Alpha 2-adrenoceptor-GTP binding regulatory protein-adenylate cyclase system in cerebral cortical membranes of adult and senescent rats

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

Reduced cortical noradrenergic neurotransmission is associated with increased neophobia and impaired spatial memory in aged rats

In the present study, young (5-month-old (mo)) and aged (24 mo) adult male Fischer-344 (F344) rats were assigned to experimental groups based upon their performance of a reference memory task in the Morris water maze and reactivity to a novel palatable taste in a gustatory neophobia task. Levels of norepinephrine (NE) and its metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) were assayed via high performance liquid chromatography (HPLC) in brain regions associated with the locus coeruleus (LC)-hippocampus-cortex system and A1/A2-hypothalamic system. Binding of ligands specific for alpha-1, alpha-2, beta-1, and beta-2 receptors was assessed in hippocampus and cortex with receptor autoradiography. Impaired acquisition and retention of the water maze task and gustatory neophobia in aged rats was primarily associated with decreased NE activity in cingulate cortex (CC) as indicated by a significant reduction in the MHPG/NE ratio coupled with increased NE content. No significant changes in adrenergic receptor binding were detected in any region sampled. The results suggest that an aging-related reduction in cortical NE neurotransmission is associated with the expression of increased neophobia and deficits in spatial learning and memory performance occurring with advanced age in rats.

Relationships between beta- and alpha2-adrenoceptors and G coupling proteins in the human brain: effects of age and suicide

Interactions between brain alpha2- and beta-adrenoceptors are of interest in physiological (aging) and pathological (major depression) processes involving both receptors. In this study, total beta-adrenoceptors and beta1/2-subtypes were quantitated in postmortem human brains to investigate their relationships with alpha2A-adrenoceptors and specific G proteins during the process of aging and in brains of suicide victims. Analysis of [3H]CGP12177 binding, in the presence of CGP20712A (beta1-antagonist), indicated that the predominant beta-adrenoceptor in the frontal cortex is the beta1-subtype (65-75%). The density of total beta- (r=-0.60, n=44) or beta1-adrenoceptors (r=-0.78, n=22), but not the beta2-subtype, declined with aging (3-80 years). The density of total beta- or beta1-adrenoceptors, but not the beta2-subtype, correlated with the number of alpha2-adrenoceptors quantitated in the same brains with the agonist [3H]UK14304 (r=0.71-0.81) or the antagonist [3H]RX821002 (r=0.61-0.66). Interestingly, the ratios alpha2/beta- or alpha2/beta1-adrenoceptors did not correlate with the age of the subject at death, indicating that the proportion of alpha2/beta-adrenoceptors in brain remains rather constant during the process of aging. The density of beta-adrenoceptors correlated with the immunodensity of G(alpha)s (r=0.55) and Gbeta (r=0.61) proteins, and that of alpha2-adrenoceptors with those of G(alpha)i1/2 (r=0.88) and Gbeta (r=0.65). In brains of suicides, compared to controls, the ratio between alpha2- and beta- or beta1-adrenoceptors (alpha2-full agonist sites/beta-sites) was greater (1.3- to 2.0-fold; P<0.05). The results demonstrate a close interdependence between brain alpha2- and beta-adrenoceptors during aging, and in brains of suicides. The quantitation of the alpha2A/beta-adrenoceptor ratio could represent a relevant neurochemical index in the study of brain pathologies in which both receptors are involved.

Evaluation of efficacies of different classes of antidepressants in the forced swimming test in mice at different ages

1. The efficacies of different classes of antidepressants were investigated using the forced swimming test with mice at different ages. 2. Imipramine (4-32 mg/kg), desipramine (2-16 mg/kg) and bupropion (32, 64 mg/kg) showed activity in all age groups. 3. The selective serotonin reuptake inhibitors (SSRIs) citalopram (16 and 32 mg) and paroxetine (4 and 8 mg) were inactive in the oldest (40 weeks) group of mice, despite showing activity at the same doses in mice ranging in age from 4-24 weeks old. 4. Both SSRIs showed anti-immobility effects at low doses, (paroxetine: 1 and 2 mg/kg; citalopram: 4 and 8 mg/kg) in the 40-week old mice. These effects were not evident in the three younger groups of mice. 5. Moclobemide, a reversible selective inhibitor of monoamine oxidase-A, showed activity only at a high dose (128 mg/kg) and only in 12-week old animals. 6. Since SSRIs have been reported to have relatively selective effects on 5-HT1B receptors, the present results suggest that further studies comparing the effectiveness of SSRIs and other antidepressants in elderly patients should be done. Studies of the effects of aging on the density and/or affinity of 5-HT1A and 5-HT1B/1D receptors are also warranted.

Dissociation of hypnotic-anesthetic actions of alpha 2 agonists from cyclic AMP in the rat

alpha 2 adrenergic agonists are used clinically for their anesthetic, analgesic, and sympatholytic actions in surgical patients. All alpha 2 adrenergic receptors, when activated by alpha 2-adrenergic agonists, are able to inhibit adenylate cyclase. We have examined the alpha 2-adrenoceptor-mediated anesthetic actions of dexmedetomidine, a highly selective alpha 2-adrenergic agonist, after pretreatment of the animals with rolipram, a cyclic AMP (cAMP)-specific phosphodiesterase inhibitor, cAMP accumulation and monoamine turnover were measured in the locus coeruleus (LC) and hippocampus (HC) following administration of rolipram [275 mg/kg, intraperitoneally (IP)] and dexmedetomidine (100-500 mg/kg, IP). The hypnotic response to dexmedetomidine was also measured in these animals. In other experiments, rats were stereotactically cannulated in the LC with an indwelling catheter, and after the second day, the tail-flick analgesic response to dexmedetomidine (3.5 mg/0.2 ml LC), following rolipram (275 mg/kg, IP) pretreatment, was assessed. In the presence of elevated cAMP levels, the hypnotic, analgesic, and sympatholytic effects of dexmedetomidine persisted. These data suggest that adenylate cyclase activity does not mediate the cellular responses to alpha 2-adrenergic agonists but instead may act in concert with other alpha 2-adrenoceptor-coupled effector mechanisms to transduce the anesthetic actions of these agents.

Membrane alterations as causes of impaired signal transduction in Alzheimer's disease and aging

Changes in cell-membrane composition in normal aging and in Alzheimer's and other age-related diseases appear to result in impaired neurotransmitter-triggered signal transduction. The impaired signal transduction seems to be related to dysfunctions in the coupling of G proteins to their receptors and effectors. Direct demonstration of altered physiochemical properties of brain tissue of patients with Alzheimer's disease has been achieved by small-angle X-ray diffraction. In this disease, thinner membranes correlate with a 30% decrease in moles of cholesterol:phospholipid. Such changes can affect directly the coupling and uncoupling properties of G proteins, and can account for signal transduction deficits. These findings offer a complementary alternative to the beta-amyloid hypothesis, and an opportunity to consider new types of therapeutic interventions.

Age-related changes in DNA synthesis stimulated by epinephrine and isoproterenol in primary cultured rat hepatocytes

We examined epinephrine- and isoproterenol-stimulated DNA synthesis in primary cultured hepatocytes from 6-, 12-, and 24-month-old rats. Epinephrine-stimulated DNA synthesis in 6-month-old rat hepatocytes began after 20 h and reached a maximum at 50 h. Similarly, isoproterenol-stimulated DNA synthesis in 6-month-old rat hepatocytes began after 10 h and reached a maximum at 45 h. In contrast, both epinephrine- and isoproterenol-stimulated DNA synthesis in 12- and 24-month-old rat hepatocytes were reduced approximately 40-60% and 80%, respectively, as compared to that at 6 months. Both epinephrine- and isoproterenol-stimulated DNA synthesis were strongly inhibited by the beta-adrenergic antagonist, propranolol, but not by the alpha 1-adrenergic antagonist, prazosin, or the alpha 2-adrenergic antagonist, yohimbine. However, in the presence of EGF, epinephrine-stimulated DNA synthesis activity was inhibited by prazosin but not by propranolol. These results indicate that stimulated DNA synthesis in rat hepatocytes declines with age and that there are two different pathways for epinephrine-stimulated DNA synthesis in the presence or absence of EGF.

Age-related changes of noradrenergic-NPY interaction in rat brain: norepinephrine, NPY levels and alpha-adrenoceptors

Noradrenergic-neuropeptide Y interaction, which is implicated in different physiological functions, was studied in senescent rats. Norepinephrine (NE) and neuropeptide Y (NPY) levels were measured in brainstem and hypothalamus, and alpha-adrenergic binding was investigated in brainstem in young (4 months) and old (34 months) Wistar rats. NE concentration was the same in senescent rats, whereas NPY concentration was decreased both in brainstem and hypothalamus compared to levels in young rats. [3H]prazosin binding to alpha 1-adrenoceptors was not modified, but [3H]rauwolscine binding to alpha 2-adrenoceptors was altered with age. In fact, the density of alpha 2-adrenoceptors (Bmax) was lower, while the binding affinity (Kd) was increased in old compared to young rats. These results suggest that the decrease of NPY levels could be one of the possible reasons for changes in [3H]rauwolscine binding to alpha 2-adrenoceptors in old rats. The G-protein-adenylate cyclase system, which is impaired in senescent rats, could be involved in the disorganization of noradrenergic-NPY interaction.

Kainic acid-induced seizures in aged rats: neurochemical correlates

This study was conducted to assess the functional integrity of the kainate receptor-mediated seizure response in aged rats. Kainic acid was administered systemically to aged female Long-Evans (LE) rats and aged male F344 rats and the proconvulsant actions of kainic acid was compared to adult controls. The effects of kainic acid on brain regional content of monoamines and amino acids was also determined in the aged female LE and adult control rats. The latency to full clonic-tonic seizures was significantly reduced in aged female LE rats, and the number of seizures was significantly increased above that of the controls. There was increased mortality and a reduction in the latency to exhibit wet dog shakes in the aged F344 rats. Studies were also conducted to evaluate the role of ovarian hormones, route of administration, and dose of kainic acid in mediating the enhanced proconvulsant actions of kainic acid in aged rats. The neurochemical studies suggested that kainic acid significantly enhanced the release of ASP, GLU, and norepinephrine (NE) in the aged rats exhibiting clonic-tonic seizures. The adult rats given the same dose of kainic acid (15 mg/kg, IP) did not exhibit any significant change in brain content of monoamines or amino acids except for a reduction in mediobasal hypothalamic NE. An in vitro study was also conducted using brain slices from adult and aged F344 and it was found that aged rats released significantly more ASP than adults in response to kainic acid. These neurochemical findings were discussed in relation to previous studies of age-related alterations in excitatory amino acids (EAAs) and the role of EAA and NE in modulating limbic seizures. This study has clearly demonstrated that aged rats may be more susceptible to the excitotoxic action of EEAs acting through kainetic receptors.

Age-induced changes in single locus coeruleus brain transplants grown in oculo: an in vivo electrochemical study

Brain stem tissue from fetal Sprague-Dawley rats containing the nucleus locus coeruleus (LC) was transplanted into the anterior chamber of the eye of young adult host rats and was studied at 4-6 months (young control) or 24-28 months after grafting (old). High-speed in vivo electrochemical measurements were used to characterize the potassium-evoked synaptic overflow of norepinephrine (NE) in both young and aged LC brain grafts. The amplitudes of potassium-evoked NE overflow were attenuated in the aged grafts as compared to the young LC grafts. In addition, the rise times of potassium-evoked responses were longer in the old LC grafts than in the young transplants. In contrast, the NE content of aged LC grafts, as determined by high-performance liquid chromatography coupled with electrochemical detection (HPLC-EC), was only slightly diminished and not significantly different from the NE levels seen in young LC grafts. However, light microscopical evaluation using tyrosine-hydroxylase immunocytochemistry revealed pyknotic cell bodies and fluorescent accumulations in aged locus coeruleus transplants which were indicative of degeneration in these grafts. The present data demonstrate a significant age-related decline in the presynaptic function of NE-containing neurons in intraocular locus coeruleus transplants of Sprague-Dawley rats.

Alpha- and beta-adrenergic receptors of the rat cerebral cortex and cerebral microvessels in aging, and their response to denervation

We studied the effects of aging and norepinephrine depletion 2 weeks after unilateral locus ceruleus lesion on alpha 1-, alpha 2- and beta-adrenergic receptors by ligand binding methods in the ipsilateral and contralateral cerebral cortex of Fischer-344 rats. We also studied the effects of aging and noradrenergic denervation on beta-adrenergic receptors in isolated cerebral microvessels. We found that specific [125I]HEAT binding to alpha 1-adrenergic receptors was not affected by aging or by norepinephrine depletion. Although aging also had no effect on the density or affinity of [3H]UK-14,304 and [125I]pindolol binding to alpha 2- and beta-adrenergic receptors, the density of receptors increased significantly in all age groups after noradrenergic denervation. beta-Adrenergic receptors of cerebral microvessels also were unaffected by aging, but increased their density after noradrenergic denervation at all ages. In all instances, there were no significant effects on the affinity of ligand binding. We conclude that aging does not affect the density or the affinity of adrenergic receptors in the cerebral cortex of Fischer-344 rats, nor does it affect the response of these receptors to norepinephrine depletion.

Inhibition of cAMP production by alpha 2-adrenoceptor stimulation in rabbit retina

Adenylate cyclase activity in rabbit retinal homogenates can be stimulated directly by forskolin or through a receptor-mediated mechanism by vasoactive intestinal peptide (VIP). In contrast the alpha 2-adrenoceptor agonists clonidine and UK-14,304 reduce the basal cAMP level slightly. This was more evident following application of forskolin and VIP where the decrease of cAMP caused by clonidine and UK-14,304 is dose-dependent. The alpha 2-adrenoceptor agonist response is blocked by pertussis toxin and is insensitive to the phosphodiesterase inhibitor, isobutylmethylxanthine, suggesting the involvement of a Gi-protein. Clonidine and UK-14,304 attenuation of elevated cAMP levels can be inhibited by the alpha 2-receptor antagonist yohimbine and phentolamine but not by the specific alpha 1-receptor antagonist, prazosin. Serotonergic, cholinergic and beta-adrenergic receptor antagonists were without effect. The results demonstrate that alpha 2-adrenergic receptors in the retina exert inhibitory effects on adenylate cyclase activity mediated by an inhibitory guanine nucleotide regulating protein.

Adrenergic receptors in aging and Alzheimer's disease: decreased alpha 2-receptors demonstrated by [3H]p-aminoclonidine binding in prefrontal cortex

Biochemical and pathological studies have described abnormalities in the brainstem locus coeruleus noradrenergic neurones in Alzheimer's disease (AD) and in aging. Loss of cortical noradrenergic fibers originating from the locus coeruleus may cause a decrease in presynaptic receptors or induce an increase in postsynaptic receptors, similar to "denervation supersensitivity" in animal models. Thus far it is unclear whether alpha 2-adrenergic receptors are affected in AD. In this study, we assessed the specific binding of [3H]p-aminoclonidine, an agonist at alpha 2-receptors and at imidazoline-preferring binding sites, to prefrontal cortex and other regions including hippocampus, temporal cortex, putamen and cerebellum from subjects with AD and aging controls. We particularly focused on the prefrontal cortex because of its relatively rich monoaminergic innervation and recent evidence suggesting involvement of noradrenergic mechanisms in cognition in aging nonhuman primates. The other regions, which are also innervated by noradrenergic fibers, were examined for comparison. Ligand binding to prefrontal cortex decreased with age of controls and was also significantly reduced by approximately 50% in AD subjects compared to age-matched controls. This change in AD was related to the maximum binding capacity (Bmax) rather than to an altered affinity of the ligand for the receptor. There were no significant changes in any of the other regions studied. Binding did not change with postmortem delay or with duration of tissue storage. We suggest that presynaptic alpha 2-receptors presumably labeled by [3H]p-aminoclonidine on noradrenergic synapses are those that are selectively decreased in the prefrontal cortex in AD and in aging.

Antinociceptive actions of alpha 2-adrenoceptor agonists in the rat spinal cord: evidence for antinociceptive alpha 2-adrenoceptor subtypes and dissociation of antinociceptive alpha 2-adrenoceptors from cyclic AMP

The antinociceptive actions of intrathecal injections of two alpha 2-adrenergic agonists, UK-14,304 and guanfacine, were investigated in rats after pretreatment of the animals with the noradrenaline neurotoxin N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) 14 days in advance. The chronic noradrenaline depletion induced by DSP4 caused a marked increase in sensitivity of the antinociceptive action of UK-14,304 in the tail-flick test. By contrast, the antinociceptive effect of guanfacine was not appreciably affected by the DSP4 treatment. The antinociceptive effects of both UK-14,304 and guanfacine were blocked by intraperitoneal injections of yohimbine, a result indicating that both drugs induced their actions by activating alpha 2-adrenoceptors. Both UK-14,304 and guanfacine were found to reduce the production of cyclic AMP (cAMP) in the spinal cord, as determined using an in vitro radioisotopic method. The cAMP inhibitory effects of both agonists were effectively blocked by yohimbine, but not by prazosin, a finding indicating the alpha 2-adrenergic nature of the response. However, the cAMP inhibitory effect of UK-14,304 was not potentiated by pretreatment with DSP4, a finding in marked contrast with the strong potentiation of the antinociceptive action of UK-14,304 induced by the chronic depletion of endogenous noradrenaline. Moreover, intrathecal injections of forskolin, which increased the endogenous levels of spinal cord cAMP fivefold, did not modify the antinociceptive effects of UK-14,304 or guanfacine in neither normal nor DSP4-treated animals. It is suggested that there exist pharmacologically differing alpha 2-adrenergic receptor pathways capable of mediating antinociceptive effects at the level of the spinal cord. The cAMP inhibitory actions of spinal cord alpha 2-adrenoceptors appear not to be directly linked with the antinociceptive actions of these receptors.

Aging does not alter muscarinic receptor-mediated inhibition of cyclic AMP formation in the striatum and hippocampus

The effects of aging on the inhibition of forskolin-stimulated cyclic AMP formation by muscarinic receptors were investigated. There were no detectable changes in the magnitude of maximal inhibition by carbamylcholine or the potency of the agonist in inducing this response in either the striatum or hippocampus obtained from young or old Fisher 344 rats.

Effect of age and monosodium-L-glutamate (MSG) treatment on neurotransmitter content in brain regions from male Fischer-344 rats

Peripheral administration of monosodium-L-glutamate (MSG) has been found to be neurotoxic in neonatal rats. When administered in an acute, subconvulsive dose (500 mg/kg i.p.), MSG altered neurotransmitter content in discrete brain regions of adult (6 month old) and aged (24 month old) male Fischer-344 rats. Norepinephrine (NE) content was reduced in both the hypothalamus (16%) and cerebellum (11%) of adult rats, but was increased in both the hypothalamus (7%) and cerebellum (14%) of aged rats after MSG treatment. MSG also altered the dopamine content in adult rats in both the posterior cortex and the striatum, causing a reduction (23%) and an increase (12%), respectively. Glycine content in the midbrain of aged rats increased (21%) after MSG injection. Of particular interest is the widespread monoamine and amino acid deficits found in the aged rats in many of the brain regions examined. NE content was decreased (11%) in the cerebellum of aged rats. Dopamine content was reduced in both the posterior cortex (35%) and striatum (10%) of aged rats compared to adult animals. Cortical serotonergic deficits were present in aged rats with reductions in both the frontal (13%) and posterior cortex (21%). Aged rats also displayed deficits in amino acids, particularly the excitatory amino acids. There were glutamate deficits (9-18% reductions) in the cortical regions (posterior and frontal) as well as midbrain and brain stem. Aspartate, the other excitatory amino acid transmitter, was reduced 10% in the brainstem of aged rats. These data indicate that an acute, subconvulsive, dose of MSG may elicit neurochemical changes in both adult and aged male Fisher-344 rats, and that there are inherent age-related deficits in particular neurotransmitters in aged male Fisher-344 rats as indicated by the reductions in both monoamines and amino acids.

Age-related changes in uptake and release on L-[3H]noradrenaline in brain slices of senescence accelerated mouse

High K+ and N-methyl-D-aspartate (NMDA) evoked L-[3H]noradrenaline (NA) release to a similar degree in the brain slices of 1-month-old senescence-accelerated resistant mice (SAM-R/1) and senescence-accelerated prone mice (SAM-P/8). However, 30 mM KCl-induced L-[3H]NA release significantly diminished in SAM-P/8 from 3 to 12 months without changing in SAM-R/1. In addition, NMDA-induced L-[3H]NA release was also reduced at 3 months and lowered to a level of spontaneous release at 12 months in SAM-P/8, but no age-related changes in SAM-R/1 were observed. It is suggested that NA release from NA nerve terminals responsive to depolarization is reduced in SAM-P/8 at an earlier stage than in SAM-R/1. Furthermore, NMDA receptors which could be localized in the soma and/or nerve terminals, seem to be involved in NA release and to be decreased with advancing age in SAM-P/8.

Increased responsiveness of the cerebral cortical phosphatidylinositol system to noradrenaline and carbachol in senescent rats

The responsiveness of cerebral cortical alpha 1-adrenoceptors and cholinergic muscarinic M1 receptors was assessed in young (3 months) and aged (24 months) male Sprague-Dawley rats. The measure of responsiveness was the accumulation of inositol phosphate (IP) formed in [3H]myo-inositol-preloaded cerebral cortical slices in the presence of lithium, following stimulation with various concentrations of noradrenaline (1-300 microM) and carbachol (5-1000 microM). In old rats the maximum response to noradrenaline was higher by 80%, and that to carbachol by 33%, indicating an increased responsiveness of the investigated receptors in senescence.

Ligand-binding characteristics of [3H]QNB, [3H]prazosin, [3H]rauwolscine, [3H]TCP and [3H]nitrendipine to cerebral cortical and hippocampal membranes of senescence accelerated mouse

The senescence accelerated mouse (SAM) is known as a murine model of aging and memory dysfunction. In the cerebral cortical membranes of male 9-month-old SAM mice, the Bmax values of [3H]rauwolscine and [3H]nitrendipine binding, and the values of both Kd and Bmax of [3H]TCP binding in the accelerated aging strain SAM-P/8, were significantly increased compared with the values in the control strain SAM-R/1. In hippocampal membranes, however, the Bmax values of [3H]quinuclidinyl benzilate and [3H]nitrendipine binding were significantly decreased in SAM-P/8 compared with those in SAM-R/1. These results suggest that muscarinic acetylcholine receptors, alpha 2-adrenoceptors, N-methyl-D-aspartate receptor channels and L-type Ca2+ channels are changed in cerebral cortex and hippocampus in SAM-P/8 at 9 months.

Aging of rat heart myocytes disrupts muscarinic receptor coupling that leads to inhibition of cAMP accumulation and alters the pathway of muscarinic-stimulated phosphoinositide hydrolysis

The biochemical responses to muscarinic stimulation (inhibition of isoproterenol-stimulated cAMP accumulation and stimulation of phosphoinositide turnover) were investigated in intact myocyte cultures prepared from the hearts of newborn rats. The studies employed young (5 days after plating) and aged (14 days old) myocyte cultures. Aging of the myocyte cultures was accompanied by marked alterations in both the inhibition of cAMP accumulation and the stimulation of the phosphoinositide metabolism via the muscarinic receptors. However, the effects on the two muscarinic responses were different. The first response was disrupted at the level of the coupling of the muscarinic receptors with adenylate cyclase through Gi. On the other hand, muscarinic stimulation of phosphoinositide hydrolysis still occurred in the aged myocyte cultures; however, the inositol trisphosphate generated was not converted to inositol 1-phosphate as in young cultures or as in aged cultures stimulated by norepinephrine. This raises the possibility that muscarinic activation of aged myocyte cultures shifts the metabolic state of the cells and alters the pathway of phosphoinositide hydrolysis. Treatment of aging cultures with phosphatidylcholine liposomes under conditions that yielded aged myocyte cultures with a lipid composition resembling that of young ones restored the muscarinic effect on cAMP accumulation, where the impairment in aged cultures was at the coupling stage (which takes place in the plasma membrane). This treatment had no effect on the response of the phosphoinositide metabolism to muscarinic stimulation.

Ontogenesis of alpha 2-adrenoceptor coupling with GTP-binding proteins in the rat telencephalon

The ontogenesis of alpha 2-adrenoceptors and GTP-binding proteins and their coupling activity were investigated in telencephalon membranes of developing rats. The manganese-induced elevation of [3H]clonidine binding was increased in an age-dependent manner but the guanosine 5'-O-(3-thio)triphosphate-induced decrease in binding did not change. The extent of the binding of [3H]clonidine at 15 nM (saturable concentration) increased in an age-dependent manner and reached the adult level at 4 days after birth. Cholera toxin and pertussis toxin catalyzed ADP-ribosylation of proteins of 46 and 41/39 kilodaltons (kDa) in solubilized cholate extracts of the membranes. The 41/39-kDa proteins ADP-ribosylated by pertussis toxin (Gi alpha + Go alpha) were increased with age and reached the adult level at day 12, whereas the 46-kDa protein (Gs alpha) reached its peak on day 12 and then decreased to the fetal level at the adult stage. The immunoblot experiments of the homogenates with antiserum (specific antibody against alpha- and beta-subunit of GTP-binding proteins) demonstrated that the 39-kDa alpha-subunit of (Go alpha) and the 36-kDa beta-subunit of GTP-binding protein (beta 36) increased with postnatal age. In contrast, 35-kDa beta-subunit (beta 35) did not change. From these results, it is suggested that the coupling activity of alpha 2-adrenoceptor with GTP-binding protein gradually develops in a manner parallel with the increase of alpha 2-adrenoceptor and pertussis toxin sensitive GTP-binding proteins, Gi, and that alpha 39 beta 36 gamma may be related to the differentiation and/or growth of nerve cells in rat telencephalon.

Age-related changes in rat serotonergic and adrenergic systems and in receptor responsiveness to subchronic desipramine treatment

Noradrenergic (NA) and serotonergic (5-HT) receptor profiles were compared in the cerebral cortex of young adult (3 months old) and aged (24 months old) male Sprague Dawley rats. Beta and alpha-1 receptors were significantly decreased in 24 month old rats, whereas alpha-2 receptors remained unchanged. 5-HT-2 Postsynaptic receptors and 5-HT high affinity uptake were reduced in aged animals; on the other hand the number of 3H-imipramine (3H-IMI) recognition sites located on serotonin nerve terminals and labelled also by 3H-paroxetine (3H-PAR), were significantly higher in the cerebral cortex of old rats. The ratio 5-hydroxyindoleacetic acid (5-HIAA)/5-HT, an index of serotonin turn-over, increased in some brain areas of senescent rats. There were no age-related changes in the responsiveness of investigated binding sites to subchronic desipramine treatment; the density of beta-noradrenergic and 5-HT-2 serotonergic receptors and that of 3H-IMI binding sites was reduced both in young adult and aged rats.

Inhibition of cyclic AMP production by alpha 2-adrenoceptor stimulation in the guinea-pig spinal cord slices

In spinal cord slices isolated from guinea-pig and preincubated with 3H-adenine, 0.3-30 microM forskolin induced a dose-dependent increase in the content of 3H-cAMP, the maximal increase being about 8-fold. The selective alpha 2-adrenergic agonist UK-14,304 (10 microM) reduced both the basal and the forskolin stimulated levels of 3H-cAMP by 18-32%. Dose response curves of the effect of UK-14,304 on cAMP production in the spinal cord slices, stimulated with 3 microM forskolin, showed an IC50 of 37 nM and a maximally inhibitory effect of 27%. A number of other alpha 2-adrenergic agonist (clonidine, guanfacine, B-HT 920 and B-HT 933) also inhibited the forskolin stimulated 3H-cAMP production; clonidine and guanfacine being almost equipotent with UK-14,304, but their maximal inhibitory effects being only about 6-7%. B-HT 920 and B-HT 933 were less potent and their maximal inhibitory effects about 16-21%. The dose response curve of UK-14,304 on inhibition of forskolin stimulated cAMP production was shifted almost 50-fold to the right by 0.3 microM yohimbine. Prazosin (0.3 microM) did not affect the UK-14,304 dose response curve. It is concluded that alpha 2-adrenoceptor stimulation mediates inhibition of cAMP production in the guinea-pig spinal cord.

Chronic membrane depolarization regulates the level of the guanine nucleotide binding protein Go alpha in cultured neuronal cells

Chronic membrane depolarization results in an increase in muscarinic acetylcholine receptor (mAChR) number in N1E-115 neuroblastoma cells. Because the mAChR interacts with the guanine nucleotide binding regulatory (G) proteins, Gi and Go, the effect of chronic membrane depolarization on the levels of subunits of these G proteins was examined. Quantitation of G protein subunit levels was performed using affinity-purified, monospecific antibodies in a quantitative immunoblot assay. Incubation with 50 microM veratridine (VTN), an activator of voltage-sensitive Na+ channels, induced a 48 +/- 15% increase in the level of the alpha subunit of Go. The effect of VTN was blocked by tetrodotoxin. On removal of VTN, the level of Go alpha decreased to control levels within 24 h. The levels of the alpha subunit of Gi and the common beta subunit were not affected by VTN treatment. These results show that in N1E-115 cells, the level of the alpha subunit of Go is regulated in a manner similar to the level of mAChR in response to chronic membrane depolarization.

Histamine increases phospholipid methylation and H2-receptor-adenylate cyclase coupling in rat brain

Histamine stimulated the enzymatic synthesis of phosphatidylcholine from phosphatidylethanolamine in crude synaptic membranes of rat brain containing the methyl donor S-adenosyl-L-methionine (SAM). In the presence of, but not in the absence of SAM, histamine increased cyclic AMP accumulation at the concentrations that stimulate phospholipid methylation. S-Adenosyl-L-homocysteine, an inhibitor of phospholipid methyltransferases, inhibited histamine-stimulated phospholipid methylation and histamine-induced cyclic AMP accumulation in the presence of SAM in a concentration-dependent manner. Histamine-induced [3H]methyl incorporation into phospholipids exhibited a marked regional heterogeneity in rat brain in the order of cortex greater than medulla oblongata greater than hippocampus greater than striatum greater than midbrain greater than hypothalamus. The regional distribution of histamine-induced cyclic AMP accumulation exactly paralleled histamine-stimulated [3H]methyl incorporation in rat brain. Histamine-induced cyclic AMP accumulation was inhibited by the addition of cimetidine or famotidine, but not by mepyramine or diphenhydramine. The accumulation of cyclic AMP in the presence of SAM was observed by the addition of impromidine or dimaprit, but not by 2-pyridylethylamine. These results indicate that phospholipid methylation is induced by histamine and may participate in H2-receptor-mediated stimulation of adenylate cyclase in rat brain.

Alpha- and beta-adrenergic receptor function in the brain during senescence

Alpha- and beta-adrenergic receptors and their second messengers play an important role in brain neurotransmission. Changes in receptor function with age may be involved in the age-related changes in arousal, mood and memory. The predominance of data indicates there is decreased beta-adrenergic receptors in all areas of the brain with the exception of the cortex. Evidence suggests a decreased rate of receptor synthesis may be contributing to this loss of receptors with age. Alpha-adrenergic receptor synthesis is also diminished with age. The modulation of receptor concentrations by hormonal factors is impaired with age, especially the time to recover from receptor down-regulation.