Test-dependent antinociceptive effect of spinal serotonin release induced by intrathecal p-chloroamphetamine in mice

The effect of direct intrathecal injection of p-chloroamphetamine (PCA) into the lumbar subarachnoid space was investigated in mice. PCA (0.6 - 20 micrograms) induced a dose-related prolongation of response latencies in the tail-flick test, but failed to affect the hind-paw lick response in a hot-plate test employing slowly rising temperature. PCA (5 micrograms) given intracerebroventricularly did, however, significantly elevate the response temperature in the hot-plate test. The antinociceptive effect of PCA in the tail-flick test was prevented by spinalization, by pretreatment with the selective serotonergic re-uptake blocker zimelidine (20 mg X kg-1 i.p.) and by the serotonin synthesis inhibitor p-chlorophenylalanine (300 + 300 + 150 mg X kg-1 i.p. 72, 48 and 24 h before test). It is concluded that PCA given intrathecally releases serotonin from spinal terminals, which may under certain conditions induce antinociception.     

Caffeine and a selective adenosine A(2B) receptor antagonist but not imidazoline receptor antagonists modulate antinociception induced by diphenyl diselenide in mice

The present study examined the antinociceptive effect of diphenyl diselenide (PhSe)2, given orally (p.o.), in the hot-plate test in mice. The administration of diphenyl diselenide (10-100 mg/kg, p.o.) caused a significant inhibition of thermal nociception induced by hot-plate test in mice. Pretreatment of animals by intraperitoneal route (i.p.) with caffeine (10 mg/kg; a non-specific adenosine receptor antagonist) and PSB1115 (1 mg/kg; an adenosine A(2B) receptor antagonist), but not DPCPX (2 mg/kg; an adenosine A(1) receptor antagonist) and SCH5826 (3 mg/kg; an adenosine A(2A) receptor antagonist) significantly blockaded the antinociceptive effect caused by diphenyl diselenide (10 mg/kg, p.o.) in the hot-plate test. Moreover, the pretreatment of animals with efaroxan (1 mg/kg, i.p.; a mixed I(1) imidazoline/alpha(2)-adrenoceptor antagonist) and idazoxan (3 mg/kg, i.p.; a mixed I(2) imidazoline/alpha(2)-adrenoceptor antagonist) did not significantly reverse the antinociception caused by oral administration of diphenyl diselenide (10 mg/kg, p.o.) in the hot-plate test. These results indicate that diphenyl diselenide produced antinociception in a thermal model of pain in mice and its effect was prevented by caffeine and by a selective adenosine A(2B) receptor, but not by imidazoline receptor antagonists in mice.     

Characterization of the human peripheral lung adenosine receptor

Because adenosine narrows asthmatic airways, is released during hypoxia and by mast cells, and is antagonized by theophylline, it may play a role in asthma. I characterized the first step in pulmonary responses to adenosine: its adenosine receptor. Plasma membranes, prepared from macroscopically normal human peripheral lung, were incubated with 10 nM 5'-N-ethylcarboxamido[3H]adenosine ([3H]NECA) and various concentrations of competing ligand under experimentally determined optimal conditions: 4 degrees C, pH 7.4, 5 mM MgCl2, 1.8 mg protein/ml, 30-min incubation time. Bound and free ligand were separated by rapid vacuum filtration, and the radioactive counts were analyzed using a weighted, non-linear, least-squares curve-fitting program, LIGAND. Analyzed together, the data from the lungs of 6 patients revealed a single binding site with a dissociation constant (Kd) for NECA of 200 nM +/- 14% and a receptor concentration of 543 fmol/mg protein +/- 13%. Analyzed separately, the individual Kds ranged from 133 to 430 nM and the receptor concentrations from 338 to 811 fmol/mg protein. Adenosine receptor ligands competed with NECA in an A2 rank order of potency: NECA greater than 8-phenyltheophylline greater than 3-isobutyl-1-methylxanthine greater than theophylline greater than N6-L-phenylisopropyladenosine greater than N6-D-phenylisopropyladenosine greater than N6-cyclohexyladenosine. Theophylline bound to the receptor with an inhibition constant (Ki = 70.9 microM +/- 28%) near the therapeutic range (28 to 56 microM). Cromolyn also bound with high affinity (Ki = 5.42 microM +/- 47%). I conclude that human lung adenosine receptors: (1) are single-site receptors, probably of the A2 subtype and (2) bind to both theophylline and cromolyn.     

Effects of intrathecal substance P and a substance P antagonist on a reflex to noxious heat are independent of changes in tail skin temperature.

Substance P or the substance P receptor antagonist (D-Arg1,D-Trp7.9,Leu11)-substance P (Spantide) was injected into the lumbar subarachnoid space in mice, and the ability to change the tail-flick reflex and the tail skin temperature was investigated. Tail-flick latency (the time needed to evoke the tail-flick reflex by noxious radiant heat) was reduced for 1-4 min after intrathecal administration of substance P (5 micrograms), but the tail skin temperature was not significantly changed. Nor was the tail skin temperature significantly changed after intrathecal injection of Spantide (5 micrograms), but this compound significantly increased tail-flick latencies 5-30 min after injection. Analysis of co-variance showed that the effects of substance P or Spantide on tail-flick latency were significant, whereas the influence of tail skin temperature on tail-flick latency was non-significant. Thus, intrathecal substance P induces a short-lasting increase in nociceptive sensitivity, and intrathecal Spantide produces an antinociceptive effect of longer duration. The results seem not to be the result of changes in tail skin temperature.     

Effects of intrathecal substance P and a substance P antagonist on a reflex to noxious heat are independent of changes in tail skin temperature

Substance P or the substance P receptor antagonist (d-Arg¹, d-Trp^{7, 9}, Leu¹¹)-substance P (Spantide) was injected into the lumbar subarachnoid space in mice, and the ability to change the tail-flick reflex and the tail skin temperature was investigated. Tail-flick latency (the time needed to evoke the tail-flick reflex by noxious radiant heat) was reduced for 1–4 min after intrathecal administration of substance P (5 μg), but the tail skin temperature was not significantly changed. Nor was the tail skin temperature significantly changed after intrathecal injection of Spantide (5 pg), but this compound significantly increased tail-flick latencies 5–30 min after injection. Analysis of co-variance showed that the effects of substance P or Spantide on tail-flick latency were significant, whereas the influence of tail skin temperature on tail-flick latency was nonsignificant. Thus, intrathecal substance P induces a short-lasting increase in nociceptive sensitivity, and intrathecal Spantide produces an antinociceptive effect of longer duration. The results seem not to be the result of changes in tail skin temperature.     

Adenosine analogues stimulate cyclic AMP formation in rabbit cerebral microvessels via adenosine A2-receptors

This study has examined the accumulation of cyclic AMP in microvessels from rabbit and feline cerebral cortex induced by a series of adenosine analogues to determine the type of receptor involved. The conversion of tritium labelled adenine nucleotides to [3H]cyclic AMP was determined in [3H]adenine labelled microvessels in the presence of an inhibitor of cyclic AMP hydrolysis, rolipram (30 microM). In microvessels from both cats and rabbits two adenosine analogues, N-5'-ethylcarboxamido adenosine (NECA) and L (S)-phenylisopropyladenosine (PIA), stimulated cyclic AMP accumulation. The response was larger and more reproducible in rabbits than in cats. In rabbit cerebral microvessels the order of potency as stimulator of cyclic AMP accumulation was NECA greater than 2-chloroadenosine greater than L-PIA greater than cyclohexyladenosine (CHA) greater than D-PIA. This order of potency defines the receptor involved as being of the A2 subtype. Although the maximal response to CHA appeared to be lower than that to NECA, CHA did not inhibit the response to NECA, suggesting that it is not a classical partial agonist. In the presence of the adenylate cyclase stimulating compound forskolin (I microM) NECA was more active than in its absence (close to 30-fold increase in EC50) and also produced a maximal effect six times higher. The maximal responses to PIA and CHA increased proportionally in the presence of forskolin. These results show that rabbit cerebral microvessels possess adenosine receptors of the A2 subtype capable of stimulating the formation of cyclic AMP. The functional significance of such receptors is not known, but may be related to regulation of vascular permeability.     

Antinociceptive and neurotoxic actions of substance P analogues in the rat's spinal cord after intrathecal administration

Intrathecal administration of both (D-Pro2,D-Trp7,9)-substance P (DPDT) and (D-Arg1,D-Trp7,9,Leu11)-substance P (DADTL) elicited antinociception in hot-plate and tail-flick test, with DADTL as the most potent. The animals injected with 2.0 micrograms DADTL, and several animals administered with DPDT at the same dose, developed bilateral motor blockade of the hind-legs, persisting for up to 3 days after DADTL. The effect of DPDT appeared to be reversible at this dose. On histopathological examination it was found that these animals with persistent paralysis had widespread neuronal necrosis in the lumbar region of the spinal cord. It is concluded that the peptides have antinociceptive effects after the intrathecal administration in rats, but that there is a small margin between the dose producing this effect and that causing irreversible toxic reactions in the spinal cord.     

Bradykinin facilitates the purinergic motor component of the rat bladder neurotransmission

The motor activity of the rat bladder elicited by transmural electrical stimulation was abolished in the presence of 200 nM tetrodotoxin but not of 1 microM atropine plus 3.4 microM guanethidine. Tissue preincubation with 20 microM, alpha, beta-methylene ATP reduced but did not obliterate the electrically-induced motor effect. Bradykinin (BK) caused a short-lasting motor response while it potentiated, in a concentration-dependent fashion, the 0.15-5 Hz-induced muscle twitching. The facilitatory action of the peptide lasted for at least 5 min and was blocked by the BK-B2 receptor antagonist D-Arg0 [Hyp3, Thi5,8, D-Phe7]-BK. The motor response caused by the exogenous application of adenosine 5'-triphosphate (ATP) was almost immediate and lasted less than 30 s; it was also potentiated by BK-B2 receptor activation, an effect that was reduced in a concentration-dependent manner by pretreatment with the BK-receptor antagonist.     

Binding of adenosine and receptor-specific analogues to lymphocytes from control subjects and patients with common variable immunodeficiency.

Studies were performed on the binding of tritiated adenosine and its analogues, 5'-N-ethylcarboxamide adenosine (NECA) and N6-phenylisopropyladenosine (PIA), to human peripheral blood lymphocytes. These revealed binding only of adenosine (Kd, 1-10 microM, 14,000 binding sites/cell), which was abolished by dipyridamole, a specific adenosine transport inhibitor, suggesting that the binding is to the nucleoside transporter. The absence of high affinity (Kd less than or equal to 1 microM) binding of adenosine or of the two analogues. NECA and PIA suggests that the previously reported effects of adenosine on cAMP formation are not mediated by cell surface specific nucleoside receptors. Binding of adenosine to the carrier in lymphocytes from patients with common variable immunodeficiency was similar to those from control subjects.     

The putative serotonin receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin antagonizes the antinociceptive effect of morphine

The effect of the selective 5-hydroxytryptamine (5-HT-1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on nociception and morphine analgesia was tested with the tail-flick method in mice. 8-OH-DPAT (0.06-1.0 mg/kg) had no apparent effect on the general behavior of the animals and did not change their reactivity to stimulation with noxious radiant heat. The compound did, however, dose-dependently attenuate the antinociception induced by administration of morphine hydrochloride (5.0 and 10.0 mg/kg). Thus, stimulation of a subpopulation of serotonin receptors may counteract the antinociceptive effect of morphine in the tail-flick test.     

The role of adenosine receptor engagement in murine fetal thymocyte development.

The role of adenosine receptor engagement in murine T-cell development was evaluated by culturing day 15-16 fetal thymic lobes in the presence of various concentrations of the adenosine receptor agonist 5'-(N-ethyl)-carboxamidoadenosine (NECA) or the adenosine receptor antagonist 8-phenyl-theophylline (8-PT) using the fetal thymic organ culture (FTOC) system. Before and 8 days after culture, thymocytes were isolated, counted, and analyzed for the expression of CD4 and CD8 T-cell differentiation molecules. Analysis of fresh thymocytes prior to culture showed that the majority of cells were of the CD4 single-positive or CD4+ CD8+ immature phenotype. Eight days after culture with media alone, 44% of cells were CD4+ and 23% were CD8+, and the number of viable thymocytes had increased from 1.7 x 10(5) to 2.2 x 10(5) cells per thymic lobe. A dose-dependent inhibition of maturation was observed in cultures with 8-PT, with greater than 85% inhibition at 50 microM. The double-positive thymocyte subset was most severely depleted. The number of cells obtained from cultures with NECA was also reduced, with about 65% inhibition at 50 microM, especially the CD8+ subset that was most severely affected. These results suggest that adenosine receptor engagement is required for normal T-cell differentiation and that adenosine receptor agonists and antagonists have distinct effects on thymocyte differentiation. An understanding of the cell-type-specific and developmental expression of adenosine receptors will help elucidate the mechanisms by which adenosine receptor engagement influences T-cell development.     

Microdialysis elevation of adenosine in the basal forebrain produces vigilance impairments in the rat psychomotor vigilance task

STUDY OBJECTIVE: The inhibitory neuromodulator adenosine has been proposed as a homeostatic sleep factor that acts potently in the basal forebrain (BF) to increase sleepiness. Here 300 microM of adenosine was dialyzed in the BF of rats, and the effect on vigilance was determined in the rat Psychomotor Vigilance Task (rPVT). DESIGN: Rats experienced all experimental conditions in a repeated-measures, cross-over design. PATIENTS OR PARTICIPANTS: Twelve young adult male Fischer-Norway rats. INTERVENTIONS: Sustained attention performance in the rPVT was evaluated following 2 hours of bilateral microdialysis perfusion of vehicle, adenosine (300 microM), or codialysis of 300 microM of adenosine with the A1 receptor antagonist 8-cyclopentyltheophylline. MEASUREMENTS AND RESULTS: During rPVT performance, response latencies and performance lapses increased significantly after adenosine dialysis when compared with baseline (no dialysis) or vehicle dialysis sessions. The codialysis of 8-cyclopentyltheophylline with adenosine completely blocked the effects produced by adenosine alone, resulting in performance equivalent to that of the vehicle sessions. CONCLUSIONS: Pharmacologic elevation of BF adenosine in rats produced vigilance impairments resembling the effect of sleep deprivation on vigilance performance in both man and rats. This effect of exogenous adenosine was completely blocked by codialysis with an adenosine A1 receptor antagonist. The results are consistent with the hypothesis that sleep loss induces elevations of BF adenosine that, acting via A1 receptors, lead to increased sleepiness and impaired vigilance.     

The role of group A5 catecholaminergic neurons in stress and morphine analgesia mechanisms in rats

The activity of antinociceptive mechanisms in cold stress and under the effect of morphine was studied in experiments on rats subjected to bilateral destruction of group A5 neurons. In cold stress in rats of the experimental and control groups the latent periods (LP) of pain reactions of limb licking differed significantly, both in initial values and between the groups; in contrast, the LP of the tail-flick reaction did not differ significantly. Under the effect of morphine the pain reactions increased authentically as compared to the initial values both in the experimental and the control groups, but there were no significant differences between the groups. Thus, group A5 noradrenergic neurons have an effect on inhibiting some pain reaction components in cold stress, but are of no importance in the mechanisms of antinociceptive actions under the effect of morphine.     

Characterization of adenosine receptors in bone. Studies on the effect of adenosine analogues on cyclic AMP formation and bone resorption in cultured mouse calvaria

The effect of different adenosine analogues on cyclic AMP (cAMP) formation and bone resorption in cultured mouse calvarial bones was investigated. 5'-N-ethylcarboxamidoadenosine (NECA), R-N6-phenylisopropyl-adenosine (PIA), N6-cyclohexyl-adenosine (CHA) and 2-chloroadenosine all stimulated cyclic AMP formation with a threshold close to 1 mumol l-1); NECA was the most potent agonist. Theophylline (10, 100 mumol l-1) inhibited the cAMP accumulation induced by NECA and 2-chloroadenosine (30 and 300 mumol l-1), dose dependently. There was no inhibition of cAMP formation by PIA and CHA in forskolin-treated bone tissue. SQ 22, 536 and 2',5'-dideoxyadenosine (100 mumol l-1) both inhibited rolipram-stimulated cAMP formation. Cyclic AMP accumulation in isolated osteoblast-like cells from neonatal mouse calvarial bones was stimulated by NECA (10 and 100 mumol l-1) and 2-chloroadenosine (100 mumol l-1). 2-chloroadenosine (10 and 30 mumol l-1), but not NECA, PIA nor CHA, caused a dose-dependent stimulation of 45Ca release in both 48- and 120-h culture. The effect of 2-chloroadenosine on 45Ca release could not be antagonized by theophylline. Neither NECA, PIA, CHA nor 2-chloroadenosine could affect PTH-stimulated 45Ca release in short term cultures (6, 24 h). By contrast, stimulation of cAMP formation by forskolin or dibutyryl cAMP caused a rapid (6 h) inhibition of PTH-stimulated bone resorption. The results demonstrate functional A2 and P-site receptors in mouse calvaria and osteoblast-like cells, but no A1-receptor was detected. These adenosine receptors regulate cAMP, but are not intimately linked to bone resorption. The calcium mobilization induced by 2-chloroadenosine appears to be unrelated to adenosine receptors.     

Effect of compound GB-115 on morphine-induced analgesia

Experiments on outbred mice showed that compound GB-115, a retropeptide analogue of the tetrapeptide cholecystokinin, produced a naloxone-dependent potentiating effect on morphine-induced analgesia in the hot-plate test, but did not modulate animal behavior in the tail-flick test in outbred mice. This potentiation of antinociceptive activity of morphine was probably related to the interaction of GB-115 with supraspinal opioidergic mechanisms. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 143, No. 6, pp. 645–647, June, 2007     

Sensory thresholds and the antinociceptive effects of GABA receptor agonists in mice lacking the beta3 subunit of the GABA(A) receptor

A line of mice was recently created in which the gabrb3 gene, which encodes the beta3 subunit of the GABA(A) receptor, was inactivated by gene-targeting. The existence of mice with a significantly reduced population of GABA(A) receptors in the CNS enabled an investigation of the role of GABA and GABA(A) receptors in nociception. The present study examined the sensory thresholds of these mice, as well as the antinociceptive effects of subcutaneously or intrathecally administered GABA(A) and GABA(B) receptor agonists. Homozygous null (beta3-/-) mice displayed enhanced responsiveness to low-intensity thermal stimuli in the tail-flick and hot-plate test compared to C57BL/6J and 129/SvJ progenitor strain mice, and their wild-type (beta3+/+) and heterozygous (beta3+/-) littermates. The beta3-/- mice also exhibited enhanced responsiveness to innocuous tactile stimuli compared to C57BL/6J, 129/SvJ and to their beta3+/+ littermates as assessed by von Frey filaments. The presence of thermal hyperalgesia and tactile allodynia in beta3-/- mice is consistent with a loss of inhibition mediated by presynaptic and postsynaptic GABA(A) receptors in the spinal cord. As expected, subcutaneous administration of the GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo-(5,4-c)pyridin-3-ol did not produce antinociception in beta3-/- mice, whereas it produced a dose-dependent increase in hot-plate latency in C57BL/6J, 129/SvJ, beta3+/+ and beta3+/- mice. However, the antinociceptive effect of the GABA(B) receptor agonist baclofen in the tail-flick and hot-plate tests was also reduced in beta3-/- mice compared to the progenitor strains, beta3+/+ or beta3+/- mice after either subcutaneous or intrathecal administration. This finding was unexpected and suggests that a reduction in GABA(A) receptors can affect the production of antinociception by other analgesic drugs as well.     

Adenosine A1 receptor-mediated presynaptic inhibition of GABAergic transmission in immature rat hippocampal CA1 neurons

In the mechanically dissociated rat hippocampal CA1 neurons with native presynaptic nerve endings, namely "synaptic bouton" preparation, the purinergic modulation of spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) was investigated using whole-cell recording mode under the voltage-clamp conditions. In immature neurons, adenosine (10 microM) reversibly decreased GABAergic mIPSC frequency without affecting the mean current amplitude. The inhibitory effect of adenosine transmission was completely blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM), a selective Alpha(1) receptor antagonist, and was mimicked by N(6)-cyclopentyladenosine (CPA, 1 microM), a selective Alpha(1) receptor agonist. However, CPA had no effect on GABAergic mIPSC frequency in postnatal 30 day neurons. N-ethylmaleimide (10 microM), a guanosine 5'-triphosphate binding protein uncoupler, and Ca(2+)-free external solution removed the CPA-induced inhibition of mIPSC frequency. K(+) channel blockers, 4-aminopyridine (100 microM) and Ba(2+) (1 mM), had no effect on the inhibitory effect of CPA on GABAergic mIPSC frequency. Stimulation of adenylyl cyclase with forskolin (10 microM) prevented the CPA action on GABAergic mIPSC frequency. Rp-cAMPS (100 microM), a selective PKA inhibitor, also blocked the CPA action. It was concluded that the activation of presynaptic Alpha(1) receptors modulates the probability of spontaneous GABA release via cAMP- and protein kinase A dependent pathway. This Alpha(1) receptor-mediated modulation of GABAergic transmission may play an important role in the regulation of excitability of immature hippocampal CA1 neurons.     

Effects of adenosine and two stable adenosine analogues on blood pressure, heart rate and colonic temperature in the rat

Adenosine exerts effects via receptors of the AI- and A2-subtype. L-phenylisopropyl adenosine (L-PIA) is more potent than N-5'-ethylcarboxamido adenosine (NECA) at the A1-subtype receptor whereas the potency order is reversed at the A2-subtype receptor. Adenosine analogues have been shown to decrease blood pressure and heart rate and to induce a marked hypothermia. In the present series of experiments adenosine, L-PIA and NECA were given i.p. or i.v. to rats, and blood pressure, ECG and colonic temperature were recorded. The NECA was the most potent of the compounds in reducing blood pressure (EC50 2 micrograms kg-1 i.v.), followed by L-PIA (EC50 approximately 30 micrograms kg-1 i.v.) and adenosine (EC50 approximately 300 micrograms kg-1 i.v.). In contrast, L-PIA and NECA were equally active in reducing heart rate (EC50 approximately 6 micrograms kg-1 i.v.). and considerably more potent than adenosine (EC50 approximately 300 micrograms kg-1 i.v.). It is suggested that simultaneous measurement of blood pressure and heart rate could be a simple in vivo model for comparison of A1- and A2-receptor subtype mediated effects. Colonic temperature was markedly reduced after i.p. administration of the adenosine analogues. Thus, 100 micrograms NECA kg-1 reduced colonic temperature from 37.8 to 26 degrees C. A 5 degrees C temperature drop was obtained by 10 micrograms kg-1 NECA, by 200 micrograms kg-1 L-PIA and by 200 mg kg-1 adenosine. The fall in colonic temperature was associated with a loss of muscular activity, as determined by needle electrodes or by palpation, indicating an inhibition of shivering.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dual effects of adenosine and adenosine analogues on motor activity of the human fallopian tube

Effects of adenosine and adenosine analogues on spontaneous contractility of the human fallopian tube during different phases of the menstrual cycle were studied. In isthmic preparations, a stimulatory effect by L-N6-phenylisopropyladenosine (L-PIA), with preference for adenosine A1-receptors, was seen mainly during the proliferative phase. In ampullary preparations, stimulation by L-PIA was seen both in the secretory and proliferative phases. Adenosine and 2-chloroadenosine exerted similar stimulatory effects. 5'-N-ethylcarboxamide-adenosine (NECA), with selectivity for adenosine A2-receptors, or D-PIA never showed a stimulatory effect. At concentrations above those needed for stimulation, adenosine, 2-chloroadenosine and L-PIA inhibited spontaneous contractions, in common with NECA and D-PIA. Here, NECA was more potent than L-PIA. The D-PIA and L-PIA were equipotent. The inhibition was seen during the whole menstrual cycle. The competitive adenosine antagonist 8-p-sulphophenyltheophylline (PS?T) reversibly antagonized the stimulatory and inhibitory effects elicited by adenosine and the analogues. The PS?T alone could exert a stimulatory or an inhibitory action on spontaneous contractility. We suggest that adenosine can modulate contractile activity in the human fallopian tube via stimulatory A1-and inhibitory A2-receptors. These receptors are located on the smooth muscle cells, and might act via cAMP. The relative receptor dominance may be influenced by cyclic hormonal changes.