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

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 β-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 β-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 β-endorphin content of the mediobasal hypothalamus and there was a positive correlation between the concentration of β-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.     

The Photoperiodic Response in Syrian Hamster Depends upon a Melatonin-Driven Circadian Rhythm of Sensitivity to Melatonin

The pineal gland, via the daily pattern of melatonin (MEL) secretion, is directly involved in the conduction of photoperiodic information. The duration of MEL secretion is proportional to the duration of the dark period and, whatever the photoperiod is, MEL synthesis occurs 3 or 4 h after the dark onset in Syrian hamsters. In order to determine the relative importance of the duration or the coincidence hypothesis, a daily infusion protocol was used in sexually active pinealectomized hamsters. Long duration of MEL infusion (10 h) completely inhibit testes whereas short duration infusion (5 h) had no effect. When the animals were infused twice within 2 h 30 min separated by 3 h, they presented a complete gonadal atrophy, similar to the one observed with the 10 h infusion. Measurement of plasma MEL during the infusion and seperation periods revealed that MEL reached physiological nighttime values during the infusion period and fell to daytime values 1 h after the end of an infusion period. Thus, the results could not be due to a time additive action of the two MEL pulses. An intermediate response was observed when the 2 signals were applied across the light/dark transition. Gonadal regression did not occur when the 2 periods of infusion were separated by 5 h 30 min. The efficiency of this type of infusion was not dependent on the ambiant photoperiod since similar results were obtained in long and short photoperiods. The infusion was also as effective during the day as well as during the night. These results suggest that there is a rhythm of sensitivity to MEL, based on the coincidence hypotheses, that are important for transmission of photoperiodic information. This rhythm of sensitivity to MEL seems to be entrained by MEL itself, since the efficiency of the two pulses of MEL is not dependent of time of application and/or of photoperiod.     

Occlusion of the Melatonin-Free Interval Blocks the Short Day Gonadal Response of the Male Syrian Hamster to Programmed Melatonin Infusions of Necessary Duration and Amplitude

Photoperiodic control of the neuroendocrine axis is mediated by changes in the duration of the nocturnal melatonin signal. This study tested the hypothesis that reading of the signal depends upon the presence of a period free of melatonin between successive signals. Adult male Syrian hamsters were pinealectomized and received chronic subcutaneous infusions of melatonin or saline for 6 weeks. Animals which received saline had large testes. Those which received a single daily infusion which lasted for 10 h (50 ng/h) followed by 14 h without infusion underwent gonadal atrophy. Other animals received a compound melatonin signal in which the melatonin-free interval was occluded by a continuous infusion (25 ng/h). Superimposed upon this was a 10 h phasic increase in infusion rate such that the maximum rate of infusion was equivalent to that observed in controls (25 ng/h increase, 50 ng/h peak rate), or the increase in rate over the baseline was the same as in controls (50 ng/h increase, 75 ng/h peak rate). In neither group did the animals undergo gonadal regression. Analysis of iodomelatonin binding sites by in vitro autoradiography failed to reveal any systematic difference between animals which did and did not respond to melatonin and so the absence of a response could not be attributed to loss of receptors. These data demonstrate that the photoperiodic system cannot identify the melatonin signal solely upon the features of nocturnal peak height or amplitude of the peak over baseline. They are consistent with the hypothesis that the melatonin-free interval plays a significant role in photoperiodic time measurement.     

The Role of N-Methyl-D-Aspartate-Type Glutamatergic Neurotransmission in the Photic Induction of Immediate-Early Gene Expression in the Suprachiasmatic Nuclei of the Syrian Hamster

This study investigated the role of N-methyl-D-aspartate (NMDA)-type glutamatergic neurotransmission in mediating the photic induction of immediate-early gene expression in the Suprachiasmatic nucleus (SCN) of the Syrian hamster. Activation of c-fos, c-jun and egr-1 was assessed by immunocytochemical detection of their protein products. To characterize the circadian basis to the inductive effects of light, hamsters were allowed to free-run in constant dim red light and received a 1 h light pulse at different circadian phases relative to activity onset (defined as CT 12). In control animals which did not receive light pulses, c-fos and egr-1 expression was absent or restricted to a small area of the dorsolateral region of the SCN, and expression of c-jun could not be detected in the SCN. In hamsters killed after presentation of a light pulse at either CT 14 or CT 20, there was a marked increase in c-fos and egr-1 immunoreactivities throughout the ventrolateral division of the SCN. In contrast, light pulses given at CT4 or CT 8 failed to activate immediate-early gene expression. Light pulses did not induce c-jun immunoreactivity at any circadian phase tested. Staining for c-fos was maximal 1 h after the start of the light pulse and had started to decline by 2 h. At this later time, c-jun expression was still undetectable. To compare the distribution of retinal afferents with that of c-fos induction, hamsters held on a light schedule of 16 h light: 8 h dark received an intraocular injection of cholera toxin-horseradish peroxidase conjugate 3 days before exposure to a 1 h light pulse given 2 h after lights off. Comparison of adjacent sections processed for c-fos immunoreactivity or for cholera toxin-horseradish peroxidase revealed that light-induced c-fos expression was precisely restricted to retinal terminal fields in the SCN. Light pulses also induced c-fos expression in the retinoreceptive ventral lateral geniculate nucleus and intergeniculate leaflet but not in the retinal fields of the dorsal lateral geniculate nucleus, indicating that the expression of cfos in response to light is spatially specific. The aim of the subsequent experiments was to investigate the role of NMDA-type glutamatergic neurotransmission in mediating the effects of light on c-fos expression in the SCN. To determine whether NMDA had the potential to activate c-fos expression in the SCN, hamsters were infused with 2.5 nmol NMDA or vehicle via an intracerebroventricular (icv) cannula positioned adjacent to the nuclei. In contrast to the effects of light, icv NMDA activated c-fos expression at both CT8 and CT 14. The distribution of immunoreactivity was more widespread than that observed after light, extending throughout the SCN and adjacent hypothalamus. To test whether NMDA receptors had a physiological role in the photic response, hamsters were treated systemically with the non-competitive NMDA antagonist MK801 (dose range 0.6 to 6.0 mg/kg body wt, ip) or vehicle prior to exposure to a 1 h light pulse given at CT 14 or CT 20. Expression of c-fos was still detectable in the dorsolateral SCN but MK801 blocked expression in the ventral portion of the retinoreceptive zone of the SCN. MK801 (10 or 100 nmol) delivered centrally (icv) also prevented light-induced c-fos expression in the ventral region of the SCN bordering the optic chiasm, though staining again persisted in the dorsolateral region. The induction of c-fos by icv NMDA, and the partial blockade of light-induced c-fos expression by the antagonist MK801, are consistent with the hypothesis that glutamate mediates the effects of light on SCN activity. However, the persistent photic induction of c-fos expression in a subfield of retinal afferents following treatment with MK801 suggests that other, non-NMDA-type mechanisms may contribute to photic entrainment.     

Effects of N-Methyl-D-Aspartate (NMDA) on Seasonal Cycles of Reproduction, Body Weight and Pelage Colour in the Male Siberian Hamster

Siberian hamsters (Phodopus sungorus) transferred from stimulatory photoperiods (long days: LD) to inhibitory photoperiods (short days: SD) undergo testicular regression within 8 weeks. This reproductive response to photoperiod was blocked by systemic daily treatment with the glutamatergic agonist N-methyl-D-aspartate (NMDA: 20 mg/kg BW, sc). This powerful effect of NMDA demonstrates the potential for endogenous glutamate to regulate reproductive function. The overall aim of the subsequent studies was to investigate the site and mechanism of action of this glutamatergic agonist in order to identify potential mechanisms through which endogenous glutamate might act. To investigate whether the effect of systemic NMDA was via an effect on the circadian timing system, alterations in gonadal regression and recrudescence, seasonal coat changes (pelage) and body weight (BW) were examined. It would be predicted that long-term cycles of all these seasonal parameters would be affected if the action of NMDA were to perturb the transduction of photoperiodic information. Daily treatments with NMDA, which initially maintained reproductive function in hamsters exposed to SD, did not influence the time course of subsequent testicular recrudescence, nor did they influence long-term cycles of pelage and BW. Moreover, treatment with NMDA induced a dose-dependent increase in serum concentrations of LH within 15 min of systemic injection. These data are consistent with the hypothesis that systemic NMDA exerts it reproductive effects not via an action on the circadian system, but via an action on secretion of GnRH. To investigate potential central sites of action of glutamate, induction of the immediate early gene c-fos, an acute marker of cellular response, was evaluated immunocytochemically (ICC) in brain areas after treatment with NMDA. Although dual-label ICC studies revealed that NMDA did not induce c-fos within GnRH neurons, NMDA did induce c-fos in many cells in the region of the organum vasculosum of the lamina terminalis (OVLT), an area containing a large number of GnRH perikarya, and in the arcuate nucleus, a region close to GnRH secretory terminals in the median eminence. The lack of c-fos induction in GnRH cells argues against a direct effect of NMDA on GnRH neurons. Thus, we examined immunocytochemically the distribution of the common NMDAR1 glutamate receptor subunit to evaluate further the potential sites of glutamatergic action. As expected, NMDAR1-ir was widespread in perikarya throughout the brain, including the region of the OVLT and the arcuate nucleus. However, NMDAR1-ir was also observed in cell processes in hypothalamic areas, including the median eminence, demonstrating the potential for actions of glutamate on neurosecretion in this structure. Collectively, these pharmacological, endocrine and neuroanatomical studies suggest that NMDA acts to release GnRH when delivered systemically, though not necessarily via a direct action on GnRH neurons. These findings are consistent with the view that glutamate is an important regulator of seasonal changes in reproductive function.     

Blockade of Glutamatergic Neurotransmission in the Suprachiasmatic Nucleus Prevents Cellular and Behavioural Responses of the Circadian System to Light

The aim of this study was to test the role of glutamatergic neurotransmission in photic entrainment of the circadian oscillator of the suprachiasmatic nuclei (SCN) in the Syrian hamster. The response of the oscillator to a brief pulse of light was assessed using two independent indices, the phase shift of the free-running activity rhythm, and the photically induced expression of the immediate-early gene c-fos within neurons of the SCN. The behavioural and the cellular responses to light were compared in animals which received intracerebroventricular (icv) infusions into the region of the SCN of either a vehicle solution or a solution of gammad-glutamyl-glycine (DGG), a competitive antagonist at both N-methyl-d-aspartate (NMDA) and non-NMDA types of glutamate receptor. Infusions of vehicle or DGG (200 nmol) were given 10 min before presentation of a 15-min light pulse at either circadian time (CT) 14 or CT20 (onset of activity defined as CT12). As anticipated, animals treated with vehicle and light at CT14 exhibited phase delays in the activity rhythm, whereas animals treated at CT20 exhibited phase advances. Central infusion of DGG prior to a light pulse at CT14 blocked the phase-delaying effect of light, and DGG delivered before a light pulse at CT20 markedly attenuated the phase-advancing effect of light. In a separate group of animals, the expression of the immediate-early gene c-fos was assessed by immunocytochemical staining for its protein product Fos. Exposure of vehicle-infused animals to light at CT14 caused extensive expression of c-fos throughout the retinorecipient region of the SCN. However, when the light pulse was preceded by icv fusion of DGG at a dose which would block the phase-shifting response to light, the total number of neurons immunopositive for Fos was significantly reduced ( approximately 50%) and the expression was confined to a restricted area of the dorsolateral SCN. The precise correlation between the effects of glutamatergic blockade upon both the behavioural and the cellular responses of the circadian system to light demonstrates that effective glutamatergic neurotransmission within or adjacent to the SCN is a necessary component of the mechanism which mediates photic entrainment of the circadian clock. The results further demonstrate a pharmacological and anatomical compartmentalization of the retinorecipient zone of the SCN, consistent with the view that retinal afferents to the ventral region employ glutamate as a transmitter, whereas more dorsal input may be dependent upon non-glutamatergic (DGG-insensitive) pathways.     

Photoperiod and androgens act independently to induce spinal nucleus of the bulbocavernosus neuromuscular plasticity in the Siberian hamster,Phodopus sungorus

In the Siberian hamster, Phodopus sungorus, short-day photoperiods induce the winter phenotype, which in males includes a decrease in the production of androgens and changes in physiology to inhibit reproduction. Motoneurones of the spinal nucleus of the bulbocavernosus (SNB) and their target muscles, the bulbocavernosus and the levator ani, a neuromuscular system involved in male copulation, also display seasonal plasticity in P. sungorus. It is not known whether the plasticity seen in the SNB system of gonadally intact hamsters is due to the effects of photoperiod per se, or to the photoperiod-induced changes in androgen production. To answer this question, we castrated adult male hamsters from long days and then implanted them with capsules containing either testosterone or blanks. Half of the hamsters from each hormone condition were moved into short photoperiod (8 : 16 h light/dark cycle) while the rest were maintained under long-day conditions (15 : 9 h light/dark cycle). After 15 weeks, many measures of the SNB system, such as somata size and weight of target muscles, responded only to androgen, not to photoperiod. However, there were effects of photoperiod on the neuromuscular junctions (NMJs) that were independent of androgen status. For example, the number of synaptic zones per NMJ and the area of the NMJs were significantly increased by short days and/or testosterone treatment. The two factors exerted an additive, rather than an interactive, effect on these measures. Another striated muscle, the extensor digitorum longus, which is present in both sexes and plays no specialized role in reproduction, displayed neither an effect of androgen nor of photoperiod on fibre size or NMJ structure. These results suggest that, in addition to androgenic effects on SNB plasticity, there is also an androgen-independent effect of photoperiod on the SNB neuromuscular system.     

Glutamatergic synaptic transmission participates in generating the hippocampal EEG

The participation of ionotropic glutamatergic synapses in the generation of hippocampal electroencephalography (EEG) of behaving rats has not been systematically studied. In this study, field potentials in hippocampal CA1 were recorded following injection of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists, or vehicle control, either into the lateral ventricles or directly into the hippocampus or the medial septum. Intraventricular (i.c.v.) AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX, 5-10 microg) decreased the commissural evoked potential and the amplitude of the hippocampal EEG, including the theta rhythm. Theta frequency was decreased by 10 microg, but not 5 microg DNQX i.c.v. Unilateral intrahippocampal injection of DNQX (5 microg) only decreased the amplitude, but not the frequency, of the theta rhythm near the site of injection, without affecting theta amplitude or frequency at the opposite hippocampus. Other than theta, the large irregular activity (with a delta frequency peak at 1-2 Hz) and gamma EEG (30-100 Hz) were also decreased by i.c.v. and intrahippocampal injections of DNQX. Intrahippocampal injection of NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid (D-APV, 2.5 microg) decreased the amplitude of the theta rhythm and, less consistently, the gamma EEG. The frequency of the theta rhythm and the peak of the commissural evoked potential were not significantly affected by intrahippocampal D-APV injection. Medial septal injections of D-APV or D,L-APV (2.24 microg in 0.4 microl), but not DNQX (10 microg in 0.4 microl), decreased the amplitude of the hippocampal theta significantly, but theta frequency was not significantly affected. It is concluded that both NMDA and AMPA receptors in the hippocampus are involved in generating the amplitude of the hippocampal EEG of theta and gamma frequencies, while NMDA receptors in the medial septum are involved in controlling the amplitude of theta and gamma EEG in the hippocampus. Excitatory glutamatergic synaptic currents, activated by afferents from the entorhinal cortex and CA3, are suggested to participate in hippocampal EEG activities.     

Effects of Stimulation of the Superior Cervical Ganglia and Local Application of Noradrenaline on Electrical Activity of the Syrian Hamster Pineal Gland

The effects of electrical stimulation of either or both superior cervical ganglia and the effects of local application of L-noradrenaline on the spontaneous electrical activity of hamster pineal cells were evaluated. Extracellular recordings from pineals of anaesthetized hamsters revealed that the spontaneous electrical activity was mainly regular with interspike intervals attributed between 12 to 20 ms during the daytime and mainly irregular with interspike intervals of 1 to 250 ms during the night. Following stimulation of the superior cervical ganglia, either unilaterally or bilaterally, or local application of noradrenaline, the responses of these pineal cells fell into three major categories: A) non-responsive, B) excited, and C) inhibited. There was no relationship between the magnitude or form of response and the source of stimulus i.e. the right superior cervical ganglia or the left superior cervical ganglia. Almost all inhibited responses from electrical stimulation of the superior cervical ganglia could be correlated with inhibited responses from the local application of noradrenaline whereas excited responses could not. In general, these results suggest that the spontaneous electrical activity of some pineal cells is influenced by inputs from the superior cervical ganglia and that the inhibitory input is likely to be mediated through the release of noradrenaline. The excitatory input from the superior cervical ganglia is probably mediated by another neurotransmitter. The heterogeneity of responses suggests that different receptors or different cell types may be involved.     

GnRH mRNA Increases with Puberty in the Male Syrian Hamster Brain

Puberty in the male Syrian hamster (Mesocricetus auratus) is characterized by decreased responsiveness to testosterone mediated negative feedback, but the neural mechanism for this change remains elusive. We hypothesized that decreased inhibition of the gonadotropin-releasing hormone (GnRH) system results in increased neurosecretory activity, which includes an increase in GnRH gene expression. This study examined GnRH mRNA in male hamsters before and after puberty, and sought to determine if any increase in mRNA was specific to particular subpopulations of GnRH neurones. Brains were collected from 21-day-old prepubertal males (n = 5) and 56-day-old postpubertal males (n = 5). Alternate 10 microm coronal sections from fresh-frozen brains were collected throughout the septo-hypothalamic region, and 25% of those sections were processed for in-situ hybridization histochemistry using an 35S-riboprobe complementary to hamster GnRH. No differences were observed in the number of GnRH mRNA expressing cells in any region, but in the diagonal band of Broca (DBB)/organum vasculosum of the lamina terminalis (OVLT) there was a significant increase in labelling intensity (defined as area of the cell occupied by silver grains) in postpubertal males. A second analysis compared the frequency distributions of cells based on labelling intensity between prepubertal and postpubertal males. This analysis revealed significant differences between the two frequency distributions in all areas analysed (DBB/OVLT, medial septum (MS), and preoptic area (POA)). Furthermore, examining the distribution of cells in these regions revealed a shift to the right in the postpubertal population of cells, which indicated an increased number of GnRH neurones with greater labelling intensity. These data clearly demonstrate increased GnRH mRNA during puberty. Furthermore, they suggest that the previous observation of brain region specific pubertal decreases in GnRH-immunoreactivity only within the DBB/OVLT and MS but not the POA are not due to differential levels of GnRH gene expression, but could indicate increased release from these neurones during puberty.     

Photoperiodic regulation of prolactin gene expression in the Syrian hamster by a pars tuberalis-derived factor

Syrian hamsters exhibit a marked seasonal variation in prolactin secretion. The aim of this study was to analyse the nature of the photoperiodic regulation of prolactin gene expression, and to define the role of melatonin and the pars tuberalis of the anterior pituitary in this process. Pituitary prolactin gene expression, restricted to the pars distalis, was increased in hamsters maintained in long daylengths (16 h : 8 h, light : dark) compared to hamsters exposed to short daylengths (8 h : 16 h, light : dark) for 8-12 weeks. Analysis of single cells by in situ hybridization showed that photoperiod had no effect on the percentage of pars distalis cells expressing prolactin mRNA, but shifted the frequency distribution of prolactin mRNA expression per cell, such that in long photoperiods a greater proportion of cells were recruited to a higher expressing population. In vitro coculture of hamster pars tuberalis fragments increased prolactin promoter-driven luciferase activity in stably transfected GH3 cells in a dose- and duration-dependent manner. Conditioned medium from hamster and ovine pars tuberalis also activated the prolactin promoter. Furthermore, basal and forskolin-stimulated conditioned medium from hamster pars tuberalis increased prolactin mRNA expression in primary cultures of pars distalis cells. Melatonin attenuated the activity of pars tuberalis-conditioned medium but had no direct effect on either prolactin mRNA expression or secretion in pars distalis cell cultures. Finally, pars tuberalis fragments from long photoperiod hamsters stimulated prolactin gene promoter activity to a greater extent than those from short photoperiod hamsters. In conclusion, this study provides the first evidence in a seasonal mammal that the synthesis of prolactin depends on photoperiodic modulation of a pars tuberalis-derived factor. Our data support further the hypothesis that seasonal modulation of prolactin gene expression depends upon a melatonin-dependent paracrine action of the pars tuberalis on pars distalis lactotrophic cells.     

A Thalamic Contribution to Arousal-induced, Non-photic Entrainment of the Circadian Clock of the Syrian Hamster

It is well established that the circadian clock of the suprachiasmatic nuclei (SCN) is entrained by light. More recently, the potent effects of arousing, non-photic cues on the clock have been recognized. The neural mediators of non-photic entrainment are yet to be identified. To examine the contribution of the thalamic intergeniculate leaflet (IGL) and its NPY-immunopositive projection, the geniculo-hypothalamic tract to non-photic entrainment by arousal, male Syrian hamsters received lesions of the IGL (IGLX) which ablated NPY-immunoreactivity in the SCN. Their circadian responses to both photic and non-photic cues were then tested. Lesions resulted in a delay in the timing of activity onset following lights out, but had no effect on the behavioural or cellular circadian responses to phase-advancing light pulses presented at circadian time (CT) CT19 (where CT12 represents the time of activity onset). Injection with a benzodiazepine (chlordiazepoxide, 100 mg/kg) at CT6 suppressed wheel-running, increased general locomotion of intact controls and induced large phase advances of the circadian rhythm of wheel-running. Chlordiazepoxide also inhibited wheel-running in lesioned animals, but there was no significant increase in general locomotion and the lesioned animals did not phase advance. Serial arousal by injection of saline at intervals of 23.5 h for 6 days entrained the circadian rhythm of wheel-running of intact hamsters and was associated with an increase in general locomotor activity. Entrainment by serial arousal was abolished by IGLX. However, the lesioned animals did show a clear behavioural response to every presentation of the non-photic cue. These results show that the IGL is a necessary component of the neural pathways mediating both arousal- and benzodiazepine-induced non-photic entrainment.     

Glutamatergic abnormalities of the thalamus in schizophrenia: a systematic review

Summary. The thalamus, a key information processing centre in facilitating sensory discrimination and cognitive processes, has been implicated in schizophrenia due to the increasing evidence showing structural and functional thalamic abnormalities. Glutamatergic abnormalities, in particular, have been examined since glutamate is one of the main neurotransmitters found in the thalamus. We aimed to review the existing literature (1978 till 2007) on post-mortem and in vivo studies of the various components of glutamatergic neurotransmission as well as studies of the glutamate receptor genes within the thalamus in schizophrenia. The literature search was done using multiple databases including Scopus, Web of Science, EBSCO host, Pubmed and ScienceDirect. Keywords used were “glutamate”, “thalamus”, “schizophrenia”, “abnormalities”, and “glutamatergic”. Further searches were made using the bibliographies in the main journals and related papers were obtained. The extant data suggest that abnormalities of the glutamate receptors as well as other molecules involved in glutamatergic neurotransmission (including glutamate transporters and associated proteins, N-methyl D-aspartate (NMDA) receptor-associated intracellular signaling proteins, and glutamatergic enzymes) are found within the thalamus in schizophrenia. There is a pressing need for more rapid replication of findings from post mortem and genetic studies as well as the promotion of multi-component or multi-modality assessments of glutamatergic anomalies within the thalamus in order to allow a better appreciation of disruptions in these molecular networks in schizophrenia. These and future findings may represent potential novel targets for antipsychotic drugs to ameliorate the symptoms of schizophrenia. Correspondence: Kang Sim, Institute of Mental Health/Woodbridge Hospital, 10 Buangkok View, Singapore 539747, Singapore     

Annual Variations of in vitro GNRH Release by Hypothalamic Explants in Intact and Castrated Male Mink: Relations with LH, FSH and Testosterone Circulating Serum Levels

In mink, a short-day breeder, testis growth begins in autumn (November), reaches a maximum in February, before matings occur, and decreases from March to very low volumes during spring and summer. To study the effects of season and testosterone feedback on gonadotrophin and GnRH secretion, the annual variations of LH, FSH, testosterone and GnRH were studied in intact and castrated mink. As portal blood sampling raised serious difficulties, an in vitro static incubation system was used for studying GnRH variations. In intact mink, serum LH concentrations did not vary significantly throughout the year, whereas FSH concentrations increased significantly between September and November then decreased to a minimum in January. Testosterone values rose significantly from November to a maximum from January to March, decreased very rapidly thereafter. Castration in November resulted in a significant increase in LH and FSH concentrations which remained higher than the values measured in intact males throughout the year. In long-term castrated mink, FSH concentrations did not fluctuate during the year, whereas LH concentrations showed an annual variation, with high values in April and August. For the study of in vitro GnRH liberation, medio-basal hypothalamic explants were incubated in Krebs-Ringer phosphate buffer for 3 periods of 15 min, and stimulated with copper complexed equimolarly with histidine (Cu/His, 200 pM) and prostaglandin E₂ (PGE₂, 10 μM). After Cu/His, the release of GnRH was 1 to 4 fold the basal release; after PGE₂, the increase was 4–7 fold the basal release. The basal release of GnRH increased significantly between September and October to reach a maximum in November, decreased significantly in December to a minimum in February then increased progressively from May. The release of GnRH stimulated by Cu/His and PGE₂ showed the same seasonal variation as the basal release. Castration 8 days before the sacrifice did not alter the release of GnRH, except in December: the release stimulated by PGE₂ was then higher in intact than in castrated mink. Taken together, these results indicate that, with an in vitro static incubation system, it is possible to study the annual variations of GnRH release and to correlate these variations with those of serum gonadotrophin and testosterone concentrations. The synthesis and release of GnRH increased slightly from May, under long days, then more rapidly from September, resulting in an increased secretion of FSH in October, responsible for testis recrudescence. The annual pattern of basal and stimulated GnRH release was similar in intact and castrated mink, suggesting a direct effect of the season on the hypothalamus, rather than a negative feedback effect of the testis; however, testosterone seemed to feedback mainly at the pituitary level.     

Stimulation of the Ventral Subiculum of the Hippocampus Evokes Glutamate Receptor-mediated Changes in Dopamine Efflux in the Rat Nucleus Accumbens

The effects of electrical stimulation of the ventral subiculum/CA1 region of the hippocampus on changes in dopamine oxidation current (corresponding to dopamine efflux) in the nucleus accumbens were examined using in vivo chronoamperometry with stearate-graphite paste electrodes in urethane-anaesthetized rats. Burst-patterned monophasic pulses (10–100 Hz/burst delivered at 0.84 Hz) evoked a three-component change in dopamine efflux in the nucleus accumbens with an initial transient increase in the dopamine signal above baseline, followed by an immediate decrease below baseline, and thereafter by a prolonged increase in the dopamine signal above baseline. 6–Hydroxydopamine lesions of the mesoaccumbens dopamine pathway or transection of the fimbria-fornix blocked all of the evoked changes in the dopamine signal. Both the first and third components of enhanced dopamine efflux were blocked by microinfusion into the nucleus accumbens of the ionotropic glutamate receptor antagonists (±)-2-amino-5-phosphonopentanoic acid, 6,7-dinitroquinoxaline-2,3-dione and kynurenate. Burst stimulation-evoked decreases in the dopamine signal were abolished following microinfusions into the nucleus accumbens of the metabotropic glutamate receptor antagonist (+)-α-methyl-4-carboxyphenylglycine. These results suggest that ventral subiculum/CA1 glutamatergic inputs to the nucleus accumbens may presynaptically modulate dopamine efflux by synaptic activation of both ionotropic and metabotropic glutamate receptors in the nucleus accumbens. These glutamate-dopamine interactions may constitute part of the mechanisms by which hippocampal signals are integrated through selective modulation of dopamine release in the nucleus accumbens in both physiological and pathological conditions.     

Melatonin Receptor Sites in the Syrian Hamster Brain and Pituitary. Localization and Characterization Using [125|]lodomelatonin*

A high-affinity, discretely localized melatonin receptor has been characterized and mapped within the brain and pituitary of the Syrian hamster using the high specific activity ligand [¹²⁵|]iodomelatonin and a combination of in vitro autoradiography and membrane homogenate receptor assays. Specific binding of radioligand was found in regions of the epithalamus and hypothalamus in the brain and the pars tuberalis of the pituitary. Excitatory amino-acid lesions destroyed [¹²⁵|]iodomelatonin binding within the brain, demonstrating that binding sites are located on neurons. Analysis of [¹²⁵|]iodomelatonin binding to membrane homogenates of the pars tuberalis revealed a linear relationship between specific ligand binding and the amount of tissue. The time-course of specific binding at 37°C reached equilibrium after 30 min and remained stable thereafter. The addition of increasing concentrations of [¹²⁵|]iodomelatonin alone and in the presence of 1 μM melatonin showed that specific binding reached equilibrium at 80 to 100 pM. Analysis of the saturation isotherm using a one-site binding model was consistent with a single receptor site with a K_d of 29.3 (±5.9 SEM) pM and B_max of 2.54 (±0.19 SEM) fmol/mg protein.     

Interleukin‐1β and the interleukin‐1 receptor antagonist act in the striatum to modify excitotoxic brain damage in the rat

The cytokine interleukin-1 (IL-1) has been implicated in ischaemic, traumatic and excitotoxic brain damage. The results presented here reveal novel actions of IL-1 in the striatum which markedly exacerbate cortical neuronal damage elicited by local excitotoxins in the striatum or cortex. Intrastriatal infusion of IL-1 receptor antagonist, IL-1ra, markedly inhibited striatal neuronal damage caused by N-methyl-d -aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor activation in the rat. In contrast, intracortical infusion of IL-1ra failed to inhibit NMDA or AMPA receptor-induced damage in the cortex. Intrastriatal co-infusion of IL-1β with the NMDA or AMPA receptor agonist did not affect local striatal damage induced by activation of either glutamate receptor subtype, but caused extensive cortical damage when administered into the striatum with AMPA. This secondary damage was significantly reduced by pretreatment with the NMDA receptor antagonist (MK-801), which did not affect local (striatal) damage caused by AMPA. Infusion of IL-1β into the striatum (but not the cortex) markedly enhanced cortical damage caused by infusion of an NMDA or AMPA receptor agonist into the cortex. These data reveal selective actions of IL-1 and IL-1ra in the striatum, which influence cortical neuronal loss and suggest that IL-1 selectively enhances damage caused by AMPA receptor activation.     

Photoperiod Regulates Genes Encoding Melanocortin 3 and Serotonin Receptors and Secretogranins in the Dorsomedial Posterior Arcuate of the Siberian Hamster

The mechanism(s) involved in the regulation of the seasonal-appropriate body weight of the Siberian hamster are currently unknown. We have identified photoperiodically regulated genes including VGF in a sub-region of the arcuate nucleus termed the dorsomedial posterior arcuate (dmpARC). Gene expression changes in this nucleus so far account for a significant number of those reported as photoperiodically regulated and are therefore likely to contribute to seasonal physiological responses of the hamsters. The present study aimed to identify additional genes expressed in the dmpARC regulated by photoperiod that could be involved in regulating the activity of this nucleus with respect to seasonal physiology of the Siberian hamster. Using laser capture microdissection coupled with a microarray analysis and a candidate gene approach, we have identified several photoperiodically regulated genes in the dmpARC that are known to have roles in secretory and intracellular signalling pathways. These include secretogranin (sg) III and SgVI (secretory pathway), melanocortin 3 receptor (MC3-R) and serotonin (5-HT) receptors 2A and 7 (signalling pathway), all of which increase in expression under a short photoperiod. The spatial relationship between receptor signalling and potential secretory pathways was investigated by dual in situ hybridisation, which revealed that 5-HT_2A and 5-HT₇ receptors are expressed in neurones expressing VGF mRNA and that a sub-population (approximately 40%) of these neurones express MC3-R. These gene expression changes in dmpARC neurones may reflect the functional requirement of these neurones for seasonal physiological responses of the hamster.