Temperature Dependency of Basal and Evoked Release of Amino Acids and Calcitonin Gene-Related Peptide from Rat Dorsal Spinal Cord

Abstract

Moderate hypothermia significantly diminishes consequences of spinal and cerebral anoxia. One component of this neuroprotection has been hypothesized to be suppression of excitotoxic transmitter release. Whether this suppression is attributable to reduced hypoxic injury that induces release or an alteration of the release process itself is unclear. We sought to characterize the temperature sensitivity (Q 10 ) of basal and evoked calcitonin gene-related peptide (CGRP) and amino acid release from dorsal horn slices of rat spinal cord over a range of temperatures from 40 to 8°C. At 40°C, potassium (60 mM) and capsaicin (10 M) evoked a 21-and 32-fold increase in basal CGRP concentrations, respectively. Capsaicin had no effect on glutamate release, but potassium evoked a 2.7-fold increase. Release evoked by either potassium or capsaicin was reduced in a biphasic fashion with declining temperature. Over the range of 40 to 34°C, the Q 10 values for evoked release for CGRP were 11.3 (potassium) and 39.7 (capsaicin) and for glutamate, 5.5 (potassium). Over the range of 34 to 8°C, Q 10 values were near unity for all evoked release (0.8 and 1.3 for CGRP and 1.2 for glutamate). Although serine, glycine, glutamine, taurine, and citrulline showed no evoked release, basal levels were reduced at temperatures below 34°C. The pronounced temperature dependency of evoked transmitter release between 40 and 34°C is consistent with the profound cerebral protection observed with mild hypothermia in which metabolic activity is only slightly depressed. Key words: hypothermia; hyperthermia; glutamate; CGRP; spinal cord superfusion; evoked transmitter release; dorsal horn; capsaicinReduction of brain or spinal temperature by 3-5°C has been shown in preclinical and clinical models to attenuate neuronal damage and dysfunction after ischemia (Vacanti and Ames, 1984;Busto et al., 1989). Several mechanisms have been proposed to account for the protection produced by such hypothermia. The first assumes that ischemic injury occurs secondary to depletion of metabolic stores and that hypothermia preserves these stores by reducing cerebral metabolic rate of oxygen consumption (CMRO 2 ). In animal models of cerebral ischemia, CMRO 2 declines monotonically by ϳ4.5% per degree over the range of 37-38°C (normothermia) to 15-18°C (profound hypothermia) (Michenfelder and Milde, 1992). These findings are clinically relevant, because recovery of normal function in humans can be observed after 60 min of circulatory arrest when the arrest occurs at 15-18°C (Tharion et al., 1982). Although such profound hypothermia has been widely acknowledged, considerable data suggest that a surprisingly moderate hypothermia (32-34°C) can provide neurological protection in the face of cerebral and spinal ischemia. Such a modest reduction in tissue temperature has little effect on CMRO 2 and does not preserve metabolic stores (Hagerdal et al., 1975;Sano et al., 1992).A second mechanism that may underlie hypothermic neuroprotection involves excitotoxicity. Activation of NMDA a...

Enhanced Spontaneous Transmitter Release Is the Earliest Consequence of Neocortical Hypoxia That Can Explain the Disruption of Normal Circuit Function

“…The study was performed in an in vitro neocortical slice preparation, which allowed us to investigate the effects of hypoxia under well controlled experimental conditions and to implement in full the patch-clamp technique to examine both intrinsic neuronal conductances and synaptic transmission. To remove possible doubts about the Q 10 of the different processes in the range between room temperature and 37°C (Stys et al, 1992;Dirig et al, 1997;Ransom et al, 2000), all experiments, including cell-attached single-channel recordings, were done at physiological temperatures. We considered the possibility that some of the hypoxia- Later, the large EPSCs disappear, leaving only small-amplitude (Ͻ20 pA), action potential-independent events (closed circles).…”

Noxious heat‐induced CGRP release from rat sciatic nerve axons in vitro

“…Noxious heat‐induced release of neuropeptides has been shown in other preparations, such as rat skin, brown adipose tissue and spinal cord slices (Dirig et al ., 1997; Osaka et al ., 1998; Kessler et al ., 1999). In the peripheral nerve we found a particularly tight correlation between stimulus temperature and response, resulting in a high temperature coefficient (Q 10 ) of ≈ 15 between 42 and 52 °C.…”

In vitro prostanoid release from spinal cord following peripheral inflammation: effects of substance P, NMDA and capsaicin

“…The spinal cord superfusion methodology has been previously published ( Dirig et al ., 1997 ). Briefly, rats were decapitated after terminal halothane anaesthesia (4%/oxygen) and spinal cords were hydraulically extruded ( Sousa & Horrocks, 1979 ).…”