The effect of cholinergic denervation and hippocampal sympathetic ingrowth on the internalization of muscarinic receptors in rat hippocampus

Following cholinergic denervation of the hippocampus by medial septal (MS) lesions, an unusual neuronal reorganization occurs in which peripheral sympathetic fibers, originating from the superior cervical ganglia, grow into the hippocampus (hippocampal sympathetic ingrowth; HSI). Previously, we have found that with MS lesions, animals with (the HSI⁽⁺⁾ group) and without (HSI⁽⁻⁾ group) ingrowth differed in carbachol stimulated PI hydrolysis, in PKC activity, and in muscarinic cholinergic receptors (mAChR). In this study, performed in hippocampal slices obtained four weeks after MS lesions, we utilized the hydrophilic muscarinic antagonist [³H] N-methylscopolamine ([³H]NMS) and hydrophobic muscarinic antagonist [³H]quinuclidinyl benzilate ([³H]QNB) in the presence of either 4-α-phorbol or phorbol 12,13-dibutyrate (PDBu) to determine the effect of MS lesions with and without ingrowth on PKC-mediated mAChR internalization. In the presence of PDBu, a group effect was observed in [³H]NMS binding, with control groups > HSI⁽⁺⁾ group > HSI⁽⁻⁾ group. However, [³H]QNB binding was similar across groups. These results suggest that the cholinergic denervation of the hippocampus enhances the internalization of mAChRs, which is modified in the presence of HSI.     

Hippocampal sympathetic ingrowth and cholinergic denervation uniquely alter muscarinic receptor subtypes in the hippocampus

Following cholinergic denervation of the hippocampus by medial septal lesions, an unusual neoronal reorganization occurs, in which peripheral sympathetic fibers, originating from the superior cervical ganglia, grow into the hippocampus. Previously, we have found that both hippocampal sympathetic ingrowth (HSI) and cholinergic denervation (CD), alone, altered the total number and affinity of muscarinic cholinergic receptors (mAChR). In this study, we utilized the muscarinic antagonist [³H]Pirenzepine, in combination with membrane radioligand binding techniques, to determine the effects of HSI and CD on hippocampal M₁ and M₁ + M₃ mAChR subtypes, 4 weeks after MS lesions. In both the dorsal and ventral hippocampus, HSI was found to markedly diminish the number of M₁ AChRs, while CD was found to increase the number of M₁ AChRs. Neither treatment affected the affinity of the M₁ AChR. However, when M₁ + M₃ binding was assessed, CD was found to decrease the affinity in both hippocampal regions, without altering the number of receptors. Neither affinity nor number of M₁ + M₃ receptors was altered by HSI. The results of this study suggest that both cholinergic denervation and hippocampal sympathetic ingrowth uniquely affect hippocampal muscarinic receptors.     

Long-lasting change in the membrane-associated protein kinase C activity in the hippocampal kindled rat

The effect of hippocampal kindling on protein kinase C (PKC) activity and protein concentration was investigated in rat amygdala/pyriform cortex (AM/PC) and right (contralateral) and left (ipsilateral) hippocampus (HIPP). There was no difference in cytosolic PKC activity between control and kindled groups in any part of the brain. The membrane-associated PKC activity was altered as follows. One week after the last seizure, it was significantly increased in both right (by 26%, P less than 0.05) and left HIPP (by 30%, P less than 0.02). Four weeks after the last seizure, it was significantly increased in the AM/PC (by 14%, P less than 0.02), right HIPP (by 37%, P less than 0.01) and left HIPP (by 24%, P less than 0.05). The protein concentrations in the crude cytosolic extracts prior to elution of PKC through DE-52 columns were significantly increased in the AM/PC (by 11%, P less than 0.05) and right HIPP (by 18%, P less than 0.02) 4 weeks after the last seizure. In the membrane extracts, there was a significant increase by 23% (P less than 0.02) in the left HIPP 1 week after the last seizure. In the fraction co-eluted with PKC, a significant increase in protein concentration of the cytosolic preparation was confirmed in the AM/PC (by 12%, P less than 0.05) as well as in the left HIPP (by 15%, P less than 0.05) 4 and 1 weeks respectively after the last seizure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long-lasting increase in protein kinase C activity in the hippocampus of amygdala-kindled rat

Previous studies have demonstrated that membrane-associated protein kinase C (PKC) activities in the right and left hippocampus of rats kindled from the left hippocampus increased significantly at 4 weeks [9] and 4 months [22] after the last seizure compared with those in matched control rats. In this study, we investigated the effect of kindling from the left amygdala on PKC activities in the amygdala/pyriform cortex and hippocampus at long seizure-free intervals (4 and 16 weeks) from the last amygdala-kindled seizure. Membrane-associated PKC activity of the kindled group increased significantly only in the left hippocampus compared with the left side control (the left hippocampus of rats subjected to a sham operation) at 4 weeks (by 34%, P < 0.03) and 16 weeks (by 24%, P < 0.05) after the last seizure. There was no significant alteration in the membrane-associated PKC activity of the kindled group in the right hippocampus or amygdala/pyriform cortex in any seizure-free interval after the last amygdala seizure. Cytosolic PKC activity did not differ between the kindled and control groups in any brain region examined in any seizure-free interval. At 16 weeks after the last seizure, the PKC activity in the P₁ fraction of the kindled group increased significantly only in the left hippocampus (by 49%, P < 0.005), but not in the right hippocampus. Neither PKC activity in the P₂ fraction nor that in the cytosolic fraction was altered in the kindled group after this seizure-free interval. The prolonged increase in activity of the membrane-associated PKC and that in the P₁ fraction in the hippocampus induced by amygdala-kindling may contribute to long-lasting seizure susceptibility induced by kindling.     

Translocation of protein kinase C in rat hippocampal slices

Tumor-promoting phorbol esters specifically activate protein kinase C and mimic the effects of neurotransmitters in certain systems. Treatment of hippocampal slices with phorbol dibutyrate caused translocation of protein kinase C activity from cytoplasm to membranes. Experiments with carbachol, norepinephrine, glutamate, KCl, and LiCl failed to demonstrate a similar translocation. Translocation more readily provides an index of protein kinase C involvement for phorbol esters than for other agents in hippocampus.     

Physical activity enhances spatial learning performance with an associated alteration in hippocampal protein kinase C activity in C57BL/6 and DBA/2 mice

The effects of physical activity on spatial learning performance and associated hippocampal functioning were examined in C57BL/6Ibg (C57) and DBA/2Ibg (DBA) mice. C57 and DBA mice, 3 months of age, were subjected to 8 weeks of a physical activity regime (consisting of moderate-pace treadmill running 5 days/week, 60 min/day, 0% grade, 12 m/min) or remained sedentary in their cages. Mice were then tested on the Morris water maze task for 6 days followed by 12 days of testing on the place learning-set task (8 trials/day with each task). Both C57 and DBA run mice showed no difference in swim speed compared to controls. Hippocampal protein kinase C (PKC) activity was measured in cytosolic, loosely bound, and membrane-bound homogenate fractions. Mice subjected to the physical activity protocol were compared to sedentary controls from the same set of litters. Physical activity produced a 2- to 12-fold enhancement in spatial learning performance on both the Morris (P < 0.0001) and place learning-set (P < 0.02) probe trials in both C57 and DBA mice. DBA mice, which characteristically perform poorly in comparison to C57 mice, were enhanced to perform similarly to C57 control mice. This physical activity-induced enhancement in spatial learning performance was accompanied by alterations in hippocampal bound PKC activity (P < 0.05). These data provide further support for our previous hypothesis of a PKC activity involvement in spatial learning and enhancement of spatial learning performance in rodents by physical activity. In addition, these data indicate that hippocampal PKC activity may be involved in the physical activity-induced enhancement of spatial learning performance.     

Enhancement of hippocampally-mediated learning and protein kinase C activity by oxiracetam in learning-impaired DBA/2 mice

The effects of oxiracetam on hippocampally-mediated learning performance and hippocampal protein kinase C (PKC) were examined in C57BL/6Ibg (C57) and DBA/2Ibg (DBA) mice. C57 and DBA mice were subjected to daily injections of oxiracetam (50 mg/kg i.p.) or vehicle (0.9% saline) for a total of 9 days. C57 and DBA mice were examined on a modified version of the Morris water maze task and the contextual fear conditioning task on the last 5 or 2 days, respectively, of the 9-day treatment schedule. When compared with controls, C57 and DBA oxiracetam-treated mice showed no difference in motor skill capability to perform these complex learning tasks (swim speed or ability to freeze). Hippocampal PKC activity was measured in cytosolic, loosely-bound, and membrane-bound homogenate fractions. Oxiracetam-treated DBA mice demonstrated a significant increase in spatial learning performance as determined by the Morris task. DBA performance was also improved in contextual learning as determined by the fear conditioning task. The increase in spatial learning performance was correlated to an increase in membrane-bound PKC. No substantial improvements in C57 mice were observed on either learning task nor did hippocampal PKC activity change in response to oxiracetam treatment. These data demonstrate that the learning impairment of DBA mice can be reversed by treatment with a nootropic agent and support previous studies suggesting that PKC may be one mechanism of action for oxiracetam.     

Protein kinase C in the hippocampus is altered by spatial but not cued discriminations: A component task analysis

The exact role of the mammalian hippocampus in memory formation remains essentially as an unanswered question for cognitive neuroscience. Experiments with humans and with animals indicate that some types of mnemonic associative processes involve hippocampal function while others do not. Support for the spatial processing hypothesis of hippocampal function has stemmed from the impaired performance of rats with hippocampal lesions in tasks that require spatial discriminations, but not cued discriminations. Previous procedures, however, have confounded the interpretation of spatial versus cued discrimination learning with the number and kinds of irrelevant stimuli present in the discrimination.An empirical set of data describing a role of protein kinase C (PKC) in different mnemonic processes is similarly being developed. Recent work has implicated the activation of this serine-threonine kinase in a variety of learning paradigms, as well as long-term potentiation (LTP), a model system for synaptic plasticity which may subserve some types of learning.The present study employs the principles of component task analysis to examine the role of membrane-associated PKC (mPKC) in hippocampal-dependent memory when all factors other than the type of learning were equivalent. The results indicate that hippocampal mPKC is altered by performance in hippocampally-dependent spatial discriminations, but not hippocampally-independent cued discriminations and provide a general experimental procedure to relate neural changes to specific behavioral changes.     

Immunohistochemical determination of protein kinase C expression and proliferative activity in human brain tumors

Summary Protein kinase C (PKC), the major receptor for phorbol ester tumor promotors, is a phospholipid- and calcium-dependent phosphorylating enzyme which plays an important role in the intracellular signal transduction necessary for a variety of basic cellular functions including the control of cell proliferation. To determine the expression of PKC in human neurogenic tumors we investigated 121 tumors of the human nervous system by means of immunohistochemistry using the monoclonal antibody C5. The results were compared with immunohistochemical staining for intermediate filament proteins, desmoplakins, and the proliferation-associated nuclear antigen Ki-67. Besides strong staining of normal and reactive astrocytes, C5 immunoreactivity was consistently observed in tumor cells of all types of gliomas. However, the fraction of C5 positive tumor cells varied between the different tumor types with astrocytomas and subependymomas demonstrating the strongest immunoreactivity. In the other gliomas, especially those of higher malignancy, a considerable heterogeneity in C5 expression could be observed. There was a tendency for the percentage of C5 immunostained tumor cells being lower in high-grade gliomas compared to low-grade ones and comparison with Ki-67 staining frequently revealed an inverse relationship between proliferative activity and C5 immunoreactivity. Besides the gliomas we found 3 of 7 neurinomas and 6 of 18 meningiomas which were partially C5 positive. All other tumors investigated including medulloblastomas and metastatic carcinomas were C5 negative. Our results thus indicate that immunohistochemistry for PKC using the monoclonal antibody C5 could be an useful aid for histopathological tumor classification in neurooncology.Supported by the Deutsche Forschungsgemeinschaft, SFB 200     

Dose-dependent phorbol ester facilitation or blockade of hippocampal long-term potentiation: relation to membrane/cytosol distribution of protein kinase C activity

We have proposed that the translocation/activation of protein kinase C (PKC) in synergism with a Ca²⁺-mediated event plays an essential role in hippocampal long-term potentiation (LTP). In a previous study, we saw no effect of PKC-activating phorbol esters alone on baseline responses, although it has been reported by others to enhance synaptic transmission. To resolve this discrepancy, we investigated the dose-response to phorbol esters of both baseline and potentiated granule cell responses elicited with perforant path stimulation. It was confirmed that iontophoretic ejection of phorbol ester to the dentate hilus, which alone had no effect on baseline responses, prolonged the persistence of potentiation produced by 2 trains of 400 Hz stimulation. These data support the proposed synergistic model in which the effects of phorbol ester and high frequency stimulation together produce a long-lasting potentiation of synaptic activation. A similar synergism was observed with ejection of a lower dose of phorbol ester into the perforant path synaptic zone in the molecular layer. Higher doses delivered to the synaptic zone without 400 Hz stimulation were sufficient to enhance baseline synaptic responses, but these doses inhibited the initial potentiation induced with 2 trains of 400 Hz stimulation delivered immediately after ejection. There was at times a slowly developing enhancement observed after the initial blockade. Thus, induction of a persistent synaptic enhancement was observed without initial potentiation. Measurement of PKC activity in membrane and cytosol indicated that PKC activation is only associated with the persistence phase of LTP. In contrast, there was no change in PKC subcellular distribution associated with the blockade of initial potentiation by higher doses of PDBu.     

Immunohistochemical distribution of protein kinase C isozymes is differentially altered in ischemic gerbil hippocampus

We used monoclonal antibodies to examine the immunohistochemical distribution of the three major Ca(2+)-dependent protein kinase C (PKC) isozymes (I, II, and III) in ischemic gerbil hippocampus. Groups of four animals were sacrificed at 15 min, 4 h, 1 day, 2 days, 3 days, and 7 days after a 10-min episode of global forebrain ischemia. In control animals, PKC-I immunoreactivity was greater in CA1 neurons than in CA3-4. Terminal-like staining was not evident. PKC-II immunoreactivity was observed in all CA fields and in the outer molecular layer of the dentate gyrus. PKC-III staining was present in the CA fields, the inner molecular layer of the dentate gyrus and the subiculum. Dentate granule cells and mossy fibers were not stained with any of the PKC antibodies. Fifteen minutes and 4 h after ischemia, PCK-I, -II and -III immunoreactivity were all increased in CA1 neurons and PKC-III immunoreactivity alone was visualized in granule cells and mossy fibers. Staining patterns returned to baseline one day after ischemia. PKC-II and -III terminal-like staining were preserved in the stratum lacunosum-moleculare for 3 days and 2 days after ischemia respectively and then disappeared. The altered patterns of PKC staining in the hippocampus may reflect activation and/or down-regulation of PKC isozymes. Ca(2+)-dependent PKC isozymes may, therefore, potentially play a role in the pathogenesis of delayed ischemic neuronal death.     

Distinct regulation of expressed calcium channels 2.3 in Xenopus oocytes by direct or indirect activation of protein kinase C

Protein kinase C (PKC)-dependent regulation of voltage-gated Ca (Ca(v); with alpha(1)beta1Balpha2/delta subunits) channel 2.3 was investigated using phorbol 12-myristate 13-acetate (PMA), or by M(1) muscarinic receptor activation in Xenopus oocytes. The inward Ca(2+)-current with Ba(2+) (I(Ba)) as the charge carrier was potentiated by PMA or acetyl-beta-methylcholine (MCh). The inactivating [I(inact)] and non-inactivating [I(noninact)] components of I(Ba) and the time constant of inactivation tau(inact) were all increased by MCh or PMA. This may be a PKC-dependent action since the effect of MCh and PMA was blocked by Ro-31-8425 or beta-pseudosubstrate. MCh effect was blocked by atropine, guanosine-5'-O-(2-thiodiphosphate) trilithium (GDPbetaS) or U-73122. The effect of MCh but not PMA was blocked by the inhibition of inositol-1,4,5-trisphosphate (IP3) receptors, intracellular Ca(2+) ([Ca(2+)](i)) or the translocation of conventional PKC (cPKC) with heparin, BAPTA and betaC2.4, respectively. While a lower concentration (25 nM) of Ro-31-8425 blocked MCh, a higher concentration (500 nM) of Ro-31-8425 was required to block PMA action. This differential susceptibility of MCh and PMA to heparin, BAPTA, betaC2.4 or Ro-31-8425 is suggestive of the involvement of Ca(2+)-dependent cPKC in MCh action, whereas cPKC and Ca(2+)-independent novel PKC (nPKC) in PMA action. PMA led to additional increase in I(Ba) that was already potentiated by preadministered MCh (1 or 10 microM), leading to the suggestion that differential phosphorylation sites for cPKC and nPKC may be present in the alpha(1)2.3 subunit of Ca(v) 2.3 channels.     

Protein kinase C activity in the hippocampus following spatial learning tasks in mice

Protein kinase C (PKC) is highly concentrated in the hippocampus and is thus a possible neural substrate of learning and memory. This study was designed to determine whether partial acquisition (i.e., the minimal amount of training leading to above-chance performance) of a spatial discrimination in an eight-arm radial maze alters hippocampal PKC activity. Mice were sacrificed at different times (5 minutes, 1 hour, 24 hours) after the second learning session, and PKC activity was measured in both cytosolic and membrane fractions of the hippocampus. In order to determine which component of the task was involved in the alterations in enzymatic activity, hippocampal PKC activity was also measured in a group of mice that was allowed to explore the maze freely. Significantly less PKC activity was found in the cytosolic fraction from the trained animals than from the quiet or active control groups. No differences were observed between the quiet and active controls. In contrast, there were no significant between-groups differences in membrane-bound PKC activity, although a negative correlation between the membrane-bound PKC activity and learning scores (accuracy) was noted. These results suggest that hippocampal PKC activity is involved essentially in the associative component of the task. The lack of learning-induced alterations in membrane-bound PKC activity and the negative correlation between this enzymatic activity and learning accuracy are discussed.     

Neuroprotective effect of protein kinase C inhibitors on oxygen/glucose free-induced decreases in 2-deoxyglucose uptake and CA1 field potentials in rat hippocampal slices

Staurosporine, a protein kinase C inhibitor, was found to produce a neuroprotective effect against an ischemic insult in both gerbils and rats in vivo. We have demonstrated that rat hippocampal slices exposed to oxygen/glucose-free medium showed decreases in 2-deoxyglucose (2-DG) uptake and CA1 field potentials elicited by the stimulation of Schaffer collaterals. Therefore we examined the effect of protein kinase C inhibitors on oxygen/glucose free-induced impairments of 2-DG uptake and CA1 field potentials. Pretreatment with staurosporine, K252a and H-7 attenuated decreases in 2-DG uptake and CA1 field potentials. Treatment with phorbol ester, a protein kinase C activator, for a long period (90 min) was found to induce a down-regulation of protein kinase C activity. Therefore we examined the effect of pretreatment with phorbol ester for 90 min on oxygen/glucose free-induced decreases in 2-DG uptake and CA1 field potentials. These decrements were not attenuated by 5-min treatment with phorbol ester but were attenuated by 90-min treatment. The present results suggest that the treatment which decreases protein kinase C activity shows a neuroprotective action against oxygen/glucose free-induced deficits of metabolic and synaptic activity in hippocampal slices.     

Effects of riluzole on N-methyl- -aspartate-induced tyrosine phosphorylation in the rat hippocampus

Since increased tyrosine phosphorylation has been observed in response to brain ischemia, we investigated whether riluzole (an inhibitor of glutamate neurotransmission with neuroprotective properties) affects tyrosine phosphorylation stimulated by N-methyl- -aspartate (NMDA) in rat hippocampal slices. Riluzole produced an extremely potent concentration-related inhibition of NMDA (1 mM)-stimulated protein tyrosine phosphorylation (IC₅₀=0.5±0.03 μM, mean±S.D.), but failed to affect that evoked by phorbol 12-myristate 13-acetate (PMA, an activator of protein kinase C, 0.1 and 1 μM). These results suggest that inhibition of tyrosine phosphorylation may contribute to the neuroprotective effects of riluzole against excitotoxic injury.     

Ca and phospholipid-dependent protein kinase activity in rat cerebral hemispheres

Ca²⁺/phospholipid-dependent protein kinase (PKC) activity was found to be asymmetrically distributed between the two cerebral hemispheres of rat brain, whereas basal protein phosphorylation was not lateralized. The left cerebral hemisphere (LCH) displayed about 50% more PKC activity in synaptosomal fractions than the right cerebral hemisphere (RCH). Polyacrylamide gel electrophoresis, autoradiography and quantitation of radioactivity in individual protein bands showed that the phosphate acceptors with major interhemispheric differences were proteins of more than 50 kDa. Cerebral lateralization was also apparent in the pattern of PKC inhibition mediated by phospholipid-interacting drugs: chlorpromazine and polymyxin B depressed activity more profoundly in LCH. A covalent protein modification usually associated with neurotransmitter receptor activation is thus unevenly distributed in rodent brain.     

Protein kinase C activity and protein levels in Alzheimer's disease

Patients with Alzheimer's disease (AD) have been reported to have abnormalities in the levels and activities of protein kinase C (PKC) in brain and other tissues. We have measured Ca²⁺-activated, phospholipid-dependent PKC activities and levels in cerebral cortex from frontal, motor, temporal and parietal regions, as well as in leukocytes and platelets from AD patients and controls. No significant differences in PKC histone H1 phosphotransferase activity were seen in frontal, motor, temporal or parietal cortex, or in leukocytes and platelets from AD patients and controls. Elevated PKC protein was present in cytosolic fractions from frontal cortex, but not in other brain regions, or in leukocytes and platelets. These data suggest that abnormalities of PKC phosphorylating activity are absent in AD.     

抑郁症患者血小板蛋白激酶C水平的变化

目的：探讨抑郁症患者血小板细胞浆和细胞膜蛋白激酶C(PKC)的变化及其意义。方法：采用[^3H]12，13-二丁酰佛波醇酯(PDBu)结合法，测定23例抑郁症患者和10名正常对照者的血小板细胞浆和细胞膜PKC水平。结果：与正常对照组比较，抑郁症组血小板胞膜PKC水平显著降低，血小板胞浆PKC水平差异无显著性。结论：蛋白激酶C的改变在抑郁症发病机制中可能起一定作用。     

Hippocampal protein kinase C activity is reduced in poor spatial learners

Activation of protein kinase C (PKC) via neurotransmitter coupling processes has been associated with long-term potentiation (LTP) or classical conditioning, but whether natural variation in PKC activity affects learning performance remains to be determined. Inbred strains of mice differ in their ability to exhibit spatial reference memory as measured by the Morris water task. C57BL/6Ibg (C57) mice perform the task better than DBA/2Ibg (DBA) mice, which show relatively little spatial preference. Hippocampal PKC activity extracted from the particulate fraction was lower in DBA mice than in C57 mice. To examine the potential relationship of PKC activity with spatial learning performance, 11 C57BL/6J × DBA/2J recombinant inbred strains (BXD RIs) were trained in the place learning version of the Morris water task. Cortical and hippocampal PKC activities were measured. Variation in spatial learning performance and PKC activity from cortex and hippocampus was observed. A positive significant correlation was observed between measures of spatial learning accuracy and hippocampal PKC in these strains. No correlation was observed between spatial learning accuracy and cortical PKC activity. These data suggest that animals with lower hippocampal PKC activity may have problems performing spatial reference memory tasks with the same degree of accuracy as those with higher hippocampal PKC activity.     

A non-radioactive assay for the cAMP-dependent protein kinase activity in rat brain homogenates and age-related changes in hippocampus and cortex

Cyclic AMP-dependent protein kinase (PKA) activity was involved in a number of brain functions such as cognitive process or aging. The measurement of PKA activity is traditionally based on the use of [(32)P]ATP in phosphorylation of specific protein. Recently non-isotopic PKA assays have been developed, but none has been tested on brain homogenates. This work aimed to adapt a fluorimetric method of PKA activity into a novel assay never applied before in brain homogenate, and to characterize the enzyme activity and ratio in hippocampus and cortex from rats of different ages. Optimal conditions of homogenization and enzyme protection were determined. The method was sensitive and reproducible (intra-assay and interassay variation was 5.0% and 9.0%, respectively). In hippocampal cytosol, PKA activity was 27+/-8 and 80+/-9 nmol/min per mg protein in basal and cAMP-stimulated activity, respectively, and accounted for 80% of total cell PKA activity. The non-PKA activity, assessed by the use of the PKA specific inhibitor (PKI) accounted for 49.0% and 65.0% of endogenous levels in cytosol and membrane, respectively. cAMP-augmenting drugs effects were measured and increase of 53%, 273% and 118% over basal by 10 microM isoproterenol, 100 microM forskolin, 1 microM Sp-AMP, respectively, was observed. With respect to the changes in animal age, PKA activity increased from newborn to the mature rats but decreased in older rats. The PKA ratio was higher in cytosol than in particulate fraction, and was decreased in hippocampal sample from old rats (P<0.05). This last result was interpreted as related to the loss of cognitive capacities in old animals.