Chronic propranolol induces deficits in retention but not acquisition performance in the water maze in mice.

Agents that alter adrenergic receptors, such as "beta-blockers," also alter memory storage. However, reports suggest that beta-adrenergic receptor antagonists, such as propranolol, have conflicting behavioral effects with acute vs chronic dosing. This study was designed to evaluate the effects of chronic propranolol on retention for a spatial learning task. Adult male ICR mice were given daily injections of propranolol (2, 4, 8, or 12 mg/kg ip) or 0. 9% NaCl for 15 days prior to, and during, trials in a Morris water maze. Mice received five massed acquisition (escape) trials in each of three daily sessions, followed by a single 60-s probe trial on the fourth day. The location of the submerged platform was constant for each animal over acquisition trials, but varied across animals; starting position varied across trials. A 5 (dose) x 3 (trial blocks) mixed factorial ANOVA for escape time yielded a significant trial blocks effect only (p <.001), showing performance improving over sessions. Time spent in the target quadrant on the probe trial was shorter under all doses of propranolol when compared to vehicle group (all p <.001), indicating poorer retention of prior platform location. This effect, however, was not dose-related. Swim speed was not significantly affected by propranolol. These data demonstrate that chronic dosing with propranolol can impair retention of spatial learning, which cannot be attributed to reduced arousal or motor function.

A nitric oxide synthase inhibitor, L-NAME, attenuates saccharin drinking in a two-choice test in water-deprived rats.

The nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) [0 (veh)], 10, 25, and 50 mg/kg s.c.) was administered to water-deprived, saccharin-preferring, rats in a 30-min two-bottle choice test of 0.1% sodium saccharin and tap water in a within subjects design. Saccharin intake was selectively attenuated in a dose-related manner with increasing dose of L-NAME, reaching statistical significance at 25 and 50 mg/kg L-NAME when compared to vehicle control condition (p < 0.01). In contrast, water intake was not appreciably affected. Total fluid intake was attenuated as well. Neither saccharin nor water intake in a second group of animals was significantly affected by the inactive isomer, D-NAME, suggesting a stereospecific action. These data suggest that a taste factor might contribute to the well-documented hypophagic action of NOS inhibitors in a number of animal species. The possibility that such effect might be mediated through a serotonergic mechanism is considered.

Chronic administration of propranolol impairs inhibitory avoidance retention in mice.

Adrenergic systems are importantly involved in memory storage processes. As such, agents that alter adrenergic receptors, such as "beta-blockers," also alter memory storage. However, the anxiety literature cautions that beta-adrenergic receptor antagonists, such as propranolol, may have different behavioral effects with acute vs chronic dosing. The effects of chronic propranolol specifically on memory modulation are unknown. This study was designed to evaluate the effects of chronic propranolol on retention for an aversive task, in which there is endogenous adrenergic activation. Adult male ICR mice were given daily injections of one of four doses of propranolol (2, 4, 8, and 12 mg/kg) or 0.9% NaCl vehicle for 15 days prior to, and continuing during, behavioral tests of exploration and retention. Exploratory behavior, as an index of anxiety level, was measured in a conventional elevated plus-maze, whereas retention of an aversive experience was measured in a step-through inhibitory avoidance apparatus. Sensitivity to aversive footshock was also evaluated. Compared to controls, propranolol-treated mice showed a dose-dependent decrease in retention for the inhibitory avoidance task, but no effect on anxiety on the plus-maze or on footshock sensitivity. Taken together with results from previous studies, it is apparent that propranolol can have different behavioral effects when administered acutely vs chronically, and its chronic effects significantly impair memory storage processes. Since these drugs are typically used chronically, and often in older adults, they could contribute to functional memory impairments.

A nitric oxide synthase inhibitor NG-nitro-L-arginine, attenuates glucoprivic feeding and deprivation-induced drinking in the mouse.

Possible involvement of nitric oxide (NO) in glucoprivic hyperphagia was investigated in nondeprived male ICR mice in independent groups designs. One pair of experiments demonstrated dose-related reductions in 2-deoxy-D-glucose (2DG)- and insulin-induced solid food intake with increasing dose (10, 25, and 50 mg/kg s.c.) of the NO-synthase (NOS) inhibitor, NG-nitro-L-arginine (L-NOARG), reaching statistical significance at 10 mg/kg L-NOARG when compared to vehicle controls. In a second pair of experiments, initial pretreatment with L-arginine (500 and 1000 mg/kg i.p.) partially or completely restored the feeding inhibitory action of an effective challenge dose (25 mg/kg) of L-NOARG; D-arginine (500 mg/kg i.p.) was ineffective, thus supporting a stereospecific action of arginine. A third set of experiments demonstrated dose-related reduction in glucoprivic feeding under delayed access (4 or 6 h) to food. These findings suggest involvement of NO in glucoprivic hyperphagia; they are consistent with and extend research linking NO and ingestive behaviors through use of NOS inhibitors. Deprivation-induced drinking was attenuated by these doses of L-NOARG as well.

N(G)-nitro-L-arginine methyl ester reduces stress-related feeding in the rat tail-pinch model.

A possible role of nitric oxide (NO) in stress-related feeding was investigated in male rats using the tail-pinch (TP) model, in within-subjects experimental designs. An initial experiment demonstrated a dose-related reduction in TP-induced solid food intake over a 10-min test period with increasing dose (10.25, and 50 mg/kg SC) of the NO-synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), reaching statistical significance at 25 mg/kg L-NAME when compared to vehicle control (p < 0.05). Pattern analysis further revealed a decrease both in total duration of food-directed oral behavior and in percentage of longer duration (> 60 s) oral behavior bouts with increasing dose of L-NAME; both measures reached statistical significance at 50 mg/kg (p < 0.01). Pretreatment with 500 mg/kg of the NO precursor, L-arginine (L-arg), resulted in partial but not significant reversal of the attenuating effect of 25 mg/kg L-NAME on food intake. Latency to begin eating or gnawing was not significantly affected by L-NAME. In a subsequent experiment, L-arg alone (500 and 750 mg/kg) did not significantly alter TP-induced food intake. It is cautiously suggested that these results implicate involvement of NO in TP-induced feeding.

Possible involvement of nitric oxide in chlordiazepoxide-induced feeding in the mouse.

Two experiments investigated a possible role of nitric oxide (NO) in chlordiazepoxide (CP)-induced feeding in nondeprived male ICR mice in independent groups designs. Experiment 1 demonstrated a dose-related decrease in CP-induced solid food intake over a 60-min test period with increasing dose (10, 25, and 50 mg/ kg SC) of the NO-synthase (NOS) inhibitor, L-NG-nitro arginine (L-NOARG), reaching statistical significance at 10 mg/kg L-NOARG when compared to vehicle control. Identical doses of L-NOARG failed to significantly affect normal feeding in vehicle treated mice. In Experiment 2, initial pretreatment with L-arginine (500 and 1000 mg/kg IP) partially or completely restored the feeding inhibitory action of a challenge dose (25 mg/kg SC) of L-NOARG; D-arginine (500 mg/kg IP) was ineffective, thus supporting a stereospecific action of arginine. Arginine isomers did not differentially affect intake in normal feeding animals. These results implicate involvement of NO in CP-induced hyperphagia; they are consistent with and extend research linking NO and ingestive behaviors.

Nitrous oxide induces feeding in the nondeprived rat that is antagonized by naltrexone.

Three experiments investigated a possible effect of nitrous oxide (N2O) on food intake in nondeprived male hooded rats in independent groups designs. Experiment 1 demonstrated a concentration-related increase in intake with increasing level of nitrous oxide (10-40% N2O), reaching statistical significance at 20% N2O when compared to room air controls (p < 0.05). In experiment 2, pretreatment with 10 and 20 mg/kg of the benzodiazepine antagonist, flumazenil, failed to significantly attenuate 30% N2O-induced hyperphagia. In Experiment 3, pretreatment with the opioid antagonist, naltrexone, effectively antagonized 30% N2O-induced hyperphagia. Pronounced attenuation (to 59% of 30% N2O-induced intake level over a 1 h period) at the lowest dose of naltrexone (0.1 mg/kg, p < 0.01) compared to vehicle level resulted in a shallow dose-response curve across the dose range tested (0.1-10.0 mg/kg). These results suggest that an endogenous opioid mechanism is prominently involved in the N2O-induced ingestive response.

Benzodiazepine receptor-mediated behavioral effects of nitrous oxide in the rat social interaction test.

The present study was conducted to ascertain whether an anxiolytic effect of nitrous oxide was demonstrable in rats using the social interaction test and whether this drug effect might be mediated by benzodiazepine receptors. Compared to behavior of vehicle-pretreated, room air-exposed rats, rat pairs exposed to nitrous oxide showed a generally inverted U-shaped dose-response curve with the maximum increase in social interaction encounters occurring at 25% and significant increase in time of active social interaction at 15-35%; higher concentrations produced a sedative effect that reduced social interaction. Treatment with 5.0 mg/kg of the anxiolytic benzodiazepine chlordiazepoxide also increased social interaction. Pretreatment with 10 mg/kg of the benzodiazepine receptor blocker flumazenil, which alone had no effect, significantly antagonized the social interaction-increasing effects of both nitrous oxide and chlordiazepoxide. In summary, these findings suggest that nitrous oxide produces a flumazenil-sensitive effect comparable to that of chlordiazepoxide and implicate central benzodiazepine mechanisms in mediation of the anxiolytic effect of nitrous oxide.

Nitrous oxide induces an anxiolytic-like effect in the conditioned defensive burying paradigm, which can be reversed with a benzodiazepine receptor blocker.

To investigate the anxiolytic effects of nitrous oxide (N2O), male hooded rats were tested in the conditioned defensive burying (CDB) test, a paradigm that exploits a propensity of rats to bury objects associated with aversive stimulation. A single, brief electrical shock was delivered to rats upon contact with an electrified prod, before exposure to one of four mixtures of N2O and oxygen (O2) (10-40% N2O) or room air (RA). Compared to RA-exposed animals, rats exposed to N2O exhibited a concentration-related reduction in duration and height of prod-directed "defensive" burying with floor bedding material; these measures reached statistical significance at 30% N2O. Pretreatment with 20 mg/kg of the benzodiazepine receptor blocker flumazenil, which alone had no effect, effectively antagonized a 30% N2O-induced decrease in burying. Horizontal locomotion and rearing were not significantly affected at concentrations of N2O that attenuated prod-directed burying. Treatment with the benzodiazepine anxiolytic standard, chlordiazepoxide (2.5-10.0 mg/kg) also resulted in dose-related attenuation of burying behavior. These findings show that N2O can induce effects similar to those of known anxiolytics in this paradigm and suggest a benzodiazepine mechanism in its mediation.

Effect of drinking on angiotensin-II-induced shifts in regional cerebral blood flow in the rat.

A map of brain regions affected by central administration of the octapeptide angiotensin II (AII) and that would further reflect the consequences of AII's well-known dipsogenic action was developed. Regional cerebral blood flow (rCBF) as an indicator of neuronal activity was measured in conscious rats shortly after an ICV bolus injection of 100 ng AII or saline vehicle (VEH). AII-treated animals were further subdivided into two groups that were either permitted to drink [AII (W+)] or whose water was removed when drinking was attempted [AII (W-)]. When compared to VEH condition, blood flow increased significantly within 1 min after AII treatment in 33 of 53 regions sampled in those rats not given an opportunity to drink. In 11 of these 33 regions, ingestion of a small amount of water was associated with a reversal of AII-induced elevation in blood flow [i.e., AII (W+) less than AII (W-)]; these regions included the organum vasculosum lamina terminalis, rostral lateral hypothalamus, supraoptic nuclei, rostral zona incerta, and median eminence. A group of similarly treated rats exhibited a significant elevation of mean arterial blood pressure following AII treatment without significant shifts in arterial blood gases, pH, or bicarbonate. These data are consistent with prominent involvement of the anteroventral third ventricular region of the rat brain. The results further indicate that rCBF may be a sensitive measure for the identification of central sites of action of AII as a dipsogenic agent and may reveal distinctions between regions associated primarily with initiation of drinking and those reflecting the results of subsequent behavioral events.

Anxiolytic effects of nitrous oxide in mice in the light-dark and holeboard exploratory tests.

To investigate anxiolytic effects of nitrous oxide (N2O), male mice were tested in two exploratory models--a two-chambered light-dark (L-D) unit and a holeboard. Tests were conducted inside a glovebag through which one of three mixtures of N2O and oxygen (25, 50 and 75% N2O) or room air (RA) was circulated at a flow rate of 10 l/min. The principal findings in the L-D unit were a concentration-related increase in number of interdepartmental transitions and a generalized increase in time spent on the light side. Nitrous oxide effectively elevated transitions in the L-D unit at a lower concentration (25% N2O) than was required to increase locomotor activity in an open field (50% N2O), suggesting that these two measures are at least partially independent; transitions might reflect a specific exploratory component of locomotor behavior. In the holeboard test, a concentration-related increase in number of head dips was observed. Pretreatment with naltrexone-HCl or saline vehicle revealed a contribution by an endogenous opioid-linked locomotor stimulant effect in some measures. A dose-related reversal by flumazenil of 50% N2O-induced shifts in number of head dips and time spent head-dipping implicates a benzodiazepine receptor. Both paradigms, in particular the holeboard, should prove useful in future N2O research.

Drinking behavior in the spiny mouse (Acomys cahirinus) following putative dipsogenic challenges.

Male spiny mice (Acomys cahirinus) were challenged with several putative dipsogenic stimulus conditions: hypertonic sodium chloride (NaCl), 24-h water deprivation, d,l-isoproterenol HCl, angiotensin II (AII) and polyethylene glycol (PEG), or control conditions, in within-subjects designs. Water intake and drinking pattern were monitored electronically in the home cage over a 2--6-h test period without food present, during the light portion of the L/D cycle. In addition, hematocrits were measured following several treatments and mean arterial blood pressure was monitored in response to several doses of AII. As expected, both water deprivation and hypertonic NaCl led to robust drinking with short latencies. PEG was also an effective dipsogen; while quite variable, latencies were often shorter than are typically reported for the rat. Isoproterenol induced a modest, but significant, dose-related drinking. Interference by AII's prominent pressor action might account, at least in part, for its relative ineffectiveness as a dipsogen. Comparisons are made with other rodent species similarly challenged.

Effect of exogenous insulin on meal patterns and stomach emptying in the spiny mouse.

Male adult spiny mice (Acomys cahirinus) were acutely challenged with a single dose of regular insulin or saline vehicle SC; either food intake, meal frequency and meal duration, or stomach emptying were then measured. Meal frequency, as well as amount eaten, was significantly higher over a 6-hr period following both 10 and 30 U/kg of insulin than following vehicle injection. Meal duration remained essentially the same across all conditions. When 30 U/kg of insulin was administered either 15 min prior to, or immediately after, a solid food meal, stomach emptying (as measured by dry weight of recovered stomach contents) was accelerated relative to vehicle controls. These data are generally consistent with and extend the comparative literature suggesting a possible link between rate of stomach emptying and insulin-induced hyperphagia in some species.

Effect of insulin and 2-deoxy-D-glucose on feeding and plasma glucose levels in the spiny mouse.

Adult male spiny mice (Acomys cahirinus) were challenged with 2-deoxy-D-glucose (2-DG) or regular insulin, and food intake or plasma glucose concentration was measured. Mice did not increase their food intake over baseline levels following treatment with 2-DG (62.5-1000 mg/kg). In contrast, regular insulin injections (1-50 U/kg) stimulated a modest, but significant increase in feeding, which was apparent within 2 hr at a low dose of 1 U/kg. However, a marked hyper- and hypoglycemia (compared to saline controls), respectively, were induced within 30 min by 2-DG (250 and 500 mg/kg) and regular insulin (1 and 3 U/kg). Reduced glucose levels may not account for the insulin-induced hyperphagia.

Opioid modulation of ingestive behaviors in the spiny mouse (Acomys cahirinus).

Feeding and drinking behavior were studied in deprived or sated spiny mice (Acomys cahirinus) at various time intervals following peripheral administration of naloxone hydrochloride and butorphanol tartrate. Naloxone attenuated both food and water intake, but not latency to respond, indicating existence of functional opioid-sensitive feeding and drinking systems in this species. Butorphanol tartrate, a mixed opioid agonist/antagonist produced a dose-related enhancement or suppression of feeding, the former naloxone reversible, but had no measureable effect on drinking.

Drinking-induced alterations in reward pathways: an in vivo autoradiographic analysis.

An in vivo autoradiographic technique permitted the visualization of discrete neuroanatomical changes in opiate receptor binding as a result of 23-h water deprivation and drinking. Two groups of rats (n = 5) were placed on a 23-h water deprivation schedule for 10 days. On the last day, one group was given access to water for 15 min. These groups, plus a matched ad libitum water control group (n = 5), received an injection of 0.002 mg/kg [3H]diprenorphine ([3H]Dpr) through chronically implanted jugular catheters followed by preparation for opiate receptor autoradiography. Relative cerebral blood flow was estimated non-quantitatively by the injection of 75 microCi/kg iodo-[14C]antipyrene into 3 additional groups identically treated. Results indicated that water-deprivation stress increased [3H]Dpr binding in the claustrum, lateral hypothalamus, amygdala and ventral tegmental area while decreasing binding in the medial frontal cortex, lateral septum, dorsolateral thalamus and central gray. All effects of water deprivation were reversed in animals receiving water. Observations of changes in relative blood flow were shown to have no correlation with changes in opiate receptor binding. It appears that water deprivation stress causes a reduction in opioid release in areas along the mesotelencephalic dopamine pathway which may contribute to a drive state. Water intake may then reduce or otherwise alter the drive state through the release of opioids along these pathways, contributing to the perception of reward.

Triethyl lead attenuates feeding and drinking, and induces a conditioned taste aversion, in adult rats.

The effect of triethyl lead (TEL) on ingestive behavior in adult male rats was studied in two experiments. In experiment 1, ad lib food and water intakes were monitored following SC injection of 1, 4, or 7 mg/kg body weight of TEL or vehicle; both were significantly attenuated at 4 and 7 mg/kg doses. In a second experiment, the same doses of TEL were given SC in a conditioned taste aversion (CTA) paradigm. Following a single pairing, a dose-related reduction in intake of a 0.1% saccharin solution was observed at all doses tested. Sensitivity of behavioral measures and potential role of discomfort in TEL-induced feeding/drinking shifts were considered.

Some aspects of feeding and locomotor activity in adult rats exposed to tetraethyl lead.

The potential relationship between concomitant changes in feeding behavior and locomotor activity in rats exposed to tetraethyl lead (TTEL) was investigated. Two groups of rats were injected IP with a dose of TTEL (7 mg/kg) that depresses food intake. One group served as a control to replicate this effect. Animals in the second lead group were food yoked to animals in a placebo group. Food intake of rats in group 2 was maintained at or near the level of their yoked control via intragastric intubation. Food loads did not maintain body weights at control levels; however, locomotor activity level in both lead treated groups was significantly elevated above controls during normophagic periods when body weights approximated control levels as well as during hypophagic, low body weight periods. These observations suggest that activity shifts are not mediated in a simple way by factors linked to reduced food intake. Two additional experiments were briefly reported in which TTEL-exposed rats were challenged with 2-deoxy-D-glucose and insulin, either during their hypophagic phase or during a later recovered (normophagic) period. Feeding in response to glucoprivic challenge was similar to that of control animals under both conditions. Results were discussed.

Bilateral cannula system for intracranial chemical microinjection in small animals.

An easily constructed and inexpensive bilateral cannula assembly for microinjection of chemicals into neural tissue in small animals is described. It reduces problems sometimes encountered with commercially available units, making it useful in both research and teaching laboratory settings. Suggestions for implant procedures and modifications for use in unique applications are suggested.

A versatile integrated circuit activity monitor for small animals.

An easily constructed and inexpensive device for use in monitoring motor activity in small animals is described, which utilizes a suspension system and integrated circuit photodetector technology. Its sensitivity is readily adjusted, making it useful in a variety of applications. It provides a reasonably natural environment with minimal disturbance to the animal subject.