Opioid peptide regulation of neurons in the bed nucleus of the stria terminalis: a microiontophoretic study

The NMDA-NR1 receptor subunit and the mu-opioid receptor are expressed in somatodendritic compartments of central nucleus of the amygdala neurons projecting to the bed nucleus of the stria terminalis

The pathway between the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST) is emerging as a critical mediator of stress-related affective processes. Evidence also indicates that exposure to drugs of abuse, like opioids, is associated with NMDA-type glutamate receptor-dependent plasticity in the CeA and BNST. However, there is little evidence that NMDA receptors are expressed in CeA neurons projecting to the BNST, or are required for opioid-induced BNST neural activation. Immunoelectron microscopy, tract tracing, and conditional gene deletion technology were used to investigate the synaptic organization of the NMDA receptor and the mu-opioid receptor (μOR) in the CeA-BNST pathway. By dual labeling electron microscopy, numerous CeA-BNST projection neurons expressed the NMDA-NR1 receptor subunit (NR1) or μOR. By triple labeling, it was also found that NR1 and μOR were co-expressed in some CeA-BNST projection neurons. Despite being colocalized in somato-dendritic compartments of CeA neurons, NR1 and μOR were rarely expressed in their axonal terminations in the BNST. Deleting the NR1 gene in CeA neurons resulted in a reduction of morphine-induced Fos protein labeling in the ventral BNST. In summary, NR1 and μOR are coexpressed in somatodendritic sites of CeA neurons, including those projecting to the BNST. In addition, expression of the NR1 gene in CeA neurons is required for morphine-induced BNST neural activation. Thus, postsynaptic NMDA receptors and μORs are positioned for the co-modulation of CeA projection neurons to the BNST, which may provide a synaptic substrate for stress-induced emotional processes critically involved in opioid addictive behaviors.

A transcriptomic analysis of type I-III neurons in the bed nucleus of the stria terminalis

The activity of neurons in the anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) plays a critical role in anxiety- and stress-related behaviors. Histochemical studies have suggested that multiple distinct neuronal phenotypes exist in the BNST(ALG). Consistent with this observation, the physiological properties of BNST(ALG) neurons are also heterogeneous, and three distinct cell types can be defined (Types I-III) based primarily on their expression of four key membrane currents, namely I(h), I(A), I(T), and I(K(IR)). Significantly, all four channels are multimeric proteins and can comprise of more than one pore-forming α subunit. Hence, differential expression of α subunits may further diversify the neuronal population. However, nothing is known about the relative expression of these ion channel α subunits in BNST(ALG) neurons. We have addressed this lacuna by combining whole-cell patch-clamp recording together with single-cell reverse transcriptase polymerase chain reaction (scRT-PCR) to assess the mRNA transcript expression for each of the subunits for the four key ion channels in Type I-III neurons of the BNST(ALG.) Here, cytosolic mRNA from single neurons was probed for the expression of transcripts for each of the α subunits of I(h) (HCN1-HCN4), I(T) (Ca(v)3.1-Ca(v)3.3), I(A) (K(v)1.4, K(v)3.4, K(v)4.1-K(v) 4.3) and I(K(IR)) (Kir2.1-Kir2.4). An unbiased hierarchical cluster analysis followed by discriminant function analysis revealed that a positive correlation exists between the physiological and genetic phenotype of BNST(ALG) neurons. Thus, the analysis segregated BNST(ALG) neurons into 3 distinct groups, based on their α subunit mRNA expression profile, which positively correlated with our existing electrophysiological classification (Types I-III). Furthermore, analysis of mRNA transcript expression in Type I-Type III neurons suggested that, whereas Type I and III neurons appear to represent genetically homologous cell populations, Type II neurons may be further subdivided into three genetically distinct subgroups. These data not only validate our original classification scheme, but further refine the classification at the molecular level, and thus identifies novel targets for potential disruption and/or pharmacotherapeutic intervention in stress-related anxiety-like behaviors.

Differential Expression of Intrinsic Membrane Currents in Defined Cell Types of the Anterolateral Bed Nucleus of the Stria Terminalis

The anterolateral group of the bed nucleus of the stria terminalis (BNSTALG) plays a critical role in a diverse array of behaviors, although little is known of the physiological properties of neurons in this region. Using whole cell patch-clamp recordings from rat BNSTALG slices in vitro, we describe three distinct physiological cell types. Type I neurons were characterized by the presence of a depolarizing sag in response to hyperpolarizing current injection that resembled activation of the hyperpolarization-activated cation current Ih and a regular firing pattern in response to depolarizing current injection. Type II neurons exhibited the same depolarizing sag in response to hyperpolarizing current injection, but burst-fired in response to depolarizing current injection, which was indicative of the activation of the low-threshold calcium current IT. Type III neurons did not exhibit a depolarizing sag in response to hyperpolarizing current injection, but instead exhibited a fast time-independent rectification that became more pronounced with increased amplitude of hyperpolarizing current injection, and was indicative of activation of the inwardly rectifying potassium current IK(IR). Type III neurons also exhibited a regular firing pattern in response to depolarizing current. Using voltage-clamp analysis we further characterized the primary active currents that shaped the physiological properties of these distinct cell types, including Ih, IT, IK(IR), the voltage-dependent potassium current IA, and the persistent sodium current INaP. The functional relevance of each cell type is discussed in relation to prior anatomical studies, as well as how these currents may interact to modulate neuronal activity within the BNSTALG.

5-hydroxytryptamine1A-like receptor activation in the bed nucleus of the stria terminalis: electrophysiological and behavioral studies

The anteriorlateral bed nucleus of the stria terminalis (BNST AL) and the serotonergic system are believed to modulate behavioral responses to stressful and/or anxiogenic stimuli. However, although the BNST AL receives heavy serotonergic innervation, the functional significance of this input is not known. Data obtained from in vitro whole-cell patch clamp recording in the rat BNST slice show that exogenous application of 5-hydroxytryptamine (5-HT) evoked a heterogeneous response in BNST AL neurons. The principal action of 5-HT in this region was inhibitory, evoking a membrane hyperpolarization (5-HTHyp) and a concomitant reduction in input resistance in the majority of neurons tested. The broad-spectrum 5-HT1 agonist, 5-carboxamindotryptamine (5-CT), but not R(+/-)8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT), mimicked the 5-HTHyp response in the BNST. Moreover, the outward current mediating 5-HTHyp was inwardly rectifying and sensitive to the G protein activated inwardly rectifying K+ (G IRK) channel blocker, tertiapin-Q. In the CNS 5-HT1A receptors are thought to couple to GIRK channels, suggesting that 5-HTHyp in BNST AL neurons was mediated by activation of 5-HT1A-like receptors. This was confirmed by the blockade of both 5-HTHyp and 5-CTHyp by the specific 5-HT1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY100635 200nM). Furthermore, an in vivo examination of the functional consequences of 5-HT1A-like induced inhibition of BNST neurons revealed that infusion of 5-CT into the BNST significantly reduced the acoustic startle response, without affecting the general motor activity of the animals. These data point to the possibility that 5-HT1A mediated inhibition of the BNST AL could contribute to an anxiolytic action. Hence, we propose that in response to stressful stimuli, enhanced levels of 5-HT in the BNST AL plays a critical homeostatic role in feedback inhibition of the anxiogenic response to these stimuli.

Convergent effects of oxytocin and a δ-opioid agonist in the bed nuclei of the stria terminalis of the peripartum rat

In the bed nuclei of the stria terminalis (BST), opioids may suppress the facilitatory effect of oxytocin on its own release pre-partum. In vitro electrophysiological recording showed that in virgin, late-pregnant, and lactating rats, a delta-opioid agonist inhibited a high proportion of BST neurons, many of which were also oxytocin responsive. Response magnitude did not differ significantly between groups, suggesting that the postulated pre-partum increase in opioid tone does not involve postsynaptic changes in neuronal sensitivity.

Chronic morphine treatment and withdrawal increase extracellular levels of norepinephrine in the rat bed nucleus of the stria terminalis

Extracellular levels of norepinephrine (NE) and glutamate (Glu) in the ventral bed nucleus of the stria terminalis (vBNST) of saline- and chronic morphine-treated rats, with or without withdrawal, were studied by means of the in vivo microdialysis technique in anesthetized rats. In addition, the tissue concentration of NE was studied at different rostrocaudal levels of the vBNST. Chronic morphine treatment significantly increased extracellular levels of NE, but not Glu, in vBNST. At 48 h after naloxone-induced morphine withdrawal there was a further significant increase in the extracellular levels of NE, but not Glu, in vBNST. The presence of UK 14304, an alpha(2)-adrenergic agonist, induced a significant decrease in NE extracellular levels in all experimental groups. In contrast, UK 14304 induced a significant decrease in Glu extracellular levels only in saline-treated rats. The results also show that the vBNST presents a rostrocaudal gradient of NE and contains 9.4% of total brain NE. The increase in NE extracellular levels in vBNST induced by chronic morphine treatment and the further increase in NE levels 48 h after naloxone-induced morphine withdrawal suggest that NE in vBNST may be involved in the pharmacological effects of chronic morphine and withdrawal.

Neuropharmacology of brain-stimulation-evoked aggression

Evidence is reviewed concerning the brain areas and neurotransmitters involved in aggressive behavior in the cat and rodent. In the cat, two distinct neural circuits involving the hypothalamus and PAG subserve two different kinds of aggression: defensive rage and predatory (quiet-biting) attack. The roles played by the neurotransmitters serotonin, GABA, glutamate, opioids, cholecystokinin, substance P, norepinephrine, dopamine, and acetylcholine in the modulation and expression of aggression are discussed. For the rat, a single area, largely coincident with the intermediate hypothalamic area, is crucial for the expression of attack; variations in the rat attack response in natural settings are due largely to environmental variables. Experimental evidence emphasizing the roles of serotonin and GABA in modulating hypothalamically evoked attack in the rat is discussed. It is concluded that significant progress has been made concerning our knowledge of the circuitry underlying the neural basis of aggression. Although new and important insights have been made concerning neurotransmitter regulation of aggressive behavior, wide gaps in our knowledge remain.

Changing pattern of oxytocin-induced excitation of neurons in the bed nuclei of the stria terminalis and ventrolateral septum in the peripartum period

Oxytocin acts within the limbic system (bed nuclei of the stria terminalis and ventrolateral septum) to induce maternal behaviour and to facilitate neuroendocrine activity at specific times during the peripartum period. Studies were undertaken to determine whether the timing of these effects arises from modulation of the oxytocin-induced excitation of limbic neurons. Extracellular activity of single units was recorded on urethane-anaesthetized rats and neurons were tested for responses to intracerebroventricular injection of 1.1 ng oxytocin. In the first part, animals were recorded on days 19 and 22 of pregnancy and on days 3 and 5 of lactation. No significant differences in the basal firing rates or in the proportion of oxytocin-responsive neurons were detected, but responses by neurons on day 22 of pregnancy occurred after a significant delay (10.7 +/- 2.0 min), resulting in a smaller overall response compared to the other groups. These differences in the pattern of response were not due to changes in density of oxytocin binding in the limbic areas studied, since autoradiographic detection of oxytocin binding sites using the iodinated antagonist [125I]d(CH2)5[Tyr(Me)2, Thr4, Orn3, Tyr-NH2(9)]-vasotocin showed no differences between the pregnant and postpartum animals. In the second part, parturient animals (day 22 of pregnancy) received intravenous injection of the long-acting opioid antagonist naltrexone, or unilateral knife-cut lesions to the stria terminalis, a source of inhibitory inputs (including enkephalinergic) to the bed nuclei of the stria terminalis and ventrolateral septum. Both treatments abolished the characteristic delay of oxytocin-induced excitation in non-treated animals on day 22 of pregnancy, and increased the overall excitatory response. Thus, during the peripartum period, a population of limbic neurons sensitive to oxytocin display a dynamically changing pattern of excitatory responses, apparently modulated by an endogenous opioid cone and independent of changes in oxytocin receptor expression. The attenuated neuronal response to central oxytocin seen on the day of parturition could account for the absence of a facilitatory effect of oxytocin on neuroendocrine activity at this time.

Angiotensin II receptor subtype antagonists can both stimulate and inhibit salt appetite in rats

In urethane-anaesthetised male Wistar rats iontophoretic application of the angiotensin II (Ang II) type 1 (AT-1) receptor specific nonpeptide antagonist losartan in the septo-preoptic continuum can produce neuronal excitation of short- and long-term duration which has been interpreted as removal of tonic Ang II-induced inhibition. Mineralocorticoid pretreatment, which increases neuronal sensitivity to Ang II to enhance salt appetite, also removes this losartan-induced long-term excitation. These results suggested steroid modulation of the AT-1 receptor and a complex involvement of Ang II in salt appetite. To investigate the role of the inhibitory action of central Ang II on salt appetite, we gave intracerebroventicular injections of a single, low dose (10 ng) of losartan in normal euhydrated rats and this produced, paradoxically, a progressive increase in salt intake (1.6 +/- 0.3 ml/day to 3.5 +/- 0.9 ml/day, n = 15, P < 0.05). Treatment of these salt enhanced rats with DOCA (0.5 mg/day, s.c., for 3 days) further increased the salt appetite, but then a second i.c.v. injection of the same dose of losartan significantly inhibited the enhanced salt appetite (4.7 +/- 0.7 to 1.3 +/- 0.4, n = 9, P < 0.05). These results provide evidence for a complex action of Ang II on the At-1 receptor mediating the generation of salt appetite that appears to involve either at least two functional subtypes of this AT-1 receptor, as already suggested by previous electrophysiological experiments, or one AT-1 receptor with several activation states.

Systemic and microiontophoretic administration of morphine differentially effect ventral pallidum/substantia innominata neuronal activity

In vivo electrophysiological recording techniques were employed to examine responses of ventral pallidum/substantia innominata (VP/SI) neurons to systemic and local administration of morphine. Using a cumulative dosing protocol, intravenous administration (0.1-30 mg/kg i.v.) produced a suppression of firing in 82% of neurons tested. The suppression was dose-related and blocked by the opioid antagonist, naloxone. In contrast, microiontophoretic applications of morphine resulted in current-related suppression (32% of neurons tested) or excitation (26%). Concurrent application of naloxone attenuated or blocked both effects of local morphine application. It was demonstrated that acute tolerance did not develop with repeated morphine exposures following either systemic or local administration. The present findings establish the sensitivity of VP/SI neurons to morphine and provide functional relevance at the level of a single neuron for opioid peptides and their receptors in this region. As reported for most other opioid-receptive brain areas, neuronal rate suppression was the predominate response observed, and it is proposed that excitations to iontophoresed morphine reflect a disinhibitory phenomenon. The differential morphine-induced rate changes, and number of responding neurons, observed with systemic vs. iontophoretic morphine administration suggest that extra-VP/SI regions that also are opioid sensitive can subsequently direct neuronal responsiveness to opioids within the VP/SI.

Enkephalinergic involvement in periaqueductal gray control of hypothalamically elicited predatory attack in the cat

The effects of central infusion of naloxone into the midbrain periaqueductal gray (PAG) upon predatory attack behavior in the cat were studied in 12 cats. Initially, quiet biting attack was elicited by electrical stimulation of sites within the lateral hypothalamus using monopolar electrodes. Then cannula-electrodes were implanted into sites within the PAG from which electrical stimulation facilitated or suppressed the attack response. Following identification of modulatory sites within the PAG, naloxone (1.0 micrograms/0.5 microliter) was microinjected into those sites and the effects upon hypothalamically elicited attack were assessed. At nine of twelve sites in the PAG where suppression was obtained, administration of naloxone served to block those effects. Similarly, at six of eight facilitatory sites within the PAG, naloxone also blocked the modulatory effects of PAG stimulation. However, vehicle (isotonic saline) alone did not alter the modulatory effects of PAG stimulation. Administration of DAME (250 ng/0.3 microliter) into PAG modulatory sites in four cats, two which facilitated and two that suppressed the attack response, reversed the effects of naloxone at these sites. These results demonstrate that opioid peptides within the PAG play a complex role in the expression of predatory attack behavior in the cat.