Microinjection Of Glutamate Research Articles

Dissociation of antinociceptive from cardiovascular effects of stimulation in the lateral reticular nucleus in the rat

The lateral reticular nucleus (LRN) in the caudal ventrolateral medulla has been implicated in the regulation of spinal nociceptive transmission and hemodynamics. Experiments were undertaken to examine the relationship between inhibition of the tail flick reflex and cardiovascular effects produced by electrical stimulation in the LRN in rats lightly anesthetized with pentobarbital. Intensity- and frequency-dependent increases in mean arterial pressure and vascular resistance in the hindquarter, mesenteric, renal and caudal arterial beds were observed. Inhibition of the tail flick reflex, however, occurred at intensities of electrical stimulation which produced no significant changes in mean arterial pressure or vascular resistance in any of the arterial beds studied. Selective stimulation of cell bodies in the LRN by microinjection of glutamate similarly inhibited the tail flick reflex but produced significant reductions in mean arterial pressure, without substantially affecting regional vascular resistances. These results suggest that the antinociceptive and depressor effects of stimulation in the LRN are mediated by activation of cell bodies, while pressor effects produced by focal electrical stimulation are mediated by activation of fibers of passage. The descending inhibition produced by stimulation in the LRN is independent of stimulation-produced cardiovascular responses.

Influence of reticular mechanisms upon hypoglossal, trigeminal and phrenic activiteis

Studies were undertaken to evaluate the hypothesis that mechanims within the reticular formation influence activities of nerves to muscles of the upper airways more than the bulbospina-phrenic system. In decerebrate, vagotomized, paralyzed and ventilated cats, activities of the phrenic, trigeminal, and the hypoglossal nerves were monitores. Activity of the pontile and medullary reticular formation was increased directly by electrical stimulation within the brainstem and indirectly by stimulating the sciatic nerve. Trigeminal and hypoglossal discharges increased more than phrenic during stimulating at many brainstem loci. Changes were typically maintained for one or more respiratory cycles following termination of stimulation. At some loci, activation of neurons by microinjections of glutamate increased trigeminal and hypoglossal activities more than phrenic. Although responses were extremely variable, activities of the trigeminal and/or hypoglossal nerves usually increased more than phrenic during stimulations of the sciatic nerve or upon termination of stimulation. The result support the conclusion that respiratory-modulated trigeminal and hypopglossal discharges are dependent upon reticular mechanimss for their expression.

Characterization of inhibition of the spinal nociceptive tail-flick reflex in the rat from the medullary lateral reticular nucleus

Inhibition of the spinal nociceptive tail-flick (TF) reflex by focal electrical stimulation in the caudal medulla was examined and characterized in lightly pentobarbital-anesthetized rats. Systematic mapping studies revealed that inhibition of the TF reflex was produced at low intensities of stimulation (12.5-25 microA) only from the lateral reticular nucleus (LRN). Areas dorsal and medial to the LRN required higher intensities of stimulation to produce descending inhibition of the TF reflex, likely reflecting spread of current to the LRN at these higher intensities of stimulation (50-100 microA). At threshold inhibitory intensities of stimulation in the LRN, changes in blood pressure were not produced. Strength-duration characterization of stimulation and the microinjection of glutamate into the LRN at the same site where focal electrical stimulation was effective suggest that the descending inhibition produced arises from activation of cell bodies in the LRN. The intrathecal administration of a variety of pharmacological antagonists revealed the descending inhibition produced by stimulation in the LRN to be mediated at least in part by spinal alpha 2-adrenoceptors. These findings, together with previous observations, suggest a role for the LRN in the centrifugal modulation of spinal nociceptive transmission.

Function in ventilatory control of respiratory neurons at the pontomedullary junction

Pevious studies have demonstrated that electrolytic lesions placed in the midline at the pontomedullary' junction result in increased respiratory frequency. The increase in frequency is greater during hypercapnia. The present study sought to determine whether the effects of the lesions were mediated, at least in part, by destruction of neurons. Alternatively, the lesions may have interrupted fiber tracts. Both destruction by kainic acid and inhibition by selective cooling of neurons on the midline at the pontomedullary junction in decerebrate, vagotomized, paralyzed and artificially ventilated cats produced results similar to those engendered by the lesions; i.e., an increased respiratory frequency. Microinjections of glutamate produced a slowing of respiration. In additional cats, extracellular single unit recordings were obtained from 48 neurons located in the medial areas (0–1 mm lateral to midline) at the pontomedullary junction. Of these, 41 neurons were tonically active and 38 were judged tobe respiratory-modulated by both the F-test and the Friedman test. The activity of these neurons was sensitive to changes in CO 2. Therefore, neurons located at the pontomedullary junction may play a primary role in the integration of central chemoreceptor afferent stimuli.

Brainstem activation of the adrenal medulla in the cat

Stimulation of the locus coeruleus in 40 cats leads to generalized activation of the sympathetic nervous system characterized by an immediate pressor response which was followed in the post-stimulus period by an increase of 49 ± 10and44 ± 6% in common carotid arterial resistance ipsilateral and contralateral to stimulation, respectively. This later response was not affected by vagotomy or bilateral cervical sympathectomy but was blocked by high spinal cord section. The post-stimulus carotid vasoconstriction response could be entirely eliminated by acute bilateral physiological adrenalectomy in the form of adrenal hilar clamping, an effect which was reversible if the clamps were removed. The carotid vasoconstrictor response was associated with a rise in the circulating level of noradrenaline (260%) and adrenaline (196%), which was prevented by clamping the adrenal hilum. This response was not mediated via the hypothalamus because it persisted in the decerebrate animal, nor was it merely excitation of fibers of passage since it was reproduced by microinjection of glutamate into the locus coeruleus. The response was blocked by phentolamine suggesting it is mediated by α-adrenoceptors. These data represent the first conclusive demonstration that cell bodies in the brainstem are capable of activating the adrenal medulla. This fact is central to our present concept of the organization of the sympatho-adrenal axis.

Evidence that vasomotor neurons in the rostral ventrolateral medulla project to the spinal sympathetic outflow via the dorsomedial pressor area

Previous work indicates that excitation of spinally projecting neurons in the rostral ventrolateral medulla evokes an increase in arterial pressure. The aim of this study was to test the hypothesis that axons of the ventrolateral neurons project to the spinal cord via a pressor region located in the dorsomedial medulla just rostral to the obex. In one group of experiments, a large quantity of horseradish peroxidase (HRP) was injected bilaterally into the spinal cord of rabbits following placement of a lesion unilaterally in the dorsomedial pressor area. This resulted in a much smaller number of HRP-labeled cells in the rostral ventrolateral medulla on the side of the lesion than on the intact side. In all other brainstem regions examined, however, the numbers of labeled cells were very similar on both sides. In other experiments, microinjections of sodium glutamate (which excites cell bodies but not axons of passage) were made into the dorsomedial pressor area. In contrast to the large pressor response evoked by electrical stimulation of this area, glutamate microinjections evoked a very small or no response. The results of both groups of experiments taken together suggest that the pressor response to electrical stimulation of the dorsomedial medulla arises from excitation of axons of passage which originate, at least in part, from the rostral ventrolateral neurons.

A method for evoking physiological responses by stimulation of cell bodies, but not axons of passage, within localized regions of the central nervous system

A method for evoking physiological responses by microinjection of sodium glutamate solution into localized regions of the central nervous system (CNS) is described. The major advantage of this method is that the cell bodies or dendritic processes of neurones within the injection site are excited, whereas axons of passage are unaffected. It was demonstrated that injections of minute volumes (50–100 nl) of 0.5 M glutamate solution into selected sites within the medulla or midbrain of anaesthetized or conscious animals, respectively, elicited marked autonomic, somatomotor or behavioural responses, depending on the injection site. In contrast, glutamate microinjection into fibre tracts failed to elicit any response, whereas electrical stimulation applied at the same sites elicited marked responses. The degree of localization of the glutamate stimulus and the relation between glutamate concentration and magnitude of evoked response are described. It is concluded that this method is a very effective means of selectively stimulating cell bodies within highly localized regions of the CNS. Further, by using this method in combination with focal electrical stimulation, it is possible in some cases to provide evidence that a response arises from excitation of axons of passage rather than cell bodies.

Role of ventrolateral medulla in vasomotor regulation: a correlative anatomical and physiological study

Two groups of experiments were carried out in rabbits. First, the ventrolateral reticular formation of the medulla oblongata was stimulated either by microinjection of sodium glutamate solution (exciting only cell bodies) or electrically (exciting cell bodies and axons). This region has been shown previously to contain a dense and compact group of bulbospinal cells. The effects of both electrical and chemical stimulation of specific sites were correlated with the density of ventrolateral bulbospinal cells at the same sites. Glutamate microinjection into the center of the group of bulbospinal cells elicited a very large and sustained increase in arterial pressure, whereas microinjection into sites outside this region elicited a very small or no response. These results suggest that it is the bulbospinal ventrolateral cells which mediate the pressor response to glutamate stimulation. Focal electrical stimulation in the ventrolateral medulla elicited increases in arterial pressure and decreases in femoral and renal vascular conductance, as well as short-latency increase in renal sympathetic nerve activity. The most effective sites for focal electrical stimulation lay within the region of greatest density of bulbospinal cells; slightly less effective sites lay just rostral and caudal to this region. It is suggested that stimulation in these latter sites predominantly excites axons of passage. Secondly, the origin of afferent fibers to the ventrolateral vasomotor area was studied using the horseradish peroxidase (HRP) method. This revealed major projections from the medial part of the nucleus tractus solitarius and the parabrachial nucleus in the pons. The physiological and anatomical studies taken together are consistent with the hypothesis that the bulbospinal ventrolateral cells are vasomotor in function, and receive afferent inputs from brain stem nuclei which are known to play a role in autonomic regulation.

Identification of neuronal cell bodies mediating components of biting attack behaviour in the cat: induction of jaw opening following microinjections of glutamate into hypothalamus

Microinjections of glutamic acid were made into the same hypothalamic and midbrain sites at which a biting attack of a cat upon a rat was elicited by electrical stimulation. It was found that: (1) caudal lateral hypothalamic and ventral midbrain tegmental glutamate injections were without behavioural effects; and (2) perifornical hypothalamic glutamate injections elicited the jaw-opening component of biting attack. As it is known that glutamate excites cell bodies but not axons, the results suggest the specific location of a population of hypothalamic neurones which mediate a major component of biting attack.

Induction of ‘page’ following microinjections of glutamate into midbrain but not hypothalamus of cats

Microinjections of glutamic acid were made into the same midbrain and hypothalamic sites of cats at which ‘defensive-rage’ behaviour was elicited by electrical stimulation. It was found that: (1) medial hypothalamic and midbrain tegmental glutamate injections were without behavioural effect; (2) midbrain periaqueductal grey injections elicited a ‘rage’ display. Since glutamate depolarizes cell bodies but not axons, the results suggest that there is a specific population of neurones in the midbrain whose excitation elicits this reaction.

Effect of chronic morphine treatment on the interaction between the periaqueductal grey and the nucleus raphe magnus of the rat

The analgesic action of morphine is, in part, mediated through activation of a descending pathway. Major components of this pathway include the periaqueductal grey (PAG) and the nucleus raphe magnus (NRM). Injection of glutamate into the PAG increased the firing rate of its cells, led to an increase in the firing rate of NRM cells, and produced analgesia. The experiments reported here were designed to determine the effect of morphine tolerance on the interactions between the PAG and the NRM. Rats were made tolerant to morphine by implanting them with morphine pellets. The response of single cells in the NRM to the injection of glutamate into the PAG was recorded before and after iontophoretic application of naloxone into the NRM. It was shown that in morphine-tolerant animals, the response of NRM cells to microinjection of glutamate into the PAG was reduced significantly. However, when naloxone was iontophoresed into the NRM, there was a small increase in the spontaneous firing rate, and the injection of glutamate into the PAG produced a significant increase in the firing rate of the cells in the NRM. It is concluded that chronic morphine treatment decreased the effectiveness of excitation of cells in the PAG in activating NRM cells, and this process may play a part in the tolerance developed to the analgesic effect of morphine.

Monosodium glutamate: increased neurotoxicity after removal of neuronal re-uptake sites

Microinjections of monosodium glutamate (MSG; 300 μg/0.5 μl) into the hippocampus of the adult rat result in only marginal damage to local neurons. Perforant path transections, removing glutamatergic afferents to hippocampal granule cells, make the latter markedly more vulnerable to a subsequent MSG injection. The principle of modulating toxic effects of MSG by interfering with its neurotransmitter role may have significant impact on our understanding of human neurodegenerative disorders.

Brain stem mechanisms in the control of arterial pressure.

Electrical stimulation of a circumscribed region within the rostral part of the ventrolateral medulla elicits a large increase in arterial pressure accompanied by intense regional vasoconstriction. Microinjection of glutamate has established that the vasomotor effects are due to excitation of cell bodies rather than axons of passage within the ventrolateral region. Bilateral localized destruction of the same region results in a profound decrease in resting arterial pressure as well as virtual abolition of reflexes controlling sympathetic vasomotor activity. Neuroanatomical studies using the method of retrograde transport of horseradish peroxidase (HRP) have shown that this region contains a dense group of neurons projecting to the thoracolumbar spinal cord and receives major monosynaptic inputs from the nucleus tractus solitarius and parabrachial nucleus. There is some evidence that the spinally-projecting ventrolateral cells contain epinephrine. The anatomical and physiological evidence taken together indicate that the bulbospinal pathway originating from the ventrolateral cells plays a major role in the tonic and phasic regulation of arterial pressure.

Antagonism of intrastriatal and intravenous kainic acid by 1-nuciferine: Comparison with various anticonvulsants and gabamimetics

1-Nuciferine has been proposed as an antagonist of kainic acid (KA) and/or glutamate on the basis of iontophoretic experimental results. Its effectiveness against KA-induced destruction of rat striatal cholinergic neurons was therefore evaluated and compared with that of diazepam, phenobarbital, baclofen, haloperidol, and related substances. Drugs were administered intraperitoneally before and after intrastriatal microinjection of KA (0.5–1.5 μg), and choline acetyltransferase activity in striatum was assessed 24 hr later. Among the substances tested, only 1-nuciferine attenuated KA-induced depletions of striatal choline acetyltransferase. This effect was not secondary to anticonvulsant activity, because (a) 1-nuciferine did not block metrazol-, maximal electroshock-, or intravenous KA-induced seizures, and (b) anticonvulsants such as phenobarbital and diazepam, which are effective in these procedures, failed to modify KA-induced striatal neurotoxicity. 1-Nuciferine antagonized certain other neurological effects of intravenous KA, but antagonism was also seen with some of the other drugs tested. Intrastriatal microinjection of KA and/or glutamate may offer a means to detect selective antagonism of KA and/or glutamate, as distinguished from simple anticonvulsant activity.

Evidence that an excitatory connection between the periaqueductal gray and nucleus raphe magnus mediates stimulation produced analgesia

Both electrical stimulation and injection of morphine into the midbrain periaqueductal gray (PAG) produce analgesia in the rat. There is evidence that this analgesic effect is mediated by a descending system that involves nucleus raphe magnus (NRM) and adjacent reticular formation. In the studies reported here, the activity of the cells in the PAG was increased by microinjection of glutamate in this area and its effect on both the activity of single cells in the NRM and on a flexion reflex elicited by noxious heat was measured. It is shown that an increase in the firing rate of the cells in the PAG is associated with a raised threshold for flexion and is also correlated with an increase in the firing rate of a majority of the cells in the NRM. This effect on the flexion reflex can be abolished by (a) lesion of the nucleus raphe magnus and a small area of the reticular formation surrounding this nucleus and (b) by nalazone 20 min after its i.v. injection. It is concluded that there is an excitatory connection between the periaqueductal gray and the nucleus raphe magnus and that activation of this system can cause analgesia.

Colliculoreticular organization in primate oculomotor system

Colliculoreticular organization in primate oculomotor system

Evidence for an insulin-sensitive receptor in the central nervous system.

Evidence for an insulin-sensitive receptor in the central nervous system.

Effects of vagal stimulation on S-A and A-V nodes.

Effects of vagal stimulation on S-A and A-V nodes.