Distribution of Fos-immunoreactive neurons in the gustatory cortex elicited by intra-oral infusion of taste solutions in conscious rats

Abstract

The location of neurons in the gustatory cortex (GC) activated by intra-oral infusion of solutions in conscious rats was mapped using Fos immunohistochemistry. Groups of adult male Wistar rats (N's = 5) received an infusion of one of the following: dH₂O, 0.1 or 1.0 M NaCl, 0.1 or 1.0 M sucrose, 0.32 M MSG (with 100 µM amiloride and 2.5 M inosine 5'-monophosphate), 0.03 M HCl, or 0.003 M QHCl delivered via an intra-oral cannula (0.233 ml/min for 5 min). Unstimulated control rats received no infusion. Taste reactivity (TR) behaviors were videotaped and scored. The number of Fos-immunoreactive (Fos-IR) neurons was counted in eight sections throughout the anterior-posterior extent of the GC in the medial and lateral halves of the granular (GI), dysgranular (DI), and dorsal (AID) and ventral (AIV) agranular insular cortices. Intra-oral infusion of dH₂O, NaCl, or sucrose altered the number of Fos-IR neurons in only specific subareas of the GC and the effects of these tastants were concentration-dependent. For example, 1.0 M NaCl increased Fos-IR neurons in the posterior lateral AID and DI and elicited more aversive TR responses than 0.1 M NaCl. Compared to dH₂O, infusions of HCl or QHCl increased the total number of Fos-IR neurons in many subareas of the GC throughout its anterior-posterior extent and increased aversive TR behaviors. Linear regression analyses suggested that neurons in the medial AID of the posterior GC may influence aversive behavioral responses to HCl and QHCl while neurons in the posterior lateral AID and DI may play a role in aversive TR responses to 1.0 M NaCl.

Keywords: Behavior; Bitter; Gustatory; Immunohistochemistry; Insular cortex; Taste stimuli.

Fig. 1

Images of coronal sections through the middle of the gustatory cortex (~0.7 mm anterior to bregma). A. Nissl-stained section outlining the granular (GI), dysgranular (DI), and dorsal and ventral agranular (AID and AIV) insular cortices. Each subarea is split into medial and lateral halves. B.-D. Fos-stained section of the same region from rats that received intra-oral infusion of dH₂O (B), HCl (C), or QHCl (D). Other abbreviations are: Cl, claustrum; ec, external capsule, Pir, piriform cortex, and rf, rhinal fissure.

Fig. 2

Graph of the mean total number of Fos-IR neurons in the whole GC for each treatment group. # indicates a significant difference from all other treatments except dH₂O and QHCl, while * indicates a significant difference from all other treatments except HCl (p’s<0.02).

Fig. 3

Graphs of the mean number of Fos-IR neurons in the medial and lateral halves of each subarea in each of the eight sections examined along the anterior-posterior axis of the GC in unstimulated (none) control rats (A) as well as in rats that received an intra-oral infusion of dH₂O (B), 0.1M sucrose (C), 1.0M sucrose (D), 0.1M NaCl (E), and 1.0M NaCl (F). A circle on the top front face or top edge of a bar indicates a significant difference from the corresponding bar on the adjacent graph (comparing dH₂O to none, 0.1M sucrose to 1.0M sucrose, and 0.1M NaCl to 1.0M NaCl). An asterisk (*) on a bar indicates a significant difference (p<0.05) from dH₂O.

Fig. 4

Graphs of the mean number of Fos-IR neurons in the medial and lateral halves of each subarea in each of the eight sections examined along the anterior-posterior axis of the GC in rats that received an intra-oral infusion of dH₂O (A), 0.32M MSG plus 100μM amiloride and 2.5 mM IMP (B), 0.03M HCl (C), and 0.003M QHCl (D). An asterisk (*) on the top front face or top edge of a bar indicates a significant difference (p<0.05) from dH₂O.

Fig. 5

Graphs of the mean number of Fos-IR neurons in different anterior-posterior regions of the lateral (A) and medial (B) halves of the GI subarea of the GC. Data in the ‘anterior 4’ group were from sections 1-4 and the ‘posterior 4’ group includes data from sections 5–8. Data in the ‘middle 4’ group were collected from sections 3–6. Each triplet of bars represents the results for the different treatment groups indicated along the x-axes. An asterisk (*) above a bar indicates a significant difference (p<0.05) from the corresponding bar for the control group (none) while a ‘w’ indicates a difference from the dH₂O group. New abbreviations are: LS (0.1M sucrose), HS (1.0M sucrose), LN (0.1M NaCl), and HN (1.0M NaCl).

Fig. 6

Graphs of the mean number of Fos-IR neurons in different anterior-posterior regions of the lateral (A) and medial (B) halves of the DI subarea of the GC. Data were combined as described in Methods and the caption for Fig. 5. Statistics are indicated as in Fig. 5 with the addition of brackets joining two bars from above indicating a significant different between those bars. Abbreviations are as in Fig. 5.

Fig. 7

Graphs of the mean number of Fos-IR neurons in different anterior-posterior regions of the lateral (A) and medial (B) halves of the AID subarea of the GC. Data were combined as described in Methods and the caption for Fig. 5. Statistics and abbreviations are indicated as in Figs. 5 and 6.

Fig. 8

Graphs of the mean number of Fos-IR neurons in different anterior-posterior regions of the lateral (A) and medial (B) halves of the AIV subarea of the GC. Data were combined as described in Methods and the caption for Fig. 5. Statistics and abbreviations are indicated as in Figs. 5 and 6.

Fig. 9

Graph of the mean number of ingestive and aversive taste reactivity (TR) behaviors performed during the 5 min behavioral trial in the control group (none) as well as in rats receiving an intra-oral infusion of dH₂O and each taste solution. An asterisk (*) above a bar indicates a significant difference from all other groups (p<0.05), while the other symbols indicate a difference from the rats that received an intra-oral infusion of dH₂O (w), 0.1M sucrose (LS), and 0.1M NaCl (LN).

Fig. 10

Results of linear regression analyses of the relationship between the mean number of Fos-IR neurons in a GC subarea within 4 sections along the anterior-posterior axis of the GC and the number of taste reactivity (TR) behaviors performed. A. The relationship between the number of Fos-IR neurons in the posterior four sections of the lateral AID subarea and the number of ingestive TR behaviors performed in rats that received intra-oral infusion of dH₂O. B. The relationship between the number of Fos-IR neurons in the anterior four sections of the medial AIV subarea and the number of aversive TR behaviors performed in rats that received intra-oral infusion of 0.1M and 1.0M sucrose. (The 4 data points above 10 aversive behaviors represent 4 of the 5 responses to 1.0M sucrose.) C. The relationship between the number of Fos-IR neurons in the posterior four sections of the lateral DI subarea and the number of aversive TR behaviors performed in rats that received intra-oral infusion of 0.1M and 1.0M NaCl. (The 5 data points above 20 aversive behaviors represent responses to 1.0M NaCl.) D. The relationship between the number of Fos-IR neurons in the posterior four sections of the medial AID subarea and the number of aversive TR behaviors performed in rats that received intra-oral infusion of 0.03M HCl and 0.003M QHCl. (The 5 data points above 90 aversive behaviors represent responses to QHCl.)