Injection of solutions into cerebral tissue: Relation between volume and diffusion

Prefrontal cortical representation of visuospatial working memory in monkeys examined by local inactivation with muscimol

In primates, dorsolateral areas of the prefrontal cortex (PFC) play a major role in visuospatial working memory. To examine the functional organization of the PFC for representing visuospatial working memory, we produced reversible local inactivation, with the local injection of muscimol (5 microg, 1 microl), at various sites (n = 100) in the dorsolateral PFC of monkeys and observed the behavioral consequences in an oculomotor delayed-response task that required memory-guided saccades for locations throughout both visual fields. At 82 sites, the local injection of muscimol induced deficits in memory-guided saccades to a few specific, usually contralateral, target locations that varied with the location of the injection site. Such deficits depended on the delay length, and longer delays were associated with larger deficits in memory-guided saccades. The injection sites and affected spatial locations of the target showed a gross topographical relationship. No deficits appeared for a control task in which the subject was required to make a visually guided saccade to a visible target. These findings suggest that a specific site in the dorsolateral PFC is responsible for the working memory process for a specific visuospatial coordinate to guide goal-directed behavior. Further, memoranda for specific visuospatial coordinates appear to be represented in a topographical memory map within the dorsolateral PFC to represent visuospatial working memory processes.

Reversible analgesia, atonia, and loss of consciousness on bilateral intracerebral microinjection of pentobarbital

Concussion, asphyxia, and systemically administered general anesthetics all induce reversible depression of the organism's response to noxious stimuli as one of the elements of loss of consciousness. This is so even for barbiturate anesthetics, which have only modest analgesic efficacy at subanesthetic doses. Little is known about the neural circuits involved in this form of antinociception, although for anesthetic agents, at least, it is usually presumed that the drugs act in widely distributed regions of the nervous system. We now report the discovery of a focal zone in the brainstem mesopontine tegmentum in rats at which microinjection of minute quantities of pentobarbital induces a transient, reversible anesthetic-like state with non-responsiveness to noxious stimuli, flaccid atonia, and absence of the righting reflex. The behavioral suppression is accompanied by slow-wave EEG and, presumably, loss of consciousness. This zone, which we refer to as the mesopontine tegmental anesthesia locus (MPTA), apparently contains a barbiturate-sensitive 'switch' for both cortical and spinal activity. The very existence of the MPTA locus has implications for an understanding of the neural circuits that control motor functions and pain sensation, and for the cerebral representation of consciousness.

Sleep inducing effects of propofol microinjection into the medial preoptic area are blocked by flumazenil

The intravenous anesthetic, propofol, has been shown to increase sleep when microinjected into the medial preoptic area (MPA) of the rat. Similar increases in sleep have also been observed with triazolam, pentobarbital and ethanol microinjection. Together, these findings implicate the MPA as an important anatomic site mediating the effects of sedatives on naturally occurring sleep. Although the molecular mechanism by which propofol in the MPA acts to induce sleep is unclear, potentiating effects on the GABA(A) receptor complex may play a role. To assess this possibility, we microinjected propofol alone, and in combination with the benzodiazepine receptor antagonist flumazenil, into the MPA. At a dose of 0.76 microg, flumazenil had no effect on sleep when given alone, and completely blocked the increase in sleep caused by a 40-ng dose of propofol although it did not affect the increase in sleep caused by an 80-ng dose of propofol. These data suggest that the sleep inducing property of propofol is in part mediated by direct or indirect actions on the GABA(A)-benzodiazepine receptor complex.

Modulation of pain by [1DMe]NPYF, a stable analogue of neuropeptide FF, in neuropathic rats

The pain modulatory effects of (D-Tyr)L(Me-Phe)QPQRF-amide ([1DMe]NPYF), a stable analogue of neuropeptide FF were studied in rats with a chronic neuropathy induced by unilateral ligation of two spinal nerves. According to behavioral assessments, intrathecal (i.t.) administration of [1DMe]NPYF induced mechanical antiallodynic and thermal antinociceptive effects in a parallel and dose-dependent fashion, whereas following administration in the periaqueductal gray (PAG) it produced only mechanical antiallodynia. I.t. or PAG administration of FLFQPQRF, a non-amidated form of NPFF, or intraplantar injection of [1DMe]NPYF into the neuropathic paw had no effects. Electrophysiological results indicated that administration of [1DMe]NPYF suppressed responses of nociceptive spinal dorsal horn neurons in a submodality selective way and without an effect on their spontaneous activity; PAG administration predominantly suppressed brush-evoked responses and i.t. administration heat-evoked responses. The descending inhibitory effect by conditioning electrical stimulation of the PAG was enhanced by i.t. administration of [1DMe]NPYF. The reversibility of [1DMe]NPYF-induced effects by naloxone (1 mg/kg subcutaneously) depended on the submodality of test stimulation and the route of drug administration. The amplitude of the innocuous H-reflex was not changed by [1DMe]NPYF administered i.t. in control rats. The present results indicate that [1DMe]NPYF produces a selective attenuation of pain in neuropathic animals due to naloxone-sensitive or -insensitive central mechanisms depending on the submodality of pain and route of drug administration. The amide-group is essential for the [1DMe]NPYF-induced attenuation of pain.

Changes in the auditory-evoked potentials induced by fear-evoking stimulations

It is long established that the inferior colliculus is involved in conveying all kinds of auditory information to higher cortical structures. Moreover, gradual increases in the electrical stimulation of this structure produces progressive aversive responses from vigilance, through freezing, until escape. Recently, we have shown that microinjections of the excitatory amino acids, N-methyl-D-aspartate (NMDA) and glutamate, into the inferior colliculus mimic these aversive effects. In the present study, we extend these observations showing that unilateral microinjections of 5 nmol of glutamate into the inferior colliculus--a dose that causes freezing behavior--in rats with bilateral recording electrodes into this structure produce an increase in the magnitude of the collicular-evoked potential in the ipsilateral side of the injection in relation to saline-injected animals. Besides, the application of two kinds of fear-evoking stimulations--light as a conditioned stimuli (CS) and ultrasound signals at the frequency of 22 kHz--also produced an increase in the amplitude of the evoked potentials recorded from the inferior colliculus in comparison to control situations without aversive stimuli presentations. These data support previous reports showing that fast-acting excitatory amino acid receptors in this midbrain region are involved in the processing of auditory information. Moreover, fear-eliciting stimulations, such as light-CS and ultrasound signals, increase acoustically evoked firing of neurons in the central nucleus of the inferior colliculus of rats.

Effects of microinjections of triazolam into the ventrolateral preoptic area on sleep in the rat

In view of interest in the ventrolateral preoptic area (VLPO), based on FOS protein accumulation during sleep as well as its output pathways to areas involved in sleep regulation, we have examined the effects of microinjections of triazolam into the VLPO. It was found that two doses of triazolam, noted previously to enhance sleep when injected into the medial preoptic area, had no significant effect on sleep or core temperature when administered into the VLPO. Although these data do not bear on the possibility that the VLPO is involved in physiological sleep regulation, they suggest that it is not a site of the pharmacologic action of hypnotic benzodiazepines.

Pre- or post-training injection of buspirone impaired retention in the inhibitory avoidance task: involvement of amygdala 5-HT1A receptors

The present study investigated the effect of buspirone on memory formation in an aversive learning task. Male Wistar rats were trained on the inhibitory avoidance task and tested for retention 1 day after training. They received peripheral or intra-amygdala administration of buspirone or other 5-HT1A drugs either before or after training. Results indicated that pretraining systemic injections of buspirone caused a dose-dependent retention deficit; 5. 0 mg/kg had a marked effect and 1.0 mg/kg had no effect. Post-training injections of the drug caused a time-dependent retention deficit, which was not due to a state-dependent effect on retrieval. When training in the inhibitory avoidance task was divided into a context-training phase and a shock-training phase, buspirone impaired retention only when administered in the shock-training phase, suggesting that the drug influenced memory processing of affective events. Further results indicated that post-training intra-amygdala infusion of buspirone or the 5-HT1A agonist 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT) caused a time-dependent and dose-dependent retention deficit. Post-training intra-amygdala infusion of the 5-HT1A antagonist WAY100635 (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)-N-(2-pyridyl) cyclohexane carboxamine maleate) attenuated the memory-impairing effects of buspirone. These findings suggest that buspirone may modulate memory storage processes in the inhibitory avoidance task through an action on amygdaloid 5-HT1A receptors.

The advantages and limitations of permanent or reversible deactivation techniques in the assessment of neural function

This review considers the different forms of reversible deactivation and how they differ from each other and from more conventional permanent deactivation methods. The different methods of chemical and cooling reversible inactivation and the limitations of each are discussed. Finally, future directions in the quest to localize brain function in the experimental animal are considered and how this work relates to emerging non-invasive imaging techniques presently being used with humans.

Pharmacological inactivation in the analysis of the central control of movement

In this review, we describe how pharmacological inactivation can be used to elucidate the central control of skilled limb movement. Local anesthetics and tetrodotoxin block neuronal cell bodies and passing fibers while gamma-aminobutyric acid (GABA) and muscimol only block cell bodies. Blockade induction time is short (several minutes) for all the agents. Blockade duration produced by local anesthetics and GABA is 15-60 min, while that of tetrodotoxin and muscimol is up to several days. We describe our drug injection system, with an integrated microelectrode and a viewing port for visually monitoring drug flow into the injection cannula. We used glucose metabolism to assess the extent of inactivation. Intracortical lidocaine or muscimol injection produces a central core of maximal hypometabolism (1 mm radius), which could be due to drug spread, surrounded by an extensive region (several millimeters) of reduced hypometabolism, possibly due to reduced synaptic activity of neurons receiving projections from the core region. Drug injection only depresses neuronal activity, which contrasts with cooling, where there can be neuronal hyperexcitability at the periphery of the inactivation site. Our experiments in behaving animals show how pharmacological inactivation is an effective analytical tool for dissecting the differential functional contributions of subcortical and cortical forelimb representations to limb movement control.

Muscarinic receptors mediate carbachol-induced inhibition of maximal electroshock seizures in the nucleus reticularis pontis oralis

PURPOSE

Previous reports from this laboratory indicated a role for N-methyl-D-aspartic acid (NMDA) and gamma-aminobutyric acid (GABA) receptors among the neuronal mechanisms of the nucleus reticularis pontis oralis (RPO) that regulate the tonic hindlimb extension (THE) component of maximal electroshock seizures (MESs) in rats. This study was intended to determine the role of cholinergic mechanisms in the RPO regulation of THE.

METHODS

Rats were surgically prepared with microinjection guide cannulas for the focal administration of drug solutions directly into the RPO. MES was induced with corneal electrodes.

RESULTS

RPO microinjection of carbachol significantly inhibited the incidence of THE. RPO microinjection of atropine by itself had no effect on the seizure response but significantly antagonized the anticonvulsant effect induced by RPO microinjection of carbachol. The selective nicotinic agonist dimethylpiperizinium (DMPP) by itself had no effect on THE. RPO microinjection of 10 ng pertussis toxin by itself had no effect on THE but significantly antagonized the anticonvulsant effect induced by RPO microinjection of carbachol.

CONCLUSIONS

RPO microinjection of carbachol inhibited the THE component of MESs in rats. The carbachol effect appeared to be mediated by muscarinic receptors as the anticonvulsant activity was antagonized by atropine, and the selective nicotinic agonist DMPP induced no anticonvulsant activity. Because pertussis toxin acts to inhibit muscarinic receptor-linked G proteins, the pertussis toxin antagonism of carbachol also supports a muscarinic mechanism of action.

Reversible inactivation of subcortical sites by drug injection

Reversible inactivation of subcortical targets by means of drug injections has been a powerful tool for revealing the contributions of discrete brain structures to behavior and the functional organization of the brain. This paper is intended to provide practical advice on this approach, including the choice of drug, means of delivering drugs, strategies for evaluating the action of injected drugs, and the application of this method to experiments with awake animals.

Extended local access fibers: adjustable treatment of deep sites in the brain

We describe a minimally invasive method for administering diffusible substances to computer-targeted regions of the primate brain. Treatments enter the brain by diffusion or under pressure from the portion of a curvilinear implant that lies within the region of interest. During the implantation surgery, a guide-tube cannula is passed through a cranial burr-hole and concentric, telescoping needles are extended from the cannula in sequence to trace a pre-planned course through brain tissue. After the leading end of the longest needle emerges through another burr-hole, the surgeon fastens a hollow dialysis fiber to its tip and draws the fiber into the brain by retracting the telescoping device in an orderly sequence. The surgery affects brain tissue only along the course of the fiber, causing about the same acute damage as the stereotactic introduction of an ordinary straight needle. The 'extended local access fiber' is relatively soft, flexible, and biocompatible. It remains permanently in the brain, with its semipermeable portion lodged in the target region. Leading and trailing portions are accessible from outside the brain. Experiments indicate a broad range of possible trajectories and confirm that substances delivered by access fiber can have physiological effects even 10 weeks after implantation.

A differential modulation of allodynia, hyperalgesia and nociception by neuropeptide FF in the periaqueductal gray of neuropathic rats: interactions with morphine and naloxone

The effect of neuropeptide FF in the periaqueductal gray on pain behaviour was studied in rats with a chronic neuropathy induced by unilateral ligation of two spinal nerves. Neuropeptide FF produced in a non-monotonic fashion a significant attenuation of tactile allodynia. The antiallodynic effect was not significantly modulated by naloxone administered systemically or intracerebrally. The dose of neuropeptide FF producing a significant antiallodynic effect was not antinociceptive in a test of mechanical or thermal nociception. The thermal antinociceptive effect induced by morphine administered in the periaqueductal gray was significantly attenuated by neuropeptide FF, whereas that induced by systemically administered morphine was not. The interaction of neuropeptide FF with intracerebrally or systemically administered morphine in a test of tactile allodynia was not significant. The results indicate that neuropeptide FF in the periaqueductal gray may produce a selective attenuation of tactile allodynia in neuropathic rats. This antiallodynic effect is at least partly independent of naloxone-sensitive opioid receptors. Furthermore, neuropeptide FF in the periaqueductal gray attenuates antinociception induced by intracerebrally but not systemically administered morphine.

Anxiolytic-like action of neurokinin substance P administered systemically or into the nucleus basalis magnocellularis region

There is evidence that the neurokinin substance P plays a role in neural mechanisms governing learning and reinforcement. Reinforcing and memory-promoting effects of substance P were found after it was injected into several parts of the brain and intraperitoneally. With regard to the close link between anxiety and memory processes for negative reinforcement learning, the aim of the present study was to gauge the effect of substance P on anxiety-related behaviors in the rat elevated plus-maze and social interaction test. Substance P was tested at injection sites where the neurokinin has been shown to promote learning and to serve as a reinforcer, namely in the periphery (after i.p. administration) and after injection into the nucleus basalis magnocellularis region. When administered i.p., substance P had a biphasic dose-response effect on behavior in the plus-maze with an anxiolytic-like action at 50 microg/kg and an anxiogenic-like one at 500 microg/kg. After unilateral microinjection into the nucleus basalis magnocellularis region, substance P (1 ng) was found to exert anxiolytic-like effects, because substance P-treated rats spent more time on the open arms of the plus-maze and showed an increase in time spent in social interaction. Furthermore, the anxiolytic effects of intrabasalis substance P were sequence-specific since injection of a compound with the inverse amino acid sequence of substance P (0.1 to 100 ng) did not influence anxiety parameters. These results show that substance P has anxiolytic-like properties in addition to its known promnestic and reinforcing effects, supporting the hypothesis of a close relationship between anxiety, memory and reinforcement processes.

Control of size and excitability of mechanosensory receptive fields in dorsal column nuclei by homolateral dorsal horn neurons

Both accidental and experimental lesions of the spinal cord suggest that neuronal processes occurring in the spinal cord modify the relay of information through the dorsal column-lemniscal pathway. How such interactions might occur has not been adequately explained. To address this issue, the receptive fields of mechanosensory neurons of the dorsal column nuclei were studied before and after manipulation of the spinal dorsal horn. After either a cervical or lumbar laminectomy and exposure of the dorsal column nuclei in anesthetized cats, the representation of the hindlimb or of the forelimb was defined by multiunit recordings in both the dorsal column nuclei and in the ipsilateral spinal cord. Next, a single cell was isolated in the dorsal column nuclei, and its receptive field carefully defined. Each cell could be activated by light mechanical stimuli from a well-defined cutaneous receptive field. Generally the adequate stimulus was movement of a few hairs or rapid skin indentation. Subsequently a pipette containing either lidocaine or cobalt chloride was lowered into the ipsilateral dorsal horn at the site in the somatosensory representation in the spinal cord corresponding to the receptive field of the neuron isolated in the dorsal column nuclei. Injection of several hundred nanoliters of either lidocaine or cobalt chloride into the dorsal horn produced an enlargement of the receptive field of the neuron being studied in the dorsal column nuclei. The experiment was repeated 16 times, and receptive field enlargements of 147-563% were observed in 15 cases. These data suggest that the dorsal horn exerts a tonic inhibitory control on the mechanosensory signals relayed through the dorsal column-lemniscal pathway. Because published data from other laboratories have shown that receptive field size is controlled by signals arising from the skin, we infer that the control of neuronal excitability, receptive field size and location for lemniscal neurons is determined by tonic afferent activity that is relayed through a synapse in the dorsal horn. This influence of dorsal horn neurons on the relay of mechanosensory information through the lemniscal pathways must modify our traditional views concerning the relative independence of these two systems.

Blockade of the oestrogen-induced luteinizing hormone surge in ovariectomized ewes by a highly selective opioid mu-receptor agonist: evidence for site of action

Endogenous opioid systems in the hypothalamus inhibit gonadotropin-releasing hormone (GnRH) secretion, and a reduction in this inhibitory input (disinhibition) is thought to be part of the neural mechanism of the preovulatory GnRH/luteinizing hormone (LH) surge. We showed previously that intracerebroventricular infusion of the highly specific opioid mu-receptor agonist DAMGO delayed the oestrogen-induced LH surge in ovariectomized (OVX) ewes, whereas both delta- and kappa-agonists were ineffective. The aim of the present study was to establish the anatomical site of this effect. The most likely hypothalamic sites of action are the medial preoptic area (MPOA), where most GnRH perikarya are located in sheep, and/or the median eminence (ME), where GnRH fibres terminate on hypophysial portal blood vessels. Conscious, unrestrained OVX ewes with permanent bilateral guide tubes implanted into either the MPOA or the mediobasal hypothalamus (MBH), close to the ME, were injected (i.m.) with oestradiol benzoate (EB) 50 microg (t = 0 h). In this model, EB elicits a time-delayed surge in LH secretion after 13-19 h. Jugular venous blood was sampled at half-hourly intervals from -2 to 0 h, and from 10 to 26 h. From 12 to 20 h, bilateral infusions of either the highly specific opioid mu-agonist DAMGO (40 nmol/h bilaterally) or saline were given into the MPOA or MBH at 2.5 microl/h. Guide tube placements were confirmed histologically. The mean (+/- SEM) time to the onset of the LH surge was significantly (p < 0.01) increased in the animals (n = 9) that received DAMGO infusion into the MPOA (20.5 +/- 1.4 vs. 15.7 +/- 0.6 h in the saline-infused controls). The effect was clearly apparent in 6/9 of the DAMGO-infused animals. The mean (+/- SEM) time to LH surge onset was also significantly (p < 0.01) increased in the animals (n = 8) that received DAMGO infusion into the MBH (19.7 +/- 1.2 vs. 14.3 +/- 0.5 h). In this case, the effect was clearly apparent in 4/8 of the DAMGO-infused animals. We conclude that bilateral infusion of DAMGO into either the MPOA or the MBH can delay the EB-induced LH surge in OVX ewes. These data provide further evidence for dual hypothalamic sites of opioid regulation of GnRH secretion, and are consistent with the hypothesis that disinhibition from opioid tone at both the MPOA and MBH/ME is permissive of the preovulatory GnRH/LH surge.

Organization of reaching and grasping movements in the primate cerebellar nuclei as revealed by focal muscimol inactivations

Two monkeys were trained to point to targets and to retrieve fruit bits from a Kluver board, bottles, and tubes. Once proficient in the tasks, the macaques underwent aseptic surgical implantation of a recording chamber over the cerebellar nuclei on the side of their preferred hand. After recovery from surgery, a series of mapping penetrations were completed to identify task-related areas within the cerebellar nuclei. Muscimol (4- 16 microgram; 1-2 microgram/microliter) was pressure injected at different sites within the forelimb zone, and the resultant deficits were observed as the monkeys performed the behavioral tasks. Quantitative measures of task performance were supplemented by direct observation of live and videotaped performance. The locations of nuclear inactivation sites were reconstructed from marking lesions and tracks visible in histological sections. Injections placed in the cerebellar interpositus nucleus and adjacent regions of dentate caused a variety of deficits in forelimb function. A prominent anteroposterior specialization was apparent within the forelimb zone of this intermediate nuclear region. Injections into the anterior interpositus nucleus and adjacent dentate impaired preshaping of the hand and the manipulation of objects, whereas injections placed more posteriorly in posterior interpositus nucleus and adjacent dentate produced deficits in the aiming of reach and the stability of the arm. During anterior injections, the monkeys failed to adequately extend their fingers in preparation for target contact, as documented for >85% of the reaches in the pointing task of monkey J. Up to 38% of the fruit bits it attempted to retrieve from the Kluver board were dropped. In comparison, during posterior inactivations, 15% were dropped and during control conditions 3% were dropped. The monkeys made significantly greater pointing errors during posterior inactivations (11 times for monkey J and 4 times for monkey C) than during anterior inactivations (8 times for monkey J and 2 times for monkey C). We refer to the region producing hand deficits as the anterior hand zone and the region producing reaching deficits as the posterior reach zone. These results are discussed in relation to the problem of achieving spatiotemporal coordination in the large population of nuclear cells that participate in any given movement. The results do not favor the hypothesis that coordination is achieved through a selection of Purkinje cells along beams of parallel fibers. Instead, it is proposed that distal and proximal musculature is coordinated by the adaptive influences of climbing fiber input to Purkinje cells. We envision a relatively nonspecific recruitment of anterior and posterior nuclear cells due to positive feedback in the limb premotor network, which then is shaped into an appropriate spatiotemporal pattern of discharge through the inhibitory input from Purkinje cells.

Modulation of memory, reinforcement and anxiety parameters by intra-amygdala injection of cholecystokinin-fragments Boc-CCK-4 and CCK-8s

This series of experiments examined the effects of the cholecystokinin (CCK) fragments Boc-CCK-4 and CCK-8s on memory, reinforcement and anxiety following unilateral injection into the central nucleus of the amygdala (CeA). In experiment 1, rats with chronically implanted cannulae were injected with CCK-8s or Boc-CCK-4 and were tested on a one-trial uphill avoidance task. Post-trial injection of 20 ng Boc-CCK-4 or 1 ng CCK-8s was found to improve the retention performance, whereas lower and higher doses had no effect. The hypermnestic effects of Boc-CCK-4 and CCK-8s were no longer evident when injection was performed 5 h, rather than immediately, after the learning trial. In experiment 2, the elevated plus-maze was used to gauge anxiogenous properties of intra-amygdala injections of Boc-CCK-4 and CCK-8s in memory-enhancing doses. The treatment with 20 ng Boc-CCK-4 and 1 ng CCK-8s did not influence the number of entries into and time spent on the open and enclosed arms of the maze as well as other anxiety-related behaviors. In experiment 3, possible reinforcing effects of the CCK-fragments were examined. After intra-amygdala injection of Boc-CCK-4 or CCK-8s in memory-enhancing doses the rats were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. Subsequent tests for conditioned corral preference revealed no evidence for reinforcing or aversive effects of the CCK-fragments. In sum, these findings indicate that Boc-CCK-4 and CCK-8s facilitate memory processing upon injection into the CeA without exerting reinforcing or anxiogenous effects.

A neuronal correlate of secondary hyperalgesia in the rat spinal dorsal horn is submodality selective and facilitated by supraspinal influence

Tissue injury produces hyperalgesia not only in the injured area (primary hyperalgesia) but also outside of it (secondary hyperalgesia). In the present investigation, the submodality selectivity and the contribution of supraspinal influence to a neural correlate of the secondary hyperalgesia induced by neurogenic inflammation was studied in the presumed pain relay neurons of the rat spinal dorsal horn. Mechanically and thermally evoked responses to wide-dynamic range (WDR) neurons of the spinal dorsal horn were recorded under sodium pentobarbital anesthesia in rats. Neurogenic inflammation was induced by application of mustard oil outside of the receptive fields of WDR neurons. To study the contribution of supraspinal influence to mustard oil-induced changes in neuronal responses, the spinal cord was transected at a midthoracic level or lidocaine was microinjected into the rostroventromedial medulla (RVM). Furthermore, the antidromically evoked compound volley in the sural nerve was determined to reveal excitability changes in the central terminals of primary afferent A-fibers induced by mustard oil. The results indicate that mustard oil adjacent to the receptive fields of spinal WDR neurons significantly enhanced their responses to mechanical but not to noxious heat stimuli, without a significant influence on their spontaneous activity. Both high- and low-threshold mechanoreceptive input to WDR neurons was equally facilitated, whereas mechanoreceptive input to spinal dorsal horn neurons mediating innocuous messages (low-threshold mechanoreceptive neurons) was not changed. Mustard oil in a remote site (forepaw) did not produce any hyperexcitability to responses evoked by hindpaw stimulation. Spinal transection or lidocaine block of the RVM significantly attenuated the mustard oil-induced mechanical hyperexcitability in spinal dorsal horn neurons. Mustard oil had no significant effect on a compound volley in the sural nerve induced by intraspinal stimulation of sural nerve terminals at a submaximal intensity. The selective mechanical hyperexcitability in spinal WDR neurons, without a change in their spontaneous activity, can be explained by a heterosynaptic facilitatory action on presynaptic terminals mediating mechanical signals to these nociceptive spinal neurons. These findings indicate that brain stem-spinal pathways, involving the RVM, do not only suppress nociception but under some pathophysiological conditions concurrent facilitatory influence may predominate and lead to enhancement of mechanical hyperexcitability. The descending facilitatory feed-back loop to nociceptive spinal neurons may help to protect the wounded tissue and thus promote healing.

Neuropeptide Y-induced operant responding for sucrose is not mediated by dopamine

Neuropeptide Y (NPY) induces feeding in previously satiated animals after injection into the hypothalamus, especially the perifornical region (PFH). NPY also appears to have rewarding properties as evidenced by its ability to produce a conditioned place preference following injection into the nucleus accumbens (N.Acc), an effect ostensibly mediated by mesolimbic dopamine (DA). Since the progressive-ratio (PR) operant schedule has also been used to assess an organism's motivation to respond for rewarding stimuli, we tested the possibility that NPY increases PR responding for sucrose pellets. Adult male rats were injected with NPY (0-235 pmol) bilaterally through cannulae aimed to terminate in the PFH. This produced a dose-dependent increase in the total number of responses made and the number of reinforcers earned. The DA receptor blocker, alpha-flupenthixol (FLU)(0-0.2 mg/kg intraperitoneally), attenuated both NPY(156 pmol)-induced and drug-free PR responding while having no effect on NPY(156 pmol)-induced free-feeding. FLU injected directly into the N.Acc (0-5 microg) also failed to reduce sucrose free-feeding. These results suggest that distinct reward mechanisms are activated during PFH NPY-induced feeding vs. PR responding, since FLU disrupted the latter but not the former.

Characterization of biphasic modulation of spinal nociceptive transmission by neurotensin in the rat rostral ventromedial medulla

Modulation of spinal nociceptive transmission by neurotensin microinjected in the rostral ventromedial medulla (RVM) was examined in anesthetized, paralyzed rats. Forty-three spinal dorsal horn neurons in the L3-L5 spinal segments responding to mechanical and noxious thermal stimulation (50 degrees C) of the plantar surface of the ipsilateral hind foot were studied. Spinal units were classified as either wide dynamic range or nociceptive specific and were located in spinal laminae I-V. Microinjection of neurotensin (0.03 pmol/0.2 microl) into the RVM produced a significant facilitation (135% of control) of spinal unit responses to noxious thermal stimulation (50 degrees C) that lasted approximately 12 min. In contrast, injection of greater doses of neurotensin (300 or 3,000 pmol) produced an inhibition of spinal unit responses to noxious heat (51.7 and 10.6% of control, respectively) that had a longer duration (60-120 min). The effects of neurotensin on wide-dynamic-range and nociceptive-specific neuron responses to noxious heat were qualitatively and quantitatively similar. Spinal unit responses to graded heating of the skin (42-50 degrees C) were completely inhibited after microinjection of 3,000 pmol of neurotensin into the RVM. Injection of a lesser dose of neurotensin (300 pmol), however, resulted in a partial inhibition of spinal unit responses and significantly reduced the slope of the stimulus-response function to graded heating of the skin. Transection of either the ipsilateral or contralateral dorsolateral funiculus (DLF) significantly reduced the inhibition of spinal nociceptive transmission produced by neurotensin (3,000 pmol) in the RVM, whereas bilateral transection of the DLFs completely blocked the effect. In contrast, bilateral transection of the DLFs had no effect on facilitation of spinal nociception by neurotensin (0.03 pmol) in the RVM. The inhibition of spinal nociceptive transmission by neurotensin (3,000 pmol) in the RVM was completely blocked by injection of the nonpeptide neurotensin receptor antagonist SR48692 (30 fmol) into the RVM 10 min before neurotensin. To confirm a specific block of neurotensin-receptor-mediated effects by the antagonist, a subsequent injection of L-glutamate into the RVM was performed. L-Glutamate (100 nmol) was found to inhibit the nociceptive responses of those spinal units whose responses were no longer inhibited by neurotensin. In contrast, injection of SR48692 (30 fmol) into the RVM failed to block the facilitation of spinal unit responses to noxious heat produced by a subsequent injection of neurotensin (0.03 pmol) into the same site. The present series of experiments demonstrate a specific role for neurotensin in the RVM in the modulation of spinal nociceptive transmission, because the peptide was found to both facilitate and inhibit spinal neuron responses to noxious thermal stimulation. Additionally, the facilitatory and inhibitory effects of neurotensin appear to occur via interaction with multiple neurotensin receptors in the RVM that activate independent systems that descend in the ventrolateral funiculi and DLFs, respectively. The results from these experiments are consistent with prior studies demonstrating that the RVM both facilitates and inhibits spinal nociceptive transmission, and they complement previous work showing that neurotensin in the RVM modulates spinal nociceptive behavioral responses.

Increase of catechol-O-methyltransferase activity in rat brain microglia after intrastriatal infusion of fluorocitrate, a glial toxin

Striatal catechol-O-methyltransferase (COMT), monoamine oxidase B (MAO-B; an astroglial enzyme), alkaline phosphodiesterase I (PDE; a microglia/macrophage marker) and tyrosine hydroxylase (TH; catecholaminergic neuron marker) activities were analyzed biochemically 1-3 days after infusion of fluorocitrate, an astrocyte damaging agent. Astrocytes, microglia and neurons were stained immunohistochemically with specific antibodies (against glial fibrillary acidic protein, OX-42 and TH, respectively) and with COMT antiserum. Three days after fluorocitrate infusion the activity of MAO-B was reduced, whereas COMT and PDE activities were increased. The elevation of COMT immunoreactivity co-localized to microglial cells, but not to astrocytes. In conclusion, this is the first report indicating that microglia contains COMT activity which may be increased in pathological conditions.

Microinfusion of the metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid into the nucleus accumbens induces dopamine-dependent locomotor activation in the rat

Although the striatum has one of the highest densities of metabotropic glutamate receptor (mGluR) binding sites in the brain, little is known about their physiological role. In this study we characterized the contribution of mGluRs located in the ventral part of the striatum (the nucleus accumbens) to the control of extrapyramidal motor function. Activation of mGluRs by local infusion of the selective agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 25, 50 and 100 nmol/0.5 microl) into the nucleus accumbens induced a dose-dependent increase in locomotor activity in rats. Intra-accumbens infusion of a selective antagonist of mGluRs, alpha-methyl-4-carboxyphenylglycine (MCPG) did not modify spontaneous locomotion but decreased the locomotor response to 1S,3R-ACPD. This effect appeared to be mediated by dopamine, since blockade of dopamine receptors with haloperidol (0.05 and 0.1 mg/kg i.p.) dose-dependently reduced 1S,3R-ACPD-induced locomotor activation. Furthermore, D-amphetamine (0.5 mg/kg, i.p.) combined with intra-accumbens infusion of 1S,3R-ACPD (100 nmol) potentiated the locomotor hyperactivity response to a higher level than that seen with a single treatment with either drug. In contrast, D-amphetamine-induced hypermotility was abolished by infusion of MCPG (100 nmol) into the nucleus accumbens. These results demonstrate that glutamate may control extrapyramidal motor function through metabotropic receptors. Furthermore, activation of metabotropic glutamate receptors appears to act in synergy with the dopamine system at the level of the nucleus accumbens to produce a motor stimulant response.

Continuous and direct infusion of drug solutions in the brain of awake animals: implementation, strengths and pitfalls

One of the best strategies for understanding an animal's behavior is to study the function of the brain by experimentally modifying brain chemistry temporarily or on a long-term basis. This can be achieved by direct manipulation of neurochemistry of a targeted brain area with various drugs whose in vitro specificity and sensitivity are known. We assume that an animal's behavior is primarily controlled by the integrated performance of neural networks, rather than the action of a "superstar" single neuron which has narrowly tuned selectivity, in a specified brain region. Therefore, the former must be regulated by a large number of combinations of various transmitter/modulator receptors, hormones, growth factors, and other biochemically identifiable and yet unidentified substances. Under certain conditions, the activation of receptor-bound second messenger systems is thought to cause the enhanced expression of particular genes. Given the wide possibilities in manipulating brain chemistry, which may otherwise result in a variety of consequences, it is crucial to have a dependable means of sustaining the steady-state action of a drug for a sufficiently long time period at a targeted area in the brain of behaving animals. In most cases the continuous application of a drug is necessary to counteract its secondary mitigating effect, which is set in action through negative feedback loops and which in effect reduces the primary action of the drug in use. We have developed a technique to answer this need, using the Alzet osmotic minipump as the source of the continuous infusion force. A drug solution is continuously and directly infused, guided through a chronically implanted cannula, into a targeted area in the brain of behaving animals. The consequences of such an infusion are assessed, during as well as after the infusion, using various types of measurements in behavior, biochemistry, neurophysiology, pharmacology and morphology. The method has been successfully applied, for example, to the study of developmentally regulated neural plasticity in cat visual cortex. A few preconditions should be satisfied for the method to be properly applied to the brains of live animals. Those are: (1) manufacturing a suitable guide system, i.e., cannula-minipump assembly, for the infusion solution; (2) stereotaxic implantation of a cannula-minipump assembly into a selected brain region; and (3) estimating the concentration gradient of the continuously infused solution. This is crucial to assess the specificity and sensitivity of a drug for its assumed effects in vivo.

Supraspinal influence on hindlimb withdrawal thresholds and mustard oil-induced secondary allodynia in rats

We examined the role of supraspinal structures in secondary allodynia induced by mustard oil in awake rats. To produce allodynia (=unpleasent sensation evoked by innocuous stimuli), mustard oil (50%) was applied for 2 min to the skin of the ankle of one hindlimb. Mechanical hypersensitivity of the skin was tested by determining the hindlimb withdrawal threshold to a series of monofilaments applied to the glabrous foot pad (=distal to the mustard oil-treated ankle). In intact rats, mustard oil produced a secondary allodynia in the mustard oil-treated hindlimb as indicated by a decreased withdrawal threshold to mechanical test stimuli applied to the glabrous skin (=outside the mustard oil-treated ankle), whereas the withdrawal threshold in the contralateral (=control) hindlimb was not changed. Following spinalization, mustard oil treatment produced no secondary allodynia, but the interpretation of this finding was complicated by a concomitant bilateral elevation of hindlimb withdrawal thresholds to mechanical skin stimulation. However, the spinalized rats had shorter tail-flick latencies to radiant heat than intact rats. Administration of an opioid antagonist, naloxone (1 mg/kg, SC), had no effect on withdrawal thresholds in spinalized animals. Importantly, microinjection of lidocaine (4%) into the nucleus raphe magnus in rats with an intact spinal cord had a selective antiallodynic effect when the injection volume was 1.0 microl but not when it was 0.5 microl. Lidocaine (4%, 0.5 microl) in the lateral reticular nucleus of the medulla also attenuated the spinal hypersensitivity, however, concomitantly with motor side effects, due to which this finding maybe artificial. It is concluded that brain stem spinal pathways, originating adjacent to but not within the raphe magnus, contribute to the behavioral expression of secondary allodynia induced by neurogenic inflammation of the skin. Furthermore, there is a differential tonic control of various spinal reflexes by the brain stem as indicated by the dissociative effects of spinalization on mechanically induced hindlimb withdrawal vs. heat-induced tail-flick reflex.

Hyperphagia induced by direct administration of midazolam into the parabrachial nucleus of the rat

Benzodiazepine receptor agonists increase food intake in many different species, yet there has been little investigation of the central site of actions of these drugs on ingestive behaviour. In the present experiments, direct administration of the benzodiazepine receptor agonist midazolam (3-30 micrograms/microliter) into the parabrachial nucleus of the pons significantly increased the consumption of a wet mash diet and a 3% sucrose solution in adult non-deprived rats. The hyperphagic response was blocked by pre-treatment with the selective benzodiazepine receptor antagonist flumazenil. Injection of midazolam into the parabrachial nucleus had no effect on locomotor activity, despite the fact that in the same animals an increase in mash intake was observed following intra-parabrachial midazolam. These data suggest that benzodiazepine receptors located in the parabrachial nucleus may be an important site of action for the effects of benzodiazepines specifically on ingestive behaviour.

Sleep induction by microinjection of pentobarbital into the medial preoptic area in rats

In a previous study we have shown that microinjection of the benzodiazepine hypnotic triazolam into the medial preoptic area increases sleep in rats. In order to determine whether this effect is specific to benzodiazepines, or whether it occurs with hypnotic medications from other pharmacologic classes, we have microinjected pentobarbital (1 and 100 micrograms) and vehicle in random sequence into rats and performed two hour sleep studies in the daytime with the lights on. Both doses significantly decreased sleep latency and increased nonREM and total sleep. The amount of REM sleep, REM latency, and intermittent waking time were not significantly altered. These data are consistent with the hypothesis that the medial preoptic area may be involved in sleep induction by both benzodiazepine and barbiturate hypnotic medications.

Apomorphine and dopamine D(1) receptor agonists increase the firing rates of subthalamic nucleus neurons

The present study investigated the regulation of spontaneous neuronal activity in the subthalamic nucleus by dopamine receptors using in vivo extracellular single unit recording techniques. Subthalamic nucleus neuronal firing rates were doubled by systemic administration of the nonselective dopamine receptor agonist apomorphine. The response to apomorphine was attenuated in animals anesthetized with chloral hydrate or ketamine. The dopamine D(2)/D(3) receptor agonist quinpirole did not alter subthalamic nucleus neuronal firing rates. Firing rates were increased by the D(1) receptor agonists SKF 38393 and SKF 82958 two- to three-fold; these increases were reversed by the D(1) receptor antagonist, SCH 23390. Autoradiographic studies using [(125)I]SCH 23982 indicated that D(1) family receptors were located along the ventral edge of the subthalamic nucleus and the dorsal aspect of the cerebral peduncle. Local administration of SKF 82958 into the subthalamic nucleus doubled neuronal firing rates; these increases were reversed by systemic administration of SCH 23390. Infusion of SCH 23390 into the subthalamic nucleus prevented systemic SKF 38393 from increasing the firing rates of subthalamic nucleus neurons. These results indicate that apomorphine and D(1) receptor agonists exert an excitatory influence on subthalamic nucleus neuronal activity. In addition, the excitation induced by D(1) receptor agonists appears to be mediated, at least in part, by D(1) receptors located in the vicinity of the subthalamic nucleus. The data suggest that basal ganglia output under conditions of increased dopamine receptor stimulation is influenced by the activation of excitatory subthalamic efferent pathways, as opposed to suppression of these pathways as predicted by current models of basal ganglia function.

Anti-nociception induced by systemic or PAG-microinjected lysine-acetylsalicylate in rats. Effects on tail-flick related activity of medullary off- and on-cells

Previous experiments using metamizol have shown that this non-steroidal anti-inflammatory drug (NSAID) produces a central anti-nociceptive effect probably through neural substrates that also support the analgesic effects of opiates, such as the periaqueductal grey matter (PAG) and the off- and on-cells of the rostral ventromedial medulla (RVM). Off- and on-cells have been postulated to respectively inhibit and facilitate nociceptive transmission, since the heat-elicited tail flick reflex (TF) occurs only after off-cells have decreased (pause), and on-cells, have increased (burst) their activity. The aim of the present study was to examine whether the effect of metamizol upon TF and off- and on-cells responses could be generalized to other NSAIDs such as, in this case, lysine-acetylsalicylate (LASA). Fifty-nine off- and on-cells of the RVM were recorded in lightly anaesthetized rats. Systemic administration (200 and 300 mg/kg) or PAG microinjection (30, 50 and 100 micrograms) of LASA caused retardation of the heat-elicited off-cell pause, on-cell burst and the corresponding TF. Neuronal responses and TF retained their mutual time relationship but shifted simultaneously toward longer latencies. This anti-nociceptive effect of LASA was dose-dependent, present 5 min after administration and reached a maximum in 30 min for both administration methods. These data confirm that analgesics typically defined as peripherally-acting, such as metamizol and LASA in this study, may also have an anti-nociceptive effect by acting directly upon PAG, and suggest that this central effect involves the RVM off- and on-cells.

The supramammillary nucleus: is it necessary for the mediation of hippocampal theta rhythm?

Recent evidence suggests that the supramammillary nucleus of the posterior hypothalamus serves as an important relay in a brainstem to septum/hippocampus pathway involved in the generation of hippocampal theta rhythm. In order to examine the role of the supramammillary nucleus as a possible relay/mediator of hippocampal theta rhythm, electrolytic lesions and procaine injections were administered to the supramammillary nucleus of freely moving and urethane-anesthetized rats, respectively. In the urethane-anesthetized rat, it was found that procaine injections attenuated both the frequency and amplitude of theta rhythm elicited by stimulation of the pontine reticular formation. These data suggest that the pontine reticular elicitation of hippocampal theta rhythm is mediated through connections with the supramammillary nucleus. However, it was found that lesions of the supramammillary nucleus failed to produce significant changes in the hippocampal electroencephalogram of freely moving animals. Several explanations concerning this apparent discrepancy are discussed. The most compelling is that multiple brainstem to septum/hippocampus pathways may serve to generate or facilitate the generation of theta rhythm in the freely moving animal. The present report demonstrates that the supramammillary nucleus plays a questionable role in the mediation of hippocampal electroencephalogram signals which are thought to be important for mnemonic processes.

Effect of intramedullary procaine injection on tracheal tone and phrenic neurogram

To map the superficial locations which are involved in the control of respiration and tracheal smooth muscle tone in ventrolateral medulla, we examined the effects of local anesthesia on phrenic activity and tracheal tone in twelve anesthetized, paralyzed, and artificially ventilated dogs. 0.5 microliter of 5% procaine was injected 0.3 to 0.5 mm below the surface unilaterally to the ventral superficial layer (from the rostral part of the trapezoid body to the caudal hypoglossal rootlets and lateral from the pyramids to 5.5 mm from the midline), which included rostral, intermediate and caudal areas, and the area lateral to the hypoglossal rootlets. The peak amplitude of the integrated phrenic neurogram was decreased by procaine injection to the intermediate area and the area lateral to the hypoglossal rootlets. Tracheal tone decreased only by procaine injection to the intermediate area. In the intermediate area, some injections decreased either phrenic output alone or tracheal tone alone. These results suggest that the two ventral medullary areas, i.e. the intermediate and caudolateral parts, contain neural structures which are involved in the shaping of phrenic output, but only the intermediate area is involved in the regulation of tracheal tone. It is also suggested that, in the intermediate area, the structures responsible for the maintenance of respiration and tracheal tone are, at least in part, separable.

GABA receptors in the dorsal motor nucleus of the vagus influence feline lower esophageal sphincter and gastric function

Gamma-aminobutyric acid (GABA) antagonist (bicuculline methiodide, BIC; picrotoxin, PIC) or agonist (muscimol, MUS) microinjections were made into the dorsal motor nucleus of the vagus nerve (DMV), and effects on lower esophageal sphincter pressure (LESP), gastric motility, and gastric acid secretion were determined in chloralose-anesthetized cats. Right or left DMV sites were microinjected with BIC, PIC, MUS, or isotonic tonic saline (140 nl) through a glass micropipette having a tip diameter of 15-21 microns. Esophageal body, LESP, and gastric fundic pressures were measured manometrically. Circular smooth muscle contractions of the antrum and pylorus were recorded with strain-gauge force transducers. Gastric acid secretion was measured every 15 min through a gastric cannula and titrated to pH 7.0. DMV microinjection sites were verified histologically. Direct BIC microinjections (0.275 or 0.550 nmol) into the DMV primarily produced a decrease in LESP (71% of all sites tested), with mean LESP changing from 23.2 +/- 1.7 mmHg to 3.7 +/- 0.7 mmHg (p < 0.01). Tonic LESP increases and phasic LESP contractile activity occurred less frequently. BIC-induced LESP responses were abolished by vagotomy or by microinjections of MUS (0.5 to 10 nmol) into the DMV. Direct PIC microinjection (0.232 nmol) into the DMV produced a pattern of responses similar to those observed with BIC (which were also abolished by vagotomy or by MUS microinjections into the DMV). The antrum and pylorus were also responsive to DMV microinjections of both GABA antagonists. Microinjections of BIC or PIC into the DMV produced increases in gastric circular muscle activity that occurred less frequently than LESP effects, but also were eliminated by vagotomy. The high (0.550 nmol) dose of BIC increased gastric motility significantly more often than the low dose of BIC (p < 0.05). In addition, BIC (0.550 nmol) microinjections into the DMV increased gastric secretory volume (from 0.6 +/- 0.2 to 6.0 +/- 2.5 ml/15 min; p < 0.01) and total titratible acid (from 34.4 +/- 8.9 to 86.0 +/- 19.1 mEq/15 min; p < 0.01), and decreased gastric pH (from 4.63 +/- 0.44 to 3.50 +/- 0.49; p < 0.05). Vagotomy also eliminated the gastric secretory effects of DMV BIC. Direct microinjections of MUS into the DMV also blocked BIC- or PIC-induced changes in gastric motility and/or gastric acid secretion. Isotonic saline microinjected into the DMV did not increase basal or decrease stimulated gastric esophageal motility or gastric secretion. These data indicate that LESP, gastric motility, and gastric secretion are influenced by a tonic DMV inhibition mediated by GABAA receptor stimulation of the DMV. Because disinhibition of these receptors clearly activates the upper gut, future work should focus on identifying the nuclei providing this synaptic input to the DMV that might be involved in the functional regulation of upper gut motor and secretory function.

Nuclei within the rostral ventromedial medulla mediating morphine antinociception from the periaqueductal gray

The relative contributions of nuclei within the rostral ventromedial medulla (RVM) involved in mediating morphine induced antinociception from the periaqueductal gray (PAG) were examined. Lidocaine injections (4%) at the time of morphine's maximal response were used to provide a localized neural block and were administered in the nucleus raphe magnus/reticularis gigantocellularis pars alpha (RMg/GiA; commonly referred to as RMg), reticularis gigantocellularis (Gi) and reticularis paragigantocellularis lateralis (LPGi). Microinjection of morphine (6 nmol; 0.5 microliter) into the PAG of awake rats produced an inhibition of the tail-flick reflex that was maximal after 30 min. This response was unaffected by a single medial lidocaine injection (0.5 microliter) into the RMg/GiA or Gi, bilateral injections into the Gi or LPGi or triple injections that included both the RMg/GiA and LPGi. A partial, non-significant block of morphine's response was observed either by bilateral injections (0.5 microliter) into both the Gi and LPGi (% inhibition = 16.4 +/- 24.8) or by bilateral injections in the LPGi and a single medial injection into the Gi (% inhibition = 41.5 +/- 29.8). However, injection of a greater volume of lidocaine (1 microliter) into the RMg/GiA or bilaterally into the LPGi affected adjacent medial and lateral tissue, and completely inhibited morphine's response. Furthermore, triple injections of lidocaine (0.5 microliter) into the Gi or bilateral injections (0.5 microliter) into the Gi and a single medial injection into the RMg/GiA completely blocked morphine's antinociceptive response. These results indicate that morphine antinociception from the PAG is mediated by a large volume of tissue in the RVM containing nuclei located both medially and laterally. Additionally, the principal nuclei involved in this response appear to be the Gi and RMg/GiA.

Localization of the antinociceptive and antianalgesic effects of neurotensin within the rostral ventromedial medulla

Triple microinjections of neurotensin (10 nmol each), which occupied a large volume of tissue within the nucleus reticularis gigantocellularis (Gi), produced an inhibition of the tail-flick reflex in awake rats. This effect was less than that previously observed by a single injection (10 nmol) into the nucleus raphe magnus (RMg) (see ref. [25]). Bilateral injections of neurotensin (10 nmol each) into the nucleus reticularis paragigantocellularis lateralis (LPGi) had no effect. The neurotensin antagonist [D-Trp11]-neurotensin (3 pmol) was previously found to enhance morphine, but not beta-endorphin antinociception from the periaqueductal gray (PAG) when injected into the RMg. A similar enhancement of morphine, but not beta-endorphin antinociception from the PAG was observed in the current study by [D-Trp11]-neurotensin injections into the bilateral LPGi, bilateral Gi, or medial Gi. These data suggest that neurotensinergic projections from the PAG function in an antianalgesic manner throughout the RVM during morphine, but not beta-endorphin antinociception. The antinociceptive effect of neurotensin, on the other hand, is more localized.

Putative role of medullary off- and on-cells in the antinociception produced by dipyrone (metamizol) administered systemically or microinjected into PAG

Recent investigations have shown that non-steroidal antiinflammatory drugs (NSAIDs) may exert an antinociceptive effect when administered at or within the central nervous system (CNS). This might be due to the engagement of CNS substrates that support the analgesic effects of opiates, including the periaqueductal gray matter (PAG) and the rostral ventromedial medulla (RVM). The off- and on-cells of the RVM have been proposed to inhibit and facilitate, respectively, nociceptive transmission. Accordingly, upon heating of a rat's tail the tail-flick (TF) reflex occurs only after off-cells have decreased, and on-cells have increased, their activity. In the present study, i.v. administration (200 and 400 mg/kg) or PAG microinjection (25, 50, 100 and 250 micrograms) of dipyrone (metamizol) to lightly anesthetized rats caused a dose-related retardation of the heat-elicited off-cell pause, on-cell discharge and corresponding TF. Neuronal response and TF retained their mutual time relationship but shifted pari passu toward longer latencies. This antinociception was apparent already 5 min post-injection and reached a maximum in 50-60 min for i.v. administration and 30-35 min for PAG microinjection. These results confirm other authors' findings of the direct antinociceptive action of NSAIDs upon PAG, and provide the first evidence for a plausible involvement of RVM off- and on-cells in such antinociceptive effect.

Dissociation of the alpha 2-adrenergic antinociception from sedation following microinjection of medetomidine into the locus coeruleus in rats

It is well established that alpha 2-adrenoceptor agonists have sedative and antinociceptive properties. In the current behavioral study we tried to find out if the alpha 2-adrenergic sedative and antinociceptive effects can be dissociated. We tested the hypothesis that alpha 2-adrenergic sedation is mediated by the locus coeruleus (LC) and antinociception by spinal alpha 2-adrenoceptors. Also, we addressed the possibility that intracerebral injection of an alpha 2-agonist might produce its antinociceptive effect by an action directly at the spinal cord. Medetomidine, an alpha 2-adrenergic agonist, or atipamezole, an alpha 2-adrenergic antagonist, were microinjected bilaterally into the LC through chronic cannulae in unanesthetized Han-Wistar rats. The effect on locomotor activity (/vigilance), tail-flick and hot-plate response, and on formalin-induced pain behavior was determined. Medetomidine microinjected into the LC (1-10 micrograms/cannula) produced dose-dependently hypolocomotion (/sedation), increase of response latencies in the hot-plate and the tail-flick tests, and a decrease in the formalin-induced pain behavior. Hypolocomotion (/sedation) was obtained at a lower medetomidine dose (1 microgram/cannula) than antinociception (3-10 micrograms/cannula). The lowest medetomidine dose used (1 microgram/cannula), which induced significant hypolocomotion (/sedation), produced either no antinociception (hot-plate and tail-flick tests) or even a slight hyperalgesia (formalin test). The hypolocomotion (/sedation) but not antinociception (tail-flick test) induced by systemic administration of medetomidine (100 micrograms/kg s.c.) could be reversed by atipamezole (10 micrograms/cannula) microinjected into the LC. Only a high systemic dose of atipamezole (1 mg/kg s.c.) reversed the antinociceptive effects of medetomidine.(ABSTRACT TRUNCATED AT 250 WORDS)

The rostroventromedial medulla is not involved in alpha 2-adrenoceptor-mediated antinociception in the rat

The aim of the current study was to investigate the role of the rostroventromedial medulla (RVM) in alpha 2-adrenoceptor-mediated antinociception. Medetomidine or clonidine, selective alpha 2-adrenoceptor agonists were microinjected into the RVM in unanesthetized rats with a chronic guide cannula. The antinociceptive effects were evaluated using the tail-flick and hot-plate tests. For comparison, medetomidine was microinjected into the cerebellum or the periaqueductal gray (PAG). To study the role of medullospinal pathways, the tail-flick latencies were also measured in spinalized rats. The reversal of the antinociception induced by intracerebral microinjections of medetomidine was attempted by s.c. atipamezole, a selective alpha 2-adrenoceptor antagonist. The reversal of the antinociception induced by systemic administration of medetomidine was attempted by microinjections of 5% lidocaine or atipamezole into the RVM. When administered into the RVM, medetomidine produced a dose-dependent (1-30 micrograms) antinociception in the tail-flick and hot-plate tests, which antinociceptive effect was completely reversed by atipamezole (1 mg/kg, s.c.). Also clonidine produced a dose-dependent (3-30 micrograms) antinociception following microinjection into the RVM. Microinjections of medetomidine into the cerebellum or the PAG produced an identical dose-response curve in the tail-flick test as that obtained following microinjection into the RVM. In spinalized rats the antinociceptive effect (tail-flick test) induced by medetomidine microinjected into the RVM was not less effective than in intact rats. Lidocaine (5%) or atipamezole (5 micrograms) microinjected into the RVM did not attenuate the antinociception induced by systemically administered medetomidine (100 micrograms/kg, s.c.).(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping the differential effects of procaine on frequency and amplitude of reticularly elicited hippocampal rhythmical slow activity

Hippocampal rhythmical slow activity (RSA, theta) was elicited in urethanized rats by high-frequency stimulation in the reticular formation. The effects of procaine infusion (0.5 microliters, 20% wt/vol) at various loci in the ascending system from pontine reticular formation to the medial septum/diagonal band area were investigated. It was found that procaine injected at points in the ascending system anterior to the supramammillary nucleus, in the region of the medial forebrain bundle or in the medial septum, reduced the amplitude of reticularly elicited RSA but had no effect on its frequency. Procaine injected at points in the ascending system from just anterior to the reticular formation stimulation site, up to, and including the supramammillary nucleus, reduced both the frequency and amplitude of reticularly elicited RSA. These results indicate that the frequency of reticularly elicited RSA is encoded in the supramammillary area, rather than in the medial septum/diagonal band as have previously been suggested.

Toward an understanding of the role of glutamate in experimental parkinsonism: agonist-sensitive sites in the basal ganglia

Increased glutamatergic transmission in the basal ganglia is implicated in the pathophysiology of Parkinson's disease. However, the mechanisms by which activation of glutamate receptors produce parkinsonism are unknown. Therefore, we examined whether the glutamate agonists N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate produce parkinsonism in rats after microapplication into different subregions of the basal ganglia. Electromyographic activity was used as a measure of parkinsonian rigidity. We found that in the rostral striatum, excitation mediated by NMDA but not by non-NMDA receptors led to parkinsonism. In the substantia nigra pars reticulata, internal pallidal segment/entopeduncular nucleus, and subthalamic nucleus, activation of AMPA/kainate and metabotropic receptors but not of NMDA receptors led to parkinsonian rigidity. Rigidity occurred also in animals bearing ibotenate-induced lesions of the posterior part of the striatum and of the external pallidal segment, but not in animals with lesions of the anterior striatum, subthalamic nucleus, internal pallidal segment/entopeduncular nucleus, or substantia nigra pars reticulata. These observations suggest that the activation of glutamate receptor subtypes in the basal ganglia may be differentially involved in the expression of parkinsonian symptoms.

Response of locus coeruleus neurons to footshock stimulation is mediated by neurons in the rostral ventral medulla

While it is well documented that locus coeruleus neurons are potently activated by foot-pinch or sciatic nerve stimulation, little is known about the circuit producing this sensory response. Previous work in our laboratory has identified the medullary nucleus paragigantocellularis as a major excitatory afferent to the locus coeruleus. Here, we use local microinjections into the paragigantocellularis to test whether this nucleus is a link in the pathway mediating the activation of locus coeruleus neurons by subcutaneous footpad stimulation, or footshock, in anesthetized rats. Lidocaine HCl microinjected into the paragigantocellularis reversibly attenuated footshock-evoked activation of 50 out of 56 locus coeruleus cells, with responses in 20 cells completely blocked. Microinjections of GABA into the paragigantocellularis reduced the footshock-evoked responses of 17 out of 27 locus coeruleus cells (seven complete blocks); microinjections of the GABAB agonist baclofen had no effect (0 out of 11 cells blocked). Microinjections of a synaptic decoupling cocktail of manganese and cadmium also attenuated locus coeruleus activation in eight out of nine cells with two complete blocks. With each agent, the most effective injection placement for complete blockade of responses was the ventromedial paragigantocellularis; injections bordering this region attenuated responses, while those outside of the paragigantocellularis (dorsal medullary reticular formation, nucleus tractus solitarius, or facial nucleus), or vehicle injections, were ineffective. These results are consistent with previous findings that pharmacologic blockade of paragigantocellularis-evoked locus coeruleus activity also blocks footshock-evoked responses of locus coeruleus neurons [Ennis and Aston-Jones (1988) J. Neurosci. 8, 3644-3657], and support the view that this somatosensory response, and perhaps other sensory-evoked responses of locus coeruleus neurons, involve the nucleus paragigantocellularis.

Rapid cessation of focally induced generalized seizures in rats through microinfusion of lidocaine hydrochloride into the focus

An experimental animal model of complex partial seizures which become secondarily generalized is produced by microinfusion of the GABA antagonist bicuculline (BIC) into the deep prepiriform cortex (DPC) of rats. In the present study, we investigated the effects of microinfusion of the local anesthetic, lidocaine hydrochloride, directly into the BIC focus in the DPC and demonstrated that direct inactivation of the focus arrested a focal seizure that was in progress. A measure of the integrated amplitude of the electrocorticogram (ECoG) and behavioral seizure scores from unanesthetized and freely moving rats were used to address this question quantitatively. Microinfusion of 2% lidocaine hydrochloride into the BIC focus significantly reduced the integrated amplitude of the ECoG to levels that did not differ from baseline in either hemisphere (mean = 112% ipsilateral, 99% contralateral), whereas saline microinfusion had no effect (mean = 175% ipsilateral, 125% contralateral). Moreover, ECoG reductions after lidocaine were present as soon as the microinfusion was complete. Behaviorally, clonic seizure severity was assessed on a rating scale of 0-5. Lidocaine microinfusion significantly reduced the seizure scores to values not different from baseline during the first postinfusion measurement period (i.e., 30 s). Microinfusion of saline alone also significantly reduced behavioral seizure severity, although to a lesser degree and not as rapidly as lidocaine. This effect suggests the need for caution in interpretation and design of studies investigating the anticonvulsant effects of various pharmacologic agents when microinfusions are used.

Cardiovascular responses to blockade of GABA synthesis in the hypothalamus of the spontaneously hypertensive rat

Previous studies have suggested that a decreased inhibitory input onto neurons within the posterior hypothalamus (PH), a known pressor area, may contribute to hypertension in the spontaneously hypertensive rat (SHR). Recent experiments from this laboratory have shown that neurons in the PH of the SHR have an altered and elevated discharge frequency compared to those in the normotensive rat. In addition, biochemical studies have reported that there is a decreased concentration of the inhibitory neurotransmitter, GABA, in the hypothalamus of the SHR. The objective of the present study was to assess any variations in GABAergic modulation of cardiovascular activity in SHRs compared to normotensive Wistar-Kyoto (WKY) rats and Sprague-Dawley (SD) rats. Arterial pressure and heart rate responses to microinjections of the GABA synthesis inhibitor 3-mercaptopropionic acid (3-MP) into the posterior hypothalamic area of anesthetized young (6-8 weeks) and mature (11-16 weeks) hypertensive and normotensive rats were recorded. Microinjection of 3-MP elicited increases in arterial pressure of 17.4 +/- 3.9 mmHg, 18.1 +/- 7.8 mmHg, 16.9 +/- 6.4 mmHg, and 10.4 +/- 3.5 mmHg in the mature WKY, mature SD, young WKY, and young SHR, respectively. In addition, heart rate was elevated by 33.2 +/- 21.9 beats/min, 70.0 +/- 25.3 beats/min, 56.3 +/- 15.0 beats/min and, 45.9 +/- 10 beats/min in the mature WKY, adult SD, young WKY, and young SHR groups, respectively. In contrast, microinjection of 3-MP into the posterior hypothalamus of adult SHRs produced no significant change in arterial pressure (-5.0 +/- 1.8 mmHg) or heart rate (+5.3 +/- 6.1 beats/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Tremor reduction by microinjection of lidocaine during stereotactic surgery

We report our experience with lidocaine microinjection into the thalamus in 10 patients undergoing stereotactic thalamotomy for the treatment of Parkinsonian or non-Parkinsonian tremor. 18 injection sites in 4 patients with Parkinson's disease and 22 sites in 6 patients with other forms of tremor have been compared with respect to the effect of microstimulation. In over two thirds of cases the test microinjection replicated the effects of microstimulation. Long term follow up will be required to determine whether lesions made on the basis of lidocaine induced tremor suppression will result in a lower rate of tremor recurrence than those based on stimulation induced tremor suppression. In those patients in whom stimulation induced tremor suppression occurs but tremor arrest cannot be produced with lidocaine microinjection, chronic thalamic stimulation may be an alternative for the long term control of tremor in these patients.

Aversive and antiaversive effects of morphine in the dorsal periaqueductal gray of rats submitted to the elevated plus-maze test

The dorsal periaqueductal gray (DPAG) is a well-known region for processing defensive behavior in the brainstem. Rats implanted with cannulae in the DPAG were submitted to the elevated plus-maze test for 5 min. The effects of morphine following systemic (0.1-1.0 mg/kg) or DPAG administration (5-30 nmol) were compared with the benzodiazepine compound midazolam injected similarly (1-10 mg/kg, IP, and 10-80 nM, DPAG). Morphine and midazolam caused dose-dependent increases in the number of entries and time spent in the open arms. A systemic injection of naloxone in doses that block mu-opioid receptors reversed the effects of centrally administered morphine. Higher doses of morphine (70 nmol) induced a non-naloxone-reversible "fearful" hyperreactivity. It is suggested that low doses of morphine inhibit the neural substrate of aversion in the DPAG, probably through activation of mu-receptors, and that microinjections of higher doses of morphine cause proaversive actions not mediated by these opioid receptors.

Effect of lesioning of medullary catecholamine neurons or the median eminence on the development of cerebral vasospasm in the squirrel monkey

Injections of blood into the interpeduncular fossa and cisterna magna in the squirrel monkey produce an angiographically demonstrable, biphasic cerebral vasospasm with a maximal acute spasm at ten minutes and a maximal late spasm at six days after the subarachnoid haemorrhage (SAH). Selective lesioning of the A2 nucleus in the medulla oblongata or the median eminence in the hypothalamus prior to the SAH prevents the development of both the acute and late cerebral vasospasm. The present data indicate that the A2 nucleus and the median eminence participate in the development of vasospasm in the squirrel monkey.

Effects of intra-amygdala injections of NMDA receptor antagonists on acquisition and retention of inhibitory avoidance

These experiments examined the effects of intra-amygdala injections of NMDA receptor antagonists on the acquisition and retention of inhibitory avoidance. In Expt. I, rats received bilateral intra-amygdala injections of the NMDA antagonists D,L-AP5 (1-10 micrograms), D-AP5 (0.03-1 micrograms), CPP (0.125 or 0.375 microgram), or MK-801 (0.2 or 0.5 microgram) prior to training in a continuous multiple-trial inhibitory avoidance (CMIA) task. Acquisition of the task was not significantly affected by any of the drug injections. In contrast, all three competitive antagonists, D,L-AP5, D-AP5 and CPP, produced dose-dependent impairment of 48 h retention performance. Although the MK-801 injections did not significantly impair retention performance, the retention scores of the 0.5 microgram MK-801 group were bimodally distributed, indicating retention impairment in a subgroup of the animals given that dose. Intra-amygdala injections of 3 or 10 micrograms D,L-AP5 did not affect footshock sensitivity (Expt. II) or locomotor activity (Expt. III) and their retention-impairing effects were not due to induction of state dependency (Expt. IV). The retention-impairing effects of intra-amygdala injections of NMDA antagonists were not due to diffusion of the drugs dorsally: injections of 1 microgram D-AP5 into the striatal area directly above the amygdala impaired acquisition but not retention performance (Expt. V). The retention-impairing effects of 1 microgram D-AP5 or 0.5 microgram MK-801 were attenuated by giving additional training to the animals shortly after receiving intra-amygdala injections (Expt. VI). The implications of these findings for hypotheses concerning amygdala function in learning and memory are discussed.