Latent inhibition and conditioning in rat strains which show differential prepulse inhibition.

Latent inhibition (LI) is the retardation of associative conditioning resulting from preexposure of the conditioned stimulus (CS) alone prior to conditioning. Schizophrenic patients show deficient prepulse inhibition (PPI) and, at least acutely, deficient LI as well. We recently found that Brown Norway (BN) rats show a PPI deficit compared to Wistar-Kyoto (WKY) rats. If PPI and LI depend on neural processes with common genetic substrates, then LI should be deficient in BN rats as well. Here, LI of a conditioned taste aversion was examined in BN and WKY rats. One group from each strain was preexposed to a saccharin-flavored solution (CS) the day prior to conditioning. For taste aversion conditioning, these two groups again consumed saccharin and were injected with lithium chloride (unconditioned stimulus) 10 min later. A second group from each strain was not preexposed to the CS and was treated identically during conditioning, while a third group was not conditioned (injected with sodium chloride). To test for taste aversion conditioning, saccharin was offered for 20 min/day for 3 days. Nonconditioned BN and WKY rats consumed equal amounts of saccharin on test days. In both strains, conditioned rats showed a saccharin aversion. However, conditioning was less robust in BN than in WKY rats. WKY rats showed good LI of the conditioned taste aversion in that preexposed WKY rats consumed significantly more saccharin on test days than conditioned, nonpreexposed WKY rats. Preexposed BN rats did not consume significantly more saccharin on test days than conditioned, nonpreexposed BN rats. The previously reported deficiency in PPI in the BN rats was confirmed here 1 week after the taste aversion experiment. These results suggest that BN rats show deficient LI as well as PPI and display poor associative learning, a trait also reported in schizophrenia.

Repeated restraint stress-induced increase in baroreceptor reflex sensitivity: role of corticotropin-releasing factor.

The effect of central administration of a corticotropin-releasing factor antagonist on repeated restraint stress-induced changes in the baroreceptor reflex response to phenylephrine was examined in male Wistar-Kyoto rats. Rats were instrumented with intracerebroventricular guide cannula and femoral arterial and venous catheters. On each of 5 consecutive days, two groups received either a central infusion of saline or an infusion of the corticotropin-releasing factor antagonist, astressin. One saline- and one astressin-treated group experienced 20 min of restraint stress 10 min after each infusion. The other saline- and astressin-treated groups served as non-stressed controls. Twenty-four hours later, each rat received 3 doses of phenylephrine which produced equivalent increases in mean arterial pressure in each of the 4 treatment groups. Reflex bradycardia was significantly greater in the saline-treated/repeated restraint group than in the saline-treated/no restraint group. This effect of repeated restraint on the baroreceptor reflex was attenuated by administration of astressin prior to each session of restraint. A single 20 min session of restraint stress failed to alter baroreceptor reflex sensitivity. However, repeated central infusions of exogenous CRF failed to alter BRR sensitivity. In a separate experiment, astressin failed to attenuate the increases in mean arterial pressure and heart rate which occurred during each session of restraint stress and, in fact, diminished habituation of the blood pressure response in the last session. The results suggest that repeated stress increases thesensitivity of the baroreceptor reflex and that corticotropin-releasing factor has a role in this stress-induced change.

Prepulse startle deficit in the Brown Norway rat: a potential genetic model.

Prepulse inhibition (PPI), an operational measure of sensorimotor gating, is deficient in schizophrenia patients. PPI was compared among 4 strains of rats: Sprague-Dawley, Spontaneously Hypertensive, Wistar Kyoto (WKY), and Brown Norway (BN). PPI was dramatically lower in BN versus the other strains, especially WKY, for both acoustic and airpuff startle stimuli, whereas startle amplitude was similar between BN and WKY. Female BN also had lower PPI than did female WKY. Response to increasing prepulse intensities showed a right shift in the BN relative to the WKY. Visual prepulses also showed deficiencies in BN versus WKY. The absence of background noise did not negate strain differences. Auditory brainstem response to clicks and tone pips revealed no differences in auditory threshold between the 2 strains. These results are the first to demonstrate that BN have impaired sensorimotor gating compared with WKY, without impaired acoustic acuity.

Maintenance of amphetamine-induced stereotypy and locomotion requires ongoing dopamine receptor activation.

Amphetamine-induced locomotion and stereotypy depend on dopamine (DA), yet, while extracellular DA concentrations peak early, and then begin to decline, intense stereotyped behaviors continue for relatively prolonged periods. These observations suggest that DA may act as a "trigger" for the entire multiphasic behavioral response. To test this hypothesis in rats, haloperidol (HAL) was injected at different times with respect to (AMPH), and automated and videotaped measures of the behavior were recorded. HAL (0.1 mg/kg, i.p.) or saline was administered either 15 min prior to AMPH (4.0 mg/kg, s.c.); 60 min following AMPH (during the phase of intense oral stereotypy); or 140 min after AMPH (during post-stereotypy locomotion). When administered prior to AMPH, HAL prevented the development of stereotypy, and an increase in locomotion was displayed in place of stereotypy. Haloperidol administration during stereotypy interrupted the response, and resulted in an increase in locomotion for the remainder of the stereotypy phase. In neither of these cases did HAL affect post-stereotypy locomotion. However, when injected during the post-stereotypy phase, HAL caused a decrease in the magnitude of the locomotor response, suggesting that both the stereotypy and locomotor components of the response remain sensitive to HAL at times when DA levels have significantly declined. These results do not support the hypothesis that the early increase in extracellular DA produced by AMPH, acts as a "trigger" for a non-dopaminergic receptor mediated expression of the later phases of the AMPH response. Instead, it appears that both stereotypy and post-stereotypy locomotion remain sensitive to DA receptor blockade when extracellular DA levels are below the levels produced by non-stereotypy-inducing doses of AMPH.

Locus coeruleus electrophysiological activity and responsivity to corticotropin-releasing factor in inbred hypertensive and normotensive rats.

The spontaneously hypertensive rat (SHR) and its normotensive progenitor, the Wistar-Kyoto rat (WKY), have been shown to be differentially responsive to the behavioral and endocrine effects of both stress and corticotropin-releasing factor (CRF), both of which increase locus coeruleus (LC) electrophysiological activity. However, the effect of central administration of CRF in these rat strains has yet to be examined. In the present studies, LC electrophysiological responsivity to intracerebroventricular infusions of CRF was assessed in SHR, an inbred strain of WKY rats (the WKY[LJ] rat), and an outbred normotensive rat strain, Sprague-Dawley (SD) rats. Spontaneous LC discharge rate, mean arterial blood pressure and heart rate were also examined. LC activity was increased to the same extent in the three rat strains in response to a 3 microg dose of CRF. However, WKY(LJ) rats showed an exaggerated LC in response to a 1 microg dose of CRF in comparison to the other rat strains tested at this dose. Spontaneous discharge rates of individual LC neurons were lower in both SHR and WKY[LJ] rats than in SD rats. Further, the variability of the discharge rates of LC neurons was greater in WKY[LJ] rats than in the other two strains. These results indicate that the WKY[LJ] rat may provide a useful model for assessing the role of sensitivity to CRF in stress responsiveness.

Reciprocal cross-desensitization of locus coeruleus electrophysiological responsivity to corticotropin-releasing factor and stress.

While acutely administered corticotropin-releasing factor (CRF) and acute stress each activate neurons of the locus coeruleus (LC), desensitization to both develops with repeated treatment. The present experiments were designed to investigate whether cross-desensitization develops between CRF and stress. Because acute hemodynamic stress caused by intravenous infusion of sodium nitroprusside increases LC electrophysiological discharge rate via a CRF-dependent mechanism, it was hypothesized that repeated CRF administration would cause desensitization to the effect of this stressor on LC. For a complementary experiment, it was hypothesized that repeated stress, which presumably results in the repeated release of endogenous CRF, would result in desensitization to subsequent exogenous CRF. The results of the first experiment showed that repeated intracerebroventricular (i.c.v.) administration of CRF caused a significant attenuation of the sodium nitroprusside-induced increase in LC discharge rate seen in naive rats, although this pretreatment actually potentiated the decrease in blood pressure produced by sodium nitroprusside. In the second experiment, either one or eight sessions of white-noise stress attenuated the effect of CRF on LC activity 24 h after the last stress exposure, and this attenuation was more pronounced following eight sessions of stress than following one session. In a test of the specificity of this effect, stress-induced desensitization did not generalize to the LC electrophysiological response to clonidine (i.c.v.). One week following the last of eight sessions of stress, LC responsivity to CRF had recovered to control levels. These experiments demonstrate reciprocal cross-desensitization between CRF and stress using LC electrophysiological responsivity as an assay. This modifiability of the interaction between CRF and the LC may represent the operation of mechanisms mediating adaptive responding to stress.

Effects of pretreatment with corticotropin-releasing factor on the electrophysiological responsivity of the locus coeruleus to subsequent corticotropin-releasing factor challenge.

Both acute central administration of exogenous, and stress-induced release of endogenous corticotropin-releasing factor result in electrophysiological activation of the noradrenergic neurons constituting the locus coeruleus. The present experiments were designed to examine whether single (1) or repeated (8) intracerebroventricular pretreatment with exogenous corticotropin-releasing factor would alter locus coeruleus electrophysiological responsivity to subsequent corticotropin-releasing factor challenge in rats. A single corticotropin-releasing factor (3 microg) pretreatment significantly attenuated challenge-induced locus coeruleus activation 24 and 72, but no 96 h later, while a single vehicle pretreatment had no significant effect on the response to subsequent challenge at any pretreatment-to-test interval. Repeated pretreatment with either corticotropin-releasing factor or vehicle completely attenuated locus coeruleus response to challenge 24 h after the final pretreatment. Seventy-two hours after the last vehicle pretreatment, challenge resulted in a significant increase in locus coeruleus activity, though the response was less than in naive controls. Challenge continued to produce no effect on locus coeruleus activity in repeated corticotropin-releasing factor-pretreated rats at this (72 h) time point. One week (168 h) after the cessation of repeated pretreatment, challenge resulted in a significant increase in locus coeruleus activity which was equal to that of naive controls in vehicle-pretreated rats, but reduced by comparison to controls in corticotropin-releasing factor-pretreated rats. Basal discharge rates of locus coeruleus neurons 24 h after the last repeated corticotropin-releasing factor pretreatment were significantly less than in naive controls. Thus, the failure of challenge to increase neuronal activity in these rats was not due to a "ceiling" effect caused by elevated tonic discharge rate. Repeated vehicle pretreatment produced a functional change similar to that produced by exogenous corticotropin-releasing factor administration. One hypothesis is that repeated vehicle pretreatment was stressful and caused the repeated release of endogenous corticotropin-releasing factor. This hypothesis was tested by determining whether locus coeruleus neurons remained responsive to challenge following repeated administration of a corticotropin-releasing factor antagonist. Thus, the effect if repeated pretreatment with the antagonist, [D-Phe, Nle, Calpha MeLeu]CRF was also examined. Challenge resulted ina significant increase in discharge rate 24 h after the final antagonist pretreatment, providing support for the hypothesis. Additionally, in rats repeatedly pretreated with vehicle, carbachol challenge induced an increase in locus coeruleus activity equal to that induced in naive controls. These results indicate that prior exposure to corticotropin-releasing factor, or the repeated mild stress of vehicle infusions, reduces locus coeruleus responsiveness to corticotropin-releasing factor, and reveal that the relationship between these two neurotransmitter systems is modifiable. This altered relationship may contribute to stress-related affective disorders in which both systems have been implicated.

Mouse strain differences in the behavioral effects of corticotropin-releasing factor (CRF) and the CRF antagonist alpha-helical CRF9-41.

The effect of the corticotropin-releasing factor (CRF) antagonist alpha-helical CRF9-41 (alpha H CRF9-41; 25 and 50 micrograms) was examined in four strains of mice (BALB/C, NIH Swiss, CF-1, and CD) in the elevated plus-maze anxiolytic test and found to significantly increase percent open arm activity in only the BALB/C mice. A marginal anxiolytic response was obtained in NIH Swiss, while no effect of the antagonist was noted in CF-1 or CD mice in this test. Diazepam (1-4 mg/kg IP) significantly increased percent open arm activity in all four mouse strains. Thus, all strains were sensitive to the effects of a known anxiolytic in this test. The locomotor-suppressing effect of the agonist CRF was assessed in the four strains of mice. While CRF suppressed locomotor activity in each of the strains, the peptide was more efficacious and more potent in the BALB/C strain than in any of the other three strains. The behavioral differences in responsiveness to CRF and the antagonist alpha H CRF9-41 could not be explained on the basis of differential binding of CRF to forebrain membranes in the four mouse strains. These data suggest that the BALB/C mouse is more sensitive to the behavioral effects of CRF and its antagonist than other strains and may be a useful strain for examining the effects of CRF and/or stress.

Footshock-induced freezing behavior in rats as a model for assessing anxiolytics.

A number of chemically distinct anxiolytics were examined for effects on defensive behavior (foot-shock-induced freezing) in rats. Central nervous system acting drugs which are not anxiolytics were also studied. Animals were injected with a drug or vehicle (IP) prior to being placed in a chamber with a grid floor through which two footshocks were delivered. Behavior was observed during the pre-shock period (2 min) and for 4 min after the second footshock. The effects of the following drugs on the duration of footshock-induced freezing were studied: diazepam (DZP); 2-amino-4,5-(1,2-cyclohexyl)-7 phosphonoheptonic acid (NPC 12626); 3-((+/-)-2-carboxypiperazine-4-yl)-propyl-l-phosphonic acid (CPP); [(+)-5-methyl-10-11,dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10- imine (MK-801); buspirone hydrochloride (BUS); DL-amphetamine sulfate (AMP); haloperidol (HAL); ethyl-beta-carboline-3 carboxylate (beta-CCE). Compounds which reduced the duration of footshock-induced freezing included DZP, BUS, and the competitive NMDA antagonists NPC 12626 and CPP. The non-competitive NMDA antagonist, MK-801, had no effect on the response. The highest dose of amphetamine tested also reduced footshock-induced freezing. However, amphetamine-treated animals did not locomote or rear after footshock, suggesting fear of the environment. Animals injected with DZP, NPC 12626, CPP or buspirone spent at least 1.4 of the 4 post shock minutes locomoting. Haloperidol had no effect on freezing at the doses tested. beta-CCE tended to increase the duration of footshock-induced freezing.(ABSTRACT TRUNCATED AT 250 WORDS)

Individual differences in aging: behavioral and neurobiological correlates.

The goal of this experiment was to determine the correlations among different behavioral and neurobiological measures in aged rats. Aged Sprague-Dawley rats were given a battery of cognitive and sensorimotor tests, followed by electrophysiological assessment of sleep and biochemical measurements of various neurotransmitter systems. The behavioral tests included the following: Activity level in an open field; short-term and long-term memory of a spatial environment as assessed by habituation: spatial navigation, discrimination reversal, and cue learning in the Morris water pool; spatial memory in a T-maze motivated by escape from water; spatial memory and reversal on the Barnes circular platform task; passive avoidance; motor skills. Sleep was assessed by electrographic cortical records. The following neurotransmitter markers were examined: Choline acetyltransferase; the density of nicotinic, benzodiazepine and glutamine receptors in the cortex and caudate nucleus; endogenous levels of norepinephrine, dopamine, and serotonin in the cortex and hippocampus. The duration of bouts of paradoxical sleep was strongly correlated with several cognitive measures and selected serotonergic markers. This finding suggests that changes in sleep patterns and brain biochemistry contribute directly to deficits in learning and memory, or that the same neurobiological defect contributes to age-related impairments in sleep and in learning and memory.

Age-related cognitive impairments as assessed with an automated repeated measures memory task: implications for the possible role of acetylcholine and norepinephrine in memory dysfunction.

Although there exists a general agreement that certain aspects of learning and memory, and certain associated neuronal systems may be impaired with aging, systematic parametric studies are needed to characterize the nature and limits of these age-related impairments and to identify the underlying neuronal mechanisms. We review a series of experiments that examined the effects of aging and experimental treatments on rats' performance of a continuous nonmatching-to-sample, working memory task. In these studies, disruption of cholinergic transmission produced robust impairments that increased with retention interval duration, but could be observed even at the shortest intervals tested. In contrast, age-related impairments were less robust. With tone and light discriminative stimuli age-related impairments were not observed under conditions that were sensitive to disruption of cholinergic transmission, but were observed with increased retention interval duration. Finally, disruption of noradrenergic transmission produced a marginal disruption of memory performance, at worst. The generality of these results, and possible implication for future studies and animal models of dementia are discussed.

Behavioral and metabolic effects of sucrose-supplemented feeding in hyperactive rats.

Two hyperactive rat strains [spontaneously hypertensive rats (SHR) and SHR-Wistar-Kyoto cross (WK-HA)] and their nonhyperactive genetic control strain (Wistar-Kyoto) were fed ad libitum sucrose-supplemented rat chow, or chow alone in controls, to determine the effects of dietary sugar on behavior. The diets were given either overnight (acute sugar) or for 14-18 days (chronic sugar), and testing was carried out on the morning after each of the dietary schedules. The metabolic studies revealed significant strain, sex, and age differences in appetite for sucrose, caloric intake, postprandial plasma levels of glucose and insulin, and weight gain after sucrose feeding. The findings indicate that sugar feeding led to increased plasma glucose and insulin levels; however, total caloric intake was decreased, and less weight gain was observed than in chow-fed controls, particularly among the hyperactive strains. In behavioral tests, sugar feeding did not alter spontaneous activity levels in any of the strains after either acute or chronic diets. There were also no significant effects of sucrose consumption on spatial learning and memory in a plus-shaped maze as determined by use of a shock-avoidance paradigm. The only significant behavioral effects of sucrose observed were an impairment in habituation and distractibility among the WK-HA females, the most hyperactive group among these strains.