Dennis L. Glanzman

Direct connectivity between pontine taste areas and gustatory neocortex in rat

Horseradish peroxidase histochemistry was used to determine the course and extent of neuronal projections from the pontine taste area (PTA) to the gustatory neocortex (GN) in rat. Two distinct findings were encountered: (1) thalamocortical projections from posterior ventromedial thalamus to GN were confirmed, and (2) direct projections from cells located in the PTA to the GN were described. This novel anatomical finding supports previous suggestions that some gustatory information may be relayed to forebrain areas without making synaptic connection in the diencephalon.

Cortical substrates of taste aversion learning: involvement of dorsolateral amygdaloid nuclei and temporal neocortex in taste aversion learning.

The amygdaloid complex is functionally implicated in conditioned taste aversion (CTA) learning. Results of previous neurobehavioral studies have provided equivocal evidence concerning the involvement of specific amygdaloid nuclei in CTA learning. The present study was conducted to examine the involvement of the central (CE), lateral (LA), and basolateral (BL) amygdaloid nuclei and the temporal neocortices (area 20) in CTA learning. To that end, distinct groups of rats received bilateral electrolytic lesion placements in the CE, LA, BL, or the temporal neocortices. Control animals received scalp and meningeal incisions only. Following recovery, animals were habituated to a restricted drinking schedule with distilled water. Animals then received CTA conditioning, with LiCl used both as the conditioned stimulus and as the unconditioned stimulus. Anterograde degeneration histologies were performed on all brain tissue to evaluate relations between CTA learning deficits and axonal pathology induced by lesion placements. Results of behavioral manipulations indicated that destruction of the CE, LA, or temporal neocortex impaired CTA acquisition, but damage induced to the basolateral amygdaloid nucleus did not. Anatomical observations indicated that degeneration of amygdalofugal and/or corticofugal projections to the convolutions of the olfactory tubercle (medial), subthalamic nucleus, and the parabrachial complex is correlated with CTA learning deficits. These results indicate that destruction of the dorsolateral amygdaloid nuclei and/or the temporal neocortices may produce CTA learning deficits by affecting olfactory, gustatory, and/or gastrointestinal processing in various portions of the forebrain.

Axon collaterals of pontine taste area neurons project to the posterior ventromedial thalamic nucleus and to the gustatory neocortex

Retrograde axonal transport of fluorescent dyes was used to demonstrate collateral projections from neurons of the pontine taste area (PTA) to gustatory-responsive areas of the posterior ventromedial thalamic nucleus (VPM), and to the gustatory neocortex (GN) of the rat. Dual-labeled PTA neurons were reliably observed following application of two different fluorescent dyes to the GN and to VPM thalamus. Dye injections into the GN and into thalamic regions surrounding the VPM nucleus, the bed nucleus of stria terminalis or the infralimbic neocortex, did not result in dual-labeled cells within the PTA. This finding suggests that gustatory information may be relayed simultaneously and specifically to VPM thalamus and to the GN via collateral axons of PTA neurons.

Thalamocortical relations in taste aversion learning:I.Involvement of gustatory thalamocortical projections in taste aversion learning

The anterior insular gustatory neocortex (AIGN) has been implicated as a functional substrate of conditioned taste aversion (CTA) learning. Results of previous neuroanatomical and neurobehavioral experiments indicate that projections from gustatory-responsive neurons in the posterior ventromedial thalamic nuclei (parvicellular division; VPMpc) may provide relevant input to the AIGN during CTA learning. In rat, gustatory thalamocortical projections from VPMpc thalamus traverse the ventrolateral neostriatum (VLS) enroute to the AIGN. In these experiments, various neuroanatomical and neurobehavioral manipulations in the VLS were used to examine the contribution of presumed gustatory thalamocortical projections to CTA learning. These experiments demonstrate that projections from VPMpc thalamus to the AIGN are essential for normal CTA learning. Because both VPMpc thalamus and the AIGN each have been implicated as functional substrates of CTA learning, the present results suggest that the gustatory thalamocortical relay per se is necessary for normal taste-illness learning.

Alterations in spontaneous miniature potential activity during habituation of a vertebrate monosynaptic pathway

Intracellular recordings of spontaneous miniature synaptic potentials were made from motoneurons of the isolated spinal cord of the frog, as a function of habituation of the monosynaptic pathway originating with lateral column fibers. The frequency and amplitude of spontaneous miniature potentials were used to assess the possibility of several proposed mechanisms underlying habituation in this system. These studies provide clear evidence in eliminating the role of transmitter depletion, incomplete vesicle filling and receptor desensitization in the habituation process occurring within this vertebrate central nervous system.

Habituation of the nictitating membrane reflex response in the intact frog

Abstract Habituation of the nictitating membrane reflex response was measured in the intact bullfrog. A stimulus map was created, and stimulating electrode pairs directly opposed across the eye proved to be the optimal loci for cutaneous electrical stimulation. The response was found to exhibit all of the parametric characteristics of habituation. To date, this represents what is perhaps the simplest intact vertebrate preparation demonstrating habituation. The response was also found to exhibit a surprising periodicity in response amplitude when unchanging stimuli were infrequently presented over long periods of time. The convenience, reliability and successful behavioral assessment strongly recommend this preparation as an ideal simple vertebrate model system for further examination of the neural basis of behavioral plasticity.

Disruption of vertebrate monosynaptic habituation by ethyl alcohol

The effects of ethyl alcohol on neuronal plasticity were examined using a model systems preparation of monosynaptic habituation: the lateral column to motoneuron pathway of the isolated frog spinal cord. Two fundamental effects of alcohol on this system were demonstrated. First, alcohol always depressed the ventral root responses to lateral column stimulation when stimulus intensity was held constant across conditions. This was accompanied by an increase in the amount of habituation produced as training progressed. Second, when stimulus intensity was increased in the alcohol condition so that responses were equated with pre-alcohol response amplitudes, the amount of habituation was markedly decreased. Results are discussed both in terms of proposed mechanisms of habituation and of possible pharmacological interactions with this simple example of response plasticity.

Ethyl alcohol iphibits habituation of the nictitating membrane reflex response in the intact frog

Habituation of the nictitating membrane reflex response in the intact frog was examined under conditions of various alcohol doses, administered by whole-body immersion into dilute alcohol solutions. Alcohol both markedly reduced the initial response amplitudes and severely disrupted the habituation process in a dose-dependent manner. A theoretical discussion is presented in which the effects of alcohol on habituation in other systems, and the interaction of alcohol with known neural processes, are examined. The results and conclusions are consistent with other reports of presynaptic inhibition modulating habituation at an invertebrate central synapse.