The ascending gustatory pathway: a Golgi analysis of the medial and lateral parabrachial complex in the adult hamster

Gustatory neural circuitry in the hamster brain stem

The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first and second central relays for the taste pathway, respectively. Taste neurons in the NST project to the PbN, which further transmits taste information to the rostral taste centers. Nevertheless, details of the neural connections among the brain stem gustatory nuclei are obscure. Here, we investigated these relationships in the hamster brain stem. Three electrode assemblies were used to record the activity of taste neurons extracellularly and then to electrically stimulate these same areas in the order: left PbN, right PbN, and right NST. A fourth electrode, a glass micropipette, was used to record from gustatory cells in the left NST. Results showed extensive bilateral communication between brain stem nuclei at the same level: 1) 10% of 96 NST neurons projected to the contralateral NST and 58% received synaptic input from the contralateral NST; and 2) 12% of 43 PbN neurons projected to the contralateral PbN and 21% received synaptic input from the contralateral PbN. Results also showed extensive communication between levels: 1) as expected, the majority of 119 NST neurons, 82%, projected to the ipsilateral PbN, but 85% of the 20 NST neurons tested received synaptic input from the ipsilateral PbN, as did 59% of 22 NST neurons that did not project to the PbN; and 2) although few, 3%, of 119 NST cells projected to the contralateral PbN and 38% received synaptic input from the contralateral PbN. These results demonstrated that taste neurons in the NST not only project to, but also receive descending input from the bilateral PbN and that gustatory neurons in the NST and PbN also communicate with the corresponding nucleus on the contralateral side.

Stimulation of Specific Regions of the Parabrachial Nucleus Elicits Ingestive Oromotor Behaviors in Conscious Rats

The "waist" area and external subnuclei of the parabrachial nucleus (PBN) have been implicated in the processing of gustatory information, yet their behavioral roles are not clearly defined. In the current study, areas within and surrounding the PBN were stimulated while oromotor behaviors were monitored in conscious rats. Electrical and chemical (100 mM glutamate) stimulation of the waist area increased ingestive oromotor behaviors over baseline (p<.01). Stimulation of external PBN subnuclei and areas medial and ventral to the PBN failed to cause a behavioral change. These data support the hypothesis that the waist area of the PBN constitutes part of the neural substrate involved in eliciting oromotor behaviors in response to taste input.

The cytoarchitecture of the adult human parabrachial nucleus: a Nissl and Golgi study

The parabrachial nucleus (PBN) plays important roles in numerous autonomic functions and in pain modulation. In different animal species, three main regions of the PBN have been identified: the m-PB, the l-PB, and the Kolliker-Fuse nucleus (KF). The KF has not been identified in humans. The present study used Nissl and Golgi-Cox material and morphoquantitative methods to investigate the cytoarchitectural organization of the adult human PBN, paying particular attention to neuronal features endowed with functional significance, i. e. the arborization of the neurons. The PBN neuron population is made up of elements which are heterogeneous in size, shape and dendritic arborization, and grouped into two regions, the lateral and medial PBN (l- and m-PB). It has been suggested that some large sized neurons located in the ventral region of the m-PB might be the counterpart of the KF. In the m-PB the fusiform neurons are the most numerous cells; in the l-PB the multipolar neurons prevail, and are particularly numerous in the dorsal l-PB. Since the dendritic arborization is generally the main target of afferent projections to a neuron, it is possible that the l-PB, and in particular its dorsal region, might be the main site for the endings of afferences to the human PBN.

The neural code for taste in the brain stem: response profiles

In the study of the neural code for taste, two theories have dominated the literature: the across neuron pattern (ANP), and the labeled line theories. Both of these theories are based on the observations that taste cells are multisensitive across a variety of different taste stimuli. Given a fixed array of taste stimuli, a cell's particular set of sensitivities defines its response profile. The characteristics of response profiles are the basis of both major theories of coding. In reviewing the literature, it is apparent that response profiles are an expression of a complex interplay of excitatory and inhibitory inputs that derive from cells with a wide variety of sensitivity patterns. These observations suggest that, in the absence of inhibition, taste cells might be potentially responsive to all taste stimuli. Several studies also suggest that response profiles can be influenced by the taste context, defined as the taste stimulus presented just before or simultaneously with another, under which they are recorded. A theory, called dynamic coding, was proposed to account for context dependency of taste response profiles. In this theory, those cells that are unaffected by taste context would provide the signal, i.e., the information-containing portion of the ANP, and those cells whose responses are context dependent would provide noise, i.e., less stimulus specific information. When singular taste stimuli are presented, noise cells would provide amplification of the signal, and when complex mixtures are presented, the responses of the noise cells would be suppressed (depending on the particular combination of tastants), and the ratio of signal to noise would be enhanced.

Differences in the intrinsic membrane characteristics of parabrachial nucleus neurons processing gustatory and visceral information

Whole-cell current-clamp recordings were made from neurons in the rat parabrachial nucleus (PBN) in three rostro-caudal brain slices. During recording the neurons were located in one of four quadrants of the PBN. Successful recordings were obtained from neurons in three of these quadrants termed the dorsolateral (DL), dorsomedial (DM) and ventromedial (VM) quadrants. Recordings were made of the intrinsic membrane properties and repetitive discharge characteristics of 58 neurons in the DL, 60 neurons in the DM, and 54 neurons in the VM-quadrants. The input resistance of the neurons in the DL quadrant was significantly lower and the membrane time constant significantly shorter than that of the neurons in the DM- and VM-quadrants. The mean action potential duration of the VM-quadrant neurons was significantly longer than that of both DL- and DM-quadrant neurons. The discharge frequency in response to a 1500 ms 100 pA current pulse of the DL quadrant neurons was significantly lower than that of the neurons in the other two quadrants. The latency of action potential initiation following a 100 pA depolarizing current pulse was significantly longer for DL quadrant neurons compared to neurons in the other two quadrants. Neurons were divided into groups based on their response to a long depolarizing current pulse immediately preceded by a hyperpolarizing current pulse. In all three rostro-caudal slices of the PBN, the largest populations of neurons were in Group II and Group III. The results demonstrate that neurons in different locations in the PBN have different membrane and repetitive discharge properties. These different PBN locations receive inputs from the visceral and gustatory regions of the NST. It is possible therefore that the differences in properties of the PBN neurons may relate to the type of sensory information that they process.

Morphology and dendritic domains of neurons in the lateral parabrachial nucleus of the rat

The present study provides a description of the dendritic morphology and the dendritic domains of neurons in the lateral parabrachial nucleus (PB) of the rat. The cells were intracellularly stained in vitro with Lucifer yellow. A subpopulation of these cells was characterized beforehand as neurons projecting to the amygdaloid complex by retrograde transport with rhodamine beads. With respect to their dendritic arborization, different types of "spatially" organized PB neurons were discriminated. One major cell type in the external lateral PB (PBel) is characterized by long, elongated dendritic trees that are preferentially oriented parallel to the superior cerebellar peduncle. The majority of their dendrites appears to respect subnuclear boundaries, yet their distal dendrites often exceed the limits of the PBel to encroach upon adjacent subnuclei located dorsally and ventrolaterally to the PBel. Another prominent cell type in the PBel has fairly small and locally restricted dendritic trees that are also elongated, running with their main axis from ventrolateral to dorsomedial. The dendrites of the majority of these neurons apparently stay within the confines of the PBel. A distinct group of neurons is found in the ventral portion of the PBel. The majority of their dendrites is mediolaterally oriented and not confined to the PBel subnucleus. In addition, we found a smaller number of neurons scattered within the lateral PB whose dendrites do not show a preferential orientation but travel across subnuclear boundaries into several different PB subnuclei. Our data show that the dendrites of a large proportion of neurons in the lateral PB either stay within the confines of a particular subnucleus or slightly extend across subnuclear limits. In any case, they appear to match with terminal territories of afferent axons and, thus, maintain the functional specificity of inputs by their relay through the PB. In contrast, PB neurons that extend their dendrites across subnuclear boundaries or known terminal territories are likely to receive inputs of different qualities from a variety of sources and therefore transmit a more general, integrated signal to the forebrain.

Gustatory function in the parabrachial nuclei: implications from lesion studies in rats

In rodents, third order gustatory neurons reside in the parabrachial nuclei of the dorsal pons. Lesions in this area of the brain have a variety of consequences on taste-related behaviors. Some behaviors are severely impaired, such as the expression of either conditioned taste aversion or depletion-induced sodium appetite. Other taste-based behaviors are less affected or not influenced at all. Although the lesion-behavior approach possesses serious methodological limitations, the constellation of findings from studies employing this experimental strategy in the PBN has promising implications. Foremost among these is the suggestion that the neural circuitry subserving performance in some of these taste-guided behavioral paradigms is dissociable. This paper critically reviews this body of behavioral research and discusses the conceptual ramifications of the results.