- Email: [email protected]
Effects of body weight loss and taste on VMH-LH electrical activity of rats
Physiology&Behavior,Vol.52, lap. 1187-1192, 1992
0031-9384/92$5.00 + .00 Copyright© 1992PergamonPressLtd.
Printedin the USA.
Effects of Body Weight Loss and Taste on VMH-LH Electrical Activity of Rats B. S. R A O ' A N D E. P R A B H A K A R
Department of Physiology, St. John's Medical College, Bangalore-560 034, India
Received 22 J a n u a r y 1992
RAO, B. S. AND E. PRABHAKAR. Effects of body weight loss and taste on VMH-LH electrical activity of rats. PHYSIOL BEHAV 52(6) 1187-1192, 1992.--Electrical responsesof ventromedial (VMH) and lateral (LH) hypothalamus to graded decrease in body weight and to taste were recorded from a conscious rat with chronically implanted macroelectrodes. Graded reduction in body weight was correlated with gradual increase in LH activity and reciprocal decrease in VMH activity, both of which were stable and specific to each gradation in body weight. On gustation of any test solution, basal activity was temporarily altered, if any. On sweet taste of calorically rich sucrose, VMH activity was enhanced and LH showed decrease. But on sweet taste of caloricallyinert saccharin, VMH activity was increased, though a reciprocal decrease in LH was not shown. Contrastingly, both bitter taste and salt taste caused increased LH activity but no change in VMH activity. Enhanced VMH activity correlated with sweet taste may be due to activation of VMH glucoreceptors. LH activation correlated with bitter and salt taste is a likelyresponse of two distinct groups of LH units; one responding to salt taste and the other to bitter aversive taste. Body weight loss
Taste
Hypothalamic activities
TASTE helps in discriminating food and nonfood and then in gating the food based on the pleasure or displeasure it produces. Though certain submodalities of taste are usually described as pleasant (ex sweet) and others as unpleasant (ex bitter), the affective component is not the inherent character of taste stimulus but an expression of response to it. Interestingly, the taste that is usually associated with a nutritive (ex salty taste of NaC1) is known to become more pleasant on bodily deficit of that specific nutritive (34,35), thus helping in increased ingestion of specific nutritive and rapid restoration of milieu interieur. Likewise, the depletion of bodily energy stores which necessarily involves a decrease in body weight (11) also is known to enhance the pleasure on the sweet taste usually associated with energy-rich carbohydrates (ex sugar). Underweight animals, in addition, showed increased aversion for bitter taste (14,33). As sweet taste is correlated with calorie (energy) content and bitterness with toxicity (16), the oral intake behaviour of calorically deprived underweight animals is in the right direction of restoring the body weight and safeguarding life. However, paradoxically, the preference for sweet taste to the underweight animals appears to be so compelling as to drive them even to death rejecting to eat bland though calorically rich food (10). In contrast to behaviour of the underweight animals, the free-feeding adult animal, presumably with normal body weight, exhibits neither excessive craving for sweet taste nor undue aversion for bitterness (33). Further, the ad lib-fed humans even evidence displeasure for sweet taste (which was pleasant for taste earlier) immediately
i Requests for reprints should be addressed to B. S. Rao.
1187
after sugar intake (5,6,21). Based on human and animal investigations, it was conceptualised (5) that taste perceptual shifts are correlated with changes in body weight from its set point (Pondersostat). On reduction in body weight below Ponderostat level, the eating is increased because the taste of food then becomes pleasant. The reverse is true when body weight surpasses the set point. In essence, eating is controlled in order to regulate the body weight (or energy balance) via alterations in taste perception. However, mechanisms that link body weight alterations with modulation of taste perception are virtually unknown, though internal signals related to body weight or energy balance were thought to be involved (5). The activities of lateral (LH) and ventromedial hypothalamus (VMH) that are classically implicated in initiation and termination of eating, respectively (2,26), are considered as probably internal signals for two reasons. One is that LH is known to be involved in reward processes that may, in part, relate specifically to taste (24,28), principally through its links with ascending and descending components of medial forebrain bundle (37). Secondly, the VMH appears to be involved in maintaining long-term nutritive state (12,17) and, thus, in maintaining the set point for body weight. A change in body weight, therefore, may affect VMH activity which, in turn, exerts its effect on LH (25). At the same time, LH may also be influenced by taste projections and their reward processes. As both LH and VMH are involved in food intake and, thus, in maintenance of body weight, it was thought logical to investigate their electrical activities to
188
RA() .AN[) t"RABHAKAR
identify the probable electrophysiological basis tbr alterations in taste perception, with the changes in body weight. METHOD Adult (3 months old) male Wistar rats (n - 60) housed in individual cages and fed ad lib were used. They were kept in an animal house with a natural light/dark cycle (roughly 12 h each) and a temperature of 24.0 _+ 2.0°C. The food and water were replenished every day at 1600 h. The food and water intake and body weight were measured daily at 1530 h. After the animals were adapted to cages to food and water intake for a period of l 0 days, they were divided into four groups based on their calorie intake via the food (1 g of food = 2.8 cat). One group (n = 14) continued on ad lib feeding and served as controls. The second (n = 14), third (n = 16), and fourth groups (n = 16) were given 75%, 50%, and 25% calories, respectively, of their ad lib intake. The amount of calories given to individual rats belonging to the second, third, and fourth groups was kept constant, irrespective of their change in body weight intake. When the animals reached an asymptote in body weight, the behavioural and electrophysiological investigations were conducted.
Behavioural Investigation The solution intake tests were administered to half the number of rats in each group. The tests were single-choice 5-min exposure to test solutions of 9.0% sucrose, 0.2% saccharin, 0.9% sodium chloride, and 0.002% quinine sulphate. Only one test solution was given randomly on any day to all the animals. The solution was given to rats in a measuring cylinder fixed to the front of the cage with the spout projecting into cage. A 2-day rest period was interposed between any 2 test days. Each solution test was repeated for a minimum of three times. Intake was computed as mean ml _+ SE/5 min/100 g body weight.
However, from rats selected at random, electricai activity k)l.. lowing taste was recorded at regular intervals (5 rain. 15 rain. 30 rain, and 60 min) in 1 b. Each record showed fluctuations in electrical activity (as in EEG). Such fluctuations are called waves. Records obtained for more than a 30-s duration were divided into segments of 30-s periods each. The 30 s of record electrical activity just before gustation is taken as basal activits. The first 30-s record obtained following 10-s gustation is considered as a response to taste. The rest of the 30-s segments of records served only as indicators of basal activity or duration of effects of taste. Between the withdrawal of spout and initiation of recording, a 1-2 min period usually elapsed. Each animal from any group participated in two to three recording sessions. Between any two recording sessions of a single animal, a 3-day period was interposed. The activity obtained before (basal) and after taste of solution is expressed as mean +_ SE frequency of waves/30 s. After the electrophysiological investigations, the location of electrodes was histologically verified. The animals were anaesthetised, and recording sites in the brain were lesioned using 24 mA for a 10-20-s period. Then the animals were intracardially perfused with normal saline followed by 10% formaldehyde solution. The electrodes were then gently pulled out and the cranium was cut to expose the brain. The slice of the brain which includes the electrode markings was taken out and sectioned (40 ~) serially to identify the location of either of the electrodes.
Statistical Analysis The data obtained from behavioural and electrophysiological investigations was analysed using analysis of variance (ANOVA). The post hoc multiple comparison was done using Scheffe's procedure excepting for the comparison of basal VMH and LH activities in individual groups, where paired t-test was used (p < 0.05 was taken as statistically significant difference).
Electrophysiological Investigation At about the time the oral intake tests were initiated, half the number of rats from each group were fixed with permanent electrodes. The stainless steel electrodes, insulated excepting at the tip (diameter 100 #, height 100 #), were stereotaxically directed to VMH and LH of anaesthetised (sodium pentabarbitone 35 mg/kg b.wt., IP) rats, using de Groot's stereotaxic coordinates for locating VMH (AP 5.0 mm, lateral 1.0 mm, and vertical 8.5 to 9.0 mm) and LH (AP 5.0, lateral 2.0 mm, and vertical 8.0 to 8.5 mm). Only one electrode was fixed on any one side of midsagittal line. A reference electrode was fixed on the skull just posterior to the active electrodes. All the electrodes were fixed permanently using dental cement. Animals were allowed a postoperative rest period of 10 days.
Recording For recording the electrical activity from VMH and LH, inkwriting, two-channel Beckman's Type RS dynograph was used. Electrical responses from at least two animals to taste of single test solution on any day were recorded at about 1600 h. Animal kept in the recording cage was connected to the dynograph and allowed a 10-15 min period of rest. When it became relatively quite, the basal activity (before the administration of taste) was recorded for a minimum period of 30 s. In some cases, basal activity alone was recorded at regular intervals (5 min, 15 min, 30 rain, and 60 min) in l h. Then the spout containing test solution was introduced into cage. After the animal licked at the spout for a lO-s period, the spout was withdrawn and electrical activity was again recorded for a minimum 30-s period.
RESULTS The graded decrease in calorie intake/100 g body weight of 75% cal group (15.0 __ 0.9) 50%° cal group (9.3 -+ 0.5) 25%° cal group (5.1 _+ 0.4) as compared to ad lib intake (21.1 _+ 0.6) resulted in gradual decrease in body weights of respective groups of rats (Table 1). The effects of body weight reduction on solution intake are shown in Table 1, and the electrophysiological correlates of body weight and taste responses are shown in Table 2. Representative records of basal electrical activity, as well as activity in response to taste, are shown in Fig. l and Fig. 2. Figure 3 shows the relation between body weight and basal hypothalamic (VMH and LH) activity, and Fig. 4 shows tasteinduced percent changes in VMH and LH activity. The three groups of rats on restricted calorie intake, which was initiated at the age of 120 days, showed asymptote in body weight at three different periods (days) of adaptation to calorie rationing. The period was increased with decrease in calorie intake (for 75% diet, 10 days; 50% diet, 25 days; for 25% diet, 40 days). Anyhow, the body weights of all groups of rats were stabilised at their age of approximately 180 days. That was the time when the behavioural and electrophysiological investigations were initiated. The solution intake tests showed (Table 1) that with graded loss in body weight the intake on sweet saccharin as well as on salt taste was increased, while on bitter taste the intake was decreased. However, on sweet sucrose the intake was not altered across the groups excepting for the sucrose intake of 75% diet group, which showed a decrease (p < 0.05).
VMH-LH ACTIVITY ALTERATIONS TO TASTE AND BODY WEIGHT
1189
TABLE 1 EFFECT OF BODY WEIGHT ON 5-M1NTEST SOLUTIONINTAKE (PER 100 g b.wt.) OF RATS Test Solution ml Intake (Mean - SE) Group
Body Weight(g)
Ad lib 75% cal 50% cal 25% cal
275.7 264.7 197.3 155.1
_+ 9.4 + 7.9 _+ 5.7 + 6.3
Sucrose 4.3 3.5 3.8 3.9
+ 0.6 _+ 0.3 _+ 0.3 _+ 0.4
Saccharin 3.0 3.7 4.8 6.5
Sodium Chloride
-+ 0.2 -+ 0.2* + 0.5* _+ 0.2*
2.0 2.0 2.7 3.5
Quinine
-+ 0.3 -+ 0.1 _+0.3* + 0.1"
0.7 0.5 0.4 0.3
___0.02 +-_0.05* ___0.06* ___0.06*
* p < 0.05.
The electrophysiological investigations evidenced (Table 2) that the basal V M H activity which was high in ad lib rats (159.5 + 4.1) showed a decrease with reduction in body weight and was minimal in 25% cal group (119.0 +_ 3.3). In contrast, the L H activity showed inverse relation to body weight. In 25% cal group with least body weight, the basal L H activity was maximal (134.4 ___4.1), which showed decrease with graded increase in body weight. The basal activity of each group of rats recorded for 1 h (not shown) was approximately stable. On gustation of sweet sucrose, an increase in V M H activity over the basal of all groups of rats was shown (Table 2). The percent increase in V M H activity across the groups was enhanced with decrease in body weight (Fig. 4). However, the increase in V M H activity in ad lib rats (161.5 ___ 3.1) and 75% cal group (163.6 ___4.2) was only slight as compared to their basal (ad lib 159.5 _ 4.1; 75% cal group 150.2 ___ 5.7), while the increase shown in 50% cal group (144.7 ___2.4) and 25% cal group (146.5 --- 3.6) over their respective basals (50% cal group 125.5 ___6.4; 25% cal group 119.0 ___3.3) was significant (p < 0.05). Following sucrose taste, the L H activity, in contrast to V M H activity, was significantly depressed in all rats. Surprisingly, after tasting another sweet solution, that is saccharin, the L H activity was undiminished, though increase in V M H activity across the groups was more that seen after sweet sucrose taste. The response to bitter quinine was in contrast to response to saccharin taste (Table 2). There was a graded increase in L H activity (ad lib 124.1 ___4.1; 75% cal group 139.9 _+ 5.7; 50% cal
group 151.2 + 3.4, and 25% cal group 168.8 + 5.9) with decrease in body weight (Fig. 4). The V M H activity was not significantly altered. The response to salt taste was almost similar to bitter taste response, differing only in magnitude. It is noteworthy that the taste-induced changes in hypothalamic activity were transient (<5 min), while the body weightrelated basal activity was approximately stable for at least 1 h. DISCUSSION The study was initiated to identify the electrophysiological basis of taste perceptual change in rats with reduced body weight. Reduced body weight was achieved by restricting food (calorie) intake. Solutions differing in taste (sweet, salty, and bitter) and calorie content (sucrose and saccharin) were used as stimuli. The 5-min intake of test solution by the rats was taken as hedonic (affective) response to taste. With decreased body weight the rats exhibited enhanced pleasure for sweet taste and increased aversiveness for bitter taste, which substantiated the earlier observations (33). The tendency to decrease intake on less sweet sucrose (as compared to saccharin) with decreasing body weight further reinforced the idea that the body weight reduction leads to decreased preference for a week sweet taste (sucrose) and enhanced preference for a strong sweet taste (saccharin). The taste-induced pleasure/displeasure expressed as intake of solution is directed by integrated electrical output from several neural assemblies. We have recorded electrical activity from the
TABLE 2 ELECTRICALACTIVITY(MEAN + SE/30 s) IN HYPOTHALAMICFEEDING AREAS BEFORE (BASAL)AND AFTER GUSTATIONOF TEST SOLUTION
Basal Test Solution Sucrose Saccharin Sodium Chloride Quinine
Nutritional Status
Hypothal Area
Ad Lib
VMH LH
159.5 ___4.1 110.8+3.6
150.2 _+ 5.7 118.0+3.2
125.5 ___6.4 126.1 +4.5
119.0 _ 3.3 134.4_+4.1
VMH LH VMH LH VMH LH VMH LH
161.5 _ 3.1 98.4 + 2.5* 177.0 ___5.9* 116.1+3.4 160.7 ___4.4 132.8 -+ 3.4* 166.5 + 2.9 124.1 _+ 4.1"
163.6 + 4.2 96.3 + 2.9* 188.0 + 4.2* 119.0+2.8 151.1 __+4.1 147.2 -+ 3.0* 155.4 ___3.3 139.9 _+ 5.7*
144.7 + 2.4* 104.8 -+ 3.5* 184.0 + 3.9* 130.4+3.5 140.2 -+ 5.4 159.3 _+ 2.1" 132.3 ___4.2 151.2 _+ 3.4*
146.5 + 3.6* 118.9 ___2.4* 194.7 + 6.2* 139.0-+4.6 127.5 ___3.7 176.1 _+ 6.0* 128.9 _+ 4.1 168.8 _+ 5.9*
75%
* Significantly (p < 0.05) altered as compared to respective basal activity.
50%
25%
1190
RA() A N I ) P R A B H A K A R BASAL
LH
2
3
~-~'~"---~'¢~-%~"
~,
~ - . ~ . . ~ ~ , _ ~_~_....~...
~*~'~-,,,,~r~
v~
i'S
LH
,.
VIvlH
,-v-",-.-~~
LH
"""-""~"~""~"~"~"~"-"
V~
4
A
at.
~.- -
~
,~.
~
LH
FIG. 1. Representative electrical activity from ventromedial (VMH) and lateral hypothalamus (LH) of 1) ad lib, 2) 75% cal, 3) 50% cal, and 4) 25% cal rats to 30-s taste of(A) sucrose and (B) saccharin. Basal records are obtained 30 s before taste.
neural assemblies in LH and VMH alone, as their activities are known to be largely responsible for initiation or cessation of eating (2,25). Electrical activities were obtained only after a lapse of 1-2 min following gustation of test solution, as reflex hyperglycemia, if any, appears at approximately 1 min after tasting (22). The electrical activity (both impulsive and nonimpulsive) aptly compared to group singing (8) is preferred over single unit study because perception depends on the simultaneous cooperative activity of million of neurons--and such global activity can be identified, measured, and explained only if one adopts a macroscopic view alongside a microscopic one (8). Further, the recording of extracellular activity from groups of neurons as is done in the present investigation is known to be largely correlated with behaviour (4). Additionally, this method minimises sam-
BASAL VMH
2VMH LH
VMH •L H
4VM H
piing error intrinsic in single-unit study (15). The basal activity showing an increase in LH and a decrease in VMH with decreased body weight (Fig. 3) confirmed the earlier electrophysiologicai evidences (32) as well as reinforced reciprocal unit discharge reports (25). It probably is the reflection of internal signals related to level of body weight. The gustation-induced transient wave-type activity despite intergroup and interindividual variations that may be due to slight variations in the placement of electrodes as well as to random movements of freely moving and behaving rats (36) evidenced patterns specific to each submodality of taste. The tasteinduced transient signalisation pattern superimposed on stable basal activity appears to help the animal to anticipate hunger or satiety that is likely to result after ingesting material associated with taste. Based on the anticipations, the animals appear either
D
C
~
~
" ~ V " ' " ' ~ ~
~
~
"~t'-~'~"
~
.,--~,¢.m,~,,,,..,--~
x.
LH FIG. 2. Representative electrical activity from ventromedial (VMH) and lateral hypothalamus (LH) of 1) ad lib, 2) 75% cal, 3) 50% cal, and 4) 25% cal rats to 30-s taste of(C) sodium chloride and (D) quinine sulphate. Basal records are obtained 30 s before taste.
VMH-LH ACTIVITY ALTERATIONS TO TASTE AND BODY WEIGHT -~ 160
The sweet taste of saccharin has its specific effects on VMHLH activity. The larger increase in VMH activity of not only 25% and 50% cal group rats, but also of 75% cal group and ad lib rats ought to be due to enhanced anticipatory release of insulin and glucose on saccharin taste (18,19). Surprisingly, despite such massive VMH stimulation, the LH activity remained approximately unaltered at basal level in contrast to clear inhibition shown after sweet sucrose. Possibly the inhibition exerted by VMH activity, as well as direct suppression of LH units by anticipatory glucose, were antagonised by some other excitatory effects on LH. Such LH activation could be due to the aversive bitter after-taste of saccharin (3,13) as a class of LH cells at the AP level of VMH is known to be maximally excited by aversive fluid stimuli and showed strong phasic as well as tonic patterns of reactivity (37). In contrast to saccharin, the sugar contains less of bitter cues (9) and, hence, less of opposition to LH inhibition after its taste. Quinine bitter taste, unlike sweet taste, is not known to cause reflex release of glucose and, hence, VMH was not activated following quinine taste. Therefore, the LH (despite unaltered VMH activity) appears to be resultant of strong stimulation of LH aversive cells by bitter taste (29,37). Incidentally, the quinine bitter taste-induced LH stimulation reinforces our earlier premise (under saccharin taste) that bitter after-taste of saccharin might have stimulated the LH. The similarity of salt taste-induced wave type activity to bitter taste responses was puzzling because of contrasting evidences. One evidence obtained 2 decades ago showed (7) that bitter taste and salt taste stimulate distinctly different quinine, best and salt, best glossopharyngeal nerve fibres, respectively, in the rat. Further behavioural intake on their taste is also different; salt solution intake is more than quinine solution intake. In contrast, the recent investigations on rabbit (37) LH units showed strongest excitatory fluid reaction to NaCI and quinine HC1 and similar units that treated NaC1 much like quinine HC1 were also found in rats (29,30), which reinforced the observations in the present investigation. The demonstration of LH single-cell activations in rabbits may be explained as due to aversion to strong solutions of NaC1 (4.4%) and quinine (0.4%) that were used as stimuli.
LH
0
140 ".>
120
2 100 ,5 ,7, 80
i
i
150
i
190
i
i
i
230 270 150 Body weiaht (g)
190
230
270
FIG. 3. Effects of body weight on VMH and LH basal activities of rats.
to enhance or to decrease ingestion. If satiety (benefit) is indicated by taste-induced wave patterns, it is increasingly ingested. The reverse is true for taste that showed changes indicating either no use or toxicity. Apart from general observation as above, there were a few interesting VMH-LH activity changes to individual submodalities of taste. The sweet sucrose taste induced increase in VMH activity of 50% cal group, and 25% cal group rats over their respective basal activity may be attributed to stimulation of VMH glucoreceptors (1) by the reflexly released anticipatory glucose (22). Absence of such VMH activation in ad lib and 75% cal group rats ought to be due to insufficiency of transient hyperglycemia to stimulate further the high VMH activity that was already present in them (18,32). In contrast, LH activity in all groups of rats was suppressed on sucrose taste. In the case of 50% cal group and 25% cal group rats, it could be a summated effect of reciprocal inhibition from activated VMH (25,26) and direct inhibition of glucosensitive LH units (27,29) by anticipatory hyperglycemia. In 75% cal group and ad lib rats, LH inhibition appears to be due to suppression o f L H glucosensitive neurons alone, for their VMH was not excited.
Sucrose
Sacchar;n
160
160 140
140 120
120 Basal 100
80
1191
BaSal I
~
o
i
80
i
I
1
I
I
0 = LFI A - - VMH Qu;nine
120 Basal
.o,_.._I-o
100 80
Salt
~, ,
Ad L;b
,
75% cal
, ,
A .
50% 25% cal
cal
Io
Basal .
/~.~'~'-~--~-A .
.
,
Ad L~ 75% cal
50% 25°/,-, cal
cal
FIG. 4. Taste-induced percent changes in VMH and LH basal activities of rats on varying calorie intake.
1192
RA(_) A N I ) P R A B H A K A R
Similar argument cannot be advanced to explain LH activation seen in the present investigation as the simulating solutions (0.9% NaC1, 0.002% quinine SO4) were weak. However, it is possible that some LH cells in rats that exhibit acceleration of previous activity on lingual stimulation with NaCI, which is sensory specific and unaffected by noxious, photic and acoustic stimulations (23) are responsible for increased LH activity seen after salt taste. Further, LH cells responding to salt taste may be different from LH cells responding to aversive quinine taste. As the surface area of the uninsulated portion of an approximately cylindrical electrode we implanted in the LH (as well as in V M H ) was large enough ( r d h = 3.14 × 100 × 100 - 31400 u:) to record activity from several hundred neurons, it may not be illogical to think that the LH cells specific to aversive taste and those
sensitive to salt taste were covered by the electrode. Stimulation of either group of cells, therefore, may be recorded as 1,t t activation in general. With the recording method employed in this investigation, it is not possible to identifx; which group of cells was responsible for increased wave-type activity in the LH. One observation that still remains unclear is the absence of well-established reciprocal relation between VMH and LH (28) after gustation of saccharin, salt, and quinine solutions. Does the activity of dorsomedial hypothalamus (20), to which both LH and V M H have interconnections, play any role in it? ACKNOWLEDGEMENT The help rendered by Dr. L. S. Piers in statistical analysis of the data is gratefully acknowledged.
REFERENCES 1. Anand B. K.; Chinna, G. S.; Sharma, K. N.; Dua, S.; Singh, B. Activity of single neurons in the hypothalamic feeding centres: Effects ofgiucose. Am. J. Physiol. 207:1146-1154; 1964. 2. Anand, B. K. Central chemosensitive mechanisms related to feeding. In: Code, C. F., ed. Handbook of physiology, sec. 6, Alimentary canal I. Washington, DC: American Physiological Society: 1967. 3. Anderson, B.; Landgren, S.; Olsson, S.; Zotterman, Y. The sweet taste fibres of dog. Acta Physiol. Scand. 21 : 105-119; 1950. 4. Bullock, T. K.; Lehniger, A. L.; Whittakar, V. P.; Schmitt, F. O. Neural coding. Neurosci. Res. Prog. Bull. 6:225-348; 1968. 5. Cabanac, M. Physiological role of pleasure. Science 173:1103-1107: 1971. 6. Cabnac, M.; Duclaux, R.; Spector, N. H. Sensory feed back in regulation of body weight. Is there a ponderostat? Nature 229:125-127; 1971. 7. Frank, M. Response patterns of rat glosso pharyngeal neurons. In: Denton, D. A.; Coghlan, J. P., eds. Olfaction and taste V. New York: Academic Press Inc.; 1975. 8. Freeman, W. J. The physiology of perception. Sci. Am. 264:34-40; 1991. 9. Gordon, G.; Kitchell, R.; Strom, L.; Zotterman, Y. The response pattern of taste fibres in the chorda tympa of monkey. Acta Physiol. Scand. 46:119-132; 1959. 10. Hamilton, C. S. Starvation induced by sucrose intake in rat: Partial protection by septal lesions. J. Comp. Physiol. Psychol. 77:59-69; 1971. 11. Harvey, G. R. Regulation of energy balance. Nature 222:629-631; 1969. 12. Harvey, G. In: Novin, D.; Wyrwicka, W.; Bray, G., eds. Hunger: Basic mechanisms and clinical implications. New York: Raven Press; 1976. 13. Iriuchijima, J.; Zotterman, Y. Conduction rates of afferent fibers to the anterior tongue of dog. Acta Physiol. Scand. 51:283-289; 1961. 14. Jacobs, H. L.; Sharma, K. N. Taste versus calories: Sensory and metabolic signals in the control of food intake. Ann. NY Acad. Sci. 157:1084-1125; 1969. 15. Lemon, R. The relationship between neural activity and behaviour: Analysis and interpretation. Methods for neuronal recording in conscious animals: Methods In neurosciences IV; 129-140; 1984. 16. Le Magnen, J. Habits and food intake. In: Code, C. F., ed. Handbook of physiology, sec. 6. Alimentary canal. Washington, DC: American Physiological Society; 1967. 17. Le Magnen, J. Gluco lipostatic mechanisms and feeding. In: Novin, D.; Wyrwicka, W.; Bray, G., eds. Hunger: Basic mechanisms and clinical implications. New York: Raven Press; 1976. 18. Le Magnen, J.; Devos, M.; Larue-Achagiotis, C. Food deprivation induced parellel changes in blood glucose, plasma free fatty acids and feeding during the two parts of the diurnal cycle in rats. Neurosci. Biobehav. Rev. 4:17-23; 1980. 19. Louis Sylvestre, J.; Le Magnen, J. Palatability and preabsorptive insulin release. Neurosci. Biobehav. Rev. 4:43-46; 1980.
20. Luiten, P. G. A.; Room, P. Interrelation between lateral, dorsomedial, and ventromedial hypothalamic nuclei in the rat: An HRP study. Brain Res. 190:321-332; 1980. 21. Moscowitz, H. R.; Kumaraiah, V.; Sharma, K. N.; Jacobs, H. L.; Dua Sharma, S. Effect of hunger, satiety and glucose load upon taste intensity and taste hedonics. Physiol. Behav. 16:471-475; 1976. 22. Nicolaidis, S. Early systemic responses to orogastric stimulation in the regulation of food and water balance: Functional and electrophysiological data. Ann. NY Acad. Sci. 157:1176-1203; 1969. 23. Nicolaidis, S. Sensory neuroendocrine reflexes and their anticilpatory optimizing role on metabolism. In: Kare, M. R.; Muller, O., eds. The chemical senses and nutrition. New York: Academic Press; 1977: 123-143. 24. Norgren, R. Taste pathways to hypothalamus and amygdala. J. Comp. Neurol. 166:17-30; 1976. 25. Oomura, Y.; Ooyama, H.; Yamamoto, Y.; Naka, F. Reciprocal relationship of the lateral and ventromedial hypothalamus in the regulation of food intake. Physiol. Behav. 2:97-115; 1967. 26. Oomura, Y. Central mechanisms of feeding. In: Advances in biophysics. Tokyo: Tokyo University Press; 1973:65-142. 27. Oomura, Y.; Ooyama, H.; Sugimori, M.; Nakamura, T.; Yamada, Y. Glucose inhibition of the gluco sensitive neurons in the lateral hypothalamus. Nature 247:284-286; 1974. 28. Oomura, Y. Input-output organisation in the hypothalamus relating to food intake behaviour. In: Morgane, P. J.; Panksepp, J., eds. Handbook of hypothalamus. 2. Physiology of Hypothalamus. New York: Marcel Dekker; 1980:557-620. 29. Ono, T.; Sasaki, K.; Nakamura, K.; Norgren, R. Integrated lateral hypothalamic neural responses to natural and artificial rewards and cue signals in the rat. Brain Res. 327:303-306; 1985. 30. Ono, T.; Nakamura, K.; Nishijo, H.; Fukuda, M. Hypothalamic neuron involvement in integration of reward aversion and cue signals. J. Neurophysiol. 56:63-79; 1986. 31. Ono, T.; Nakamura, K. Learning and integration of rewarding and aversive stimuli in the rat lateral hypothalamus. Brain Res. 346: 368-373; 1985. 32. Rao, B. S.; Sharma, K. N. Electrophysiological correlates of diurnal rhythm disruption in rats. Ind. J. Physiol. Pharmacol. 25:308-318; 1981. 33. Rao, B. S. Effects of dynamic and static body weight loss on taste responses. Ind. J. Physiol. Pharmacol. 27:25-31; 1983. 34. Richter, C. P. Salt taste thresholds of normal and adrenalectomised rats. Endocrinology 24:367-371; 1939. 35. Rozin, P. Thiamine specific hunger. In: Code, C. F., ed. Handbook of physiology, see. 6. Alimentary canal. Washington, 19(?:American Physiological Society; 1967. 36. Rolls, E. T. Activity of hypothalamic and related neurons in the alert animal. In: Morgane, P. J.; Panksepp, J., eds. Handbook of hypothalamus, vol. 3. Behavioural studies, part A. New York: Marcel Dekker; 1980:439-466. 37. Schwartzbaum, J. S. Electrophysiology of taste feeding and reward in lateral hypothalamus of rabbit. Physiol. Behav. 44:507-526; 1988.
0031-9384/92$5.00 + .00 Copyright© 1992PergamonPressLtd.
Printedin the USA.
Effects of Body Weight Loss and Taste on VMH-LH Electrical Activity of Rats B. S. R A O ' A N D E. P R A B H A K A R
Department of Physiology, St. John's Medical College, Bangalore-560 034, India
Received 22 J a n u a r y 1992
RAO, B. S. AND E. PRABHAKAR. Effects of body weight loss and taste on VMH-LH electrical activity of rats. PHYSIOL BEHAV 52(6) 1187-1192, 1992.--Electrical responsesof ventromedial (VMH) and lateral (LH) hypothalamus to graded decrease in body weight and to taste were recorded from a conscious rat with chronically implanted macroelectrodes. Graded reduction in body weight was correlated with gradual increase in LH activity and reciprocal decrease in VMH activity, both of which were stable and specific to each gradation in body weight. On gustation of any test solution, basal activity was temporarily altered, if any. On sweet taste of calorically rich sucrose, VMH activity was enhanced and LH showed decrease. But on sweet taste of caloricallyinert saccharin, VMH activity was increased, though a reciprocal decrease in LH was not shown. Contrastingly, both bitter taste and salt taste caused increased LH activity but no change in VMH activity. Enhanced VMH activity correlated with sweet taste may be due to activation of VMH glucoreceptors. LH activation correlated with bitter and salt taste is a likelyresponse of two distinct groups of LH units; one responding to salt taste and the other to bitter aversive taste. Body weight loss
Taste
Hypothalamic activities
TASTE helps in discriminating food and nonfood and then in gating the food based on the pleasure or displeasure it produces. Though certain submodalities of taste are usually described as pleasant (ex sweet) and others as unpleasant (ex bitter), the affective component is not the inherent character of taste stimulus but an expression of response to it. Interestingly, the taste that is usually associated with a nutritive (ex salty taste of NaC1) is known to become more pleasant on bodily deficit of that specific nutritive (34,35), thus helping in increased ingestion of specific nutritive and rapid restoration of milieu interieur. Likewise, the depletion of bodily energy stores which necessarily involves a decrease in body weight (11) also is known to enhance the pleasure on the sweet taste usually associated with energy-rich carbohydrates (ex sugar). Underweight animals, in addition, showed increased aversion for bitter taste (14,33). As sweet taste is correlated with calorie (energy) content and bitterness with toxicity (16), the oral intake behaviour of calorically deprived underweight animals is in the right direction of restoring the body weight and safeguarding life. However, paradoxically, the preference for sweet taste to the underweight animals appears to be so compelling as to drive them even to death rejecting to eat bland though calorically rich food (10). In contrast to behaviour of the underweight animals, the free-feeding adult animal, presumably with normal body weight, exhibits neither excessive craving for sweet taste nor undue aversion for bitterness (33). Further, the ad lib-fed humans even evidence displeasure for sweet taste (which was pleasant for taste earlier) immediately
i Requests for reprints should be addressed to B. S. Rao.
1187
after sugar intake (5,6,21). Based on human and animal investigations, it was conceptualised (5) that taste perceptual shifts are correlated with changes in body weight from its set point (Pondersostat). On reduction in body weight below Ponderostat level, the eating is increased because the taste of food then becomes pleasant. The reverse is true when body weight surpasses the set point. In essence, eating is controlled in order to regulate the body weight (or energy balance) via alterations in taste perception. However, mechanisms that link body weight alterations with modulation of taste perception are virtually unknown, though internal signals related to body weight or energy balance were thought to be involved (5). The activities of lateral (LH) and ventromedial hypothalamus (VMH) that are classically implicated in initiation and termination of eating, respectively (2,26), are considered as probably internal signals for two reasons. One is that LH is known to be involved in reward processes that may, in part, relate specifically to taste (24,28), principally through its links with ascending and descending components of medial forebrain bundle (37). Secondly, the VMH appears to be involved in maintaining long-term nutritive state (12,17) and, thus, in maintaining the set point for body weight. A change in body weight, therefore, may affect VMH activity which, in turn, exerts its effect on LH (25). At the same time, LH may also be influenced by taste projections and their reward processes. As both LH and VMH are involved in food intake and, thus, in maintenance of body weight, it was thought logical to investigate their electrical activities to
188
RA() .AN[) t"RABHAKAR
identify the probable electrophysiological basis tbr alterations in taste perception, with the changes in body weight. METHOD Adult (3 months old) male Wistar rats (n - 60) housed in individual cages and fed ad lib were used. They were kept in an animal house with a natural light/dark cycle (roughly 12 h each) and a temperature of 24.0 _+ 2.0°C. The food and water were replenished every day at 1600 h. The food and water intake and body weight were measured daily at 1530 h. After the animals were adapted to cages to food and water intake for a period of l 0 days, they were divided into four groups based on their calorie intake via the food (1 g of food = 2.8 cat). One group (n = 14) continued on ad lib feeding and served as controls. The second (n = 14), third (n = 16), and fourth groups (n = 16) were given 75%, 50%, and 25% calories, respectively, of their ad lib intake. The amount of calories given to individual rats belonging to the second, third, and fourth groups was kept constant, irrespective of their change in body weight intake. When the animals reached an asymptote in body weight, the behavioural and electrophysiological investigations were conducted.
Behavioural Investigation The solution intake tests were administered to half the number of rats in each group. The tests were single-choice 5-min exposure to test solutions of 9.0% sucrose, 0.2% saccharin, 0.9% sodium chloride, and 0.002% quinine sulphate. Only one test solution was given randomly on any day to all the animals. The solution was given to rats in a measuring cylinder fixed to the front of the cage with the spout projecting into cage. A 2-day rest period was interposed between any 2 test days. Each solution test was repeated for a minimum of three times. Intake was computed as mean ml _+ SE/5 min/100 g body weight.
However, from rats selected at random, electricai activity k)l.. lowing taste was recorded at regular intervals (5 rain. 15 rain. 30 rain, and 60 min) in 1 b. Each record showed fluctuations in electrical activity (as in EEG). Such fluctuations are called waves. Records obtained for more than a 30-s duration were divided into segments of 30-s periods each. The 30 s of record electrical activity just before gustation is taken as basal activits. The first 30-s record obtained following 10-s gustation is considered as a response to taste. The rest of the 30-s segments of records served only as indicators of basal activity or duration of effects of taste. Between the withdrawal of spout and initiation of recording, a 1-2 min period usually elapsed. Each animal from any group participated in two to three recording sessions. Between any two recording sessions of a single animal, a 3-day period was interposed. The activity obtained before (basal) and after taste of solution is expressed as mean +_ SE frequency of waves/30 s. After the electrophysiological investigations, the location of electrodes was histologically verified. The animals were anaesthetised, and recording sites in the brain were lesioned using 24 mA for a 10-20-s period. Then the animals were intracardially perfused with normal saline followed by 10% formaldehyde solution. The electrodes were then gently pulled out and the cranium was cut to expose the brain. The slice of the brain which includes the electrode markings was taken out and sectioned (40 ~) serially to identify the location of either of the electrodes.
Statistical Analysis The data obtained from behavioural and electrophysiological investigations was analysed using analysis of variance (ANOVA). The post hoc multiple comparison was done using Scheffe's procedure excepting for the comparison of basal VMH and LH activities in individual groups, where paired t-test was used (p < 0.05 was taken as statistically significant difference).
Electrophysiological Investigation At about the time the oral intake tests were initiated, half the number of rats from each group were fixed with permanent electrodes. The stainless steel electrodes, insulated excepting at the tip (diameter 100 #, height 100 #), were stereotaxically directed to VMH and LH of anaesthetised (sodium pentabarbitone 35 mg/kg b.wt., IP) rats, using de Groot's stereotaxic coordinates for locating VMH (AP 5.0 mm, lateral 1.0 mm, and vertical 8.5 to 9.0 mm) and LH (AP 5.0, lateral 2.0 mm, and vertical 8.0 to 8.5 mm). Only one electrode was fixed on any one side of midsagittal line. A reference electrode was fixed on the skull just posterior to the active electrodes. All the electrodes were fixed permanently using dental cement. Animals were allowed a postoperative rest period of 10 days.
Recording For recording the electrical activity from VMH and LH, inkwriting, two-channel Beckman's Type RS dynograph was used. Electrical responses from at least two animals to taste of single test solution on any day were recorded at about 1600 h. Animal kept in the recording cage was connected to the dynograph and allowed a 10-15 min period of rest. When it became relatively quite, the basal activity (before the administration of taste) was recorded for a minimum period of 30 s. In some cases, basal activity alone was recorded at regular intervals (5 min, 15 min, 30 rain, and 60 min) in l h. Then the spout containing test solution was introduced into cage. After the animal licked at the spout for a lO-s period, the spout was withdrawn and electrical activity was again recorded for a minimum 30-s period.
RESULTS The graded decrease in calorie intake/100 g body weight of 75% cal group (15.0 __ 0.9) 50%° cal group (9.3 -+ 0.5) 25%° cal group (5.1 _+ 0.4) as compared to ad lib intake (21.1 _+ 0.6) resulted in gradual decrease in body weights of respective groups of rats (Table 1). The effects of body weight reduction on solution intake are shown in Table 1, and the electrophysiological correlates of body weight and taste responses are shown in Table 2. Representative records of basal electrical activity, as well as activity in response to taste, are shown in Fig. l and Fig. 2. Figure 3 shows the relation between body weight and basal hypothalamic (VMH and LH) activity, and Fig. 4 shows tasteinduced percent changes in VMH and LH activity. The three groups of rats on restricted calorie intake, which was initiated at the age of 120 days, showed asymptote in body weight at three different periods (days) of adaptation to calorie rationing. The period was increased with decrease in calorie intake (for 75% diet, 10 days; 50% diet, 25 days; for 25% diet, 40 days). Anyhow, the body weights of all groups of rats were stabilised at their age of approximately 180 days. That was the time when the behavioural and electrophysiological investigations were initiated. The solution intake tests showed (Table 1) that with graded loss in body weight the intake on sweet saccharin as well as on salt taste was increased, while on bitter taste the intake was decreased. However, on sweet sucrose the intake was not altered across the groups excepting for the sucrose intake of 75% diet group, which showed a decrease (p < 0.05).
VMH-LH ACTIVITY ALTERATIONS TO TASTE AND BODY WEIGHT
1189
TABLE 1 EFFECT OF BODY WEIGHT ON 5-M1NTEST SOLUTIONINTAKE (PER 100 g b.wt.) OF RATS Test Solution ml Intake (Mean - SE) Group
Body Weight(g)
Ad lib 75% cal 50% cal 25% cal
275.7 264.7 197.3 155.1
_+ 9.4 + 7.9 _+ 5.7 + 6.3
Sucrose 4.3 3.5 3.8 3.9
+ 0.6 _+ 0.3 _+ 0.3 _+ 0.4
Saccharin 3.0 3.7 4.8 6.5
Sodium Chloride
-+ 0.2 -+ 0.2* + 0.5* _+ 0.2*
2.0 2.0 2.7 3.5
Quinine
-+ 0.3 -+ 0.1 _+0.3* + 0.1"
0.7 0.5 0.4 0.3
___0.02 +-_0.05* ___0.06* ___0.06*
* p < 0.05.
The electrophysiological investigations evidenced (Table 2) that the basal V M H activity which was high in ad lib rats (159.5 + 4.1) showed a decrease with reduction in body weight and was minimal in 25% cal group (119.0 +_ 3.3). In contrast, the L H activity showed inverse relation to body weight. In 25% cal group with least body weight, the basal L H activity was maximal (134.4 ___4.1), which showed decrease with graded increase in body weight. The basal activity of each group of rats recorded for 1 h (not shown) was approximately stable. On gustation of sweet sucrose, an increase in V M H activity over the basal of all groups of rats was shown (Table 2). The percent increase in V M H activity across the groups was enhanced with decrease in body weight (Fig. 4). However, the increase in V M H activity in ad lib rats (161.5 ___ 3.1) and 75% cal group (163.6 ___4.2) was only slight as compared to their basal (ad lib 159.5 _ 4.1; 75% cal group 150.2 ___ 5.7), while the increase shown in 50% cal group (144.7 ___2.4) and 25% cal group (146.5 --- 3.6) over their respective basals (50% cal group 125.5 ___6.4; 25% cal group 119.0 ___3.3) was significant (p < 0.05). Following sucrose taste, the L H activity, in contrast to V M H activity, was significantly depressed in all rats. Surprisingly, after tasting another sweet solution, that is saccharin, the L H activity was undiminished, though increase in V M H activity across the groups was more that seen after sweet sucrose taste. The response to bitter quinine was in contrast to response to saccharin taste (Table 2). There was a graded increase in L H activity (ad lib 124.1 ___4.1; 75% cal group 139.9 _+ 5.7; 50% cal
group 151.2 + 3.4, and 25% cal group 168.8 + 5.9) with decrease in body weight (Fig. 4). The V M H activity was not significantly altered. The response to salt taste was almost similar to bitter taste response, differing only in magnitude. It is noteworthy that the taste-induced changes in hypothalamic activity were transient (<5 min), while the body weightrelated basal activity was approximately stable for at least 1 h. DISCUSSION The study was initiated to identify the electrophysiological basis of taste perceptual change in rats with reduced body weight. Reduced body weight was achieved by restricting food (calorie) intake. Solutions differing in taste (sweet, salty, and bitter) and calorie content (sucrose and saccharin) were used as stimuli. The 5-min intake of test solution by the rats was taken as hedonic (affective) response to taste. With decreased body weight the rats exhibited enhanced pleasure for sweet taste and increased aversiveness for bitter taste, which substantiated the earlier observations (33). The tendency to decrease intake on less sweet sucrose (as compared to saccharin) with decreasing body weight further reinforced the idea that the body weight reduction leads to decreased preference for a week sweet taste (sucrose) and enhanced preference for a strong sweet taste (saccharin). The taste-induced pleasure/displeasure expressed as intake of solution is directed by integrated electrical output from several neural assemblies. We have recorded electrical activity from the
TABLE 2 ELECTRICALACTIVITY(MEAN + SE/30 s) IN HYPOTHALAMICFEEDING AREAS BEFORE (BASAL)AND AFTER GUSTATIONOF TEST SOLUTION
Basal Test Solution Sucrose Saccharin Sodium Chloride Quinine
Nutritional Status
Hypothal Area
Ad Lib
VMH LH
159.5 ___4.1 110.8+3.6
150.2 _+ 5.7 118.0+3.2
125.5 ___6.4 126.1 +4.5
119.0 _ 3.3 134.4_+4.1
VMH LH VMH LH VMH LH VMH LH
161.5 _ 3.1 98.4 + 2.5* 177.0 ___5.9* 116.1+3.4 160.7 ___4.4 132.8 -+ 3.4* 166.5 + 2.9 124.1 _+ 4.1"
163.6 + 4.2 96.3 + 2.9* 188.0 + 4.2* 119.0+2.8 151.1 __+4.1 147.2 -+ 3.0* 155.4 ___3.3 139.9 _+ 5.7*
144.7 + 2.4* 104.8 -+ 3.5* 184.0 + 3.9* 130.4+3.5 140.2 -+ 5.4 159.3 _+ 2.1" 132.3 ___4.2 151.2 _+ 3.4*
146.5 + 3.6* 118.9 ___2.4* 194.7 + 6.2* 139.0-+4.6 127.5 ___3.7 176.1 _+ 6.0* 128.9 _+ 4.1 168.8 _+ 5.9*
75%
* Significantly (p < 0.05) altered as compared to respective basal activity.
50%
25%
1190
RA() A N I ) P R A B H A K A R BASAL
LH
2
3
~-~'~"---~'¢~-%~"
~,
~ - . ~ . . ~ ~ , _ ~_~_....~...
~*~'~-,,,,~r~
v~
i'S
LH
,.
VIvlH
,-v-",-.-~~
LH
"""-""~"~""~"~"~"~"-"
V~
4
A
at.
~.- -
~
,~.
~
LH
FIG. 1. Representative electrical activity from ventromedial (VMH) and lateral hypothalamus (LH) of 1) ad lib, 2) 75% cal, 3) 50% cal, and 4) 25% cal rats to 30-s taste of(A) sucrose and (B) saccharin. Basal records are obtained 30 s before taste.
neural assemblies in LH and VMH alone, as their activities are known to be largely responsible for initiation or cessation of eating (2,25). Electrical activities were obtained only after a lapse of 1-2 min following gustation of test solution, as reflex hyperglycemia, if any, appears at approximately 1 min after tasting (22). The electrical activity (both impulsive and nonimpulsive) aptly compared to group singing (8) is preferred over single unit study because perception depends on the simultaneous cooperative activity of million of neurons--and such global activity can be identified, measured, and explained only if one adopts a macroscopic view alongside a microscopic one (8). Further, the recording of extracellular activity from groups of neurons as is done in the present investigation is known to be largely correlated with behaviour (4). Additionally, this method minimises sam-
BASAL VMH
2VMH LH
VMH •L H
4VM H
piing error intrinsic in single-unit study (15). The basal activity showing an increase in LH and a decrease in VMH with decreased body weight (Fig. 3) confirmed the earlier electrophysiologicai evidences (32) as well as reinforced reciprocal unit discharge reports (25). It probably is the reflection of internal signals related to level of body weight. The gustation-induced transient wave-type activity despite intergroup and interindividual variations that may be due to slight variations in the placement of electrodes as well as to random movements of freely moving and behaving rats (36) evidenced patterns specific to each submodality of taste. The tasteinduced transient signalisation pattern superimposed on stable basal activity appears to help the animal to anticipate hunger or satiety that is likely to result after ingesting material associated with taste. Based on the anticipations, the animals appear either
D
C
~
~
" ~ V " ' " ' ~ ~
~
~
"~t'-~'~"
~
.,--~,¢.m,~,,,,..,--~
x.
LH FIG. 2. Representative electrical activity from ventromedial (VMH) and lateral hypothalamus (LH) of 1) ad lib, 2) 75% cal, 3) 50% cal, and 4) 25% cal rats to 30-s taste of(C) sodium chloride and (D) quinine sulphate. Basal records are obtained 30 s before taste.
VMH-LH ACTIVITY ALTERATIONS TO TASTE AND BODY WEIGHT -~ 160
The sweet taste of saccharin has its specific effects on VMHLH activity. The larger increase in VMH activity of not only 25% and 50% cal group rats, but also of 75% cal group and ad lib rats ought to be due to enhanced anticipatory release of insulin and glucose on saccharin taste (18,19). Surprisingly, despite such massive VMH stimulation, the LH activity remained approximately unaltered at basal level in contrast to clear inhibition shown after sweet sucrose. Possibly the inhibition exerted by VMH activity, as well as direct suppression of LH units by anticipatory glucose, were antagonised by some other excitatory effects on LH. Such LH activation could be due to the aversive bitter after-taste of saccharin (3,13) as a class of LH cells at the AP level of VMH is known to be maximally excited by aversive fluid stimuli and showed strong phasic as well as tonic patterns of reactivity (37). In contrast to saccharin, the sugar contains less of bitter cues (9) and, hence, less of opposition to LH inhibition after its taste. Quinine bitter taste, unlike sweet taste, is not known to cause reflex release of glucose and, hence, VMH was not activated following quinine taste. Therefore, the LH (despite unaltered VMH activity) appears to be resultant of strong stimulation of LH aversive cells by bitter taste (29,37). Incidentally, the quinine bitter taste-induced LH stimulation reinforces our earlier premise (under saccharin taste) that bitter after-taste of saccharin might have stimulated the LH. The similarity of salt taste-induced wave type activity to bitter taste responses was puzzling because of contrasting evidences. One evidence obtained 2 decades ago showed (7) that bitter taste and salt taste stimulate distinctly different quinine, best and salt, best glossopharyngeal nerve fibres, respectively, in the rat. Further behavioural intake on their taste is also different; salt solution intake is more than quinine solution intake. In contrast, the recent investigations on rabbit (37) LH units showed strongest excitatory fluid reaction to NaCI and quinine HC1 and similar units that treated NaC1 much like quinine HC1 were also found in rats (29,30), which reinforced the observations in the present investigation. The demonstration of LH single-cell activations in rabbits may be explained as due to aversion to strong solutions of NaC1 (4.4%) and quinine (0.4%) that were used as stimuli.
LH
0
140 ".>
120
2 100 ,5 ,7, 80
i
i
150
i
190
i
i
i
230 270 150 Body weiaht (g)
190
230
270
FIG. 3. Effects of body weight on VMH and LH basal activities of rats.
to enhance or to decrease ingestion. If satiety (benefit) is indicated by taste-induced wave patterns, it is increasingly ingested. The reverse is true for taste that showed changes indicating either no use or toxicity. Apart from general observation as above, there were a few interesting VMH-LH activity changes to individual submodalities of taste. The sweet sucrose taste induced increase in VMH activity of 50% cal group, and 25% cal group rats over their respective basal activity may be attributed to stimulation of VMH glucoreceptors (1) by the reflexly released anticipatory glucose (22). Absence of such VMH activation in ad lib and 75% cal group rats ought to be due to insufficiency of transient hyperglycemia to stimulate further the high VMH activity that was already present in them (18,32). In contrast, LH activity in all groups of rats was suppressed on sucrose taste. In the case of 50% cal group and 25% cal group rats, it could be a summated effect of reciprocal inhibition from activated VMH (25,26) and direct inhibition of glucosensitive LH units (27,29) by anticipatory hyperglycemia. In 75% cal group and ad lib rats, LH inhibition appears to be due to suppression o f L H glucosensitive neurons alone, for their VMH was not excited.
Sucrose
Sacchar;n
160
160 140
140 120
120 Basal 100
80
1191
BaSal I
~
o
i
80
i
I
1
I
I
0 = LFI A - - VMH Qu;nine
120 Basal
.o,_.._I-o
100 80
Salt
~, ,
Ad L;b
,
75% cal
, ,
A .
50% 25% cal
cal
Io
Basal .
/~.~'~'-~--~-A .
.
,
Ad L~ 75% cal
50% 25°/,-, cal
cal
FIG. 4. Taste-induced percent changes in VMH and LH basal activities of rats on varying calorie intake.
1192
RA(_) A N I ) P R A B H A K A R
Similar argument cannot be advanced to explain LH activation seen in the present investigation as the simulating solutions (0.9% NaC1, 0.002% quinine SO4) were weak. However, it is possible that some LH cells in rats that exhibit acceleration of previous activity on lingual stimulation with NaCI, which is sensory specific and unaffected by noxious, photic and acoustic stimulations (23) are responsible for increased LH activity seen after salt taste. Further, LH cells responding to salt taste may be different from LH cells responding to aversive quinine taste. As the surface area of the uninsulated portion of an approximately cylindrical electrode we implanted in the LH (as well as in V M H ) was large enough ( r d h = 3.14 × 100 × 100 - 31400 u:) to record activity from several hundred neurons, it may not be illogical to think that the LH cells specific to aversive taste and those
sensitive to salt taste were covered by the electrode. Stimulation of either group of cells, therefore, may be recorded as 1,t t activation in general. With the recording method employed in this investigation, it is not possible to identifx; which group of cells was responsible for increased wave-type activity in the LH. One observation that still remains unclear is the absence of well-established reciprocal relation between VMH and LH (28) after gustation of saccharin, salt, and quinine solutions. Does the activity of dorsomedial hypothalamus (20), to which both LH and V M H have interconnections, play any role in it? ACKNOWLEDGEMENT The help rendered by Dr. L. S. Piers in statistical analysis of the data is gratefully acknowledged.
REFERENCES 1. Anand B. K.; Chinna, G. S.; Sharma, K. N.; Dua, S.; Singh, B. Activity of single neurons in the hypothalamic feeding centres: Effects ofgiucose. Am. J. Physiol. 207:1146-1154; 1964. 2. Anand, B. K. Central chemosensitive mechanisms related to feeding. In: Code, C. F., ed. Handbook of physiology, sec. 6, Alimentary canal I. Washington, DC: American Physiological Society: 1967. 3. Anderson, B.; Landgren, S.; Olsson, S.; Zotterman, Y. The sweet taste fibres of dog. Acta Physiol. Scand. 21 : 105-119; 1950. 4. Bullock, T. K.; Lehniger, A. L.; Whittakar, V. P.; Schmitt, F. O. Neural coding. Neurosci. Res. Prog. Bull. 6:225-348; 1968. 5. Cabanac, M. Physiological role of pleasure. Science 173:1103-1107: 1971. 6. Cabnac, M.; Duclaux, R.; Spector, N. H. Sensory feed back in regulation of body weight. Is there a ponderostat? Nature 229:125-127; 1971. 7. Frank, M. Response patterns of rat glosso pharyngeal neurons. In: Denton, D. A.; Coghlan, J. P., eds. Olfaction and taste V. New York: Academic Press Inc.; 1975. 8. Freeman, W. J. The physiology of perception. Sci. Am. 264:34-40; 1991. 9. Gordon, G.; Kitchell, R.; Strom, L.; Zotterman, Y. The response pattern of taste fibres in the chorda tympa of monkey. Acta Physiol. Scand. 46:119-132; 1959. 10. Hamilton, C. S. Starvation induced by sucrose intake in rat: Partial protection by septal lesions. J. Comp. Physiol. Psychol. 77:59-69; 1971. 11. Harvey, G. R. Regulation of energy balance. Nature 222:629-631; 1969. 12. Harvey, G. In: Novin, D.; Wyrwicka, W.; Bray, G., eds. Hunger: Basic mechanisms and clinical implications. New York: Raven Press; 1976. 13. Iriuchijima, J.; Zotterman, Y. Conduction rates of afferent fibers to the anterior tongue of dog. Acta Physiol. Scand. 51:283-289; 1961. 14. Jacobs, H. L.; Sharma, K. N. Taste versus calories: Sensory and metabolic signals in the control of food intake. Ann. NY Acad. Sci. 157:1084-1125; 1969. 15. Lemon, R. The relationship between neural activity and behaviour: Analysis and interpretation. Methods for neuronal recording in conscious animals: Methods In neurosciences IV; 129-140; 1984. 16. Le Magnen, J. Habits and food intake. In: Code, C. F., ed. Handbook of physiology, sec. 6. Alimentary canal. Washington, DC: American Physiological Society; 1967. 17. Le Magnen, J. Gluco lipostatic mechanisms and feeding. In: Novin, D.; Wyrwicka, W.; Bray, G., eds. Hunger: Basic mechanisms and clinical implications. New York: Raven Press; 1976. 18. Le Magnen, J.; Devos, M.; Larue-Achagiotis, C. Food deprivation induced parellel changes in blood glucose, plasma free fatty acids and feeding during the two parts of the diurnal cycle in rats. Neurosci. Biobehav. Rev. 4:17-23; 1980. 19. Louis Sylvestre, J.; Le Magnen, J. Palatability and preabsorptive insulin release. Neurosci. Biobehav. Rev. 4:43-46; 1980.
20. Luiten, P. G. A.; Room, P. Interrelation between lateral, dorsomedial, and ventromedial hypothalamic nuclei in the rat: An HRP study. Brain Res. 190:321-332; 1980. 21. Moscowitz, H. R.; Kumaraiah, V.; Sharma, K. N.; Jacobs, H. L.; Dua Sharma, S. Effect of hunger, satiety and glucose load upon taste intensity and taste hedonics. Physiol. Behav. 16:471-475; 1976. 22. Nicolaidis, S. Early systemic responses to orogastric stimulation in the regulation of food and water balance: Functional and electrophysiological data. Ann. NY Acad. Sci. 157:1176-1203; 1969. 23. Nicolaidis, S. Sensory neuroendocrine reflexes and their anticilpatory optimizing role on metabolism. In: Kare, M. R.; Muller, O., eds. The chemical senses and nutrition. New York: Academic Press; 1977: 123-143. 24. Norgren, R. Taste pathways to hypothalamus and amygdala. J. Comp. Neurol. 166:17-30; 1976. 25. Oomura, Y.; Ooyama, H.; Yamamoto, Y.; Naka, F. Reciprocal relationship of the lateral and ventromedial hypothalamus in the regulation of food intake. Physiol. Behav. 2:97-115; 1967. 26. Oomura, Y. Central mechanisms of feeding. In: Advances in biophysics. Tokyo: Tokyo University Press; 1973:65-142. 27. Oomura, Y.; Ooyama, H.; Sugimori, M.; Nakamura, T.; Yamada, Y. Glucose inhibition of the gluco sensitive neurons in the lateral hypothalamus. Nature 247:284-286; 1974. 28. Oomura, Y. Input-output organisation in the hypothalamus relating to food intake behaviour. In: Morgane, P. J.; Panksepp, J., eds. Handbook of hypothalamus. 2. Physiology of Hypothalamus. New York: Marcel Dekker; 1980:557-620. 29. Ono, T.; Sasaki, K.; Nakamura, K.; Norgren, R. Integrated lateral hypothalamic neural responses to natural and artificial rewards and cue signals in the rat. Brain Res. 327:303-306; 1985. 30. Ono, T.; Nakamura, K.; Nishijo, H.; Fukuda, M. Hypothalamic neuron involvement in integration of reward aversion and cue signals. J. Neurophysiol. 56:63-79; 1986. 31. Ono, T.; Nakamura, K. Learning and integration of rewarding and aversive stimuli in the rat lateral hypothalamus. Brain Res. 346: 368-373; 1985. 32. Rao, B. S.; Sharma, K. N. Electrophysiological correlates of diurnal rhythm disruption in rats. Ind. J. Physiol. Pharmacol. 25:308-318; 1981. 33. Rao, B. S. Effects of dynamic and static body weight loss on taste responses. Ind. J. Physiol. Pharmacol. 27:25-31; 1983. 34. Richter, C. P. Salt taste thresholds of normal and adrenalectomised rats. Endocrinology 24:367-371; 1939. 35. Rozin, P. Thiamine specific hunger. In: Code, C. F., ed. Handbook of physiology, see. 6. Alimentary canal. Washington, 19(?:American Physiological Society; 1967. 36. Rolls, E. T. Activity of hypothalamic and related neurons in the alert animal. In: Morgane, P. J.; Panksepp, J., eds. Handbook of hypothalamus, vol. 3. Behavioural studies, part A. New York: Marcel Dekker; 1980:439-466. 37. Schwartzbaum, J. S. Electrophysiology of taste feeding and reward in lateral hypothalamus of rabbit. Physiol. Behav. 44:507-526; 1988.