Latent inhibition of a conditioned taste aversion in fetal rats.

The etiology of schizophrenia's cognitive symptoms may have its basis in prenatal alterations of glutamate N-methyl-D-aspartate (NMDA) receptor functioning. Therefore, the current study investigated the effects of ketamine (an NMDA receptor blocking drug) on both a conditioned taste aversion (CTA) and latent inhibition (LI; a model of attentional capacity) in rat fetuses. We first sought to determine if a CTA could be diminished by nonreinforced preexposure to a CS in fetal rats (i.e., LI). We injected E18 pregnant Sprague-Dawley rats with 100% allicin (garlic taste) or an equal volume of saline. Some of the pregnant dams also received ketamine (100 mg/kg, i.p.). One day later (E19), the dams received a second injection of the CS, followed by either lithium chloride (the US) or saline. Finally, on E21 pups received oral lavage with allicin and observations of ingestive orofacial motor responses were recorded. When allicin had been paired with LiCl in utero, E21 fetuses exhibited a conditioned suppression of orofacial movements, indicative of an aversion to this taste. Preexposure to the garlic taste on E18 produced a LI of this CTA. Ketamine significantly disrupted the formation of the CTA and had some impact on LI. However, the direct effect of ketamine on LI is less certain since the drug also blocked the original CTA.

Developmental changes in brainstem neurons regulating lower airway caliber.

Premature infants are at risk for lower airway obstruction; however, maturation of reflex pathways regulating lower airway patency is inadequately studied. We hypothesized that postnatal maturation causes developmental change in brainstem efferent airway-related vagal preganglionic neurons (AVPNs) within the rostral nucleus ambiguus (rNA) that project to the airways and in pulmonary afferent fibers that terminate in the nucleus tractus solitarius (NTS). Ferrets aged 7, 14, 21, and 42 d received intrapulmonary injection of cholera toxin (CT)-beta subunit, a transganglionic retrograde tracer. Five days later, their brainstem was processed for dual immunolabeling of CT-beta and the cholinergic marker, choline acetyl transferase. CT-beta-labeled AVPNs and CT-beta-labeled afferent fiber optical density (OD) were analyzed. There was a significantly higher CT-beta-labeled cell number within the rNA at the youngest compared with older ages. All efferent CT-beta-labeled cells expressed choline acetyl transferase. OD of CT-beta-labeled afferent fibers was also higher at 7 d compared with 14 d. We conclude that the number of efferent AVPNs and afferent fiber OD both diminish over the second postnatal week. We speculate that exposure to injurious agents in early postnatal life may inhibit natural remodeling and thereby enhance later vulnerability to airway hyperreactivity.

Time-dependent retrograde amnesic effects of muscimol on conditioned taste aversion extinction.

We explored how stimulation of GABA(A) receptors at different times during conditioned taste aversion (CTA) acquisition or extinction influenced extinction. In Experiment 1, rats acquired a CTA to 0.3% saccharin-flavored water (SAC) when it followed an injection of lithium chloride (LiCl; 81.0 mg/kg, i.p.). Following conditioning, rats received extinction training in which the GABA(A) agonist muscimol (1.0 mg/kg, i.p.), or control (saline) injections, were administered either before or after each extinction trial. Muscimol hindered extinction when administered after extinction trials. Muscimol's inhibitory effects may have impeded extinction learning by disrupting synaptic mechanisms required to consolidate information experienced during extinction training. In Experiment 2, we studied the effects of muscimol on CTA acquisition and subsequent extinction. Rats received muscimol (1.0 mg/kg, i.p.) either before or after CTA conditioning trials. Following CTA acquisition, all rats were given CTA extinction training without muscimol administration. All groups developed CTA, but the group that received muscimol before CTA conditioning trials extinguished rapidly in comparison to other treatment groups. Differences between muscimol's effects on CTA conditioning and CTA extinction indicate that fear conditioning and extinction involve, to some degree, different neuronal mechanisms.

Spontaneous recovery of a conditioned taste aversion differentially alters extinction-induced changes in c-Fos protein expression in rat amygdala and neocortex.

Conditioned taste aversions (CTAs) may be acquired when an animal consumes a novel taste (conditioned stimulus; CS) and then experiences the symptoms of poisoning (unconditioned stimulus; US). Animals will later avoid the taste that was previously associated with malaise. Extinction of a CTA is observed following repeated, non-reinforced exposures to the CS and represents itself as a resumption of eating/drinking the once-avoided tastant. Spontaneous recovery (SR) of a CTA (a revival of the taste avoidance) occurs when the CS is offered after a latency period in which the CS was not presented. An initial study explored the experimental parameters required to produce a reliable SR following acquisition and extinction of a robust CTA in rats. A CTA was formed through 3 pairings of 0.3% oral saccharin (SAC) and 81 mg/kg i.p. lithium chloride (LiCl) followed by extinction training resulting in 90% reacceptance of SAC. After extinction training, some of the animals were also tested for SR of the CTA upon exposure to SAC following a 15-, 30-, or 60-day latency period of water drinking. We report here that latencies of 15, 30, or 60 days produced small, but reliable, SRs of the CTA--with longer latencies producing progressively more suppression of SAC consumption. A second study investigated changes in the amygdala (AMY), gustatory neocortex (GNC), and medial prefrontal cortex (mPFC) functioning during SR of a CTA. Using immunohistochemical methods, brain c-Fos protein expression was analyzed in rats that extinguished the CTA as well as those that exhibited SR of the CTA after a 30-day latency. Our previous studies indicated that the numbers of c-Fos-labeled neurons in GNC and mPFC is low following CTA acquisition and increase dramatically as rats fully extinguished the aversion. Here we report that cortical c-Fos protein expression declines significantly following SR of the CTA. Expression of c-Fos in basolateral AMY decreased significantly from EXT to SR, but control animals with an intact CTA also decreased significantly from a short-term CTA test to a long-term CTA test. Low levels of c-Fos expression in the central nucleus of the amygdala (CE) were observed throughout EXT with little change in expression detectable following SR. These measurements reflect the dynamic nature of brain activity during acquisition and extinction of a CTA and highlight an important role for cortical neurons in the brain reorganization that occurs during SR of a CTA. The data also suggest that certain sub-nuclei of the AMY may play a relatively minor role in SR of this defensive reaction to a learned fear.

Vagal afferents modulate cytokine-mediated respiratory control at the neonatal medulla oblongata.

Perinatal sepsis and inflammation trigger lung and brain injury in preterm infants, and associated apnea of prematurity. We hypothesized that endotoxin exposure in the immature lung would upregulate proinflammatory cytokine mRNA expression in the medulla oblongata and be associated with impaired respiratory control. Lipopolysaccharide (LPS, 0.1mg/kg) or saline was administered intratracheally to rat pups and medulla oblongatas were harvested for quantifying expression of mRNA for proinflammatory cytokines. LPS-exposure significantly increased medullary mRNA for IL-1ß and IL-6, and vagotomy blunted this increase in IL-1ß, but not IL-6. Whole-body flow plethysmography revealed that LPS-exposed pups had an attenuated ventilatory response to hypoxia both before and after carotid sinus nerve transection. Immunochemical expression of IL-1ß within the nucleus of the solitary tract and area postrema was increased after LPS-exposure. In summary, intratracheal endotoxin-exposure in rat pups is associated with upregulation of proinflammatory cytokines in the medulla oblongata that is vagally mediated for IL-1ß and associated with an impaired hypoxic ventilatory response.

Explicit disassociation of a conditioned stimulus and unconditioned stimulus during extinction training reduces both time to asymptotic extinction and spontaneous recovery of a conditioned taste aversion.

Conditioned taste aversions (CTAs) may be acquired when an animal consumes a novel taste (CS) and then experiences the symptoms of poisoning (US). This aversion may be extinguished by repeated exposure to the CS alone. However, following a latency period in which the CS is not presented, the CTA will spontaneously recover (SR). In the current study we employed an explicitly unpaired extinction procedure (EU-EXT) to determine if it could thwart SR of a CTA. Sprague-Dawley rats acquired a strong CTA after 3 pairings of saccharin (SAC the CS) and Lithium Chloride (LiCl the US). CTA acquisition was followed by extinction (EXT) training consisting of either (a) CS-only exposure (CSO) or, (b) exposure to saccharin and Lithium Chloride on alternate days (i.e., explicitly unpaired: EU). Both extinction procedures resulted in >/= 90% reacceptance of SAC, although the EU extinction procedure (EU-EXT) significantly decreased the time necessary for rats to reach this criterion (compared to CSO controls). Rats were subsequently tested for SR of the CTA upon re-exposure to SAC following a 30-day latency period of water drinking. Rats that acquired a CTA and then underwent the CSO extinction procedure exhibited a significant suppression of SAC drinking during the SR test (as compared to their SAC drinking at the end of extinction). However, animals in the EU-EXT group did not show such suppression in drinking compared to CSO controls. These data suggest that the EU-EXT procedure may be useful in reducing both time to extinction and the spontaneous recovery of fears.

Acetaminophen self-administered in the drinking water increases the pain threshold of rats (Rattus norvegicus).

Previous studies have suggested that the addition of flavored acetaminophen suspension (for example, Children's Tylenol) in the drinking water of rats may not be effective in producing postoperative analgesia because of low levels of consumption. However, these investigations neither measured analgesia nor compared the consumption by rats that had undergone surgery with that by unmanipulated rats. The present study reports that although unmanipulated rats naive to the taste of flavored acetaminophen do indeed drink significantly less of this liquid than tap water, they drank sufficient amounts of the acetaminophen-containing solution to significantly raise pain thresholds, as measured by the hot-plate test. Moreover, rats that had undergone surgery drank significantly more acetaminophen solution than did those that had no surgery. These data suggest that oral self-administration of flavored acetaminophen by rats may be an appropriate means to reduce pain.