Inhibiting progesterone metabolism in the hippocampus of rats in behavioral estrus decreases anxiolytic behaviors and enhances exploratory and antinociceptive behaviors

Progesterone attenuates corticotropin-releasing factor-enhanced but not fear-potentiated startle via the activity of its neuroactive metabolite, allopregnanolone

Intact female rats and ovariectomized (OVX) rats with different ovarian steroid replacement regimens were tested for changes in corticotropin-releasing factor (CRF)-enhanced startle (increased acoustic startle amplitude after intracerebroventricular infusion of 1 mug of CRF). OVX rats injected with estradiol (E) followed by progesterone (P) showed a blunted CRF-enhanced startle effect compared with OVX and E-injected rats. CRF-enhanced startle also was reduced significantly in lactating females (high endogenous P levels) compared with cycling rats (low to moderate P levels), as well as in non-E-primed rats when P was administered acutely (4 hr before testing) or chronically (7 d P replacement). The ability of P to attenuate CRF-enhanced startle was probably mediated by its metabolite allopregnanolone [tetrahydroprogesterone (THP)], because THP itself had a similar effect, and chronic administration of medroxyprogesterone, which is not metabolized to THP, did not blunt CRF-enhanced startle but instead slightly increased it. These data suggest that P blunts CRF-enhanced startle through a mechanism involving its neuroactive metabolite THP, although a role for the P receptor cannot be completely ruled out. Finally, neither chronic P replacement nor acute THP affected fear-potentiated startle, suggesting that P metabolites have an effect on the bed nucleus of the stria terminalis and anxiety rather than on the amygdala and stimulus-specific fear.

Differences in affective behaviors and hippocampal allopregnanolone levels in adult rats of lines selectively bred for infantile vocalizations

Allopregnanolone, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-THP), a progesterone metabolite, is an endogenous neurosteroid mediating affective behaviors via its positive modulation of GABA(A) receptors. In order to better understand the role of this neurosteroid in individual differences in affective behavior, we used an animal model based on selective breeding for an infantile affective trait, ultrasonic vocalizations (USV). Adult male and female (in either proestrus or diestrus) rats that had been bred for low (low line) or high (high line) rates of USV after maternal separation were tested in a series of affective behavioral tests: open field, emergence, social interaction, defensive freezing, and the Porsolt forced swim task. Concentrations of allopregnanolone in combined hippocampus and amygdala tissue were then measured. low line subjects showed significantly lower anxiety and depression responses in the emergence, open field, and Porsolt forced swim tasks than did high line subjects. Proestrus females exhibited less affective behaviors than diestrus females or males. Allopregnanolone levels in hippocampus/amygdala were significantly higher in low line subjects compared to high line subjects, and in proestrus females compared to diestrus females and males. These data indicate that: (1) affective behaviors in lines selectively bred for an infantile anxiety trait exhibit selection persistence into adulthood; and (2) levels of allopregnanolone in the limbic system parallel selected disparities in affective behavior, suggesting a selection for alterations in the neurosteroid/GABA(A) receptor system in these lines.

Attenuating 5alpha-pregnane-3alpha-ol-20-one formation in the hippocampus of female rats increases pentylenetetrazole-induced seizures

Progesterone has antiseizure effects, which may be due to the actions of its 5alpha-reduced metabolite, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP). Whether metabolism of progesterone to 3alpha,5alpha-THP in the hippocampus is essential for its antiseizure effects was investigated. In Experiment 1, ovariectomized rats were administered subcutaneous progesterone (500 microg) or vehicle (sesame oil), followed 1 hour later by subcutaneous administration of an inhibitor of the 5alpha-reductase enzyme, finasteride (50 mg/kg), or vehicle (90% sesame oil, 10% ethanol). Administration of progesterone increased the latency to, and decreased the number of, tonic seizures and increased hippocampal 3alpha,5alpha-THP levels, compared with vehicle. Administration of finasteride with progesterone attenuated progesterone's antiseizure effects and decreased levels of 3alpha,5alpha-THP in the hippocampus. Finasteride administration alone did not alter ictal behavior or 3alpha,5alpha-THP levels compared with vehicle. In Experiment 2, ovariectomized rats were administered subcutaneous progesterone (500 microg) or vehicle (sesame oil), followed 1 hour later by bilateral infusions of finasteride (10 microg) or vehicle (beta-cyclodextran) into the hippocampus. Administration of finasteride to the hippocampus of progesterone-primed rats significantly increased ictal activity and decreased hippocampal 3alpha,5alpha-THP levels, compared with progesterone administration alone. These data suggest that formation of 3alpha,5alpha-THP in the hippocampus is important for progesterone's antiseizure effects.

Progesterone's 5α-reduced metabolite, 3α,5α-THP, mediates lateral displacement of hamsters

5 alpha-Pregnan-3 alpha-ol-20-one (3 alpha,5 alpha-THP), progesterone (P4)'s 5 alpha-reduced, 3 alpha-hydroxysteroid oxidoreduced product, facilitates lordosis of rodents in part via agonist-like actions at GABA(A)/benzodiazepine receptor complexes in the ventral tegmental area (VTA). Whether 3 alpha,5 alpha-THP influences another reproductively-relevant behavior, lateral displacement, of hamsters was investigated. Lateral displacement is the movement that female hamsters make with their perineum towards male-like tactile stimulation. This behavior facilitates, and is essential for, successful mating. Hamsters in behavioral estrus had greater lateral displacement responses when endogenous progestin levels were elevated compared to when progestin levels were lower. Administration of P4, a prohormone for 3 alpha,5 alpha-THP, dose-dependently (500 > 200 > 100, 50, or 0 microg) enhanced lateral displacement of ovariectomized hamsters that had been primed with SC estradiol benzoate (5 or 10 microg). Inhibiting P4's metabolism to 3 alpha,5 alpha-THP by co-administering finasteride, a 5 alpha-reductase inhibitor, or indomethacin, a 3 alpha-hydroxysteroid oxidoreductase inhibitor, either systemically or to the VTA, significantly decreased lateral displacement and midbrain progestin levels of naturally receptive or hormone-primed hamsters compared to controls. These data suggest that lateral displacement is progestin-sensitive and requires the formation of 3 alpha,5 alpha-THP in the midbrain VTA.

Ethanol-induced elevation of 3 alpha-hydroxy-5 alpha-pregnan-20-one does not modulate motor incoordination in rats

BACKGROUND

Ethanol administration elevates the levels of GABAergic neuroactive steroids in brain and contributes to some of its behavioral actions. In the present study, we investigated whether such elevation of GABAergic neuroactive steroids contributes to the motor incoordinating effects of ethanol.

METHODS

Sprague-Dawley rats were administered ethanol (2 g/kg intraperitoneally) or saline, and the level of 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP) was measured across time in cerebral cortex and in various brain regions at the peak time by radioimmunoassay. To study whether increases in GABAergic neuroactive steroids are responsible for the motor incoordinating actions of ethanol, rats were subjected to chemical (5alpha-reductase inhibitor, finasteride) and surgical (adrenalectomy) manipulations before receiving ethanol (2 g/kg intraperitoneally) injections. The rats were then subjected to different paradigms to evaluate motor impairment including the Majchrowicz motor intoxication rating scale, Rotarod test, and aerial righting reflex task at different time points.

RESULTS

The radioimmunoassay of 3alpha,5alpha-THP in different brain regions showed that ethanol increases 3alpha,5alpha-THP levels by 3- and 9-fold in cerebral cortex and hippocampus, respectively. There was no change in 3alpha,5alpha-THP levels in cerebellum and midbrain. The time course of 3alpha,5alpha-THP elevations in the cerebral cortex showed significant increases 20-min after ethanol injection with a peak at 60 min. In contrast, motor toxicity peaked between 5 and 10 min after ethanol injections and gradually decreased over time. Furthermore, adrenalectomy or pretreatment with finasteride (2 x 50 mg/kg, subcutaneously) did not reduce motor incoordinating effects of ethanol as assessed by the Majchrowicz intoxication rating score, Rotarod test, or aerial righting reflex task.

CONCLUSIONS

Ethanol increases GABAergic neuroactive steroids in a time- and brain region-selective manner. The role of neuroactive steroids in alcohol action is specific for certain behaviors. Alcohol-induced deficits in motor coordination are not mediated by elevated neuroactive steroid biosynthesis.

Inter-individual vs line/strain differences in psychogenetically selected Roman High-(RHA) and Low-(RLA) Avoidance rats: neuroendocrine and behavioural aspects

Inter-individual differences in neuroendocrine and behavioural responses to environmental challenges will be considered within the context of psychogenetic selection, using the Roman High-(RHA) and Low-(RLA) Avoidance rat lines as an example. We assume that the selected genotypes, by interacting with environmental factors, determine specific 'biobehavioural profiles'. Practical and theoretical problems regarding the measurement of inter-individual vs line/strain differences, the definition of 'traits' vs experimental variables, and possible correlations between physiological and behavioural parameters will be discussed. We will argue that environmental influences are the main cause of inter-individual variability, and that the genotype only constitutes a 'blueprint' from which typical biobehavioural profiles are established, notably under the influence of early environmental factors. These biobehavioural profiles may correspond in part to human categories known as 'types', 'temperaments' or 'personality traits'. Within each category (including those which can be obtained by psychogenetic selection), more individual personality traits can evolve, notably as a result of social interactions and particular life events.

Progestins in the hippocampus of female rats have antiseizure effects in a pentylenetetrazole seizure model

PURPOSE

Progestins can have profound effects on seizure processes. However, the effects and mechanisms of progestins to modulate seizures have not been systematically investigated. The present studies were designed to characterize the effects of progestins to modulate pentylenetetrazole (PTZ)-induced seizures in female rats.

METHODS

In Experiment 1, ictal activity and plasma and hippocampal progesterone (P) and 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) levels of proestrous rats were compared with those of diestrous and ovariectomized (ovx) rats. Experiments 2 and 3 examined effects of ovx and replacement with vehicle, P, or 3alpha,5alpha-THP, systemically (Experiment 2) or to the hippocampus (Experiment 3) on seizures and plasma and hippocampal P and 3alpha,5alpha-THP concentrations.

RESULTS

Proestrous rats had reduced ictal activity and increased levels of P and 3alpha,5alpha-THP in plasma and hippocampus compared with diestrous or ovx rats (Experiment 1). Rats administered systemic P or 3alpha,5alpha-THP had significantly reduced ictal activity and increased plasma and hippocampal P and 3alpha,5alpha-THP levels compared with vehicle-administered rats (Experiment 2). Administration of P or 3alpha,5alpha-THP to the hippocampus of ovx rats significantly reduced seizure activity and increased hippocampal, but not plasma, levels of P and 3alpha,5alpha-THP compared with vehicle administration (Experiment 3).

CONCLUSIONS

Together, these data suggest that P can have antiseizure effects, and these effects may be due in part to actions of its metabolite, 3alpha,5alpha-THP, in the hippocampus.

Estrogen and/or progesterone administered systemically or to the amygdala can have anxiety-, fear-, and pain-reducing effects in ovariectomized rats

Estrogen (E2) and/or progesterone (P) in the amygdala may influence anxiety, fear, and pain behaviors. Ovariectomized rats were administered subcutaneous or intra-amygdala vehicle, E2, P, or E2 + P: Effects on open field, elevated plus-maze, defensive freezing, and hot-plate task performance were observed. Subcutaneous E2 + P or intra-amygdala E2, P, or E2 + P increased open field central entries and open arm time in the plus-maze compared with vehicle. Subcutaneous or intra-amygdala E2, P, or E2 + P decreased time spent freezing postshock compared with vehicle. Subcutaneous or intra-amygdala E2 + P increased latencies to lick paws compared with vehicle. Thus, E2 and P may have effects in the amygdala to decrease anxiety, fear, and/or pain responses.

Hippocampal 3α,5α-THP may alter depressive behavior of pregnant and lactating rats

The 5alpha-reduced metabolite of progesterone (P), 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), may mediate progestins' effects to reduce depressive behavior of female rats in part through actions in the hippocampus. To investigate, forced swim test behavior and plasma and hippocampal progestin levels were assessed in groups of rats expected to differ in their 3alpha,5alpha-THP levels due to endogenous differences (pregnant and postpartum), administration of a 5alpha-reductase inhibitor (finasteride; 50 mg/kg sc), and/or gestational stress [prenatal stress (PNS)], an animal model of depression. Pregnant rats had higher plasma and hippocampal 3alpha,5alpha-THP levels and less depressive behavior (decreased immobility, increased struggling and swimming) in the forced swim test than did postpartum rats. Finasteride, compared to vehicle-administration, reduced plasma and hippocampal 3alpha,5alpha-THP levels and increased depressive behavior (increased immobility, decreased struggling and swimming). PNS was associated with lower hippocampal, but not plasma, 3alpha,5alpha-THP levels and increased swimming compared to that observed in control rats. Together, these data suggest that 3alpha,5alpha-THP in the hippocampus may mediate antidepressive behavior of female rats.

Sexually dimorphic effects of postnatal allopregnanolone on the development of anxiety behavior after early deprivation

Stress early in life exerts persistent detrimental effects on brain development. In this experiment, a rodent model of child neglect, early deprivation (ED), was used to investigate the role of the neurosteroid allopregnanolone [AlloP; 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP)] in the development of anxiety behavior. Subjects were either undisturbed controls or ED: separated individually for 6 h per day from postnatal day (PN) 2 to 6. Control and ED subjects were also either noninjected, vehicle-injected or injected with 5 mg/kg AlloP prior to the isolation. At PN 7, responses to 2.5 or 5 microg icv injections of AlloP were determined for separation-induced ultrasonic vocalizations (USVs). Tolerance to the USV-reducing effect of daily AlloP was seen in control but not ED pups, and daily AlloP reversed the expected ED suppression of USVs. As adults, controls treated with postnatal AlloP were less anxious than all other groups on the elevated plus maze. ED counteracted this effect. Male controls showed a reversal of the typical sex difference. There were no effects on open-field activity. These results suggest that the neonatal brain is responsive to alterations in AlloP levels, and that neuroactive progesterone metabolites may play a role in mediating the development of stress-related sex differences.

Progesterone enhances motor, anxiolytic, analgesic, and antidepressive behavior of wild-type mice, but not those deficient in type 1 5 alpha-reductase

The importance of progesterone's (P(4)) metabolism by the 5 alpha-reductase type I enzyme was examined in homozygous and heterozygous 5 alpha-reductase type I knockout mice and their wild-type siblings. P(4) (1.0 mg) or vehicle was administered and effects on motor, anxiety, nociceptive, and depression behavior were observed. After testing, whole-brain progesterone and 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha,5 alpha-THP) levels were determined by radioimmunoassay. Motor behavior in the horizontal crossing and open field tasks of 5 alpha-reductase-deficient mice administered P(4) was similar to vehicle control mice and significantly reduced compared to wild-type mice administered P(4). In the open field, 5 alpha-reductase-deficient mice administered P(4) had a similar number of central entries as did vehicle control mice, both were lower than central entries of P(4)-administered wild-type mice. However, in the plus maze, P(4) to 5 alpha-reductase-deficient or wild-type mice significantly increased open arm activity compared to vehicle-administered control mice. P(4) to wild-type, but not 5 alpha-reductase-deficient mice, significantly increased latencies to lick front and back paws in response to radiant heat stimuli compared to vehicle administration to control mice. In the forced swim test, 5 alpha-reductase-deficient mice administered P(4) were similar to vehicle control mice and the latency to immobility was significantly decreased, and the duration of immobility was significantly increased, compared to wild-type mice administered P(4). Thus, these data suggest metabolism by the 5 alpha-reductase type I enzyme may mitigate P(4)'s effects on some tasks of motor, anxiety, nociception, and depression behavior.

Gender differences in the regulation of 3 alpha-hydroxysteroid dehydrogenase in rat brain and sensitivity to neurosteroid-mediated stress protection

The enzyme 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) is involved in the generation of neuroactive steroids through ring-A-reduction of hormonal precursors. We examined the developmental regulation of, gender differences in, and effects of hormonal manipulations on the expression of 3 alpha-HSD in the rat hippocampus. High levels of 3 alpha-HSD mRNA were found on postnatal day 7, coinciding with the stress hyporesponsive period in the rat. Gender differences in 3 alpha-HSD expression were documented during puberty, but not in adulthood. Adrenalectomy and gonadectomy, and supplementation with individual steroid hormones influenced 3 alpha-HSD expression in a gender-specific mode. We also demonstrate that the manifestation of behavioral and endocrine consequences of early life stress depends on the individual's gender and gonadal status. Males are liable to aftereffects of neonatal maternal deprivation, regardless of their adult gonadal status. In females, however, anxiogenic aftereffects of neonatal stress become apparent only after gonadectomy. These data suggest that (i) transient increase of neurosteroid biosynthesis may contribute to stress hyporesponsiveness during early infancy; (ii) gonadal steroids regulate 3 alpha-HSD expression in the hippocampus in a sex-specific mode; (iii) physiological sex steroid secretions in females may mask behavioral consequences of adverse early life events, and (iv) concomitant treatment with the neurosteroid THP counteracts behavioral and endocrine dysregulation induced by neonatal stress in both genders.

Enhancing effects of estrogen on inhibitory avoidance performance may be in part independent of intracellular estrogen receptors in the hippocampus

Estradiol (E(2)) can have classical actions via intracellular estrogen receptors (ERs) in the dorsal hippocampus, as well as effects independent of ERs ('non-genomic' mechanisms). These experiments investigated whether E(2)'s cognitive enhancing effects in the inhibitory avoidance task require actions at ERs in the dorsal hippocampus. Ovariectomized (ovx) rats were administered E(2) (s.c. or to the dorsal hippocampus), an E(2) conjugate (E(2):BSA), or vehicle and/or an ER antagonist, tamoxifen (10 mg/kg s.c.) or ICI 182,780 (10 microg intrahippocampally), or vehicle for 2 days prior to training (Day 3) and testing (Day 4) in the inhibitory avoidance task. Exp 1: crossover latencies in the inhibitory avoidance task were significantly increased in ovx rats with s.c. E(2) silastic capsules or s.c. injections of 1000 or 10 microg E(2) compared to vehicle-administered rats. Exp 2: bilateral inserts of E(2) to the dorsal hippocampus significantly increased crossover latencies compared to vehicle. Exp 3: s.c. tamoxifen, the ER antagonist, did not block the increased crossover latencies produced by 10 microg E(2) s.c. (compared to vehicle). Exp 4: s.c. tamoxifen did not block the increased crossover latencies produced by intrahippocampal E(2) (compared to vehicle). Exp 5: ICI 182,780 was unable to attenuate the increased crossover latencies produced by intrahippocampal E(2). Exp 6: E(2):BSA administered to the dorsal hippocampus significantly enhanced performance on the inhibitory avoidance task compared to control implants to the hippocampus. The ability of systemic and intrahippocampal E(2) to similarly enhance inhibitory avoidance performance suggests that actions of E(2) in the dorsal hippocampus are sufficient to enhance cognitive performance. Further, that neither tamoxifen nor ICI 182,780 blocked E(2)'s enhancing effects on inhibitory avoidance and that E(2):BSA was able to enhance performance suggest that non-genomic mechanisms may in part mediate E(2)'s cognitive enhancing performance in this task.

Changes in progesterone metabolites in the hippocampus can modulate open field and forced swim test behavior of proestrous rats

The purpose of these experiments was to test the hypothesis that attenuating the endogenous increase of the 5alpha-reduced progesterone metabolite 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) in the hippocampus will alter anxiety and depression behavior of proestrous rats. In Experiment 1, anxiety (open field) and depression (forced swim test) behavior was compared of rats that should have high (proestrous) and low (diestrous and male rats) endogenous hippocampal 3alpha,5alpha-THP. Proestrous rats exhibited more anxiolytic-like (increased central entries in the open field) and anti-depressant-like (less immobility in the forced swim test) behavior than diestrous or male rats. In Experiments 2 and 3, respectively, systemic and intrahippocampal finasteride, a 5alpha-reductase inhibitor which attenuates progesterone's metabolism to 3alpha,5alpha-THP, versus vehicle administration to proestrous rats was compared for effects on open field and forced swim test behavior. Systemic or intrahippocampal finasteride decreased central entries in the open field and increased immobility in the forced swim tests compared to vehicle administration. In Experiment 4, the effects of systemic and intrahippocampal finasteride vs vehicle administration on hippocampal 3alpha,5alpha-THP of proestrous rats was examined. Finasteride, SC or intrahippocampally, reduced 3alpha,5alpha-THP in the hippocampus compared to vehicle administration. Together these data suggest that variations in 3alpha,5alpha-THP levels in the hippocampus may mitigate proestrous changes in anxiety and depressive behavior of cycling rats.