Inhibitory effects evoked from the anterior hypothalamus are selective for the nociceptive responses of dorsal horn neurons with high- and low-threshold inputs

The Periaqueductal Gray Orchestrates Sensory and Motor Circuits at Multiple Levels of the Neuraxis

The periaqueductal gray (PAG) coordinates behaviors essential to survival, including striking changes in movement and posture (e.g., escape behaviors in response to noxious stimuli vs freezing in response to fear-evoking stimuli). However, the neural circuits underlying the expression of these behaviors remain poorly understood. We demonstrate in vivo in rats that activation of the ventrolateral PAG (vlPAG) affects motor systems at multiple levels of the neuraxis through the following: (1) differential control of spinal neurons that forward sensory information to the cerebellum via spino-olivo-cerebellar pathways (nociceptive signals are reduced while proprioceptive signals are enhanced); (2) alterations in cerebellar nuclear output as revealed by changes in expression of Fos-like immunoreactivity; and (3) regulation of spinal reflex circuits, as shown by an increase in α-motoneuron excitability. The capacity to coordinate sensory and motor functions is demonstrated in awake, behaving rats, in which natural activation of the vlPAG in fear-conditioned animals reduced transmission in spino-olivo-cerebellar pathways during periods of freezing that were associated with increased muscle tone and thus motor outflow. The increase in spinal motor reflex excitability and reduction in transmission of ascending sensory signals via spino-olivo-cerebellar pathways occurred simultaneously. We suggest that the interactions revealed in the present study between the vlPAG and sensorimotor circuits could form the neural substrate for survival behaviors associated with vlPAG activation.

SIGNIFICANCE STATEMENT

Neural circuits that coordinate survival behaviors remain poorly understood. We demonstrate in rats that the periaqueductal gray (PAG) affects motor systems at the following multiple levels of the neuraxis: (1) through altering transmission in spino-olivary pathways that forward sensory signals to the cerebellum, reducing and enhancing transmission of nociceptive and proprioceptive information, respectively; (2) by alterations in cerebellar output; and (3) through enhancement of spinal motor reflex pathways. The sensory and motor effects occurred at the same time and were present in both anesthetized animals and behavioral experiments in which fear conditioning naturally activated the PAG. The results provide insights into the neural circuits that enable an animal to be ready and able to react to danger, thus assisting in survival.

Neurons of the dopaminergic/calcitonin gene-related peptide A11 cell group modulate neuronal firing in the trigeminocervical complex: an electrophysiological and immunohistochemical study

Activation of spinal trigeminal afferents innervating the cranial vasculature is likely to play a role in migraine, although some parts of the clinical presentation may have a dopaminergic basis. The A11 nucleus, located in the posterior hypothalamus, provides the only known source of descending dopaminergic innervation for the spinal gray matter. Extracellular recordings were made in the trigeminocervical complex (TCC) in response to electrical stimulation of the dura mater. Receptive fields were characterized by mechanical noxious and innocuous stimulation of the ipsilateral ophthalmic dermatome. Stimulation of the A11 significantly inhibited peri-middle meningeal artery dural and noxious pinch evoked firing of neurons in the TCC. This inhibition was reversed by the D(2) receptor antagonist eticlopride. Lesioning of the A11 significantly facilitated dural and noxious pinch and innocuous brush evoked firing from the TCC. In previous work using immunohistofluorescence, it was shown that D(1) and D(2) receptors were found in the rat TCC, and here we report, in addition, that D(4) and D(5) dopamine receptors are also present, whereas D(3) receptors are not. No dopamine receptors were present in the A11 nucleus itself. However, the A11 does contain dopamine and calcitonin gene-related peptide (CGRP) and, by this combination, is distinct from the neighboring CGRPergic subparafascicular nucleus. Exploration of dopaminergic influences and mechanisms in migraine may open up an almost untapped opportunity to pursue potential new therapeutic options for the disorder.

Hypothalamic deep-brain stimulation: target and potential mechanism for the treatment of cluster headache

Recently, functional imaging data have underscored the crucial role of the hypothalamus in trigemino-autonomic headaches, a group of severe primary headaches. This prompted the application of hypothalamic deep-brain stimulation (DBS), with the intention to preventing cluster headache (CH) attacks in selected severe therapy-refractory cases. To date, a total of 50 operated intractable CH patients, one patient with short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing and three with atypical facial pain, have been reported. However, it is not apparent why the spontaneous bursts of activation in the inferior posterior hypothalamus result in excruciating head pain, whereas continuous electrical stimulation of the identical area is able to prevent these attacks. Recently, this issue has been addressed by examining 10 operated chronic CH patients, using H(2)(15)O-positron emission tomography and alternately switching the hypothalamic stimulator on and off. The stimulation-induced activation in the ipsilateral posterior inferior hypothalamic grey (the site of the stimulator tip) as well as activation and de-activation in several cerebral structures belonging to neuronal circuits usually activated in pain transmission. These data argue against an unspecific antinociceptive effect or pure inhibition of hypothalamic activity as the mode of action of hypothalamic DBS and suggest functional modulation of the pain-processing network.

Selective inhibition from the anterior hypothalamus of C- versus A-fibre mediated spinal nociception

Modulation of spinal nociception from the anterior hypothalamus/preoptic area (AH/POA), and consequent alterations in the pain experience may contribute to integrated responses brought into play during fear or stress and as part of the sickness response. This study was designed to compare the effects of descending control from AH/POA on A- versus C-fibre-evoked spinal nociception, since any differential control is of behavioural and clinical importance given that A-fibre and C-fibre nociceptors convey different qualities of the pain signal (first and second pain, respectively), and play different roles in the development and maintenance of chronic pain states. In anaesthetised rats, electromyographic responses were recorded to monitor thresholds of withdrawal to slow (2.5 degrees Cs(-1)) or fast (7.5 degrees Cs(-1)) rates of skin heating of the hindpaw, to preferentially activate C- or A-nociceptors, respectively. Neuronal activation by microinjection of dl-homocysteic acid at sites within a specific region of AH/POA, lateral area of the anterior hypothalamus (LAAH), significantly increased response thresholds to slow heating rates (p<0.02, n=11), but not those to fast rates of heating (p=0.48, n=10). Injection of DLH adjacent to LAAH (n=9) had no significant effect on responses to slow (n=8) or fast (n=9) rates of skin heating. The functional significance of differential descending control of spinal processing of C- and A-nociceptive inputs is discussed with respect to roles both of the LAAH in pain processing, and of C- and A-nociceptive inputs in acute and chronic pain.

The Hypothalamic Orexinergic System: Pain and Primary Headaches

The primary headaches are a group of distinct individually characterized attack forms, which although varying in presentation, share some common anatomical basis responsible for the pain component of the attack. The hypothalamus is known to modulate a multitude of functions and has been shown to be involved in the pathophysiology of a variety of primary headaches including cluster headache and chronic migraine. It seems likely that it may be involved in other primary headache disorders due to their episodic nature and may underlie many of their diverse symptoms. We discuss the hypothalamic involvement in the modulation of trigeminovascular processing and examine the involvement of the hypothalamic orexinergic system as a key regulator of this function.

Hypothalamic deep brain stimulation in positron emission tomography

Recently, functional imaging data have underscored the crucial role the hypothalamus plays in cluster headache, one of the most severe forms of primary headache. This prompted the application of hypothalamic deep brain stimulation. Yet, it is not apparent how stimulation of an area that is thought to act as a pace-maker for acute headache attacks is able to prevent these attacks from occurring. We addressed this issue by examining 10 operated chronic cluster headache patients, using H2(15O)-positron emission tomography and alternately switching the hypothalamic stimulator on and off. The stimulation induced activation in the ipsilateral hypothalamic gray (the site of the stimulator tip), the ipsilateral thalamus, somatosensory cortex and praecuneus, the anterior cingulate cortex, and the ipsilateral trigeminal nucleus and ganglion. We additionally observed deactivation in the middle temporal gyrus, posterior cingulate cortex, and contralateral anterior insula. Both activation and deactivation are situated in cerebral structures belonging to neuronal circuits usually activated in pain transmission and notably in acute cluster headache attacks. Our data argue against an unspecific antinociceptive effect or pure inhibition of hypothalamic activity. Instead, the data suggest a hitherto unrecognized functional modulation of the pain processing network as the mode of action of hypothalamic deep brain stimulation in cluster headache.

Variation and homogeneity in affective responses to physical activity of varying intensities: an alternative perspective on dose-response based on evolutionary considerations

A model for systematic changes in patterns of inter-individual variation in affective responses to physical activity of varying intensities is presented, as a conceptual alternative to the search for a global dose-response curve. It is theorized that trends towards universality will emerge in response to activities that are either generally adaptive, such as moderate walking, or generally maladaptive, such as strenuous running that requires anaerobic metabolism and precludes the maintenance of a physiological steady state. At the former intensity the dominant response will be pleasure, whereas at the latter intensity the dominant response will be displeasure. In contrast, affective responses will be highly variable, involving pleasure or displeasure, when the intensity of physical activity approximates the transition from aerobic to anaerobic metabolism, since activity performed at this intensity entails a trade-off between benefits and risks. Preliminary evidence in support of this model is presented, based on a reanalysis of data from a series of studies.

Prostaglandin E2 in the medial preoptic area produces hyperalgesia and activates pain-modulating circuitry in the rostral ventromedial medulla

Prostaglandin E2 (PGE2) produced in the medial preoptic region (MPO) in response to immune signals is generally accepted to play a major role in triggering the illness response, a complex of physiological and behavioral changes induced by infection or injury. Hyperalgesia is now thought to be an important component of the illness response, yet the specific mechanisms through which the MPO acts to facilitate nociception have not been established. However, the MPO does project to the rostral ventromedial medulla (RVM), a region with a well-documented role in pain modulation, both directly and indirectly via the periaqueductal gray. To test whether PGE2 in the MPO produces thermal hyperalgesia by recruiting nociceptive modulating neurons in the RVM, we recorded the effects of focal application of PGE2 in the MPO on paw withdrawal latency and activity of identified nociceptive modulating neurons in the RVM of lightly anesthetized rats. Microinjection of a sub-pyrogenic dose of PGE2 (50 fg in 200 nl) into the MPO produced thermal hyperalgesia, as measured by a significant decrease in paw withdrawal latency. In animals displaying behavioral hyperalgesia, the PGE2 microinjection activated on-cells, RVM neurons thought to facilitate nociception, and suppressed the firing of off-cells, RVM neurons believed to have an inhibitory effect on nociception. A large body of evidence has implicated prostaglandins in the MPO in generation of the illness response, especially fever. The present study indicates that the MPO also contributes to the hyperalgesic component of the illness response, most likely by recruiting the nociceptive modulating circuitry of the RVM.

Actions of oxytocin and interactions with glutamate on spontaneous and evoked dorsal spinal cord neuronal activities

Among the numerous pain control mechanisms that have been proposed, those acting at the spinal cord have been broadly studied, but little is known about how neuropeptides originating in supraspinal structures may relate to pain and analgesic mechanisms. Oxytocin (OT), in addition to its well known hormonal action, produces neuronal effects in various regions of the central nervous system. Indeed, some parvocellular neurons in the hypothalamic paraventricular nucleus (PVN) are oxytocinergic and project to the caudal part of the brain and the spinal cord. Moreover, the rat spinal cord shows a good overlap between the oxytocinergic hypothalamo-spinal neuron projections and the distribution of OT binding sites. However, the physiological significance of these binding sites is largely unknown. Extracellular unit activity of spinal cord neurons was recorded at the T13-L1 levels in male rats anesthetized with halotane. Somatic stimulation was applied to the inner and outer thigh of the ipsilateral hindpaw, and glutamate (GLU) and OT were locally delivered by pressure using pipettes coupled to recording electrodes. Our results show that spinal cord neurons, mainly located in the dorsal horn, in the intermediolateral cell column (IML) and in the intermediomedial gray matter (IMM), respond to the application of OT (71.5%) with activation (48%) or inhibition (52%). In some cases, opposite OT effects were observed during simultaneous recordings of two cells, suggesting OT activation of an inhibitory interneuron followed by the inhibition of the second recorded neuron. Increases in neuronal firing rate produced by GLU could be blocked by prior OT application. Finally, OT could reduce or partially block the responses to tactile and nociceptive somatic stimulation. We found that spinal cord neurons are sensitive to OT indicating that OT binding sites are functionally active. OT effects suggest the activation of inhibitory interneurons acting on a second order projecting cells to modulate afferent tactile and nociceptive information.

Biphasic modulation of spinal visceral nociceptive transmission from the rostroventral medial medulla in the rat

Descending inhibitory and facilitatory influences from the rostroventral medulla (RVM) on responses of lumbosacral spinal neurons to noxious colorectal distension (CRD, 80 mmHg, 20 s) were studied. At 25 sites in the RVM, electrical stimulation produced biphasic effects, facilitating responses of spinal neurons to CRD at lesser intensities of stimulation (5-25 microA) and inhibiting responses of the same neurons at greater intensities of stimulation (50-100 microA). At 38 other sites in the RVM, electrical stimulation produced only intensity-dependent inhibition of neuron responses to CRD. At another 13 sites in the RVM, electrical stimulation (5-100 microA) produced only facilitatory effects on responses to CRD. Descending modulatory effects were selective for distension-evoked activity; spontaneous activities of the same spinal neurons were not significantly affected by electrical stimulation that either facilitated or inhibited neuron responses to CRD. Neuron responses to graded CRD (20-100 mmHg) were positively accelerating functions that were shifted leftward or rightward, respectively, by lesser, facilitatory intensities or greater, inhibitory intensities of RVM stimulation. L-glutamate microinjection into the RVM replicated the effects of electrical stimulation, producing similar biphasic modulatory effects as produced by electrical stimulation. Microinjection of glutamate into the RVM at a low dose (5 nmoles) facilitated responses of spinal neurons to CRD and inhibited responses of the same neurons at a greater dose (50 nmoles). In some experiments, microinjection of lidocaine (0.5 microl of 4% solution) or the neurotoxin ibotenic acid (0.5 microl, 10 microg) into the RVM produced reversible or long-lasting, respectively, decreases in spontaneous activity and responses of spinal neurons to CRD. These results reveal that spinal visceral nociceptive transmission is subject to a tonic descending excitatory influence from the RVM and that descending modulatory effects from the RVM on visceral nociceptive transmission are qualitatively similar to modulation of cutaneous nociceptive transmission.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Extracellular signal-regulated kinases 1 and 2 phosphorylated neurons in the tele- and diencephalon of rat after visceral pain stimulation: an immunocytochemical study

We aimed at verifying whether extracellular signal-regulated kinases (erks) 1 and 2 are activated, i.e. phosphorylated, in forebrain neurons after visceral pain stimulation (VPS). Ether and urethane anaesthetized rats received an intraperitoneal injection of acetic acid or were left untreated (ECT, UCT). After 2 h the animals were perfused. Paraffin embedded brain sections immunoreacted with an antibody selective for the phosphorylated erks. The light microscope analysis revealed only a few labelled neurons in ECT, while in UCT, positive cells were detected. In VPS rats (VPSR) positive cells were mainly distributed in regions, such as the hypothalamic anterior and thalamic paraventricular midline nuclei, amygdala, hippocampal and parahippocampal, insular and perirhinal cortex, involved in nociception and/or visceral activities. Our data suggest an association of erks activation with the emotional component of nociception; moreover, they show that erks activation is not suppressed by anaesthesia.

Inhibitory effects evoked from the rostral ventrolateral medulla are selective for the nociceptive responses of spinal dorsal horn neurons

The aim of the present study was to determine whether or not descending control of spinal dorsal horn neuronal responsiveness following neuronal activation at pressor sites in the rostral ventrolateral medulla is selective for nociceptive information. Extracellular single-unit activity was recorded from 49 dorsal horn neurons in the lower lumbar spinal cord of anaesthetized rats. The 30 Class 2 neurons selected for investigation responded to noxious (pinch and radiant heat) and non-noxious (prod, stroke and/or brush) stimulation within their cutaneous receptive fields on the ipsilateral hindpaw. The excitatory amino acid, DL-homocysteic acid, was microinjected into either the rostral or the caudal rostral ventrolateral medulla at sites that evoked increases in arterial blood pressure. Effects of neuronal activation at these sites were then tested on the responses of Class 2 neurons to noxious and non-noxious stimulation within their excitatory receptive fields. The noxious pinch and radiant heat responses of Class 2 neurons were depressed, respectively to 13+/-3.8% (n=23) and to 16+/-3.7% (n=18) of control, following stimulation at sites in the rostral rostral ventrolateral medulla. In contrast, the low-threshold (prod) responses of eight Class 2 neurons tested were not depressed following neuronal activation at the same sites. When tested, control injections of the inhibitory amino acid, GABA, at the same sites in the rostral rostral ventrolateral medulla had no significant effects on neuronal activity. Neither intravenous administration of noradrenaline (to mimic the pressor responses evoked by DL-homocysteic acid microinjections in the rostral ventrolateral medulla) nor activation at pressor sites in the caudal rostral ventrolateral medulla had any significant effect on neuronal responsiveness. With regard to sensory processing in the spinal cord, these data suggest that descending inhibitory control that originates from neurons in pressor regions of the rostral rostral ventrolateral medulla is highly selective for nociceptive inputs to Class 2 neurons. These data are discussed in relation to the role of the rostral ventrolateral medulla in executing the changes in autonomic and sensory functions that are co-ordinated by higher centres in the CNS.

Visceral inputs to neurons in the anterior hypothalamus including those that project to the periaqueductal gray: a functional anatomical and electrophysiological study

The present study was designed to examine peripheral, in particular noxious visceral, inputs to neurons in the hypothalamus that project to the midbrain periaqueductal gray. The induction of Fos protein was used to localize hypothalamic neurons that were activated by noxious visceral stimulation. This was combined with retrograde transport of fluorescent latex microspheres from identified "pressor" and "depressor" sites in the dorsolateral/lateral or ventrolateral columns of the periaqueductal gray. A second series of electrophysiological experiments examined the receptive field characteristics, including the incidence of viscerosomatic convergence, of neurons in the ventral part of the anterior hypothalamus. Noxious visceral stimulation (intraperitoneal acetic acid) induced Fos-like immunoreactivity in significantly more neurons in the hypothalamus than control stimuli (intraperitoneal saline and intravenous phenylephrine). Particularly high numbers of Fos-positive neurons were found in the paraventricular nucleus, the supraoptic nucleus and ventral regions of the anterior hypothalamus. When combined with retrograde tracing from "depressor" sites in the ventrolateral periaqueductal gray, the highest numbers of double-labelled neurons were localized in the paraventricular nucleus and the lateral area of the anterior hypothalamus. However, the regions that contained the greatest proportions of Fos-positive neurons that projected to "depressor" sites in the ventrolateral periaqueductal gray were the lateral area of the anterior hypothalamus and its rostral extension, the lateral preoptic area. Fewer double-labelled neurons were localized in the hypothalamus after retrograde transport from sites in the dorsolateral/lateral periaqueductal gray compared to the results obtained from injections of tracer in the ventrolateral periaqueductal gray. Furthermore, the numbers of Fos-positive hypothalamic neurons that projected to the dorsolateral/lateral periaqueductal gray were very similar in experimental and control animals. The electrophysiological study confirmed that a large proportion of neurons in and around the lateral area of the anterior hypothalamus can be driven by noxious visceral stimulation and demonstrated a high incidence of viscerosomatic convergence in these cells (66% of cells driven from somatic structures were also driven by electrical stimulation of the splanchnic nerve). Somatic receptive fields of these neurons were generally large, often including all four limbs and the face. The results of the functional anatomical and electrophysiological studies have identified neurons in an area of the ventral anterior hypothalamus that are a focus of nociceptive visceral input and which project to the midbrain periaqueductal gray, in particular to its ventrolateral column. These results are discussed in relation to the roles of the anterior hypothalamus and the different longitudinal columns of the periaqueductal gray in co-ordinating autonomic and sensory functions in response to visceral pain.

Excitatory projections from the anterior hypothalamus to periaqueductal gray neurons that project to the medulla: a functional anatomical study

The present study was designed to investigate the organization of excitatory projections from regions of the anterior hypothalamus that are known to co-ordinate autonomic and sensory functions to medullo-output neurons in the periaqueductal gray. The induction of Fos protein was used to identify neurons in the periaqueductal gray that were activated synaptically by chemical stimulation at sites in the anterior hypothalamus from which either increases or decreases in arterial blood pressure were evoked (pressor sites and depressor sites, respectively). This was combined with retrograde tracing using fluorescent latex microspheres from sites in the medulla. When compared to control animals, neuronal activation at pressor sites in the anterior hypothalamus evoked Fos-like immunoreactivity in significantly more neurons in all but one sub-division of the periaqueductal gray (P at least < 0.05). The majority of Fos-positive neurons following a pressor response were located in the caudal half of the periaqueductal gray where significantly more neurons contained Fos-like immunoreactivity in lateral than in any other sub-division (P < 0.01). In all but two of 14 subdivisions of the periaqueductal gray, the numbers of neurons that expressed Fos-like immunoreactivity following stimulation at depressor sites in the anterior hypothalamus were not significantly different from controls. When neuronal activation at pressor or depressor sites in the anterior hypothalamus was combined with retrograde tracing from the rostral ventrolateral medulla, nucleus raphe magnus and/or nucleus raphe obscurus the majority of double-labelled neurons were located in the caudal half of the periaqueductal gray. Comparisons between the numbers of double-labelled neurons that resulted from different combinations of hypothalamic and medullary injection sites revealed that neuronal activation at pressor sites in the anterior hypothalamus combined with retrograde tracing from the rostral ventrolateral medulla resulted in the greatest numbers of double-labelled neurons. The identification of double-labelled neurons indicates that medullo-output neurons in the periaqueductal gray receive excitatory inputs predominantly from pressor compared to depressor sites in the anterior hypothalamus. These results are discussed in relation to the roles of the different longitudinal columns of the periaqueductal gray, and the organisation of their projections to the medulla, in the co-ordination of autonomic and sensory functions.

Distribution of neurons in the anterior hypothalamic nucleus activated by blood pressure changes in the rat

Electrophysiological and Fos-like protein immunocytochemical methods were used to identify the number and distribution of anterior hypothalamic neurons that are activated by changes in arterial pressure. First, in anesthetized, male Sprague-Dawley rats, arterial pressure increases and decreases led to differential activation of neurons in the anterior hypothalamic nucleus. Most of the units that responded to a rise in arterial pressure with a decrease in activity (pressor units) were located in the central part of the anterior hypothalamic nucleus, whereas units that increased firing when arterial pressure rose (the depressor units) were found throughout the nucleus. Second, in awake, male Sprague-Dawley rats, Fos-like protein immunoreactivity was mapped following sustained arterial pressure changes. Within the anterior hypothalamus, reduction in arterial pressure increased the number of Fos-labeled neurons primarily in the paraventricular nucleus and to a lesser extent in the anterior half of the anterior hypothalamic nucleus. In contrast, elevation in arterial pressure increased Fos labeling throughout the anterior hypothalamic nucleus and to a lesser extent in the paraventricular nucleus.

Oxytocin modulates glutamatergic synaptic transmission between cultured neonatal spinal cord dorsal horn neurons

The functional characteristics of binding sites for the neuropeptide oxytocin (OT) detected by radioautography in laminae I and II of the dorsal horn (DH) and on cultured neonatal DH neurons were studied on the latter using perforated patch-clamp recordings. The neurons were identified by their spike discharge properties and on the basis of the presence of met-enkephalin-like and glutamate decarboxylase-like immunoreactivities. OT (100 nM) never induced any membrane current at a holding potential of -60 mV but increased the frequency of spontaneously occurring AMPA receptor-mediated EPSCs or the mean amplitude of electrically evoked EPSCs in a subset (35%) of neurons. The frequency of miniature EPSCs (m-EPSCs) recorded in the presence of 0.5 microM tetrodotoxin was also increased by OT (100 nM) without any change in their mean amplitude, indicating an action at a site close to the presynaptic terminal. The decay kinetics of any type of EPSC were never modified by OT. The effect of OT was reproduced by [Thr4, Gly7]-OT (100 nM), a selective OT receptor agonist, and blocked by d(CH2)5-[Tyr(Me)2,Thr4,Tyr-NH29]-ornithine vasotocin (100 nM), a specific OT receptor antagonist. Reducing the extracellular Ca2+ concentration from 2.5 to 0.3 mM in the presence of Cd2+ (100 microM) reversibly blocked the effect of OT on m-EPSCs. The OT receptors described here may represent the substrate for modulatory actions of descending hypothalamo-spinal OT-containing pathways on the nociceptive system.