Alpha-chloralose suppression of neuronal activity

Development of a decerebrate model for investigating mechanisms mediating viscero-sympathetic reflexes in the spinalized rat

Autonomic dysreflexia (AD) often occurs in individuals living with spinal cord injury (SCI) and is characterized by uncontrolled hypertension in response to otherwise innocuous stimuli originating below the level of the spinal lesion. Visceral stimulation is a predominant cause of AD in humans and effectively replicates the phenotype in rodent models of SCI. Direct assessment of sympathetic responses to viscerosensory stimulation in spinalized animals is challenging and requires invasive surgical procedures necessitating the use of anesthesia. However, administration of anesthesia markedly affects viscerosensory reactivity, and the effects are exacerbated following spinal cord injury (SCI). Therefore, the major goal of the present study was to develop a decerebrate rodent preparation to facilitate quantification of sympathetic responses to visceral stimulation in the spinalized rat. Such a preparation enables the confounding effect of anesthesia to be eliminated. Sprague-Dawley rats were subjected to SCI at the fourth thoracic segment. Four weeks later, renal sympathetic nerve activity (RSNA) responses to visceral stimuli were quantified in urethane/chloralose-anesthetized and decerebrate preparations. Visceral stimulation was elicited via colorectal distension (CRD) for 1 min. In the decerebrate preparation, CRD produced dose-dependent increases in mean arterial pressure (MAP) and RSNA and dose-dependent decreases in heart rate (HR). These responses were significantly greater in magnitude among decerebrate animals when compared with urethane/chloralose-anesthetized controls and were markedly attenuated by the administration of urethane/chloralose anesthesia after decerebration. We conclude that the decerebrate preparation enables high-fidelity quantification of neuronal reactivity to visceral stimulation in spinalized rats. NEW & NOTEWORTHY In animal models commonly used to study spinal cord injury, quantification of sympathetic responses is particularly challenging due to the increased susceptibility of spinal reflex circuits to the anesthetic agents generally required for experimentation. This constitutes a major limitation to understanding the mechanisms mediating regionally specific neuronal responses to visceral activation in chronically spinalized animals. In the present study, we describe a spinalized, decerebrate rodent preparation that facilitates quantification of sympathetic reactivity in response to visceral stimuli following spinal cord injury. This preparation enables reliable and reproducible quantification of viscero-sympathetic reflex responses resembling those elicited in conscious animals and may provide added utility for preclinical evaluation of neuropharmacological agents for the management of autonomic dysreflexia.

CNS animal fMRI in pain and analgesia

Animal imaging of brain systems offers exciting opportunities to better understand the neurobiology of pain and analgesia. Overall functional studies have lagged behind human studies as a result of technical issues including the use of anesthesia. Now that many of these issues have been overcome including the possibility of imaging awake animals, there are new opportunities to study whole brain systems neurobiology of acute and chronic pain as well as analgesic effects on brain systems de novo (using pharmacological MRI) or testing in animal models of pain. Understanding brain networks in these areas may provide new insights into translational science, and use neural networks as a "language of translation" between preclinical to clinical models. In this review we evaluate the role of functional and anatomical imaging in furthering our understanding in pain and analgesia.

(R)-3,4,5-Tride-oxy-5,6-didehydro-1,2-O-(2,2,2-trichloro-ethyl-idene)-α-d-gluco-furan-ose-6,3-carbolactone: a new derivative of α-chloralose

THE TITLE COMPOUND [SYSTEMATIC NAME: (R)-2-trichloro-methyl-3a,3b,7a,8a-tetra-hydro-5H-pyrano[2',3':4,5]furano[2,3-d][1,3]dioxol-5-one], C(9)H(7)Cl(3)O(5), a triyclic system that contains a central α-d-furan-ose ring cis-fused with a dioxolane ring as well as a δ-lactone ring, exhibits a twisted conformation. The CCl(3) group has an axial orientation. The furan-ose ring approximates an envelope conformation due to the α,β-unsaturated lactone functionality. The asymmetric unit contains two independent mol-ecules with almost identical geometries.

On the reduction of spontaneous and glutamate-driven spinocerebellar and spinoreticular tract neuronal activity during active sleep

The present study was performed to provide evidence that dynamic neural processes underlie the reduction in dorsal spinocerebellar tract and spinoreticular tract neuron activity that occurs during active sleep. To ascertain the effect of local inhibition on the spontaneous and glutamate-evoked spike discharge of sensory tract neurons, preliminary control tests were performed during the state of quiet wakefulness, where GABA or glycine was co-administered in a sustained fashion during pulsatile release of glutamate to dorsal spinocerebellar tract (n=3) or spinoreticular tract (n=2) neurons. Co-administration of GABA or glycine also resulted in a significant marked suppression of spontaneous spike activity and glutamate-evoked responses of these cells. Extracellular recording experiments combined with juxtacellular application of glutamate were then performed on 20 antidromically identified dorsal spinocerebellar tract and spinoreticular tract neurons in the chronic intact cat as a function of sleep and wakefulness. The glutamate-evoked activity of a group of 10 sensory tract neurons (seven dorsal spinocerebellar tract, three spinoreticular tract), which exhibited a significant decrease in their spontaneous spike activity during active sleep, was examined. Glutamate-evoked activity in these cells was significantly attenuated during active sleep compared with wakefulness. In contrast, the glutamate-evoked activity of a second group of eight sensory tract neurons (four dorsal spinocerebellar tract, four spinoreticular tract), which exhibited a significant increase in their spontaneous spike activity during active sleep, was not significantly altered in a state-dependent manner. These data indicate that, during natural active sleep, a dynamic neural process is engaged onto certain dorsal spinocerebellar tract and spinoreticular tract neurons, which in turn dampens sensory throughput to higher brain centers.

Kainic acid administration in the fastigial nucleus produces differential cardiovascular effects in awake and anesthetized rats

Kainic acid was microinjected or microdialyzed into the rostral medial aspect of the fastigial nucleus to determine its effect on mean arterial pressure and heart rate. This was carried out in both the awake and the anesthetized (alpha-chloralose) rat. In awake animals, kainic acid elicited an initial phasic pressor response which was followed by a long-term elevation of mean arterial pressure that lasted for the duration of the experiment (2 h). Rats anesthetized with alpha-chloralose exhibited only a tonic depressor response. This converted to a pressor response as the rats began to emerge from anesthesia after 2 h. Both the awake and the anesthetized rats exhibited regular phasic changes in mean arterial pressure that was superimposed on the longer term changes in the mean arterial pressure. Similar results were obtained in both the microinjected and the microdialyzed animals. Thus, stimulation of the intrinsic fastigial neurons by kainic acid evokes an elevation of the mean arterial pressure in the awake rat. This is manifested as a decrease in pressure in the anesthetized animal. Thus, stimulation of the cardiovascular region of the fastigial nucleus can increase or decrease mean arterial pressure. It is possible that the direction of the change in mean arterial pressure is dependent on the level of afferent or intrinsic fastigial neural activity.

Anesthesia affects respiratory and sympathetic nerve activities differentially

Phrenic and cervical sympathetic nerve responses to hypercapnia were examined before and after anesthesia in twelve midcollicularly decerebrated, vagotomized, glomectomized, paralyzed and ventilated cats. We measured responses of integrated phrenic and cervical sympathetic nerve activities to increases in end-tidal PCO2 (PETCO2) from apneic threshold to approximately 30 torr above threshold. All cats were studied first in the unanesthetized state. Six cats were then restudied after a quarter of a usual dose of chloralose/urethane (10 mg/kg and 62.5 mg/kg, respectively) and then after half the usual dose of chloralose/urethane (20 mg/kg and 125 mg/kg). The other six animals were restudied after quarter of a standard dose of pentobarbital (9 mg/kg), after half the standard dose (18 mg/kg) and then after the full (35 mg/kg) dose. Both anesthetic agents led to significant increases in apneic thresholds for both phrenic and sympathetic nerve activities. These agents also caused dose-dependent decreases in peak, tonic and respiratory-related sympathetic nerve activities. Peak (tidal) phrenic nerve activities, in comparison, were much less affected by the anesthetic agents. CO2 response curves showed that both of these anesthetic agents depressed, at any given level of PETCO2, respiratory-related sympathetic nerve responses more than the responses found in the phrenic nerve. We conclude that the relations between peak, tonic (i.e. between phasic bursts) and respiratory-related sympathetic nerve activities and phrenic nerve activity can be altered by anesthesia.

Spinal effects of four injectable anaesthetics on nociceptive reflexes in rats: a comparison of electrophysiological and behavioural measurements

1. To assess the direct spinal contributions to the depression of reflexes caused by general anaesthetics, the intravenous potency of four injectable anaesthetics has been compared in two preparations: in decerebrate, spinalised rats, using a novel preparation requiring little surgical intervention, and in intact rats with chronically implanted i.v. cannulae. 2. Methohexitone (1-8 mg kg-1 i.v.), alphaxalone/alphadolone (0.5-8 mg kg-1 i.v.), alpha-chloralose (20-80 mg kg-1 i.v.) and ketamine (0.5-16 mg kg-1 i.v.) all produced a dose-dependent depression of single motor unit activity evoked by controlled noxious mechanical stimuli in decerebrate, spinalised animals. 3. The sedative and motor effects brought about by equivalent doses to those used in the electrophysiological experiments were assessed in intact rats. Methohexitone, alphaxalone/alphadolone and alpha-chloralose all caused similar levels of behavioural sedation at the doses that caused depression of spinal reflexes. Ketamine required relatively much higher doses to cause sedation. 4. To determine whether background anaesthesia modulated the potency with which these compounds affected spinal reflex activity, depressant effects in decerebrate, unanaesthetized rats were compared with those in animals maintained under anaesthesia with either alpha-chloralose or the steroid mixture of alphaxalone/alphadolone. The presence of either of these two agents as maintenance anaesthetics did not influence the effectiveness with which other compounds depressed nociceptive responses. However, additional doses of the maintenance anaesthetics were less effective than the same doses tested in decerebrate animals. 5. All the anaesthetics tested produced a significant depression of spinal reflex responses to noxious stimuli at doses well below those required for anaesthesia. Whilst the presence of maintenance anaesthetics appears not to distort pharmacological tests of other agents, there may nonetheless be a biasing of the samples of cells recorded.

Alpha-chloralose anesthesia inhibits the somato-sympathetic reflex response in cats more effectively than halothane

The effect of halothane anesthesia (H; 0.8 Vol%) or alpha-chloralose anesthesia (AC; 60 mg/kg i.v.) on the somato-sympatho-adrenal reflex response evoked by supramaximal bilateral sciatic nerve stimulation, were examined in two groups of cats (H: n = 6; AC: n = 4). Blood samples were collected simultaneously from the adrenal vein and femoral artery at baseline (S1) and during bilateral sciatic nerve stimulation (S2) for the measurement of norepinephrine, epinephrine, neuropeptide Y, and Metenkephalin, while mean arterial blood pressure (MABP) and heart rate (HR) were recorded. There were no differences between groups at baseline. In halothane anesthetized cats, sciatic nerve stimulation caused significant increases in MABP (S1: 113 +/- 8 mm Hg, S2: 178 +/- 10 mm Hg; mean +/- SE), HR (S1: 223 +/- 15 bpm, S2: 278 +/- 22 bpm), and adrenal vein plasma levels of norepinephrine (S1: 3.1 +/- 0.98 ng/ml, S2: 19.53 +/- 11.5 ng/ml), epinephrine (S1: 15.5 +/- 4.76 ng/ml, S2: 67.31 +/- 14.9 ng/ml), neuropeptide Y (S1: 1.3 +/- 0.12 ng/ml, S2: 2.16 +/- 0.42 ng/ml), and Met-enkephalin (S1: 107 +/- 35.7 pg/ml, S2: 200 +/- 76.5 pg/ml). In contrast, sciatic nerve stimulation in alpha-chloralose anesthetized cats, caused a significant increase only in MABP during sciatic nerve stimulation (S1: 115 +/- 10 mm Hg, S2: 171 +/- 7 mm Hg), while HR and adrenal vein plasma levels of catecholamines, neuropeptide Y and Met-enkephalin remained unchanged from baseline. Adrenal vein epinephrine levels measured during stimulation in the alpha-chloralose group (S2: 6.17 +/- 0.86 ng/ml), were significantly lower as compared to values observed during halothane anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation by chloralose of the cardiovascular and cerebrovascular responses evoked from the cerebellar fastigial nucleus

We studied the effects of chloralose anesthesia on the elevation in arterial pressure (AP), heart rate (HR), and regional CBF (rCBF) elicited by stimulation of the cerebellar fastigial nucleus (FN). Rats were anesthetized with an initial dose of chloralose (40 mg/kg s.c.), paralyzed, and artificially ventilated. The FN was stimulated (50-100 microA, 50 Hz, 1 s on/1 s off) with microelectrodes stereotaxically implanted. During the stimulation AP was carefully maintained within cerebrovascular autoregulation. CBF was measured by the [14C]iodoantipyrine technique with regional dissection. In rats that received only the initial dose of chloralose, FN stimulation elevated rCBF in brain and spinal cord, up to 209 +/- 13% of control in frontal cortex (n = 5; p less than 0.01, analysis of variance). Administration of additional chloralose (10 mg/kg i.v., 30 min prior to measurement of CBF) did not affect resting rCBF (n = 5), the EEG, or the elevation in AP and HR elicited by FN stimulation (n = 4). However, the additional chloralose abolished the elevations in rCBF (n = 5; p greater than 0.05). Thus, the cerebrovasodilation elicited from the FN is more susceptible to the effects of additional anesthesia than the elevation in AP and HR. These results indicate that the cerebrovascular and cardiovascular responses elicited from the FN are functionally distinct and provide additional evidence for the notion that these responses are mediated by different neural pathways and transmitters.

Spinal antinociceptive actions of mu- and kappa-opioids: the importance of stimulus intensity in determining 'selectivity' between reflexes to different modalities of noxious stimulus

1. In electrophysiological experiments in spinalized rats, mu- and kappa-opioids were tested intravenously on the responses of single motoneurones to electronically controlled, alternating noxious heat and noxious pinch stimuli. The effects of mu- and kappa-opioids were compared with those of the general anaesthetic alpha-chloralose and the dissociative anaesthetic/PCP ligand ketamine. 2. The kappa-opioids U-50,488 (0.5-16 mgkg-1 i.v.) and tifluadom (0.05-1.6 mgkg-1 i.v.) had very similar actions to the mu-opioid fentanyl (0.5-16 micrograms kg-1 i.v.). Thus all three agonists reduced thermal and mechanical nociceptive reflexes in parallel and in a dose-dependent manner, but only so long as neuronal responses to the alternating stimuli elicited similar excitability levels in the neurone under study. Ketamine (0.5-16 mgkg-1 i.v.) had similar actions to the opioids whereas alpha-chloralose (20 mgkg-1 i.v.) had very little effect on neuronal responsiveness. 3. Apparently 'selective' depressions by both mu- and kappa-opioids could be orchestrated by a deliberate mismatch of the intensities of alternating noxious heat and pinch stimuli; as measured by neuronal firing rate, the weaker of the responses to either type of stimulus was invariably reduced to a greater degree. 4. Similar 'selectivity' could be demonstrated for both mu- and kappa-ligands when the weaker and stronger responses were of the same modality, being applied by the same pincher device but with alternating applied force. 5. It is concluded that the 'selective' spinal actions of kappa-opioids seen in non-thermal over thermal behavioural models of nociception is likely to be related to the relative intensities, rather than the modalities, of the noxious stimuli used. The validity of the interpretation of results obtained in such behavioural studies is discussed.

Thermal salivation in rats, anesthetized with barbiturates, chloralose, urethane and ketamine

1. The relationship between thermal salivation (TS) and thermoregulation was studied in anesthetized rats. 2. Of the 6 anesthetics used, ketamine-anesthetized rats secreted the largest amount of saliva. Salivation, however, was thermal and not induced by ketamine itself. 3. Ketamine-anesthetized rats readily secreted saliva at core temperatures less than 40 degrees C but TS was remarkably enhanced by hyperthermia of 40-42.5 degrees C. 4. The equilibrium phase in the triphasic heat response of core temperature was a consequence of equilibrium between heat gain and heat loss by salivation.

Effects of chloralose-urethane anesthesia on single-axon reciprocal Ia IPSPs in the cat

Reciprocal Ia inhibitory postsynaptic potentials (IPSPs) generated by single afferents have been recorded with signal averaging in unanesthetized ischemic-decapitate cats for comparison with measurements previously obtained from preparations anesthetized with a mixture of chloralose and urethane. The results are similar to those which we obtained recently for single-axon recurrent IPSPs. Together, the studies show that chloralose-urethane anesthesia has a depressant effect on two widely studied circuits in the mammalian spinal cord.

A descriptive study of spinal dorsal horn neurons in the physiologically intact, awake, drug-free cat

The extracellular recording of single neuron activity from the spinal dorsal horn of physiologically intact, awake, drug-free cats (n = 14) revealed two potentially important differences between the intact preparation and data obtained from acute preparations. The first of the differences, as previously reported in intact rat and monkey, is that low-threshold (LT) neurons had very low rates of spontaneous activity. Of 127 LT neurons that were studied in the intact animal, 82% had spontaneous rates less than 1 impulse/s (IPS). The second difference was the low number (n = 13) of wide dynamic range (WDR, multireceptive, convergent) neurons that were encountered in the intact animal. Three chronic intact animals were also recorded from during light barbiturate anesthesia. The presence of barbiturate anesthesia significantly increased both the number of LT neurons with spontaneous rates greater than 1 IPS and the likelihood of encountering dorsal horn neurons with WDR response profiles. In spite of these two differences, many similarities between the chronic preparations and previous acute studies were observed. For example, receptive field size and response properties to natural stimulation for LT, WDR, and proprioceptive neurons were found to be similar to those reported in acute preparations. The results of this study suggest that, although there are many similarities in dorsal horn neurophysiology in acute and chronic cats, important differences that may be attributed to the preparation itself may also exist.

WDR response profiles of spinal dorsal horn neurons may be unmasked by barbiturate anesthesia

The number of WDR (convergent, multireceptive) neurons encountered in the spinal dorsal horn of physiologically intact, awake, drug-free cats has been much smaller than expected (9% in intact, drug-free animals). Control studies in barbiturate-anesthetized or spinal cord transected animals indicate that the dearth of WDR neurons was not just an artifact of the chronic recording technique. In those preparations WDR neurons represented 34% and 61% of the sample, respectively. Initial studies in which the effects of light barbiturate anesthesia on spinal dorsal horn neurons (n = 12) have been examined revealed that a 20 mg/kg dose of pentobarbital can, in some neurons (n = 4), unmask thermally evoked activity that was not present in the intact, drug-free animal. Responses to noxious mechanical stimuli were also enhanced following barbiturate administration. These changes resulted in a reclassification of neural type from low threshold in the intact, awake, drug-free animal to WDR in the anesthetized animal.