Specific [3H]UK 14,304 binding in human cortex occurs at multiple high affinity states with alpha 2-adrenergic selectivity and differing affinities for GTP

Protein kinase Calpha mediates a novel form of plasticity in the accessory olfactory bulb

Modification of synapses in the accessory olfactory bulb (AOB) is believed to underlie pheromonal memory that enables mate recognition in mice. The memory, which is acquired with single-trial learning, forms only with coincident noradrenergic and glutamatergic inputs to the AOB. The mechanisms by which glutamate and norepinephrine (NE) alter the AOB synapses are not well understood. Here we present results that not only reconcile the earlier, seemingly contradictory, observations on the role of glutamate and NE in changing the AOB synapses, but also reveal novel mechanisms of plasticity. Our studies suggest that initially, glutamate acting at Group II metabotropic receptors and NE acting at alpha(2)-adrenergic receptors inhibit N-type and R-type Ca(2+) channels in mitral cells via a G-protein. The N-type and R-type Ca(2+) channel inhibition is reversed by activation of alpha(1)-adrenergic receptors and protein kinase Calpha (PKCalpha). Based on these results, we propose a hypothetical model for a new kind of synaptic plasticity in the AOB that accounts for the previous behavioral data on pheromonal memory. According to this model, initial inhibition of the Ca(2+) channels suppresses the GABAergic inhibitory feedback to mitral cells, causing disinhibition and Ca(2+) influx. NE also activates phospholipase C (PLC) through alpha(1)-adrenergic receptors generating inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Calcium and DAG together activate PKCalpha which switches the disinhibition to increased inhibition of mitral cells. Thus, PKCalpha is likely to be a coincidence detector integrating glutamate and NE input in the AOB and bridging the short-term signaling to long-term structural changes resulting in enhanced inhibition of mitral cells that is thought to underlie memory formation.

Alpha 1-noradrenergic system role in increased motivation for cocaine intake in rats with prolonged access

In rodents, extended access to cocaine produces an escalation in cocaine self-administration that has face and construct validity for human compulsive drug intake. Here we report that rats with six-hour access (long access, LgA) to cocaine self-administration produced a higher breakpoint for cocaine using a progressive-ratio schedule than rats with one-hour access (short access, ShA), and prazosin (alpha 1 receptor antagonist) reduced the higher breakpoint for cocaine in LgA rats. Additionally, the number of neurons with alpha 1-adrenergic receptor-like immunoreactivity in the bed nucleus of stria terminalis (BNST) was found to be much lower in LgA rats than in ShA and drug-naive rats. In contrast, UK14304 (alpha 2 receptor agonist) and betaxolol (beta 1 receptor antagonist) had no effect on cocaine self-administration in either group. The data suggest that activation of the alpha 1-noradrenergic system, perhaps in the BNST, is associated with increased motivation for cocaine in rats with extended access.

Noradrenergic modulation of XII motoneuron inspiratory activity does not involve α2-receptor inhibition of the Ih current or presynaptic glutamate release

Norepinephrine has powerful and diverse modulatory effects on hypoglossal (XII) motoneuron activity, which is important in maintaining airway patency. The objective was to test two hypotheses that α2-adrenoceptor-mediated, presynaptic inhibition of glutamatergic inspiratory drive (Selvaratnam SR, Parkis MA, and Funk GD. Brain Res 805: 104–115, 1998) and postsynaptic inhibition of the hyperpolarization-activated inward current ( Ih) (Parkis MA and Berger AJ. Brain Res 769: 108–118, 1997) modulate XII inspiratory activity. Nerve and whole cell recordings were applied to rhythmic medullary slice preparations from neonatal rats ( postnatal days 0–4) to monitor XII inspiratory burst amplitude and motoneuron properties. Application of an α2-receptor agonist (clonidine, 1 mM) to the XII nucleus reduced inspiratory burst amplitude to 71 ± 3% of control but had no effect on inspiratory synaptic currents. It also reduced the Ih current by ∼40%, but an Ih current blocker (ZD7288), at concentrations that blocked ∼80% of Ih, had no effect on inspiratory burst amplitude. The clonidine inhibition was unaffected by the GABAA antagonist (+)bicuculline but attenuated by the α2-antagonist rauwolscine and the imidazoline 1 (I1) antagonist efaroxan. The I1 agonist rilmenidine, but not the α2-agonist UK14304, inhibited XII output. Clonidine also reduced action potential amplitude or impaired repetitive firing. Although a contribution from α2, and in particular I1, receptors remains possible, results demonstrate that 1) noradrenergic modulation of XII inspiratory activity is unlikely to involve α2-receptor-mediated presynaptic inhibition of glutamate release or modulation of Ih; 2) inhibition of repetitive firing is a major factor underlying the inhibition of XII output by clonidine; and 3) Ih is present in neonatal XII motoneurons but does not contribute to shaping their inspiratory activity.

Prolonged activation of mesolimbic dopaminergic neurons by morphine withdrawal following clonidine: participation of imidazoline and norepinephrine receptors

The alpha2 adrenoceptor (alpha2R) agonist clonidine is used as a treatment for heroin addiction. Substantial evidence indicates that dopaminergic and noradrenergic systems have key roles in opiate dependence and withdrawal but the possible interactions between these two pathways remain unclear. The objective of this study was to establish the effects of clonidine pretreatment on ventral tegmental area dopaminergic (VTA DA) neuronal activity during morphine withdrawal. Responses of VTA DA neurons to withdrawal precipitated by naltrexone were characterized in anesthetized rats using extracellular recordings. As expected, withdrawal produced a marked inhibition of VTA DA neuronal activity. However, pretreatment with clonidine prevented this inhibition induced by withdrawal, and instead produced a long-lasting activation of firing rate (+50%) and burst firing (+19%). In contrast, pretreatment with a more selective alpha2R agonist, UK14304, did not prevent the inhibition of VTA DA neuron activity during withdrawal. We tested whether the high affinity of clonidine for imidazoline-1 receptors (I1Rs) was responsible for the difference between these two alpha2R agonists. In morphine-dependent rats pretreated with rilmenidine (mixed alpha2R/I1R agonist), precipitation of withdrawal elicited a 22% increase of VTA DA impulse activity. The action of clonidine on I1Rs was studied by coadministering clonidine with RX821002, a specific alpha2R antagonist. Pretreatment with RX821002 plus clonidine prevented the inhibition of VTA DA activity during withdrawal but failed to produce excitation. These results indicate that the pharmacological effects of clonidine on VTA DA neurons during morphine withdrawal is related to actions on I1Rs as well as alpha2Rs.

Receptor subtype mediating the adrenergic sensitivity of pain behavior and ectopic discharges in neuropathic Lewis rats

Receptor subtype mediating the adrenergic sensitivity of pain behavior and ectopic discharges in neuropathic Lewis rats. We attempted to identify the subtype of alpha-adrenergic receptor (alpha-AR) that is responsible for the sympathetic (adrenergic) dependency of neuropathic pain in the segmental spinal injury (SSI) model in the Lewis strain of rat. This model was chosen because our previous study showed that pain behaviors in this condition are particularly sensitive to systemic injection of phentolamine (PTL), a general alpha-AR blocker. We examined the effects of specific alpha1- and alpha2-AR blockers on 1) behavioral signs of mechanical allodynia, 2) ectopic discharges recorded in the in vivo condition, and 3) ectopic discharges recorded in an in vitro setup. One week after tight ligation of the L5 and L6 spinal nerves, mechanical thresholds of the paw for foot withdrawals were drastically lowered; we interpreted this change as a sign of mechanical allodynia. Signs of mechanical allodynia were significantly relieved by a systemic injection of PTL (a mixed alpha1- and alpha2-AR antagonist) or terazosin (TRZ, an alpha1-AR antagonist) but not by various alpha2-AR antagonists (idazoxan, rauwolscine, or yohimbine), suggesting that the alpha1-AR is in part the mediator of the signs of mechanical allodynia. Ongoing ectopic discharges were recorded from injured afferents in fascicles of the L5 dorsal root of the neuropathic rat with an in vivo recording setup. Ongoing discharge rate was significantly reduced after intraperitoneal injection of PTL or TRZ but not by idazoxan. In addition, by using an in vitro recording setup, spontaneous activity was recorded from teased dorsal root fibers in a segment in which the spinal nerve was previously ligated. Application of epinephrine to the perfusion bath enhanced ongoing discharges. This evoked activity was blocked by pretreatment with TRZ but not with idazoxan. This study demonstrated that both behavioral signs of mechanical allodynia and ectopic discharges of injured afferents in the Lewis neuropathic rat are in part mediated by mechanisms involving alpha1-ARs. These results suggest that the sympathetic dependency of neuropathic pain in the Lewis strain of the rat is mediated by the alpha1 subtype of AR.

A thermodynamic analysis of estrogen regulation of alpha 2-adrenoceptor binding

We previously demonstrated that in vivo estradiol treatment markedly attenuates alpha 2-adrenoceptor function and coupling to G-proteins in the hypothalamus of female rats. Ligand binding studies indicated that 48 h exposure to estradiol decreases the number of alpha 2-adrenergic receptors in the agonist high affinity state. In the present studies, when [3H]RX821002 was used to label brain alpha 2-adrenoceptors, the density of binding sites significantly increased in the hypothalamus and preoptic area 48 h after estrogen treatment. Moreover, the thermodynamics of ligand binding to alpha 2-adrenergic receptors in membranes of female rat hypothalamus were modified by the same estradiol treatments that reduce alpha 2-adrenoceptor function. In hypothalamic membranes from ovariectomized control rats, antagonist (RX821002)-receptor binding was primarily entropy-driven while agonist (oxymetazoline) binding had a higher enthalpy component. In membranes from estradiol-exposed animals, the entropic contribution to both agonist and antagonist bindings was markedly increased, and the enthalpy component was reduced. Since the thermodynamic characteristics of ligand-receptor binding are strongly correlated with efficacy in activating signal transduction [36], these data raise the intriguing possibility that steroids regulate transmembrane signaling by stabilization of a receptor conformation with reduced intrinsic efficacy.

Increased imidazoline and alpha 2 adrenergic binding in platelets of women with dysphoric premenstrual syndromes

An association between dysphoric premenstrual syndromes (PMS) and a lifetime history of major depressive disorders has previously been documented. Other studies have demonstrated an increase in the binding of radiolabeled imidazoline compounds to platelets of depressed patients. Clonidine and related imidazoline compounds interact with alpha 2 adrenoceptors to inhibit neuronal noradrenergic activity and in higher concentrations, they stimulate noradrenergic activity through their interaction with imidazoline receptors. Here we report increased 3H para-aminoclonidine binding to high affinity alpha 2 adrenoceptor sites as well as to nonadrenergic imidazoline binding sites in platelets of women with dysphoric PMS. This higher binding was most pronounced during the late-luteal-symptomatic phase of the menstrual cycle and, to a lesser degree, during the non-symptomatic mid-follicular phase. Binding to the imidazoline site distinguished women with dysphoric PMS from women with no such symptoms, was highly positively correlated with the severity of symptoms, and was negatively correlated with plasma levels of progesterone. These findings suggest that platelet imidazoline binding sites might be a biological marker for dysphoric states in PMS or for the vulnerability to develop them. These findings also point to a possible biological link between dysphoric PMS and major depressive disorders.

Alpha 2-adrenoceptors in the brain of suicide victims: increased receptor density associated with major depression

To examine directly in the brain the status of the alpha 2-adrenoceptor in major depression, the specific binding of the agonists [3H]clonidine and [3H]UK 14304 was quantitated in various brain regions of suicide victims with a retrospective diagnosis of depression or other psychiatric disorders. In depressed suicides, the binding capacity of [3H]clonidine was found to be increased in the hypothalamus (Bmax 35%-55% greater), and to a lesser extent in the frontal cortex, as compared with that in matched controls, schizophrenic suicides, or suicides with various diagnosis. The binding capacity of [3H]UK 14304 also was found increased in the frontal cortex (Bmax 30% greater), and to a lesser extent in the hypothalamus, of depressed suicides. In other brain regions such as the amygdala, hippocampus, and cerebellum there also was a tendency for an increased receptor density associated with suicide. Moreover, in the frontal cortex of suicides, the potency of norepinephrine in displacing the binding of the antagonist [3H]idazoxan also was found increased (Ki decreased eight-fold). The results indicate that the density and affinity of alpha 2A-adrenoceptors in the high-affinity state are increased in the brain of depressed suicides.

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.

3H-spiroperidol binding in human temporal cortex (Brodmann areas 41-42) occurs at multiple high affinity states with serotonergic selectivity

3H-Spiroperidol labels multiple high affinity states with serotonergic selectivity in human prefrontal cortex and with dopaminergic selectivity in human caudate and putamen. The characteristics of the binding of this ligand in human temporal cortex have not been previously described. Brodmann areas 41-42 in the temporal cortex are associated with primary auditory sensation and, in epileptics, with auditory hallucinatory experiences. We found that in this region of the human brain, antipsychotic ligands bind at multiple high affinity states, the majority of which exhibit serotonergic, rather than dopaminergic, selectivity. Dose-response data is best resolved by a three-site fit. Results of the co-analysis of dose-response data with saturation data indicates that two of the 3H-spiroperidol affinity states may represent the high and low affinity states of the serotonin (5HT)2 receptor, while a third affinity state may represent the 5-HT1A receptor.