beta(2)-Adrenoceptor-mediated facilitation of glutamatergic transmission in rat ventromedial hypothalamic neurons

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Repeated Activation of Noradrenergic Receptors in the Ventromedial Hypothalamus Suppresses the Response to Hypoglycemia

Activation of the adrenergic system in response to hypoglycemia is important for proper recovery from low glucose levels. However, it has been suggested that repeated adrenergic stimulation may also contribute to counterregulatory failure, but the underlying mechanisms are not known. The aim of this study was to establish whether repeated activation of noradrenergic receptors in the ventromedial hypothalamus (VMH) contributes to blunting of the counterregulatory response by enhancing local lactate production. The VMH of nondiabetic rats were infused with either artificial extracellular fluid, norepinephrine (NE), or salbutamol for 3 hours/day for 3 consecutive days before they underwent a hypoglycemic clamp with microdialysis to monitor changes in VMH lactate levels. Repeated exposure to NE or salbutamol suppressed both the glucagon and epinephrine responses to hypoglycemia compared to controls. Furthermore, antecedent NE and salbutamol treatments raised extracellular lactate levels in the VMH. To determine whether the elevated lactate levels were responsible for impairing the hormone response, we pharmacologically inhibited neuronal lactate transport in a subgroup of NE-treated rats during the clamp. Blocking neuronal lactate utilization improved the counterregulatory hormone responses in NE-treated animals, suggesting that repeated activation of VMH β2-adrenergic receptors increases local lactate levels which in turn, suppresses the counterregulatory hormone response to hypoglycemia.

Carvedilol prevents counterregulatory failure and impaired hypoglycaemia awareness in non-diabetic recurrently hypoglycaemic rats

AIMS/HYPOTHESIS

This study evaluates whether the non-selective β-blocker, carvedilol, can be used to prevent counterregulatory failure and the development of impaired awareness of hypoglycaemia (IAH) in recurrently hypoglycaemic rats.

METHODS

Sprague Dawley rats were implanted with vascular catheters and intracranial guide cannulas targeting the ventromedial hypothalamus (VMH). These animals underwent either three bouts of insulin-induced hypoglycaemia or received three saline injections (control group) over 3 days. A subgroup of recurrently hypoglycaemic animals was treated with carvedilol. The next day, the animals underwent a hypoglycaemic clamp with microdialysis without carvedilol treatment to evaluate changes in central lactate and hormone levels. To assess whether carvedilol prevented IAH, we treated rats that had received repeated 2-deoxyglucose (2DG) injections to impair their awareness of hypoglycaemia with carvedilol and measured food intake in response to insulin-induced hypoglycaemia as a surrogate marker for hypoglycaemia awareness.

RESULTS

Compared with the control group, recurrently hypoglycaemic rats had a ~1.7-fold increase in VMH lactate and this was associated with a 75% reduction in the sympathoadrenal response to hypoglycaemia. Treatment with carvedilol restored VMH lactate levels and improved the adrenaline (epinephrine) responses. In 2DG-treated rats compared with control animals receiving saline, food intake was reduced in response to hypoglycaemia and increased with carvedilol treatment.

CONCLUSIONS/INTERPRETATION

We conclude that carvedilol may be a useful therapy to prevent counterregulatory failure and improve IAH.

Influence of VMH fuel sensing on hypoglycemic responses

Hypoglycemia produces complex neural and hormonal responses that restore glucose levels to normal. Glucose, metabolic substrates and their transporters, neuropeptides and neurotransmitters alter the firing rate of glucose-sensing neurons in the ventromedial hypothalamus (VMH); these monitor energy status and regulate the release of neurotransmitters that instigate a suitable counter-regulatory response. Under normal physiological conditions, these mechanisms maintain blood glucose concentrations within narrow margins. However, antecedent hypoglycemia and diabetes can lead to adaptations within the brain that impair counter-regulatory responses. Clearly, the mechanisms employed to detect and regulate the response to hypoglycemia, and the pathophysiology of defective counter-regulation in diabetes, are complex and need to be elucidated to permit the development of therapies that prevent or reduce the risk of hypoglycemia.

α(2A) adrenoceptor-mediated presynaptic inhibition of GABAergic transmission in rat tuberomammillary nucleus neurons

Histaminergic neurons within the tuberomammillary nucleus (TMN) play an important role in the regulation of sleep-wakefulness. Here, we report the adrenergic modulation of GABAergic transmission in rat TMN histaminergic neurons using a conventional whole-cell patch clamp technique. Norepinephrine (NE) reversibly decreased the amplitude of action potential-dependent GABAergic inhibitory post-synaptic currents (IPSCs) and increased the paired pulse ratio. The NE-induced inhibition of GABAergic IPSCs was mimicked by clonidine, a selective α2 adrenoceptor agonist. However, cirazoline and isoproterenol, nonselective α1 and β adrenoceptor agonists, respectively, had no effect on GABAergic IPSCs. The NE-induced inhibition of GABAergic IPSCs was significantly blocked by BRL44408, a selective α2A adrenoceptor antagonist, but not imiloxan or JP1302, a selective α2B and α2C adrenoceptor antagonists. The extent of NE-induced inhibition of GABAergic IPSCs was inversely proportional to the extracellular Ca(2+) concentration. Pharmacological agents affecting the activities of adenylyl cyclase or G-protein-coupled inwardly rectifying K(+) channels did not affect the NE-induced inhibition of GABAergic IPSCs. However, NE had no effect on the frequency and amplitude of GABAergic miniature IPSCs. These results suggest that NE acts on presynaptic α2A adrenoceptor to inhibit action potential-dependent GABA release via the inhibition of Ca(2+) influx from the extracellular space to GABAergic nerve terminals, and that this α2A adrenoceptor-mediated modulation of GABAergic transmission may be involved in regulating the excitability of TMN histaminergic neurons.

Adenosine modulates the excitability of layer II stellate neurons in entorhinal cortex through A1 receptors

Stellate neurons in layer II entorhinal cortex (EC) provide the main output from the EC to the hippocampus. It is believed that adenosine plays a crucial role in neuronal excitability and synaptic transmission in the CNS, however, the function of adenosine in the EC is still elusive. Here, the data reported showed that adenosine hyperpolarized stellate neurons in a concentration-dependent manner, accompanied by a decrease in firing frequency. This effect corresponded to the inhibition of the hyperpolarization-activated, cation nonselective (HCN) channels. Surprisingly, the adenosine-induced inhibition was blocked by 3 μM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) receptor antagonists, but not by 10 μM 3,7-dimethyl-1-propargylxanthine (DMPX), a selective A(2) receptor antagonists, indicating that activation of adenosine A(1) receptors were responsible for the direct inhibition. In addition, adenosine reduced the frequency but not the amplitude of miniature EPSCs and IPSCs, suggesting that the global depression of glutamatergic and GABAergic transmission is mediated by a decrease in glutamate and GABA release, respectively. Again the presynaptic site of action was mediated by adenosine A(1) receptors. Furthermore, inhibition of spontaneous glutamate and GABA release by adenosine A(1) receptor activation was mediated by voltage-dependent Ca(2+) channels and extracellular Ca(2+) . Therefore, these findings revealed direct and indirect mechanisms by which activation of adenosine A(1) receptors on the cell bodies of stellate neurons and on the presynaptic terminals could regulate the excitability of these neurons.

Modulation of β-adrenergic receptors in the ventromedial hypothalamus influences counterregulatory responses to hypoglycemia

OBJECTIVE

Norepinephrine is locally released into the ventromedial hypothalamus (VMH), a key brain glucose-sensing region in the response to hypoglycemia. As a result, this neurotransmitter may play a role in modulating counterregulatory responses. This study examines whether norepinephrine acts to promote glucose counterregulation via specific VMH β-adrenergic receptors (BAR).

RESEARCH DESIGN AND METHODS

Awake male Sprague-Dawley rats received, via implanted guide cannulae, bilateral VMH microinjections of 1) artificial extracellular fluid, 2) B2AR agonist, or 3) B2AR antagonist. Subsequently, a hyperinsulinemic-hypoglycemic clamp study was performed. The same protocol was also used to assess the effect of VMH delivery of a selective B1AR or B3AR antagonist.

RESULTS

Despite similar insulin and glucose concentrations during the clamp, activation of B2AR in the VMH significantly lowered by 32% (P < 0.01), whereas VMH B2AR blockade raised by 27% exogenous glucose requirements during hypoglycemia (P < 0.05) compared with the control study. These changes were associated with alternations in counterregulatory hormone release. Epinephrine responses throughout hypoglycemia were significantly increased by 50% when the B2AR agonist was delivered to the VMH (P < 0.01) and suppressed by 32% with the B2AR antagonist (P < 0.05). The glucagon response was also increased by B2AR activation by 63% (P < 0.01). Neither blockade of VMH B1AR nor B3AR suppressed counterregulatory responses to hypoglycemia. Indeed, the B1AR antagonist increased rather than decreased epinephrine release (P < 0.05).

CONCLUSIONS

Local catecholamine release into the VMH enhances counterregulatory responses to hypoglycemia via stimulation of B2AR. These observations suggest that B2AR agonists might have therapeutic benefit in diabetic patients with defective glucose counterregulation.

Compound K, a metabolite of ginsenosides, facilitates spontaneous GABA release onto CA3 pyramidal neurons

Ginsenoside Rb1, a major ingredient of ginseng saponins, can affect various brain functions, including learning and memory. When ingested orally, ginsenoside Rb1 is not found in plasma as well as urine, but its metabolite compound K (ComK) reaches the systemic circulation in animals and human. Nevertheless, the pharmacological actions of ComK are still poorly known. In the present study, we investigated the effect of ComK on GABAergic spontaneous miniature inhibitory post-synaptic currents (mIPSCs) in acutely isolated rat hippocampal CA3 pyramidal neurons using a conventional whole-cell patch-clamp technique. While ComK significantly increased mIPSC frequency in a concentration-dependent manner, it had no effect on the current amplitude, suggesting that ComK acts pre-synaptically to increase the probability of spontaneous GABA release. ComK still increased mIPSC frequency even in a Ca(2+) -free external solution, suggesting that the ComK-induced increase spontaneous GABA release is not related to Ca(2+) influx from the extracellular space. However, the ComK-induced increase mIPSC frequency was significantly decreased after the blockade of either sarcoplasmic/endoplasmic reticulum Ca(2+) -ATPase or Ca(2+) release channels. These results strongly suggest that ComK enhances spontaneous GABA release by increasing intraterminal Ca(2+) concentration via Ca(2+) release from pre-synaptic Ca(2+) stores. The ComK-induced modulation of inhibitory transmission onto CA3 pyramidal neurons could have a broad impact on the excitability of CA3 pyramidal neurons and affect the physiological functions mediated by the hippocampus.

Regional distribution and cellular localization of beta2-adrenoceptors in the adult zebrafish brain (Danio rerio)

The beta(2)-adrenergic receptors (ARs) are G-protein-coupled receptors that mediate the physiological responses to adrenaline and noradrenaline. The present study aimed to determine the regional distribution of beta(2)-ARs in the adult zebrafish (Danio rerio) brain by means of in vitro autoradiographic and immunohistochemical methods. The immunohistochemical localization of beta(2)-ARs, in agreement with the quantitative beta-adrenoceptor autoradiography, showed a wide distribution of beta(2)-ARs in the adult zebrafish brain. The cerebellum and the dorsal zone of periventricular hypothalamus exhibited the highest density of [(3)H]CGP-12177 binding sites and beta(2)-AR immunoreactivity. Neuronal cells strongly stained for beta(2)-ARs were found in the periventricular ventral telencephalic area, magnocellular and parvocellular superficial pretectal nuclei (PSm, PSp), occulomotor nucleus (NIII), locus coeruleus (LC), medial octavolateral nucleus (MON), magnocellular octaval nucleus (MaON) reticular formation (SRF, IMRF, IRF), and ganglionic cell layer of cerebellum. Interestingly, in most cases (NIII, LC, MON, MaON, SRF, IMRF, ganglionic cerebellar layer) beta(2)-ARs were colocalized with alpha(2A)-ARs in the same neuron, suggesting their interaction for mediating the physiological functions of nor/adrenaline. Moderate to low labeling of beta(2)-ARs was found in neurons in dorsal telencephalic area, optic tectum (TeO), torus semicircularis (TS), and periventricular gray zone of optic tectum (PGZ). In addition to neuronal, glial expression of beta(2)-ARs was found in astrocytic fibers located in the central gray and dorsal rhombencephalic midline, in close relation to the ventricle. The autoradiographic and immunohistochemical distribution pattern of beta(2)-ARs in the adult zebrafish brain further support the conserved profile of adrenergic/noradrenergic system through vertebrate brain evolution.

The infralimbic cortex regulates the consolidation of extinction after cocaine self-administration

The infralimbic cortex (IL) regulates the consolidation of extinction learning for fear conditioning. Whether the IL influences the consolidation of extinction learning for cocaine self-administration is unknown. To address this issue, male Sprague-Dawley rats underwent 2 wk of cocaine self-administration followed by extinction training. On the first 5 d of extinction, rats underwent brief (15- or 30-min) extinction sessions and received intra-IL microinjections immediately after each extinction session. On days 6-12 of extinction, rats underwent full-length (2-h) extinction sessions that were used to assess the retention of the extinction learning from the short sessions. IL inactivation via microinjections of the GABA agonists baclofen and muscimol (BM) immediately after the extinction sessions (days 1-5) impaired the retention of extinction learning. Control experiments demonstrated that this effect was not due to inactivation of the prelimbic cortex or due to effects of the drugs on the subsequent day's behavior. In contrast, post-training intra-IL microinjections of the allosteric AMPA receptor potentiator 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide (PEPA) enhanced retention of the extinction learning. As evidence suggests a role for the beta-adrenergic receptors in memory consolidation, other rats received microinjections of the beta(2)-adrenergic receptor agonist clenbuterol or antagonist ICI-118,551 (ICI). Post-training intra-IL administration of clenbuterol or pre-training administration of ICI enhanced or impaired, respectively, the retention of extinction learning. These data indicate that the IL, and specifically the glutamatergic and beta-adrenergic systems in the IL, regulates the consolidation of extinction of cocaine self-administration and that the IL can be manipulated to influence the retention of extinction.

Relationship of arousal to circadian anticipatory behavior: ventromedial hypothalamus: one node in a hunger-arousal network

The mechanisms by which animals adapt to an ever-changing environment have long fascinated scientists. Different forces, conveying information regarding various aspects of the internal and external environment, interact with each other to modulate behavioral arousal. These forces can act in concert or, at times, in opposite directions. These signals eventually converge and are integrated to influence a common arousal pathway which, depending on all the information received from the environment, supports the activation of the most appropriate behavioral response. In this review we propose that the ventromedial hypothalamic nucleus (VMN) is part of the circuitry that controls food anticipation. It is the first nucleus activated when there is a change in the time of food availability, silencing of VMN ghrelin receptors decreases food-anticipatory activity (FAA) and, although lesions of the VMN do not abolish FAA, parts of the response are often altered. In proposing this model it is not our intention to exclude parallel, redundant and possibly interacting pathways that may ultimately communicate with, or work in concert with, the proposed network, but rather to describe the neuroanatomical requirements for this circuit and to illustrate how the VMN is strategically placed and connected to mediate this complex behavioral adaptation.