Effects of amygdaloid stimulation upon trigeminal sensory fields of the LIP that are established during hypothalamically-elicited quiet biting attack in the cat

Btam Research, 197 (1980) 39-55

39

( Elsevier/North-Holland Bmmedical Press

EFFECTS OF A M Y G D A L O I D S T I M U L A T I O N UPON T R I G E M I N A L SENSORY FIELDS OF T H E LIP T H A T ARE E S T A B L I S H E D D U R I N G H Y P O T H A L A M I C A L L Y - E L I C I T E D Q U I E T BITING A T T A C K I N T H E CAT

CHRISTINE H BLOCK, ALLAN SIEGEL and HENRY EDINGER Depal tment~ ofPh v stoh~gl' and Neurone fence, Neu Jer.sey Medical School, Newat k, N J 07103 ( U.S.A )

(Accepted February 28th, 1980) Key woJd~ aggression - - quiet biting attack - - amygdala - - tngemmal sensory field

SUMMARY An experiment was performed m order to determine the role of the amygdala and surrounding cortex in quiet biting attack ehcited from electrical stimulation of the hypothalamus Stimulation of basal, cortical, and anterior amygdala as well as pyriform cortex and parahlppocampal gyrus resulted in a suppression of the attack response and m a constriction of trigemmal sensory fields that are estabhshed during hypothalamic stlmulat~on. Stimulation of lateral and central amygdala resulted m a faclhtation of the quiet biting attack response and an expansion of the trigemmal sensory fields, and a decreased latency for the occurrence of jaw opemng when the sensory field was held constant. These studies suggest that the amygdala modulated quiet b~ting attack behavior generated by hypothalamzc stimulation at least, m part, by ~lrtue of its control over sensory fields.

INTRODUCTION That the amygdala plays a central role m the control of aggressive behaviors has been well documented in the literatureg, 10,12 14,21,25,32,49,53,54. Perhaps the clearest demonstration of amygdaloid involvement in hypothalamically elicited aggresswe behavior was provided by Egger and Flynn 1~' 14. Their studies indicated a differentiatlon of function within the amygdala with regard to its effects upon the quiet biting form of attack m the cat. In particular, these authors have shown that the dorsolateral dJvJszon of the lateral nucleus facilitates the attack response, whereas the magnocellular division of the basal nucleus inhibits this response. Of primary Importance are the possible mechanisms underlying amygdaloid control of hypothalamically elicited aggressive behavior. It is clear that the quiet biting

40 attack response does not merely represent motor activation in response t(, lateral hypothalamic stimulation ; it is also under sensory control inasmuch as visual, tactilc, and possibly olfactory cues play a major role in elicltatlon of the total response patterll in the cat t.2,16,z6'av,4s and m the rat 4.1'~,44 46 Evidence for this point of view was generated principally from the studies by MacDonnell and Fiynn 3~ who showed that an intact trigeminal sensory input to the perloral region of the cat was necessary l~r the biting attack response to occur Later studies have supported thl~ OositlOq Bugbee and Eichelman 4, Thor and Ghlselh 45 and Thor et al. ~ demonstrated an increased occurrence in fighting behavior in rats with intact vibnssa wheJl compared with devibrissaed or facially anesthetized animals. Bandler and Flynn e demonstrated 'paw-striking' in response to tactile stimulation of the paw during hypothalamlcally ehcited quiet biting attack in cats. In addition to these studies which suggest that sensory modaht~eb play a significant role in the successful completion of the attack sequence, the result~ of othel Investigations suggest the further posslbiht2¢ that the amygdala may alst, modulate sensory mechanisms 2°.29'z4.4I'az'-l~ For example, Turner 4s showed an ~ncrease H~ placidity in rats with amygdalold lesions that were associated with contralateral visual impairment and proprioceptlve loss O'Keefe and Bouma 41 demonstrated alterations in amygdaloid cellular activity in response to auditory, visual, and pr~pNoceptlvC stimuli, and Bonvallet and Gary Bobo z observed alterations in reflexive jaw activit~ during stimulation from basal amygdala that was associated with spinal trlgeminal (pars orahs) activity. The following experiments were undertaken to test the hypothesis that amygdaloid control of attack behavior involves modulation of the sensory components of the total response pattern. Evidence in support of such a notion would thus provide further insight into some of the possible mechanisms by which modification of the attack behavior by the amygdala is achieved METHODS Eleven cats of both sexes which did not spontaneously attack rats were selected for the experiment. They were maintained on an ad hbltum feeding and watering schedule throughout the experiment. Electrolytically sharpened stainless-steel stylets, insulated with an oil-base paint and exposed 0.5 m m from the tip, were used for both stimulating and recording electrodes. Cats were anesthetized with sodium pentobarbital (40 mg/kg body weight). Under aseptic conditions, a set of 3 electrodes was implanted stereotaxically and bilaterally in the amygdala. Six stainless-steel 18-gauge guide tubes were mounted over the hypothalamic sites for later electrode implantation. Three uninsulated stylets were implanted in the skull as indifferent electrodes Stimuli, generated by two independent Grass S-8 stimulators, were led through stimulus isolation units to the cat. Pairs of 40,000 ~ resistors in series with the cat approximated constant current conditions. Pairs of leads were connected through relays to the electrodes as well as to input of a Grass 78 EEG m order that stimulation

41 and recording was possible through the same electrode. Currents dehvered to the hypothalamus were in the range of 0.16-0.80 mA and those delivered to the amygdala varied between 0.08 and 0.25 mA, depending upon the seizure threshold present in that structure for each animal. When both structures were stimulated concomlttantly, balanced biphasic pulses were set such that stimulation of one brain structure followed onset of the other by 4 msec. Frequency of stimulation was maintained at 60 Hz and the pulse width at 1 msec per half-cycle duration. The peak-to-peak current was measured by a Tektronix dual beam oscilloscope with differential inputs. One week postoperatively, the hypothalamic electrodes were lowered through the guide tubes in 0.5 mm steps At each level, the cat was stimulated. At that level where the desired behavior was obtained and consistently repeated, the electrode was cemented m place

Behavioral experiment The purpose of these experiments was to determine the nature of the amygdaloid site upon the quiet biting attack. The behavioral trials were carried out m an observation box (24 in. >< 24 m. < 24 in.) with a plexiglas door An anesthetized rat was always present in the behavioral chamber. The experimental paradigm for these trials consisted of paired trials of single stimulation (S) of the hypothalamus alone which elicited the attack response and dual stimulation (D) of the hypothalamus and the amygdala. Currents applied to the hypothalamus produced quiet biting reliably within 15 sec. In subsequent dual stimulation of the hypothalamus and the amygdala, current intensities applied to the hypothalamus were identical to those used during paired trials of the hypothalamus alone. A single-dual-dual-single ( S - D - D - S ) pattern was employed for determining the order of presentation of each palr of stimuli to the cat Response latencies to initial movement (l e. transposition of the forepaws following stimulus onset) were recorded. On all trials revolving dual electrical stimulation of the amygdala the EEG was recorded and those trials m which afterdlscharges were observed were not included in the statistical treatment of the data. A t-test for paired observations was employed to analyze the differences in attack and movement latencies between single and dual stimulation

Sensory l~eld expermlents After an amygdalold site was identified as either inhibiting or facIhtating quiet biting attack, the effect of this site upon the trlgeminal sensory field was evaluated. The cat was suspended in a restrainer (Alice King-Chatham Products). Prior to electrical brain stimulation, the entire extent of the hpline was probed with a blunt probe at constant pressure from the lateral extent to the middle of the lip. Such probing did not produce any jaw movements and constituted a control trial. Then, this procedure was applied during single stimulation of a hypothalamxc site from which attack could be ehclted. The distance from the mldhne of the lip on the side ipsllateral to stimulation that elicited a j a w opening response following tactile stimulation was recorded. On the next trial, a similar probing procedure of the ipsllateral hphne was utlhzed during dual stimulation of the hypothalamus and amygdala. On the following trial, the contra-

42 lateral lipline was probed during dual stimulation. On the final trial of the sequence the contralateral lipline was probed during single stimulation of the hypothalamus. Thus. an S - D - D - S paradigm was applied for presentation of the stimuli to the cat in which an S-D pair of trials involving the ipsilateral side was followed by a D-S pmr of trials involving the contralateral side of the hphne. The current intensities dehvered to the hypothalamus and amygdala on these trials were the same as those used during thc behavioral trmls. The distance from the midline of the lip, which when probed ehcited jaw opening, was recorded after each trial, t-Tests for paired observations were utilized to determine the significance of the differences between the effects of smgte and dual stimulation upon the lateral extent of the hpline from which jaw opening could be produced.

Jan' opening latencies It was noted that at the current intensities used for either single or dual trials, an area of about 0.5 cm at the midline of the lip consistently elicited jaw opening when probed, thus yielding a constant sensory field through which possible variations m latencies for the jaw opening reflex could be compared between single and dual stimulation. This midline region was probed during single stimulation and latency for the jaw opening response to occur was recorded. Then, this region was probed during dual stimulation and the latency for this response to occur was recorded The differences in latency to jaw opening for paired trials of single and dual stimulation were analyzed by a t-test for paired observations. At the end of the experiments, lesions were made at the effective s~te,~ and the animals were perfused transcardially with 0.9°,0 saline and I 5°/,, potassium ferncyamde m 10 o~ formalin. The brain was cut on a freezing microtome at 60 t~m and stained with cresyl violet. RESU LTS

Stimulation of the hypothalamus The effects of amygdaloid stimulation upon hypothalamlcall2¢ ehcated quiet

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Fig t. Map ofhypothalamic sites from whlchelecmcalstimulatlonehclted quiet bJtingattack behavior (filled diamonds). Sections are taken from the atlas of Jasper and Ajmone-Marsan2s, Abbreviations Bin, basal nucleus (pars magnocellular); Bp, basal nucleus (pars parvocellular); Ce, central nucleus. Co, cortical nucleus; f, fornix; GP, globus pallidus; Hpc, hippocampus; 1C, internal capsule; LH, lateral hypothalamus; M, medial nucleus; Op, optic tract; P, posterior hypothalamus

43

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F,g. 2 Electrode sites in the amygdala and surrounding cortex that modulated quiet biting attack and 'effectwe sensory fields' Filled triangles, inhibition of qmet biting attack and constriction of'effective sensory fields', filled squares, faohtation of quiet biting attack and expansion of 'effective sensory fields'. The number at the bottom right-hand corner indicates the frontal plane of the section m stereotaxic coordinatese8. Abbreviations: AA, anterior amygdala, CI, claustrum, E, external capsule, La, lateral nucleus, PH, parahippocampal gyrus ; Put, putamen ; pyr, pyriform cortex See Fig. 1 for further abbrewat]ons biting attack were considered in these studies. This response was first described by W a s m a n and Flynn 52. The attack was generated in the present study by electrical stimulation o f regions o f lateral and posterior h y p o t h a l a m u s at m i n i m a l current intensities to achieve a consistent response wxthin 15 sec and was characterized by an initial stalking and a p p r o a c h to the prey, rubbing o f the cat's muzzle over the rat, and c u l m i n a t i n g in the bite to the back or the neck o f the rat. The mean latencies for biting occurred between 3.6 a n d 9 5 sec, while those latencies for initial m o v e m e n t o f the forepaws ranged from 1.2 to 7.0 sec. A m a p of electrode sites in the h y p o t h a l a m u s from which attack was elicited is shown m Fig 1.

Effect.s of ano'gdaloid stimulation upon quiet biting attack The effects o f electrical stimulation o f s~tes in the basal, cortical, and anterior a m y g d a l a , pyriform cortex, and p a r a h I p p o c a m p a l gyrus (Fig. 2) resulted in a suppression o f the quiet biting a t t a c k response (Fig. 3A, Table I). in general, the b e h a v i o r observed during dual stimulation o f these inhibitory sites was s o m e w h a t s l m d a r to the a t t a c k sequence observed during h y p o t h a l a l n i c stimulation alone The cat stalked, a p p r o a c h e d , and r u b b e d its muzzle over the rat d u r m g trials o f dual stimulation. However, in m a r k e d contrast to single stimulation o f the h y p o t h a l a m u s alone, the attack response would typically not occur following initial facial sensory contact with the rat. Generally, the cat would circle the rat one or two more t~mes

44 TABLE 1

Effect ~ oJ concurt ent amygdaloid atimulattan upon attack ehctted from the hypothalamu~ %, refers to the mean value of attack latenc~es resulting from stimulation of hypothalamus alone. XD

refers to the mean value of attack latencles resulting from stimulation of both hypothalamus and amygdala S~ refers to the standard error of the mean of the distribution of the difference scores of paired trmls of single and dual stimulation. I refers to inhibition of the response; F refers to facditat~on of the response

Ca~e

Electrode atte

Numbe~ of patted

Mean dtfferences m attack latenctes

Effe~ t

1 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17

Basal-magnocellular Basal-magnocellular

23 20 12 19 24 24 24 24 24 24 10 20 20 24 22 24 23

- 2 08/0.75 --0.72/0.28 - 2 80/1.58 --6.38/0.65 --2.05/0.69 -2.54/0.67 --5.38/0 8l - - 2 10/0.59 - 2 31/I 03 - I t0/0.59 - 1.45/0.73 5.15/1.08 - 2 87/0 79 1 33/0.66 I 26/0.28 I 53/0 16 3 82/0 69

1* * I** 19 I *** I* * * 1* * * 1"** I*** l* I~ I9 1"** I*** F* F*** F*** F***

* P

Basal-parvocellular Basal-parvocellular

Basal-parvocellular Basal-parvocellular Basal-parvocellular Cortical Anterior Anterior

Parahippocampal gyrus P y r i f o r m cortex Pyriform cortex Lateral Lateral Central Central _ 005;

** P

001;

*** P

0001,

~ P - 0.10

before finally biting the prey. Thus, tt appears that following dual stimulation involving an inhibitory amygdaloid site, the initial sensory input to the cat's lipline had become ineffective for triggering the biting response. The latencies for initial movement of the forepaws during dual stimulation of the amygdala and hypothalamic attack site ranged from 1.2 to 10.0 sec and mean latencles for the attack occurred between 6.5 and 15.0 sec Ten of the inhibitory sites (cases 1, 2, 4-9, 12, 13) produced statistically significant suppression of quiet biting attack (P ~ 0.05). Stimulation from 3 other sites (cases 3, 10, 11) significantly suppressed hypothalamic attack at P <: 0.10. In 10 of the cases (1-3, 5, 6, 8, 10-13) the differences m mean latencies to imtml movement of the forepaws between dual and single stimulation were not significant (P ~ 0.10" Fig. 3). Statistically significant facilitation of quiet biting attack occurred during dual stimulation from 3 electrode sites in the lateral and central nuclei of the amygdala at P 0.001 and from one lateral site at P ~ 0.05 (cases 14-17; Figs. 2, 3B, Table I). The behavioral response following dual stimulation occurred between 3 6 and 5.4 sec compared with a latency range of 3.6-9.5 sec during single stimulation of the hypothalamus. Because the difference between the latency for initial movement and that for quiet biting attack is suggestive of the 'stalking time', it is evtdent that th~s

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Fag. 3 Bar graphs indicate that concomitant stimulation of the amygdala and hypothalamus alter latencJes to attack (open histogram) whereas, this snmulat~on has no effect upon latencles to initial movement of the forepaws (hatched histogram) For each pair of histograms, the first bar corresponds to the mean attack latency for single stlmulanon of the hypothalamus alone; the second bar represents the mean attack latency for dual stimulation Numbers centered below pairs of bar graphs identify the cats descnbed m Table l; +P ~ 0 IO:*P 0.05;**P : 001;***P < 0001 A:lnh]bltorysltes B facd~tatory sites.

phase o f the attack sequence was shortened following dual stlmulatxon of inhibitory amygdaloid sites. It should be noted, though, that the actual pattern of biting occurred m a manner similar to that observed during single stimulation; l e. the cat rubbed its muzzle over the rat and then bit the back or the neck o f the rat at the completion of this approach. The latencies for initial movement of the forepaws between trials of single and dual stimulation did not differ significantly (P > 0.10) from 4 sites tested from which facilitation of attack was produced (cases 14-17). The range of these latencies vaned between 1.0 and 1 8 sec (Fig. 3)

Effects of hypothalamic stimulatton upon trigeminal sensory,l~elds MacDonnell and Flynn 37 established that the extent of the lipline, when probed during hypothalamic stimulation, could produce a jaw opening response and was a function of the current intensity delivered to that hypothalamic site. We have observed that if either the ipsilateral or the contralateral hpline was probed during single stimulation of the hypothalamus and if the procedure was coupled with increasing current intensities delivered to that slte, the extent of the lipline from which jaw opening was elicited upon probing expanded laterally from the midhne (Fig. 4). We tentatively define the lateral extent o f the lipline from which jaw opening can be ehcited in response to probing during hypothalamlc stimulation as the 'effective sensory field'.

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Effect oJ stimulation of the amygdata upon the hypothalamically elictted "~ective sensory field" The effects upon the "effective sensory field' following stimulation of sites in the amygdala, which clearly modulated quiet biting attack elicited by the hypothalamus, are described below. Preliminary experiments suggested that inhibitory sites in the A

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Fig. 5. Bar graphs indicate that concomitant stimulation of the amygdala and hypothalamus (hatched histogram) alters the size of the 'effective sensory field' established during single stimulation of the hypothalamus (open histogram). A ' construction following stimulation of inhibitory sites. B ' expansion following stimulation of facilitatory sites for both apsdateral and contralateral sensory fields. Numbers centered below pairs of bar graphs adentify the cats described m Table II. *P 0 05; **P - 0 01, ' * * P - 0.001

47 amygdala constricted the 'effective sensory field' while facilitatory sites expanded them. Accordingly, m order to assess the possible effects of the amygdala upon the 'effective sensory field' current intensities delivered to the hypothalamus were selected which provided the optimal conditions for such testing, i.e. during trials involving inhibitory sites in the amygdala, the current delivered to the hypothalamus was raised to such a level that the entire extent of the liphne, when probed, would elicit jaw opening. During trials involving facilitatory sites in the amygdala, the current dehvered to the hypothalamus was decreased in order that the expanding effects upon the hphne following stimulation of this facilitatory site would not be obscured. In general, however, these current intensities delivered to the hypothalamus during the sensory field experiments were the same as those employed during the behavioral trials for each of the ammals examined. hthibitory ~sites. Those amygdaloid sites that produced statistically significant inhibition of quiet biting attack following dual electrical stimulation, also produced a statistically significant constriction (P -, 0.05) of the "effective sensory field' when tested during dual stimulation relative to that seen during single stimulation of the hypothalamus (Fig. 5A, Table ll). In 12 cases (1, 2, 4-13) the contralateral extent of the hpline was sigmficantly constricted (P ~ 0.05) following stimulation of behavioral-

TABLE 11

EJ/e¢ t,s o f eoncm ient amygdalotd ~timulatton upon the tp6ilateral and contralateral e ~tent o f the ' effectl ve sen,wry fwld,C C refers to constmchon of the sensory field of the hphne; E refers to expansion of the sensory field N S., not slgmficant

Ca~e

I

Electrode ~tte

lp~ tlatel al hphne Number Effe¢t o f pat t e d trtala

Contt alateral liphne Number Effect o f paired trtal~

Basal-magnocellular Basal-magnocellular Basal-parvocellular

6 20 9 16 20 20 20 20 20 16 16 15 12 16 16 16 16

7 20 10 16 20 20 20 20 20 16 17 16 14 16 16 16 16

2 3 4 5 6 7 8 9 [0 I1 12 13 14 15 16 17

Antemor Anterior

* P

0.05,

Basal-parvocellular

Basal-parvocellular Basal-parvocellular Basal-parvoce[lular Cortical

Parahlppocampal gyrus Pyriform cortex Pyriform cortex Lateral Lateral Central Central ** P

001;

*** P -

C** C*** C** C*** C*** C*** C*** C*** C*** N S. N.S C*** C*** E*** E **~ E*** E*** 0.001.

C* C*** N.S C*** C*** C*** C*** C*** C*** C*** C* C*** C*** E*** E*** E*** E***

48 ly identified inhibitory sites, in 11 cases (1-9, 12, 13) a significant constriction o f the ipsilateral liphne was clearly seen (P ~-. 0.01 ). It was noted that the most potent points which suppressed quiet biting attack (cases 1, 2, 4-9, 12, 13) were associated with bilateral constriction o f the 'effective sensory field'. For example, in case 5 (Fig. 6) the differences in latencies for the occurrence o f attack between dual stimulation and single stimulation o f the hypothalamus were highly significant (P ~ 0.001 ). Here, both the ipsllateral and contralateral 'effective sensory fields' were sigmficantly constricted during dual stimulation of the hypothalamus and a m y g d a l a (P : 0.00l). In those cases where the differences in attack latencies between single and dual stimulation were marginally significant (P ~_ 0.10; cases 3, 10, 11), unilateral constrictzon of the 'effective sensory field' was observed. For example, in case 3, the difference in the latency to attack between single and dual stimulation was significant at P : 0.10; ipsilateral constriction of the 'effective sensory field' was significant at P ~ 0.01, but no constriction o f the contralateral side of the lipline was noted (P > 0.40). Facilitatory sites. Stimulation of the 4 amygdalold sites that produced faclhtation of the attack response (cases 14-17) also produced an expansion o f the "effective sensory field' (Fig. 5B, Table 11). In each of these cases, the expansion of the "effective sensory fields' was observed to occur bilaterally (P - 0.001). This phenomena is

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Fig. 7 Individual profiles dlustratmg the facdltatory effects of stimulation of the lateral nucleus of the amygdala upon : (A) latency to lmt~al movement of the forepaws ; (B) latency to quiet b~ting attack, (C) the ~psdateral and contralateral 'effective sensory fields'; and (D) latency to jaw opening. Open h~stograms, single stimulation of the hypothalamus; hatched histograms, dual stimulation of the hypothalamus and lateral nucleus. *P ~ 0 05; ***P < 0.001, N S., not significant

illustrated in case 14 (Fig. 7). Here, the site in the lateral nucleus of the amygdala sigmficantly facilitated the quiet biting attack response (P <" 0.05); the ipsllateral and contralateral ~effective sensory fields' were also significantly expanded relative to that observed following single stimulation of the hypothalamus (P < 0.001).

Effect of dual stimulation upon the latency for the jaw opening response In the following experiment a constant sensory field was probed along the region of the midline of the lip during both conditions of single and dual stimulation in order to assess the effects of dual stimulation upon the motor component of th~s response. Inhibitory sites. Sites in the amygdala that produced significant suppression of attack behavior from the hypothalamus produced no change in the latencies for the occurrence of the jaw opening response as indicated in Fig. 8A and Table lII. The latencles for the occurrence of th~s response during either single or dual stimulation ranged from 2.6 to 8.0 sec. Facihtatory sttes. In 3 cases, the latency for the occurrence of the jaw opemng response following probing of the midline of the lip, was significantly shorter during dual stimulation than during single stimulation (P < 0.05, cases 15-17). The mean latency for jaw opening ranged from 3.8 to 7.3 sec during single stimulation of the

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Fig 8. Bar graphs indicate that concomitant stimulation of the amygdala and hypothalamus: (A) has no effect upon the latency to jaw opening when the amygdaloid site is inhibitory, and (B) decreases the latency to jaw opening when the amygdaloid s~te is facilitatory upon quiet biting attack. Open h:stogram, single stlmulatmn of the hypothalamus; hatched histogram, dual stimulation of the hypothalamus and amygdala. Numbers centered below pairs of bar graphs ~dentify the cats described m Table II! *P 0.05; **P -~ 001

TABLE 11l

Effects o f concurrent amygdaloid stimulation upon the latency to jarr opening during mtd-liplme ptobtng F refers to facd:tation of the jaw opening m response to probing Ks refers to the mean value of jaw opening latencies resulting from stimulation of hypothalamus alone. ~D refers to the mean value of jaw opening latencies resulting from stimulation of both hypothalamus and amygdala. Sg refers to the standard error of the mean of the d:stribution of the difference scores of paired trials of single and dual stimulation N.A., data for jaw opening latenc:es not available for these cases N S , not significant

Case

Electrode site

Numbel o[ pan'ed tt ta[,~

Mean differences m Effect law opening latenctes ( se(") O?s --,~l,/ S~ )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Basal-magnocellular Basal-magnocellular Basal-parvocellular Basal-parvocellular Basal-parvoceUular Basal-parvocellular Basal-parvocellular Cortical Anterior Anter:or Parahippocampal gyrus Pyriform cortex Pyriform cortex Lateral Lateral Central Central

N,A 24 N A 20 20 24 24 20 24 12 N A. 20 20 20 20 20 20

N A -0 30/0 24 N,A - 0.06/0.53 - - 0 24/0.27 -1.05/0.39 --0.86/0 45 0.14/0 59 0 09/0 23 0 04/0.26 N A -- 0 83/0 45 0.09/0 48 0 49/0 82 I 22/0.43 1 68/0 48 0 70/0 31

* P < 0.05;

** P < : 0 . 0 1 .

N A. N S N A NS NS N.S N S N S NS N S NA N S N S. N S F** F** F*

51 hypothalamus while the latency for this response during dual stimulation of hypothalamus and facllitatory sites in the amygdala ranged from 2.6 to 6.8 sec (Fig. 8B, Table Ill) In sites that were studied that showed no significant modulation of quiet biting attack (P ,~ 0.10), there was no subsequent change in the 'effective sensory field' during dual stimulation nor was there any change in the latency for the jaw opening response to occur. In neither the behavioral nor in the sensory field and mld-hphne probing experiments did stimulation of the amygdala or perlamygdalold cortex alone produce any organized behavioral response or movements of any kind. DISCUSSION The findings m this study confirm and extend those reported by Egger and Flynn v-'-14 and suggest a possible mechamsm underlying amygdalold modulation of the hypothalamus. Specifically, electrical stimulation of sites m the basal, cortical, and anterior amygdalold nuclei, pyriform cortex and parahlppocampal gyrus suppressed the occurrence of hypothalamically elicited attack, while electrical stimulation of sites dorsal and lateral to these inhibitory sites in the lateral and central nuclei of the amygdala resulted in facilitation of the occurrence of this response. Stimulation from sites in the amygdala and adjacent regions that inhibited quiet biting attack also constricted the sensory fields of the liphne that resulted m a jaw opening response when probed during single stimulation of hypothalamlc attack sites. The results of this study point to the likelihood that suppression of quiet biting attack by the amygdala is accomphshed, at least in part, through a sensory modulatory mechanism that is associated with reflexive jaw responses. That this inhlbltlon suggests principally a modification of behaviorally established reflexes is evidenced by the lack of inhibitory amygdalold Influence upon the latency to Initial movement of the forepaws and latency to the jaw opening response to probing, both of which may be viewed as indices of motor function. Similarly, other behavioral responses closely associated with motor activation and sympathetic arousal, such as affectlve d~splay, a form of aggressive behavior generated from ventromedlal hypothalamlc stimulation, and ongoing movements are not altered by amygdalo~d stimulation of these sites (author's observations). Consistent with this view are the observations of Egger and Flynn 14 and Fonberg and Delgado 17 who noted that the animal's behavioral patterns were normal and that the animal remained responsive during stimulation of these inhibitory sites. SItes in the lateral and central nuclei that enhanced qmet biting attack and expanded the 'effective sensory fields' estabhshed during stimulation of these attack sites, are not as clearly delineated in terms of motor and sensory modulation because of the presence of significant facilitation of both modahtles during stimulation of these amygdaloid sites, as observed in its effects upon latency to jaw opening as well as upon the expansion of the sensory fields. One possible interpretation of the effects of amygdaloid stimulation upon the latency to jaw opening is suggested from the studies of Kawamura and Tsukamoto30 who demonstrated jaw movement in rabbits during stimulation of the lateral nucleus of the amygdala, while stimulation of other

52 amygdalold nuclei had no effect upon jaw movement activity. Accordingly, this study implies that facilitation of attack by the lateral nucleus is mamfested through a mechanism which involves both sensory and motor components of the .law openmg reflex in contrast to the observations noted during suppression of attack generated from other amygdaloid nuclear groups At the present time it ~s not possible to state w~th certamty how the amygdala generates its effects upon the 'effective sensory fields' operative during qmet biting attack. It is possible that its effects upon 'sensorimotor" mechanisms occur d~rectly at the pontme level upon the tngeminal nucle~ or perhaps indirectly through the neocortex A possible clue to our understanding of these potential neural mechamsms underlying amygdalold control of the 'effective sensory fields' may be gamed from our knowledge of the anatomical connections of the regions m question. A number ol different fiber systems appear to be logical candidates. A most hkely poss~bdity representing the initial link in such a fiber chain are the separate known tiber systems of the stria termlnalis and ventral amygdalofugal pathway which connect the amygdala and adjacent pyriform corte~ with both the medial and lateral hypothalamus respectivelyS,18,22,23,3a,3s,50,sL Presumably, if the fibers of amygdaloid origin synapsing in the lateral hypothalamus constitute the first-order neurons m this chain, then fibers descending from the lateral hypothalamus to the ventral and central tegmental areas would represent the second-order neurons of this pathway~ 7,:~9,40.4~ Possible third-order neurons of this chain have been described by Chl et al. ,5 and appear to arise from sites in the ventral tegmentum from which biting attack can be elicited. Most interestingly, these fibers are distributed to the spinal and mare sensory nuclei of NV as well as to the specml visceral efferents (i.e motor nuclei) of NV and NVll. Thus, such a multisynaptic chain would constitute the most direct route governing amygdalold modulation of the size of the 'effective sensory fields' present during biting attack. We can only speculate about the possible synaptic regions along this chain where significant interactions from the amygdala might occur to produce ~ts effects upon sensory and motor mechamsms present during biting attack, It seems plausible that some interactions occur at the level of the lateral hypothalamus as a result of either amygdaloid neurons directly terminating m this region or indirectly from amygdaloid fibers which synapse first in the medial preoptico-hypothalamus t~,ee Since fibers from the amygdala have recently been shown to project to ~ome extent directly into the brain stem°-6.38,ag, there may be some interactions from the amygdala which occur there as well. Whde it remains somewhat of a more remote possibdlty, it ~s also conceivable that some modulation of the 'effechve sensory fields' occurs at the level of sensorimotor cortex. Such an effect could be manifested either through ascending retlculo-thalamo-cortical fibers 4~, amygdalo-cortical fibers ~7,:~;~.or even by ventral tegmental-cmgulate fibers z4 which could form the first neuron m a d~synaptlc pathway, ultimately reaching sensonmotor cortex From the discussion noted above it appears that the pathway(s) subserwng amygdaloid modulation of the trigeminal system during biting attack ~s not a s~mple one, but probably involves a number of synapses. Evidence in support ol this notion stems from an experiment by Kawamura and Tsukamoto 3a. It was shown that the

53 latency for onset of the masseteric reflex was shorter following cerebral cortical stimulation than it was following stlmulatlon of the amygdala, suggesting that the functional connections between the amygdala and trigeminal system are probably mult~synapt~c. In summary, we have defined the sensory field of the perioral region from which a jaw opening response can be elicited following tactile p r o b i n g a n d c o n c u r r e n t electrical stimulation of h y p o t h a l a m i c sites associated w~th quiet biting attack as the "effective sensory field'. In the present study, we have found that stimulation from sites m the amygdala which suppress biting attack behavior also constricts the 'effective sensory field', while stimulation from other sites assocmted with facilitation of attack produces an expansion of the 'effective sensory fields', as well as faclhtatlon of the occurrence of the jaw opening reflex. These results suggest that amygdaloM m o d u l a tion of aggressive behavior ts mamfested, in part, by ItS direct or indirect effects upon the tr~gemlnal system, thus estabhshmg for the first t~me a new potential m e c h a n i s m underlying such control by an i m p o r t a n t c o m p o n e n t of the limbic system. ACKNOWLEDGEMENTS This research was supported by National Institutes of Health G r a n t NS 0794111 awarded by the N a t i o n a l Institutes of Neurological and C o m m u m c a t i v e Disorders and Stroke.

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