Activation of basolateral amygdala corticotropin-releasing factor 1 receptors modulates the consolidation of contextual fear.

The basolateral amygdala complex (BLA) and central amygdala nucleus (CeA) are involved in fear and anxiety. In addition, the BLA contains a high density of corticotropin-releasing factor 1 (CRF(1)) receptors in comparison to the CeA. However, the role of BLA CRF(1) receptors in contextual fear conditioning is poorly understood. In the present study, we first demonstrated in rats that oral administration of DMP696, the selective CRF(1) receptor antagonist, had no significant effects on the acquisition of contextual fear but produced a subsequent impairment in contextual freezing suggesting a role of CRF(1) receptors in the fear memory consolidation process. In addition, oral administration of DMP696 significantly reduced phosphorylation of cyclic AMP response element-binding protein (pCREB) in the lateral and basolateral amygdala nuclei, but not in the CeA, during the post-fear conditioning period. We then demonstrated that bilateral microinjections of DMP696 into the BLA produced no significant effects on the acquisition of conditioned fear but reduced contextual freezing in a subsequent drug-free conditioned fear test. Importantly, bilateral microinjections of DMP696 into the BLA at 5 min or 3 h, but not 9 h, after exposure to contextual fear conditioning was also effective in reducing contextual freezing in the conditioned fear test. Finally, microinfusions of either DMP696 into the CeA or a specific corticotropin-releasing factor 2 receptor antagonist in the BLA were shown to have no major effects on disrupting either contextual fear conditioning or performance of contextual freezing in the drug-free conditioned fear test. Collectively, results implicate a role of BLA CRF(1) receptors in activating the fear memory consolidation process, which may involve BLA pCREB-induced synaptic plasticity.

Antagonism of CRF(2) receptors produces anxiolytic behavior in animal models of anxiety.

Two pharmacologically distinct CRF receptors are distributed in different brain regions and peripheral tissues. Studies suggest that CRF(1) receptors play an important role in mediating the anxiety provoking effects of CRF. In contrast, far less functional information is available on CRF(2) receptors. Therefore, we conducted dose response studies using antisauvagine-30 (anti-SVG-30, 0-20 microg, 20-min pretreatment, i.c.v.), a potent CRF(2) peptide antagonist, and tested rats in three models of anxiety - the conditioned freezing, the elevated plus maze, and the defensive-withdrawal test. Anti-SVG-30 produced a significant dose-dependent reduction in conditioned freezing. In the elevated plus maze test, administration of anti-SVG-30 effectively increased the number of entries and time spent in the open arms. In the defensive-withdrawal test, anti-SVG-30 treatment facilitated exploratory activity in a large illuminated open field. Thus, in all three animal models, administration of anti-SVG-30 was consistent in producing an anxiolytic-like behavioral effect. In addition, a dose of anti-SVG-30 (10 microg) that produced anxiolytic-like behavior had no significant effects on locomotor activity measured in an automated activity box. This latter finding suggests that antagonism of CRF(2) receptors is not associated with a non-specific increase in behavioral movements. These results provide evidence that, in addition to CRF(1) receptors, CRF(2) receptors may play an important role in the mediation of anxiety behavior.

Attenuation of fear conditioning by antisense inhibition of brain corticotropin releasing factor-2 receptor.

Corticotropin releasing factor (CRF) is an important regulator of the endocrine, behavioral, autonomic and immune responses to stress. Two high affinity CRF receptors have been identified, which are distributed in distinct anatomical regions. CRF(1) receptors have been relatively well characterized and antagonists to this receptor effectively block stress-induced behaviors in rodents. The function of CRF(2) receptors, which are highly expressed in limbic brain regions, is less well understood. Therefore, an antisense oligonucleotide approach was used to study the role of CRF(2) receptors in the lateral septum in rats. An antisense oligonucleotide directed against the CRF(2) receptor mRNA reduced expression of CRF(2) receptors by 60--80%. In shock-induced freezing tests, animals administered the antisense oligonucleotide exhibited a significant reduction in freezing duration. However, pain sensitivity and locomotor activity were unaltered. A four-base mismatch of the antisense sequence had no significant effects on CRF(2) receptor density and on freezing behavior. These data support the involvement of CRF(2) receptors in fear conditioning. CRF(1) receptor antagonists also reduce freezing in this test. Additional studies to determine the effects of simultaneous inhibition of both receptor subtypes show that rats receiving both CRF(2) receptor antisense oligonucleotide and CRF(1) receptor antagonist froze significantly less than animals treated with either agent alone. These results provide additional evidence for the role of CRF(2) receptors in mediating the stress-induced actions of endogenous CRF.

Role of CRF(1) and CRF(2) receptors in fear and anxiety.

Fear and anxiety are common emotions that can be triggered by stress. This paper reviews the work examining the role played by specific corticotropin-releasing factor (CRF) receptors in mediating the expression of these emotions. Several lines of evidence taken from CRF(1) transgenic knockout mice, CRF(1) antisense oligonucleotide studies, and CRF(1) receptor antagonist work suggest that the anxiety inducing effects of CRF are mediated by the CRF(1) receptor. Of these three methodological approaches, the work using transgenic CRF(1) knockout mice appears to be the most consistent. In contrast, the work using specific CRF(1) antagonists has produced somewhat varied results that may be explained, in part, by the testing method. When animals are stressed prior to behavioral testing, CRF(1) receptor antagonists appear to have anxiolytic-like effects. In addition, chronic dosing with CRF(1) antagonists may have more potent anxiolytic-like effects, especially in animal models of spontaneous anxiety, than acute dosing procedures. Spontaneous anxiety is defined as behavior that is elicited entirely by the testing situation without current or prior aversive or explicitly induced stress. CRF(1) antisense oligonucleotide work is difficult to interpret because of potential toxicological side effects produced by the antisense oligonucleotide and, in some cases, the absence of verifiable reductions in CRF(1) receptor densities after treatment. Similar methods-CRF(2) knockouts, CRF(2) antisense oligonucleotides, and CRF(2) antagonists-were used to evaluate the function of CRF(2) receptors in emotionality. In comparison to the large number of CRF(1) receptor studies, fewer CRF(2) receptor investigations have been conducted and these studies have yielded mixed results. However, recent work demonstrating a robust reduction in CRF(2) receptors using a CRF(2) antisense oligonucleotide with minimal toxicity, and dose response studies using a peptide CRF(2) antagonist suggest that CRF(2) receptors play a role in stress-induced and spontaneous anxiety. Furthermore, inhibiting the actions of both CRF(1) and CRF(2) receptors produces a greater reduction in stress-induced behavior than inhibition of either receptor alone. Thus, current data suggest that CRF(1) and CRF(2) receptors are involved in the mediation of fear and anxiety behavior.

Prolonged stress-induced elevation in plasma corticosterone during pregnancy in the rat: implications for prenatal stress studies.

Experiments were conducted to test the hypothesis that exposure to uncontrollable stress during pregnancy results in a heightened elevation of plasma glucocorticoids. Rats were exposed to uncontrollable electric tail shocks every other day during the 3 weeks of pregnancy. Plasma corticosterone concentrations in stressed dams increased significantly from gestation days 4 to 20. Importantly, this increase in plasma corticosterone occurred 24- and 48-h after exposure to stress suggesting a prolonged elevation in stress-induced glucocorticoid secretion. In addition, the stress-induced rise in plasma corticosterone was accompanied by a significant decrease in maternal levels of corticosteroid binding globulin which suggests increased circulating levels of free corticosterone. Significant stress-induced elevations in plasma corticosterone also occurred in fetuses that were examined on gestation day 20. Furthermore, a significant positive correlation was found between maternal and fetal plasma corticosterone. Results demonstrate that repeated exposure to uncontrollable stress increases plasma concentrations of glucocorticoids throughout pregnancy. In the unbound state, corticosterone may be highly effective in producing alterations in brain development of offspring. These data have important implications for understanding the process underlying the effects of prenatal stress.

Prenatal stress: consequences of glucocorticoids on hippocampal development and function.

Prenatally stressed offspring exhibit a variety of physiological and behavioral alterations. This paper highlights those alterations associated with prenatal stress-induced elevations in glucocorticoid secretion. Three major alterations are identified that may be produced by glucocorticoid-induced actions on the developing hippocampus. Changes include reductions in steroid receptors that bind endogenous glucocorticoids, enhanced secretion of stress hormones and increased reactivity or emotionality in stressful situations. Some of these alterations may be ameliorated by early postnatal environmental manipulations such as adoption and handling procedures. These latter results suggest that prenatal stress-induced effects of glucocorticoids extend into the early postnatal period to produce long-term hippocampal and behavioral alterations. Support for this hypothesis is based on studies demonstrating that the hippocampus undergoes considerable maturational changes during the early postnatal period such as establishing the regional distribution of corticosteroid receptor densities and development of hippocampal dentate gyrus cells as well as cholinergic systems. Hippocampal corticosteroid receptors are involved in the regulation of glucocorticoid negative feedback and hippocampal dentate gyrus and cholinergic development are influenced by endogenous glucocorticoids and are implicated in the development of defensive or stress-induced behavior. The developing hippocampus appears especially vulnerable to alterations induced by prenatal stress-induced elevations in glucocorticoids that continue to produce their effects throughout the early postnatal period.

Rapid stress-induced elevations in corticotropin-releasing hormone mRNA in rat central amygdala nucleus and hypothalamic paraventricular nucleus: an in situ hybridization analysis.

High densities of nerve cells containing corticotropin-releasing hormone (CRH) are located in the central nucleus of the amygdala (CeA) and paraventricular nucleus (PVN) of the hypothalamus. These brain regions play an important role in activating autonomic, behavioral, and endocrine responses to stress. This study was conducted to provide needed information concerning the acute effects of stress on CeA and PVN CRH mRNA expression. Rats were exposed to restraint stress for 1 h and brains collected after a 1-h post-stress interval. CRH mRNA expression occurring in the CeA and PVN was examined using in situ hybridization techniques. Densitometric analysis revealed that acute restraint stress produced significant increases in CRH mRNA levels in the PVN and in the rostral CeA region. In addition, the area in the rostral CeA encompassing high CRH mRNA signals increased significantly after stress. Results provide clear evidence that CRH neurons in the CeA and PVN exhibit rapid increases in CRH mRNA expression after exposure to stress.

Glucocorticoid facilitation of cholinergic development in the rat hippocampus.

The role of endogenous glucocorticoids in facilitating the postnatal innervation of septohippocampal cholinergic projections was examined. Septohippocampal cholinergic innervation was determined using two methods. One method involved measuring the optical density of acetylcholinesterase, a marker of cholinergic fibres in the hippocampus. In the other method, acetylcholinesterase-positive fibre counts were made in the hippocampus. Both methods revealed that 14-day-old rats adrenalectomized at 10 days of age have significantly lower densities of acetylcholinesterase in the hippocampal dentate gyrus molecular layer and in the regio inferior when compared to sham-operated control rats. This reduction in hippocampal acetylcholinesterase did not occur when 10-day-old adrenalectomized rats were either injected daily with exogenous corticosterone (0.3 mg/100 g body weight) or when adrenalectomy was conducted at later postnatal ages. In addition, unlike the developing hippocampus, the basolateral nucleus of the amygdala, which is also highly innervated by cholinergic fibres, showed no significant changes in acetylcholinesterase density after adrenalectomy. These observations suggest that glucocorticoids play an important role in supporting the development of cholinergic projections to the hippocampus. Cholinergic innervation of the hippocampus appears especially sensitive to the action of glucocorticoids occurring before the conclusion of the second postnatal week. Furthermore, this glucocorticoid influence is directed rather specifically to the hippocampus in comparison to the basolateral amygdala.

Glucocorticoids and the hippocampus. Developmental interactions facilitating the expression of behavioral inhibition.

When threatened, the rapid induction of fear and anxiety responses is adaptive. This article summarizes the current knowledge of the neurobiological development of behavioral inhibition, a prominent response occurring in fear and anxiety-provoking situations. In the rat, behavioral inhibition as exemplified by freezing first appears near the end of the second postnatal week. This emergence of freezing coincides with the developmental period marked by the rapid increase in plasma concentrations of glucocorticoids. Studies show that removal of glucocorticoids at this time severely impairs the age-dependent appearance of freezing. This behavioral impairment produced by adrenalectomy, however, is prevented by exogenous glucocorticoid administration. The effectiveness of glucocorticoids in facilitating the development of freezing appears to be caused by its actions in the hippocampus. In particular, glucocorticoids appear to play a vital role in the postnatal cellular development of the hippocampal dentate gyrus. Doses of glucocorticoids shown to reverse the behavioral inhibitory deficits occurring after adrenalectomy are ineffective when hippocampal dentate granule neurons are destroyed by neurotoxins. Notably, site-specific administration of glucocorticoids to the dorsal hippocampus is successful in promoting the occurrence of freezing in the adrenalectomized rat pup. It is hypothesized that glucocorticoids exert their behavioral inhibitory effects by influencing the development of the septohippocampal cholinergic system. Support for this hypothesis is derived from work demonstrating the importance of glucocorticoids on nerve growth factor systems that play a critical role in septohippocampal cholinergic survival.

Presynaptic muscarinic cholinergic receptors in the dorsal hippocampus regulate behavioral inhibition of preweanling rats.

The aim of this research was to determine whether early maturation of the dorsal hippocampal cholinergic system mediates behavior exhibited by preweanling rats in the presence or absence of an unfamiliar adult male rat, a threatening stimulus. The behavioral responses that were examined included behavioral inhibition or freezing which emerges at 2 weeks of age and ultrasonic vocalizations. Prior to behavioral testing, oxotremorine, an M2 muscarinic receptor agonist that reduces cholinergic release from presynaptic terminals, was infused into the dorsal hippocampal dentate gyrus. Results demonstrated that 14-day-old rats with bilateral hippocampal infusions of a 1 microgram dose of oxotremorine exhibited significant deficits in freezing when exposed to the adult male rat. Importantly, oxotremorine had no significant effects on ultrasound emission and ambulatory activity when rat pups were tested in social isolation. Thus, effects of oxotremorine in the hippocampal dentate gyrus do not produce global changes in behavior. Results suggest that cholinergic release into the dorsal hippocampus facilitates the display of behavioral inhibition at the end of the second postnatal week. Behavioral deficits in freezing may reflect an oxotremorine-induced disruption of hippocampal cholinergic function underlying the processing of biologically relevant olfactory stimuli as well as mechanisms associated with attention.

Glucocorticoids, the hippocampus, and behavioral inhibition in the preweanling rat.

Endogenous corticosteroids influence brain development and behavioral expression. In rat pups, a corticosteroid-dependent developmental response is behavioral inhibition, which occurs in situations involving threat. Behavioral inhibition consists of freezing and a reduction in ongoing behavior. It is presently unknown which brain region(s) that bind corticosterone (CORT) is involved in the development of freezing. The hippocampus (HC), however, is the principal target site of CORT that regulates the postnatal development of HC dentate granule cells. Therefore, this study examined whether the HC, and in particular, the dentate granule cells, plays a major role in the early appearance of behavioral inhibition. On postnatal day 9, rat pups received bilateral HC electrolytic lesions, or bilateral HC infusions of colchicine, a neurotoxin selective for dentate granule cells, or bilateral HC infusions of kainic acid, a neurotoxin selective for pyramidal cells in the CA3 field. Control rats received sham operations. After the operations, all rats were adrenalectomized (ADX) and injected daily with 3.0 mg/kg CORT, except on the day of the behavioral test. On day 14, all pups were tested for behavioral inhibition, which consisted of removing the pup from the nest box and placing it in a temperature-controlled enclosure subdivided into two compartments by a wire-mesh partition. The pup was placed in one compartment and an unfamiliar anesthetized adult male rat was placed in the adjacent compartment. Results indicated that preweanling rats with electrolytic lesions ranging from the dorsal to the ventral HC exhibited significant deficits in freezing. Importantly, similar deficits in freezing were present in pups treated with colchicine but not KA. Hence, administration of exogenous CORT is not effective in facilitating the occurrence of freezing in preweanling pups lacking dentate granule cells. To determine whether the dorsal HC dentate gyrus is an essential target site of CORT in facilitating freezing, 9-d-old rats were implanted bilaterally with 30 gauge cannula filled with either CORT or cholesterol. After the operation, all rats were ADX and tested for behavioral inhibition on day 14. During testing, ADX pups with CORT-filled cannulae showed significantly higher levels of freezing than ADX control pups. Taken together, results suggest that during the early postnatal period, the action of endogenous CORT in the HC influences the development of dentate granule cells that play an essential role in mediating the appearance of behavioral inhibition.

Relative contributions of pituitary-adrenal hormones to the ontogeny of behavioral inhibition in the rat.

Recent investigations revealed that adrenalectomized (ADX) rat pups exhibit deficits in behavioral inhibition. Furthermore, administration of exogenous corticosterone (CORT) restores behavioral inhibition in ADX pups. Although these studies suggest that CORT has an important role in the development of behavioral inhibition, the relative behavioral effects of elevated pituitary hormone secretion induced by ADX are not known. Therefore, experiments were conducted to assess the potential behavioral effects of elevated adrenocorticotropin (ACTH) secretion induced by ADX and to further evaluate the contribution of endogenous CORT to the development of behavioral inhibition. In Experiment 1., we verified that 10-day-old ADX rats exhibit high levels of plasma ACTH throughout the preweaning period associated with the development of behavioral inhibition. In Experiment 2, 10-day-old pups were hypophysectomized (HYPOX) and ADX and were compared behaviorally to sham-operated controls on day 14. When tested in the presence of an anesthetized unfamiliar adult male rat, HYPOX + ADX pups exhibited low levels of freezing accompanied by ultrasonic vocalizations. These pups also had reduced concentrations of plasma ACTH and CORT. In Experiment 3, 10-day-old pups were HYPOX and tested for behavioral inhibition on day 14. In comparison to sham-operated controls, HYPOX rats exhibited significantly lower levels of freezing and had reduced plasma concentrations of ACTH and CORT. Results demonstrate clearly that deficits in freezing occur even in the presence of low plasma ACTH concentrations. Therefore, elevated secretion of pituitary hormones is not a major factor that contributes to the ADX-induced deficits in behavioral inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Restraint stress increases corticotropin-releasing hormone mRNA content in the amygdala and paraventricular nucleus.

Corticotropin-releasing hormone (CRH) neurons located in the paraventricular nucleus (PVN) of the hypothalamus are implicated in regulating the endocrine response to stress. The amygdala is an established component of the neural circuitry mediating the stress response. To obtain information concerning the effects of stress on amygdala CRH neurons, a time-course study was conducted to examine, in rats, whether a 1-h restraint period increases CRH mRNA levels. The effects of restraint were also measured in the PVN. Using a sensitive RNase protection assay, we found that CRH mRNA levels in both the amygdala and paraventricular nucleus were significantly elevated 1 h after cessation of restraint. CRH mRNA levels in the paraventricular nucleus, but not the amygdala, remained elevated at the 3-h post-stress interval. 48 h after the termination of restraint, CRH mRNA levels in both brain structures returned to control levels. These data provide the first direct evidence that stress activates amygdala CRH neurons.

Intracranial action of corticosterone facilitates the development of behavioral inhibition in the adrenalectomized preweanling rat.

We tested the hypothesis that in preweanling rats central administration of exogenous corticosterone (CORT) is sufficient to facilitate the development of behavioral inhibition. 28-gauge cannulae containing varying concentrations of CORT (0, 25, 50 and 100%) were implanted unilaterally into the lateral ventricles of 9-day-old rat pups. After a 24-h postoperative recovery period, pups were adrenalectomized. At 14 days of age, pups were tested for behavioral inhibition which consisted of removing the pup from the nest and exposing it to an unfamiliar adult male rat. Pups implanted with cannulae containing 0, 25 and 50% concentrations of CORT spent significantly less time in freezing postures than pups implanted with cannulae containing 100% CORT. These freezing pups also tended to emit fewer ultrasonic vocalizations than pups in the other three implant conditions, albeit the level obtained was not statistically significant. RIAs indicated that, in general, hormone-filled cannulae produced no detectable concentrations of plasma CORT on the day of the test or on days preceding testing. Results suggest that in the early postnatal period endogenous CORT acts centrally to facilitate the development of neural pathways involved in the ontogenetic expression of behavioral inhibition.

Organizing action of corticosterone on the development of behavioral inhibition in the preweanling rat.

Altricial rat pups develop the ability to freeze and to terminate their emission of ultrasonic vocalizations when exposed to an unfamiliar adult male rat. This developmental competence in expressing behavioral inhibition is impaired when rat pups are adrenalectomized (ADX) on postnatal day 10, a period prior to the emergence of behavioral inhibition. Adrenalectomy, however, fails to induce similar behavioral deficits when performed after behavioral inhibition has developed. Results suggest that adrenal steroids are involved in promoting the development but not the activation of behavioral inhibition. To critically test this hypothesis, four groups of rats were adrenalectomized on day 10 and tested for behavioral inhibition on day 18. Prior to testing, one group of rats received daily s.c. injections of vehicle whereas another group was treated with daily injections of 3.0 mg/kg of corticosterone (B). The other two groups of rats received daily B injections on only days 10-13 or days 14-17. Results indicated that ADX rats treated with B only on days 10-13 as well as throughout exhibited significantly higher levels of freezing than the other two treatment groups. In order to evaluate whether the behavioral inhibitory deficits produced by ADX at 10 days of age are due to a delayed insensitivity to the 3.0 mg/kg dose of B, day 10 pups were ADX and injected on days 14-17 with doses of B as high as 12 mg/kg. When tested for behavioral inhibition on day 18, these high doses of B were found to be ineffective in potentiating freezing above the level of vehicle-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulus control of behavioral inhibition in the preweanling rat.

Previous studies demonstrate that 14-day-old rats reduce their emission of ultrasonic vocalizations and freeze when exposed to an unfamiliar adult male rat. This study sought to identify the stimulus characteristics of conspecific males that potentiate the display of behavioral inhibition. In Experiment 1, day 14 rats were isolated from the nest and exposed to either an unfamiliar prepubescent male rat or an unfamiliar adult male rat. Pups exposed to the unfamiliar adult male rat exhibited significantly elevated levels of freezing and reduced their emission of ultrasounds. In Experiment 2, pups were exposed to either a familiar or an unfamiliar adult male rat. Although several pups exposed to the familiar adult male rat exhibited freezing, pups tested with the unfamiliar adult rat showed a reliably higher duration of freezing and made fewer vocalizations. Results suggest that neither the unfamiliar factor nor cues associated with male adulthood are sufficient to account for the occurrence of behavioral inhibition when presented separately. However, the combination of unfamiliarity and adult male stimuli are highly potent stimulus features that elicit behavioral inhibition in preweanling rats.

Attenuation of stress-induced behavior by antagonism of corticotropin-releasing factor receptors in the central amygdala in the rat.

Research suggests that endogenous corticotropin-releasing factor (CRF) in the amygdala plays a role in the expression of stress-induced behavior. This study examined in rats whether antagonism of CRF receptors in the central amygdala (CA) region using alpha-helical CRF9-41, a CRF antagonist, was effective in attenuating the occurrence of stress-induced freezing. Bilateral infusions of 50, 100, or 200 ng of the CRF antagonist were made in the CA region using 33-gauge cannula immediately prior to testing. Freezing was measured in two test conditions. In one condition, the effects of the CRF antagonist on freezing was assessed immediately after exposure to electric foot shock. In the other condition, freezing was examined in shock-experienced rats that were re-exposed to the shock environment. Results suggested that 50 and 100 ng of the CRF antagonist were effective in reducing the duration of freezing in the immediate post-shock period. In addition, the 100 ng dose produced a significant reduction in freezing duration after rats were re-exposed to the shock environment. Collectively, data suggest that antagonizing the action of endogenous CRF in the CA region contributes to a general alleviation of stress-induced freezing.

Corticosteroid induction of threat-induced behavioral inhibition in preweanling rats.

Termination of ongoing behavior and assumption of defensive postures when threatened are adaptive characteristics of vertebrates. Altricial rat pups develop these characteristics by 14 days of age. At this time, pups inhibit their ultrasonic vocalizations and freeze when threatened. This emergence of behavioral inhibition is impaired when rats are adrenalectomized (ADX) at 10 days of age. That is, 14-day-old ADX pups exhibit deficits in freezing and continue to emit ultrasounds when confronted by an adult male rat. Studies also showed that removal of adrenal hormones does not potentiate vocalizations or render pups incapable of reducing their ultrasounds. More important, 3.0 mg/kg of corticosterone (CORT), but not lower doses, administered daily to ADX pups restored freezing, with lesser effects on ultrasound inhibition. Disrupting the developmental action of endogenous CORT appears to impair the ontogenetic expression of behavioral inhibition.

Ontogeny of behavioral inhibition induced by unfamiliar adult male conspecifics in preweanling rats.

Previous studies showed that when socially isolated at 22 degrees C, postnatal day 14 rats, but not younger day 7 rats, reduce their emission of ultrasonic vocalizations when exposed to an unfamiliar adult male rat, a naturalistic threat. Because ultrasound production is associated with factors such as age and body temperature, this study examined in age-appropriate thermoneutral temperature ranges whether preweanling rats of different ages are equally capable of inhibiting their emission of ultrasounds when threatened. In Experiment 1, 7- and 14-day-old rats were socially isolated and exposed to unfamiliar anesthetized adult male rats in a thermoneutral environment. Only 14-day-old rats significantly reduced their emission of ultrasounds. This reduction in ultrasound production was accompanied by freezing. In Experiment 2, additional ages were examined under identical test conditions. At 3, 6, and 9 days of age, pups frequently emitted ultrasounds when exposed to the anesthetized male rat. However, at 12 days of age, rat pups responded to the anesthetized male rat by freezing and significantly reducing their emission of ultrasounds. Results indicate clearly that under the present testing conditions the ability of rat pups to inhibit ultrasounds and freeze when threatened is not present at birth but emerges by the end of the second postnatal week.

Antagonism of corticotropin-releasing factor receptors in the locus coeruleus attenuates shock-induced freezing in rats.

Intracerebroventricularly administered alpha-helical CRF9-41, a corticotropin-releasing factor (CRF) receptor antagonist, is known to reduce a variety of stress-induced behavioral responses. This study examined in rats whether antagonism of CRF receptors in the region of locus coeruleus (LC) plays a role in reducing freezing induced by electric foot shock. Freezing is a well-characterized defensive response to stress and has been demonstrated to index an animal's degree of fear. A CRF-receptor antagonist, alpha-helical CRF9-41, bilaterally infused into the LC significantly reduced the duration of freezing at a dose as low as 0.20 micrograms. Additional experiments confirmed that 0.20 micrograms of alpha-helical CRF9-41 significantly reduced the duration of freezing only when cannulae were within the LC or in regions bordering the nucleus. Antagonist-treated rats with cannulae that did not impinge on the LC exhibited freezing at levels not different from vehicle-treated animals. These results strongly implicate CRF receptors located in the LC region in influencing the display of stress-induced behavior.