Preclinical characterization of selective phosphodiesterase 10A inhibitors: a new therapeutic approach to the treatment of schizophrenia

We have recently proposed the hypothesis that inhibition of the cyclic nucleotide phosphodiesterase (PDE) 10A may represent a new pharmacological approach to the treatment of schizophrenia (Curr Opin Invest Drug 8:54-59, 2007). PDE10A is highly expressed in the medium spiny neurons of the mammalian striatum (Brain Res 985:113-126, 2003; J Histochem Cytochem 54:1205-1213, 2006; Neuroscience 139:597-607, 2006), where the enzyme is hypothesized to regulate both cAMP and cGMP signaling cascades to impact early signal processing in the corticostriatothalamic circuit (Neuropharmacology 51:374-385, 2006; Neuropharmacology 51:386-396, 2006). Our current understanding of the physiological role of PDE10A and the therapeutic utility of PDE10A inhibitors derives in part from studies with papaverine, the only pharmacological tool for this target extensively profiled to date. However, this agent has significant limitations in this regard, namely, relatively poor potency and selectivity and a very short exposure half-life after systemic administration. In the present report, we describe the discovery of a new class of PDE10A inhibitors exemplified by TP-10 (2-{4-[-pyridin-4-yl-1-(2,2,2-trifluoro-ethyl)-1H-pyrazol-3-yl]-phenoxymethyl}-quinoline succinic acid), an agent with greatly improved potency, selectivity, and pharmaceutical properties. These new pharmacological tools enabled studies that provide further evidence that inhibition of PDE10A represents an important new target for the treatment of schizophrenia and related disorders of basal ganglia function.

Behavioral and neurochemical characterization of mice deficient in the phosphodiesterase-1B (PDE1B) enzyme

PDE1B is a calcium-dependent cyclic nucleotide phosphodiesterase that is highly expressed in the striatum. In order to investigate the physiological role of PDE1B in the central nervous system, PDE1B knockout mice (C57BL/6N background) were assessed in behavioral tests and their brains were assayed for monoamine content. In a variety of well-characterized behavioral tasks, including the elevated plus maze (anxiety-like behavior), forced swim test (depression-like behavior), hot plate (nociception) and two cognition models (passive avoidance and acquisition of conditioned avoidance responding), PDE1B knockout mice performed similarly to wild-type mice. PDE1B knockout mice showed increased baseline exploratory activity when compared to wild-type mice. When challenged with amphetamine (AMPH) and methamphetamine (METH), male and female PDE1B knockout mice showed an exaggerated locomotor response. Male PDE1B knockout mice also showed increased locomotor responses to higher doses of phencyclidine (PCP) and MK-801; however, this effect was not consistently observed in female knockout mice. In the striatum, increased dopamine turnover (DOPAC/DA and HVA/DA ratios) was found in both male and female PDE1B knockout mice. Striatal serotonin (5-HT) levels were also decreased in PDE1B knockout mice, although levels of the metabolite, 5HIAA, were unchanged. The present studies demonstrate increased striatal dopamine turnover in PDE1B knockout mice associated with increased baseline motor activity and an exaggerated locomotor response to dopaminergic stimulants such as methamphetamine and amphetamine. These data further support a role for PDE1B in striatal function.

Performance of heterozygous brain-derived neurotrophic factor knockout mice on behavioral analogues of anxiety, nociception, and depression

Evidence suggests that brain-derived neurotrophic factor (BDNF) may be important in the pathophysiology of depression, in addition to its role as a neurotrophic factor for sensory neurons. The authors conducted a series of experiments examining the behavioral profile of BDNF heterozygous knockout and wild-type mice. The heterozygous and wild-type mice did not differ on measures of activity, exploration, or hedonic sensitivity, or in the forced swim test. When assessed in the learned helplessness paradigm, heterozygous mice were slower to escape after training than were wild-type mice (p = .02). This effect may be accounted for by the fact that these mice demonstrate a reduced sensitivity to centrally mediated pain, apparent on the hot plate and Formalin injection tests of nociception. Overall, heterozygous mice were not more likely to display anxious or depressive-like behaviors and, consequently, may not constitute a murine model of genetic vulnerability to mood and anxiety disorders.

Performance of heterozygous brain-derived neurotrophic factor knockout mice on behavioral analogues of anxiety, nociception, and depression

Evidence suggests that brain-derived neurotrophic factor (BDNF) may be important in the pathophysiology of depression, in addition to its role as a neurotrophic factor for sensory neurons. The authors conducted a series of experiments examining the behavioral profile of BDNF heterozygous knockout and wild-type mice. The heterozygous and wild-type mice did not differ on measures of activity, exploration, or hedonic sensitivity, or in the forced swim test. When assessed in the learned helplessness paradigm, heterozygous mice were slower to escape after training than were wild-type mice (p = .02). This effect may be accounted for by the fact that these mice demonstrate a reduced sensitivity to centrally mediated pain, apparent on the hot plate and Formalin injection tests of nociception. Overall, heterozygous mice were not more likely to display anxious or depressive-like behaviors and, consequently, may not constitute a murine model of genetic vulnerability to mood and anxiety disorders.

Hippocampal mossy fiber sprouting induced by chronic electroconvulsive seizures

Stress, which can precipitate and exacerbate depression, causes atrophy and in severe cases death of hippocampal neurons. Atrophy of the hippocampus has also been observed in patients suffering from recurrent major depression. The present study examines the influence of electroconvulsive seizures, one of the most effective treatments for depression, on the morphology and survival of hippocampal neurons. The results demonstrate that chronic administration of electroconvulsive seizures induces sprouting of the granule cell mossy fiber pathway in the hippocampus. This sprouting is dependent on repeated administration of electroconvulsive seizures, reaches a maximum 12 days after the last treatment and is long lasting (i.e. up to six months). Electroconvulsive seizure-induced sprouting occurs in the absence of neuronal loss, indicating that sprouting is not a compensatory response to cell death. This is different from the sprouting induced by kindling or excitotoxin treatment, which induce cell death along with recurrent seizures. Electroconvulsive seizure-induced sprouting is significantly diminished in brain-derived neurotrophic factor heterozygote knockout mice, indicating that this neurotrophic factor contributes to mossy fiber sprouting. However, infusion of brain-derived neurotrophic factor into the hippocampus does not induce sprouting of the mossy fiber pathway. The results demonstrate that chronic administration of electroconvulsive seizures induces mossy fiber sprouting and suggest that increased expression of brain-derived neurotrophic factor is necessary, but not sufficient for the induction of this sprouting. Although the functional consequences remain unclear, sprouting of the mossy fiber pathway would appear to oppose the actions of stress and could thereby contribute to the therapeutic actions of electroconvulsive seizure therapy.

Co-infusion with a TrkB-Fc receptor body carrier enhances BDNF distribution in the adult rat brain

Fusion proteins comprising the Fc domain of human IgG and extracellular domains of receptor tyrosine kinases can neutralize the activity of their cognate ligands when administered in molar excess. We have generated a fusion protein using the ectodomain of TrkB (TrkB-Fc). Although the ability of TrkB-Fc to neutralize the activity of brain-derived neurotrophic factor (BDNF) in vitro has been demonstrated, there have been no conclusive demonstrations of its ability to neutralize the activity of BDNF in vivo. We co-infused TrkB-Fc with BDNF into the cortex and hippocampus of adult rats to determine whether TrkB-Fc would interfere with the ability of BDNF to upregulate neuropeptide Y (NPY). We report here that rather than neutralizing the activity of exogenous BDNF, co-infusion with the TrkB-Fc fusion protein greatly increased the volume of tissue in which neuropeptide Y immunostaining was upregulated. In addition, TrkB-Fc greatly enhanced BDNF's distribution through adult brain parenchyma. TrkB-Fc also markedly increased the otherwise limited diffusion of BDNF into brain parenchyma following intraventricular infusion. These results show that rather than neutralizing or sequestering BDNF, the TrkB-Fc, at close to molar equivalence to BDNF, can function as a carrier for BDNF and thus enhance the delivery or penetration of this polypeptide into the brain.

BDNF induction of tryptophan hydroxylase mRNA levels in the rat brain

We have previously demonstrated an augmentation of serotonergic activity within various brain areas following infusion of brain-derived neurotrophic factor (BDNF) into the midbrain near the periaqueductal gray and dorsal and median raphe nuclei (PAG/DR). However, the mechanism of this BDNF-induced modulatory effect on serotonergic systems was unclear. The aim of the present work was to study the regulation of tryptophan hydroxylase (TPH) mRNA levels after chronic BDNF administration in vivo. TPH mRNA levels were measured using a quantitative competitive reverse transcription polymerase chain reaction (RT-PCR) assay. A significant increase in the expression of TPH mRNA (13-fold) was found within the PAG/DR as early as 24 hr after onset of BDNF infusion and was sustained throughout the duration of infusion (11 days). This was accompanied by increased serotonin (5-hydroxytryptamine, 5-HT) levels and decreased nociceptive responsiveness assessed by tail-flick latency. BDNF induction of TPH mRNA levels was also observed in a serotonergic cell line derived from raphe neurons, indicating that BDNF can directly regulate TPH mRNA levels. These results suggest that BDNF augments 5-HT synthesis in vivo by directly enhancing steady-state TPH mRNA levels, and subsequently leading to marked behavioral alterations.

Physiological characterization of Taxol-induced large-fiber sensory neuropathy in the rat

The cancer chemotherapeutic agent Taxol (paclitaxel) causes a dose-related peripheral neuropathy in humans. We produced a dose-dependent large-fiber sensory neuropathy, without detrimental effects on general health, in mature rats by using two intravenous injections 3 days apart. Tests of other dosing schedules demonstrated the dependence of the severity of the neuropathy and of animal health on both the dose and the frequency of dosing. Pathologically, severe axonal degeneration and hypomyelination were observed in sections of dorsal roots, whereas ventral roots remained intact. Electrophysiologically, H-wave amplitudes in the hindlimb and amplitudes of predominantly sensory compound nerve action potentials in the tail were reduced. These effects persisted for at least 4 months after treatment. Motor amplitudes were not affected, but both motor and sensory conduction velocities decreased. The ability of rats to remain balanced on a narrow beam was impaired, indicating proprioceptive deficits. Muscle strength, measured by hindlimb and forelimb grip strength, and heat nociception, measured by tail-flick and hindlimb withdrawal tests, were not affected by Taxol. This model of Taxol-induced neuropathy in mature rats, with minimal effects on general health, parallels closely the clinical syndrome observed after Taxol treatment in humans.

Effects of BDNF infusion on the regulation of TrkB protein and message in adult rat brain

Exposure of embryonic CNS neurons to BDNF in vitro causes down-regulation of TrkB protein and mRNA, and an attenuation of functional responses to acute neurotrophin stimulation. In order to investigate ligand-mediated regulation of TrkB in vivo, we infused BDNF into the midbrain, near the periaquaductal grey-dorsal raphe (PAG-DR), or into the olfactory bulb of adult rats. Midbrain infusion of BDNF produced analgesia that was sustained for the duration of BDNF delivery. Analysis of TrkB receptor levels revealed that at the point when the maximal analgesic effect of BDNF was obtained, there was a concommitant 75% decrease in full-length TrkB protein at the infusion site. After discontinuation of infusion, levels of TrkB recovered toward base line. Interestingly, TrkB protein levels were not accompanied by decreased trkB mRNA levels. To determine if BDNF infusion decreased TrkB protein levels in other brain areas and whether trkB mRNA might be down-regulated in the cell bodies of neurons projecting to the infusion site, we infused BDNF into the olfactory bulb. Following a 12-day infusion of BDNF, TrkB protein levels decreased within the bulb to a similar extent as in the PAG-DR. This decrease in receptor protein, however, was not accompanied by decreased trkB mRNA levels in the olfactory cortex, which is afferent to the bulb. Taken together, our data suggest that decreases in TrkB receptor protein at the site of BDNF infusions in the adult brain represent receptor turnover, but this is not associated with altered expression of trkB mRNA or attenuation of the pharmacological effects of BDNF.

Antidepressant-like effect of brain-derived neurotrophic factor (BDNF)

Previous studies have shown that infusion of brain-derived neurotrophic factor (BDNF) into the midbrain, near the PAG and dorsal/median raphe nuclei, produced analgesia and increased activity in monoaminergic systems. Alterations in monoaminergic activity have also been implicated in the pathogenesis and treatment of depression. The present studies examined the ability of centrally administered BDNF to produce antidepressant-like activity in two animal models of depression, learned helplessness following exposure to inescapable shock and the forced swim test. In the learned helplessness paradigm, vehicle-infused rats pre-exposed to inescapable shock (veh/shock) showed severe impairments in escape behavior during subsequent conditioned avoidance trials, including a 47% decrease in the number of escapes and a 5 fold increase in escape latency, as compared to vehicle-infused rats which received no pre-shock treatment (veh/no shock). Midbrain BDNF infusion (12-24 micrograms/day) reversed these deficits, and in fact, BDNF-infused rats pre-exposed to inescapable shock (BDNF/shock) showed escape latencies similar to veh/no shock and BDNF/no shock rats. In the forced swim test, BDNF infusion decreased the immobility time by 70% as compared to vehicle-infused controls. Non-specific increases in activity could not account for these effects since general locomotor activity of BDNF- and vehicle-infused animals was not different. These findings demonstrate an antidepressant-like property of BDNF in two animal models of depression, which may be mediated by increased activity in monoaminergic systems.

Local infusion of brain-derived neurotrophic factor modifies the firing pattern of dorsal raphé serotonergic neurons

Previous studies have reported a neuromodulatory effect of brain-derived neurotrophic factor (BDNF) on serotonin neurons in the central nervous system. In the present study, we examined the effects of local infusion of BDNF on the electrophysiological activity of serotonergic neurons in the rat dorsal raphé nucleus with extracellular single unit recording in vivo. Compared with vehicle-infused rats, chronic administration of BDNF (10-14 days) caused serotonergic neurons to fire in a significantly less regular pattern, without altering the mean firing rate or other measures of electrical activity. These results suggest that the ability of similar infusions of BDNF to produce behavioral effects (i.e. analgesia and an antidepressant-like effect) associated with elevated serotonin turnover may be in part the result of more irregular firing patterns of dorsal raphé neurons.

BDNF increases monoaminergic activity in rat brain following intracerebroventricular or intraparenchymal administration

We have previously demonstrated alterations in serotonin metabolism within descending pathways following infusion of brain-derived neurotrophic factor (BDNF) into the midbrain, near the periaqueductal gray and dorsal and median raphe nuclei. The aim of the present study was to extend these studies to include a comprehensive regional examination of monoamine (serotonin, dopamine and norepinephrine) and metabolite levels in discrete areas of the intact, adult rat forebrain following direct intraparenchymal midbrain BDNF infusion. We have compared neurochemical changes following midbrain infusion of BDNF to those obtained following intracerebroventricular (i.c.v.) infusion. Significant increases in levels of 5-HIAA and/or the 5-HIAA/5-HT ratio were found in all areas examined including the hippocampus, cortex, striatum, n. accumbens, substantia nigra and hypothalamus following both midbrain and i.c.v. infusion. Changes in dopaminergic activity were also observed, but displayed more regional specificity, i.e. changes were found primarily within the striatum and cortex. The two infusion sites produced similar patterns of neurochemical effects although the magnitude of the changes did vary in some areas. These results suggest that BDNF increased synthesis and/or turnover of serotonin, and to a lesser extent dopamine, in the mature rat forebrain. Furthermore, these data point to possible functional roles for BDNF in neuropsychiatric and neurodegenerative conditions which involve a dysregulation of these monoamine systems.

Localization of 2-[125I]iodomelatonin binding sites in visual areas of the turtle brain

The hormone melatonin is believed to play an important role in the regulation of both circadian and circannual rhythms. In mammalian vertebrates melatonin receptors are discretely localized, with broader distributions reported in avians and reptiles. To examine the sites at which melatonin may act in the turtle brain, 2-[125I]iodomelatonin binding sites were assessed using quantitative autoradiography. Specific binding sites were primarily restricted to forebrain structures with a wide distribution in visual recipient areas. The distribution of melatonin sensitive sites within the turtle visual system suggests that the ability to transduce received photoperiodic signals in the reptilian brain is broadly distributed within the central nervous system.

Brain-derived neurotrophic factor promotes the survival and sprouting of serotonergic axons in rat brain

A pathology of brain serotonergic (5-HT) systems has been found in psychiatric disturbances, normal aging and in neurodegenerative disorders including Alzheimer's and Parkinson's disease. Despite the clinical importance of 5-HT, little is known about the endogenous factors that have neurotrophic influences upon 5-HT neurons. The present study examined whether chronic pain parenchymal administration of the neurotrophins brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or NGF could prevent the severe degenerative loss of serotonergic axons normally caused by the selective 5-HT neurotoxin p-chloroamphetamine (PCA). The neurotrophins (5-12 micrograms/d) or the control substances (cytochrome c or PBS vehicle) were continuously infused into the rat frontoparietal cortex using an osmotic minipump. One week later, rats were subcutaneously administered PCA (10 mg/kg) or vehicle, and the 5-HT innervation was evaluated after two more weeks of neurotrophin infusion. As revealed with 5-HT immunocytochemistry, BDNF infusions into the neocortex of intact (non-PCA-lesioned) rats caused a substantial increase in 5-HT axon density in a 3 mm diameter region surrounding the cannula tip. In PCA-lesioned rats, intracortical infusions of BDNF completely prevented the severe neurotoxin-induced loss of 5-HT axons near the infusion cannula. In contrast, cortical infusions of vehicle or the control protein cytochrome c did not alter the density of serotonergic axons in intact animals, nor did control infusions prevent the loss of 5-HT axons in PCA-treated rats. NT-3 caused only a modest sparing of the 5-HT innervation in PCA-treated rats, and NGF failed to prevent the loss of 5-HT axon density. The immunocytochemical data were supported by neurochemical evaluations which showed that BDNF attenuated the PCA-induced loss of 5-HT and 5-HIAA contents and 3H-5-HT uptake near the infusion cannula. Thus, BDNF can promote the sprouting of mature, uninjured serotonergic axons and dramatically enhance the survival or sprouting of 5-HT axons normally damaged by the serotonergic neurotoxin PCA.

BDNF produces analgesia in the formalin test and modifies neuropeptide levels in rat brain and spinal cord areas associated with nociception

Previous studies have demonstrated an antinociceptive effect of brain-derived neurotrophic factor (BDNF) following infusion into the midbrain, near the periaqueductal grey and dorsal raphe nuclei. BDNF administration attenuated the behavioural response in the tail-flick and hot-plate tests, two models employing a phasic, thermal high-intensity nociceptive stimulus; the present studies extend our previous findings to include a model of moderate, continuous pain resulting from a chemical stimulus, the formalin test. Midbrain infusion of BDNF decreased the behavioural paw flinch response to subcutaneous formalin injection in both the early and late phases of the test. As our previous studies showed that BDNF-induced analgesia was reversible by naloxone, we have examined the effects of BDNF administration on brain and spinal cord levels of neuropeptides involved in the modulation of nociceptive information, including the endogenous opioid peptides, met-enkephalin and beta-endorphin, as well as substance P and neuropeptide Y (NPY). At the site of infusion, within the PAG and dorsal raphe, BDNF increased the level of beta-endorphin by 63%, but had no effect on substance P, metenkephalin or NPY levels. In the dorsal spinal cord, substance P (113% increase), beta-endorphin (97% increase) and NPY (64% increase) were elevated, although ventral spinal cord levels of these peptides remained unchanged. These studies demonstrate a modulatory effect of BDNF on relevant neuropeptides within areas of the brain and spinal cord involved in the processing of nociceptive information.

In situ hybridization of trkB and trkC receptor mRNA in rat forebrain and association with high-affinity binding of [125I]BDNF, [125I]NT-4/5 and [125I]NT-3

The TrkB and TrkC receptor tyrosine kinases have been identified as high-affinity receptors for the neurotrophic factors brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5) and NT-3 respectively. These receptor classes were identified and mapped by the in situ hybridization of antisense riboprobes complementary to portions of the intracellular (tyrosine kinase) or extracellular (ligand-binding) domains of trkB and trkC mRNA, and by the distribution of high-affinity [125I]BDNF, [125I]NT-4/5 and [125I]NT-3 binding sites in adjacent rat brain sections. Both methods showed that TrkB and TrkC receptors are abundant and widely expressed throughout the brain. Kinase or extracellular domain trkC probes labelled neuronal somata in a qualitatively similar manner in virtually every major area of the forebrain. Neither trkC probe labelled non-neuronal cells except for elements within cerebral arteries and arterioles. The kinase domain trkB probe hybridized exclusively to neurons. Neurons expressing trkB were even more widely distributed than those expressing trkC. The extracellular domain trkB probe labelled neurons with the same relative distribution as the trkB kinase domain probe, but also hybridized extensively with non-neural cells, particularly astrocytes, ependyma and choroid epithelium cells. The distribution of [125I]NT-3 binding sites generally resembled that of trkC hybridization, particularly in the neocortex, striatum and thalamus. [125I]BDNF and [125I]NT-4/5 binding sites were more widely distributed and denser than those for [125I]NT-3, and resembled the trkB hybridization pattern. These patterns are consistent with the preferential binding in the brain of TrkC receptors by [125I]NT-3 and of TrkB receptors by [125I]BDNF and [125I]NT-4/5. That the predominantly neuronal patterns of hybridization obtained with kinase and extracellular domain probes for trkC are qualitatively indistinguishable suggests that truncated and full-length forms of TrkC are expressed within extensively overlapping populations of neurons. In marked contrast to TrkC, expression of the full-length and truncated forms of TrkB appears to be largely segregated, being expressed principally on neurons and non-neuronal cells respectively. The abundant and widespread neuronal distribution of full-length, signal-transducing forms of TrkB and TrkC predict that their cognate ligands, BDNF, NT-4/5 and NT-3, may exert direct effects on a large proportion of neurons within the mature brain.

Unilateral optic nerve transection decreases 2-[125I]-iodomelatonin binding in retinorecipient areas and visual pathways of chick brain

In chick brain, specific 2-[125I]-iodomelatonin-binding was localized primarily in the visual system, i.e., retinorecipient and relay nuclei and fiber tracts of the tectofugal, thalamofugal, circadian and accessory visual pathways. Unilateral transection of the optic nerve (ONX) significantly reduced the binding of 2-[125I]-iodomelatonin (75 pM) in many, but not all, primary retinal targets and visual pathways at 7 and 14 days, but not 1 day, postlesion. As measured using quantitative autoradiography, 2-[125I]-iodomelatonin binding was decreased by 90% in both the central portion of the lesioned optic tract and one of its targets, the nucleus of the basal optic root (nBOR). Other retinorecipient areas exhibiting substantial decreases (60%) in 2-[125I]-iodomelatonin-binding included the optic tectum, lateroventral and dorsolateral geniculate nuclei and tectal gray area contralateral to the lesion. These findings are consistent with the hypothesis that melatonin receptors are located presynaptically on incoming optic nerve terminals in many retinorecipient areas. This localization may account for most of the binding sites in nBOR. In other primary visual areas, however, melatonin receptors also appear to be located on postsynaptic cells and/or non-retinal afferents. ONX had no significant effect on 2-[125I] -iodomelatonin binding in two retinorecipient areas, the visual suprachiasmatic nucleus and the dorsolateral anterior thalamus, which are part of the circadian/oculomotor and thalamofugal pathways, respectively. An unexpected consequence of ONX was that 2-[125I]- iodomelatonin binding was decreased in certain secondary (nucleus rotundus, isthmi nuclei) and tertiary level (ectostriatum) nuclei along the prominent tectofugal visual pathway. Binding in the tectorecipient nucleus triangularis was not significantly altered, however. Analysis of secondary level relay nuclei in the oculomotor pathway revealed that binding after ONX was decreased in the ipsilateral Edinger-Westphal nucleus but not in the oculomotor nuclei. Selective transsynaptic changes in 2-[125I]-iodomelatonin binding after lesion of the visual input most likely reflect activity-dependent regulation and functional plasticity of central melatonin receptors.

Antinociceptive effect of brain-derived neurotrophic factor and neurotrophin-3

Infusions of brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3), but not nerve-growth factor, into the rat midbrain significantly elevated the tail-flick response latency. Analgesia was observed as soon as 24 h after the onset of infusion, reached maximum levels by day 5 and remained constant for at least an additional 6 days, suggesting no development of tolerance. BDNF infusion also increased latency in the hot-plate test. Naloxone administration reversed the BDNF-induced increase in the tail-flick latency. The antinociceptive effect of BDNF infusion was accompanied by an augmentation in serotonergic activity within the brain and spinal cord. These data demonstrate both an effect of BDNF and NT-3 on serotonergic neurons and an analgesic property of these neurotrophins which appears to involve both serotonin and opioid mechanisms.

Effect of pinealectomy and the light/dark cycle on 2-[125I]iodomelatonin binding in the chick optic tectum

1. These studies investigated the regulation of melatonin receptor sites within the chick brain by the light/dark cycle and by endogenous melatonin using quantitative autoradiography. Saturation analyses performed in coronal brain sections of the optic tectum revealed a single class of high-affinity binding sites. 2. For diurnal rhythm studies, chicks were maintained on a 14:10 L:D cycle (light on 0400) and sacrificed at 4-h intervals. No significant variation was observed in either the affinity (KD = 67.8-76.4 pM) or the density (BMAX = 88.9-118.9 fmol/mg protein) of 2-[125I]iodomelatonin binding sites as a function of time of day. 3. In pinealectomized or age-matched sham-operated chicks, no changes were found in either the affinity (KD = 66.7-89.1 pM) or the density (BMAX = 112.8-180.4 fmol/mg protein) of 2-[125I]iodomelatonin binding sites at either 1, 7, or 14 days following surgery. These data suggest that endogenous pineal melatonin may not play a role in the regulation of melatonin receptor site affinity or density in the chick optic tectum.

Optic nerve transection decreases 2-[125I]iodomelatonin binding in the chick optic tectum

The distribution of specific 2-[125I]iodomelatonin binding sites in the various layers of the chick optic tectum was analyzed using quantitative receptor autoradiography. Following unilateral optic nerve transection, binding in the optic fiber layer and superficial retinorecipient layers of the contralateral tectum was significantly decreased at 7 and 14 days, but not at 1 day, following transection. The results are consistent with the presence of presynaptic melatonin receptors on axon terminals of retinotectal fibers.

Monoamines and their precursors and metabolites in the chicken brain, pineal, and retina: regional distribution and day/night variations

Levels of norepinephrine, epinephrine, dopamine, and serotonin (5-HT) and their precursors [tyrosine, L-3,4-dihydroxyphenylalanine, tryptophan, and 5-hydroxytryptophan (5-HTP)] and metabolites [3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT), homovanillic acid, 3-methoxy-4-hydroxyphenylglycol, and 5-hydroxyindoleacetic acid (5-HIAA)] were determined concurrently in samples of chick retina, pineal gland, and nine selected areas of the brain (optic lobes, thalamus, hypothalamus, optic chiasm, pons/medulla, cerebellum, neostriatum/ectostriatum, hyperstriatum, and basal forebrain) using HPLC coupled with a coulometric electrode array detection system. The norepinephrine level was highest in the pineal gland, but it was also widely distributed throughout the chick brain, with the thalamus and hypothalamus showing substantial levels. The dopamine level was highest in the basal forebrain. The epinephrine level was highest in the hypothalamus. The thalamus and hypothalamus showed the highest levels of 5-HT. Daytime levels (1100 h) of these compounds were compared with levels in chicks killed in the middle of the dark phase (2300 h). In the brain areas examined, no day/night variations in levels of norepinephrine, epinephrine, dopamine, or 5-HT were seen, although significant nocturnal changes in levels of their metabolites were observed in some areas. Pineal levels of 5-HIAA decreased significantly at night. The retina showed significant nocturnal increases in 5-HTP, 5-HT, and 5-HIAA levels. Retinal levels of 3-MT and DOPAC were significantly decreased at night.

Quantitative pharmacological analysis of 2-125I-iodomelatonin binding sites in discrete areas of the chicken brain

We have localized and characterized 2-125I-iodomelatonin binding sites in the chicken brain using in vitro quantitative autoradiography. Binding sites were widely distributed throughout the chicken brain, predominantly in regions associated with the visual system. The specific binding of 2-125I-iodomelatonin to discrete chicken brain areas was found to be saturable, reversible, and of high affinity. The specific binding of 2-125I-iodomelatonin (75 pm) was quantitated for 40 identifiable brain regions. Eight brain regions were chosen for binding characterization and pharmacological analysis: optic tectum, Edinger-Westphal nucleus, oculomotor nucleus, nucleus rotundus, ventral supraoptic decussation, ventrolateral geniculate nucleus, neostriatum, and ectostriatum. These regions showed no rostral-caudal gradient in 2-125I-iodomelatonin specific binding, and saturation analysis revealed a single class of high-affinity sites with KD values in the range of 33-48 pM and receptor site density (Bmax) ranging from 31 to 58 fmol/mg protein. Competition experiments carried out with various indoles revealed a similar order of pharmacological affinities in these areas: melatonin greater than 6-chloromelatonin greater than methoxyluzindole greater than N-acetylserotonin greater than luzindole much greater than 5-HT greater than 5-methoxytryptamine. The affinity constants determined by quantitative autoradiography for these compounds to compete for 2-125I-iodomelatonin binding in the optic tectum correlated well with the affinities in chicken brain membranes at 25 degrees C (r = 0.966; slope = 0.845; n = 7) and 0 degree C (r = 0.946; slope = 0.379; n = 7), chicken retinal membranes (r = 0.973; slope = 0.759; n = 7), and the potency or affinity of these compounds to affect the calcium-dependent release of 3H-dopamine from the rabbit retina (r = 0.902; slope = 0.506; n = 6). We conclude that the high-affinity sites labeled by 2-125I-iodomelatonin in various chicken brain areas have identical binding and pharmacological characteristics to the ML-1 melatonin binding site previously described in chicken brain and retinal membranes and to the ML-1 melatonin receptor modulating dopamine release from the retina. In the chicken brain, the ML-1 receptor site may mediate functional responses regulated by melatonin.

The area of 2-[125I] iodomelatonin binding in the pars tuberalis of the ground squirrel is decreased during hibernation

Quantitative receptor autoradiography was used to analyze 2-[125I]iodomelatonin binding sites in the brains of golden-mantled ground squirrels (Citellus lateralis). Specific binding appeared to be discretely localized to the pars tuberalis region of the pituitary that surrounds the medial basal hypothalamus. The total area of binding was significantly decreased in brains of hibernating squirrels as compared to those of awake, euthermic animals. These findings support a role for melatonin receptors of the pars tuberalis in seasonal behavior.

Autoradiographic localization of 2-[125I]iodomelatonin binding sites in the brains of C3H/HeN and C57BL/6J strains of mice

The present study uses in vitro autoradiography to localize 2-[125I]iodomelatonin binding sites in the brains of two strains of mice, the C3H/HeN and the C57BL/6J, which have been shown to exhibit differences in pineal melatonin content. We found a differential pattern of distribution of 2-[125I]iodomelatonin binding sites between the two strains with the suprachiasmatic nucleus, paraventricular nucleus of the thalamus, and the median eminence/pars tuberalis regions labelled in both strains. These studies should help to interpret behavioral changes due to activation of melatonin receptors in these strains of mice.

Antinociceptive effect of intrathecal morphine in tolerant and nontolerant spinal rats

The antinociceptive effect of intrathecal morphine on the tail-flick (TF) reflex of rats was significantly enhanced within one day after spinal transection (ED50 = 0.125 microgram) relative to the effect obtained in intact rats (ED50 = 5.9 micrograms). By 20-30 days after spinalization the potency of intrathecally administered morphine had substantially declined. Intact rats, made tolerant to the antinociceptive effect of systemic morphine (3.0 mg/kg, SC on each of seven consecutive days), were not tolerant to intrathecal morphine (ED50 = 6.5 micrograms). In contrast, rats that were pretreated with either morphine alone, repeated TF tests alone, or both of these treatments, were tolerant to intrathecal morphine when tested one day after spinal transection. The results suggest first, that the antinociceptive effect of intrathecal morphine in intact rats is tonically inhibited by descending supraspinal input and that removal of this input is responsible for the enhanced antinociceptive effect of intrathecal morphine in spinal rats. Second, the data suggest that tolerance to the antinociceptive effect of intrathecal morphine in intact rats may also be tonically inhibited by supraspinal input, because spinal opiate tolerance is expressed after spinal transection.

The synergistic effect of concurrent spinal and supraspinal opiate agonisms is reduced by both nociceptive and morphine pretreatment

The antinociceptive effect of morphine administered into the periaqueductal gray (PAG), the intrathecal space (ITH) and concurrently, into both sites (in a 1:1 dose ratio), was assessed in 1) nontolerant rats, 2) rats made tolerant to the effect of morphine on the tail-flick (TF) test and 3) rats that were tested on the TF during chronic saline administration. In nontolerant rats, concurrent morphine injections produced a multiplicative antinociceptive effect (ED50 = 0.392 microgram, total dose) relative to that obtained after separate PAG (ED50 = 2.8 micrograms) or ITH (ED50 = 6.7 micrograms) injections. The multiplicative effect of concurrent morphine administration was significantly reduced in rats made tolerant to morphine (one 3 mg/kg SC injection and TF test per day for six days). Opiate synergy was also reduced but to a smaller extent in rats that were repeatedly tested on the TF during chronic saline administration (one SC injection and TF test per day for six days). Neither chronic morphine nor saline pretreatment altered the dose-response function to intrathecal morphine. However, both morphine and saline pretreatment significantly reduced the antinociceptive effect of morphine administered into the PAG. The data indicate that concurrent morphine administration into the PAG and ITH space results in a synergistic antinociceptive action which is reduced by performance of the nociceptive response, even in the absence of opiate administration. We suggest that the decrease in opiate synergism produced by nociceptive assessment (behavioral tolerance) is mediated supraspinally, while the additional decline resulting from morphine administered in conjunction with the nociceptive tests (opiate tolerance) is mediated by a combined action at spinal and supraspinal sites.

Tolerance to morphine microinjections in the periaqueductal gray (PAG) induces tolerance to systemic, but not intrathecal morphine

The acquisition and retention of tolerance to the antinociceptive effect of supraspinal morphine on the tail withdrawal reflex was assessed in rats implanted with unilateral cannulae in the periaqueductal gray (PAG). Development of tolerance to daily microinjections of morphine was indicated by the return of the tail flick response within 4 days, followed by the recovery of analgesic sensitivity one week later. After tolerance had developed, the effect of an acute systemic (1.5-4.5 mg/kg) or intrathecal (5-15 micrograms) morphine injection was determined. 'Cross-tolerance' was observed between systemic and supraspinal morphine but not between intrathecal and supraspinal morphine. The data indicate that tolerance to chronic intracerebral morphine produces the same behavioral consequences as tolerance to systemic morphine.