Myelinated (Aδ-fibre) and unmyelinated (C-fibre) nociceptors convey different qualities of the pain signal (first and second pain, respectively) and they are believed to play different roles in the development and maintenance of chronic pain. As such, studies that use differential activation of Aδ- and C-nociceptors should provide important insights into the central processing and modulation of nociceptive inputs. Here we demonstrate a technique originally described by Yeomans et al. (1996a, b), and further modified in this laboratory (McMullan et al. 2004), that uses different rates of skin heating to preferentially activate either Aδ- or C-heat nociceptors. We use this approach to study the effects of descending control from the midbrain periaqueductal grey (PAG) on withdrawal and autonomic reflex responses evoked by Aδ- or C- heat nociceptors. A male Wistar rat (280-300g) will be deeply anaesthetised using halothane (2-3% in O₂; Merial Animal Health Ltd, UK) and the right jugular vein and carotid artery cannulated. Following preparatory surgery, anaesthesia will be maintained by continuous intravenous infusion of sodium pentobarbital (31mg kg^-1 h^-1; Sigma, UK). The animal will be placed in a stereotaxic frame and a small craniotomy performed. Electromyographic activity (EMG) will be recorded from the biceps femoris via an intramuscular bipolar electrode (two Teflon-coated stainless steel wires, 0.075mm in diameter; Advent Research Materials, UK). EMG will be used to monitor the thresholds and magnitudes of withdrawal responses to the different rates of skin heating. Arterial blood pressure recorded from the carotid artery will be used to monitor effects of peripheral and central stimulation on cardiovascular responses. A T-type thermocouple will be used to monitor skin surface temperature at the centre of a copper plate positioned at the focus of a radiant heat source. Fast (7.5°C s^-1) or slow (2.5°C s^-1) rates of skin heating will be applied to the dorsal surface of the hind paw to preferentially activate Aδ- or C-heat nociceptors, respectively. These rates have been shown previously to produce linear subsurface increases of 2.5 and 0.6°C s^-1, respectively. Output from the thermocouple, the filtered and amplified EMG, and blood pressure signals will be digitised and displayed on a PC running Spike2 software (CED, UK). A glass micropipette (Harvard Apparatus Ltd, UK), containing 50mM D,L-homocysteic acid (DLH; Sigma, UK), will be driven vertically into the brain to a depth of between 4.25 and 5.7mm below the cortical surface, at approximately 7.6mm caudal to bregma and 0.9mm lateral to the midline in order to reach the dorsolateral or ventrolateral PAG. The pipette will be connected to a 1μl glass syringe (SGE, Australia) in order to administer 30nl microinjections of DLH . Cardiovascular and reflex withdrawal responses to fast or slow rates of skin heating will be compared before and after DLH-evoked neuronal activation in the PAG in order to determine the extent to which descending control modulates reflex responses to Aδ- and C-nociceptive inputs.