17beta-estradiol activates estrogen receptor beta-signalling and inhibits transient receptor potential vanilloid receptor 1 activation by capsaicin in adult rat nociceptor neurons

There is mounting evidence that estrogens act directly on the nervous system to affect the severity of pain. Estrogen receptors (ERs) are expressed by sensory neurons, and in trigeminal ganglia, 17beta-estradiol can indirectly enhance nociception by stimulating expression and release of prolactin, which increases phosphorylation of the nociceptor transducer transient receptor potential vanilloid receptor 1 (TRPV1). Here, we show that 17beta-estradiol acts directly on dorsal root ganglion (DRG) sensory neurons to reduce TRPV1 activation by capsaicin. Capsaicin-induced cobalt uptake and the maximum TRPV1 current induced by capsaicin were inhibited when isolated cultured DRGs neurons from adult female rats were exposed to 17beta-estradiol (10-100 nm) overnight. There was no effect of 17beta-estradiol on capsaicin potency, TRPV1 activation by protons (pH 6-4), and P2X currents induced by alpha,beta-methylene-ATP. Diarylpropionitrile (ERbeta agonist) also inhibited capsaicin-induced TRPV1 currents, whereas propylpyrazole triol (ERalpha agonist) and 17alpha-estradiol (inactive analog) were inactive, and 17beta-estradiol conjugated to BSA (membrane-impermeable agonist) caused a small increase. TRPV1 inhibition was antagonized by tamoxifen (1 microm), but ICI182870 (10 microm) was a potent agonist and mimicked 17beta-estradiol. We conclude that TRPV1 in DRG sensory neurons can be inhibited by a nonclassical estrogen-signalling pathway that is downstream of intracellular ERbeta. This affects the vanilloid binding site targeted by capsaicin but not the TRPV1 activation site targeted by protons. These actions could curtail the nociceptive transducer functions of TRPV1 and limit chemically induced nociceptor sensitization during inflammation. They are consistent with clinical reports that female pelvic pain can increase after reductions in circulating estrogens.

Acute and chronic changes in dorsal horn innervation by primary afferents and descending supraspinal pathways after spinal cord injury

Sprouting of peptidergic nociceptive and descending supraspinal projections to the dorsal horn following spinal cord injury (SCI) has been proposed as a mechanism of neuropathic pain. To identify structural changes that could initiate or maintain SCI pain, we used a complete transection model in rats to examine how structural remodeling in the dorsal horn rostral to the lesion relates to distance from injury, laminar region, and duration of injury. The major classes of C-fiber primary afferents differed greatly in their susceptibility to structural and chemical changes and their ability to undergo plasticity. Peptidergic primary afferents showed a widespread loss throughout the dorsal horn of segments approaching the injury site. Some of this loss may have been due to decreased neuropeptide expression. The reduction in peptidergic fibers was transient, indicating compensatory sprouting and perhaps also increased neuropeptide expression within the cord. Nonpeptidergic afferents expressing GFRalpha1 were largely unaffected by SCI. In contrast, in GFRalpha2-expressing nonpeptidergic afferents SCI caused a permanent loss of dorsal horn innervation. Unexpectedly, GFRalpha2 was transiently induced throughout deeper laminae but this was not due to upregulation of GFRalpha2 in dorsal root ganglia. We also observed permanent sprouting of catecholamine terminals of supraspinal origin. This was restricted to the superficial laminae. Our results show that SCI caused a loss of sensory input as well as structural remodeling such that the balance of nociceptive inputs and descending modulation was permanently altered. These changes may contribute to mechanisms rostral to the site of SCI that trigger and maintain neuropathic pain.

Induction of c-Fos and zif268 in the nociceptive amygdala parallel abstinence hyperalgesia in rats briefly exposed to morphine

Opioid-induced analgesia can be followed by spontaneous pain in humans, and hyperalgesia in rodents. In this study, opioid-induced hyperalgesia was measured by the tail-flick test when acute abstinence was precipitated by administering naloxone to drug naive rats that had experienced morphine analgesia for only 30 min. In a further experiment, the drug treatment that previously caused opioid-induced hyperalgesia was found to increase neurons expressing nuclear c-Fos or zif268 proteins in extended amygdalar regions targeted by projections of the ascending spino-parabrachio-amygdaloid nociceptive pathway. Transcription factor induction, however, was not detected in multiple brain regions known to respond in parallel with the same extended amygdalar structures when (1) rats are exposed to interoceptive/physical stressors, or (2) naloxone is used to precipitate abstinence in opioid dependent rats. Surprisingly, in many regions c-Fos induction by morphine was reduced or blocked by naloxone, even though these subjects had also experienced the effects of morphine for 30 min prior to antagonist administration. It is suggested transcription factor induction during opioid hyperalgesia in non-dependent rats could support the induction or consolidation of neural plasticity in nociceptive amygdaloid circuitry previously suggested to function in bi-directional control of pain and expression of pain-related behaviors.

Coexpression of prodynorphin and corticotrophin-releasing hormone in the rat central amygdala: Evidence of two distinct endogenous opioid systems in the lateral division

The lateral subdivision of the central nucleus of the amygdala (CeA) comprises two groups of gamma-aminobutyric acid (GABA) neurons that express corticotrophin-releasing hormone (CRH) and enkephalin. Regulation of the expression and release of these neuropeptides by glucocorticoids and other factors has been suggested to have a regulatory function on the diverse somatic, autonomic, and neuroendocrine responses that are coordinated by the CeA. Because another opioid peptide, dynorphin, has been reported to be also expressed by neurons in the lateral CeA, this study examined the neuronal expression of this kappa-opioid (KOP) receptor-preferring ligand by using immunohistochemistry for the precursor peptide prodynorphin. Prodynorphin neurons in the extended amygdala were observed mostly in the medial and central regions of the lateral CeA and the oval of the bed nucleus of the stria terminalis (BST). About one-third of the prodynorphin neurons in the CeA coexpressed CRH, whereas no coexpression with CRH was detected in the BST. Prodynorphin was not expressed by calbindin neurons in the medial part of the lateral CeA, and indirect evidence suggested that it was not expressed by enkephalin neurons. Coexpression of prodynorphin in extrahypothalamic CRH neurons in the CeA could provide an anatomical basis for regulation of the stress responses and other CRH-related functions by the brain dynorphin/KOP receptor system.

Characterization of neurons in the rat central nucleus of the amygdala: cellular physiology, morphology, and opioid sensitivity

The central nucleus of the amygdala (CeA) orchestrates autonomic and other behavioral and physiological responses to conditioned stimuli that are aversive or elicit fear. As a related CeA function is the expression of hypoalgesia induced by conditioned stimuli or systemic morphine administration, we examined postsynaptic opioid modulation of neurons in each major CeA subdivision. Following electrophysiological recording, biocytin-filled neurons were precisely located in CeA regions identified by chemoarchitecture (enkephalin-immunoreactivity) and cytoarchitecture (DAPI nuclear staining) in fixed adult rat brain slices. This revealed a striking distribution of physiological types, as 92% of neurons in capsular CeA were classified as late-firing, whereas no neurons in the medial CeA were of this class. In contrast, 60% or more of neurons in the lateral and medial CeA were low-threshold bursting neurons. Mu-opioid receptor (MOPR) agonists induced postsynaptic inhibitory potassium currents in 61% of CeA cells, and this ratio was maintained in each subdivision and for each physiological class of neuron. However, MOPR agonists more frequently inhibited bipolar/fusiform cells than triangular or multipolar neurons. A subpopulation of MOPR-expressing neurons were also inhibited by delta opioid receptor agonists, whereas a separate population were inhibited kappa opioid receptors (KOPR). The MOPR agonist DAMGO inhibited 9/9 CeM neurons with projections to the parabrachial nucleus identified by retrograde tracer injection. These data support models of striatopallidal organization that have identified striatal-like and pallidal-like CeA regions. Opioids can directly inhibit output from each subdivision by activating postsynaptic MOPRs or KOPRs on distinct subpopulations of opioid-sensitive neurons.

Dissection of peripheral and central endogenous opioid modulation of systemic interleukin-1beta responses using c-fos expression in the rat brain

In opiate addicts or patients receiving morphine treatment, it has been reported that the immune system is often compromised. The mechanisms responsible for the adverse effects of opioids on responses to infection are not clear but it is possible that central and/or peripheral opioid receptors may be important. We have utilised an experimental immune challenge model in rats, the systemic administration of the human pro-inflammatory cytokine interleukin-1beta (IL-1beta) to study the effects of selectively blocking peripheral opioid receptors only (using naloxone methiodide) or after blocking both central and peripheral opioid receptors (using naloxone). Pre-treatment with naloxone methiodide decreased (15%) IL-1beta-induced Fos-immunoreactivity (Fos-IR) in medial parvocellular paraventricular nucleus (mPVN) corticotropin-releasing hormone (CRH) neurons but increased responses in the ventrolateral medulla (VLM) C1 (65%) and nucleus tractus solitarius (NTS) A2 (110%) catecholamine cell groups and area postrema (136%). However no effect of blocking peripheral opioid receptors was detected in the central nucleus of the amygdala (CeA) or dorsal bed nucleus of the stria terminalis (BNST). We next determined the effect of blocking both central and peripheral opioid receptors with naloxone and, when compared to the naloxone methiodide pre-treated group, a further 60% decrease in Fos-IR mPVN CRH neurons induced by IL-1beta was detected, which was attributed to block of central opioid receptors. Similar comparisons also detected decreases in Fos-IR neurons induced by IL-1beta in the VLM A1, VLM C1 and NTS A2 catecholamine cell groups, area postrema, and parabrachial nucleus. In contrast, pre-treatment with naloxone increased Fos-IR neurons in CeA (98%) and dorsal BNST (72%). These results provide novel evidence that endogenous opioids can influence central neural responses to systemic IL-1beta and also suggest that the differential patterns of activation may arise because of actions at central and/or peripheral opioid receptors that might be important in regulating behavioural, hypothalamic-pituitary-adrenal axis and sympathetic nervous system responses during an immune challenge.

Localization of immunoreactivity for Deleted in Colorectal Cancer (DCC), the receptor for the guidance factor netrin-1, in ventral tier dopamine projection pathways in adult rodents

DCC (deleted in colorectal cancer)-the receptor of the netrin-1 neuronal guidance factor-is expressed and is active in the central nervous system (CNS) during development, but is down-regulated during maturation. The substantia nigra contains the highest level of netrin-1 mRNA in the adult rodent brain, and corresponding mRNA for DCC has also been detected in this region but has not been localized to any particular neuron type. In this study, an antibody raised against DCC was used to determine if the protein was expressed by adult dopamine neurons, and identify their distribution and projections. Significant DCC-immunoreactivity was detected in midbrain, where it was localized to ventrally displaced A9 dopamine neurons in the substantia nigra, and ventromedial A10 dopamine neurons predominantly situated in and around the interfascicular nucleus. Strong immunoreactivity was not detected in dopamine neurons found elsewhere, or in non-dopamine-containing neurons in the midbrain. Terminal fields selectively labeled with DCC antibody corresponded to known nigrostriatal projections to the dorsolateral striatal patches and dorsomedial shell of the accumbens, and were also detected in prefrontal cortex, septum, lateral habenular and ventral pallidum. The unique distribution of DCC-immunoreactivity in adult ventral midbrain dopamine neurons suggests that netrin-1/DCC signaling could function in plasticity and remodeling previously identified in dopamine projection pathways. In particular, a recent report that DCC is regulated through the ubiquitin-proteosome system via Siah/Sina proteins, is consistent with a potential involvement in genetic and sporadic forms of Parkinson's disease.

Mu-opioid receptor desensitization: is morphine different?

Opioid tolerance and dependence are important phenomena. The contribution of acute mu-opioid receptor regulatory mechanisms to the development of analgesic tolerance or physical dependence are unknown, and even the mechanisms underlying relatively rapid receptor desensitization in single cells are unresolved. To a large degree, the uncertainty surrounding the mechanisms and consequences of short-term regulation of tau-opioid receptors in single cells arises from the limitations in the experimental design in many of the studies that have investigated these events. Receptor overexpression and use of assays in which regulatory mechanisms are likely to blunt control determinations have led to measurements of opioid receptor activity that are likely to be insensitive to receptor uncoupling. Together with uncertainties concerning molecular details of tau-opioid receptor interactions with potential regulatory molecules such as G protein-coupled receptor kinases and arrestins, we are left with an incomplete picture crudely copied from the well-worked-out regulatory schema for beta(2)-adrenoceptors. As a consequence, suggestions that clinically relevant tau-opioid receptor agonists may have different propensities to produce tolerance and dependence that arise from their differential recruitment of regulatory mechanisms are premature, and have not yet been appropriately assessed, nor explained in the context of a thoroughly established regulatory scheme. In this commentary, we outline the experimental limitations that have given rise to conflicting ideas about how mu-opioid receptors are regulated, and identify the issues we feel still need to be addressed before we can understand why morphine promotes receptor trafficking differently to other opioids.

Opposing Electrophysiological Actions of 5-HT on Noncholinergic and Cholinergic Neurons in the Rat Ventral Pallidum In Vitro

The ventral pallidum in rat is a basal forebrain structure that contains neurons that project in the limbic striatopallidal circuitry and magnocellular cholinergic corticopetal neurons. Because 5-hydroxytryptamine (5-HT) terminals on dorsal raphe projections form close appositions with these neurons, we made patch-clamp recordings in immature rat brain slices to determine whether they are modulated by postsynaptic 5-HT receptors. Inward currents were predominantly induced by 5-HT in noncholinergic neurons, which were distinguished from cholinergic neurons by immunohistochemical and electrophysiological criteria. The inward current induced by 5-HT was mimicked and occluded when adenylyl cyclase was stimulated with forskolin, and was almost abolished when h-currents in noncholinergic neurons were blocked with cesium. Consistent with 5-HT7 receptor activation of h-curents by cAMP in other brain regions, we found inward currents were mimicked by the mixed 5-HT1/5-HT7 agonists 5-methoxytryptamine, and by 5-carboxamidotryptamine (5-CT), which was more potent than 5-HT. In contrast, 5-HT1 preferring 8-OH-DPAT was a weak partial agonist, and the 5-HT1–selective antagonist pindolol had no effect. However, despite this profile, antagonists that bind at the 5-HT7 receptor only partly reduced the agonist inward current (SB-269970 and clozapine), or had no effect (mianserin and pimozide). We found in cholinergic neurons that 5-HT predominantly induced hyperpolarizing currents, which were carried by potassium channels, and were smaller than currents induced by 8-OH-DPAT and 5-CT. We conclude from this study that ascending 5-HT projections from the dorsal raphe could have direct and opposite effects on the activities of neurons within the limbic striatopallidal and cholinergic corticopetal circuitry in the ventral pallidum.

Effect of naloxone-precipitated morphine withdrawal on c-fos expression in rat corticotropin-releasing hormone neurons in the paraventricular hypothalamus and extended amygdala

Morphine withdrawal is characterized by physical symptoms and a negative affective state. The 41 amino acid polypeptide corticotropin-releasing hormone (CRH) is hypothesized to mediate, in part, both the negative affective state and the physical withdrawal syndrome. Here, by means of dual-immunohistochemical methodology, we examined the co-expression of the c-Fos protein and CRH following naloxone-precipitated morphine withdrawal. Rats were treated with slow-release morphine 50 mg/kg (subcutaneous, s.c.) or vehicle every 48 h for 5 days, then withdrawn with naloxone 5 mg/kg (s.c.) or saline 48 h after the final morphine injection. Two hours after withdrawal rats were perfused transcardially and their brains were removed and processed for immunohistochemistry. We found that naloxone-precipitated withdrawal of morphine-dependent rats increased c-Fos immunoreactivity (IR) in CRH positive neurons in the paraventricular hypothalamus. Withdrawal of morphine-dependent rats also increased c-Fos-IR in the central amygdala and bed nucleus of the stria terminalis, however these were in CRH negative neurons.

Opioid agonists have different efficacy profiles for G protein activation, rapid desensitization, and endocytosis of mu-opioid receptors

The differential ability of various mu-opioid receptor (MOP) agonists to induce rapid receptor desensitization and endocytosis of MOP could arise simply from differences in their efficacy to activate G proteins or, alternatively, be due to differential capacity for activation of other signaling processes. We used AtT20 cells stably expressing a low density of FLAG-tagged MOP to compare the efficacies of a range of agonists to 1) activate G proteins using inhibition of calcium channel currents (ICa) as a reporter before and after inactivation of a fraction of receptors by beta-chlornaltrexamine, 2) produce rapid, homologous desensitization of ICa inhibition, and 3) internalize receptors. Relative efficacies determined for G protein coupling were [Tyr-D-Ala-Gly-MePhe-Glyol]enkephalin (DAMGO) (1) > or = methadone (0.98) > morphine (0.58) > pentazocine (0.15). The same rank order of efficacies for rapid desensitization of MOP was observed, but greater concentrations of agonist were required than for G protein activation. By contrast, relative efficacies for promoting endocytosis of MOP were DAMGO (1) > methadone (0.59) >> morphine (0.07) > or = pentazocine (0.03). These results indicate that the efficacy of opioids to produce activation of G proteins and rapid desensitization is distinct from their capacity to internalize mu-opioid receptors but that, contrary to some previous reports, morphine can produce rapid, homologous desensitization of MOP.

Postnatal maturational changes in rat pelvic autonomic ganglion cells: a mixture of steroid-dependent and -independent effects

Androgens have potent effects on the maturation and maintenance of a number of neural pathways involved in reproductive behaviors in males. Most studies in this area have focused on central pathways, but androgen receptors are expressed by many peripheral neurons innervating reproductive organs, and previous studies have demonstrated structural and chemical changes in these neurons at puberty and after castration. We have performed the first electrophysiological comparison of pelvic autonomic ganglion neurons in male rats before and after puberty and following pre- or postpubertal castration. Studies were performed in vitro on intact ganglia with hypogastric and pelvic nerves attached to allow synaptic activation of sympathetic or parasympathetic neurons, respectively. Pelvic ganglion neurons underwent many changes in their passive and active membrane properties over the pubertal period, and some of these changes were dependent on exposure to circulating androgens. The most pronounced steroid-dependent effects were on membrane capacitance (soma size) in sympathetic neurons and duration of the action potential afterhyperpolarization in tonic neurons. Our study also showed that rat pelvic ganglion cells and their synaptic inputs were more diverse than previously reported. In conclusion, this study demonstrated that rat pelvic ganglion neurons undergo considerable postnatal changes in their electrophysiological properties. The steroid dependence of some of these changes indicates that circulating androgens may influence reproductive behaviors at many locations within the nervous system not just in the brain and spinal cord.

Expression of mRNA and functional alpha(1)-adrenoceptors that suppress the GIRK conductance in adult rat locus coeruleus neurons

1. Locus coeruleus neurons in adult rats express binding sites and mRNA for alpha(1)-adrenoceptors even though the depolarizing effect of alpha(1)-adrenoceptor agonists on neonatal neurons disappears during development. 2. In this study intracellular microelectrodes were used to record from locus coeruleus neurons in brain slices of adult rats and reverse transcription-polymerase chain reaction (RT - PCR) was used to investigate the mRNA expression of alpha(1)- and alpha(2)-adrenoceptors in juvenile and adult rats. 3. The alpha(1)-adrenoceptor agonist phenylephrine had no effect on the membrane conductance of locus coeruleus neurons (V(hold) -60 mV) but decreased the G protein coupled, inward rectifier potassium (GIRK) conductance induced by alpha(2)-adrenoceptor or mu-opioid agonists. The GIRK conductance induced by noradrenaline was increased in amplitude when alpha(1)-adrenoceptors were blocked with prazosin. 4. RT - PCR of total cellular RNA isolated from microdissected locus coeruleus tissue demonstrated strong mRNA expression of alpha(1a)-, alpha(1b)- and alpha(1d)-adrenoceptors in both juvenile and adult rats. However, only mRNA transcripts for the alpha(1b)-adrenoceptors were consistently detected in cytoplasmic samples taken from single locus coeruleus neurons of juvenile rats, suggesting that this subtype may be responsible for the physiological effects seen in juvenile rats. 5. Juvenile and adult locus coeruleus tissue expressed mRNA for the alpha(2a)- and alpha(2c)-adrenoceptors while the alpha(2b)-adrenoceptor was only weakly expressed in juveniles and was not detected in adults. 6. The results of this study show that alpha(1)-adrenoceptors expressed in adult locus coeruleus neurons function to suppress the GIRK conductance that is activated by mu-opioid and alpha(2)-adrenoceptors.

Peripheral withdrawal recruits distinct central nuclei in morphine-dependent rats

This study examined if brain pathways in morphine-dependent rats are activated by opioid withdrawal precipitated outside the central nervous system. Withdrawal precipitated with a peripherally acting quaternary opioid antagonist (naloxone methiodide) increased Fos expression but caused a more restricted pattern of neuronal activation than systemic withdrawal (precipitated with naloxone which enters the brain). There was no effect on locus coeruleus and significantly smaller increases in Fos neurons were produced in most other areas. However in the ventrolateral medulla (A1/C1 catecholamine neurons), nucleus of the solitary tract (A2/C2 catecholamine neurons), lateral parabrachial nucleus, supramamillary nucleus, bed nucleus of the stria terminalis, accumbens core and medial prefrontal cortex no differences in the withdrawal treatments were detected. We have shown that peripheral opioid withdrawal can affect central nervous system pathways.

Morphine-6 beta-glucuronide has a higher efficacy than morphine as a mu-opioid receptor agonist in the rat locus coeruleus

1. The pharmacological properties of the active morphine metabolite, morphine-6 beta-D-glucuronide (M6G), and the parent compound were compared in rat locus coeruleus neurons by electrophysiological recording in brain slices. 2. M6G and morphine activated potassium currents in voltage clamped neurons, which were blocked by the opioid receptor antagonist naloxone. 3. Both M6G and morphine behaved as partial agonists that produced maximal responses smaller than the system maximum, which was measured using [Met(5)]-enkephalin. M6G produced a larger maximal response (78%) than morphine (62%), which we estimated was due to a 2 - 4 fold difference in the relative efficacy of the agonists. 4. 3-O-methoxynaltrexone, which has been reported to behave as a selective antagonist of a M6G preferring receptor, was equally effective at blocking currents produced by M6G and the selective mu-opioid receptor agonist DAMGO. 5. M6G currents were occluded by a prior application of morphine, and were reduced when mu-opioid receptors were desensitized by using [Met(5)]-enkephalin. 6. Morphine-3 beta-D-glucuronide did not affect action potential firing or membrane currents in locus coeruleus neurons and had no effect on currents produced by M6G. 7. These results show that the relative efficacy of M6G is higher than morphine in locus coeruleus neurons, contrary to what has been shown using mu-opioid receptors expressed in cell clones.

Electrophysiological properties of cholinergic and noncholinergic neurons in the ventral pallidal region of the nucleus basalis in rat brain slices

The ventral pallidum is a major source of output for ventral corticobasal ganglia circuits that function in translating motivationally relevant stimuli into adaptive behavioral responses. In this study, whole cell patch-clamp recordings were made from ventral pallidal neurons in brain slices from 6- to 18-day-old rats. Intracellular filling with biocytin was used to correlate the electrophysiological and morphological properties of cholinergic and noncholinergic neurons identified by choline acetyltransferase immunohistochemistry. Most cholinergic neurons had a large whole cell conductance and exhibited marked fast (i.e., anomalous) inward rectification. These cells typically did not fire spontaneously, had a hyperpolarized resting membrane potential, and also exhibited a prominent spike afterhyperpolarization (AHP) and strong spike accommodation. Noncholinergic neurons had a smaller whole cell conductance, and the majority of these cells exhibited marked time-dependent inward rectification that was due to an h-current. This current activated slowly over several hundred milliseconds at potentials more negative than -80 mV. Noncholinergic neurons fired tonically in regular or intermittent patterns, and two-thirds of the cells fired spontaneously. Depolarizing current injection in current clamp did not cause spike accommodation but markedly increased the firing frequency and in some cells also altered the pattern of firing. Spontaneous tetrodotoxin-sensitive GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) were frequently recorded in noncholinergic neurons. These results show that cholinergic pallidal neurons have similar properties to magnocellular cholinergic neurons in other parts of the forebrain, except that they exhibit strong spike accommodation. Noncholinergic ventral pallidal neurons have large h-currents that could have a physiological role in determining the rate or pattern of firing of these cells.

Where is the locus in opioid withdrawal?

Identification of neuroadaptations in specific brain regions that generate withdrawal is crucial for understanding and perhaps treating opioid dependence. It has been widely proposed that the locus coeruleus (LC) is the nucleus that plays the primary causal role in the expression of the opioid withdrawal syndrome. MacDonald Christie, John Williams, Peregrine Osborne and Clare Bellchambers believe that this view and the interpretation of the literature on which it is based are at best controversial. Here, they suggest an alternative view in which regions close to the LC such as the periaqueductal grey, as well as other brain structures which are independent of the LC noradrenergic system, play a more important role in the expression of the opioid withdrawal syndrome.

Forskolin enhancement of opioid currents in rat locus coeruleus neurons

1. Opioids are known to hyperpolarize all neurons in the nucleus locus coeruleus (LC) and to inhibit adenylyl cyclase. Recent work has shown that activation of adenylyl cyclase with forskolin increased the amplitude of the opioid hyperpolarization in LC cells. The aim of the present study was to determine the mechanism of this augmented hyperpolarization. 2. Agonist-induced currents were studied in LC cells in brain slices using both intracellular and whole cell recordings. Forskolin increased the amplitude of mu-opioid- and alpha 2-adrenoceptor-mediated currents by approximately 30% of control measured at -60 mV. This effect of forskolin was dependent on the concentration having a threshold of approximately 1 microM and a peak effect at approximately 30 microM. Dideoxyforskolin (30 microM) caused a small reduction (-52 +/- 28 pA, mean +/- SE) in the amplitude of the opioid current. 3. Forskolin increased the agonist current in the outward direction over the entire potential range between -140 and -50 mV when recordings were made from neurons in cells recorded from slices cut in the horizontal plane. This augmented current produced a shift of the apparent reversal potential to more negative values. 4. Both the forskolin augmentation of the opioid current and the opoid current itself were reduced when the space clamp was improved by cutting the slice in the coronal plane, increasing the extracellular potassium concentration, and treating the slice with carbenoxolone. In addition, forskolin did not change the reversal potential of the opoid current. When expressed as a percentage change from control, forskolin had no significant effect on the opioid current in carbenoxolone (-13 +/- 13%) but produced a small augmentation in high extracellular potassium (15 +/- 4%) and coronoal slices (31 +/- 12%). 5. Two models were tested to explain the action of forskolin, one where cells are coupled electronically by a forskolin-sensitive conductance (coupled-cell model) and a second where opioids mediate an inhibition of a forskolin-induced cation conductance (2-conductance model). The experimental results were fit well only by the coupled-cell model, which predicted that the opioid/forskolin interaction is indirect and occurs primarily in response to forskolin increasing the degree of electrotonic coupling between LC neurons. The consequence of increased coupling would be to augment synchronous activity within the nucleus.

Opioid inhibition of rat periaqueductal grey neurones with identified projections to rostral ventromedial medulla in vitro

1. Rat caudal periaqueductal grey (PAG) output neurones containing rhodamine microspheres, retrogradely transported from an injection site in the rostral ventromedial medulla (RVM), were visualized in brain slices and recorded from using whole-cell patch clamp techniques. 2. The specific GABAB receptor agonist baclofen (10 microM) produced an outward current or hyperpolarization in fifty out of fifty-six caudal PAG output neurones. In 44% of these baclofen-sensitive neurones, the opioid agonist methionine enkephalin (30 microM) also produced an outward current or hyperpolarization. The opioid current reversed polarity at -104 mV and could also be produced by DAMGO, an agonist selective for the mu-subtype of opioid receptor. 3. Opioid-responding output neurones were not distributed uniformly in the caudal PAG. In horizontal slices containing lateral PAG, 56% of output neurones were inhibited by opioids, as compared with only 14% of the output neurones in slices containing ventrolateral PAG. 4. These observations are consistent with opioid disinhibition of ventrolateral PAG neurones projecting to the RVM as the predominant mechanism underlying opioid-induced analgesia in the PAG. The role of opioid receptors found on a major proportion of the output neurones in the lateral PAG remains to be established, but is assumed not be related to modulation of nociceptive function.

Tetrahydro-9-aminoacridine has mixed actions on muscarinic currents and blocks opioid currents in rat locus ceruleus neurons

Actions of tetrahydro-9-aminoacridine (THA) on membrane properties of locus ceruleus neurons were examined using intracellular recording in superfused brain slices. Low concentrations of THA (300 nM-3 microM) caused a small inward current and a 10-fold increase in the potency of ACh to produce inward (excitatory) currents. No effect was seen on currents activated by carbachol, a muscarinic agonist not degraded by cholinesterases. High concentrations of THA (30-300 microM) caused larger inward currents and a decrease in cell conductance. At these concentrations THA inhibited inward currents induced by carbachol (IC50 = 33 microM) and by substance P, which reportedly excites locus ceruleus neurons via the same ionic mechanism as muscarinic agonists. Furthermore, outward currents activated by opioids could be completely blocked (IC50 = 15 microM). Also affected was the action potential waveform, which was slower to rise, longer in duration and smaller in amplitude. The results suggest that THA has predominantly excitatory effects on locus ceruleus neurons--both by greatly enhancing the actions of ACh and by producing a small inward current. At high concentrations effects are mixed and include inhibition of muscarinic currents, as well as of resting and agonist-induced inwardly rectifying potassium currents. The block of opioid currents by THA was not consistent with inhibition of a cationic conductance as recently proposed.

Characterization of acute homologous desensitization of mu-opioid receptor-induced currents in locus coeruleus neurones

1. Acute homologous desensitization of mu-opioid receptor-induced currents was pharmacologically characterized in locus coeruleus (LC) neurones by use of intracellular and whole cell recording in superfused brain slices. 2. Following desensitization of opioid receptors by perfusion with a high concentration of [Met5] enkephalin (ME) for 5 min, there was a reduction in the maximum response and a rightward shift of the concentration-response curves for ME, [D-Ala2, N-MePhe4, Gly-ol]enkephalin (DAMGO) and normorphine. 3. By simultaneously fitting the operational model to the paired pre- and post-desensitization concentration-response data for each agonist, estimates of the level of desensitization were obtained. The values obtained for the three agonists (between 88% and 96%) were similar and did not vary according to the efficacy of the agonist used. 4. Use of whole cell patch recording techniques caused a slow rundown in the amplitude of ME currents (approx. 40% reduction over 60 min) but did not greatly affect the expression of acute desensitization of opioid currents. 5. When included in the patch recording solution, the phosphatase inhibitors, microcystin (50 nM-4 microM) and okadaic acid (1 microM) had no effect on the induction of desensitization or the normal ability of opioid or alpha 2-adrenoceptors to produce currents. Microcystin decreased the rate of recovery of the ME (300 nM) currents following desensitization; however, okadaic acid had little effect on the rate of recovery from desensitization. 6. Strong calcium buffering with BAPTA (10-20 mM) had no effect on desensitization or the recovery from desensitization. 6. Strong calcium buffering with BAPTA (10-20 mM) had no effect on desensitization or the recovery from desensitization.7 These results suggest that acute homologous desensitization of micro-opioid receptors in LC neurones entails a rapid loss of responsiveness that involves a majority of the receptor population. The mechanism by which desensitization is reversed may involve a non-calcium-dependent protein phosphatase but the processess that cause desensitization remain unclear.