Tyrosine hydroxylase-like and dopamine β-hydroxylase-like immunoreactivity in the gustatory zone of the nucleus of the solitary tract in the hamster: Light- and electron-microscopic studies

Abstract A quantitative electron-microscopic analysis has been conducted on the neurons within the gustatory zone of the nucleus of the solitary tract of the hamster. The most common group of neurons within the gustatory zone contains both large (X1) and small (X3) members that possess deeply invaginated nuclear profiles. These neurons have somal areas that average 113μm 2 (range 34–281 μm 2 ) and a value of somal area/nuclear area that averages 2.2. Other large and small neurons that have non-invaginated nuclear profiles are also observed. The larger (X2) neurons average 151μm 2 (range 49–487μm 2 ) and have much cytoplasm and associated membranous organelles that is reflected in a mean value of somal area/nuclear area of 2.6. Members of the X2 group are the largest neurons in the gustatory zone. The smaller (X4) group contains the smallest neurons in the gustatory zone of the nucleus of the solitary tract, averages 5μm 2 (range 16–103μm 2 ), shows almost no perinuclear cytoplasm and has a mean value of somal area/nuclear area of only 1.5. These findings are consistent with and expand upon the results of similar studies at the light-microscopic level. This grouping has been used to explore the association of tyrosine hyroxylase-like and dopamine β-hydroxylase-like immunoreactivities with specific populations of neurons that are known to be distributed across the various levels of the gustatory zone. At the light-microscopic level, numerous well-defined and intensely labelled tyrosine hydroxylase-like immunoreactive somata of various morphologies and sizes are observed. Quantification at the electron-microscopic level indicates that 10–15% of the neurons encountered in the dorsal and intermediate levels of the gustatory zone are immunoreactive. The ventral level of the gustatory zone contains few immunoreactive neurons. Tyrosine hydroxylase-like immunoreactive neurons possess either non-invaginated or invaginated nuclear profiles and their somal areas average 106 and 142μm 2 , respectively. On the bases of size and ultrastructural features, these immunoreactive somata are assigned to the two groups (X1 and X2) of large neurons within the gustatory portion of the nucleus of the solitary tract. In general, small neurons are not immunoreactive. The distribution of dopamine β-hydroxylase-like immunoreactivity has also been examined in adjacent sections in order to reveal the presence of any putative noradrenergic neurons in the gustatory zone. Such immunoreactive neurons are uncommon, implying a low level of conversion of dopamine to norepinephrine. These results are compared to those from the rat because the locations of catecholamine cell groups in the rat's brainstem have been well documented. The rat expresses fewer tyrosine hydroxylase-like immunoreactive neurons in the gustatory zone than the hamster but similar and also low numbers of dopamine β -hydroxylase-positive neurons. In the rat and hamster, the caudal portion of the nucleus of the solitary tract that is included in the A2 noradrenergic cell group contains a substantial number of tyrosine hydroxylase-like immunoreactive somata and more modest numbers of dopamine β -hydroxylase-like immunoreactive somata. Caudally, both types of immunoreactivities are greater than that seen in the gustatory zone and are greater in the rat than in the hamster. These studies quantify for the first time in the hamster the ultrastructural features of morphologically distinct groups of neurons that are in a position to process peripheral gustatory activity. In addition, a unique distribution of tyrosine hydroxylase-like immunoreactive somata in the gustatory zone of the nucleus of the solitary that may reflect dopamine has been associated with particular groups of neurons. This population of putative dopaminergic neurons is more substantial in the hamster than in the rat and is located in a portion of the nucleus of the solitary tract that is not included in the A2 catecholamine cell group of the dorsomedial medulla. The finding that putative dopaminergic neurons are located in the rostral pole of the nucleus of the solitary tract, together with previous findings that many neurons of the gustatory zone have intrinsic caudal projections and that the caudal part of the nucleus also contains dopaminergic neurons, suggests interactions between adjacent regions concerned primarily with ingestional and autonomic functions.