Rat Primary Somatosensory Cortex Research Articles

Development and lesion induced reorganization of the cortical representation of the rat's body surface as revealed by immunocytochemistry for serotonin

Immunocytochemistry with an antiserum directed against serotonin (5-HT) was used to assess the development of the representation of the body surface in the rat's primary somatosensory cortex (S-I). Within 1 hour of birth (P-O), 5-HT-positive fibers were present in the marginal zone, the cortical plate, and developing layers V and VI. Immunoreactivity in the marginal zone consisted of a thin band of coarse fibers oriented parallel to the pia. Only a small number of isolated fibers were visible in the cortical plate. A denser network of both coarse and fine fibers could be seen in presumptive layers V and VI. By the first hour of P-I, 5-HT-positive axons in the deeper cortical plate were organized into a crude representation of the rat's body surface. At this age, aggregates of fibers corresponding to the head, lower jaw, trunk, and forepaw could be clearly distinguished. These regions of dense 5-HT immunoreactivity consisted primarily of fine caliber axons that had invaded the lower part of the cortical plate. Dense aggregates of fine caliber axons were also visible in developing layers V and VI. Coarse 5-HT-positive fibers were visible in all layers, but they did not appear to contribute to the pattern that corresponded to the body surface. By the first hour of P-2, the map of the body surface in S-I was more refined and a row-related organization of 5-HT-immunoreactive fibers was visible in the portion of the cortex representing the vibrissa pad. The laminar distributions of coarse and fine caliber serotoninergic axons at this age were essentially the same as on P-I. By P-2.5 (60 hours after birth), patches of 5-HT-positive fibers corresponding to individual vibrissa follicles were clearly evident. These consisted of dense aggregates of fine caliber axons that were centered in presumptive layer IV, but which also extended above and below this lamina. Over the next 3 days, the pattern continued to mature. By P-4, dense 5-HT labelling was also visible in the secondary somatosensory cortex (S-II). By the beginning of P-5, clusters of fibers corresponding to more rostral facial hairs and individual digits within the forepaw representation could also be discerned. By P-12, the differential distribution of 5-HT fibers in S-I was no longer visible. Thus, immunocytochemistry for serotonin showed a representation in S-I homeomorphic with the body surface prior to the age at which it can be discerned with other methods thought to reveal thalamocortical axons. Transection of the infraorbital nerve (ION) on the day of birth altered the organization of the vibrissal representation in the contralateral cortex from the earliest age at which it could be detected by 5-HT immunocytochemistry in normal animals. However, the departure from the normal organization was gradual. Row-related organization was clearly visible in the cortices of rats sacrificed on P-3, but not in those of rats that were killed on P-5. These results suggested that the organization of the 5-HT innervation of the cortex may be guided by thalamic afferents and further that some aspects of this guidance persist, albeit temporarily, after ION transection on P-0. The 5-HT immunoreactivity that we observed in the developing somatosensory cortex was not contained in thalamocortical axons. Unilateral electrocautery of the ventrobasal thalamus on P-4 did not reduce the density or alter the pattern of the 5-HT innervation of the cortex in rats that were examined on P-6.

Two inhibitory postsynaptic potentials, and GABAA and GABAB receptor-mediated responses in neocortex of rat and cat.

1. Pyramidal neurones from layers II and III of the rat primary somatosensory cortex and cat primary visual cortex were studied in vitro. Inhibitory postsynaptic potentials (IPSPs) and responses to exogenously applied gamma-aminobutyric acid (GABA) and its analogue baclofen were characterized. The results from rats and cats were very similar. 2. Single electrical stimuli to deep cortical layers evoked a sequence of PSPs in the resting neurone: (a) an initial, brief excitation (EPSP), (b) a short-latency, fast inhibition (the f-IPSP) and (c) a long-latency, more prolonged inhibition (the l-IPSP). The f-IPSP was accompanied by a large conductance increase (about 70-90 nS) and reversed polarity at -75 mV; the l-IPSP displayed a relatively small conductance increase (about 10-20 nS) and reversed at greater than -90 mV. 3. Focal application of GABA near the soma evoked a triphasic response when measured near the threshold voltage for action potentials: (a) the GABAhf (hyperpolarizing, fast) phase was very brief and was generated by a large conductance increase with a reversal potential of -78 mV, (b) the GABAd (depolarizing) phase also had a high conductance but reversed at -51 mV, (c) the GABAhl (hyperpolarizing, long-lasting) phase had a relatively low conductance and reversed at -70 mV. The GABAhf response was specifically localized to the soma, whereas the apical or basilar dendrites generated predominantly GABAd responses. 4. Baclofen, a selective GABAB receptor agonist, caused a small (about 2 mV), slow hyperpolarization of the resting potential, which reversed at -90 mV. Saturating baclofen doses increased membrane conductance by a maximum of about 12 nS. Baclofen depressed the amplitude and conductance of PSPs; when baclofen was focally applied near the soma. IPSPs were selectively depressed. 5. The GABAA receptor antagonists bicuculline methiodide or picrotoxin (10 microM) greatly depressed f-IPSPs, but either enhanced or did not affect l-IPSPs. Concomitantly, GABAhf and GABAd responses were antagonized, leaving a more prominent GABAhl response that reversed polarity at a more negative level of -87 mV. Baclofen responses were unaffected by bicuculline and picrotoxin. Extracellular barium abolished the baclofen response, and shifted the reversal potentials of the GABAd and GABAhl responses in the positive direction; the GABAhf response was unaffected. 6. Both focal GABA and f-IPSPs strongly depressed the intrinsic excitability of pyramidal neurones. Each greatly increased spike threshold and abolished or vastly reduced the capacity of the cells to fire repetitively during intense stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Corticocortical connections within the primary somatosensory cortex of the rat.

Corticocortical connections within the primary somatosensory (SI) cortex of rat were investigated by using discrete injections of retro- and orthogradely transported neuroanatomical tracers (including HRP, WGA, PHA-L, and 3H-leucine). Tangential and vertical connections were defined with respect to the cytoarchitectonic divisions within the rat SI, specifically: (1) the "granular zones" (GZs), characterized by their dense layer IV granular aggregates, which receive the majority of direct ventroposterior (VP) thalamocortical terminations, (2) the "perigranular zones" (PGZs), the less-granular cortical matrix just surrounding the GZs, and (3) the "dysgranular zones" (DZs), the larger dysgranular regions lying centrally within and just lateral to the SI. Receptive fields recorded in the granular zones are small and discrete, whereas in the perigranular zones and especially in dysgranular zones they exhibit complex sensory convergence. A major aim of this study was to determine whether the pattern of intracortical connectivity within the SI is compatible with these observed physiological differences. In general, the perigranular and dysgranular zones contained more profuse systems of corticocortical connections than did the granular zones. For example, discrete tracer injections in the perigranular zones produced "walls" of labelling throughout the adjacent perigranular zones, while adjacent granular zones were relatively empty. Nevertheless, the granular zones were filled with dendritic branches of neurons in adjacent perigranular zones. Since these dendrites could presumably receive direct VP thalamocortical contacts, they represent one path through which this thalamic sensory information might be transmitted to the perigranular zones. Further transmission to the dysgranular zones might be subserved by a topographically organized system of reciprocal interconnections that was found between the perigranular zones and dysgranular zones. In coronal sections, labelling produced by relatively distant injections of either retro or orthograde tracers generally appeared in a columnar distribution, and was localized in perigranular zones and dysgranular zones. Within these zones, orthograde labelling consisted of vertically oriented axons emitting collateral sprays of terminals in all layers. Retrograde neuronal labelling (composed almost exclusively of pyramidal cells) was greatest in supragranular layers. Proximal to the injection site, labelling tended to spread out from these columns into supra- and infragranular layers in adjacent granular zones.(ABSTRACT TRUNCATED AT 400 WORDS)

Field potentials evoked in rat primary somatosensory cortex (SI) by impulses in cutaneous Aβ- and C-fibres

A projection of cutaneous C-afferent fibres to the contralateral primary somatosensory cortex (SI) was examined in the halothaneanaesthetized rat. Field potentials evoked by electrical stimulation of the right sural nerve were recorded at the surface of and intracortically within the left SI cortex. A late surface positive potential (latency 110–190 ms, mean 136 ms) was evoked by sural stimulation at a strength that activated A- and C-fibres. A selective anodal block of impulse conduction in A-fibres proximal to the stimulating electrodes showed that impulses in C-fibres generate the late potential also in the absence of a preceding A-fibre input. Stimulation of the sural nerve at two sites caused a shift in latency of the late potential corresponding to a conduction velocity of the primary afferent fibres of less than 1.0 m/s. The distribution of the C-fibre-evoked potential in SI was similar to that of the Aβ-fibre evoked ‘primary’ potential suggesting that the investigated projection of cutaneous C-fibres to SI has a somatotopic organization.

Effects of ionophoresed noradrenaline on the spontaneous activity of neurones in rat primary somatosensory cortex.

Changes in spontaneous activity of rat S1 cortical neurones with identified receptive fields were investigated in reply to ionophoresed noradrenaline (NA). Extracellular levels of NA were maintained constant by continuous electrochemical analysis at the carbon fibre recording tip of the multibarrel micro-electrode. In the absence of NA there were clear differences in spike amplitude, firing rate and pattern of firing of deep (800-1400 micron) and superficial (0-800 micron) cells. Superficial cells responded to low (5 X 10(-8) to 5 X 10(-7) M) NA concentrations with simple inhibition. Recovery occurred within a minute or so of extracellular NA concentrations falling below detectable (10(-8) M) levels. Increases in local concentration merely stopped cells firing. In contrast, cells located in the deep zone could often be excited by very low NA concentrations (less than 10(-8) M), with inhibition occurring at levels 10-100 times greater. Most cells, however, were inhibited, with threshold doses for a 50% change in firing rate much higher than for superficial cells. Some cells in the deep zone showed sustained increases in firing rate following an ionophoretic trial. This could occur for periods of up to 1 h after ceasing a trial. Such effects could be produced by levels as low as 10(-7) M-NA. Interspike interval analysis for deep cells suggested that their spontaneous activity resembled that established for slow-wave sleep. During and after excitation by NA the pattern of firing of small groups of these cells changed to that established for the waking state. The effect could persist for up to 1 h following a short (2-5 min) ionophoretic trial.

The functional status and columnar organization of single cells responding to cutaneous stimulation in neonatal rat somatosensory cortex S1.

1. An investigation was carried out on single cells in 7 day old rat primary somatosensory cortex, which responded to cutaneous stimulation using mechanical pulses. 3 percent of cells encountered showed stable spontaneous activity, whereas 88 percent were silent in the absence of intentional stimulation. The remainder showed unstable spontaneous activity. In contrast, the great majority of adult cells were spontaneously active in the absence of stimulation, under similar conditions of urethane anaesthesia. 2. The distribution within cortical layers of cutaneously driven cells was similar in adult and 7 day old rats, and similar to that found in adult mammalian cortex by other workers. 3. 7 day old cells showed diminished excitability to cutaneous stimulation with stimuli at intervals below 10-15 sec, whereas adult cells could be successfully repetitively driven with stimuli at intervals of 500 msec. The low ability of the immature cells to follow repetitive cutaneous stimulation is not due to an overall depression of these cells excitability per se. Latencies of unitary responses in these immature cells were about sixfold those found in equivalent cells at maturity. 4. Columnar organization at seven days of age was similar in outline to that of the adult, but much less discrete. Receptive fields were considerably larger at 7 days and evidence is given that this may be due to inadequate surround inhibition. Immature vibrissae-driven units were directionally selective. 5. At 7 days of age, long inter-spike intervals were rare in spontaneously active cells with the result that inter-spike interval histogram distributions (i.h.s.) were approximately normal. Corresponding i.h.s. of adult cells invariably showed skew distributions. 6. Tactile stimulation of centre receptive fields produced an increase in short and long intervals from spontaneously active cells at each age. In contrast to adult cells, the immature cells commonly responded cyclically, with alternating phases of increased and decreased firing rate for periods of up to 3 sec following punctate stimulation. 7. Decrease in spontaneous firing rate, following the first phase of excitation, was profound in 7 day old cells, and implied that inhibitory mechanisms operate at an early age in the rat somatosensory system. These mechanisms also appear to contribute to cyclical activity of 7 day old cells when driven by punctate cutaneous stimulation.

A SENSORY CORTICAL REPRESENTATION OF THE VAGUS NERVE: WITH A NOTE ON THE EFFECTS OF LOW BLOOD PRESSURE ON THE CORTICAL ELECTROGRAM

A SENSORY CORTICAL REPRESENTATION OF THE VAGUS NERVE: WITH A NOTE ON THE EFFECTS OF LOW BLOOD PRESSURE ON THE CORTICAL ELECTROGRAM