Alpha 1-adrenoceptor denervation supersensitivity in brain: physiological and receptor binding studies

Sleep induction and temperature lowering by medial preoptic α1 adrenergic receptors

Changes in sleep-wakefulness (S-W) and body temperature (T(b)) on administration of alpha(1) agonist (methoxamine) and antagonist (prazosin) into the medial preoptic area (mPOA) were studied in rats. Presynaptic catecholaminergic terminals of the mPOA were destroyed by injecting 6-hydroxydopamine at the ventral noradrenergic bundle (VNA), before administration of the drugs. Microinjection of 0.05 microg methoxamine induced sleep, though 0.1 microg prazosin produced no change in S-W. On the other hand, in normal rats, the same dose of methoxamine produced no change, while prazosin produced arousal. Denervation hypersensitivity may be responsible for the appearance of hypnogenic response on methoxamine administration, in the VNA-lesioned rats. The VNA-lesioned animals (before administration of any drug) had higher pre-injection values of wake period than the normal rats. A reduction in the tonic activity of noradrenergic fibers to the mPOA, and resulting reduced activity of alpha(1) receptors, may be responsible for increased wake period in the VNA-lesioned rats. The action of prazosin was probably abolished in the absence of tonic activity of alpha(1) receptor in the VNA-lesioned rats. Reduction and increase in T(b) produced by methoxamine and prazosin, respectively, confirm the involvement of alpha(1) receptors in the thermal changes. Methoxamine was less effective, than in normal rats, in reducing T(b). So, the possibility of involvement of presynaptic receptors in the thermal response is suggested. The results suggest the involvement of separate sets of alpha(1) receptors (and neurons) in hypnogenesis and in lowering T(b). As sleep is associated with fall in T(b), the alpha(1) adrenergic receptors may be involved in interlinking sleep regulation and thermoregulation.

Propranolol for disruptive behaviors in nursing home residents with probable or possible Alzheimer disease: a placebo-controlled study

OBJECTIVE

Enhanced behavioral responsiveness to central nervous system (CNS) norepinephrine (NE) in Alzheimer disease (AD) may contribute to the pathophysiology of disruptive behaviors such as aggression, uncooperativeness with necessary care, irritability, and pressured pacing. We evaluated the efficacy of the beta-adrenergic antagonist propranolol for treatment-resistant disruptive behaviors and overall behavioral status in nursing home residents with probable or possible AD.

METHODS

Thirty-one subjects (age 85 +/- 8 [SD]) with probable or possible AD and persistent disruptive behaviors that interfered with necessary care were randomized to propranolol (n = 17) or placebo (n = 14) in a double-blind study. Stable doses of previously prescribed psychotropics were maintained at pre-study dose during the study. Following a propranolol or placebo dose titration period of up to 9 days (per a dosing algorithm), subjects were maintained on maximum achieved dose for 6 weeks. Primary outcome measures were the Neuropsychiatric Inventory (NPI) and the Clinical Global Impression of Change (CGIC).

RESULTS

Propranolol augmentation (mean achieved dose 106 +/- 38 mg/d) was significantly more effective than placebo for improving overall behavioral status on the total NPI score and CGIC. Improvement in individual NPI items within propranolol subjects was significant only for "agitation/aggression" and "anxiety," and reached borderline statistical significance favoring propranolol over placebo only for "agitation/aggression." Pressured pacing and irritability did not appear responsive to propranolol. In propranolol subjects rated "moderately improved" or "markedly improved" on the CGIC at the end of the double-blind study phase, improvement of overall behavioral status had diminished substantially after 6 months of open-label propranolol treatment.

CONCLUSION

Short-term propranolol augmentation treatment appeared modestly effective and well tolerated for overall behavioral status in nursing home residents with probable or possible AD complicated by disruptive behaviors. Propranolol may be helpful specifically for aggression and uncooperativeness (the behaviors assessed by the NPI "agitation/aggressiveness" item). However, the usefulness of propranolol in this very old and frail population was limited by the high frequency of relative contraindications to beta-adrenergic antagonist treatment and diminution of initial behavioral improvements over time.

Complex regional pain syndrome

Complex regional pain syndrome (CRPS) may develop after limb trauma and is characterized by pain, sensory-motor and autonomic symptoms. Most important for the understanding of the pathophysiology of CRPS are recent results of neurophysiological research. Major mechanism for CRPS symptoms, which might be present subsequently or in parallel during the course of CRPS, are trauma-related cytokine release, exaggerated neurogenic inflammation, sympathetically maintained pain and cortical reorganisation in response to chronic pain (neuroplasticity). The recognition of these mechanisms in individual CRPS patients is the prerequisite for a mechanism-oriented treatment.

Prazosin reduces trauma-related nightmares in older men with chronic posttraumatic stress disorder

Trauma-related nightmares in posttraumatic stress disorder (PTSD) rarely respond to pharmacologic treatment. Neurobiologic data suggest that enhanced brain responsiveness to adrenergic stimulation may contribute to the pathophysiology of trauma-related nightmares in PTSD. Nine older men with chronic PTSD secondary to military or Holocaust trauma were prescribed the lipophilic alpha-1 adrenergic antagonist prazosin for treatment-resistant trauma-related nightmares. Prazosin 2 mg to 4 mg 1 hour before bedtime substantially reduced nightmares and moderately or markedly reduced overall PTSD severity in 8 of 9 subjects. Prazosin was well tolerated. These open-label results are consistent with demonstrated therapeutic efficacy of prazosin for PTSD nightmares and sleep disturbance in a recent placebo-controlled trial in Vietnam veterans.

Denervation supersensitivity to phenylephrine in guinea-pig vas deferens in vivo and in vitro: functional studies on alpha1-adrenoceptors

The objective was to estimate alterations in adrenergic receptor sites of guinea pig vas deferens, in vivo and in vitro, induced by chronic denervation. The denervation process induced an increased sensitivity (3-fold at the EC50 level) without alteration in the maximum response to phenylephrine in vitro. The sensitivity alteration was characterized by the decrease in the dissociation constant of phenylephrine for alpha-adrenoceptor [K(A): normal tissue 3.50 (0.75-16.21) x 10(-5) and denervated tissue 0.43 (0.11-1.67) x 10(-5) M, p < 0.05] without changing the dissociation constant of prazosin. A decrease in pD'2 value for phenylephrine-phenoxybenzamine, probably due to a qualitative rather than a quantitative alteration in the alpha-adrenoceptor, was also shown in vitro [pD'2: normal tissue (8.2776+/-0.0402) and denervated tissue (8.0051+/-0.0442), p < 0.05]. No change in sensitivity and maximum response to phenylephrine was observed in vivo after denervation, although an increased resistance of vas deferens to phenoxybenzamine blockade has been evidenced in this condition.

Changes in receptor levels for thyrotropin releasing hormone, serotonin, and substance P in cervical spinal cord of Wobbler mouse: a quantitative autoradiography study during early and late stages of the motoneuron disease

Receptor levels for thyrotropin releasing hormone (TRH) measured by quantitative autoradiography in the Wobbler mouse cervical spinal cord show receptor losses that may relate to the inherited loss of motoneurons, most pronounced late (at Stage 4) in the motoneuron disease. An age-related decrease of TRH and serotonin (5-HT) receptors can be seen in the ventral horn of the control specimens (normal phenotype littermate and wild-type alike). However, this pattern is missing for substance P (SP) receptors from the wild-type specimens. Therefore the age-related decrease of SP receptors detected in the Wobbler mouse strain may identify a strain-related defect in SP neuronal/receptor developmental patterns. A higher level of TRH receptors was measured in the Wobbler dorsal horn at an early stage (Stage 1) in the motoneuron disease compared with the control specimens. The data are discussed in relation to an aberrant neuronal sprouting that occurs around the degenerating motoneurons in the ventral horn during the course of the motoneuron disease.

Sustained increase in rat myocardial alpha 1A-adrenoceptors induced by 6-hydroxydopamine treatment involves a decelerated receptor turnover

The biochemical mechanisms involved in the alpha 1-adrenoceptor up-regulation and possible changes in subtypes of adrenoceptors in the rat heart after chemical denervation were investigated. The effects of acute 6-hydroxydopamine treatment (two increasing doses 24 h apart) on the pseudo-steady state densities and turnover rates of alpha 1-adrenoceptors were studied in ventricular myocardium of the rat. We have assessed the repopulation kinetics of [3H]prazosin binding sites after irreversible inactivation of alpha 1-adrenoceptors induced by a single dose of phenoxybenzamine (1 mg/kg i.p.) in rats acutely treated either with 6-hydroxy-dopamine or with vehicle (control animals). Seven days after the last administration of 6-hydroxydopamine an enhanced density of [3H]prazosin binding sites (Bmax 58.7 +/- 3.6 fmol/mg protein vehicle-treated rats versus 82.6 +/- 5.3 fmol/mg protein 6-hydroxydopamine-treated rats) was observed. This was not accompanied by changes in the dissociation constant value. Furthermore, the proportion of high affinity sites for WB-4101 was altered (21 +/- 2% versus 72 +/- 3% for animals treated with vehicle and 6-hydroxydopamine, respectively). In rat myocardium, alpha 1-adrenoceptor turnover, evaluated during the 6-hydroxydopamine-induced up-regulation (7-19 days after the completion of treatment with 6-hydroxydopamine) revealed an increase in the half-life of the alpha 1-adrenoceptor (t1/2 of 67.2 h versus 38.7 h in control animals). The present study confirms an increase in alpha 1-adrenoceptors in rat myocardium after chemical denervation and reveals that the effect is almost completely confined to the alpha 1A-adrenoceptor subtype. Furthermore, the up-regulation of alpha 1A-adrenoceptors is the result of a decrease in the cellular processes that control the rate of receptor degradation.

Effects of surgical sympathetic denervation on G-protein levels, alpha and beta-adrenergic receptors, cAMP production and adenylate cyclase activity in the smooth muscles of rabbit iris

The relative densities of a number of G protein subunits were quantified in membranes prepared from iris sphincter and dilator muscles of rabbits that have undergone sympathetic denervation and from contralateral innervated controls by immunoblotting with specific polyclonal antibodies against Gs alpha, Gi alpha and Gq alpha protein subunits. In addition, alpha and beta-adrenergic receptor densities, basal and isoproterenol (ISO)-stimulated cAMP production, and basal and ISO+GTP gamma S-stimulated adenylate cyclase (AC) activities were measured in the same tissues. Densitometric analysis of the immunoblot data revealed a 32% reduction in the level of Gi alpha in the denervated sphincter, a 26% increase in the level of Gi alpha in the denervated dilator, and no changes due to denervation were found in the levels of Gs alpha and Gq alpha. Sympathetic denervation had no effect on the densities of alpha- and beta-adrenergic receptors in these tissues, however, it did induce a significant decrease in the KD values of alpha-adrenergic receptors in both dilator and sphincter, and in beta-adrenergic receptors in the sphincter. The basal- and ISO-stimulated cAMP production in the sphincter was 4-fold as high as that of the dilator, and at 0.05 microM ISO the denervation supersensitivity for cAMP production in the sphincter increased by 118% as compared to 36% in the dilator. Sympathetic denervation increased by 19-47% the basal- and ISO-stimulated activity of AC in these tissues, however, it had no effect on the phospholipase C activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Antidepressant treatment and chemical sympathectomy fail to modulate alpha 1-adrenoceptor sensitivity in mouse eye

The mydriatic response to alpha 1-adrenergic agonists was used as a functional index of postsynaptic alpha 1-adrenoceptors in mouse iris dilator muscle. Topical ocular application of methoxamine or phenylephrine caused dose-related mydriasis which was inhibited by pretreatment with prazosin or phentolamine. Chemical sympathectomy with topical 6-hydroxydopamine (6-OHDA) produced supersensitivity to phenylephrine but not methoxamine. Daily antidepressant treatment for 14 days with desipramine (10 mg/kg, i.p.), amitriptyline (10 mg/kg, i.p.), fluoxetine (2 mg/kg, i.p.), or moclobemide (40 mg/kg, i.p.) did not alter the response to methoxamine. Central alpha 1-adrenoceptors labelled with [3H]prazosin were similarly unaffected except for a modest downregulation produced by fluoxetine. These results demonstrate that postsynaptic alpha 1-adrenoceptors in mouse CNS and iris dilator muscle are refractory to manipulations known to alter their sensitivity in other tissues.

Alpha 1-adrenoceptors in the adult rat barrel field: effects of deafferentation and norepinephrine removal

Norepinephrine (NE), acting on brain adrenoceptors, plays an important role in barrel field neuronal activity and plasticity. For this reason, the distribution of alpha 1- and alpha 2-adrenoceptors in the somatosensory cortex barrel field was studied by autoradiographic techniques in rats undergoing plastic change or NE depletion. In layers IV and V of the cortex, the pattern of alpha 1-adrenoceptors (assessed by [3H]prazosin binding) varied across the barrel field. There was relatively low binding within the barrels themselves, with 21% higher binding in the surrounding septa. alpha 2-Adrenoceptor binding (assessed with [3H]paraminoclonidine) was almost homogeneous across the entire barrel field. Two weeks after noradrenergic deafferentation by unilateral lesioning of the locus coeruleus, there was a 16% upregulation of [3H]prazosin binding. This then returned to control levels of by 8 weeks. Peripheral deafferentation of sensory input to the barrel field produced the opposite effect on alpha 1-adrenoceptors. Unilateral removal of all but the central (C3) vibrissa (which induces plastic changes in the cortical representation of the spared virbrissa) caused a 12% decrease in [3H]prazosin binding in the whole barrel field at 2 weeks after surgery which returned to normal by 8 weeks. Therefore, alpha 1-adrenoceptors in the barrel field of the rat are affected in opposite ways by changes in NE content and afferent sensory input. We hypothesize that alpha 1-adrenoceptor levels are modulated after vibrissectomy through either an indirect reaction to reduced cortical gamma-aminobutyric acid levels, or by a reordering of metabolic priorities during plastic change of the cortical neuronal network.

Altered alpha 1-adrenoceptor binding in intact and adrenalectomized obese Zucker rats (fa/fa)

While many autonomic and metabolic defects associated with genetic obesity in the Zucker rat are corrected by adrenalectomy (Adx), brain adrenoceptor function has not been examined in this context. Here, 3 weeks after Adx or sham surgery, brains of 11 weeks old lean (Fa/Fa) and obese (fa/fa) male Zucker rats were assayed for alpha 1-([3H]prazosin; [3H]PRZ) and alpha 2-adrenoceptor ([3H]paraminoclonidine; [3H]PAC) binding by autoradiography. By genotype, obese rats had 19-256% higher [3H]PRZ binding than lean rats in the amygdala (central [ACN], basolateral [ABL], basomedial [ABM] and medial [MAN] nuclei [n.]), hypothalamus (dorsomedial n. [DMN] and lateral [LH]) and somatosensory cortex. In the ABL and ACN, increased maximal binding (Bmax) in obese rats was associated with decreased affinity (increased Kd). Three weeks after surgery, sham-operated obese rats gained 27% more weight than lean rats but lean and obese Adx rats gained the same amount of weight. Adx reduced [3H]PRZ binding in both lean and obese rats by 37-70% in the amygdala (ABM, ACN, MAN) compared to sham-operated rats. But, Adx selectively reduced [3H]PRZ binding only in lean rats in the ABL, DMN, ventromedial hypothalamic n. (VMN) and ventroposteromedial thalamic n. In most areas, decreases in maximal binding (Bmax) associated with Adx were accompanied by decreases in Kd. Unlike [3H]PRZ binding, there was no consistent genotype difference in [3H]PAC binding although Adx was followed by increased binding in obese and decreased binding in lean rats in the ABL. In only the VMN, obese rats had a 21% higher alpha 2- to alpha 1-adrenoceptor ratio than lean rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Central noradrenergic lesion impairs the adrenocorticotrophin response to release of endogenous catecholamines

Activation of hypothalamic α(1) -adrenoceptors stimulates the secretion of corticotrophin-releasing factors which in turn stimulate pituitary adrenocorticotrophin (ACTH). This mechanism is important in the physiological control of ACTH secretion. This study assesses the feasibility of using the ACTH response to release of endogenous catecholamines as a means of detecting a hypothalamic noradrenergic lesion in vivo. Intracerebroventricular infusion of the catecholamine neurotoxin, 6-hydroxydopamine, was used to destroy noradrenergic nerve endings in rats, with the purpose of producing a model that could be used to study alterations in ACTH responses that may result from a lesion involving central noradrenergic neurons. 6-Hydroxydopamine (250 μg icv) significantly reduced hypothalamic noradrenaline content, indicating damage to noradrenergic nerve endings, without affecting postsynaptic receptor function, as judged by preservation of the effect of a selective α(1) -adrenergic agonist. Pharmacological release of endogenous catecholamines, effected by combined administration of a catecholamine precursor and an α(2) -adrenergic antagonist, stimulated the secretion of ACTH in control, but not in 6-hydroxydopamine-treated rats. Degeneration of hypothalamic noradrenergic nerve endings is not followed by denervation hypersensitivity, and is therefore accompanied by impairment of the ACTH response to release of endogenous catecholamines.

The locus coeruleus and dopaminergic function in rat brain: implications to parkinsonism

Clinical and experimental evidence suggests that degeneration of the locus coeruleus (LC) may be responsible for certain symptoms of Parkinson's disease (PD). We have, therefore, studied the effects of LC lesion on dopamine (DA) metabolism in the rat striatum. Unilateral depletion of norepinephrine (NE) was obtained by stereotaxic injection of 6-hydroxydopamine (6-OHDA) into the dorsal NE bundle (DNEB). Rats were sacrificed 1 or 3 weeks after lesioning. 6-OHDA induced approximately 50% depletion of NE in the ipsilateral hippocampus at 1 week postinjection, and over 75% depletion after 3 weeks. DNEB lesions had no effect on DA or DOPAC levels in the ipsi- or contralateral striatum at either time point. Lesions also failed to affect DA synthesis or utilization in either striatum. The metabolism of exogenous levodopa in the striatum was also unaffected. It is suggested that any possible effect of the LC on DA transmission in the striatum is not mediated by the DNEB.

Role of catecholamines in the courtship behavior of male ring doves

The role of catecholamines in the expression of male courtship behavior in ring doves was examined using central administration of pharmacological agents. Males treated with 6-hydroxydopamine or U-14,624, which depleted norepinephrine (NE) levels in the preoptic-hypothalamic area, showed increased levels of bow-coo and nest-coo displays. Conversely, males treated with tyramine or desipramine, which elevated NE levels in the preoptic-hypothalamic area, showed decreased levels of bow-coo and nest-coo displays. Drug-induced changes in dopamine levels were not consistent with any changes in behavior. This suggests that in the male ring dove NE in the preoptic-hypothalamic area is important in the expression of courtship displays.

Changes in adrenoceptors and monoamine metabolism in neonatal and adult rat brain after postnatal exposure to the antihypertensive labetalol

1. The purpose of the present study was to investigate the acute (single injection), direct (chronic treatment) and the long-lasting effects after exposure to the alpha 1/beta-adrenoceptor antagonist labetalol during rat brain development on adrenoceptors and monoamine metabolism. 2. In 10-day-old rat pups, subcutaneously administered labetalol (10 mg kg-1) passed the blood-brain barrier, reaching a level of 2.1 micrograms g-1 tissue in the brain 90 min after injection. 3. Chronic labetalol treatment (10 mg kg-1, s.c., twice daily) during the first 10 days of life significantly increased alpha 1-adrenoceptor binding in the hypothalamus (+39%), but not in the occipital cortex. 4. This chronic postnatal labetalol treatment did not result in long-lasting changes in alpha 1- and beta-receptors measured on day 60. 5. A single labetalol injection (10 mg kg-1, s.c.) on postnatal day 10 significantly increased noradrenaline (NA) metabolism in all brain regions tested (+25 to 105%), but had no effects on 5-hydroxytryptamine (5-HT) or dopamine metabolism. 6. Chronic labetalol treatment between postnatal (PN) days 1 and 10 also increased NA metabolism on PN 10 (3-methoxy-4-hydroxyphenylglycol (MHPG)/NA, +20 to 100%), suggesting that tolerance to the acute effect of labetalol did not occur. A slight increase in 5-HT metabolism (20%) was induced by the chronic labetalol treatment in the hippocampus and meso-limbic system. 7. In general, long-lasting effects on NA metabolism could not be detected on day 60 more than one month after the treatment. However, 5-HT metabolism was significantly increased in all four brain regions measured (+20 to 70%). 8. We conclude that chronic labetalol exposure during early postnatal rat brain development does not cause long-lasting changes in beta-receptor number or NA metabolism, but appears to be critical for the rate of 5-HT metabolism in later life.

Iontophoresis of norepinephrine onto neurons of the pigeon's lateral geniculate nucleus: characterization of an inhibitory response

A group of neurons in the pigeon's lateral geniculate equivalent nucleus (LGNe) shows associative enhancement of their response to light during visually conditioned heart rate change. The source of the relevant unconditioned stimulus input to LGNe for this enhancement has been identified as the locus coeruleus (LC). Thus, we have begun to examine neurotransmitters synthesized in LC for possible involvement in associative modification of neuronal discharge in LGNe. As a first step, we have examined the responses of LGNe neurons to iontophoretic application of norepinephrine (NE) and identified the receptor mediating one response class. The majority of neurons in LGNe show inhibition of maintained activity in response to iontophoretic application of NE or its agonists. The potency of the NE agonists is alpha-methyl NE greater than epinephrine greater than NE greater than phenylephrine greater than isoproterenol. This profile is characteristic of an alpha 2-adrenoceptor. The alpha 2-agonist clonidine also induces inhibition of maintained activity. The alpha 2-antagonists WB-4101 and yohimbine block the NE-inhibition while the alpha 1-antagonist prazosin and beta-antagonist sotalol do not. It is thus suggested that the receptor mediating the NE-inhibition of maintained activity has the characteristics of an alpha 2-adrenoceptor.

Alpha-adrenergic influences on neuronal responses to visceral afferent input in the nucleus tractus solitarius

Studies were made of the effects of the alpha 1-adrenoceptor agonist methoxamine on spontaneous and synaptically evoked activity in the solitary tract nucleus in isolated, perfused slices of the brain of the rat and those effects were compared to the effects of the alpha 2-agonist clonidine. Methoxamine had no effect on the spike activity of 7 out of 8 spontaneously firing neurones, with no response to electrical stimulation of the solitary tract. Among neurones that responded to tract stimulation, those which were otherwise silent (N = 38) showed a decrease of responsiveness to tract input during the infusion of methoxamine, whereas those with spontaneous activity (N = 10) showed mostly increases of both firing rate and responsiveness. The effects of activation of alpha 2-receptors on responsiveness to tract stimulation were the opposite of previously demonstrated effects of activation of alpha 2-receptors, and these opposing effects could be demonstrated in the same neurone. These results suggest that the transmission of visceral sensory information within the nucleus tractus solitarius may be controlled by opposing alpha 1- and alpha 2-receptor-mediated neural systems.

Involvement of norepinephrine activity in the regulation of alpha 1 adrenergic receptors in the medial preoptic nucleus of estradiol-treated rats

To establish whether the diurnal decrease in the density of alpha 1 receptors observed in the medial preoptic nucleus (MPN) of estrogen (E2)-treated rats is related to the concomitant diurnal increase in norepinephrine (NE) turnover rates, we quantitated the density of [3H]-Prazosin binding to alpha 1 receptors after blockade of NE turnover with alpha-methyl-paratyrosine (alpha MPT). A series of preliminary studies was performed to rule out an interference of this drug with [3H]-Prazosin binding to alpha 1 adrenergic receptors in vitro and in vivo. Incubation of brain slices with alpha MPT produced a dose-dependent inhibition of [3H]-Prazosin binding to alpha 1 adrenergic receptors with an IC50 of approximately 6 mM. Scatchard analysis demonstrated that alpha MPT exhibited a simple competitive interaction with [3H]-Prazosin binding sites as shown by an increase in the apparent dissociation constant (Kd) of the ligand and no change in the number of alpha 1 receptors (Bmax). In contrast, preincubation of brain slices with alpha MPT and prior in vivo administration of alpha MPT did not affect [3H]-Prazosin binding to alpha 1 adrenergic receptors. Once we established that alpha MPT could be used to suppress NE turnover without interfering with the measurement of alpha 1 receptor densities, we repeatedly injected this drug to ovariectomized (OVX) and E2-implanted rats. The density of alpha 1 adrenergic receptors in MPN was quantitated autoradiographically. Blockade of NE turnover with alpha MPT only partially prevented the reduction in alpha 1 receptor density observed in the E2-treated rats, suggesting that the decrease in the level of [3H]-Prazosin binding sites cannot be completely ascribed to increased NE turnover rates.

Modulation of second messenger function in rat brain by in vivo alteration of receptor sensitivity: relevance to the mechanism of action of electroconvulsive therapy and antidepressants

1. The second messengers cyclic AMP and inositol triphosphate are the intracellular mediators for a number of neurotransmitters for which receptors exist on brain neurons. 2. Up- or down-regulation of these receptors in general produce corresponding changes in the associated second messenger systems. 3. Chronic administration of antidepressants including electroconvulsive shock to rats produces a number of changes in cerebral receptors, notably down-regulation of beta-adrenergic and serotonin 5-HT2 receptors and up-regulation of alpha-1 adrenergic receptors. 4. The changes in receptor number induced by such antidepressant treatments are in general accompanied by corresponding changes in the associated second messenger reactions. 5. Antidepressant administration has also been shown to induce increased post-receptor mediated adenylate cyclase activity in cortical membranes, and similar effects have also been reported in striatum after chronic administration of neuroleptics. The relevance of these effects to the mechanism of action of the drugs is discussed.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. II. Receptors mediating the effect of synaptically released norepinephrine

The present studies were undertaken to determine the nature of the receptors mediating the effects of endogenous norepinephrine (NE) released by stimulation of the locus coeruleus (LC) on the firing activity of dorsal hippocampus pyramidal neurons in the rat. Unitary activity of CA3 pyramidal neurons was recorded extracellularly. In most neurons, the LC stimulation produced a period of suppression, followed by a period of activation. The suppression was selectively blocked by prazosin, an alpha 1-adrenoceptor antagonist, whereas the activation was selectively blocked by propranolol, a beta-adrenoceptor antagonist. Idazoxan, an alpha 2-adrenoceptor antagonist, increased the period of suppression without affecting the period of activation. The effectiveness of microiontophoretic applications of NE on the same neurons was reduced by idazoxan, but was modified neither by propranolol nor prazosin. Lesion of the central noradrenergic system by intracerebroventricular 6-hydroxydopamine markedly decreased the NE content in the hippocampus in all rats but the effectiveness of the LC stimulation was reduced only in rats with a depletion greater than 90%. These results demonstrate that the suppressant effect of endogenous NE released by LC stimulation on hippocampus pyramidal neurons is mediated by an alpha 1-adrenoceptor and suggest that its late excitatory effect might involve beta-adrenoceptors. Since the effect of microiontophoretically applied NE on the same neurons is mediated by alpha 2-adrenoceptors, these data provide evidence that, in the rat hippocampus, postsynaptic alpha 1-adrenoceptors are intrasynaptic, whereas postsynaptic alpha 2-adrenoceptors are extrasynaptic.

A function for REM sleep: regulation of noradrenergic receptor sensitivity

We hypothesize that REM sleep serves to upregulate and/or prevent downregulation of brain norepinephrine (NE) receptors. This hypothesis is based on the following observations: (1) NE neurons of the locus coeruleus (LC) are tonically active in waking and non-REM sleep, but the entire population of LC NE neurons is inactive during REM sleep. (2) Continuous presence of NE or adrenoceptor agonists downregulates NE receptors, while a reduction in NE availability upregulates these receptors. (3) The effects of REM sleep deprivation are similar to those of NE receptor downregulation. Recent biochemical studies of NE receptor sensitivity provide strong experimental support for this hypothesis. The functional consequence of enhanced NE receptor 'tone' brought about by REM sleep would be improved signal processing in diverse brain systems, thus endowing the organism with a selective advantage. This hypothesis makes a number of specific predictions which can be tested with currently available techniques, and suggests new ways of understanding the evolution and postnatal development of REM sleep.

Noradrenergic regulation of alpha 1-receptors during the postnatal development of the guinea pig

In guinea pig brain, alpha 1-noradrenergic receptor concentrations undergo region-specific fluctuations during the first weeks of postnatal life. However, the factors involved in the regulation of these receptors have yet to be identified. In this study, the ontogeny of one possible regulatory factor, norepinephrine, was examined in relation to postnatal changes in alpha 1-receptor levels in several different regions of guinea pig brain. Results from these studies showed that while the activity of the noradrenergic system increased throughout the first weeks of postnatal development in each brain area examined, the concentration of alpha 1-receptors decreased in preoptic area and hypothalamus and increased in cortex. In subsequent experiments, the effects of noradrenergic lesions with 6-hydroxydopamine on alpha 1-receptor levels were assessed to examine the possibility that alpha 1-receptors are differentially sensitive to noradrenergic stimulation in cortex and preoptic area/hypothalamus in immature guinea pigs. Noradrenergic lesions which reduced norepinephrine levels by 87-94% resulted in significant elevations in alpha 1-receptors in all regions examined. These results are discussed with reference to the anatomical distribution of alpha 1-receptors and their regulation by norepinephrine.

Role of ascending and descending noradrenergic pathways in the antinociceptive effect of baclofen and clonidine

Baclofen and clonidine interact with central noradrenaline (NA) pathways by a variety of mechanisms. The specific role of ascending and descending pathways in antinociception produced by these agents was examined by lesioning the dorsal bundle (DB), locus coeruleus (LC) and descending NA pathways by the microinjection of the neurotoxin 6-hydroxydopamine (6-OHDA). Lesions were verified using high-performance liquid chromatography analysis of NA. Both baclofen and clonidine were injected intraperitoneally in all experiments. The antinociceptive effect of baclofen in the tail-flick test was inhibited 7-21 days after DB lesions. This manipulation decreased NA levels in cortex, hippocampus and hypothalamus but did not alter spinal cord levels. Lesions of the LC potentiated the effect of baclofen 12-16 days postlesion. NA levels were reduced in all the regions just mentioned. DB lesions produced a transient decrease in the effect of clonidine, being observed 7 but not 12-16 days postlesion. Neither acute depletion of NA levels with alpha-methyl-p-tyrosine (alpha-MPT), nor LC lesions significantly affected antinociception produced by clonidine. Intraspinal 6-OHDA potentiated the antinociceptive action of clonidine in the tail-flick test. This treatment markedly reduced spinal cord NA levels, but had minimal effects on brain NA. The results of this and previous studies in this laboratory suggest that the antinociceptive effect of baclofen is mediated by interactions with both ascending and descending NA pathways. These pathways appear to interact in a complex manner. Interpretation of data for clonidine is complicated because lesions can both deplete endogenous NA as well as inducing postsynaptic supersensitivity of alpha 2-receptors. Clonidine does not depend on endogenous NA pathways for producing antinociception because acute depletion of NA with alpha-MPT does not alter its action. Spinal sites of action are of importance following systemic clonidine because intraspinal 6-OHDA produces supersensitivity. Altering NA activity in ascending pathways alone produces a transient inhibition of the effect of clonidine, but supersensitivity is not apparent. Simultaneous lesions of both ascending and descending pathways do not produce supersensitivity, again suggesting important interactions between such pathways can occur.

Alpha-adrenoceptors and monoamine contents in the cerebral cortex of the rodent Jaculus orientalis: effects of acute cold exposure

The tritiated adrenergic antagonists prazosin ([3H]PRZ) and idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity to alpha 1- and alpha 2-adrenoceptors respectively, and were used to measure adrenoceptors in membrane preparations obtained from the cerebral cortex of Jaculus orientalis. Membrane preparations were also obtained from a group of cold exposed animals, to determine whether these adrenoceptors could be modified by a thermic stress. The density of receptors (Bmax; maximum binding capacity) and the dissociation constant (Kd 25 degrees C) were estimated by iterative modelling, and by using the procedure of Hill. After acute cold exposure (16 hr, 5 degrees C) there was a decrease in the affinity of the alpha 1-adrenoceptors, as judged by the Kd 25 degrees C for [3H]PRZ, with no changes in the Bmax. The alpha 2-sites did not show any significant changes, as revealed by [3H]IDA binding. Pretreatment of the membrane preparations from control animals with the disulfide and sulfhydryl reactives DL-dithiothreitol, 5,5'-dithiobis-(2-nitrobenzoic acid) and N-ethylmaleimide decreased specific [3H]PRZ and [3H]IDA binding, with minor changes in non-specific counts, indicating that the fixation of these ligands was to the receptor proteins. The endogenous cortical monoamine contents were also determined in the frontal cerebral cortex of these same animals, using high performance liquid chromatography with electrochemical detection. The catecholamine levels and their major metabolites were found to be stable in the cortex after the acute thermic stress, but there was a marked reduction in serotonin with a normal content in 5-hydroxyindole-3-acetic acid.

Decreased effectiveness of GABA-mediated inhibition in the inferior colliculus of the genetically epilepsy-prone rat

The inferior colliculus (IC) is a critical site for induction of audiogenic seizures in the genetically epilepsy-prone rat (GEPR). Abnormal response properties observed in inferior colliculus neurons of that strain include a high incidence of onset-offset responses which may be a form of afterdischarge. These response abnormalities may involve altered actions of neurotransmitters in that region of the brain. GABA is implicated as a transmitter in endogenous sound-induced inhibition in the inferior colliculus. Endogenous inhibition and the actions of agents that affect GABA receptors were examined in inferior colliculus neurons in epileptic and normal rats. The iontophoretic dose (current) of GABA required to suppress neuronal firing in the epilepsy-prone rat was significantly greater than that required in neurons of the normal rat. A form of endogenous (binaural) inhibition in inferior colliculus neurons, which is proposed to be GABA-mediated, was also significantly reduced in the epilepsy-prone rat as compared with the normal rat. A benzodiazepine (flurazepam) which enhances the action of GABA in many brain sites including the inferior colliculus was significantly less effective than normal when applied iontophoretically onto the same neurons of the epilepsy-prone rat. The GABAA antagonist, bicuculline, which blocks the effect of GABA on inferior colliculus neurons, frequently induced the onset-offset response in neurons not previously exhibiting this pattern. These findings suggest that the reduction of GABA-mediated inhibition in the genetically epilepsy-prone rat may result in the increased incidence of afterdischarges in neurons of the inferior colliculus and may serve as an important mechanism of epileptogenesis in audiogenic seizures in this genetic form of epilepsy.

DSP4-induced noradrenergic lesions increase beta-adrenergic receptors and hippocampal electrophysiological responsiveness

Following profound (greater than 90%) depletions of norepinephrine (NE) by the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4), the numbers of beta-adrenergic receptors were significantly increased (20-25%) in rat hippocampal and somatosensory cortical membranes; however, the numbers of alpha 1-adrenergic receptors and the affinities of both types of receptors were unaffected. This selective up-regulation of beta-adrenergic receptors was evident 1 week after DSP4 administration and was maintained for at least 2 more weeks. In electrophysiological experiments in the hippocampal slice preparation, responses to threshold as well as maximal concentrations of isoproterenol were enhanced 150% and 33%, respectively, in the DSP4-lesioned animals. The results demonstrate that nearly complete depletion of brain NE produced by administration of DSP4, like that produced by 6-hydroxydopamine, results in increased numbers of beta- but not alpha-adrenergic receptors, and suggest that the density of the former are regulated by afferent noradrenergic fibers. Furthermore, the functional significance of the increased number of hippocampal beta-adrenergic receptors is directly manifested in a greater electrophysiological responsiveness to an exogenously administered beta-adrenergic receptor agonist.

Surgical sympathetic denervation increases alpha 1-adrenoceptor-mediated accumulation of myo-inositol trisphosphate and muscle contraction in rabbit iris dilator smooth muscle

Sympathetic denervation of the iris muscle produces increases in both the breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) and in muscle contraction in response to norepinephrine (NE). To shed more light on the biochemical basis underlying this supersensitivity we investigated: the effects of NE on PIP2 breakdown, measured as myo-inositol trisphosphate (IP3) accumulation, and on muscle contraction in normal and denervated rabbit iris dilator; and the effects of denervation on selected biochemical properties of this muscle. The data obtained from these studies can be summarized as follows: The EC50 values (microM) for NE-induced IP3 accumulation in normal and denervated dilators were 14 and 3, respectively. This accumulation of IP3 was blocked by prazosin (1 microM). The EC50 values (microM) for NE-induced contraction for the normal and denervated muscles were 10 and 0.6, respectively. The NE-induced muscle contraction was blocked by prazosin (1 microM). The t1/2 values (s) for IP3 accumulation in normal and denervated muscles were 31 and 11, respectively, and for contraction the values were 19 and 9, respectively. Denervation increased significantly (15-18%) the basal labelling of phosphoinositides from myo-[3H]inositol, but not from 32P or [14C]arachidonic acid. Denervation had little effect on the activities of the enzymes involved in phosphoinositide metabolism. However, the activities of protein kinase C and Ca2+-ATPase increased in the denervated muscle. It is concluded that sympathetic denervation of the iris dilator renders the coupling between alpha1 receptors and PIP2 breakdown into IP3 and 1,2-diacylglycerol (DG) more efficient.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of neurotransmitter receptor sensitivity in cat visual cortex during the critical period

We have examined the characteristics of various receptors in cat visual cortex during postnatal development. These included beta-adrenergic, GABA, benzodiazepine and acetylcholine receptors. For each population of receptor the number (Bmax) and affinity (Kd) were examined as a function of postnatal age (3 days-adult). For all receptors examined, the Bmax increased during development from low early values to a peak within the critical period. The Kd also changed during development for most receptors. The simultaneous alterations in Bmax and Kd necessitate defining a term which takes both of these receptor properties into consideration. This term, called receptor sensitivity (RS), provides a more comprehensive measure of receptor function than either Bmax or Kd alone. Using this measure, we find that receptor sensitivity is low near birth for the 4 receptor populations studied, rises to a peak within the first two months of life, and then declines to near-neonatal levels for 3 of the 4 receptor populations.

The autoradiographic localization of substance P receptors in the rat and bovine spinal cord and the rat and cat spinal trigeminal nucleus pars caudalis and the effects of neonatal capsaicin

Substance P (SP) is a putative neurotransmitter in the central nervous system. In the present report we have used autoradiographic receptor binding techniques to investigate the distribution of SP receptor binding sites in the rat and bovine spinal cord and in the rat and cat spinal trigeminal nucleus pars caudalis. Although some quantitative differences were evident, all species appeared to have a similar distribution of SP receptor binding sites in both the spinal cord and in the spinal trigeminal nucleus pars caudalis. In the spinal cord the heaviest concentration of SP receptors is located in lamina X, while moderate to heavy concentrations were found in laminae I, II and V-IX. Very low concentrations of SP receptors were present in laminae III and IV. Examination of the cat and rat spinal trigeminal nucleus pars caudalis revealed a moderate density of SP receptor binding sites in laminae I and II, very low concentrations in laminae III and IV, and low to moderate concentrations in lamina V. Rats treated neonatally with capsaicin showed a small (11%) but significant (P less than 0.02) increase in the levels of SP receptor binding sites in laminae I and II of the cervical and lumbar spinal cord while in all other laminae the levels remained unchanged.

alpha 1- and beta-adrenergic receptors are co-regulated during both noradrenergic denervation and hyperinnervation

Changes in the density of alpha 1- and beta-adrenergic receptors were studied following denervation of rat cerebral cortex and hyperinnervation of cerebellum and motor trigeminal nucleus, caused by neonatal 6-hydroxydopamine treatment. Four well-defined thalamic projection zones to cortex were studied separately using tissue punch methodology. Both alpha 1- and beta-adrenergic receptors were unevenly distributed in motor, sensory, visual and auditory cortex. The density of alpha 1-adrenergic receptors correlated better with the norepinephrine content of the punches (r = 0.62) than did the density of beta-adrenergic receptors (r = 0.38). Noradrenergic denervation increased both alpha 1- and beta-adrenergic receptor density in almost all cortical areas studied, however the percentage increase was larger for beta- than alpha 1-adrenergic receptors. The change in receptor density was largest in visual cortex and smallest in somatosensory cortex for both receptor sub-types. Noradrenergic hyperinnervation caused a 15-18% decrease in both alpha 1- and beta-adrenergic receptor density in the motor trigeminal nucleus of the pons, but did not change the density of either receptor type in the cerebellum. In general, following either noradrenergic denervation or hyperinnervation the change in alpha 1-adrenergic receptor density was correlated (r = 0.64, P less than 0.005) with the change in beta-adrenergic receptor density in each region, suggesting that these different receptor sub-types are under similar control mechanisms.

Alpha 1- and alpha 2-adrenoceptor binding in the Dahl rat model of hypertension

Dahl sensitive rats on a high salt diet (DSH group) developed significant elevations in blood pressure (BP). Sensitive rats maintained on a low salt diet (DSL group) and Dahl resistant rats on a high or low salt diet (DRH and DRL groups, respectively) remained normotensive. The DSH and DRH groups displayed a lower density of alpha 2-adrenoceptors (as measured with [3H]-clonidine) in the cerebral cortex than normotensive DSL and DRL groups. In contrast, the density of alpha 2-adrenoceptors in the medulla was significantly lower in the DSH group than the DSL group, but significantly higher in the DRH group compared to the DRL group. The density of alpha 1-adrenoceptors (as measured with [3H]-WB4101) in the hypothalamus was lower in the DSH group than the DSL group but greater in the DRH group than the DRL group. The results suggest that the sensitive and resistant lines can be distinguished by the density of alpha 1- adrenoceptors in the hypothalamus and medulla, respectively. The interactive effects of dietary NaCl and susceptibility to hypertension on adrenoceptors lend further support to the hypothesis that the genetic predisposition to hypertension is associated with a disruption in central adrenergic activity.