Noradrenergic Blockade Research Articles

Locus Coeruleus Activation Patterns Differentially Modulate Odor Discrimination Learning and Odor Valence in Rats.

ABSTRACTThe locus coeruleus (LC) produces phasic and tonic firing patterns that are theorized to have distinct functional consequences. However, how different firing modes affect learning and valence encoding of sensory information are unknown. Here, we show bilateral optogenetic activation of rat LC neurons using 10-Hz phasic trains of either 300 ms or 10 s accelerated acquisition of a similar odor discrimination. Similar odor discrimination learning was impaired by noradrenergic blockade in the piriform cortex (PC). However, 10-Hz phasic light-mediated learning facilitation was prevented by a dopaminergic antagonist in the PC, or by ventral tegmental area (VTA) silencing with lidocaine, suggesting a LC–VTA–PC dopamine circuitry involvement. Ten-hertz tonic stimulation did not alter odor discrimination acquisition, and was ineffective in activating VTA DA neurons. For valence encoding, tonic stimulation at 25 Hz induced conditioned odor aversion, whereas 10-Hz phasic stimulations produced an odor preference. Both conditionings were prevented by noradrenergic blockade in the basolateral amygdala (BLA). Cholera Toxin B retro-labeling showed larger engagement of nucleus accumbens-projecting neurons in the BLA with 10-Hz phasic activation, and larger engagement of central amygdala projecting cells with 25-Hz tonic light. These outcomes argue that the LC activation patterns differentially influence both target networks and behavior.

Noradrenaline blockade specifically enhances metacognitive performance.

Impairments in metacognition, the ability to accurately report one's performance, are common in patients with psychiatric disorders, where a putative neuromodulatory dysregulation provides the rationale for pharmacological interventions. Previously, we have shown how unexpected arousal modulates metacognition (Allen et al., 2016). Here, we report a double-blind, placebo-controlled, study that examined specific effects of noradrenaline and dopamine on both metacognition and perceptual decision making. Signal theoretic analysis of a global motion discrimination task with adaptive performance staircasing revealed that noradrenergic blockade (40 mg propranolol) significantly increased metacognitive performance (type-II area under the curve, AUROC2), but had no impact on perceptual decision making performance. Blockade of dopamine D2/3 receptors (400 mg amisulpride) had no effect on either metacognition or perceptual decision making. Our study is the first to show a pharmacological enhancement of metacognitive performance, in the absence of any effect on perceptual decision making. This enhancement points to a regulatory role for noradrenergic neurotransmission in perceptual metacognition.

Effect of Noradrenergic Blockade on Low‐frequency Oscillations of Human Skin Microcirculation.

Under normothermic conditions, the skin of the leg has higher sympathetic activity than the skin of the forearm; by contrast, after locally heating the skin to 42°C, the skin of the forearm has higher sympathetic activity. Work in human skin free flaps suggested that the vasomotion waves around 0.021–0.52 Hz corresponded to sympathetic activity.PURPOSENo study has verified the low‐frequency oscillations corresponding to sympathetic activity (0.021–0.052 Hz) in intact, healthy human skin. We hypothesized that sympathetic blockade would reduce the power in this low‐frequency band under normothermic conditions as well as in response to local skin heating. We examined the effects of acute, localized sympathetic nerve blockade on the low‐frequency oscillations of skin blood flow (SkBF) of the forearm and leg.METHODSNine volunteers (6 male, 3 female) had 2 sites on each limb prepared with microdialysis fibers, one perfused with Lactated Ringer's (study vehicle, CTRL) the other bretylium tosylate (BT). Local skin heaters and laser‐Doppler flux (LDF) probes were used to control local temperature and assess SkBF. Skin vasomotion data were analysed using wavelet analysis of the LDF signal. LDF data were collected for 10 min at 33°C and for 35 min after locally heating to 42°C. Data are presented as wavelet amplitude in arbitrary units (au).RESULTSBaselineIn the leg, neurogenic activity was reduced at BT treated sites (0.401 ± 0.049 au) compared to CTRL sites (0.569 ± 0.077 au; P = 0.04). There was no difference between the forearm CTRL and BT sites (P = 0.09). Nor were there differences between the forearm and leg CTRL (P = 0.69) and BT treated sites (P = 0.89).Onset of heatingNeurogenic activity was significantly higher at CTRL sites of the forearm (0.812 ± 0.091 au) and leg (0.854 ± 0.112 au) than at BT treated forearm (0.471 ± 0.064 au; P = 0.03) or leg (0.503 ± 0.094 au; P = 0.01). There was no difference between forearm and leg CTRL (P = 0.78) or BT treated sites (P = 0.75).Prolonged HeatingNeurogenic activity was higher at leg CTRL sites (1.272 ± 0.195 au) compared to BT treated sites (0.776 ± 0.152 au; P < 0.01). Forearm CTRL and the BT treated sites were not different (P = 0.08). Forearm CTRL and leg CTRL sites did not differ (P = 0.3) and neither did the BT treated sites (P = 0.57)CONCLUSIONThese data indicate that acute pharmacological blockade of the sympathetic nerves reduced the low‐frequency oscillations of SkBF of the forearm and leg reported to be associated with neural activity. During normothermia, neural activity was only reduced in the skin of the leg and not the forearm. Neurogenic activity was reduced at the onset of heating for both the forearm and leg at BT treated sites. Following prolonged local skin heating at 42°C, we found that neural activity of only the leg was reduced by noradrenergic blockade. These data suggest that the neural frequency band is representative of sympathetic activity. Furthermore, these data indicate that the skin of the leg has a higher noradrenergic component than that of the forearm which is congruent with previous in vivo human work.Support or Funding InformationBall State University Department of Kinesiology

Sleep-Wake Cycle Dysfunction in the TgCRND8 Mouse Model of Alzheimer's Disease: From Early to Advanced Pathological Stages.

In addition to cognitive decline, individuals affected by Alzheimer’s disease (AD) can experience important neuropsychiatric symptoms including sleep disturbances. We characterized the sleep-wake cycle in the TgCRND8 mouse model of AD, which overexpresses a mutant human form of amyloid precursor protein resulting in high levels of β-amyloid and plaque formation by 3 months of age. Polysomnographic recordings in freely-moving mice were conducted to study sleep-wake cycle architecture at 3, 7 and 11 months of age and corresponding levels of β-amyloid in brain regions regulating sleep-wake states were measured. At all ages, TgCRND8 mice showed increased wakefulness and reduced non-rapid eye movement (NREM) sleep during the resting and active phases. Increased wakefulness in TgCRND8 mice was accompanied by a shift in the waking power spectrum towards fast frequency oscillations in the beta (14-20 Hz) and low gamma range (20-50 Hz). Given the phenotype of hyperarousal observed in TgCRND8 mice, the role of noradrenergic transmission in the promotion of arousal, and previous work reporting an early disruption of the noradrenergic system in TgCRND8, we tested the effects of the alpha-1-adrenoreceptor antagonist, prazosin, on sleep-wake patterns in TgCRND8 and non-transgenic (NTg) mice. We found that a lower dose (2 mg/kg) of prazosin increased NREM sleep in NTg but not in TgCRND8 mice, whereas a higher dose (5 mg/kg) increased NREM sleep in both genotypes, suggesting altered sensitivity to noradrenergic blockade in TgCRND8 mice. Collectively our results demonstrate that amyloidosis in TgCRND8 mice is associated with sleep-wake cycle dysfunction, characterized by hyperarousal, validating this model as a tool towards understanding the relationship between β-amyloid overproduction and disrupted sleep-wake patterns in AD.

Noradrenergic blockade of memory reconsolidation: a failure to reduce conditioned fear responding.

Upon recall, a memory can enter a labile state in which it requires new protein synthesis to restabilize. This two-phased reconsolidation process raises the prospect to directly target excessive fear memory as opposed to the formation of inhibitory memory following extinction training. In our previous studies, we convincingly demonstrated that 40 mg propranolol HCl administration before or after memory reactivation eliminated the emotional expression of fear memory indexed by the fear potentiated startle reflex. To apply this procedure in clinical practice it is important to understand the optimal and boundary conditions of this procedure. As part of a large project aimed at unraveling putative boundary conditions of disrupting reconsolidation of associative fear memory with propranolol HCl, we again tested our memory reconsolidation procedure. Participants (N = 44) underwent a three-day differential fear conditioning procedure. Twenty-four hours after fear acquisition, participants received 40 mg propranolol HCl prior to memory reactivation. The next day, participants were subjected to extinction training and reinstatement testing. In sharp contrast to our previous findings, propranolol HCl before memory reactivation did not attenuate the startle fear response. Remarkably, the startle fear response even persisted during extinction training and did not show the usually observed gradual decline in conditioned physiological responding (startle potentiation and skin conductance) upon repeated unreinforced trials. We discuss these unexpected findings and propose some potential explanations. It remains, however, unclear why we observed a resistance to reduce conditioned fear responding by either disrupting reconsolidation or extinction training. The present results underscore that the success of human fear conditioning research may depend on subtle manipulations and instructions.

Noradrenergic blockade and memory in patients with major depression and healthy participants

Patients with major depressive disorder (MDD) often suffer from impaired declarative, episodic and working memory. Further, MDD is associated with alterations in the noradrenergic system. There is evidence that presynaptic α2 receptors that inhibit release of noradrenaline are upregulated in MDD. Results from our recent study demonstrated that increasing noradrenergic activity by blocking the α2 receptor with yohimbine leads to stronger memory consolidation in MDD patients. In the current study, we further examined the role of noradrenaline on memory in MDD by administering clonidine that activates presynaptic α2 receptors and thereby globally suppresses the noradrenergic output. In a placebo-controlled, within-subject crossover design, 20 patients with MDD and 20 healthy controls received either 0.15 mg of clonidine or placebo orally before memory testing. A word list paradigm (memory consolidation), an autobiographical memory test (retrieval) and a working memory test were applied. Salivary alpha-amylase and blood pressure were measured. Across groups, clonidine decreased blood pressure and alpha-amylase. Clonidine impaired memory consolidation (word list learning) in depressed patients and controls. Memory retrieval and working memory were not affected by clonidine. Reducing noradrenergic activity had a specific effect on memory consolidation in patients with MDD and healthy controls. The underlying mechanisms need further scrutiny.

Retrieval per se is not sufficient to trigger reconsolidation of human fear memory

Ample evidence suggests that consolidated memories, upon their retrieval, enter a labile state, in which they might be susceptible to change. It has been proposed that memory labilization allows for the integration of relevant information in the established memory trace (memory updating). Memory labilization and reconsolidation do not necessarily occur when a memory is being reactivated, but only when there is something to be learned during memory retrieval (prediction error). Thus, updating of a fear memory trace should not occur under retrieval conditions in which the outcome is fully predictable (no prediction error). Here, we addressed this issue, using a human differential fear conditioning procedure, by eliminating the very possibility of reinforcement of the reminder cue. A previously established fear memory (picture-shock pairings) was reactivated with shock-electrodes attached (Propranolol group, n=18) or unattached (Propranolol No-Shock Expectation group, n=19). We additionally tested a placebo-control group with the shock-electrodes attached (Placebo group, n=18). Reconsolidation was not triggered when nothing could be learned during the reminder trial, as noradrenergic blockade did not affect expression of the fear memory 24h later in the Propranolol No-Shock Expectation group. Only when the outcome of the retrieval cue was not fully predictable, propranolol, contrary to placebo, reduced the startle fear response and prevented the return of fear (reinstatement) the following day. In line with previous studies, skin conductance response and shock expectancies were not affected by propranolol. Remarkably, a double dissociation emerged between the emotional (startle response) and more cognitive expression (expectancies, SCR) of the fear memory. Our findings have important implications for reconsolidation blockade as treatment strategy for emotional disorders. First, fear reducing procedures that target the emotional component of fear memory do not necessarily affect the cognitive component and vice versa. Second, mere retrieval of the fear memory is not sufficient to induce its labilization and reconsolidation.

The effects of noradrenergic blockade on extinction in humans

The process of reconsolidation has attracted much attention because of its potential application for the treatment of psychiatric disorders. Here, we investigate a possible boundary condition of disrupting reconsolidation with the noradrenergic antagonist propranolol in humans. Reconsolidation can be initiated by retrieval of an acquired fear memory, which is in procedure equivalent to extinction training. If memory retrieval promotes the formation of a novel extinction memory trace, propranolol may interfere with extinction rather than with reconsolidation. Using a differential fear conditioning paradigm, we demonstrate that administration of propranolol (double-blind placebo controlled) prior to repetitive unreinforced CS presentations did not affect extinction at a physiological level (startle reflex and skin conductance). At a cognitive level, propranolol directly impaired extinction learning. These findings indicate that careful selection of timing parameters is essential to ensure that pharmacological agents interfere with the intended memory process to reduce fear.

Noradrenergic enhancement of reconsolidation in the amygdala impairs extinction of conditioned fear in rats-a possible mechanism for the persistence of traumatic memories in PTSD

Posttraumatic stress disorder (PTSD) is associated with enhanced noradrenergic activity. Animal and human studies demonstrate that noradrenergic stimulation augments consolidation of fear learning. Retrieval of well-established memories by presenting a learned fear cue triggers reconsolidation processes during which memories may be updated, weakened, or strengthened. We previously reported that noradrenergic blockade in the rat amygdala impairs reconsolidation of fear memories. Here we investigated the effects of noradrenergic enhancement on reconsolidation of learned fear. Using auditory fear conditioning in rats, we tested the effects of postretrieval intraamygdala infusion of the β-adrenergic receptor agonist isoproterenol or the antagonist propranolol on conditioned fear in the amygdala. A single intraamygdala infusion of isoproterenol following a retrieval of a well-consolidated memory enhanced fear memory elicited by the learned fear stimulus and impaired extinction of this memory 48 hr later. Intraamygdala infusion of the β-adrenergic receptor antagonist propranolol following a consecutive retrieval trial blocked the enhancing effects of isoproterenol on fear memory. Postretrieval β-adrenergic stimulation in the amygdala enhances reconsolidation of fear memories, making them resistant to extinction. Noradrenergic augmentation during retrieval of fear memories may thus contribute to persistence and severity of traumatic memories. Reconsolidation may be a useful tool in understanding the pathology of PTSD and may thus help in developing new and in modifying existing treatments of traumatic memories.

Effect of Acute Posttrauma Propranolol on PTSD Outcome and Physiological Responses During Script‐Driven Imagery

Animal and human research suggests that the development of posttraumatic stress disorder (PTSD) may involve the overconsolidation of memories of a traumatic experience. Previous studies have attempted to use pharmaceutical agents, especially the β-adrenergic blocker propranolol, to reduce this overconsolidation. In this randomized, placebo-controlled study of the efficacy of propranolol in reducing the development of PTSD, we optimized dosages and conducted both psychophysiological and clinical assessments 1 and 3 months after the traumatic event. Forty-one emergency department patients who had experienced a qualifying acute psychological trauma were randomized to receive up to 240 mg/day of propranolol or placebo for 19 days. At 4 and 12 weeks post-trauma, PTSD symptoms were assessed. One week later, participants engaged in script-driven imagery of their traumatic event while psychophysiological responses were measured. Physiological reactivity during script-driven traumatic imagery, severity of PTSD symptoms, and the rate of the PTSD diagnostic outcome were not significantly different between the two groups. However, post hoc subgroup analyses showed that in participants with high drug adherence, at the 5-week posttrauma assessment, physiological reactivity was significantly lower during script-driven imagery in the propranolol than in the placebo subjects. The physiological results provide some limited support for a model of PTSD in which a traumatic conditioned response is reduced by posttrauma propranolol. However, the clinical results from this study do not support the preventive use of propranolol in the acute aftermath of a traumatic event.

Dissociating response systems: Erasing fear from memory

In addition to the extensive evidence in animals, we previously showed that disrupting reconsolidation by noradrenergic blockade produced amnesia for the original fear response in humans. Interestingly, the declarative memory for the fear association remained intact. These results asked for a solid replication. Moreover, given the constructive nature of memories, the intact recollection of the fear association could eventually ‘rebuild’ the fear memory, resulting in the spontaneous recovery of the fear response. Yet, perseverance of the amnesic effects would have substantial clinical implications, as even the most effective treatments for psychiatric disorders display high percentages of relapse. Using a differential fear conditioning procedure in humans, we replicated our previous findings by showing that administering propranolol ( 40 mg ) prior to memory reactivation eliminated the startle fear response 24 h later. But most importantly, this effect persisted at one month follow-up. Notably, the propranolol manipulation not only left the declarative memory for the acquired contingency untouched, but also skin conductance discrimination. In addition, a close association between declarative knowledge and skin conductance responses was found. These findings are in line with the supposed double dissociation of fear conditioning and declarative knowledge relative to the amygdala and hippocampus in humans. They support the view that skin conductance conditioning primarily reflects contingency learning, whereas the startle response is a rather specific measure of fear. Furthermore, the results indicate the absence of a causal link between the actual knowledge of a fear association and its fear response, even though they often operate in parallel. Interventions targeting the amygdalar fear memory may be essential in specifically and persistently dampening the emotional impact of fear. From a clinical and ethical perspective, disrupting reconsolidation points to promising interventions persistently erasing fear responses from trauma memory without affecting the actual recollection.

Noradrenergic Signaling in the Amygdala Contributes to the Reconsolidation of Fear Memory

Intrusive memories resulting from an emotional trauma are a defining feature of posttraumatic stress disorder (PTSD). Existing studies demonstrate that an increase of noradrenergic activity during a life-threatening event contributes to strengthening or "overconsolidation" of the memory for trauma. The lateral nucleus of the amygdala (LA) is critical for fear learning. Using classical fear conditioning in rats, we have recently demonstrated that noradrenergic blockade in the LA following reactivation of fear memory by retrieval disrupts memory reconsolidation and lastingly impairs fear memory. This suggests that noradrenergic blockade may be useful in attenuating traumatic memories, even well-consolidated old memories, in PTSD.

Hypothalamic release of nitric oxide and interaction with amino acid neurotransmitters in chronically uraemic rats

The activity of the hypothalamic gonadotrophin-releasing hormone (GnRH) pulse generator is diminished in uraemia. Since GnRH release is influenced by nitric oxide (NO) neurotransmission, we examined the integrity of hypothalamic NO neurotransmission in the chronically uraemic rat model. Adult male castrated rats were rendered uraemic by two-stage 5/6 nephrectomy. Basal, N-methyl-D-aspartate (NMDA)-stimulated and DL-2-amino-5-phosphonovaleric acid (AP-5)-inhibited NO outflow was measured in uraemic and sham-nephrectomized control animals via a microdialysis probe in the medial preoptic area (MPOA). The influence of the noradrenergic system was evaluated by blocking noradrenergic neurons with N-(2-chloroethyl)-N-ethyl 2-bromobenzylamine (DSP-4). The activity of different NO synthase (NOS) isoforms was investigated by administration of the isoform-specific NOS inhibitors S-methyl-L-thiocitrulline (SMLT) and L-N6-(1-iminoethyl)-lysine (L-NIL). Moreover, hypothalamic mRNA expression of the individual NOS isoforms was quantitated by real-time reverse transcriptase-polymerase chain reaction. Effects of NO on amino acid outflow were assessed by addition of the NO donor S-nitroso-N-acetyl-penicillamine (SNAP). The expression of different NOS species and basal NO outflow did not differ between uraemic and control animals. Administration of the NO donor SNAP increased local NO production and amino acid outflow similarly in both groups. SMLT but not L-NIL, an inhibitor of the inducible NOS isoform, reduced NO outflow in both groups. AP-5 equally decreased, and noradrenergic blockade increased NMDA-stimulated NO outflow in both groups. NO is produced locally and may interfere with amino acid neurotransmission in the rat MPOA. Uraemia did not interfere with NO neurotransmission in our study.

Noradrenergic blockade prevents attacks in a model of episodic dysfunction caused by a channelopathy

Episodic neurological dysfunction often results from ion channel gene mutations. Despite knowledge of the mutations, the factors that precipitate attacks in channelopathies are not clear. In humans, mutations of the calcium channel gene CACNA1A are associated with attacks of neurological dysfunction in familial hemiplegic migraine and episodic ataxia type-2. In tottering mice, a mutation in the same gene causes attacks resembling paroxysmal dyskinesia. Stress, a trigger associated with human episodic disorders, reliably elicits attacks in tottering mice. Because noradrenergic neurotransmission is critical to the stress response and because noradrenergic hyperinnervation is observed in tottering mice, the role of norepinephrine in stress-induced attacks was investigated. Drugs that act at alpha-adrenergic receptors to block noradrenergic transmission prevented attacks. However, agents that facilitate noradrenergic neurotransmission failed to induce attacks. These results suggest that, while noradrenergic neurotransmission may be necessary for attacks, an increase in norepinephrine is not sufficient to induce attacks.

Noradrenergic blockade and numeric working memory in humans

To investigate the noradrenergic modulation of working memory in humans single doses of two beta-blockers [either 25 mg of propranolol (lipophilic) or 50 mg of atenolol (hydrophilic)] or placebo were administered to young healthy volunteers (16 subjects per drug condition) performing a numerical working memory task that requires either short-term maintenance or maintenance plus manipulation of visually presented four-number sequences. Higher manipulation costs (i.e. process-specific slowing of reaction times in the manipulation conditions compared to the control condition) were observed after propranolol but not after atenolol. The propranolol effect was mainly observed in subjects with low emotional arousal (i.e. low state anxiety rating at baseline). Because both beta-blockers induced a comparable decrease of blood pressure and pulse, the propranolol effect on the 'working component' of working memory is considered to be a central, presumably prefrontal one.

Disruption of reconsolidation but not consolidation of auditory fear conditioning by noradrenergic blockade in the amygdala

Consolidation is a process through which labile memories are made persistent [Science 287 (2000) 248]; [Annu Rev Psychol 55 (2004) 51]. When retrieved, a consolidated memory is rendered labile again and undergoes reconsolidation [Learn Mem 7 (2000) 73]; [Trends Neurosci 26 (2003) 65]). Reconsolidation thus offers the opportunity to manipulate memory after it is formed, and may therefore provide a means of treating intrusive memories associated with post-traumatic stress disorder (PTSD). Reconsolidation is most usually studied using protein synthesis inhibitors, which is not practical in humans. However, the β adrenergic receptor antagonist propranolol impairs consolidation of declarative memory in humans [Science 287 (2000) 248]; [Nature 371 (1994) 702] and consolidation and reconsolidation of inhibitory avoidance learning in rats [Brain Res 368 (1986) 125]; [J Neurosci 19 (1999) 6623]. Here, we show that systemic or intra-amygdala infused propranolol blocks reconsolidation but not consolidation. If the effects on reconsolidation are verified in humans, the results would suggest the possibility that propranolol after memory retrieval might be an effective way of treatment of intrusive memories in PTSD. That the systemic effects of propranolol on reconsolidation are achieved via an action in the amygdala is especially important in light of the fact that PTSD involves alterations in the amygdala [Arch Gen Psychiatry 53 (1996) 380].

Voltage-operated Ca 2+ channels involved in K +-evoked release of vasoactive intestinal polypeptide from the rat hypothalamus

Rat brain hypothalami were exposed to various depolarizing stimuli and vasoactive intestinal polypeptide-like immunoreactivity (VIP-LI) release was measured by means of a radioimmunoassay (RIA) procedure. Under conditions of noradrenergic blockade, exposure to high K + (40–100 mM) produced dose-dependent increases in the VIP-LI release in a Ca 2+-dependent manner. Exposure to veratridine (3–100 μM) also induced concentration-dependent increases in VIP-LI release, an effect that was Ca 2+-dependent and tetrodotoxin (TTX)-sensitive. Specific ligands for the L, N, and P/Q-type voltage-operated Ca 2+ channels (VOCCs) were used to determine which channel subtypes were involved in the K +-evoked VIP-LI release. The L-type VOCC ligand, nifedipine (10 μM), had no effect on release. In contrast, the N-type VOCC blocker, ω-conotoxin GVIA (ω-CgTx GVIA) (0.1–100 nM), markedly reduced the K +-evoked response, with maximal inhibition of approximately 60±8%. ω-Agatoxin IVA (ω-Aga IVA) (1–50 nM), which binds P-type and, at high doses, also Q-type VOCCs, produced dose-dependent inhibition of up to 25±3%, while the maximal inhibition observed with the non-selective VOCCs ligand, ω-conotoxin MVIIC (ω-CmTx MVIIC) (1 nM–3 μM), amounted to 85±8%. These findings indicate that N and P-type Ca 2+ channels play predominant roles in the high K +-evoked release of VIP-LI from the rat hypothalamus.

348 Roles of somatic calcium increases in induction of long-term depression in the visual cortex

Rat brain hypothalami were exposed to various depolarizing stimuli and vasoactive intestinal polypeptide-like immunoreactivity (VIP-LI) release was measured by means of a radioimmunoassay (RIA) procedure. Under conditions of noradrenergic blockade, exposure to high K+ (40–100 mM) produced dose-dependent increases in the VIP-LI release in a Ca2+-dependent manner. Exposure to veratridine (3–100 μM) also induced concentration-dependent increases in VIP-LI release, an effect that was Ca2+-dependent and tetrodotoxin (TTX)-sensitive. Specific ligands for the L, N, and P/Q-type voltage-operated Ca2+ channels (VOCCs) were used to determine which channel subtypes were involved in the K+-evoked VIP-LI release. The L-type VOCC ligand, nifedipine (10 μM), had no effect on release. In contrast, the N-type VOCC blocker, ω-conotoxin GVIA (ω-CgTx GVIA) (0.1–100 nM), markedly reduced the K+-evoked response, with maximal inhibition of approximately 60±8%. ω-Agatoxin IVA (ω-Aga IVA) (1–50 nM), which binds P-type and, at high doses, also Q-type VOCCs, produced dose-dependent inhibition of up to 25±3%, while the maximal inhibition observed with the non-selective VOCCs ligand, ω-conotoxin MVIIC (ω-CmTx MVIIC) (1 nM–3 μM), amounted to 85±8%. These findings indicate that N and P-type Ca2+ channels play predominant roles in the high K+-evoked release of VIP-LI from the rat hypothalamus.

Roles of somatic calcium increases in induction of long-term depression in the visual cortex

Rat brain hypothalami were exposed to various depolarizing stimuli and vasoactive intestinal polypeptide-like immunoreactivity (VIP-LI) release was measured by means of a radioimmunoassay (RIA) procedure. Under conditions of noradrenergic blockade, exposure to high K+ (40–100 mM) produced dose-dependent increases in the VIP-LI release in a Ca2+-dependent manner. Exposure to veratridine (3–100 μM) also induced concentration-dependent increases in VIP-LI release, an effect that was Ca2+-dependent and tetrodotoxin (TTX)-sensitive. Specific ligands for the L, N, and P/Q-type voltage-operated Ca2+ channels (VOCCs) were used to determine which channel subtypes were involved in the K+-evoked VIP-LI release. The L-type VOCC ligand, nifedipine (10 μM), had no effect on release. In contrast, the N-type VOCC blocker, ω-conotoxin GVIA (ω-CgTx GVIA) (0.1–100 nM), markedly reduced the K+-evoked response, with maximal inhibition of approximately 60±8%. ω-Agatoxin IVA (ω-Aga IVA) (1–50 nM), which binds P-type and, at high doses, also Q-type VOCCs, produced dose-dependent inhibition of up to 25±3%, while the maximal inhibition observed with the non-selective VOCCs ligand, ω-conotoxin MVIIC (ω-CmTx MVIIC) (1 nM–3 μM), amounted to 85±8%. These findings indicate that N and P-type Ca2+ channels play predominant roles in the high K+-evoked release of VIP-LI from the rat hypothalamus.

The effect of pelvic nerve stimulation on recto-anal motility in the cat

Rectal and anal motility responses to pharmacological manipulation of neuro-transmission and graded efferent electrical pelvic nerve stimulation were investigated in alpha-1-chloralose anaesthetized cats. N ω -nitro- l-arginine ( l-NNA), a competitive inhibitor of nitric oxide synthase, did not influence spontaneous rectal and anal motility. No significant change in anal pressure or rectal tone was observed after sectioning the pelvic nerves in animals pretreated with l-NNA. The effect of pelvic nerve stimulation on anal tone was varying and depended upon the intensity of stimulation and the prevailing anal tone. A reduction of anal tone on pelvic nerve stimulation was consistently converted to an increase of anal tone after pretreatment with l-NNA. The rectal response to pelvic nerve stimulation was unchanged by l-NNA. Residual increase of anal tone observed on pelvic nerve stimulation after l-NNA and noradrenergic blockade was partly sensitive to hexamethonium and abolished by atropine. The results suggest that there is no tonic influence on rectal and anal motility via nitric oxide mechanisms. On the other hand, the reduction of anal tone on high intensity pelvic nerve stimulation seemed to be dependent on the release of nitric oxide. An excitatory cholinergic component of the smooth muscle contractility of the feline anal canal, partly sensitive to hexamethonium, was demonstrated to be conveyed in the pelvic nerves.