Lordosis Reflex In Female Rats Research Articles

The role of nitric oxide (NO) in control of LHRH release that mediates gonadotropin release and sexual behavior.

Nitric oxide (NO) plays a crucial role in reproduction at every level in the organism. In the brain, it activates the release of luteinizing hormone-releasing hormone (LHRH). The axons of the LHRH neurons project to the mating centers in the brain stem and by efferent pathways, evoke the lordosis reflex in female rats. In males, there is activation of NOergic terminals that release NO in the corpora cavernosa penis to induce erection by generation of cyclic guanosine monophosphate (cGMP). NO also activates the release of LHRH which reaches the pituitary and activates the release of gonadotropins by activating neural NO synthase (NOS) in the pituitary gland. Follicle stimulating hormone (FSH)RH selectively releases FSH also by activating NOS. Leptin releases LHRH by activating NOS to release FSH and LH with the same potency as LHRH. These actions are mediated by specific receptors on the gonadotropes for LHRH, FSHRH and leptin. The responsiveness of the pituitary is controlled by gonadal steroids. In the gonad, NO plays an important role inducing ovulation and in causing luteolysis; whereas in the reproductive tract, it relaxes uterine muscle via cGMP and constricts it by prostaglandins.

The role of nitric oxide in reproduction.

Nitric oxide (NO) plays a crucial role in reproduction at every level in the organism. In the brain, it activates the release of luteinizing hormone-releasing hormone (LHRH). The axons of the LHRH neurons project to the mating centers in the brain stem and by afferent pathways evoke the lordosis reflex in female rats. In males, there is activation of NOergic terminals that release NO in the corpora cavernosa penis to induce erection by generation of cyclic guanosine monophosphate (cGMP). NO also activates the release of LHRH which reaches the pituitary and activates the release of gonadotropins by activating neural NO synthase (nNOS) in the pituitary gland. In the gonad, NO plays an important role in inducing ovulation and in causing luteolysis, whereas in the reproductive tract, it relaxes uterine muscle via cGMP and constricts it via prostaglandins (PG).

Biphasic augmentation of the lordosis reflex of female rats from the ventral premammillary nucleus

Biphasic augmentation of the lordosis reflex of female rats from the ventral premammillary nucleus

Suppression of the lordosis reflex of female rats by efferents of the medial preoptic area

In freely moving, estrogen- and progesterone-treated ovariectomized female rats, electrical stimulation of the medial preoptic area, which had been isolated dorsally from the septum by a horizontal knife cut (roof cut), caused immediate interruption of the lordosis reflex, a principal component of female reproductive behavior in this species. Neither the knife cut nor the stimulation affected their proceptive interactions with the males. Lordosis was interrupted in a graded manner in response to increased stimulus intensity, with a threshold at 30 μA. The optimal frequency was at 50–100 Hz. Lordosis was reinstated promptly by the termination of current application. The rapid time course distinguished the stimulation effect in the rats with the roof cut from that in the intact control animals, which was slow in its onset and recovery. In the shamoperated animals, an adjunct bilateral cut of the stria terminalis was sufficient to restore the rapid response as in the roof-cut animals. The roof-cut animals were sensitive to lower doses of estrogen than those without the knife cut; therefore, the preoptic area appears to be a separate entity from the septum in the inhibitory control of lordosis. The elimination of facilitatory neural components for this reflex, which enter the preoptic area dorsally via the stria terminalis, might be responsible for the prompt and exaggerated stimulation effect in the roof-cut animals.

Effects of anterior roof deafferentation on lordosis behavior and estrogen receptors in various brain regions of female rats

This study investigated whether the estrogen receptors (ER), located at different brain areas and anterior pituitary (AP), changed after anterior roof deafferentation (ARD), and on the effects of facilitating the lordosis reflex in female rats. Female rats were ovariectomized and implanted with estradiol capsules. ARD or sham operation was performed with a Halász knife. All animals were tested for lordosis both before and after surgery. One day after the last test they were sacrificed. Cytosol and nuclear ER in the AP, medial preoptic area (MPOA), basal medial hypothalamus (BMH), amygdala (AMYG), septum (SEP), hippocampus (HPC), and cortex (CTX) were measured using an in vitro exchange assay. Rats with ARD showed significantly higher mean levels of lordosis quotient than the control and the sham groups before ARD surgery. An increase of both cytosol and nuclear ER in the BMH area compared to the control was observed, whereas the ER, in the SEP was reduced. ER in other areas were not affected by ARD. Serum estradiol and progesterone levels were not altered by the operation. These data suggest that the dorsal inhibitory pathway from the extrahypothalamus to the preoptic area and hypothalamus may modulate the estrogen receptor and the display of lordosis in female rats. Change of ER level in the BMH area may influence the hormonal sensitivity of lordosis in female rats.

Interruption of the lordosis reflex of female rats by ventral midbrain stimulation

The lordosis reflex, dorsiflexion of the vertebral column, is an estrogen-dependent, essential element of female sexual behavior in rodents. Unilateral electrical stimulation of the midbrain ventral tegmental area through a chronically implanted electrode in freely moving, estrogen-primed ovariectomized female rats caused a rapid and strong suppression of the lordosis reflex in response to either male mounts or manual cutaneous stimuli. The interruption occurred in a graded manner to increased stimulus intensity, with a threshold at 30 μA. The optimal frequency was at 75–125 Hz. After the termination of electrical stimulation, lordosis performance returned promptly to the prestimulation level. No aversive response accompanied the blockade of lordosis. Electrical stimulation specifically blocked lordosis, without disrupting the proceptive components of female sexual behavior. In 10 animals tested, concomitant injection of dopamine receptor blocker pimozide tended to offset the effects of electrical stimulation in 2 cases. Interruption of the lordosis reflex might be mediated by projections from the ventral tegmental area, which activate a descending pathway inhibitory to the lordosis reflex arc at or below the lower brain stem.

Suppression of the lordosis reflex in female rats by chronic central melatonin implants

Pellets with beeswax:melatonin concentrations of 1:0, 5:1, and 2:1 were implanted near the suprachiasmatic nucleus of ovariectomized female rats. In repeated standard measures of the lordosis reflex after estrogen replacement, females with pellets containing either melatonin concentration were consistently less sexually receptive than were females with pure beeswax implants. Suppression of behavioral receptivity by melatonin is consistent with other antireproductive effects of the hormone. Melatonergic action could account for certain pharmacological findings previously attributed to serotonergic action.

Midbrain lesions eliminate sexual receptivity but spare sexual motivation in female rats

To measure sexual motivation we have used a preference test paradigm which involves testing a female with a sexually active male and a sexually inactive castrate male at the same time. On the assumption that a sexually motivated female will choose to spend more time with a partner who can provide her with stimulation appropriate to her motivational state, a female can be said to be sexually motivated to the extent that she spends more time with the active male than with the castrate. We find that bilateral destruction of the midbrain peripeduncular region eliminates the lordosis reflex in female rats, and abolishes sexual soliciting darting responses. Lesioned females treated with ovarian hormones continue, however, to show a strong preference for a sexually active male over a castrate. Thus, although peripeduncular destruction eliminates copulatory behavior, such destruction appears to spare hormone dependent systems for sexual motivation.

Modulation of the lordosis reflex of female rats by LHRH, its antiserum and analogs in the mesencephalic central gray.

Effects of intracerebral infusion of LHRH and related compounds on the lordosis reflex were examined in ovariectomized, estrogen-treated female rats. Lordosis, triggered either by manual cutaneous stimulation or by male mounting, was facilitated by the infusion of LHRH dissolved in saline into the mesencephalic central gray at midcollicular levels (CG). Infusion of an anti-LHRH globulin into the CG disrupted lordosis. Pretreatment with estrogen was necessary to obtain the behavioral facilitation by LHRH infusion. In separate experiments, placement of cannulae containing LHRH in an agar-saline gel in the CG also facilitated lordosis behavior. LHRH analogs applied to the CG by this method showed a clear dissociation between their behavioral effects and their reported effects on LH release. Cellular mechanisms involved in the mediation of LHRH effect in midbrain for the facilitation of lordosis may be different from those in gonadotrophs for LH release.

Inhibition of lordosis behavior in the female rat by intraventricular infusion of prolactin and by chronic hyperprolactinemia.

The role of prolactin (PRL) in the control of the lordosis reflex of female rats was investigated. In the initial series of experiments, the normal high level of sexual receptivity observed in the ovariectomized, estrogen-progesterone (E-P) primed female rat was suppressed by intraventricular infusion of 100 ng PRL. Mating behavior remained suppressed 2, 3, and 5 hours following a single infusion of PRL into the third ventricle. In contrast, infusions of either an equal volume of the solvent vehicle (saline) or 100 ng of adrenocorticotropic hormone (ACTH) were ineffective in modulating the level of mating behavior in hormone-primed female rats. In a second series of experiments, chronic hyperprolactinemia was induced by pituitary transplants under the renal capsule in intact, normal cycle diestrus rats (N=12). A significant decrement in E-P induced mating behavior was observed at 12 and 14 weeks posttransplantation but not at 4 weeks. Sham-operated animals (N=12) displayed the characteristic pattern of behavior normally observed under exogenous E-P therapy. In summary, transient exposure as well as chronic exposure to high levels of PRL can suppress mating behavior, thus suggesting a possible role for PRL in the mediation of reproductive behavior in the female rat.

Dispensability of spinal monoaminergic systems in mediating the lordosis reflex of the female rat

The possibility that bulbospinal monoaminergic systems are essential for expression of the estrogen-induced lordosis reflex in female rats was tested by studying the effects of reserpine and monoamine antagonists on the lordosis responses to mating and manual stimulation. Lordosis performance of five groups of ovariectomized rats, either treated (groups II through V) or not treated (group I) with estrogen, were measured before and after pharmacological treatments. Immediately after post-treatment measurements, females (except group V) were decapitated and the lumbosacral cord quickly removed, frozen, and later assayed for norepinephrine, dopamine and serotonin. Group I females (n=7) did not show any lordosis before or after a control saline injection, and their spinal monoamine contents were not different from those of group II (n=8), who showed near maximal levels of lordosis both before and after a saline injection. Reserpine at low (2 mg/kg; group III, n=6) or high (10 mg/kg, group IV, n=10) doses depleted spinal monoamines severely but had no significant effect on lordosis, except for a slight decline of lordosis intensity after the high dose treatment. A combination treatment of reserpine (10 mg/kg) and a serotonin receptor blocker, methiothepin (10 mg/kg), did not affect lordosis performance (group V, n=6). Further injection of an α-adrenergic antagonist, phenoxybenzamine (20 mg/kg), in addition to this combination, had no effect on the lordosis reflex in response to manual stimulation, although the response to mating stimulation was reduced. The reduction was probably a result of a drug-induced abnormal posture, which made most of the reflexogenic skin area of the female inaccessible to mating stimuli. These results showed that estrogen treatment sufficient to induce a near maximal level of lordosis performance did not alter spinal monoamine contents, and that the monoamines in the spinal cord are not necessary for mediating the estrogen-stimulated lordosis reflex.

Effects of acute central and peripheral ACTH 1–24 administration on lordosis behavior

The role of adrenocorticotropic hormone (ACTH) in the control of the lordosis reflex of female rats was investigated. Experiment 1 examined the effects of ACTH 1–24 infused into the ventricular system of the brain. ACTH treatment lowered the performance of both ovariectomized (ovx) and adrenalectomized-ovariectomized (adx-ovx) extrogen-primed females. ACTH-treated females also showed evidence of spontaneous stretching, yawning, and grooming. Experiment 2 examined the effects of ACTH 1–24 administered subcutaneously. Here ACTH elevated performance of ovx but not adx-ovx estrogen-primed females. There was no evidence of spontaneous stretching, yawning, and grooming movements in females given peripheral ACTH. In both experiments, untreated adx-ovx females showed a higher level of lordosis than untreated ovx females. These results suggest that the central effect of acute ACTH elevation differs from the peripheral effect, and that the peripheral effect is probably mediated by the adrenal.

LH-RH in the mesencephalic central grey can potentiate lordosis reflex of female rats.

The lordosis reflex, which characterises female reproductive behaviour in rodents, can be potentiated in oestrogen-primed ovariectomised female rats by electrical stimulation of the ventromedial nucleus of the hypothalamus (VMN). Subsequent demonstration of an immediate and large facilitation of the lordosis reflex from the mesencephalic central grey (CG) prompted us to speculate that the VMN may exert an oestrogen-dependent tonic bias on the mesencephalic circuitry for lordosis via its heavy descending projections. Moreover, we considered that luteinising hormone-releasing hormone (LH-RH) could be involved in this system. LH-RH can potentiate lordosis when given systemically to oestrogen-primed ovariectomised rats, and neurotropic effects of this peptide have been shown by microiontophoresis. Also, some axons in the CG have been stained with antiserum to LH-RH. The present study examined a possible role of LH-RH in the CG in the regulation of the lordosis reflex of oestrogen-primed ovariectomised female rats. Infusion of exogenous LH-RH in the CG had an immediate facilitative effect on the lordosis reflex, whereas passive immunisation against endogenous LH-RH by anti-LH-RH gamma-globulin diminished the reflex.

Responses of lumbosacral spinal units to mechanical stimuli related to analysis of lordosis reflex in female rats.

1. To analyze further the sensory mechanisms for triggering the lordosis reflex, single-unit (n = 345) activity was recorded extracellularly from spinal segments L5-S1 of urethan-anesthetized female rats. Unit responses to pressure on the skin (necessary and sufficient for evoking lordosis) and other mechanical stimuli were studied. 2. Units were classified according to their responses to the battery of mechanical stimuli: 16% of the units responded only to pressure. The majority of these pressure-responsive units were excited, while a few were inhibited or responded differently, depending on the site stimulated; 52% did not respond to pressure, but responded to brushing, muscle-joint, and/or visceral stimulation, or did not respond at all. The remaining 32% responded to pressure plus other forms of stimulation. 3. Units responding only to the movement of individual types of hair tended to be located in the dorsal horn, more dorsal than units responding only to pressure (found primarily in the intermediate gray). Units responding to subdermal stimulation were usually found at greater depths. Segmental and somatotopic distributions of spinal units observed were very similar to those reported for cat (3) and monkey (5). 4. Compared to primary sensory units (28), spinal units had higher resting firing rates, more complicated responses to a given stimulus, a wider variety of unit types, and much larger receptive fields. These comparisons tend to indicate convergence of primary sensory neurons onto individual spinal units. 5. The range of pressure thresholds of pressure-responsive units is comparable to the range effective for triggering lordosis. We postulate that excitation of units responding only to pressure is centrally involved in triggering the lordosis reflex. Those units not responding to pressure are probably irrelevant for this behavior. Presently undetermined are the roles of units with complex or mixed-response types.

Medullary reticular formation lesions and lordosis reflex in female rats

Medullary reticular formation lesions and lordosis reflex in female rats

Facilitation of the lordosis reflex of female rats from the ventromedial nucleus of the hypothalamus.

1. Effects of electrical stimulation of hypothalamic ventromedial nucleus (v.m.n.) on the lordosis reflex of female rats were examined in ovariectomized and oestrogen-primed animals with chronically implanted electrodes. 2. Lordosis triggered either by manual cutaneous stimulation or by male mounting, was facilitated by electrical stimulation of the v.m.n. 3 A gradual increase in lordosis performance followed a relatively long period of stimulation; never less than 15 min and usually about 1 hr of stimulation was necessary for maximum facilitation. Following the termination of stimulation, the performance returned gradually to the control level during a 5--8 hr period. 4. The optimal frequency of stimulation was between 10 and 30 Hz. Threshold for effective facilitation was, on the average, 12.5 microA. 5. Stimulation tended to induce larger facilitation when applied to the lateral side of v.m.n. 6. Pre-treatment with oestrogen was necessary to obtain facilitation by v.m.n. stimulation. The threshold dosage of oestrogen was 2.5 microgram per animal. 7. Stimulation was effective in adrenalectomized rats, in dexamethasone-primed animals, and in rats pre-treated with exogenous progesterone. Thus, adrenal prodesterone release is not required for the v.m.n. facilitation of lordosis. 8. Medial preoptic stimulation with the same parameters suppressed the lordosis reflex. 9. The v.m.n. participates in the control of lordosis by a facilitatory output. The delay before facilitation implies that the v.m.n. is not in the direct reflex-arc for the execution of lordosis. Rather, a summation or interaction process with an unusually long time course is involved.

Deficit in the lordosis reflex of female rats caused by lesions in the ventromedial nucleus of the hypothalamus.

1. The effect of electrolytic lesions of the ventromedial nucleus of the hypothalamus (v.m.n.) on the lordosis reflex has been investigated on ovariectomized female rats. Lesions were made through chronically implanted platinum-iridium electrodes. 2. V.m.n. lesions did not disrupt lordosis immediately, but induced a gradual decline in the reflex. Lordosis performance reached it minimum no less than 12 hr after the lesion, and typically after 36--60 hr. 3. The magnitude of the lordosis deficit was related to the amount of v.m.n. damage. Destruction of other hypothalamic regions was without appreciable relation to the deficit. Within v.m.n., lesion size in the lateral, but not medial portion was significantly correlated with lordosis deficit. 4. Because of the slow time courses of v.m.n. lesions and stimulation (Pfaff & Sakuma, 1978) effects, it is postulated that the v.m.n. is not part of the direct reflex-arc for lordosis. Rather, neurones in v.m.n. are likely to exert a tonic hormone-dependent bias on brain stem reflex paths for this behaviour.

Responses of single units in sixth lumbar dorsal root ganglion of female rats to mechanostimulation relevant for lordosis reflex

1. The activity of single units (n = 312) was recorded from dorsal root ganglion L6 in female rats; 84% received input from cutaneous mechanoreceptors. 2. Among cutaneous mechanoreceptive units (n = 263), 22% responded to individual guard hair movement, 18% to down hair, 11% to clitoral hair, and 14% to stimulation of tylotrich hair-haarscheibe (type I) receptors, 5% responded to punctate skin deformation, but not hair movement of pressure, and 18% responded to pressure. 3. No significant differences between estrogen-treated and untreated ovariectomized rats were observed in receptive-field size, or von Frey hair, or pressure threshold of any given type of unit. 4. The lordosis reflex of female rats is triggered by pressure, but not hair deflection, on skin regions innervated by dorsal root L6 (10). Stimulation from the male rat mounting would cause a barrage of action potentials from most of the unit types identified here, but only pressure units and type I units gave sustained responses to a lordosis-triggering type of pressure stimulation.

Facilitation of the lordosis reflex in female rats by electrical stimulation of the lateral vestibular nucleus

(1) Electrical stimulation of the lateral vestibular nucleus (LVN) facilitated lordosis reflex responses to somatosensory stimulation, in estrogen-primed female rats. (2) Unilateral LVN stimulation was sufficient for this effect. Currents as low as 10muA could be used. (3) With the stimulus parameters used, the effect develops within 5 min of LVN stimulation and decays within 10 min following cessation of stimulation. (4) Together with previous evidence that LVN lesions disrupt lordosis, these results suggest LVN facilitation of spinal reflex mechanisms for lordosis.

Effects of spinal cord transections on lordosis reflex in female rats

(1) Female rats with complete transections of spinal cord at low thoratic levels were not able to perform the lordosis reflex. (2) Females with bilateral transections of dorsal columns, dorsolateral columns or entire dorsal half of spinal cord performed lordosis in a normal way. (3) Bilateral transection of fibers in the ventromedial columns, added to transection of dorsal columns or entire dorsal half of the cord still allowed lordosis to be performed. (4) Large bilateral transections of the anterolateral columns caused severe loss in the strength and frequency of the lordosis reflex. Effective transections often included some ventromedial or dorsolateral column fibers, and were accompanied by abnormalities of locomotion. (5) We conclude that supraspinal control is required for the normal lordosis reflex, and that fibers necessary and sufficient for lordosis run in the anterolateral columns. These fibers are likely to be dispersed throughout the anterolateral columns, since large transections were required to eliminate lordosis, and also to be involved in (or interspersed with other fibers involved in) other aspects of motor control. Candidate ascending systems are the anterolateral fibers of Mehler 14. Candidate descending systems are the lateral vestibulospinal and reticulospinal tracts.