Influence of light irradiance on hydroxyindole-O-methyltransferase activity, serotonin-N-acetyltransferase activity, and radioimmunoassayable melatonin levels in the pineal gland of the diurnally active Richardson's ground squirrel

When Richardson's ground squirrels were kept under light:dark cycles of 14:10 h there was no nocturnal rise in pineal hydroxyindole-O-methyltransferase (HIOMT) activity. Conversely, the 10 h dark period was associated with large nocturnal rises in both pineal serotonin-N-acetyltransferase (NAT) activity and radioimmunoassayable melatonin levels. The nighttime rises in pineal NAT and melatonin were not suppressed by the exposure of the animals to a light irradiance of 925 mu W/cm2 during the normal dark period. On the other hand, when the light irradiance was increased to 1850 mu W/cm2 the rise in pineal NAT activity was eliminated while the melatonin rise was greatly reduced. When ground squirrels were acutely exposed to a light irradiance of 1850 mu W/cm2 for 30 min beginning at 5.5 h after lights out, pineal NAT activity and melatonin levels were reduced to daytime values within 30 min. The half-time (t 1/2) for each constituent was less than 10 min. Exposure to a light irradiance of either 5 s or 5 min (beginning at 5.5 h into dark period) was equally as effective as 30 min light exposure in inhibiting pineal NAT activity and melatonin levels. When animals were returned to darkness after a 30 min exposure to a light irradiance of 1850 mu W/cm2 at night, both pineal NAT activity and melatonin levels were restored to high nighttime levels within 2 h of their return to darkness. The results indicate that the pineal gland of the wild-captured, diurnal Richardson's ground squirrel is 9000 X less sensitive to light at night than is the pineal gland of the laboratory raised, nocturnal Syrian hamster.     

The depression in rat pineal melatonin production after saline injection at night may be elicited by corticosterone

A hind-leg subcutaneous saline injection into rats at night elicits a decrease in N-acetyltransferase (NAT) activity and melatonin content of the pineal gland. The decrement in pineal melatonin production after saline injection is prevented by adrenalectomy. The present studies were undertaken to determine what factor(s) from the adrenal gland cause(s) the drop in pineal melatonin production after saline injection at night. In the first study, groups of intact and adrenal-demedullated male rats were given a saline injection at 23.10 h (3 h, 10 min after lights off) and their pineals were collected 15 or 30 min later. Pineal NAT activity was depressed in both intact and adrenal-demedullated rats at 15 min postinjection as compared to their respective control animals. Pineal melatonin levels exhibited a drop in intact animals at 15 min and in adrenal-demedullated rats at 30 min. In a second study, hypophysectomy was found to prevent the drop in nocturnal pineal NAT activity and melatonin levels normally associated with a hind leg injection of saline. Finally, in a third experiment, groups of hypophysectomized rats were injected i.p. with corticosterone at 23.10 h and killed 10, 25 or 40 min postinjection. Corticosterone injection in hypophysectomized rats produced a response similar to that caused by saline injection in intact animals: NAT activity was depressed at 10 min and melatonin content was lowered at 25 min. These results suggest that the adrenal-mediated depression in melatonin synthesis after saline injection at night in rats may be elicited by an adrenal cortical hormone (corticosterone) and apparently does not involve the release of factors from the adrenal medulla.     

Individual pineal cells in chick possess photoreceptive, circadian clock and melatonin-synthesizing capacities in vitro

Chick pineal cells express a circadian rhythm of melatonin release under light–dark (LD) cycles, with an increase during the dark period and a decrease during the light period, and this rhythm persists under constant darkness (DD). We cultured individual single pineal cells with 15 μl of medium per well in a Terasaki plate and measured melatonin secretion every 12 h under LD, DL and DD. Individual cells secreted more melatonin during the dark period than during the light period under both LD and DL conditions, and those rhythmic secretions persisted under DD. These results suggest that individual pineal cells in chick have photoreceptive, circadian clock and melatonin-synthesizing capacities.     

Changes in Pineal Indoleamines in Rats after Single Melatonin Injections: Evidence for a Diurnal Sensitivity to Melatonin

We recently determined that melatonin stimulated serotonin (5-HT) secretion from rat pineal glands by increasing 5-HT release from the pinealocytes (μM melatonin concentrations) and by inhibiting 5-HT uptake in the pineal sympathetic nerve endings (mM melatonin concentrations). The present study investigated whether a single melatonin injection could alter the content of indoleamines in the rat pineal gland, as well as its possible dependence on the daytime of administration. Melatonin (150 μg/kg) was i.p. injected at 8 time points (11.00 h, 14.00 h, 17.00 h, 20.00 h, 23.00 h, 02.00 h, 05.00 h and 08.00 h) to rats kept in 12:12 h light:dark cycle (lights on at 07.00 h). Melatonin injections in the afternoon (17:00 h) and late in the nighttime (02.00 h and 05.00 h) decreased pineal 5-HT content 90 min later. The levels of 5-hydroxyindoleacetic acid (5-HIAA) were also decreased 90 min after the melatonin treatment at 14.00 h, 17.00 h and 02.00 h. The effect of melatonin on 5-HT content was a long-lasting effect (still evident after 180 min) only when injected at 02.00 h, whereas 5-HIAA levels were found to be decreased 180 min after melatonin treatment at 14.00 h and 23.00 h. No changes in these compounds were detected 240 min after melatonin treatment. Moreover, melatonin did not change 5-hydroxytryptophan levels at any of the daytime points studied. By contrast, 90 min after the injection of melatonin at 20.00 h, an increased content of pineal N-acetylserotonin was observed. This effect of melatonin could be mediated through a phase alteration of the pineal N-acetyltransferase activity rhythm by acting on the suprachiasmatic clock, althought a direct melatonin effect on the pineal rhythmic function cannot be excluded. The effects of the hormone on 5-HT and 5-HIAA contents agree with previous findings on the inhibitory effect of pharmacological doses of melatonin on pineal 5-HT uptake, which presumably would result in a decreased intraneuronal content of 5-HT and its acid metabolite. These data point to an acute regulatory action of exogenous melatonin on the pineal melatonin synthesis pathway which seems to be limited to two daytime phases: the afternoon-early evening period and the second half of the night.     

Behaviorally conditioned suppression of mitogen-induced proliferation and immunoglobulin production: effect of time span between conditioning and reexposure to the conditioning stimulus

Rats were subjected to taste aversion conditioning using the immunosuppressive drug cyclophosphamide (CY) as the unconditioned stimulus (UCS) paired with saccharin, the conditioned stimulus (CS), and were reexposed to the CS at 2, 5, or 10 days after a single conditioning trial. Twenty-four hours after reexposure the rats were sacrificed and spleen cells assayed for mitogen-induced proliferation and immunoglobulin production. A robust conditioned taste aversion (CTA) was observed irrespective of the day of CS reexposure. However, only conditioned rats reexposed to the CS 2 days after training displayed a conditioned reduction in proliferative responses to PHA and PWM. These rats also exhibited a reduction in the synthesis of IgM, but not IgG or IgA, by spleen cells cultured with PWM. These effects were not observed in conditioned rats reexposed 5 or 10 days after conditioning. In another experiment, rats were subjected to a backward conditioning (UCS prior to CS) training trial, tested 2 days later for the presence of CTA, and sacrificed 24 h later for assessment of immune function as described above. The results of this experiment demonstrated that rats do not develop an aversion to saccharin when it is first presented 4 h after CY, and no alterations in spleen cell proliferation and immunoglobulin production were noted. The data show that the CTA response established by explicit association between CY and saccharin depresses in vitro spleen cell proliferation and IgM production only when elicited shortly after the conditioning trial.     

Urinary 6-sulphatoxymelatonin excretory rhythms in laboratory rats: effects of photoperiod and light

The excretion rhythm of the melatonin metabolite, 6-sulphatoxymelatonin, was determined in rats maintained on contrasting14h L: 10h D and10h L: 14h D photoperiods. The novel use of a high protein liquid diet together with an automatic urine collection system facilitated the monitoring of the onset, offset and total production of metabolite before, during and after a 57.5% reduction in the dark period (5.75 h and 8 h delay in lights off). In14L: 10D, melatonin metabolite excretion increased2.7±0.2h after lights off, whereas in10L: 14D, the onset occurred5.1±0.2h after lights off. Duration of melatonin metabolite excretion was not different between the two photoperiods. Reduction of the dark period by 57.5% delayed the appearance of melatonin metabolite excretion until2.15±0.4h and2.9±0.4h after darkness in both long and short photoperiods. Upon restoration of the original photoperiod on the third day, the onset of metabolite excretion was significantly delayed by1.4±0.4h(P < 0.05) in long day animals and2.2±0.2h(P < 0.05) in short day-length compared to the forst day. This study highlights the utility of urinary 6-sulphatoxymelatonin determinations in rats and provides new information on the acute effects of extended evening light on pineal melatonin secretion.     

Conditioned changes in dopamine oxidation currents in the nucleus accumbens of rats by stimuli paired with self‐administration or yoked‐administration of d‐amphetamine

In vivo chronoamperometry was used to monitor changes in dopamine oxidation currents corresponding to dopamine efflux in the nucleus accumbens of rats after presentation of a conditioned light stimulus repeatedly paired with either yoked- or self-administered intravenous injections of the psychostimulant d-amphetamine. Daily conditioning trials began with a non-contingent drug injection, paired with a conditioned stimulus consisting of a 5 s flashing light and 30 s lights out, after which a house light was illuminated during the 3 h session, signalling drug availability. Each subsequent injection of d-amphetamine was paired with the conditioned stimulus. Electrochemical measures were taken on conditioning trials 4–7, and on each trial, intravenous d-amphetamine (0.25 mg/kg per injection) self-administration produced a significant maximal increase in mean dopamine oxidation currents of ≈ 8 nA above baseline. Dopamine oxidation currents in rats receiving yoked d-amphetamine were ≈ 5 nA above baseline by the fourth day of drug administration and reached ≈ 8 nA on the seventh and final day of drug administration. On day 9 the first presentation of the vehicle injection and conditioned stimulus, in combination with illumination of the house lights, induced an immediate increase in nucleus accumbens dopamine oxidation currents in all rats that had previously received d-amphetamine. Subsequent presentations of the conditioned stimulus at 30 min intervals induced further increases in extracellular dopamine oxidation currents in both drug-treated groups. By the end of the 3 h session, both groups had similar maximal conditioned increases in dopamine oxidation currents of ≈ 6 nA. These data are discussed with relation to the neurochemistry of drug craving.     

Orbitofrontal cortex and representation of incentive value in associative learning.

Clinical evidence indicates that damage to ventromedial prefrontal cortex disrupts goal-directed actions that are guided by motivational and emotional factors. As a consequence, patients with such damage characteristically engage in maladaptive behaviors. Other research has shown that neurons in the corresponding orbital region of prefrontal cortex in laboratory animals encode information regarding the incentive properties of goals or expected events. The present study investigates the effect of neurotoxic orbitofrontal cortex (OFC) lesions in the rat on responses that are normally influenced by associations between a conditioned stimulus (CS) and the incentive value of reinforcement. Rats were first trained to associate a visual CS with delivery of food pellets to a food cup. As a consequence of learning, rats approached the food cup during the CS in anticipation of reinforcement. In a second training phase, injection of LiCl followed consumption of the food unconditioned stimulus (US) in the home cage, a procedure used to alter the incentive value of the US. Subsequently, rats were returned to the conditioning chamber, and their responding to the CS in the absence of the food US was tested. Lesions of OFC did not affect either the initial acquisition of a conditioned response to the light CS in the first training phase or taste aversion learning in the second training phase. In the test for devaluation, however, OFC rats exhibited no change in conditioned responding to the visual CS. This outcome contrasts with the behavior of control rats; after devaluation of the US a significant decrease occurred in approach to the food cup during presentation of the CS. The results reveal an inability of a cue to access representational information about the incentive value of associated reinforcement after OFC damage.     

Effect of NMDA Receptor Blockade on Melatonin and Activity Rhythm Responses to a Light Pulse in Rats

The possible role of the excitatory amino acids as mediators of the acute suppression and subsequent delay by light of pineal melatonin production was studied in rats using the NMDA receptor antagonist MK-801. Saline or MK-801 in doses up to 3 mg/kg (IP), was administered 15 min before a 15-min light pulse (200 lx), 4 h after dark onset, and the excretion of 6-sulphatoxymelatonin (aMT.6S) determined. Under these conditions saline injected/light exposed animals exhibited an acute, total but transient suppression of urinary aMT.6S excretion and a delay in the onset of aMT.6S the following night of 1.5 ± 0.2 h. MK-801 failed to block either the acute or phase delaying effect of light (onset delayed by 2.2 ± 0.4 h). Pretreatment with MK-801 (3 mg/kg) failed to block the effects of shorter, less intense light pulses 15 min before the pulse (e.g., 1 min/2 lx; onset delayed by 2.0 ± 0.4 h following saline, 1.5 ± 0.1 h following MK-801) or 60 min before a short duration low intensity pulse. In other experiments MK-801 (1 and 3 mg/kg) failed to affect aMT.6S excretion when injected in the dark at the time of lights out or 4 h after dark onset. NMDA (10 and 30 mg/kg) injection at the time of lights out or 4 h after darkness did not mimic the effects of a light pulse by decreasing aMT.6S excretion or causing a delay in the onset of excretion the following night. Finally MK-801 (3 mg/kg) injected 4 h after dark failed to block the phase delaying effects of a 15 min light pulse (200 lx) on running activity in rats. These results do not support the hypothesis that excitatory amino acids in the retino-hypothalamic tract acting on the NMDA receptor subtype and terminating in the suprachiasmatic nucleus mediate the photic influences upon rat pineal melatonin and activity rhythms. Copyright © 1996 Elsevier Science Inc.     

Studies on pineal melatonin levels in a diurnal species,the Eastern chipmunk (Tamias striatus): Effects of light at night,propranolol administration or superior cervical ganglionectomy

Summary Five experiments were carried out on the control of melatonin levels in the pineal gland of a diurnal species, the Eastern chipmunk (Tamias striatus). We confirmed that the exposure of chipmunks to fluorescent white light of 3,981–4,304 lux during the normal dark period does not prevent the rise in pineal melatonin levels normally associated with darkness. Also, the administration of propranolol (20mg/kg) at 8 p.m. did not block the rise in pineal melatonin in animals exposed to either dark or light at night. Similarly, if chipmunks received propranolol 4 hours into the dark phase, pineal melatonin levels were not depressed 2 hours later. When animals were superior cervical ganglionectomized, however, the pineal content of melatonin remained low regardless of whether the animals were exposed to darkness or light at night. The exposure of chipmunks acutely to light at midnight (4 hours after darkness onset) had only a slight depressive effect on pineal melatonin 30 min later; by comparison, when chipmunks were acutely exposed to light at 3 a.m. (7 hours after darkness onset) daytime pineal melatonin levels were reached within 15 min after light onset. These findings in a diurnal species, the Eastern chipmunk, differ markedly when compared to previously reported observations on nocturnal laboratory rodents.     

Response of pineal serotonin N-acetyltransferase activity in male guinea pigs exposed to light pulses at night

Summary Serotonin N-acetyltransferase (NAT), which is crucial for the formation of melatonin, undergoes a typical day/night rhythm in the pineal gland with low levels during daytime and high levels at night. Short pulses of light given at night have been shown to rapidly depress NAT activity in some species, but not in others, the reasons for this difference being unclear. As diurnality and nocturnality of the experimental animals may play a role and since diurnally active animals have been little investigated in this respect, in the present study the diurnally active guinea pig was investigated. Male guinea pigs kept under a lighting regimen of LD 1212 (lights off at 1700 hrs) were killed between 1200 or 1300 hrs and between 0000 and 0200 hrs, at night in the dark or after exposure to 10 or 45 min of light. The results obtained show that the day/night difference of NAT activity is about 2-fold. 10 min or 45 min of light given at night significantly depress pineal NAT activity. Re-exposure to darkness for 1 hr of animals previously given light for 10 min leads to restoration of NAT activity. These findings together with data from the literature suggest that it does not appear to be the activity pattern (diurnality versus nocturnality) of an animal nor the amplitude of the day/night difference of pineal NAT activity that account for the suppressibility or non-suppressibility of pineal NAT activity by light at night.     

The interstimulus interval and classical conditioning in the marine snail Hermissenda crassicornis

We examined the role of the interstimulus interval for the conditioned association between light and rotation stimuli in the marine snail Hermissenda. This interval between the conditioned stimulus (CS) and the unconditioned stimulus (US) is an important and widespread property of vertebrate associative learning. We demonstrated that with a forward CS-US delay of 1.0 s we were able to produce significant 24-h retention of an associative memory after 50 training trials. Other paired treatments providing intervals of 1.5 s, 0.5 s, simultaneous, and backward arrangements did not support retention at 24 h.     

Entrainment of the rat circadian clock controlling the pineal N-acetyltransferase rhythm depends on photoperiod.

Entrainment of the circadian clock as a function of time when a light stimulus is presented has been studied in detail while little attention has been paid to a role photoperiod may play in the resetting. To find out whether and how photoperiod affects the entrainment, resetting of the rat circadian pacemaker by delays in the evening light offset and by advances in the morning light onset, respectively, was studied in rats maintained either under a short photoperiod, with 8 h of light and 16 h of darkness per day (LD 8:16) or under a long, LD 18:6 photoperiod. To assess phase shifts of the clock, the suprachiasmatic nucleus controlled rhythm in the pineal N-acetyltransferase (NAT), namely the time of the evening NAT rise and the time of the morning decline, were followed. One day after a delay in lights off, on LD 8:16 the NAT rhythm with a normal amplitude was retained following longer delays of the light offset and the maximum phase delay of the NAT rise was 3 times larger than on LD 18:6. One day after an advance in lights on, the NAT decline was phase advanced under both photoperiods; on LD 8:16 the maximum shift was 3 times as large as on LD 18:6. On LD 8:16, the NAT rise was not shifted after shorter advances in lights on and became phase delayed only when the light onset was brought forward to before midnight while on LD 18:6 the NAT rise was phase delayed after any, even a mere 1 h, advance in lights on. The data show that magnitude and direction of phase shifts of the NAT rhythm depend not only on the time of light presentation but on photoperiod as well. Difference in resetting of the rhythm under various photoperiods may reflect photoperiod-dependent changes of an underlying pacemaker.     

The effect of different light intensities on pineal melatonin content

In Syrian hamsters, elevated night-time pineal melatonin levels are quickly reduced to low daytime levels by exposing the animals to light. The purpose of this study was to determine the lowest light intensity capable of causing a large reduction in night-time levels of pineal melatonin in the male hamster. During the dark phase of the light: dark cycle, groups of hamsters were exposed to one of 8 different intensities of white fluorescent light: 5380, 2798, 151, 20.44, 5.38, 1.08, 0.11 and 0.01 lux. For each light intensity, pineals were collected from 8 hamsters each at 2 min before and at 2, 8 and 32 min after the lights were turned on. Pineal melatonin content was determined by radioimmunoassay. Light intensities of 1.08 lux or greater depressed pineal melatonin content significantly (P < 0.001). Light intensities of 0.11 or 0.01 lux failed to depress pineal melatonin levels. Thus, the apparent threshold for the action of white fluorescent light on hamster pineal melatonin content lies between 1.08 and 0.11 lux.     

Regulation of short-term associative memory by calcium-dependent protein kinase

Neural and behavioral correlates of an associative memory in Hermissenda were examined during induction and/or formation of the memory. Hermissenda received either light (conditioned stimulus or CS) and rotation (unconditioned stimulus or US) paired (i.e., Pavlovian conditioning), light and rotation unpaired (pseudoconditioning), or no exposure to light and rotation. Following 9 pairings in a 6 min session, conditioned animals exhibited a contraction of the foot in response to a test CS presented 2 min after the last conditioning trial, whereas pseudoconditioned and untreated animals exhibited a foot extension to the same CS. In addition, both an associative and a nonassociative reduction in light-induced locomotion was observed. To examine neural correlates of this learning within minutes of acquisition, the isolated nervous system of the Hermissenda (containing the visual and vestibular organs) was trained with stimulus conditions identical to those used for the intact animal. Prior isolation and preparation of the nervous system permitted immediate intracellular recording following the final conditioning trial. Relative to pseudoconditioned and untreated animals, the B photoreceptors in conditioned nervous systems were found to have elevated input resistance (inversely related to K+ channel conductance and positively related to excitability) and exhibited increased steady-state depolarization in response to the light CS, as well as a prolonged depolarization after the CS offset. These neural correlates of the associative memory were attenuated if the protein kinase inhibitor H7 was present in the extracellular bath during conditioning, demonstrating in the reduced preparation that antagonism of protein kinase activity blocks the induction of membrane alterations of identified neurons that correlate with memory storage.(ABSTRACT TRUNCATED AT 250 WORDS)     

A quantitative method for assessing of the affective component of the pain: conditioned response associated with CO2 laser-induced nocifensive reaction

Sensory and affective components are included in the overall behavioral manifestation in a nocifensive reaction. We have developed a behavioral model using classical conditioning to differentiate the affective component from the sensory responses following a thermal noxious stimulus. In laser-pain conditioning, free moving rats were trained to associate a tone (conditioned stimulus, CS) and short CO2 laser pulsation (unconditioned stimulus, US). A monotonous tone (800 Hz, 0.6 s) was delivered through a loudspeaker as the CS. CO(2) laser pulses (5 W at 100 ms in duration) applied to the hind paw were adopted as the US. The CS-US interval was 0.5 s. The conditioned responses as quantitatively measured by their body movement were developed over a period of 40 CS-US pairings. These conditioned responses were found retained when the rats were tested by presenting CS alone, immediate to and 24 h subsequent to training. The conditioned responses however diminished significantly following both morphine and buspirone treatment. This method demonstrated that neutral auditory stimuli could form association with unlearned nocifensive responses evoked by noxious CO2 laser pulses stimuli. Thus, the assessment of conditioned response may be a valuable tool for the measurement of the affective component of nociception.     

Fos-like immunoreactivity in the circadian timing system of calorie-restricted rats fed at dawn: daily rhythms and light pulse-induced changes

Daily rhythms of pineal melatonin, body temperature, and locomotor activity are synchronized to the light–dark cycle (LD) via a circadian clock located in the suprachiasmatic nuclei (SCN). A timed caloric restriction in rats fed at dawn induces phase-advances and further phase-stabilization of these rhythms, suggesting that the circadian clock can integrate conflicting daily photic and non-photic cues. The present study investigated the daily expression of Fos-like immunoreactivity (Fos-ir) and light pulse-induced Fos-ir in the SCN, the intergeniculate leaflet (IGL) and the paraventricular thalamic nucleus (PVT) in calorie-restricted rats fed 2 h after the onset of light and in controls fed ad libitum. A daily rhythm of Fos-ir in the SCN was confirmed in control rats, with a peak approximately 2 h after lights on. At this time point (i.e. just prior to the feeding time), the level of SCN Fos-ir was lowered in calorie-restricted rats. Concomitantly, IGL Fos-ir was higher in calorie-restricted vs. control rats. In response to a light pulse during darkness, Fos-ir induction was found to be specifically (i.e. phase-dependently) lowered in the SCN and IGL of calorie-restricted rats. Observed changes of Fos-ir in the PVT were possibly related to the wake state of the animals. This study shows that repetitive non-photic cues presented in addition to a LD cycle affect the Fos expression in the circadian timing system.     

Conditioned inhibition of the nictitating membrane response in decorticate rabbits

Rabbits with substantial neocortical ablation were trained in a Pavlovian conditioned inhibition task using a light as the reinforced conditioned stimulus (CS) and the same light compounded with a tone as the non-reinforced CS. The conditioned response was the nictitating membrane response, and the unconditioned stimulus was paraorbital shock. Training began 4–14 months posteperatively. With a number of procedural variations across 4 experiments, there was little evidence of impaired conditioned inhibition by decorticates compared to control animals. These results are consistent with earlier reports on Pavlovian discrimination learning and reversal involving single-element CSs from different sense modalities. Hence, they extend the basic conclusion that neocortex is not essential for Pavlovian inhibition to the conditioned inhibition paradigm. Consideration of these findings in the light of previous investigations of conditioned inhibition in rabbits, involving mesencephalic or hippocampal lesions, supports the hypothesis that the crucial neural elements for Pavlovian inhibition may be located in the brain stem.     

Subhypnotic doses of propofol accelerate extinction of conditioned taste aversion

Subhypnotic doses of propofol accelerate extinction of conditioned taste aversion. Some intravenous anesthetic agents including propofol is known to induce anterograde and retrograde amnesia. We evaluated whether propofol affect the long-term memory formed by the conditioned taste aversion (CTA) paradigm. Rats were allowed a 4h access to water through the experiments. After preconditioning water intake, the rats were offered 0.1% sodium saccharin (Sac) as conditioned stimulus (CS) for 20 min. An intraperitoneal (i.p.) injection of several concentrations (0.5-100 mg/kg) of propofol 10 min after Sac exposure was followed by an i.p. injection of 0.15M LiCl (2% of body weight) as unconditioned stimulus (US) 30 min after CS-exposure. The volumes of intake of Sac for 20 min were measured on the successive 4 days. The rats, which acquired CTA by every CS-US paradigm, strongly avoided Sac on the first test day after conditioning and maintained the avoidance for 3 days. However, when subhypnotic dose of propofol was injected before LiCl-injection, Sac intake abruptly increased on the second test day and the almost complete extinction occurred on the third test day after conditioning. The extinction process of CTA was barely affected by hypnotic dose of propofol. These results suggest that propofol affects the retention mechanism of the CTA memory in a dose-dependent manner. Subhypnotic dose of propofol may affect the sub-cellular process of the memory consolidation in CTA.     

Hippocampal correlates of a conditioned sniffing response

Eight white rats were trained on a discriminative classical conditioning paradigm and then on a counter-conditioning paradigm using ESB (electrical stimulation of the brain) as the UCS (unconditioned stimulus) and two incandescent lights as the rewarded (S_D) and non-rewarded (S_Δ) stimulus. Hippocampal EEG correlates of both stimuli in the CS (conditioned stimulus) position were analyzed in Early Conditioning, Late Conditioning, and Early and Late Counter-conditioning.