A subset of recorded cells was labeled with neurobiotin to observe their morphology, axonal projection patterns, and neurotransmitter distribution. Most of the neurobiotin-labeled neurons (ten out of 12) had widely branched neuropils near the surface of the dorsal
telencephalon (Figures 4F and 4H). Some axons diverged from these neuropils and grew toward the dorsal nucleus of the ventral telencephalic area (Vd) (Figures 4F–4I), which may correspond to the mammalian striatum by the expression of genetic markers (Figures S4F–S4J; Mueller et al., 2008; Mueller and Wullimann, 2009). Most of these neurons (nine out of 10) also had projections directed toward the slightly more posteriorly located dorsal part of the entopeduncular nucleus (ENd) that may be homologous to the primate globus pallidus (Figures 4H and 4J) and, in some INK1197 ic50 cases, this projection did not terminate at the ENd but entered into the anterior commissure (AC) (data not shown). The dorsal part of the telencephalon contains numerous glutamatergic neurons and sparse GABAergic neurons, whereas neurons in the ventral part of the telencephalon are http://www.selleckchem.com/HIF.html mainly GABAergic (Figure S4E). Two-color in situ hybridization to the labeled neurons
with vglut1/2.1/2.2 and gad65/67 revealed mostly glutamatergic neurons ( Figure 4D, vglut1/2.1/2.2 n = 5, gad65/67 n = 0, neither n = 2). In a few cases (two out of 12 cells), the labeled neurons showed less the developed neuropils without clear long projections (data not shown) with no particular relationship between these neurons and electrophysiological features. Altogether, these findings indicate that glutamatergic afferents from the activated area project to putative striatum (Figure 4E). To challenge the fixed pattern of telencephalic activity in response to the cue presentation, we changed the behavioral rule once fish had learned the original active avoidance paradigm. In this alternate paradigm, fish must remain in the initial compartment during the cue presentation to avoid the electric shock, instead of swimming to the opposite compartment. We named this modified paradigm the “stay task” and the original
paradigm as the “avoidance task” (Figure 5A1). We were particularly interested in testing whether the pattern of telencephalic neural activity observed during the avoidance task represented simple motor commands or encoded the appropriate behavioral program for active avoidance. The former possibility would predict disappearance of the activity once fish were retrained to stay still after cue presentation in the stay task. Learner fish trained for the avoidance task on the first day were tested for retrieval of the avoidance response on the next day. After a 20 min resting period, the same fish were further trained for the stay task (Figure 5A2). By the third session of the stay task, the rate of trials in which the fish stayed in the initial compartment reached over 80% (Figure 5B, stay success rate).