Monoaminergic mediation of the antinociceptive effect of vaginal stimulation in rats

A progestin antagonist blocks vaginocervical stimulation-induced fos expression in neurones containing progestin receptors in the rostral medial preoptic area

Vaginocervical stimulation (VCS) has a variety of effects on the brain, physiology and behaviour. Previous work demonstrated that a progestin antagonist blocked neuronal response to VCS (i.e. Fos expression) in the absence of progesterone in some neurones, and suggested that some of the effects of VCS on the brain are mediated by ligand-independent activation of progestin receptors (PRs). Although it had been reported previously that some of the cells in which VCS induces Fos expression also contain PRs, it had not been determined if a progestin antagonist blocked Fos expression in these particular neurones. The purpose of this experiment was to determine if a progestin antagonist decreases Fos expression specifically in cells that also express PRs in the preoptic area and ventromedial hypothalamus. As has been shown previously, VCS increased Fos-immunoreactive (ir) expression in the particular areas studied. In the rostral medial preoptic area, VCS increased Fos expression in cells that coexpressed PRs, as well as in cells that do not. However, in the caudal medial preoptic area, VCS only increased Fos expression in cells that did not coexpress PRs. Injection of the progestin antagonist, RU 486, decreased Fos expression in the rostral, but not caudal medial preoptic area, and it decreased Fos expression only in cells that coexpressed PR-ir. In contrast to a previous report, in the present study, the progestin antagonist did not inhibit VCS-induced Fos expression in the ventromedial hypothalamic area. The results of this experiment suggest that the progestin antagonist inhibits VCS-induced Fos expression in some neurones by blocking PRs, and they provide further support for the idea that VCS influences neuronal response in some cells by ligand-independent activation of PRs in those cells.

Sex-related hormonal influences on pain and analgesic responses

Considerable evidence indicates sex-related differences in pain responses and in the effectiveness of various analgesic agents. Specifically, females are at greater risk for experiencing many forms of clinical pain and are more sensitive to experimentally induced pain relative to males. Regarding analgesic responses, nonhuman animal studies indicate greater opioid analgesia for males, while a limited human literature suggests the opposite. Though the mechanisms underlying these effects remain unclear, the influence of gonadal hormones on nociceptive processing represents one plausible pathway whereby such sex differences could emerge. The present article reviews the complex literature concerning sex steroid effects on pain responses and analgesia. First, nonhuman animal research related to hormonal effects on nociceptive sensitivity and analgesic responses is presented. Next, human studies regarding gonadal hormonal influences on experimental pain responses are reviewed. Several potential mechanisms underlying hormonal effects on nociceptive processing are discussed, including hormonal effects to both peripheral and central nervous system pathways involved in pain transmission. Finally, based on these findings we draw several conclusions and make specific recommendations that will guide future research as it attempts to elucidate the magnitude and importance of sex-related hormonal effects on the experience of pain.

Gestational and ovarian sex steroid antinociception: relevance of uterine afferent and spinal alpha(2)-noradrenergic activity

Pregnancy is associated with an antinociception that is multifactorial and results from spinal (kappa/delta) opioid antinociceptive pathways as well as peripheral processes (ovarian sex steroids, uterine afferent neurotransmission). The present results provide the first indication that the full manifestation of pregnancy-induced analgesia also requires a supraspinal component. The analgesia of gestation or its hormonal simulation (via estrogen and progesterone administration; HSP) is substantially attenuated (>/=60%) following blockade of spinal alpha(2) (but not alpha(1)) adrenergic receptors. HSP antinociception is also attenuated by transection of the hypogastric nerve, the magnitude of which is indistinguishable from that produced by spinal alpha(2) receptor blockade. Additionally, hypogastric neurectomy abolishes the component of the antinociception associated with HSP that is mediated by spinal alpha(2) receptors. This suggests that the augmented spinal noradrenergic activity during HSP is not due to activation at the terminal of noradrenergic spinal projection neurons but requires supraspinal activity. It is suggested that enhanced spinal noradrenergic activity amplifies ongoing spinal kappa/delta antinociception as has been observed following the concomitant intrathecal application of alpha(2) and opioid agonists. The current observations underscore the importance of visceral afferent activity as well as its modulation by a female-specific hormonal milieu to the efficacy of endogenous spinal opioid antinociception.

Hormonal integration of neurochemical and sensory signals governing female reproductive behavior

This review focuses on findings from our laboratory regarding mechanisms by which the ovarian steroid hormones, estradiol (E2) and progesterone (P), act in the hypothalamus (HYP) to regulate the expression of lordosis, an important component of female reproductive behavior in rats and many other species. The first section summarizes recent work suggesting that cGMP, perhaps via P-receptor activation, may be an intracellular mediator of the facilitatory actions of a variety of hormones and neurotransmitters on lordosis behavior in E2-primed rats. In the second section, we focus on E2 and P regulation of norepinephrine (NE) neurotransmission in the HYP. We review evidence that ovarian hormones act both peripherally and centrally to determine whether NE is released in the HYP in response to copulatory stimuli. We also suggest that the steroid milieu determines the cellular responses of hypothalamic neurons to released NE, favoring the activation of pathways implicated in the facilitation of both lordosis behavior and the preovulatory gonadotropin surge. It is likely that E2 and P have similar actions on other neurotransmitter and neuromodulator systems, thereby maximizing the probability that females are sexually receptive during the periovulatory period.

Vaginocervical stimulation inhibits female-female mounting in laboratory rats

The objective of this study was to examine how vaginocervical stimulation (VCS) affects female-female mounting in laboratory rats. After receiving sexual stimulation from the male, the latency for the female to mount another female was significantly longer than that of control females. In the absence of any copulatory stimulation, the latency to initiate mounting of another female was about 3 min. However, following three mounts or three intromissions by a male, the latency for the experimental female to initiate mounting increased to about 10 min, and ejaculation abolished mounting for almost 2 h. Once females began mounting, regardless of the copulatory stimulation they received prior to testing, their mounting rate (mounting frequency/2 h) did not differ from stimulus control females. Artificial VCS also inhibited female mounting and anesthetization of the vaginocervical area diminished the inhibiting effect of ejaculation. Taken together, the present results provide evidence that VCS can temporarily inhibit female mounting, and that the duration of the inhibition is related to the amount of VCS received. These data are interpreted within the perspective that female mounting behavior is not a sexual behavior and is consequently suppressed within the context of normal copulation.

Analgesic synergism between AP5 (an NMDA receptor antagonist) and vaginocervical stimulation in the rat

Vaginocervical stimulation (VS) releases multiple neurotransmitters into superfusates of the spinal cord; these can stimulate both nociceptive (e.g., glutamate, and glycine acting at the NMDA site), and antinociceptive (e.g., GABA, norepinephrine, 5-HT, and glycine acting at the strychnine-sensitive receptor) systems. Although the balance between these two opposing systems can determine the nature, magnitude, and duration of the response to VS, the characteristic prevailing response to VS is analgesia. We hypothesized that by counteracting the nociceptive component of this system, the magnitude and duration of the response to VS would be augmented. In the present study, the NMDA receptor antagonist AP5 [10 microg injected intrathecally (i.t.)] significantly increased the magnitude and duration of the analgesia (measured as tail flick latency to radiant heat) produced by VS (200 g force). At several time points the analgesic effect of AP5 combined with VS was greater than the sum of the effects of AP5 and VS separately, suggesting that they act synergistically. We propose that AP5 potentiates the analgesic effect of VS by two mechanisms: (a) antagonizing the putative pain-producing action of glutamate and glycine acting jointly at the NMDA receptor, and consequently, (b) permitting the unimpeded expression of the analgesic action of inhibitory neurotransmitters released by VS (e.g., glycine at the strychnine-sensitive receptor, and GABA).

The activity of ON and OFF cells at the rostroventromedial medulla is modulated by vagino-cervical stimulation

In anesthetized rats it was tested whether or not the activity of the ON and OFF cells within the rostral ventromedial medulla (RVM) is modulated by the mechanical stimulation of the uterine cervix (VS). ON cells were identified by an abrupt increase in their firing rate before the tail flick in response to a noxious heat. OFF cells were identified by a sudden decrease in their firing rate before the tail flick. All (27 out of 27) identified ON cells decreased their firing rate immediately after VS was applied. The effect of VS on the activity of the cells persisted for the entire stimulation period. On the other hand, all (19 out of 19) identified OFF cells increased their firing rate immediately after VS. The effect of VS on the activity of these cells also persisted for the entire stimulation period. The activity of the neutral cells showed no change, neither during the application of noxious heat, nor during VS. These results suggest that the analgesic-like effect produced by VS can be mediated by the activity of the antinociceptive circuit at the RVM. Alternatively, it can be suggested that the afferent inflow from the genital tract can induce the activity of the antinociceptive circuit at RVM, either by projections to the periaqueductal gray matter or by direct projections to RVM.

Vaginocervical stimulation suppresses the expression of c-fos induced by mating in thoracic, lumbar and sacral segments of the female rat

In female rats, vaginocervical stimulation induces neuroendocrine responses necessary for pregnancy as well as analgesia to a variety of noxious stimuli. In this study, Fos immunocytochemistry was used to detect vaginocervical stimulation-induced changes in the activity of spinal neurons at levels T11-S3, segments known to receive afferent input from nerves which innervate the reproductive tract. Adult ovariectomized estrogen and progesterone-treated rats were killed 1 h after receiving mating stimulation from males, which included five or 15 intromissions, mounts-without-intromission by use of either vaginal masks or genitally-anaesthetized males, or immediately after being removed from their home cages. At all spinal levels, Fos labelling was lowest in the home cage group (50 +/- 22 cells), intermediate in the groups receiving intromissions (84 +/- 8 and 118 +/- 22 cells) and highest in groups receiving mounts-without-intromission stimulation (187 +/- 21 and 218 +/- 35 cells). Significant increases above control levels following intromissive stimulation were observed at levels L6, S1 and S2. Surprisingly, both groups receiving mounts-without-intromission showed significantly higher numbers of Fos-positive cells than did the fully mated groups at all levels. Analysis of selected spinal segments by Rexed's laminae revealed that intromissive stimulation increased Fos labelling above control levels in laminae II-V and X at L6, and laminae I, II, V and X at S1; vaginocervical stimulation did not increase labelling at L1. The greater Fos responses seen in mounts-without-intromission animals than in control or intromitted animals were apparent at L1, L6 and S1 within the same laminae (II-V and X). These results suggest that stimulation of the uterine cervix initiates activity within L6-S2 neurons which receive pelvic nerve afferents and that such stimulation suppresses activity at all levels within populations of neurons normally activated by cutaneous somatic inputs received from male mounts. As antinociceptive agents are known to suppress c-fos expression, vaginocervical stimulation received during natural mating may be capable of initiating spinal and/or brain mechanisms of analgesia.

Oxytocin does not act locally in the hypothalamus to elicit norepinephrine release

An in vitro brain slice preparation was used to determine whether facilitatory effects of systemically administered oxytocin on hypothalamic norepinephrine release result from direct action of the neuropeptide on noradrenergic terminals. When oxytocin was superfused onto hypothalamic slices from ovariectomized control and ovariectomized, hormone-treated female rats, the neuropeptide failed to influence either basal or electrically evoked release of 3H-norepinephrine. Thus, it is likely that oxytocin facilitates hypothalamic norepinephrine release by a peripheral mechanism, perhaps vaginocervical contraction.

Vaginocervical stimulation attenuates hindpaw shock-induced substance P release into spinal cord superfusates in rats

The present study was designed to elucidate the mechanism in the spinal cord by which vaginocervical stimulation (VS) attenuates responses to noxious stimulation. This was accomplished by testing the hypothesis that VS reduces noxious stimulation-induced release of substance P at the level of the spinal cord. Noxious foot shock significantly increased the release of substance P (measured using radioimmunoassay) into superfusates of the lumbosacral spinal cord region in urethane-anesthetized rats. VS applied concurrently with foot shock significantly attenuated the release of substance P compared to the foot shock-only condition. In addition, substance P levels were significantly lower after the VS-only condition than after the no stimulation or foot shock-only conditions. These findings indicate that VS may produce analgesia, at least in part, by suppressing the release of substance P within the spinal cord.

Estradiol and progesterone influence the response of ventromedial hypothalamic neurons to tactile stimuli associated with female reproduction

Stimulation of the vagina and cervix, provided by the male during copulation or manually with a probe, causes many behavioral and endocrine changes associated with female reproduction in rats. Previously, we found that vaginal-cervical stimulation (VCS), by mating or manual probing, increases the expression of Fos-immunoreactivity (Fos-IR) in discrete populations of neurons in the preoptic area, mediobasal hypothalamus and midbrain, suggesting that these neurons respond to VCS. The purpose of the present study was to determine if hormonal priming would increase the number of Fos-IR cells following VCS. Contrary to our hypothesis, in Experiment 1 priming animals with a behaviorally effective dose of 17 beta-estradiol benzoate followed 48 h later by progesterone caused a trend towards a decrease in the number of VCS-induced Fos-IR cells in the ventromedial hypothalamus. In Experiment 2, which was done to confirm this decrease in VCS-induced Fos-IR neurons by hormones, this effect was found to be statistically significant. Furthermore, this hormone-induced decrease in VCS-responsive cells was localized to the ventromedial nucleus of the hypothalamus, an area rich in estrogen and progestin receptors. No effects of hormone treatment on VCS-induced Fos-IR were observed in any other brain regions analyzed. These findings suggest that steroid hormones may elicit some of their effects on female reproductive behavior and physiology by altering the responsiveness of ventromedial nucleus neurons to vaginal-cervical stimulation.

Release of amino acids into regional superfusates of the spinal cord by mechano-stimulation of the reproductive tract

Based on pharmacological evidence that inhibitory amino acids mediate vaginocervical mechano-stimulation produced analgesia (VSPA), we hypothesized that inhibitory amino acids would be released endogenously in the spinal cord in response to vaginocervical mechano-stimulation (VS). This hypothesis was tested by HPLC analysis of the amino acid content of 5-min superfusates of the spinal cord before, during and after VS (400 g force applied against the cervix) in urethane-anesthetized rats. Utilizing an in vivo push-pull superfusion method, artificial cerebrospinal fluid was continuously superfused over the spinal cord through the intrathecal space surrounding the sacral-lower thoracic region. In addition, concentrations of amino acids in the superfusate were measured in response to KCl stimulation (increasing the superfusion medium from 3.4 to 40.0 mM KCl to produce non-specific depolarization), and noxious hind paw mechano-stimulation (pinching the hind paw to produce a sustained flexor response in ipsilateral hind leg). There was a significant increase in the concentration of Gly, Tau, Asp, Glu and Lys in the superfusate in response to VS (n = 8) and to KCl (n = 8), but not to hind paw stimulation (n = 5). Also, GABA concentrations increased in response to KCl, and the concentration of Ala, Ser, Gln, Thr, Arg and Phe increased in response to VS, however, GABA levels were sometimes below the limits of detection. In contrast, there was no significant change in any amino acid concentration in response to hind paw pinch stimulation, and VS did not significantly affect the concentrations of Tyr, His, Ile, Leu, Met, Trp or Val. The present findings support our hypothesis that VS releases inhibitory amino acids in the spinal cord. Moreover, other amino acids, including 'excitatory' amino acids, are released into the superfusate. The profile of amino acid release in response to VS differs from that in response to paw pinch or KCl administration.

Lordosis response components can be elicited in decerebrate rats by combined flank and cervix stimulation

Previous research has indicated that chronically decerebrate adult female rats are incapable of exhibiting lordosis in response to lumbosacral somatic stimuli, suggesting a dependency of this response on forebrain influence. Since vaginocervical stimulation facilitates lordosis to lumbosacral stimuli in rats, the present study investigated the possibility that chronically decerebrate rats could show lordosis in response to flank palpation if vaginocervical pressure was applied concurrently. Six of nine ovariectomized, decerebrate rats exhibited a coordinated display of lordosis-like responses to flank palpation if this stimulus was accompanied by vaginocervical stimulation, but not in response to flank and perineal pressure alone. Estradiol-progesterone treatment increased the magnitude of the response in five of six animals. As in intact rats, cervix pressure in the decerebrates also produced immobilization and blocked responses to nociceptive stimuli. Thus, the caudal neuraxis can mediate expression of essential components of the lordosis response in the absence of forebrain influence.

Prolonged inhibition of the flexor reflex by probing the cervix uteri in the cat

In decerebrate or spinal cats, sustained mechanical stimulation of the cervix uteri inhibited the flexor reflex elicited by electrical stimulation of the foot pad during the probing period (160 s). After probing, 3-15 min were required for reflex recovery. No additional inhibition was produced if probing was repeated before recovery, but instead the reflex was facilitated. When probing was applied 5-10 min after reflex recovery the reflex was again abolished. The recovery, however, occurred earlier and was followed by facilitation. Probing the cervix with single mechanical pulses inhibited transiently (140-200 ms) the short latency reflex components, but the components with longer latencies are unaffected or facilitated. Distension of the vaginal wall with a balloon also inhibited the flexor reflex, but a transient, mild facilitation appeared several seconds after the distension. In general, whenever the inhibition decreases, the facilitation predominates. Our findings suggest that cervical probing or vaginal distension triggers both a long-lasting inhibition and a concomitant facilitation in different intraspinal flexor reflex pathways.

Neonatal capsaicin treatment attenuates sensory-induced analgesia and nociception

Neonatal capsaicin treatment (50 mg/kg SC in two-day-old rats) increased thermal pain thresholds in both sexes when measured at different ages, decreased the responsiveness of adult females to specific noxious stimulation, and differentially decreased the magnitude of vaginocervical stimulation (VS)-produced analgesia in nociceptive tests. When adult, "capsaicin" females (n = 37) were significantly greater than controls (n = 24) in vocalization threshold (VT) to electrical tail shock (55.4%) and in paw lick (PL) latency to a hot plate (75.9%). In contrast, neither tail flick (TF) latency nor the leg withdrawal reflex (LWR) to mechanical pressure of the ipsilateral hind paw was affected by neonatal capsaicin. In response to VS, the controls showed a significant increase in thermal (TF, 279%; PL, 411%), mechanical pressure (LWR, 100%) and electrical (VT, 86.8%) pain thresholds. The "capsaicin" females response to VS was significantly less than controls in TF (26.1%), PL (26.0%), and LWR (54.1%) measures, and surprisingly, during VS their VT was significantly decreased below baseline levels 12.2% +/- 4.3. These results suggest that neonatal capsaicin treatment differentially attenuates the analgesia-producing component of VS, while sparing a nociception-inducing component of this stimulus. That is, after neonatal capsaicin treatment, the ability of VS to produce analgesia is reduced; moreover, VS lowers the VT, suggesting that it actually becomes a noxious stimulus in and of itself.

Intrahypothalamic implants of noradrenergic antagonists disrupt lordosis behavior in female rats

There is considerable experimental evidence that hormonal activation of lordosis in female rats involves norepinephrine (NE) neurotransmission. However, no clear picture has emerged regarding either: 1) the neural sites at which NE influences lordosis, or 2) the NE receptor subtype(s) mediating NE effects on lordosis. To address these two issues, the behavioral effects of antagonists with relative specificity for alpha 1, alpha 2, or beta adrenergic receptors were examined. Drugs were administered via bilateral crystalline implants directly into the ventromedial nucleus of the hypothalamus (VMN) or medial preoptic area (MPOA) of ovariectomized female rats primed for 48 hr with 3 micrograms of estradiol benzoate (EB) and given 200 micrograms of progesterone (P) 3.5-4 hr before testing. When applied to the VMN 1 hr before the P injection, the alpha 1 receptor antagonist prazosin reduced lordosis behavior in 86% of animals but in only 10% of rats when applied to the MPOA. However, prazosin did not inhibit lordosis when implanted into the VMN just prior to EB administration. Yohimbine, an alpha 2 receptor antagonist with low affinity for alpha 1 receptors, also suppressed lordosis in 41% of animals with VMN implants and in 37% of rats with MPOA implants. By contrast, the alpha 2 antagonist idazoxan, which has little activity at alpha 1 receptors, did not significantly affect estrous responding when implanted into either the VMN or MPOA. VMN implants of the beta receptor antagonists propranolol and pindolol reduced lordosis behavior in 50% and 86% of rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Inverse relationship between intensity of vaginal self-stimulation-produced analgesia and level of chronic intake of a dietary source of capsaicin

Women who chronically ingest a diet rich in capsaicin, the pungent ingredient in hot chili peppers, showed a significantly lower magnitude of analgesia in response to vaginal self-stimulation than women with relatively low or medium levels of ingestion. Vaginal self-stimulation-produced analgesia was quantified by measuring (on the hand) pain detection thresholds, pain tolerance thresholds and tactile thresholds. Whereas vaginal self-stimulation produced a 32.6-43.8% increase in pain detection and pain tolerance thresholds in the low chili diet group, it produced only a 2.3-7.3% increase in these measures in the high chili diet group. The medium chili diet group showed an intermediate effect on the pain thresholds. Tactile thresholds were not increased by the vaginal self-stimulation. Baseline (no stimulation) pain thresholds did not differ significantly among the three groups. These findings are consistent with earlier studies in laboratory rats, in which capsaicin administered neonatally abolished vaginal stimulation-produced analgesia, but did not affect baseline pain thresholds to mechanostimulation.

Abolition of vagino-cervical stimulation-induced analgesia by capsaicin administered to neonatal, but not adult rats

We have previously reported that vagino-cervical mechanical stimulation (VS or probing) produces analgesia in rats. Neonatal treatment with capsaicin (CAP) has been shown to reduce the concentrations of several neuropeptides in dorsal root ganglia, spinal cord, and autonomic ganglia, via a neurotoxic effect. In the present study, we report that CAP administered in the neonatal period abolishes the analgesic effect of probing in adulthood. In addition, we report that the ability of VS to potentiate the lordosis response to manual stimulation of the flanks is abolished by neonatal CAP treatment. By contrast, rats treated as adults with CAP show the typical VS-produced effects of analgesia and potentiation of the lordosis response. Our results suggest that neonatal, but not adult, CAP treatment depletes a neuropeptide(s) that mediates the analgesia and lordosis-inducing effects of VS.

Differential effects of p-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. I. Biochemical and behavioral analysis

The involvement of endogenous serotonergic pathways in the mediation of antinociception has been indicated by electrophysiological, pharmacological and behavioral experiments. However, manipulation of the indole pathway, either by lesioning of raphe nuclei or drug intervention, often produces disparate results. In particular, serotonin (5-HT) synthesis inhibition with p-chlorophenylalanine (PCPA) has been reported to produce either hyperalgesia or analgesia, depending upon the type of pain measurement examined. In the present study, we sought to evaluate the effects of PCPA on (1) behavioral responses to noxious stimulation, and (2) levels of serotonin, tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in raphe nuclei (pallidus, obscurus, magnus and dorsalis) and spinal cord regions by HPLC with electrochemical detection. Treatment of rats with 400 or 600 mg/kg of PCPA for 3 consecutive days resulted in significant elevations in pain thresholds assessed by tail withdrawal from radiant heat as well as vocalization to electric shock of the tail. The effect of PCPA on vocalization threshold was particularly striking, for the majority of animals showed a nociceptive-specific attenuation of this response. Although the PCPA induced changes in indole content of the various raphe nuclei were not unequivocally dose-dependent, differential reductions of serotonin and 5-HIAA were clearly detected in the various raphe regions. Nuclei raphe pallidus and obscurus were depleted of 5-HT and 5-HIAA to the greatest extent, whereas levels detected in nuclei raphe magnus and dorsalis were reduced by 30-40% from control values. Metabolism of 5-HT and 5-HIAA appeared unaffected by PCPA in all regions examined except the dorsal portion of the spinal cord. These findings collectively suggest that the effects of PCPA are not uniform throughout the central nervous system and raise the possibility that discrepancies in the behavior literature may be attributed to drug-induced changes in some, but not all serotonergic pathways.

Inhibition of dorsal-horn cell responses by stimulation of the Kölliker-Fuse nucleus

The Kölliker-Fuse nucleus (KF) in the dorsolateral pons has been shown to be the major source of catecholamine innervation of the spinal cord. This has important implications in terms of pain control mechanisms, since catecholamine-mediated mechanisms are essential for the expression of opiate and other varieties of antinociception. This study examines the effects of KF stimulation on responses of dorsal-horn cells to innocuous and noxious cutaneous stimuli in anesthetized cats. Stimulation of the KF potently inhibits the responses of dorsal-horn cells to both noxious and innocuous stimuli. The threshold for the inhibitory effect is significantly lower for responses to noxious stimuli as opposed to innocuous stimuli. The inhibitory effect is specific to the stimulus site, as evidenced by a marked decrease in the effect following small changes in the position of the stimulating electrode in the brain stem. The latency of the effects indicates a bulbospinal conduction velocity of 4 to 5 m/sec, which is much slower than usual reticulospinal effects and is consistent with a catecholamine-mediated system. The dependence of KF-spinal inhibition on intact biogenic amines was tested by depleting the animals of these amines with reserpine pretreatment. Depletion of biogenic amines resulted in a significant decrease in the KF spinal inhibitory effects, suggesting their dependence on intact noradrenergic stores. The results of these studies are consistent with the idea that the KF-spinal system plays an important noradrenergic-dependent role in the brain-stem modulation of spinal processing of noxious, potentially painful stimuli.

Sexual responsiveness and its relationship to vaginal stimulation-produced analgesia in the rat

Increasing amounts of pressure applied to the cervix produce a dose-response-like elevation of pain threshold in rats. This vaginal stimulation-produced analgesia (VSPA) is facilitated in animals given estrogen (E) doses sufficient to induce high levels of sexual receptivity. It has been proposed that enhancement of VSPA may serve to decrease any noxious input associated with multiple intromissions by the male. In this study, the anti-nociceptive effect of VSPA was compared in animals given E doses insufficient to increase receptivity with animals made receptive using subthreshold E levels + progesterone (P) in an attempt to determine if enhancement of VSPA is associated with the receptive state of the animal or the dose of E used. Tail flick latencies and tail shock vocalization thresholds were measured in groups of E, E + P and oil-treated rats during application of 0, 100 and 200 g of force on the cervix. Within oil, E and E + P-treated animals, significant increases in tail flick latencies were observed at 100 and 200 g with respect to baseline (0 g). Moreover, at 100 g of force E treated animals displayed a significant increase in tail flick latency over oil and E + P treated rats. In contrast, tail shock vocalization was increased at 100 and 200 g levels of probing in oil and E + P groups but was not facilitated by E. In the present study, as in previous work, VSPA was potentiated by E; however, this potentiation was not correlated with steroid-induced receptivity.(ABSTRACT TRUNCATED AT 250 WORDS)

A fictive tail flick reflex in the rat

The discharges of motor axons participating in the tail flick reflex were recorded from nerve filaments innervating the medial longissimus muscles of anesthetized rats. The reflex discharges evoked by stimulation of the tail with either noxious radiant heat or pinch were recorded before and after paralysis of the animals. Nociceptive discharges recorded from motor axons in the paralyzed state showed a strong correlation with those observed in the absence of the paralytic agent. For this reason, the electrophysiological response triggered by noxious input was termed a 'fictive tail flick reflex'. To evaluate the potential applicability of this model in the analysis of pain blocking mechanisms, vaginal stimulation was found to produce a profound reduction to the nociceptive discharges of the fictive tail flick. By eliminating movement artifacts from the experimental paradigm, this model expands our ability to study a basic nociceptive response which was previously limited to behavioral observation.

Effects of prenatal exposure to morphine on the development of sexual behavior in rats

Females exposed to morphine sulfate in utero (5-10 mg/kg twice a day on days 11-18 of gestation) displayed precocious vaginal opening and had increased body weight from the 8th week after weaning. In addition, there was a substantial inhibition in adult feminine sexual behavior. Male rats that received either morphine or saline prenatally did not show any body weight differences, and most of the measures of masculine sexual behavior did not differ between the two groups. However, the male rats exposed to morphine had a significantly shorter post-ejaculatory intromission latency than the saline controls. Examination of cytosol estrogen receptor levels in the hypothalamus-preoptic area (HPOA) of both saline and morphine sulfate-treated female rats revealed essentially identical patterns of depletion and replenishment. Additionally, estrogen treatment was equally effective at inducing HPOA progestin receptor synthesis in both groups. These results show that prenatal morphine treatment at the times and dose level administered disrupts the development of reproductive function in females but has only minor effects on male reproductive function.

Dorsolateral funiculus and intraspinal pathways mediate vaginal stimulation-induced suppression of nociceptive responding in rats

In rats, stimulation of the vaginal cervix with a glass rod reliably produces analgesia, as measured by the tail-flick test. The present studies sought to identify the neural substrates underlying this potent pain inhibition by examining the effects of decerebration, spinalization and bilateral dorsolateral funiculus (DLF) lesions on vaginal stimulation-produced analgesia (VSPA). These studies indicate that the neural circuitry mediating VSPA is contained within the caudal brainstem and spinal cord, since decerebration did not reduce VSPA when compared with sham-operated controls. A significant though markedly reduced level of analgesia was induced in spinalized rats, indicating that VSPA involves both intraspinal and descending pathways. This descending pathway, originating within supraspinal nuclei of the caudal brainstem, projects to the spinal cord via the DLF, since DLF lesions and spinalization produced equivalent reductions in VSPA compared to sham-operated controls. These results, considered in the light of previous electrophysiological and anatomical findings, indicate that the ventral medullary region may be the source of the descending DLF projection mediating VSPA.

A study of the catecholaminergic systems of the lateral reticular nuclei and the serotoninergic systems of the raphe magnus in various types of analgesia

The effects of catecholaminergic A-1 lateral reticular nuclei and serotoninergic neurons of NRM on pain reactions before and after various types of stimulation are presented. It was established that specific lesions of catecholaminergic (NE) neurons in A-1 nuclei using 6-hydroxydopamine, and of serotoninergic (5-HT) neurons of the nucleus raphe magnus using 5,7-dihydroxytryptamine caused a decrease in the respective levels of epinephrine and serotonin in the spinal cord. The baseline pain sensitivity did not change following surgery. Analgesia induced by cold swimming stress (CSS), auricular electroacupuncture (AEA) and vaginal probe (VP) was less in A-1-lesioned rats. Using stimulation of high intensity, such as CSS and VP, a decrease in pain sensitivity was determined, compared to the baseline. The AEA did not produce such an effect. The data obtained suggest that catecholaminergic systems of A-1 play an important role in pain regulation when CSS, AEA and VP are used. Other neurochemical mechanisms, as well as A-1 nuclei systems, are involved in analgesia, induced by CSS and VP. It was shown that lesion of 5-HT-ergic systems of NRM did not have any influence of antinociceptive mechanisms, when activated by AEA and VP.

Estrogen accelerates the recovery of the lordosis response after its exhaustion induced by cervical probing

Repetitive flank-perineum palpation combined with probing the vaginal cervix in ovariectomized hormonally untreated rats elicited a mean of 4.2 +/- 0.2 (SEM) successive lordosis responses before the response was exhausted. Two days after a single injection of estradiol benzoate (100 micrograms/kg body wt) in ovariectomized rats, a significantly greater number (7.8 +/- 0.2) of lordosis responses could be elicited before the response was exhausted (P less than 0.01). After this exhaustion, recovery of lordosis responsiveness to this stimulation occurred significantly earlier in the estrogen-treated rats (6 hr) than in the oil-treated rats (60 hr) (P less than 0.01). Several possible mechanisms mediating these effects are proposed.

Role of prolactin-opiate interactions in the central regulation of pain threshold

Endogenous hyperprolactinaemia induced by anterior pituitary transplantation under the kidney capsule has been found to reduce the behavioural responsiveness to electrical footshock and to increase morphine-induced analgesia. The apparent analgesic effect of prolactin has been related to the stimulation of nigro-striatal dopaminergic transmission, as suggested by the increase in striatal dopamine turnover observed in hyperprolactinaemic rats. It seems likely that central opiate system is involved in the behavioural effects of prolactin. Thus, naloxone prevents the effects of hyperprolactinaemia on footshock responsiveness and heroin self-administration is decreased in hyperprolactinaemic rats.

Bulbar catecholaminergic neurons projecting to the thoracic spinal cord of the chicken. Evans Blue labeling study in combination with catecholamine histofluorescence

This study was undertaken to map the location of cell groups in the chicken brain stem that project to the thoracic spinal cord by retrogradely transported Evans Blue (EB). The majority of EB-labeled neurons are located in an area between the nucleus vagus motorius (NVMD) and hypoglossus ventralis dorsomedialis, the nucleus raphis, the nucleus vestibularis ventrolateralis, the nucleus tegmentalis dorsalis (NTD) and the reticular formation. However, this procedure demonstrates that no cell bodies within the nucleus tractus solitarii project to the spinal cord, unlike those of mammals. It is possible that this difference may be involved in the existence of the diaphragma. Additionally, catecholamine (CA)-containing cells in the brain stem projecting to the thoracic spinal cord were investigated with the aid of a technique demonstrating both CA and EB fluorescence in the same neuron. The doubly labeled cells are constantly found to be located within such areas as the lateral reticular formation at the level of the NVMD, the ventrolateral reticular formation at the level of the nucleus abducens major, the NTD and the ventrolateral reticular formation at the level of the NTD. The doubly labeled neurons are most numerous in the medial part of the NTD in transverse section, reflecting a similar tendency in the mammalian locus coeruleus.

Spinal cord monoamines modulate the antinociceptive effects of vaginal stimulation in rats

Perispinal administration (into the lumbar intrathecal space) of phentolamine (40 micrograms), an alpha-adrenergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 39.7% (measured by tail flick latency) and 57.1% (measured by vocalization threshold) as compared to controls. Perispinal administration of methysergide (10 micrograms), a serotoninergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 48.5% (measured by vocalization threshold), although it did not significantly affect the tail flick measure. In a separate experiment, vaginal stimulation activated the release of norepinephrine and serotonin into a superfusate of the spinal cord. During vaginal stimulation, levels of norepinephrine and serotonin increased about 2-fold above resting levels. These findings indicate that vaginal stimulation releases norepinephrine and serotonin into the spinal cord, thereby exerting an analgesic effect.

Kölliker-Fuse nucleus: the principal source of pontine catecholaminergic cells projecting to the lumbar spinal cord of cat

Using retrograde transport of the fluorescent dye Evans Blue (EB), in combination with glyoxylic acid histofluorescence, the ponto-spinal catecholaminergic pathways were investigated. The cells which contain catecholamine and project to the lumbar spinal cord of the cat are most densely concentrated in the Kölliker-Fuse nucleus. Locus coeruleus, the subcoeruleus area, and the parabrachial nuclei were found to have relatively few cells that both contain catecholamine and project to the lumbar spinal cord.

Modulation of antinociceptive responses following tail pinch stress

Mild, non-noxious, oscillating pinches to a rat's tail elicits hyperphagia. The present study examined whether tail-pinch (TP) would exert hyperalgesic and hyperactive effects in rats that also exhibit the overeating response. The first experiment assessed TP effects upon reactivity to electric shock as measured by flinch-jump thresholds. Significant decreases in jump thresholds were observed 0 and 15, but not 30, min following TP. This effect persisted regardless of whether food was present or absent during TP. The second experiment assessed TP effects upon reactivity to heat as measured by hot-plate latencies. In contrast to jump thresholds, the shortened hot-plate latencies observed following TP persisted into the recovery period. In examining TP effects upon activity levels (Experiment 3), it was found that animals display similar patterns of temporally-declining activity regardless of whether TP was administered or not. Finally, TP selectively decreased the analgesic responses to two different doses of morphine and two different cold-water swim temperatures (Experiment 4). The TP-induced reductions occurred when TP was administered either before or after the analgesic manipulation. These data are discussed in terms of the nociceptive selectivity of the TP effect, and its influences upon analgesic processes.

Effects of peripheral stimulation on the activity of neurons in the ventral tegmental area, substantia nigra and midbrain reticular formation of rats

Extracellular recordings were obtained from neurons in the ventral tegmental area (VTA), the substantia nigra, including the zona compacta (SNC) and the zona reticulata (SNR), and the midbrain reticular formation (FOR) of adult female albino rats anesthetized with urethane and chloral hydrate. Based on electrophysiological characteristics the neurons were divided into two types. Type I neurons, with relatively long spike durations and slow discharge rates, were confined to the VTA and SNC. Type II neurons, with shorter spike durations and faster discharge rates, were observed in the SNR and FOR as well as the VTA and SNC. The effects of foot pinch (FP), tail pinch (TP) and stimulation of the vaginal cervix (VC) on the activity of the two types of neurons were investigated. Previously it was demonstrated that FP was aversive, TP elicited locomotion, sniffing and gnawing responses and VC lordosis response, vocalization and immobility. For approximately two-thirds of the neurons the effects of the three peripheral stimuli were similar; either they were activated or suppressed. Approximately 8 percent of the neurons were suppressed by FP and TP and activated by VC whereas a similar number were activated by FP and TP and suppressed by VC. Type 1 and Type II neurons in the VTA and SN were activated and suppressed by the peripheral stimuli with suppression being the most common response to FP and TP. The results are consistent with the view that VTA and SN neurons integrate a number of central and peripheral inputs.

Activation of striatal dopamine receptors induces pain inhibition in rats

In the rat, elevating dopamine content in corpus striatum with electrical stimulation of substantia nigra or direct administration of apomorphine (50-200 micrograms) into the lateral cerebral ventricle or apomorphine (2-10 microgram) into the caudate-putamen complex decreased pain sensitivity (as shown by an increase in the latency to hind-paw lick in the hot plate test). Furthermore, the decreased pain sensitivity after the central administration of apomorphine was antagonized by pretreatment with haloperidol (a dopamine antagonist). On the other hand, lowering dopamine content in corpus striatum with electrolytic destruction of substantia nigra and 6-hydroxydopamine lesions to the substantia nigra, as well as direct injection of haloperidol into the lateral cerebral ventricle or caudate-putamen complex increased pain sensitivity. The data indicate that activation of striatal dopamine receptors in rat brain induces pain inhibition.

The antinociceptive effect of vaginal stimulation in the rat is reduced by naloxone

Mechanical stimulation of the vagina produces a powerful antinociceptive effect against a variety of noxious stimuli. In rats tested in either the radiant heat tail flick or the warm water tail immersion test the antinociceptive effect of vaginal stimulation was found to be significantly reduced, but not abolished, following the administration of 10 mg/kg of naloxone. These results are in contrast to those of an earlier study in which naloxone was ineffective against the antinociceptive action of vaginal stimulation in a tailshock vocalization paradigm. It therefore appears that the nature of the noxious stimulus used may influence the type of antinociceptive mechanism triggered by vaginal stimulation.

Morphine versus haloperidol catalepsy in the rat: a behavioral analysis of postural support mechanisms

Our experiments demonstrate that morphine and haloperidol produce two distinct and contrasting behavioral states, which can be thought of as exaggerated, isolated, and simplified forms of organized adaptive behavioral states functioning as components of normal motivated behavior. Haloperidol catalepsy constitutes an organized state in which tonic reactions subserving the maintenance of stable static equilibrium prevail, at the expense of phasic locomotor reactions. In contrast, morphine produces an immobility state characterized by inhibition of the postural support subsystem, and compatible with or preparatory to locomotor rather than static postural reactions. haloperidol-treated rats (1, 2.5, 5, 10 mg/kg i.p.) display exaggeraged bracing reactions to passive displacement as well as to stimuli which do not actively challenge stable equilibrium. In contrast, rats treated with morphine sulfate (10, 20, 40, 80 mg/kg i.p.) show a dose-dependent suppression of bracing and an exaggerated tendency to run in response to stimuli which produce bracing in haloperidol-treated rats. Further evidence that haloperidol-treated rats are organized to stand still in stable equilibrium includes their typical posture during akinesia (i.e. broad-based support), bradykinesia, tonic grasping and enhanced postural components of contact- and air-righting. Under morphine, however, the postural support subsystem is dispensed with, as evidenced by the posture of akinesia (i.e. a frozen phase of the locomotor step cycle associated with loss of limb support), absence of tonic grasping, and nature of the deficits in contact- and air-righting. Furthermore, the opiate-induced immobility state is accompanied by an increased readiness to locomote. Morphine produces an alternation between two extreme behavioral states: complete immobility (inhibition of the postural support subsystem) versus locomotor paroxysms (varying degrees of 'explosive motor behavior'). We suggest that the postures or actions adopted by morphine-treated rats involve movement subsystems concerned with the adaptive behavioral state known as the 'immobility reflex' ('tonic immobility', 'animal hypnosis').

Copulation-induced abbreviation of estrus in the female guinea pig: block by clonidine

In the female guinea pig, copulation produces a rapid inhibition of subsequent sexual receptivity. Injection of the alpha adrenergic agonist clonidine before mating blocked the early inhibitory effects of coital stimulation, while the serotonergic antagonist methysergide, and the dopaminergic antagonist pimozide were without effect. The effects of clonidine were prevented by pretreatment with the alpha adrenergic antagonist yohimbine. These results suggest that an adrenergic system is involved in the copulation-induced inhibition of receptivity and provide further evidence that such a system has an important facilitative role in the control of female sexual behavior in the guinea pig.

Noradrenalin, serotonin, and the dorsal horn

There is anatomical, pharmacological, and physiological evidence that descending systems from the brain stem using noradrenalin and serotonin (5-hydroxytryptamine, 5-HT) participate in the control of segmental sensory processing. Study of these systems is described in this paper. L-dopa was given intravenously to cats to cause the release of noradrenalin and 5-HT. The resultant effects on the responses of dorsal horn cells to innocuous and noxious thermal stimulation of skin were determined. Using the catecholamine cell neurotoxin, 6-hydroxydopamine, the dopamine beta-hydroxylase inhibitor, fusaric acid, and the tryptophan hydroxylase inhibitor, parachlorophenylalanine, the available central stores of noradrenalin or 5-HT were altered, thus allowing separation of the effects of noradrenalin and 5-HT release on dorsal horn cells. The results indicate that noradrenalin facilitates the responses of dorsal horn cells to innocuous cutaneous stimuli, and has an inhibitory effect on the responses to noxious stimuli. Serotonin inhibits the responses of dorsal horn cells that respond to noxious stimuli, to both innocuous and noxious stimuli. The results are discussed in light of current developments concerning aminergic control of segmental sensory processing.

Dissociation of active from immobility components of sexual behavior in female rats by central 6-hydroxydopamine: implications for CA involvement in sexual behavior and sensorimotor responsiveness

Ovariectomized female rats were given a hormone treatment (2 X 8 micrograms/kg estradiol benzoate) that normally supports only low levels of lordosis responding and no soliciting behavior in tests with sexually active males. When subjected to an intraventricular 6-hydroxydopamine (6-OHDA) procedure (with pargyline pretreatment) that produced 85% and 95% depletions of caudate dopamine and cortical norepinephrine respectively, these females exhibited a dramatic increase in the intensity and frequency of lordotic responding but no soliciting behavior over 3 weekly tests. The increase in lordosis was not due to a drug- or stress-induced release of adrenal progesterone, since dexamethasone suppressed the progesterone levels, as documented by radioimmunoassay, but not the higher receptivity of 6-OHDA treated females. In other ovariectomized females given a hormone regimen (2 X 50 micrograms/kg estradiol benzoate plus 500 micrograms progesterone) that supported maximal levels of lordosis and soliciting, the same 6-OHDA treatment prolonged the average duration of lordosis while actually decreasing the incidence and duration of soliciting. The hypothesis is put forward that the differential effects of interfering with catecholamine, and more likely dopamine function on the soliciting and lordosis components of female sexual behavior might best be understood as a dissociation between mutually antagonistic behavior patterns such that responsiveness involving active orientation and forward locomotion is suppressed, whereas responses requiring immobility are augmented.

Analgesia induced by vaginal stimulation in rats is apparently independent of a morphine-sensitive process

Previous studies have suggested that in rats probing against the vaginal cervix with a glass of is analgesic, for this stimulus elevates the threshold for eliciting vocalization in response to tail shock. In the present studies pretreatment with naloxone HCl (1 or 10 mg/kg), a potent narcotic antagonist did not antagonize this vaginal stimulation-induced analgesia. Furthermore, vaginal stimulation was found to exert its analgesic effect even in rats made tolerant to, and dependent upon, morphine sulfate. These results suggest that the analgesic effect of vaginal stimulation is not necessarily mediated by an opiate-sensitive neural system. However, we hypothesize that even though vaginal stimulation and other analgesic manipulations may act via different neural substrates, they may nevertheless converge onto a final common mechanism for pain suppression.