The effects of nerve growth factor and anti-nerve growth factor antibody on the neuroendocrine reproductive system in the European starling Sturnus vulgaris.

Thyroidectomy of starlings causes them to remain in breeding condition indefinitely; deactivation of gonadotrophin-releasing hormone neurons that is characteristic of photorefractoriness does not occur. We hypothesise that a neurotrophin, whose presence or ability to function is dependent upon thyroid hormones, is somehow involved in this termination of gonadotrophin-releasing hormone release. Nerve growth factor is one such candidate. Mouse 7S-nerve growth factor dissolved in artificial cerebro-spinal fluid was therefore infused into the lateral ventricle of thyroidectomised male starlings held on long days four times daily for 21 days and 31 days, in separate experiments, to see if photorefractoriness would occur. The result was significant gonadal regression in the treatment groups during the infusion period, with no change in testicular volume in the control groups. Testicular recrudescence occurred after the end of the treatment period. To see if this was a non-specific effect, or progression towards photorefractoriness per se, we used castrated, photorefractory starlings held on long days. Anti-nerve growth factor antibody was infused into the lateral ventricle at increasing concentrations and frequency. There was a significant rise in circulating luteinising hormone levels in the treatment groups as compared to controls, increasing with antibody dosage.

Does unusual entrainment of the circadian system under T36h photocycles reduce the critical daylength for photoperiodic induction in Japanese quail?

In photoperiodic species, short daylength resonance cycles of modulo t + 1/2 (t = 24 h) behave like long days because they entrain the circadian system so that alternate light pulses coincide with the photoinducible phase (Oi) in castrated quail. However, while a long-day response after exposure to a single long daylength is readily detected by a rise in plasma LH (photoinduction), long-term exposure to LD 6:30 is ineffective in this respect. To discover whether this occurs because of unusual entrainment, circadian rhythms in quail and starlings were investigated. Whereas starlings entrained in the expected way with alternate pulses falling at different circadian phases, activity bouts in quail appeared to follow 24 h after successive light pulses. Because of this, activity was examined in free-running conditions to confirm that the pacemaker in quail was indeed being reset to a constant phase (reset to circadian time [CT] 0) by successive pulses. Examination of the circadian rhythms of plasma melatonin secretion under LD 6:30 also showed a resetting to CT 0. The positioning of all light pulses at the same circadian phase in the early subjective day explains the lack of photoinduction in quail since Oi in the early subjective night phase remains unilluminated. A second feature in quail is that when the length of the photophase is gradually increased within T36h cycles, there is a progressive increase in the degree of photoinduction although the photophase length remains well below the critical daylength for induction in normal T24h cycles. We therefore tested whether Oi is reset to a constant phase by successive pulses in LD 6:30, and that this phase is also advanced relative to light onset so that photophases shorter than the critical daylength can interact with Oi to cause induction. Such a reduction in critical daylength relative to successive LD 6:30 pulses was confirmed by transferring quail to various types of long day and measuring the change in LH secretion. When the long-day test was replaced with continuous light, stimulation of LH secretion occurred 5-7 h earlier in quail pretreated with LD 6:30 and LD 6:54 compared to quail pretreated with LD 6:18 or LD 6:42, implying that Oi had been markedly phase advanced under resonance cycle.

Suppression of suprachiasmatic nucleus neurone activity with a vasopressin receptor antagonist: possible role for endogenous vasopressin in circadian activity cycles in vitro.

Neurones of the rat suprachiasmatic nucleus (SCN) were tested with [Arg8]vasopressin (AVP) and the AVP receptor antagonist, [d(CH2)5,d-Tyr(OEt)2,Val4,Cit8]-vasopressin in vitro. 52% of AVP-responsive neurones showed an antagonist-induced decrease in activity, indicative of the presence of an endogenous excitatory tone. The magnitude of this effect declined significantly between subjective light and dark phases, consistent with the possibility that circadian fluctuations in endogenous AVP excitation contribute to the cycle of electrical activity within the SCN. However, similar fluctuations in basal activity between the light and dark phases was observed for both antagonist-sensitive and -insensitive neurones, indicating that endogenous AVP was not the only factor determining the circadian cycle.

Melatonin and photoperiodic time measurement in Japanese quail (Coturnix coturnix japonica).

Artificial extension of the duration of nocturnally secreted circulating melatonin with exogenous injections produces a short day effect in the reproductive status of mammals, and this paradigm has been applied to Japanese quail to test the hypothesis that birds are similar to mammals in this respect. Male quail reared on non-stimulatory short days (8L:16D) were switched to mildly stimulatory 12L:12D and given daily melatonin injections at dusk (10 micrograms 2 h before dusk and 10 micrograms at dusk) or at dawn (10 micrograms 2 h before dawn and 10 micrograms at dawn) for about 3 weeks. Although assay of circulating melatonin suggested that injections had extended the melatonin signal, there was no short day effect, i.e. reproductive stimulation was not prevented. This reinforces the view that, unlike mammals, birds do not read the duration of the melatonin signal to measure scotoperiod. Paradoxically, however, the injections resulted in a small but significant stimulation. The results are discussed in view of the postulated role for melatonin as an internal Zeitgeber, which is coupled to the external photic Zeitgeber, to regulate the circadian system.

Circadian nature of the photoperiodic clock in Japanese quail.

The photoperiodic clock in quail (Coturnix coturnix japonica) is based upon a rhythm of photoinducibility (phi i) but the extent to which this rhythm is circadian remains unclear. Two types of experiment investigated this situation. In the first, gonadectomized quail were adapted to live in periods of darkness by training them on a schedule containing one short day and 3 days of darkness (SD/DD/DD/DD). They were then exposed to a single pulse of 6 or 10 h of light at different times across 3 days of darkness. The photoperiodic response, measured by the increase in LH secretion, showed clear rhythmicity, demonstrating unequivocally the circadian nature of phi i. The second set of experiments employed Nanda-Hamner cycles and varied the length of the photoperiod from 6 to 11 h. Responsiveness in a 36 h or a 60 h cycle was highly dependent upon the length of the photoperiod, something not predicted from theory. For instance, LD 6:30 was not photoperiodically inductive but LD 10:26 was clearly inductive. Close analysis of patterns of LH secretion indicated an unexpected delay before induction occurred and then a rapid rise to a stable level of induction. When LH was measured in every pulse under LD 10:26 there was no evidence that LH levels alternately increased and decreased. This is not consistent with the simplest interpretation of Nanda-Hamner experiments where alternate pulses of light are thought to "entrain" the rhythm or "induce" a photoperiodic response by coinciding with phi i. It is concluded that the quail's photoinducible rhythm is indeed based on a circadian rhythm but one that is only weakly self-sustaining.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of photoperiod on the hypothalamic content of Beta-endorphin and the luteinizing hormone responses to naloxone and to steroid withdrawal in the male Syrian hamster.

Abstract The aim of this study was to investigate the influence of inhibitory photoperiods upon opioidergic function, as determined by changes in the hypothalamic content of beta-endorphin and the luteinizing hormone response to opioidergic receptor blockade, in the male Syrian hamster over the course of gonadal involution and spontaneous gonadal recrudescence. Animals exposed to an 8 h light: 16 h dark cycle (8L: 16D) for 14 weeks underwent gonadal regression. Regression was also observed in animals held for 7 weeks on one of a range of short daylengths of between 11.5 h and 13.5 h, the degree of atrophy being greatest in those animals on the shortest daylength. The tissue concentration of beta-endorphin within the mediobasal hypothalamus was significantly higher in animals exposed to 8L: 16D for 14 weeks than in gonadally active controls held on long days (16L: 8D). Exposure to photoperiods of less than 13.5 h for 7 weeks also caused a significant increase in the beta-endorphin content of the mediobasal hypothalamus and there was a positive correlation between the concentration of beta-endorphin, the degree of gonadal atrophy and the shortness of the photoperiod. Endorphin levels within the preoptic area were not affected by photoperiodic treatments. Exposure of intact animals to 8L: 16D for 12 weeks caused gonadal atrophy and an associated loss of the luteinizing hormone responses to both naloxone and castration. Castrated animals receiving testosterone replacement (cast + T) also exhibited photoinhibition, in the form of reduced serum levels of luteinizing hormone, and this was similarly accompanied by a loss of sensitivity to naloxone and to withdrawal of steroid. Prolonged exposure to 8L:16D led to spontaneous reactivation of the gonadotrophic axis as a consequence of the development of scotorefractoriness. In both gondally intact animals and in cast + T groups, this was associated with a restoration, in parallel, of the luteinizing hormone responses to naloxone and to castration/ steroid withdrawal. The time-course of the restoration of the response to steroid withdrawal in castrates was not significantly different to that observed in intact animals. The luteinizing hormone response to naloxone took significantly longer to redevelop in cast + T groups than it did in gonadally intact animals. The data demonstrate that central opioid systems are sensitive to photoperiod and are consistent with the hypothesis that opioids are involved in the neuroendocrine regulation of reproductive responses to daylength.

Role of the paraventricular nucleus in controlling the frequency of milk ejection and the facilitatory effect of centrally administered oxytocin in the suckled rat.

The milk-ejection reflex was studied in anaesthetized, lactating Wistar rats in order to evaluate the contribution of the paraventricular nucleus (PVN) to the patterning of milk ejection and the facilitatory action of centrally administered oxytocin. In the first series of experiments, radiofrequency lesions were performed and centred: (1) antero-dorsal to the PVN, damaging parts of the medial septum and anterior hypothalamus; (2) in the PVN, such that much of the parvocellular division was destroyed, but parts of the magnocellular division remained intact; or (3) in the PVN, destroying both parvocellular and magnocellular divisions. Suckling tests performed before and after lesioning showed that the milk-ejection interval was significantly increased (decreased frequency) after lesioning in groups 2 and 3, but that milk-ejection amplitude was significantly decreased only in group 3. These results suggest that damage to the parvocellular division of the PVN affects milk-ejection frequency, but that damage to the magnocellular PVN only affects amplitude. Subsequent tests on rats injected into the PVN with the neurotoxin N-methyl-D,L-aspartate revealed a fall in the amplitude and frequency of milk ejection, similar to that after complete radiofrequency lesions of the PVN. In the second series of experiments, the facilitatory action of centrally administered oxytocin (1 mU, 2.2 ng) was examined in animals bearing either sham or complete PVN lesions. In both groups, intracerebroventricular injection of oxytocin was able to increase the frequency of milk ejections, although the incidence of milk ejection was lower in the pre- and post-injection period in the PVN-lesioned animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The hypothalamus and photoperiodic control of FSH secretion by melatonin in the male Syrian hamster.

In the photoinhibited castrated male Syrian hamster, removal of the pineal gland or transfer to long photoperiods was followed by a rapid increase in the serum concentration of FSH. Levels were significantly above those of controls within 10 days. Central passive immunization of pineal-intact photoinhibited castrated animals against melatonin had a stimulatory effect on serum FSH levels, comparable with that observed following pinealectomy or transfer to short days. The effects of pinealectomy were blocked by programmed s.c. infusions of melatonin in a time-dependent manner. Serum FSH levels remained low in animals receiving 100 ng melatonin delivered over 10 h but the same mass of melatonin delivered over 4 h had no effect on the response to pinealectomy. Lesions of the anterior hypothalamus had no effect on the pinealectomy-induced increase of serum FSH in animals receiving saline infusions. However, in lesioned animals, programmed infusions of melatonin were no longer able to suppress the rise in FSH following pinealectomy. These results demonstrated that pineal melatonin is the mediator of central photoperiodic control of FSH secretion. The duration of the melatonin signal determines its effectiveness and an intact anterior hypothalamus is necessary for the signal to be read and/or the appropriate neuroendocrine response expressed.

Prolonged electrical stimulation of the nipples evokes intermittent milk ejection in the anaesthetised lactating rat.

Intermittent and continuous electrical stimulation of the nipples elicited milk-ejection responses in the lactating rat. The responses occurred intermittently, with similar amplitudes and periodicity as those seen in suckled rats. The responses were always associated with synchronization of the electroencephalogram (EEG), but some rats with synchronized EEG activity did not milk eject during stimulation. Since continuous stimulation also resulted in intermittent milk ejection it seems unlikely that the periodicity of the reflex in suckled rats depends upon changes in the intensity of sensory stimulation. The techniques of nipple stimulation may be a useful method with which to study neural pathways and other phenomena such as gating involved in oxytocin release and milk ejection. The success of the technique depends on a variety of factors such as parameters of the stimulation and state of anaesthesia.