Intense inhibitory avoidance training increases nuclear-phosphorylated glucocorticoid receptors in neurons of CA1 of hippocampus and ventral caudate putamen.

Corticosterone (CORT), the principal glucocorticoid in rodents, is released after stressful experiences such as training with high foot-shock intensities in the inhibitory avoidance task (IA). CORT reaches the glucocorticoid receptor (GR) located in almost all brain cells; the GR is subsequently phosphorylated at serine 232 (pGRser232). This has been reported as an indicator of ligand-dependent activation of the GR, as well as a requirement for its translocation into the nucleus for its transcription factor activity. The GR is present in the hippocampus with a high concentration in CA1 and dentate gyrus (DG), and a smaller proportion in CA3, and sparsely present in the caudate putamen (CPu); both structures are involved in memory consolidation of IA. To study the participation of CORT in IA, we quantified the ratio of pGR-positive neurons in both dorsal hippocampus (CA1, CA3 and DG) and dorsal and ventral regions of CPu of rats trained in IA, using different foot-shock intensities. Brains were dissected 60 min after training for immunodetection of pGRser232 positive cells. The results show that the groups trained with 1.0 and 2.0 mA had higher retention latencies than the 0.0 mA or 0.5 mA groups. An increase in the ratio of pGR-positive neurons was found in CA1 and ventral region of CPu only for the 2.0 mA trained group. These findings suggest that activation of GRs in CA1 and ventral CPu is involved in the consolidation of a stronger memory of IA, possibly through the modulation of gene expression.

Corticosterone in the dorsolateral striatum facilitates the extinction of stimulus-response memory.

Glucocorticoid hormones are crucially involved in modulating mnemonic processing of stressful or emotionally arousing experiences. They are known to enhance the consolidation of new memories, including those that extinguish older memories. In this study, we investigated whether glucocorticoids facilitate the extinction of a striatum-dependent, and behaviorally more rigid, stimulus-response memory. For this, male rats were initially trained for six days on a stimulus-response task in a T-maze to obtain a reward after making an egocentric right-turn body response, regardless of the starting position in this maze. This training phase was followed by three extinction sessions in which right-turn body responses were not reinforced. Corticosterone administration into the dorsolateral region of the striatum after the first extinction session dose-dependently enhanced the consolidation of extinction memory: Rats administered the higher dose of corticosterone (30 ng), but not lower doses (5 or 10 ng), exhibited significantly fewer right-turn body responses and had longer latencies compared to vehicle-treated animals on the second and third extinction sessions. Co-administration of the glucocorticoid receptor antagonist RU 486 (10 ng) prevented the corticosterone effect, indicating that glucocorticoids enhance the extinction of stimulus-response memory via activation of the glucocorticoid receptor. Corticosterone administration into the dorsomedial striatum did not affect extinction memory. These findings indicate that stress-response mechanisms involving corticosterone actions in the dorsolateral striatum facilitate the extinction of stimulus-response memory that might allow for the development of an opportune behavioral strategy.

Differential Phosphorylation of the Glucocorticoid Receptor in Hippocampal Subregions Induced by Contextual Fear Conditioning Training.

Aversive events induce the release of glucocorticoid stress hormones that facilitate long-term memory consolidation, an effect that depends on the activation of glucocorticoid receptors (GRs). GRs are distributed widely in the hippocampus. The dorsal region of the hippocampus has been related to cognitive functions and the ventral region to stress and emotion. GR acts as a transcription factor which after hormone binding becomes phosphorylated, affecting its cellular distribution and transcriptional activity. Two functionally well-described GR phosphorylation sites are serine 232 (pSer232), which enhances gene expression, and serine 246 (pSer246), having the opposite effect. Since gene expression is one of the plastic mechanisms needed for memory consolidation, we investigated if an aversive learning task would induce GR phosphorylation in the dorsal (DH) and the ventral (VH) hippocampus. We trained rats in contextual fear conditioning (CFC) using different foot-shock intensities (0.0, 0.5, or 1.5 mA). One subgroup of animals trained with each intensity was sacrificed 15 min after training and blood was collected to quantify corticosterone (CORT) levels in serum. Another subgroup was sacrificed 1 h after training and brains were collected to evaluate the immunoreactivity (IR) to GR, pSer232 and pSer246 by SDS-PAGE/Western blot in DH and VH, and by immunohistochemistry in dorsal and ventral CA1, CA2, CA3, and dentate gyrus (DG) hippocampal regions. The conditioned freezing response increased in animals trained with 0.5 and 1.5 mA during training and extinction sessions. The degree of retention and CORT levels were directly related to the intensity of the foot-shock. Although total GR-IR remained unaffected after conditioning, we observed a significant increase of pSer246-IR in the dorsal region of CA1 and in both dorsal and ventral DG. The only region in which pSer232-IR was significantly elevated was ventral CA3. Our results indicate that fear conditioning training is related to GR phosphorylation in specific subregions of the hippocampus, suggesting that its transcriptional activity for gene expression is favored in ventral CA3, whereas its repressor activity for gene-silencing is increased in dorsal CA1 and in both dorsal and ventral DG.

Effects of anisomycin infusions into the dorsal striatum on memory consolidation of intense training and neurotransmitter activity.

The most influential hypothesis about the neurobiological basis of memory consolidation posits that this process is dependent upon de novo protein synthesis. Strong support for this proposition has been provided by a multitude of experiments showing that protein synthesis inhibitors (PSIs) interfere with consolidation. However, this hypothesis has been challenged by the results of studies showing that PSIs also produce a host of side effects that, by themselves, could account for their amnestic effects. It has been demonstrated that amnestic treatments become innocuous when administered to animals that have been subjected to intense training in a variety of learning tasks. We now report that while infusion of anisomycin (ANI), a PSI, into the dorsal striatum (DS) impairs memory consolidation of inhibitory avoidance learning in response to moderate aversive stimuli, such impairment by ANI is overcome by application of an intense stimulus. We also confirmed that ANI induces inhibition of protein synthesis in the DS, as evidenced by a reduction of the activity-regulated cytoskeletal associated protein (Arc). We found, for the first time, that ANI also induces an increased concentration of serotonin in the DS, which, by itself, may account for the interference with memory consolidation. These findings suggest that de novo protein synthesis in the dorsal striatum is not necessary for the consolidation of intense emotionally arousing experiences. The possibility of a non-genomic-dependent mechanism of memory consolidation is discussed.

Glucocorticoid interactions with the dorsal striatal endocannabinoid system in regulating inhibitory avoidance memory.

The endocannabinoid (eCB) system is highly stress sensitive and known to modulate memory formation of emotionally arousing experiences across different corticolimbic structures. eCB signaling within these circuits is also essentially involved in regulating non-genomically mediated glucocorticoid hormone effects on memory. It has long been thought that the dorsal striatum, which plays a major role in procedural memory and habit formation, is considerably less impacted by stressful experiences; however, recent findings indicate that stress and glucocorticoids also affect striatal-dependent memory processes. Yet, to what extent eCB signaling within the dorsal striatum may mediate such glucocorticoid effects on memory consolidation is currently unknown. Here we show, in male Wistar rats, that the cannabinoid agonist WIN55,212-2 administered into the dorsal striatum immediately after an inhibitory avoidance training experience dose-dependently enhanced 48-h retention performance. Conversely, the cannabinoid type 1 receptor (CB1R) antagonist AM251 impaired retention when administered into the dorsal striatum after inhibitory avoidance training. Most importantly, antagonism of striatal CB1R activity with AM251 completely abolished the effect of corticosterone or of the membrane-impermeable ligand corticosterone:BSA administered posttraining into the dorsal striatum or injected systemically on enhancement of inhibitory avoidance memory. Further, suppression of glucocorticoid signaling by systemic injection of the corticosterone-synthesis inhibitor metyrapone also impaired the memory-enhancing effect of intra-striatal WIN55, 212-2 administration. These findings indicate that the eCB system, in close interaction with glucocorticoid signaling, is involved in modulating plasticity changes underlying memory consolidation not only in corticolimbic structures but also within the dorsal striatum.

Glucocorticoid administration into the dorsolateral but not dorsomedial striatum accelerates the shift from a spatial toward procedural memory.

Glucocorticoid stress hormones are known to enhance the consolidation of hippocampus-dependent spatial and contextual memory. Recent findings indicate that glucocorticoids also enhance the consolidation of procedural memory that relies on the dorsal striatum. The dorsal striatum can be functionally subdivided into the dorsolateral striatum (DLS), which is primarily implicated in shaping procedural memories, and the dorsomedial striatum (DMS), which is engaged in spatial memory. Here, we investigated the hypothesis that posttraining glucocorticoid administration into the DLS promotes the formation of a procedural memory that will normally take place only with extensive training. Male Wistar rats were trained to find a reward in a cross maze that can be solved through either place or response learning. Rats received four trials per day for 5days, a probe trial on Day 6, further training on Days 7-13, and an additional probe trial on Day 14. On Days 2-4 of training, they received posttraining infusions of corticosterone (10 or 30ng) or vehicle into either the DLS or DMS. Rats treated with vehicle into either the DLS or DMS displayed place learning on Day 6 and response learning on Day 14, indicating a shift in control of learned behavior toward a habit-like procedural strategy with extended training. Rats administered corticosterone (10ng) into the DLS displayed response learning on both Days 6 and 14, indicating an accelerated shift to response learning. In contrast, corticosterone administered posttraining into the DMS did not significantly alter the shift from place to response learning. These findings indicate that glucocorticoid administration into the DLS enhances memory consolidation of procedural learning and thereby influences the timing of the switch from the use of spatial/contextual memory to habit-like procedural memory to guide behavior.

Glucocorticoids in the dorsomedial striatum modulate the consolidation of spatial but not procedural memory.

Glucocorticoid hormones are known to influence widely interconnected brain networks, thereby enhancing the consolidation of memory of several types of training experiences. In this network, the dorsal striatum plays an important role in transforming goal-directed behavior into habitual behavior. Many studies have shown that the dorsolateral striatum (DLS) enables the formation of stimulus-response associations that are needed for procedural learning. In contrast, the dorsomedial striatum (DMS) is predominantly involved in influencing goal-directed behaviors via interactions with the dorsal hippocampus and medial prefrontal cortex. To date, most studies that have supported a functional dissociation of the dorsal striatum in memory have focused on the behavioral deficits produced by lesions or temporary inactivation of different striatal regions. Few studies have investigated the effect of pharmacological activation of the DMS in modulating memory of distinct kinds of spatial navigation. Therefore, in the present study corticosterone (CORT) was administered into the DMS immediately after training on either a place or cue water-maze task to investigate possible effects on the consolidation of spatial and procedural memory. Our findings indicate that CORT (5, 10 and 20 ng) enhanced 24-h retention of place training, without affecting retention of cue training. However, CORT administration after place and cue training did not shift the selection from a procedural to a spatial navigation strategy in a place-cue competition test. These findings support the functional heterogeneity of the dorsal striatum and suggest that the DMS can modulate the consolidation of allocentric spatial information via glucocorticoid action.

Glucocorticoid-cholinergic interactions in the dorsal striatum in memory consolidation of inhibitory avoidance training.

Extensive evidence indicates that glucocorticoid hormones act in a variety of brain regions to enhance the consolidation of memory of emotionally motivated training experiences. We previously reported that corticosterone, the major glucocorticoid in the rat, administered into the dorsal striatum immediately after inhibitory avoidance training dose-dependently enhances memory consolidation of this training. There is also abundant evidence that the intrinsic cholinergic system of the dorsal striatum is importantly involved in memory consolidation of inhibitory avoidance training. However, it is presently unknown whether these two neuromodulatory systems interact within the dorsal striatum in the formation of long-term memory. To address this issue, we first investigated in male Wistar rats whether the muscarinic receptor agonist oxotremorine administered into the dorsal striatum immediately after inhibitory avoidance training enhances 48 h retention of the training. Subsequently, we examined whether an attenuation of glucocorticoid signaling by either a systemic administration of the corticosterone-synthesis inhibitor metyrapone or an intra-striatal infusion of the glucocorticoid receptor (GR) antagonist RU 38486 would block the memory enhancement induced by oxotremorine. Our findings indicate that oxotremorine dose-dependently enhanced 48 h retention latencies, but that the administration of either metyrapone or RU 38486 prevented the memory-enhancing effect of oxotremorine. In the last experiment, corticosterone was infused into the dorsal striatum together with the muscarinic receptor antagonist scopolamine immediately after inhibitory avoidance training. Scopolamine blocked the enhancing effect of corticosterone on 48 h retention performance. These findings indicate that there are mutual interactions between glucocorticoids and the striatal cholinergic system in enhancing the consolidation of memory of inhibitory avoidance training.

Corticosterone infused into the dorsal striatum selectively enhances memory consolidation of cued water-maze training.

Glucocorticoid hormones enhance memory consolidation of hippocampus-dependent spatial/contextual learning, but little is known about their possible influence on the consolidation of procedural/implicit memory. Therefore, in this study we examined the effect of corticosterone (2, 5, or 10 ng) infused into the dorsal striatum of male Wistar rats immediately after training on either a cued or spatial version of the water maze. We found that corticosterone dose-dependently enhanced 48-h retention of the cued training without affecting the retention of the spatial training. These findings indicate that corticosterone acts within the dorsal striatum to enhance memory consolidation of procedural/implicit training.

Acquisition and retention of enhanced active avoidance are unaffected by interference with serotonergic activity.

Pre-training administration of p-chloroamphetamine (PCA) produces reliable deficits of avoidance learning. When animals are trained in inhibitory avoidance with relatively high foot-shock intensities, other amnesic treatments have no effect. The present experiment was conducted to determine if this protective effect of high foot shock is also observed after administration of PCA (10mg/kg, i.p., injected 7 days before training; this dose produces a lesion of central serotonin neurons). Rats were trained in active avoidance (a single 20-trial session), administering shocks of 0.6, 0.8, 1.0, or 1.4 mA to independent groups of rats. When compared to saline-injected groups trained with the same intensities, PCA produced a significant learning deficit in the low foot-shock groups, but not in the high foot-shock animals. These results indicate that the dose of PCA administered, which is known to deplete cerebral serotonin, does not interfere with acquisition and retention of enhanced active avoidance training.

Sensory and physiological determinants of maternal behavior in the goat (Capra hircus).

Maternal behavior in the goat appears at the time of parturition, partly under the activating influence of vaginocervical stimulation. Mothers actively lick their neonate and rapidly establish a selective bond with their kid through olfactory recognition. They also develop visual and acoustic recognition of the kid within 4 h following birth. Acoustic recognition is present at 48 h. The establishment of maternal recognition can be impaired by underfeeding during the second half of pregnancy. There is no indication that the mechanisms controlling the onset of maternal behavior and bonding are different from those reported in sheep, despite the fact that lambs start to follow their mother within a few hours after birth and kids hide for about a week. During lactation, the cues provided by the kid are necessary for the maintenance of maternal responsiveness, but suckling itself does not appear of primary importance. The presence of the kid also modulates the hormonal response to udder stimulation and influences recovery of postpartum sexual activity when kidding (i.e. birthing) takes place in autumn. Finally, the rapid establishment of mutual attachment between mother goats (does) and their kids offers the possibility to investigate an aspect of mother-young affiliation that is not present in many laboratory species.

Prepartum peripherally-induced hyposmia does not reduce postpartum anoestrus duration in nursing goats.

Parturient goats rapidly develop exclusive nursing of their own litter that relies on olfactory recognition of the young. They also show a period of postpartum anoestrus whose duration depends on the presence of the kid. In cattle, maternal selectivity is one of the factors that delays the recovery of sexual activity. To investigate the possible influence of maternal selectivity on the duration of postpartum anoestrus in goats, we compared the recovery of estrus behavior by daily estrus detection with an active buck in intact and selective nursing goats (n = 24) with that of dams rendered non-selective by peripheral hyposmia with ZnSO4 (n = 18). Postpartum anoestrus duration was shorter in intact (68+/-7 days) than in hyposmic mothers (93+/-7 days; P < 0.05). However, the cycles of normal duration were less frequent in intact goats (P = 0.03). We conclude that in nursing goats, preventing the establishment of selective nursing by prepartum peripheral hyposmia does not reduce postpartum anoestrus duration. Our results suggest that daily exposure to the buck may result in an earlier recovery of ovarian activity in intact mothers.

Early recognition of newborn goat kids by their mother: I. Nonolfactory discrimination.

Mother sheep and goats develop an early bond with their neonate on the basis of olfactory recognition. We investigated whether goats were also able to show early (<24 hr postpartum) nonolfactory discrimination of their kids, as already reported in sheep. In a first experiment, we found that goats are not able to recognize their kid at 1 m away on the basis of olfactory cues alone. By contrast, they showed a significant preference for their own kid in a two-choice test as early as 4 hr postpartum, and prepartum maternal anosmia did not impede the ability of mothers to show discrimination. We conclude that goats, like sheep, are fully able to discriminate their neonate without the help of olfactory cues very early after parturition. The difference in the early spatial mother-young relationship between the two species due to the different behavior of the young (kids = hiders, lambs = followers), is not associated with marked differences in the dynamics or mechanisms controlling the development of recognition of the neonate by its mother.

Early recognition of newborn goat kids by their mother: II. Auditory recognition and evidence of an individual acoustic signature in the neonate.

The vocal recognition of newborn kids by their mother at 2 days postpartum and the possible existence of interindividual differences in the voice structure of newborn kids were investigated in two separate studies. The ability of goats to discriminate between the bleats of their own versus an alien kid was tested at 2 days postpartum in mothers being prevented access to visual and olfactory cues from the young. Goats spent significantly more time on the side of the enclosure from which their own kid was bleating, looked in its direction for longer, and responded more frequently to the bleats of their own than to those of the alien kid (p < 0.05). In the second study, the sonograms of 13 kids, studied from Days 1 to 5, showed significant interindividual differences for the five variables taken into account and on each of the 5 days (duration of bleat, fundamental frequency, peak frequency, and numbers of segments and of harmonics). The potential for individual coding ranged between 1.1 and 4.1, indicating that for some variables variations between individuals were greater than intraindividual variations. Furthermore, when considering the five parameters together, the discriminating scores showed an average of 95% in the 78 combinations of any 2 kids for any given day. Finally, some significant intraindividual differences also were found between days, suggesting ontogenic changes in the characteristics of the kid's voice in early life. Therefore, mother goats are likely to recognize the vocalizations of their 48-hr-old kids, as they show sufficient interindividual variability to allow the existence of individual vocal signatures, even though some of the characteristics of the bleats change rapidly over time.

Maternal olfaction differentially modulates oxytocin and prolactin release during suckling in goats.

In postparturient goats, olfactory recognition of the young allows the establishment of a selective bond between the mother and her kids. Once this bond is formed, the mother rejects alien young that attempt to suckle. We tested whether the development of the maternal selective bond in goats modulates prolactin (PRL) and oxytocin (OT) release in response to suckling. On day 37 of lactation, serial blood samples were taken during nursing of the mother's own or alien kid(s) in 10 intact/selective goats and in 10 goats rendered anosmic/nonselective through prepartum peripheral ZnSO(4) irrigation. Spontaneous nursing behavior was also studied weekly from day 7 to 30 of lactation, at which time milk production was measured. Maternal selectivity had no effect on PRL release, in contrast to OT release, which was significantly affected by this factor. Intact mothers released OT only when nursing their own kids, but not with aliens, while anosmic/nonselective dams showed an increase in OT levels regardless of the identity of the kids. In addition to these effects on maternal selectivity, the amplitude of the response of both hormones was lower in anosmic mothers than in intact mothers. Finally, nursing behavior and milk production were not significantly affected by anosmia. We conclude that maternal selective behavior in goats, which relies on the individual olfactory signature of the kid, modulates the OT, but not the PRL, response to suckling. In addition, perception of the smell of the young appears to have a general facilitatory effect, independent of the kid's identity, on the release of both hormones.