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The distribution of neurons in the medullary reticular formation (RF) activated by the ingestion of sucrose or rejection of quinine was examined using standard immunohistochemical techniques to detect the expression of the Fos protein product of the immediate-early gene c-fos. Double-labeling techniques were used to gain further insight into the possible functional significance of RF neurons exhibiting Fos-like immunoreactivity (FLI). Compared with sucrose and unstimulated controls, quinine elicited significantly more FLI neurons in three specific RF subdivisions: parvocellular reticular nucleus (PCRt), intermediate reticular nucleus (IRt), and dorsal medullary reticular nucleus (MdD). Moreover, the number of FLI neurons in the RF of quinine-stimulated animals was significantly correlated with the degree of oromotor activity. Thus, the distinct distribution of FLI neurons throughout the RF after quinine may reflect the activation of a specific oral rejection circuit. The double-labeling results indicated a high degree of segregation between FLI neurons and premotor projection neurons to the hypoglossal nucleus (mXII) retrogradely labeled with Fluorogold. Thus, although there were a significant number of double-labeled neurons in the RF, the major concentration of premotor projection neurons to mXII in IRt were medial to the preponderance of FLI neurons in the PCRt. In contrast, there was substantial overlap between FLI neurons in the RF and labeled fibers after injections of the anterograde tracer, biotinylated dextran into the rostral (gustatory) portion of the nucleus of the solitary tract. These results support a medial (premotor)/lateral (sensory) functional topography of the medullary RF. Key words: c-fos; medullary reticular formation; brainstem; ingestion; rejection; ratOne of the fundamental roles of gustation is to discriminate palatable from unpalatable, often toxic substances. In the rat (Woods, 1964;Grill and Norgren, 1978b) as well as other species (Steiner, 1973;Berntson and Micco, 1976), the underlying circuitry for this discrimination is located in the caudal brainstem, because decerebrate animals respond appropriately with stereotyped ingestion and rejection behaviors after gustatory stimulation. Although the location of the "switch" from ingestion to rejection is unknown, a role for the medullary reticular formation (RF) is suggested. Both the first-order central gustatory relay [the rostral nucleus of the solitary tract (NST)] and the second order gustatory relay [the parabrachial nucleus (PBN)] project to specific regions

The consummatory components of ingestion and rejection, organized in the caudal brainstem, include licking, swallowing, and the oral phase of rejection (gaping). Studies employing electrical-stimulation induced motor activity have localized interneurons controlling these complex motor patterns to the medullary reticular formation (RF), but the characteristics of these neurons during more naturally induced behavior are unknown. The purpose of the present study was to record the activity profiles of RF neurons during licking, swallowing, and oral rejection in response to gustatory stimulation. Two-hundred and two neurons recorded from awake, freely moving rats were broadly classified as orally related (67%) or non-orally related (33%). Orally related neurons included a large number that were rhythmically active during licking (n = 76; 38%). These "lick-rhythmic" neurons were widely distributed in the RF, but were concentrated in the caudal medullary reticular formation adjacent to the hypoglossal nucleus (Probst's region) and further rostral in the intermediate zone (IRt) of the RF. An analysis of autocorrelations determined that lick-rhythmic neurons in these regions were more closely coupled to licking than to lick-rhythmic neurons more lateral in the parvocellular RF (PCRt). In addition to neurons with weak lick-rhythmic activity, the PCRt also contained a disproportionate number of neurons with orosensory or mixed oro-sensorimotor properties. These data provide evidence for functional specialization within different regions of the medullary RF. A high proportion of lick-rhythmic neurons also showed differential activity associated with swallowing (41%) and/or gaping (75%), further suggesting that the different components of ingestion and rejection share brainstem substrates instead of being produced by unique subsets of interneurons.

Serotonin (5-HT) plays important regulatory roles in mammalian circadian timekeeping; however, little is known concerning the regulation of serotonergic activity in the circadian clock located in the suprachiasmatic nuclei (SCN). By using in vivo microdialysis to measure 5-HT release we demonstrated that electrical or pharmacological stimulations of the dorsal or median raphe nuclei (DRN and MRN, respectively) can alter basal release of 5-HT in the hamster SCN. There were similar increases in SCN 5-HT release after electrical stimulation of either the MRN or DRN, indicating that both could contribute to the serotonergic activity in the SCN. Systemic pretreatment with the 5-HT antagonist metergoline abolished DRN-induced SCN 5-HT release but had little effect on MRN-induced SCN 5-HT release, suggesting different pathways for these nuclei in regulating 5-HT output in the SCN. Microinjections of the 5-HT1A autoreceptor agonist 8-OH-DPAT or antagonist WAY 100635 into the MRN caused significant inhibition and stimulation of SCN 5-HT release, respectively. Both drugs had substantially less effect in the DRN. These differential drug actions indicate that somatodendritic 5-HT1A autoreceptors on MRN neurons provide the prominent raphe autoregulation of 5-HT output in the SCN. Collectively the current results are evidence that DRN as well as MRN neurons can contribute to the regulation of 5-HT release in the hamster SCN. On the basis of the current observations and those from recent anatomic tracing studies of serotonergic projections to SCN it is hypothesized that DRN input to the SCN could be mediated by a DRN --> MRN --> SCN pathway involving a 5-HT-sensitive multisynaptic interaction between the DRN and MRN neurons.

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1. The activity of 34 hypoglossal (mXII) neurons was characterized during the ingestion and rejection of gustatory stimuli in the awake rat. Intraoral infusions of water, sucrose, sodium chloride, or hydrochloric acid initiated ingestion responses; infusions of quinine monohydrochloride initiated rejection responses. Electromyographic (EMG) activity from three oropharyngeal muscles monitored the occurrence of lick cycles and swallows (ingestion) and gape cycles (rejection). In addition, the orofacial region was videotaped to provide an independent assessment of lingual and jaw movements in relation to neural activity. 2. EMG activity during lick and gape cycles was quantified by calculating the duration, magnitude, and peak time of muscle contractions. Lick and gape cycles produced highly differentiated patterns of activity from jaw-opener (anterior digastric, AD), lingual protrudor (geniohyoid, GEN), and lingual retractor (styloglossus, STY) muscles. Lick cycles were characterized by an alternating two-phase sequence of protrusion-retraction; gape cycles by an initial coactivation of both lingual muscles (phase I), followed by a sequence of protrusion (phase II) and retraction (phase III). Contraction durations were significantly longer during gape cycles compared with lick cycles for the AD (Xlick +/- 59 ms; Xgape +/- 134 ms, means +/- SD), GEN (Xlick +/- 77 ms; Xgape +/- 200 ms), and STY (Xlick +/- 93 ms; Xgape +/- 220 ms) muscles. 3. Thirty-one out of 34 mXII neurons were functionally classified as protrudor- or retractor-related by cross-correlating anterior digastric EMG activity with neural activity during licking. Fourteen out of 34 neurons were protrudor-related, 17/34 were retractor-related. These classifications were largely consistent with the results from an analysis of a subset of cells (n = 14) that directly compared neural activity with videotaped records of visible tongue movements. 4. The magnitude of mXII activity during ingestion and rejection was compared by determining the mean number of spikes per lick, gape, and swallow for each neuron. Five out of 14 (36%) protrudor-related and 10/17 (59%) retractor-related cells had significant increases in activity during gape responses compared with the number of spikes per lick cycle. This increased activity of mXII neurons was consistent with the more robust lingual motor activity during the gape response. Two protrudor-related and three retractor-related neurons showed significant decreases in activity during gape responses. Although a similar proportion of mXII neurons exhibited decreases in activity during swallows compared with licks (3 protrudor- and 1 retractor-related), fewer mXII neurons (1 protrudor- and 1 retractor-related) showed increased activity during swallows.(ABSTRACT TRUNCATED AT 400 WORDS)

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