Neonatal gonadal hormone manipulation and emotionality following septal lesions in weanling rats

When tested at 26 days of age, female rats displayed increased emotional reactivity to capture and tactile stimulation after ablation of the septal nuclei, whereas male littermates receiving comparable lesions show no change in emotional reactivity. Sexual differentiation of hypothalamic structures in rats is subserved by the presence or absence of androgen during the perinatal period and this process is thought to underlie sex differences in many forms of behavior. In an attempt to determine whether a similar process of differentiation is involved in the observed sex differnces in emotionality after septal lesions, male rats were castrated at 2 days of age and female littermates injected with 1.25 mg testosterone proprionate. Following neonatal gonadal hormone manipulation castrated males became hyperreactive after septal lesions at 25 days of age, while androgenized females showed no change. Control females displayed emotional hyperreactivity but control males were unaffected. Behavioral changes following septal lesions may therefore be due to the functional disruption of a system whose neural development is influenced by the presence or absence of androgen at critical periods.     

Neural pathways mediating the prolactin secretory response to acute neurogenic stress in the male rat

Adult male rats, intact (N) or bearing complete, anterior or posterior hypothalamic deafferentations (CHD, AHD or PHD, respectively), or bilateral medial forebrain bundle (MFB) lesions, were acutely exposed to visual or audiogenic stimulation. At 2, 4,10 or 30 min following stress onset the animals were decapitated and trunk blood was collected for prolactin (PRL) determinations. Basal serum PRL levels were found to be similar in all groups. In N animals, exposure to both modalities resulted in rapid and marked PRL secretory responses. These responses were totally abolished in the CDH group. In AHD rats, no significant elevation in serum PRL concentration was found upon stress exposure. In PHD animals, the PRL secretory responses were only slightly attenuated when compared with the N group. In MFB-lesioned rats, a marked elevation in serum PRL concentrations was recorded following visual stimulation; contrary thereto, the PRL secretory response following audiogenic stress was markedly attenuated. These data (1) describe the temporal aspects of the PRL secretory response to acute exposure to neurogenic stresses in the male rat, and (2) demonstrate that these PRL responses are elicited via a neural pathway impinging upon the medial basal hypothalamus from the rostral direction.     

Comparison of the metabolic and behavioral disturbances following paraventricular- and ventromedial-hypothalamic lesions

Lesions of the ventromedial hypothalamus (VMH) result in an obesity syndrome with several metabolic and behavioral manifestations. It has also been reported that damage to the paraventricular hypothalamus (PVH) leads to changes characteristic of obesity. However, little is known about the consequence of PVH lesions, especially in contrast to the extensive documentation of VMH lesion-induced effects. To assess the basic features of the two hypothalamic obesity syndromes, rats underwent VMH, PVH, or sham lesions and, for 15 weeks, were maintained ad lib on a series of test diets. Both lesions groups were hyperphagic and showed similar weight gains. Although both lesion groups became obese (measured by % carcass fat), VMH rats were fatter than PVH animals. Similarly, only VMH rats were hyperinsulinemic. Further tests were conducted in PVH and VMH rats restricted to control body weights. VMH, but not PVH, rats developed a persisting elevation in basal gastric acid secretion. As well, only VMH, and not PVH, animals developed an obesity when restricted to normal weights. These data indicate similarities in PVH and VMH rats maintained ad lib but experiments on restricted animals reveal fundamental differences in the two obesities and point to different etiologies.     

The role of the medial septal nucleus in mediating adrenocortical responses to somatosensory stimulation

In view of the demonstrated involvement of the hippocampus in the mediation of adrenocortical responses following sciatic nerve stimulation, the role of the septum and the preoptic area in the transmission of this response was investigated. Changes in plasma corticosterone following ether stress and photic, acoustic, or sciatic nerve stimulation were studied in intact rats and in animals with lesions in the medial septal nucleus and the preoptic area. The response to ether stress and to photic and acoustic stimulation was normal in these animals. However, the adrenocortical response to sciatic nerve stimulation was partially reduced in the rats with lesions in the superior, but not in the inferior preoptic area, and it was completely blocked in those with medial septal lesions. Our previous and present experiments indicate that the ventral hippocampus, the fimbria, the septum, and anterior hypothalamic afferents mediate the adrenocortical response to somatosensory stimulation.     

Genetic relation between the performance in a two-way avoidance task and increased emotionality following septal lesions

Large radio frequency septal lesions were performed on male Roman low (RLA/Verh) and high (RHA/Verh) avoidance rats. It was found that the genetic low-avoiders displayed higher acute postoperative irritability than the genetic high-avoiders. This result had been predicted a-priori from an independent set of data, relating the ability to master active avoidance inversely to general emotionality and readiness to engage in intracranial self-stimulation (ICSS), and from research demonstrating a genetic correlation between the severity of the septsal syndrome, and the readiness to engage in ICSS.On the sixth postoperative day, all animals were tested for their performance in a standard shuttle box avoidance task. It was found that septal lesions did not affect the performance of either the low or high genetic avoiders, which behaved as expected from their respective genotypes.     

Effects of hypothalamic and preoptic lesions on reproductive behavior in male rats

Adult male rats, which were selected on the basis of showing complete patterns of male copulatory behavior on two prior tests, were castrated six weeks prior to brain surgery. Animals were divided into three groups and given small bilateral lesions in the dorsomedial preoptic area (POA), ventromedial hypothalamus (VMH), or sham operations. Starting 10 days postsurgery, all animals were injected with estrogen alone and estrogen plus progesterone, and tested twice for lordosis behavior. Ten days following the female behavior tests, animals were injected daily with testosterone propionate for 13 days and tested for masculine sexual behavior on injection days 5, 9 and 13. Low levels of lordotic behavior were observed for POA and VMH animals on both tests for female sex behavior and were comparable to sham operated animals. However, in terms of all dependent measures of male copulatory behavior, animals with VMH lesions showed significantly higher levels of male sex behavior with shorter latencies than sham animals across all three behavior tests. In contrast, POA lesioned rats showed little or no male sex behavior on any test and were significantly inferior to sham operated animals. Thus, the POA and VMH appear to exert excitatory and inhibitory control, respectively, over male copulatory behavior in male rats.     

Hypothalamic obesity: Comparison of radio-frequency and electrolytic lesions in male and female rats

Obesity-inducing lesions of the ventromedial hypothalamus (VMH) have generally been produced by passing anodal electrolytic current through a stainless steel electrode, which in addition to tissue ablation leaves deposits of metallic ions that might chronically irritate surrounding tissue. Although a few early investigators reported that nonirritative lesions (i.e., those that do not leave deposits) resulted in little or no obesity, their results were largely ignored when others observed substantial weight gain in animals after using the same techniques. The results of the present experiment suggest that these discrepancies were the result of a sex difference (the former group of studies all used male rats, while the latter group all used females). Male rats in the present study gained less than 80% of the weight gained by female rats regardless of lesion type, while rats with nonirritative radio-frequency lesions gained about 60% of the weight gained by rats with electrolytic lesions regardless of sex. Thus, female rats with radio-frequency lesions displayed substantial (albeit suppressed) weight gain, whereas male rats with identical lesions gained only slightly more weight than sham-lesioned controls. It is concluded that obesity resulting from electrolytic lesions of the VMH is due in part to tissue ablation and in part to chronic irritation (possibly of parasympathetic efferents mediating insulin release).     

Modulation of single-unit activity in the rat medial amygdala by neurotransmitters, estrogen priming, and synaptic inputs from the hypothalamus and midbrain.

The medial amygdala (m-AMG) appears to act as an integrative center for sensory, synaptic, and endocrine signals important in the regulation of reproductive function. Extracellular single-unit recordings from anesthetized, ovariectomized female rats were used to investigate neuropharmacological, hormonal, and synaptic modulation of neurons in the m-AMG. Electrical stimulation of the ventromedial hypothalamus (VMH) elicited excitatory or inhibitory orthodromic responses in 72% and antidromic responses in 7% of m-AMG neurons, whereas stimulation of the midbrain central gray (MCG) induced orthodromic responses in 43% of m-AMG neurons. Interestingly, most cells that were influenced by MCG stimulation were also orthodromically driven by the VMH, as 40% of all m-AMG cells responded orthodromically to both the VMH and MCG. Furthermore, the majority of these cells tended to be modulated by both areas in the same direction. Iontophoretic application of glutamate, GABA, ACh, and LHRH could modulate the spontaneous firing rate of m-AMG neurons. In particular, ACh had a predominantly excitatory action, which was more effective on m-AMG neurons that were orthodromically driven by the VMH and that were from estrogen-primed animals. In addition to increasing chemical responsiveness to ACh, estrogen priming of ovariectomized animals also increased the spontaneous firing rate of m-AMG neurons and decreased the number of silent cells. These modulatory actions on m-AMG neurons may be important in the medial amygdala's regulation of the behavioral and endocrine aspects of reproductive function in the female rat.     

Fos-like immunoreactive neurons following electrical stimulation of the dorsal periaqueductal gray at freezing and escape thresholds

Electrical stimulation of the dorsal regions of the periaqueductal gray (PAG) leads to defensive reactions characterized as freezing and escape responses. Until recently it was thought that this freezing behavior could be due to the recruitment of neural circuits in the ventrolateral periaqueductal gray (vlPAG), while escape would be mediated by other pathways. Nowadays, this view has been changing mainly because of evidence that freezing and escape behaviors thus elicited are not altered after lesions of the vlPAG. It has been suggested that there are at least two pathways for periaqueductal gray-mediated defensive responses, one involving the hypothalamus and the cuneiform nucleus (CnF) which mediates responses to immediate danger and another one involving the amygdala and vlPAG which mediates cue-elicited responses, either learned or innate. To examine this issue further we measured Fos protein expression in brain areas activated by electrical stimulation of the dorsolateral PAG (dlPAG) at the freezing and escape thresholds. The data obtained showed that freezing-provoking stimulation caused increases in Fos expression in the dorsomedial PAG (dmPAG), while escape-provoking stimulation led to increases at both dmPAG and dlPAG. Surprisingly, neither escape- nor freezing-provoking stimulations altered Fos expression in the central nucleus of amygdala (CeA). Escape-provoking stimulation caused increased Fos expression in the ventromedial hypothalamus (VMH), dorsal premammilary nucleus (PMd) and in the cuneiform nucleus. Significant increases in Fos labeling were found in the dmPAG and PMd following freezing-provoking stimulation. Therefore, the present data support the notion of a neural segregation for defensive behaviors in the dorsal columns of PAG, with increased Fos expression in the dmPAG following freezing, while dlPAG is affected by both freezing and escape responses. dlPAG, CnF, VMH and PMd are part of a brain aversion network activated by fear unconditioned stimuli. The present data also suggests that the defensive responses generated at the dlPAG level do not recruit the neural circuits of the vlPAG and CeA usually activated by conditioned fear stimuli.     

Prevention of renal hypertension and of the central pressor effect of angiotensin by ventromedial hypothalamic ablation

Various lines of research have implicated the central nervous system in the development of renal hypertension. The ablation of a periventricular region surrounding the anteroventral third ventricle (AV3V) has been shown to block the development of renal hypertension. Because the hemodynamic effects produced by AV3V electrical stimulation can be abolished by a midline lesion of the ventromedial hypothalamic-median eminence region (VMH-ME), the effect of VMH-ME ablation on the development of renal hypertension was studied. Following recovery from surgery that destroyed the VMH-ME region the lesioned rats and controls were subjected to unilateral nephrectomy and figure-of-eight wrapping of the remaining kidney. Control animals developed renal hypertension but those with VMH-ME lesions did not. When the pressor response produced by intracerebroventricular injections of angiotensin II was studied, it was found that rats with VMH-ME lesions, as compared to neurologically intact animals, showed significantly attenuated responses. The data suggest that a neural system related to cardiovascular control descends through the VMH-ME region and that the integrity of this pathway is necessary for the development of renal hypertension.     

Fetal hypothalamic brain grafts reduce the obesity produced by ventromedial hypothalamic lesions

Bilateral destruction of the rat ventromedial hypothalamus (VMH) produces a syndrome characterized by hyperphagia and obesity. In the present study we examined whether grafts of fetal hypothalamus could reverse the effects of this lesion. Three groups of adult rats received bilateral electrolytic lesions of the VMH. The first group of animals was then implanted with embryonic day 14–16 hypothalamic tissue by stereotaxic injection into the lesion sites. The second series of animals received comparable-sized grafts from a variety of non-hypothalamic regions of the fetal CNS. The third group experienced similar VMH lesions but did not receive any tissue grafts. After surgery, body weight and food consumption were recorded daily for up to 8 weeks. These measures were compared with similar ones obtained from non-operated rats. Hyperphagia and obesity were consistently observed in all of the lesioned animals not bearing transplants. An initial period of weight gain was also observed in animals receiving hypothalamic grafts, but the duration of the ‘dynamic’ phase of this syndrome was reduced. Consequently, these graft recipients exhibited significantly less weight gain. This depressed weight gain, however, did not coincide with a statistically significant decrease in hyperphagia. Transplantation of non-hypothalamic tissue also caused an attenuation of the VMH-lesion effect but this was more modest than that induced by homotopic grafts.The results of this experiment show that homotopic transplants can alter the dynamics of weight gain induced by bilateral VMH lesions. However, lesion-induced hyperphagia was not completely reversed in these grafted animals. The fact that other regions can exert a similar effect, though of lesser magnitude, suggests that a more general property of fetal CNS tissue may be involved.     

Hyperprolactinemia: its electrophysiologic and pharmacologic effect on neurons of the ventromedial nucleus of the hypothalamus

Extracellular action potentials were recorded from 691 neurons in the ventromedial hypothalamus (VMH) of urethane anesthetized female rats under acute and chronic sham-operated and hyperprolactinemic conditions. Hyperprolactinemia was produced by transplanting pituitaries under the kidney capsules. Neuronal excitability was recorded and analyzed during spontaneous, baseline activity and following the iontophoretic application of prolactin, dopamine (DA) and luteinizing hormone-releasing hormone (LH-RH). No statistically significant changes were observed in the spontaneous electrical activity of VMH neurons under the conditions tested. Of the responsive neurons, approximately 90% of the neurons recorded and tested with prolactin displayed an increase in firing activity while DA produced a decrease, independent of the endogenous plasma prolactin levels (basal or elevated). However, the response to LH-RH was modified in the chronic hyperprolactinemic animal. The predominant response of VMH neurons to LH-RH in acute sham-operated and hyperprolactinemic as well as in chronic sham-operated animals was one of inhibition, while in the chronic hyperprolactinemic animal, the application of LH-RH initiated excitation rather than inhibition. These results provide evidence that chronic (long-term) exposure to elevated prolactin levels is a sufficient stimulus to modify the neuronal response pattern of VMH nerve cells to iontophoretically applied LH-RH but not to prolactin nor to DA.     

Dual estradiol action in diencephalon and the regulation of sociosexual behavior in female golden hamsters

Previously, we found that single implants of estradiol (E2) placed in the ventromedial hypothalamus (VMH) but not the anterior hypothalamus (AH) facilitated precopulatory, i.e. vaginal scent marking, and copulatory, i.e. lordosis, behavior following progesterone administration. However, the duration of lordosis was markedly attenuated in comparison to the duration shown by intact cycling females. This study was designed to examine whether dual implantation of E2 in the diencephalon would facilitate patterns of precopulatory and copulatory behaviors similar to those shown by intact cycling females. One E2 implant was placed in either the medial preoptic area (MPO) or AH and a second E2 implant was placed in the VMH. Control females were tested following E2 application at only the MPO or AH region in conjunction with a cholesterol implant in the VMH. An additional control group was tested with females implanted with cholesterol at both MPO-AH regions and the VMH. During a 2-day postimplantation test period, vaginal marking scores were elevated for both single and dual E2 implanted females and agonistic response patterns toward males declined significantly. In addition, a significant inverse relationship was found between the number of vaginal marks and bites exhibited by females with single E2 implants in the MPO, whereas these two response patterns were positively correlated in females with E2 stimulation occurring only in the AH region. No significant relationship was found between vaginal marking and biting attack for females receiving dual E2 stimulation. Systemic progesterone administration on the third postimplantation day facilitated sexual receptivity in the majority of females with dual E2 implants (greater than 90%). These receptive females displayed lordotic responsiveness that closely matches the full display of sexual receptivity shown by intact cycling females. In contrast, only one female with a single E2 implant in the AH region showed sexual responsiveness. The results demonstrate that: precopulatory vaginal marking and biting attack are mediated by E2 action in the MPO and AH but in a different manner; additional action of E2 in the VMH diminishes the distinctive precopulatory behavioral effects of E2 in the MPO and AH suggesting an influential role of the VMH in regulating sociosexual activities; and E2 action in either the MPO or AH region in conjunction with E2 action in the VMH may be necessary in order to facilitate the species-typical display of lordotic responsiveness.     

High-frequency stimulation of the dorsolateral periaqueductal gray and ventromedial hypothalamus fails to inhibit panic-like behaviour

Electrical stimulation of the dorsolateral periaqueductal gray (dlPAG) and one of its target structures, the ventromedial hypothalamus (VMH), produces a typical behaviour in rats consisting of vigorous running and jumping which is known as “escape behaviour”. Escape behaviour in rodents closely mimics panic attacks in humans. Since electrical stimulation at higher frequencies generally inhibits the stimulated region, we tested in this study the hypothesis that deep brain stimulation (DBS) of the dlPAG and VMH at higher frequencies (>100 Hz) would not induce escape behaviour. More specifically, we evaluated whether experimental DBS could be used to inhibit panic-like behaviour. Rats underwent implantation of DBS-electrodes at the level of the dlPAG and VMH and the effects of various stimulation parameters were assessed. In addition, we studied the neural activation pattern resulting from DBS of the dlPAG and VMH using c-Fos immunohistochemistry. We found that stimulation amplitude is the most important stimulation parameter in the induction of escape behaviour. Remarkably, stimulation frequency (1–300 Hz) had no effect on stimulation-induced escape behaviour and therefore it was not possible to prevent the induction of escape behaviour with higher frequencies. The neuronal activation pattern resulting from dlPAG and VMH DBS was similar. These findings suggest that DBS of the dlPAG and VMH induces panic-related behaviours even at higher frequencies.     

Higher inherent fearfulness potentiates the effects of chronic stress in the Japanese quail

There is considerable variability in the susceptibility of individuals to the adverse effects of chronic stress. In humans and other mammals, individual traits such as high anxiety are proposed as a vulnerability factor for the development of stress-related disorders. In the present study, we tested whether a similar behavioural trait in birds, higher emotional reactivity, also favours the occurrence of chronic stress-related behavioural and physiological dysfunction.For this, lines of Japanese quail divergently selected for a typical fear response in birds, the duration of tonic immobility, were subjected to unpredictable aversive stimulation over 2 weeks. Previous studies demonstrate that the selection program modifies the general underlying emotionality of the birds rather than exerting its effect only on tonic immobility.Interestingly, only birds selected for their higher emotionality exhibited significantly enhanced latency to first step and decreased locomotor activity in the open-field test after exposure to chronic stress compared to non-stressed control birds. This effect of chronic stress was selective for the tested dimension of bird emotional reactivity because there was no observed effect on the tonic immobility response. Moreover, chronically stressed birds selected for their higher emotionality exhibited significantly decreased basal corticosterone levels, a physiological marker of stress. These findings show that chronic stress is associated with changes in emotional reactivity and related physiological markers in birds. They also highlight emotional reactivity as an important predisposing factor for the occurrence of the adverse effects of chronic stress in birds.     

Effect of septal and fimbrial stimulation on auditory and visual cortical evoked potentials in the cat

The effects of septal and fimbrial stimulation on auditory and visual cortical evoked potentials were determined. The cortical responses were recorded from the ectosylvian and marginal gyri respectively with both acutely and chronically implanted electrodes. After all recordings were completed, lesions were made in the septum in three of the cats and both auditory and visual cortical evoked potentials were again studied. Depending on the location within the septum, septal stimulation in acute preparations either attenuated or facilitated both auditory and visual cortical evoked potentials; stimulation of the fimbria had a facilitatory effect. With chronically implanted electrodes, septal stimulation led to decreases and fimbrial stimulation to increases in evoked response amplitudes. Certain of the previous effects were reversed after placement of the lesion in the septum. Auditory and visual cortical evoked potentials previously attenuated by septal stimulation were facilitated and fimbrial stimulation after the septal lesion resulted in the attenuation of previously facilitated cortical evoked potentials. These findings implicate the septum and fimbria in processing sensory information and give evidence that a uniprocess concept of septal functioning is no longer adequate.     

Electrical stimulation of the ventromedial hypothalamus enhances both fat utilization and metabolic rate that precede and parallel the inhibition of feeding behavior

The effects of ventromedial hypothalamic (VMH) stimulation on various metabolic parameters in freely moving animals were measured using a specific indirect calorimetric chamber associated with a quantitative measurement of locomotor activity, which allows the separate measurement of locomotor energy expenditure from that of background metabolism, BM (free from expenses due to locomotion). To obtain circumscribed VMH stimulation, a slight-intensity (20-25 microA) bipolar, constant current was applied for 15 min at the beginning of the dark phase on ad libitum fed rats. The VMH stimulation suppressed feeding for 40 min, then animals progressively recovered within the subsequent 60 min as shown by comparison with the control group. On different days, the same stimulation parameters were applied while food was unavailable, and metabolic parameters were recorded. An increase in BM lasting 30 min was observed. This increase in metabolic rate was sustained by means of a recruitment of lipid stores as indicated by a concomitant drop in respiratory quotient. These observations indicate that the VMH is part of the sympathetic nervous system, capable of inducing lipolysis. The sequence of metabolic and feeding events may then in part be due to VMH-induced lipolysis that provides more fuel to the metabolic economy, raising the BM, which in turn decreases hunger.     

Characteristics in Emotional Behavior of the Rat with Bilateral Olfactory Bulb Ablations

The general activity and emotional responses to various stimuli of the rats with olfactory bulb ablations (O.B. rat) were investigated in comparison with those of the rats with either septal or amygdaloid lesions. The results are summarized as follows: 1. The rat gradually became to exert hyperemotional responses to various stimuli, following bilateral removal of the olfactory bulb. This hyperemotionality remained unchanged for a long period of time. The rat with septal lesions, on the contrary, exhibited hyperemotionality immediately after lesioning, but the hyperemotionality disappeared within 2 weeks. Amygdaloid lesions caused no change in the emotional responses to stimuli of intact rats but abolished the hyperemotionality of the O.B. rats. 2. In the O.B. rats, muricidal behavior and attack response to a rod were more prominent tiian the other responses, whereas in the septal rat, the hyperemotional responses such as startle, flight, jump and squeak were more marked, and muricidal behavior was scarcely observed. Hyperemotionality of the O.B. rat is regarded as active aggressiveness in nature and that of the septal rat would be better expressed as hyperreactivity or hyperirrit-ability. 3. Exploratory behavior such as ambulation and rearing of the O.B. rat in an open-field situation markedly increased, while that of the septal rat decreased instead even after the hyperemotionality disappeared. Amygdaloid lesions increased exploratory behavior in intact as well as in the O.B. rats, despite of reducing hyperemotionality of the latter. (This study was supported in part by the grant for the research of science from the Department of Education of the Government of Japan.)     

Do early-life events permanently alter behavioral and hormonal responses to stressors?

Early-life stimulation (e.g. brief handling) attenuates the behavioral and neuroendocrine responses to stressors encountered in adulthood, particularly with respect to activation of hypothalamic-pituitary-adrenal (HPA) activity. In contrast, if neonates were subjected to a more severe stressor, such as protracted separation from the dam or exposure to an endotoxin, then the adult response to a stressor was exaggerated. These early-life experiences program HPA functioning, including negative feedback derived from stimulation of hippocampal glucocorticoid receptors, and corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) coexpression in PVN neurons, to modify the response to subsequent stressor experiences. The persistent variations of HPA activity observed in handled/stimulated animals may stem from alterations in dam–pup interactions (e.g. increased arched-back feeding, licking, grooming). In addition genetic makeup is critical in determining stress reactivity. For instance, BALB/cByJ mice are more reactive to stressors than C57BL/6ByJ mice, exhibiting greater HPA hormonal alterations and behavioral disturbances. BALB/cByJ also fail to acquire a spatial learning response in a Morris water-maze paradigm, which has been shown to be correlated with hippocampal cell loss associated with aging. Early-life handling of BALB/cByJ mice prevented these performance deficits and attenuated the hypersecretion of ACTH and corticosterone elicited by stressors. The stressor reactivity may have been related to maternal and genetic factors. When BALB/cByJ mice were raised by a C57BL/6ByJ dam, the excessive stress-elicited HPA activity was reduced, as were the behavioral impairments. However, cross-fostering the more resilient C57BL/6ByJ mice to a BALB/cByJ dam failed to elicit the behavioral disturbances. It is suggested that genetic factors may influence dam–pup interactive styles and may thus proactively influence the response to subsequent stressors among vulnerable animals. In contrast, in relatively hardy animals the early-life manipulations may have less obvious effects.