxins in the LH. Janas and Stellar [23] found an increase in threshold for LH self-stimulation after ipsilateral lesions, posterior to the lateral preoptic area (LPOA) or anterior to the ventral tegmentum (VT). Waraczynski [62] noted no effect on LH threshold after lesioning some of the origins of the MFB, such as the septum, the diagonal band and the anterior preoptic area (APOA). However, an effect was noted after lesioning the LPOA or the MFB, anterior to the stimulation electrode. She concluded that the majority of reward fibers do not originate in telencephalic nuclei and that intrinsic neurons may be responsible for LH selfstimulation. Nevertheless, Arvanitogiannis and Shizgal [ 1] made excitotoxic lesions of basal forebrain regions and found an increase in VT and LH thresholds. Stellar et al. [50] made excitotoxic lesions near the LH and concluded that intrinsic LH neurons are not a major substrate of MFB self-stimulation. Waranczinski et al. [64] destroyed the amygdala, a region known for its rich connections with the LH, and found no change in VT or LH self-stimulation threshold. Stellar and Neely [52] made lesions in anterior or posterior MFB sites and found that the latter had more effect on MFB self-stimulation. On the other hand, Murray and Shizgal [34] found permanent threshold changes in LH and VT thresholds after an ipsilateral lesion of the anterolateral MFB. Waraczynski et al. [63] destroyed the dorsomedial hypothalamic nucleus and found no effect on caudal ipsilateral MFB placements. Leon and Gallistel [26] lesioned the MFB at the diencephalic-mesencephalic border and noted an increase in LH threshold. Sim et al. [48] destroyed the LH and noted no change in threshold at posterior MFB sites. Finally, excitotoxic lesions of the VT and ventral pallidum by Tehrani and Stellar [56] or electrolytic lesions of the parabrachial region by Waraczynski and Shizgal [65] were found to have minimal effect on MFB threshold. These recent studies, which add to the inconsistency of a considerable number of older data, do little to help untying the Gordian knot of brain stimulation reward. The critical locus which, once damaged, would abolish ipsilateral self-stimulation remains undiscovered despite 40 years of persisting effort. An alternative view according to which the reward substrate would include a significant number of decussating neurons has not yet been proposed explicitly as a working hypothesis nor has it been the subject of persistent investigation. Lesion studies examining possible interhemispheric links in reward have been conducted but the measurements were most often based on rates of selfstimulation. Some of the most recent studies deserve discussion. Following a study using unilateral precollicular ablations and bilateral self-stimulation electrodes in the peribrachial region, Huston et al. [20] concluded that self-

stimulation of this region is relatively independent of ipsilateral fibers going to or coming from the telencephalon. Rostral regions such as the LH are interconnected with the deep mesencephalic nucleus (DpMe) and the tegmental pedunculopontine nucleus (PPTg) through interhemispheric fibers [ 19,31,66]. Huston et al. [20] proposed that the persistence of self-stimulation following unilateral precollicular ablation could be explained by the presence of self-stimulation fibers decusating near the DpMe-PPTg region (see also Huston and Tomaz [22]). Buscher et al. [5] and Lepore and Franklin [28] found that electrolytic or excitotoxic lesions of the PPTg increased the threshold more in the MFB of the contralateral side. According to Buscher et al. [5], the findings could be explained by the presence of ascending and descending fibers decussating at the levels of preoptic commissura or the supramammillary decussation. Finally, Colle and Wise [9] reported that unilateral forebrain ablations had opposite effects on ipsilateral and contralateral LH thresholds. Support for the existence of interhemispheric links in reward can also be found in experiments using 2-DG utilisation and other tracing methods. For example, Porrino et al. [41] injected 2-DG during VT self-stimulation and found an increase in local glucose utilisation in several nuclei on both sides of the brain. The same experiment performed with LH electrodes [42] showed mainly unilateral MFB effects, in sites rostral as well as caudal to the stimulating electrode. However, bilateral effects were also noted for sites beyond the MFB. Pritzel et al. [43] ablated both sides of the telencephalon and injected HRP in the LH. HRP transport was detected at the levels of the thalamic commissura and the pars Ganser of the supraoptic decussation, a locus known to link the two lateral hypothalami [31,37]. According to Pritzel et al. [43], the grey commissura and the supramammilary decussation may convey the reward signal of the LH. The hypothesis that the reward substrate may comprise a consistent proportion of decussating neurons was investigated in the present study, using the paired-pulse stimulation technique. Pulses presented in pairs can be delivered through a single electrode in order to estimate the neural post-stimulation excitability cycle (refractory period test) or through different electrodes in order to demonstrate the presence of connectivity between neural sites (collision test). These old neurophysiological techniques have been adapted for behavioral measurements by Deutsch [ 10], Yeomans [67] and Shizgal et at. [47]. Conclusions from the behavioral version of the collision test are based on the observed changes in threshold frequency (that is, the number of pulse pairs needed to maintain self-stimulation) when the intra-pair interval is varied: at short intervals, the potentials generated by the two electrodes meet between the stimulated sites and cancel each

J. Malette. E. Miliaressis / Behavioural Brain Research 68 (1995) II 7-137

other, thus elevating the threshold frequency. At long intervals, collision is avoided and, consequently, the threshold frequency decreases. In their pioneer study, Shizgal et al. [47] used the collision test with pairs of ipsilateral MFB electrodes and a few pairs of contralateral LH electrodes. A collision effect was noted using exclusively ipsilateral self-stimulation sites. With contralateral sites, the threshold frequency was found to be the same regardless of the intra-pair interval. In our study, pairs of moveable monopolar electrodes were implanted in ipsilateral or contralateral MFB locations. In a first experiment, the electrodes were implanted bilaterally in the LH. In a second experiment, one electrode was implanted in the LH and another in the contralateral VT. In a third experiment, the electrodes were implanted in the LH and the ipsilateral VT. Finally, the consequences of a LH lesion on the LH and contralateral VT self-stimulation thresholds were measured separately.

2. Materials and methods 2. I. Subjects and surgery

Sprague-Dawley rats weighing 275-300 g were implanted stereotaxically (under general anesthesia with sodium pentobarbital, 40 mg/kg, i.p.) with two monopolar electrodes [30] made of a plastic body and a 0.25 mm dia. moveable wire which was insulated except for its conically shaped tip. The current returned through a miniature plug connected to four 0-80 skull screws fixed on the calvarium and imbedded in dental cement. For the first experiment, the electrodes were implanted contralaterally in the lateral hypothalami (LH) or the posterior lateral hypothalami (PLH). For the second experiment, one electrode was aimed at the LH and a second at the contralateral ventral tegmentum (VT). For the last experiment, the electrodes were aimed at the LH and VT of the same brain side. 2.2. Apparatus and procedure

Rats self-stimulated by pressing a bar in an operant box made of transparent acrylic. Each barpress triggered a twin constant-current generator [32] that delivered a 0.3 s train of twin or single cathodal rectangular pulses of fixed duration (0.1 or 0.3 ms) and of variable frequency and intensity. A delay of 0.4 s was imposed between the start of two consecutive stimulation trains. Before each self-stimulation trial, the animals were primed with three stimulation trains, using the parameters destined for that trial. The procedure common to all experiments consisted in finding the threshold, that is, the pulse frequency required

119

for criterial self-stimulation rate, under various stimulation conditions. The threshold, inferred from the rate-frequency function, was defined as the number of pulses required for half-maximal self-stimulation. To obtain a rate-frequency function, the frequency of constant-intensity pulses was varied between trials from values sustaining little or no response to values producing maximum self-stimulation. A rate-frequency function consisted in a series of 60 s self-stimulation trials, separated by a time-out interval of 30 s. A complete rate-frequency function was based on the delivery of at least two ascending and two descending series of frequency values. A frequency threshold was obtained separately for trains of single pulses and then for trains of paired pulses at variable intra-pair intervals. In the paired-pulse condition, the first and second pulse of each pair (the so-called C and T pulses) were delivered either through the same electrode (refractory period test) or through separate electrodes (collision test). The range of C - T intervals varied from 0.2 to 5.0 ms. In the refractory-period test, the thresholds obtained with single and paired pulses were used in Yeoman's formula [67] to calculate the effectiveness (E) of T as a function of the C - T interval. In the formula E = Nsp/ Not - 1, Nsp is the frequency threshold for single pulses and Net that for paired pulses, for the C - T interval under consideration. In the test involving two electrodes, the stimulation effectiveness was calculated according to Shizgal et al.'s equation [47] E - Nspl/Nct - 1

Nsp,/Nsp2 where Nsp I and Nsp 2 are the frequency threshold for the electrode showing the lower and higher threshold, respectively and Net, the threshold for paired pulses. In the test involving two electrodes, the pulse intensities were adjusted individually so as to obtain similar thresholds for both structures. Depending on brain site and stimulation condition, the pulse intensity varied from 74 to 943 #A. Following completion of the refractory period and collision tests, the electrodes were moved ventrally by 0.16 mm and testing resumed the day after. An electrolytic lesion was also made through the LH electrode of one animal previously tested with pairs of LH and contralateral VT pulses. With the LH electrode connected to the anode of the stimulator, a current of 600 gA was Passed for 40 s while the animal was under light anesthesia with Halothane. Following the lesion, the frequency thresholds for LH and VT self-stimulation were obtained separately, as a function of the pulse intensity and post-lesion day. Four weeks after the lesion, the effectiveness of L H - V T paired pulses was also reevaluated.

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J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

2.3. Histology At the end of the experiments, the animals were put under deep anesthesia and the tissue surrounding the electrode tips was marked electrochemically according to a procedure described elsewhere [45]. The brains were subsequently removed and stored in 10~o Formalin for 24 h. Slices (40 #m thick) containing the electrode tracks were then obtained under -20 °C and stained with thionine. The successive dorsoventral positions of the electrodes were inferred from the final marked site and reported on a plate redrawn from Paxinos and Watson's stereotaxic atlas [38].

3. Results and discussion

3.1. Interactions between contralateral lateral hypothalamic impulses Figs. 1-3 show the effectiveness of paired pulses delivered through contralateral LH or PLH electrodes, as a function of the intra-pair (C-T) interval, for several subjects. In one condition, the first and second pulse of each pair was delivered through the left and right electrode, respectively (1LH-rLH or 1PLH-rPLH, filled squares), whereas in a second condition, the presentation order was reversed (rLH-1LH or rPLH-1PLH, filled circles). The number beside each structure's label refers to the dorsoventral position of the moveable electrode, with the implantation position numbered zero. The number shown at left of each label refers to the pulse intensity, in mAmperes. A value of zero in the ordinates indicates that the combined effect of the two pulses was not better than that obtained with one of the pulses omitted whereas, a value of 1.0 indicates that the rewarding effects of the two pulses summated perfectly. The refractory period data are also shown in Fig. 6 for some brain sites under discussion. Fig. 1 shows data obtained from subjects 112 and 55. Nineteen pairs of contralateral PLH sites were tested in each subject. Due to space limitations, the data from only 12 pairs of sites are shown. Panel A shows the data from subject 112, using electrode sites labelled ILH1 and rLH5. It can be noted that the level of summation between the effects of contralateral LH pulses was independent of the C - T interval, an effect believed to indicate no direct linkage between these sites. However, the data in panels B - F , obtained for more ventral sites, show a strikingly different profile, resembling a collision effect. A collision effect, indicating that the two electrodes excited common axons, is characterized by a lower plateau, a dynamic segment which lasts for the duration of the refractory periods and an upper plateau. The duration of the lower plateau repre-

sents the sum of the absolute refractory period of the fastest reacting neurons and the inter-electrode conduction time. Although the overall profile of these data suggests the presence of collision, the timing fails to do so: depending on the pair of sites, the shortest C - T interval before any rise occurs, varied from 0.4 to 0.5 ms, an estimate similar to that noted for the refractory periods (shown in Fig. 6). In other words, no inter-electrode conduction time can be estimated from the present data. Finally, note that the magnitude of the collision-like effect varied as a function of the electrode sites. Representative data from six pairs of sites tested in subject 55 are shown in panels G - L . The results replicate closely those obtained with subject 112 above: dorsal pairs of sites showed a constant effectiveness level, whereas more ventral sites exhibited a collision-like effect. Note that the magnitude of this effect varied (increased then decreased) depending on the dorsoventral locations of sites involved. The shortest C - T interval before any rise occurs was again near identical to that noted on the refractory-period curves (Fig. 6). As in the case of the previous subject, no conduction time was apparent in the data. Fig. 2 shows data obtained from subjects 27, 195 and 25. A total of 13 pairs of sites were tested in these subjects and the data from nine sites are reported. The data from all three subjects replicate very closely the findings described above: in subject 27, a collision-like effect of variable magnitude was noted for all pairs of sites, whereas in subject 195, a collision-like effect was detected only for the two deepest sites. In subjects 27 and 195, the shortest interval before any rise occurs (from 0.4 to 0.7 ms) was near identical to that noted in the refractory period data (Fig. 6). This interval was longer in subject 25 (approximately 0.9 ms, panel I) but it also corresponded to a longer interval in the refractory-period curve (0.9 ms, not shown). Fig. 3 shows data obtained with subjects 207 and 210. Eight pairs of brain sites were tested and the data for three pairs of sites from each subject are reported. A collisionlike effect was noted for one pair of sites in subject 207, whereas no such effect was detected in subject 210. The interval before recovery (approximately 0.5 ms, panel C) was again identical to that noted in the refractory-period curve (Fig. 6). The histological plates presented in Fig. 7 show the successive positions of the contralateral electrodes, as inferred from the final marked location (see Materials and methods). The number inside the plate identifies the subject. The number at the upper left or right part of the plate designates the corresponding plate in Paxinos and Watson's Atlas [38]. By convention, the implantation site of each electrode was numbered zero. Filled circles refer to brain sites involved in the paired-pulse experiment. The

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

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electrodes of subjects 112 and 55 were found in the posterior hypothalamus, at the frontal level passing through

the supramammillary decussation. The electrodes of the remaining subjects were found more rostraUy, at a frontal

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level passing through the lateral hypothalamic nuclei. Brain loci represented by large circles refer to the combination of sites that gave the largest collision-like effect. Note that in most cases, the maximum effect was detected with electrode tips in or near homologous L H structures. Figs. 1-3 revealed the presence of a time-dependent interaction between contralateral L H stimuli. The contention that this interaction was the result of some stimulation artefact is inconsistent with the observation that the effect was dependent on subtle changes in electrode place-

ments. Although the overall profile of this interaction resembles that of a collision effect, the timing fails to do so, since conduction time was not apparent in any but a single case. A delay of 0.1 ms was noted between the pairedpulse and refractory period data for some sites in subject 27. If this delay represents conduction time, the velocity estimate (based on the inter-electrode distance of 4 mm), must then be at least 40 m/s, that is, a value falling clearly outside the range of velocity estimates for self-stimulation neurons [2,3,12,18,47].

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

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0.5

-

0.5

0.40 0.42

• 1LH3->rLH2 • rLH2->ILH3

0.7

-

0.6

-

0.5

0.3-

0.3-

0.2

0.2

-

0.2

0.1

0.1

-

0.1

0.0

0.0

0.1

1.0

10.0

' 2'.o'

10.0

F 0.40 0.42

,, 1 L H 3 - > r L H 3

-

rLH3->ILH3

0.4

0.4-

2'.0' '5'6 '''~.

1LH2->rLH5 rLH5->ILH2

1.0

#21o

0.9

0.4-

0'.2'0'2' ' ' ' 1 .

0.4

. . . . . .

• •

0.0

0.70.0

0.. 2. 0.1

1.0

#210

0.8-

0.0

10.0

m

0.9-

0.8

0.7

0.0

0.63 0.20

0.7

0.6

.

C

0.6 0.60 0.25

0.7

0.5

0 .. 2.

#207

0.9

0.0

0.0

-e-4

1.0

B

0.0

0.0-

~>

#207

0.9

123

0.3

0'2'0',~.. .... i 1.0

0.1

C-T

Interval

z'.0' . %'6 '''F. 10.0

0.0

. 0.1

0'.2'6~. .... i 1.0

~0'

%:6 '''i IO.O

(ms)

Fig. 3. P a i r e d - p u l s e effectiveness as a f u n c t i o n o f the C - T interval for c o n t r a l a t e r a l L H electrodes. E a c h p a n e l s h o w s the results o b t a i n e d u s i n g a different p a i r o f b r a i n sites. F o r f u r t h e r specifications, see Fig. 1 a n d text.

3.2. Interactions between lateral hypothalamic and contralateral ventral tegmentum impulses Fig. 4 shows the effectiveness of paired pulses delivered through contralateral L H and VT electrodes, as a function of the C - T interval for four subjects. The refractory period data for some brain sites under discussion are also shown in Fig. 6. Representative data from 6 (out of 14) L H - V T sites tested in subject 190 are shown in panels A - F . The first panel shows the presence of a moderate but steady summation effect. However, as the electrodes moved more ventrally in the brain, an asymmetric collision-like effect progressively took place. An asymmetric collision effect, characterized by the presence of a delay between the two curves, is normally expected with electrodes lying at variable distance from each side of a synapse. The duration of the lower plateau in the late-rising curve is determined by the sum of (a) the absolute refractory period of the fastest reacting neurons, excited by the electrode located at the front of the synapse and (b) the inter-electrode conduction time, which includes the synaptic delay (usually 0.5 ms). For MFB self-stimulation neurons, the sum of these variables (with an inter-electrode distance of

4 mm) should amount to at least 1.7 ms. The duration of the lower plateau in the early-rising curve should be shorter (can be as short as zero ms) because the antidromic potential cannot invade the presynaptic axon. Although the presence of a delay in the above data suggests an asymmetric collision effect, the timing fails to do so in at least four out of five cases: In these cases, the lower plateau of the late-rising curves lasted approximately 0.5-0.6 ms, a value considerably shorter than the sum of the three variables given above. The duration of the lower plateau shown in panel E, although appreciably longer (1.1 ms), cannot easily be reconciled with the presence of a transynaptic effect because the time left for inter-electrode conduction (approximately 0.1-0.2 ms) is much too short. Panels G - L present data for six (out of 15) pairs of L H - V T sites tested in subjects 194, 164 and 27. The data of subject 194 demonstrate the presence of an asymmetric collision-like effect which is site dependent. Note again that the duration of the longer lower plateau (0.6-0.8 ms) is barely longer than that noted in the refractory period data (Fig. 6). The same observation can be made for the data of subject 164. In subject 27, the lower plateau of the late-rising curve (1.0 ms, panel K) was approximately 0.3 ms longer than that of the refractory-period curves, a

J. Malette. E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

124 1.0 0.9 0.0

0.8

-

0.6

-

0.7

0.7

-

0.7

-

0.6

0.6 -

0.5

0.5

0.4

0.4 -

0.3

0.3 0.2

-

0.2

0.09

0.1

.

#TO->lLH2 KHZ->rVTO

’’

2:o ’ X6’

n

0.10

0.0

0.2

o’.c’

1.0

0.1

-

0.2 0.1 0.0

1 10.0

0.1

0.07 0.16 0.2

.m rVTS->lLH6 ILEt?->rVT3

’

0.4

2.0

1.0

5.0

1 10.0

0.0 0.1

Cn m

1.0

Q

0.9

F1

0.6

0.0 -

0.6

-

kf

0.7

0.7 -

0.7

-

0.6 -

0.6

-

0.5 -

0.5

-

0.4 -

0.4 -

0.3 -

0.3

0.2 -

0.2 -

.rl 4

::: ;*:

0 Q;,

0.2 n

0.10 z

l

rVT5->lLH7 lLH7->rVT5

0.0 0.1

0.0 0.1

w 1.0 aJ

0.9

ul

0.0

0.11 0.10

0.1 0.2

rVT5->lLHB ILBB->rVT6

l l

’

0.4

2.0

1.0

#194 G

n

0.25

l

0.1

5.d ‘1

rVT3->lLH7 lLB7->rVTS

0.1

0.7

-

0.6

-

e,

0:5

0.5

-

0.4

-

0.3

-

0.2

-

0.1

-

E .A

n l

rVT2->lLH3 lLH3->rVT2

0.1

o:2’o’.a”i”

1.0

2l.o’

‘61d”l 10.0

1.0 0.9

5.0

1 10.0

0.2

rVTS->lLHB lLH9->rVTI

l

.

’

0.4(

2.0

1.0

6.6

1 10.0

2

n

rVTS->lLHlO

l

0.0

cd I&

0.12 0.20

0.1 0.2

2.0

1.0

0.8-

:‘;

:::

’

0.4

0.14 0.10

-

0.0

10.0

0.2

rVTS->lLH6 lLEB->rVTS

n l

-

7

a

0.07 0.14

0.1 -

0.1 ;‘z

_#I584

1.0

10.0

0.6

0.8 -

0.7

0.7 -

0.1 ;‘;

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0.2 ,

0.0

0.6 0.7

I 0.4 I”“‘, 0.21

2:o lLHlO->rVT3 ’ ‘5W’l 10.0

1.0

-#27 - L -

0.6 -

0.8 0.6

0.5 -

0.4

0.4

-

0.3 0.2 0.1

0.10

0.0 0.1

I I I 0.2 0.4 III,,

l

1.0

lVT4- > rLH4 rLE4->lVT4

0.1 -

I I , ,111, 5.0 ’

2.0

10.0

0.0

C-T

0.1

0.13 0.12 0.2

0:r I

* l

I I”,

1.0

lVT3->rLHB rLHB->lVTS 2’. 0

Interval

’

0.14

‘5’.61 10.0

0.2 1

0.0

0.1

I 0.4 I “I,,

1.0

n l

lVT3->rLHB rLH9->lVTS

do ’ WI ‘1

10.0

(ms)

Fig. 4. Paired-pulse effectiveness as a function of the C-T interval for contralateral LH-VT electrodes. In one condition, the C pulse was delivered to the lateral hypothalamus (1LH or rLH) and the T pulse to the contralateral ventral tegmentum (NT or rVT). In a second condition, the presentation order was reversed. Each panel shows the results obtained using a different pair of brain sites. The number beside the label of each structure refers to the location of the electrode, shown in the histological plates of Fig. 8. Other details as for Fig. 1.

delay still too short to be accounted for by a transynaptic effect. The successive locations of the LH and contralateral

VT electrodes are shown in Fig. 8. Filled circles refer to brain sites involved in the paired-pulse experiment. Pairs of brain loci represented by large open circles refer to the

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

sites that gave the largest delay between the two curves. The sites tested with the anterior electrodes were found in the LH or neighbouring areas. Most of the posterior sites were found in the rostral part of the VT area. Note that the longest delay was obtained with mostly ventral electrode placements (shown by open circles).

3.3. Interactions between lateral hypothalamic and ipsilateral ventral tegmentum impulses Fig. 5 shows the effectiveness of paired pulses delivered through ipsilateral LH and VT electrodes, as a function of the C - T interval for three subjects. The refractory-period data for some brain sites under discussion are also shown in Fig. 6. The data for five (out of six) pairs of L H - V T sites tested with subject 981 are shown in panels A-E. Note that the VT electrode was held fixed. The data show that the effectiveness profile depended on the position of the LH electrode: LH sites 2 and 7 (panels A and E) produced a steady stimulation effectiveness, whereas intermediate positions produced a collision or collision-like effect. Note that a single electrode move (compare panels A and B) resulted in a typical collision profile, characterized by the absence of delay between the two curves and the presence of a measurable conduction time. The latter (approximately 0.5 ms) provides a velocity estimate of 7.6 mm/s, a value which is consistent with previous findings [2,3,12,18,47]. Thus, the results showing summation and collision effects are in agreement with previous data obtained with MFB electrodes. On the other hand, panel C shows, for the first time, an asymmetric collision-like effect similar to that reported above with contralateral electrodes. However, unlike for contralateral placements, the V T - L H curve rose first. Panel F reports the data for one pair of L H - V T sites tested in subject 27. Note that this subject had three electrodes and that the results using contralateral L H - L H and L H - V T placements have been shown in Figs. 2 and 4. As for subject 981, the results showed a typical collision profile. Panels G - L present the data obtained from six pairs of L H - V T ipsilateral sites in subject 984. Note that the VT electrode was held fixed. Stimulation using the most dorsal LH site (panel G) produced an unexpected, U-shaped profile, whereas the site immediately below produced a seemingly composite effect. More ventral sites produced a typical collision effect. The difference between the two curves shown in panel L is better interpreted assuming an horizontal shift, rather than the presence of delay in recovery time. The successive positions of the ipsilateral LH and VT electrodes are shown in Fig. 9. Filled circles represent the brain sites involved in the experiment. Brain loci repre-

125

sented by large open circles refer to sites where a maximum collision effect was noted. Some of the effects shown in Fig. 5 (collision effects) are in agreement with previous studies, suggesting the presence of reward neurons linking the LH and VT [2,12,18,47]. Note, however, that other types of unexpected effects were also obtained. The relative success of the present study in revealing such atypical effects may be attributed to the moveable electrodes and the use of a high C - T interval resolution. The asymmetric effect shown in panel C may not be interpreted unambiguously, in terms of transynaptic collision. Note that the lower plateau of the late curve lasted approximately 1.0 ms, a time considerably shorter than the sum of the three variables explained earlier. A U-shaped profile can theoretically be predicted for one of the curves in transynaptic collision. Therefore, the presence of this profile on both curves calls for a different interpretation (see Neural modelling and general discussion). The data profile noted in panel H may reflect the combined effect of the U-shaped effect and collision. Note finally the presence of a narrow negative peak immediately preceding recovery (most apparent in panels D and E). This peak, which is present in most of the collision profiles, has already been noted by us [12] but has not yet received satisfactory explanation. It occurs slightly after the U-shaped effect and, therefore, cannot be considered as a residue of this effect.

3.4. The effects of lesions The effects oflesioning the LH in subject 164 are shown in Figs. 10 and 11. The LH and VT electrodes of this subject were placed into a different hemisphere. The prelesion paired-pulse effectiveness data, revealing an asymmetric collision-like effect between rLH4 and 1VT4, were presented in Fig. 4. After the lesion, the threshold frequency for self-stimulation was recorded separately for each structure, as a function of the pulse intensity and post-lesion day. At the end of these measurements, the paired-pulse effectiveness profile was re-evaluated. Fig. 10 shows this profile before (panel A) and 4 weeks following the LH lesion (panel B). Note that the lesion abolished the delay between the curves and that the magnitude of the collision-like effect was reduced. The threshold data are presented in Fig. 11. Panels A - F show the 1°log change in LH frequency threshold as a function of the pulse intensity and post-lesion day. Varying the pulse intensity enables one to examine the extent of neural destruction, whereas varying post-lesion time provides information about recovery. Note that the lesion increased the frequency threshold and that the magnitude of this effect was inversely proportional to the pulse intensity. With the smallest current, the threshold increased by approximately

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

126

1.0 0.9

1.0

- -

-

#981 A

0.8

0.0

0.7

0.7-

0.6

-

0.6

0.0-

0.5-

0.5

0.5-

0.4-

0.4

0.4-

0.3-

0.3

0.3

0.2

0.2-

-

0.47 0.60

0.1 0.0

• 1VT2->ILH2 • ILHg->IVT2

o'.2'o'.~ .... i

1.0 -

o.0-

2'.o'

1.0

o.1

0.~I 0.40

0.1

'5'.b '''t

0.0

10.0 0.9

• IVT2->ILH3 • ILH3->IVT2

o'.~'o'.~ .... '

1.0

0.1

1.0

#981 D

........ 2.0 6.0

I

1.0

#9o1 E

0.6

0..~

0.5

0.5

0.4

0.4

0.4

0.3

0.3

0.3

0.2

0.2

0.2

0.1

0.1

0.0

0.0

o'.2'o'.~ . . . . '

1.0

2'.o' '6'.b'"'

10.0

0.20 0.39

1.0

0.9

0.9

• IVT2->ILH7 • ILHT->IVT2

o'.z' d . i . . . . '

~.o'

1.0

0.1

1.0

'6'.6'"'

0.6

0.5

0.5

0.4

0.4

0.4

0.3

0.3

~

• IVT3->ILH2 • ILH2->IVT3

0.1

2'.o

1.0

'5'.6'"'

0.0

o'.z 'o'.i .... i 0.1

10.0

2'.o'

1.0

'6'.b'"'

o.o

0.2

0.7

0.7

0 . 6 --

0.6

0.6

0.5

0.5

0.5

0.4 -

0.4

0.4

0.3 --

0,3

0.3

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1.0

2'.o'

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10.0

2'.0'

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o'.z'o'.~ . . . . J o.1

0.8

0.2 --

0.49 0.45

0.0

10.0

1.0

1.0

z'.o' '5'.6'"'

1.0

0.1

0.1

....

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0.3 0.45 0.45

0.2

o'.2'o'.

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0.9 0.7

o.o

l

10.0

1.0

#984

0.6

0.1

0.18 0.41 0.1

10.0

0.8

0.0

2'.0 5.0

0.0

0.7

#9

0.7

1.0

0.7

0.8

0.8

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0.8

0.6

0.1

• IVT2->ILH5 • ILH5->IVT2

#

0.9

0.7

0.0

0.20 0.31 0.1

0.8

• IVT2->ILH0 • ILH6->IVTZ

-

0.0

10.0

0.7

0.41 0.41

-

0.I-

0.8

o.1

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-

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~

0.7

0.8-

I~

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0.1

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1.0

Interval

'5'.b'"' 10.0

~

0.2 0.1

"

0.26 0.25

~

• 1VT3->ILH6 • ILH0->IVT3

o'.z'o'.4' .... J

o.o 0.1

1.0

2'.o' T.b '''~ I0.0

(ms)

Fig. 5. Paired-pulse effectiveness as a function of the C - T interval for ipsilateral L H - V T electrodes. In the condition shown by circles, the C pulse w a s delivered to the left lateral h y p o t h a l a m u s (1LH) and the T pulse to the left ventral t e g m e n t u m (1VT). In the condition s h o w n by squares, the pulse p r e s e n t a t i o n order w a s reversed. E a c h panel s h o w s the results o b t a i n e d using a different pair of brain sites. The n u m b e r beside the label of each structure refers to the location of the electrode, s h o w n in the histological plates of Fig. 9. Other details as for Fig. 1.

0.3-0.38 logfrequency units throughout the testing period, indicating that 50-60~o of the stimulation effect was lost.

With the two largest pulse intensities, the effect was considerably smaller and decayed with time. The intensity-

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137 1.0

1.0

_;#112

0.9

0.7

-

0.'7-

0.6

0.0

-

0.6-

0.5

0.5-

0.5-

0.4

0.4-

0.4-

0.3

0.8

0.2

0.2

-

0.1

-

~

~

0.34 0.81

o'.2'o'.~ ....

1.0

'

1.0

IPLH5 * rPLH7 •

e'.o'

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>

0.4 -

• ~-q

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"~

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0.1

(~)

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|-

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0.29

'

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10.0

o.'7-

~-4

0.6 0.5 0.4

l

~--4

1-r~ X -

0.8

-

0:2 'o'.4; .... '

1.0

z'.o'

E d '''=

0.0

I

o'.2'o:4' .... 0.1

•

0.25

'

I ~

0.63 0.28

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1.0

0.1

0.9

H

0.6

0.5

0.5

0.4

0.4

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.... I

0.1

• •

0.1

0.8

0.8

0.7

0.7

0.6

0.6

0.6

0.5

0.5

0.5

0.4

0.4

0.4

0.3

0.3

0.3

0.0

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0.1

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i 1.0

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2'.o' ,5,.6,,, I

Interval

10.0

%

0.0

10.0

1.0

0.7

'

1.0

0.1

0.8

2:o

o'.z '0:4..... J 2:0 ' '5'.6''"

0.2

1.o

1.0

T5 ILH0

10.0

0.3 0.27 0.10

o.0

0:2 'o:,t .... ~

•

0.0

K

0.1

0.14

,2,i

1.0

#164

-

]~F

o.o

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#27

0.0

I

10.0

F

0.1

0.7

0.2

lllll

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0.2

0.6

• IVT2 * 1LH8

I

0.3

1.0

0.21 0.40

I

0.4

0.9

-.L~J

=

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0.6

1.0

0.1

1Ill

0.5

0.9

0.2

J

0.7

0.7

10.0

t

0.2 0.4

ILH13 * rLH13

#19o

0.8

0.1

z:o ' '5:~' ''=

1.0

i

0.1 1.0

0.2

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0.0

10.0

0.3

0.30.2 0.1 -

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o.1-

0.9

0.1

0.9

G

0.87 , , F ~ 5

7

#2

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0.1

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0.7

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0.8

0.1

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127

0:2 'o:~

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"1

0.13 0.12

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2'.0 . . . 5. ...0.

1.0

I

10.0

#984-

0.49 0.48

0.2 0.1

0.0

o'.z '0'.~ .... 0.1

'

1.0

• IVT3 * ILH3

2,.o, ,5,.6,,,i I0.0

(ms)

Fig. 6, T-pulse effectiveness as a function of the C - T interval, w h e n the C and T pulses were delivered through the s a m e e l e c t r o d e . Each panel shows the d a t a o b t a i n e d from selected L H a n d VT sites involved in the two-electrode experiments (Figs. 1-4). Other details in text.

dependent effect indicates that the number o f neurons damaged decreased as a function o f the distance from the electrode tip, as normally expected. The presence o f a time-dependent effect for the largest currents m a y indicate

the presence of functional recovery in the neurons most distant from the electrode tip. If a current-distance constant of 1300-2000 # A / m m 2 is assumed for the hypothalamic reward fibers [16], the smallest intensity excited

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

28

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neurons within a radius of approximately 0.23-0.28 mm. The fact that only 50-60~o of the stimulation effect was lost from within such a short radius and that the effect decreased dramatically for currents above 0.21 mA suggest the presence of a very small lesion, a contention that was subsequently verified histologically. Panels G - L present the effects of lesioning the L H on the contralateral VT threshold. Note that the lesion increased the frequency threshold and that the magnitude of this effect was substantially larger for intermediate currents. Note also that unlike the LH, the VT threshold increased with time. The presence of a non-monotonic relationship between current and threshold frequency change is consistent with the view that the output of a significant number of neurons lying within a short radius from the VT electrode was impeded following the L H lesion. Finally, the fact that the effect was delayed in time is consistent with the presence of retrograde degeneration.

4. Neural modelling and general discussion Some of the data shown in Figs. 1-3 confirmed the presence of summation between contralateral LH reward signals, a phenomenon known to indicate convergence onto a common integrator [47]. However, some data revealed the presence of a strikingly new collision-like effect, whose magnitude depended on subtle changes in electrode placements. In a genuine collision profile, the dynamic segment (reflecting the recovery from refractoriness below the electrodes), is delayed by a fraction equal to the interelectrode conduction time. For example, recovery using our ipsilateral MFB placements began not earlier than 1.2 ms, allowing for an inter-electrode conduction time of approximately 0.5-0.7 ms. Our data cannot, therefore, be interpreted in terms of collision because the time at which the stimulation effectiveness began recovering often was not longer than that seen in the refractory-period curves.

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In other words, inter-electrode conduction time could not be measured. The contention that the conduction time was below detectability must be considered. In most of our curves the standard errors were small. In these conditions, the presence of a systematic condunction time of as short as 0.1-0.2 ms would have been easily detected. These values translate into velocities of 40 and 20 m/s, respectively. If the data reflect a genuine collision effect, we must then be dealing with some extraordinary type of reward neurons. The time-dependent interaction between contralateral L H signals might, alternatively, be ascribed to a synaptic effect. According to this contention, the summation of contralateral LH reward impulses would be more efficient at long C - T intervals. However, spatiotemporal summation at post-synaptic elements is classically believed to be more efficient at short intervals, which directly contradicts this contention. Ungerleider and Coons [57] investigated

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the presence of summation of homologous contralateral MFB reward impulses. They found that the latency to barpress increased as the C - T interval was varied from 0.1 ms to 60 ms. In another study, German and Holloway [ 17] used an electrode in the anterior preoptic area and a second in the posterior contralateral hypothalamus. With the C pulse delivered through the posterior electrode, barpressing rates decreased by approximately 10~o as the C - T interval was varied from 0.1 to 5 ms. With the C pulse delivered through the anterior electrode, the rates were maximal for a C - T interval of 1.0-1.5 ms and decreased by approximately 25~o for C - T intervals of 0.1 and 5.0 ms. The slight variations in barpressing rates noted in this experiment cannot be attributed unambiguously to the presence of spatiotemporal synaptic integration. For the reasons discussed above, synaptic or collision effects may not be held responsible for the large effectiveness changes noted with our contralateral LH electrodes. Therefore, in the following section we propose an alternative neural model involving the activation of collaterals (Fig. 12). With this model, the data can be predicted conditional to the following assumptions: (1) each LH electrode activated a different collateral of the same neuron and (2) the antidromic potential evoked by each electrode failed to invade the collateral excited by the second electrode. The lower part of each panel shows a sketchy representation of such a putative reward neuron possessing at least three branches. Note that the antidromic potentials of the collaterals are assumed to invade the branch labelled C. The lines labelled el and e2 represent the location of the two hypothetical electrodes, relative to the junction. The number below each collateral represents a proximity index, showing the relative distance between the junction and each electrode. For example, a proximity value of 0.25 for electrode el (panel A) means that this electrode lies 3 times closer to the junction compared to e2, whereas a value of 0.5 (panel C) indicates that the two electrodes lie symmetrically around the junction. The curves shown in each panel are computer printouts, as generated using the assumptions described above. They show the delay predicted between the curves, as a function of the electrode placements. The labels near the curves refer to the pulse presentation order. For convenience, the range of refractory periods at the junction was set at 0.51.0 ms, whereas the conduction velocity (set at 6 m/s) was assumed to be the same for the two stimulated branches. Non-symmetrical electrodes (panels A and B) generated two curves reflecting the refractory periods at the junction with, however, a delay that decreased as the electrode asymmetry became less pronounced. On the other hand, symmetrical electrodes (panel C) generated two superimposed curves, a phenomenon that matches the empirical data.

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134

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

V T - L H condition. If the model is accurate, this finding would indicate that the conduction time from the stimulation point to the junction was shorter for the LH branch. In other words, the junction would lie closer to the LH than the VT, assuming the same velocity for the two branches. At first glance, this observation may lead to the contention that the junction lay rostrally to our LH electrodes. However, with a junction rostral to the LH (that is, at least 4 mm from the VT), the V T - L H condition (the condition that rose second) should not rise before 1.2 ms (that is, the time noted with ipsilateral V T - L H placements). The fact that most of our V T - L H curves began rising before this interval indicates that the junction must necessarily lie between the two structures. Its precise location can further be speculated from the magnitude of delay between the two curves. In our data, the delay varied from 0.2 to 0.6 ms, depending on the pair of stimulated sites. The equations printed in panel D show the delay predicted by the model as a function of the total conduction time and relative proximity of the LH electrode to the junction. The proximity values correspond to the electrode locations shown by lines of same length above the neuron's collaterals. Conduction time refers to the sum of the individual conduction times from each electrode to the junction. With a conduction time of 0.6 ms (the average estimate for our ipsilateral L H - V T sites) the model predicts delays of 0.2 and 0.6 ms for proximity values of approximately 0.42 and 0.26, respectively. A proximity value of 0.42 is not substantially different from that (0.5) defining equidistant electrode placements. On the other hand, a value of 0.26 translates into a situation where the LH electrode would lie approximately three times closer to the junction, compared to the VT electrode (see panel A). In conclusion, our data with contralateral L H - V T placements can be accounted for by the presence of a junction lying closer to the LH electrode. The speculations about the junction's precise location depend on the validity of the assumptions about conduction velocity. The latter was set at the value estimated from ipsilateral sites and was assumed to be the same for the two collaterals. We believe that our lesion findings represent a critical test for the model. Following the LH lesion, the collisionlike effect between this site and the contralateral VT was reduced and the delay between the curves, abolished. In addition, the threshold frequency increased for both electrodes. Remember that the maximum change in VT threshold amounted to 0.47 logfrequency units, indicating a near 3-fold reduction in stimulation effectiveness. It should be emphasised that such a drastic effect has rarely been reported, even with ipsilateral placements. The most parsimonious explanation of these two effects is that some reward-related fibers linked the LH and contralateral VT. However, the signals did not seem to have travelled along

the entire distance separating these sites for if they did, a typical collision effect would have been observed. The data rather suggest the presence of conduction failure at some point between the electrodes. We suggested that the electrodes excited a different branch of the same neuron and that conduction failure occurred at the branchpoint. Some MFB neurons are known to cross the midline through the mammillary and the dorsal and ventral tegmental decussations. Most relevant to the present discussion is the finding by Rompr6 and Trudeau [46] that pontine neurons could be antidromically activated by bilateral LH selfstimulation electrodes. Thus, anatomical evidence favouring our neural model does exist. Conduction failure at the branchpoint (the second assumption in our model) has been predicted from simulations using axonal inhomogeneity concepts and verified experimentally on both invertebrate and vertebrate preparations. Conduction of action potentials across axonal branches depends on the safety factor [27,54,55] which is determined by differences in axonal diameter, myelinisation, internodal length and other local conditions. Thus, conduction failure is expected to occur at points of impedance mismatch, such as branchpoints. Other, non-morphological factors, such as previous impulse history, play a role in conduction failure. For example, it was shown that antidromic volleys, induced by electrical stimulation of one branch, may fail to invade a parent branch or the soma, depending on the frequency of pulses [55]. Similarly, a second impulse, travelling early in the refractory period of a prior impulse, would be blocked as it passed the branchpoint [ 11 ]. According to Swadlow, Kocsis and Waxman [53], impulses that fail to invade one daughter branch while they successfully invade the other branch form a three-position switch that may have physiological significance. Two important effects of the lesion differentiated the LH and VT data: first, the change in VT threshold was larger for intermediate currents, indicating that the neural elements affected by the LH lesion had a precise topography below the VT electrode. Second, unlike for the LH, the VT threshold increased with time. These observations are consistent with the view that the LH lesion induced retrograde degeneration which affected the output of fibers coursing through a short, identifiable distance from the contralateral VT electrode. According to the model, a reduction in VT output can be expected as soon as the degenerative process reaches the branchpoint. Retrograde degeneration over a few millimetres is known to occur over several days [ 15,24], a fact consistent with our data. The typical collision effects obtained with most of our ipsilateral L H - V T placements are consistent with previous data [2,12,18,47]. However, U-shaped and asymmetric effects were also noted. The former can theoretically be predicted for o n e of the curves in transynaptic collision,

J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-137

with, however, a different time of occurrence. This double discrepancy has no easy explanation. According to Ranck [44], fibers running immediately below the stimulating electrode may, if stimulated with high current, fail to fire due to the presence of inward current at the next node. If some of the fibers activated by the C pulse coursed through this silent region, the U-shaped effect may then be attributed to the failure of C impulse to travel beyond the second electrode. According to this schema, moving the electrode by a small step should push this set of fibers outside the silent region and result in a collision effect, a prediction met by the data. Note also that the maximum U-effect was seen for a C - T interval of approximately 0.8 ms, corresponding closely to the time required for the signal to reach the region below the second electrode. On the other hand, the finding of an identical U-effect on both curves would require the presence of neurons with both ascending and descending branches or, alternatively, two different sets of fibers coursing to opposing directions. The second atypical profile, showing a delay between the two curves, was similar to that seen with contralateral electrodes and might therefore reflect the activation of branches running to opposing directions along the same hemisphere. Note that this effect was found only once. The fact that the VT electrode of this subject (981) was found near the midline (see Fig. 9) may indicate that neurons 'b' and 'c' (shown in Fig. 13) are actually the same, but the VT electrode hit the descending branch at a point just before decussation. In short, the model proposes that a significant number of MFB reward neurons send branches to the contralateral hemisphere. According to our data from 101 pairs of sites, interhemispheric MFB links may be as frequent as homologous ipsilateral links. In effect, a link was detected for 69 ~o of ipsilateral sites and for 61.4 ~o of contralaterai sites. A sketchy representation of the model's main elements is shown in Fig. 13. The contralateral L H - L H data were accounted for by the activation of neurons 'a' assumed to send branches to both hypothalami. Neurons 'b', sending an ascending branch to the L H and a second descending branch to the VT of the other hemisphere, were proposed to explain the contralateral L H - V T data. Neurons 'c', with both ascending and descending branches on the same hemisphere, were also proposed to explain a single atypical result obtained with ipsilateral electrodes. The presence of neurons 'd', projecting long branches along the MFB is consistent with the typical collision effect obtained with ipsilateral electrodes in the present and previous studies [2,3,12,18,47]. Finally, neurons 'e' are proposed to account for a transynaptic effect reported recently by Bushnik-Harris and Bielajew [6]. These authors found evidence for transynaptic collision with seven pairs of ipsilateral V T - L P O A electrodes and inferred a caudorostral direction for six out of the seven electrode

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pairs. The model predicts that the collaterals of neuron 'b' branch out at a point between the L H and VT, probably closer to the former. On the other hand, the model makes no assumptions about the precise location of the somata of these or any other neuron shown. A probable location for neurons 'a' is the pedunculopontine nucleus which, as reported earlier, contains neurons that can be activated antidromicaily by bilateral LH electrodes [46]. Finally, note that the main trunk is cut, indicating that no assumptions are made about the course and destination of this branch. If the model is accurate, lesions in one hemisphere should increase the threshold for contraiateral selfstimulation, a prediction met in the present study. On the other hand, lesions at various placements along the MFB should (depending on the electrode placements) have variable effects on ipsilateral self-stimulation, another prediction matched by a large body of older data. For example, a LH lesion damaging mostly 'b' collaterals would have little effect on ipsilaterai VT self-stimulation. In summary, the present study revealed the presence of, as of yet, unknown interhemispheric interactions between reward-related impulses. The neural model proposed to account for these effects relies on realistic anatomical grounds, is testable, makes clear-cut predictions and has

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J. Malette, E. Miliaressis / Behavioural Brain Research 68 (1995) 117-I 37

already received preliminary support from our lesion findings. This model, which sheds new light on the anatomy of a reward pathway, enables one to reconcile a large body of lesion data and opens new avenues for future investigations. One prediction that may be tested in the immediate is that knife cuts at some point of the interhemispheric line should abolish self-stimulation obtained from contralateral MFB placements.

Acknowledgements This study was supported by a research grant from the National Science and Engineering Research Council of Canada. We thank Roberta Anderson for assistance in testing subject No. 55.

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