D1 agonist SKF 38393 antagonizes pentobarbital-induced narcosis and depression of hippocampal and cortical cholinergic activity in rats

Historical and Modern Evidence for the Role of Reward Circuitry in Emergence

Increasing evidence supports a role for brain reward circuitry in modulating arousal along with emergence from anesthesia. Emergence remains an important frontier for investigation, since no drug exists in clinical practice to initiate rapid and smooth emergence. This review discusses clinical and preclinical evidence indicating a role for two brain regions classically considered integral components of the mesolimbic brain reward circuitry, the ventral tegmental area and the nucleus accumbens, in emergence from propofol and volatile anesthesia. Then there is a description of modern systems neuroscience approaches to neural circuit investigations that will help span the large gap between preclinical and clinical investigation with the shared aim of developing therapies to promote rapid emergence without agitation or delirium. This article proposes that neuroscientists include models of whole-brain network activity in future studies to inform the translational value of preclinical investigations and foster productive dialogues with clinician anesthesiologists.

Escape From Oblivion: Neural Mechanisms of Emergence From General Anesthesia

The question of how general anesthetics suppress consciousness has persisted since the mid-19th century, but it is only relatively recently that the field has turned its focus to a systematic understanding of emergence. Once assumed to be a purely passive process, spontaneously occurring as residual levels of anesthetics dwindle below a critical value, emergence from general anesthesia has been reconsidered as an active and controllable process. Emergence is driven by mechanisms that can be distinct from entry to the anesthetized state. In this narrative review, we focus on the burgeoning scientific understanding of anesthetic emergence, summarizing current knowledge of the neurotransmitter, neuromodulators, and neuronal groups that prime the brain as it prepares for its journey back from oblivion. We also review evidence for possible strategies that may actively bias the brain back toward the wakeful state.

Activation of D1 dopamine receptors induces emergence from isoflurane general anesthesia

BACKGROUND

A recent study showed that methylphenidate induces emergence from isoflurane anesthesia. Methylphenidate inhibits dopamine and norepinephrine reuptake transporters. The objective of this study was to test the hypothesis that selective dopamine receptor activation induces emergence from isoflurane anesthesia.

METHODS

In adult rats, we tested the effects of chloro-APB (D1 agonist) and quinpirole (D2 agonist) on time to emergence from isoflurane general anesthesia. We then performed a dose-response study to test for chloro-APB-induced restoration of righting during continuous isoflurane anesthesia. SCH-23390 (D1 antagonist) was used to confirm that the effects induced by chloro-APB are specifically mediated by D1 receptors. In a separate group of animals, spectral analysis was performed on surface electroencephalogram recordings to assess neurophysiologic changes induced by chloro-APB and quinpirole during isoflurane general anesthesia.

RESULTS

Chloro-APB decreased median time to emergence from 330 to 50 s. The median difference in time to emergence between the saline control group (n = 6) and the chloro-APB group (n = 6) was 222 s (95% CI: 77-534 s, Mann-Whitney test). This difference was statistically significant (P = 0.0082). During continuous isoflurane anesthesia, chloro-APB dose-dependently restored righting (n = 6) and decreased electroencephalogram δ power (n = 4). These effects were inhibited by pretreatment with SCH-23390. Quinpirole did not restore righting (n = 6) and had no significant effect on the electroencephalogram (n = 4) during continuous isoflurane anesthesia.

CONCLUSIONS

Activation of D1 receptors by chloro-APB decreases time to emergence from isoflurane anesthesia and produces behavioral and neurophysiologic evidence of arousal during continuous isoflurane anesthesia. These findings suggest that selective activation of a D1 receptor-mediated arousal mechanism is sufficient to induce emergence from isoflurane general anesthesia.

The TRH neuron: a hypothalamic integrator of energy metabolism

Thyrotropin-releasing hormone (TRH) has an important role in the regulation of energy homeostasis not only through effects on thyroid function orchestrated through hypophysiotropic neurons in the hypothalamic paraventricular nucleus (PVN), but also through central effects on feeding behavior, thermogenesis, locomotor activation and autonomic regulation. Hypophysiotropic TRH neurons are located in the medial and periventricular parvocellular subdivisions of the PVN and receive direct monosynaptic projections from two, separate, populations of leptin-responsive neurons in the hypothalamic arcuate nucleus containing either alpha-melanocyte-stimulating hormone (alpha-MSH) and cocaine- and amphetamine-regulated transcript (CART), peptides that promote weight loss and increase energy expenditure, or neuropeptide Y (NPY) and agouti-related protein (AGRP), peptides that promote weight gain and reduce energy expenditure. During fasting, the reduction in TRH mRNA in hypophysiotropic neurons mediated by suppression of alpha-MSH/CART simultaneously with an increase in NPY/AGRP gene expression in arcuate nucleus neurons contributes to the fall in circulating thyroid hormone levels, presumably by increasing the sensitivity of the TRH gene to negative feedback inhibition by thyroid hormone. Endotoxin administration, however, has the paradoxical effect of increasing circulating levels of leptin and melanocortin signaling and CART gene expression in arcuate nucleus neurons, but inhibiting TRH gene expression in hypophysiotropic neurons. This may be explained by an overriding inhibitory effect of endotoxin to increase type 2 iodothyroine deiodinase (D2) in a population of specialized glial cells, tanycytes, located in the base and infralateral walls of the third ventricle. By increasing the conversion of T4 into T3, tanycytes may increase local tissue concenetrations of thyroid hormone, and thereby induce a state of local tissue hyperthyroidism in the region of hypophysisotrophic TRH neurons. Other regions of the brain may also serve as metabolic sensors for hypophysiostropic TRH neurons including the ventrolateral medulla and dorsomedial nucleus of the hypothalamus that have direct monosynaptic projections to the PVN. TRH also exerts a number of effects within the central nervous system that may contribute to the regulation of energy homeostasis. Included are an increase in core body temperature mediated through neurons in the anterior hypothalamic-preoptic area that coordinate a variety of autonomic responses; arousal and locomotor activation through cholinergic and dopaminergic mechanisms on the septum and nucleus accumbens, respectively; and regulation of the cephalic phase of digestion. While the latter responses are largely mediated through cholinergic mechanisms via TRH neurons in the brainstem medullary raphe and dorsal motor nucleus of the vagus, effects of TRH on autonomic loci in the hypothalamic PVN may also be important. Contrary to the actions of T3 to increase appetite, TRH has central effects to reduce food intake in normal, fasting and stressed animals. The precise locus where TRH mediates this response is unknown. However, evidence that an anatomically separate population of nonhypophysiotropic TRH neurons in the anterior parvocellular subdivision of the PVN is integrated into the leptin regulatory control system by the same arcuate nucleus neuronal populations that innervate hypophysiotropic TRH neurons, raises the possibility that anterior parvocellular TRH neurons may be involved, possibly through interactions with the limbic nervous system.

Synthesis and biological evaluation of GABA derivatives able to cross the blood–Brain barrier in rats

Two new GABA derivatives, 1 and 2, were synthesized and tested for their capacity to display CNS activity, which was assessed by determining the effects on the duration of pentobarbital-induced hypnosis in rats. Compound 1, peripherally injected, significantly prolonged the hypnosis time, a typical GABA-mimetic effect, while both intracerebroventricular and intravenous administration of compound 2 surprisingly shortened the hypnotic effect in an atropine-sensitive way. The study was extended also to compounds 1a, 1b and 2a, putative oxidative/hydrolytic metabolites of 1 and 2.

Catecholamines participate in the induction of ornithine decarboxylase gene expression in normal and hyperplastic mouse kidney

In the quinazoline antifolate (CB 3717)-induced hyperplastic kidney model, a remarkable increase of ornithine decarboxylase (ODC) activity was paralleled by a smaller, but highly significant augmentation of the ODC transcript level. Catecholamine depletion, evoked by reserpine, strongly impaired antifolate-induced ODC expression; the enzyme activity was almost completely abolished while the mRNA level decreased by 60%. Moreover, under conditions of a depleted catecholamine pool, kidney enlargement was significantly reduced confirming our earlier reports on the indispensability of ODC induction for renal hyperplasia (M. Manteuffel-Cymborowska et al. , Biochim. Biophys. Acta, 1182 (1993) 133-141[1]). In normal mouse kidney catecholamines appeared to be inducers of ODC expression. Use of selective agonists of catecholamine receptors demonstrated the importance of dopamine D2 receptors, and to a lower extent beta adrenoreceptors, in the catecholamine mediation of induction of ODC activity and of ODC mRNA levels. These increases were not abolished by an antiandrogen, casodex, suggesting that catecholamine control of ODC expression is an androgen receptor-independent process. The results obtained point to the critical role of renal catecholamines; these biogenic amines are not only involved in the regulation of ODC expression in normal kidney but are also required for the induction of ODC in hyperplastic kidney evoked by antifolate and, as shown recently (M. Manteuffel-Cymborowska et al., Biochim. Biophys. Acta, 1356 (1997) 292-298[2]), in testosterone-induced hypertrophic kidney.

An update on the CNS actions of TRH and its analogs

This brief review will discuss the recent literature on several of the central actions of TRH and its analogs. The most prominent of these actions include: (1) the arousal or analeptic effect in drug narcotized animals or in concussion models; (2) the reversal of cognitive deficits produced by various drugs or procedures, and (3) the improvement of several neurological deficits produced in animal models of spinal and/or cerebellar injury. The mediation of these TRH effects by neurotransmitters is discussed. While little has been published on the human neuropsychopharmacology of TRH, and especially of its analogs, the future holds considerable therapeutic promise for these interesting drugs.

Dopaminergic regulation of cortical acetylcholine release: effects of dopamine receptor agonists

The regulation of the basal forebrain cholinergic system by D1 and D2 dopamine receptors was assessed in the rat using in vivo microdialysis of cortical acetylcholine. The D1 agonist CY 208-243 significantly increased cortical acetylcholine release; in contrast, the D2 agonists quinpirole and (+)-4-propyl-9-hydroxynaphthoxazine were without significant effects. Moreover, when administered in combination with CY 208-243, quinpirole failed to potentiate the D1 agonist-induced increases in cortical acetylcholine release. The non-selective dopamine receptor agonist apomorphine also increased cortical acetylcholine release, and this was completely blocked by the selective D1 receptor antagonist SCH 23390 and slightly, but not significantly attenuated by the D2 antagonist raclopride. The present results indicate that stimulation of D1 receptors activates cortically-projecting cholinergic neurons; however, a minor contribution of D2 receptors cannot be excluded.

The effects of SCH 23390 and raclopride on cocaine-induced analepsis and EEG arousal in rabbits

The effects of the dopamine D1 and D2 receptor antagonists on cocaine-induced, cholinergically-mediated analeptic and hippocampal theta activity in anesthetized rabbits were investigated. Cocaine (2 mg/kg, i.v.) reduced by 35% the duration of loss of righting reflex produced by a 25 mg/kg dose of pentobarbital. This shortening of narcosis time was blocked by pretreating the animals with the D1 antagonist, SCH 23390 (0.1 mg/kg) but not with the D2 antagonist raclopride (1-2 mg/kg). Cocaine (5 mg/kg, i.v.) also produced a short burst of increased hippocampal theta activity in urethane-anesthetized rabbits, which was also blocked by SCH 23390 but not by raclopride. An unexpected finding was that raclopride itself, at 2 mg/kg (i.v.), produced a marked activation of theta activity that lasted for 15-20 min. When cocaine was administered after this time it produced a potentiated theta response, both in duration and in magnitude. These results suggest that in the rabbit, cocaine exerts a cholinergically-mediated behavioral and EEG arousal through a D1 dopamine mechanism. The role of the D2 system is less clear but appears to be antagonistic to the D1-mediated response.

Dopaminergic regulation of cortical acetylcholine release

The extent to which the activity of basal forebrain cholinergic neurons is influenced by dopamine (DA) was investigated using in vivo microdialysis of cortical acetylcholine (ACh). Systemic administration of the DA receptor agonist apomorphine significantly increased dialysate concentrations of ACh. Systemic, but not local, administration of d-amphetamine produced similar effects. Both D1 (SCH 23390) and D2 (haloperidol, raclopride) DA receptor antagonists attenuated the amphetamine-induced increase in cortical ACh release; however, only the D1 antagonist significantly reduced basal output of cortical ACh. These findings suggest that the activity of cortically projecting cholinergic neurons in the nucleus basalis is regulated in an excitatory manner by central dopaminergic neurons and that both D1 and D2 receptors are involved.