Ultrasound Vocalization Research Articles

Early social enrichment affects responsiveness to different social cues in female mice

Communal nesting (CN), an early social enrichment procedure in which multiple females rear the offspring in a single nest, increases maternal care levels received by offspring and interaction with peers. It has been shown that male mice reared under CN conditions show increased social competence and propensity to social interactions at adulthood. In the present study we investigated long-term behavioural effects of CN on female offspring. Mouse pups were reared under two different experimental conditions: standard nesting (SN, where single mother rears her pups) and CN (three females rearing their pups in a single nest). At adulthood CN and SN virgin females underwent three different behavioural tests: (i) maternal induction following presentation of foster pups; (ii) social recognition test in which ultrasound vocalizations (USVs) and social investigation behaviour emitted by a resident female in the presence of a female partner were recorded; (iii) zero-maze to analyze anxiety profiles. CN females showed (i) decreased licking response in the maternal induction test accompanied by an increased sniffing response; (ii) decreased of social interest towards a novel partner (during the Retest Different phase), and decreased USV emission rate in the social recognition test; CN and SN females did not differ in the emotional responses measured in the zero-maze apparatus. As a whole these data suggest that CN rearing render female mice less reactive to social novelty.

Tactile-Induced Ultrasonic Vocalization in the Rat: A Novel Assay to Assess Anti-Migraine Therapies in vivo

A pharmacological model of migraine is described using ultrasound vocalization (USV) of rats following central inflammation-induced sensitization to tactile stimulation. Central inflammation induced by intracerebroventricular injection of lipopolysaccharide (LPS) increased USV induced by an air current focused on the head and this was abolished by morphine and ketorolac, suggesting a nociceptive component. USV in naive rats were unaffected. Diazepam reduced USV in both inflamed and naive rats. The triptans, zolmitriptan and sumatriptan, both reduced USV in inflamed but not in naive rats, as did dihydroergotamine, and the calcitonin gene-related peptide (CGRP) antagonists alphaCGRP(8-37) and BIBN4096BS. The neurokinin-1 antagonist L-733-060 had no effect in either inflamed or naive rats when given after induction of inflammation, but when given with the LPS it prevented the augmentation of USV. This profile of activity of agents proven to be effective in the clinic suggests this model can be used to predict novel therapeutic agents for migraine.

Anxiogenic effects of activation of NK-1 receptors of the dorsal periaqueductal gray as assessed by the elevated plus-maze, ultrasound vocalizations and tail-flick tests

Ultrasound vocalizations (USVs) known as 22kHz are usual components of the defensive responses of rats exposed to threatening conditions. The amount of emission of 22kHz USVs depends on the intensity of the aversive stimuli. While moderate fear causes an anxiolytic-sensitive enhancement of the defensive responses, high fear tended to reduce the defensive performance of the animals to aversive stimuli. The dorsal periaqueductal gray (dPAG) is an important vocal center and a crucial structure for the expression of defensive responses. Substance P (SP) is involved in the modulation of the defensive response at this midbrain level, but the type of neurokinin receptors involved in this action is not completely understood. In this study we examined whether local injections of the selective NK-1 agonist SAR-MET-SP (10-100 pmol/0.2microL) into the dPAG (i) cause anxiogenic effects in the elevated plus-maze (EPM) (Exp. I), (ii) influence the novelty-induced 22kHz USVs recorded within the frequency range of 20-26kHz (Exp. II) and (iii) change the nociceptive reactivity to heat applied to the rat's tail (Exp III). The data obtained showed that SAR-MET-SP elicited significant "anxiety-like" behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM. These anxiogenic effects were accompanied with antinociception and disruption of the novelty-induced increase in the number and duration of 22kHz USVs. These findings are in agreement with the notion that NK-1 receptors of the dPAG may be an important neurochemical target for new selective drugs aimed at the control of pathological anxiety states.

Sound localization by echolocating bats: Are auditory signals enough?

Echolocating bats localize objects by producing ultrasound vocalizations and listening to sonar returns. Interaural differences in the echoes arriving at the bats’ ears contribute to the process of object localization. The spatial properties of sonar emissions and the mobility of the head and pinna also contribute to the process of sound localization in bats. Measurements of sonar emission patterns from unrestrained, freely vocalizing bats have shown that sonar beam shape varies across individual vocalizations. Consequently, the bat auditory system needs to adapt its computations to accurately localize objects using changing acoustic inputs. Extra-auditory signals that carry information about pinna position and beam shape are required for auditory localization of sound sources. The auditory system must learn associations between extra-auditory signals (e.g., head and pinna position) and acoustic spatial cues. These demands on the nervous system suggest that sound localization is achieved through the interaction of behavioral control and acoustic inputs. A sensorimotor model that demonstrates how an organism can learn space through auditory-motor contingencies is presented. The model also reveals how different aspects of sound localization, e.g., experience-dependent acquisition, adaptation, and extra-auditory influences, can be joined under a comprehensive framework.

Effects of low doses of atrazine on the neurobehavioural development of mice

Recent studies suggest that atrazine (ATZ), a triazine herbicide widely used and frequently detected in ground and surface water, is able to affect estrogen production by inducing aromatase, the enzyme that converts androgen into estrogen, an essential transformation occurring at the CNS level for maturation and expression of behaviour. In the present study we evaluated the effects of environmentally relevant doses of ATZ on somatic growth and early behavioural ontogeny, a crucial stage in shaping future behaviour. For this purpose we observed mice born to mothers exposed to 1 or 100 µg/kg ATZ during pregnancy and lactation. We studied, between postnatal day 2 to 15, the righting reflex, cliff aversion, forepaw grasping, auditory startle, eyelid and ear opening, and ultrasound vocalizations. In both sexes ATZ effects were evident, on body weight at birth, on the maturation of righting and grasping reflexes, and on the rate of emission and the spectrographic characteristics of ultrasound vocalizations. Dosage level appeared to be particularly relevant because, in some cases, the lower ATZ exposure was more effective than the higher one in modifying behavioural response. The latter finding suggests that this compound, similarly to many others endocrine disruptors, does not follow a linear dose-response curve, and that, as a consequence, its effects should be studied carefully at very low doses. Our results, compatible with ATZ properties, suggest caution in the use of a chemical agent that may, even at low doses, interfere with brain development and differentiation, inducing alterations of the developmental trajectories of behaviours.

Neonatal basal forebrain cholinergic hypofunction affects ultrasonic vocalizations and fear conditioning responses in preweaning rats

The present study investigated the effects of intracebroventricular injections of 192 IgG-saporin in 7-day-old rats on (i) ultrasound vocalizations (USVs) on postnatal day (pnd) 13 following isolation and reunion with the mother and (ii) fear conditioning on pnd 18–19 recording both freezing and other behavioural responses as well as USVs. On pnd 13 lesioned and control pups showed comparable USV baseline values; a brief reunion with the mother induced a significant increase in USVs in all rats (maternal potentiation). On pnd 18, during the fear conditioning training, 192 IgG-saporin rats emitted a lower number of USVs. On pnd 19 all rats showed a stronger conditioned response (with full inhibition of locomotion) to auditory than to contextual cues. Surprisingly, lesioned rats showed a stronger fear-conditioned response to contextual cues than controls. These results suggest that early selective removal of the cholinergic basal forebrain paradoxically enhances hippocampally dependent fear-conditioned responses on pnd 19.

Substance P injected into the dorsal periaqueductal gray causes anxiogenic effects similar to the long-term isolation as assessed by ultrasound vocalizations measurements

Housing conditions change the emotional state of the animals. Ultrasound vocalizations (USVs) termed as 22 kHz are the usual components of the defensive responses of rats exposed to threatening conditions such as isolation. The amount of emission of 22 kHz USVs depends on the intensity of the aversive stimuli. While short periods of isolation caused an anxiolytic-sensitive enhancement of the defensive responses, long-term isolation tended to reduce the defensive performance of the animals to aversive stimuli. The dorsal periaqueductal gray (dPAG) is an important vocal center and a crucial structure for the expression of defensive response. While it has been shown that Substance P (SP) at this midbrain level is involved in the modulation of the defensive response, its role in the emission of ultrasound vocalizations has not been evaluated. In this study we examined whether the resocialization and local injections of SP into the dPAG have an influence on the isolation-induced 22 kHz USVs recorded within the frequency range of 18–26 kHz. Rats isolated for 1 day showed a significant increase in the number and duration of USVs, which were reversed by resocialization. On the other hand, 2-week isolation reduced the number and duration of 22 kHz USVs, which could not be reversed by resocialization. SP injections into the dPAG (35 pmol/0.2 μL) caused a reduction in the 22 kHz USVs. Pretreatment with the NK-1 receptor antagonist spantide (100 pmol/0.2 μL) blocked these effects but exhibited no effect when given alone. These findings suggest that 1-day and 2-week isolation recruit distinct brain defensive systems. Also, in agreement with the notion that intense fear is associated with the neural substrates of fear of the dPAG, activation of NK-1 receptors of this midbrain structure reduces the 22 kHz USVs.

Paradoxical effects of prenatal acetylcholinesterase blockade on neuro-behavioral development and drug-induced stereotypies in reeler mutant mice

Epidemiological and experimental studies support a link between genetic and epigenetic factors in vulnerability to develop enduring neurobehavioral alterations. We studied the interplay between genetic vulnerability and the prenatal exposure to a neurotoxic compound. Chlorpyrifos, a potent and reversible acetylcholinesterase blocker used as a pesticide, and the "reeler" mouse, lacking the extracellular-matrix protein Reelin, were used. Homozygous reeler (RL), heterozygous (HZ), and wild-type (WT) mice were prenatally exposed to chlorpyrifos-oxon (CPF-O), the active metabolite of chlorpyrifos, or to vehicle (prenatal controls) on gestation days 14-16, that is, during a peak period of neurogenesis in the cerebral cortex. The offspring was reared by the natural dam and tested during infancy and at adulthood for global consequences of the prenatal exposure. The results are consistent with complex interactions between genetic (reeler genotype) and epigenetic (prenatal exposure to CPF-O) factors. In the case of some "genetically modulated" parameters (ultrasound vocalization, amphetamine-induced locomotion, and stereotypy), exposure to CPF-O paradoxically reverted the effects produced by progressive reelin absence. Conversely, for an "epigenetically modulated" parameter (grasping reflex maturation), the effects of CPF-O exposure were counteracted by progressive reelin absence. Finally, for parameters apparently untouched by either factor alone (righting reflex latency, scopolamine-induced locomotor activity), prenatal CPF-O exposure unmasked an otherwise latent genotype dependency. This complex picture also points to reciprocal adaptations within cholinergic and dopaminergic systems during development. Data are interesting in view of recently discovered cholinergic abnormalities in autism and schizophrenia, and may suggest new avenues for early intervention.

A social recognition test for female mice reveals behavioral effects of developmental chlorpyrifos exposure

CD-1 mice were exposed to the organophosphate pesticide chlorpyrifos (CPF) both prenatally (gestational days 15–18; doses 0, 3 or 6 mg/kg) and postnatally (postnatal days 11–14, doses 0, 1 or 3 mg/kg). When four-month-olds, females underwent a social recognition test in which ultrasound vocalizations (USVs) and social investigation behavior emitted by a resident female in the presence of a female partner were measured during two subsequent 3 min sessions (interval between the two sessions 45 min). Throughout the social recognition test a marked increase in USVs was found in females prenatally treated with the highest CPF dose; USV increase was also paralleled by a selective increase in frequency and not in duration of social investigation. These results confirm that developmental exposure to CPF induces long-lasting alterations in the social behavior repertoire of the mouse, thus extending our previous observations on the effects of postnatal CPF on male agonistic behavior to the female sex. They also suggest that social recognition can be easily and rapidly assessed in the female mouse making it possible to evaluate, primarily by means of USV emission, even subtle alteration of social behavioral patterns dissociated from cognitive components of individual recognition.

Genetic predisposition to anxiety-related behavior determines coping style, neuroendocrine responses, and neuronal activation during social defeat.

Genetic background may influence an individual's susceptibility to, and subsequent coping strategy for, an acute stressor. When exposed to social defeat (SD), rats bred for high (HAB) or low (LAB) trait anxiety, which also differ in depression-like behavior, showed highly divergent passive and active coping behaviors, respectively. HABs spent more time freezing and emitted more ultrasound vocalization calls during SD than LABs, which spent more time rearing and grooming. Although the behavioral data confirmed the prediction that heightened trait anxiety would make rats more prone to experience stress, adrenocorticotropin and corticosterone were secreted to a higher extent in LABs than in HABs. In the latter, Fos expression upon SD was enhanced in the amygdala and hypothalamic areas compared with LABs, whereas it was diminished in prefrontal and brainstem areas.

Opposite effects of short- and long-duration isolation on ultrasonic vocalization, startle and prepulse inhibition in rats

Housing conditions change the sensorimotor gating and the emotional state of rats. The prepulse inhibition (PPI) is a reduction in the startle reflex to loud sounds when they are preceded by acoustic stimuli of low-intensity, and this test has been considered to be a useful measurement of the functioning of sensory gating in animals and man. Rats reared under conditions of isolation from the time of weaning, but not for 1 week at adult age, display clear deficits in prepulse inhibition and in sensorimotor gating. Ultrasound vocalizations (USVs) at 20–24 kHz are the usual defensive responses of rats exposed to threatening conditions such as novel situations. The amount of emissions of ultrasound vocalizations at these frequencies depends on whether the aversive stimuli are presented either alone or in combination. Given this background we evaluated the prepulse inhibition and the emission of ultrasound vocalizations in response to novelty in rats isolated for 1 day or 2 weeks and compared the results to those in grouped rats. We also examined whether the anxiolytic agent midazolam (0.5 and 1.0 mg/kg) could reverse the effects of isolation under the experimental conditions used. Rats isolated for 1 day showed a significant increase in the number and duration of USVs together with an enhancement in the startle response to loud sounds, which were antagonized in a dose-dependent manner by midazolam. On the other hand, 2-week isolation had the effect of reducing the number of USVs emitted at 20–24 kHz without changing the startle response. The PPI was not changed by isolation, irrespective of the duration of isolation (1 day or 2 weeks). The results suggest that 1 day and 2 weeks of isolation have opposite effects on the emotional state of the animals. While short periods of isolation cause an anxiolytic-sensitive enhancement of the defensive responses, longer periods tend to reduce the defensive reaction of the animals to aversive stimuli. Based on these results, this work presents a novel method for induction of two different modes of defensive response, which are proposed to be mediated by separate neural substrates in rats. Also, isolation from 1 day to 2 weeks has no effect on the expression of prepulse inhibition and, by extension, on the functioning of the sensory gating.

Isolation-Induced Changes in Ultrasonic Vocalization, Fear-Potentiated Startle and Prepulse Inhibition in Rats

Isolation causes important changes in the behavioral reactivity of rats to environmental stimuli. These changes include deficit in sensorimotor gating and altered fear-like responses to aversive stimuli. Measures of ultrasound vocalizations at 20–22 kHz when rats are exposed to threatening conditions, such as novelty, have been taken as a good measure of fear. The fear-potentiated startle to loud sounds and the prepulse inhibition tests have been considered reliable indicators of anxiety and attention impairments, respectively. Rats reared under conditions of isolation from weaning display clear deficits in prepulse inhibition. Taking into account that housing condition changes the emotional state of the animals, we evaluated in this work the performance of rats in the fear-potentiated startle test, prepulse inhibition and emission of ultrasound vocalizations to novelty when isolated for 10 days and after resocialization for 1 week in comparison to grouped rats. Isolated rats showed greater reactivity to loud sounds in the fear-potentiated startle test than grouped animals. They also emitted less ultrasound vocalizations at 20–22 kHz than grouped animals when exposed to a novel environment. In contrast to the well-known deficit in prepulse inhibition displayed by isolation-reared animals, in the present study isolation for 10 days caused a significant increase in prepulse inhibition. Resocialization was not able to counteract the effects of isolation in all three tests. The results suggest that the emotional state of the animals is altered by 10 days of isolation; they do not vocalize characteristically as grouped rats when submitted to novelty; unconditioned responses to loud sounds are enhanced and increased prepulse inhibition is shown rather than a deficit as largely documented in studies with isolation-reared animals. It is suggested that the assessment of the emotional state of the animals is a prerequisite in the evaluation of prepulse inhibition. The level of defensive reactivity displayed by isolated animals is crucial for the functioning of sensory gating and, by extension, to the expression of prepulse inhibition.

Identification of Glyoxalase-I as a Protein Marker in a Mouse Model of Extremes in Trait Anxiety

For >15 generations, CD1 mice have been selectively and bidirectionally bred for either high-anxiety-related behavior (HAB-M) or low-anxiety-related behavior (LAB-M) on the elevated plus-maze. Independent of gender, HAB-M were more anxious than LAB-M animals in a variety of additional tests, including those reflecting risk assessment behaviors and ultrasound vocalization, with unselected CD1 "normal" control (NAB-M) and cross-mated (CM-M) mice displaying intermediate behavioral scores in most cases. Furthermore, in both the forced-swim and tail-suspension tests, LAB-M animals showed lower scores of immobility than did HAB-M and NAB-M animals, indicative of a reduced depression-like behavior. Using proteomic and microarray analyses, glyoxalase-I was identified as a protein marker, which is consistently expressed to a higher extent in LAB-M than in HAB-M mice in several brain areas. The same phenotype-dependent difference was found in red blood cells with NAB-M and CM-M animals showing intermediate expression profiles of glyoxalase-I. Additional studies will examine whether glyoxalase-I has an impact beyond that of a biomarker to predict the genetic predisposition to anxiety- and depression-like behavior.

Acoustic variability and distinguishability among mouse ultrasound vocalizations.

Auditory neurobiology has benefited significantly from ethological approaches using acoustic communication signals. Developing an ethological model in a genetically manipulable system such as the mouse would enhance the ability to investigate the processing, learning, and recognition of sounds. Characterizing the basic acoustic structure of mouse vocalizations would help lay a foundation for such a future study. Towards this goal, ultrasound vocalizations emitted by isolated mouse pups and pairs of adult males and females have been digitally recorded and examined. Previous work suggests that these calls may have communicative significance. An analysis of the natural variability in their spectral content, median frequency, duration, and repetition period reveals acoustic structure that could be used for recognizing the calls. Other parameters, like the rate of frequency modulation, may also be informative, but have not been examined. Pup isolation calls develop systematically between postnatal day 5 and 12 towards a more stereotyped vocalization--contracting from a wide range of values into narrower clusters of frequency and duration, and shifting from longer to shorter repetition periods. Most significantly, pup isolation and adult encounter calls fall into two distinct spectral and temporal categories, making it possible for a receiver to acoustically distinguish between them, and to potentially categorically perceive them along those dimensions.

Analysis of freezing behavior and ultrasonic vocalization in response to foot-shocks, ultrasound signals and GABAergic inhibition in the inferior colliculus: effects of muscimol and midazolam

Freezing behavior is a common response to distal threatening stimuli. It has been reported that experimenter-presented 20-kHz tones produce freezing in Wistar rats. The present studies were designed to determine the acoustic specificity for induction of the emission of ultrasound vocalizations (USV) and freezing of rats to either ultrasound tones or to a standard stressor, foot-shocks. We also examined whether GABA–benzodiazepine mechanisms, known for modulating anxiety-related processes, are involved in the regulation of defensive responses to these two aversive unconditioned stimuli. It was found that emission of USV was only observed with foot-shocks. Moreover, rats exhibited freezing when they were exposed to foot-shocks and 20–25-kHz ultrasound signals. . Likewise, removal of the GABAergic inhibitory control on the neural substrates of aversion in the inferior colliculus by local microinjections of semicarbazide, a blocker of the glutamic acid decarboxylase, caused freezing behavior without emission of USV. All these responses were significantly reduced by midazolam and muscimol. It is suggested that the neural substrates of aversion in the inferior colliculus are under inhibitory control of GABAergic mechanisms and are different from those triggered by foot-shocks.

Effects of perinatal octylphenol on ultrasound vocalization, behavior and reproductive physiology in rats

A rodent diet containing paraffin wax was designed to administer the environmental estrogen octylphenol (OP) to nonpregnant, pregnant and lactating rats. The estrogenic activity of OP via this diet was first confirmed in ovariectomized adult animals: 20 mg OP/kg/day increased the mitoses in the vaginal epithelium, and 60 mg OP/kg/day stimulated mitoses in the uterine luminal epithelium. The effects on a variety of reproductive and nonreproductive parameters were then investigated in the offspring of dams fed OP (100–250 mg/kg/day during gestation and lactation). A number of modest reproductive and morphological effects observed in the offspring including decreased body weights in adults of both sexes, disrupted vaginal cyclicity and decreases in seminiferous tubule diameter and testis, kidney, spleen and ovary weights. Behavioral effects included increased sexual arousal in males, increased sexual motivation in females towards a female teaser and increased motor activity by females. Ultrasonic vocalizations by pups at Postnatal Day (PND) 7 were reduced in number and duration in both sexes. There were no effects of perinatal OP on ano-genital distance, prepuce separation, aggressive behavior or adult ultrasound vocalization. These observations confirm that the dietary intake of estrogenic amounts of OP during pregnancy and lactation can have a wide variety of effects in the offspring.

Effects of home-nest odors on the startle response in preweanling rats

RICHARDSON, R. AND M. DEFINA. Effects of Home-Nest Odors on the Startle Response in Preweanling Rats. PHYSIOL BEHAV 64(5) 621–624, 1998.—The present study examined the effects of a highly preferred, biologically relevant olfactory stimulus (i.e., home-nest odors) on the startle response in preweanling rats. Although the rats detected the odor (as revealed by reductions in ultrasound vocalizations) it had no effect on either the initial magnitude of the startle response or habituation of this response across 80 test trials. These results are discussed in relation to recent studies on modulation of the startle response in developing rats, the effects of olfactory stimuli on the startle response, and the pleasure-attenuation of startle in nonhuman subjects.

Preclinical pharmacology of FG5893: a potential anxiolytic drug with high affinity for both 5-HT 1A and 5-HT 2A receptors

The effects of FG5893 were evaluated by several different methods; rats were used as experimental animals. Receptor binding studies revealed that FG5893 (2-(4-(4,4-bis(4-fluorophenyl)butyl)-1-piperazinyl)-3-pyridinecarboxylic acid methyl ester) binds with high affinity to both 5-HT 1A ( K i = 0.7 nM) and 5-HT 2A receptors ( K i = 4.0 nM) but has only low affinity for the 5-HT 2C receptor ( K i = 170 nM). FG5893 dose dependently reduced body temperature, and this effect was inhibited by pretreatment with (±)-pindolol. FG5893 (0.1 mg/kg) significantly inhibited head twitch behaviour induced by DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) and FG5893 was also a potent inhibitor of ultrasound vocalization in rat pups (0.3 mg/kg) and of a passive avoidance response (0.1 mg/kg) in mature animals. FG5893 inhibited the cage-leaving response and induced part of the 5-HT behavioural syndrome, but only at very high doses (5 and 10 mg/kg, respectively). At increased doses (1 mg/kg), FG5893 also elicited corticosterone release and reduced the immobility time in the forced-swim test (1 mg/kg). Together, these data indicate that the mixed 5-HT 1A receptor agonist/5-HT 2A receptor antagonist FG5983 is a potent stimulator of presynaptic 5-HT 1A receptors but is less active at the postsynaptic site. FG5893 had potent anxiolytic-like effects both on separation-induced ultrasound vocalization in rat pups and on a passive avoidance response. At increased doses, FG5893 possessed an antidepressant-like property.

Effects of cholecystokinin tetrapeptide and sulfated cholecystokinin octapeptide in rat models of anxiety

The effects of the acutely administered cholecystokinin (CCK) agonists CCK tetrapeptide (BOC-CCK-4) and sulfated CCK octapeptide (CCK-8S) were examined in four animal models of anxiety in rats. In the elevated plus maze, BOC-CCK-4 reduced the time spent in the open arms and the number of entries into the open arms. BOC-CCK-4 but not the anorectic acting CCK-8S increased the suppression of feeding in a conflict paradigm based on novelty suppressed feeding in hungry rats. In the two-compartment black-and-white box, BOC-CCK-4 decreased the time spent and locomotor activity in the white compartment. In the ultrasound vocalization test, using rat pups separated from the mother, BOC-CCK-4 increased the number of distress calls. No evidence was found for inducing anxiety-like behaviour by CCK-8S.

Ultrasound vocalization is not related to corticosterone response in isolated rat pups

Isolated from their mother, rat pups respond with changes in ultrasound vocalization (USV), a paradigm that can be used as a test for a large range of anxiolytics. Because the relation between corticosterone (CORT) and putative stress responses like USV is not clear, we examined the effects of the benzodiazepine ]rugs chlordiazepoxide and diazepam vs. the nonbenzodiazepine drugs buspirone and 8-hydroxy-2-(di- n-propylamino) tetralin (8-OH-DPAT) on USV and plasma CORT concentrations. All drugs caused in dose-related decrease in USV, but only buspirone and 8-OH-DPAT, that is, the decrease in USV and the concomitant increase in plasma CORT, are due to the fact that these two drugs act as full agonists at both pre- and postsynaptic 5-HT 1A receptors. Our results indicate that, when measured as an increase in the activity of the pituitary adrenocortical axis, the stress response can be interpreted in markedly different ways, depending on whether the increased activity is elicited by an environmental stressor or by pharmacological manipulation.