Effects of pinealectomy on the secretion of luteinizing hormone, testosterone and prolactin in rams exposed to various lighting régimes.

Two experiments were carried out to study the effects of controlled lighting régimes on plasma levels of LH were low in all groups of rams, which made the detection of significant effects of any treatment very unlikely. Pinealectomy reduced the effects of changes in the daily photoperiod on the patterns of secretion of testosterone and prolactin. These findings establish the pineal gland as an organ which influences the endocrine responses of rams to photoperiodic stimuli and it is concluded that the pineal gland is probably important as a mediator of seasonal reproductive changes in these animals.

FSH secretion in pinealectomized rams subjected to artificial photoperiods.

Eight mature Romney rams which had been pinealectomized and another eight sham-operated rams were allocated equally to one of two contrasting lighting regimes. They were housed indoors with the temperature held constant, offered a constant diet and plasma samples were collected each week for 9 months. Changes in plasma FSH in pinealectomized and sham-operated rams were similar when the rams were subjected to normal seasonal changes in photoperiod, with lower FSH levels recorded under increasing daily photoperiods than during the phase of decreasing (stimulatory) photoperiods. In contrast a reversed pattern of changes in lighting advanced the increase of plasma levels of FSH in the case of sham-operated rams, because the period of decreased daily photoperiods occurred earlier in this situation. In this case, however, pinealectomized rams did not behave like the control animals, but had raised plasma FSH levels at the same time as rams on the normal lighting regime. These results demonstrate that the pineal-mediated effects of photoperiod on reproduction in sheep directly involve FSH secretion and that seasonal cyclic changes in FSH secretion persist even after pinealectomy.

Temporal changes in LH and testosterone and their relationship with the first antler in red deer (Cervus elaphus) stags from 3 to 15 months of age.

Blood samples were taken from six tame red deer stags at 3-15 months of age once a month from a jugular catheter every 30 min for 24 h to investigate hormonal secretion during puberty and during growth of the pedicle and first antler. All plasma samples were analysed for LH and testosterone concentrations and the resultant data were analysed using the PULSAR pulse detection routine. In addition each stag was injected wih gonadotrophin-releasing hormone (GnRH; 20 ng/kg body weight) after the above samples had been taken and the bleeding regimen was continued for a further 2 h. Body weight, antler size and status (i.e. whether the stags had a pedicle or antler) were also recorded. The pulsatile secretion of LH could be considered to have occurred in three phases. The first of these was one of development, with the LH pulse frequency increasing to 8 pulses/24 h, the second a phase of regression, with a decrease in LH pulse frequency to 2 pulses/24 h, and finally a second phase of development characterized by increased LH pulse frequency to 12 pulses/24 h. Testosterone secretion generally followed the same pattern. During the period before the permanent bony pedicles grew, there were less than five LH pulses/24 h. When the pedicles were growing, LH and testosterone pulsatile secretion increased but the pulse frequency of both hormones fell during velvet antler growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma LH and testosterone responses to gonadotrophin-releasing hormone in adult red deer (Cervus elaphus) stags during the annual antler cycle.

Eight adult red deer stags were given an i.v. injection of synthetic gonadotrophin-releasing hormone (GnRH) on seven occasions at various stages of the antler cycle, namely hard antler in late winter, casting, mid-velvet growth, full velvet growth, antler cleaning and hard antler both during the rut and in mid-winter. The stags were allocated at random on each occasion to one of four doses, i.e. 1, 3, 10 or 95 micrograms GnRH. Blood samples were taken before GnRH injection and for up to 2 h after injection. Pituitary and testicular responses were recorded in terms of plasma LH and testosterone concentrations. There was an increase in plasma concentration of LH after the GnRH injection in all stags at all stages of the antler cycle. Dose-dependent responses of LH to GnRH in terms of area under the curve were apparent at all stages of the antler cycle. The lowest responses were recorded at casting, during velvet antler growth and at the rut sampling. The pattern of testosterone response reflected the inter-relationship of the antler and sexual cycles with very low testosterone responses occurring at casting and during velvet antler growth. The responses were higher at antler cleaning and then increased to a maximum at the rut before declining to reach their nadir at casting. The results are consistent with a hypothesis that the antler cycle, as a male secondary sexual characteristic, is closely linked to the sexual cycle and its timing is controlled by reproductive hormones. Low plasma concentrations of testosterone, even after LH stimulation, are consistent with the hypothesis that testosterone is unnecessary as an antler growth stimulant during growth.

Effects of cranial cervical ganglionectomy and castration of male lambs. II. 24 hour plasma profiles of luteinizing hormone, testosterone and prolactin.

Male lambs were utilized in an experiment designed to evaluate the effects of cranial cervical ganglionectomy (GX), castration and age on hormone secretion profiles. Blood plasma samples were collected at hourly intervals for 24 hours from 24 lambs aged 101 days and 20 lambs aged 277 days, then assayed for concentrations of luteinizing hormone (LH), testosterone and prolactin. At both ages pulsatile secretion of LH and testosterone was confirmed, but no circadian rhythm of LH testosterone or prolactin secretion was detected. Castration elevated LH levels significantly at both ages. GX and its interaction with castration had no effect on LH secretion at 101 days, but at 277 days these factors were significant, largely due to elevated levels being recorded from GX castrates. GX did not affect testosterone levels in entire animals at either age, while plasma from castrates contained no detectable testosterone. GX reduced prolactin concentrations at 101 days of age (summer) but elevated them at 277 days of age (winter). Castration and the interaction of castration with GX had no significant influence on plasma prolactin levels at either age. This study confirmed that the pineal gland of sheep is involved in the regulation of prolactin secretion, and probably influences LH secretion as well.

Effects of cranial cervical ganglionectomy and castration of male lambs. III. Hormonal responses following administration to gonadotrophin releasing hormone (GnRH).

Entire and castrate male lambs, which were cranial cervical ganglionectomized (GX) or untreated, were utilized in a study of responses to intravenous GnRH; 24 animals were treated at both 101 and 277 days of age. GX caused a reduction in basal LH concentrations of both wethers and rams at the first sampling, but increased pre-injection levels of this hormone in 277 day old wethers. Basal LH levels of castrates were substantially higher than those of entires, but GX had no significant influence on pretreatment testosterone secretion in rams. GnRH treatment elevated plasma LH levels in all animals, while in entires increases in testosterone concentrations also occurred. Castration significantly increased peak LH levels together with total LH output. At neither age were the LH or testosterone reponses influenced significantly by GX, nor was the interaction of castration and GX significant for LH response data. The major effect of age at GnRH treatment was that markedly higher testosterone responses were recorded from the older rams.

Changes in ovine pineal gland neuron-specific enolase immunoreactivity following bilateral, but not unilateral, superior cervical ganglionectomy.

Pineal gland tissue from control and from unilaterally or bilaterally superior cervical ganglionectomized (SCGX) sheep was found to contain neuron-specific enolase immunoreactive cells and nerve fibers. Morphological characteristics of pineal cells exhibiting immunoreactivity indicated that they were predominantly pinealocytes, while other cell types were nonimmunoreactive. Whereas bilateral SCGX resulted in a reduction in the size, and possibly number, of immunoreactive cells in the pineal, unilateral denervation did not result in any significant effects when compared with control pineals. Concomitant with the reduction in immunoreactivity in bilaterally denervated pineals was a significant increase in the volume of interstitial space, but not the number of nonimmunoreactive cells. These results suggest that sympathetic nerve fibers innervating the pineal of unilaterally sympathectomized sheep exhibited a degree of neural plasticity that resulted in denervated pinealocytes being reinnervated by remaining intact nerve terminals, thus preventing the occurrence of degenerative changes normally associated with complete loss of neural input through bilateral denervation. The fact that in unilaterally denervated sheep neither left nor right SCGX produced any discernible effects in either half of the pineal indicates that nerve fibers from each of the ganglia cross over to innervate the contralateral as well as the ipsilateral pineal half. In the stalk of the pineal an extensive network of immunoreactive nerve fibers was found in both the caudal and habenular commissures, and occasionally these fibers were observed to enter the body of both intact and sympathetically denervated pineals. This latter result suggests that the sympathetic innervation enters the pineal over its surface and not via the stalk.

Effects of various lighting regimens and pinealectomy on semen production in Romney rams.

Sixteen Romney rams were utilized in a 2 x 2 factorial experiment designed to evaluate the effects on semen production of (i) contrasting lighting regimens, and (ii) pinealectomy or sham-operation. Reversal of the annual photoperiodic cycle advanced the seasonal peak of fructose levels in semen by 4 months and also reduced percentages of unstained and morphologically normal spermatozoa. Pinealectomy abolished this advancement of peak fructose production and reduced overall concentrations and motility of spermatozoa. Moreover, towards the end of the study pinealectomized rams had higher ejaculate volumes than did sham-operated rams. Testicular and accessory sex gland data obtained after the rams were killed showed a marked influence of the lighting regimen: high values from sham-operated rams in short daily photoperiods contrasted with depressed values from those in long daily photoperiods. Pinealectomized rams provided values midway between these extremes, a finding which suggested both anti- and pro-gonadotrophic roles for the pineal gland in rams.

Effects of pinealectomy of rams on secretory profiles of luteinizing hormone, testosterone, prolactin and cortisol.

In a study of pineal gland influences on hormone secretory profiles, blood samples were collected at 20 min intervals for 26 h from 4 pinealectomized and 4 sham-operated rams which were subjected to a 14.5 h daily photoperiod. Plasma levels of luteinizing hormone (LH) and testosterone (T) underwent rapid pulsatile fluctuations in all rams. Pinealectomized rams produced more LH than the sham-operated rams and a similar trend was recorded for plasma T data, since T secretion closely followed that of LH. A nocturnal elevation of plasma prolactin (Prl) levels displayed by sham-operated rams was substantially abolished by pinealectomy. Also a rise in cortisol secretion during the morning hours recorded from 3 of the sham-operated rams was absent in 2 of the pinealectomized rams. The possible significance of the pineal gland in the regulation of secretion of these hormones is discussed.

Prolonged service intervals in cattle.

A trial based on progesterone radioimmunoassay of milk samples is described. Samples from 2274 cows in 14 herds were collected on the day of insemination and 7, 23 and 30 days later, unless a return to service occurred before the designated sample date. Two additional samples were collected from cows which returned to service more than 35 days after the first service, one on the day of the return and another seven days later. All six samples from these cows were assayed for progesterone concentration. Late returns, defined as returns to oestrus 36 or more days after mating, occurred in 8.6% of the cows. Milk progesterone assay results indicate that of these apparently late-returning cows, 55.9% suffered a loss of the conceptus, 22.1% had not been detected in oestrus around 21 days after first insemination, 11.8% were in anoestrus at insemination, 5.6% conceived to the insemination and were pregnant when thought to have had a late return, 2.6% were in prooestrus or dioestrus at insemination and 2.1% went into anoestrus after an oestrus insemination. The average prevalence of late returns after the 35th day (8.6%) and the estimated incidence of losses of concepta (4.8%) are lower than those reported in comparable studies elsewhere. Although many of the late-return cows were mated again and conceived, this syndrome nevertheless contributes significantly to the wastage rate, at least in New Zealand dairy herds with their strictly seasonal calving pattern. The implications of these findings are discussed and recommendations for their prevention are made where appropriate.

Analgesic effects on endogenous melatonin secretion.

Animal welfare concerns have led to increased demands for use of post-operative analgesia in association with experimental surgery. Such treatment is appropriate provided it has no effect on experimental parameters. In order to assess any effects of analgesics on endogenous melatonin production, groups of six ewes were bled at 30 min intervals for the first 4 hr of darkness a) without analgesic treatment and b) immediately after analgesic administration. Analgesics tested were ketoprofen, phenylbutazone (both non-steroidal anti-inflammatory drugs or NSAIDs) and buprenorphine (an opioid). Plasma melatonin was measured by radioimmunoassay and 4 hr secretion profiles computed for each animal. Ketoprofen and buprenorphine treatment reduced mean four hour melatonin secretion profiles by 50.6% (from 1,347.3 to 665.1 pg/ml.hr) and 38.6% (from 287.8 to 171.3 pg/ml.hr), respectively. Only in the case of phenylbutazone was the response not statistically significant; hence this drug has been selected as a post-operative analgesic for future experimental studies involving measurement of melatonin levels.

Effects of oxytocin and vasopressin on sperm transport from the cauda epididymis in sheep.

This study was performed to determine whether oxytocin or vasopressin affect the transport of spermatozoa from the epididymis of rams in vivo. Under general anaesthesia, cannulae were inserted into each ductus deferens and passed into the cauda epididymis of 24 Oxford Down cross rams and the luminal fluid was collected at 10 min intervals for 2-3 h. Animals were divided into seven groups and received either (i) 2 ml 0.9% saline, (ii) 10 micrograms oxytocin, (iii) 100 micrograms oxytocin, (iv) 100 micrograms oxytocin antagonist, (v) 300 micrograms oxytocin antagonist followed by 100 micrograms oxytocin, (vi) 100 micrograms vasopressin, or (vii) 100 micrograms vasopressin followed by 100 micrograms oxytocin, all by i.v. injection. The mass of fluid and number of spermatozoa in each 10 min sample was measured and the motility of the spermatozoa was assessed. Treatment with saline did not affect the mass or the number of spermatozoa in the fluid collected. Oxytocin at 10 micrograms significantly increased both the output of fluid and the number of spermatozoa by twofold. Oxytocin at 100 micrograms produced a greater increase in both fluid output and the number of spermatozoa within 10 min of administration of the peptide. Treatment with oxytocin antagonist had no immediate effect, but subsequently caused a significant reduction in both fluid output and the number of spermatozoa. Pretreatment with oxytocin antagonist inhibited the stimulatory effect of oxytocin. Vasopressin did not increase the number or concentration of spermatozoa in the fluid and appeared to decrease fluid output. No significant changes in the morphology or motility of the spermatozoa collected was observed in any of the samples. These data demonstrate that oxytocin has specific actions on the epididymis to increase sperm transport. They indicate that local oxytocin may be involved in regulating basal contractility of the cauda epididymidis and that augmentation by the peptide in the peripheral circulation, as occurs around the time of ejaculation, may promote a significant increase in the transport of spermatozoa into the vas deferens and ejaculate.

The reproductive response of rams to thyroidectomy: mediation by impaired inhibin feedback rather than a change in LH pulsatility.

A series of experiments was conducted to examine the mechanism by which removal of the thyroid glands in seasonally suppressed rams brings about rapid testicular growth. In the first experiment, thyroidectomy at the nadir of the testicular cycle (late winter) initiated testis growth without any detectable change in the extent of spermatogenesis compared with sham-operated controls. The serum concentration of FSH, but not LH, was also markedly increased by thyroidectomy. In the second experiment, serum FSH concentration was again increased by thyroidectomy in late winter but there was no effect of thyroidectomy on LH concentration, LH pulses (measured in frequent blood samples) or testosterone concentration. Furthermore, there was no evidence of a change in central dopaminergic inhibition of GnRH, as measured by the pulsatile LH response to an i.m. injection of the dopaminergic D(2) agonist bromocriptine or antagonist sulpiride. The rapid increase in FSH concentration occurred despite a markedly increased serum inhibin A concentration in thyroidectomized rams. Therefore, the efficacy of inhibin feedback was examined by testing the FSH-suppressive effect of an inhibin preparation (5 ml charcoal-stripped bovine follicular fluid i.v.) in long-term thyroidectomized and thyroid intact castrated rams. Bovine follicular fluid suppressed FSH concentrations in control rams as expected but in marked contrast, was completely without effect in thyroidectomized animals. In castrated rams, the FSH concentration was only marginally increased by thyroidectomy, indicating that there is a major component of the mediation of the effects of thyroidectomy that is testicular in origin. It was concluded that a reduction in the ability of endogenous inhibin to inhibit FSH release at the pituitary, rather than a hypothalamic mechanism, is the primary cause of the stimulation of testis growth by thyroidectomy.

Function and localization of oxytocin receptors in the reproductive tissue of rams.

Oxytocin is present in the male reproductive tract and has been shown to increase contractility in the epididymis and to modulate steroidogenesis. This study investigated the effects of oxytocin in the testis in vivo, and the presence and cellular localization of oxytocin receptors in the reproductive tract of rams. During the breeding season, mature rams underwent efferent duct ligation before injection of either oxytocin (20 microg) or oxytocin plus an oxytocin antagonist (20 microg) into the testicular artery; the contralateral testicular artery received saline. Injection of oxytocin caused a significant increase (P < 0.05) in the concentration of spermatozoa collected from the rete testis. This effect was not observed after treatment with the oxytocin antagonist plus oxytocin. Western blot analysis performed using a specific oxytocin receptor antibody (020) identified a single immunoreactive band of 66 kDa in testicular and epididymal tissue. This band was present in uterine tissue but not in liver or muscle. Immunocytochemistry identified oxytocin receptors on Leydig and Sertoli cells of the testis, on epithelial cells throughout the epididymis, on peritubular smooth muscle cells in the cauda epididymidis, and on the epithelial cells and circular smooth muscle layer of the ductus deferens. These findings indicate that oxytocin can modulate sperm transport in the ram testis. A role for oxytocin in promoting sperm transit is supported by the localization of oxytocin receptors in the cauda epididymis and ductus deferens, and the presence of receptors on Leydig, Sertoli and epididymal epithelial cells provides further evidence that oxytocin may be involved in the local regulation of steroidogenesis.

Role of steroids in antler growth of red deer stags.

A series of six studies were carried out in red deer stags to test hypotheses concerning the importance of steroid control of velvet antler growth and to investigate mechanisms by which these hormones exert their effects. Medroxyprogesterone acetate (MPA) an LH inhibitor administered to stags during hard antler caused premature antler casting, reduced subsequent antler weight and caused a reduction in the LH and testosterone responses to GnRH. In two separate studies blockade of testosterone receptors with cyproterone acetate (CPA) administered to stags, either during early velvet antler growth or during the hard antler stage, significantly reduced LH and testosterone responses to GnRH. In both studies antler length, but not weight, was increased by CPA treatment. In another study testosterone implants were used to prevent the gradual decline in plasma testosterone levels normally observed during winter. Implants were removed 3 weeks before the anticipated date of antler casting. The implants significantly increased plasma testosterone levels and subsequent antler growth (expressed as a proportional increase compared with the previous year) compared with untreated controls. To determine whether the annual cycle of plasma testosterone response following GnRH stimulation was due simply to a lack of LH stimulation, ovine LH was injected on six occasions at defined stages of the antler cycle to red deer stags and the testosterone response measured. The testosterone responses were low at antler casting and during velvet antler growth compared with antler cleaning and peak rut. It appears low testosterone levels are due, in part, to a loss of responsiveness by the testes to LH as well as a low level of secretion of LH during the antler growing season. Finally synthetic ACTH was injected at the same defined stages of antler growth as in the previous study to determine whether cortisol and adrenal androgen production altered with the stage of the antler cycle. No significant differences were found in the dehydroepiandrosterone (DHEA) response, but cortisol responses were higher from late velvet antler growth to peak rut, compared with the times of antler casting and early velvet growth. Overall it was concluded that velvet antler growth can occur without testosterone stimulation during the period of velvet growth, but the data reinforce the concept that the timing of antler growth is linked to the annual cycle of testosterone.

Effects of oestradiol benzoate treatment on the reproductive performance and endocrine status of sows after lactations of 10 or 35 days.

Forty-two Landrace X Large White sows were used in 3 studies of oestradiol benzoate treatment 24 h (E1) or 48 h (E2) after piglet removal, following lactations of 10 (S) or 35 (L) days, or were untreated controls (C). For Exp. 1, oestradiol benzoate doses were 4, 16 and 64 micrograms/kg body weight; since highest mean LH peaks followed use of the 16 micrograms/kg dose, this treatment was used in later studies. Treatments for Exp. 2 were S-E1 and L-E1 (N = 3). Pregnancy rates were 1/3 and 2/3, respectively, while 10-day weaned sows tended to have longer weaning-remating intervals, shorter oestrous periods and lower post-injection oestradiol peaks. Post-treatment LH peaks were recorded from all L-E1 sows, but only one S-E1 animal. Treatments S-E2, L-E2, S-C and L-C (N = 6) were used in Exp. 3; pregnancy rates were 1/6, 6/6, 5/6 and 6/6, respectively, while peak oestradiol levels were 49.5 +/- 8.3, 74.8 +/- 15.1, 21.7 +/- 2.0 and 51.8 +/- 26.5 pg/ml, respectively. Weaning-remating intervals were extended in S-C sows (P less than 0.05), while oestradiol treatment prolonged behavioural oestrus. LH peak values were reduced after short lactations, but not affected by oestradiol treatment. Plasma prolactin concentrations at weaning were higher in sows in treatment S than in treatment L, while at the subsequent oestrus in oestradiol-treated sows they were elevated more and for longer periods than in controls. These results demonstrate that reduced gonadotrophin secretion and fertility after short lactations are not overcome by treatment with oestradiol benzoate.

Plasma concentrations of LH, prolactin, oestradiol-17 beta and progesterone in sows weaned after lactation for 10 or 35 days.

Piglets were weaned from multiparous sows at 10 days (Group E; N = 7) or 35 days (Group C; N = 8) of lactation. Blood samples were collected at 8 h intervals from the day before weaning (Day--1) until and including the day of first mating, then once daily until 10 days post coitum. Additionally, 5 sows in each group were sampled at 30 min intervals for 12 h on Days--1, 0 and 1, then at 30 min intervals for 6 h daily until mating, finally at 30 min intervals for 2 h daily until 5 days post coitum. Group E sows had relatively longer weaning to remating intervals (8.3 +/- 0.8 compared with 5.0 +/- 0.7 days; P less than 0.01) and tended, but not significantly, to produce smaller subsequent litters (10.2 +/- 1.9 compared with 12.0 +/- 0.6). Sows in group E had lower lactational and post-weaning plasma LH levels (P less than 0.001). They also had greatly attenuated preovulatory LH rises and the area under the peak was reduced (P less than 0.01 and P less than 0.001, respectively). Preweaning plasma prolactin levels were higher at 9 days of lactation than at 34 days and levels in both groups dropped precipitously subsequent to piglet removal. Although peak levels of prolactin at oestrus did not differ between treatments, they tended to occur before the LH peak in sows of Group C and after the LH peak in Group E.(ABSTRACT TRUNCATED AT 250 WORDS)

LH and testosterone responses to GnRH in red deer (Cervus elaphus) stags kept in a manipulated photoperiod.

Six red deer stags from age 4 months were kept in a light-proof room under an artificial photoperiod consisting of 5.5 cycles of alternate 2-month periods of 16 h light and 8 h dark (16L:8D) and 8L:16D. At 2 or 3 weekly intervals from 10 months of age through 4 cycles, the stags were anaesthetized with xylazine and challenged i.v. with 10 micrograms GnRH. Blood samples were withdrawn immediately before and 10 and 60 min after injection. LH and testosterone concentrations were measured in all samples by RIA. Antler status was recorded daily. Peak LH values on each sampling day occurred in the sample taken 10 min after GnRH stimulation while peak testosterone occurred in the sample taken at 60 min. There were 4 cycles of LH and testosterone secretion accompanied by 4 antler cycles in the stags. The highest LH responses were detected during short days (8L:16D), and the highest testosterone responses were detected around the time of the change from short to long days. The responses of both hormones were lowest at the end of periods of long days or the beginning of short days. The increased pituitary LH response to GnRH was evident 4 weeks after the change to short days which are stimulatory for gonadal development. Antler casting occurred at the end of long days and cleaning at the end of short days. It is considered that antler cycles were due to the ability of the stags to vary release of LH and testosterone in response to changes in the artificial photoperiod.

The myocardial effects of fenoterol, isoprenaline and salbutamol in normoxic and hypoxic sheep.

Three groups, each of four sheep, were randomly allocated under blinded conditions to receive the beta-adrenoceptor agonists, salbutamol, fenoterol or isoprenaline, in doses of 0.5, 2, 8, 32 and 128 micrograms/kg intravenously at 15-minute intervals. A separate group of four animals received equal volumes of saline. Heart rate was recorded immediately prior to each drug administration and serum potassium was measured. Three to 4 days later the experiment was repeated during induced systemic hypoxia and the animals then necropsied. All the agonists produced significant increases in heart rate. During hypoxia, lower heart rates were recorded than in the normoxic experiments. Under conditions of hypoxia, all the beta-agonists produced significant hypokalaemia. Sterile saline had no effect on either heart rate or serum potassium levels. At necropsy, myocardial lesions were found in animals receiving all three beta-agonists. Subendocardial haemorrhage was consistently seen in all animals receiving fenoterol. Multifocal myonecrosis of 3-4 days duration was present in the left ventricle of all animals receiving fenoterol, in two receiving isoprenaline and in two receiving salbutamol. The lesions were most severe in the papillary muscle and were visible grossly in one animal given fenoterol and one given isoprenaline. No lesions were found in the control animals.

Luteinizing hormone release in the anaesthetised cat following electrical stimulation of limbic structures.

The release of luteinizing hormone (LH) in response to electrical stimulation of limbic centres, namely the medial preoptic region (MPO) medial basal hypothalamus (MBH) and the medial amygdala (AME) has been studied in the anaesthetised gonadectomized cat. Chronically gonadectomized cats were anaesthetised with pentobarbitone or Althesin and paired bipolar stimulating electrodes were aimed at the MPO, AME or MBH. The effect of electrical stimulation of these regions on the secretion of LH was studied by radio-immunoassay of LH in serial blood samples taken before, during and after stimulation. No change in plasma LH in response to electrical stimulation was ever recorded during pentobarbitone anaesthesia. During Althesin anaesthesia stimulation in the MPO more often than not resulted in a peak of LH release during stimulation. A peak release of LH during stimulation was also recorded when electrodes were placed in the arcuate-median eminence region of the MBH. The time-course of these peaks in LH secretion was similar to the time-course of the plasma LH responses recorded following a single intravenous injection of gonadotrophin-releasing hormone (GnRH). In contrast, electrodes placed in AME had no effect on plasma LH during electrical stimulation, but immediately after stopping it, a small LH peak was recorded. The time-course of these responses suggests a pulse release of GnRH, the rapid response to MPO and MBH stimulation possibly being the result of a direct action on GnRH neurons while the delayed AME response may be produced by AME projections to the GnRH release system. These responses could be likened to the surge of LH which in the cat occurs post-coitus.