Neurotoxicity of adjuvants used in perineural anesthesia and analgesia in comparison with ropivacaine.

BACKGROUND AND OBJECTIVES: Clonidine, buprenorphine, dexamethasone, and midazolam (C, B, D, M) have been used to prolong perineural local anesthesia in the absence of data on the influence of these adjuvants on local anesthetic-induced neurotoxicity. Therefore, the impact of these adjuvants on ropivacaine (R)-induced death of isolated sensory neurons was assessed. METHODS: The trypan blue exclusion assay was used to assess death of sensory neurons isolated from adult male Sprague-Dawley rats. Drugs were applied, alone or in combination, for 2 or 24 hrs at 37°C. RESULTS: Neuronal viability was halved by 24-hr exposure to R (2.5 mg/mL), far exceeding the neurotoxicity of C, B, D, or M (at 2-100 times estimated clinical concentrations). Plain M at twice the estimated clinical concentration produced a small but significant increase in neurotoxicity at 24 hrs. After 2-hr exposure, high concentrations of B, C, and M increased the neurotoxicity of R; the combination of R + M killed more than 90% of neurons. Estimated clinical concentrations of C + B (plus 66 µg/mL D) had no influence on (i) R-induced neurotoxicity, (ii) the increased neurotoxicity associated with the combination of R + M, or (iii) the neurotoxicity associated with estimated clinical concentrations of M. There was increased neurotoxicity with 133 µg/mL D combined with R + C + B. CONCLUSIONS: Results with R reaffirm the need to identify ways to mitigate local anesthetic-induced neurotoxicity. While having no protective effect on R-induced neurotoxicity in vitro, future research with adjuvants should address if the C + B + D combination can enable reducing R concentrations needed to achieve equianalgesia (and/or provide equal or superior duration, in preclinical in vivo models).

Characterization and opioid modulation of inflammatory temporomandibular joint pain in the rat.

PURPOSE: Experimental inflammation of the rat temporomandibular joint (TMJ) is commonly used to study trigeminal nociceptive processing. This study describes spontaneous pain-related behaviors following TMJ inflammation in the rat. The ability of preemptive systemic morphine to attenuate behaviors as well as immediate-early gene expression in the trigeminal nucleus is described. MATERIALS AND METHODS: Adult male Sprague-Dawley rats received an intra-articular injection of mustard oil (0% to 20%, 50 microL) and were observed for behavioral changes. Morphine sulfate (0 to 10 mg/kg SC) was given 30 minutes before mustard oil; this was reversed in one group with naltrexone hydrochloride (5 mg/kg SC). Two hours after injection rats were killed and perfused. Immunohistochemistry for the protein product of the immediate-early gene c-fos was performed, and brain stem sections including the trigeminal subnucleus caudalis were examined for positive nuclei. RESULTS: Mustard oil inflammation of the rat TMJ induces dose-dependent, morphine-sensitive behaviors. Behaviors observed included excessive grooming of the region, a chewing-like behavior, and head shaking. Fos expression in the trigeminal subnucleus caudalis parallels changes in behaviors. Morphine dose dependently attenuates the number of behaviors, as well as Fos expression; this effect is reversed by the micro-opioid receptor antagonist naltrexone. CONCLUSIONS: Mustard oil inflammation of the rat TMJ causes reliable behavioral changes, which may be quantified and, together with Fos expression, used to assess various experimental TMJ treatment modalities.

Involvement of excitatory amino acid receptors and nitric oxide in the rostral ventromedial medulla in modulating secondary hyperalgesia produced by mustard oil.

We have recently reported a model of secondary hyperalgesia in which facilitation of the thermal nociceptive tail-flick reflex following topical mustard oil is largely dependent on descending influences from the rostral ventromedial medulla (RVM). The current study was designed to examine a potential role for excitatory amino acid receptors and nitric oxide in the RVM in modulating this hyperalgesia. Topical application of mustard oil (100%) to the lateral surface of the hind leg of awake rats produced a short-lived (60 min) facilitation of the tail-flick reflex that was dose-dependently attenuated by microinjection of the selective N-methyl-D-aspartate (NMDA) receptor antagonist APV (1-100 fmol) into the RVM. Microinjection of a greater dose of APV (1000 fmol) into the RVM produced a significant inhibition of the tail-flick reflex in the presence, but not absence, of mustard oil. In contrast, microinjection of the non-NMDA receptor antagonist DNQX (10 nmol) into the RVM further enhanced the magnitude and duration of the hyperalgesic response, and produced a facilitation of the tail-flick reflex following injection into the RVM of naive animals. Similar to APV, microinjection of the nitric oxide synthase inhibitor L-NAME (100-1000 nmol) into the RVM attenuated mustard oil hyperalgesia, while the greatest dose (1000 nmol) produced a significant inhibition of the tail-flick reflex in the presence, but not absence, of mustard oil. A role for nitric oxide synthase in the RVM in mustard oil hyperalgesia was further demonstrated by a significant increase in the number of NADPH-d labeled cells in the RVM at the time of maximal hyperalgesia. Involvement of NMDA receptors and nitric oxide in the RVM in descending nociceptive facilitation was supported by the observation that microinjection of either NMDA or the NO* donor GEA 5024 into the RVM of naive animals dose-dependently facilitated the tail-flick reflex. The hyperalgesia produced by NMDA injection into the RVM was blocked by prior intra-RVM injection of either APV or L-NAME. These results support the notion that secondary hyperalgesia produced by mustard oil involves concurrent activation of dominant descending facilitatory, as well as masked inhibitory systems from the RVM. Additionally, the data suggest that descending facilitation involves activation of NMDA receptors and production NO* in the RVM, whereas inhibition involves activation of non-NMDA receptors in the RVM.

Descending facilitatory influences from the rostral medial medulla mediate secondary, but not primary hyperalgesia in the rat.

Prolonged nociceptive input following peripheral injury results in hyperalgesia (enhanced response to a noxious stimulus), which is thought to occur as a consequence of sensitization of primary afferent nociceptors and enhanced excitability of spinal dorsal horn nociceptive neurons (central sensitization). Since there is often an expansion of hyperalgesia to tissue adjacent, and even distant from the site of injury (secondary hyperalgesia), it is thought that this phenomenon primarily involves mechanisms of central modulation/plasticity. In contrast, hyperalgesia observed at the site of tissue injury (primary hyperalgesia) involves peripheral mechanisms. In the current study, we examined the relative contribution of descending nociceptive facilitatory systems from the rostral medial medulla to enhanced behavioral nociceptive responses in models of primary and secondary hyperalgesia in awake rats. The effect of bilateral rostral medial medulla lesions produced by the soma-selective neurotoxin ibotenic acid was determined in three different models of cutaneous thermal hyperalgesia following peripheral inflammation: (i) intraplantar injection of carrageenan into the hindpaw (model of primary hyperalgesia); (ii) intra-articular injection of carrageenan/kaolin into the knee of the hind leg (model of secondary hyperalgesia); and (iii) topical application of mustard oil to the hind leg (model of secondary hyperalgesia). Compared with sham lesion animals, a bilateral lesion of the rostral medial medulla completely blocked thermal hyperalgesia in the two models of secondary hyperalgesia (intra-articular carrageenan/kaolin injection into the knee and topical mustard oil application to the hind leg), but was ineffective in blocking facilitation of the thermal paw withdrawal response in the model of primary hyperalgesia (intraplantar carrageenan injection into the hindpaw). These results suggest that primary and secondary hyperalgesia are differentially modulated in the CNS, and support the notion that descending nociceptive facilitatory influences from the rostral medial medulla significantly contribute to secondary, but not primary, hyperalgesia.

Thalamic NMDA receptors modulate inflammation-produced hyperalgesia in the rat.

The effects of inhibition of thalamic NMDA receptor function and synthesis on thermal and mechanical hyperalgesia induced by hindlimb intraplantar injection of carrageenan in the rat were studied in the 'acute' phase (within 3-5 h) and the 'subacute' phase (24 h) after carrageenan administration. Blockade of NMDA receptors was produced by intrathalamic injection of D,2-amino-5-phosphonovaleric acid (D-APV) and NMDA receptor synthesis was decreased (or not) by pretreatment of rats with intrathalamic (hindlimb representation area) injections of antisense, sense or missense oligodeoxynucleotides (ODNs) directed against the NR1 subunit of the NMDA receptor complex. Treatment with D-APV, but not saline, in the contralateral (but not ipsilateral) thalamus significantly reduced both the acute thermal and mechanical hyperalgesia in the injected paw; these same rats demonstrated significantly less thermal and mechanical hyperalgesia in the sub-acute phase than rats that had received saline or D-APV in the ipsilateral thalamus. None of the treatments had any effect on withdrawal responses of the uninjected hindpaw. Rats pretreated with NR1 sense or missense ODNs developed both thermal and mechanical hyperalgesia that was equivalent in magnitude and duration to rats that received intrathalamic saline injections. In contrast, rats pretreated with NR1 antisense ODN did not develop either acute or subacute thermal hyperalgesia; they developed less mechanical hyperalgesia than saline, sense or missense ODN-treated rats. Antisense ODN-treated rats also displayed a decrease in the number of thalamic NMDA receptors as determined by receptor binding assay. These results suggest an involvement of thalamic NMDA receptors in the development and maintenance of hyperalgesia associated with neurogenic inflammation in a model of tonic pain.

Contributions of glutamate receptors to the maintenance of mustard oil-induced hyperalgesia in spinalized rats.

N-methyl-d-aspartate (NMDA) receptors, which are widely distributed throughout the central nervous system, appear to play a critical role in several types of plasticity and long-term potentiation. In the pain system, increased sensitivity to somatosensory stimuli, known as hyperalgesia and allodynia, can arise from tissue damage or excessive C-fiber nociceptor activation. Previously, NMDA, non-NMDA ionotropic, and metabotropic glutamate receptors have been proposed to contribute to the sustained hyperalgesia following tissue injury or nociceptor activation. Although non-NMDA receptors appear to mediate both hyperalgesia and normal (nonhyperalgesic) responses and behavior, NMDA receptors have been reported to participate only in hyperalgesic responses. In contrast, other studies have implicated NMDA receptors in both hyperalgesic and normal responses. The aim of this study was to critically compare the effects of the glutamate receptor antagonists ketamine and 2-amino-5-phosphonovaleric acid (APV; NMDA receptor antagonists), 6,7-dinitroquinoxaline-2,3-dione (DNQX; non-NMDA ionotropic receptor antagonist), and 2-amino-3-phosphonopropionic acid (AP3; metabotropic receptor antagonist) on intra-articular mustard oil-induced facilitation of flexion withdrawal reflexes in spinalized rats. Our results showed that, as expected from previous studies, ketamine, APV, and DNQX dose-dependently inhibited the flexion withdrawal reflex evoked by C-fiber electrical stimulation of the sciatic nerve. Surprisingly, however, ketamine, APV, and DNQX also inhibited flexion withdrawal reflexes in normal (nonhyperalgesic) rats with similar ED50s. In contrast, AP3 had no effect in either hyperalgesic or normal rats. These results demonstrate that NMDA and non-NMDA ionotropic, but not metabotropic, glutamate receptors contribute without preference to both facilitated and normal flexion withdrawal reflexes evoked by high-intensity electrical stimulation in the spinalized rat. Thus, the apparent preference of NMDA receptors for hyperalgesic states seen in some previous studies on nociception, as well as in other model systems, may have arisen from differences in experimental paradigm, such as the intensity of sensory stimulation or excitability of the spinal cord, coupled with the voltage dependency of the NMDA conductance.

Participation of central descending nociceptive facilitatory systems in secondary hyperalgesia produced by mustard oil.

The present series of experiments were designed to examine a potential role for central descending pain facilitatory systems in mediating secondary hyperalgesia produced by topical application of mustard oil and measuring the nociceptive tail-flick reflex in awake rats. Topical application of mustard oil (100%) to the lateral surface of the hind leg produced a facilitation of the tail-flick reflex that was significantly reduced in spinal transected animals. Mustard oil hyperalgesia was also inhibited in animals that had received electrolytic lesions in the rostral ventromedial medulla (RVM). Intrathecal (i.t.) administration of the non-selective cholecystokinin (CCK) receptor antagonist proglumide (10 micrograms) prior to mustard oil application completely blocked both the lesser and greater hyperalgesic responses observed in spinal transected and normal animals, respectively, and produced an inhibition of the tail-flick reflex in normal animals. Administration of the selective CCKB receptor antagonist L-365260 i.t. dose-dependently inhibited mustard oil hyperalgesia (ID50 = 364 ng) at doses approximately 5-fold less than the CCKA receptor antagonist devazepide (ID50 = 1760 ng). Similar to spinal proglumide, microinjection of the neurotensin antagonist SR48692 (3.5 micrograms) into the RVM blocked mustard oil hyperalgesia and inhibited the tail-flick reflex. These data suggest that secondary hyperalgesia produced by mustard oil is mediated largely by a central, centrifugal descending pain facilitatory system which involves neurotensin in the RVM and spinal CCK (via CCKB receptors). The inhibition of the tail-flick reflex produced by mustard oil following spinal or supraspinal administration of receptor antagonists suggests concurrent activation of central descending facilitatory and inhibitory systems.

A model of cardiac nociception in chronically instrumented rats: behavioral and electrophysiological effects of pericardial administration of algogenic substances.

This report presents evidence that pericardial administration of a mixture of algogenic substances is a potentially useful model of cardiac nociception. In awake rats in which a looped silicone catheter had been placed in the pericardial sac at least 5 days previous, administration of a mixture containing equal concentrations of bradykinin (BK), acetylcholine (ACh), adenosine (ADEN), histamine (HIST), serotonin (5-HT) and prostaglandin E2 (PGE2) (total 25 nmol in 25 microliters) led to rapid acquisition of a passive avoidance behavior. In contrast, neither BK alone (5-25 nmol) nor the same mixture of ACh, ADEN, HIST, 5-HT and PGE2 without BK led to acquisition of the behavior or produced effects significantly different than produced by saline given into the pericardial sac in the same volume (25 microliters). Both BK and the mixture containing BK produced dose-dependent cardiovascular responses (pressor response and tachycardia) of similar magnitude. Neither saline nor the mixture without BK produced significant changes in mean arterial blood pressure or heart rate. In electrophysiological experiments in the same rats, thoracic spinal cord neurons responded dose-dependently to the mixture and, except for one neuron, responded also to BK in a dose-dependent manner. However, responses to BK, when compared to a similar dosage of BK contained in the mixture, were significantly less in magnitude and duration. All units received convergent somatic input from the thorax and all neurons also received convergent input from the esophagus. Balloon distension of the esophagus excited all units. Results of the behavioral characterization of algogenic substances administered into the pericardial sac of awake rats gave evidence of differences between the effects of BK and a mixture of six substances, including BK. BK in either of two dosages tested produced effects not different than saline while the mixture containing BK was aversive. In complementary electrophysiological studies, both BK and the mixture containing BK excited thoracic spinal cord neurons, suggesting that neuron responses to putative algogenic substances are not necessarily reliable measures of cardiac nociception.

Effects of neonatal capsaicin treatment on descending modulation of spinal nociception from the rostral, medial medulla in adult rat.

Stimulation-produced modulation from the rostral, medial medulla (RMM) on the spinal nociceptive tail-flick (TF) reflex and on lumbar spinal dorsal horn neuron responses to noxious cutaneous stimuli was studied in adult rats treated as neonates with capsaicin or vehicle. In vehicle-treated rats (n = 7), both descending facilitatory and inhibitory influences on the TF reflex were produced from the RMM. At 11/23 sites in the RMM, electrical stimulation produced biphasic modulatory effects. Electrical stimulation facilitated the spinal nociceptive TF reflex at low intensities (5-25 microA) and inhibited the TF reflex at greater intensities (50-200 microA). The mean threshold intensity of stimulation to inhibit the TF reflex (cut-off time = 7.0 s) was 66 microA (n = 11). At 11 of 23 sites, electrical stimulation only inhibited the TF reflex; the mean threshold intensity of stimulation to inhibit the TF reflex was 50 microA (n = 11). At one stimulation site, electrical stimulation only facilitated the TF reflex at the intensities tested (5-100 microA). In capsaicin-treated rats (n = 6), the proportion of sites from which electrical stimulation only inhibited the TF reflex was significantly less (3/27 sites = 11%) than in vehicle-treated rats (11/23 = 48%). The threshold intensity of stimulation to inhibit the TF reflex from these three sites was 50 microA. The number of sites in RMM from which electrical stimulation only facilitated the TF reflex was significantly greater in capsaicin-treated rats (15/27 = 56%) than in vehicle-treated rats (1/23 = 4%). Neither the number of sites in RMM from which electrical stimulation produced biphasic modulatory effects on the TF reflex (48% and 33%, respectively) nor the intensities of stimulation or magnitudes of facilitation or inhibition of the TF reflex significantly differed between vehicle- and capsaicin-treated rats. In electrophysiological experiments, all units studied responded to non-noxious and noxious intensities of mechanical stimulation applied to the glabrous skin of the plantar surface of the ipsilateral hind foot and also to noxious heating of the skin (50 degrees C). The number of sites where electrical stimulation produced only facilitatory effects on responses of spinal dorsal horn neurons to noxious stimulation (thermal or mechanical) of the skin was significantly increased from 13% of the total sites in vehicle-treated rats to 40% in capsaicin-treated rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Pentobarbital prevents the development of C-fiber-induced hyperalgesia in the rat.

Noxious stimuli applied to the skin can produce long-lasting, C-fiber-dependent, secondary hyperalgesia that is mediated by central mechanisms. NMDA receptor antagonists and low doses of morphine can preferentially block the development of hyperalgesia without significantly altering unpotentiated responses to nociceptive stimuli. The aim of our study was to determine if low doses of pentobarbital can also preferentially alter either hyperalgesic or unpotentiated responses to nociceptive heat stimuli in spinalized and intact rats. Our results demonstrate the following. (1) Mustard oil applied above the ankle joint or electrical stimulation of the sciatic nerve at C-fiber intensity in spinalized, unanesthetized rats decreased the latency to withdrawal of the foot from water maintained at 47-49 degrees C. This secondary hyperalgesia to thermal stimulation persisted for at least 1 h and was most likely mediated by central mechanisms. (2) Pentobarbital in both spinalized and spinal cord-intact rats prevented the development of the late component (42-120 min) but only partially decreased the early (2-6 min) component of hyperalgesia. In contrast, pentobarbital had relatively minimal effects on unpotentiated withdrawal responses. Thus, pentobarbital is similar to morphine in its ability to prevent hyperalgesia, but may differ from the anesthetic isoflurane, which does not interfere with the development of hyperalgesia.

Neonatal capsaicin treatment prevents the development of the thermal hyperalgesia produced in a model of neuropathic pain in the rat.

Loose ligation of the sciatic nerve with 4-0 chromic gut sutures in rats produces behavioral evidence of neuropathic pain. In the present experiments we examined the involvement of capsaicin-sensitive afferents in mediating the thermal hyperalgesia produced by this model. Male Sprague-Dawley rats, treated as neonates (within 48 h of birth) with capsaicin (50 mg/kg, s.c.) or vehicle, were used at 16-18 weeks of age. Chromic gut sutures (4-0) were tied around the left sciatic nerve and withdrawal latencies of both hind paws to radiant heat were determined on postoperative days 3, 5, 10 and 20. Whereas there was a pronounced thermal hyperalgesia which lasted for up to 20 days in vehicle-treated rats, there was no evidence of thermal hyperalgesia in capsaicin-treated rats. There was no difference in baseline (pre-surgery) withdrawal latencies between the two groups. Radioimmunoassay revealed that there was a significant depletion of substance P (43.8%) and calcitonin-gene-related peptide (72.6%) in the lumbar spinal cord of neonatal capsaicin-treated rats compared to vehicle-treated rats. These results demonstrate that the chromic gut-induced thermal hyperalgesia is mediated by capsaicin-sensitive afferents and suggest that central mechanisms which process and control the reflex response to heat are different than mechanisms involved in thermal hyperalgesia.

Modulation of cortical evoked potentials by stimulation of nucleus raphe magnus in rats.

1. In pentobarbital sodium-anesthetized rats, we evaluated changes in cortical evoked potentials (EPs) associated with electrical and chemical stimulation of nucleus raphe magnus (NRM). A condition-test (C-T) paradigm was used. Cortical EPs were produced by test stimuli delivered to a hindpaw or the thalamic ventral posterior lateral nucleus (VPL; electrical stimulation), or by photic stimulation of the eyes or electrical stimulation of contralateral homotopical cortex (transcallosal EPs). These test stimuli were then preceded by electrical or chemical conditioning stimulation (CS) delivered to NRM through a stereotaxically implanted electrode or injection cannula, respectively. Effects of CS on EPs produced by the test stimuli were characterized. 2. Electrical CS preceding a test stimulus delivered to the foot reduced the amplitude of EPs at thresholds as low as 10-25 microA. The magnitude of EP reduction was dependent on CS intensity, frequency, and the C-T interval. Optimal parameters were trains of 10 pulses (400 Hz) delivered at a C-T interval of 5-10 ms. Injection of glutamate and lidocaine into NRM demonstrated that these effects were due to activation of NRM neurons and not to current spread to medial lemniscus (ML). NRM CS also reduced cortical EPs produced by test stimulation in VPL but did not alter EPs from visual stimulation or from electrical stimulation of contralateral homotopical cortex. 3. These findings suggest that NRM CS attenuates EPs by inhibiting thalamic or thalamocortical afferent activity. Because NRM CS affected all components of the cortical EPs, the effect appears to involve alteration of general sensory activity and is not nociception specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal capsaicin treatment abolishes the nociceptive responses to intravenous 5-HT in the rat.

The intravenous (i.v.) administration of serotonin (5-HT) to lightly pentobarbital-anesthetized rats is known to produce a triad of reflex cardiovascular responses, distinct afferent-mediated pseudaffective reactions, and a vagally mediated inhibition of the nociceptive tail-flick (TF) reflex consistent with 5-HT acting as a noxious stimulus. In the present experiments we examined the involvement of capsaicin-sensitive afferents in mediating these responses. Lightly pentobarbital-anesthetized 16-week-old male Sprague-Dawley rats which had been treated as neonates (in the first 48 h of life) with capsaicin (50 micrograms/kg, s.c.) were compared to age-matched neonatal vehicle-treated controls. Whereas the i.v. administration of 5-HT produced a dose-dependent (6-96 micrograms/kg, i.v.) inhibition of the nociceptive TF reflex (ED50 = 48.1 +/- 11.3 micrograms/kg; n = 7) and distinct pseudaffective responses (usually by 24-48 micrograms/kg) in vehicle-treated rats, 5-HT (6-192 micrograms/kg, i.v.) failed to significantly alter TF latency or produce pseudaffective behaviors in the capsaicin-treated rats (n = 10). There was no difference in baseline TF latencies between the two groups. There were essentially no differences between vehicle- and capsaicin-treated rats with respect to the initial cardiopulmonary vagal afferent-mediated (Bezold-Jarisch reflex) decreases in heart rate and arterial blood pressure or the subsequent pressor phase. However, the magnitude of the late hypotensive phase was significantly greater in capsaicin-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of electrical stimulation of vagal afferents on spinothalamic tract cells in the rat.

The effects of electrical stimulation of cervical vagal afferents (VAS) on the background activity and on the responses of 25 spinothalamic tract (STT) neurons to noxious stimuli were studied in anesthetized rats. Background (spontaneous) activity of 9 (36%) STT neurons was inhibited by all intensities of VAS. 6 (24%) units were facilitated at lesser and inhibited at greater intensities of VAS, 5 (20%) units were only facilitated by all intensities of VAS, and 5 (20%) units were not affected by VAS. Responses of 8 (36%) STT neurons to noxious stimuli were only inhibited by VAS, 9 (41%) were facilitated at lesser and inhibited at greater intensities of VAS, and 5 units (23%) were only facilitated by VAS. There were no significant differences in VAS-produced modulatory effects between STT neurons and 16 unidentified lumbar spinal dorsal horn neurons studied under the same conditions. These results reveal that descending facilitatory and inhibitory pathways engaged by activation of vagal afferents modulate rostrally projecting nociceptive transmission neurons in the spinal cord, constituting an important regulatory network for nociception.

Coronary vasoconstriction during stimulation in hypothalamic defense region.

Previous studies have identified a site in lateral hypothalamus (LH) in which electrical stimulation elicits coronary vasoconstriction. We injected the retrogradely transported tracer Fast Blue to determine which brain regions project to LH. Projections to or through LH were found from the paraventricular nucleus (PVN) of the hypothalamus, bed nucleus of the stria terminalis (BNST), and dorsal raphe nucleus (DRN). In chloralose-anesthetized cats, electrical stimulation in DRN and BNST failed to increase coronary vascular resistance (CVR). However, stimulation lateral to PVN in the anterior hypothalamic area (AHA), a region not labeled by the tracer, caused a transient decrease in coronary blood flow similar to that elicited from LH. The increase in CVR was accompanied by hemodynamic changes that are characteristic of the defense reaction including a cholinergically mediated decrease in hindquarter vascular resistance. This response is likely due to activation of fibers of passage and not cell bodies, since cell bodies in the region were not retrogradely labeled and coronary vasoconstriction was not seen following microinjection of several excitatory amino acids into AHA. These data suggest that coronary vasoconstriction may be a component of the defense reaction elicited by electrical activation of AHA.

Connections between hypothalamus and medullary reticular formation mediate coronary vasoconstriction.

We have recently identified discrete sites within the lateral hypothalamus and medullary reticular formation that, when stimulated electrically, produce neurally mediated coronary vasoconstriction. This study examined whether these sites are part of the same coronary vasomotor pathway. The neuronal tracing dye fast blue was injected in cats into the coronary vasoconstrictor site within medullary reticular formation. Fluorescence microscopy revealed major afferent projections originating from within the same region of midbrain ventrolateral periaqueductal gray that receives projections from lateral hypothalamus. To determine the functional importance of the proposed connections between the hypothalamic and medullary sites, anesthetized cats were prepared for continuous hemodynamic measurements. Constant current electrical stimulation within lateral hypothalamus produced significant increases in heart rate (21 +/- 6%), arterial pressure (11 +/- 3%), and femoral (36 +/- 18%) and coronary resistances (14 +/- 9%) with no change in coronary flow velocity (-1.1 +/- 2.5%). After beta-adrenoreceptor blockade, significantly greater increases in arterial pressure (35 +/- 8%) and coronary resistance (39 +/- 5%) with transient decreases in coronary flow velocity (21 +/- 6%) were seen. Microinjections of lidocaine into the medullary site blocked coronary constriction produced by lateral hypothalamic stimulation (39 +/- 5% increase in coronary resistance to electrical stimulation before and 2.4 +/- 2% increase after lidocaine in medullary reticular formation). These data provide evidence that specific regions of lateral hypothalamus and medullary reticular formation are part of a common central vasomotor projection that mediates coronary vasoconstriction in addition to other hemodynamic effects.

Brain-stem relays mediating stimulation-produced antinociception from the lateral hypothalamus in the rat.

Several lines of evidence have demonstrated a role for the lateral hypothalamus (LH) in an endogenous system of descending inhibition. The present study, in rats lightly anesthetized with pentobarbital, was undertaken to examine systematically the organization in the brain stem of pathways mediating descending inhibition of the nociceptive tail flick (TF) reflex produced by focal electrical stimulation in the LH. The microinjection of lidocaine into the midbrain, dorsolateral pons, or medial medulla resulted in significant increases in stimulation thresholds in the LH for inhibition of the TF reflex (89.1, 67.4, and 73.6%, respectively). Selective lesions of cell bodies in the midbrain or medulla by the neurotoxin ibotenic acid also produced significant increases in stimulation thresholds in the LH for inhibition of the TF reflex (31.6 and 131.6%, respectively), thus revealing relays in the periaqueductal gray and the nucleus raphe magnus located between the LH and the lumbar spinal cord. The failure of ibotenic acid to affect LH-produced descending inhibition when microinjected into the dorsolateral pons, and the significant effect produced by lidocaine microinjected into the same area, implicates fibers of passage in the dorsolateral pons in descending inhibition of the TF reflex produced by focal electrical stimulation in the LH. The fluorescent dye Fast blue and HRP conjugated to wheat germ agglutinin were used to confirm that the area stimulated in the LH has reciprocal connections with the periaqueductal gray and nucleus raphe magnus.

Electrical stimulation in perifornical lateral hypothalamus decreases coronary blood flow in cats.

Based on evidence implicating the central nervous system in the regulation of coronary vascular resistance and the knowledge that the hypothalamus is a central site for integration of cardiovascular control, studies were undertaken to determine if electrical stimulation in the hypothalamus produced coronary vasoconstriction. In anesthetized cats, following beta-adrenergic receptor blockade, stimulation in perifornical lateral hypothalamus produced a transient decrease in coronary blood flow velocity (30 +/- 5%), a small pressor effect (7 +/- 2 mmHg), and an initial decrease in hindquarter blood flow velocity (51 +/- 5%). The decrease in coronary flow velocity, which had an onset latency of 1-3 s and a duration of 5-15 s, was abolished by ipsilateral stellate ganglionectomy and by intravenous and intracoronary prazosin. The coronary vasoconstriction produced by hypothalamic stimulation was not different from that produced by cardioaccelerator nerve stimulation. These results suggest that electrical stimulation of a hypothalamic site produces an alpha-adrenergic receptor-mediated decrease in coronary blood flow that is unmasked by beta-adrenergic receptor blockade, requires the integrity of ipsilateral cardiac sympathetic innervation, and mimics the coronary response to cardioaccelerator nerve stimulation.

Capsaicin treatment in adult Wistar-Kyoto and spontaneously hypertensive rats: neurochemical effects in the spinal cord.

The effects of s.c. capsaicin treatment on the thoracic spinal cord contents of immunoreactive substance P (ISP), norepinephrine (NE) and serotonin (5-HT) were investigated in adult male Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Capsaicin administered s.c. significantly reduced the concentration of ISP in the thoracic spinal cords from rats of both the WKY and SHR strains. The s.c. administration of capsaicin significantly increased the NE contents of the thoracic spinal cords in rats of both the WKY and SHR strains, and both vehicle- and capsaicin-treated SHR animals exhibited significantly greater thoracic spinal cord NE concentrations than did rats from the corresponding WKY treatment groups. In contrast, s.c. capsaicin treatment significantly increased the concentration of 5-HT only in the thoracic spinal cords of SHR subjects. These results strongly suggest that the neurochemical effects of s.c. capsaicin administration on the spinal cord were not specific to ISP-containing neurons since significant changes in the concentrations of NE and 5-HT were also produced by capsaicin treatment. These capsaicin induced changes in the concentrations of putative neurotransmitter substances in the spinal cord are related to previously reported alterations in nociception and cardiovascular regulation produced by s.c. capsaicin administration. Physiological and pharmacological mechanisms possibly relating the neurochemical changes produced by capsaicin to the behavioral and cardiovascular effects of the drug previously reported are advanced and discussed.