Electrophysiological analysis of retrieval of conditioned taste aversion. Physiological techniques and data processing.

New techniques suitable for electrophysiological investigation of retrieval of conditioned taste aversion (CTA) were developed. The gustatory discrimination apparatus consisted of two parallel drinking spouts (20 mm apart) equipped with photoelectric lick sensors. The spouts contained water or the aversive fluid (0.15 M LiCl), respectively, and their position could be rapidly (50 ms) interchanged with a reversive electromotor. Licking at either spout and the position of the fluids were recorded on one channel of a tape recorder. Unit activity was picked up with tungsten microelectrodes (40 micrometers) inserted into the brain of a freely moving rat with a head-mounted microdrive system (weight 2 g), fixed to an implanted guiding cannula. The electrode was connected through a head-carried FET signal follower to a wide band integrated circuit amplifier and the unit activity was recorded in the other channel of the tape recorder. The records were evaluated using an off-line computer program (LINC 8) consisting of a spike detection subroutine followed by amplitude histogram analysis. The mean (M) and SD values were computed for uni- or bimodal amplitude distributions of the principal spike components. Spike falling within the M+/-2 SD range of the selected parameter were identified as single units and used for construction of post stimulus histograms triggered by licking or by spout switching.     

Brain regions responsible for the expression of conditioned taste aversion in rats

Conditioned taste aversion (CTA) is acquired when the ingestion of a food is followed by malaise. CTA is a kind of fear learning making animals avoid subsequent intake of the food and show aversive behavior to the taste of the food. To elucidate the brain regions responsible for the expression of CTA, our previous electrophysiological and recent c-fos immunohistochemical studies have been reviewed. Among a variety of brain regions including the parabrachial nucleus, amygdala, insular cortex, supramammillary nucleus, nucleus accumbens, and ventral pallidum that are involved in different phases of CTA expression, the enhanced taste sensitivity to facilitate detection of the conditioned stimulus may originate in the central nucleus of the amygdala and the hedonic shift, from positive to negative, may originate in the basolateral nucleus of the amygdala.     

Some critical factors involved in formation of conditioned taste aversion to sodium chloride in rats

Some factors concerning acquisition and retention of conditionedtaste aversions (CTAs) were behaviorally examined in the rat.In the CTA paradigm, aqueous solution of 0.1 M NaCl was usedas the conditioned stimulus (CS) and an intraperitoneal (i.p.)injection of 0.15 M LiCl was employed as the unconditioned stimulus(US). In experiment 1, CTAs to 0.1 M NaCl were examined in bothforward (CS–US) and backward (US–CS) conditioningparadigms. Reliable CTAs were produced in the US–CS conditioningparadigm when the US–CS interval was less than 10 min,as well as in the CS–US conditioning paradigm. In experiment2, strong CTAs to 0.1 M NaCl were established when water-deprivedrats made at least 500 continuous licks, corresponding to 2.5ml intake and 2 min of drinking time. In experiment 3, effectsof gustatory deafferentation on CTA formation were studied.Only the chorda tympani played an important role in acquisitionand retention of CTAs to NaCl solutions. These results ipHimMthat strong CTAs can be acquired to 0.1 M NaCl, if its tasteinformation which is conveyed via the chorda tympani duringthe 500 continuous licks is followed by LiCl-induced sickness.     

Gamma-L-glutamyl-taurine (Litoralon) affects conditioned taste aversion in rats

The dipeptide gamma-L-glutamyl-taurine (Litoralon) reduced neophobia of rats at a dose of 5.0 mg/kg (i.p.) in a "one-bottle forced choice paradigm" for conditioned taste aversion (CTA), but did not significantly affect the rats' "memory" of intoxication following chronic treatment at doses of 0.05, 0.50 and 5.00 mg/kg (i.p.). Acute treatment with Litoralon (10-1000 micrograms/kg, i.p.) did not affect CTA checked in a "two-bottle test", when administered immediately following the unconditioned stimulus (LiCl injection). In contrast, when given 90 min prior to the retention test, the injection of Litoralon (50.0 micrograms/kg) and gamma-aminobutyryl ethanolamine phosphate (100 and 500 micrograms/kg) resulted in a significantly higher intake of saccharin solution by the rats. This effect is comparable to the action of diazepam tested in the same experimental procedure. The results support our hypothesis about the anti-conflict potencies of these dipeptides, exerted by reducing aversion of phobia and/or the anxiety level of the animals in the experimental situation.     

Genetic differences in nicotine-induced conditioned taste aversion

Genetic differences in nicotine-induced conditioned taste aversion were examined using inbred mice. Adult male C57BL/6J, DBA/2J, BALB/cJ and C3H/heJ mice were adapted to a 2-h per day water access regimen. Subsequently, mice received nicotine injections (0.5, 1.0 or 2.0 mg/kg) immediately after 1-h access to a NaCl flavored solution. DBA and C3H mice developed dosedependent aversions to the nicotine-paired flavor. BALB mice showed only minor reductions in intake with no difference between the nicotine dose groups. C57BL mice did not show development of nicotine-induced conditioned taste aversion. These results demonstrate that nicotine's aversive motivational effect is strongly influenced by genotype. Further, genetic sensitivity (DBA mice) or insensitivity (C57BL mice) to nicotine-induced conditioned taste aversion was similar to reports of genetic sensitivity to ethanol's aversive effect measured in this design.     

Genetic differences in ethanol-induced hyperglycemia and conditioned taste aversion.

Genetic differences in the hyperglycemic response to acute ethanol exposure and ethanol-induced conditioned taste aversion were examined using inbred mice. Adult male C57BL/6J and DBA/2J mice were injected with ethanol (0-6 g/kg, I.P.) and blood glucose levels determined over 4 h. C57 mice demonstrated greater dose-dependent elevations in blood glucose compared to DBA mice. In a conditioned taste aversion procedure, water deprived mice received ethanol injections (1-4 g/kg, I.P.) immediately after access to a NaCl flavored solution. DBA mice developed aversion to the ethanol-paired flavor at a lower dose (2 g/kg) than C57 mice. These results provide further support for a possible inverse genetic relationship between sensitivity to ethanol-induced hyperglycemia and sensitivity to conditioned taste aversion.     

Acquisition and retrieval of conditioned taste aversion is impaired by brain damage caused by two hours of pilocarpine-induced status epilepticus

The effect of Cavalheiro's pilocarpine model of epileptogenesis upon conditioned taste aversion (CTA), an important example of nondeclarative memory, was studied in adult Long Evans rats. Deterioration of CTA was studied during the silent period between pilocarpine-induced status epilepticus (SE) and delayed spontaneous recurrent seizures. SE was elicited by i.p. injection of pilocarpine (320 mg/kg ) and interrupted after 2 hours by clonazepame (1 mg/kg i.p.). Peripheral cholinergic symptoms were suppressed by methylscopolamine (1 mg/kg i.p.), administered together with pilocarpine. CTA was formed against the salty taste of isotonic LiCl. In the experiment of CTA acquisition, the CTA was formed and tested during the silent period after SE. In the experiment of CTA retrieval, the CTA was acquired before SE and the retrieval itself was tested during the silent period. Retrieval of CTA acquired before SE was impaired more than the retrieval of CTA formed during the silent period. Our findings indicate that epileptic seizures can disrupt even non-declarative memory but that CTA formed by the damaged brain can use its better preserved parts for memory trace formation. Ketamine (50 mg/kg i.p.) applied 2 min after the onset of pilocarpine-induced status epilepticus protected memory deterioration.     

Monosodium glutamate and sweet taste: generalization of conditioned taste aversion between glutamate and sweet stimuli in rats

Even though monosodium glutamate (MSG) is a prototypical umami substance, previous studies reported that a conditioned taste aversion (CTA) to MSG, mixed with amiloride to block the taste of sodium, generalizes to sucrose. These findings suggest that the taste of glutamate mimics the taste of sucrose and raise the question of whether glutamate has a broadly tuned sweet taste component. To test this hypothesis, CTA experiments were conducted to test for generalization between MSG and several sweet stimuli: sucrose, glucose, maltose, saccharin and SC-45647. Strong bidirectional generalization was seen between MSG mixed with amiloride and sucrose, glucose, saccharin and SC-45647. Weak generalization was seen between MSG and maltose, and sucrose and maltose. None of the CTAs generalized to NMDA. These findings support the hypothesis that the taste of MSG has broadly tuned, sweet-like characteristics, possibly due to the convergence of afferent signals for MSG, natural sugars and artificial sweeteners.     

咸味觉厌恶模型大鼠鼓索神经对味觉刺激的电生理反应特性

目的:探索大鼠咸味觉厌恶建立后外周鼓索神经(CT)对咸味觉及其他味觉刺激的电生理反应特性的改变。方法:将14只SD成年雄性大鼠分为咸味觉厌恶模型组(CTA)和对照组(n=7/group)。实验第1日给予大鼠30min的0.1mol/LNaCl饮食,随后CTA组和对照组大鼠分别腹腔注射2ml0.15mol/LLiCl和同等量生理盐水。在第2、3和4日,测量两组大鼠每天30min内对NaCl和蒸馏水饮用量。于第4日行为学测试后,分别记录CTA组大鼠和对照组大鼠CT对口内给予系列浓度NaCl溶液、0.3mol/LNaCl与0.1mmol/L阿米洛利(一种舌上皮钠通道阻断剂)混合液和其他四种基本味觉刺激溶液的电生理反应。结果:与对照组相比,CTA组大鼠CT对系列浓度NaCl和其他4种基本味觉刺激的电生理反应特性没有发生明显变化(P>0.05);舌上皮钠通道阻断剂阿米洛利强烈抑制CTA大鼠对NaCl的反应(P<0.01)。结论:条件性咸味觉厌恶模型大鼠CT对各种味觉刺激的电生理反应特性没有发生明显改变。     

Dose-dependent effects of peptide YY(3-36) on conditioned taste aversion in rats

We used a conditioned taste aversion test to assess whether PYY(3-36) reduces food intake by producing malaise. Two-hour IV infusion of PYY(3-36) (8, 15, and 30 pmol/kg/min) at dark onset in non-food-deprived rats produced a dose-dependent inhibition of feeding and a conditioned aversion to the flavored chow paired with PYY(3-36) infusion. In food-deprived rats, PYY(3-36) at 2 and 4 pmol/kg/min inhibited intake of a flavored saccharin solution without producing conditioned taste aversion, whereas higher doses (8 and 15 pmol/kg/min) inhibited saccharin intake and produced taste aversion. These results suggest that anorexic doses of PYY(3-36) may produce a dose-dependent malaise in rats, which is similar to that reported for PYY(3-36) infusion in humans. Previous studies have shown that PYY(3-36) potently inhibits gastric emptying, and that gut distention can produce a conditioned taste aversion. Thus, PYY(3-36) may produce conditioned taste aversion in part by slowing gastric emptying.     

Stimulus specificity in the acquisition and extinction of conditioned taste aversion

An experiment evaluated whether the acquisition and extinction of conditioned taste aversion in the rat is stimulus-specific by testing the degree of response transfer between sweet and salty tastes. Animals in the paired-same and paired-different groups received a presentation of a gustatory CS and a cyclophosphamide injection US. Nonconditioned control groups received unpaired CS /US presentations or the CS followed by a vehicle injection. Taste avoidance was evaluated in three nonreinforced test sessions. In the paired-same, unpaired and vehicle groups, all test sessions were conducted with the same flavor as originally used in training, whereas the paired-different group was tested with a novel flavor on the first and second sessions and with the originally trained flavor in last session. Stimulus specific acquisition was apparent in the first test session, when the animals in the group paired-same exhibited lower fluid intake than the other three groups. Evidence of specificity of extinction was apparent in the last test session, when animals in the group paired-different exhibited lower fluid intake than the other three groups. These results provide further evidence of stimulus specificity in acquisition and extinction of conditioned taste aversion, supporting the associative interpretation of these phenomena.     

Parabrachial unit activities after the acquisition of conditioned taste aversion to a non-preferred HCl solution in rats

Abstract In a behavioral experiment, rats reliably acquired a taste aversion to non-preferred 0.01 M HCl that had been previously paired with intraperitoneal injection of 0.15 M LiCl. These rats showed aversions to other acidic solutions such as malic acid and tartaric acid. In a neurophysiological experiment, the neuronal activities of the parabrachial nucleus (PBN) were recorded after the acquisition of conditioned taste aversion (CTA) to 0.01 M HCl in urethane-anesthetized rats. Neuronal responses to the conditioned stimulus (CS) did not change on the whole but decreased in the dorsal region to the brachium conjunctivum. The proportion of HCl-best to NaCl-best units was lower in the CTA group than in controls. The spontaneous firing rate was lower in the CTA group than in controls. Correlation coefficients between the HCl CS and normally preferred tastes (sucrose and NaCl) were more negative and those between HCl and quinine were more positive in the CTA group than in the controls. These results may be explained by the notion that gustatory responses of PBN neurons are concerned with alterations in taste hedonics after the acquisition of conditioned taste aversions.     

Tests of adipsia and conditioned taste aversion following the intrahypothalamic injection of amylin.

Intrahypothalamic injection of amylin (AMY) was shown to reduce the intake of rat chow and water for 8 and 4 h, respectively, in schedule-fed rats. Amylin also reduced water intake to a much lesser degree in 24-h water-deprived rats. A test of the ability of AMY to form a conditioned taste aversion yielded no change in saccharin preference, as compared to controls treated with vehicle. These results suggest that although AMY has adipsic effects, the reduction in water is not of sufficient magnitude to cause the anorexia. In addition, the failure of AMY to support a conditioned taste aversion suggests that AMY does not cause anorexia by inducing malaise. Therefore, in addition to other metabolic effects, AMY may be involved in the control of food and water intake.