Electrophysiological characterization of excitatory amino acid responses in rat lateral parabrachial neurons in vitro

Parabrachial complex glutamate receptors modulate the cardiorespiratory response evoked from hypothalamic defense area

To characterize the possible role of glutamate in the interaction between Hypothalamic Defense Area (HDA) and Parabrachial complex (PBc) nuclei, cardiorespiratory changes were analyzed in response to electrical stimulation of the HDA (1 ms pulses, 30-50 μA given at 100 Hz for 5s) before and after the microinjection of the nonspecific glutamate receptor antagonist kynurenic acid (50 nl, 5 nmol), NMDA receptor antagonist MK-801 (50 nl, 50 nmol), non-NMDA receptor antagonist CNQX (50 nl, 50 nmol) or metabotropic glutamate receptor antagonist MCPG (50 nl, 5 nmol) within the PBc. HDA stimulation evoked an inspiratory facilitatory response, consisting of an increase in respiratory rate (p<0.001) due to a decrease in expiratory time (p<0.01). The respiratory response was accompanied by a pressor (p<0.001) and a tachycardic response (p<0.001). Kynurenic acid within the lateral parabrachial region (lPB) abolished the tachycardia (p<0.001) and decreased the magnitude of blood pressure response (p<0.001) to HDA stimulation. Similarly, the magnitude of the tachycardia and the pressor response was decreased after the microinjection of MK-801 (p<0.01 and p<0.001, respectively) and CNQX (p<0.05 in both cases) into the lPB. Kynurenic acid microinjection in this region produced an inhibition of the tachypnea (p<0.001) to HDA stimulation but the respiratory response persisted unchanged after MK-801 or CNQX microinjection into the lPB. Kynurenic acid within the medial parabrachial region (mPB) abolished the tachycardia (p<0.01) and decreased the magnitude of the pressor response (p<0.001) to HDA stimulation. MK-801 and CNQX microinjection in this region decreased the magnitude of the tachycardia (p<0.05, in both cases) and pressor response (p<0.05, in both cases). The respiratory response evoked by HDA stimulation was not changed after the microinjection of kynurenic acid, MK-801 or CNQX within the mPB. No changes were observed in the cardiorespiratory response evoked to HDA stimulation after MCPG microinjection within lPB and mPB. These results indicate that glutamate PBc receptors are involved in the cardiorespiratory response evoked from the HDA. The possible mechanisms involved in these interactions are discussed.

Ionotropic glutamate receptors in the external lateral parabrachial nucleus participate in processing cardiac sympathoexcitatory reflexes

Stimulation of cardiac sympathetic afferents during myocardial ischemia with metabolites such as bradykinin (BK) evokes sympathoexcitatory reflex responses and activates neurons in the external lateral parabrachial nucleus (elPBN). The present study tested the hypothesis that this region in the pons processes sympathoexcitatory cardiac reflexes through an ionotropic glutamate receptor mechanism. The ischemic metabolite BK (0.1-1 μg) was injected into the pericardial space of anesthetized and bilaterally vagotomized or intact cats. Hemodynamic and renal sympathetic nerve activity (RSNA) responses to repeated administration of BK before and after unilateral 50-nl microinjections of kynurenic acid (Kyn; 25 mM), 2-amino-5-phosphonopentanoic acid (AP5; 25 mM), and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol(F)quinoxaline (NBQX; 10 mM) into the elPBN were recorded. Intrapericardial BK evoked significant increases in mean arterial pressure (MAP) and RSNA in seven vagotomized cats. After blockade of glutamate receptors with the nonselective glutamate receptor antagonist Kyn, the BK-evoked reflex increases in MAP (50 ± 6 vs. 29 ± 2 mmHg) and RSNA (59 ± 8.6 vs. 29 ± 4.7%, before vs. after) were significantly attenuated. The BK-evoked responses returned to pre-Kyn levels 85 min after the application of Kyn. Similarly, BK-evoked reflex responses were reversibly attenuated by blockade of glutamate N-methyl-d-aspartate (NMDA) receptors with AP5 (n = 5) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with NBQX (n = 5). In contrast, we observed that the repetitive administration of BK evoked consistent reflex responses including MAP and RSNA before and after microinjection of 50 nl of the artificial cerebrospinal fluid vehicle into the elPBN in five animals. Microinjection of glutamate receptor antagonists into regions outside the elPBN did not alter BK-induced reflex responses. Microinjection of Kyn into the elPBN reversibly attenuated BK-induced reflex responses in four vagus intact animals. These data are the first to show that NMDA and AMPA ionotropic glutamate receptors in the elPBN play an important role in processing cardiac excitatory reflex responses.

Non-NMDA receptors in the lateral parabrachial nucleus modulate sodium appetite

Glutamatergic mechanisms have been implicated in the control of fluid ingestion. In the present study, we investigated whether non-N-methyl-d-aspartate (NMDA) glutamatergic receptors in the lateral parabrachial nucleus (LPBN) are involved in the control of water and sodium intake. Male Sprague-Dawley rats had cannulas implanted bilaterally into the LPBN. They were acutely depleted of water and sodium by injections of the diuretic furosemide (Furo; 10 mg/kg, bw) and given a low dose of the angiotensin-converting enzyme inhibitor, captopril (Cap; 5 mg/kg, bw). Bilateral LPBN injections of the non-NMDA receptor antagonist DNQX (2 and 5 nmol/0.2 microl) increased the ingestion of 0.3 M NaCl and water of Furo/Cap treated rats. The increased ingestion produced by DNQX was abolished by pretreating the LPBN with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), a non-NMDA receptor agonist. AMPA injected alone into the LPBN reduced water and 0.3 M NaCl intake. Injections of DNQX (5 nmol/0.2 microl) into the LPBN also produced ingestion of 0.3 M NaCl after sc injections of the beta-adrenoceptor agonist, isoproterenol, a hypotensive drug that typically produces only water intake. Food intake, arterial blood pressure and heart rate were not altered by DNQX LPBN injections. We conclude that agonists acting on non-NMDA receptors in the LPBN exert an inhibitory influence on sodium intake during acute fluid depletion with hypotension and after isoproterenol treatment. A possible interaction of serotonin with glutamate within the LPBN is discussed.

Effects of intertrigeminal region NMDA and non-NMDA receptors on respiratory responses in rats

Respiratory disturbance, including apnea, can be induced by microinjection of glutamate into the intertrigeminal region (ITR) of the lateral pons, a region that is anatomically coupled to both the dorsal and ventral respiratory groups of the medulla. We showed that the ITR plays a functional role in regulating both vagal reflex apnea and spontaneous sleep-related apnea in rats, but the mechanisms have not been determined. This study shows that functional NMDA receptors are expressed in the ITR since the blockade of these receptors by AP5, a specific NMDA receptor antagonist, was fully effective in blocking apnea induced by glutamate injection within this region. Selective blockade of ITR NMDA receptors had no effect on the immediate apnea evoked by an intravenous 5-HT bolus, whereas the nonspecific glutamate receptor antagonist kynurenic acid significantly increased the duration of this vagal reflex apnea. These findings are of interest because pontine NMDA receptors participate in inspiratory off-switch mechanisms and have been implicated in various short- and long-term potentiation and depression phenomena. These data support the involvement of ITR non-NMDA receptors in modulation of reflex apnea per se, whereas NMDA receptors play a role in damping respiratory responses to transient disturbances.

Parabrachial neurons mediate dorsal periaqueductal gray evoked respiratory responses in the rat

The neural substrates mediating autonomic components of the behavioral defense response reside in the periaqueductal gray (PAG). The cardiovascular components of the defense response evoked from the dorsal PAG (DPAG) have been well described and are dependent, in part, on the integrity of neurons in the region of the parabrachial nucleus as well as the rostral ventrolateral medulla. Descending pathways mediating the ventilatory response associated with activation of DPAG neurons are unknown. The present study was undertaken to test the hypothesis that parabrachial area neurons are also involved in mediating the respiratory response to DPAG stimulation. In urethane-anesthetized, spontaneously breathing rats, electrical stimulation of the DPAG significantly increased respiratory rate, arterial pressure, and heart rate. Changes in respiratory frequency were associated with significant decreases in inspiratory and expiratory durations. After bilateral inhibition of neurons in the lateral parabrachial nucleus (LPBN) region with 5 mM muscimol ( n = 6), DPAG-evoked increases in respiration and heart rate were attenuated by 90 ± 6 and 72 ± 13%, respectively. The pressor response evoked by DPAG stimulation, however, was attenuated by only 57 ± 6%. Bilateral blockade of glutamate receptors with 20 mM kynurenic acid ( n = 6) in the LPBN also markedly attenuated DPAG-evoked increases in respiration and heart rate (65 ± 15 and 53 ± 9% reduction, respectively) but only modestly changed the DPAG-evoked pressor response (34 ± 16% reduction). These results demonstrate that LPBN neurons play a significant role in the DPAG-mediated respiratory component of behavioral defense responses. This finding supports previous work demonstrating that the dorsolateral pons plays a significant role in mediating most physiological adjustments associated with activation of the DPAG.

Dopamine-induced synaptic depression in the parabrachial nucleus is independent of CTX- and PTX-sensitive G-proteins, PKA and PLC signalling pathways

We have previously reported that dopamine (DA) depresses non-NMDA receptor-mediated glutamatergic transmission in the rat parabrachial nucleus (PBN), an interface between brainstem and forebrain that is implicated in autonomic regulation. This work examined cellular signalling pathways that might underlie this DA-induced synaptic depression. Direct activation of adenylyl cyclase with 10 microM forskolin increased the evoked EPSC but did not occlude DA-induced EPSC depression. Similarly, a preferential protein kinase A inhibitor, H-7 (10 microM), did not block DA's synaptic effects. Incubation of slices with cholera toxin (CTX; 1 microgram/ml) or pertussis toxin (PTX; 0.5 microgram/ml) for 20 h, procedures used to irreversibly activate or disable the G(s) and G(i) proteins, respectively, did not change DA's effects. The putative phospholipase C inhibitor, U-73122 (10 microM) and its inactive analogue U-73343 (10 microM) did not alter DA-induced reduction in the EPSCs. Alterations in signalling molecules downstream of phospholipase C including depleting internal calcium stores by thapsigargin and cyclopiazonic acid and blocking protein kinase C with chelerythrine, had no effect on DA-induced synaptic depression. Furthermore, DA's depression of the non-NMDA response was not blocked by APV, an NMDA receptor antagonist. Finally, DA depressed evoked, pharmacologically isolated NMDA receptor-mediated synaptic responses while increasing NMDA-induced inward currents in the PBN. These results indicate that DA-induced synaptic effects in the PBN are not through the activation of cholera or pertussis toxin sensitive G proteins. Furthermore, it does not employ the adenylyl cyclase-cAMP-PKA cascade, the phospholipase C signalling pathway and NMDA receptor-coupled mechanisms to depress excitatory synaptic transmission in the PBN.

Dendritic domains of nociceptive-responsive parabrachial neurons match terminal fields of lamina I neurons in the rat

This study investigates, in the anesthetized rat, the dendritic extent of parabrachial (PB) neurons whose nociceptive response to noxious stimuli has been previously recorded with an extracellular micropipette. The PB neurons were then injected with biocytin through the recording micropipette, via a juxtacellular technique. The dendritic arborization of individual PB neurons was carefully compared with the projections of medullary (trigeminal) and spinal lamina I neurons. The latter projections were labeled in separate animals that received injections of Phaseolus vulgaris-leucoagglutinin restricted to the superficial layers of spinal or medullary dorsal horn. We report here that: 1) PB neurons excited chiefly by noxious stimulation of the face have their dendritic tree located primarily within the field of lamina I trigeminal projections, i.e., in the caudal portion of PB area, around the external medial and the caudal part of the external lateral subnuclei; and 2) PB neurons excited chiefly by noxious stimulation of the paw or the tail have their dendritic tree located primarily within the field of lamina I spinal projections, i.e., in PB mid-extent, around the borderline between the external lateral and both the lateral crescent and the superior lateral subnuclei. Our results suggest the presence of an extensive excitatory axodendritic link between lamina I projections and PB nociceptive neurons around the lateral crescent and the external medial subnuclei. These findings strengthen the possibility of involvement of a subgroup of PB neurons in nociceptive processes.

Increased c-Fos expression in select lateral parabrachial subnuclei following chemical versus electrical stimulation of the dorsal periaqueductal gray in rats

The parabrachial nucleus (PBN) is located in the rostral dorsolateral pons and has been identified as a critical relay for cardiovascular responses (sympathoexcitation and baroreflex attenuation) evoked by the dorsal periaqueductal gray (PAG). We examined the pattern of c-Fos protein immunoreactivity throughout the rostral-caudal extent of the PBN in four groups of anesthetized male Sprague-Dawley rats to identify the specific PBN regions activated by dorsal PAG stimulation. Both electrical stimulation and chemical (0.3 mM bicuculline methobromide) activation of the dorsal PAG elicited a selective increase in Fos-like immunoreactivity (FLI) in the superior lateral and central lateral subnuclei of the rostral lateral PBN (LPBN) relative to surgery and blood pressure control groups. In the middle LPBN chemical stimulation of the dorsal PAG selectively increased FLI in the central lateral subnucleus while electrical stimulation increased FLI in the Kolliker-Fuse area only. Finally, in the caudal LPBN only electrical stimulation of the dorsal PAG induced significant changes in FLI above control. Significant changes in FLI in the medial PBN were not observed under any experimental conditions. These results confirm neuroanatomical data demonstrating that neurons in superior lateral and central lateral subnuclei of the rostral and middle LPBN are the primary targets of the dorsal PAG. Our results also demonstrate that this descending projection to the central lateral and superior lateral subnuclei of the LPBN is in part excitatory. Finally, our results raise the possibility that neurons in the central lateral subnucleus of the middle and rostral LPBN are integrally involved in descending modulation of sympathetic drive associated with dorsal PAG activation.

Estrogen-induced autonomic effects are mediated by NMDA and GABAA receptors in the parabrachial nucleus

The present study was done to determine if estrogen interacts with excitatory and/or inhibitory amino acid neurotransmitters to alter neuronal excitability within the parabrachial nucleus (PBN) and modulate autonomic tone. First, the role of estrogen in modulating autonomic tone was investigated in male rats anesthetized with Inactin (100 mg/kg). Animals were instrumented to record blood pressure, heart rate, vagal parasympathetic and renal sympathetic nerve activities as well as baroreflex sensitivity. Direct, bilateral injection of 17beta-estradiol (0.5 microM; 200 nl/side) into the PBN resulted in a significant decrease in blood pressure (17+/-4 mmHg), sympathetic tone (20+/-5%) and heart rate (22+/-5 beats/min) while increasing parasympathetic tone (34+/-4%) 30 min post-injection. These estrogen-induced effects were completely blocked by the co-injection of estrogen with the estrogen receptor antagonist, ICI 182,780 (20 microM; 200 nl/side). Co-injection of the NMDA receptor antagonist, (+/-)-3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid (CPP; 10 microM; 200 nl/side), with estradiol resulted in complete blockade of the estrogen-induced decrease in heart rate and increase in parasympathetic tone only. Co-injection of estradiol with the GABA(A) receptor antagonist, (+)-bicuculline (0.1 microM; 200 nl/side), resulted in complete blockade of the estrogen-induced decrease in blood pressure and sympathetic nerve activity only. These results suggest that estrogen acts on estrogen receptors on neurons in the PBN to modulate GABA(A)-receptor mediated inhibitory neurotransmission to alter sympathetic tone and blood pressure and on neurons in a separate, parallel pathway to modulate NMDA-receptor mediated neurotransmission to alter parasympathetic tone and heart rate.

In situ hybridization analysis of flip/flop splice variants of AMPA-type glutamate receptor subunits in the rat parabrachial and Kölliker-Fuse nuclei

The aim of the present study was to analyze the occurrence and distribution of flip/flop splice variants of AMPA-type glutamate receptors (GluRA-D) in the rat parabrachial and Kölliker-Fuse nuclei (PB/KF). We performed in situ hybridization experiments on sections through different rostro-caudal levels of the PB/KF and analyzed the subunit expression semiquantitatively by means of grain counts for each probe in eight PB nuclei and in the KF. Our experiments revealed that the splice variants of the AMPA receptor subunit mRNAs are expressed differentially in the distinct nuclei of the PB/KF. The flip splice variants were predominantly expressed (GluRB-D flip) while the flop splice variants (GluRA flop and C flop) were expressed considerably weaker. Within the PB/KF, several nuclei expressed transcripts of GluRB flip (superior, central, dorsal, external, and ventral lateral PB, waist area, medial PB, KF) and GluRC flip (internal, superior, central, dorsal, external, and ventral lateral PB, waist area, KF). GluRB transcripts were not found in neurons of the internal lateral PB and in only 50% of the neurons in the KF. A more restricted expression in the PB/KF was observed for the GluRD flip (internal lateral PB), GluRA flop (medial PB, KF) and GluRC flop mRNA (superior lateral PB, KF). The present data demonstrate that the nuclei of the PB/KF show a differential expression of AMPA receptor subunits. This suggests that the anatomically and functionally distinct nuclei might make use of AMPA-type glutamate receptors with different physiological properties and ion selectivities.

Electrophysiological studies of neurohypophysial neurons and peptides

We have used hypothalamic slices of the supraoptic nucleus (SON) to investigate synaptic control of magnocellular vasopressinergic and oxytocinergic neurons. With the use of perforated patch recording techniques we identified and isolated excitatory or inhibitory postsynaptic currents elicited by electrical stimulation of afferent fibers. Both inhibitory and excitatory afferent fibers displayed presynaptic GABAB receptors; the GABAB agonist, baclofen caused a dose-dependent suppression of the evoked potentials in the absence of any effects on postsynaptic input resistance. Further evidence for a presynaptic locus included an increase in paired pulse ratio and a lack of effect on currents elicited by exogenously applied muscimol (a GABAA receptor agonist) or AMPA (a glutamate agonist). With the use of an GABAB receptor antagonist we demonstrated an action of endogenously released GABA, acting at GABAB receptors on excitatory terminals, to reduce excitatory transmission. In addition to presynaptic modulation by GABA of afferent inputs, we also observed actions of vasopressin and oxytocin, released from dendrites of magnocellular SON neurons, to gate afferent, excitatory transmission in the SON. Exogenously applied vasopressin and oxytocin, or these peptides when released by depolarizing stimuli of magnocellular neurons, reduced the size of evoked excitatory postsynaptic potentials at a presynaptic locus. We have also observed actions of arginine vasopressin to modulate the action of glutamate in slices of the ventral septal area and to attenuate a glutamate-mediated excitatory postsynaptic current in slices of the parabrachial nucleus.

Distribution of metabotropic glutamate receptors in the parabrachial and Kölliker-Fuse nuclei of the rat

In the present study, we analysed the distribution and cellular localization of metabotropic glutamate receptors (1alpha, 2/3, 5) in parabrachial and Kölliker-Fuse nuclei using subtype-specific antisera. Immunolabelling revealed that different nuclei express different sets of metabotropic glutamate receptors. Metabotropic glutamate receptor la immunoreactivity was found in the Kölliker-Fuse nucleus and in several parabrachial nuclei, including the waist area, lateral crescent, medial, external medial and ventral lateral nuclei. The external lateral and internal lateral parabrachial nuclei were devoid of metabotropic glutamate receptor 1alpha immunoreactivity. Metabotropic glutamate receptor 5 immunoreactivity was observed in the Kölliker-Fuse and in the medial parabrachial nuclei, while in the remaining nuclei the staining was very weak. Again, the external lateral nucleus was devoid of metabotropic glutamate receptor 5 immunoreactivity. The metabotropic glutamate receptor 2/3 antisera stained all lateral parabrachial nuclei as well as the Kölliker-Fuse nucleus, while staining in the medial parabrachial nucleus was weak. Metabotropic glutamate receptor 1alpha immunoreactivity was observed on presumed dendritic profiles, while metabotropic glutamate receptor 5 immunoreactivity was found predominantly on neuronal cell bodies. Metabotropic glutamate receptor 2/3 immunoreactivity was present as a fine, punctate immunostaining in the neuropil. Our data suggest that glutamate release in the parabrachial and Kölliker-Fuse nuclei might induce a variety of second messenger cascades, as indicated by the presence or absence of certain types of metabotropic glutamate receptors.

Electrophysiological properties of rat lateral parabrachial neurons in vitro

Anatomical studies have demonstrated that the lateral parabrachial nucleus (LPBN) is composed of at least seven separate subnuclei distinguished by cell morphology, spatial clustering, and afferent and efferent connectivity. We hypothesized that neurons within the subnuclear clusters of the LPBN might have distinct electrophysiological properties that correlate with cellular morphology. An in vitro slice preparation was used to intracellularly record the intrinsic properties of 64 neurons located within the external lateral (EL) and central lateral (CL) subnuclei of the LPBN in adult rats. Analysis of intrinsic properties revealed that neurons in the EL subnucleus had significantly wider action potentials and on the average demonstrated more spike frequency adaptation during 2 s of depolarization compared with CL neurons. The majority of both EL and CL area neurons expressed delayed excitation (DE) after membrane hyperpolarization. DE was eliminated with the A-current blocker 4-aminopyridine (1.5-5 mM). Postinhibitory rebound was also observed in a subpopulation of EL and CL neurons. Morphological analysis of 11 LPBN neurons, which were electrophysiologically characterized and filled with 2% biocytin, failed to demonstrate an association between morphology and the electrophysiological profiles of LPBN neurons. The lack of distinct "type" of neuron within a single subnucleus of the LPBN is in agreement with recent findings reported from the neonatal rat.

Characterization of a hyperpolarizing-activated current in rat lateral parabrachial neurons

The present study examined the electrophysiological and kinetic properties of a hyperpolarizing-activated current in neurons located in the lateral parabrachial nucleus. We investigated whether differences observed in the shape of action potential afterhyperpolarizations in lateral parabrachial nucleus neurons, and the ability of these neurons to accommodate, correlated with the presence of this current. A voltage-activated inwardly rectifying current that increased in amplitude with hyperpolarization was observed in 83% of the neurons examined. Under voltage-clamp recording conditions, this current activated at about -70 mV, was half-activated at -96.5 mV and was blocked by bath application of 2 mM cesium, but not by 100 microM barium. In the current-clamp mode, activation of this current resulted in a transient increase in neuronal excitability at the termination of the more negative current injections. The presence of this current did not correlate with specific action potential characteristics or the ability of lateral parabrachial nucleus neurons to accommodate, as the kinetics and voltage-dependent characteristics are such that this hyperpolarizing-activated current does not affect neuronal excitability at or near the resting membrane potential. However, it may act as an important depolarizing mechanism that prevents neurons from becoming unresponsive when they are excessively hyperpolarized, These results provide evidence that the majority of neurons located in the lateral parabrachial nucleus exhibit a mixed cationic current, which is consistent with the H-current or Q-current. This current may function as a negative feedback mechanism that is activated under conditions of intense hyperpolarization so as to ensure that lateral parabrachial nucleus neurons are in a more suitable state of readiness to respond appropriately to afferent input.

Differences in the intrinsic membrane characteristics of parabrachial nucleus neurons processing gustatory and visceral information

Whole-cell current-clamp recordings were made from neurons in the rat parabrachial nucleus (PBN) in three rostro-caudal brain slices. During recording the neurons were located in one of four quadrants of the PBN. Successful recordings were obtained from neurons in three of these quadrants termed the dorsolateral (DL), dorsomedial (DM) and ventromedial (VM) quadrants. Recordings were made of the intrinsic membrane properties and repetitive discharge characteristics of 58 neurons in the DL, 60 neurons in the DM, and 54 neurons in the VM-quadrants. The input resistance of the neurons in the DL quadrant was significantly lower and the membrane time constant significantly shorter than that of the neurons in the DM- and VM-quadrants. The mean action potential duration of the VM-quadrant neurons was significantly longer than that of both DL- and DM-quadrant neurons. The discharge frequency in response to a 1500 ms 100 pA current pulse of the DL quadrant neurons was significantly lower than that of the neurons in the other two quadrants. The latency of action potential initiation following a 100 pA depolarizing current pulse was significantly longer for DL quadrant neurons compared to neurons in the other two quadrants. Neurons were divided into groups based on their response to a long depolarizing current pulse immediately preceded by a hyperpolarizing current pulse. In all three rostro-caudal slices of the PBN, the largest populations of neurons were in Group II and Group III. The results demonstrate that neurons in different locations in the PBN have different membrane and repetitive discharge properties. These different PBN locations receive inputs from the visceral and gustatory regions of the NST. It is possible therefore that the differences in properties of the PBN neurons may relate to the type of sensory information that they process.

Ibogaine and a total alkaloidal extract of Voacanga africana modulate neuronal excitability and synaptic transmission in the rat parabrachial nucleus in vitro

Ibogaine is a natural alkaloid of Voacanga africana that is effective in the treatment of withdrawal symptoms and craving in drug addicts. As the synaptic and cellular basis of ibogaine's actions are not well understood, this study tested the hypothesis that ibogaine and Voacanga africana extract modulate neuronal excitability and synaptic transmission in the parabrachial nucleus using the nystatin perforated patch-recording technique. Ibogaine and Voacanga africana extract dose dependently, reversibly, and consistently attenuate evoked excitatory synaptic currents recorded in parabrachial neurons. The ED50 of ibogaine's effect is 5 microM, while that of Voacanga africana extract is 170 micrograms/ml. At higher concentrations, ibogaine and Voacanga africana extract induce inward currents or depolarization that are accompanied by increases in evoked and spontaneous firing rate. The depolarization or inward current is also accompanied by an increase in input resistance and reverses polarity around 0 mV. The depolarization and synaptic depression were blocked by the dopamine receptor antagonist haloperidol. These results indicate that ibogaine and Voacanga africana extract 1) depolarize parabrachial neurons with increased excitability and firing rate; 2) depress non-NMDA receptor-mediated fast synaptic transmission; 3) involve dopamine receptor activation in their actions. These results further reveal that the Voacanga africana extract has one-hundredth the activity of ibogaine in depressing synaptic responses. Thus, ibogaine and Voacanga africana extract may produce their central effects by altering dopaminergic and glutamatergic processes.

Cholecystokinin and neurotensin inversely modulate excitatory synaptic transmission in the parabrachial nucleus in vitro

Cholecystokinin and neurotensin are present in fibres innervating the parabrachial nucleus and have previously been shown to modulate the flow of visceral afferent information through the parabrachial nucleus to the cortex in the rat. This study examined the effects of cholecystokinin and neurotensin on synaptic transmission in the parabrachial nucleus using a pontine slice preparation and the nystatin perforated-patch recording technique. Stimulation of the ventral, external lateral portion of the parabrachial nucleus elicited glutamate-mediated, excitatory postsynaptic currents in cells recorded in the parabrachial nucleus. Bath application of neurotensin dose-dependently and reversibly enhanced, while cholecystokinin attenuated, the evoked excitatory postsynaptic current. In addition, the frequency of spontaneous, miniature excitatory postsynaptic currents recorded in parabrachial nucleus cells was significantly increased by neurotensin and decreased by cholecystokinin application. Paired-pulse depression was also enhanced and decreased by neurotensin and cholecystokinin, respectively. These synaptic changes induced by neurotensin and cholecystokinin were not accompanied by changes in input resistance of parabrachial nucleus cells over a wide voltage range (although neurotensin reduced an outwardly rectifying conductance at potentials positive to -20 mV), nor did these peptides alter the inward current induced by a brief bath application of the glutamate agonist, alpha-amino-3-hydroxy-methylisoxazole-4-propionate. The neurotensin antagonist, SR48692 (100 microM), completely and reversibly blocked the neurotensin-induced enhancement of the excitatory postsynaptic current. The non-selective cholecystokinin receptor antagonist, proglumide (100 microM), completely and reversibly blocked the cholecystokinin-induced attenuation of the excitatory postsynaptic current. In addition, the selective cholecystokinin-A receptor antagonist, L-364,718 (10 microM), but not the selective cholecystokinin-B receptor antagonist, L-365,260 (100 microM), blocked the effect of cholecystokinin on synaptic transmission. These results suggest that neurotensin and cholecystokinin act at presynaptic neurotensin and cholecystokinin-A receptors, respectively, to modulate excitatory synaptic transmission in the parabrachial nucleus.

An electrophysiological study of neurons in the horizontal limb of the diagonal band of Broca

We examined the morphological and electrophysiological properties of neurons within the horizontal limb of the diagonal band of Broca (hDBB) and investigated the role of excitatory amino acid mediated synaptic transmission in this region. Whole cell patch-clamp recordings were obtained from hDBB neurons in rat forebrain slices. The hDBB cells examined in this study display a morphological and electrophysiological profile that is consistent with the type B, noncholinergic cell type. Cable analysis reveals that hDBB neurons are electrotonically compact and may therefore function as efficient relays for transmission of inputs to other forebrain target sites. Application of agonists for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate, N-methyl-D-aspartate (NMDA), and metabotropic receptors all evoke inward currents in hDBB neurons. Pharmacological analyses of synaptic events indicate that evoked excitatory postsynaptic currents (EPSC) are either mediated by non-NMDA receptors alone or a combination of non-NMDA and NMDA receptors. In some neurons, the metabotropic receptor agonist, 1-aminocyclopentane-trans-1, 3-dicarboxylic acid, reduced EPSC amplitude without altering postsynaptic input conductance, thus suggesting a presynaptic locus of action. The electrical and pharmacological properties described for hDBB neurons may be physiologically relevant for the effective transmission of excitatory synaptic inputs to sites that receive projections from the hDBB.

Peptidergic modulation of synaptic transmission in the parabrachial nucleus in vitro: importance of degradative enzymes in regulating synaptic efficacy

This study examined the effects of substance P (SP) and calcitonin gene-related peptide (CGRP) on synaptic transmission in a pontine slice containing the parabrachial nucleus (PBN). Stimulation of the ventral, external lateral portion of the PBN elicited glutamate-mediated EPSCs in cells recorded using the nystatin perforated-patch recording technique in the external lateral, external medial, and central lateral subnuclei of the PBN. Bath application of SP or CGRP dose-dependently and reversibly attenuated the evoked EPSC. The attenuation of the EPSC induced by both of these peptides was not accompanied by changes in input resistance of PBN cells over a wide voltage range, nor did these peptides alter the inward current induced by a brief bath application of AMPA. The combined application of subthreshold concentrations of these peptides revealed a synergistic interaction in reducing the evoked EPSC. The substance P neurokinin-1 receptor antagonist CGP49823 completely and reversibly blocked both the SP- and the CGRP-induced attenuation of the EPSC. However, the rat CGRP receptor antagonist human-CGRP8-37 did not block the actions of CGRP or SP on the EPSC. Using a metabolically stable analog of SP, SP (5-11), or an endopeptidase inhibitor, phosphoramidon, we were able to demonstrate that CGRP enhances the SP effect by inhibiting an SP endopeptidase. Application of phosphoramidon also revealed an endogenous SP "tone" apparently made effective by blockade of the endopeptidase. These results suggest that SP (and CGRP indirectly through an inhibition of the SP endopeptidase) acts on presynaptic NK-1 receptors to cause an inhibition of excitatory transmission in the PBN. These results indicate an important role of endopeptidases in regulating synaptic modulation by peptides.

Differential distribution of AMPA-selective glutamate receptor subunits in the parabrachial nucleus of the rat

The parabrachial complex is made up of at least 11 cytoarchitectonically distinct subnuclei which differ in their anatomical connections and neurotransmitter content, as well as the functions they subserve. To determine whether parabrachial subnuclei also express different types of glutamate receptors, we undertook a light microscopic examination of the regional distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunits within the parabrachial complex using antibodies directed against synthetic peptides corresponding to the C-terminal parts of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunits. Antibodies that recognize GluR1 subunits stained cells mainly in the central lateral parabrachial subnucleus, whereas GluR4 antibodies selectively stained cells in the internal lateral subnucleus. In contrast, antibodies directed against the GLuR2/3 subunits stained neurons in every parabrachial subnucleus, although the most dense labelling was seen in the external lateral cell group. These differences in expression of alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate receptor subtypes may eventually allow selective activation or inhibition of specific subsets of neurons in the parabrachial complex.