Rat pup isolation distress and the brain benzodiazepine receptor

Rat Ultrasonic Vocalizations and Behavioral Neuropharmacology: From the Screening of Drugs to the Study of Disease

Several lines of evidence indicate that rats emit ultrasonic vocalizations (USVs) in response to a wide range of stimuli that are capable of producing either euphoric (positive) or dysphoric (negative) emotional states. On these bases, recordings of USVs are extensively used in preclinical studies of affect, motivation, and social behavior. Rat USVs are sensitive to the effects of certain classes of psychoactive drugs, suggesting that emission of rat USVs can have relevance not only to neurobiology, but also to neuropharmacology and psychopharmacology. This review summarizes three types of rat USVs, namely 40-kHz USVs emitted by pups, 22-kHz USVs and 50-kHz USVs emitted by young and adult animals, and relevance of these vocalizations to neuropharmacological studies. Attention will be focused on the issues of how rat USVs can be used to evaluate the pharmacological properties of different classes of drugs, and how rat USVs can be combined with other behavioral models used in neuropharmacology. The strengths and limitations of experimental paradigms based on the evaluation of rat USVs will also be discussed.

Social stress, therapeutics and drug abuse: preclinical models of escalated and depressed intake

The impact of ostensibly aversive social stresses on triggering, amplifying and prolonging intensely rewarding drug taking is an apparent contradiction in need of resolution. Social stress encompasses various types of significant life events ranging from maternal separation stress, brief episodes of social confrontations in adolescence and adulthood, to continuous subordination stress, each with its own behavioral and physiological profile. The neural circuit comprising the VTA-accumbens-PFC-amygdala is activated by brief episodes of social stress, which is critical for the DA-mediated behavioral sensitization and increased stimulant consumption. A second neural circuit comprising the raphe-PFC-hippocampus is activated by continuous subordination stress and other types of uncontrollable stress. In terms of the development of therapeutics, brief maternal separation stress has proven useful in characterizing compounds acting on subtypes of GABA, glutamate, serotonin and opioid receptors with anxiolytic potential. While large increases in alcohol and cocaine intake during adulthood have been seen after prolonged maternal separation experiences during the first two weeks of rodent life, these effects may be modulated by additional yet to be identified factors. Brief episodes of defeat stress can engender behavioral sensitization that is relevant to escalated and prolonged self-administration of stimulants and possibly opioids, whereas continuous subordination stress leads to anhedonia-like effects. Understanding the intracellular cascade of events for the transition from episodic to continuous social stress in infancy and adulthood may provide insight into the modulation of basic reward processes that are critical for addictive and affective disorders.

Inhibition of ultrasonic vocalizations by beta-adrenoceptor agonists

Infant rat ultrasonic vocalizations (USVs) are widely believed to result from the induction of an emotional state of anxiety or distress. This perspective, however, is not easily reconciled with the demonstration by W. J. Farrell and J. R. Alberts 2000 that norepinephrine, a nonselective beta-adrenoceptor agonist with anxiogenic properties, inhibits production of USVs. Here, Farrell and Alberts' finding was replicated and extended with 12-day-old rats using a conventional isolation paradigm. First, treatment with norepinephrine (1 mg/kg) significantly inhibited ultrasound production while also increasing body temperature. Next, treatment with the beta-2 agonist terbutaline (1 mg/kg) and the beta-3 agonist CL-316243 (1 mg/kg), but not the beta-1 agonist dobutamine (1 mg/kg), inhibited ultrasound production; only CL-316243 increased body temperature. The unexpected inhibition of USVs by terbutaline, a potent bronchodilator, was replicated using a slightly modified procedure; again, body temperature was unaffected by terbutaline administration. In no experiment was inhibition of USVs related to changes in motor activity. Altogether, these results suggest either that ultrasound production is not a valid indicator of anxiety or that anxiety in infant rats is produced by neuropharmacological mechanisms that differ fundamentally from those in adults.

Effects of antihypertensive drugs on ultrasound production and cardiovascular responses in 15-day-old rats

When exposed to extreme cold or injected with the alpha(2)-adrenoceptor agonist, clonidine, infant rats emit ultrasonic vocalizations (USVs). Based upon the cardiovascular changes that accompany these two manipulations, especially decreased venous return, it was hypothesized that USVs are the acoustic by-product of the abdominal compression reaction (ACR), a maneuver that increases venous return. If this hypothesis is correct, then other anithypertensive drugs that decrease venous return should evoke USVs. In Experiment 1, sodium nitroprusside (SNP, 400 microg/kg), a direct-acting dilator of arteries and veins, was administered to 15-day-old rats under thermoneutral conditions while cardiac rate and ultrasound production were monitored. In Experiment 2, femoral artery pressure was monitored after SNP administration. Infants responded to SNP administration with decreased arterial pressure and tachycardia and, in addition, significantly increased ultrasound production. In Experiment 3, chlorisondamine (5 mg/kg), a ganglionic blocker that causes vasodilation and bradycardia, and hydralazine (20 mg/kg), a selective dilator of arteries, was administered to 15-day-olds. As predicted, chlorisondamine evoked ultrasound production and hydralazine did not. These results introduce SNP and chlorisondamine as only the second and third known agents capable of independently evoking USVs in thermoneutral conditions, and provide further support for the notion that ultrasound production is triggered by decreased venous return.

Do infant rats cry?

In the current revival of interest in the emotional and mental lives of animals, many investigators have focused attention on mammalian infants that emit distress vocalizations when separated from the home environment. Perhaps the most intensively studied distress vocalization is the ultrasonic vocalization of infant rats. Since its discovery, this vocalization has been interpreted both as a communicatory signal for the elicitation of maternal retrieval and as the manifestation of emotional distress. In contrast, the authors examined the cardiovascular causes and consequences of the vocalization, and on the basis of this work, they hypothesized that the vocalization is the acoustic by-product of the abdominal compression reaction (ACR), a maneuver that results in increased venous return to the heart. Therefore, the vocalization may be analogous to a sneeze, serving a physiological function while incidentally producing sound.

Maternal deprivation of neonatal rats produces enduring changes in dopamine function

Isolation-rearing of weanling rats produces a syndrome of behavioral and neurochemical effects that are indicative of enhanced ventrostriatal dopamine function observed in adulthood. By contrast, maternal deprivation of neonatal rats decreases behavioral responses to dopamine agonists when tested in adults, which may indicate the opposite situation. However, in the present study it is reported that in vivo microdialysis of the nucleus accumbens (NAC) revealed enhanced release of dopamine (DA) in response to both d-amphetamine and high K+ perfusate in maternally deprived subjects. Thus, behavioral responses to d-amphetamine are diminished in maternally deprived rats despite apparent increases in presynaptic dopaminergic function in the NAC.

Rat pup ultrasonic vocalization: effects of benzodiazepine receptor ligands

The involvement of the GABA(A)-benzodiazepine receptor complex in rat pup ultrasonic vocalisations was studied by testing benzodiazepine receptor ligands with varying intrinsic activity and selectivity for benzodiazepine subtype receptors. Ultrasonic vocalisations were recorded under two temperature conditions (37 degrees C and 18 degrees C), presumably reflecting a low and high stress state. The latency to the negative geotaxis response, a measure of motor coordination and the rectal temperature were determined to assess putative side effects of drugs. The full, non-selective benzodiazepine receptor agonists diazepam, chlordiazepoxide, alprazolam and oxazepam suppressed ultrasonic vocalisations both at 37 degrees C and 18 degrees C conditions, although more efficaciously at 37 degrees C. The partial, non-selective benzodiazepine receptor agonist bretazenil and the partial benzodiazepine, selective receptor agonist alpidem significantly reduced ultrasonic vocalisations at 37 degrees C, but not at 18 degrees C. The full benzodiazepine, selective receptor agonist zolpidem behaved like other full, non-selective benzodiazepine receptor agonists by reducing ultrasonic vocalisations under both high and low temperature. The effects of zolpidem indicate that activation of benzodiazepine, receptors alone already suffices to suppress ultrasonic vocalisations. The non-selective, benzodiazepine receptor antagonist flumazenil and the partial, non-selective benzodiazepine receptor inverse agonist FG 7142 (N'-methyl-beta-carboline-3-carboxamide) and the full, non-selective benzodiazepine receptor inverse agonist DMCM (6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) had no significant effect on ultrasonic vocalisations under both temperature conditions. The involvement of benzodiazepine receptors in rat pup ultrasonic vocalisations (37 degrees C-condition) was confirmed by antagonism of the ultrasonic vocalisations reducing effects of chlordiazepoxide by flumazenil (1 or 3 mg/kg). Using the rat pup ultrasonic vocalisations paradigm under 18 degrees C and 37 degrees C conditions combined with measurements of negative geotaxis-latencies and rectal temperatures it is possible to (1) distinguish benzodiazepine receptor agonists from other anxiolytics because of dissimilar dose response curves at 37 degrees C and 18 degrees C, (2) differentiate partial from full receptor agonists by absence of effects at the 18 degrees C condition, (3) suggest a key role for benzodiazepine, receptors in the modulation of ultrasonic vocalisations. These data contribute to the predictive validity of pup vocalizations as an animal model of anxiety.

Brain substrates of infant-mother attachment: contributions of opioids, oxytocin, and norepinephrine

The aim of this paper is to review recent work concerning the psychobiological substrates of social bonding, focusing on the literature attributed to opioids, oxytocin and norepinephrine in rats. Existing evidence and thinking about the biological foundations of attachment in young mammalian species and the neurobiology of several other affiliative behaviors including maternal behavior, sexual behavior and social memory is reviewed. We postulate the existence of social motivation circuitry which is common to all mammals and consistent across development. Oxytocin, vasopressin, endogenous opioids and catecholamines appear to participate in a wide variety of affiliative behaviors and are likely to be important components in this circuitry. It is proposed that these same neurochemical and neuroanatomical patterns will emerge as key substrates in the neurobiology of infant attachments to their caregivers.

Multiple regulators of ultrasonic vocalization in the infant rat

This paper reviews our growing understanding of the external environmental and the internal neuroregulatory mechanisms for the infant rat's vocal response to abrupt isolation from nest, littermates and mother. Interactions of the rat pup with specific components of its animate and inanimate environment have been identified and found to exert a tonic regulatory action on the neural systems mediating ultrasonic vocalization (USV). In isolation, withdrawal of these regulators produces an acute outburst of calling. Changes in rate of ultrasonic calling in response to administration of receptor agonist and antagonist agents generally correspond to the effects of those agents on measures of anxiety in adult animals and in human clinical populations, suggesting a conservation of neural substrates for anxiety in evolution and in development. The inhibition of isolation calling in response to predator cues and its potentiation by brief exposure to maternal cues, represent novel forms of regulation with implications for neuroendocrine development and for the neurobiology of early cognitive-emotional processes.

MDL 100,458 and MDL 102,288: two potent and selective glycine receptor antagonists with different functional profiles

Glycine receptor antagonists have been proposed to have multiple therapeutic applications, including the treatment of stroke, epilepsy, and anxiety. The present study compared the biochemical and behavioral profiles of two strychnine-insensitive glycine receptor antagonists, MDL 100,458 (3-(benzoylmethylamino)-6-chloro-1H-indole-2- carboxylic acid) and MDL 102,288 (5,7-dichloro-1,4-dihydro-4-[[[4- [(methoxycarbonyl)amino]phenyl]sulfonyl]imino]-2-quinolinecarboxylic acid monohydrate). Both compounds potently inhibited [3H]glycine binding to rat cortical/hippocampal membranes (Ki = 136, 167 nM, respectively) without showing significant activity in 18 other receptor binding assays. In an in vitro functional assay, both compounds completely antagonized N-methyl-D-aspartate (NMDA)-stimulated cGMP accumulation in rat cerebellar slices. However, in contrast to their equipotency in the glycine receptor assay, MDL 100,458 was approximately 6-fold more potent than MDL 102,288 in the cGMP assay (IC50 values = 1.25, 7.8 microM, respectively). Behavioral tests demonstrated that MDL 102,288 and MDL 100,458 exhibited strikingly different in vivo profiles. MDL 100,458 antagonized audiogenic seizures in DBA/2J mice (ED50 = 20.8 mg/kg i.p.), whereas MDL 102,288 was without effect in the dose range tested (ED50 > 300 mg/kg i.p.). Central nervous system penetration did not appear to account for this difference. For example, MDL 102,288 was not active following direct intracerebroventricular administration (ED50 > 16 micrograms; vs. 0.78 microgram for MDL 100,458). In a test of anxiolytic activity, MDL 102,288 reduced separation-induced ultrasonic vocalizations in rat pups (ED50 = 6.3 mg/kg i.p.) whereas MDL 100,458 was only weakly active (ED50 = 80.8 mg/kg i.p.). Furthermore, the anxiolytic effect of MDL 102,288 was selective in that it occurred at doses that did not produce motoric disruption as measured by an inclined-plane test (ED50 > 160 mg/kg; therapeutic index > 25.4). In contrast, the anxiolytic activity of MDL 100,458 was non-selective in that it occurred at doses that also produced motoric disruption (ED50 = 57.7 mg/kg; therapeutic index = 0.7). Thus, two glycine receptor antagonists which have similar in vitro binding profiles as selective ligands for the strychnine-insensitive glycine receptor, demonstrate different in vitro and in vivo functional profiles. The reason for these differences is not clear, though one possibility could be that the compounds may act on different NMDA receptor subtypes. These data support the possibility that different glycine receptor antagonists may have different therapeutic targets.

Aggression, anxiety and vocalizations in animals: GABAA and 5-HT anxiolytics

A continuing challenge for preclinical research on anxiolytic drugs is to capture the affective dimension that characterizes anxiety and aggression, either in their adaptive forms or when they become of clinical concern. Experimental protocols for the preclinical study of anxiolytic drugs typically involve the suppression of conditioned or unconditioned social and exploratory behavior (e.g., punished drinking or social interactions) and demonstrate the reversal of this behavioral suppression by drugs acting on the benzodiazepine-GABAA complex. Less frequently, aversive events engender increases in conditioned or unconditioned behavior that are reversed by anxiolytic drugs (e.g., fear-potentiated startle). More recently, putative anxiolytics which target 5-HT receptor subtypes produced effects in these traditional protocols that often are not systematic and robust. We propose ethological studies of vocal expressions in rodents and primates during social confrontations, separation from social companions, or exposure to aversive environmental events as promising sources of information on the affective features of behavior. This approach focuses on vocal and other display behavior with clear functional validity and homology. Drugs with anxiolytic effects that act on the benzodiazepine-GABAA receptor complex and on 5-HT1A receptors systematically and potently alter specific vocalizations in rodents and primates in a pharmacologically reversible manner; the specificity of these effects on vocalizations is evident due to the effectiveness of low doses that do not compromise other physiological and behavioral processes. Antagonists at the benzodiazepine receptor reverse the effects of full agonists on vocalizations, particularly when these occur in threatening, startling and distressing contexts. With the development of antagonists at 5-HT receptor subtypes, it can be anticipated that similar receptor-specificity can be established for the effects of 5-HT anxiolytics.

The effects of melatonin on isolation distress in chickens

Melatonin (0.1-1.0 mg/kg) reduced isolation-induced distress vocalizations (DVs) in young domestic chickens in a dose-dependent manner. This effect was unaffected by the administration of d-amphetamine (1.0 mg/kg) suggesting that melatonin's effects were not merely due to fatigue. The melatonin reduction in DVs was not naloxone reversible, indicating an action independent of the endogenous opioid system. However, chronic pretreatment with naltrexone facilitated the melatonin effect, suggesting a complex relationship between melatonin and the endogenous opioids in regulating distress vocalizations. Chickens exhibited a marked reduction in DVs when isolation chambers were darkened, suggesting endogenous, as well as exogenous, melatonin mediation of isolation distress; however; pinealectomy only partially reversed the darkness effect. Pinealectomized animals, like control animals, exhibited a reduction in DVs following melatonin treatment; however, the melatonin effect was shorter lasting. The implications that these results may have for socialization and emotional distress are discussed.

Behavioral and hormonal responses to stress in the newborn mouse: effects of maternal deprivation and chlordiazepoxide

These studies investigated behavioral and hormonal responses to stress in developing mice. Experiment 1 examined the effects of 24-hr maternal deprivation on corticosterone (CORT) secretion and ultrasonic vocalization (UVZ) rate in 4-, 8-, and 12-day-old mice. At these ages, exposure to a novel environment resulted in minimal changes in CORT secretion. Maternal deprivation increased pups' CORT secretion in an age-dependent fashion but did not affect their UVZ rate. The aim of experiment 2 was to test the effects of chlordiazepoxide (CDP), an anxiolytic compound, on CORT secretion and UVZ in both normally reared and in maternally deprived 8-day-old mice. CDP administration elevated CORT secretion in a dose-dependent fashion, producing larger CORT increases in deprived (DEP) animals. CDP affected UVZ only in nondeprived (NDEP) animals: UVZ rate was decreased by high CDP doses. Overall, these findings demonstrate that the infant mouse shows a period of stress hyporesponsiveness similar to the rat and that maternal presence contributes to inhibit adrenocortical activity. CDP administration, but not novelty exposure, increased CORT secretion in 8-day-old normally reared mice suggesting that during the stress hyporesponsive period, the HPA axis is capable of responding only to specific stimuli. Changes in HPA axis activity and UVZ rate resulting from maternal deprivation and/or CDP challenge do not seem to be directly related.

Diazepam withdrawal: effects of diazepam and gepirone on acoustic startle-induced 22 kHz ultrasonic vocalizations

It has proven difficult to demonstrate and study the "anxiogenic" quality of drug withdrawal states in animals. Ultrasonic vocalizations (USV) in response to acoustic startle stimuli have shown promise as a measure of affect and may represent "distress" responses during diazepam withdrawal. Three experiments evaluated the association between USV and "distress" by comparing the effects of diazepam as a prototypic benzodiazepine agonist and the putative anxiolytic gepirone with affinity for 5-hydroxytryptamine (5-HT1A) receptors in naive and diazepam-withdrawn subjects. Adult male Long-Evans rats were exposed to acoustic startle sessions consisting of nine 105 dB and nine 115 dB stimuli. USV at 20-30 kHz were readily emitted during startle and often commenced after the third or fourth stimulus presentation. Acutely, intraperitoneal (IP) administration of diazepam (0.1-3 mg/kg) and gepirone (0.1-1 mg/kg) decreased USV dose-dependently without affecting the startle reflex; gepirone also decreased tail flick latency. Startle-induced USV were also sensitive to the "anxiogenic" effects of withdrawal from diazepam exposure (0, 2.5, 5, 10 mg/kg b.i.d. IP x 5 days). Twenty-four hours after the last diazepam injection, rats were hyperreactive to startle stimuli and doubled their rate of USV over vehicle-treated controls. Gepirone (0.1-1 mg/kg IP), but not diazepam (3-20 mg/kg IP) antagonized the increased rate of USV in rats withdrawn from 10 mg/kg b.i.d. diazepam. Diazepam (2.5-10 mg/kg IP) antagonized the increased rate of USV in rats withdrawn from 2.5 mg/kg b.i.d. diazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of 5-HT receptor ligands on ultrasonic calling in mouse pups

The influence of a range of drugs acting at various 5-HT receptor sites on the ultrasonic calling of mouse pups was assessed. Calling was decreased by the novel anxiolytics buspirone, ipsapirone and gepirone, and by TFMPP, spiperone, ritanserin and GR 38032F. In contrast, 8-OH-DPAT, DOI and quipazine increased the rate of calling. These effects on ultrasonic calling were independent of sedative or thermoregulatory actions of these drugs. Present data provide further support for the view that ultrasonic calling can be used to assess novel compounds for possible anxiolytic or anxiogenic properties.

NMDA receptor complex antagonists have potential anxiolytic effects as measured with separation-induced ultrasonic vocalizations

Pre-weaning rat pups emit ultrasonic vocalizations when removed from the litter. These 'separation-induced vocalizations' (SIV) are suppressed by classical benzodiazepine anxiolytics and by non-benzodiazepine anxiolytics which lack muscle relaxant and sedative properties. The present study used the SIV model to assess potential anxiolytic properties of compounds which target different sites associated with the NMDA receptor complex. Comparison was made to drugs which affect benzodiazepine or serotonin (5-HT) receptors. Muscle relaxant potential was assessed using 'TIP' (time on an inclined plane), the amount of time a pup was able to retain its position on a steeply inclined surface. Mephenesin, a centrally acting muscle relaxant, significantly suppressed TIP but not SIV. The benzodiazepine agonist diazepam suppressed both SIV and TIP, whereas the 5-HT1A partial agonists, buspirone and MDL 73,005EF, suppressed SIV without affecting TIP. The 5-HT2 antagonist MDL 11,939 suppressed TIP but not SIV, whereas neither measure was affected by the 5-HT3 antagonist MDL 73,147EF. SIV was suppressed by NMDA antagonists including those acting at the glutamate recognition site (D,L-amino-phosphonovaleric acid (AP5) and MDL 100,453) or at the ion channel (MK-801), or by the strychnine-insensitive glycine antagonist 5,7-dichlorokynurenic acid (5,7-DCKA). TIP was suppressed even more potently by AP5, MDL 100,453 and MK-801, whereas 5,7-DCKA was inactive on this measure. Thus, antagonists acting at different sites present on the glutamate recognition site exhibit potential anxiolytic activity, but the glycine antagonist was unusual in its lack of prominent muscle relaxant side effects.

Infant rat separation is a sensitive test for novel anxiolytics

1. Rat pups emit ultrasonic calls during brief episodes of social separation. These calls have been variously described as "distress" calls and may be related to the separation cries expressed by the young of many mammalian species. 2. Ultrasonic call of rat pups are modulated by environmental stimuli such as ambient temperature, olfactory and tactile stimuli associated with the nest. 3. Calls are also sensitive to a variety of purported anxiolytic and anxiogenic drugs, including the benzodiazepines, serotonin agonists, and ligands at the NMDA-glycine receptor complex. 4. In addition to providing a simple test for the anxiolytic properties of drugs, this model may also provide new insights about the development and neurobiology of anxiety.

The effects of opioid and benzodiazepine antagonists on dam-induced reductions in rat pup isolation distress

In 10-day-old pups isolated in a novel environment, the presence of the dam reduced the rate of ultrasonic vocalization by 93%. The opiate antagonist naltrexone caused a dose-related increase in the rate of vocalization that was significant only at 5.0 mg/kg. This suggests that the dam exerts comforting effects through endogenous opioid mechanisms. Although the benzodiazepine agonist chlordiazepoxide is as effective as morphine in the quieting of isolation distress, its antagonist, Ro 15-1788, at doses up to 20 mg/kg did not alter the lowered rate of vocalization associated with the presence of the dam. Instead, it facilitated quiet contact with the companion. This raises the possibility that the presence of a social companion precludes the release of an endogenous anxiogen that binds to the benzodiazepine receptor.