Excitation of locus coeruleus neurons by vasoactive intestinal peptide: role of a cAMP and protein kinase A

In accord with previous studies, it was found that vasoactive intestinal peptide (VIP), a powerful activator of adenylate cyclase, and cAMP-active agents (i.e., 8-Br-cAMP, forskolin, and Ro20-1724) increased the firing rate of noradrenergic neurons in the locus coeruleus (LC) by inducing an inward current. The response to VIP was usually more rapid and larger in a subpopulation of LC neurons with subthreshold rhythmic oscillations in membrane potential (oscillatory cells) as compared to nonoscillatory cells. In either case, the inward currents elicited by VIP and cAMP-active agents were found to be nonadditive, suggesting the action of VIP, at least in part, is via the same mechanism as that of cAMP-active agents. Intracellular application of a specific protein (or related peptide) inhibitor of cAMP-dependent protein kinase markedly attenuated the activation induced by either cAMP-active agents or VIP, suggesting that cAMP-dependent protein kinase (protein kinase A), presumably through protein phosphorylation, plays a role in the action of VIP. Taken together, the results provide evidence that cAMP and protein kinase A are involved in mediating the electrophysiological actions of VIP on LC neurons.     

Excitation of locus coeruleus neurons by an adenosine 3',5'-cyclic monophosphate-activated inward current: extracellular and intracellular studies in rat brain slices

The firing rate of locus coeruleus (LC) neurons in rat brain slices was increased reversibly by agents that either elevate intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) or mimic its actions (e.g., forskolin, and activator of adenylate cyclase, 8-Br-cAMP, a membrane permeable analog of cAMP, and Ro20-1724, a preferential inhibitor of cAMP-phosphodiesterase). Intracellular recordings showed that 8-Br-cAMP and forskolin induce a depolarization of LC neurons, accompanied by a decrease in input resistance. The 8-Br-cAMP- and forskolin-elicited depolarization persisted in the presence of cobalt, a calcium channel blocker. Steady-state current-voltage curves revealed that in the voltage range of -50 to -120 mV, 8-Br-cAMP and forskolin induced an inward current, which did not reverse at the potassium equilibrium potential and could not be blocked by tetrodotoxin. Partial replacement of sodium with Tris or choline markedly reduced the depolarization elicited by 8-Br-cAMP. We conclude that 8-Br-cAMP and forskolin act through a common mechanism to increase the firing rate of locus coeruleus neurons by inducing a cAMP-activated inward current, carried out at least in part by sodium ions.     

Opiate withdrawal and the rat locus coeruleus: behavioral, electrophysiological, and biochemical correlates

We have compared the time course of the behavioral manifestations of opiate withdrawal to the in vivo activity of locus coeruleus (LC) neurons and to increases in the levels of G-proteins, adenylate cyclase, and cAMP-dependent protein kinase known to occur in the LC in opiate-dependent animals. Rats were given morphine by daily subcutaneous implantation of morphine pellets for 5 d. On the sixth day, morphine withdrawal was induced by subcutaneous administration of naltrexone, an opiate receptor antagonist, with additional doses given 6 and 24 hr later, conditions that resulted in sustained, maximal levels of withdrawal over the duration of the experiment. We found a striking parallel between the time courses of the behavioral signs and the increased activity of LC neurons during withdrawal, both of which appeared to follow 2 phases. There was an early, rapid phase, during which withdrawal signs and increased LC activity became most pronounced within 15-30 min after naltrexone administration, and then recovered rapidly by over 50% within 4 hr of withdrawal. Subsequently, there was a slower phase, during which the persisting withdrawal signs and elevated LC activity remained roughly constant from 4 to 24 hr and did not recover completely until after 72 hr of continuous withdrawal. Adenylate cyclase and cAMP-dependent protein kinase activities in isolated LC subcellular fractions, both elevated in dependent animals, recovered to control levels after 6 hr of withdrawal, in parallel with the rapid phase of withdrawal. Levels of G1 and Go, also elevated in dependent animals, remained only slightly elevated at 6 hr and returned to normal by 24 hr. Taken together, these data suggest that increased neuronal activity in the LC is associated temporally with the behavioral morphine withdrawal syndrome and that increased levels of G-proteins and an up-regulated cAMP system may contribute to the early withdrawal activation of these neurons.     

The activity of constitutive nitric oxide synthase is increased by the pathway cAMP/cAMP-activated protein kinase in human platelets. New insights into the antiaggregating effects of cAMP-elevating agents

Human platelets synthesize nitric oxide (NO) through an endothelial-type NO synthase (ecNOS) activated also by substances enhancing 3',5'-cyclic adenosine monophosphate (cAMP) concentrations, such as catecholamines, beta-adrenoceptor agonists and adenosine. To verify whether cAMP directly activates ecNOS through the cAMP-dependent protein kinase A (PKA), we evaluated (i) the influence of 8-Br-cAMP, adenosine and forskolin on ecNOS activity and phosphorylation at Ser(1177) and (ii) the effect of PKA inhibition on ecNOS activity. Platelets from 10 healthy male volunteers were used for aggregation studies and measurement of NOS activity (conversion of L-[(3)H]-arginine to L-[(3)H]-citrulline) following exposure to 8-Br-cAMP, adenosine and forskolin, both in the absence and in the presence of the PKA inhibitor Rp-cAMPS (100 micromol/l). The phosphorylation of the PKA substrate vasodilator-stimulated phosphoprotein (VASP) at Ser(157) and Ser(239) and of ecNOS at Ser(1177) was evaluated by Western blot. NOS activity (pmol L-citrulline/10(8) platelets) increased from 0.090+/-0.002 to 0.148+/-0.013 with 500 micromol/l 8-Br-cAMP (p<0.0001), to 0.140+/-0.008 with 30 micromol/l adenosine (p<0.0001) and to 0.140+/-0.009 with 10 micromol/l forskolin (p<0.0001). Rp-cAMPS decreased baseline NOS activity from 0.093+/-0.001 to 0.075+/-0.006 (p<0.02) and prevented the stimulation by 8-Br-cAMP, adenosine and forskolin. Platelet exposure to 8-Br-cAMP and forskolin, beside the phosphorylation of the specific PKA substrate VASP, markedly increased the expression of ecNOS protein phosphorylated at Ser(1177). The study shows that NOS activity of human platelets is increased by the cAMP/PKA pathway which is involved in NO synthesis induced by adenosine, forskolin and potentially by every antiaggregating substance enhancing intraplatelet cAMP via receptor-dependent and -independent mechanisms.     

Phosphorylation of voltage-gated ion channels in rat olfactory receptor neurons

In olfactory receptor neurons (ORNs), ligand-odorant receptor interactions cause G protein-mediated activation of adenylate cyclase and a subsequent increase in concentration of the intracellular messenger cAMP. Odorant-evoked elevation in cAMP is thought to directly activate a cation-selective cyclic nucleotide-gated channel, which causes external Ca2+ influx, leading to membrane depolarization and the generation of action potentials. Our data show that in freshly dissociated rat ORNs, odorant-induced elevation in cAMP also activates cAMP-dependent protein kinase (PKA), which is then able to phosphorylate various protein targets in the olfactory signal transduction pathway, specifically voltage-gated sodium and calcium channels. The presence of PKI (PKA inhibitor peptide) blocked the modulatory action of cAMP on voltage-gated ion channels. By modulating the input/output properties of the sensory neurons, this mechanism could take part in the complex adaptation process in odorant perception. In addition, we found modulation of voltage-gated sodium and calcium channel currents by 5-hydroxytryptamine and the dopamine D1 receptor agonist SKF 38393. These findings suggest that in situ ORNs might also be a target for efferent modulation.     

Imidazoquinoline derivatives: potent inhibitors of platelet cAMP phosphodiesterase which elevate cAMP levels and activate protein kinase in platelets.

Compounds containing the imidazoquinoline nucleus are a new class of potent, broad-spectrum inhibitors of platelet aggregation. This report describes studies with a simply-substituted imidazoquinoline (BMY 20844) and several new ether-linked side chain derivatives (BMY 21638 and BMY 43351). These compounds are potent inhibitors of platelet cAMP phosphodiesterase (IC50 values: BMY 20844, 1.3 X 10(-8); BMY 21638, 2 X 10(-10); and BMY 43351, 1 X 10(-10) M, measured using 0.15 microM cAMP) but have little effect on platelet homogenate cGMP phosphodiesterase (IC50 greater than 10(-5) M). Inhibition of different cAMP phosphodiesterase isozymes was tested to determine if the compounds inhibited similar isozymes in other tissues. Rabbit heart cAMP phosphodiesterase isozymes were resolved by ion-exchange chromatography and three peaks of activity were obtained. BMY 20844 inhibited only fraction III (a "cGMP-inhibitable, low Km" cAMP-specific phosphodiesterase) with an IC50 value of 5 X 10(-8) M. These compounds also inhibited canine cardiac sarcoplasmic reticulum membrane-bound "cGMP-inhibitable, low Km" cAMP-specific phosphodiesterase with virtually the same potency as inhibition of cAMP phosphodiesterase in platelet homogenate. In washed platelets these compounds elevated cAMP levels and activated the platelet cAMP dependent protein kinase. Activation of cAMP-dependent protein kinase was determined by cAMP-dependent protein kinase ratio measurements and phosphorylation of intracellular proteins. These studies suggest that this potent new class of agents inhibits platelet phosphodiesterase activity in intact platelets causing an elevation in cAMP levels sufficient to activate the cAMP-dependent protein kinase and stimulate protein phosphorylation. This mechanism is, at least in part, responsible for the ability of these compounds to prevent platelet aggregation and thrombosis in experimental animal models.     

Opposing effects by pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide on hippocampal synaptic transmission

Pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal peptide (VIP), and their receptors have been localized within the hippocampus but their physiological function on synaptic transmission is still unclear. We investigated the effects of PACAP and VIP on evoked excitatory postsynaptic currents (EPSCs) recorded with patch clamp from CA1 pyramidal neurons in rat hippocampal slices. Bath application of PACAP reversibly reduced EPSC amplitude. This effect was partly prevented by intracellular addition of (R)-adenosine, cyclic 3',5'-hydrogenphosphorothioate (cAMPS-Rp), a cAMP antagonist inhibiting protein kinase A, but not by the calcium chelator 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Application of VIP induced a long-lasting increase of EPSC amplitude that was completely abolished when cAMPS-Rp was included in the intracellular solution. PACAP and VIP effects on EPSCs were mimicked by the cAMP agonist 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP). The differing abilities of PACAP and VIP to modulate transmission efficiency over long periods of time, through the cAMP/PKA pathway, suggest that these neuropeptides may exert opposing roles in synaptic plasticity.     

Functional and pharmacological characterization of the modulatory role of serotonin on the firing activity of locus coeruleus norepinephrine neurons

Previous studies, using in vivo extracellular unitary recordings in anaesthetized rats, have shown that the selective 5-HT(1A) receptor antagonist WAY 100,635 suppressed the firing rate of locus coeruleus (LC) norepinephrine (NE) neurons and that this effect was abolished by lesioning 5-HT neurons. In the present experiments, the selective 5-HT(2A) receptor antagonist MDL 100,907, while having no effect on the spontaneous firing activity of LC neurons in controls, was able to restore NE neuronal discharges following the injection of WAY 100,635. The 5-HT(1A) receptor agonist 8-OH-DPAT enhanced the firing activity of NE neurons and this action was entirely dependent on intact 5-HT neurons, unlike the inhibitory effect of the 5-HT(2) receptor agonist DOI. Taken together, these data indicate that 5-HT(2A) but not 5-HT(1A) receptors controlling LC firing activity are postsynaptic to 5-HT neurons. Prolonged, but not subacute, administration of selective 5-HT reuptake inhibitors (SSRIs) produces a decrease in the spontaneous firing activity of LC NE neurons. MDL 100,907 partially reversed this suppressed firing activity of LC neurons in paroxetine-treated rats. Although the alpha(2)-adrenoceptor antagonist idazoxan also enhanced the firing activity of NE neurons in paroxetine-treated rats, this increase was similar to that obtained in controls. In conclusion, prolonged SSRI treatment enhances a tonic inhibitory influence by 5-HT on LC neurons through postsynaptic 5-HT(2A) receptors that are not located on NE neurons. A speculative neuronal circuitry accounting for these phenomena on LC NE activity is proposed.     

Distribution of cAMP and cAMP-dependent protein kinases in Aplysia sensory neurons

Sensitization of the gill- and siphon-withdrawal reflex in Aplysia is considered a simple form of learning. Previous work has provided physiological and pharmacological evidence that cAMP-dependent protein phosphorylation within identified sensory neurons of the abdominal ganglion underlies the short-term form of this behavioral modification. Our main goal in this paper is to determine the subcellular distribution of cAMP and to measure the amounts and properties of the 2 types of subunits (regulatory and catalytic) that constitute the cAMP-dependent protein kinase. Do these biochemical parameters differ in sensory cells from those in other parts of nervous tissue? We found that the increased cAMP synthesized under conditions of sensitization is distributed in 3 compartments in the neuron: most of it is free in the cytoplasm; the remainder is bound either to cytoplasmic or to particulate proteins, which are believed to be regulatory subunits of the cAMP-dependent protein kinase. Binding of cAMP within the neurons is a measure of activation of the kinase. At rest, 17% of the binding sites in sensory cells were occupied. After brief electrical stimulation of the connective, which released endogenous transmitter, occupancy increased to 34%. This treatment increased the amount of cAMP bound to the various binding proteins differentially. The biochemical characteristics of cAMP binding were found to be the same in sensory neurons as in the rest of the nervous system but different from those in muscle. Thus, memory and learning are likely to be mediated by enzymes that are shared by other nerve cells. We found that sensory neurons have greater cAMP-dependent protein kinase activity than other neurons, however, and as a result may be more sensitive to small increases of cAMP.     

Muscarinic Modulation of Intrinsic Burst Firing in Rat Hippocampal Neurons

Intracellular recordings in rat hippocampal slices were used to examine how exogenous and endogenous cholinergic agonists modulate the firing pattern of intrinsically burst-firing pyramidal cells. About 24% of CA1 pyramidal cells generated all-or-none, high-frequency bursts of fast action potentials in response to intracellular injection of long positive current pulses. Application of carbachol (5 μM) converted burst firing in these neurons into regular trains of independent spikes. Acetylcholine (5 μM) exerted a similar effect, provided acetylcholine esterase activity was blocked with neostigmine (2 μM). Atropine (1 μM) reversed this cholinergic effect, indicating its mediation by muscarinic receptors. Cholinergic agonists also caused mild neuronal depolarization but the block of intrinsic burst firing was independent of this effect. Repetitive stimulation of cholinergic fibres in the presence of neostigmine (2 μM) evoked a slow cholinergic excitatory postsynaptic potential (EPSP) lasting about a minute. During the slow EPSP, burst firing could not be evoked by depolarizing pulses and the neurons fired in regular mode. These effects were prevented by pretreatment with atropine (1 μM). Exogenously applied cholinergic agonists and endogenously released acetylcholine also reduced spike frequency accommodation and suppressed the long-duration afterhyperpolarization in burst-firing pyramidal cells in an atropine-sensitive manner. A membrane-permeable cAMP analogue (8-bromo-cAMP; 1 mM) also reduced frequency accommodation and blocked the long-duration afterhyperpolarization, but did not affect intrinsic burst firing at all. Taken together, the data show that muscarinic receptor stimulation transforms the stereotyped, phasic response of burst-firing neurons into stimulus-graded, tonic discharge.     

Inhibitory Effects of Natriuretic Peptides on Vasopressin Neurons Mediated through cGMP and cGMP-Dependent Protein Kinase in vitro

The effects of natriuretic peptides on electrical activity and cellular cGMP levels were studied in neurons of the supraoptic nucleus (SON) of rat hypothalamic slice preparations. Intracellular and extracellular recordings showed that bath application of A type natriuretic peptide (ANP) at 100 nM or B type natriuretic peptide (BNP) at 100 to 300 nM decreased the firing rate and hyperpolarized the membrane potential in phasically firing (putative vasopressin) neurons. Non-phasically firing (putative oxytocin) neurons did not respond to these natriuretic peptides in firing rate or membrane potential. The membrane-permeable cGMP analogue 8-bromo cGMP at 0.5 mM and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) at 50 μM mimicked the inhibitory effects of ANP and BNP. The specific inhibitor of cGMP phosphodiesterase 1-(3-chloroanilino)-4-phenylphthalazine (MY5445) at 30 μM also decreased the firing rate of SON neurons. The cGMP-dependent protein kinase inhibitor N-(2-(methylamino)ethyl)-5-isoquinoline-sulfonamide dihydrochloride (H8) at 1 μM abolished the inhibition by natriuretic peptides. We measured cGMP and cAMP contents in discrete SON regions and compared the change of the contents before and after application of ANP and BNP. The increases in cellular cGMP accumulation were 430% for ANP and 120% for BNP, although they did not cause significant change of cAMP accumulation. The results suggest that the inhibitory effects of natriuretic peptides on putative vasopressin neurons are mediated through cGMP and cGMP-dependent protein kinase.     

Cyclic AMP, but not basic FGF, increases the in vitro survival of mesencephalic dopaminergic neurons and protects them from MPP(+)-induced degeneration.

We studied how stimulation of protein kinase C and cAMP-dependent protein kinases affect the development of mesencephalic dopaminergic neurons in primary cell cultures derived from fetal rats at embryonic day E14. The effects of compounds which activate these second messenger systems were compared to those of basic fibroblast growth factor (bFGF) and insulin-like growth factor I (IGF-I). In mesencephalic cultures, there was a continuous loss of dopaminergic neurons. Despite this decline in cell number, neurotransmitter uptake per neuron increased with time, indicating that the surviving dopaminergic neurons continued their biochemical differentiation while others degenerated. IGF-I and bFGF did not affect the number of dopaminergic neurons. However, dopamine uptake per neuron was significantly higher in bFGF and IGF-I treated cultures, suggesting that these factors stimulated differentiation. Protein kinase C and cAMP-dependent protein kinases were not involved in mediating the effects of bFGF and IGF-I. Treatment of cultures with phorbol esters did not affect dopamine uptake, whereas elevated levels of intracellular cAMP resulted in an increase in dopamine uptake which was additive to that elicited by bFGF or IGF-I. Further analysis revealed that exposure of mesencephalic cultures to dibutyryl cAMP (dbcAMP) during the first 3 days after plating increased the survival of dopaminergic neurons, whereas prolonged treatment attenuated the development of the dopamine uptake system. Moreover, cyclic AMP, but not bFGF, was able to prevent the degeneration of dopaminergic neurons induced by 1-methyl-4-phenyl-pyridinium ion (MPP+), the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The results suggest that increased intracellular levels of cAMP protect dopaminergic neurons in situations of stress like the process of dissociation and plating or the exposure to neurotoxic compounds. Our results reveal novel possibilities for the treatment of Parkinson's disease.     

Both phasic sensory stimulation and tonic pharmacological activation increase Fos-like immunoreactivity in the rat locus coeruleus.

Neuronal activation increases levels of Fos protein, the product of the early immediate gene c-fos. Since most studies used stimuli that evoke sustained elevations in activity; the present study examined Fos-like immunoreactivity (Fos-LI) in the nucleus locus coeruleus (LC). Halothane-anesthetized rats were given either footshock to elicit phasic activation of LC neurons or yohimbine (10 mg/kg, i.p.) to induce a tonic increase in firing. Both treatments markedly increased Fos-LI in a subpopulation of LC neurons. These results demonstrate that c-fos induction does not require high tonic levels of neuronal activity and that Fos-LI may underestimate the proportion of LC neurons neurophysiologically activated by a given stimulus and suggest that factors beyond neuronal activity per se contributes to c-fos expression.     

Lidocaine suppresses the sodium current inEuhadra neurons which is mediated by cAMP-dependent protein phosphorylation

The action of a local anesthetic, lidocaine, in association with the cyclic AMP (cAMP)-mediated intracellular biochemical process, was examined in identifiedEuhadra neurons. Lidocaine dose-dependently inhibited the inward current which was elicited by dibutyryl cAMP (db-cAMP) and isobutylmethylxanthine (IBMX). This inhibitory effect was transiently reversed by the intracellular injection of a catalytic subunit of a cAMP-dependent protein kinase. The inward current elicited by db-cAMP and IBMX was abolished by Na⁺-free saline but not by Ca²⁺-free saline. The data suggest that lidocaine is not acting directly on the Na⁺ channel, but acts at a level proximal to the catalytic subunit of cAMP-dependent protein kinase.     

Effects of cAMP on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rat

Involvement of cAMP in the generation of respiratory rhythm was studied in newborn rat brainstem-spinal cord preparations. The respiratory rhythm was monitored by C4 inspiratory activity and/or pre-inspiratory (Pre-I) activity of neurons in the rostral ventrolateral medulla; previously suggested to be primary rhythm generating neurons which have pacemaker properties. The effects of four cAMP-increasing agents (forskolin, IBMX, Db-cAMP, and 8-Br-cAMP) on this neuronal activity were examined. Perfusion with forskolin (3–10 μM) increased the burst rate of C4 inspiratory activity in 20 of 32 preparations, but in 8 of those the increase was preceded by transient depression. The facilitation of the respiratory rhythm was greater whenever the burst rate before forskolin treatment was lower. The Pre-I neuron burst rate, which was recorded together with C4 activity, predominantly increased with forskolin treatment. The effects of IBMX, Db-cAMP and 8-Br-cAMP were similar to those of forskolin, but they were slightly less potent. Long-lasting depression of the respiratory rhythm (C4 and Pre-I activity) by clonidine, which might decrease intracellular cAMP level viaμ₂-receptors, was reversed by forskolin. To investigate the direct effects of the cAMP-increasing agents on Pre-I neurons, Pre-I activity was isolated by blocking the chemical synaptic transmission by incubation in a low Ca solution (0.2 mM Ca²⁺, 5 mM Mg²⁺). Forskolin (5–10 μM), IBMX (5–10 μM), Db-cAMP (0.2–0.4 mM), and 8-Br-cAMP (0.4–0.75 mM) all enhanced the burst rate of isolated Pre-I neurons. Results suggest that cAMP is important in respiratory rhythm generation; this may be due to its regulation of the intrinsic burst generating properties of Pre-I neurons.     

Cyclic AMP analog activates Na(+)-dependent inward currents in dissociated frog motoneurons.

Effects of intracellular accumulation of 3',5'-cyclic adenosine monophosphate (cAMP) were studied using 3',5'-cyclic 8-bromoadenosine monophosphate (8-Br-cAMP) and forskolin on single motoneurons acutely dissociated from adult bullfrog spinal cord. 8-Br-cAMP (10(-3) M) and forskolin (1.5 x 10(-6) M) activated inward currents under K(+)-free conditions at a holding potential of -70 mV. The currents were dependent on extracellular Na+ concentration, and were never reversed within the range of membrane potentials tested (-130 to 30 mV). These results indicate that accumulation of intracellular cAMP induces Na(+)-dependent inward currents in frog motoneurons.     

Molecular control of locus coeruleus neurotransmission.

The locus coeruleus (LC) is the major noradrenergic nucleus in the brain and innervates large segments of the neuraxis. LC neurons are thought to regulate states of attention and vigilance as well as activity of the sympathetic nervous system. These neurons also have been implicated in the actions of stress, antidepressants, and opiates on the brain. Aided in part by the fact that the LC is relatively homogeneous, it has been possible to understand some of the cellular and molecular mechanisms that control their functional state. This review focuses on the role played by the cAMP pathway in regulation of LC neurons, particularly after chronic perturbations. Thus, several components of this intracellular signaling pathway are upregulated in the LC after chronic stress or chronic opiate treatment, but downregulated after chronic antidepressant treatment. LC neurons exhibit a pacemaker activity, which appears to be mediated, at least in part, by a nonspecific cation current that is activated by protein kinase A. As a result, stimuli that upregulate the cAMP pathway after chronic administration (e.g., stress or opiates) increase the excitability of LC neurons, whereas stimuli that downregulate the cAMP pathway (e.g., antidepressants) exert the opposite effect. Such molecular adaptations could contribute to the behavioral plasticity that is associated with these various conditions.     

(3)H]cAMP binding sites and protein kinase a activity in the prefrontal cortex of suicide victims.

OBJECTIVE: The cAMP-dependent enzyme protein kinase A phosphorylates intracellular proteins upon activation and thereby plays a major role in mediating various physiological functions in the brain. To examine the role of this enzyme in suicidal behavior, the authors examined the catalytic and regulatory activities of protein kinase A in the postmortem brain of suicide victims. METHOD: Brain tissues were collected from 17 suicide victims and 17 nonpsychiatric comparison subjects. Regulatory activity was determined by examining [(3)H]cAMP binding to protein kinase A, while catalytic activity was determined by enzymatic assay in the presence (total activity) and the absence (endogenous activity) of cAMP in the membrane and cytosol fractions of the prefrontal cortex. RESULTS: The number (B(max)) of [(3)H]cAMP binding sites to protein kinase A was significantly lower in the suicide victims without any changes in affinity in either the membrane or cytosol fractions of the prefrontal cortex. Further, significantly less protein kinase A activity, both in the presence and the absence of cAMP, was seen in the membrane and cytosol fractions of the prefrontal cortex of suicide victims; however, the difference in total protein kinase A activity was much more pronounced. CONCLUSIONS: The results suggest that cAMP binding to the regulatory subunits of protein kinase A, as well as the phosphotransfer catalytic activity of protein kinase A, are lower in the prefrontal cortex of suicide victims than in nonpsychiatric comparison subjects, which may be of clinical relevance in the pathophysiology of suicidal behavior.