Social conflict analgesia: studies on naloxone antagonism and morphine cross-tolerance in male DBA/2 mice

It has recently been reported that male mice exhibit pronounced analgesia in response to attack from aggressive conspecifics. Although several studies indicate that this reaction can be blocked by opiate antagonist pretreatment, unequivocal evidence of opioid involvement is very much more limited. In the present study, the phenomenon of conflict analgesia has been studied in male DBA/2 intruder mice following exposure to a criterion level of attack from aggressive BKW residents. Our findings indicate that this analgesia is blocked and reversed by naloxone unaltered by methyl naloxone, except at high doses (75 mg/kg) and fully cross-tolerant with morphine. This profile confirms and extends earlier findings with B6AF1 mice, indicating that the opioid mediation of this biologically-relevant form of environmental analgesia is not strain specific.

Do small and large luteal cells of the sheep interact in the production of progesterone?

Corpora lutea from cyclic ewes were dissociated by collagenase and trypsin/EGTA treatments, and enriched fractions of small and large luteal cells were prepared on gradients of Ficoll. These fractions were incubated separately or remixed before incubation. Colchicine, cytochalasin B and the calcium channel-blocker verapamil significantly reduced progesterone production by both small and large luteal cell fractions, while isoprenaline stimulated an increase in progesterone production by large luteal cell fractions only. When fractions of small and large luteal cells were remixed, no more and no less progesterone was produced than would have been predicted from equivalent fractions incubated separately. There was therefore no evidence of synergism between small and large luteal cells in the production of progesterone. Prostaglandin F-2 alpha, which can inhibit LH-stimulated progesterone production by ovine luteal tissue in vitro, had no effect on LH-stimulated progesterone production by small luteal cell fractions, but significantly inhibited that by enriched fractions of large luteal cells. Since large luteal cell fractions were contaminated with small luteal cells, which are probably responsible for the progesterone-secretory response of these fractions to LH, it was concluded that the inhibition of LH-stimulated progesterone production by small luteal cells is dependent on the presence of large luteal cells. Oxytocin added to large and small luteal cell fractions did not affect progesterone production by either fraction. It was therefore concluded that the inhibitory action of PGF-2 alpha on LH-stimulated progesterone production may require the interaction of large and small luteal cells, but that oxytocin is not likely to be an intermediary in this interaction.

Naloxone potentiates the depressant effect of chlordiazepoxide on spontaneous activity in mice

Recent reports suggest that at least some of the behavioural effects of benzodiazepines may involve interaction with endogenous opioid mechanisms. In the present study, chlordiazepoxide (CDP, 10-20 mg/kg, i.p.) induced dose-dependent reductions in spontaneous locomotor activity and rearing in mice. Although naloxone (1 mg/kg) was itself without effect, and did not alter behavioural changes induced by the lower dose of CDP, it enhanced the depressant effects of 20 mg/kg CDP on locomotor activity. Results are discussed in relation to previous studies in which opiate antagonists have been found to block or to have no effect upon behavioural actions of benzodiazepines.

Hot-plate learning in mice is unaltered by immediate post-training administration of naloxone, naltrexone or morphine

Repeated exposure to the hot-plate assay results in shorter response latencies in both rats and mice. In view of the postulated role of endorphins in memory processes, the present study examined the effects of immediate post-trial opiate-induced manipulations on this phenomenon of hot-plate learning. In experiment 1, adult male mice were injected with naloxone (0.1-10.0 mg/kg, i.p.) immediately after initial hot-plate testing and were reassessed 24 and 48 hr later. In experiment 2, mice were similarly treated with either naltrexone (0.1-10.0 mg/kg) or morphine (0.5-5.0 mg/kg) and retested 48 and 72 hr later. Results indicated a potent training effect in all groups but failed to reveal any significant effects of drugs. These data are inconsistent with a role for endorphins in hot-plate learning and, further, may question the generality of the involvement of opioids in memory processes.

Benzodiazepines and their antagonists: a pharmacoethological analysis with particular reference to effects on "aggression"

A review of the literature on chlordiazepoxide indicates that the traditional view of benzodiazepines as antiaggressive drugs represents an inaccurate generalization. In fact, highly variable findings have been reported with the most significant factor in the variability being the type of aggression studied. Furthermore, considerable controversy surrounds the nature of chlordiazepoxide's influence on social conflict in rodents, with opinion divided regarding the selectivity of its inhibitory effect on offensive responding. It is argued that inappropriate behavioural methodology may have substantially contributed to this controversy. A pharmacoethological approach to the analysis of drug effects on social behaviour is described and exemplified by new data on chlordiazepoxide and midazolam. Both agents inhibit offense at doses which do not result in general response inhibition, yet their overall behavioural profiles are somewhat different. Evidence is also presented indicating low dose behavioural activity of the benzodiazepine antagonist Ro15-1788 in two test situations and suggesting possible differences in the effects of Ro15-1788 and CGS8216 on novelty-related responding. Several avenues of research are discussed which may yield insights into the manner whereby benzodiazepines influence social patterns and the significance of benzodiazepine-GABA interactions in such processes.

CGS 8216 fails to modify novelty-related behaviour in mice: further evidence for differential actions of benzodiazepine antagonists

Recent evidence has indicated that, in rats, benzodiazepine antagonists exhibit low-dose intrinsic behavioural properties. In this study, the influence of CGS 8216 (1-20 mg/kg, i.p.) on the responsiveness of male mice to environmental novelty was examined. Unlike Ro 15-1788, which has previously been reported to enhance rearing under the present test conditions, analysis revealed that CGS 8216 was without effect on novelty-related responding. Results are discussed in relation to possible differences in the mode of action of various benzodiazepine antagonists.

Effects of the benzodiazepine antagonist Ro 15-1788 on social and agonistic behaviour in male albino mice

In view of recently reported low-dose behavioural activity of Ro 15-1788, the present study examined the effects of this benzodiazepine antagonist on social and agonistic behaviours in adult male albino mice. Using a resident-intruder paradigm, independent pharmacological manipulation of interactants and pharmaco-ethological analysis, our data demonstrate significant behavioural effects of Ro 15-1788 in benzodiazepine-naive animals. In residents, treatment with the antagonist (1.25, 2.5, 10 and 20 mg/kg, IP) resulted in dose-related increases in offensive threat behaviour and reduced olfactory investigation. However, 5 mg/kg exerted no detectable behavioural action in these animals. In intruders, behavioural effects were observed only with 1.25 mg/kg Ro 15-1788, and consisted of a profile suggestive of reduced defensiveness. In both experiments, the behaviour of untreated opponents confirmed the existence of drug-induced behavioural changes in their partners. It is argued that present data are not inconsistent with the existence of putative endogenous benzodiazepine-like ligands and that the differential effects of Ro 15-1788 in residents (singly-housed) and intruders (grouped) suggest one possible explanation for previous failures to detect low-dose behavioural activity with this compound.

Morphometry of the functional and regressing corpus luteum of the guinea pig

A morphometric study of functional and regressing corpora lutea (CL) of guinea pigs (n = 5 per day) was performed on days 9, 12, and 16 of the estrous cycle. On day 9 the functional CL contained congruent to 750,000 cells, which included 565,200 +/- 56,700 (S.D.) endothelial cells or pericytes and 137,300 +/- 7,700 luteal cells. Between days 9 and 12 the only significant change suggesting the onset of regression was a reduction in vascular luminal surface area. During this time the number of luteal cells per CL increased to 204,400 +/- 34,800 (P less than .05), with an accompanying reduction in luteal cell volume from 19.8 +/- 1.8 to 14.4 +/- 2.4 pl/cell (P less than .01). The increase in cell numbers was explicable by cell division, with mitotic indices of 0.83% and 0.97% on days 9 and 12, respectively. Luteal volume was unaltered. Between days 12 and 16, the mean volume of a single CL fell from 3.98 +/- 0.2 to 1.42 +/- 0.3 mm3 (P less than .01) and luteal cell volume was reduced to 5.3 +/- 1.1 pl/cell (P less than .01). Between these 2 days the number of endothelial cells per CL fell from 539,900 +/- 75,500 to 144,400 +/- 63,300 (P less than .01), with an accompanying reduction in vascular luminal surface area and in the volume occupied by vascular lumina. The total number of luteal cells per CL was not reduced significantly. It was concluded that luteal cell numbers in the guinea pig increase up to the time of onset of luteal regression, and that during regression up to day 16, shrinkage of luteal cells is the major cause of loss of luteal volume. During regression, endothelial cell loss occurs much more rapidly than loss of luteal cells.

Morphometric analysis and function in vivo and in vitro of corpora lutea from ewes treated with LHRH during seasonal anoestrus

Ovulation was induced by 3 X 30 micrograms LHRH i.v. at 90-min intervals in anoestrous Corriedale ewes. Plasma LH surges occurred in all of 31 ewes given LHRH, but ovulation occurred in only 16 of these ewes. Luteal weight and plasma progesterone concentration were lower in ewes in which ovulation was induced during anoestrus than in cyclic control ewes in the breeding season, and when data from induced and control ewes were pooled luteal weight was strongly correlated with plasma progesterone concentration (r = +0.612, P less than 0.01). Five mature corpora lutea (CL) resulting from ovulation induced during anoestrus were compared by morphometric methods with 5 CL from cyclic control ewes. When data from induced and control CL were pooled, luteal volume was positively correlated with total number of cells per CL (r = +0.869, P less than 0.01) but negatively correlated with number of cells per mm3 luteal tissue (r = -0.676, P less than 0.05), i.e. smaller CL contained fewer cells, but more cells per unit volume. Relative numbers of large to small luteal cells, at approximately equal to 1:6, were similar in LHRH-induced and cyclic control CL. Large and small luteal cells were smaller in induced CL than in control CL, but cytoplasmic:nuclear ratio did not differ between induced and control CL. Basal and LH-stimulated progesterone production by dispersed luteal cells in vitro were lower for CL from LHRH-treated ewes than from controls. However, percentage increase in progesterone production in response to LH was not different between LHRH-treated and control ewes at any dose rate of LH used. It is concluded that the small size of CL induced by LHRH is due primarily to the low numbers and small volumes of the luteal cells in these CL, and that subnormal luteal weight and subnormal progesterone production per luteal cell contribute to the low plasma progesterone concentrations in ewes treated with LHRH during seasonal anoestrus.

Morphometric analysis of the cellular composition of the ovine corpus luteum

An electron microscopical, morphometric study has been made of the cellular and non-cellular composition of the ovine corpus luteum fixed by perfusion, together with an estimate of total cell numbers in the corpus luteum based on measurement of cell and tissue DNA content. Normal cyclical corpora lutea from six ewes were studied by each method. Total cell numbers per corpus luteum were estimated at 258 X 10(6) by morphometry and 238 X 10(6) by DNA measurement. The existence of four structurally distinct major cell populations in ovine luteal tissue was confirmed. Percentages of the total cell number were as follows: large luteal cells - 4.2%; small luteal cells - 18.5%; fibrocytes - 16.8%; endothelial cells and pericytes - 52.6%. The remaining 7.9% comprised other miscellaneous cell types. Of the four major cell types, cell volume (X 10(3) micron 3 +/- standard error of the mean) was greatest for large luteal cells (13.08 +/- 2.10) and least for endothelial cells and pericytes (0.52 +/- 0.05), with small luteal cells (2.08 +/- 0.19) and fibrocytes (1.61 +/- 0.27) intermediate in size. Large luteal cells occupied 25.4% of luteal volume density, small luteal cells 17.5%, fibrocytes 11.2% and endothelial cells and pericytes 12.7%. Other cell types occupied 5.4%, with the remaining volume occupied by vascular lumina (10.8%) and intercellular space (17.1%). The data on cell volume support morphological findings suggesting that large and small luteal cells of the ovine corpus luteum are distinct populations rather than the upper and lower limits of a single population whose sizes are normally distributed. Of these two types of luteal cell, small luteal cells outnumbered large luteal cells by more than 4:1, while the individual large luteal cells were more than six times the mean volume of the small luteal cells. These facts need to be considered when assessing the relative contributions of the two types of luteal cell to overall luteal steroidogenic function.

Naloxone administration following brief exposure to novelty reduces activity and rearing in mice upon 24-h retest: a conditioned aversion?

It has recently been reported that naloxone treatment, prior to initial exposure to a novel arena, results in significant behavioural change when animals are retested 24 h later. In an attempt to clarify the nature of this delayed action of the opiate antagonist, three further studies have been performed. In the first experiment, male mice were injected with naloxone hydrochloride (0-10 mg/kg, IP) immediately after their initial experience of the test arena. When retested 24 h later, all groups that had previously received naloxone exhibited greatly reduced activity and rearing, with no evidence of a dose-response relationship. In the second experiment, naloxone (0-10 mg/kg) failed to induce a conditioned place aversion when administered according to the above regimen. In the final experiment, no evidence for a naloxone-induced taste aversion to saccharin was observed. It is concluded that the behavioural changes observed in the open-field study may reflect either (a) subtle aversive properties of naloxone which are insensitive to traditional one-trial paradigms or (b) opioid modulation of memory for non-painful experiences.

Large luteal cells the source of luteal oxytocin in the sheep

To determine the cellular origin of oxytocin produced by the cyclical corpus luteum (CL) of the sheep, enriched fractions of enzymatically dispersed small and large luteal cells from 12 CL were prepared on a Ficoll 400 gradient. Oxytocin was measured by RIA. Large luteal cells contained 1.08 +/- (SD) 0.70 fg/cell oxytocin, which was congruent to 30 X the content of small luteal cells. Endothelial cells contained little if any oxytocin. During a 12-h incubation, large luteal cells produced 0.28 fg/cell.h oxytocin: small luteal cells did not produce measurable amounts of oxytocin. It is concluded that the large luteal cells are the source of the oxytocin produced by the CL of the sheep.

Evidence for intrinsic behavioural activity of the benzodiazepine antagonist, Ro15-1788, in male mice

It would be predicted that putative benzodiazepine should be released under anxiety-provoking conditions and that behavioural changes should be observed following pretreatment with selective antagonists of the benzodiazepine receptor. To test this hypothesis, adult male albino mice were briefly exposed to a novel, brightly-illuminated arena during the dark phase of their LD cycle. Under these test conditions, Ro15-1788 (10 mg/kg) enhanced total rearing whilst, a 5-10 mg/kg, it significantly altered the normal pattern of rearing over the test session. However, at the highest dose tested (20 mg/kg), such behavioural changes were no longer apparent. A similar, though non-significant, trend was observed for locomotor activity. These data, the first to demonstrate-low-dose intrinsic activity of Ro15-1788 in mice, suggest that benzodiazepine antagonists may prove to be powerful tools in the study of the behavioural significance of the benzodiazepine receptor.

Progesterone production in vitro by small and large ovine luteal cells

Corpora lutea from cyclic ewes were dissociated by collagenase digestion and trypsin/EGTA treatment. Enriched fractions of endothelial cells, small luteal cells and large luteal cells were prepared on a stepped gradient of Ficoll 400. Progesterone was measured by radioimmunoassay and the results corrected so that progesterone production by each cell type could be determined. Endothelial cells did not produce significant amounts of progesterone, with or without LH stimulation, and endothelial cell contamination of small and large luteal cell fractions did not influence progesterone production by these fractions. Mean +/- s.e.m. basal progesterone production (n = 10) by large luteal cells was greater (P less than 0.001) on a per cell basis than that by small luteal cells (1.16 +/- 0.16 compared with 0.25 +/- 0.06 pg/h/cell). However LH, which stimulated a maximal 3-4-fold increase in progesterone production by small luteal cells (LH ED50 = 0.14 ng/ml), had no significant effect on production by large luteal cells, when contamination by small luteal cells was taken into account. The response of small luteal cells was specific to LH, other hormones having had no significant effect. Basal progesterone production by small luteal cells (0.12 +/- 0.03 fg/h/micron3) calculated per unit volume of cell was not significantly different from that of large luteal cells (0.17 +/- 0.02 fg/h/micron3). After LH stimulation, small luteal cells produced more progesterone than did large luteal cells (0.40 +/- 0.09 compared with 0.18 +/- 0.03 fg/h/micron3) (P less than 0.05). When the amounts of progesterone produced per cell were multiplied by the absolute numbers of large luteal (1 X 10(7] and small luteal (5 X 10(7] cells in the intact corpus luteum, basal progesterone production by large luteal cells (11.6 +/- 1.6 micrograms/h) was similar to that by small luteal cells (12.3 +/- 3.0 micrograms/h). However, under LH stimulation, progesterone production by the small luteal cell type (39.9 +/- 9.5 micrograms/h) was approximately 3 times greater than that by the large luteal cell type (12.3 +/- 1.6 micrograms/h) (P less than 0.05). We therefore conclude that small luteal cells may be the principal source of luteal progesterone production in the sheep.

Social conflict activates status-dependent endogenous analgesic or hyperalgesic mechanisms in male mice: effects of naloxone on nociception and behaviour

The concept of environmentally-induced activation of endogenous analgesia mechanisms rests, almost exclusively, upon studies which have involved the use of rather intense artificial stimuli. The current study was therefore designed to assess the validity of this concept under the more naturalistic conditions of social conflict between isolated resident mice and group-housed intruders. Agonistic experience was found to result in a potent, naloxone-reversible (10 mg/kg) analgesia in intruder mice while, in residents, it produced a moderate hyperalgesic reaction which was very sensitive to naloxone antagonism (0.1 mg/kg). Detailed videotape analyses revealed that only the behaviour of residents was significantly altered by naloxone treatment, with a highly selective inhibition of attack observed at 10 mg/kg. These data suggest that (1) social conflict in mice is a potent, and biologically-relevant, stimulus in the activation of endogenous naloxone-sensitive pain control mechanisms, (2) social status is an important determinant of nociceptive response to such experience and (3) inescapability from attack may be a critical factor in the development of encounter-induced analgesia.

Naloxone partially antagonizes post-encounter analgesia and enhances defensive responding in male rats exposed to attack from lactating conspecifics

Recent evidence indicates that inter-male conflict in mice, but not rats, results in opiate-like analgesia in defeated animals. To test the hypothesis that differences in attack intensity/controllability may underlie this apparent species difference, male rats were exposed to non-ritualized attack from lactating conspecifics. In response to such encounters, male intruders exhibited significant elevations in tail-flick latency. Pretreatment with low (0.1-1.0 mg/kg), but not high (10 mg/kg), doses of naloxone partially antagonized the development of post-encounter analgesia and enhanced several components of male defensiveness during encounters. These results suggest that exposure to attack from lactating conspecifics activates both opioid and nonopioid analgesia mechanisms in male intruders. Correlational analyses revealed, however, that intruder analgesia was not related to the number of attacks received or the display of submissive postures. Data are discussed with reference to the possible importance of psychological factors in the activation of endogenous pain inhibitory mechanisms in rats.

Differential effects of naloxone and diprenorphine on defensive behaviour in rats

Two opiate antagonists, naloxone and diprenorphine, were used to examine the proposed involvement of endogenous opioid mechanisms in the modulation of shock-induced defensive behaviour patterns in rats. Naloxone was found to exert a biphasic influence on defensive fighting, with small dose (0.1 mg/kg) facilitation and large dose (10 mg/kg) inhibition. This compound also induced a dose-dependent inhibitory effect on shock-elicited threat whilst facilitated escape/avoidance behaviour was observed with the largest doses only. None of these influences could be attributed to alterations in electric shock thresholds. In contrast to the effects of naloxone, diprenorphine (0.1-10 mg/kg) was largely ineffective in modifying shock-induced defensive behaviour. These data suggest that endogenous opioid mechanisms may not be involved in the effects of naloxone on defensive fighting and cast some doubt upon the general hypothesis of the involvement of endorphins in the regulation of innate defence patterns.

Effects of a synthetic prostaglandin analogue, cloprostenol, on the corpus luteum of the guinea pig

The effects of a synthetic prostaglandin analogue, cloprostenol, on luteal function in the guinea pig were studied. At a dose of 250 micrograms, cloprostenol administered I-P on day 9 of the cestrous cycle caused a reduction in the length of the oestrous cycle from 17.4 +/- s.d.0.9 to 14.5 +/- 1.1 days (p less than 0.01). Lower doses were ineffective, and post-treatment cycles were not different in length from pre-treatment cycles. Cloprostenol also caused a dose-dependent reduction in luteal weight, which fell from 3.52 +/- 0.82 to 1.82 +/- 0.41mg (p less than 0.01) 48 h after administration of a 250 micrograms dose on day 9. Plasma progesterone, measured by radioimmunoassay, was reduced from 4.67 +/- 0.59 to 2.69 +/- 0.66 ng ml-1 (p less than 0.01) 48 h after administration of 250 micrograms cloprostenol on day 9. 250 micrograms cloprostenol also reduced blood flow per corpus luteum, measured by 85Sr-labelled 15 microns microspheres, both at 3 h (20.20 +/- 10.36 to 9.40 +/- 4.20 microliters min-1; p less than 0.05) and at 48 h (18.47 +/- 8.27 to 5.23 +/- 1.90 microliters min-1; p less than 0.01) after administration on day 9. No adverse side-effects were observed at any dose level of cloprostenol used. It was concluded that cloprostenol is a useful experimental luteolysin in the guinea pig.

GABAergic influences on defensive fighting in rats

The involvement of GABAergic mechanisms in shock-induced defensive fighting in rats was investigated in a series of three experiments. In Experiment 1, sodium n-dipropylacetate (100-200 mg/kg) failed to produce significant behavioural change whilst gamma-vinyl-GABA (100-200 mg/kg) induced a selective and dose-dependent reduction in fighting. In Experiment 2, although inconsistent behavioural effects were obtained with (+)-bicuculline (0.25-4 mg/kg), a biphasic influence on defensive fighting was observed with picrotoxin (0.125-2 mg/kg). The inhibitory effect on fighting, induced by the highest dose of picrotoxin, was related to motor impairment. In Experiment 3, muscimol reduced fighting at doses above 0.25 mg/kg with motor disruption evident only at the highest dose used (1 mg/kg). A dose-dependent inhibition of defensive fighting was observed with 1-baclofen (0.15-1.2 mg/kg) which, at the highest dose tested, also impaired motor coordination. None of the compounds tested significantly altered shock thresholds. Results are discussed in relation to the hypothesized inhibitory role of GABA in the mediation of aggressive behaviours.

Agonistic behaviour in rats: evidence for non-involvement of opioid mechanisms

It has recently been proposed that a stress-activated, endogenous analgesia mechanism would be adaptive in situations in which pain perception might otherwise disrupt effective behavioural performance. In a semi-natural test situation, the current study examined two predictions arising from this hypothesis: (1) in a manner analogous to other stressors, agonistic experience should produce analgesia and, if naloxone-sensitive opioid mechanisms are implicated, then (2) pretreatment with naloxone should block the development of this response and alter the displayed behaviour patterns. Neither prediction was substantiated by the data. Experience of an agonistic encounter failed to produce analgesia in either resident or intruder animals. Furthermore, naloxone hydrochloride (1-25 mg/kg) was also without effect on patterns of offense or defense. Data are discussed in relation to the critical nature of the stimulus factors involved in the activation of endogenous analgesic mechanisms and the postulated involvement of such mechanisms in biologically-adaptive behaviours.

Purification, Morphology, and Progesterone Production and Content of Three Cell Types Isolated from the Corpus Luteum of the Sheep

A method is presented for the isolation and purification of three cell types, endothelial cells, small luteal cells and large luteal cells, from the ovine corpus luteum. The method involves enzymatic dispersion of luteal tissue followed by centrifugation of separated cells on a Ficoll gradient. The three purified cell types and others, particularly fibrocytes and smooth muscle cells, that were removed during purification, were identified by their morphology. The cell yield, the cellular composition and cellular progesterone content of each fraction from the Ficoll gradient were measured. The endothelial cell fractions were relatively free of contamination by other cell types and had negligible progesterone. Fractions of small luteal cells and those of large luteal cells contained endothelial cells but were relatively free of other cell types. Large luteal cells contained significantly more progesterone, produced more progesterone when incubated in culture, but were less responsive to luteinizing hormone than small luteal cells.

Delayed effects of naloxone on responsiveness to environmental novelty in rats

Two experiments were conducted to examine further the hypothesized involvement of endorphins in responsiveness to environmental novelty. In Experiment 1, rats were treated with naloxone hydrochloride (0.5--5.0 mg/kg, SC) before initial exposure to a novel arena (Day 1) and then retested in the area 24 h later (Day 2). Only naloxone (5 mg/kg) significantly affected Day 1 performance, producing a selective reduction in locomotor activity. However, compared to saline controls, all groups that had previously received naloxone showed marked reductions in both locomotor activity and rearing upon Day 2 retest. In Experiment 2, naloxone (0.5--5.0 mg/kg) was without significant effect on performance in naive animals which had been injected on Day 1 but not exposed to the arena until Day 2. These data suggest that the delayed effects of naloxone relate specifically to the initial experience of environmental novelty rather than receptor changes or metabolite influences resulting from acute antagonist treatment. Results are discussed in relation to a possible action of naloxone upon mechanisms of attention and/or memory.

Plasma LH and FSH in ewes that were either fertile or infertile after long-term grazing of oestrogenic pasture

The levels of plasma LH and FSH were measured in serial blood samples taken at 15-min intervals for 6 h from ewes that had remained fertile after grazing oestrogenic pasture (clover-fertile ewes), from ewes that were permanently affected by clover disease (clover-infertile ewes) and from normal ewes. Two flocks of ewes from different locations were studied. In flock 1, tonic LH secretion (total area under the curve of LH concentration versus time, 1 area unit = 1 ng ml-1 x 1 h) was significantly (P < 0.05) greater in clover-infertile ewes (10.4 area units) during anoestrus than in ewes that had remained fertile after prolonged grazing of oestrogenic clover (5.4 area units). Tonic LH and FSH secretions during the bleeding season and FSH secretion during anoestrus were not significantly different. In flock 2, LH levels during the breeding season were significantly (P < 0.05) elevated in clover-infertile ewes (10.9 area units) compared to normal ewes (5.4 area units) that had never grazed oestrogenic clover. LH secretion in clover-infertile ewes (7.8 area units) was intermediate to that found in infertile and control ewes. Concentrations of FSH, progesterone and ovarian vein oestradiol-17 beta (E2) during the breeding season were similar in the three groups. In another experiment, the positive feedback release of LH following administration of E2 (12.5, 25 or 50 micrograms per ewe) was measured in anoestrous ewes of flock 2. Significantly (P < 0.01) more clover-infertile ewes demonstrated a positive feedback effect than control ewes when given 12.5 micrograms E2 but not when given higher doses. The elevation of LH secretion in permanently affected clover-infertile ewes is inconsistent with the hypothesis that the hypothalamo-pituitary axis of these ewes is less responsive to the negative feedback effect of oestrogen. Furthermore, the patency of the positive feedback loop is consistent with the ability to ovulate.

Exploratory behaviour and aversive thresholds following intra-amygdaloid application of opiates in rats

Rats were bilaterally implanted with guide cannulae aimed at the central or medial nucleus of the amygdala. Microinjections of morphine (10 microgram) at both sites significantly elevated the threshold of response in the flinch-jump test; but only at medial sites did naloxone (1 microgram) antagonise this effect. However, in the hole-board test, an opposite pattern of results emerged. Morphine injections into the central nucleus produced naloxone-reversible reductions in both exploration and activity whilst, in the medial nucleus, the morphine-induced decrease in exploration was not reversed by naloxone. It is concluded that (1) the presence or absence of naloxone-sensitive opiate receptors cannot always be deduced on the basis of a single behavioural test and (2) within the amygdaloid complex, two distinct naloxone-sensitive opiate systems appear to be involved in the modulation of behavioural responses to different forms of stimulation.

Effect of naloxone on the behaviour of rats exposed to a novel environment

It has recently been suggested that endogenous opiates may play a general role in stress responding. To test this hypothesis, naloxone hydrochloride (0.5-4.0 mg/kg SC) was administered to rats exposed to an open field situation. Naloxone treatment produced a decrease in locomotor activity and rearing, and an increase in defaecation. A simple dose-response relationship was not observed, with the most potent effects exerted by the 1 mg/kg dose. Nvertheless, these results indicate that naloxone increases emotionality in the rat and suggest that opioid peptides may be released under conditions of nonpainful stress.

Partial anxiolytic action of morphine sulphate following microinjection into the central nucleus of the amygdala in rats

In the social interaction test of anxiety, bilateral microinjections of morphine sulphate (10 microgram) into the central nucleus of the amygdala counteracted the reduction in social interaction normally seen when the test arena is unfamiliar to rats. However, these injections did not counteract the decrease in social interaction that is observed as illuminance of the arena is increased. Morphine injections into the medial site depressed social interaction below the levels shown by control animals. In the open field test, morphine produced a facilitation of peripheral activity when injected into the central nucleus whilst a decrease in rearing was observed following similar injections into the medial nucleus. Overall, these data indicate a partial anxiolytic action of morphine in the central amygdaloid nucleus. Results are discussed in relation to possible differences in opioid peptide innervation of these two amygdaloid nuclei.

Effects of nicotine, mecamylamine, and hexamethonium on shock-induced fighting, pain reactivity, and locomotor behaviour in rats

Three series of experiments were performed to evaluate possible nicotinic cholinergic influences on fighting behaviour in rats. Each series consisted of three tests (naive animals in each test); shock-induced fighting, pain threshold estimation and locomotor activity. In the first series, nicotine (0.25 -- 1.00 mg/kg) was found to produce a dose-dependent inhibition of fighting without altering shock thresholds. However, the highest dose used also significantly reduced rearing in the activity test. In the second series, mecamylamine (a centrally active antinicotinic) produced a facilitation of fighting at low doses (2.5 mg/kg) and an inhibition at higher doses (10 mg/kg). Whilst these effects were unrelated to changes in shock thresholds, the high dose resulted in a reduction in both horizontal activity and rearing. Finally, as a control for possible peripheral effects of nicotinic blockage, a third series examined the behavioural effects of hexamethonium. Low doses of this compound (2.25 -- 4.5 mg/kg) had little effect on fighting whilst higher doses (9 -- 18 mg/kg) attenuated these responses. Interestingly, although hexamethonium had no effect on shock thresholds, the highest dose (18 mg/kg) produced a facilitation of horizontal activity. Results are discussed in relation to the hypothesis of central nicotinic cholinergic inhibition of agonistic behaviour.

Elevation of aversive threshold in rats by intra-amygdaloid injection of morphine sulphate

Bilateral micro-injection of morphine sulphate (10 microgram, 20 microgram) into the cortico-medial amygdala produced a dose-dependent increase in aversive threshold. Similar injections into the basolateral amygdala or caudate-putamen failed to have any consistent effect on aversive thresholds. Whilst overall activity levels remained unaffected by morphine injection into either amygdaloid site, caudate animals exhibited a significant decrement in total activity in response to both morphine and control injections. Results are discussed with reference to a possible role for limbic mechanisms in morphine analgesia.