Ontogenesis of receptive field characteristics in the dorsal lateral geniculate nucleus of the rabbit

Summary The responses of rabbit dorsal lateral geniculate neurons to light or optic nerve shock were tested for 415 units in 43 rabbit pups 2–20 days of age. Units were driven by optic nerve shock at the youngest ages tested, but could not be driven by light until postnatal day six. Examples of each of the three prominent categories of receptive fields found in the adult were first observed at 8 days of age. Cells with receptive field properties not characteristic of the dorsal lateral geniculate nucleus of the adult were encountered until 17 days of age. The percentage of neurons with uniform and motion sensitive receptive fields approached adult levels soon after eye opening (11–12 days) but the percentage of cells with concentric receptive fields showed a steady increase throughout the neonatal period studied. The relevance of our data to the development of the visual response in the dorsal lateral geniculate nucleus and striate cortex is discussed.     

Neuronal activity in rabbit neostriatum during classical eyelid conditioning

Extracellular multiple- and single-unit recordings were made from the neostriatum of rabbits during classical eyelid conditioning. Neostriatal neurons processed information regarding the conditioned auditory stimulus (CS) and conditioned eyelid response (CR) as well as the unconditioned stimulus/response (US/UR). These data are consistent with previous reports that neostriatal neurons respond to movement and movement-related sensory stimuli. In most cases, neostriatal neurons increased activity to the US during the early phase of training, but to the CR as training progressed. A close temporal correlation was found between neuronal activity and CR onset with unit discharges typically preceding CR onset by 10–50 ms. The activity of some multiple and single units was monitored after injection of haloperidol, a neuroleptic and dopamine antagonist known to disrupt neostriatal function. Interestingly, haloperidol caused a greater disruption of CRs at low-intensity than at high-intensity CSs, but conditioning-related neuronal activity was disrupted equally at both intensities. These data are discussed in terms of a possible role for the neostriatum in eyelid conditioning.     

A pragmatic comparison of noise burst and electric shock unconditioned stimuli for fear conditioning research with many trials

Several methods that are promising for studying the neurophysiology of fear conditioning (e.g., EEG, MEG) require a high number of trials to achieve an adequate signal‐to‐noise ratio. While electric shock and white noise burst are among the most commonly used unconditioned stimuli (US) in conventional fear conditioning studies with few trials, it is unknown whether these stimuli are equally well suited for paradigms with many trials. Here, N = 32 participants underwent a 260‐trial differential fear conditioning and extinction paradigm with a 240‐trial recall test 24 h later and neutral faces as conditioned stimuli. In a between‐subjects design, either white noise bursts (n = 16) or electric shocks (n = 16) served as US, and intensities were determined using the most common procedure for each US (i.e., a fixed 95 dB noise burst and a work‐up procedure for electric shocks, respectively). In addition to differing US types, groups also differed in closely linked US‐associated characteristics (e.g., calibration methods, stimulus intensities, timing). Subjective ratings (arousal/valence), skin conductance, and evoked heart period changes (i.e., fear bradycardia) indicated more reliable, extinction‐resistant, and stable conditioning in the white noise burst versus electric shock group. In fear conditioning experiments where many trials are presented, white noise burst should serve as US.     

Stimulation at a site of auditory-somatosensory convergence in the medial geniculate nucleus is an effective unconditioned stimulus for fear conditioning.

The medial division of the medial geniculate nucleus (MGm) and the posterior intralaminar nucleus (PIN) are necessary for fear conditioning to an auditory conditioned stimulus (CS), receive both auditory and somatosensory input, and project to the amygdala, which is involved in production of fear conditioned responses. If CS-unconditioned stimulus (US) convergence in the MGm-PIN is critical for fear conditioning, then microstimulation of this area should serve as an effective US during classical conditioning, in place of standard footshock. Guinea pigs underwent conditioning (40-60 trials) using a tone as the CS and medial geniculate complex microstimulation as the US. Conditioned bradycardia developed when the US electrodes were in the PIN. However, microstimulation was not an effective US for conditioning in other parts of the medial geniculate or for sensitization training in the PIN or elsewhere. Learning curves were similar to those found previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for fear conditioning to acoustic stimuli.     

Neural activity in the medial geniculate nucleus during auditory trace conditioning

 In classical trace conditioning the acquisition of a conditioned response (CR) is possible even though an interval (the trace interval) elapses between the conditioned stimulus (CS) and unconditioned stimulus (US). This implies that some neural representation of the CS (the stimulus trace) is able to support association between the two stimuli. The medial geniculate nucleus (MGN), particularly the medial division (mMGN), has been identified as one site in the auditory pathway where associative related changes in neural activity occur. If neurons in the MGN are involved in such a sensory trace and in acquisition of a CR, then it is expected that activity following an acoustic CS should be related to both stimulus and response. This study examined the extracellular activity of single units in the MGN during differential auditory trace conditioning of the rabbit nictitating membrane response (NMR). Two 150-ms tones (600 Hz and 1200 Hz) served as CS+ and CS–, and the US was periorbital electrostimulation. Changes in activity during the stimulus and trace interval were largest in the medial and dorsal MGN divisions on CS+ trials and on trials in which a CR was made. Examination of probe stimuli of short (50 ms) and long (600 ms) duration suggested that both CR latency and activity changes in the trace interval were related to stimulus duration and time-locked to stimulus offset. Comparisons of neural activity on the basis of fast or slow CR responses revealed different patterns of response – activity on fast CR trials was generally greater and tended to occur earlier. These results suggest that MGN neurons are involved in the maintenance of a sensory memory trace and possibly play a part in CR generation and timing. Received: 2 June 1995 / Accepted: 30 August 1996     

Cell analog of the conditioned reflex to cortical electrical stimulation

Conclusion The possibility of obtaining a cellular analog of a CR was investigated on the sensory motor cortex of the unanesthetized rabbit. Single unit activity was analyzed. Stimulation through surface cortical electrodes 2.5–12mm away from the recording microelectrode was used as the conditional stimulus (CS). The unconditional stimulus (US) was stimulation through two closely situated electrodes.With short (1–5 sec) inter-trial intervals, and with a US of sufficient strength and duration, a well-marked augmentation of the excitatory response to CS was obtained in most neurons. However, it was short (less than 30 sec) and, as a rule, it was the analog not of a CR but of a pseudo conditioned reflex, for it did not require pairing of the CS and US, but appeared after one or a few presentations of the US alone.In 9 of the 21 neurons tested with longer intervals between trials (7–120 sec) conditioned-reflex changes were observed. These changes consisted of the appearance (or modification) of a response to the CS during conditioning trials and disappearance during extinction trials, or of the appearance of a response at the time corresponding to the missing US when the US was intermittently omitted. The character of the conditioned-reflex changes (activation or inhibition) was the same as that of the cell's response to the US. Changes appeared after the first 10–20 conditioning trials and recovery after extinction took place more rapidly than initial conditioning, i.e., in 5–10 combinations.     

Arousal-related associative response characteristics of dorsal lateral geniculate nucleus neurons during acoustic Pavlovian fear conditioning

This research determined whether fear-conditioned, acoustic stimuli induce thalamic arousal reflected in associative responses in dorsal lateral geniculate nucleus (dLGN) neurons. Rabbits received a Pavlovian discriminative fear conditioning procedure in which one tone conditioned stimulus (CS +) was always paired with an aversive unconditioned stimulus (US) and another tone (CS-) was never paired with the US. Responses of single dLGN neurons to random CS+ and CS- presentations were then recorded. Nine of 15 recorded neurons demonstrated significantly greater firing during the CS+ versus the CS-. Their spontaneous activity demonstrated tonic firing during increased neocortical arousal and burst firing during decreased neocortical arousal. The results demonstrate that dLGN neurons show associative responses to fear-conditioned, acoustic stimuli and present a model for investigating the neural circuits by which such stimuli affect sensory processing at the thalamic level.     

Single neurons in the dentate gyrus and CA1 of the hippocampus exhibit inverse patterns of encoding during trace fear conditioning

Trace fear conditioning is a hippocampus-dependent learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single-neuron activity was recorded simultaneously from the dentate gyrus (DG) and CA1 of rats during unpaired pseudoconditioning and subsequent trace fear conditioning. Single neurons in DG showed a progressive increase in learning-related activity to the CS and US across trace fear conditioning. Single neurons in CA1 showed an early increase in responding to the CS, which developed into a decrease in firing later in trace conditioning. Correlation analyses showed that DG and CA1 units exhibit inverse patterns of responding to the CS during trace fear conditioning.     

Does activation of the baroreceptors reinforce differential Pavlovian conditioning of heart rate responses?

High- and low-pitched tones (CS+ and CS?) signalled baroreceptor stimulation or inhibition (US+ and US-) on 6-s conditioning trials (n= 128). Baroreceptor stimulation was induced by the phase-related external suction (PRES) method of Rau et al. (1992) in which a brief pulse of negative external pressure is applied to the neck at systole and one of positive pressure at diastole within each cardiac cycle (the reverse sequence is used for baroreceptor inhibition). Changes in heart period (R-R intervals) confirmed that PRES manipulated the baroreceptors in the presence of CS+ and CS? without habituation over conditioning trials. However, conditioned heart period responses were not observed on test trials (n= 32) in which CS+ and CS? were presented with the baroreceptor manipulation removed. Subjects were unable to state which CS had signalled baroreceptor stimulation and inhibition when given PRES-alone trials after the conditioning phase (differential attention thus controlled). These results (a) confirm that the differential effect of the two PRES stimuli was specific to the baroreceptors and (b) support earlier studies that have found that differential conditioning is impaired when CS?US relations are not processed in attention. We discuss implications regarding when baroreceptor firing might be discriminable and reinforcing.     

Dynamics of single unit activity in the association cortex of waking cats during defensive conditioning

Responses of neurons in association area 5 during defensive conditioning to acoustic stimulation were studied in chronic experiments on cats. As a rule the neurons responded by excitation to presentation of conditioned and unconditioned stimuli. During the conditioned reflex unit responses usually appeared in the first 50 msec after the beginning of acoustic stimulation,i.e., they were connected with the action of the conditioned stimulus and not with manifestations of conditioned-reflex motion. The most significant changes in responses of cortical association units were observed in the initial period of conditioning. During stabilization of the conditioned reflex, responses of some neurons became stabilized, whereas in other neurons the spontaneous activity and intensity of responses increased, and in a third group the response to one of the stimuli disappeared. This last result indicates a switch during conditioning from polysensory unit responses to monosensory specialized responses. Extinctive inhibition was found to consist of a gradual decrease in the level of the spike discharge and its approximation to spontaneous activity, i.e., to be passive in character.Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 563–572, November–December, 1978.     

Luxotonic responses of units in macaque striate cortex.

1. Single units in striate cortex were studied in alert macaques while they viewed a ganzfeld. Of the 385 well-isolated units studied for 10 min to 2 h, 24% gave "luxotonic" responses, i.e., their rate of discharge for 1 min or more in diffuse, featureless, wideangle illumination (20-450 cd/m2) was at least double that during a comparable period in darkness, or vice versa, and not attributable to eye movements of blinking. Those discharging faster in the light, "photergic" units, outnumber those responding to darkness, "scotergic" units 1 by 4:1. 2. In the lateral geniculate nucleus, on the other hand, among 46 units studied, 28% were luxotonic, but scotergic units were the more common. Both types were present in both magno- and parvocellular laminae. 3. For striate cortex two-thirds of the luxotonic units were binocular. Some showed highly similar response for either eye alone, and essentially no summation binocularly; others had grossly differing responses from each eye, and complex binocular interaction. 4. Many units of all types at striate cortex showed significant modulation of their activity consequent to saccadic eye movements made in darkness, whereas comparable modulation was not observed at the lateral geniculate nucleus. 5. On the basis of these and other findings it is concluded that luxotonic cortical activity is prominent probably only in alert primates, and that this is a consequence of the fact that all retinal ganglion cells in primates synapse in the lateral geniculate nucleus (Ref. 9). Possible functions range from mere trophic input to providing a veridical image or a scaling factor for maintenance of perceptual constancy in the face of varying levels of general illumination.     

Electrical properties affecting discharge of units of the mid and posterolateral thalamus of conscious cats

Discharge properties in response to intracellularly applied, rectangular currents were measured in units of the mid (lateralis dorsalis and centrolateral nuclei) and posterolateral (lateralis posterior and pulvinar nuclei) thalamus of conscious cats. A separate aim was to determine if neuronal excitability changed in association with changes in stimulus-evoked activity after the animals were trained to discriminate between two acoustic stimuli when performing a conditioned motor response. Low threshold spike (l.t.s.) discharges were observed in three of 272 cells given 1 nA intracellular, hyperpolarizing current pulses of 40 ms duration. This finding supports the view that thalamic neurons of conscious animals operate mainly in the relay as opposed to the oscillatory mode. Application of larger and longer hyperpolarizing currents in the cells produced rebound l.t.s. discharges, supporting the expectation that most thalamic neurons are capable of producing this type of discharge. Decrements of spike afterhyperpolarizations (AHP) and broadening of spike bases upon repeated discharge also were observed in each area of the thalamus studied. After conditioning, changes were found in the posterolateral thalamus (but not in the mid-thalamus) in the proportions of cells with spontaneous, rapid (>/=50 Hz), repetitive, discharges (RRD) and rapid, sustained discharges at rates >/=100 Hz during application of depolarizing current (RSD). In the posterolateral thalamus the percentage of units responding to 1 nA depolarization with RSD fell from 71% before conditioning to 45% after conditioning. The percentage of cells with RRD decreased from 69% to 46%. The changes were accompanied by a 3 mV hyperpolarization of the membrane potentials of the cells and a decrease in baseline activity. After conditioning, increases in excitability were found in cells of the mid thalamus that responded selectively to the click conditioned stimulus (CS) that elicited the conditioned response, and decreases in excitability were found in cells of the posterolateral thalamus that responded to the discriminative acoustic stimulus (DS) to which the animals were trained not to respond. An earlier study showed a potentiation of discharge in response to the CS in units of the midthalamus after similar conditioning and a reduction of the proportion of DS responsive units and peak discharge to the DS in units of the posterolateral thalamus. We conclude that the discharge properties of units of the mid and posterolateral thalamus can change to support discrimination between acoustic stimuli of different functional significance after conditioning.     

The formation of auditory fear memory requires the synthesis of protein and mRNA in the auditory thalamus

The medial geniculate nucleus of the thalamus responds to auditory information and is a critical part of the neural circuitry underlying aversive conditioning with auditory signals for shock. Prior work has shown that lesions of this brain area selectively disrupt conditioning with auditory stimuli and that neurons in the medial geniculate demonstrate plastic changes during fear conditioning. However, recent evidence is less clear as to whether or not this area plays a role in the storage of auditory fear memories. In the current set of experiments rats were given infusions of protein or messenger RNA (mRNA) synthesis inhibitors into the medial geniculate nucleus of the thalamus 30 min prior to auditory fear conditioning. The next day animals were tested to the auditory cue and conditioning context. Results showed that rats infused with either inhibitor demonstrated less freezing to the auditory cue 24 h after training, while freezing to the context was normal. Autoradiography confirmed that the doses used were effective in disrupting synthesis. Taken together with prior work, these data suggest that the formation of fear memory requires the synthesis of new protein and mRNA at multiple brain sites across the neural circuit that supports fear conditioning.     

Electrophysiological evidence for formation of new corticorubral synapses associated with classical conditioning in the cat

The present study was performed to clarify whether or not structural plasticity of synaptic connections underlies classical conditioning mediated by the red nucleus (RN) in the cat. Conditioned forelimb flexion is established by pairing electrical conditioned stimuli (CS), applied to corticorubral fibers at the cerebral peduncle (CP), with a forelimb skin shock (the unconditioned stimulus, US), but not by applying the CS alone or by pairing the CS and US at random intervals. In our previous study, it was shown that the firing probability of rubrospinal neurons (RN neurons) in response to the CS was well correlated with acquisition of the conditioned forelimb flexion and that the primary site of neural change underlying establishment of the conditioned forelimb flexion was suggested to be at corticorubral synapses. In the present study, we investigated corticorubral excitatory postsynaptic potentials evoked by CP stimulation (CP-EPSPs), in order to identify the neuronal mechanism underlying establishment of classical conditioning. In normal cats, CP-EPSPs had a typical slow-rising phase, which has been attributed to the distal location of corticorubral synapses on the dendrites of RN neurons. In contrast, in animals that received paired conditioning, subsequent CP stimulation evoked potentials with a fast-rising time course. In control groups of cats that received CS alone, CS randomly paired with the US, or only the same surgical operations as the conditioned animals, most of the CP-EPSPs displayed slow-rising EPSPs that similar to those observed in normal cats. The mean time from onset to peak of the potentials in the conditioned animals was significantly shorter than that seen in other groups. Therefore, the appearance of a fast-rising potential correlates well with acquisition of the conditioned forelimb flexion. The amplitude of the fast-rising potential was gradually changed with stimulus intensity. It had a short onset latency following CP stimulation (0.9 ms), which was similar to that of the slow-rising EPSP in normal cats. It followed high-frequency stimulation up to 100 Hz. These results suggest that the newly appearing, fast-rising potential was a monosynaptically evoked EPSP. Fast-rising EPSPs were also induced by stimulation of the sensorimotor cortex (SM). Since the SM-EPSP was occluded by the CP-EPSP, the SM cortex is, at least in part, a likely source of fast-rising EPSPs. Fast-rising SM-EPSPs were also observed at the unitary level. The SM-EPSPs in the conditioned animals exhibited somatotopical representation in their cortical origin, as has been described in normal cats. The electrotonic length was calculated from the voltage transient responses to current steps injected into the RN neurons. There was no concomitant change in the electrotonic length following the classical conditioning. Furthermore, the fastrising EPSPs were often observed as if they were superposed on the slow-rising EPSPs that were observed in normal animals. These observations suggest that the appearance of fast-rising EPSPs is due to the formation of new corticorubral synapses on the somata or the proximal dendrites of the RN neurons, and not as a result of a reduction in the electrotonic length of the RN neurons. The present study provides further evidence that this type of structural plasticity of synaptic connections underlies establishment of the classically conditioned forelimb flexion.     

Computation of color and brightness differences by neurons in the lateral geniculate nucleus of the rabbit

The responses of 51 neurons in the lateral geniculate nucleus of the rabbit to substitution of colored stimuli different brightness and stimuli differing only in intensity were studied. Neurons in the geniculate nucleus, like neurons in the visual cortex, were found to respond with initial phasic discharges at 50–90 msec after stimulus substitution, the magnitudes of these responses correlating with the interstimulus differences; neurons also showed prolonged tonic responses in which the spike frequency depended on the intensity of the stimulus presented. Analysis of phasic responses allowed two groups of neurons to be identified: some were specialized to discriminate stimulus intensity only, while others were specialized to discriminate both the intensity and the color tone of the stimulus. Use of the magnitude of the early phasic discharge as a measure of the difference between stimuli yielded a sensory space for lateral geniculate nucleus neurons. The responses of neurons in the first group (44 cells, 86%) produced a two-dimensional achromatic space with two axes-brightness and darkness; this structure appeared independently of whether stimuli were of the same or different color tones. The phasic responses of neurons in the second group (seven of 51, 14%) generated a four-dimensional space with two color and two achromatic axes. The color and achromatic spaces of lateral geniculate nucleus neurons were analogous to the spaces previously identified for neurons in the rabbit visual cortex using the same stimulation conditions. The sensory spaces reconstructed on the basis of neuron phasic discharges essentially coincided with the spaces obtained from analysis of the N85 component of visual evoked potentials in rabbits, which provides support for the vector information coding principle in the visual analyzer. The tonic discharges of most lateral geniculate nucleus neurons correlated linearly with changes in stimulus intensity and can be regarded as reflecting a pre-detector function for the visual cortex detector neurons. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 56, No. 1, pp. 75–85, January–February, 2006.     

Modulations of the lateral geniculate nucleus cell responses by a second discrete conditioning stimulus: implications of the superior colliculus in rabbits

Summary This study analyzes the interactions between two discrete stimuli located in the visual field of the rabbit at the lateral geniculate level. Single unit recordings were carried out simultaneously from the superior colliculus (SC) and lateral geniculate nucleus (LGN) in anesthetized and paralyzed rabbits. A first conditioning stimulus (most often a moving target) was positioned in the receptive field of the collicular cell to ensure activation of the retino-collicular path. A second test stimulus was introduced into the receptive field of the LGN cell. The presentation of this latter stimulus was timed so as to fire the geniculate cell at various delays after the collicular neuron had responded to its own stimulus. The spontaneous firing of each cell was unaffected by the stimulus appropriate to the complementary unit. The conditioning collicular stimulus produced increases or decreases in geniculate responses. This modulation may eventually reduce the direction specificity of a geniculate unit. The fluctuations of the geniculate responses peaked 200 to 300 ms after collicular cells had responded. In a separate series of experiments the influence of the conditioning stimulus on geniculate responses was abolished when the SC was locally inactivated. These results suggest that the well documented colliculo-geniculate system mediates the interactions of several stimuli in the visual field. The outcome of this processing results in a modulation of geniculate responses.     

Synaptic organization of the nucleus of the optic tract in the rabbit: A combined Golgi-electron microscopic study

Summary The organization of the nucleus of the optic tract was investigated with light and electron microscopy in combination with Golgi impregnation.In Golgi material, neurons ranged in size from 10 to 25 m with three to seven principal dendrites extending predominantly parallel to the fibres of the optic tract, irrespective of their location within the nucleus. In some areas dendrites extended into the neuropil of the adjacent dorsal terminal nucleus of the accessory optic system and the posterior pretectal nucleus. Occasionally spines and appendages were observed. The fine structure of the nuclei, perikarya and the dendritic arborization did not allow a well-defined distinction between interneurons and projection neurons.The synaptic organization of the nucleus of the optic tract showed great resemblance to the neuropil of the lateral geniculate nucleus and the superior colliculus. Similar types of presynaptic terminals were noticed: (i) R-terminals were either large and scalloped or small and regular in outline with spherical vesicles and electron-lucent mitochondria, and showed asymmetric contact zones; (ii) F-terminals with flattened vesicles, opaque mitochondria and symmetric contact zones; (iii) RLD-terminals with spherical vesicles and electron-dense mitochondria and asymmetric contact zones; (iv) P-terminals with pleomorphic vesicles and electron-lucent or opaque mitochondria and asymmetric synaptic thickenings. These different types of terminal were found isolated in the neuropil or in clusters of synapses.The most striking differences between the nucleus of the optic tract and the lateral geniculate nucleus were the relative scarcity of F-terminals in the clusters, the paucity of triadic arrangements and the relatively small size of the R-terminals. The differences in ultrastructure may be related to retinal W-type ganglion cells, which form the main retinal input to the nucleus of the optic tract and could also be related to the physiologically identified direction-selective units within the nucleus of the optic tract.     

Processing of amplitude modulated sounds in the medial geniculate body of squirrel monkeys

Summary The responses of single and multi units in the medial geniculate body of the squirrel monkey (Saimiri sciureus) to modulation frequency, modulation depth and changes in absolute intensity of sinusoidally amplitude modulated (AM) sounds were studied. Both spike-frequency and spike rate modulation were used as a measure for neuronal response. Spike rate modulation was derived from FFT (Fast-Fourier-Transformation) analysis of the PSTHs. In all cases (n = 133) spike rate modulation was shown to be dependent on the stimulus modulation frequency: Most neurons responded best to one modulation frequency, i.e., they showed a modulation transfer function with bandpass characteristic; only a few displayed a low pass or multiple peaked transfer characteristic. The majority of the neurons responded best in a range from 4 to 64 Hz, with a peak at 32 Hz and a median at 16 Hz. Such modulation frequencies are common in parts of the species vocal repertoire.     

Extracellular matrix and development of lamination in the dorsal lateral geniculate nucleus in the tree shrew (Tupaia belangeri).

 In the tree shrew (Tupaia belangeri), the cytoarchitectonic lamination of the lateral geniculate nucleus cannot be detected at birth; it only appears during the early postnatal period. However, a laminated pattern was revealed with rapid Golgi staining and retinal afferents were segregated into the appropriate laminae well before cytoarchitectonic lamination could be seen. Both observations indicate that the extracellular matrix may play a role in the separation of lateral geniculate nucleus cells into laminae. In the present study, the organization of the extracellular matrix was investigated during development using immunohistochemical and in situ hybridization techniques. For immunohistochemistry, peanut agglutinin (PNA) lectin and antibodies against tenascin (TN) were chosen, while for in situ hybridization, mTN riboprobes were used, simultaneously, with antibodies against Vimentin (Vim) and microtubule associated protein (MAP-2). The results showed that the pattern of PNA-binding glycoproteins and that of tenascin were relatively similar, although tenascin appeared later and disappeared earlier. The first interlaminar spaces to be detected were those between layers innervated by opposite eyes. The TN specific mRNA was detected in the lateral geniculate nucleus at P0, but was no longer visible at P7. By comparing TN mRNA and Vim or MAP-2 stainings a correspondence could be observed. The extracellular matrix lamination therefore seems to precede cytoarchitectonic lamination, suggesting that the extracellular matrix may play a role in the development of laminated structures. The TN-producing cells seem to be developing astrocytes and neurons. Accepted: 23 November 1998     

The unconditioned stimulus pre‐exposure effect in preweanling rats in taste aversion learning: Role of the training context and injection cues

The unconditioned stimulus pre‐exposure effect (US‐PE) refers to the interference paradigm in which acquisition of the conditioned response is retarded due to prior experience with the US. Most studies analyzing the psychological mechanisms underlying this effect have been conducted with adult rats. The most widely accepted hypothesis explains this effect as a contextual blocking effect. Contextual cues associated with the US block the conditioned stimulus (CS)‐US association during conditioning. The modulatory role of a context devoid of distinctive olfactory attributes is not observable until approximately PD23 in rats, including modulation of interference paradigms such as latent inhibition or extinction. In this study, we analyzed US‐PE in preweanling rats along with the role of the training context in this effect in terms of conditioned taste aversion preparation. Pre‐exposure to LiCl before conditioning retarded the acquisition of taste aversion. The US‐PE was observed in preweanling rats when, during pre‐exposure, subjects were exposed to the conditioning context, and this effect was not attenuated either by the administration of the US in a familiar environment (Experiment 1a), or by the presence of an alternative, more salient context during pre‐exposure (Experiment 1b). Additionally, the US‐PE was still observed when the route by which the US was administered was changed between the pre‐exposure and conditioning phases (Experiment 2a) as well as when the injection cues were removed during conditioning (Experiment 2b). These experiments show a strong US‐PE in preweanling rats and fail to support the contextual blocking hypothesis, at least in this stage of ontogeny. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 193–204, 2013