Evidence for a cephalic site of action of high magnetic fields on the behavioral responses of rats

Static high magnetic fields (MFs) from 7 T to 9 T can elicit behavioral responses in rodents such as suppression of rearing, locomotor circling, and acquisition of a conditioned taste aversion (CTA). MF exposure also induces c-Fos expression in the visceral and vestibular nuclei of the brainstem, suggesting the stimulation of some sensory pathways. It is not clear, however, if the effects of the MF are caused by exposure to the uniform maximal field at the center of the magnet, or by exposure to the steep field gradients along the bore of the magnet during the rat's placement. In addition, the site of action within the rat is unknown. In an attempt to limit MF exposure to rostral or caudal portions of the rats' body, we exposed male and female rats at different positions within the bore of a 14.1-T superconducting magnet ranging from 2 cm (1.6 T at the head) to 155 cm (0.05 T at the head), with the center of the bore at 65 cm (14.1 T across the whole body). This approach also allowed us to expose rats to the maximal field strength (14.1 T) vs. the maximal field gradients (54 T/m). To assess both immediate and delayed behavioral effects, locomotor and CTA responses were recorded. A small but significant CTA was seen after exposure of the head to the lowest MF tested (0.05 T at 155 cm). Graded effects were seen, however, with greater circling and CTA acquisition as the MF strength increased at the rostral end of the rat. This suggests a cephalic site of action. Furthermore, maximal circling and CTA were induced after exposure to the uniform center field, and not after exposure to high field gradients on either side of the center. This suggests that the behavioral responses seen after MF exposure are a consequence of the uniform static field at the center of the magnet, and are not caused by passage through, or exposure to, the vertical field gradients. Female rats responded similarly to male rats, although magnet-induced CTA appeared resistant to extinction in female rats.     

N(omega)-nitro-L-arginine methyl ester attenuates lithium-induced c-Fos, but not conditioned taste aversion, in rats

Lithium chloride (LiCl) at doses sufficient to induce conditioned taste aversion (CTA) causes c-Fos expression in the relevant brain regions and activates the hypothalamic-pituitary-adrenal (HPA) axis. It has been suggested that nitric oxide (NO) in the central nervous system may play a role not only in the activation of HPA axis but also in CTA learning, and that LiCl may activate the brain NO system. To determine the role of NO in lithium-induced CTA, we examined the lithium-induced CTA, brain c-Fos expression, and plasma corticosterone level with Nomega-nitro-L-arginine methyl ester (L-NAME) pretreatment. Intraperitoneal L-NAME (30 mg/kg) given 30 min prior to LiCl significantly decreased lithium-induced c-Fos expression in the brain regions implicated in CTA learning, such as the hypothalamic paraventricular nucleus (PVN), central nucleus of amygdala (CeA), and nucleus tractus of solitarius. However, either the lithium-induced CTA acquisition or the increase in plasma corticosterone was not attenuated by l-NAME pretreatment. These results suggest that NO may be involved in lithium-induced neuronal activation of the brain regions, but not in the CTA acquisition or the HPA axis activation.     

Recall of a previously acquired conditioned taste aversion in rats following lesions of the area postrema

A conditioned taste aversion was produced by pairing a novel sucrose solution with either 3 mEq lithium chloride or with 100 rad gamma radiation in rats with the area postrema intact. Lesions of the area postrema were then made in half of the rats exposed to each treatment and in rats that were not treated with the unconditioned stimulus. When tested for a conditioned taste aversion, all treated rats showed a significant aversion to the sucrose solution compared to the untreated control rats. There were no significant differences between rats with area postrema lesions and those with the area postrema intact, indicating that the lesions had no effect on the recall of the previously acquired aversion. The results are interpreted as being consistent with the hypothesis that the role of the area postrema in taste aversion learning is to monitor blood and cerebrospinal fluid for potential toxins and to transmit that information to the central nervous system.     

Algebraic summation of the affective properties of a rewarding and an aversive stimulus in the rat

The presentation of a novel flavor followed by an injection of lithium chloride, produced conditioned taste aversions that were attenuated or blocked by rewarding brain stimulation (BSR) at low and high current intensities, respectively. Conversely, flavor/BSR pairings produced conditioned taste preferences that were attenuated in a dose-dependent manner by lithium chloride. These data support the contention of Young (20–23) for the algebraic summation of the positive and negative properties of affective stimuli. The implications of this work for the measurement of brain-stimulation reward and the aversive effects of drugs are also discussed.     

Disruption of conditioned taste aversion by the combined effects of LiCl and ECS

Rats were taught a conditioned taste aversion by allowing them to drink sucrose (CS) for 5 min and 30 min later poisoning them with LiCl (UCS). Experimental animals were given ECS (80 mA for 250 msec) at 0, 15, 20, 25, 27.5, 30, 32.5, 35, 40, or 45 min after the CS. Only animals given ECS both within the CS-UCS interval and in close temporal proximity to the UCS (within 5 min) showed a significant, although limited, disruption of learning. At least two explanations are possible. The first is that apart from its toxicity, LiCl may also possess amnesic properties which interact with those of the ECS. Alternatively, ECS may have impaired the ability of the animals to fully experience the lithium-induced illness.     

Interactions of temperature and taste in conditioned aversions

The influence of temperature on taste cues and the ability to discriminate and learn about different temperatures of foods are important factors regulating ingestion. The goal of this research was to demonstrate that thermal orosensory input can serve as a salient stimulus to guide ingestive behavior in the rat, and also that it interacts with gustatory input during choice and conditioned aversion experiments. A novel apparatus with Peltier refrigerators was used to control the temperature of solutions in 10-min, 2-bottle tests. It was determined that naive rats preferred cold water (10 °C) to warm water (40°). When cold water was paired with a toxic LiCl injection, rats avoided cold water and drank warm water, thus demonstrating that cold water could serve as the conditioned stimulus in a conditioned temperature aversion. Rats conditioned against cold water could discriminate 10 °C water from 16 °C water, but not from 13 °C water, thus showing an ability to discriminate orosensory thermal cues to within 3-6 °C. Rats also generalized conditioned aversions from cold water to cold saccharin and cold sucrose solutions. However, if rats were conditioned against a compound taste and thermal stimulus (10 °C, 0.125% saccharin), the rats could distinguish and avoid each component individually, i.e., by avoiding cold water or warm saccharin. Finally, daily 2-bottle extinction tests were used to assess the strength of aversions conditioned against a taste cue (0.25 M sucrose), a thermal cue (10 °C water), or the combination. Aversions to taste or temperature alone persisted for 7-14 days of extinction testing, but the combined taste-temperature aversion was stronger and did not extinguish after 20 days of extinction testing. These results demonstrate that temperature can serve as a salient cue in conditioned aversions that affect ingestion independent of taste cues or by potentiating taste cues.     

Labyrinthectomy abolishes the behavioral and neural response of rats to a high-strength static magnetic field

Vertigo is a commonly-reported side effect of exposure to the high magnetic fields found in magnetic resonance imaging machines. Although it has been hypothesized that high magnetic fields interact with the vestibular apparatus of the inner ear, there has been no direct evidence establishing its role in magnet-induced vertigo. Our laboratory has shown that following exposure to high magnetic fields, rats walk in circles, acquire a conditioned taste aversion (CTA), and express c-Fos in vestibular and visceral relays of the brainstem, consistent with vestibular stimulation and vertigo or motion sickness. To determine if the inner ear is required for these effects, rats were chemically labyrinthectomized with sodium arsanilate and tested for locomotor circling, CTA acquisition, and c-Fos induction after exposure within a 14.1 T magnet. Intact rats circled counterclockwise after 30-min exposure to 14.1 T, but labyrinthectomized rats showed no increase in circling after magnetic field exposure. After 3 pairings of 0.125% saccharin with 30-min exposure at 14.1 T, intact rats acquired a profound CTA that persisted for 14 days of extinction testing; labyrinthectomized rats, however, did not acquire a CTA and showed a high preference for saccharin similar to sham-exposed rats. Finally, significant c-Fos was induced in the brainstem of intact rats by 30-min exposure to 14.1 T, but magnetic field exposure did not elevate c-Fos in labyrinthectomized rats above sham-exposed levels. These results demonstrate that an intact inner ear is necessary for all the observed effects of exposure to high magnetic fields in rats.     

Functional relations between the cortical gustatory area and the amygdala: Electrophysiological and behavioral studies in rats

Summary The functional interconnections between the cortical gustatory area (CGA) and the amygdala were examined by electrophysiological and behavioral experiments in rats. The cortical neurons responsive to taste stimuli applied to the anterior part of the tongue were located in the Vth layer of the agranular insular cortex. Of a total of 27 cortical neurons recorded, 10 showed facilitatory and/or inhibitory responses with the mean onset latency of about 20 msec to electrical stimulation of the ipsilateral amygdala. On the other hand, of 18 amygdaloid neurons responsive to taste stimuli, 13 showed facilitatory and/or inhibitory responses to electrical shocks to the ipsilateral CGA, with a mean latency of about 16 ms. No cortical and amygdaloid neurons sampled responded antidromically to the electrical stimulation. These results suggest the existence of mutual polysynaptic fiber connections between the CGA and the amygdala. The behavioral experiment was performed by means of a conditioned taste aversion (CTA) technique. After acquisition of CTA to sucrose solution by pairing it with an i.p. injection of LiCl which produces sickness, bilateral small knife cuts between the CGA and the amygdala in the perirhinal region disrupted retention of CTA. Thus, these interconnections may play some role in association of taste-related cognitive processes with feeding behavior.     

Conditioned taste aversion suppresses induction of delayed-type hypersensitivity immune reactions

Conditioned taste aversion was induced in mice by pairing saccharin drinking with an intraperitoneal injection of lithium chloride, a toxic but nonimmunosuppressive drug. Conditioned mice showed not only suppressed saccharin drinking but also a 75% reduction in the induction of delayed-type hypersensitivity immune responses to low doses of sheep erythrocytes. This effect was observed with doses of lithium chloride which had no effect of their own on immune functions. In addition, a reduction in water consumption was not responsible for the reduced immune response of conditioned mice since the immune responses of water deprived mice did not differ from those of nondeprived mice. Conditioned mice exposed to saccharin had higher plasma levels of glucocorticoids than nonconditioned mice, suggesting that the experience of being reexposed to a taste paired with lithium chloride was perceived as aversive. These data demonstrate that alterations in immune functions can be induced by a conditioned taste aversion procedure independently of any immunosuppressive drug.     

The antiaggressive effect of lithium is abolished by area postrema lesion

Lithium (Li) suppresses intraspecific aggression in rats and other mammals. However, antiaggressive doses of Li have been shown to produce signs of illness and to serve as an unconditioned stimulus for conditioned taste aversions (CTA). Therefore, it is possible that the antiaggressive effects of Li result from Li-induced malaise. Malaise and CTA's produced by some drugs and toxins are mediated by chemoreceptors in the area postrema. In this paper we report [1] that thermal lesion of the area postrema abolishes Li's ability to condition taste aversions and [2] that area postrema lesion abolishes the antiaggressive effect of Li as measured by attack latency in a shock-induced aggression test. Taken together, these findings suggest that some or all of Li's antiaggressive action in rats may result from malaise attributable to Li's action on the area postrema.     

Estrogen increases the taste threshold for sucrose in rats

Anecdotal and empirical evidence suggests that females' preferences for sweet foods are affected by hormonal fluctuations across the reproductive cycle. In rats, the preference for sweet foods may involve estrogen-mediated changes in response to the taste of sweets. Our recent work showed that ovariectomized female rats lick less to dilute sucrose solutions when given estrogen than when given the oil vehicle. These findings suggest that estrogen decreases the preference for less concentrated sucrose solutions; however, an alternative explanation is that estrogen interferes with the ability to detect dilute sucrose solutions. To distinguish between these possibilities, we conditioned a taste aversion to 0.2 M sucrose in ovariectomized rats by pairing it with injection of LiCl and then examined the generalization of that taste aversion to 0.075 and 0.025 M sucrose solutions during estrogen or oil treatment. Oil-treated rats generalized the LiCl-induced aversion conditioned to 0.2 M sucrose to both 0.075 and 0.025 M sucrose. Estrogen-treated rats generalized the LiCl-induced taste aversion to 0.075 M sucrose but not to 0.025 M sucrose. Moreover, two weeks later, when estrogen had cleared the system, both groups generalized the aversion to both 0.075 and 0.025 M sucrose. These results show that estrogen affects the ability to discriminate dilute sucrose from water and suggest that estrogen may have short-term effects on the detection threshold for sucrose taste in rats.     

Effect of medial septal lesions on conditioned taste aversion in the rat.

Rats that had sustained lesions of the medial septum developed a conditioned food aversion to sweetened condensed milk at the same rate as controls. This aversion was produced by post-ingestion administration of lithium chloride in ascending doses given on alternate days. Although development of the aversion was the same in lesioned animals as in controls, recovery from it was more rapid in the lesioned animals.     

Conditioned aversion to amino acid flavors in the catfish, Ictalurus punctatus

Naive catfish readily accepted foof flavored with L-cysteine but generally avoided foods flavored with other amino acids. The aversions to these latter amino acids quickly disappeared after the fish had feeding experience with each. When illness induced by lithium chloride was paired with an amino acid, that flavor strongly avoided. Amino acids structurally similar to that were also avoided while other amino acid flavors were readily consumed. My studies on anosmic fish indicate that olfaction does not play a role in conditioning amino acid taste aversions. Observations of the sequential units of the feeding response suggest that sensory input to the vagal lobe of the brain is primarily responsible for taste aversion discrimination.     

High doses of vasopressin delay the onset of extinction and strengthen acquisition of LiCl-induced conditioned taste avoidance

When low doses of vasopressin are given 50 min after pairing sucrose consumption with a high dose of LiCl, extinction of the LiCl-induced conditioned taste avoidance is accelerated. These low doses of vasopressin do not themselves induce conditioned taste avoidance when paired with sucrose consumption. Predicated on previous studies administering two avoidance-inducing agents after sucrose consumption, studies were designed to determine whether high doses of vasopressin capable of inducing conditioned taste avoidance would (1) delay rather than accelerate extinction of a conditioned taste avoidance induced by a high dose of LiCl and (2) strengthen acquisition of a conditioned taste avoidance induced by a low dose of LiCl. The results of three studies showed that doses of 9 and 18 microg/kg of vasopressin induced a conditioned taste avoidance when injected 50 min after sucrose consumption, delayed the onset of extinction when injected 50 min after pairing sucrose consumption with a high dose of LiCl, and strengthened acquisition of a conditioned taste avoidance when injected 50 min after pairing sucrose consumption with a low dose of LiCl. Taken together, these data suggest that the delay in onset of extinction is due to a strengthening of acquisition. It has been suggested that vasopressin is a mnemonic neuropeptide that delays extinction of learned tasks. However, for conditioned taste avoidance, the evidence for the effects of low doses of vasopressin on extinction do not support this hypothesis and the evidence for high doses of vasopressin can be accounted for by the avoidance-inducing properties of vasopressin.     

The pituitary-adrenal response to “minimized” schedule-induced drinking

Previous research has indicated that hungry rats engaging in polydipsic drinking while on an intermittent food-delivery schedule exhibit a significant drop in plasma corticosterone levels during a 30-min experimental session; rats that do not have water available during sessions maintain their presession corticosterone levels, however. This effect of schedule-induced drinking has been interpreted as reflecting primarily psychological factors rather than peripheral physiological factors. This interpretation was supported in the present experiments. In Experiment 1, corticosterone levels in polydipsic rats were determined at 0, 10, 20 and 30 min during an intermittent-feeding session in order to assess the time course of the effect. In Experiment 2, fluid consumption by polydipsic rats during a 30-min session was reduced by presenting a saccharine solution previously paired with illness in a conditioned taste aversion procedure. The results of both experiments indicate that significant reductions in corticosterone from presession levels are found even when the amount of fluid consumed during a session is relatively small.     

Failure of ACTH to prolong extinction of a conditioned taste aversion in the absence of the testes

Both corticotropin (ACTH) and testosterone prolong the extinction of a conditioned taste aversion in water-deprived intact male rats. An investigation was made to determine whether ACTH affects extinction in the absence of the testes and also to determine the effect of ACTH on serum testosterone levels. Water-deprived intact males showed prolonged extinction after ACTH injections; water-deprived gonadectomized males and intact females did not. All three of these groups showed elevated testosterone levels after ACTH administration, but testosterone levels were higher in the intact males than in the gonadectomized males or intact females. These results clearly show that in the absence of the testes ACTH is unable to prolong extinction. It is proposed that the increased level of testosterone following ACTH injection in water-deprived intact males is responsible for the prolonged extinction of a conditioned taste aversion. Although testosterone levels may increase in females and castrated males following ACTH injection, the increase is not sufficient to prolong extinction in these water-deprived animals.     

Investigations into the nature of the dexamethasone and ACTH effects upon learned taste aversion

Dexamethasone injected prior to the conditioning trial will attenuate a learned taste aversion. ACTH injections will prolong the recovery from a taste aversion when administered prior to recovery trials. Two experiments are reported here. In the first, the interaction between these effects was studied. Using a factorial design, four groups of rats received dexamethasone or ACTH prior to conditioning and prior to recovery trials. An additional group received isotonic saline throughout training and recovery. The primary finding was that animals receiving dexamethasone prior to conditioning and ACTH prior to recovery trials did not differ from the group pretreated with ACTH during both conditioning and recovery. That is, given the parameters of these experiments, the ACTH effect was prepotent. In the second experiment it was shown that the ACTH effect upon the performance of a taste aversion can occur in one trial. Taken together, the results of the two experiments support the idea that ACTH influences memory retrieval when administered prior to a test for retention of an avoidance response.     

Differential effect of paradoxical sleep deprivation on acquisition and retrieval of conditioned taste aversion in rats.

Paradoxical sleep deprivation (PSD) was used to interfere with acquisition or retrieval of conditioned taste aversion (CTA). PSD was achieved by confining rats to small pedestals placed on an electrified grid floor. Fifteen-min access to 0.1% sodium saccharin (conditioned stimulus-CS) by water deprived rats was followed 30–120 min later by intraperitoneal injection of lithium chloride (unconditioned stimulus-US, 0.15 M, 2% to 4% body weight). Retention was tested 1 to 5 days later by offering the animal saccharin again. A 24-hr PSD preceding saccharin drinking prevented CTA acquisition. CTA disruption was diminished by a 2-hr, and completely abolished by an 8-hr interval of home cage recovery inserted between the 24-hr PSD and saccharin presentation. CTA was slightly facilitated by 2-hr PSD in the CS-US interval. The 24-hr PSD preceding CS caused the same CTA disruption when followed by free sleep opportunity or by continued PSD in the 2-hr CS-US interval. Twenty-four-hr PSD preceding retention testing slightly improved retrieval of well established CTA. It is concluded that PSD interferes with the formation of the short-term gustatory trace of CTA but does not affect retrieval of CTA engrams. Gradual compensation for the PSD effect on CTA learning by pre-acquisition sleep suggests that the processes responsible for CTA disruption and recovery correspond to depletion and repletion of the same neurotransmitter stores.     

Conditioned reflex responses of the sympathetico-adrenal system to an aversive taste stimulus

Studies were carried out on male and female rats in which the effects of isolated presentation of a conditioned stimulus (a saccharine solution) to which the animals had previously developed conditioned reflex taste aversion (RTA) on the level of urinary catecholamine secretion were determined. The studies showed that presentation of an aversive taste stimulus without reinforcement by a negative stimulus increased the levels of urinary adrenaline, noradrenaline, and dopamine secretion; this repeated, albeit more weakly, the effects of the negative reinforcement (angular acceleration) used for development of RTA. After presentation of the isolated aversive taste stimulus, the greatest increase in catecholamine excretion affected adrenaline, which indicates an anxiety state (fear). There was also a significant increase in noradrenaline excretion in these conditions. The accompanying increase in dopamine excretion in experimental and control animals showed this change to be largely nonspecific in nature, and to result from the experimental procedures. Isolated presentation of the conditioned taste stimulus elicited significantly greater increases in urinary catecholamine excretion in males than in females. It is suggested that the time for which the RTA is retained could be increased by activation of the sympathetico-adrenal system resulting from presentation of the nonreinforced taste stimulus which had acquired aversive properties. Laboratory for the Ontogenesis of Higher Nervous Activity, I. P. Pavlov Institute of Physiology, Russian Academy of Sciences, 199034 St. Petersburg. Translated from Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 82, No. 3, pp. 57–65, March, 1996.     

Rapid extinction of a conditioned taste aversion following unreinforced intraperitoneal injection of the fluid CS

After drinking a novel, saccharin-cyclamate solution, two groups of rats were poisoned with LiCl injection in order to establish a conditioned taste aversion, while a control group was not poisoned. Eighteen hours later, one of the poisoned groups of rats received differential unreinforced exposure to the sweet solution via an intraperitoneal injection. The group injected with the concentrated form of the saccharin-cyclamate solution showed subsequent rapid extinction of the conditioned taste aversion. An analogy was made to the technique of flooding (response prevention) used to hasten extinction of active, shock-motivated avoidance behavior.