Increased corticotropin-releasing factor receptors in rat cerebral cortex following chronic atropine treatment

Alternative Pharmacological Strategies for the Treatment of Alzheimer's Disease: Focus on Neuromodulator Function

Alzheimer's disease (AD) is a neurodegenerative disorder, comprising 70% of dementia diagnoses worldwide and affecting 1 in 9 people over the age of 65. However, the majority of its treatments, which predominantly target the cholinergic system, remain insufficient at reversing pathology and act simply to slow the inevitable progression of the disease. The most recent neurotransmitter-targeting drug for AD was approved in 2003, strongly suggesting that targeting neurotransmitter systems alone is unlikely to be sufficient, and that research into alternate treatment avenues is urgently required. Neuromodulators are substances released by neurons which influence neurotransmitter release and signal transmission across synapses. Neuromodulators including neuropeptides, hormones, neurotrophins, ATP and metal ions display altered function in AD, which underlies aberrant neuronal activity and pathology. However, research into how the manipulation of neuromodulators may be useful in the treatment of AD is relatively understudied. Combining neuromodulator targeting with more novel methods of drug delivery, such as the use of multi-targeted directed ligands, combinatorial drugs and encapsulated nanoparticle delivery systems, may help to overcome limitations of conventional treatments. These include difficulty crossing the blood-brain-barrier and the exertion of effects on a single target only. This review aims to highlight the ways in which neuromodulator functions are altered in AD and investigate how future therapies targeting such substances, which act upstream to classical neurotransmitter systems, may be of potential therapeutic benefit in the sustained search for more effective treatments.

Interaction between the cholinergic system and CRH in the modulation of spatial discrimination learning in mice

Both cholinergic and CRH systems have been linked to cognitive processes such as learning and memory, and neuroanatomical as well as neurochemical evidence suggests important interactions between these two systems. Moreover, recent reports of pro-mnestic effects of CRH open the possibility that CRH could have beneficial effects in animals with cholinergic dysfunction. In a first experiment, spatial discrimination of C57BL/6 mice treated with various doses of scopolamine (0.5--2.0 mg/kg IP) was tested in a two-choice water maze task. Scopolamine, but not methylscopolamine, impaired accuracy and decreased responsivity. In contrast, similar doses of the nicotinic antagonist mecamylamine had no effect on choice accuracy but altered responsivity, as indicated by increased errors of omission and a reduction in swim speed during early experimental stages. ICV CRH (0.5--1.0 microg) also failed to significantly affect accuracy, but a strong tendency was observed to impair percentage correct responses. Measures of responsivity, such as errors of omission, choice latency and distance traveled, and of thigmotaxis were not significantly affected by CRH. However, initial swim speed was reduced by the peptide. Combined treatment with scopolamine (0.5 mg/kg IP) and CRH (0.5 microg ICV) had only mild, and primarily independent, effects, but overall suggested that concomitant blockade of muscarinic receptors and activation of the CRH system would rather act synergistically to disrupt spatial discrimination learning. Synergistic effects were also observed when animals receiving a combination of mecamylamine (2.0 mg/kg IP) and CRH (0.5 microg ICV) were tested, both in terms of responsivity and thigmotaxis, and there was limited evidence that part of these effects were potentiating. Thus, the cholinergic and CRH systems interact in the modulation of learning, but CRH, contrary to prediction, worsens the impairment caused by cholinergic blockade.

Corticotropin-releasing hormone receptor subtypes and emotion

Preclinical data indicate that corticotropin-releasing hormone (CRH) has anxiogenic properties and a dysregulation in CRH systems has been suggested to play a role in a variety of stress-related psychiatric disorders, such as anxiety, depression, and eating disorders. Two CRH receptor subtypes have been identified, termed CRH1 receptor (CRH1) and CRH2 receptor (CRH2), with its splice variants CRH2 alpha and CRH2 beta. These receptor subtypes differ in their pharmacology and expression pattern in the brain. Mouse mutants in which the CRH1 receptor subtype has been deleted show an impaired stress response, reduced anxiety-related behavior, and cognitive deficits. Studies using antisense oligodeoxynucleotides directed against CRH1 or CRH2 alpha identified the CRH1 receptor as the main target for CRH in mediating anxiogenesis, although recent data also suggest a possible role for CRH2 alpha. More clearly, CRH2 alpha is involved in the CRH effects on food intake. Moreover, local injection of CRH into areas rich in CRH2 alpha also result in altered sexual female behavior. Therefore, it is suggested that the CRH2 alpha may primarily influence a system concerned with implicit processes necessary for survival, i.e., with motivational types of behavior including feeding, reproduction, and possibly defense, whereas the CRH1 may be more concerned with explicit processes, including attention, executive functions, the conscious experience of emotions, and possibly learning and memory related to these emotions. This also suggests that patients suffering from anxiety and depression may benefit from treatment with CRH1 antagonistic drugs, while drugs targeting CRH2 alpha may be of particular benefit for patients with eating disorders.

Physiological and neurochemical aspects of corticotropin-releasing factor actions in the brain: the role of the locus coeruleus

Corticotropin-releasing factor (CRF) is both a major regulator of the hypothalamo-pituitary-adrenal (HPA) axis and the activity of the autonomic nervous system. Besides, it exerts numerous effects on other physiological functions such as appetite control, motor and cognitive behavior and immune function. The basis for these effects is constituted by its distribution in hypothalamic and extra-hypothalamic brain areas, the latter being represented by limbic structures such as the central nucleus of the amygdala or by brain stem neurons such as the locus coeruleus (LC) or nucleus of the solitary tract (NTS). The effects of CRF are mediated through recently described CRF-receptor subtypes, whose molecular biology, biochemistry and pharmacological regulation are discussed in detail. In the second part of this review, we will focus on the physiology of CRF-systems in the brain, with a particular emphasis on cardiovascular regulation, respiration, appetite control and stress-related behavior. Finally, the role of the locus coeruleus in the control of CRF-mediated behavioral activities is discussed. The interaction of noradrenergic and CRF-neurons clearly implies that CRF appears to directly activate LC neurons in a stressful situation, thus ultimately coordinating the bodily response to a stressful stimulus.

Corticotropin-releasing factor receptors: physiology, pharmacology, biochemistry and role in central nervous system and immune disorders

Corticotropin-releasing factor (CRF) plays a major role in coordinating the endocrine, autonomic, behavioral and immune responses to stress through actions in the brain and the periphery. CRF receptors identified in brain, pituitary and spleen have comparable kinetic and pharmacological characteristics, guanine nucleotide sensitivity and adenylate cyclase-stimulating activity. Differences were observed in the molecular mass of the CRF receptor complex between the brain (58,000 Da) and the pituitary and spleen (75,000 Da), which appeared to be due to differential glycosylation of the receptor proteins. The recently cloned CRF receptor in the pituitary and the brain (designated as CRF1) encodes a 415 amino acid protein comprising seven putative membrane-spanning domains and is structurally related to the calcitonin/vasoactive intestinal peptide/growth hormone-releasing hormone subfamily of G-protein-coupled receptors. A second member of the CRF receptor family encoding a 411 amino acid rat brain protein with approximately 70% homology to CRF1 has recently been identified (designated as CRF2); there exists an additional splice variant of the CRF2 receptor with a different N-terminal domain encoding a protein of 431 amino acids. In autoradiographic studies, CRF receptors were localized in highest densities in the anterior and intermediate lobes of the pituitary gland, olfactory bulb, cerebral cortex, amygdala, cerebellum and the macrophage-enriched zones and red pulp regions of the spleen. CRF can modulate the number of CRF receptors in a reciprocal manner. For example, stress and adrenalectomy increase hypothalamic CRF secretion which, in turn, down-regulates CRF receptors in the anterior pituitary. CRF receptors in the brain and pituitary are also altered as a consequence of the development and aging processes. In addition to a physiological role for CRF in integrating the responses of the brain, endocrine and immune systems to physiological, psychological and immunological stimuli, recent clinical data implicate CRF in the etiology and pathophysiology of various endocrine, psychiatric, neurologic and inflammatory illnesses. Hypersecretion of CRF in the brain may contribute to the symptomatology seen in neuropsychiatric disorders, such as depression, anxiety-related disorders and anorexia nervosa. Furthermore, overproduction of CRF at peripheral inflammatory sites, such as synovial joints may contribute to autoimmune diseases such as rheumatoid arthritis. In contrast, deficits in brain CRF are apparent in neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease and Huntington's disease, as they relate to dysfunction of CRF neurons in the brain areas affected in the particular disorder. Strategies directed at developing CRF-related agents may hold promise for novel therapies for the treatment of these various disorders.

High intracerebral levels of CRH result in CRH receptor downregulation in the amygdala and neuroimmune desensitization

Corticotropin-releasing hormone (CRH) acts at the pituitary level to increase ACTH secretion and, within the central nervous system, to stimulate the sympathoadrenomedullary axis and behavioral activity. In addition, the central administration of CRH has been reported to reduce cellular immunity in the periphery. This study investigated the temporal relationship between CRH receptor regulation and the changes in splenic natural killer (NK) cell and pituitary-adrenocortical hormone responses to a single intracisternal (IC) CRH challenge (acute CRH) 24 h after chronic CRH pretreatment (5 nmol/day IC CRH for 4 days). Chronic CRH significantly decreased by 44.2 +/- 7.8% (P < 0.01) the CRH receptor concentration (beta max) in the amygdala. In contrast, the CRH receptor concentration of the anterior pituitary in the chronic CRH group was similar to the pituitary CRH receptor concentration in chronic saline controls. The immunoreactive-CRH concentration of the amygdala measured 24 h after the last IC CRH injection was not different from brain CRH levels in controls receiving chronic saline pretreatment. Consequently, the downregulation of amygdala CRH receptors occurred after transient increases in intracerebral CRH levels and did not result from ex vivo receptor occupancy by residual exogenous CRH sequestrated in brain tissue at the time of the CRH binding assay. Pretreatment with chronic CRH completely abolished the ability of a central CRH injection to suppress splenic NK activity; whereas, pituitary-adrenal responses to a superimposed acute CRH challenge were not significantly altered by chronic CRH pretreatment. These results suggest that the desensitization of the brain processes mediating CRH-induced suppression of splenic NK cytotoxicity is temporally correlated with CRH receptor downregulation in the amygdala but independent of pituitary-adrenal activation. These findings represent the first in vivo demonstration of homologous downregulation of extrahypothalamic CRH receptors and provide further evidence for the role of central CRH in the modulation of immune function.

Effect of acute and chronic diisopropylfluorophosphate and atropine administration on somatostatin binding in the rat frontoparietal cortex and hippocampus

The acute and chronic administration of diisopropylfluorophosphate (DFP), an inhibitor of acetylcholinesterase (AChE), and of atropine, a blocker of muscarinic cholinergic receptors, did not affect somatostatin-like immunoreactivity (SLI) content in the frontoparietal cortex and hippocampus of rats. Acute and chronic DFP administration increased the number of specific 125I-Tyr11-somatostatin (125I-Tyr11-SS) receptors in synaptosomes from the frontoparietal cortex but not in those from the hippocampus and did not change the affinity constant. This increase in 125I-Tyr11-SS binding was not due to a direct effect of DFP on somatostatin (SS) receptors since no rise of binding was produced by high concentrations of DFP (10(-5) M) when added in vitro. The increase could be blocked by pretreatment with atropine. The acute administration of atropine alone had no observable effect on the number of SS receptors. However, repeated atropine administration produced a significant decrease in the 125I-Tyr11-SS binding in synaptosomes from the frontoparietal cortex but not in those from the hippocampus although the affinity constant was unchanged. The results suggest that interactions between somatostatinergic and cholinergic receptors may be important in the rat frontoparietal cortex.

Cerebrospinal fluid corticotropin-releasing hormone and ACTH, and peripherally circulating choline-containing phospholipid in senile dementia

Cerebrospinal fluid (CSF) levels of corticotropin-releasing hormone (CRH) and ACTH, plasma levels of ACTH and cortisol, and serum levels of phospholipid and its fractions were determined in samples taken simultaneously from patients with senile dementia of the Alzheimer type (SDAT), multi-infarct dementia (MID) or dementia following a cerebrovascular accident (CVD), and the borderline-to-normal control subjects. CRH levels in CSF were significantly reduced in patients with SDAT and CVD but not with MID compared to the borderline-to-normal controls. ACTH levels in CSF were significantly reduced in SDAT compared to MID. The levels of circulating lecithin (phosphatidyl-choline) were depressed in a similar fashion to the levels of CRH in CSF in the SDAT patients and the group of severe dementia. Dementia and its severity did not affect the morning plasma levels of ACTH and cortisol. CSF CRH was positively correlated with CSF ACTH, while CSF ACTH was negatively correlated with plasma cortisol. No significant correlations were found between serum lecithin and CSF CRH or ACTH. These findings suggest that: 1) abnormalities in the extrahypothalamic CRH system play a role in the pathophysiology of senile dementia, which may not be specific to SDAT; 2) the CRH system and the ACTH system correlate with each other within the brain; 3) CSF ACTH is subject to the feedback inhibition by circulating cortisol; and 4) in the SDAT patients and the severe dementia group CSF CRH and serum lecithin are reduced probably via independent mechanisms.

Effect of various neurotransmitters and neuropeptides on the release of corticotropin-releasing hormone from the rat cortex in vitro

Corticotropin-releasing hormone (CRH), in addition to its neuroendocrine role, may act as a central neurotransmitter. Cerebral cortical CRH may have an important role in behavioral and neurodegenerative disorders. To gain an understanding of factors that may influence cortical CRH, we investigated the effect of several neurotransmitters and neuropeptides on the release of immunoreactive CRH (iCRH) from various cerebral cortical regions [frontal (FC), parietal (PC), temporal (TC), and occipital (OC)] in vitro. The hypothalamic release of iCRH was also evaluated under the same experimental conditions. Basal release of iCRH was approximately 2-fold, and KCl-stimulated iCRH release was approximately 4-fold higher in the hypothalamus than in any of the cortical regions. Cortical iCRH release was stimulated by 10 nM somatostatin (SRIF) in PC and 1 nM neuropeptide Y (NPY) in TC. Cortical iCRH release was inhibited by 1 and 10 nM acetylcholine (ACh), 0.1 microM glutamate, and 10 nM NPY. These effects were confined to the FC and/or PC. Hypothalamic iCRH release was stimulated by 1 and 10 nM ACh, 10 microM GABA, and 1 and 10 nM serotonin but was inhibited by 10 nM SRIF and 1 microM GABA. Growth hormone-releasing hormone did not affect cortical or hypothalamic iCRH release. These results demonstrate that CRH release from the cerebral cortex and the hypothalamus are under different regulatory mechanism(s). Furthermore, they indicate that the release of CRH in various cortical regions may be regulated differentially by the same neurotransmitter.

Somatostatin receptor elevation in rat striatum after diisopropylfluorophosphate administration

The acute and chronic administration of diisopropylfluorophosphate (DFP), an inhibitor of acetylcholinesterase or of atropine, a blocker of muscarinic cholinergic receptors, did not affect somatostatin-like immunoreactivity (SLI) content in the striatum of rats. Acute and chronic DFP administration increased the number of specific 125I-Tyr11-somatostatin (125I-Tyr11-SS) receptors in cells dissociated from the striatum without changing the affinity constant. Although the increase could be blocked by pretreatment with atropine, it was not due to a direct effect by DFP on somatostatin (SS) receptors, because no rise in 125I-Tyr11-SS binding was produced by high concentrations of DFP (10(-5) M) when added in vitro. The acute administration of atropine alone had no observable effect on the number of SS receptors. However, repeated atropine administration produced a significant decrease in the 125I-Tyr11-SS binding in cells dissociated from the striatum, although the affinity constant was unchanged. The results suggest that interactions between somatostatinergic and cholinergic receptors may be of importance in the rat striatum.

Effects of atropine treatment on in vitro and in vivo binding of 4-[125I]-dexetimide to central and myocardial muscarinic receptors

Upregulation of muscarinic cholinergic receptors (mAChR) after chronic atropine treatment has been described previously. The present study was designed to evaluate 4-iodine-125 dexetimide as an agent to determine changes in the number of mAChR. Rats were injected subcutaneously with atropine (500 mg/kg) either once or chronically, once daily for 10 days, and sacrificed 24 h later. In vitro binding assays with 4-[125I]-dexetimide showed significant increases in the number of mAChR in cerebra (21%) and ventricles (45%) after chronic atropine treatment but not after acute treatment. The affinity of binding to cerebral and ventricular mAChR declined after acute and chronic atropine treatment. In vivo studies were carried out involving intravenous injection of 4-[125I]-dexetimide 24 h after atropine treatment. Binding was markedly reduced in the brain and heart. Upregulation of mAChR, as seen in in vitro studies, could not be observed because of the remaining atropine. Occupancy of mAChR by atropine persisted as long as 7 days after one dose. The results of these studies indicate that 4-[125I]-dexetimide binding reflects the effects of atropine on central and peripheral muscarinic cholinergic receptors in vitro and in vivo.

Effects of noise on high-affinity choline uptake in the frontal cortex and hippocampus of the rat are blocked by intracerebroventricular injection of corticotropin-releasing factor antagonist

Acute exposure (20 min) to loud noise (100 dB) decreased sodium-dependent high-affinity choline uptake activities in the frontal cortex and hippocampus of the rat. These effects were blocked by intracerebroventricular (i.c.v.) administration of the corticotropin-releasing factor (CRF) antagonist alpha-helical-CRF9-41 (alpha-HCRF) immediately before noise exposure. Intracerebroventricular injection of CRF (1 microgram) also decreased high-affinity choline uptake in the frontal cortex and the hippocampus of the rat, and these effects of CRF could be blocked by pretreating the animal with the narcotic antagonist naltrexone (1 mg/kg, i.p.). These results indicate that the effects of noise on central cholinergic systems are mediated by CRF and suggest a stressor-CRF-endogenous opioid-acetylcholine sequence of effects in the brain.

Effects of nucleus basalis lesions on cerebral cortical concentrations of corticotropin-releasing hormone (CRH)-like immunoreactivity in the rat

The present study was designed to determine the effects of partial cholinergic denervation on parietal cortical corticotropin-releasing hormone-like immunoreactivity (CRH-LI) in the rat at different ages. Young adult rats received either unilateral or bilateral ibotenic acid infusions into their nucleus basalis, destroying most of the acetylcholinesterase-positive neurons in that region. Parietal cortical levels of CRH-LI were assayed 2.5, 10, 14 and 19 months after placement of nucleus basalis lesions. Parietal CRH-LI was elevated at 10, 14 and 19 months in bilaterally lesioned animals, while unilateral lesions had no effect on CRH-LI.

Distribution of corticotropin-releasing factor in the cerebellum and precerebellar nuclei of the opossum: a study utilizing immunohistochemistry, in situ hybridization histochemistry, and receptor autoradiography

This study reports 1) a nonhomogeneous distribution of three morphologically distinct, corticotropin-releasing factor (CRF)-immunoreactive axonal phenotypes within the cerebellum of the opossum: climbing fibers, mossy fibers, and beaded fibers within the ganglionic plexus; 2) the existence of CRF binding sites within the cerebellar cortex; and 3) the distribution of CRF-containing neurons in brainstem precerebellar nuclei identified with immunohistochemistry and in situ hybridization histochemistry. CRF-immunoreactive climbing and/or mossy fibers were identified within all cerebellar lobules. The density of CRF-immunoreactive fibers was greatest in the vermis, where longitudinal bands of intensely immunoreactive climbing and mossy fibers were interspersed with regions containing fibers demonstrating lower levels of immunolabeling. CRF-immunoreactive fibers were present within all deep cerebellar nuclei. The topography of CRF-containing cerebellar fibers is discussed with respect to possible sites of origin within the brainstem. CRF-immunoreactive neurons were identified in all nuclei of the inferior olivary complex, although the number and intensity of immunostaining of CRF-containing cells varied within and among individual nuclei. CRF-immunoreactive somata were also present in brainstem nuclei known to give rise to cerebellar mossy fibers. In situ hybridization histochemistry utilizing an 35S-labeled synthetic 48-base oligodeoxynucleotide complementary to amino acids 22-37 of rat CRF proper revealed that CRF mRNA is transcribed in precerebellar nuclei. Variation in the level of CRF mRNA was detected among inferior olivary nuclei, in correspondence with variations detected in the levels of immunostaining. Data from this study suggest that variation in the level of CRF immunoreactivity detected within cerebellar afferent fibers may correlate with the level of CRF mRNA within cell bodies of origin of the projections. In vitro receptor autoradiography, utilizing 125I-Tyro-ovine CRF, revealed correspondence between CRF binding sites and CRF-immunoreactive fibers in the cerebellar cortex. Results of this study support suggestions for CRF-mediated circuitry in the cerebellum.

Characterization of corticotropin-releasing factor receptors in dissociated brain cell cultures

Although corticotropin-releasing factor (CRF) receptors have been identified throughout the brain, relatively little is known about the regulation of CRF receptors. Recent investigations aimed at developing an in vitro model for studying the regulation of CRF receptors demonstrated CRF binding in brain cell cultures. To test the hypothesis that dissociated brain cell cultures contain CRF receptors and may provide a model for studying their regulation, studies characterizing binding of labeled CRF were performed. Dissociated cells derived from hypothalamus and extrahypothalamic forebrain (predominantly cortex) of day 17 fetal rats were maintained in chemically defined medium. We used a stable 125I-labeled analog of ovine CRF, 125I-Tyro-ovine CRF (125I-oCRF), to identify and characterize CRF receptors. Although specific binding of 125I-oCRF was demonstrated in both hypothalamic and extrahypothalamic cell cultures, the concentration of CRF receptors was much greater (3-5 fold) in extrahypothalamic cells. Binding of 125I-oCRF in extrahypothalamic cells was saturable and was composed of high affinity (Kd = 0.51 nM) and low affinity (Kd = 17.25 nM) sites. Pharmacological displacement of labeled CRF from cells with a variety of CRF fragments and analogs was similar to that in studies of pituitary and brain homogenates. Extrahypothalamic cells studied at several times between 4 and 13 days in culture revealed an increase in the number of CRF receptors; the concentration of CRF receptors at 13 days was 3.5 times that observed at 4 days. Studies directed toward determining whether CRF receptor concentration could be modulated by CRF, adrenocorticotropic hormone, atropine or a CRF antagonist showed a change (36% decrease) only in response to chronic exposure with CRF.

CONCLUSIONS

(1) dissociated fetal rat brain cell cultures derived from extrahypothalamic forebrain and hypothalamus contain CRF receptors; (2) CRF receptors in brain cells exhibit a differential distribution and characteristics similar to those previously reported in brain and pituitary; (3) dissociated fetal rat brain cell cultures may provide a relatively simplified in vitro model for studying the regulation of CRF receptors; and (4) CRF down-regulates its own receptor in extrahypothalamic forebrain cells.

Autonomic and cardiovascular effects of corticotropin-releasing factor in the spontaneously hypertensive rat

Corticotropin-releasing factor (CRF) administered intracerebroventricularly (i.c.v.) produced a greater increase of plasma epinephrine and glucose concentrations in spontaneously hypertensive rats (SHR) than in Wistar-Kyoto (WKY) or Sprague-Dawley (SD) rats. In contrast, CRF given i.c.v. produced significant elevations of mean arterial pressure (MAP) and heart rate (HR) in WKY and SD rats, but not in SHRs. To determine whether the prominent rise of plasma epinephrine levels following CRF administration to SHRs was a unique response to this peptide, two other stimuli for epinephrine secretion were evaluated, i.e. bombesin given i.c.v., and insulin given intravenously (i.v.). In contrast to the apparent enhanced responsiveness of the SHR to CRF, plasma epinephrine levels following either bombesin or insulin administration were similar in SHR, WKY and SD rats. These results demonstrate that the adrenomedullary response to CRF administration in the SHR is of greater magnitude than in WKY or SD rats. In an effort to identify the mechanisms responsible for the differential cardiovascular and adrenomedullary responses to CRF in the SHR versus WKY rat, CRF binding studies were performed. No difference in binding affinity of [125I]CRF or CRF receptor number could be identified in brains from SHR and WKY rats. Thus, CRF influences cardiovascular and adrenomedullary functions in a qualitatively dissimilar fashion in SHR and WKY rats. These differences are not secondary to any measurable alteration in CRF receptor affinity and number in SHR and WKY rats.

Corticotropin-releasing hormone (CRH) receptors in brain

Studies with iodine-125-labeled analogues of CRH have identified, characterized, and localized binding sites for CRH in rat and human brain. In addition, we have demonstrated that CRH stimulates cAMP production in various regions of the rat CNS. The significant regional differences in the stoichiometric relationship between receptor number and receptor-mediated cAMP production suggests that some populations of CRH receptors in brain may be functionally coupled to alternative signal transduction mechanisms. The autoradiographic data demonstrate that the distribution of CRH binding sites in the rat CNS corresponds well with the immunohistochemical distribution of CRH pathways and pharmacological sites of action of CRH in some areas of brain but not in others. The demonstration of an upregulation of CRH receptors following a decrease in CRH-IR in the cerebral cortex in Alzheimer's disease indicates a physiological relevance of the receptor site and is consistent with the concept that CRH acts as a neurotransmitter in regulating normal cortical functions. In addition, the data suggest that disease of this peptidergic system may be important in certain clinical manifestations of Alzheimer's disease. The effects of chronic atropine treatment to selectively upregulate CRH receptors in the cerebral cortex suggests an interaction between CRH and acetylcholine. These data provide further support for the proposed role of CRH as a neurotransmitter in the CNS. Furthermore, these studies demonstrating the characteristics of CRH receptors and CRH receptor-mediated signal transduction mechanisms in brain provide a means for better understanding the various functions of this neuropeptide under physiological and pathological conditions.

Time course of physostigmine effects on neuroendocrine responding at varying environmental temperatures

1. Hypothermia was found to be related to both the dose of physostigmine and the environmental temperature. 2. Plasma corticosterone levels were elevated above controls regardless of dose of physostigmine or environmental temperature. 3. Plasma free fatty acid levels appeared to be inversely related to physostigmine-induced hypothermia. 4. A hyperglycemic response was observed under all experimental conditions at 0.5 hours and 1.0 hour post injection. 5. Significant inhibition of brain acetylcholinesterase was observed, whereas, plasma and erythrocyte acetylcholinesterase activity was inconsistent.

Abnormalities in corticotropin-releasing hormone (CRH) in Alzheimer's disease and other human disorders

CRH-IR is significantly reduced in the cerebral cortex of individuals with AD, PD and PSP. Furthermore, we report that the decreases in CRH-IR in AD are accompanied by reciprocal increases in CRH receptors in affected cortical areas. The changes in pre- and postsynaptic markers for CRH are significantly correlated with decrements in ChAT activity. The demonstration of an up regulation of CRH receptors following a decrease in CRH-IR indicates a physiological relevance of the receptor site and is consistent with the concept that CRH acts as a neurotransmitter in normal cortical functions and that disease of this peptidergic systems may be important in certain clinical manifestations of dementia. While the clinical consequences of the changes in CRH in these various disorders are unclear, future therapies directed at increasing CRH levels in brain may prove useful for treatment.

Characterization of corticotropin-releasing factor receptor-mediated adenylate cyclase activity in the rat central nervous system

We report here that corticotropin-releasing factor (CRF) stimulates adenylate cyclase activity in the rat central nervous system (CNS). In frontoparietal cortex homogenates, the stimulation by CRF was dependent on time, temperature, tissue protein concentration, and guanine nucleotides. The rank order of potency for CRF analogs and fragments in stimulating adenylate cyclase activity [(Nle21,38) rat CRF greater than rat CRF approximately equal to acetyl ovine CRF (4-41) approximately equal to alpha helical ovine CRF greater than ovine CRF much greater than ovine CRF (1-39) approximately equal to ovine CRF (7-41)] was consistent with their affinities for CRF receptors in the brain and their relative potencies in stimulating pituitary adrenocorticotropic hormone secretion in vitro. The putative CRF receptor antagonist, alpha helical ovine CRF (9-41), did not stimulate adenosine 3',5'-cyclic monophosphate (cAMP) production but was able to attenuate the stimulation by various concentrations of rat CRF. The regional distribution of 125I-Tyr(o)-ovine CRF binding (olfactory bulb greater than frontoparietal cortex approximately equal to cerebellum greater than hypothalamus greater than striatum greater than or equal to midbrain greater than hippocampus greater than or equal to spinal cord) did not correspond with the regional degree of CRF receptor-mediated stimulation of adenylate cyclase (frontoparietal cortex greater than olfactory bulb greater than or equal to cerebellum greater than midbrain greater than or equal to hippocampus greater than striatum greater than or equal to hypothalamus greater than spinal cord). In addition, marked differences were observed in the ability of forskolin to potentiate CRF-stimulated cAMP production in the various brain areas examined. In summary, these data demonstrate that at least one of the second-messenger systems mediating the effects of CRF in the CNS involves stimulation of cAMP production and provides further support for a neurotransmitter role for this neuropeptide in the brain. Significant differences in the regulation of CRF-stimulated cAMP production and the disparity between CRF receptor number and receptor-mediated adenylate cyclase activity in discrete regions of the rat CNS suggest that some populations of CRF receptors in the brain may be functionally coupled to alternative signal transduction mechanisms.