Cat Somatosensory Cortex Research Articles

Response characteristics of the cat somatosensory cortex following the mechanical stimulation to endodontically treated teeth with overextension

The overextension of filling materials may take place accidentally during an endodontic root canal treatment. Previous studies to understand the correlation between overfilling and paresthesia of teeth were inconsistent. In this study, an intrinsic signal optical imaging technique was employed as the objective tool to compare the response characteristics of the cat somatosensory cortex following mechanical stimulation applied to endodontically treated teeth with overextension (ETTWO) and natural teeth. Based on the evoked cortical response, the signal strength of the ETTWO was found to be significantly higher than that of the natural teeth. However, the tactile threshold of the ETTWO was significantly lower than that of the natural teeth. It was concluded that the tactile function of ETTWO is more sensitive than that of natural teeth, and that the overextension of filling materials can cause hyperesthesia of teeth after root canal treatment.

Response characteristics of the cat somatosensory cortex following the mechanical stimulation to non-vital and vital canine

Patients sometimes complain that non-vital teeth after root canal treatment (RCT) are paresthesia compared with vital teeth, and previous psychological studies on the tactile sensibility of non-vital teeth remained controversial. In the present study, intrinsic signal optical imaging, which served as an objective tool, was employed to compare the cortex response characteristics following forces applied to the cat non-vital and vital canines. Based on the evoked cortical responses, the response threshold, signal strength, spatial pattern, temporal dynamics and the preference of force direction, they were not significantly different between vital and non-vital canines. It seemed that the tactile sensibility of vital and non-vital teeth was comparable at the cortical response level, and pulpal receptors were not concerned in tactile function.

Modulation of Synaptic Reactions of the “Nociceptive” Neurons in the Cat Cortex upon Stimulation of the Periaqueductal Gray and Application of Some Pharmacological Agents

We examined the effects of electrical stimulation of the periaqueductal gray (PAG) and systemic morphine injections on postsynaptic processes in neurons of the cat somatosensory cortex, which were activated by nociceptive influences. Stimulation of the PAG resulted in long-lasting suppression of synaptic reactions induced by stimulation of dental pulp nociceptors. There was certain parallelism within the conditioning effects of PAG stimulation and effects of systemic introduction of morphine. Ionophoretic application of strychnine on such pyramidal cortical neurons did not evoke paroxysmal depolarization shifts (PDSh) of the membrane potential in such neurons. At the same time, surface application of this agent on the cortex (which influenced extensive populations of cortical neurons) resulted in the development of considerable PDShs. This observation confirms the synaptic nature of the above shifts. Applications of strychnine using both techniques resulted in the blockade of, first of all, early IPSP components in pyramidal neurons but did not affect significantly late components of these potentials; this is indicative of different geneses of the above components. It is supposed that the early IPSP component is generated due to activation of axo-somatic inhibitory synapses, while the late component of such reactions is related to activation of inhibitory synapses localized in the dendrites. The mechanisms of modulation of postsynaptic reactions of somatosensory cortex neurons developing upon activation of high-threshold (nociceptive) afferent inputs are discussed. Such modulation is probably based on changes in both pre- and post-synaptic intracortical cell structures.

Population response characteristics of intrinsic signals in the cat somatosensory cortex following canine mechanical stimulation

Intrinsic signal optical imaging has been widely used to measure functional maps in various sensory cortices due to better spatial resolution and sensitivity for detecting cortical neuroplasticity. However, application of this technique in dentistry has not been reported. In this study, intrinsic signal optical imaging was used to investigate mechanically driven responses in the cat somatosensory cortex, when punctate mechanical stimuli were applied to maxillary canines. The global signal and its spatial organization pattern were obtained. Global signal strength gradually increased with stimulus strength. There was no significant difference in response strength between contralateral and ipsilateral mechanical stimulation. A slightly greater response was recorded in the sigmoidal gyrus than in the coronal gyrus. The cat somatosensory cortex activated by sensory inputs from mechanical stimulation of canines lacks both topographical and functional organization. It is not organized into columns that represent sensory input from each tooth or direction of stimulation. These results demonstrate that intrinsic signal optical imaging is a valid tool for investigating neural responses and neuroplasticity in the somatosensory cortex that represents teeth.

Modulation of Synaptic Processes in Cortical Neurons in Response to Painful Stimulation and Analgesia

We studied effects of electrical stimulation of the substantia nigra (SN), locus coeruleus (LC), raphe nuclei (RN), substantia innominata (SIn), nucleus caudatus (NC) and central grey (CG) on postsynaptic processes evoked in neurons of the cat somatosensory cortex by excitation of nociceptive and non-nociceptive afferent inputs (intense stimulation of the dental pulp and moderate stimulation of the thalamic ventroposteromedial nucleus, VPMN, respectively). We analyzed intracellularly recorded activity of cortical cells activated exclusively by stimulation of nociceptors and cells activated by both nociceptive and non-nociceptive influences (“nociceptive” and convergent neurons). In neurons of both groups, stimulation of both nociceptive afferents and thalamic VPMN resulted in the development of successions of EPSP – action potential (AP) or their series – IPSP (IPSP duration 200–300 msec). Conditioning electrical stimulation the above-mentioned nuclei induced suppression of synaptic reactions that occur in cortical neurons in response to stimulation of nociceptive inputs. The maximum of inhibition was observed at test intervals of 600 to 800 msec. We observed certain parallelism between conditioning influentions of PAG activation and effects of systemic injections of morphine. Decrease in the amplitude or complete postsynaptic inhibition of IPSP in cortical neurons under different treatments associate with the occurrence of convulsive epileptic activity, and at the painful action with analgesic effect. Discussed are the mechanisms for modulatory influences exerted by conditioning stimulation of SN, LC, RN, NC, SIn and CG on the somatosensory neurons, activated upon excitation of high-threshold (nociceptive) afferent inputs.

Effects of Stimulation of the Substantia Innominata and Caudate Nucleus on Postsynaptic Reactions of Neurons of the Cat Somatosensory Cortex Activated by Excitation of Nociceptors

We studied effects of electrical stimulation of the substantia innominata (SIn) and nucleus caudatus (NC) on postsynaptic processes evoked in neurons of the cat somatosensory cortex by excitation of nociceptive and non-nociceptive afferent inputs (intense stimulation of the dental pulp and moderate stimulation of the thalamic ventroposteromedial nucleus, VPMN, respectively). We analyzed intracellularly recorded activity of seven cortical cells activated exclusively by stimulation of nociceptors and nine cells activated by both nociceptive and non-nociceptive influences (“nociceptive” and convergent neurons). In neurons of both groups, stimulation of both nociceptive afferents and thalamic VPMN resulted in the development of successions of EPSP – action potential (AP) or their series – IPSP (IPSP duration 200–300 msec). After isolated stimulations of the SIn and NC by short high-frequency series of stimuli, cortical neurons generated APs accompanied by long-lasting high-amplitude IPSPs. Conditioning stimulations of the NC and SIn preceding testing stimulations of the tooth pulp or thalamic VPMN with intervals from 100 to 900 msec induced intense suppression of impulse responses to test stimulations of the above-mentioned afferent inputs during an initial period of IPSP (100–150 msec) after conditioning stimulation. Partial recovery of the test responses was observed during the development of the second (GABAB-mediated) IPSP component. Complete recovery of the above responses developed within the borders of the latest segment of hyperpolarization evoked by conditioning stimulation and after termination of this potential. Stimulations of the NC and SIn result in the release of acetylcholine (ACh) in the cortex; ACh influences target neurons via muscarinic and nicotinic receptors and provides modulation of activity of the respective populations of nociceptive and non-nociceptice cortical neurons. Such modulation is probably based on changes realized in both pre- and post-synaptic cortical mechanisms.

Effect of stimulation of β adrenoreceptors on conditioned reflex-related spike activity of neurons of the cat somatosensory cortex

Using extracellular recording of impulse activity of pyramidal neurons of the cat somatosensory cortex generated in the course of realization of an operant conditioned reflex, we examined effects of microiontophoretic applications of an agonist of β2 adrenoreceptors, metaproterenol, and of an antagonist of β adrenoreceptors, propranolol, on the frequency of this impulsation. Applications of propranolol induced intensification of both background and reflex-related spiking. The effect of metaproterenol was manifested as moderate intensification of background discharges, a noticeable increase in the frequency of evoked impulsation, and an increase in the latency of impulse reactions. It is supposed that noradrenergic projections to the neocortex intensify neuronal activity via activation of β2 adrenoreceptors and suppress this activity via influencing mostly β1 adrenoreceptors. Such effects of synaptically active agents should probably be taken into account in the treatment of neurological diseases, namely epilepsy, neurodegenerative disorders, psychoemotional shifts, etc., which are accompanied by emotional and cognitive disorders.

Raphe Stimulation-Evoked Modulation of Postsynaptic Responses by Neurons of the Cat Somatosensory Cortex Activated by Stimulation of Nociceptors

We studied the effects of electrical stimulation of the raphe nuclei (RN) of the cat brain on postsynaptic potentials developing in somatosensory cortex neurons activated by nociceptive influences. Intracellular records were obtained from 15 cells, which were either selectively excited by stimulation of nociceptors (intense electrical stimulation of the dental pulp) or activated by both the above nociceptive and non-nociceptive (moderate stimulations of the infraorbital nerve or thalamic ventroposteromedial nucleus, VPMN) influences. In neurons of both groups, stimulation of both nociceptive afferents and the VPMN evoked complex responses (EPSP–AP–IPSP; IPSPs were 200 to 300 msec long). In some studied cortical neurons, isolated electrical stimulation of the RN (which caused the release of serotonin, 5-HT, in the cortex) resulted in relatively short-latency synaptic excitation, while inhibition was observed in other cells. In the case where stimulation of the RN was used as conditioning influence, such stimulation (independently of the kind of the initial response to RN stimulation) led to long-latency and long-lasting suppression of all components of the synaptic reactions evoked by excitation of nociceptors. The maximum of inhibition was observed at test intervals of 300 to 800 msec. The mechanisms underlying modulatory influences coming from the 5-HT-ergic brainstem system to neurons of the somatosensory cortex, which are activated by excitation of high-threshold (nociceptive) afferent inputs, are discussed.

Modulation of Postsynaptic Responses of Nociceptor-Activated Neurons of the Cat Somatosensory Cortex by Stimulation of the Substantia Nigra

We studied the effect of electrical stimulation of the substantia nigra (SN) on postsynaptic processes evoked in neurons of the somatosensory cortex of the cat by stimulation of nociceptive (intense stimulation of the dental pulp) and non-nociceptive (moderate, in strength, stimulations of the n. infraorbitalis and ventroposteromedial nucleus, VPM, of the thalamus) afferent inputs. The intracellularly recorded 7 cells activated exclusively by stimulation of dental nociceptors and 10 cells activated by both nociceptive and non-nociceptive afferent volleys (nociceptive and convergent cortical neurons, respectively) were examined in detail. Complexes EPSP–action potential–long-lasting IPSP (up to 200–300 msec) developed in neurons of both groups after stimulations of both nociceptive afferents and the thalamic VPM. Conditioning stimulation of the SN preceding test stimulation of the dental pulp or VPM with intervals from 100 to 700 msec resulted in selective suppression of IPSPs; their amplitudes decreased by 40–60%. The maximum effect in both cases was observed at conditioning/test intervals of 300 to 700 msec. Under conditions of SN stimulation, the release of dopamine, by affecting receptors and secondary messengers, modulates postsynaptic activity in the cortical GABA-ergic system and decreases the amplitude of IPSPs induced by stimulations of both nociceptors (dental pulp) and low-threshold afferent inputs (VPM of the thalamus). This process can be mediated by both pre-synaptic and post-synaptic mechanisms.

Effects of stimulation of the basal forebrain on cerebral blood flow in the primary somatosensory cortex in cats

Vasodilative regulation of the cerebral cortex by intracranial cholinergic nerves originating in the nucleus basalis of Meynert of the basal forebrain and projecting broadly to the cerebral cortex has been demonstrated in rats [see a review by Sato A & Sato Y: Neurosci Res 14: 242, 1992]. Although similar cholinergic projections exist in the cerebral cortex of cats, it has not as yet been determined whether they mediate vasodilation. We have recently showed that when blood pressure changes were prevented by section of the spinal cord at the upper thoracic level, cerebral cortical blood flow response induced by somatosensory stimulation of a forelimb in anesthetized cats was quite different from that reported in rats: vasodilative responses in cats were restricted to the primary somatosensory cortex contralateral to the stimulated forelimb, whereas responses in rats were bilateral and widespread.

Cerebral cortical blood flow response during basal forebrain stimulation in cats

We examined whether stimulation of the basal forebrain affects regional cerebral blood flow in the primary somatosensory cortex in cats. In anesthetized cats with spinal cord transection at the T1 level, focal electrical stimulation of the unilateral basal forebrain increased the blood flow of the ipsilateral primary somatosensory cortex that was increased by stimulation of the contralateral forepaw, without any change in blood pressure. The response was the largest when the tip of the electrode was located within the area known to contain the basal forebrain neurons projecting to the primary somatosensory cortex. These results suggest that basal forebrain neurons projecting to the primary somatosensory cortex have a vasodilative function in cats, as previously found in rats.

Effects of Stimulation of the Periaqueductal Gray and Locus Coeruleus on Postsynaptic Reactions of Cat Somatosensory Cortex Neurons Activated by Nociceptors

In cats, we studied the influences of stimulation of the periaqueductal gray (PAG) and locus coeruleus (LC) on postsynaptic processes evoked in neurons of the somatosensory cortex by stimulation of nociceptive (intensive stimulation of the tooth pulp) and non-nociceptive (moderate stimulations of the infraorbital nerve and ventroposteromedial nucleus of the thalamus) afferent inputs. Twelve cells activated exclusively by nociceptors and 16 cells activated by both nociceptive and non-nociceptive influences (hereafter, nociceptive and convergent neurons, respectively) were recorded intracellularly. In neurons of both groups, responses to nociceptive stimulation (of sufficient intensity) looked like an EPSP-spike-IPSP (the latter, of significant duration, up to 200 msec) complex. Electrical stimulation of the PAG (which could itself evoke activation of the cortical neurons under study) resulted in long-term suppression of synaptic responses evoked by excitation of nociceptors (inhibition reached its maximum at a test interval of 600 to 800 msec). We observed a certain parallelism between conditioning influences of PAG activation and effects of systemic injections of morphine. Isolated stimulation of LC by a short high-frequency train of stimuli evoked primary excitatory responses (complex EPSPs) in a part of the examined cortical neurons, while in other cells high-amplitude and long-lasting IPSP (up to 120 msec) were observed. Independently of the type of the primary response to PAG stimulation, the latter resulted in long-term (several seconds) suppression of the responses evoked in cortical cells by stimulation of the nociceptive inputs. The mechanisms of modulatory influences coming from opioidergic and noradrenergic brain systems to somatosensory cortex neurons activated due to excitation of high-threshold (nociceptive) afferent inputs are discussed.

Pervasive synchronization of local neural networks in the secondary somatosensory cortex of cats during focal cutaneous stimulation.

Extracellular discharges were recorded from 205 neurons in the secondary somatosensory (SII) cortex of isoflurane-anesthetized cats. Cross-correlation analysis was used to characterize the temporal coordination of SII neurons recorded during cutaneous stimulation with a focal air jet that moved back-and-forth across the distal forelimb. Over two-thirds of the recorded neuron pairs ( n=357) displayed significant levels of synchronized activity during one or both directions of air-jet movement. The probability of detecting correlated activity varied according to the distance separating the neurons. Whereas synchronized responses were observed in 82.3% of the pairs in which the neurons were separated by 200-300 micro m, the incidence of synchronization declined to 52.3% for neurons that were separated by 600-800 micro m. The distance between neurons also had a significant effect on the temporal precision of correlated activity. For neurons that were separated by 200-300 micro m, synchronized responses in the cross-correlograms (CCGs) were characterized by narrow (0.5-1.0 ms) peaks at time zero. For SII neurons that were more widely separated, the peak half-widths were substantially broader and more likely to be displaced from time zero. Analysis of directional sensitivity indicated that only 14.2% of the correlated neurons displayed a directional preference for synchronized activity. By comparison, 63.4% of the neurons displayed a directional preference in their discharge rate. These results indicate that stimulus-induced synchronization is a prominent feature among local populations of SII neurons, but synchronization does not appear to play a critical role in coding the direction of stimulus movement. A comparison of these results with those obtained from similar experiments conducted in primary somatosensory (SI) cortex indicates that neuronal synchronization is more likely in SII cortex. This finding is discussed with respect to the known functional differences between the SI and SII cortical areas.

Long-term increases in neuronal activity in the motor cortex evoked by simultaneous stimulation of the thalamus and somatosensory cortex in cats

Experiments on anesthetized cats were used to study the activity of motor cortex neurons (field 4 gamma) in response to separate and simultaneous stimulation of the ventrolateral nucleus of the thalamus and the somatosensory cortex (field 2) of the brain. Long-term potentiation of motor cortex neuron activity in response to simultaneous stimulation of the ventrolateral nucleus and somatosensory cortex arose only in regions receiving corticocortical projections from the stimulation site in the somatosensory cortex of the brain, while regions lacking corticocortical projections from the somatosensory cortex showed no such effect. Experiments demonstrated that the duration of increased motor cortex neuron activity following stimulation of the ventrolateral nucleus of the thalamus and somatosensory cortex was greater than one hour after recording was started. These data led to the conclusion that simultaneous stimulation of corticocortical and thalamocortical afferents can alter the level of neuronal activity in the motor cortex only in regions with convergent sensory inputs from the thalamus and somatosensory cortex of the brain.

Temporally structured impulse activity in spontaneously discharging somatosensory cortical neurons in the awake cat:: recognition and quantitative description of four different patterns of bursts, post-recording GFAP immunohistology and computer reconstruction of the studied cortical surface

We elaborated two methods used in two previous publications [J. Martinson, H.H. Webster, A.A. Myasnikov, R.W. Dykes, Recognition of temporally structured activity in spontaneously discharging neurons in the somatosensory cortex in waking cats, Brain Res. 750 (1997) 129–140 [16]; H.H. Webster, I. Salimi, A.A. Myasnikov, R.W. Dykes, The effects of peripheral deafferentation on spontaneously bursting neurons in the somatosensory cortex of waking cats, Brain Res. 750 (1997) 109–121 [21]]: (A) a procedure for detecting and classifying brief epochs of high-frequency extracellular impulse activity (bursts) recorded chronically in the somatosensory cortex of the awake cat, and (B) a modification of an immunohistochemical technique [L.A. Bevento, L.B. McCleary, An immunochemical method for marking microelectrode tracks following single-unit recordings in long surviving, awake monkeys, J. Neurosci. Meth. 41 (1992) 199–204 [5]] for visualization of electrode tracks and electrolytic lesions around the tip of tungsten-in-glass microelectrodes [D.M.D. Landis, The early reactions of non-neuronal cells to brain injury, Annu. Rev. Neurosci. 17 (1994) 133–151 [15]] weeks after lesions were made in cortex. The burst recognition and classification method uses an interval threshold to determine the beginning and end of one epoch [M. Armstrong-James, K. Fox, Effects of ionophoresed noradrenaline on spontaneous activity of neurons in rat primary somatosensory cortex, J. Physiol. (London), 335 (1983) 427–447 [3]] in the original sequence of interspike intervals (ISIs) to segregate and analyze separately a burst. The threshold is based on the duration of the shortest modal ISI found in the autocorrelogram [J. Martinson, H.H. Webster, A.A. Myasnikov, R.W. Dykes, Recognition of temporally structured activity in spontaneously discharging neurons in the somatosensory cortex in waking cats, Brain Res. 750 (1997) 129–140 [16]]. The technique allowed recognition of bursts with several distinctive patterns: (i) an initial, longer ISI followed by progressively shorter ones; (ii) an initially shorter ISI followed by progressively longer ones; (iii) patterns where the intermediate ISI could be either longer or shorter than surrounding ones; and (iv) consecutive ISIs of relatively equal duration. Among the cells discharging in bursts with equal ISIs, the technique allows recognition of cells generating only short (up to three to five intervals) bursts, and others generating mixtures of a short and long (up to six or more intervals) bursts. Finally, frequency distributions of the probability of encountering bursts having intervals of a stated length is described. The visualization of tracks from chronic recording experiments is important for relating neuronal function to a specific cytoarchitectural region and a specific cortical layer. Several modifications of the procedure of immunostaining for GFAP allows identification of recording sites in clearer relationship to the cytoarchitectonic details of cat somatosensory cortex. Themes: Sensory systems Topics: Somatosensory cortex and thalamocortical relationships

Effects of Conditioning Stimulation of the Central Amygdaloid Nucleus on Tooth Pulp-Driven Neurons in the Cat Somatosensory Cortex(SI).

To study the limbic control of nociception, we examined the effect of conditioning stimulation of the central amygdaloid nucleus (ACE) on tooth pulp-driven (TPD) neurons in the first somatosensory cortex (SI). Cats were anesthetized with N(2)O-O(2) (2:1) and 0.5% halothane, and immobilized with tubocurarine chloride. The tooth pulp test stimulus was applied by a single rectangular pulse (0.5 ms in duration and 3-5 times the threshold intensity for the jaw-opening reflex). Conditioning stimuli to the ACE consisted of trains of 33 pulses (300 microA) delivered at 330 Hz at intervals of 8-10 s. In 35 out of 61 of the slow (S)-type TPD neurons with latencies of more than 20 ms, conditioning stimulation in the ACE, especially in the medial division, markedly reduced the firing response to the pulpal stimulation. The inhibition of the firing rate in the S-type neurons was 74% of the control. In these S-type neurons, the neurons that were inhibited had significantly longer latencies compared to the non-inhibited neurons (45.0 +/- 17.6 ms, n = 32 vs. 34.8 +/- 10.5 ms, n = 26). In contrast, the ACE conditioning stimulation affected only one out of 18 fast-type TPD neurons with latencies of less than 20 ms. In addition, ACE stimulation had no effect on the spontaneous discharges of either S-type or F-type neurons. The ACE inhibitory effect on S-type neurons was not diminished by naloxone administration (1 mg/kg, I.V. ), while the blockade of histamine H(1)-receptor by diphenhydramine hydrochloride (0.5 mg/kg, I.V.) partially reversed the inhibitory effect. These results suggest that the ACE inhibits ascending nociceptive information to the SI and that this inhibition is mediated in part by histamine (H(1)) receptors. It seems likely that the antinociceptive effect is a neurophysiological basis for stress-induced analgesia (SIA).

Development of intrinsic connections in cat somatosensory cortex

We explored the development of clustered connections in cat somatosensory cortex by using intracortical injections of biocytin and carbocyanine dye (DiI). Biocytin injections in adults revealed clusters of retrogradely labeled cells, affirming earlier reports of the patchy nature of corticocortical connectivity in the adult cat somatosensory cortex. On postnatal days (PNDs) 1 and 3, a diffuse distribution of axons and labeled cells were found after DiI injections. A dramatic rearrangement of connections had taken place by PND 6. Well-defined clusters of labeled cells surrounded the injection site. Intercluster zones were relatively free of labeled cells and contained few labeled axons. At later ages (PNDs 12 and 15), a clear patchy distribution of intrinsic connections was seen. We analyzed neuronal clustering by using spatial point process analysis, which corroborated our qualitative observations. The density of labeled neurons was significantly higher in clusters than in the intercluster zones on PND 6. Autocorrelations run on profile plots of optical density values, along paths parallel to the edge of the injection sites (reflecting axonal distributions), revealed significant periodicity (P < .05; center-to-center approximately 750 pm) by PND 6. These data demonstrate that corticocortical connections in cat somatosensory cortex develop from a diffuse network at birth, which transforms into a patchy network by the end of the 1st postnatal week. This is substantially earlier than the development of local circuits in the cat visual cortex and may reflect fundamental differences in the organization of the two cortices.

Development of intrinsic connections in cat somatosensory cortex

Development of intrinsic connections in cat somatosensory cortex

Correlating cytoarchitecture and function in cat primary somatosensory cortex: the challenge of individual differences

Principles of organization for the primary somatosensory cortex are generalizations derived by examining data obtained in different individuals. The manner in which these data are combined influences the conclusions derived. We found the line representing the widest anteroposterior distance across the sigmoid gyrus to be a useful reference in the cat somatosensory cortex for combining and comparing electrophysiological and cytoarchitectonic data from different individuals when we constructed cytoarchitectonic and functional maps of the bank of the medial ansate sulcus; maps prepared from combined data sets had boundaries similar to those found among individuals. Nevertheless, we argue that, for reasons inherent to the nature of the cerebral hemispheres and cortical maps, such references will never allow combinations of data capable of defining a unique high resolution prototypical map of individual body parts; the somatotopic order of body representations is, as are certain other attributes of somatosensory cortex, idiosyncratic. The genetic, developmental and use-dependent reasons for this situation are discussed.

Transient and long-lasting effects of iontophoretically administered norepinephrine on somatosensory cortical neurons in halothane-anesthetized cats

We examined the modulatory effects of iontophoretically administered norepinephrine (NE) on the excitability of 117 neurons in cat somatosensory cortex. NE was released in the vicinity of neurons with receptive fields (31/117) while they were excited by somatic stimuli and near neurons without receptive fields (86/117) while they were excited by glutamate. In 54% of the neurons (n = 63) the effects were inhibitory, decreasing both the spontaneous and the driven activity. Most of these cells were found in the middle layers of cortex. In 36% of the neurons (n = 42), mostly located in either the upper or lower layers, the effects were excitatory, enhancing either or both driven and spontaneous activity, but 52% (n = 22) of this group displayed a transient phase of inhibition. NE usually had a proportionately greater effect on the spontaneous activity than on the evoked activity. Effects of specific NE agonists and antagonists indicated that alpha 2- and beta-receptors mediated the inhibition, but that alpha 1-receptors mediated the excitation. We hypothesize that when NE is released in cat somatosensory cortex, it modulates neuronal responses to afferent activity by generally reducing the excitability of cells in the middle layers and by increasing their signal-to-noise ratios. However, in the upper and lower layers NE will enhance neuronal activity, encouraging exchanges with other cortical areas. In addition, we tested and confirmed the hypothesis that short treatments with NE can modify the excitability of neurons in adult cat somatosensory cortex for long periods of time. Of 69 cells, 59% showed effects of NE lasting for more than 5 min. Response enhancement lasting for the duration of the recording session (5 to 36 min) occurred in 82% (18/22) of silent cells without a receptive field and in 63% (17/27) of spontaneously active cells without a receptive field, but only in 14% (2/14) of cells with a receptive field, suggesting an inverse relationship between cell excitability and this effect of NE. The magnitude of the enhancement followed the same relationship, being greatest for silent cells and least for cells with receptive fields (125, 58, and 49%, respectively). The long-lasting enhancement was blocked by an alpha 1-receptor antagonist in 6 of 9 neurons tested, while the administration of alpha 2- and beta-receptor agonists never produced a long-lasting effect, suggesting that the effect is mediated by alpha 1-adrenoceptors.